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Probing charge transfer and electric fields at the bulk and interface using vibrational spectroscopy
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Content
Probing Charge Transfer and Electric Fields at the Bulk and Interface Using Vibrational
Spectroscopy
by
Cindy Tseng
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA (USC)
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
December 2022
Copyright 2022 Cindy Tseng
ii
TABLE OF CONTENTS
List of Figures ......................................................................................................................v
Abstract ....................................................................................................................xv
Chapter 1: Introduction to Concepts ...................................................................................1
1.1 Interfacial Electric Field and Stark Shift Spectroscopy .....................................1
1.2 Surface Enhanced IR Absorption Spectroscopy (SEIRAS) ...............................3
Chapter 2: A Direct Determination of SEIRAS Enhancement Factor and Penetration
Depths in Surface Enhanced IR Absorption Spectroscopy ..............................................10
2.1 Introduction ......................................................................................................10
2.2 Experimental Methods ....................................................................................12
2.3 Results and Discussion ....................................................................................14
2.4 Conclusion and Future Directions ..................................................................21
Chapter 3: Interplay Between Charge Transfer and Interfacial Electric Fields .................22
3.1 Introduction ......................................................................................................22
3.2 Experimental and Computational Methods ....................................................24
3.3 Results and Discussion ....................................................................................30
3.4 Conclusion and Future Directions ..................................................................38
Chapter 4: Quinone Proton-Coupled Electron Transfer in Bulk and Surfaces ..................39
4.1 Introduction ......................................................................................................39
4.2 Experimental Methods ....................................................................................40
4.3 Results and Discussion ....................................................................................43
4.4 A Fundamental Study of Quinones and Future Directions ..............................49
Chapter 5: Long-Lived Transient Responses from Photoexcitation of Prussian Blue and
its Analogs .........................................................................................................................52
5.1 Introduction ......................................................................................................52
5.2 Questions We Wanted to Answer ....................................................................53
5.3 Experimental Methods .....................................................................................54
5.4 Results and Discussion ....................................................................................58
5.5 Conclusion and Future Work ...........................................................................72
Chapter 6: Conclusion and Future Work ...........................................................................73
iii
References ..........................................................................................................................75
Appendices .........................................................................................................................87
Appendix A Further Explanation of SEIRAS ........................................................87
A.1 What is Surface Plasmons? ..................................................................87
A.2 How to prepare SEIRAS substrates ...................................................102
A.3 Setting Up an Electrochemical ATR-SEIRAS Experiment ...............104
Appendix B Vis Pump IR Probe Setup: Reading Signals with a PC Detector ....112
Appendix C Stark Shift Spectroscopy of Functionalized Metal Oxides ..............117
C.1 Motivation ..........................................................................................117
C.2 Introduction ........................................................................................117
C.3 Rate Law Analysis on Hematite Films ...............................................120
C.4 Attempts to Functionalize Metal Oxides ............................................128
C.5 Conclusion and Future Work .............................................................132
Appendix D Supporting Information for A Direct Determination of SEIRAS
Enhancement Factor and Penetration Depth ........................................................134
D.1 Linear Dependence of Peak Currents to Scan Rate for 6-FCHT .......134
D.2 The Interference Fringe Pattern of the FTIR Spacer ..........................135
D.3 The Linear Fits for Bulk Absorption of 6-FCHT ...............................136
D.4 The Baseline Corrected 6-FCHT SEIRAS Spectrum ........................137
D.5 Estimation of Penetration Depth Based on a Homogeneous f ...........137
D.6 The Calculation for PB Electrodeposition Rate .................................138
D.7 Representative Chronocoulometry Data for PB Film ........................139
D.8 SEIRAS Spectra and Penetration Depth for Another Gold ZnSe ......140
Appendix E Supporting Information for Interplay Between Charge Transfer and
Interfacial Electric Fields .....................................................................................141
E.1 Cyclic Voltammogram of Mixed Monolayer of 6-FCHT:4-MBN .....141
E.2 Representative Spectra of Mixed Monolayer of 6-FCHT:4-MBN .....141
E.3 The Reversibility of the Spectroelectrochemical Scans .....................142
E.4 Frequency Calculations Based on the Multipole Based Model ..........144
E.5 QM Frequency Calculations Based on MD Snapshots ......................144
iv
Appendix F Supporting Information for Quinone Proton-Coupled Electron
Transfer in Bulk and Surfaces ..............................................................................145
F.1 Electrochemical Characteristic of 11-(ferrocenyl)undecanethiol .......145
F.2 Surfactant Solvation Effects on the CV of 11-FCUT .........................145
Appendix G Supporting Information for Long-Lived Transient Responses from
Photoexcitation of Prussian Blue and its Analogs ...............................................147
G.1 The Synthesis of Other PB Analog Films Using SILAR Method .....147
G.2 The Detector Response of the MCT Detector ....................................148
G.3 FTIR Spectra of CoFe and FeFe Films on Different Substrates ........148
G.4 Spectroelectrochemical Raman Data for a CoFe film .......................149
G.5 UV-Vis of CoFe and FeFe PB Films .................................................155
G.6 Additional Pump Probe Data for CoFe Film .....................................155
G.7 Additional Pump Probe Spectrum of CoFe Film ...............................156
v
List of Figures
Figure 1.1 General concept of Vibrational Stark Shift Spectroscopy ..................................2
Figure 1.2 A normalized IR spectrum of 4-mercaptobenzonitrile on a gold wafer .............3
Figure 1.3 Cartoon schematic of ATR at various incident angles .......................................5
Figure 2.1 A general concept schematic of the methods used to determine the
enhancement factor of a SEIRAS substrate using a monolayer system ............................12
Figure 2.2 A representative cyclic voltammogram of the 6-FCHT monolayer after
subtraction of the capacitive response at scan rate of 200 mV/s .......................................15
Figure 2.3 Bulk FTIR spectra corresponding to the C-H stretches of 6-FCHT. (a) The raw
absorption of the C-H region as a function of 6-FCHT concentrations. (b) The peak
heights of the 2853 cm
-1
and (c) 2929 cm
-1
peak plotted as function of 6-FCHT
concentration. .....................................................................................................................16
Figure 2.4 SEIRAS spectrum of a 6-FCHT monolayer on gold (C-H region) ..................17
Figure 2.5 Integrated CN stretch peak areas of a Prussian Blue film electrodeposited on
the SEIRAS ....................................................................................................................20
Figure 3.1 Concept figure of the competing fields from charge transfer of surface-
tethered 6-FCHT ................................................................................................................23
Figure 3.2 A representative diagram of the sample systems of control and mixed
monolayers: two controls (4-MBN, alkanethiol:4-MBN) and two samples (short chain
ferrocene:4-MBN, long chain ferrocene:4-MBN) .............................................................25
vi
Figure 3.3 Cartoon of the ATR-SEIRAS setup and images of Si prism substrates ...........26
Figure 3.4 Functionalized gold surfaces generated by CHARMM-GUI decorated with
4-MBN and other ligands to simulate the mixed monolayers of the experimental setup ..26
Figure 3.5 A figure of the restraints used to generate the ferrocene moiety ......................27
Figure 3.6 The cubic water of 1 nm edge dimensions with the ferrocene inside for
BOMD calculations ...........................................................................................................29
Figure 3.7 The systems of 4-MBN on gold studied in both vacuum (left) and in
water (right) ....................................................................................................................30
Figure 3.8 A representative cyclic voltammogram of the 6-FCHT:4-MBN mixed
monolayer ..........................................................................................................................31
Figure 3.9 (Left) The spectra for the different mixed monolayer systems at open circuit
potential and (right) the center nitrile frequencies plotted as a function of voltage ..........32
Figure 3.10 The densities of different atoms relative to the surface of the gold electrode.
