Close
About
FAQ
Home
Collections
Login
USC Login
Register
0
Selected
Invert selection
Deselect all
Deselect all
Click here to refresh results
Click here to refresh results
USC
/
Digital Library
/
University of Southern California Dissertations and Theses
/
Low temperature synthesis of functional metal oxide nanocrystals using a vapor diffusion sol-gel method
(USC Thesis Other)
Low temperature synthesis of functional metal oxide nanocrystals using a vapor diffusion sol-gel method
PDF
Download
Share
Open document
Flip pages
Contact Us
Contact Us
Copy asset link
Request this asset
Transcript (if available)
Content
LOW TEMPERATURE SYNTHESIS OF FUNCTIONAL METAL OXIDE
NANOCRYSTALS USING A VAPOR DIFFUSION SOL–GEL METHOD
by
Sean P. Culver
__________________________________________________________________
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
December 2016
Copyright 2016 Sean P. Culver
ii
Acknowledgements
The completion of this thesis would not have been possible without the assistance
of so many generous individuals whose names I may not touch upon, however their
contributions are very much appreciated.
First and foremost, I would like to thank my mother and father, Patty and Ray
Culver, who provided me with unwavering love and support over the last five years.
There is no way that I would have attained this accomplishment without them.
I would like to sincerely thank Prof. Dr. Mark Thompson, Prof. Dr. Smaranda
Marinescu, Prof. Dr. Aiichiro Nakano, and Prof. Dr. Brent Melot for taking the time to be
on my committee and for providing guidance over the years. I would also like to thank
the entirety of the Brutchey group (i.e., Dr. Matthew Greaney, Haipeng Lu, Blair Combs,
Carrie McCarthy, Gozde Barim, Emily Roberts, Dr. Patrick Cottingham, Dr. David
Webber, John Lee, and Antonio Tinoco) for their constant support throughout my
graduate studies and for some incredible times both inside and outside of lab.
I would especially like to thank Prof. Dr. Federico Rabuffetti for mentoring me
both as a scientist and as an individual. To say that I would not have learned nearly as
much as I did without him would be a gross understatement.
iii
Finally, I would like to thank Prof. Dr. Richard Brutchey who truly made this
possible. Richard has developed an ideal environment for both scientific and personal
growth, and I am grateful for having been a part of it. Over the last five years Richard
was not just my boss, but also a great friend and I cannot fully express my appreciation
for everything he has taught me and done for me along the way. Thank you, Richard.
Thank you all for everything.
iv
Table of Contents
Acknowledgements ii
List of Tables vii
List of Figures
Abstract
viii
xii
Chapter 1. Compositionally Dependent Surface Chemistry of Colloidal
Ba
x
Sr
1-x
TiO
3
Perovskite Nanocrystals
1.1. Introduction
1.2. Results and Discussion
1.3. Experimental
1.3.1. General Considerations
1.3.2. Apparatus
1.3.3. Synthesis of Ba
1-x
Sr
x
TiO
3
nanocrystals
1.3.4. Material Characterization
1.3.5. Surface Characterization (ssNMR/FT-IR) of the
Nanocrystals Dosed with CO
2
1.3.6. CO
2
Temperature Programmed Desorption (TPD)
1.4. References
1
1
1
2
12
12
12
12
13
14
14
15
Chapter 2. Low Temperature Synthesis and Characterization of Lanthanide-
Doped BaTiO
3
Nanocrystals
2.1. Abstract
2.2. Introduction
2.3. Results and Discussion
2.4. Experimental
2.4.1. Nanocrystal Synthesis
2.4.2. Materials Characterization
2.5. Conclusions
2.6. References
Chapter 3. Surface Modification of BaTiO3 Inclusions in
Polydicyclopentadiene Nanocomposites for Energy Storage
3.1. Abstract
3.2. Introduction
3.3. Results and Discussion
17
17
17
18
27
27
28
29
30
32
32
32
34
v
3.3.1. BaTiO3 Nanocrystal Surface Modification
3.3.2. Nanocomposite Preparation and Physical Properties
3.3.3. Nanocomposite Dielectric Properties
3.4. Experimental
3.4.1. General Considerations
3.4.2. Surface Modification of BaTiO3 Nanocrystals
3.4.3. Material Characterization
3.4.4. Nanocomposite Preparation
3.4.5. Nanocomposite Characterization
3.5. Conclusions
3.6. References
Chapter 4. Low Temperature Synthesis of AMoO4 (A = Ca, Sr, Ba)
Scheelite Nanocrystal
4.1. Abstract
4.2. Introduction
4.3. Results and Discussion
4.4. Experimental
4.4.1. Nanocrystal Synthesis
4.4.2. Material Characterization
4.5. Conclusions
4.6. References
Chapter 5. Low Temperature Synthesis of Homogeneous Solid Solutions
of Scheelite-Structured Ca
1-x
Sr
x
WO
4
and Sr
1-x
Ba
x
WO
4
Nanocrystals
5.1. Abstract
5.2. Introduction
5.3. Results and Discussion
5.4. Experimental
5.4.1. General Considerations
5.4.2. Nanocrystal Synthesis
5.4.3. Material Characterization
5.5. Conclusions
5.6. References
Chapter 6. Thermally Activated Rotational Disorder in CaMoO4
Nanocrystals
6.1. Abstract
6.2. Introduction
6.3. Results and Discussion
6.4. Experimental
6.4.1. Nanocrystal Synthesis
34
36
38
41
41
41
41
42
43
44
45
47
47
47
48
60
60
61
63
64
66
66
66
69
78
78
78
79
81
82
84
84
84
86
124
124
vi
6.4.2. Material Characterization
6.5. Conclusions
6.6. References
Chapter 7. Lanthanide-Activated CaWO
4
Nanocrystal Phosphors by the
Low-Temperature Vapor Diffusion Sol–Gel Method
7.1. Abstract
7.2. Introduction
7.3. Results and Discussion
7.4. Experimental
7.4.1. General Considerations
7.4.2. Nanocrystal Synthesis
7.4.3 Material Characterization
7.5. Conclusions
7.6. References
Bibliography
125
126
127
129
129
129
131
144
144
144
145
146
147
149
vii
List of Tables
Table 1.1: Physical features of the Ba
1–x
Sr
x
TiO
3
nanocrystals before and
after calcination
4
Table 4.1: Rietveld analysis of XRD data of AMoO
4
nanocrystals 54
Table 5.1: Rietveld Analysis of XRD Data for AWO
4
Nanocrystals 72
Table 6.1: Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K
Extracted From Rietveld Analysis
106
Table 6.2: Structural Paramters of CaMoO
4
Nanocrystals From 90–480 K
Extracted From PDF Analysis
Table 7.1: Structural Parameters of As-synthesized Ln-Doped CaWO
4
Nanocrystals
Table 7.2: Structural Parameters of Calcined Ln-Doped CaWO
4
Nanocrystals
113
135
136
viii
List of Figures
Figure 1.1: Powder XRD of the as-prepared perovskite nanocrystals. 3
Figure 1.2: TEM images of the as-prepared perovskite nanocrystals. 3
Figure 1.3: TGA/DSC-MS of the as-prepared perovskite nanocrystals run at
a heating rate of 10 ˚C min
–1
under flowing Ar.
4
Figure 1.4: FT-IR spectra of the as-prepared and calcined perovskite
nanocrystals with material diluted in KBr (1/10 vol/vol).
5
Figure 1.5: A plot of n(O–H) stretching frequency as a function of %Ba for
Ba
1–x
Sr
x
TiO
3
nanocrystals.
6
Figure 1.6: FT-IR spectra of CO
2
adsorbed on the calcined perovskite
nanocrystals.
8
Figure 1.7: Low-temperature CO
2
TPD of the calcined Ba
1–x
Sr
x
TiO
3
nanocrystals tracked by TCD analysis (150 mg/CO
2
adsorption at -50 ˚C and desorption in He).
10
Figure 1.8:
13
CO
2
adsorption of perovskite nanocrystals tracked by HPDEC
13
C ssNMR spectroscopy.
11
Figure 2.1: TGA thermograms for La:BaTiO
3
and Dy:BaTiO
3
nanocrystals. 19
Figure 2.2: Powder XRD patterns of xLn:BaTiO
3
nanocrystals synthesized
at room temperature.
20
Figure 2.3: TEM images of 0.6 mol% Dy:BaTiO
3
and 0.8 mol% La:BaTiO
3
nanocrystals. High-resolution TEM images are provided in the
insets.
21
Figure 2.4: SAED patterns for xLn:BaTiO
3
nanocrystals with their
corresponding crystal planes indexed.
22
Figure 2.5: Relative permittivity (ε) and dielectric loss tangent (tan δ) of the
Ln:BaTiO
3
nanocrystals as a function of frequency.
23
ix
Figure 2.6: EPR spectra for undoped BaTiO
3
, 1.6 mol% La:BaTiO
3
, and
1.2 mol% Dy:BaTiO
3
nanocrystal powder samples at 78 K.
26
Figure 3.1: XRD patterns of BT and mBT nanocrystals, and pDCPD/BT
and pDCPD/mBT nanocomposite films, as well as a
representative TEM image of BT nanocrystals.
35
Figure 3.2: FT-IR spectra of BT nanocrystals and TGA curves for BT
nanocrystals and corresponding nanocomposites.
36
Figure 3.3: High magnification cross-sectional SEM images of pDCPD/BT
and pDCPD/mBT free-standing films.
37
Figure 3.4: Relative permittivity and dielectric loss tangent (tan δ) of
pDCPD, PDCPD/BT, and pDCPD/mBT free-standing films as
a function of frequency (25 °C).
38
Figure 3.5: Two-parameter Weibull plots of pDCPD, pDCPD/BT, and
pDCPD/mBT free-standing films.
40
Figure 4.1: TGA thermorgrams of AMoO
4
nanocrystals.
49
Figure 4.2: XRD patterns and TEM images of AMoO
4
nanocrystals before
and after thermal aging step.
50
Figure 4.3: Rietveld analysis of powder XRD patterns of AMoO
4
nanocrystals.
51
Figure 4.4: Lattice parameters a and c and unit cell volume V (top panel),
and metal−oxygen distances (bottom panel) in AMoO
4
nanocrystals as a function of the ionic radius of the A
2+
ion.
53
Figure 4.5: Raman spectra of the AMoO
4
nanocrystals. 55
Figure 4.6: TEM images of the AMoO
4
nanocrystals. 56
Figure 4.7: EDS and XPS spectra of AMoO
4
nanocrystals.
Figure 4.8: Electrochemical testing of CaMoO
4
nanocrystals.
57
59
x
Figure 5.1: Powder XRD patterns for Sr
1–x
Ba
x
WO
4
and Ca
1–x
Sr
x
WO
4
nanocrystal solid solutions. Lattice parameters (a and c) and
unit cell volumes for Sr
1–x
Ba
x
WO
4
and Ca
1-x
Sr
x
WO
4
nanocrystal solid solutions.
Figure 5.2: Raman spectra of the A
1–x
A´
x
WO
4
nanocrystal solid solutions.
Figure 5.3: XPS spectra for select A
1–x
A´
x
WO
4
nanocrystal solid solution
compositions.
Figure 5.4: STEM-EDX maps for clusters of select A
1–x
A´
x
WO
4
nanocrystal solid solution compositions.
Figure 5.5: TEM images of AWO
4
nanocrystals.
Figure 5.6: Representative TEM image of Sr
0.75
Ba
0.25
WO
4
nanocrystals
thermally aged at 100 °C for 24 h under flowing dry nitrogen.
Figure 6.1: Rietveld and PDF analysis of the X-ray total scattering data for
CaMoO
4
nanocrystals collected at 90 K.
Figure 6.2: Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Figure 6.3: PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Figure 6.4: Lattice constants (a and c) and unit cell volumes extracted
from Rietveld and PDF analysis of the X-ray total scattering
data for CaMoO
4
nanocrystals from 90–480 K.
Figure 6.5: Bond distances and distortion indices derived from Rietveld
and PDF for the CaMoO
4
nanocrystals from 90–480 K.
Figure 7.1: XRD patterns for the as-synthesized and calcined
CaWO
4
:1%Ln nanocrystals.
Figure 7.2: Rietveld analysis of powder XRD patterns for the as-
synthesized CaWO
4
:Ln nanocrystals.
Figure 7.3: Rietveld analysis of powder XRD patterns for the calcined
CaWO
4
:Ln nanocrystals.
71
74
75
76
77
78
87
88
97
120
122
132
133
134
xi
Figure 7.4: TEM images of as-synthesized and calcined CaWO
4
:1%Eu
nanocrystals.
Figure 7.5: Assigned room temperature excitation and emission spectra of
CaWO
4
:1%Eu, CaWO
4
:1%Tb, and CaWO
4
:1%Tm
nanocrystals.
Figure 7.6: Room temperature luminescence lifetime curves of
CaWO
4
:1%Eu, CaWO
4
:1%Tb, and CaWO
4
:1%Tm
nanocrystals.
Figure 7.7: Room temperature emission spectrum of CaWO
4
:1%Eu,1%Tb
nanocrystals.
Figure 7.8: Room temperature emission spectra for calcined
CaWO
4
:1%Eu, CaWO
4
:1%Tb, and CaWO
4
:(1%Eu,1%Tb)
nanocrystals upon excitation at 366 nm. Photoluminescence
decay curves for CaWO
4
:(1%Eu,1%Tb) nanocrystals
monitored at 615 and 544 nm.
138
140
141
143
143
xii
Abstract
Perovskite oxides are an exceptionally useful class of materials in the areas of energy
storage and conversion, which are typically fabricated at very high temperatures (i.e., >
1000 °C). As such, the advancement of low temperature techniques that can allow for the
precise tailoring of composition and associated properties is of paramount importance.
Over the last decade, the Brutchey group has developed a vapor diffusion sol–gel
(VDSG) method that provides a facile, low-temperature route to nanocrystalline
perovskite oxides with a high degree of compositional control. Our group has exploited
this method to generate a series of alkaline–earth perovskite oxide ABO
3
type
nanocrystals (A = Sr, Ba; B = Ti, Zr) and solid solutions thereof. More importantly, we
have been able to leverage a multitude of characterization techniques against these oxides
to probe their unique surface chemistry and structural diversity towards elucidating
critical structure-property relationships. Herein, we investigate the compositional
dependence of CO
2
chemisorption on colloidal Ba
1-x
Sr
x
TiO
3
nanocrystal surfaces and
also demonstrate that the energy storage properties of perovskite oxide nanocrystals can
in fact be enhanced through compositional tuning with low concentrations of lanthanides.
Given that our novel VDSG technique provides an ultrabenign (i.e., atmospheric
pressure, low temperature, surfactant–free, and near neutral pH) synthetic pathway to
crystalline metal oxides, we were highly motivated to investigate the extension of our
method beyond perovskite oxides, thereby broadening the ubiquity of this method. The
search brought us to the scheelite family of materials. The intrinsic properties of
scheelite–structured oxide materials with the formula ABO
4
(A = Ca, Sr, Ba; B = Mo, W)
make them highly useful in the areas of energy conversion (cryogenic phonon
scintillation detectors and solid state phosphor hosts) and storage (anode materials for
lithium ion batteries), as well as in heterogeneous catalysis (supports for noble metals).
Previously, these materials have been synthesized using solid state, solution precipitation,
microwave radiation, and hydrothermal techniques, among others, which are often energy
intensive and require contaminating chemical agents, or further processing via high
temperature, post-synthetic calcination treatments to achieve a crystalline product. As an
alternative, we have applied our VDSG technique to crystallize a series of alkaline-earth
molybdates and tungstates. Gas–liquid rather than liquid–liquid hydrolysis and the
compositional flexibility of this method afford the facile preparation of sub–30 nm
nanocrystals. Of note, the sub–30 nm regime of scheelite-structured oxides was
previously unexplored.
By coupling high-resolution structural techniques (e.g., Rietveld and pair distribution
function analyses), we were able to identify local phenomena within CaMoO
4
nanocrystals, which could not be seen in the more average structure, thereby highlighting
the importance of employing a dual-space structural approach. Additionally, the excellent
compositional control associated with the VDSG method has allowed us to generate a
variety of Ca
1-x
Sr
x
WO
4
and Sr
1-x
Ba
x
WO
4
solid solutions, as well as a series of lanthanide
xiii
doped CaWO
4
nanocrystals. Interestingly, the composition of the CaWO
4
nanocrystals
could be modulated to fabricate a white-emitting phosphor upon UV excitation.
1
Chapter 1. Compositionally Dependent Surface Chemistry of Colloidal Ba
x
Sr
1-x
TiO
3
Perovskite Nanocrystals
1.1. Introduction
Colloidal nanocrystals of alkaline earth perovskite oxides having the formula ABO
3
(A = Ca, Sr,
Ba; B = Ti, Zr), and their corresponding quaternary A
1–x
Aʹ
x
BO
3
solid solutions, exhibit a wide
range of technologically important properties. The versatility of this family of complex oxide
nanocrystals stems from the compositional dependence of their physical properties, which allows
the functionality of these materials to be synthetically tuned via chemical composition.
1,2
Perovskite oxide nanocrystals have demonstrated utility in energy conversion and storage (as
dielectric spacers in capacitors and electrolytes in proton-conducting solid oxide fuel cells),
3,4
display technologies (as phosphor hosts),
5
and heterogeneous catalysis (as supports for metal and
metal oxide catalysts).
6,7
While a great deal of effort has gone into the synthesis of well-defined
colloidal ABO
3
and A
1–x
Aʹ
x
BO
3
nanocrystals,
8–10
and elucidation of their corresponding
composition-structure-property relationships,
10–13
comparatively little effort has gone into
understanding the compositional dependence of their surface chemistry.
We have previously reported the low-temperature synthesis of colloidal Ba
1–x
Sr
x
TiO
3
nanocrystals (0 ≤ x ≤ 1) by a vapor diffusion sol-gel (VDSG) method.
10,15
This method relies on
the slow delivery of water via vapor phase to induce the kinetically controlled hydrolysis and
polycondensation of an alcohol solution of a bimetallic alkoxide, or a mixture of bimetallic
alkoxides, for the nucleation and growth of multinary perovskite oxide nanocrystals. The
resulting nanocrystals are synthesized in good yield and are colloidally stable at moderate to high
concentrations in polar solvents as a result of residual alkoxide functionality on their surfaces.
2
While it has been empirically observed that the nanocrystals react with atmospheric CO
2
to form
carbonates, no systematic study on the effect of nanocrystal composition on surface affinity for
CO
2
has been performed to this point. Such CO
2
adsorbate-surface interactions have the potential
to impact a range of applications, including photocatalysis,
16
fuel cells,
17
and the dielectric
properties of perovskite oxide thin film capacitors.
18
Herein, we investigate the compositionally
dependent chemisorption of CO
2
on colloidal Ba
1-x
Sr
x
TiO
3
nanocrystal surfaces using in-situ FT-
IR, temperature programmed desportion (TPD), and solid state
13
C NMR spectroscopy.
1.2 Results and Discussion
A series of colloidal Ba
1–x
Sr
x
TiO
3
nanocrystals were synthesized by the VDSG method with x =
0, 0.3, and 1. The nanocrystal compositions were verified by inductively coupled plasma atomic
emission spectroscopy (ICP-AES), and powder X-ray diffraction (XRD) was used to confirm
that the nanocrystals are phase pure and crystallize in the expected cubic Pm3 m space group
(Figure 1.1). The mean diameters of the quasi-spherical, as-prepared nanocrystals were
determined by transmission electron microscopy (TEM) to be 8.9 ± 1.4, 11.6 ± 2.1 and 14.8 ±
1.9 nm for BaTiO
3
, Ba
0.7
Sr
0.3
TiO
3
and SrTiO
3
, respectively (Figure 1.2). The corresponding BET
surface areas measured by nitrogen adsorption were 21,900, 21,300, and 18,100 m
2
mol
–1
.
Coupled thermogravimetric analysis/differential scanning calorimetry-mass spectrometry
(TGA/DSC-MS) of the colloidal nanocrystals showed a mass loss in the range of 6-10 wt% from
residual surface alkoxide combustion occurring at an exothermic event at 300 ˚C with
concomitant evolution of CO
2
(Figure 1.3). Calcination of the nanocrystals to 600 ˚C under
synthetic air to free the surface of residual organics for further surface analysis resulted in the
expected grain growth of the nanocrystals to give slightly larger mean diameters (13-17 nm) by
3
TEM analysis and lower BET surface areas (10,000-14,600 m
2
mol
–1
), as summarized in Table 1.
The nanocrystals remain phase pure after calcination, as assessed by powder XRD.
Figure 1.1. Powder XRD of the as-prepared nanocrystals. Key: black (BaTiO
3
), red (Ba
0.7
Sr
0.3
TiO
3
), blue
(SrTiO
3
).
Figure 1.2. TEM images of the as-prepared nanocrystals. Key: left (BaTiO
3
), middle (Ba
0.7
Sr
0.3
TiO
3
),
right (SrTiO
3
).
4
Figure 1.3. TGA/DSC-MS of the as-prepared nanocrystals run at a heating rate of 10 ˚C min
–1
under
flowing Ar. Key: left (BaTiO
3
), middle (Ba
0.7
Sr
0.3
TiO
3
), right (SrTiO
3
).
Table 1.1. Physical features of the Ba
1–x
Sr
x
TiO
3
nanocrystals before and after calcination
The FT-IR spectra of the as-prepared Ba
1–x
Sr
x
TiO
3
nanocrystals show a broad n(O–H)
stretching band centered at 3456 cm
–1
for SrTiO
3
, 3492 cm
–1
for Ba
0.7
Sr
0.3
TiO
3
, and 3512 cm
–1
for BaTiO
3
. This is consistent with the presence of hydrogen-bonded surface hydroxyls on the
three oxide materials. In addition, sp
3
n(C–H) stretches and intense bands in the C–O region
As#synthesized,material
d
,, Calcined,material,
χ size
a
nm
dp size
b
nm
SA
c
m
2
.g
-1
χ size
a
nm
dp size
b
nm
S
c
m
2
.mol
-1
BaTiO
3
8.0 5.9 ± 1.2 14.7 17.4 ± 6.0 10027
Ba
0.7
Sr
0.3
TiO
3
11.0 10.0 ± 1.3 13.9 12.6 ± 3.0 14619
SrTiO
3
6.4 5.9 ± 1.2 17.5 16.2 ± 3.7 10275
a: Estimate by Scherrer analysis
b: Mean diameter by TEM
c: Surface area calculated by BET method
d: Data extract from Brutchey, R. L. et al. Journal of Materials Chemistry 2010, 20, 5074.
BaTiO
3
Ba
0.7
Sr
0.3
TiO
3
SrTiO
3
5
(1200-1700 cm
–1
) were observed, which are assigned to surface-bound organics and carbonate
by-products, respectively (Figure 1.4a).
19,20
After calcination to 600 ˚C under synthetic air, there
is a complete disappearance of IR bands associated with the organic functionality, a significant
decrease of the bands in the C–O region, and a decrease of the n(O–H) stretching intensity from
dehydration, yielding two distinct bands in the range of 3400-3700 cm
–1
(Figure 1.4c). The major
intensity IR band position depends on A-site cation composition – shifting from 3403 cm
–1
for
SrTiO
3
to 3455 cm
–1
for Ba
0.7
Sr
0.3
TiO
3
and 3477 cm
–1
for BaTiO
3
.
Figure 1.4. (a) FT-IR spectra of the as-prepared nanocrystals with material diluted in KBr (1/10 vol/vol);
(b) zoom in of 2300-3800 cm
–1
region. (c) FT-IR spectra of the nanocrystals dehydrated and calcined to
600 ˚C; (d) zoom in 3100-3700 cm
–1
region. Key: black (BaTiO
3
), red (Ba
0.7
Sr
0.3
TiO
3
), blue (SrTiO
3
).
As shown in Figure 1.5, a linear correlation was observed between the A-site composition and
the n(O–H) stretching frequency for the as-prepared and calcined materials. This is consistent
with BaO being more basic than SrO; therefore, as the Ba content increases in the perovskite, the
O–H bond strength and the n(O–H) stretching frequency increases. The higher frequency n(O–
6
H) stretching band at ca. 3675 cm
–1
is more or less invariant with A-site composition, which is
suggestive of this band corresponding to a titanol n(O–H) stretch.
21
Figure. 1.5. A plot of n(O–H) stretching frequency as a function of %Ba for Ba
1–x
Sr
x
TiO
3
nanocrystals.
Key: black (calcined), red (as-synthesized). FT–IR spectra of the calcined nanocrystals in the range of
3360-3560 cm
–1
are given as the inset. Inset key: black (BaTiO
3
), red (Ba
0.7
Sr
0.3
TiO
3
), blue (SrTiO
3
).
The exchange of Ba for Sr in the perovskite structure appears to affect the O–H bond strength,
and hence the basicity of the nanocrystal surface. To further understand the Ba
1–x
Sr
x
TiO
3
nanocrystal surface chemistry, CO
2
was used as a diagnostic acidic probe molecule. An excess of
CO
2
was introduced to each of the calcined nanocrystal samples at 25 ˚C, followed by degassing
under high vacuum at room temperature for 30 min. FT-IR spectra were measured for the
degassed samples. Prior to CO
2
exposure, the FT-IR spectra of the calcined nanocrystals all show
a feature in the C–O region of 1200-1700 cm
–1
(vide supra), suggesting that amorphous alkaline
earth carbonates are present in each of the materials. After addition of CO
2
to the perovskite
nanocrystals, the bands in the same C–O spectral region become more complex and increase in
intensity (Figure 1.6). The FT-IR spectra each possess an intense band at ca. 1020 cm
–1
that is
indicative of the symmetric (n
1
) stretch of the carbonate ion.
20,22
There is a broad band extending
3560
3540
3520
3500
3480
3460
3440
3420
3400
1.0 0.8 0.6 0.4 0.2 0.0
%Ba in Ba
x
Sr
1-x
TiO
3
As-synthesized
Calcined
νO-H vibration – cm
-1
Absorbance - a.u.
3520 3480 3440 3400 3360
Wavenumber - cm
-1
BaTiO 3
Ba 0.3 Sr 0.7 TiO 3
SrTiO 3
νO-H area for calcined
R
2
=0.99
'
R
2
=0.99
'
7
from 1250 to 1640 cm
–1
that is in the region of the asymmetric (n
3
) carbonate stretch;
20,22
the
multiple local maxima on this band are suggestive of various coordination geometries of surface
coordinated CO
3
2–
.
23
Moreover, a weak band exists in all three spectra at 1710-1735 cm
–1
that
can be assigned to a combination n
1
+ n
4
(in-plane deformation) mode.
20,22
The formation of
carbonate upon exposure of the nanocrystals to CO
2
is an obvious conclusion, but their precise
structure elucidation is problematic due to the presence of multiple species having different
vibrational modes.
8
Figure 1.6. FT-IR spectra of CO
2
adsorbed on the calcined nanocrystals. (a) BaTiO
3
, (b) Ba
0.7
Sr
0.3
TiO
3
,
and (c) SrTiO
3
nanocrystals. Key: black (calcined nanocrystals prior to exposure to CO
2
), red (after CO
2
adsorption at 25 ˚C and degassing under high vacuum).
Investigation into the interaction of CO
2
with the calcined perovskite nanocrystals by
temperature programmed desorption (TPD) suggests the presence of weakly adsorbed and
chemisorbed CO
2
as both surface and bulk carbonate species. After contacting the Ba
1–x
Sr
x
TiO
3
nanocrystals with CO
2
at low temperature (-50 ˚C), the temperature was increased to 950 ˚C
under a flow of He. For all three A-site compositions, three major TPD peaks were observed at
Absorbance - a.u.
3500 3000 2500 2000 1500 1000
Wavenumber - cm
-1
Calcined 600
o
C
CO
2
adsorbed at 25
o
C and then degassed under high vacuum
(c) SrTiO
3!
4000 3500 3000 2500 2000 1500 1000
Wavenumber - cm
-1
Absorbance - a.u.
Calcined 600
o
C
CO
2
adsorbed at 25
o
C and then degassed under high vacuum
(b) Ba
0.7
Sr
0.3
TiO
3!
Absorbance - a.u.
3500 3000 2500 2000 1500 1000
Wavenumber - cm
-1
Calcined 600
o
C
CO
2
adsorbed at 25
o
C and then degassed under high vacuum (a) BaTiO
3!
9
low, intermediate, and high temperatures (Figure 1.7). The broad peak at low temperature (-30 to
300 ˚C) corresponds to desorption of CO
2
(m/z = 44), with CO (m/z = 28) as an ionization
fragment of the parent CO
2
molecule. Deconvolution of the this broad, low-temperature TPD
peak yields three constituent peaks for each of the Ba
1–x
Sr
x
TiO
3
nanocrystals, roughly centered at
-7 to 19 ˚C, 67-71 ˚C, and 174-182 ˚C. The first two desorption events in the low temperature
envelope (-7 to 71 ˚C) are suggestive of physisorbed and weakly chemisorbed CO
2
. The presence
of Sr in the Ba
1–x
Sr
x
TiO
3
nanocrystals allows for a higher uptake of physisorbed CO
2
, with
SrTiO
3
having an integrated TPD peak area fraction of 40% for these species as compared to 26
and 19% for Ba
0.7
Sr
0.3
TiO
3
and BaTiO
3
, respectively. The third desorption peak in the low
temperature envelope (174-182 ˚C) appears in the temperature range that has been assigned in
the literature to CO
2
chemisorbed to lattice oxygens near irregular AO and TiO
2
surfaces.
24,25
The intermediate temperature desorption peak (477–494 ˚C) also corresponds to CO
2
desorption
(m/z = 28 and 44), which has been previously assigned to CO
2
chemisorbed to oxygen
vacancies.
24,25
Interestingly, another mass signal (m/z = 15) assigned to CH
4
is also observed in
the same temperature range. Methane is a common decomposition product of organic moieties,
and could be considered a result of the decomposition of residual surface alkoxides; however, the
temperature of this desorption event is inconsistent with that of the surface alkoxide
decomposition observed by TGA/DSC-MS (vide supra). Finally, the high temperature TPD peak
located in the range of 700-1000 ˚C corresponds to the decomposition of bulk carbonates.
26
Substitution of Ba for Sr in the perovskite structure creates a more basic surface and causes CO
2
to chemisorb more strongly as carbonate, corresponding to the higher temperature TPD peaks
beginning at 174 ˚C having a higher integrated TPD peak area fraction of 81 and 74% for
BaTiO
3
and Ba
0.7
Sr
0.3
TiO
3
, respectively, as compared to 60% for SrTiO
3
. Spectroscopic
10
investigation into the adsorbed CO
2
by ssNMR was conducted in order to further corroborate
these assignments.
Figure 1.7. Low-temperature CO
2
TPD of the calcined Ba
1–x
Sr
x
TiO
3
nanocrystals tracked by TCD
analysis (150 mg/CO
2
adsorption at -50 ˚C and desorption in He). Key: black (BaTiO
3
), red
(Ba
0.3
Sr
0.7
TiO
3
), blue (SrTiO
3
).