This is shown as atomic partial number densities and charge density profiles of key
atoms (water, nitrogen of 4-MBN, iron of 6-FCHT, potassium of KCl, chloride of KCl,
and gold of the electrode) ..................................................................................................34
Figure 3.11 The three zones of potential where we hypothesize explain the Stark shift
behavior of 4-MBN in different environments. The zones are: before the onset current
of 6-FCHT oxidation, during the redox activity region of 6-FCHT, and after the full
oxidation of 6-FCHT where all the ferrocenes have been oxidized to ferrocenium ..........36
vii
Figure 4.1 The molecular structures for TCBQ and H
2
TCBQ, showing the two
PCET processes that they undergo when they are in their charged/discharged forms ......40
Figure 4.2 A cartoon of the active material preparation and diagram of the assembly of
the coin cell ........................................................................................................................41
Figure 4.3 A picture of the in-situ Raman setup. The coin cell battery is hooked up to the
potentiostat with red copper wires and taped to a microscope slide for stability on the
microscope stage. The 532 nm Raman laser is hooked up to a microscope where the
light can be focused onto the top of the hole drilled on the coin cell battery ....................42
Figure 4.4 Raman spectra of the controls (TCBQ and H
2
TCBQ) .....................................44
Figure 4.5 The battery performance shown as the specific capacity plotted as a function
of measured voltage for the discharging and charging cycle .............................................45
Figure 4.6 The spectroelectrochemical data for the discharging cycle ..............................46
Figure 4.7 The spectroelectrochemical data for the charging cycle ..................................47
Figure 4.8 The “nine-membered square scheme” of benzoquinone/hydroquinone ...........49
Figure 4.9 (Left) The CV of 11-MUHQ on the gold wafer substrate at 200 mV/s and
(right) the CV at various scan rates ....................................................................................50
Figure 4.10 Various CVs of 11-MUHQ as a function of water titration in bulk
acetonitrile ....................................................................................................................51
Figure 5.1 A cartoon diagram of the lattice structure of PB ..............................................52
viii
Figure 5.2 A cartoon schematic of the Visible pump IR probe experimental setup ..........56
Figure 5.3 The FTIR spectrum of (red) CoFe PB film and (blue) FeFe PB film in the
reflectance geometry ..........................................................................................................58
Figure 5.4 The pump probe data for FeFe film pumped at 800 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same film
is plotted next to the data with matching wavenumbers accordingly ................................60
Figure 5.5 A few representative time slices for three different wavenumbers of the FeFe
sample pumped at 800 nm .................................................................................................63
Figure 5.6 The pump probe data for FeFe film pumped at 400 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same
film is plotted next to the data with matching wavenumbers accordingly ........................64
Figure 5.7 A few representative time slices for three different wavenumbers of the FeFe
sample pumped at 400 nm .................................................................................................65
Figure 5.8 The pump probe data for CoFe film pumped at 800 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same
film is plotted next to the data with matching wavenumbers accordingly ........................66
Figure 5.9 A few representative time slices for four different wavenumbers of the CoFe
sample pumped at 800 nm .................................................................................................67
Figure 5.10 The pump probe data for CoFe film pumped at 400 nm. The color bar
shows the signs and intensities of the signals. The steady state FTIR spectrum for the
same film is plotted next to the data with matching wavenumbers accordingly ...............68
ix
Figure 5.11 A few representative time slices for four different wavenumbers of the
CoFe sample pumped at 400 nm ........................................................................................69
Figure 5.12 The spectra of CoFe film as a function of heating at different temperatures.
The left panel shows the raw voltage data read from the lock-in and the right panel
shows differential spectra subtracted from the room temperature spectrum .....................70
Figure 5.13 The differential spectrum of CoFe film at t = 25 𝜇s pumped at 800 nm
(red) and 400 nm (blue). ....................................................................................................71
Figure A.1 A concept figure of the big picture for all the steps to solve the dispersion
relation for surface plasmons .............................................................................................88
Figure A.2 The dispersion relation for light in vacuum and different media ....................89
Figure A.3 A diagram showing the k vectors and their various components at the
interface of two different materials with refractive indices n
1
and n
2
................................90
Figure A.4 A diagram that sets up the problems to understand Gauss’ Law .....................91
Figure A.5 A representation diagram to explain Gauss’ Law in the integral form ...........92
Figure A.6 A concept figure of Faraday’s Law .................................................................93
Figure A.7 The three boundary conditions at the interface of a dielectric and metal ........94
Figure A.8 The reflectivity plotted as a function of frequency and wavelength for
different metals to show their respective plasma frequencies ...........................................99
x
Figure A.9 The reflectivity plotted as a function of frequency and wavelength for a
semiconductor ..................................................................................................................100
Figure A.10 The plot for the dispersion relation of a surface plasmon and the MATLAB
code that generated this plot with the parameters chosen ................................................101
Figure A.11 A photo of the ZnSe ATR crystals before and after electroless deposition 103
Figure A.12 A SEIRAS spectrum of ethanol and the calibration bar of 5 mO.D. ...........105
Figure A.13 A picture of the VeeMAX III accessory inserted into our FTIR sample
chamber, with the Teflon electrochemical cell accessory attached on top ......................106
Figure A.14 A cartoon representing the three electrode system setup in the ATR
geometry ..................................................................................................................107
Figure A.15 The typical parameters that are set for collecting a SEIRAS spectrum .......110
Figure B.1 A representative of the signal picked up by the oscilloscope if the MCT
detector sees a CW IR light source and if there is a change in the light ..........................113
Figure B.2 A representative diagram of the signals that can be picked up by the
oscilloscope if there is transient response to a pulsed pump source ................................113
Figure B.3 A representative set of transient data of a silicon optical cavity that was
pumped with 17 𝜇𝐽 of energy from a 800 nm pulsed laser and probed with CW IR light
from a QCL ......................................................................................................................114
Figure B.4 A representative chart of all the different types of differential signals that can
xi
be picked up from the lock-in ..........................................................................................115
Figure C.1 The linear sweep voltammogram of hematite film for non-illuminated (solid)
and illuminated sample (dashed) .....................................................................................118
Figure C.2 The absorption spectrum of hematite films prepared by our collaborators ...120
Figure C.3 The experimental setup for the TA of hematite films ....................................120
Figure C.4 A concept figure of the energy diagram of the TA experiment and the signs
of the TA signals for all the different possible processes that can be probed ..................121
Figure C.5 The TA data shown as a function of (left) wavelength and time response held
at 1.5 V vs RHE and (right) time and external bias applied at probe 𝜆 = 700 𝑛𝑚 .........122
Figure C.6 The experimental setup for PIA .....................................................................123
Figure C.7 (Left) The PI absorption signal that was probed at 700 nm as a function of
increasing pump intensity. (Right) The long time scale transient photocurrent also as a
function of increasing pump intensity ..............................................................................125
Figure C.8 The rate law analysis plotted as a relationship between the photocurrent and
density of accumulated surface holes (extrapolated from the calibration curve) for the
hematite photoanodes. The sample was held at 1.5 V vs RHE ........................................126
Figure C.9 The DRIFTS spectra of 3,4 dihydroxybenzonitrile functionalized on
different metal oxides (left) and a general schematic of the starting materials and the
basic mechanism of DRIFTS ...........................................................................................129
xii
Figure C.10 A cartoon schematic of the SILAR method for preparing hematite films ...130
Figure C.11 A picture of the hematite deposited on etched Ag substrate via the SILAR
method and the SERS spectrum of the sample ................................................................130
Figure C.12 The SERS spectrum (zoomed in at the nitrile region) of the hematite film on
Ag substrate that is functionalized with 3, 4 DHBN and a cartoon depiction of what the
sample looks like once prepared ......................................................................................131
Figure C.13 SERS spectra of the hematite film that was functionalized with 3,4 DHBN
(blue) and not functionalized with 3,4 DHBN (red). There is a small peak at 2225 cm
-1
for the functionalized spectrum that corresponds to the nitrile group of 3,4 DHBN .......132
Figure D.1 (Left) The CV from Fig 1 in the main text that is baseline corrected. (Right)
The peak currents of the reductive (black) and oxidative (red) peaks as a function of scan
rate. The data points were each fitted to a linear line, and the R
2
is shown for each fit ..134
Figure D.2 The interference fringe pattern obtained with the FTIR transmission cell
with the spacer filled with air ...........................................................................................135
Figure D.3 The linear fits for the bulk adsorption of 6-FCHT showing the (0,0) point
and best fit R
2
values ........................................................................................................136
Figure D.4 The SEIRAS spectrum shown here is the baseline corrected version of the
spectrum in the main text Figure 3. The spectrum, excluding the region of interest
(2811 cm
-1
– 2975 cm
-1
), were fit to a polynomial, and then subtracted from the
polynomial to extract the absorption peak heights that are baseline subtracted ..............137
Figure D.5 Left panel: dilution of acetonitrile (ACN) with ethanol on the SEIRAS substrate.
xiii
The molar absorptivity of bulk ACN is 45 M
-1
cm
-1
. The SEIRAS absorption of pure
acetonitrile was measured to be 16 mOD. Assuming the molar absorptivity decays
exponentially from the SEIRAS substrate into the bulk (middle panel), we can use Aacn
with the measured enhancement factor from 6-FCHT to calculate the penetration depth
of the IR. This yields unphysical numbers (right panel) due to treating every surface site
as having the same spatial molar absorptivity decay profile ...........................................137
Figure D.6 A cartoon depiction of the unit cell of the Prussian Blue film and the calculation
for the estimation of ~ 14 nm deposition increments for electrodepositing the film at
- 40 𝜇𝐴 for 145 seconds. One electron transfer was estimated to correspond to a unit cell
of volume 125 Å
!