In order to further elucidate the binding mode of CO
2
to the perovskite nanocrystal surface,
13
C ssNMR was used. The high-power decoupled (HPDEC)
13
C spectrum of
13
CO
2
-SrTiO
3
obtained at a 10 kHz spinning frequency contains 3 peaks at 123.5, 160.0 and 164.4 ppm. The
low signal-to-noise ratio for this measurement is caused by a low uptake of
13
CO
2
onto SrTiO
3
,
and hence, the background contribution was taken into account for this fitting. In the case of
13
CO
2
- Ba
0.7
Sr
0.3
TiO
3
, 3 peaks are also observed at 126.4, 161.9 and 165.2 ppm, whereas for the
13
CO
2
-BaTiO
3
sample, only 2 peaks are observed at 162.1 and 166.1 ppm. The proportion of the
three different species appears to be strongly dependent of the type of alkaline earth metal
present in the perovskite matrix. The high-shielded peaks, only present for Sr-containing
nanocrystals, have a chemical shift nearly identical to free CO
2
, indicating weak, linear binding
of CO
2
to the perovskite nanocrystal surface. The peaks in the 160-166 pm range can be assigned
to alkaline earth carbonate species.
28
Rakotovelo et al. have simulated the adsorption of CO
2
Normalized TCD signal - uV.g
sample
-1
1000 900 800 700 600 500 400 300 200 100 0
Sample Temperature -
o
C
(c)$SrTiO
3$
(a)$BaTiO
3$
(b)$Ba
0.7
Sr
0.3
TiO
3$
11
onto a cubic BaTiO
3
unit cell and found the formation of one major carbonate species.
29
The CO
2
possesses an angle of 134˚, with the carbon binding to a surface oxygen. A neighboring Ba is
also in interaction with one of the oxygens of the bound CO
2
molecule. The lower quantity of
carbonate formation on SrTiO
3
is again rationalized by a less basic surface, leading to a greater
propensity for physisorption in the Sr containing nanocrystals. The differentiation of the two
carbonate species is not currently possible.
Figure 1.8.
13
CO
2
adsorption tracked by HPDEC
13
C ssNMR spectroscopy.
13
CO
2
was adsorbed onto the
calcined nanocrystal surface at 25 ˚C, followed by degassing under high vacuum prior to measurement.
Key: black (experimental spectrum), grey (fit), * (spinning side bands).
ppm 2 5 0 2 0 0 1 5 0 1 0 0
ppm 250 200 150 100
p p m 2 5 0 2 0 0 1 5 0 1 0 0 5 0
ppm 2 5 0 2 0 0 1 5 0 1 0 0
Fit !
Experiment!
*: spinning side band!
(a)$BaTiO
3$
(b)$Ba
0.7
Sr
0.3
TiO
3$
(c)$SrTiO
3$
100! 150! 200! 250!
ppm!
*"
*"
*" *"
*"
*"
12
1.3 Experimental
1.3.1 General Procedures
All manipulations were conducted under nitrogen atmosphere using standard Schlenk
techniques. All reagents were used as received. Solutions of the double metal alkoxides
BaTi(OR)
6
and SrTi(OR)
6
(R = CH
2
CHCH
3
(OCH
3
)) in n-butanol/2-methoxypropanol (1:3 v/v,
bp 117 ˚C) from Gelest, Inc. were employed as precursors for the synthesis of colloidal Ba
1–
x
Sr
x
TiO
3
nanocrystals. The molarities of the bimetallic alkoxide solutions were 0.50 and 0.70 M
for BaTi(OR)
6
and SrTi(OR)
6
, respectively.
1.3.2 Apparatus
The apparatus employed for the synthesis of colloidal Ba
1–x
Sr
x
TiO
3
nanocrystals via vapor
diffusion sol-gel (VDSG) consists of a 100-mL three-neck reaction flask and a glass bubbler
containing a 0.75 M HCl solution. The bubbler is connected to the nitrogen manifold via a
needle-valve rotameter. Nitrogen gas is bubbled through the 0.75 M HCl solution for 30 min,
while one or more bimetallic alkoxide precursors are transferred to the reaction flask via syringe.
Then, the bubbler is connected to the reaction flask in order to allow N
2
/HCl/H
2
O vapor to
continuously flow over the precursor solution at room temperature and atmospheric pressure.
1.3.3 Synthesis of Ba
1–x
Sr
x
TiO
3
nanocrystals
One or more bimetallic alkoxide precursors are added to the reaction flask to obtain the desired
stoichiometry and a total volume of 2.00 mL (approximate exposed liquid area: 3 cm
2
). For
example, 2.0 mL (1.0 mmol) of BaTi(OR)
6
were employed in the synthesis of BaTiO
3
nanocrystals. Mixtures of two bimetallic alkoxides were magnetically stirred for 20 min to
13
ensure intimate mixing; then, the stir bar was removed. The one-step synthesis consisted of
flowing N
2
/HCl/H
2
O vapor over the precursor solution for 36 h. The solution turned into a fully
rigid gel a few hours (~7 h) after starting the vapor flow, followed by cracks and the expulsion of
a clear supernatant. After 36 h, the vapor flow is stopped, the reaction flask opened, and pieces
of the gel collected. These were washed with 5 mL of absolute ethanol, sonicated for 10 min,
and centrifuged at 6000 rpm for 25 min. This washing step was repeated three times. The
resulting nanocrystals were dried under vacuum at room temperature for 4 h, yielding a fine, off-
white powder. Inside an inert atmosphere glovebox, the resulting powder was transferred to a
flow tubular quartz reactor. The sample was calcined at 600
˚
C under a synthetic air stream (0.2
O
2
/0.8 N
2
) with a temperature ramp of 1
˚
C min
–1
. The powder was then degassed under high
vacuum (10
–5
mbar) for 30 min and transferred to the glovebox.
1.3.4 Material Characterization
Elemental analysis: The chemical composition of the Ba
1-x
Sr
x
TiO
3
nanocrystals was determined
using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Galbraith Labs,
Knoxville, TN). Thermal analysis: Thermal analysis measurements were made on a Netzsch
DSC/TGA - MS/IR STA449 F1 using sample sizes of ~20 mg. Argon was used as a protective
and flow gas. Mass flow controllers were set to 40 mL min
–1
for the protective gas and 10 mL
min
–1
for the purge gas. The thermal range examined was 50-500 ˚C with a heating rate of 10 ˚C
min
–1
. N
2
adsorption at 77 K: Nitrogen adsorption–desorption at 77 K was measured on a Bel-
Mini from BelJAPAN. Before the measurement, the samples were transferred from the
glovebox to the Bel-Mini using an airtight cell. Total surface area was calculated using the BET
14
method and the pore volume using the BJH adsorption method. Transmission Electron
Microscopy (TEM): TEM images were collected with a Philips CM12 transmission electron
microscope. All samples were dispersed in absolute ethanol using ultrasound prior to the grid
preparation. The particle size distributions were obtained from a minimum of 150 nanocrystals.
Powder X-ray diffraction (XRD): XRD patterns were collected with STOE STADI P powder
diffractometer, operating in transmission mode. A germanium monochromator, Cu Kα
1
irradiation, and Dectris Mythen silicon strip detector were used. ssNMR measurement: 4-mm
rotor, spinning rate10 kHz, HPDEC.
1.3.5 Surface characterization (ssNMR/FT-IR) of the nanocrystals dosed with CO
2
Inside the inert atmosphere glovebox, transmission FT-IR spectra of freshly calcined Ba
1–
x
Sr
x
TiO
3
was measured and 150 mg of this material is loaded inside a Schlenk cell (V ≈ 20 mL).
The sample was degassed under high vacuum (10
–5
mbar) for 30 min at 25 ˚C. At 25 ˚C, an
excess of carbon dioxide (≈ 4 CO
2
nm
–2
) was introduced inside the Schlenk cell, and the CO
2
was left in contact with the sample for 30 min. The sample was then degassed under high
vacuum (10
–5
mbar) for 30 min at 25 ˚C. A transmission FT-IR of spectrum of the CO
2
-
Ba
1-x
Sr
x
TiO
3
measurement was carried out under inert conditions. Samples for ssNMR
characterization were similarly prepared, except with
13
CO
2
.
1.3.6 CO
2
temperature programmed desorption (TPD)
These experiments were carried out using a BELCAT-B from BEL JAPAN equipped with a
CATCryo system allowing a linear temperature control from 223–1213 K. CO
2
-TPD was
performed on 100 mg of calcined Ba
1-x
Sr
x
TiO
3
that was pre-treated at 300
˚
C in He (30 mL min
–
15
1
) for 2 h using a ramp of 5 ˚C min
–1
. The sample was then cooled down to 25 ˚C under He for
45 min and then to –50 ˚C under CO
2
for 1 h. The TPD measurement was performed from –50
˚C to 900 ˚C under He using a ramp of 10 K min
–1
. The CO
2
uptake was followed using a
calibrated TCD detector and a mass spectrometer set with m/z = 15, 18, 28 and 44.
1.4. References
(1) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074.
(2) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
(3) Beier, C. W.; Sanders, J. M.; Brutchey, R. L. J. Phys. Chem. C 2013, 117, 6958.
(4) Fabbri, E.; Bi, L.; Tanaka, H.; Pergolesi, D.; Traversa, E. Adv. Funct. Mater. 2011, 21,
158.
(5) Zhang, H.; Fu, X.; Xin, Q. J. Alloys Compd., 2008, 459, 103.
(6) Enterkin, J. A.; Setthapun, W.; Elam, J. W.; Christensen, S. T.; Rabuffetti, F. A.; Marks,
L. D.; Stair, P. C.; Poeppelmeier, K. R.; Marshall, C. L. ACS Catal. 2011, 1, 629.
(7) Townsend, T. K.; Browning, N. D.; Osterloh, F. E. ACS Nano 2012, 6, 7420.
(8) Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004,
126, 9120.
(9) Demirörs, A. F.; Imhof, A. Chem. Mater. 2009, 21, 3002.
(10) Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
(11) Smith, M. B.; Page, K.; Siegrist, T.; Richmond, P. L.; Walter, E. C.; Seshadri, R.; Brus,
L. E.; Steigerwald, M. L. J. Am. Chem. Soc. 2008, 130, 6955.
(12) Petkov, V.; Gateshki, M.; Niederberger, M.; Ren, Y. Chem. Mater. 2006, 18, 814.
(13) Rabuffetti, F. A.; Brutchey, R. L. ACS Nano 2013, 7, 11435.
(14) Rabuffetti, F. A.; Brutchey, R. L. J. Am. Chem. Soc. 2012, 134, 9475.
16
(15) Rabuffetti, F. A.; Brutchey, R. L. Dalton Trans. 2014, 43, 14499.
(16) Wagner, F. T.; Ferrer, S.; Somorjai, G. A. Surf. Sci. 1980, 101, 462.
(17) Fabbri, E.; D’Epifanio, A.; Di Bartolomeo, E.; Licoccia, S.; Traversa, E. Solid State
Ionics 2008, 179, 558.
(18) Im, J.; Steiffer, S. K.; Auciello, O.; Krauss, A. R. Appl. Phys. Lett. 2000, 77, 2593.
(19) Frey, M. H.; Payne, D. A. Phys. Rev. B, 1996, 54, 3158.
(20) Rabuffetti, F. A.; Stair, P. C.; Poeppelmeier, K. R. J. Phys. Chem. C 2010, 114, 11056.
(21) Lin, W.; Frei, H. J. Am. Chem. Soc. 2002, 124, 9292.
(22) Huang, C. K.; Kerr, P. F. Am. Mineral. 1960, 45, 311.
(23) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966.
(24) Garra, J.; Vohs, J. M.; Bonnell, D. A. J. Vac. Sci. Technol. A 2009, 27, L13.
(25) Baniecki, J. D.; Ishii, M.; Kurihara, K.; Yamanaka, K.; Yano, T.; Shinozaki, K.; Imada,
T.; Kobayashi, K. J. Appl. Phys. 2009, 106, 054109.
(26) Ding, W.; Chen, Y.; Fu, X. Appl. Catal. A 1993, 104, 61.
(27)
(28) Durán, P.; Gutierrez, D.; Tartaj, J.; Bañares, M. A.; Moure, C. J. Eur. Ceram. Soc. 2002,
22, 797.
(29) Rakotovelo, G.; Moussounda, P. S.; Haroun, M. F.; Légaré, P.; Rakotomahevitra, A.;
Rakotomalala, M.; Parlebas, J. C. Surf. Sci. 2009, 603, 1221.
17
Chapter 2. Low Temperature Synthesis and Characterization of Lanthanide-Doped
BaTiO
3
Nanocrystals*
*Published in Chem. Commun. 2014, 50, 3480-3483.
2.1. Abstract
The vapor diffusion sol-gel (VDSG) method was employed for the room-temperature synthesis
of ~10 nm, aliovalently doped 0.4, 0.8, and 1.6 mol% La:BaTiO
3
and 0.4, 0.6, and 1.2 mol%
Dy:BaTiO
3
nanocrystals. Maximum ensemble relative permittivities of 176 and 208 were
observed in the 0.8 mol% La:BaTiO
3
and the 1.2 mol% Dy:BaTiO
3
nanocrystals, respectively,
relative to 89 for undoped BaTiO
3
(at 1 MHz, 25 °C) due to local disorder induced by aliovalent
substitution.
2.2. Introduction
Doping small concentrations of lanthanide (Ln) ions into bulk ABO
3
perovskites (e.g., BaTiO
3
)
has proven useful in modulating the associated dielectric
1,2
, electrical,
3
and optical properties
4,5
in these materials. Three modes of aliovalent substitution within BaTiO
3
are believed to be
possible based on the ionic radius. Studies have demonstrated that large ions (e.g., La
3+
)
6
substitute exclusively at the 12-coordinate A-site, while small ions (e.g., Yb
3+
)
6
solely occupy
the 6-coordinate B-site. Ions with intermediate ionic radii (e.g., Eu
3+
, Gd
3+
, Dy
3+
) are said to be
amphoteric, whereby these intermediate sized Ln
3+
can substitute at the A-site, the B-site, or both
sites.
7
Upon substitution, the charge imbalance introduced by the lattice defects must be
compensated for in order to maintain charge neutrality. Thus, Ln
3+
substitutions at the A-site can
generate B-site vacancies and B-site substitutions can result in oxygen deficiency.
8
18
Incorporation of La
3+
within bulk BaTiO
3
has been shown to occupy the A-site, induce B-site
vacancies, and enhance the room-temperature permittivity.
9
Additionally, given the intermediate
ionic radius of Dy
3+
compared to the host lattice atoms (i.e., r(Ba
2+
) = 1.61 Å and r(Ti
4+
) = 0.61
Å for 12- and 6-coordinate environments, respectively
6
), it is believed to exhibit amphoteric
character, while also elevating the associated room-temperature permittivity.
10,11
Generally, the
aforementioned substitutions are achieved through high temperature (> 1000 °C) solid state
synthesis. Though effective, such high temperature routes cause particle sintering and micron-
sized grains.
12
As electronic devices become smaller and smaller, the ability to achieve
functional and processable nanocrystals with tunable properties is becoming more important.
13
Therefore, the ability to aliovalently dope nanocrystals of BaTiO
3
and related perovskites at low
temperatures to prevent sintering is highly desirable.
The vapor diffusion sol-gel method allows for the low-temperature synthesis of functional
metal oxide nanocrystals under ultra-benign conditions.
14-16
The controlled flow of water vapor
over an alcohol solution of metal alkoxide and metal acetylacetonate (acac) precursors induces
their hydrolysis and polycondensation to nucleate and grow nanocrystals. Herein, the synthesis
of xDy:BaTiO
3
and xLa:BaTiO
3
(0 ≤ x < 2 mol%) occurs via kinetically controlled hydrolysis
and cross polycondensation within alcohol solutions of BaTi(OR)
6
(R = CH
2
CHCH
3
OCH
3
) and
Ln(acac)
3
precursors to yield the resulting nanocrystals at room temperature.
2.3. Results and Discussion
Overall ceramic yields for the vapor diffusion sol-gel reactions were estimated by mass balance
and thermal gravimetric analysis (TGA) to be 68, 73, and 76% for nominal 0.5, 1.0, and 2.0
mol% Dy:BaTiO
3
and 69, 69, and 76% for nominal 0.5, 1.0, and 2.0 mol% La:BaTiO
3
,
respectively. The organic content of the isolated nanocrystals was determined to be < 8 wt% by
19
TGA and can be attributed to unreacted surface alkoxy groups (Figure 2.1).
Figure 2.1. TGA thermograms for (a) La:BaTiO
3
and (b) Dy:BaTiO
3
nanocrystals.
The crystallinity and phase purity of the resulting Dy:BaTiO
3
and La:BaTiO
3
nanocrystals were
confirmed by powder X-ray diffraction (XRD). As shown in Figure 2.2, all reflections can be
indexed to the cubic perovskite structure with lattice constant a ~ 4.03 Å, belonging to the
paraelectric Pm m space group (JCPDS no. 31-0174). Segregation of secondary crystalline
carbonate phases, or Ln
2
Ti
2
O
7
pyrochlore phases typically observed in Ln
3+
-doped BaTiO
3
ceramics synthesized via high-temperature solid-state reactions,
7
were not observed.
3
20
Figure 2.2. Powder XRD patterns of xLn:BaTiO
3
nanocrystals synthesized at room temperature.
Furthermore, no differences in the crystallinity or lattice constants were observed in any of the
studied compositions, as indicated by the XRD patterns. Post-synthetic thermal treatment of the
nanocrystals synthesized under ambient conditions was not required to induce crystallization and
incorporate the Ln
3+
ions. We have recently demonstrated by synchrotron X-ray diffraction and
total scattering that lanthanide dopants incorporate into the pervoskite lattice (as opposed to
forming core/shell structures) under these synthesis conditions, as evidenced by slight changes in
the lattice parameter at low dopant concentrations.
17
Herein, both inductively coupled plasma–
optical emission spectroscopy (ICP–OES) and dielectric characterization (vide infra) suggest that
the lanthanide is incorporated into the BaTiO
3
nanocrystal host. Elemental analysis was
performed with ICP–OES to quantify the La
3+
and Dy
3+
concentrations in the La:BaTiO
3
and
Dy:BaTiO
3
nanocrystals, respectively. The La
3+
and Dy
3+
concentrations were found to be 0.4,
0.8, and 1.6 mol% and 0.4, 0.6, and 1.2 mol%, respectively, for nominal 0.5, 1.0, and 2.0 mol%
Ln:BaTiO
3
nanocrystals. Both of the lanthanide dopants exhibited less than unity incorporation
into the host lattice against the nominal doping concentration, achieving doping efficiencies of
60-80% relative to nominal under these benign synthesis conditions.
21
The morphology of the Dy:BaTiO
3
and La:BaTiO
3
nanocrystals was investigated by
transmission electron microscopy (TEM). The nanocrystals possess a quasispherical shape with
relatively monodisperse size distributions (σ = 16–19%, N = 100) for nanocrystals synthesized at
low temperature (Figure 2.3)
Figure 2.3. TEM images of (a) 0.6 mol% Dy:BaTiO
3
and (b) 0.8 mol% La:BaTiO
3
nanocrystals. High-
resolution TEM images are provided in the insets; the corresponding lattice fringe d-spacing and lattice
planes are indicated.
The average diameters were found to be 9.8 ± 1.8, 10.1 ± 1.6, and 10.2 ± 1.9 nm for the 0.4, 0.6,
and 1.2 mol% Dy:BaTiO
3
nanocrystals and 9.8 ± 1.9, 10.1 ± 1.7, and 9.9 ± 1.7 nm for the 0.4,
0.8, and 1.6 mol% La:BaTiO
3
nanocrystals. The lanthanide dopants did not have a significant
effect on the size or shape of the resulting nanocrystals. The presence of well-defined lattice
fringes corresponding to the (100) and/or (110) lattice planes in the high-resolution TEM images
suggest that the nanocrystals are composed of single crystalline domains (Figure 2.3 insets).
Additionally, selected area electron diffraction patterns were collected for all compositions and
indexed to the cubic perovskite phase (Figure 2.4), corroborating the XRD data.
22
Figure 2.4. SAED patterns for (a) 0.4, (b) 0.6, and (c) 1.2 mol% Dy:BaTiO
3
and (d) 0.4, (e) 0.8, and (f)
1.6 mol% La:BaTiO
3
nanocrystals with their corresponding crystal planes indexed.
The ensemble dielectric properties of the Dy:BaTiO
3
and La:BaTiO
3
nanocrystals were
measured from composite parallel plate capacitors obtained by pressing a mixture of the
Ln:BaTiO
3
nanocrystals with polyvinyl alcohol (PVA) into cylindrical pellets and coating both
sides with silver contacts. Bruggeman’s effective medium model
18
was employed to extrapolate
the dielectric constant of the unsintered nanocrystal component:
v
ε
i
−ε
eff
ε
i
+(n−1)ε
eff i=1
n
∑
=0
The effective dielectric constant was modeled as a three component system (i.e., n = 3)
consisting of volume fractions (v) of air, PVA, and Dy:BaTiO
3
or La:BaTiO
3
nanocrystals. The
dielectric constants used for air and PVA were 1.00 and 1.95, respectively. It should be noted
that high temperature annealing was not performed on the pellets in order to prevent nanocrystal
23
sintering and to more accurately estimate the true ensemble dielectric constant of the ~10-nm
nanocrystals prepared at room temperature.
Dielectric permittivity and loss tangent measurements on the Ln:BaTiO
3
nanocrystals were
conducted in a frequency range of 1 kHz to 2 MHz at 25 °C under nitrogen (Figure 2.5). The
undoped BaTiO
3
nanocrystals exhibited a relative permittivity of 89 and a loss tangent of 0.017
at 1 MHz. Incorporation of 0.4 mol% La
3+
did not significantly affect the nanocrystal
permittivity, which exhibited a value of 103 at 1 MHz (Fig. 2.5a). The onset of the effect of La
3+
concentration on the permittivity was found to occur at 0.8 mol%. Here, the permittivity is two
times greater than undoped BaTiO
3
, with a relative permittivity of 176 at 1 MHz.
Figure 2.5. Relative permittivity (ε) and dielectric loss tangent (tan δ) of the Ln:BaTiO
3
nanocrystals as a function of frequency. All measurements were conducted at 25 °C under a
nitrogen atmosphere in a frequency range of 1 kHz to 2 MHz.
As the concentration of La
3+
is further increased to 1.2 mol%, the relative permittivity slightly
decreases by 16% to 148. For the Dy:BaTiO
3
nanocrystals, Dy
3+
doping does not have a
substantial effect on the relative permittivity below a concentration of 0.6 mol% (Fig. 2.5b), in
agreement with previous reports on Dy:BaTiO
3
bulk ceramics.
10,11
The relative permittivity for
the 0.4 and 0.6 mol% Dy:BaTiO
3
compositions were 114 and 104, respectively, at 1 MHz. Upon
24
increasing the Dy
3+
concentration to 1.2 mol%, the relative permittivity increased to 208, which
is nearly two and a half times greater than that of undoped BaTiO
3
. To test against the formation
of amorphous Ln
2
O
3
shells, or related species, being the cause of the observed dielectric
properties, we subjected pre-formed, undoped BaTiO
3
nanocrystals to a second vapor diffusion
sol-gel reaction in the presence of Dy(acac)
3
(nominal 2.0 mol%; 25 ˚C, 48 h). The dielectric
constant of the BaTiO
3
nanocrystals surface treated with Dy(acac)
3
decreased by 18% relative to
the untreated BaTiO
3
nanocrystals, suggesting that the surface species resulting from such a
reaction are not the cause of the observed dielectric effects. The dielectric loss tangents were
between 0.02–0.05 for all Ln:BaTiO
3
nanocrystals (i.e., greater than the undoped BaTiO
3
nanocrystals for all compositions).
The increase in permittivity may be attributed to local disorder promoted by aliovalent
substitution. It has been computationally modeled for bulk ceramics that the titanium octahedra
distort to electrostatically stabilize the lattice because of B-site vacancies, generating local
polarizations that enhance the relative permittivity.
19
As previously mentioned, La
3+
is known to
substitute at the A-site, whereas Dy
3+
can substitute either the A- or B-site exclusively or both
the A- and B-sites. Introducing trivalent ions into the A-site generates Ti
4+
vacancies to
accommodate the charge imbalance caused by the defect, according to:
20
4 La
Ba
•
→ V
Ti
ʹʹʹʹ
The Ti
4+
vacancies distort the Ti–O bonding in the surrounding octahedra.
19
Another possible
charge compensation mechanism involves the reduction of Ti
4+
to Ti
3+
. In order to elucidate
which mechanism is occurring, electron paramagnetic resonance (EPR) spectra were collected at
78 K on undoped BaTiO
3
, 1.6 mol% La:BaTiO
3
, and 1.2 mol% Dy:BaTiO
3
nanocrystals (Figure
2.6). Beyond the observed sextet indicative of Mn
2+
, the absence of any paramagnetic centers in
25
the g-value range of 1.907–1.974 belonging to Ti
3+
suggests that the concentration of reduced
titanium is below the EPR detection limit. Paramagnetic centers were observed at g-values of
2.004–2.005 for all three samples, which correspond to the presence of titanium vacancies
(V
Ti
).
21,22
Thus, the formation of titanium vacancies upon A-site substitution is the dominant
charge compensation mechanism occurring in this system. Regarding Dy
3+
doping, a recent
study by Rabuffetti et al. on Eu
3+
aliovalently doped into BaTiO
3
nanocrystals reported that Eu
3+
(r = 1.29 Å for a 12-coordinate environment; extrapolated value
6
) substitutes onto the A-site for
concentrations ≤ 1 mol%, with a transition to A- and B-site substitution at > 2 mol%.
17
Due to
the similar ionic radii of Eu
3+
and Dy
3+
, these results suggest that Dy
3+
may be preferentially
occupying the A-site at the doping level investigated here; however, high resolution structural
characterization is required to verify this hypothesis.
26
Figure 2.6. EPR spectra for undoped BaTiO
3
, 1.6 mol% La:BaTiO
3
, and 1.2 mol% Dy:BaTiO
3
nanocrystal powder samples at 78 K. The observed sextet corresponds to Mn
2+
EPR signals. The g-
values for the signals resulting from V
Ti
are shown.
With regards to the frequency dependent dielectric properties, the undoped BaTiO
3
showed a
permittivity of 103 at 1 kHz, which decreased to 91 by 100 kHz. Such permitivitty frequency
dependence results from interfacial polarization at lower frequencies.
23
All compositions
exhibited minimal frequency dependence, with reductions in permittivity ranging from 13-20%
by 100 kHz. Following the low frequency dependence, the nanocrystals were stable in
permittivity up to 2 MHz. Dielectric loss tangents between 0.05–0.08 were observed in the
Dy:BaTiO
3
and La:BaTiO
3
nanocrystals at 1 kHz, which decreases to between 0.02–0.05 by 100
kHz.
27
2.4. Experimental
2.4.1. Nanocrystal Synthesis
All manipulations were conducted under a nitrogen atmosphere at ambient pressure using
standard Schlenk techniques. Dy(acac)
3
·yH
2
O and La(acac)
3
·yH
2
O (acac = acetylacetonate,
C
5
H
7
O
2
) from Sigma Aldrich and a 0.5 M solution of BaTi(OR)
6
(R = CH
2
CHCH
3
OCH
3
) in n-
butanol/2-methoxypropanol from Gelest, Inc. were used as precursors. All reagents were used as
received. The synthetic apparatus applied herein is described in detail elsewhere.
1
In short, a
rotameter controls the flow of the carrier gas (nitrogen) through a bubbler containing 0.75 M
aqueous HCl, which is connected to a 100 mL, 3-neck round bottom flask containing the
precursor solution. Dy:BaTiO
3
and La:BaTiO
3
(x = 0–2 mol%) nanocrystals were synthesized
by first dissolving the appropriate mass of Ln(acac)
3
·yH
2
O (Ln = lanthanide) in 4 mL (2 mmol)
of BaTi(OR)
6
with stirring at 60 ˚C for 2 h under flowing nitrogen. For example, 4.8 mg (0.010
mmol) of Dy(acac)
3
·yH
2
O was used for the synthesis of 0.4 mol% Dy:BaTiO
3
nanocrystals.
After complete dissolution of the reagents, a translucent reddish-brown solution was observed.
After cooling to 25 ˚C, stirring was stopped and nitrogen/HCl/H
2
O vapor was allowed to flow
over the reaction solution, resulting in the formation of a rigid, translucent gel within 12 h.
Water vapor was allowed to flow for a total of 72 h. At this point, the flow of water vapor was
stopped and the resulting gel was isolated and added to 10 mL of absolute ethanol. The mixture
was sonicated for 20 min and centrifuged at 6500 rpm for 30 min to collect the solid.
For the surface treatment of undoped BaTiO
3
nanocrystal with Dy
3+
, the vapor diffusion sol-
gel method was performed on two 4 mL aliquots of the 0.5 M BaTi(OR)
6
precursor for 72 h.
Following completion, the two products were sonicated in 4 mL of ethanol until dispersed. To
sample 1, 1 mL of pure ethanol was added (5 mL total volume) and to sample 2, 1 mL of ethanol
28
containing 9.6 mg of Dy(acac)
3
•yH
2
O was added (5 mL total volume). Vapor diffusion sol-gel
was then performed on the two aliquots for an additional 48 h. The products were washed with
10 mL of pure ethanol and centrifuged at 6000 rpm for 15 min. The recovered nanocrystals were
dried under flowing nitrogen overnight. Dielectric characterization was then performed as
described in the experimental; however, the single point measurements provided below were
conducted on a GWInstek LCR-817 LCR meter at 1 kHz and 25 ˚C under nitrogen.
2.4.2. Materials Characterization
Thermogravimetry (TG): TG analyses were performed using a thermogravimetric analyzer TA
Q50 (TA Instruments) under a high-purity air flow (60 mL min
−1
). Samples were heated from 25
to 100 °C, held isothermal at 100 ˚C for 15 min to remove moisture, and subsequently ramped
from 100 to 800 ˚C at a linear rate of 10 ˚C min
−1
. Powder X-ray Diffraction (XRD): XRD
patterns were collected in the 20–80° 2θ range using a Rigaku Ultima IV diffractometer operated
at 44 mA and 40 kV. Cu Kα radiation (λ = 1.5406 Å) was employed. The step size and
collection time were 0.025˚ and 1 s per step, respectively. All diffraction patterns were collected
under ambient conditions. Elemental Analysis: Elemental analyses of were performed on all
samples by inductively coupled plasma optical emission spectroscopy (ICP-OES) at Galbraith
Laboratories (Knoxville, TN). Transmission Electron Microscopy (TEM): TEM images were
obtained using a JEOL JEM2100F (JEOL Ltd.) transmission electron microscope operating at
200 kV. Samples for TEM studies were prepared by drop-casting a stable suspension of
nanocrystals in ethanol on a 400 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.).
Selected Area Electron Diffraction (SAED): SAED patterns were obtained using a JEOL
JEM2100F (JEOL Ltd.) electron microscope operating at 200 kV. Samples for SAED studies
29
were prepared by drop-casting a stable suspension of nanocrystals in ethanol on a 400 mesh Cu
grid coated with an ultrathin lacey carbon film (Ted Pella, Inc.). Grids were previously cleaned
via ozone treatment for 60 min prior to sample deposition. Dielectric Characterization: Pellets
for dielectric studies were prepared by grinding ~200 mg of Dy:BaTiO
3
or La:BaTiO
3
nanocrystals with 1 mL of a 1 mg mL
‒1
aqueous solution of polyvinyl alcohol (PVA, 10 kDa).
The resulting slurry was allowed to dry for 12 h under a flowing nitrogen atmosphere. The dry
powder was pressed into a 13 mm diameter pellet by applying ~5 metric tons of pressure for 3
min in vacuo. Then, the pressed pellet was thermally treated at 150 ˚C for 2.5 h under flowing
nitrogen. The resulting pellet had a thickness of ~0.50 mm. Colloidal silver paint (Electron
Microscopy Sciences) was applied to both sides of the pellet to form a simple cylindrical
capacitor. Finally, the pellet was thermally treated at 100 ˚C for 2 h under flowing nitrogen and
subsequently stored under a nitrogen atmosphere. Capacitance and loss tangents were measured
using an Agilent 4294A Impedance Analyzer in a frequency range of 1 kHz to 2 MHz; all
measurements were carried out at 25 ˚C under a nitrogen atmosphere. Electron Paramagnetic
Resonance Spectroscopy (EPR): EPR spectra were collected at 78 K on a Bruker ELEXSYS
E580 X-band spectrometer equipped with an MS3 resonator. Measurements were performed on
~40 mg of powder in a frequency range of 9.6–9.7 GHz and at a constant microwave power of
0.6 mW.
2.5. Conclusions
In summary, the VDSG method was employed to synthesize aliovalently doped La:BaTiO
3
and Dy:BaTiO
3
nanocrystals at room temperature under ultra-benign conditions. Dielectric
properties and ICP-OES confirmed that the lanthanide ions could be incorporated at room
30
temperature without the need for post-synthetic annealing. The size-controlled, quasispherical
nanocrystals appear to be single crystalline, as indicated by high-resolution TEM. Maximum
relative permittivities of 176 and 206 were obtained for the 0.8 mol% La:BaTiO
3
and 1.2 mol%
Dy:BaTiO
3
nanocrystals, respectively, at 25 ˚C and 1 MHz. While we have previously
demonstrated that the relative permittivity of BaTiO
3
nanocrystals can be tuned by isovalent
substitution of Ba
2+
and Ti
4+
with Sr
2+
and Zr
4+
, respectively, these solid solutions require
substitution of ca. 33 mol% Sr
2+
for Ba
2+
or 15 mol% Zr
4+
for Ti
4+
to achieve maximum relative
permittivity.
24-26
Therefore, much lower aliovalent substitution levels can achieve similar effects
on the dielectric properties of BaTiO
3
nanocrystals using the vapor-diffusion sol-gel route.
2.6. References
(1) West, A. R.; Adams, T. B.; Morrison, F. D.; Sinclair, D. C. J. Eur. Ceram. Soc. 2004, 24,
1439.
(2) Moulson, A. J.; Herbert, J. M. Electroceramics. Chapman and Hall: London, 1990.
(3) Glinchuk, M. D.; Bykov, I. P.; Kornienko, S. M.; Laguta, V. V.; Slipenyuk, A. M.;
Bilous, A. G.; V’yunov, O. I.; Yanchevskii, O. Z. J. Mater. Chem. 2000, 10, 941.
(4) Vernon, J. P.; Hobbs, N.; Cai, Y.; Lethbridge, A.; Vukusic, P.; Deheyn, D. D.; Sandhage,
K. H. J. Mater. Chem. 2012, 22, 10435.
(5) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Adv. Mater. 2012, 24, 1434.
(6) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(7) Lu, D. Y.; Koda, T.; Suzuki, H.; Toda, M. J. Ceram. Soc. Jpn. 2005, 113, 721.
(8) Freeman, C. L.; Dawson, J. A.; Chen, H. R.; Harding, J. H.; Ben, L. B.; Sinclair, D. C. J.
Mater. Chem. 2011, 21, 4861.
(9) Morrison, F. D.; Sinclair, D. C.; Skakle, J. M. S.; West, A. R. J. Am. Ceram. Soc. 1998,
81, 1957.
31
(10) Kang, D. W.; Park, T. G.; Kim, J. W.; Kim, J. S.; Lee, H. S.; Cho, H. Electron. Mater.
Lett. 2010, 6, 145.
(11) Pu, Y.; Chen, W.; Chen, S.; Langhammer, H. T. Cerâmica 2005, 51, 214.
(12) Morrison, F. D.; Sinclair, D. C.; West, A. R. J. Appl. Phys. 1999, 86, 6355.
(13) Testino, A. Int. J. Appl. Ceram. Technol. 2013, 10, 723.
(14) Culver, S. P.; Rabuffetti, F. A.; Zhou, S.; Mecklenburg, M.; Song, Y.; Melot, B. C.;
Brutchey, R. L. Chem. Mater. 2013, 25, 4129.
(15) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
(16) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
(17) Rabuffetti, F. A.; Culver, S. P.; Lee, J. S.; and R. L. Brutchey, R. L. Nanoscale 2014, 6,
2909.
(18) Tinga, W. R.; Voss, W. A.; Blossey, D. F. J. Appl. Phys. 1973, 44, 3897.
(19) Freeman, C. L.; Dawson, J. A.; Harding, J. H.; Ben, L. B.; Sinclair, D. C. Adv. Funct.
Mater. 2012, 23, 491.
(20) Morrison, F. D.; Sinclair, D. C.; West, A. R. J. Am. Ceram. Soc. 2001, 84, 531.
(21) Kolodiazhnyi, T.; Petric, A. J. Phys. Chem. Solids 2003, 64, 953.
(22) Dunbar, T. D.; Warren, W. L.; Tuttle, B. A.; Randall, C. A.; Tsur, Y. J. Phys. Chem. B
2004, 108, 908.
(23) Hou, R. Z.; Ferreira, P.; Vilarinho, P. M. Chem. Mater. 2009, 21, 3536.
(24) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074.
(25) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
(26) Rabuffetti, F. A.; Brutchey, R. L. ACS Nano 2013, 7, 11435.
32
Chapter 3. Surface Modification of BaTiO
3
Inclusions in Polydicyclopentadiene
Nanocomposites for Energy Storage*
*Published in J. Appl. Polym. Sci. 2014, 131, 40290.
3.1. Abstract
A new nanocomposite system displaying high breakdown strength, improved permittivity, low
dielectric loss, and high thermal stability is presented. Free-standing nanocomposite films were
prepared via a solvent-free in-situ polymerization technique whereby 5 vol% BaTiO
3
nanocrystals with tailored surface chemistry were dispersed in dicyclopentadiene (DCPD) prior
to initiation of ring opening metathesis polymerization by a second generation Grubbs catalyst.
The relative permittivity was enhanced from 1.7 in the neat poly(DCPD) film to a maximum of
2.4 in the composite, while the dielectric loss tangent was minimized below 0.7%. Surface
modification of the BaTiO
3
nanocrystals mitigated reduction in breakdown strength of the
resulting nanocomposites such that only a 13% reduction in breakdown strength was observed
relative to the neat polymer films.
3.2. Introduction
Recently, there has been significant interest in the development of capacitors that can meet
energy storage needs that require high breakdown strength, low dielectric loss, and high thermal
stability. By using a nanocomposite approach, it has become possible to take advantage of the
solution processability and high breakdown strength of polymers combined with the high
permittivity of inorganic nanoparticles towards achieving this goal. Among the available
inorganic filler materials for nanocomposite capacitors, BaTiO
3
(BT) has become prevalent
33
because of its high dielectric constant.
1,2
As a result, the addition of BT nanocrystals into
polymer matrices has been shown to systematically increase composite permittivity. The main
drawback of using inorganic fillers is that their inclusion, even at low volume loadings (≤ 5
vol%), often leads to a cataclysmic reduction of the nanocomposite breakdown strength,
lessening any benefit achieved from increased permittivity in terms of energy storage
capability.
3–6
Polydicyclopentadiene (pDCPD) is a crosslinked thermoset polymer that can be prepared via
ring opening metathesis polymerization of dicyclopentadiene (DCPD) with Grubbs catalyst.
7–9
Polymerization occurs within the neat monomer (i.e., under solvent-free conditions) and is
initiated by very low catalyst loadings (< 1 wt%). pDCPD has found widespread application as a
result of its high thermal stability and chemical resistance, low water uptake, and excellent
mechanical properties.
8,10
In comparison to the well-known thermoplastics polypropylene and
polystyrene, pDCPD displays similar permittivity but far better thermal stability (i.e., up to 500
°C), thereby extending its use to a variety of high temperature applications.
11,12
While several
studies have explored the physical and mechanical attributes of various pDCPD
nanocomposites,
13–15
there has been very little work on investigating the dielectric properties.
Yin and coworkers recently published the first known study on the dielectric properties of
pDCPD-based nanocomposites, using fumed silica inclusions (10 wt% loading).
12
In their work,
neat pDCPD films exhibited breakdown strengths as high as 750 V µm
-1
with very low dielectric
loss (tan d = 0.5%). Although the pDCPD/SiO
2
composites demonstrated improvements in the
relative permittivity and corona resistance under an AC bias relative to the neat polymer, the DC
breakdown strengths were not reported.
34
To date there have been no studies on incorporating BT inclusions into pDCPD
nanocomposites. Herein we investigate the dielectric properties of a model pDCPD/BT
nanocomposite. Nanocrystals of BT were surface modified with 10-undecenoic acid to affect the
nanocrystal-polymer interface and mitigate losses in breakdown strength. It should be noted that
while several studies have probed the effects of phosphonic acid modified inclusions,
4,6
the
utility of carboxylic acid ligands has received far less attention. A distinct ligand-dependent
effect on the measured breakdown strength was observed, while enhancements in composite
permittivity and lowered dielectric loss tangents were maintained.
3.3. Results and Discussion
3.3.1. BaTiO
3
Nanocrystal Surface Modification
Powder X-ray diffraction (XRD) confirmed the crystallinity and phase purity of the BT
nanocrystals, which were determined to be in the cubic perovskite phase with a measured lattice
constant of a = 4.04 ± 0.05 Å (JCPDS no. 75-0215; Figure 3.1a). The nanocrystals were
observed to be quasispherical with some anisotropy throughout and possessed an average size of
84 ± 16 nm (Figure 3.1b).
35
Figure 3.1. (a) XRD patterns of BT and mBT nanocrystals, and pDCPD/BT and pDCPD/mBT
nanocomposite films. (b) Representative image of BT nanocrystals. Inset depicts the associated size
distribution histogram with N = 100.
Surface modification of the BT nanocrystals via sonication with 10-undecenoic acid (an
unsaturated carboxylic acid) was employed toward affecting the resultant nanocomposite
properties, hereafter referred to as mBT. Sonication allows for a more rapid and lower
temperature surface modification procedure as compared to traditional methods.
16–19
Functionalization of the nanocrystals was confirmed by FT-IR spectroscopy (Figure 3.2.). The
presence of strong alkyl v(C–H) stretching bands at 2960 and 2860 cm
−1
and a weaker alkenyl
v(C–H) band at 3080 cm
−1
verified binding of the carboxylic acid (see inset). The symmetric
v(CO
2
) stretching band of the carboxylic acid was located at 1421 cm
−1
, while the asymmetric
v(CO
2
) stretching band was found at 1548 cm
−1
. The associated separation between the
symmetric and asymmetric bands can be used to reveal the carboxylate binding mode.
20-22
The
observed difference of 127 cm
−1
suggests bidentate chelation by the carboxylate moieties on the
nanocrystal surface. After extensive solvent rinsing, there remained approximately 81% of a
theoretical monolayer of 10-undeceneoic acid on the surface, as determined by
36
thermogravimetric analysis (TGA, Figure 3.2b) and calculated surface area values, assuming an
area of ~0.21 nm
2
for each carboxylate group.
23,24
Figure 3.2. (a) FT-IR spectrum of mBT nanocrystals. Inset shows the zoomed FT-IR spectra of BT and
mBT nanocrystals with the alkenyl C–H stretching band highlighted for clarity. TGA cuvers for (b) BT
and mBT nanocrystals and (c) neat pDCPD, pDCPD/BT, and pDCPD/mBT free-standing films are also
provided.
3.3.2. Nanocomposite Preparation and Physical Properties
Cross sectional SEM micrographs of the nanocomposite films confirm the measured film
thicknesses – all films were determined to be between 20−40 µm (Figure 3.3). The neat pDCPD
films were extremely flexible, optically clear, and displayed a faint yellow hue as a result of the
ruthenium catalyst. Upon addition of 5 vol% BT and mBT, the nanocomposites became opaque
yet maintained their excellent mechanical properties, as the composites remained both flexible
and fracture free (Figure 3.3e). The very low viscosity of the DCPD monomer ensured effective
mixing of the nanocrystal inclusions. Once surface modified, the BT nanocrystals demonstrated
improved processability and a reduction in agglomerate size in the resultant films was observed
by SEM (Figure 3.3). No gross agglomerates on the order of 10‒25 µm were noted in the
pDCPD/mBT films, as were found in the pDCPD/BT films. Furthermore, XRD patterns of the
37
nanocomposite films confirmed that the BT nanocrystals were phase pure after processing, with
no indication of BaCO
3
formation.
Figure 3.3. High magnification cross-sectional SEM images of (a) pDCPD/BT and (c) pDCPD/mBT
free-standing films. Lower magnification cross-sectional SEM images of (b) pDCPD/BT and (d)
pDCPD/mBT free-standing films. Gross agglomerates observed in the pDCPD/BT free-standing films are
circled for clarity. (e) Digital photograph of pDCPD/BT nanocomposite film demonstrating film
flexibility.
In regards to thermal stability, the pDCPD/BT composites (referring to both the modified and
unmodified BT nanocomposites) were quite thermally robust, as confirmed by TGA. Figure 3.2c
demonstrates that addition of the unmodified nanocrystals had no effect on the thermal properties
relative to the neat pDCPD films, which possessed a decomposition onset at 450 °C under
nitrogen. On the other hand, the pDCPD/mBT nanocomposite displayed slightly reduced
thermal stability, with a decomposition onset occurring at 390 °C. The lower temperature mass
loss is largely attributed to the ω-olefin functionalized BT nanocrystals crosslinking with the
pDCPD matrix during polymerization.
25‒27
The crosslinking disrupts the extended pDCPD
crosslinked network, likely producing regions with reduced thermal stability. Additionally, a
small amount of mass loss stems from the decomposition of the unreacted surface bound ligands,
consistent with the decomposition onset obtained for the mBT nanocrystals (Figure 3.2b).
38
3.3.3. Nanocomposite Dielectric Properties
The relative permittivity and dielectric loss of the free-standing nanocomposite films were
measured in a frequency range of 1 kHz to 2 MHz (Figure 3.4). The permittivity of the
pDCPD/BT nanocomposites was higher than the neat pDCPD films at all frequencies tested. For
example, the relative permittivity of pDCPD was 1.7 at 1.0 MHz and increased to 2.4 after the
addition of 5 vol% BT. Upon addition of 5 vol% mBT, the nanocomposite retained a
permittivity of 2.3 at 1 MHz. Importantly, dielectric loss tangents remained below 0.7% for all
free-standing films at frequencies up to 2 MHz. Dielectric loss tangent below 1% is generally
desirable for capacitor applications, as it reduces the likelihood for leakage currents that facilitate
premature breakdown at low electric fields.
28
Therefore, the low dielectric losses observed for
all pDCPD nanocomposites in this work are practically competitive for low loss applications.
Figure 3.4. Relative permittivity and dielectric loss tangent (tan δ) of pDCPD, PDCPD/BT, and
pDCPD/mBT free-standing films as a function of frequency (25 °C).
Dielectric breakdown strengths were calculated using a two-parameter Weibull distribution
function with the cumulative probability of breakdown, P, defined as:
39
where V is the measured breakdown voltage, E
BD
is the breakdown strength at 63.2% cumulative
probability, and β is the shape parameter which indicates the width of the distribution.
Distributions are provided in Figure 3.5. The breakdown strength of the neat pDCPD film was
found to be 541 V µm
−1
. Upon inclusion of 5 vol% of BT nanocrystals, the breakdown strength
drops more than 25% to 405 V µm
−1
, which is consistent with previous reports of BT-based
nanocomposites.
3,29,30
Traditionally, BT-based nanocomposites display a significant decrease in
breakdown strength even at low loadings.
29
For example, Dang and coworkers prepared
polyimide/BT nanocomposites and observed a 40% decrease in breakdown strength at 5 vol%
loading, with improved energy storage properties only achieved at loadings exceeding 30
vol%.
3,30
Almadhoun et al. found that the incorporation of <100 nm BaTiO
3
in
poly(vinylidenefluoride-co-trifluoroethylene) caused the breakdown strength to fall 25% from
225 to 170 V µm
‒1
at 5 vol% loading.
31
In their work, by hydroxylating the surface of the
nanoinclusions, the authors were able to mitigate the decrease to 200 V µm
‒1
at the same loading
via weak interfacial dipole interactions. Furthermore, Kim and coworkers were able to limit the
decrease in breakdown strength at 5 vol% in a similar ferroelectric polymer system (i.e.,
poly(vinylidenefluoride-co-hexafluoropropylene)) by surface modifying the BaTiO
3
inclusions
with pentafluorobenzyl phosphonic acid.
6
More recently, Beier et al. demonstrated that the
incorporation of a homogenous dispersion of ~10 nm Ba
0.7
Sr
0.3
TiO
3
nanocrystals in a polyimide
matrix enhanced the breakdown strength from 240 to ~290 V µm
‒1
at 5 vol% loading.
32
Herein,
surface modificiation of BT nanocrystals with 10-undecenoic acid resulted in nanocomposites
with improved high voltage endurance relative to the unmodified inclusions and exceptionally
P(V)=1−exp −
V
E
BD
"
#
$
%
&
'
β
(
)
*
*
+
,
-
-
40
high breakdown strength overall. The pDCPD/mBT nanocomposite displayed a breakdown
strength of 468 V µm
−1
, representing only a 13% reduction relative to the neat polymer films,
which corresponds to a competitive reduction in breakdown strength as compared to previous
nanocomposite systems with comparable volume loadings. The presence of a non-polar organic
shell not only improves nanocrystal dispersion, but likely mitigates the permittivity offset
between the host matrix and the inclusions, thereby dampening the effects of localized field
enhancements.
4,5,33,34
In this system, surface modification has effectively been shown to mitigate
substantial drops in breakdown strength at low loadings, while retaining the beneficial dielectric
properties induced by the high permittivity BT nanocrystals.
Figure 3.5. Two-parameter Weibull plots of pDCPD, pDCPD/BT, and pDCPD/mBT free-standing films.
Gray line represents the Weibull breakdown strength at 63.2% cumulative probability.
41
3.4. Experimental
3.4.1. General Considerations
All manipulations were performed under a nitrogen atmosphere using dry, air-free solvents
throughout. BaTiO
3
nanocrystals (BT) (≥ 99.9% trace metals basis; U.S. Research
Nanomaterials, Inc.), 10-undeceneoic acid (99%; Alfa Aesar), dicyclopentadiene (DCPD) (95%;
Alfa Aesar), [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(phenylmethylene)
(tricyclohexylphosphine)ruthenium (Grubbs catalyst, second generation; Materia, Inc.),
potassium permanganate (98%; Alfa Aesar), oxalic acid dihydrate (98%; Alfa Aesar), and
concentrated sulfuric acid (EMD Chemicals, Inc.) were all used as received.
3.4.2. Surface Modification of BaTiO
3
Nanocrystals
In order to modify the nanocrystal surfaces, 1.58 g (8.60 mmol) of 10-undeceneoic acid was
dissolved in 50 mL of dry toluene and added directly to a 100 mL round bottom flask containing
1.0 g of BT nanocrystals. The flask was placed under flowing nitrogen and sonicated for 1 h at
30 °C before the nanocrystals were isolated via centrifugation (6000 rpm for 15 min). The
resulting product was washed with toluene (3 × 30 mL) before being dried in vacuo (20 °C, 0.05
mmHg) for 24 h, and stored in a dry, air-free environment.
3.4.3. Material Characterization
Transmission electron microscopy (TEM) images were collected on a JEOL JEM-2100
microscope at an operating voltage of 200 kV, which was equipped with a Gatan Orius CCD
camera. Samples for TEM analysis were prepared by drop casting suspensions of the
nanocrystals in toluene onto ultrathin carbon film supported on 400 mesh copper grids (Ted
42
Pella, Inc.). Scanning electron microscopy (SEM) images were collected on a JEOL JSM-
6610LV microscope in high-vacuum mode using an accelerating voltage of 10 kV. Cross-
sectional samples were prepared by freeze-fracturing a film, pressing it between two glass plates,
and mounting it on top of an aluminum stub. To prevent film charging, a thin layer of carbon
was deposited on all films. Fourier transform infrared (FT-IR) spectra were collected on a
Bruker Vertex 80v at a scanning interval of 0.5 cm
-1
with a resolution of 2 cm
-1
in vacuo.
Powder X-Ray diffraction (XRD) was performed on a Rigaku Ultima IV diffractometer using Cu
Kα (λ = 1.54 Å) radiation. For XRD samples of nanocrystalline powder and pDCPD/BaTiO
3
films, the powder or film was placed directly onto a zero diffraction single crystalline silicon
substrate (MTI Corporation, Inc.) and patterns were collected in the 20−80 2θ range. All
samples for thermogravimetric analysis (TGA) were run on a TA instruments Q50
thermogravimetric analyzer and were dried within the instrument for 30 min under flowing
nitrogen at 100 °C, followed by ramping to 650 °C at a rate of 10 °C min
−1
.
3.4.4. Nanocomposite Preparation
The pDCPD/BT nanocomposites were prepared via an in situ polymerization route. In a typical
experiment, BT nanocrystals (5 vol%) were added to 1.0 g DCPD. The DCPD was heated to
35−40 °C to melt the monomer, and then the mixture was sonicated and vortexed. To initiate
polymerization, 2 mg (0.2 wt%) of second generation Grubbs catalyst was dissolved in 0.1 mL
dry dichloromethane immediately prior to being added to the monomer/nanocrystal mixture.
The solution was continually mixed until a slight increase in viscosity was evident (ca. 1 min),
and then 0.4 mL of the reaction mixture was removed and immediately cast between two 6” × 6”
cleaned glass plates and pressed with 50 N of force for 1 h at 20 °C. Film thicknesses were
controlled by placing 25.4 µm thick polymer shims (Practi-Shim; Accutrex Products, Inc.)
43
between the glass plates prior to casting. The pressed film was then placed into an oven at 100
°C for 24 h under flowing nitrogen to ensure complete polymerization. The film was slowly
cooled to room temperature under flowing nitrogen for 2 h before removal from the oven and
lifting the film. Upon reaching room temperature, the free-standing film was lifted from the
glass plates by immersing them into deionized water. The film was blotted dry and cut into
squares 1.27 × 1.27 cm
2
in size and stored under nitrogen. Film thickness ranged between 20−40
µm as measured by a custom metrology tool with ±1 µm accuracy. Glass plates were cleaned by
immersion in a very dilute solution of KMnO
4
/H
2
SO
4
(1:1 approximate molar ratio) for a
minimum of 48 h. The plates were removed from the solution, rinsed with deionized water, and
then soaked in a dilute aqueous solution of oxalic acid for 30 min. The plates were removed
from the oxalic acid bath, rinsed with deionized water, dried under flowing nitrogen, and used
immediately.
3.4.3. Nanocomposite Characterization
In order to measure the capacitance and dielectric loss of the as-prepared free standing films, a
custom dielectric testing station comprised of flat circular aluminum electrodes was used. The
film was placed directly between the two electrodes and pressed, forming a parallel plate
geometry (17 mm
2
plate area). Capacitance and loss tangents were measured on an Agilent
4294A impedance analyzer with a frequency sweep of 1 kHz – 2 MHz. Dielectric breakdown
measurements were performed using a gold plated hemispherical rod (1 mm in diameter) that
made direct contact with the pDCPD/BaTiO
3
film. Upon contact, the film was immersed into a
mineral oil bath, then subjected to a DC negative polarity bias at a linear ramp rate of 2500 V s
−1
(Bertan 225, 20 kV DC power supply), connected to a high-voltage probe (Tektronix P6015A;
Tektronix, Inc.), and monitored by an oscilloscope (Tektronix TDS 2004C; Tektronix, Inc).
44
Breakdown events were indicated by a spontaneous increase in current. One breakdown test was
performed per 1.6 cm
2
square, with 15−20 independent measurements conducted for each
nanocomposite. The points for breakdown tests were chosen at random, and the thickness was
measured after each breakdown event within close proximity to the failure site.
3.5. Conclusions
Nanocrystals of BT were successfully dispersed in pDCPD films via an in-situ polymerization
route to investigate the effects of inclusion surface modification on the dielectric properties of a
novel pDCPD/BT nanocomposite. At 5 vol% BT nanocrystal loading, the relative permittivity
of the nanocomposites increased from 1.7 in the neat pDCPD film to 2.4 in the pDCPD/BT
composite, while low dielectric loss tangents (< 0.7%) were obtained for all compositions up to 2
MHz. Furthermore, the pDCPD/mBT nanocomposite exhibited a breakdown strength of 468 V
µm
−1
, which represented only a 13% reduction from that of the neat pDCPD films. Therefore,
surface modification of the BT nanocrystal inclusions with 10-undecenoic acid was shown to
enhance breakdown strengths relative to the unmodified inclusions, while maintaining increased
permittivity, low dielectric loss, and excellent thermal stability. Though further optimization of
the polymer dielectric properties is required to achieve higher filler loadings, pDCPD represents
a promising polymer matrix towards improving nanocomposite-based energy storage devices.
45
3.6. References
(1) Lines, M. E.; Glass, A. M. Principles and applications of ferroelectrics and related
materials, Oxford classic texts in the physical sciences, Oxford University Press: New
York, 2001.
(2) Newnham, R. E.; Cross, L. E. MRS Bull. 2011, 30, 845.
(3) Dang, Z. M.; Lin, Y. Q.; Xu, H. P.; Shi, C. Y.; Li, Y. S.; Bai, J. Adv. Funct. Mater. 2008,
18, 1509.
(4) Kim, P.; Jones, S. C.; Hotchkiss, P. J.; Haddock, J. N.; Kippelen, B.; Marder, S. R.;
Perry, J. W. Adv. Mater. 2007, 19, 1001.
(5) Barber, P.; Balasubramanian, S.; Anguchamy, Y.; Gong, S.; Wibowo, A.; Gao, H.;
Ploehn, H. J.; Zur Loye, H. C. Materials 2009, 2, 1697.
(6) Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M. J.; Marder, S. R.; Li, J.;
Calame, J. P.; Perry, J. W. ACS Nano 2009, 3, 2581.
(7) Slugovc, C. Macromol. Rapid Comm. 2004, 25, 1283.
(8) Grubbs, R. H. Handbook of metathesis, Wiley-VCH: Weinheim, Germany, 2003.
(9) Kissin, Y. V. Kirk-Othmer Encyclopedia of Chemical Technology, Wiley: New Jersey,
2005.
(10) Grubbs, R. H. Tetrahedron 2004, 60, 7117.
(11) Rabuffi, M.; Picci, G. IEEE T. Plasma Sci. 2002, 30, 1939.
(12) Yin, W.; Kniajanski, S.; Amm, B. IEEE 2010, 1.
(13) Simons, R.; Guntari, S. N.; Goh, T. K.; Qiao, G. G.; Bateman, S. A. J. Polym. Sci. Part A.
2012, 50, 89.
(14) Jeong, W.; Kessler, M. R. Chem. Mater. 2008, 20, 7060.
(15) Yoonessi, M.; Toghiani, H.; Kingery, W. L.; Pittman, C. U. Macromolecules 2004, 37,
2511.
(16) Xu, C.; Xu, K.; Gu, H.; Zheng, R.; Liu, H.; Zhang, X.; Guo, Z.; Xu, B. J. Am. Chem. Soc.
2004, 126, 9938.
(17) Shultz, M. D.; Reveles, J. U.; Khanna, S. N.; Carpenter, E. E. J. Am. Chem. Soc. 2007,
129, 2482.
46
(18) Huang, W.; Lin, Y.; Taylor, S.; Gaillard, J.; Rao, A. M.; Sun, Y. P. Nano Lett. 2002, 2,
231.
(19) Koshio, A.; Yudasaka, M.; Zhang, M.; Iijima, S. Nano Lett. 2001, 1, 361.
(20) Zhang, L.; He, R.; Gu, H. C. Appl. Surf. Sci. 2006, 253, 2611.
(21) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delon, J. F. Langmuir 1993, 9,
2370.
(22) Mielczarski, J. A.; Cases, J. M. Langmuir 1995, 11, 3275.
(23) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.
(24) Sahoo, Y.; Goodarzi, A.; Swihart, M. T.; Ohulchanskyy, T. Y.; Kaur, N.; Furlani, E. P.;
Prasad, P. N. J. Phys. Chem. B 2005, 109, 3879.
(25) Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. J. Am. Chem. Soc. 2002, 124, 5729.
(26) Tavasoli, E.; Guo, Y.; Kunal, P.; Grajeda, J.; Gerber, A.; Vela, J. Chem. Mater. 2012, 24,
4231.
(27) Liu, X.; Basu, A. J. Organomet. Chem. 2006, 691, 5148.
(28) Dissado, L. A.; Fothergill, J. C. Electrical degradation and breakdown in polymers, IEE
materials and devices series 9, Peter Peregrinus Ltd.: London, United Kingdom, 1992.
(29) Khalil, M. S. IEEE T. Dielect. El. In. 2000, 7, 261.
(30) Feng, Y.; Yin, J.; Chen, M.; Liu, X.; Li, G. IEEE 2011, 226.
(31) Almadhoun, M. N.; Bhansali, U. S.; Alshareef, H. N. J. Mater. Chem. 2012, 22, 11196.
(32) Beier, C. W.; Sanders, J. M.; Brutchey, R. L. J. Phys. Chem. C 2013, 117, 6958.
(33) Jung, H. M.; Kang, J. H.; Yang, S. Y.; Won, J. C.; Kim, Y. S. Chem. Mater. 2010, 22,
450.
(34) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. Langmuir 2009, 26, 5067.
47
Chapter 4. Low Temperature Synthesis of AMoO
4
(A = Ca, Sr, Ba) Scheelite
Nanocrystals*
*Published in Chem. Mater. 2013, 25, 4129–4135.
4.1. Abstract
An extension of the vapor diffusion sol-gel method to the synthesis of the AMoO
4
(A = Ca, Sr,
Ba) scheelite family of materials is reported. Sub-30 nm quasispherical nanocrystals were
obtained after vapor diffusion at room temperature, followed by thermal aging at 80 °C.
Rietveld analysis of X-ray diffraction data, Raman spectroscopy, transmission electron
microscopy and energy dispersive X-ray spectroscopy demonstrated that the vapor diffusion sol-
gel method affords crystalline and phase pure AMoO
4
nanocrystal with excellent compositional
control. The potential lithium storage capacity of the CaMoO
4
nanocrystals versus Li at a rate of
C/4 was also investigated. The nanocrystals exhibited an extremely large first discharge capacity
of 1300 mA h g
−1
, which stabilized at 250 mA h g
−1
after 25 cycles.
4.2. Introduction
Scheelite-structured alkaline earth molybdates with the formula AMoO
4
(A = Ca, Sr, Ba) have
been employed as functional materials in energy storage and conversion applications (e.g., solid
state phosphors, cryogenic scintillation detectors, and Li-ion batteries, among others).
1–3
In order
to harness these applications, a myriad of synthetic techniques have been exploited. Classic solid
state, sol-gel, molten salt, and hydrothermal routes have previously been used;
4–7
these
approaches require high temperature and/or pressure to achieve a crystalline and phase pure
product. Recently, more benign preparations have been developed, including microemulsion,
48
solution precipitation, and aqueous mineralization techniques, but such methods require the use
of surfactants, complexing agents and/or mineralizers.
8–10
Moreover, while these low
temperature techniques allow the preparation of scheelite micro- and nanocrystals spanning
several microns to 30 nm in size, the sub-30 nm regime remains largely unexplored.
Over the past few years, our group has developed a vapor diffusion sol-gel (VDSG) method
that affords phase pure metal oxide nanocrystals under ultrabenign conditions (low temperature,
ambient pressure, and near neutral pH).