.
4
The total charge in Coulombs was calculated for holding the potential
at - 40 𝜇𝐴 for 145 seconds and then converted to the volume it would take up, assuming
one electron transfer occurs per unit cell. This volume was then divided by the surface
area of the substrate to estimate the depth (length) of the deposition ..............................138
Figure D.7 A representative set of chronopotentiometry data for the deposition of a
~ 14 nm film of Prussian Blue. This was repeated multiple times with SEIRAS spectra
being collected between consecutive chronopotentiometry scans ...................................139
Figure D.8 (Left) The SEIRAS spectra for PB film as a function of thickness and (right)
The integrated peak area of an electrodeposited Prussian Blue film on top of the SEIRAS
substrate as a function of film thickness. This is data corresponding to a different
SEIRAS substrate than the one in the main text (Figure 4). We provide a qualitative
measure of the gold thickness by measuring the resistance across a 1 cm distance on the
gold deposited substrates. This substrate had a resistance of ~ 5 Ω 𝑐𝑚
"#
while the
substrate in Figure 4 had a resistance of ~ 15 Ω 𝑐𝑚
"#
. This SEIRAS substrate was
thicker, and more conductive than the substrate in the main text ....................................140
Figure E.1 (Left) the cyclic voltammogram of the mixed monolayer 6-FCHT:4-MBN in
1 M KCl in aqueous solution as a function of scan rate. The half wave potential
xiv
remains approximately around + 0.47 V vs Ag/AgCl. (Right) The peak currents for the
anodic and cathodic waves plotted as a function of scan rate to show the linearity
dependence on scan rate, a characteristic of adsorbed species ........................................141
Figure E.2 Representative spectra zoomed in on the nitrile region of the mixed
monolayer of 6-FCHT:4-MBN as a function of potential ...............................................141
Figure E.3 The forward and backward scan of (top) the short chain ferrocene and 4-MBN
mixed monolayer (middle) the long chain ferrocene and 4-MBN mixed monolayer and
(bottom) the alkanethiol and 4-MBN mixed monolayer. The nitrile stretches were each
fit to a Gaussian and the center frequency with the fitting standard deviation were plotted
as a function of applied potential .....................................................................................143
Figure E.4 Frequency calculations at 0V vs Ag/AgCl that were based on the multipole
based model for four different geometry setups (one uniformly distributed ligands and
three random distributions). Qualitatively, the trends match experimental results .........144
Figure E.5 QM Frequency calculations using the snapshots generated with MD
simulations (4-MBN with and without ferrocene moiety nearby). The 4-MBN
vibrational frequency depends mostly on the surface charge of the electrode and
not the charge of the ferrocene, resulting in a minimal effect near the redox potential
of ferrocene/ferrocenium .................................................................................................144
Figure F.1 (Left) The CV of surface tethered ferrocene, 11-(ferrocenyl)undecanethiol
tethered on a gold wafer electrode at various scan rates and (right) the linear dependence
of anodic/cathodic peak currents on scan rate .................................................................145
Figure F.2 The solvation effects of surfactants on 11-FCUT that manifests as shifts in
the CV. The solid lines represent the 11-FCUT CV in aqueous 100 mM KCl solution
and the dotted lines represent the same sample in the same electrolyte with the addition
xv
of 1 mM surfactants. (Left) A cationic surfactant, CTAB, that shifts the CV of
11-FCUT in the oxidative direction and (right) an anionic surfactant, SDS, that shifts
the CV of 11-FCUT in the reductive direction ................................................................146
Figure G.1 A picture of the starting materials in aqueous solutions to make Prussian Blue
and its analogs and the films of types of films that were deposited on ITO via the
SILAR method .................................................................................................................147
Figure G.2 The response of the MCT detector that was used in the Visible pump IR
probe experiment. This was done by directing the 800 nm pump pulse (~ 50 fs pulse
width) light onto the detector and then fitting the decay to an exponential. The time
extracted time was ~ 2 𝜇𝑠 ................................................................................................148
Figure G.3 FTIR reflectance spectra, zoomed in on the nitrile region, of different PB
films on varying substrates (ITO, gold, and bulk). The spectra vary depending on the
type of substrate the films are deposited on .....................................................................149
Figure G.4 (Left) A cartoon diagram of the three-electrode setup in a cuvette and (right)
a CV of the CoFe film at 200 mV/s .................................................................................150
Figure G.5 The forward and backward scan for the spectroelectrochemical experiment
of the CoFe film from 0.6 V to 1 V vs RHE. ...................................................................151
Figure G.6 The forward and backward scan for the spectroelectrochemical experiment
of the CoFe film from 0.6 V to 1.5 V vs RHE .................................................................152
Figure G.7 The forward and backward scan for the spectroelectrochemical experiment
of the CoFe film from 0.6 V to 2 V vs RHE ....................................................................154
xvi
Figure G.8 A picture of the CoFe and FeFe films deposited by the SILAR method on
gold (right) and their UV-Vis absorption spectra ............................................................155
Figure G.9 (Left) Another set of 400 pump IR probe data for another CoFe film and
(right) representative time slices from five different wavelengths ..................................155
Figure G.10 (Left) Another set of 800 pump IR probe data for another CoFe film and
(right) representative time slices from five different wavelengths ..................................156
Figure G.11 Another set of spectra for the CoFe film pumped at 400 nm and 800 nm ..156
xvii
Abstract
If I had to categorize my PhD projects into big themes and questions I wanted to answer,
it would be the following two. First is probing chemical reactions and measuring electric fields at
the interface. Surface chemistry is significant because a lot of the important chemistry occurs at
the electrode-electrolyte boundaries. However, it is often difficult to probe reactions at the
surface. Complications arise because at the surface, models that are used to describe phenomena
in the bulk no longer apply. This can be overcome by using various spectroscopic tools and
models, which is discussed throughout this thesis. The second theme is probing molecular
species in the bulk while they undergo operando chemical transformations. My PhD work
focuses on investigating these two umbrella themes by utilization of vibrational spectroscopy.
This thesis is arranged as follows. I first explain the main concepts that relate to the systems of
interest that I study. Then, I discuss a technique that I heavily used to study surface
spectroelectrochemistry, Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS), and a
quantitative method I used to measure the enhancement factor and penetration depth for different
substrates. Next, I use the application of SEIRAS to discuss the interplay between surface
electrostatics and a one electron charge transfer process by incorporating a mixed monolayer
system of a molecular Stark probe and a surface tethered ferrocene. I then study a two-electron
two-proton transfer reaction by probing the PCET process of benzoquinone/hydroquinone
crystals utilized in batteries. Lastly, I extend these studies to a thin film system where I probed
the lifetime of photoinduced charge-separated species of a Prussian Blue film by tracking the
vibrational features of the CN ligands.
1
Chapter 1
Introduction to Concepts
1.1 Interfacial Electric Field and Stark Shift Spectroscopy
Interfacial electric fields play a critical role in a wide range of important chemical
reactions. Specifically, electrostatic interactions at the electrode-electrolyte interfaces play a
huge role in molecular orientation and dynamics at these boundaries, which in turn affect the
thermodynamics and kinetics of chemical reactions.
1, 2
Some examples of these chemical
reactions include, but are not limited to, protein shuttling at biological membranes, water
splitting using electrocatalysts, water structure at the surface, and surfactant solvation. Therefore,
probing the local electric fields in molecular systems is important for both fundamental
understanding of these chemical systems as well as modifying and designing improved systems.