11–15
The VDSG method relies on the interfacial
hydrolysis and condensation of alkoxide precursors upon diffusion of water vapor into the
alkoxide solution. Previously, this method has been exclusively applied to the synthesis of
perovskite oxide nanocrystals of formula A
1-x
A´
x
B
1-y
B´
y
O
3
(A = Ba, Sr and B = Ti, Zr; 0 ≤ x ≤ 1,
0 ≤ y ≤ 1).
11,12
Herein, we report the extension of the VDSG method to the low temperature
synthesis of sub-30 nm scheelite-structured AMoO
4
nanocrystals. Chemical, structural, and
morphological characterization, as well as preliminary results on the potential utility of these
nanocrystals as a Li-ion battery electrode material are provided.
4.3. Results and Discussion
The crystallization of metal oxides using the VDSG method progresses through kinetically
controlled hydrolysis and condensation of metal alkoxides upon diffusion of water vapor.
13
As
previously stated, the VDSG method has been exclusively applied to the synthesis of perovskite
nanocrystals (e.g., BaTiO
3
, SrTiO
3
, and BaZrO
3
).
11,12
For this family of perovskite nanocrystals,
the desired crystalline oxide products could only be obtained using bimetallic alkoxide
precursors under room temperature conditions.
15
Herein, crystalline and phase pure AMoO
4
nanocrystals were synthesized using individual metal precursors (e.g., MoO
2
(acac)
2
mixed with
49
monometallic alkaline earth alkoxides), thereby expanding the versatility of the VDSG method
to the synthesis of other families of functional materials. Upon dissolution of MoO
2
(acac)
2
into
the appropriate alcohol solution of alkaline earth alkoxide, diffusion of water vapor allows for
the kinetically controlled hydrolysis and cross polycondensation at the liquid-vapor interface
within the precursor solution. Thermal aging of the resulting gel at 80 °C affords sub-30 nm
AMoO
4
nanocrystals. Ceramic yields were estimated to be 82, 88, and 90% for CaMoO
4
,
SrMoO
4
and BaMoO
4
, respectively. The organic content was estimated by TGA to be ≤ 5 wt%
for each material, and can be attributed to unreacted surface alkoxy groups retained from the
starting precursors (Figure 4.1).
Figure 4.1. TGA thermorgrams of AMoO
4
nanocrystals.
Furthermore, while the VDSG method afforded BaTiO
3
, SrTiO
3
, and BaZrO
3
nanocrystals at
room temperature without the need for a thermal aging step, XRD and TEM results collected on
AMoO
4
samples prior to thermal aging indicated that this step was required for the scheelite
materials (Figure 4.2). XRD patterns of AMoO
4
before and after the thermal aging step at 80 °C
illustrate an increase in intensity and sharpness of the diffraction peaks upon thermal aging,
50
which is suggestive of an increase in crystallinity. Indeed, the coexistence of amorphous and
crystalline material was found in the AMoO
4
samples prior to thermal aging insomuch as
crystalline nuclei embedded in an amorphous matrix were observed by TEM. Additionally, the
ratio of the crystalline to amorphous fraction increases upon going from CaMoO
4
to SrMoO
4
and
finally to BaMoO
4
. In other words, though thernal aging was not required to initiate nucleation,
it was essential to drive the amorphous to crystalline phase transition to completion, especially in
the case of CaMoO
4
.
Figure 4.2. (top) XRD patterns of AMoO
4
nanocrystals before and after the thermal aging step. (bottom)
TEM Images of (a) CaMoO
4
, (b) SrMoO
4
, and (c) BaMoO
4
nanocrystals before the thermal aging step.
XRD patterns confirming the crystallinity and phase purity of the products obtained using the
VDSG method are shown in Figure 4.3. The diffraction maxima can all be indexed to the
51
tetragonal scheelite structure (JCPDS, No. 29−0351, 85−0809, and 29−0193 for CaMoO
4
,
SrMoO
4
and BaMoO
4
, respectively) with no impurities present. All AMoO
4
nanocrystals are
isostructural, belonging to the I4
1
/a (no. 88) space group. Upon going from CaMoO
4
to SrMoO
4
and finally to BaMoO
4
, a shift of the diffraction maxima to lower 2θ is observed, indicating an
expansion of the unit cell with increasing A-site cation radius (rA
2+
= 1.12, 1.26 and 1.42 Å for
Ca, Sr and Ba, respectively).
22
Figure 4.3. Rietveld analysis of powder XRD patterns of AMoO
4
nanocrystals. Experimental (×) and
calculated (⎯) patterns are shown for each sample along with the difference curve (⎯) and tickmarks (⏐)
corresponding to the phase refined.
Rietveld analysis of the XRD patterns of AMoO
4
samples was carried out using the tetragonal
I4
1
/a space group. Structural parameters extracted from Rietveld analysis are plotted in Figure
52
4.4 and summarized in Table 4.1. Visual inspection of the fits to the experimental data, as well
as the low values of the R
wp
and χ
2
goodness-of-fit indicators, further confirm that all three
AMoO
4
samples under study are isostructural and that the crystal structure is adequately
described by a tetragonal scheelite structure. The lattice constants and unit cell volume increase
linearly upon going from Ca to Sr and finally to Ba.
3,7,23
The atomic arrangement within the
tetragonal unit cell consists of AO
8
dodecahedra and MoO
4
tetrahedra; the latter are isolated from
each other and each A
2+
ion shares corners with eight adjacent MoO
4
tetrahedra. The
dodecahedral oxygen coordination environment of A
2+
can be described with two sets of A−O
distances, each set corresponding to the bond to four oxygen atoms (4 + 4). The tetrahedral
oxygen coordination environment of Mo
4+
can be described with a single Mo−O distance;
however, two sets of O−Mo−O angles (2 + 2) were found for all three AMoO
4
structures. These
observations indicate that the coordination polyhedra of both A
2+
and Mo
4+
ions are not regular;
further structural characterization is underway to clarify the dependence of the polyehdra shape
on chemical composition. Regarding the variation of metal−oxygen distances with chemical
composition, it should be noted that A−O and Mo−O distances increase and decrease,
respectively, upon going from Ca to Sr and finally to Ba; this bond length variation is
significantly more pronounced for A−O bonds, in agreement with the previously reported
quasirigidity of Mo−O bonds in scheelites.
24
Therefore, the observed expansion of the unit cell
upon increasing the ionic radius of the alkaline-earth cation is driven by the expansion of the
AO
8
dodecahedra.
53
Figure 4.4. Lattice parameters a and c and unit cell volume V (top panel), and metal−oxygen distances
(bottom panel) in AMoO
4
nanocrystals as a function of the ionic radius of the A
2+
ion; two sets of A−O
distances are depicted with closed and open symbols. Dotted lines are a guide-to-the-eye.
54
Table 4.1. Rietveld analysis of XRD data of AMoO
4
nanocrystals
CaMoO
4
SrMoO
4
BaMoO
4
a (Å) 5.2253(5) 5.3931(7) 5.5785(6)
c (Å) 11.4326(11) 12.0063(17) 12.8000(15)
V (Å
3
) 312.16(9) 349.21(15) 398.33(14)
x, y, z O
0.6461(2),
0.5095(2),
0.20873(8)
0.6354(3),
0.5188(3),
0.20460(11)
0.6184(4),
0.5242(5),
0.20106(19)
U
A
(Å
2
)
a
0.78(3) 1.31(3) 0.58(4)
U
Mo
(Å
2
)
a
0.98(2) 0.41(3) 1.30(4)
U
O
(Å
2
)
a
1.86(5) 1.37(6) 3.06(13)
R
wp
7.8 7.1 9.1
χ
2
1.16 1.14 1.16
a
Given as 100×U.
The crystal structure of the AMoO
4
scheelites was further probed using Raman spectroscopy.
Corresponding spectra, along with tentative assignments of the vibrational bands, are shown in
Figure 4.5. All vibrational bands can be assigned to the AMoO
4
scheelite structure, which
further confirmed the phase purity of the oxide nanocrystals.
25–27
Vibrational bands are divided
into three groups for clarity. The first group (< 250 cm
−1
) is comprised of translational (T) and
rotational (R) bands associated with the interactions between the A-site cations and the [MoO
4
]
2−
tetrahedra (T(E
g
), T(B
g
), R(A
g
), and R(E
g
)).
25
The second (250–450 cm
−1
) and third groups (750–
900 cm
−1
) of vibrational bands arise from bending (A
g
, B
g
, B
g
, and E
g
) and stretching modes (B
g
,
E
g
, and A
g
) of the [MoO
4
]
2−
tetrahedra, respectively.
26,27
55
Figure 4.5. Raman spectra of the AMoO
4
nanocrystals. Bands arising from the scheelite phase are
denoted with the ⏐ symbol and their assignments are provided.
The morphology of the AMoO
4
scheelites was investigated via TEM, and corresponding
images are shown in Figure 4.6. The nanocrystals all exhibit quasispherical shapes with an
average diameter of 9.3 ± 2.7, 7.9 ± 1.9, and 12.3 ± 2.8 nm for CaMoO
4
, SrMoO
4
, and BaMoO
4
,
respectively. Furthermore, the nanocrystals appear to be single crystalline as indicated by the
well-defined lattice fringes observed by high resolution TEM imaging. Lattice fringes from
CaMoO
4
with a d-spacing of 0.29 nm corresponding to the (004) lattice planes are provided in
Figure 4.5d.
56
Figure 4.6. TEM images of (a) CaMoO
4
, (b) SrMoO
4
, and (c) BaMoO
4
nanocrystals. (d) High resolution
TEM image showing lattice fringes from a single CaMoO
4
nanocrystal corresponding to the (004) lattice
planes.
Several groups have recently reported on the synthesis of AMoO
4
nanocrystals with
interesting morphologies. Gong et al. exploited microemulsions to synthesize crystalline
BaMoO
4
with a variety of shapes (i.e., particles, microrods, and dendrites) ranging from 100 nm
to 15 μm in size using cetyltrimethylammonium bromide (CTAB) as the surfactant.
28
Employing
a similar approach, Shi et al. replaced CTAB with a mixture of undecylic acid and decylamine to
achieve BaMoO
4
nanobelts with lengths on the order of tens of microns.
29
Chen et al. prepared
5–6 μm CaMoO
4
and SrMoO
4
dumbbells, rods and spherules using an aqueous mineralization
technique in alkaline media.
10
In contrast, the VDSG method affords sub-30 nm, quasispherical
AMoO
4
nanocrystals at low temperature and near neutral pH without the use of surfactants,
complexing agents, or mineralizers.
The chemical composition of the AMoO
4
nanocrystals prepared by the VDSG method was
probed with EDS and XPS techniques. EDS spectra from averaged groups of 50–100
57
nanocrystals are shown in Figure 4.7a and from their analysis a 47:53, 51:49, and 51:49 A:Mo
ratio was extracted for Ca:Mo, Sr:Mo, and Ba:Mo, respectively, which are in good agreement
with the nominal composition of the nanocrystals. These results highlight the significantly better
compositional control afforded by the VDSG method over high temperature preparations, which
have been shown to yield Mo-deficient oxide products as a result of the volatility of MoO
3
.
30
Additionally, the Mo3d
5/2
state was observed at 233 eV in the XPS spectra, which confirmed the
hexavalent oxidation state of the Mo atom in all three compositions studied in this work (Figure
4.7b).
31
Figure 4.7. (a) EDS spectra of AMoO
4
nanocrystals. (b) XPS spectra for AMoO
4
nanocrystals. Doublet
with maxima at 233 and 236 eV corresponds to the Mo3d
5/2
and Mo3d
3/2
states, respectively, and is
assigned to Mo
6+
. Blue, red, and green represent BaMoO
4
, SrMoO
4
, and CaMoO
4
, respectively.
Given previous reports that CaMoO
4
can be used as a conversion electrode in Li–ion batteries,
the electrochemical performance of the high surface area nanocrystals prepared here (surface
area extracted from Brunauer–Emmett–Teller method was 70 m
2
g
–1
) was characterized using
galvanostatic cycling at a rate of C/4 with the resulting voltage versus capacity profiles shown in
58
Figure 4.8a. The (dis)charge curves followed the typical profile for a conversion process, with
the first discharge curve displaying a rapid drop in voltage to a sloping voltage plateau,
beginning around 1 V and continuing to a capacity of approximately 750 mA h g
−1
. This plateau
is followed by a steady decrease to the cut off voltage yielding an initial discharge capacity of
1300 mA h g
−1
. This first discharge process results in the decomposition of CaMoO
4
and has
been suggested to correspond to the formation of an inert Li–Ca–O matrix and Li
x
MoO
y
, which
serves as the active material in the subsequent cycles.
3
The first discharge capacity of 1300 mA
h g
−1
compares favorably against the value of 1082 mA h g
−1
obtained by Liang et al. at a rate of
C/5 using roughly 70 nm diameter CaMoO
4
nanorods.
32
Here, the charging curves show a rapid
increase to 0.5 V followed by a smooth increase to 3.8 V, indicating a constant contribution to
the capacity over the entire voltage window. Following the first cycle, a reversible capacity of
504 mA h g
−1
is obtained. The reversible capacity following the first cycle is not quite as high as
the ~800 mA h g
−1
achieved by Liang et al.; however it does exceed the as-prepared solution
precipitation (7–10 μm) and sol-gel (50–80 nm) derived CaMoO
4
micro- and nanophase material
capacities reported by Sharma et al. (200–350 mA h g
−1
).
3,32
59
Figure 4.8. Electrochemical testing of the CaMoO
4
nanocrystals. (a) Voltage versus capacity profile for
the 10th, 30th and 50th cycles. Inset: voltage versus capacity profile for first cycle. (b) Discharge
capacities as a function of cycle number. All cycles were conducted in the voltage window of 0–3.8 V at
a rate of C/4.
The irreversible capacity of the first cycle in conversion electrodes has been suggested to
result from the large energy penalty incurred during the decomposition of the starting material
into nanoscale redox active phases.
32,33
However, for the high surface area, 8-nm CaMoO
4
nanocrystals employed in this work, typical conversion electrode capacity losses are still
observed in the first cycle, suggesting that the energy penalty is not resulting from conversion to
small particulates. Here, the irreversible capacity in the first cycle is likely due to the combined
effects of the formation of the Li–Ca–O matrix and the solid electrolyte interface.
3
The
reversible capacity fades in subsequent charge–discharge curves, but stabilizes at 250 mA h g
−1
within 25 cycles. Work is underway to optimize the reversible performance of this system using
more efficient battery architectures.
60
4.4. Experimental
4.4.1. Nanocrystal Synthesis
All manipulations were conducted under nitrogen atmosphere at ambient pressure using standard
Schlenk techniques. MoO
2
(acac)
2
(95%, acac = C
5
H
7
O
2
) from Sigma Aldrich and alkoxide
solutions of Ca(OCH
2
CH
2
OCH
3
)
2
(20 wt% in methoxyethanol), Sr(OCH
2
CHCH
3
OCH
3
)
2
(19
wt% in methoxypropanol) and Ba(OCH
2
CHCH
3
OCH
3
)
2
(25 wt% in methoxypropanol) from
Gelest, Inc. were used as precursors. The solvents 2-methoxyethanol and 2-methoxypropanol
were purchased from Sigma Aldrich. All reagents were used as received. The synthetic
apparatus utilized herein is described in detail elsewhere.
16
Briefly, a rotameter controls the flow
of the carrier gas (N
2
) through a bubbler housing 0.75 M aqueous HCl, which is connected via
tygon tubing to a 100 mL, 3-neck round bottom flask containing the precursor solution. Using
CaMoO
4
as an example target material, 0.79 mL (1.0 mmol) of Ca(OCH
2
CH
2
OCH
3
)
2
was
transferred into the reaction flask containing 326 mg (1.00 mmol) of MoO
2
(acac)
2
. The resulting
mixture was diluted to 2.0 mL with 2-methoxyethanol and stirred (60 °C) for 2 h under nitrogen,
after which complete dissolution of the reagents was observed, resulting in a dark reddish-brown
solution. Once cool, stirring was stopped and N
2
/HCl/H
2
O vapor was allowed to flow over the
reaction solution, which resulted in the formation of an opaque, off-white gel within 12 h. Vapor
was allowed to flow for a total of 48 h. At this point, the flow of water vapor was stopped and
the gel was thermally aged at 80 °C for 24 h under nitrogen. The resulting gel was then collected
and washed 3 times with 10 mL of absolute ethanol; the mixture was sonicated for 5 min and
centrifuged at 6500 rpm for 15 min between each wash. Ceramic yields were estimated to be 82,
88, and 90% for CaMoO
4
, SrMoO
4
and BaMoO
4
, respectively. TGA results reveal the resulting
nanocrystals possess ≤ 5 wt% organic content (Figure 4.1).
61
4.4.2. Material Characterization
Powder X-ray diffraction (XRD): XRD patterns were collected in the 10–80° 2θ range using a
Rigaku Ultima IV diffractometer operated at 44 mA and 40 kV. Cu Kα radiation (λ = 1.5406 Å)
was employed. The step size and collection time were 0.0075° and 1 s per step, respectively.
All patterns were recorded under ambient conditions. Thermogravimetry (TG): TG analyses
were performed using a thermogravimetric analyzer TA Q50 (TA Instruments) under a high-
purity air flow (60 mL min
−1
). Samples were heated from 25 to 100 °C, held isothermal at 100
°C for 15 min to remove moisture, and subsequently ramped from 100 to 800 °C at a linear rate
of 10 °C min
−1
. Rietveld analysis: Rietveld structural refinements
17,18
was carried out using the
General Structure Analysis System (GSAS) software.
19
The following parameters were refined:
(1) scale factor, (2) background, which was modeled using a shifted Chebyschev polynomial
function, (3) sample displacement, (4) peak shape, which was modeled using a modified
Thomson−Cox−Hasting pseudo-Voigt,
20
(5) lattice constants, (6) fractional atomic coordinates
of the oxygen atom, and (7) an isotropic thermal parameter for each chemical species (i.e., U
A
,
U
Mo
, and U
O
). The usual R
wp
and χ
2
indicators were employed to assess the quality of the refined
structural models.
21
Raman spectroscopy: Raman spectra were recorded in the 110–1000 cm
−1
range using a Horiba Xplora Raman microscope (Horiba Scientific). Laser radiation of 785 nm
wavelength was employed as the excitation source and the power at the sample level was 50
mW. Sulfur and 4-acetamidophenol were employed as frequency standards for calibration of
Raman shifts. The absolute accuracy of Raman shifts was estimated to be ±1 cm
−1
. All spectra
were recorded under ambient conditions. Transmission electron microscopy (TEM): TEM
images were obtained using a JEOL JEM2100F (JEOL Ltd.) electron microscope operating at
62
200 kV. Samples for TEM studies were prepared by drop-casting a stable suspension of
nanocrystals in ethanol on a 200 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.).
Selected Area Electron Diffraction (SAED): SAED patterns were obtained using a JEOL
JEM2100F (JEOL Ltd.) electron microscope operating at 200 kV. Samples for SAED studies
were prepared by drop-casting a stable suspension of nanocrystals in ethanol on a 400 mesh Cu
grid coated with an ultrathin lacey carbon film (Ted Pella, Inc.). Grids were previously cleaned
via ozone treatment for 60 min prior to sample deposition. X-ray photoelectron spectroscopy
(XPS): XPS spectra were collected using a Surface Science M-Probe Instruments controlled by
Hawk Data Collection software (Service Physics, Bend OR; V7.03.04). The monochromatic X-
ray source was the Al Kα line at 1486.6 eV, directed at 35° to the sample surface (55° off
normal). Emitted photoelectrons were collected at an angle of 35° with respect to the sample
surface (55° off normal) by a hemispherical analyzer. Low-resolution survey spectra were
acquired between binding energies of 0–1000 eV. Higher-resolution detailed scans, with a
resolution of ~0.8 eV, were collected for Mo 3d region. The sample chamber was maintained at
< 2 × 10
−9
Torr. The XPS data were analyzed using the Hawk Data Analysis software
(V7.03.04). Samples were prepared by pressing ~75 mg of dry powders into 13 mm diameter
pellets using ~5 metric tons of pressure for 3 min and applying the pellets onto conductive
carbon tape. Gas sorption analysis: BET measurements were performed on a Nova 2200e
surface area and pore size analyzer (Quantachrome Instruments, Inc.). Samples were degassed
for 2 h at 150 °C in vacuo prior to measurements. Energy dispersive X-ray spectroscopy (EDS):
EDS spectra were obtained using a JEOL JEM2100F (JEOL Ltd.) electron microscope operating
at 200 kV. Samples for EDS studies were prepared by drop-casting a stable suspension of
nanocrystals in ethanol on a 400 mesh Cu grid coated with an ultrathin lacey carbon film (Ted
63
Pella, Inc.). Grids were previously cleaned via ozone treatment for 60 min prior to sample
deposition. Quantification was achieved through the use of the Cliff-Lorimer K edge k-factors
for CaMoO
4
and SrMoO
4
. Due to the inability to collect the K edge of BaMoO
4
, the Ba to Mo
ratio was calculated by comparing the L edges and the corresponding k-factor. Electrochemical
testing: Galvanostatic cycling was performed using standard Swagelok-type cells. The positive
electrode was prepared by mixing the active material
with 30% by weight Ketjenblack (EC60JD,
AczoNobel) and ball milling for 20 min using a Spex 8000M mill. Lithium metal foil was used
as the negative electrode. The electrodes were separated by two sheets of Whatman GF/D
borosilicate glass fiber saturated with 1 M LiPF
6
in a 1:1 wt/wt mixture of ethylene
carbonate/dimethyl carbonate. Cells were assembled in an argon-filled glove box and typically
cycled at 25 °C between 0 and 3.8 V versus Li at a rate of 1 Li
+
per formula unit over 4 h (C/4).
Galvanostatic cycling was carried out on a VMP3 potentiostat (BioLogic).
4.5. Conclusions
In summary, the utility of the VDSG method has been extended to the scheelite family of
materials with the synthesis of phase pure sub-30 nm AMoO
4
nanocrystals. The low temperature
synthetic scheme allows for excellent control over composition as compared to previously
applied high temperature routes. Moreover, the successful preparation of scheelite nanocrystals
using monometallic precursors adds a degree of synthetic flexibility to the VDSG method,
thereby expanding the potential utility towards the synthesis of other useful metal oxides. The
lithium storage capacity of the CaMoO
4
nanocrystals versus Li was assessed and demonstrated a
large first discharge capacity of 1300 mA h g
−1
, which stabilized at a capacity of 250 mA h g
−1
after 25 cycles.
64
4.6. References
(1) Parchur, A. K.; Prasad, A. I.; Ansari, A. A.; Rai, S. B.; Ningthoujam, R. S. Dalton Trans.
2012, 41, 11032.
(2) Mikhailik, V. B.; Kraus, H.; Miller, G.; Mykhaylyk, M. S.; Wahl, D. J. Appl. Phys. 2005,
97, 083523.
(3) Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R.; Dong, Z. L.; White, T.
J. Chem. Mater. 2004, 16, 504.
(4) Sleight, A. W. Acta Crystallogr. 1972, B28, 2899.
(5) Marques, A. P. A.; Melo, D. M. A.; Paskocimas, C. A.; Pizani, P. S.; Joya, M. R.; Leite,
E. R.; Longo, E. J. Solid State Chem. 2006, 179, 671.
(6) Afanasiev, P. Mater. Lett. 2007, 61, 4622.
(7) Luo, Y. S.; Zhang, W. D.; Dai, X. J.; Yang, Y.; Fu, S. Y. J. Phys. Chem. C 2009, 113,
4856.
(8) Gong, Q.; Qian, X.; Ma, X.; Zhu, Z. Cryst. Growth Des. 2006, 6, 1821.
(9) Ahmad, G.; Dickerson, M. B.; Church, B. C.; Cai, Y.; Jones, S. E.; Naik, R. R.; King, J.
S.; Summers, C. J.; Kröger, N.; Sandhage, K. H. Adv. Mater. 2006, 18, 1759.
(10) Chen, D.; Tang, K.; Li, F.; Zheng, H. Cryst. Growth Des. 2006, 6, 247.
(11) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074.
(12) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
(13) Rabuffetti, F. A.; Brutchey, R. L. J. Am. Chem. Soc. 2012, 134, 9475.
(14) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
(15) Brutchey, R. L.; Morse, D. E. Angew. Chem., Int. Ed. 2006, 45, 6564.
(16) Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2011, 23, 4063.
(17) Rietveld, H. M. Acta Crystallogr. 1967, 22, 151−152.
(18) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65−71.
(19) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos
National Laboratory, 2000.
65
(20) Thompson, P.; Cox, D. E.; Hastings, J. M. J. Appl. Crystallogr. 1987, 20, 79−83.
(21) Young, R. A.; Oxford University Press: New York, 1993.
(22) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(23) Mi, Y.; Huang, Z.; Hu, F.; Li, Y.; Jiang, J. J. Phys. Chem. C 2009, 113, 20795.
(24) Errandonea, D.; Kumar, R. S.; Ma, X.; Tu, C. J. Solid State Chem. 2008, 181, 355.
(25) Liegeois-Duyckaerts, M.; Tarte, P. Spectrochim. Acta 1972, 28A, 2037.
(26) Porto, S. P. S.; Scott, J. F. Phys. Rev. 1967, 157, 716.
(27) Panchal, V.; Garg, N.; Sharma, S. M. J. Phys.: Condens. Matter 2006, 18, 3917.
(28) Gong, Q.; Qian, X.; Cao, H.; Du, W.; Ma, X.; Mo, M. J. Phys. Chem. B 2006, 110,
19295.
(29) Shi, H.; Qi, L.; Ma, J.; Wu, N. Adv. Funct. Mater. 2005, 15, 442.
(30) Cho, W. S.; Yashima, M.; Kakihana, M.; Tudo, A.; Sakata, T.; Yoshimura, M. J. Am.
Ceram. Soc. 1997, 80, 765.
(31) Gao, D.; Lai, X.; Cui, C.; Cheng, P.; Bi, J.; Lin, D. Thin Solid Films 2010, 518, 3151.
(32) Liang, Y.; Han, X.; Yi, Z.; Tang, W.; Zhou, L.; Sun, J.; Yang, S.; Zhou, Y. J. Solid State
Electrochem. 2007, 11, 1127.
(33) Cabana, J.; Monconduit, L.; Larcher, D.; Palacín, M. R. Adv. Energy Mater. 2010, 22,
E170.
66
Chapter 5. Low Temperature Synthesis of Homogeneous Solid Solutions of
Scheelite-Structured Ca
1-x
Sr
x
WO
4
and Sr
1-x
Ba
x
WO
4
Nanocrystals*
*Published in Dalton Trans. 2015, 44, 15042–15048.
5.1. Abstract
A series of compositionally complex scheelite-structured nanocrystals of the formula A
1-
x
A´
x
WO
4
(A = Ca, Sr, Ba) have been prepared under benign synthesis conditions using the vapor
diffusion sol-gel method. Discrete nanocrystals with sub-20 nm mean diameters were obtained
after kinetically controlled hydrolysis and polycondensation at room temperature, followed by
composition-dependent thermal aging at or below 60 °C. Rietveld analysis of X-ray diffraction
data and Raman spectroscopy verified the synthesis of continuous and phase-pure nanocrystal
solid solutions across the entire composition space for both Ca
1-x
Sr
x
WO
4
and Sr
1-x
Ba
x
WO
4
,
where 0 ≤ x ≤ 1. Elemental analysis by X-ray photoelectron and energy dispersive X-ray
spectroscopies demonstrated excellent agreement between the nominal and experimentally
determined elemental stoichiometries and illustrated good spatial elemental homogeneity within
these nanocrystals synthesized under benign conditions.
5.2. Introduction
Over the last several decades, the scheelite-structured ABO
4
(A = Ca, Sr, Ba, Pb and B = Mo, W)
family of materials has attracted considerable interest in the areas of energy storage and
conversion, in part because of their excellent chemical and thermal stability. Applications have
grown to include electrodes in Li-ion batteries, host materials for solid-state phosphors,
scintillation detectors, photocatalysts, and ionic conductors.
1–5
The classic solid-state synthesis
of these materials, where crystallization is induced via mechanical mixing of ACO
3
and BO
3
67
followed by high temperature sintering, is energy intensive and typically requires temperatures
≥ 1000 °C.
6
Furthermore, poor mixing can result in phase segregation in such multinary
systems, and the intrinsically high vapor pressure of the MoO
3
and WO
3
precursors can lead to
compositional inhomogeneity and nonstoichiometry at reaction temperatures exceeding 800 °C.
7
To overcome the abovementioned issues, lower temperature methods have been developed as
routes to this materials family.
For example, combinatorial microwave-hydrothermal methods (i.e., high pressures and
temperatures > 100 °C) have recently been employed to generate quasispherical alkaline earth
tungstate microcrystals.
8,9
Similar morphologies were also observed by Thangadurai et al.
through a metathetic room-temperature technique.
10
More sophisticated morphologies have
since been obtained with polymer-directed, catanionic reverse micelle type procedures, where
BaWO
4
penniform architectures were successfully prepared.
11
Here, the relevant metal salt
precursors were reacted in an undecyl acid and decylamine surfactant mixture to generate
“feather-like” structures with lengths and widths on the order of 50 and 3.5 µm, respectively.
Additionally, Mao and co-workers have been able to achieve various alkaline earth tungstate
nanorods (ca. 200 nm in diameter and several microns in length), and solid solutions thereof,
using a modified template synthesis.
12–14
Despite the interesting scheelite morphologies
currently available, the small nanoscale regime of compositionally complex scheelites still
remains largely unexplored. This highlights the need to develop new types of low-temperature
synthetic preparations that can yield small, discrete, and compositionally controlled scheelite-
structured nanocrystals.
Over the past decade, our group has developed a vapor diffusion sol-gel method that allows
phase-pure, multinary metal oxide nanocrystals to be synthesized under ultrabenign conditions
68
(i.e., low-temperature, ambient pressure, near neutral pH).
15
This synthesis method was
originally applied towards the synthesis of various perovskite-structured ABO
3
(A = Sr, Ba and
B = Zr, Ti) nanocrystals.
16–19
The success of this method relied upon the kinetically controlled
hydrolysis and polycondensation of bimetallic alkoxide precursors for the nucleation and growth
of the perovskite ABO
3
nanocrystals. Recently, we extended the scope of our vapor diffusion
sol-gel method into the scheelite family of materials with the synthesis of small, ternary AMoO
4
nanocrystals.
20,21
Interestingly, this class of oxide nanocrystals could be accessed under benign
synthesis conditions using a stoichiometric mixture of monometallic precursors rather than with
a bimetallic precursor. It was therefore of interest to see if the vapor diffusion sol-gel method
could next be extended to the other major class of scheelite-structured materials (i.e., AWO
4
,
where A = Ca, Sr, and Ba), and explore if, under comparable synthetic conditions, continuous
series of substitutional solid solutions of A
1–x
A´
x
WO
4
nanocrystals may be accessed. It has
previously been shown that useful properties of scheelite-structured materials can be tuned by
varying the A-site cation in A
1–x
A´
x
BO
4
solid solutions;
22–25
however, under certain high
temperature synthesis conditions, some systems (e.g., CaWO
4
-SrWO
4
) have been reported to
have narrow regions of one phase, solid solution stability upon cooling.
26
This provides further
impetus to explore the synthesis and compositional homogeneity of A
1-x
A´
x
WO
4
solid solutions
under kinetically controlled, low temperature conditions. Herein, we report the synthesis of a
series of homogeneous A
1–x
A´
x
WO
4
scheelite-structured nanocrystal solid solutions, where 0 ≤ x
≤ 1. Chemical, structural, and morphological characterization is provided, which showcases the
high degree of compositional control associated with the vapor diffusion sol-gel method.
69
5.2. Results and Discussion
The nucleation and growth of scheelite-structured AWO
4
(A = Ca, Sr, Ba) nanocrystals using the
vapor diffusion sol-gel method relies on the kinetically controlled delivery of water vapor into a
compositionally tailored precursor solution.
15
Crystalline and phase-pure nanocrystals were
synthesized at low temperature and ambient pressure from a 1:1 stoichiometric mixture of
monometallic alkoxide precursors (i.e., W(OEt)
6
mixed with an appropriate alkaline earth
alkoxide). Upon dissolution of the W(OEt)
6
precursor into a 2-methoxypropanol solution of
alkaline earth alkoxide, diffusion of water vapor into the alcohol solution initiates a series of
hydrolysis and polycondensation reactions at the gas–liquid interface that induces gel formation.
As previously noted for the molybdate nanocrystals fabricated by the vapor diffusion sol-gel
method, no post-synthetic, high-temperature calcination is required to obtain crystalline
products.
20
Low-temperature aging of the CaWO
4
and SrWO
4
gels is, however, necessary to
drive the completion of the amorphous-to-crystalline phase transition, as was previously
observed for the AMoO
4
nanocrystals.
20
The CaWO
4
and SrWO
4
products were thermally aged
at 60 °C under flowing nitrogen following gelation, while the BaWO
4
was aged at 22 °C.
Isolation of the aged gel, followed by an ethanol wash and vacuum drying, yields an off-white
powder comprised of discrete AWO
4
nanocrystals. Ceramic yields were consistently found to be
in the range of 80-90%, with residual organic content of ~5 wt% as determined by
thermogravimetric analysis. The resulting nanocrystals can be dispersed in ethanol with
sonication to form colloidally stable suspensions at concentrations up to ~10 mg mL
–1
.
When rapid hydrolysis and polycondensation is induced by the fast injection of water into the
1:1 stoichiometric mixture of monometallic alkoxide precursors, an amorphous oxide precipitate
results. This highlights the need for the slow, kinetically controlled delivery of water via vapor
70
phase to the alcohol solution in order to induce nucleation and growth of crystalline AWO
4
scheelites at low temperatures. Moreover, the vapor diffusion sol-gel method allows for the low-
temperature synthesis of compositionally complex and controlled quaternary A
1–x
A´
x
WO
4
nanocrystals for alkaline earth A-site cation pairs of Ca
1–x
Sr
x
WO
4
and Sr
1–x
Ba
x
WO
4
(0 ≤ x ≤ 1).
While the vapor diffusion sol-gel method had previously been applied to the synthesis of
quaternary and quinary A
1–x
A´
x
B
1–y
B´
y
O
3
perovskite oxides (0 ≤ x ≤ 1, 0 ≤ y ≤ 1),
17-19
the
synthesis of the scheelite structured nanocrystals has heretofore been limited to ternary AMoO
4
using this route.
20
To synthesize a continuous series of A
1-x
A´
x
WO
4
solid solutions,
stoichiometric mixtures of the two alkaline earth alkoxides are simply combined in the proper
ratio to synthetically control the A:A´ ratio in the resulting nanocrystals (vide infra).
Powder X-ray diffraction (XRD) analysis of the resulting nanocrystals confirms the
crystallinity of the Ca
1–x
Sr
x
WO
4
and Sr
1-x
Ba
x
WO
4
nanocrystals prepared under low temperature
conditions (Figure 5.1). The diffraction maxima can be indexed to the tetragonal scheelite
structure (PDF No. 08–0457, 08–0490, and 07–0210 for BaWO
4
, SrWO
4
, and CaWO
4
,
respectively), with all of the nanocrystal structures being isostructural and belonging to the I4
1
/a
(No. 88) space group. A clear shift in the diffraction maxima towards lower 2θ values with
increasing A-site cation radius (rA
2+
= 1.12, 1.26, 1.42 Å for Ca, Sr, and Ba, respectively
33
) was
observed, along with a concomitant expansion in unit cell volume. No traces of crystalline
impurities were seen in any of the diffraction patterns, demonstrating that the as-prepared
nanocrystals are phase pure.
71
Figure 5.1. Powder XRD patterns for (a) Sr
1–x
Ba
x
WO
4
and (b) Ca
1–x
Sr
x
WO
4
nanocrystal solid solutions.
Lattice parameters (a and c) and unit cell volumes for (c) Sr
1–x
Ba
x
WO
4
and (d) Ca
1-x
Sr
x
WO
4
nanocrystal
solid solutions. Dotted lines are a guides-to-the-eye.
Rietveld analysis of the XRD patterns for the A
1–x
A´
x
WO
4
solid solutions was conducted using
the tetragonal I4
1
/a space group to further verify the structure as well as the nominal nanocrystal
stoichiometries. Extracted lattice parameters (a and c) and unit cell volumes for the Sr
1–
x
Ba
x
WO
4
and Ca
1–x
Sr
x
WO
4
nanocrystals are plotted in Figure 5.1c and d, respectively. Lattice
parameters ranged from a = 5.2485(8) Å and c = 11.3846(18) Å for CaWO
4
, to a = 5.4268(9) Å
and c = 11.9688(21) Å for SrWO
4
, to a = 5.6065(7) Å and c = 12.7084(16) Å for BaWO
4
. A
summary of the calculated structural parameters for the ternary AWO
4
nanocrystals is given in
Table 5.1. The low values for the R
wp
and χ
2
goodness-of-fit indicators, along with the minimal
72
differences between the least-squares fits and the experimental diffraction patterns, provide
further confirmation that the A
1–x
A´
x
WO
4
nanocrystals are all isostructural and belong to the
tetragonal scheelite family.
Table 5.1. Rietveld Analysis of XRD Data for AWO
4
Nanocrystals
CaWO4 SrWO4 BaWO4
a (Å) 5.2485(8) 5.4268(9) 5.6065(7)
c (Å) 11.3846(18) 11.9688(21) 12.7084(16)
V (Å
3
) 313.61(15) 352.48(18) 399.46(15)
x, y, z O
0.6545(6),
0.4918(4),
0.2100(2)
0.6467(6),
0.4977(5),
0.2079(2)
0.6204(8),
0.5052(7),
0.2029(3)
UA (Å
2
)
a
0.83(2) 1.14(2) 1.32(2)
UW (Å
2
)
a
0.66(2) 0.91(2) 1.06(2)
UO (Å
2
)
a
1.46(13) 4.14(16) 4.25(24)
Rwp 6.7 5.6 7.2
χ
2
1.98 1.54 1.78
a
Given as 100×U.
The composition of the A
1–x
A´
x
WO
4
nanocrystals was confirmed by Rietveld analysis of their
XRD pattern. In each case, a single scheelite phase with fixed stoichiometry defined by the
nominal composition of the reaction mixture was refined. Linear fits to the lattice parameters
and unit cell volumes as a function of composition, with near unity residual R
2
values (i.e.,
0.9811 – 0.9999), verify the validity of Vegard’s law (Figure 5.1c and d). The monotonic linear
dependence of lattice parameters and unit cell volume with composition suggests the formation
of true solid solutions within the A
1–x
A´
x
WO
4
nanocrystals under low temperature synthesis
conditions, with a homogeneous distribution of the two A-site cations over the length scale of
73
Bragg diffraction. This stands in contrast to solid-state methods, for which limited solubilities
are observed unless high temperature synthesis conditions are employed. For example, Chang
reported that continuous solid solutions could not be obtained at temperatures under 825 ˚C for
the CaWO
4
-SrWO
4
system.
26
Here, the observed linearity of the lattice parameters and unit cell
volume with composition not only suggests continuous solid solution formation, but also
suggests strong agreement between the nominal and experimental nanocrystal stoichiometries.
This demonstrates the ability of the vapor diffusion sol-gel method to accurately achieve
compositional control by simply tuning the ratio of alkaline earth alkoxides within the reaction
mixture.
The structure of the A
1–x
A´
x
WO
4
solid solutions was also investigated using Raman
spectroscopy. Raman spectra, with the corresponding band assignments, are given in Fig. 5.2.
All Raman bands can be assigned to the tetragonal scheelite structure, corroborating the phase
purity observed by XRD. Two distinct groups of tungstate Raman bands are present in the
spectra. The first group of bands (300-400 cm
–1
) arise from bending modes (A
g
, B
g
, B
g
) of the
[WO
4
]
2–
tetrahedra, while the second group (750-950 cm
–1
) corresponds to stretching modes (E
g
,
B
g
, A
g
) of the [WO
4
]
2–
tetrahedra.
34
74
Figure 5.2. Raman spectra of the A
1–x
A´
x
WO
4
nanocrystal solid solutions. Intensity multipliers were
applied to some spectra and are shown below the associated spectrum. Raman bands arising from the
scheelite phase are denoted with the “⏐” symbol with their corresponding assignments.
The chemical composition of the A
1–x
A´
x
WO
4
solid solution nanocrystals was investigated by
both X-ray photoelectron microscopy (XPS) and energy dispersive X-ray spectroscopy (EDX).
Importantly, the W4f doublet observed at ca. 37.0 and 35.0 eV in all XPS spectra confirms the
hexavalent oxidation state of the tungsten cation (Figure 5.3).
14
Moreover, the relevant binding
energies for the alkaline earth cations (Ca2p = 350.2 and 346.6 eV; Sr3d = 134.5 and 132.8 eV;
75
and Ba3d = 793.5 and 778.1 eV) are consistent for these divalent cations in the scheelite
structure. Surface elemental composition measured by XPS correlates well with the nominal
bulk compositions, further validating the conclusion drawn from Rietveld analysis.
Experimentally determined surface compositions for the Ca
1–x
Sr
x
WO
4
nanocrystal series were
49:51 Ca/W, 78:22 Ca/Sr, 51:49 Ca/Sr, 25:75 Ca/Sr, and 49:51 Sr/W for nominal x = 0, 0.25,
0.5, 0.75, and 1.0, respectively. For the Sr
1–x
Ba
x
WO
4
nanocrystal series, the experimentally
determined surface elemental compositions were found to be 79:21 Sr/Ba, 52:48 Sr/Ba, 28:72
Sr/Ba, and 47:53 Ba/W for nominal x = 0.25, 0.5, 0.75, and 1.0.
Figure 5.3. XPS spectra for select A
1–x
A´
x
WO
4
nanocrystal solid solution compositions. A doublet with
maxima at 37 and 35 eV corresponds to the W4f
5/2
and W4f
7/2
states, respectively, and is assigned to
hexavalent tungsten in the scheelite structure.
STEM-EDX was used to map the spatial distribution of the elements for the A
1–x
A´
x
WO
4
solid
solution nanocrystals on the nanoscale. EDX maps illustrating the relevant atomic distributions
76
for Ca
0.5
Sr
0.5
WO
4
and Sr
0.5
Ba
0.5
WO
4
nanocrystals can be found in Figure 5.4. Indeed, the EDX
maps qualitatively confirm a well-mixed, homogeneous dispersion of the alkaline earth elements
and tungsten within small clusters of nanocrystals for both solid solution compositions.
Figure 5.4. STEM-EDX maps for clusters of select A
1–x
A´
x
WO
4
nanocrystal solid solution compositions.
(a) STEM image for a cluster of Ca
0.5
Sr
0.5
WO
4
nanocrystals; (b–d) corresponding net intensity elemental
maps for Ca (K-line), Sr (L-line), and W (L-line), respectively; and (e) STEM image for a cluster of
Sr
0.5
Ba
0.5
WO
4
nanocrystals; (f–h) corresponding net intensity elemental maps for Sr (L-line), Ba (L-line),
and W (L-line), respectively.
The morphology of the A
1–x
A´
x
WO
4
nanocrystals was probed by transmission electron
microscopy (TEM). Representative TEM images depicting the end member AWO
4
compositions are given in Figure 5.5. The nanocrystals are quasispherical in shape and exhibited
no compositional size or shape dependence. Average nanocrystal diameters obtained from the
particle size distribution analysis (N = 100) were found to be 10.6 ± 2.0 (s/
€
d = 19%), 8.5 ± 1.8 (s/
€
d = 21%), 8.2 ± 2.0 (s/
€
d = 24%), 8.2 ± 1.5 (s/
€
d = 18%), and 9.4 ± 2.0 nm (s/
€
d = 21%) for the Ca
1–
x
Sr
x
WO
4
nanocrystals for nominal x = 0, 0.25, 0.5, 0.75 and 1.0, respectively. The Sr
1-x
Ba
x
WO
4
77
nanocrystals possessed average diameters of 7.8 ± 1.5 (s/
€
d = 19%), 9.5 ± 1.8 (s/
€
d = 18%), 7.3 ±
1.5 (s/
€
d = 21%), and 19.7 ± 3.6 nm (s/
€
d = 21%) with respect to nominal x = 0.25, 0.5, 0.75 and
1.0, respectively. The well-defined representative lattice fringes seen in the high-resolution
TEM image for SrWO
4
are consistent with the tetragonal scheelite structure (Fig. 5.5d), and
suggest that the nanocrystals are single crystalline. It should be noted that when higher aging
temperatures (e.g., 100 °C) were explored after gelation, the resulting nanocrystal morphologies
were found to be more irregular and polydisperse (Figure 5.6).
Figure 5.5. TEM images of (a) CaWO
4
, (b) SrWO
4
, and (c) BaWO
4
nanocrystals. (d) High-resolution
TEM image displaying lattice fringes for an apparently single crystalline SrWO
4
nanocrystal.
78
Figure 5.6. Representative TEM image of Sr
0.75
Ba
0.25
WO
4
nanocrystals thermally aged at 100 °C for 24
h under flowing dry nitrogen.
5.3. Experimental
5.3.1. General Considerations
All manipulations were conducted under a nitrogen atmosphere at ambient pressure using
standard Schlenk techniques. W(OEt)
6
(Et = CH
2
CH
3
) from Alfa Aesar and alcoholic solutions
of Ca(OCH
2
CH
2
OCH
3
)
2
(20 wt% in methoxyethanol), Sr(OCH
2
CHCH
3
OCH
3
)
2
(19 wt% in 2-
methoxypropanol) and Ba(OCH
2
CHCH
3
OCH
3
)
2
(25 wt% in 2-methoxypropanol) from Gelest,
Inc. were used as precursors. 2-methoxypropanol was purchased from Sigma Aldrich. All
reagents were used as received.
5.3.2. Nanocrystal Synthesis
The synthetic apparatus utilized for vapor diffusion sol-gel is described in detail elsewhere.
27
79
Briefly, a rotameter controls the flow of the nitrogen carrier gas through glass bubbler filled with
0.75 M aqueous HCl, connected via Tygon tubing to a 100-mL, 3-neck round bottom flask
containing the precursor solution. Using a stoichiometry of Ca
0.5
Sr
0.5
WO
4
as an example, 0.5
mL (0.5 mmol) of Ca(OCH
2
CH
2
OCH
3
) and 0.8 mL (0.5 mmol) of Sr(OCH
2
CHCH
3
OCH
3
)
2
were
transferred into the reaction flask containing 454 mg (1.00 mmol) of W(OCH
2
CH
3
)
6
. The
resulting reaction mixture was diluted to 2.0 mL total volume with 2-methoxypropanol and
stirred at 80 °C for 2 h under flowing dry nitrogen, after which complete dissolution of the
reagents gave a reddish-brown solution. Once cooled to room temperature, stirring was stopped
and water vapor was allowed to flow over the reaction solution, which resulted in the formation
of an opaque, off-white gel within 12 h. Water vapor was allowed to flow for a total of 48 h. At
this point, the flow of water vapor was stopped and the gel was aged at 60 °C for 24 h under dry
nitrogen. Two different thermal aging procedures were applied depending on the composition.
For Ca
1-x
Sr
x
WO
4
(0 ≤ x ≤ 1), a temperature of 60 °C was used during the aging segment of the
procedure, while all other compositions were aged at 22 °C under dry nitrogen. The resulting gel
was then collected and washed 3 times with 10 mL of absolute ethanol; the product was
sonicated for 5 min and centrifuged at 6000 rpm for 15 min between each wash. The final
precipitate was dried under vacuum overnight, yielding an off-white powder. Ceramic yields
were estimated to be 80-90%, as determined by thermogravimetric analysis.
5.3.3. Material Characterization
Powder X-ray diffraction (XRD): XRD patterns were collected in the 10-80° 2θ range using a
Rigaku Ultima IV diffractometer operated at 44 mA and 40 kV. Cu Kα radiation (λ = 1.5406 Å)
80
was employed. The step size and collection time were 0.0075° and 5 s per step, respectively.
All diffraction patterns were recorded under ambient conditions. Rietveld analysis: Rietveld
structural refinements
28,29
were carried out using the General Structure Analysis System (GSAS)
software.
30
The following parameters were refined: (1) scale factor, (2) background, which was
modeled using a shifted Chebyshev polynomial function, (3) sample displacement, (4) peak
shape, which was modeled using a modified Thompson−Cox−Hastings pseudo-Voigt,
31
(5)
lattice constants, (6) fractional atomic coordinates of the oxygen atom, and (7) an isotropic
thermal parameter for each chemical species (i.e., U
A
, U
W
, and U
O
). The usual R
wp
and χ
2
indicators were employed to assess the quality of the refined structural models.
32
Raman
spectroscopy: Raman spectra were recorded in the 200-1000 cm
−1
range using a Horiba Xplora
Raman microscope (Horiba Scientific). Laser irradiation of 785 nm wavelength was employed
as the excitation source and the power at the sample level was 50 mW. Sulfur and 4-
acetamidophenol were employed as frequency standards for calibration of Raman shifts. The
absolute accuracy of Raman shifts was estimated to be ±1 cm
−1
. All spectra were recorded under
ambient conditions. Transmission electron microscopy (TEM): TEM images were obtained
using a JEOL JEM2100F (JEOL Ltd.) transmission electron microscope operating at 200 kV.
Samples for TEM studies were prepared by drop-casting a stable suspension of nanocrystals in
ethanol on a 400 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.). X-ray
photoelectron spectroscopy (XPS): XPS spectra were acquired using a Kratos Axis Ultra X-ray
photoelectron spectrometer with the analyzer lens in hybrid mode. High-resolution scans were
performed using a monochromatic aluminum anode with an operating current of 5 mA and
voltage of 10 kV using a step size of 0.1 eV, a pass energy of 40 eV, and a pressure range
between 1-3 × 10
–8
Torr. The binding energies for all spectra were referenced to the C1s core
81
level at 284.6 eV. Scanning transmission electron microscopy – energy-dispersive X-ray
spectroscopy (STEM-EDX): Elemental maps were acquired using a JEOL JEM2100F (JEOL
Ltd.) transmission electron microscope operating at 200 kV, equipped with an EDAX Octane T
Plus silicon drift detector. Elemental maps were acquired by signal averaging over a period of
30-45 min. Samples for STEM-EDX studies were prepared by drop-casting a stable suspension
of nanocrystals in ethanol on a 400 mesh Cu grid coated with a lacey carbon film (Ted Pella,
Inc.) and dried under vacuum overnight prior to imaging.
5.4. Conclusions
In summary, a series of homogeneous, scheelite-structured Ca
1-x
Sr
x
WO
4
and Sr
1–x
Ba
x
WO
4
solid solution nanocrystals have been synthesized under very benign conditions (i.e., low
temperature and ambient pressure) with the vapor diffusion sol-gel method. The resulting
nanocrystals were found to be single crystalline, sub-20 nm in mean diameter, and phase pure.
The synthetic flexibility of the vapor diffusion sol-gel method allows for the preparation of
complex ternary A
1–x
A´
x
WO
4
nanocrystals of arbitrary and well-defined elemental stoichiometry.
Achieving good agreement between nominal and experimental elemental stoichiometries can be
difficult with high temperature synthesis methods, and thus the development of facile low-
temperature methods towards compositional control is important. Moreover, the formation of
continuous solid solutions for Ca
1-x
Sr
x
WO
4
and Sr
1–x
Ba
x
WO
4
highlights the ability of the low-
temperature vapor diffusion sol-gel method to achieve complete miscibility across the whole
composition range.
82
5.5. References
(1) Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R.; Dong, Z. L.; White, T.
J. Chem. Mater. 2004, 16, 504.
(2) Su, Y.; Li, L.; Li, G. Chem. Mater. 2008, 20, 6060.
(3) Mikhailik, V. B.; Kraus, H.; Miller, G.; Mykhaylyk, M. S.; Wahl, D. J. Appl. Phys. 2005,
97, 083523.
(4) Kato, H.; Matsudo, N.; Kudo, A. Chem. Lett. 2004, 33, 1216.
(5) Esaka, T.; Tachibana, R.; Takai, S. Solid State Ionics 1996, 92, 129.
(6) Blasse, G.; Brixner, L. H. Chem. Phys. Lett. 1990, 173, 409.
(7) Wöhler, L.; Balz, O. Z. Elektrochem. 1921, 27, 415.
(8) Cavalcante, L. S.; Longo, V. M.; Sczancoski, J. C.; Almeida, M. A. P.; Batista, A. A.;
Varela, J. A.; Orlandi, M. O.; Longo, E.; Siu Li, M. Cryst. Eng. Comm. 2012, 14, 853.
(9) Siqueira, K. P. F.; Moreira, R. L.; Valadares, M.; Dias, A. J. Mater. Sci. 2010, 45, 6083.
(10) Thangadurai, V.; Knittlmayer, C.; Weppner, W. Mat. Sci. Eng. B 2004, 106, 228.
(11) Shi, H.; Qi, L.; Ma, J.; Cheng, H. J. Am. Chem. Soc. 2003, 125, 3450.
(12) Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2004, 126, 15245.
(13) Zhang, F.; Yiu, Y.; Aronson, M. C.; Wong, S. S. J. Phys. Chem. C 2008, 112, 14816.
(14) Zhang, F.; Sfeir, M. Y.; Misewich, J. A.; Wong, S. S. Chem. Mater. 2008, 20, 5500.
(15) Rabuffetti, F. A.; Brutchey, R. L. Dalton Trans. 2014, 43, 14499.
(16) Brutchey, R. L.; Morse, D. E. Angew. Chem. Int. Ed. 2006, 45, 6564.
(17) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074.
(18) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
(19) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
(20) Culver, S. P.; Rabuffetti, F. A.; Zhou, S.; Mecklenburg, M.; Song, Y.; Melot, B. C.;
Brutchey, R. L. Chem. Mater. 2013, 25, 4129.
83
(21) Rabuffetti, F. A.; Culver, S. P.; Suescun, L.; Brutchey, R. L. Inorg. Chem. 2014, 53,
1056.
(22) Kotera, Y.; Sekine, T. J. Phys. Chem. Solids 1964, 25, 353.
(23) Longo, V. M.; Orhan, E.; Cavalcante, L. S.; Pôrto, S. L.; Espinoza, J. W. M.; Valera, J.
A.; Longo, E. Chem. Phys. 2007, 334, 180.
(24) Pôrto, S. L.; Longo, E.; Pizani, P. S.; Boschi, T. M.; Simőes, L. G. P.; Lima, S. J. G.;
Ferreira, J. M.; Soledade, L. E. B.; Espinoza, J. W. M.; Cássia-Santos, M. R.; Maurera,
M. A. M. A.; Paskocimas, C. A.; Santos, I. M. G.; Souza, A. G. J. Solid State Chem.
2008, 181, 1876.
(25) Oeder, R.; Sharmann, A.; Schaw, D. J. Cryst. Growth 1980, 49, 349.
(26) Chang, L. L. Y. Am. Mineral. 1967, 52, 427.
(27) Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2011, 23, 4063.
(28) Rietveld, H. M. Acta Crystallogr. 1967, 22, 151.
(29) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65.
(30) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos
National Laboratory Report LAUR 86-748; Los Alamos National Laboratory: Los
Alamos, NM, 2000.
(31) Thompson, P.; Cox, D. E.; Hastings, J. M. J. Appl. Crystallogr. 1987, 20, 79.
(32) Young, R. A. The Rietveld Method; IUCr Monographs on Crystallography (Book 5);
Oxford University Press: New York, 1993.
(33) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(34) Manjón, F. J.; Errandonea, D.; Garro, N.; Pellicer-Porres, J.; Rodríguez-Hernández, P.;
Radescu, S.; López-Solano, J.; Mujica, A.; Muñoz, A. Phys. Rev. B 2006, 74, 144111.
84
Chapter 6. Thermally Activated Rotational Disorder in CaMoO
4
Nanocrystals*
*Published in Cryst. Eng. Commun. 2016, 18, 4485–4488.
6.1. Abstract
A dual-space approach, combining Rietveld and pair distribution function (PDF) analyses, has
been applied to temperature-dependent synchrotron X-ray total scattering data collected on vapor
diffusion sol-gel derived CaMoO
4
nanocrystals. A sharp transition in Ca–O bond distances in
the range of 151–163 K was identified by PDF analysis, which is attributed to the thermal
activation of rotational disorder associated with the rigid MoO
4
tetrahedra.
6.2. Introduction
Scheelite-structured oxides of the formula AMO
4
(A = Ca, Sr, Ba; M = Mo, W) have proven
to be a significant class of functional materials in the areas of energy conversion and storage
(e.g., solid state phosphors, lithium ion batteries, cryogenic scintillation detectors).
1–3
Belonging
to the I4
1
/a space group, the scheelite structure features a tetragonal unit cell (Z = 4, Figure 6.1).
4
The group II and VI atoms occupy special positions 4a (0, 1/4, 1/8) and 4b (0, 1/4, 5/8),
respectively, while the oxygen atoms lie in a general position 16f (x, y, z). The AMO
4
structure
is comprised of AO
8
bisdisphenoid polyhedra (i.e., dodecahedra) and MO
4
tetrahedra.
Moreover, the oxygen coordination environment surrounding the group II atoms is best
described by two distinct A–O distances (4 + 4), however the more rigid MO
4
units exhibit only
one M–O distance. Though the MO
4
tetrahedra are isolated from each other within the lattice,
they share corners with the surrounding AO
8
dodecahedra, which share edges with four other
85
dodecahedra. Importantly, the corner-sharing polyhedra form A–O–M bridges that act as
“hinges,” allowing the MO
4
tetrahedra more rotational freedom.
5
Structural studies performed on scheelite-structured oxides have primarily focused on the
effects of high temperature and/or pressure on bulk materials (e.g., phase transitions, thermal
expansion, compressibility).
6–8
To date, very little is known about the structure of AMO
4
materials at low temperatures, or on the nanoscale for that matter. In a study by Simon et al.,
electron paramagnetic resonance and dielectric measurements were used to show that bulk
CaMoO
4
undergoes a second-order ferroelastic phase transition at 52 K.
9
Later, Senyshyn and
co-workers investigated bulk CaMoO
4
via Rietveld analysis of synchrotron X-ray diffraction
(XRD) data, but they did not detect any structural anomalies in the range of 12–300 K.
10
While
Rietveld analysis provides structural information on the longer length scale of Bragg diffraction,
pair distribution function (PDF) analysis is a total scattering technique (i.e., Bragg and diffuse
scattering) that can resolve atomic pair arrangements on the local Å scale.
11
By combining
multiple techniques, structures can be more accurately described in certain instances.
Recently, our group reported the synthesis of a series of sub-30 nm AMO
4
nanocrystals using
the vapor diffusion sol-gel (VDSG) method.
12,13
Structural characterization by Rietveld analysis
revealed a substantial, but anomalous, contraction of the Mo–O bonds (~2.8%) upon chemical
substitution of Ca
2+
by Ba
2+
(rCa
2+
= 1.12 Å and rBa
2+
= 1.42 Å).
14
This finding provided the
impetus for a more thorough investigation, as it contradicted preexisting theory regarding the
rigidity of MoO
4
tetrahedra within scheelite structured oxides.
15–17
It has been shown in these
structures that changes brought on by chemical substitution, pressure, and/or temperature should
be accommodated for through geometric distortions of the AO
8
dodecahedra, as opposed to
affecting the rigid MoO
4
units. To elucidate the origin of this irregularity, our group used a dual-
86
space approach, whereby Rietveld and PDF analysis were applied to synchrotron X-ray total
scattering data.
18
Employing both techniques in concert, the anomalous contraction of Mo–O
distances was found to be the result of orientational disorder induced by random rotations of the
MoO
4
tetrahedra (i.e., rotational disorder). Herein, we have exploited this dual-space approach
on temperature-dependent synchrotron X-ray diffraction data collected on 9.7 ± 2.3 nm VDSG-
derived CaMoO
4
nanocrystals to gain insight into the nature/temperature-dependence of the
observed disorder.
6.3. Results and Discussion
Exemplary Rietveld and PDF fits to the experimental XRD patterns collected at 90 K are shown
in Figure 6.1. A total of 33 diffraction patterns were collected from 90–480 K (i.e., every 12 K).
All calculated structural parameters and the corresponding Rietveld and PDF fits are provided in
Tables 6.1–6.2 and Figures 6.2–6.3. Low values for the fit residuals, R
wp
and R
w
, as well as a
qualitative assessment of the refinements indicate that both the average and local crystal
structures are adequately described by a tetragonal scheelite structure, with space group I4
1
/a
(no. 88). Notably, the values of R
wp
and R
w
remained within 3.6–4.1% and 8.8–12.1%,
respectively, over the entire temperature range for Rietveld and PDF. Given that literature
values for R
w
are conventionally in the range of 10–25% for fits to experimental PDFs, the
values obtained herein illustrate the exceptional quality of fit. Lattice constants (a, c) and unit
cell volumes are plotted in Figure 6.4 as a function of temperature. As expected, the unit cell
parameters expand nearly linearly with increasing temperature. Furthermore, parameters
extracted from the structural analysis correspond well with literature values for bulk CaMoO
4
.
87
Figure 6.1. (a) Rietveld and (b) PDF analysis of the X-ray total scattering data for CaMoO
4
nanocrystals
collected at 90 K. Experimental (○) and calculated (⎯) patterns are shown along with the difference
curve (⎯). Tickmarks (⏐) corresponding to the phase refined are given in (a). Inset: CaMoO
4
scheelite
structure. Blue, purple, and red spheres represent calcium, molybdenum, and oxygen atoms, respectively.
88
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
89
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
90
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
91
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
92
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
93
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
94
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
95
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
96
Figure 6.2. Rietveld analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯) and
tickmarks (⏐) corresponding to the phase refined. The temperature at which the pattern was collected is
indicated in the top right of each pattern, along with the associated R
wp
. ⎯Continues on the following
page.
97
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
98
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
99
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
100
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
101
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
102
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
103
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
104
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
. ⎯Continues on the following page.
105
Figure 6.3. PDF analysis of X-ray total scattering data for CaMoO
4
nanocrystals from 90–480 K.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). The
temperature at which the pattern was collected is indicated in the top right of each pattern, along with the
associated R
w
.