Measuring interfacial electric field is no trivial task, however, because it is difficult to precisely
measure and challenging to model.
One powerful tool that can be used to measure interfacial electric fields is vibrational
Stark Shift Spectroscopy. In a nutshell, vibrational Stark spectroscopy utilizes inserting a
molecular probe in an environment and back tracking the local electric field based on the
vibrational frequency shifts arising from the influences of the microenvironment that is
surrounding the molecular probe (Figure 1.1). The Stark effect arises when there is an external
electric field perturbation to a system that changes the system’s molecular energy levels that will
either stabilize or destabilize the ground and excited state dipoles, resulting in a frequency shift.
The change in frequency, the Stark Shift, is proportional to the external electric field and the
Stark tuning rate, which is pre-determined through experiments or calculations for a molecule
2
before it is used as a molecular Stark probe. Details of how the Stark tuning rate is measured is
described by Boxer et al.
3, 4
It’s also noteworthy to mention that the Stark effect is present in
both electronic and vibrational transitions. Electronic Stark shifts will result in larger change in
dipole moments in molecules like organic dyes. The limitation with using electronic Stark probes
is that the spatial resolution for measuring the electric field is limited by the molecule size and
inhomogeneous fields may be averaged and blurred out over the length scale of the molecule.
Because of this, vibrational Stark probes are used when we are interested in probing local electric
fields in the molecular scale.
In the Dawlaty lab, and during my PhD, we use 4-mercaptobenzonitrile (4-MBN) as our
vibrational probe to measure surface electric fields. 4-MBN is an excellent Stark probe for a few
reasons. First, the nitrile frequency stretch of unperturbed 4-MBN is at 2230 cm
-1
as shown in
Figure 1.2, which is well isolated from other common vibrational stretches of organic molecules.
Second, its frequencies and linewidths are very sensitive to its surrounding environments,
Figure 1.1. General concept of Vibrational Stark Shift Spectroscopy.
3
providing accurate field measurements. Third, the molecule is relatively unreactive, acting as a
stable, well behaved molecular
voltmeter. And finally, 4-MBN has a
thiol bond that covalently binds to
gold, which can form self-assembled
monolayers for interfacial Stark
measurements. The hydrogen
bonding and Lewis interactions of the
nitrile lone pair do influence the
vibrational frequency, however, they
can all be accounted for. An in depth review of vibrational Stark Shift spectroscopy and the
utilizations of 4-MBN as a Stark probe can be read in a book chapter that has been previously
published by our work.
5
I will discuss in Chapter 3 how I used a mixed monolayer of 4-MBN
and surface-tethered ferrocene to measure and track the electric field before, during, and after
interfacial charge transfer.
1.2 Surface Enhanced IR Absorption Spectroscopy (SEIRAS)
Studying molecular reactions at interfaces can be a challenging task because you are
measuring a very small amount of surface molecules, which oftentimes means a low sample
signal-to-noise ratio. When this happens, you can either increase the concentration of your
sample or use techniques that will enhance the signal. When we are studying interfaces, we are
limited by the molecules that are close to or right at the surface and cannot increase the sample
concentration, so we resort to the latter solution.
Figure 1.2. A normalized IR spectrum of
4-mercaptobenzonitrile on a gold wafer.
4
Surface enhanced IR Absorption Spectroscopy (SEIRAS) is a very useful surface
sensitive technique that enhances the infrared absorption signal of molecules adsorbed at or close
to the surface. This effect is due to the collective electronic oscillations of confined metal
nanostructures on the metal films that act as nanoantennas, which enhances the electric field
experienced by molecules nearby. Depending on the surface morphology and material,
molecules on SEIRAS surfaces show orders of magnitudes more intense infrared absorption
signals compared to without the metal. There are ways to quantify this enhancement factor,
which is discussed in Chapter 2. Here, I will discuss the general ideas and concepts of SEIRAS.
First, I will discuss the concept of attenuated total reflection (ATR) because SEIRAS
builds on ATR. Figure 1.3 is a general schematic of ATR. Light travels differently in different
media of varying refractive indices. At the boundary between two different materials (n
1
and n
2
),
the angle of incidence of light (𝜃
#
) can be related to the angle of refraction of light (𝜃
$
) by
Snell’s law which states that
𝑛
#
𝑠𝑖𝑛𝜃
#
= 𝑛
$
𝑠𝑖𝑛𝜃
$
5
When light is traveling from a medium of larger refractive index to a smaller one, it will bend
further away from the normal axis (the dotted line).
And it will continue to do so until it hits the critical
angle, which is the angle at which the refracted light
is 90 degrees from the normal axis. At the critical
angle, the light skids across the plane of the
interface. This is the point in which as the incidence
angle continues to increase, there will be no refracted
light and all of it will be reflected. This is called total
internal reflection because all the light is completely
reflected. In theory, past the critical angle, all the
incident light should be completely reflected, but what people found out experimentally was that
there is a small leakage of light that exists at the boundary between the two materials. This is
known as an evanescent wave or evanescent field. An evanescent wave is a concentrated
oscillating electric field that exists at the boundary of the interface during total internal
reflection. It does not propagate across the surface as electromagnetic waves do and can be
thought of as the tail of the electromagnetic field of light that pokes through the surface of an
ATR crystal during total internal reflection. The penetration depth of the evanescent wave is
described by
penetration depth =
λ
2πn
$
?sin
$
(θ)−(
n
$
n
#
)
$
Figure 1.3. Cartoon schematic of ATR
at various incident angles.
6
where λ is the wavelength of incident light, θ is the angle of incidence, and n
#
and n
$
are the
refractive indices of the media. It exists as part of the incident light, so it will poke through the
crystal and then follow the propagation of the incident light when it is completely reflected.
Evanescent waves can be utilized in Fourier-transform infrared spectroscopy (FTIR)
experiments to probe samples. Do to so, appropriate crystal materials that have high refractive
indices that are cut at certain angles are chosen so total internal reflection can occur when correct
incident angle conditions are matched. These materials are oftentimes things like Silicon and
ZnSe that are cut at 60
%
. Samples that are placed on top of the crystal can then be characterized
by absorption spectroscopy. You tune the incident angle so it is past the critical angle of your
experimental setup to get total internal reflection. The evanescent waves will poke through the
crystal and interact with the sample. Any portion of the incident light that is absorbed by the
sample will be absorbed, and everything else will be reflected, which can be picked up by a
detector. The instrument then performs a Fourier transform operation to convert the
interferogram to an infrared absorption spectrum. This is why the technique is called attenuated
total reflection because any part of the light that is absorbed by the sample will be attenuated and
reflected.
In addition to characterizing samples, we are oftentimes interested in applying an external
potential to the system and probing electrochemical processes while measuring the spectroscopic
signatures. This requires an electrode, which is often done by depositing a layer of metal on top
of the ATR crystal. The metal serves as a conductive material to make electrical contact with the
sample. The thickness of the layer of metal deposited is critical because it needs to be thick
enough to be conductive, but thin enough so IR light can penetrate through to interact with the
sample of interest. This thickness is oftentimes in the order of tens of nanometers. When metal of
7
this order of magnitude of thickness is deposited on an ATR crystal, it produces discontinuous
gold islands on the surface that act as gold antennas which enhances local electric fields. This
phenomena is believed to be a large contributor of the enhancement effect of SEIRAS, in which
molecules adsorbed on these metal surfaces show IR absorption that are orders of magnitude
times enhanced. The enhancement greatly depends on the morphology and density of the metal
particles. In general, the largest SEIRAS enhancement is produced when the metal islands are
densely packed at the surface but not touching each other.
6
It has also been shown that the
surface selection rules are different in SEIRAS compared to transmission or ATR (REF). In
SEIRAS, only molecular vibrational modes that produce dipole changes in the direction
perpendicular to the metal surface are IR active and enhanced. Therefore most, if not all, of
monolayer SEIRAS signals arise from p-polarized IR light and not s-polarized light.