106
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
90 K 102 K 114 K 127 K 139 K
a (Å) 5.218 5.218 5.219 5.219 5.220
c (Å) 11.404 11.406 11.407 11.408 11.410
V (Å
3
) 310.5 310.6 310.7 310.8 310.9
x
O
0.647 0.647 0.647 0.647 0.647
y
O
0.512 0.512 0.513 0.513 0.513
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.3, 0.7 0.3, 0.7 0.4, 0.7 0.4, 0.7 0.4, 0.8
U
eq
(Å
2
) 0.4 0.4 0.5 0.5 0.3
Mo: U
11
, U
33
(Å
2
)
0.2, 0.4 0.3, 0.4 0.3, 0.4 0.3, 0.5 0.3, 0.5
U
eq
(Å
2
) 0.3 0.3 0.3 0.4 0.4
O: U
11
, U
22
, U
33
(Å
2
) 0.6, 0.4, 1.5 0.7, 0.4, 1.5 0.7, 0.5, 1.5 0.7, 0.5, 1.6 0.7, 0.5, 1.6
U
12
, U
13
, U
23
(Å
2
) −0.3, 0.3, −0.1 −0.3, 0.3, −0.1 −0.3, 0.3, −0.2 −0.3, 0.3, −0.1 −0.3, 0.3, −0.2
U
eq
(Å
2
) 0.8 0.9 0.9 0.9 0.9
Ca−O (1) (Å) 2.459 2.459 2.460 2.460 2.461
Ca−O (2) (Å) 2.487 2.487 2.487 2.488 2.489
V
CaO
8
(Å
3
)
26.88 26.89 26.90 26.92 26.93
Δ
AO
8
(×10
3
)
5.6 5.6 5.5 5.7 5.8
Mo−O (Å) 1.745 1.745 1.744 1.744 1.744
V
MoO
4
(Å
3
)
2.71 2.71 2.71 2.71 2.71
Ca−O−Mo (1) (°) 120.4 120.4 120.4 120.4 120.3
Ca−O−Mo (2) (°) 132.9 132.9 132.9 133.0 133.0
a
Atomic displacements parameters
are given as 100×U
ij
.
107
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
151 K 163 K 175 K 188 K 200 K
a (Å) 5.220 5.221 5.221 5.222 5.222
c (Å) 11.411 11.413 11.415 11.417 11.419
V (Å
3
) 311.0 311.1 311.2 311.3 311.4
x
O
0.647 0.647 0.647 0.647 0.647
y
O
0.513 0.513 0.513 0.513 0.513
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.4, 0.8 0.4, 0.8 0.4, 0.8 0.4, 0.9 0.5, 0.9
U
eq
(Å
2
) 0.5 0.5 0.5 0.6 0.6
Mo: U
11
, U
33
(Å
2
)
0.3, 0.5 0.3, 0.5 0.4, 0.6 0.4, 0.6 0.4, 0.6
U
eq
(Å
2
) 0.4 0.4 0.5 0.5 0.5
O: U
11
, U
22
, U
33
(Å
2
) 0.7, 0.5, 1.6 0.8, 0.5, 1.7 0.8, 0.5, 1.7 0.9, 0.5, 1.7 0.9, 0.6, 1.8
U
12
, U
13
, U
23
(Å
2
) −0.3, 0.3, −0.2 −0.3, 0.3 , −0.1 −0.3, 0.3 , −0.1 −0.3, 0.3 , −0.1 −0.3, 0.3 , −0.1
U
eq
(Å
2
) 0.9 1.0 1.0 1.0 1.1
Ca−O (1) (Å) 2.461 2.461 2.462 2.462 2.463
Ca−O (2) (Å) 2.489 2.490 2.491 2.491 2.492
V
CaO
8
(Å
3
)
26.94 26.96 26.98
27.00
27.01
Δ
AO
8
(×10
3
)
5.7 5.9 5.9
5.9
5.8
Mo−O (Å) 1.744 1.744 1.744 1.744 1.744
V
MoO
4
(Å
3
)
2.71 2.71 2.71
2.71
2.71
Ca−O−Mo (1) (°) 120.4 120.3 120.3 120.3 120.3
Ca−O−Mo (2) (°) 133.0 133.0 133.0 133.1 133.1
a
Atomic displacements parameters
are given as 100×U
ij
.
108
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
212 K 224 K 236 K 248 K 261 K
a (Å) 5.223 5.224 5.224 5.225 5.225
c (Å) 11.421 11.423 11.425 11.427 11.429
V (Å
3
) 311.6 311.7 311.8 311.9 312.1
x
O
0.646 0.646 0.646 0.646 0.646
y
O
0.513 0.513 0.513 0.513 0.513
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.5, 0.9 0.5, 1.0 0.5, 1.0 0.5, 1.0 0.5, 1.0
U
eq
(Å
2
) 0.6 0.7 0.7 0.7 0.7
Mo: U
11
, U
33
(Å
2
)
0.4, 0.6 0.5, 0.6 0.5, 0.7 0.5, 0.7 0.5, 0.7
U
eq
(Å
2
) 0.5 0.5 0.6 0.6 0.6
O: U
11
, U
22
, U
33
(Å
2
) 0.9, 0.6, 1.8 0.9, 0.7, 1.8 1.0, 0.7, 1.8 1.0, 0.7, 1.9 1.0, 0.7, 1.9
U
12
, U
13
, U
23
(Å
2
) −0.3, 0.3 , −0.1 −0.2, 0.3 , −0.1 −0.2, 0.3 , −0.1 −0.2, 0.3 , 0.0 −0.2, 0.3 , 0.0
U
eq
(Å
2
) 1.1 1.1 1.2 1.2 1.2
Ca−O (1) (Å) 2.463 2.464 2.464 2.465 2.465
Ca−O (2) (Å) 2.493 2.493 2.494 2.495 2.496
V
CaO
8
(Å
3
)
27.04 27.05 27.08 27.10 27.12
Δ
AO
8
(×10
3
)
6.0 6.0 6.1 6.0 6.1
Mo−O (Å) 1.743 1.743 1.743 1.743 1.742
V
MoO
4
(Å
3
)
2.71 2.71 2.71 2.71 2.71
Ca−O−Mo (1) (°) 120.3 120.3 120.3 120.3 120.3
Ca−O−Mo (2) (°) 133.1 133.1 133.1 133.1 133.2
a
Atomic displacements parameters
are given as 100×U
ij
.
109
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
273 K 285 K 297 K 309 K 322 K
a (Å) 5.226 5.227 5.227 5.228 5.228
c (Å) 11.431 11.433 11.435 11.437 11.440
V (Å
3
) 312.2 312.3 312.4 312.6 312.7
x
O
0.646 0.646 0.646 0.646 0.646
y
O
0.513 0.513 0.513 0.513 0.513
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.6, 1.1 0.6, 1.1 0.6, 1.1 0.6, 1.2 0.6, 1.2
U
eq
(Å
2
) 0.8 0.8 0.8 0.8 0.8
Mo: U
11
, U
33
(Å
2
)
0.5, 0.7 0.5, 0.8 0.6, 0.8 0.6, 0.8 0.6, 0.8
U
eq
(Å
2
) 0.6 0.6 0.7 0.7 0.8
O: U
11
, U
22
, U
33
(Å
2
) 1.0, 0.7, 1.9 1.1, 0.8, 2.0 1.1, 0.8, 2.0 1.2, 0.8, 2.0 1.2, 0.8, 2.1
U
12
, U
13
, U
23
(Å
2
) −0.2, 0.3 , 0.0 −0.2, 0.3 , 0.0 −0.2, 0.3 , 0.0 −0.2, 0.3 , 0.0 −0.1, 0.3 , 0.1
U
eq
(Å
2
) 1.2 1.3 1.3 1.3 1.4
Ca−O (1) (Å) 2.466 2.466 2.466 2.467 2.468
Ca−O (2) (Å) 2.496 2.497 2.497 2.498 2.499
V
CaO
8
(Å
3
)
27.14 27.15 27.17 27.19 27.21
Δ
AO
8
(×10
3
)
6.1 6.1 6.2 6.2 6.3
Mo−O (Å) 1.742 1.742 1.742 1.742 1.742
V
MoO
4
(Å
3
)
2.70 2.70 2.71 2.70 2.70
Ca−O−Mo (1) (°) 120.3 120.3 120.3 120.3 120.3
Ca−O−Mo (2) (°) 133.2 133.2 133.2 133.2 133.3
a
Atomic displacements parameters
are given as 100×U
ij
.
110
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
334 K 346 K 358 K 370 K 383 K
a (Å) 5.229 5.230 5.230 5.231 5.231
c (Å) 11.442 11.445 11.447 11.450 11.452
V (Å
3
) 312.9 313.0 313.1 313.3 313.4
x
O
0.646 0.646 0.646 0.646 0.646
y
O
0.513 0.514 0.514 0.514 0.514
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.7, 1.2 0.7, 1.3 0.7, 1.3 0.7, 1.4 0.8, 1.4
U
eq
(Å
2
) 0.9 0.9 0.9 0.9 1.0
Mo: U
11
, U
33
(Å
2
)
0.6, 0.9 0.7, 0.9 0.7, 0.9 0.7, 0.9 0.7, 1.0
U
eq
(Å
2
) 0.7 0.8 0.8 0.8 0.8
O: U
11
, U
22
, U
33
(Å
2
) 1.3, 0.8, 2.1 1.3, 0.9, 2.2 1.4, 0.9, 2.2 1.4, 0.9, 2.2 1.4, 0.9, 2.3
U
12
, U
13
, U
23
(Å
2
) −0.1, 0.3 , 0.1 −0.1, 0.3 , 0.1 −0.1, 0.3 , 0.1 0.0, 0.3 , 0.2 0.0, 0.3 , 0.2
U
eq
(Å
2
) 1.4 1.5 1.5 1.5 1.5
Ca−O (1) (Å) 2.468 2.470 2.470 2.471 2.472
Ca−O (2) (Å) 2.500 2.500 2.501 2.502 2.504
V
AO
8
(Å
3
)
27.24 27.27 27.30 27.33 27.36
Δ
AO
8
(×10
3
)
6.3 6.2 6.3 6.3 6.4
Mo−O (Å) 1.742 1.741 1.740 1.740 1.739
V
MoO
4
(Å
3
)
2.70 2.70 2.70 2.69 2.69
Ca−O−Mo (1) (°) 120.3 120.3 120.3 120.3 120.3
Ca−O−Mo (2) (°) 133.3 133.3 133.3 133.3 133.4
a
Atomic displacements parameters
are given as 100×U
ij
.
111
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis ⎯Continues on the following page
395 K 407 K 419 K 431 K 444 K
a (Å) 5.232 5.233 5.233 5.234 5.234
c (Å) 11.454 11.457 11.459 11.461 11.463
V (Å
3
) 313.5 313.7 313.8 313.9 314.0
x
O
0.646 0.645 0.645 0.645 0.645
y
O
0.514 0.515 0.515 0.515 0.515
z
O
0.209 0.209 0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.8, 1.4 0.8, 1.5 0.9, 1.5 0.9, 1.5 0.9, 1.6
U
eq
(Å
2
) 1.0 1.0 1.1 1.1 1.1
Mo: U
11
, U
33
(Å
2
)
0.7, 1.0 0.8, 1.0 0.8, 1.0 0.8, 1.0 0.8, 1.1
U
eq
(Å
2
) 0.8 0.9 0.9 0.9 0.9
O: U
11
, U
22
, U
33
(Å
2
) 1.5, 0.9, 2.4 1.6, 1.0, 2.4 1.6, 1.0, 2.5 1.7, 1.0, 2.5 1.7, 1.0, 2.5
U
12
, U
13
, U
23
(Å
2
) 0.0, 0.3 , 0.2 0.0, 0.3 , 0.3 0.0, 0.3 , 0.3 0.0, 0.3 , 0.3 0.0, 0.3 , 0.3
U
eq
(Å
2
) 1.6 1.7 1.7 1.7 1.7
Ca−O (1) (Å) 2.473 2.474 2.475 2.476 2.476
Ca−O (2) (Å) 2.505 2.506 2.507 2.508 2.510
V
AO
8
(Å
3
)
27.39 27.43 27.46 27.50 27.53
Δ
AO
8
(×10
3
)
6.4 6.5 6.4 6.5 6.7
Mo−O (Å) 1.738 1.738 1.737 1.736 1.736
V
MoO
4
(Å
3
)
2.69 2.69 2.68 2.68 2.68
Ca−O−Mo (1) (°) 120.3 120.3 120.3 120.3 120.2
Ca−O−Mo (2) (°) 133.4 133.4 133.4 133.4 133.5
a
Atomic displacements parameters
are given as 100×U
ij
.
112
Table 6.1. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From Rietveld
Analysis
456 K 468 K 480 K
a (Å) 5.234 5.235 5.235
c (Å) 11.466 11.468 11.469
V (Å
3
) 314.1 314.2 314.3
x
O
0.645 0.645 0.645
y
O
0.515 0.516 0.516
z
O
0.209 0.209 0.209
Ca: U
11
, U
33
(Å
2
)
a
0.9, 1.6 0.9, 1.6 0.9, 1.7
U
eq
(Å
2
) 1.1 1.1 1.2
Mo: U
11
, U
33
(Å
2
)
0.8, 1.1 0.9, 1.1 0.9, 1.1
U
eq
(Å
2
) 0.9 1.0 1.0
O: U
11
, U
22
, U
33
(Å
2
) 1.8, 1.0, 2.6 1.8, 1.0, 2.6 1.9, 1.0, 2.6
U
12
, U
13
, U
23
(Å
2
) 0.0, 0.2 , 0.3 0.0, 0.2 , 0.3 0.0, 0.2 , 0.4
U
eq
(Å
2
) 1.8 1.8 1.8
Ca−O (1) (Å) 2.477 2.477 2.478
Ca−O (2) (Å) 2.511 2.512 2.513
V
AO
8
(Å
3
)
27.56 27.58 27.60
Δ
AO
8
(×10
3
)
6.9 6.9 7.1
Mo−O (Å) 1.735 1.734 1.734
V
MoO
4
(Å
3
)
2.67 2.67 2.67
Ca−O−Mo (1) (°) 120.2 120.2 120.2
Ca−O−Mo (2) (°) 133.6 133.6 133.6
a
Atomic displacements parameters
are given as 100×U
ij
.
113
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
90 K 102 K 114 K 127 K 139 K
a (Å) 5.213 5.213 5.213 5.213 5.214
c (Å) 11.373 11.374 11.376 11.377 11.379
V (Å
3
) 309.0 309.1 309.1 309.2 309.3
x
O
0.650 0.649 0.649 0.648 0.648
y
O
0.506 0.506 0.506 0.506 0.506
z
O
0.211 0.211 0.211 0.211 0.211
Ca: U
11
, U
33
(Å
2
)
a
0.6, 0.9 0.6, 0.9 0.6, 1.0 0.6, 1.0 0.6, 1.0
U
eq
(Å
2
) 0.7 0.7 0.7 0.7 0.7
Mo: U
11
, U
33
(Å
2
)
0.4, 0.5 0.4, 0.5 0.4, 0.5 0.4, 0.5 0.4, 0.6
U
eq
(Å
2
) 0.4 0.4 0.5 0.5 0.5
O: U
11
, U
22
, U
33
(Å
2
) 1.0, 3.0, 1.5 1.1, 3.1, 1.6 1.3, 3.2, 1.6 1.4, 3.2, 1.6 1.5, 3.3, 1.6
U
12
, U
13
, U
23
(Å
2
) −0.2, 0.1, 1.3 −0.2, 0.0, 1.4 −0.2, 0.0, 1.4 −0.2, −0.1, 1.5 −0.2, −0.1, 1.5
U
eq
(Å
2
) 1.9 1.9 2.0 2.1 2.1
Ca−O (1) (Å) 2.426 2.425 2.424 2.422 2.421
Ca−O (2) (Å) 2.460 2.463 2.466 2.470 2.472
V
AO
8
(Å
3
)
25.91 25.94 25.97 26.01 26.03
Δ
AO
8
(×10
3
)
7.0 7.7 8.7 9.8 10.5
Mo−O (Å) 1.783 1.782 1.781 1.781 1.780
V
MoO
4
(Å
3
)
2.89 2.89 2.89 2.89 2.89
Ca−O−Mo (1) (°) 119.8 119.7 119.5 119.4 119.3
Ca−O−Mo (2) (°) 132.2 132.3 132.5 132.7 132.8
a
Atomic displacements parameters
are given as 100×U
ij
.
114
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
151 K 163 K 175 K 188 K 200 K
a (Å) 5.214 5.214 5.214 5.215 5.216
c (Å) 11.381 11.384 11.386 11.388 11.390
V (Å
3
) 309.4 309.5 309.6 309.7 309.8
x
O
0.647 0.644 0.644 0.643 0.643
y
O
0.507 0.508 0.508 0.508 0.508
z
O
0.211 0.213 0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
0.6, 1.0 0.6, 1.1 0.7, 1.1 0.7, 1.1 0.7, 1.2
U
eq
(Å
2
) 0.8 0.8 0.8 0.8 0.8
Mo: U
11
, U
33
(Å
2
)
0.4, 0.6 0.5, 0.6 0.5, 0.6 0.5, 0.7 0.5, 0.7
U
eq
(Å
2
) 0.5 0.5 0.5 0.5 0.6
O: U
11
, U
22
, U
33
(Å
2
) 1.7, 3.3, 1.7 2.4, 3.2, 1.7 2.5, 3.3, 1.7 2.7, 3.4, 1.7 2.8, 3.4, 1.7
U
12
, U
13
, U
23
(Å
2
) −0.2, −0.2, 1.5 −0.1, −0.4, 1.6 −0.1, −0.4, 1.6 −0.1, −0.5, 1.7 0.0, −0.5, 1.7
U
eq
(Å
2
) 2.2 2.4 2.5 2.6 2.6
Ca−O (1) (Å) 2.419 2.407 2.405 2.404 2.403
Ca−O (2) (Å) 2.477 2.496 2.499 2.503 2.505
V
AO
8
(Å
3
)
26.08 26.20 26.23 26.27 26.30
Δ
AO
8
(×10
3
)
11.9 18.3 19.2 20.3 20.8
Mo−O (Å) 1.779 1.777 1.777 1.776 1.776
V
MoO
4
(Å
3
)
2.88 2.88 2.88 2.87 2.87
Ca−O−Mo (1) (°) 119.1 118.3 118.2 118.0 118.0
Ca−O−Mo (2) (°) 133.0 134.0 134.2 134.4 134.5
a
Atomic displacements parameters
are given as 100×U
ij
.
115
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
212 K 224 K 236 K 248 K 261 K
a (Å) 5.216 5.217 5.217 5.218 5.219
c (Å) 11.390 11.393 11.395 11.397 11.399
V (Å
3
) 309.8 310.1 310.2 310.3 310.4
x
O
0.642 0.642 0.642 0.642 0.641
y
O
0.508 0.508 0.508 0.508 0.508
z
O
0.213 0.213 0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
0.7, 1.2 0.7, 1.2 0.8, 1.3 0.8, 1.3 0.8, 1.3
U
eq
(Å
2
) 0.9 0.9 0.9 0.9 1.0
Mo: U
11
, U
33
(Å
2
)
0.5, 0.7 0.5, 0.7 0.5, 0.8 0.6, 0.8 0.6, 0.8
U
eq
(Å
2
) 0.6 0.6 0.6 0.6 0.6
O: U
11
, U
22
, U
33
(Å
2
) 2.9, 3.5, 1.8 3.0, 3.6, 1.8 3.1, 3.6, 1.8 3.2, 3.7, 1.9 3.2, 3.7, 1.9
U
12
, U
13
, U
23
(Å
2
) 0.0, −0.5, 1.7 0.0, −0.5, 1.8 0.0, −0.5, 1.8 0.0, −0.5, 1.8 0.0, −0.6, 1.8
U
eq
(Å
2
) 2.7 2.8 2.8 2.9 3.0
Ca−O (1) (Å) 2.402 2.401 2.401 2.401 2.401
Ca−O (2) (Å) 2.509 2.511 2.513 2.514 2.516
V
AO
8
(Å
3
)
26.34 26.37 26.39 26.42 26.44
Δ
AO
8
(×10
3
)
21.8 22.3 22.8 23.2 23.4
Mo−O (Å) 1.775 1.775 1.774 1.774 1.774
V
MoO
4
(Å
3
)
2.87 2.87 2.87 2.86 2.86
Ca−O−Mo (1) (°) 117.8 117.8 117.7 117.7 117.7
Ca−O−Mo (2) (°) 134.7 134.8 134.8 134.9 135.0
a
Atomic displacements parameters
are given as 100×U
ij
.
116
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
273 K 285 K 297 K 309 K 322 K
a (Å) 5.219 5.220 5.221 5.221 5.222
c (Å) 11.401 11.403 11.404 11.406 11.409
V (Å
3
) 310.6 310.7 310.8 311.0 311.1
x
O
0.641 0.641 0.641 0.641 0.641
y
O
0.508 0.508 0.508 0.508 0.508
z
O
0.213 0.213 0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
0.8, 1.3 0.8, 1.4 0.9, 1.4 0.9, 1.4 0.9, 1.5
U
eq
(Å
2
) 1.0 1.0 1.0 1.1 1.1
Mo: U
11
, U
33
(Å
2
)
0.6, 0.8 0.6, 0.8 0.6, 0.9 0.6, 0.9 0.6, 0.9
U
eq
(Å
2
) 0.7 0.7 0.7 0.7 0.7
O: U
11
, U
22
, U
33
(Å
2
) 3.3, 3.7, 2.0 3.4, 3.8, 2.0 3.4, 3.8, 2.0 3.5, 3.8, 2.1 3.6, 3.9, 2.2
U
12
, U
13
, U
23
(Å
2
) 0.0, −0.6, 1.8 0.0, −0.6, 1.9 0.0, −0.6, 1.9 0.1, −0.6, 1.9 0.1, −0.7, 1.9
U
eq
(Å
2
) 3.0 3.0 3.1 3.1 3.2
Ca−O (1) (Å) 2.401 2.401 2.401 2.401 2.402
Ca−O (2) (Å) 2.516 2.517 2.518 2.519 2.520
V
AO
8
(Å
3
)
26.46 26.48 26.50 26.52 26.55
Δ
AO
8
(×10
3
)
23.4 23.7 23.8 24.0 23.9
Mo−O (Å) 1.774 1.773 1.773 1.773 1.773
V
MoO
4
(Å
3
)
2.86 2.86 2.86 2.86 2.86
Ca−O−Mo (1) (°) 117.7 117.6 117.6 117.6 117.6
Ca−O−Mo (2) (°) 135.0 135.1 135.1 135.1 135.2
a
Atomic displacements parameters
are given as 100×U
ij
.
117
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
334 K 346 K 358 K 370 K 383 K
a (Å) 5.223 5.224 5.224 5.225 5.226
c (Å) 11.411 11.413 11.415 11.418 11.420
V (Å
3
) 311.3 311.4 311.6 311.7 311.8
x
O
0.640 0.640 0.640 0.640 0.640
y
O
0.508 0.508 0.508 0.508 0.508
z
O
0.213 0.213 0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
1.0, 1.5 1.0, 1.5 1.0, 1.5 1.0, 1.6 1.1, 1.6
U
eq
(Å
2
) 1.1 1.2 1.2 1.2 1.3
Mo: U
11
, U
33
(Å
2
)
0.7, 0.9 0.7, 1.0 0.7, 1.0 0.7, 1.0 0.7, 1.0
U
eq
(Å
2
) 0.8 0.8 0.8 0.8 0.8
O: U
11
, U
22
, U
33
(Å
2
) 3.6, 3.9, 2.2 3.7, 4.0, 2.3 3.7, 4.0, 2.4 3.8, 4.0, 2.4 3.9, 4.1, 2.5
U
12
, U
13
, U
23
(Å
2
) 0.1, −0.7, 1.9 0.1, −0.7, 2.0 0.1, −0.8, 1.9 0.1, −0.8, 2.0 0.1, −0.8, 2.0
U
eq
(Å
2
) 3.3 3.3 3.4 3.4 3.5
Ca−O (1) (Å) 2.403 2.403 2.405 2.405 2.405
Ca−O (2) (Å) 2.521 2.521 2.521 2.522 2.523
V
AO
8
(Å
3
)
26.57 26.59 26.61 26.63 26.65
Δ
AO
8
(×10
3
)
23.9 24.0 23.7 23.7 23.8
Mo−O (Å) 1.772 1.772 1.772 1.772 1.772
V
MoO
4
(Å
3
)
2.86 2.86 2.85 2.85 2.85
Ca−O−Mo (1) (°) 117.6 117.6 117.7 117.7 117.7
Ca−O−Mo (2) (°) 135.2 135.2 135.2 135.2 135.3
a
Atomic displacements parameters
are given as 100×U
ij
.
118
Table 6.2. Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF
Analysis ⎯Continues on the following page
395 K 407 K 419 K 431 K 444 K
a (Å) 5.226 5.227 5.228 5.228 5.229
c (Å) 11.422 11.424 11.426 11.429 11.431
V (Å
3
) 312.0 312.1 312.3 312.4 312.5
x
O
0.640 0.640 0.640 0.640 0.640
y
O
0.508 0.508 0.508 0.508 0.508
z
O
0.213 0.213 0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
1.1, 1.6 1.1, 1.6 1.2, 1.7 1.2, 1.7 1.2, 1.7
U
eq
(Å
2
) 1.3 1.3 1.3 1.4 1.4
Mo: U
11
, U
33
(Å
2
)
0.7, 1.1 0.8, 1.1 0.8, 1.1 0.8, 1.1 0.8, 1.2
U
eq
(Å
2
) 0.9 0.9 0.9 0.9 0.9
O: U
11
, U
22
, U
33
(Å
2
) 3.9, 4.1, 2.6 4.0, 4.0, 2.6 4.0, 4.1, 2.7 4.1, 4.1, 2.7 4.1, 4.1, 2.8
U
12
, U
13
, U
23
(Å
2
) 0.1, −0.8, 2.0 0.2, −0.9, 2.0 0.2, −0.9, 2.0 0.2, −0.9, 2.0 0.2, −0.9, 2.0
U
eq
(Å
2
) 3.5 3.5 3.6 3.6 3.7
Ca−O (1) (Å) 2.406 2.407 2.408 2.408 2.408
Ca−O (2) (Å) 2.523 2.523 2.524 2.524 2.525
V
AO
8
(Å
3
)
26.67 26.68 26.70 26.72 26.74
Δ
AO
8
(×10
3
)
23.6 23.7 23.5 23.5 23.7
Mo−O (Å) 1.771 1.771 1.771 1.771 1.771
V
MoO
4
(Å
3
)
2.85 2.85 2.85 2.85 2.85
Ca−O−Mo (1) (°) 117.7 117.7 117.7 117.7 117.7
Ca−O−Mo (2) (°) 135.2 135.3 135.3 135.3 135.3
a
Atomic displacements parameters
are given as 100×U
ij
.
119
Table 6.2 Structural Parameters of CaMoO
4
Nanocrystals From 90–480 K Extracted From PDF Analysis
456 K 468 K 480 K
a (Å) 5.229 5.230 5.230
c (Å) 11.433 11.435 11.436
V (Å
3
) 312.6 312.8 312.9
x
O
0.640 0.640 0.640
y
O
0.508 0.508 0.508
z
O
0.213 0.213 0.213
Ca: U
11
, U
33
(Å
2
)
a
1.3, 1.7 1.3, 1.8 1.3, 1.8
U
eq
(Å
2
) 1.4 1.5 1.5
Mo: U
11
, U
33
(Å
2
)
0.8, 1.2 0.8, 1.2 0.8, 1.2
U
eq
(Å
2
) 0.9 1.0 1.0
O: U
11
, U
22
, U
33
(Å
2
) 4.2, 4.1, 2.8 4.2, 4.1, 2.9 4.2, 4.1, 2.9
U
12
, U
13
, U
23
(Å
2
) 0.2, −1.0, 2.0 0.2, −1.0, 2.0 0.3, −1.0, 2.0
U
eq
(Å
2
) 3.7 3.7 3.8
Ca−O (1) (Å) 2.409 2.409 2.409
Ca−O (2) (Å) 2.525 2.527 2.527
V
AO
8
(Å
3
)
26.76 26.78 26.79
Δ
AO
8
(×10
3
)
23.5 23.9 23.8
Mo−O (Å) 1.771 1.771 1.770
V
MoO
4
(Å
3
)
2.85 2.85 2.85
Ca−O−Mo (1) (°) 117.7 117.7 117.7
Ca−O−Mo (2) (°) 135.3 135.4 135.4
a
Atomic displacements parameters
are given as 100×U
ij
.
120
Figure 6.4. Lattice constants (a and c) and unit cell volumes extracted from Rietveld (black symbols)
and PDF (red symbols) analysis of the X-ray total scattering data for CaMoO
4
nanocrystals from 90–480
K.
Previous investigations into the lattice thermal expansion behavior of bulk CaMoO
4
have been
conducted in the temperature range of 25–1000 °C.
16,19
Coefficients of thermal expansion were
reported to be 13.5 × 10
-6
°C
−1
and 22.8 × 10
-6
°C
−1
along the a and c-axes (i.e., α
a
and α
c
),
respectively. The α
a
and α
c
values for the VDSG derived CaMoO
4
nanocrystals, as determined
by Rietveld analysis, were 8.8 × 10
-6
°C
−1
and 15.1 × 10
-6
°C
−1
, respectively. Additionally, the
values obtained by PDF analysis were quite similar: 9.6 × 10
-6
°C
−1
and 14.5 × 10
-6
°C
−1
for α
a
and α
c
, respectively. The larger α
c
relative to α
a
indicates that the expansion of the unit cell is
driven by the distortion of the CaO
8
dodecahedra, which stagger along the c-axis of the unit
cell.
20
Meanwhile, the more invariable tetrahedral units suppress expansion along the a-axis
with increasing temperature.
In accordance with the known structural rigidity of the MoO
4
tetrahedra, the Mo–O bond
distances exhibit only a slight contraction from 90–480 K (Fig. 6.5a). A reduction in M–O bond
121
distance with increasing temperature has been observed in other AMO
4
systems and was shown
to be associated with correlated thermal motion initiated by the MO
4
tetrahedra.
21
As seen by
Rabuffetti et al., the Mo–O bond distances for CaMoO
4
nanocrystals determined by PDF
analysis more closely resemble literature values for bulk CaMoO
4
.
18
In the present work,
Rietveld analysis yields bond distances that are much shorter than expected for MoO
4
tetrahedra
in scheelite-structured molybdates (i.e., 1.7445–1.7343 Å from 90 to 480 K).
16
The inability of
Rietveld analysis to accurately describe the spatial arrangement of atoms within CaMoO
4
nanocrytals highlights the importance of employing a dual-space approach, especially when
evaluating materials with reduced structural coherence, such as nanocrystals.
122
Figure 6.5. (a) Mo–O distances, (b) Ca–O distances, and (d) bond length distortion indices extracted
from Rietveld (black symbols) and PDF (red symbols) analysis of the X-ray total scattering data for
CaMoO
4
nanocrystals from 90–480 K. (c) Evolution of the CaMoO
4
structure with increasing
temperature, as described by PDF analysis. Blue, purple, and red spheres represent calcium,
molybdenum, and oxygen, respectively. Each unique Ca–O bond distance in (b) and (c) is denoted with a
number in parentheses.
Upon examining the variation of the two unique Ca–O bond distances with temperature, a
critical disagreement between Rietveld and PDF analysis arises (Figure 6.5b). Rietveld analysis
suggests that the two sets of Ca–O bond distances increase linearly from 90–480 K. However,
PDF analysis reveals a sharp transition between 151 and 163 K. On the local scale, the Ca–O
bond distances evolve incongruously with increasing temperature. The dependence of Ca–O
bond distances on increasing temperature, as determined by PDF analysis, is illustrated in Figure
6.3c. At 90 K, Ca–O (1) is 2.4261 Å and Ca–O (2) is 2.4603 Å. By 163 K, the bond length of
123
Ca–O (1) is reduced by 0.8% to 2.4065 Å, while Ca–O (2) is elongated by 1.5% to 2.4963 Å.
This phenomenon manifests in the thermal parameters derived from PDF analysis as well. In
this temperature range, a substantial increase is noted in the U
11
, while the others remain largely
unchanged. Contrastingly, Rietveld analysis suggests that the two unique Ca–O bond distances
and thermal parameters of the oxygen atoms are nearly constant. Overall, PDF analysis indicates
that as the shorter Ca–O bonds contract, the longer Ca–O bonds expand from 90–480 K,
resulting in a substantial distortion of the CaO
8
polyhedra. As such, the dimensionless bond
distance distortion index (Δ
CaO
8
)
22
was employed to quantitatively assess geometric distortions of
the CaO
8
dodecahedra. Δ
CaO
8
is defined as:
𝛥
CaO
8
=
1
8
d
Ca!O
i
− d
Ca!O
d
Ca!O
8
i = 1
where d
Ca!O
is the average Ca–O bond distance. Figure 6.5d shows the change in the Δ
CaO
8
with temperature. At 90 K, the bond length distortion values derived from Rietveld and PDF are
nearly identical (~5–7); however, these values develop quite differently as the temperature is
elevated. Indices extracted from the Rietveld analysis show a slight increase by 480 K, while
those from the PDF analysis increase to approximately 24, with a marked increase in the range of
151–163 K.
The difference in behavior of the Ca–O distances extracted from Rietveld and PDF point
towards structural phenomena occurring locally, that are not directly observable on the longer
scale of Bragg diffraction. As previously mentioned, the MoO
4
tetrahedra within these AMO
4
nanocrystals are known to be randomly rotated throughout the scheelite lattice; an effect
124
attributed to the presence of orientational disorder and accommodated for by AO
8
dodecahedral
distortions.
18
The variation of the Ca–O bond distances with temperature, as determined by
PDF, suggests that this process is thermally promoted. Below 151 K, the MoO
4
tetrahedra are in
more of a rotationally locked position, however, as the temperature progresses through 163 K, an
activation barrier is surmounted that allows the rigid tetrahedral units to more freely rotate about
the Ca–O–Mo “hinges” (Figure 6.5c). It should be noted that while only one rotational scenario,
involving two oxygen atoms, is depicted in Figure 6.5c, all four tetrahedral oxygens induce
distortions upon the corner-sharing CaO
8
polyhedra with each rotation. Accordingly, the gap
between the two unique Ca–O distances widens significantly, with a corresponding rise in the
bond distance distortion index. Meanwhile, on the average scale, the Ca–O distances exhibit a
linear increase as the temperature increases from 90 to 480 K, further establishing that these are
not concerted rotations.
6.4. Experimental
6.4.1. Nanocrystal Synthesis
CaMoO
4
nanocrystals were synthesized via a vapor diffusion sol−gel method described in detail
elsewhere. Briefly, MoO
2
(acac)
2
(95%, Strem Chemicals, Inc.) was dissolved in a
Ca(OCH
2
CH
2
OCH
3
)
2
alkoxide solution (19−25 wt. % in methoxypropanol, Gelest, Inc.) in a 1:1
molar ratio. The resulting solution was exposed to a controlled flow of water vapor for 48 h at
room temperature and atmospheric pressure. Diffusion of water vapor into the solution resulted
in the formation of a highly contracted gel, which was subsequently aged under nitrogen
atmosphere for 24 h at 80 °C. The resulting gel was collected, washed with absolute ethanol (3 ×
125
10 mL), and vacuum dried at room temperature to recover an off-white fine powder consisting of
CaMoO
4
nanocrystals. These exhibited quasispherical shape with an average diameter of 9.3 ±
2.7 nm, as determined by transmission electron microscopy analysis (N = 250).
6.4.2. Material Characterization
Synchrotron X-ray Diffraction: X-ray diffraction patterns were collected at the 11−ID−B
beamline of the Advanced Photon Source at Argonne National Laboratory. An incident photon
energy of 90.484 keV (λ = 0.137024 Å) was employed. The sample was loaded in a Kapton tube
and diffraction data were collected in transmission mode from 90–480 K at a rate of 6 K/min
using the Oxford cryosystems cryostream 700 plus. Rietveld Analysis: Rietveld structural
refinements were carried out using the GSAS software. Experimental data and atomic X-ray
scattering factors were corrected for sample absorption and anomalous scattering, respectively.
The average crystal structure of AMoO
4
nanocrystals was refined with the tetragonal I4
1
/a (no.
88) space group. The following parameters were refined: (1) scale factor, (2) background, which
was modeled using a shifted Chebyschev polynomial function, (3) peak shape, which was
modeled using a modified Thomson−Cox−Hastings pseudo-Voigt function, (4) lattice constants
(a and c), (5) fractional atomic coordinates of the oxygen atom (x
O
, y
O
, z
O
), and (6) atomic
anisotropic displacement parameters constrained by the site symmetry (U
11
and U
33
for Ca and
Mo, and U
11
, U
22
, U
33
, U
12
, U
13
, and U
23
for O). The R
wp
indicator was employed to assess the
quality of the refined structural models. Pair Distribution Function Analysis: The pair
distribution function G(r) defined as:
126
G(r) = 4πr[ρ(r) − ρ
0
] = (2 / π) Q S Q − 1 sin Qr dQ
Q
max
Q
was employed for structural analysis. Here, r is the radial distance, ρ(r) and ρ
0
are the local and
average atomic number density, respectively, and Q is the magnitude of scattering vector. The
RAD software was employed to extract G(r) from the raw diffraction data. These were first
corrected for background, sample absorption, and Compton scattering. Then, normalized
structure functions S(Q) were obtained. Finally, S(Q) was Fourier-transformed to yield G(r). A
maximum scattering vector (Q
max
) of 24.5 Å
–1
was employed in the Fourier transform. Structural
refinements were carried out using the PDFgui software. The local crystal structure of CaMoO
4
nanocrystals was refined with the tetragonal I4
1
/a space group. Fits of this structural model to the
experimental PDFs were performed in the 1.5−13 Å interatomic distance range in order to
account for all atom−atom pairs along the largest dimension of the unit cell. The following
parameters were refined: (1) scale factor, (2) lattice constants (a and c), (3) fractional atomic
coordinates of the oxygen atom (x
O
, y
O
, z
O
), and (4) atomic anisotropic displacement parameters
constrained by the site symmetry (U
11
and U
33
for Ca and Mo, and U
11
, U
22
, U
33
, U
12
, U
13
, and
U
23
for O). The R
w
indicator was employed to assess the quality of the refined structural models.
6.5. Conclusions
In conclusion, a dual-space structural investigation was conducted on VDSG derived
CaMoO
4
nanocrystals from 90–480 K to gain a better understanding of their orientational
disorder as a function of temperature. Despite all relevant parameters associated with the
average structure (Rietveld) showing nearly linear behavior with temperature, the local structure
(PDF) exhibited a pronounced transition between 151 and 163 K. In this region, the two distinct
127
Ca–O bond distances were found to strongly diverge from each other, resulting in a significant
increase in the bond distance distortion index. The origin of this divergence is attributed to the
thermal activation of random MoO
4
tetrahedral rotations across the scheelite lattice. Given the
growing interest in applying materials of this type as cryogenic scintillation detectors, as well as
the impact of structure on optoelectronic properties, the results presented herein could be of
interest for the development of next generation scintillating crystals.
6.6. References
(1) Parchur, A. K.; Prasad, A. I.; Ansari, A. A.; Rai, S. B.; Ningthoujam, R. S. Dalton Trans.
2012, 41, 11032.
(2) Mikhailik, V. B.; Kraus, H.; Miller, G.; Mykhaylyk, M. S.; Wahl, D. J. Appl. Phys. 2005,
97, 083523.
(3) Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R.; Dong, Z. L.; White, T.
J. Chem. Mater. 2004, 16, 504.
(4) Sleight, A. W. Acta Crystallogr. 1972, B28, 2899.
(5) Miller, W.; Smith, C. W.; Mackenzie, D. S.; Evans, K. E. J. Mater. Sci. 2009, 44, 5441.
(6) Christofilos, D.; Kourouklis, G. A.; Ves, S. J. Phys. Chem. Solids 1995, 56, 1125.
(7) Errandonea, D.; Pellicer-Porres, J.; Manjón, F. J.; Segura, A.; Ferrer-Roca, Ch.; Kumar,
R. S.; Tschauner, O.; Rodríguez-Hernández, P.; López-Solano, J.; Radescu, S.; Mujica,
A.; Muñoz, A.; Aquilanti, G. Phys. Rev. B 2005, 72, 174106.
(8) Saha, D.; Ranjan, R.; Swain, D.; Narayana, C.; Guru Row, T. N. Dalton Trans. 2013, 42,
7672.
(9) Simon, J.; Banys, J.; Hoentsch, J.; Völkel, G.; Böttcher, R.; Hofstaetter, A.; Scharmann,
A. J. Phys. Condens. Matter 1996, 8, L359.
(10) Senyshyn, A.; Kraus, H.; Mikhailik, V. B.; Vasylechko, L.; Knapp, M. Phys. Rev. B
2006, 73, 014104.
128
(11) Proffen, T.; Billinge, S. J. L.; Egami, T.; Louca, D. Z. Kristallogr. 2003, 218, 132.
(12) Culver, S. P.; Rabuffetti, F. A.; Zhou, S.; Mecklenburg, M.; Song, Y.; Melot, B. C.;
Brutchey, R. L. Chem. Mater. 2013, 25, 4129.
(13) Culver, S. P.; Greaney, M. J.; Tinoco, A.; Brutchey, R. L. Dalton Trans. 2015, 44, 15042.
(14) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
(15) Hazen, R. M.; Finger, L. W.; Mariathasan, J. W. E. J. Phys. Chem. Solids 1985, 46, 253.
(16) Achary, S. N.; Patwe, S. J.; Mathews, M. D.; Tyagi, A. K. J. Phys. Chem. Solids 2006,
67, 774.
(17) Errandonea, D.; Kumar, R. S.; Ma, X.; Tu, C. J. Solid State Chem. 2008, 181, 355.
(18) Rabuffetti, F. A.; Culver, S. P.; Suescun, L.; Brutchey, R. L. Inorg. Chem. 2014, 53,
1056.
(19) Im, H.-N.; Choi, M.-B.; Jeon, S.-Y.; Song, S.-J. Ceram. Int. 2011, 37, 49.
(20) Trots, D. M.; Senyshyn, A.; Schwarz, B. C. J. Solid State Chem. 2010, 183, 1245.
(21) Evans, J. S. O.; Mary, T. A.; Vogt, T.; Subramanian, M. A.; Sleight, A. W. Chem. Mater.
1996, 8, 2809.
(22) Baur, W. H. Acta Crystallogr. 1974, B30, 1195.
129
Chapter 7. Lanthanide-Activated CaWO4 Nanocrystal Phosphors Prepared by the
Low-Temperature Vapor Diffusion Sol–Gel Method
7.1. Abstract
A series of Eu
3+
-, Tb
3+
-, and Tm
3+
-doped CaWO
4
phosphor nanocrystals have been synthesized
under benign conditions using the vapor diffusion sol–gel method. The high degree of synthetic
flexibility inherent to this approach has enabled the synthesis of a CaWO
4
:Eu,Tb dual-sensitized
white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long
lifetime exceeding 1 ms.
7.2. Introduction
The development of inorganic luminescent materials has garnered substantial interest in recent
years due to their diverse applicability (e.g., bioimaging, display technologies, solid state
lighting).
1–3
The success of these materials has in a large part been dependent on a subclass of
materials reliant upon the tunable emission harnessed from rare earth lanthanide (Ln) elements.
More specifically, the incorporation of small concentrations of Ln dopants into inorganic oxide
hosts (e.g., phosphates, vanadates, tungstates) has led to a variety of phosphors exhibiting
emission across the visible spectrum.
4–7
Among the available oxides, scheelite-structured
tungstates have emerged as attractive hosts owing to their excellent chemical and thermal
stability, in addition to their unique self-activated luminescence properties.
8,9
In particular, the CaWO
4
scheelite has proven to be an effective host matrix for a multitude of
Ln ions.
10–14
For this reason, numerous studies have exploited CaWO
4
hosts towards achieving
130
down conversion solid-state phosphors. Recently, Wang and co-workers fabricated luminescent
films of Eu
3+
- and Tb
3+
-doped CaWO
4
via the Pechini method, in order to make use of their
characteristic red and green emission, respectively.
12
Additionally, Liao and co-workers
hydrothermally generated CaWO
4
microspheres with a range of Tb
3+
concentrations to yield
green emitting phosphors.
13
In yet another study, Zhang et al. incorporated various
concentrations of Dy
3+
into CaWO
4
using a traditional solid state synthesis to ultimately produce
a white light emitting phosphor through host sensitization coupled with subsequent emissive
energy transfer to Dy
3+
.
14
Unfortunately, all of the aforementioned techniques, including
previously used methods for preparing Ln
3+
-activated scheelite nanocrystals, use high
temperature (≥130 ˚C), high pressure, and/or non-neutral pH to induce crystallization and
incorporate Ln
3+
ions,
15–17
which often inhibits morphological control and leads to compositional
inhomogeneities that negatively affect the targeted emission properties.
Alternatively, the vapor diffusion sol–gel (VDSG) method allows for the synthesis of
compositionally controlled, crystalline metal oxide nanopartilces under ultrabenign conditions
(low temperature, ambient pressure, and neutral pH) by providing kinetic control over the
delivery of water vapor to the gas–liquid interface of an alcohol solution containing essential
metal alkoxides.
18–20
Diffusion of water vapor into the compositionally tailored precursor
solution initiates hydrolysis and polycondensation, resulting in the slow nucleation and growth of
sub-30 nm metal oxide nanocrystals. This method has previously been applied to the synthesis
of many complex multinary metal oxides, including a series of red emitting Eu
3+
-doped BaZrO
3
and BaTiO
3
nanocrystals.
21,22
Herein, three Ln
3+
ions with intrinsic primary color emission (i.e.,
Eu
3+
, Tb
3+
, and Tm
3+
) have been doped into sub-30 nm CaWO
4
nanocrystals under benign
conditions by employing the VDSG method. Structural and morphological characterizations are
131
provided, along with steady-state photoluminescence (PL) spectra, chromaticity coordinates, and
PL lifetimes to elucidate the resultant emission properties. Furthermore, a white light emitting
CaWO
4
nanocrystalline phosphor with a long lifetime was synthesized through dual activator ion
incorporation for the first time.
7.3. Results and Discussion
Powder X-ray diffraction (XRD) was used to probe the crystallinity and phase purity of the
CaWO
4
:xLn (x = 0–1 mol.%) nanocrystals (Figure 7.1). All diffraction maxima can be indexed
to the tetragonal scheelite structure (PDF no. 07-0210), belonging to the I4
1
/a space group.
Moreover, the absence of any secondary phases (e.g., Ln
2
W
2
O
9
) indicates that the nanocrystals
are indeed phase pure. Incorporation of the Ln
3+
ions into the host lattice via the VDSG method
was verified by performing inductively coupled plasma – atomic emission spectroscopy (ICP-
AES) and spectrofluorometry (vide infra). According to ICP-AES, the dopant concentrations of
Eu
3+
, Tb
3+
, and Tm
3+
were 0.54, 0.70, and 0.65 mol.% for nominally CaWO
4
:1%Eu,
CaWO
4
:1%Tb, and CaWO
4
:1%Tm nanocrystals, respectively. Though post-synthetic thermal
treatment (i.e., calcination) was not required to induce crystallization and incorporate the Ln
3+
ions, the as-synthesized nanocrystals were calcined at 600 ˚C for 15 min in air to remove organic
moieties from the surface (~10 wt.% by thermogravimetric analysis) that otherwise quench the
desired luminescence.
132
Figure 7.1. Rietveld analysis of powder XRD patterns of (a) as-synthesized and (b) calcined CaWO
4
nanocrystals. (c) XRD patterns of as-synthesized and calcined CaWO
4
:1%Ln nanocrystals.
Rietveld analysis of the powder XRD patterns was conducted to further probe the structure of
the as-synthesized and calcined CaWO
4
:1%Ln nanocrystals. As shown in Figure 7.1, the
calcined nanocrystals exhibit an increase in crystallinity, but remain phase pure; no segregation
of secondary phases such as CaCO
3
, WO
3
, Ln
2
O
3
, or Ln
2
W
2
O
9
is observed. All calculated
structural parameters and the corresponding Rietveld fits are provided (Tables 7.1–7.2 and
Figures 7.2–7.3). Visual assessment of the fits to the experimental patterns, as well as low
values of the goodness-of-fit indicators (R
wp
and χ
2
) suggest that the structure is adequately
described by the tetragonal scheelite framework. The lattice parameters and unit cell volumes
extracted from the structural analysis correspond well with the literature values for bulk
CaWO
4
.
23
Notably, slight contractions in the unit cell volumes are observed for all
CaWO
4
:1%Ln nanocrystals, further confirming Ln
3+
incorporation and rationalized as follows:
(1) Ln
3+
is known to substitute onto Ca
2+
lattice positions in this system,
24
(2) Ln
3+
ions have
appreciably smaller ionic radii relative to Ca
2+
(rLn
3+
= 1.07, 1.04, and 0.99 Å for 8-coordinate
Eu, Tb, and Tm, respectively and rCa
2+
= 1.12 Å),
25
and (3) substitution of Ca
2+
by Ln
3+
creates
a charge imbalance that is compensated through calcium vacancy formation (V´´
Ca
).
26
133
Figure 7.2. Rietveld analysis of powder XRD patterns for the as-synthesized CaWO
4
:Ln nanocrystals.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯). Tickmarks
(⏐) corresponding to the phase refined are provided.
134
Figure 7.3. Rietveld analysis of powder XRD patterns for the calcined CaWO
4
:Ln nanocrystals.
Experimental (○) and calculated (⎯) patterns are shown, along with the difference curve (⎯).
Tickmarks (⏐) corresponding to the phase refined are provided.
135
Table 7.1. Structural Parameters of As-synthesized Ln-Doped CaWO
4
Nanocrystals
CaWO
4
CaWO
4
:1%Eu CaWO
4
:1%Tb CaWO
4
:1%Tm CaWO
4
:(1%Eu,1%Tb)
a (Å) 5.248(1) 5.245(1) 5.246(1) 5.246(1) 5.244(2)
c (Å) 11.390(3) 11.381(3) 11.384(3) 11.378(3) 11.378(4)
V (Å
3
) 313.6(2) 313.1(2) 313.3(2) 313.1(2) 312.9(3)
x
O
0.6543(8) 0.6567(9) 0.6554(9) 0.6604(10) 0.6608(11)
y
O
0.4969(7) 0.4906(7) 0.4948(7) 0.4908(7) 0.4816(8)
z
O
0.2097(3) 0.2109(4) 0.2103(4) 0.2110(4) 0.2116(4)
U
Ca
(Å
2
)
a
0.46 0.97 0.86 1.30 1.53
U
W
(Å
2
)
1.51 0.96 1.05 0.89 0.74
U
O
(Å
2
) 0.99 1.17 0.89 1.40 1.59
A−O (1) (Å) 2.425(4) 2.399(4) 2.414(4) 2.392(5) 2.369(6)
A−O (2) (Å) 2.429(4) 2.406(5) 2.421(5) 2.406(5) 2.375(5)
W−O (Å) 1.830(3) 1.866(4) 1.844(4) 1.875(4) 1.914(4)
V
AO
8
(Å
3
)
25.39 24.60 25.08 24.43 23.58
V
WoO
4
(Å
3
)
3.11 3.29 3.18 3.33 3.54
R
wp
7.09 7.65 7.49 7.65 8.42
χ
2
1.42 1.48 1.43 1.51 1.68
a
Atomic displacements
parameters are given as
100×U
ij
.
136
Table 7.2. Structural Parameters of Calcined Ln-Doped CaWO
4
Nanocrystals
CaWO
4
CaWO
4
:1%Eu CaWO
4
:1%Tb CaWO
4
:1%Tm CaWO
4
:(1%Eu,1%Tb)
a (Å) 5.240(1) 5.240(1) 5.239(1) 5.238(1) 5.239(1)
c (Å) 11.378(2) 11.373(2) 11.370(2) 11.368(2) 11.367(3)
V (Å
3
) 312.5(2) 312.2(2) 312.0(2) 311.8(2) 312.0(2)
x
O
0.6531(9) 0.6518(9) 0.6556(9) 0.6525(8) 0.6541(9)
y
O
0.5028(8) 0.4998(7) 0.4957(8) 0.4944(7) 0.4890(7)
z
O
0.2108(4) 0.2108(4) 0.2105(4) 0.2106(3) 0.2115(4)
U
Ca
(Å
2
)
a
0.33 0.92 0.83 0.61 0.95
U
W
(Å
2
)
0.92 1.21 0.90 1.27 1.13
U
O
(Å
2
) 0.96 0.67 1.36 0.33 1.19
A−O (1) (Å) 2.427(4) 2.415(4) 2.413(4) 2.401(4) 2.382(4)
A−O (2) (Å) 2.452(5) 2.448(4) 2.420(5) 2.429(4) 2.412(5)
W−O (Å) 1.810(4) 1.818(4) 1.839(4) 1.838(3) 1.867(4)
V
AO
8
(Å
3
)
25.77 25.55 25.03 25.04 24.45
V
WoO
4
(Å
3
)
3.02 3.06 3.16 3.15 3.30
R
wp
8.25 8.10 8.73 7.83 8.07
χ
2
1.55 1.75 1.99 1.66 1.75
a
Atomic displacements
parameters are given as
100×U
ij
.
137
The morphology of the as-synthesized and calcined CaWO
4
:1%Ln nanocrystals was
investigated by transmission electron microscopy (TEM). Representative TEM images depicting
the as-synthesized and calcined CaWO
4
:1%Eu nanocrystals are shown in Figure 7.4. The
nanocrystals exhibit a quasispherical shape with no compositional size or shape dependence.
Mean diameters were found to be 12.5 ± 3.2, 10.9 ± 2.9, 10.2 ± 2.7, and 8.8 ± 2.6 nm for the as-
synthesized CaWO
4
, CaWO
4
:1%Eu, CaWO
4
:1%Tb, and CaWO
4
:1%Tm nanocrystals,
respectively. Upon calcination at 600 ˚C for 15 min in air, the nanocrystals show an increase in
both the mean diameter and the degree of agglomeration; however, the nanoscale morphology is
retained with all particle sizes remaining sub-30 nm. Mean diameters of 21.8 ± 5.8, 22.3 ± 5.5,
22.1 ± 5.5, and 16.8 ± 4.1 nm were found for the calcined CaWO
4
, CaWO
4
:1%Eu,
CaWO
4
:1%Tb, and CaWO
4
:1%Tm nanocrystals, respectively.
138
Figure 7.4. TEM images of (a) as-synthesized and (b) calcined CaWO
4
:1%Eu nanocrystals.
The optical properties of the calcined xLn:CaWO
4
powders were probed using steady-state
and time-resolved spectrofluorometry. Room temperature excitation and emission spectra
displaying the characteristic f–f transitions of the Ln
3+
ions are given in Figure 7.5. With regards
to the nominal CaWO
4
:1%Eu nanocrystals, the excitation spectrum monitored at 615 nm exhibits
peaks corresponding to excitation of the
7
F
0
ground state to the
5
D
4
(359 nm),
5
L
7
(380 nm),
5
L
6
(393 nm),
5
D
3
(414 nm),
5
D
2
(462 nm), and
5
D
1
(471 nm) states (Figure 7.5a).
27
Upon excitation
at 393 nm, emission peaks stemming from the relaxation of the
5
D
0
excited state to the
7
F
1
(590
139
nm),
7
F
2
(615 nm), and
7
F
3
(653 nm) states are observed. The calculated Commission
Internationale de l’Élairage (CIE) coordinates for this composition are (0.595, 0.399), which
corresponds to a reddish orange color. Importantly, the most dominant emission intensity
belongs to the
5
D
0
7
F
2
(electric dipole) transition, indicating that the Eu
3+
ions occupy sites
that lack inversion symmetry (i.e., Ca
2+
sites; S
4
point symmetry without an inversion center),
further corroborating that the Ln
3+
ions are being incorporated into the host lattice at the Ca
2+
lattice position.
27
Turning to the excitation spectrum of the nominal CaWO
4
:1%Tb nanocrystals
monitored at 544 nm, peaks stemming from the excitation of the
7
F
6
ground state to the
5
D
2
(357
nm),
5
L
10
(375 nm), and
5
D
4
(485 nm) states are observed (Figure 7.5b).
13
The associated
emission spectrum reveals peaks corresponding to the relaxation of the
5
D
4
excited state to the
7
F
5
(544 nm),
7
F
4
(588 nm), and
7
F
3
(620 nm) states when excited at 485 nm. The CIE
coordinates for this composition are (0.442, 0.548), which corresponds to a yellowish green
color. Finally, the excitation spectrum of the nominal CaWO
4
:1%Tm nanocrystals was obtained
by monitoring at 454 nm, where a peak arising from the excitation of the
3
H
6
ground state to the
1
D
2
(358 nm) state is present (Figure 7.5c).
28
Meanwhile, the corresponding emission spectrum
exhibits a peak pertaining to the relaxation of the
1
D
2
excited state to the
3
F
4
(454 nm) state. The
CIE coordinates for this composition are (0.183, 0.168), which corresponds to a blue color. It
should also be noted that self-activated blue emission from the host was negligible at all of the
aforementioned excitation wavelengths.
→
140
Figure 7.5. Assigned room temperature excitation and emission spectra of (a) CaWO
4
:1%Eu, (b)
CaWO
4
:1%Tb, and (c) CaWO
4
:1%Tm nanocrystals.
Photoluminescence lifetime measurements corresponding to the nominal CaWO
4
:1%Eu,
CaWO
4
:1%Tb, and CaWO
4
:1%Tm nanocrystals are given in Figure 7.6. All PL lifetime curves
were fit well with a monoexponential function according to I = I
0
exp(-t/τ), where I
0
is the initial
intensity, t is the time, and τ is the associated lifetime. Lifetimes of 1.01, 1.12, and 0.15 ms were
obtained for the nominal CaWO
4
:1%Eu (λ
exc
= 393 nm, λ
em
= 614 nm,
5
D
0
7
F
2
),
CaWO
4
:1%Tb (λ
exc
= 485 nm, λ
em
= 544 nm,
5
D
4
7
F
5
), and CaWO
4
:1%Tm (λ
exc
= 358 nm, λ
em
= 454 nm,
1
D
2
3
F
4
) nanocrystals, respectively. These results compare favorably against
previous studies on lanthanide-doped CaWO
4
(e.g., τ = 0.5 ms for hydrothermally prepared
CaWO
4
:Eu submicron phosphors
29
and τ = 0.3 ms for sonochemically prepared CaWO
4
:Eu
submicron phosphors
30
), and demonstrate the efficacy of radiative lanthanide emission
associated with these VDSG-prepared, sub-30 nm nanocrystals.
13,28
→
→
→
141
Figure 7.6. Room temperature luminescence lifetime curves of (a) CaWO
4
:1%Eu, (b) CaWO
4
:1%Tb,
and (c) CaWO
4
:1%Tm nanocrystals. Monoxponential fits are depicted as solid lines; the associated
excitation and emission wavelengths, and lifetimes (τ) are also provided.
In order to attain a white light emitting phosphor using the VDSG method, Eu
3+
and Tb
3+
were co-doped within a single CaWO
4
host. Elemental analysis by ICP-AES revealed Eu
3+
and
Tb
3+
concentrations to be 0.62 and 0.75 mol.%, respectively, in the nominal
CaWO
4
:1%Eu,1%Tb nanocrystals. The increased overall Ln
3+
concentration had no observable
effect on nanocrystal morphology. The emission spectrum of the nominal CaWO
4
:1%Eu,1%Tb
nanocrystals exhibits the characteristic f–f transitions of Eu
3+
and Tb
3+
upon excitation at 366 nm
(i.e., commercial UV excitation; Figure 7.7). In addition to the previously assigned radiative
142
transitions from Eu
3+
and Tb
3+
, higher energy emission peaks arising from the
5
D
4
7
F
6
(488
nm),
5
D
3
7
F
4
(437 nm), and
5
D
3
7
F
5
(413 nm) Tb
3+
relaxations are also observed.
31
Addtionally, excitation wavelengths of 393 and 485 nm can be applied to the CaWO
4
:Eu,Tb
nanocrystals in order to isolate the characteristic individual emission profiles of Eu
3+
and Tb
3+
,
respectively. The emission spectrum of CaWO
4
:Eu,Tb nanocrystals excited at 485 nm does
however show apprciable intensity at 615 nm (i.e., the
5
D
0
7
F
2
transition from Eu
3+
), owing to
the established energy transfer from Tb
3+
to Eu
3+
that occurs when both ions are present within
the same host matrix.
32,33
Photoluminescence lifetimes related to the relaxation of Eu
3+
(λ
em
=
615 nm) and Tb
3+
(λ
em
= 544 nm) excited states were 1.09 and 1.18 ms with an excitation
wavelength of 366 nm, respectively (Figure 7.8.), commensurate with the lifetimes of the singly
activated CaWO
4
:1%Eu and CaWO
4
:1%Tb nanocrystal phosphors. The CIE coordinates for this
composition are (0.349,0.356), which corresponds to white light with a color temperature of
4850 K.
→
→ →
→
143
Figure 7.7. Room temperature emission spectrum of CaWO
4
:1%Eu,1%Tb nanocrystals. Inset depicts
CIE chromaticity coordinates.
Figure 7.8. (a) Room temperature emission spectra for calcined CaWO
4
:1%Eu, CaWO
4
:1%Tb, and
CaWO
4
:(1%Eu,1%Tb) nanocrystals upon excitation at 366 nm. Photoluminescence decay curves for
CaWO
4
:(1%Eu,1%Tb) nanocrystals monitored at (b) 615 and (c) 544 nm. The associated excitation and
emission wavelengths, and lifetimes (τ) are also provided.
144
7.4. Experimental
7.4.1. General Considerations
All manipulations were conducted under a nitrogen atmosphere at ambient pressure using
standard Schlenk techniques. W(OEt)
6
(Et = CH
2
CH
3
) from Alfa Aesar, Eu(acac)
3
(acac =
acetylacetonate, C
5
H
7
O
2
), Tb(acac)
3
, Tm(acac)
3
from Sigma Aldrich, and an alcoholic solution
of Ca(OCH
2
CH
2
OCH
3
)
2
(20 wt% in methoxyethanol) from Gelest, Inc. were used as precursors.
Methoxyethanol was purchased from Sigma Aldrich. All reagents were used as received.
7.4.2. Nanocrystal Synthesis
Lanthanide (Ln)-doped CaWO
4
nanocrystals were synthesized via a vapor diffusion sol−gel
method described in detail elsewhere. Briefly, a rotameter controls the flow of the nitrogen
carrier gas through a glass bubbler filled with 0.75 M aqueous HCl, connected via Tygon tubing
to a 100 mL, 3-neck round bottom flask containing the precursor solution. In a typical synthesis,
454 mg (1.0 mmol) W(OEt)
6
and the appropriate mass of Ln(acac)
3
(e.g., 4.5 mg Eu(acac)
3
(0.01
mmol) were employed for the synthesis of nominal CaWO
4
:1%Eu) were added to 1.0 mL (1.0
mmol) Ca(OCH
2
CH
2
OCH
3
)
2
. The resulting mixture was diluted to 2.0 mL total volume with
methoxyethanol and stirred at 80 °C under flowing dry nitrogen for 2 h, after which complete
dissolution of the reagents was observed. Once cooled, the solution was exposed to a controlled
flow of water vapor for 48 h at room temperature and atmospheric pressure. Diffusion of water
vapor into the solution resulted in the formation of a cracked gel, which was subsequently aged
under a nitrogen atmosphere for 24 h at 60 °C. The resulting gel was collected, washed with
145
absolute ethanol (3 × 10 mL), and vacuum dried at room temperature to recover an off-white fine
powder consisting of CaWO
4
:Ln nanocrystals.