The infrared absorption, A, can be described as
A = F
&'
&(
F
$
|E|
$
cos
$
𝜃
where
&'
&(
is a change in the dipole moment of the molecule as a function of some space
coordinate Q, E is the electric field that is exciting the molecule, and 𝜃 is the angle between
&'
&(
and E. There are two mechanisms that people believe to contribute to the overall enhancement of
SEIRAS. One is the enhancement of the electric field, E, and the other is a chemical
enhancement of
&'
&(
. I will first discuss the electromagnetic mechanism, followed by the chemical
mechanism.
As mentioned earlier, metal islands are formed when metal films are deposited on
SEIRAS substrates. These metal islands are polarized by the incident light via the excitation of
8
localized plasmons, and the dipole induced in these islands generate an enhanced confined
electric field. In other words, the metal islands are believed to act as an amplifier in SEIRAS. We
can model the enhanced signal intensity of adsorbed molecules in SEIRAS by using the
effective-medium approximation (EMA), which relates the dielectric function of the SEIRAS
composite film to the polarization susceptibility of the metal islands. There are several models
that describe EMA, and the Bruggeman (BR) model is one of most popular ones that describe
densely packed metal islands embedded in an effective medium. The BR model states that
𝜀
)*
= 𝜀
+
3(1−𝐹)+𝐹𝛼
,
3(1−𝐹)−2𝐹𝛼
,
where 𝛼
,
is the polarization susceptibility of the metal islands, F is the filling factor of the metal
in the composite layer, and 𝜀
+
is the dielectric constant of the surrounding host medium.
7
If we
assume the gold islands can be modeled by ellipsoids coated with a layer of adsorbed molecules,
we can work out the value of 𝛼
,
and express it as a function of the dielectric functions of the
metal (𝜀
-
) and adsorbed molecules.
6
The dielectric function of the metal can be calculated from
Drude model
𝜀
-
(𝜔) = 1−
𝜔
.
$
𝜔(𝜔+
𝑖
𝜏
)
where 𝜔
.
is the plasma frequency, 𝜏 is the relaxation time, and 𝜔 is the frequency of the incident
light. The dielectric function of the adsorbed molecules can be approximated by a damped
harmonic oscillator given by
𝜀
&
(𝜈) = 𝜀
,
(𝜈)+ 𝜀
,,
(𝜈)
9
𝜀
,
(𝜈) = 𝑛
/
$
+
𝐵(𝑛
0
$
−𝜈
$
)
(𝜈
0
$
−𝜈
$
)
$
+𝛾
$
𝜈
$
𝜀
,,
(𝜈) =
𝐵𝛾𝜈
(𝜈
0
$
−𝜈
$
)
$
+𝛾
$
𝜈
$
where 𝜈 is the frequency, 𝜈
0
is the band center, 𝑛
/
is the refractive index at a frequency far from
𝜈
0
, 𝛾 is the bandwidth, and B is a constant that is related to the band intensity. With the dielectric
functions for the metal and adsorbed molecules, they can be inserted into the BR model to
simulate SEIRAS spectra for adsorbed molecules on different metal substrates and compare that
to substrates without deposited metal. For an 8 nm thick Ag film, the estimated enhancement
factor using the BR model is calculated to be around 45-450, comparable to experimental results
in other literature.
Described earlier is the electromagnetic mechanism of SEIRAS enhancement. Some
people believe there is an additional chemical mechanism as well that produces further
enhancement of SEIRAS. Some studies have shown that chemisorbed molecules on a metal
surface possess larger signal enhancement than physiosorbed molecules,
8
suggesting some kind
of “intensity borrowing” from the charge oscillations between molecular orbitals and the metal.
There are also some simulations that suggest coupling between the molecular vibrations and
metal electronic transitions.
9
The chemical mechanism, as far as I know, is still being explored
and further evidence is required to support this theory.
10
Chapter 2
A Direct Determination of Plasmon Enhancement Factor and Penetration Depths in Surface
Enhanced IR Absorption Spectroscopy
2.1 Introduction
Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) is a powerful tool for
studying a wide range of surface and electrochemical phenomena. Since its development in the
1980s,
10
Surface Enhanced IR Absorption Spectroscopy (SEIRAS) has opened several avenues
to study spectroelectrochemical processes at interfaces. SEIRAS is an excellent tool for probing
surface species in various electrochemical processes such as CO
2
reduction,
11-18
alcohol
oxidation,
19-21
hydrogen evolution,
22-24
and water structure at surfaces.
25-28
For most
electrochemical experiments the evanescent field of an IR beam partially penetrates through a
thin metal electrode deposited on top of an attenuated total reflection (ATR) crystal to interact
with molecules of interest. In a nutshell, SEIRAS involves depositing a thin layer of metal on a
substrate through which IR light can partially penetrate and interact with molecules. Unlike
surface plasmon resonance (SPR) spectroscopy, exact momentum matching is not required for
SEIRAS due to surface roughness. SEIRAS is often performed in an attenuated total reflection
(ATR) geometry where the metal is directly deposited on top of the ATR crystal. Without the
deposited metal layer, the theory to describe ATR is well developed, especially if the dielectric
constant of the medium above the crystal is known.
29, 30
However, for electrochemical
measurements, metal deposition is necessary for electrical contact, which complicates the theory
of ATR-SEIRAS.
11
Two phenomena emerge as a result of the metal deposition. First is the enhancement
arising from interaction of the IR with the roughened metal and thereby producing an antenna
effect that effectively increases the absorption cross section of the near-surface species, as
described in Section 1.2.
31
Second, after metal deposition, the penetration depth of the IR as
understood from the dielectric theory of ATR no longer applies. In fact, if the metal is very thick,
no light can interact with the near-surface molecules. Previously, the enhancement factor has
been estimated by comparing absorption cross sections for the two scenarios – with and without
roughened metal.
31-33
This, unfortunately, is not conclusive since the penetration depth is affected
by the metal. Enhancement factor of SEIRAS, which fall in the range of 10-10,000, have been
reported previously.
6, 33-37
Lack of quantitative information for the enhancement factor makes
interpretation of some experiments difficult and limits the utility of the technique.
To resolve this issue, we developed a systematic method for measuring this, which relies
upon independent determination of surface coverage by Coulometry of a surface-bound redox-
active species. A general schematic is shown in Figure 2.1 Following that, we measure the
SEIRAS spectrum of the surface bound species, and from the knowledge of surface coverage,
retrieve the effective molar absorptivity, 𝜀
123*41
. Comparing this to the independently
determined bulk molar absorptivity leads us to the enhancement factor 𝑓 = 𝜀
123*41
/ 𝜀
5678
. We
report enhancement factors in excess of 1,000 for the C-H stretches of surface bound ferrocene
molecules. We additionally developed a methodical approach to measure the penetration depth
of the evanescent field from the metal electrode into a thin film. Such systematic measure of the
12
enhancement factor and penetration depth will help SEIRAS advance from a qualitative to a
more quantitative method.
2.2 Experimental
Methods
Gold was deposited on
60
°
cut ZnSe ATR prisms (Pike
Technologies) via an electroless
deposition method adapted from
Bao et al.
38
In summary, the
ZnSe prism is heated on a hot
plate at 100° C, and is then
exposed to a 10 mM HAuCl
4
solution for ~ 30 s. Prior to metal deposition, the ZnSe substrates
were polished with alumina slurry (30 um, 15 um, 1 um, 0.05 um, consecutively) and then cleaned
in an ultrasonic bath with ultrapure water for 5-10 min. This method produces a rough morphology
as seen in the Scanning Electron Microscopy images reported.
38
The gold thicknesses obtained
from this method are in the order of 30 nm, as reported previously based on AFM measurements.
38
The exact determination of the gold thickness is not a critical part of our analysis and outside the
scope of this work, but it is estimated to be also of the same order.
Self-assembled monolayers (SAMs) were prepared by immersing the gold deposited ZnSe
crystals in a 20 mM solution of 6-ferrocenyl hexanethiol (6-FCHT) in ethanol for 15-30 minutes
to establish a dilute monolayer. The cyclic voltammograms for the SAMs were measured using a
potentiostat (Gamry 1010B) from 0 V to + 0.6 V vs Ag/AgCl. The linear dependence of peak
Figure 2.1. A general concept schematic of the methods
used to determine the enhancement factor of a SEIRAS
substrate using a monolayer system.
13
current versus scan rate establishes that the 6-FCHT was chemisorbed to the gold (see Figure D.1).