7.4.3. Material Characterization
Thermogravimetric Analysis (TGA): TGA analyses were performed using a thermogravimetric
analyzer TA Q50 (TA Instruments) under a high-purity air flow (60 mL min
−1
). Samples were
heated from 25 to 600 °C at a linear rate of 15 °C min
−1
and held isothermal for 15 min. Powder
X-ray Diffraction (XRD): XRD patterns were collected in the 10–80° 2θ range using a Rigaku
Ultima IV diffractometer operated at 44 mA and 40 kV. Cu Kα radiation (λ = 1.5406 Å) was
employed. The step size and collection time were 0.0075° and 5 s per step, respectively. All
diffraction patterns were recorded under ambient conditions. Rietveld Analysis: Rietveld
structural refinements were carried out using the General Structure Analysis System (GSAS)
software. The average crystal structure of AWO
4
nanocrystals was refined with the tetragonal
I4
1
/a (no. 88) space group. The following parameters were refined: (1) scale factor, (2)
background, which was modeled using a shifted Chebyschev polynomial function, (3) peak
shape, which was modeled using a modified Thomson−Cox−Hasting pseudo-Voight function,
(4) lattice constants (a and c), (5) fractional atomic coordinates of the oxygen atom (x
O
, y
O
, z
O
),
and (6) an isotropic thermal parameter for each chemical species (i.e., U
A
, U
W
, and U
O
). The
usual R
wp
and χ
2
indicators were employed to assess the quality of the refined structural models.
Transmission Electron Microscopy (TEM): TEM images were obtained using a JEOL
JEM2100F (JEOL Ltd.) electron microscope operating at 200 kV. Samples for TEM studies
were prepared by drop-casting a stable suspension of nanocrystals in ethanol on a 400 mesh Cu
146
grid coated with a lacey carbon film (Ted Pella, Inc.). Inductively Coupled Plasma – Atomic
Emission Spectroscopy (ICP-AES): Elemental analyses were conducted at Galbraith Laboratories
(Knoxville, TN). Spectrofluorometry: Excitation and emission spectra of CaWO
4
:Ln powders
were recorded using a Horiba Nanolog spectrofluorometer equipped with a 450 W Xe lamp as
the excitation source and a photomultiplier tube as the detector. All spectra were collected under
ambient conditions.
7.5. Conclusions
In conclusion, a series of three prototypical Ln
3+
ions have been incorporated into sub-30 nm
CaWO
4
nanocrystals using the VDSG method. The ability of this approach to realize highly
crystalline luminescent phosphors under benign conditions with doping flexibility has been
demonstrated. Moreover, as a result of facile compositional tailoring, a phosphor exhibiting
white light emission has been achieved using an excitation wavelength equivalent to commercial
UV excitation. Given the rich diversity of Ln ions (i.e., emission profiles) and scheelite host
compositions available, further luminescence tuning may be easily accomplished with the VDSG
method. Importantly, this work highlights the synthetic potential of the VDSG method towards
the fabrication of nanocrystals for white light emitting applications.
147
7.6. References
(1) Tsang, M.-K.; Chan, C.-F.; Wong, K.-L.; Hao, J. J. Lumin. 2015, 157, 172.
(2) Saraf, R.; Shivakumara, C.; Dhananjaya, N.; Behera, S.; Nagabhushana, H. J. Mater. Sci.
2015, 50, 287.
(3) Peter, A. J.; Banu, I. B. S. J. Mater. Sci: Mater. Electron. 2014, 25, 2771.
(4) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater.
2003, 15, 4606.
(5) Stouwdam, J. W.; Raudsepp, M.; van Veggel, F. C. J. M. Langmuir 2005, 21, 7003.
(6) Lee, G.-H.; Kim, T.-H.; Yoon, C.; Kang, S. J. Lumin. 2008, 128, 1922.
(7) Wang, F.; X. Liu, X. Acc. Chem. Res. 2014, 47, 1378.
(8) Yang, X.; Liu, J.; Yang, H.; Yu, X.; Guo, Y.; Zhou, Y.; Liu, J. J. Mater. Chem. 2009, 19,
3771.
(9) van Loo, W. Phys. Stat. Sol. A 1975, 28, 227.
(10) Nazarov, M. V.; Jeon, D. Y.; Kang, J. H.; Popvici, E.-J.; Muresan, L.-E.;
Zamoryanskaya, M. V.; Tsukerblat, B. S. Solid State Commun. 2004, 131, 307.
(11) Zhang, Y.; Gong, W.; Yu, J.; Cheng, Z.; Ning, G. RSC Adv. 2016, 6, 30886.
(12) Wang, W.; Yang, P.; Cheng, Z.; Hou, Z.; Li, C.; Lin, J. ACS Appl. Mater. Interfaces
2011, 3, 3921.
(13) Liao, J.; Qiu, B.; Wen, H.; You, W. Opt. Mater. 2009, 31, 1513.
(14) Zhang, Y.; Gong, W.; Yu, J.; Pang, H.; Song, Q.; Ning, G. RSC Adv. 2015, 5, 62527.
(15) Maheshwary; Singh, B. P.; Singh, R. A. New J. Chem. 2015, 39, 4494.
(16) Sharma, K. G.; Singh, N. R. New J. Chem. 2013, 37, 2784.
(17) Sharma, K. G.; Singh, Th. P.; Singh, N. R. J. Alloy Compd. 2014, 602, 275.
(18) Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2011, 23, 4063.
(19) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
148
(20) Culver, S. P.; Greaney, M. J.; Tinoco, A.; Brutchey, R. L. Dalton Trans. 2015, 44, 15042.
(21) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Adv. Mater. 2012, 24, 1434.
(22) Rabuffetti, F. A.; Culver, S. P.; Lee, J. S.; Brutchey, R. L. Nanoscale 2014, 6, 2909.
(23) Errandonea, D.; Pellicer-Porres, J.; Manjón, F. J.; Segura, A.; Ferrer-Roca, Ch.; Kumar,
R. S.; Tschauner, O.; Rodríguez-Hernández, P.; López-Solano, J.; Radescu, S.; Mujica,
A.; Muñoz, A.; Aquilanti, G. Phys. Rev. B 2005, 72, 174106.
(24) Gonçalves, R. F.; Cavalcante, L. S.; Nogueira, I. C.; Longo, E.; Godinho, M. J.;
Sczancoski, J. C.; Mastelaro, V. R.; Pinatti, I. M.; Rosa, I. L. V.; Marques, A. P. A. Cryst.
Eng. Commun. 2015, 17, 1654.
(25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(26) Shi, S.; Gao, J.; Zhou, J. Opt. Mater. 2008, 30, 1616.
(27) Wang, W.; Yang, P.; Gai, S.; Niu, N.; He, F.; Lin, J. J. Nanopart. Res. 2010, 12, 2295.
(28) Liao, J.; Qiu, B.; Wen, H.; Chen, J.; You, W.; Liu, L. J. Alloy Compd. 2009, 487, 758.
(29) Lei, F.; Yan, B. J. Solid State Chem. 2008, 181, 855.
(30) Li, C.; Lin, C.; Liu, X.; Lin, J. J. Nanosci. Nanotech. 2008, 8, 1183.
(31) Cavalli, E.; Boutinaud, P.; Mahiou, R.; Bettinelli, M.; Dorenbos, P. Inorg. Chem. 2010,
49, 4916.
(32) van Uitert, L. G. J. Electrochem. Soc. 1967, 114, 1048.
(33) Nazarov, M.; Noh, D. Y. J. Rare Earths 2010, 28, 1.
149
Bibliography
Afanasiev, P. Mater. Lett. 2007, 61, 4622.
Ahmad, G.; Dickerson, M. B.; Church, B. C.; Cai, Y.; Jones, S. E.; Naik, R. R.; King, J. S.;
Summers, C. J.; Kröger, N.; Sandhage, K. H. Adv. Mater. 2006, 18, 1759.
Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.
Almadhoun, M. N.; Bhansali, U. S.; Alshareef, H. N. J. Mater. Chem. 2012, 22, 11196.
Baniecki, J. D.; Ishii, M.; Kurihara, K.; Yamanaka, K.; Yano, T.; Shinozaki, K.; Imada, T.;
Kobayashi, K. J. Appl. Phys. 2009, 106, 054109.
Barber, P.; Balasubramanian, S.; Anguchamy, Y.; Gong, S.; Wibowo, A.; Gao, H.; Ploehn, H. J.;
Zur Loye, H. C. Materials 2009, 2, 1697.
Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074.
Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. Langmuir 2009, 26, 5067.
Beier, C. W.; Sanders, J. M.; Brutchey, R. L. J. Phys. Chem. C 2013, 117, 6958.
Blasse, G.; Brixner, L. H. Chem. Phys. Lett. 1990, 173, 409.
Brutchey, R. L.; Morse, D. E. Angew. Chem., Int. Ed. 2006, 45, 6564.
Cabana, J.; Monconduit, L.; Larcher, D.; Palacín, M. R. Adv. Energy Mater. 2010, 22, E170.
Cavalcante, L. S.; Longo, V. M.; Sczancoski, J. C.; Almeida, M. A. P.; Batista, A. A.; Varela, J.
A.; Orlandi, M. O.; Longo, E.; Siu Li, M. Cryst. Eng. Comm. 2012, 14, 853.
Cavalli, E.; Boutinaud, P.; Mahiou, R.; Bettinelli, M.; Dorenbos, P. Inorg. Chem. 2010, 49, 4916.
Chang, L. L. Y. Am. Mineral. 1967, 52, 427.
Chen, D.; Tang, K.; Li, F.; Zheng, H. Cryst. Growth Des. 2006, 6, 247.
Cho, W. S.; Yashima, M.; Kakihana, M.; Tudo, A.; Sakata, T.; Yoshimura, M. J. Am. Ceram.
Soc. 1997, 80, 765.
Culver, S. P.; Greaney, M. J.; Tinoco, A.; Brutchey, R. L. Dalton Trans. 2015, 44, 15042.
Culver, S. P.; Rabuffetti, F. A.; Zhou, S.; Mecklenburg, M.; Song, Y.; Melot, B. C.; Brutchey, R.
L. Chem. Mater. 2013, 25, 4129.
150
Dang, Z. M.; Lin, Y. Q.; Xu, H. P.; Shi, C. Y.; Li, Y. S.; Bai, J. Adv. Funct. Mater. 2008, 18,
1509.
Demirörs, A. F.; Imhof, A. Chem. Mater. 2009, 21, 3002.
Ding, W.; Chen, Y.; Fu, X. Appl. Catal. A 1993, 104, 61.
Dissado, L. A.; Fothergill, J. C. Electrical degradation and breakdown in polymers, IEE materials
and devices series 9, Peter Peregrinus Ltd.: London, United Kingdom, 1992.
Dunbar, T. D.; Warren, W. L.; Tuttle, B. A.; Randall, C. A.; Tsur, Y. J. Phys. Chem. B 2004,
108, 908.
Durán, P.; Gutierrez, D.; Tartaj, J.; Bañares, M. A.; Moure, C. J. Eur. Ceram. Soc. 2002, 22, 797.
Enterkin, J. A.; Setthapun, W.; Elam, J. W.; Christensen, S. T.; Rabuffetti, F. A.; Marks, L. D.;
Stair, P. C.; Poeppelmeier, K. R.; Marshall, C. L. ACS Catal. 2011, 1, 629.
Errandonea, D.; Kumar, R. S.; Ma, X.; Tu, C. J. Solid State Chem. 2008, 181, 355.
Errandonea, D.; Pellicer-Porres, J.; Manjón, F. J.; Segura, A.; Ferrer-Roca, Ch.; Kumar, R. S.;
Tschauner, O.; Rodríguez-Hernández, P.; López-Solano, J.; Radescu, S.; Mujica, A.; Muñoz, A.;
Aquilanti, G. Phys. Rev. B 2005, 72, 174106.
Esaka, T.; Tachibana, R.; Takai, S. Solid State Ionics 1996, 92, 129.
Fabbri, E.; Bi, L.; Tanaka, H.; Pergolesi, D.; Traversa, E. Adv. Funct. Mater. 2011, 21, 158.
Fabbri, E.; D’Epifanio, A.; Di Bartolomeo, E.; Licoccia, S.; Traversa, E. Solid State Ionics 2008,
179, 558.
Feng, Y.; Yin, J.; Chen, M.; Liu, X.; Li, G. IEEE 2011, 226.
Freeman, C. L.; Dawson, J. A.; Chen, H. R.; Harding, J. H.; Ben, L. B.; Sinclair, D. C. J. Mater.
Chem. 2011, 21, 4861.
Freeman, C. L.; Dawson, J. A.; Harding, J. H.; Ben, L. B.; Sinclair, D. C. Adv. Funct. Mater.
2012, 23, 491.
Frey, M. H.; Payne, D. A. Phys. Rev. B, 1996, 54, 3158.
Gao, D.; Lai, X.; Cui, C.; Cheng, P.; Bi, J.; Lin, D. Thin Solid Films 2010, 518, 3151.
Garra, J.; Vohs, J. M.; Bonnell, D. A. J. Vac. Sci. Technol. A 2009, 27, L13.
151
Glinchuk, M. D.; Bykov, I. P.; Kornienko, S. M.; Laguta, V. V.; Slipenyuk, A. M.; Bilous, A. G.;
V’yunov, O. I.; Yanchevskii, O. Z. J. Mater. Chem. 2000, 10, 941.
Gonçalves, R. F.; Cavalcante, L. S.; Nogueira, I. C.; Longo, E.; Godinho, M. J.; Sczancoski, J.
C.; Mastelaro, V. R.; Pinatti, I. M.; Rosa, I. L. V.; Marques, A. P. A. Cryst. Eng. Commun. 2015,
17, 1654.
Gong, Q.; Qian, X.; Cao, H.; Du, W.; Ma, X.; Mo, M. J. Phys. Chem. B 2006, 110, 19295.
Gong, Q.; Qian, X.; Ma, X.; Zhu, Z. Cryst. Growth Des. 2006, 6, 1821.
Grubbs, R. H. Handbook of metathesis, Wiley-VCH: Weinheim, Germany, 2003.
Grubbs, R. H. Tetrahedron 2004, 60, 7117.
Hou, R. Z.; Ferreira, P.; Vilarinho, P. M. Chem. Mater. 2009, 21, 3536.
Huang, C. K.; Kerr, P. F. Am. Mineral. 1960, 45, 311.
Huang, W.; Lin, Y.; Taylor, S.; Gaillard, J.; Rao, A. M.; Sun, Y. P. Nano Lett. 2002, 2, 231.
Im, J.; Steiffer, S. K.; Auciello, O.; Krauss, A. R. Appl. Phys. Lett. 2000, 77, 2593.
Jeong, W.; Kessler, M. R. Chem. Mater. 2008, 20, 7060.
Jung, H. M.; Kang, J. H.; Yang, S. Y.; Won, J. C.; Kim, Y. S. Chem. Mater. 2010, 22, 450.
Kang, D. W.; Park, T. G.; Kim, J. W.; Kim, J. S.; Lee, H. S.; Cho, H. Electron. Mater. Lett.
2010, 6, 145.
Kato, H.; Matsudo, N.; Kudo, A. Chem. Lett. 2004, 33, 1216.
Khalil, M. S. IEEE T. Dielect. El. In. 2000, 7, 261.
Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M. J.; Marder, S. R.; Li, J.; Calame,
J. P.; Perry, J. W. ACS Nano 2009, 3, 2581.
Kim, P.; Jones, S. C.; Hotchkiss, P. J.; Haddock, J. N.; Kippelen, B.; Marder, S. R.; Perry, J. W.
Adv. Mater. 2007, 19, 1001.
Kissin, Y. V. Kirk-Othmer Encyclopedia of Chemical Technology, Wiley: New Jersey, 2005.
Kolodiazhnyi, T.; Petric, A. J. Phys. Chem. Solids 2003, 64, 953.
152
Koshio, A.; Yudasaka, M.; Zhang, M.; Iijima, S. Nano Lett. 2001, 1, 361.
Kotera, Y.; Sekine, T. J. Phys. Chem. Solids 1964, 25, 353.
Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos
National Laboratory, 2000.
Lee, G.-H.; Kim, T.-H.; Yoon, C.; Kang, S. J. Lumin. 2008, 128, 1922.
Lei, F.; Yan, B. J. Solid State Chem. 2008, 181, 855.
Li, C.; Lin, C.; Liu, X.; Lin, J. J. Nanosci. Nanotech. 2008, 8, 1183.
Liang, Y.; Han, X.; Yi, Z.; Tang, W.; Zhou, L.; Sun, J.; Yang, S.; Zhou, Y. J. Solid State
Electrochem. 2007, 11, 1127.
Liao, J.; Qiu, B.; Wen, H.; Chen, J.; You, W.; Liu, L. J. Alloy Compd. 2009, 487, 758.
Liao, J.; Qiu, B.; Wen, H.; You, W. Opt. Mater. 2009, 31, 1513.
Liegeois-Duyckaerts, M.; Tarte, P. Spectrochim. Acta 1972, 28A, 2037.
Lin, W.; Frei, H. J. Am. Chem. Soc. 2002, 124, 9292.
Lines, M. E.; Glass, A. M. Principles and applications of ferroelectrics and related materials,
Oxford classic texts in the physical sciences, Oxford University Press: New York, 2001.
Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966.
Liu, X.; Basu, A. J. Organomet. Chem. 2006, 691, 5148.
Longo, V. M.; Orhan, E.; Cavalcante, L. S.; Pôrto, S. L.; Espinoza, J. W. M.; Valera, J. A.;
Longo, E. Chem. Phys. 2007, 334, 180.
Lu, D. Y.; Koda, T.; Suzuki, H.; Toda, M. J. Ceram. Soc. Jpn. 2005, 113, 721.
Luo, Y. S.; Zhang, W. D.; Dai, X. J.; Yang, Y.; Fu, S. Y. J. Phys. Chem. C 2009, 113, 4856.
Maheshwary; Singh, B. P.; Singh, R. A. New J. Chem. 2015, 39, 4494.
Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2004, 126, 15245.
Marques, A. P. A.; Melo, D. M. A.; Paskocimas, C. A.; Pizani, P. S.; Joya, M. R.; Leite, E. R.;
Longo, E. J. Solid State Chem. 2006, 179, 671.
153
Mi, Y.; Huang, Z.; Hu, F.; Li, Y.; Jiang, J. J. Phys. Chem. C 2009, 113, 20795.
Mielczarski, J. A.; Cases, J. M. Langmuir 1995, 11, 3275.
Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delon, J. F. Langmuir 1993, 9, 2370.
Mikhailik, V. B.; Kraus, H.; Miller, G.; Mykhaylyk, M. S.; Wahl, D. J. Appl. Phys. 2005, 97,
083523.
Morrison, F. D.; Sinclair, D. C.; Skakle, J. M. S.; West, A. R. J. Am. Ceram. Soc. 1998, 81,
1957.
Morrison, F. D.; Sinclair, D. C.; West, A. R. J. Appl. Phys. 1999, 86, 6355.
Morrison, F. D.; Sinclair, D. C.; West, A. R. J. Am. Ceram. Soc. 2001, 84, 531.
Moulson, A. J.; Herbert, J. M. Electroceramics. Chapman and Hall: London, 1990.
Nazarov, M. V.; Jeon, D. Y.; Kang, J. H.; Popvici, E.-J.; Muresan, L.-E.; Zamoryanskaya, M. V.;
Tsukerblat, B. S. Solid State Commun. 2004, 131, 307.
Nazarov, M.; Noh, D. Y. J. Rare Earths 2010, 28, 1.
Newnham, R. E.; Cross, L. E. MRS Bull. 2011, 30, 845.
Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004, 126, 9120.
Oeder, R.; Sharmann, A.; Schaw, D. J. Cryst. Growth 1980, 49, 349.
Panchal, V.; Garg, N.; Sharma, S. M. J. Phys.: Condens. Matter 2006, 18, 3917.
Parchur, A. K.; Prasad, A. I.; Ansari, A. A.; Rai, S. B.; Ningthoujam, R. S. Dalton Trans. 2012,
41, 11032.
Peter, A. J.; Banu, I. B. S. J. Mater. Sci: Mater. Electron. 2014, 25, 2771.
Petkov, V.; Gateshki, M.; Niederberger, M.; Ren, Y. Chem. Mater. 2006, 18, 814.
Pôrto, S. L.; Longo, E.; Pizani, P. S.; Boschi, T. M.; Simőes, L. G. P.; Lima, S. J. G.; Ferreira, J.
M.; Soledade, L. E. B.; Espinoza, J. W. M.; Cássia-Santos, M. R.; Maurera, M. A. M. A.;
Paskocimas, C. A.; Santos, I. M. G.; Souza, A. G. J. Solid State Chem. 2008, 181, 1876.
Porto, S. P. S.; Scott, J. F. Phys. Rev. 1967, 157, 716.
Pu, Y.; Chen, W.; Chen, S.; Langhammer, H. T. Cerâmica 2005, 51, 214.
154
Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437.
Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2011, 23, 4063.
Rabuffetti, F. A.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
Rabuffetti, F. A.; Brutchey, R. L. J. Am. Chem. Soc. 2012, 134, 9475.
Rabuffetti, F. A.; Brutchey, R. L. ACS Nano 2013, 7, 11435.
Rabuffetti, F. A.; Brutchey, R. L. Dalton Trans. 2014, 43, 14499.
Rabuffetti, F. A.; Culver, S. P.; Lee, J. S.; and R. L. Brutchey, R. L. Nanoscale 2014, 6, 2909.
Rabuffetti, F. A.; Culver, S. P.; Suescun, L.; Brutchey, R. L. Inorg. Chem. 2014, 53, 1056.
Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Adv. Mater. 2012, 24, 1434.
Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Chem. Mater. 2012, 24, 3114.
Rabuffetti, F. A.; Stair, P. C.; Poeppelmeier, K. R. J. Phys. Chem. C 2010, 114, 11056.
Rabuffi, M.; Picci, G. IEEE T. Plasma Sci. 2002, 30, 1939.
Rakotovelo, G.; Moussounda, P. S.; Haroun, M. F.; Légaré, P.; Rakotomahevitra, A.;
Rakotomalala, M.; Parlebas, J. C. Surf. Sci. 2009, 603, 1221.
Rietveld, H. M. Acta Crystallogr. 1967, 22, 151152.
Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 6571.
Sahoo, Y.; Goodarzi, A.; Swihart, M. T.; Ohulchanskyy, T. Y.; Kaur, N.; Furlani, E. P.; Prasad,
P. N. J. Phys. Chem. B 2005, 109, 3879.
Saraf, R.; Shivakumara, C.; Dhananjaya, N.; Behera, S.; Nagabhushana, H. J. Mater. Sci. 2015,
50, 287.
Sharma, K. G.; Singh, N. R. New J. Chem. 2013, 37, 2784.
Sharma, K. G.; Singh, Th. P.; Singh, N. R. J. Alloy Compd. 2014, 602, 275.
Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R.; Dong, Z. L.; White, T. J.
Chem. Mater. 2004, 16, 504.
155
Shi, H.; Qi, L.; Ma, J.; Cheng, H. J. Am. Chem. Soc. 2003, 125, 3450.
Shi, H.; Qi, L.; Ma, J.; Wu, N. Adv. Funct. Mater. 2005, 15, 442.
Shi, S.; Gao, J.; Zhou, J. Opt. Mater. 2008, 30, 1616.
Shultz, M. D.; Reveles, J. U.; Khanna, S. N.; Carpenter, E. E. J. Am. Chem. Soc. 2007, 129,
2482.
Simons, R.; Guntari, S. N.; Goh, T. K.; Qiao, G. G.; Bateman, S. A. J. Polym. Sci. Part A. 2012,
50, 89.
Siqueira, K. P. F.; Moreira, R. L.; Valadares, M.; Dias, A. J. Mater. Sci. 2010, 45, 6083.
Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. J. Am. Chem. Soc. 2002, 124, 5729.
Sleight, A. W. Acta Crystallogr. 1972, B28, 2899.
Slugovc, C. Macromol. Rapid Comm. 2004, 25, 1283.
Smith, M. B.; Page, K.; Siegrist, T.; Richmond, P. L.; Walter, E. C.; Seshadri, R.; Brus, L. E.;
Steigerwald, M. L. J. Am. Chem. Soc. 2008, 130, 6955.
Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15,
4606.
Stouwdam, J. W.; Raudsepp, M.; van Veggel, F. C. J. M. Langmuir 2005, 21, 7003.
Su, Y.; Li, L.; Li, G. Chem. Mater. 2008, 20, 6060.
Tavasoli, E.; Guo, Y.; Kunal, P.; Grajeda, J.; Gerber, A.; Vela, J. Chem. Mater. 2012, 24, 4231.
Testino, A. Int. J. Appl. Ceram. Technol. 2013, 10, 723.
Thangadurai, V.; Knittlmayer, C.; Weppner, W. Mat. Sci. Eng. B 2004, 106, 228.
Thompson, P.; Cox, D. E.; Hastings, J. M. J. Appl. Crystallogr. 1987, 20, 7983.
Tinga, W. R.; Voss, W. A.; Blossey, D. F. J. Appl. Phys. 1973, 44, 3897.
Townsend, T. K.; Browning, N. D.; Osterloh, F. E. ACS Nano 2012, 6, 7420.
Tsang, M.-K.; Chan, C.-F.; Wong, K.-L.; Hao, J. J. Lumin. 2015, 157, 172.
van Loo, W. Phys. Stat. Sol. A 1975, 28, 227.
156
van Uitert, L. G. J. Electrochem. Soc. 1967, 114, 1048.
Vernon, J. P.; Hobbs, N.; Cai, Y.; Lethbridge, A.; Vukusic, P.; Deheyn, D. D.; Sandhage, K. H.
J. Mater. Chem. 2012, 22, 10435.
Wagner, F. T.; Ferrer, S.; Somorjai, G. A. Surf. Sci. 1980, 101, 462.
Wang, F.; X. Liu, X. Acc. Chem. Res. 2014, 47, 1378.
Wang, W.; Yang, P.; Cheng, Z.; Hou, Z.; Li, C.; Lin, J. ACS Appl. Mater. Interfaces 2011, 3,
3921.
Wang, W.; Yang, P.; Gai, S.; Niu, N.; He, F.; Lin, J. J. Nanopart. Res. 2010, 12, 2295.
West, A. R.; Adams, T. B.; Morrison, F. D.; Sinclair, D. C. J. Eur. Ceram. Soc. 2004, 24, 1439.
Wöhler, L.; Balz, O. Z. Elektrochem. 1921, 27, 415.
Xu, C.; Xu, K.; Gu, H.; Zheng, R.; Liu, H.; Zhang, X.; Guo, Z.; Xu, B. J. Am. Chem. Soc. 2004,
126, 9938.
Yang, X.; Liu, J.; Yang, H.; Yu, X.; Guo, Y.; Zhou, Y.; Liu, J. J. Mater. Chem. 2009, 19, 3771.
Yin, W.; Kniajanski, S.; Amm, B. IEEE 2010, 1.
Yoonessi, M.; Toghiani, H.; Kingery, W. L.; Pittman, C. U. Macromolecules 2004, 37, 2511.
Young, R. A.; Oxford University Press: New York, 1993.
Zhang, F.; Sfeir, M. Y.; Misewich, J. A.; Wong, S. S. Chem. Mater. 2008, 20, 5500.
Zhang, F.; Yiu, Y.; Aronson, M. C.; Wong, S. S. J. Phys. Chem. C 2008, 112, 14816.
Zhang, H.; Fu, X.; Xin, Q. J. Alloys Compd., 2008, 459, 103.
Zhang, L.; He, R.; Gu, H. C. Appl. Surf. Sci. 2006, 253, 2611.
Zhang, Y.; Gong, W.; Yu, J.; Cheng, Z.; Ning, G. RSC Adv. 2016, 6, 30886.
Abstract (if available)
Abstract
Perovskite oxides are an exceptionally useful class of materials in the areas of energy storage and conversion, which are typically fabricated at very high temperatures (i.e., > 1000 ℃). As such, the advancement of low temperature techniques that can allow for the precise tailoring of composition and associated properties is of paramount importance. Over the last decade, the Brutchey group has developed a vapor diffusion sol–gel (VDSG) method that provides a facile, low-temperature route to nanocrystalline perovskite oxides with a high degree of compositional control. Our group has exploited this method to generate a series of alkaline–earth perovskite oxide ABO₃ type nanocrystals (A = Sr, Ba
Linked assets
University of Southern California Dissertations and Theses
Conceptually similar
PDF
Synthesis and surface modification of perovskite-based nanocrystals for use in high energy density nanocomposites
PDF
Expanding the library of surface ligands for semiconductor nanocrystal synthesis and photovoltaic applications
PDF
Semiconductor inks for solution processed electronic thin‐films
PDF
Synthesis and characterization of metal chalcogenide semiconductor nanocrystals using dialkyl dichalcogenide precursors
PDF
Solution phase synthesis routes to functional nanomaterials for energy storage
PDF
Understanding intercalation-driven structural transformations in energy storage materials
PDF
Solution‐phase synthesis and deposition of earth‐abundant metal chalcogenide semiconductors
PDF
Synthesis of high-quality nanoparticles using microfluidic platforms
PDF
Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates
PDF
Direct C−H arylation for the synthesis of conjugated polymers
PDF
Simple complexes: synthesis and photophysical studies of luminescent, monovalent, 2-coordinate carbene-coinage metal complexes and higher coordination geometries
PDF
Modification of electrode materials for lithium ion batteries
PDF
Understanding the mechanism of oxygen reduction and oxygen evolution on transition metal oxide electrocatalysts and applications in iron-air rechargeable battery
PDF
Synthesis and characterization of 3-hexylesterthiophene based random and semi-random polymers and their use in ternary blend solar cells
PDF
Sustainable manufacturing of out-of-autoclave (OoA) prepreg: processing challenges
PDF
Carbon-hydrogen bond activation: radical methane functionalization; unactivated alkene coupling; saccharide degradation; and carbon dioxide hydrogenation
PDF
Computational analysis of the spatial and temporal organization of the cellular environment
PDF
Synthesis, characterization, and device application of two-dimensional materials beyond graphene
PDF
Solution processing of chalcogenide functional materials using thiol–amine “alkahest” solvent systems
PDF
Fabrication and study of organic and inorganic optoelectronics using a vapor phase deposition (VPD)
Asset Metadata
Creator
Culver, Sean P.
(author)
Core Title
Low temperature synthesis of functional metal oxide nanocrystals using a vapor diffusion sol-gel method
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
10/27/2016
Defense Date
08/19/2016
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
dielectric,disorder,Li-ion battery,nanocrystal,OAI-PMH Harvest,perovskite,phosphor,scheelite,sol-gel,vapor diffusion
Format
application/pdf
(imt)
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Brutchey, Richard (
committee chair
), Melot, Brent (
committee member
), Nakano, Aiichiro (
committee member
)
Creator Email
sculver@usc.edu,sp.culver@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c40-316042
Unique identifier
UC11213610
Identifier
etd-CulverSean-4894.pdf (filename),usctheses-c40-316042 (legacy record id)
Legacy Identifier
etd-CulverSean-4894.pdf
Dmrecord
316042
Document Type
Dissertation
Format
application/pdf (imt)
Rights
Culver, Sean P.
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Tags
dielectric
disorder
Li-ion battery
nanocrystal
perovskite
phosphor
scheelite
sol-gel
vapor diffusion