39
The working and counter electrodes were the deposited gold contacted with a gold wire (99.9%
pure) and a Pt mesh, respectively. Prior to electrochemical measurements, both the gold wire and
the Pt mesh were flame annealed, cleaned in piranha solution, and sonicated in ultrapure water.
The electrolyte was 1 M KCl in aqueous solution.
Each ferrocene molecule undergoes one electron transfer in these potential regions, which
allows calculation of surface coverage. The coverage of the monolayer was calculated by
integrating the area under the curve of the anodic wave of the cyclic voltammogram. Prior to
integration, to remove the influence of the substrate’s capacitive current from the voltammogram,
the baseline was fit to a polynomial and subtracted from the anodic curve to extract the Faradaic
current from the surface-bound ferrocene. The geometric area of the electrode was 3.14 cm
2
.
The films of Prussian Blue (PB) were electrochemically deposited using methods followed
by Roig et al.
40
The ZnSe gold substrate working electrode was held at a constant cathodic current
(-40 𝜇A) immersed in a 0.02 M in K
3
Fe(CN)
6
, 0.02 M FeCl
3
, 0.01 M HCl solution. Each ~ 14 nm
layer was deposited by applying the current for 145 seconds, followed by taking a SEIRAS
spectrum.
All spectra were collected using a ThermoFisher Nicolet iS50 FTIR spectrometer and a
spectroelectrochemical Teflon cell that is compatible with the VeeMAX III (Pike Technologies)
in the ATR geometry. The spectra were averaged for 128 scans with a resolution of 4 cm
"#
. The
6-FCHT monolayer SEIRAS spectra and PB film spectra were collected with a liquid nitrogen-
cooled mercury cadmium telluride (MCT) detector and the bulk transmission FTIR spectra were
collected with a deuterated tri-glycine sulfate (DTGS) room temperature detector. For bulk
14
measurements, the molecules were dissolved in deuterated DMSO (dDMSO) to avoid overlap with
the C-H stretch region of 6-FCHT. A CaF
2
FTIR cell with a 100 𝜇𝑚 spacer was used for the bulk
measurements. The actual thickness of the spacer was measured to be 97 µm. This was obtained
with the interference fringes with the transmission cell filled with air, as shown in the Figure D.2.
For surface measurements, the SEIRAS spectra were backgrounded with respect to spectra of gold
deposited on ZnSe in air, before their respective monolayers were assembled. We did not see any
SEIRAS signal of 6-FCHT when the incident IR light was s polarized. Therefore, we took all the
measurements with p polarized light. Note that this observation does not imply a perfectly ordered
monolayer. The roughness of the gold surface implies that there are many orientations of the
monolayer with respect to the polarization of light. However, as reported previously in the
literature, the p-polarization sensitivity of SEIRAS arises from the response of the metal, rather
than the orientation of the molecules.
41
2.3 Results and Discussion
Surface Coverage of 6-FCHT Measured by Cyclic Voltammetry
Cyclic voltammograms of the SAM covered substrates were measured and the
anodic/cathodic peaks were observed at + 0.49 V and + 0.39 V vs Ag/AgCl, respectively, as
shown in Figure 2.2. We note a peak-to-peak separation of 100 mV that deviates from that of the
ideal surface-active voltammogram.
42, 43
This could be due to differences in the solvation of local
15
environments that lead to a difference in the energetics for oxidizing and reducing the
monolayer, as has been reported in the literature
as well.
44-46
Note that since we are interested in
the integrated area, this offset does not affect
our analysis. The raw data is shown here, and a
baseline corrected version of this CV is seen in
Figure D.1, along with the linear dependence of
the peak currents on scan rate. The surface
coverage for a series of samples ranged
between 0.21 – 1.53 molecules nm
-2
(Table 1).
Molar Absorptivity of 6-FCHT Measured by Bulk FTIR
The bulk absorption spectra of 6-FCHT in the C-H stretch region as a function of various
concentrations in dDMSO is shown in Figure 2.3. For determination of the molar absorptivity of
the hydrocarbon stretches of 6-FCHT, we used the peaks at 2853 cm
-1
and 2929 cm
-1
, which
correspond to the symmetric and asymmetric CH
2
stretches of the hydrocarbon chain.
47-50
The
baseline corrected spectra and the absorbance values at 2853 cm
-1
and 2929 cm
-1
are plotted against
the concentration of 6-FCHT as shown in Figure 2.3. The slope obtained from the absorbance
verses concentration plot is the product of molar absorptivity and path length (97 μm in our case).
Figure D.3 shows the linear fits that include the (0,0) point and the R
2
values of the linear fits. The
Figure 2.2. A representative cyclic
voltammogram of the 6-FCHT monolayer
at scan rate of 50 mV/s.
16
molar absorptivity 𝜀
5678
of the symmetric and asymmetric stretches were found to be 71 M
-1
cm
-1
and 126 M
-1
cm
-1
.
Enhancement factor from SEIRAS
To calculate 𝜀
123*41
, we measured spectra of monolayers of 6-FCHT on the electrodes.
The SEIRAS spectrum of the same sample from Figure 2.2 is shown in Figure 2.4. The two
peaks at 2853 cm
-1
and 2929 cm
-1
match those observed in the bulk 6-FCHT spectrum. The
absorption values at these same wavelengths were used for the calculation of the enhancement
factors after the spectrum was background subtracted (Figure D.4). The SEIRAS absorption
based on an effective Beer’s law for the surface is 𝐴
123*41
= 𝜀
123*41
𝑙 𝑐, where 𝑙 and 𝑐 are the
effective path length and concentration. For a surface tethered species, the product of 𝑙 and 𝑐 is
Figure 2.3. Bulk FTIR spectra corresponding to the C-H stretches of 6-FCHT. (a) The raw
absorption of the C-H region as a function of 6-FCHT concentrations. (b) The peak heights
of the 2853 cm
-1
and (c) 2929 cm
-1
peak plotted as function of 6-FCHT concentration.
17
the surface density, 𝜌, leading to
𝐴
123*41
= 𝜀
123*41
𝜌. The
enhancement factor 𝑓 is defined as
the ratio of the molar absorptivity of
SEIRAS to that of the bulk 𝑓 =
𝜀
123*41
/𝜀
5678
. This leads to:
𝑓 =
𝐴
123*41
𝜌 𝜀
5678
Table 1 shows the enhancement
factors for four different samples using the two SEIRAS absorption peaks at 2853 cm
-1
and 2929
cm
-1
. The average enhancement factor for the analysis at 2853 cm
-1
is 1267 ± 115 and for 2929 cm
-
1
is 1360 ± 129. In other words, the aliphatic C-H stretches of 6-FCHT is enhanced by three orders
of magnitude when it is close to the metal surface of a SEIRAS substrate.
It has been shown that organic groups with less than 16 CH
2
groups form disordered layers.
51
Even if there was a perfect orientation of the molecules over the surface, the absorption
Table 1. A summary of the four samples of 6-FCHT monolayers, showing surface density,
adsorption for the two C-H stretch nodes, and their retrieved enhancement factors.
Figure 2.4. SEIRAS spectrum of a 6-FCHT
monolayer on gold in the C-H region. This is
the same substrate used in Figure 1.
18
coefficients would only change by a factor of 3. This is because isotropic averaging of a vector
quantity (the transition dipole moment) results into introduction of a factor 1/3.
52
Furthermore,
given the roughness of the gold substrate surfaces from the SEM images of our references, even
if all the molecules preferentially adsorbed in one orientation, those orientations will still be
randomized given the scale of roughness of the substrates being much larger than the scale of a
single molecule adsorbed at the surface. Therefore, we do not consider the orientation of the
monolayers in evaluating their interaction with light. Instead, we take the calculated
enhancement factors as an averaged value over all orientations at the surface. Note that the
enhancement factor retrieved based on the two peaks are quite close and within the error bars. It
would be a nontrivial task to remove the chemisorbed monolayers after the calculation of the
enhancement factor to reuse the substrates for another experiment. However, even though there
is variability in the substrate preparation, they are similar enough to give similar enhancement
factors within errors presented in Table 1. Therefore, one can assume the substrates are
comparable enough from one substrate to another, as long as they are prepared in similar
conditions. We also note that unlike thermally deposited gold, these gold films are more robust.
They can withstand multiple rounds of ultrasonication without flaking off or showing spectral
changes, something that thermally deposited gold films could not do based on our previous
experiences on Silicon and ITO. The explanation for the more robust substrate is because of a
strong gold-selenium bond that forms upon deposition.
38, 53, 54
Penetration Depth
Literature suggests that the surface enhancement is not uniformly distributed over the metal
substrate, but rather arises from distinct “hot spots”.
34
It is relatively straightforward to test this
based on our estimated enhancement factor. We first assume that the enhancement factor of ~
19
1,000 is uniformly distributed across the metal film. Second, we assume that the enhancement
decays exponentially into the solution. With these assumption in hand, we measured a spectrum
of pure bulk acetonitrile above the SEIRAS substrate. Since we know the bulk molar absorptivity
of acetonitrile, from the knowledge of SEIRAS absorption and enhancement factor, we can work
out the penetration depth (see Figure D.5). The retrieved penetration depth from this analysis is ~
0.2 nm. This is non-physical since it is in the order of the length of a bond, and we know the IR
light can penetrate through more than that from observation of spectral signatures of entities farther
away. Therefore, the assumption of uniformly distributed enhancement factor is not valid. Rather,
the measured ~ 1,000 enhancement factor is an average over the entire surface where the hot spots
necessarily have a much larger enhancement factor locally. Our experiment does not allow
determination of the density of hot spots and the local enhancement factor. Therefore, our reported
enhancement factor should be taken as an effective or averaged value.
Nonetheless, the question of effective penetration depth of the IR field into the sample
remains. We resorted to another experiment to answer this question. We electrodeposited Prussian
Blue (PB) thin films on the SEIRAS substrate at increments of ~ 14 nm at a time while measuring
the absorbance value of the nitrile stretches of PB. Figure D.6 shows the calculation of for the
estimate of ~ 14 nm per deposition increment. As shown in Figure 2.5, the absorption signal of
the Prussian Blue film continues to grow with the thickness of the film until it reaches a saturation
point around 126 nm. From that point forward, the SEIRAS signal does not grow significantly
while the electrodeposition continues as evidenced from Coulometry that shows current can still
be sustained even after the SIERAS signal stops to grow. Since the method of electrodeposition
relies on electron transfer from the electrode to cause the precipitation of the PB film and no other
reactions can occur at this current, this means the film is still growing. A representative
20
Coulometry is shown in Figure D.7. This means that the penetration depth of the IR into the
substrate was ~ 126 nm. Note that penetration depth of the IR field into the film is also a function
of the thickness of the gold film, with thicker gold films resulting into smaller penetration depths
into the film (See Figure D.8). The penetration depth of the IR field into the sample is expected to
be a function of the refractive index of the film. The refractive index of Prussian Blue is reported
to be ~ 1.44.
55
Therefore, for materials and solvents of similar optical properties, a similar
penetration depth is expected. Exact knowledge of the refractive index over a broad range
frequency is quite rare, except for very common thin films used more broadly in industry.
Additionally, the refractive index varies across an absorption. Many materials with strong visible
absorptions, derive most of their IR refractive index from their response in the visible range. Often
the absorption cross sections in the IR are much smaller than the visible cross sections, which is
also true for Prussian Blue as it is a vivid pigment in the visible range. Therefore, the dominant
contribution will arise from the background refractive index as dictated by the response in the
visible range, and not from the smaller absorption peaks in the IR range.
Figure 2.5. (Left) The SEIRAS spectrum of the PB film at different thicknesses and the
(right) integrated peak area of the nitrile stretch of the PB film electrodeposited on the
SEIRAS substrate ~ 14 nm increments.
2000 2100 2200 2300 2400
Wavenumber (cm
-1
)
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
Absorbance (OD)
14 nm
28 nm
42 nm
56 nm
70 nm
84 nm
98 nm
112 nm
126 nm
140 nm
168 nm
196 nm
21
2.4 Conclusion and Future Direction
Knowledge of the plasmon enhancement factor for electrochemical ATR is necessary to
elevate the observations of this technique from qualitative to quantitative. Here we presented a
systematic approach which relies upon independent measurement of surface coverage, and yields
enhancement factors in excess of 1,000 for electroless deposited gold on ZnSe. Note that the
enhancement factor will vary for different substrates and will depend on the metal. However, this
method is generalizable and can be applied to other substrates and metals. A future direction that
this project could be expanded on is looking at various different types of materials that can be
electrodeposited on the ZnSe substrates and tracking the penetration depth as a function of the
refractive index of various materials. It would be interesting and useful if we could come up with
an equation to represent the penetration depth and the refractive index of materials for gold
deposited ZnSe substrates. Once you know the penetration depth very well, I believe this would
be a very powerful tool that can be used to perform population analysis for surface
electrochemistry experiments.
22
Chapter 3
Interplay Between Charge Transfer and Interfacial Electric Fields
3.1 Introduction
In this Chapter, I will discuss a project that utilized Stark Shift Spectroscopy, as
described in Chapter 1, to measure the surface electric field during interfacial charge transfer.
Electrochemical charge transfer processes at electrode-electrolyte boundaries are significant to a
wide range of applications such as electrochemical energy conversion reactions, solid-aqueous
interfaces, ion intercalations in batteries, and interfacial electrocatalysis.
56-62
A central quantity
for understanding these processes is interfacial electric field and polarization. Interfacial field is
intimately tied to charge transfer, reaction barriers, solvation, molecular orientation, and
concentration gradients. However, it is often difficult to measure and model this quantity, in
particular during electrochemical charge transfer. This problem requires a well-calibrated
molecular probe for measuring the local electric fields along with a reversible and controllable
redox active entity. To address this challenge, we combined these two ideas for unraveling the
intricacies of interfacial fields during charge transfer. We prepared mixed self-assembled
monolayers (SAMs) with two components comprising a vibrational Stark shift probe (4-MBN)
for measuring electric field and a well-defined redox probe (surface tethered ferrocene) for
driving charge transfer. Ferrocene is a well-characterized redox active species that can be
charged and discharge by externally controlling the potential applied to the electrode.
43
The 4-
MBN is a well calibrated Stark probe that can act as a molecular field-reporter.
4, 5, 63
Our main
goal is to understand how the interfacial field responds to charge transfer using this mixed
monolayer system.
23
Vibrational Stark shift spectroscopy is a useful tool for measuring local electric fields. It
involves installing a pre-calibrated molecular probe in an environment and inferring the local
electric field from the frequency shift of the probe. More explanation and in-depth discussions
are given in Chapter 1.1 and other references.
3-5, 64
There are many molecules that can be used as
a Stark probe, but benzonitrile is a great candidate for various reasons also discussed in Chapter
1.1.
Ferrocene monolayers are often used as model systems to study charge transfer processes
because they are well-understood and extensively characterized.
43, 65, 66
The kinetics of charge
transfer from the electrode and a surface bound ferrocene moiety have been extensively studied
and shown to follow an outer-sphere
tunneling mechanism.
66
The half-wave
redox potential of SAM-bound ferrocene is
often ~ + 0.45 V vs Ag/AgCl and its redox
full width at half maximum for dilute
monolayers is 90 mV, consistent with ideal,
one-electron redox processes in non-
interacting surface-adsorbed molecules.
43, 67
In a mixed monolayer of ferrocene
and 4-MBN, upon oxidation of ferrocene to ferrocenium, two competing fields are expected to
appear at the interface- one emanating from the electrode due to its oxidative potential, and
another from the ferrocenium, which becomes positively charged, as shown in Figure 3.1. We
also know that when we have a field oriented along the nitrile molecule as shown on the left of
Figure 3.1, we observe a blue shift of the 4-MBN nitrile frequency. And when we have a field
Figure 3.1. Concept figure of the competing
fields from charge transfer of surface-tethered
6-FCHT.
24
oriented in the opposite direction as shown on the right of Figure 3.2, we observe a red shift. The
origin of these molecular shifts is discussed elsewhere,
3-5
but we wanted to understand the
competition of these two opposing forces. The 4-MBN Stark probe molecules are expected to
report on these two competing effects when we observe the overall Stark effect of the probe in
various mixed monolayer environments. One may assume that the field emanating from the
ferrocenium will be larger because a nearby positive charge would have a greater effect than a
planar electrode. However, our experiments show a counterintuitive result. In brief, we observed
that during charge transfer, the Stark probe does not respond strongly to the applied potential.
I first present the experimental results on vibrational frequency shifts as a function of
potential. This is followed by molecular dynamics (MD) simulation of the molecules at the
interface, revealing the relative geometries of the redox active ferrocene and the nitrile probe.
These geometries are used in periodic DFT calculations for computing frequencies and redox
states. Finally, I provide a hypothesis of the observed results.
3.2 Experimental and Computational Methods
Gold thin-film electrodes were prepared by depositing 20 nm of gold onto silicon
substrates (VeeMAX Si Crystal, 60
o
) using argon sputtering (Denton Vacuum). Prior to metal
deposition, the Si substrates were polished with alumina powder (30 um, 15 um, 1 um, 0.05 um)
and then cleaned in an ultrasonic bath with isopropyl alcohol and ethanol.
Self-assembled monolayers (SAM) were prepared by immersing the gold deposited Si
crystals in a 20 mM solution of 4-MBN in ethanol for at least 24 hours to ensure full surface
coverage. Mixed SAMs were prepared with a 1:1 mole ratio of each molecule in a 20 mM
solution in ethanol. After soaking, the solutions were removed, and the Si crystals were rinsed
25
and sonicated with ethanol to remove any physiosorbed molecules. Figure 3.2 shows a
representative diagram of the different mixed monolayer samples we prepared.
Spectroelectrochemical experiments were carried out in a Teflon electrochemical cell that
was compatible with a VeeMAX III (PIKE Technologies) accessory for reflectance IR
measurements. A three-electrode system was set up for electrochemical measurements: a thin
gold wire was used for electrical contact with the gold deposited Si prism working electrode, a Pt
wire was used as a counter electrode, and a Ag/AgCl electrode was used as a reference electrode.
Figure 3.3 shows a cartoon diagram of the spectroelectrochemical cell and photos of the gold-
deposited Si crystals. The electrolyte was a 1 M KCl aqueous solution. Spectra using surface
enhanced infrared reflection absorption spectroscopy under attenuated total reflection conditions
(ATR-SEIRAS) were taken with a Vertex Bruker 80 FTIR instrument using a VeeMAX III
accessory, with an incident angle of 60
o
. The potentials applied ranged from -1 V to + 0.6 V (vs
Figure 3.2. A representative diagram of the sample systems of control and mixed
monolayers: two controls (4-MBN, alkanethiol:4-MBN) and two samples (short chain
ferrocene:4-MBN, long chain ferrocene:4-MBN).
26
Ag/AgCl) and were scanned in a stepwise chronoamperometric method, where the potential was
ramped in increments of 0.2 V, and held at each potential while a spectrum was taken.
Molecular dynamics simulations were run by
Tanmoy Pal and Professor Qiang Cui at Boston
University. The functionalized gold surface was generated
using CHARMM-GUI,
68
as seen in Figure 3.4. The
nanomaterial modeler
69
input generator was used to
construct an Au (100) surface of dimension 4 nm x 4 nm x
2 nm with two types of ligands (-SCH
2
CH
3
and -
SCH
2
NHCH
2
CH
3
at 1:1 ratio). The surface density of
ligands at the time of packing was kept at 2 nm
-2
(32 of
100 ligand sites were occupied) to approximate the experimental density. We generated three
such setups with random placement of ligands and one setup with uniform spacing between two
kinds of ligands. Next the –SCH
2
CH
3
type ligands were patched
70
with 4-benzonitrile group to
obtain 4-MBN functionalization. Subsequently the –SCH
2
NHCH
2
CH
3
ligands were patched with
Figure 3.4. Functionalized gold surfaces generated by CHARMM-GUI decorated with 4-
MBN and other ligands to simulate the mixed monolayers of the experimental setup.
Figure 3.3. Cartoon of the ATR-
SEIRAS setup and images of Si
prism substrates.
27
one of three types of residues: a short-chain alkyl ferrocene (–SC
6
H
12
FEC), a long-chain alkyl
ferrocene (–SC
11
H
22
FEC) and an alkanethiol without the redox center (–SC
12
H
25
) (control). The
ferrocene moiety was constructed by first building one C
5
ring for each ligand, and then
translating all C
5
rings in the direction of ring normal (hence an eclipsed conformation) using
VMD.
71
Further refinement of the ferrocene structure was performed downstream using
harmonic restraints during energy minimization and molecular dynamics. With 4-MBN and one
of the three types of ligands placed, we appropriately oriented and reflected the functionalized
Au surface to obtain a 4 nm x 4 nm x 4 nm Au slab containing functionalization on both +Z and -
Z directions. Finally, we replaced all alkanethiol ligands from the surface (from 4- MBN +
alkanethiol system) to obtain another control that has only 4-MBN ligands with same surface
density.
The Au surface was described using INTERFACE force field.
72, 73
Topology of the
ferrocene moiety has been handled in different ways in literature.
74
In this work, a non-bonded
approach
75
was used – where the cyclopentadienyl (Cp) aromatic rings and Fe
2+
were held
together using two kinds of harmonic restraints, equilibrium
distances of which were obtained from CCSD(T) calculations
of equilibrium (eclipsed) structure of ferrocene.
65
The first
restraint of strength 100 kcal mol
-1
Å
-1
and equilibrium length
3.31 Å (2 x Fe—Cp distance) was applied between every pair
of aligned Cp carbon atoms (each belonging to different Cp
rings) along the Cp plane normal. The second restraint of
same strength was applied to restrict flexibility of Fe—C
Cp
distance at 2.05 Å. A representative diagram of these
Figure 3.5. A figure of the
restraints used to generate the
ferrocene moiety.
28
restraints is shown in Figure 3.5. Each carbon and hydrogen atom in cyclopentadienyl rings were
assigned a partial charge of -0.10 e and the Fe atom had a partial charge of +2.0 e. Topologies
and force field parameters for ligands were either taken directly or adapted for same atom types
from CHARMM36
76, 77
and CHARMM general force field.
78-81
After building the surface functionalized gold layer, we ran molecular dynamics
simulations using the GPU version of GROMACS
82-88
to obtain structural insights. First, every
ligand setup (4-MBN, 4-MBN + alkanethiol, 4-MBN + short-chain alkyl-ferrocene, 4-MBN +
long-chain alkyl-ferrocene) for all four setups (in terms of ligand distribution, 3 x random, 1 x
uniform) was contained in a box of X or Y dimension of 4 nm (equal to the X or Y dimensions
of Au surface). The Z dimension of the box was set to be at least 5 times that of X or Y
dimension. Then it was solvated using TIP3P
89, 90
waters, and energy minimization was
performed with frozen solute. 1 M KCl ions were added thereafter, followed by another iteration
of energy minimization. Next, 10 ns of equilibrium MD was performed in NPT ensemble. We
use 1 fs timestep for alkyl-ferrocene functionalized setups and 0.5 fs timestep for control setups
to ensure stability of MD simulations. Then the simulation boxes were repacked with water and
ions to avoid any cavity formation. Subsequently, energy minimization followed by a short 0.5
ns equilibration was performed. Finally, at least 200 ns of production MD in NPT ensemble was
run for each system.
Equilibrium and production MD simulations were performed keeping all Au atoms
harmonically restrained with a force constant of 2 x 10
5
KJ mol
-1
nm
-1
. LINCS
91, 92
algorithm was
used to constrain all bonds containing hydrogen atoms. Periodic boundary conditions were used
in all three dimensions. Electrostatic interactions were treated with particle-mesh-Ewald method
with a Fourier spacing of 0.12 nm and a Coulomb cut-off distance of 1.2 nm. A Van der Walls
29
cut-off of 1.2 nm is used with force-switch modifier for smooth switching of forces. For
temperature control during MD, we used Nosè-Hoover thermostat
93, 94
with a time constant of 1
ps and reference temperature of 300 K. Parrinello-Rahman
95, 96
extended ensemble pressure
coupling is used in a semi-isotropic fashion. We used a compressibility of 4.5 × 10
−5
bar
-1
and a
10 ps time constant for pressure coupling.
Using CP2K
97
package we performed ab-initio
molecular dynamics simulations (AIMD) to validate our
structural observations from non-polarizable force-field based
classical MD simulations. We probed the water structure
around one ferrocene by simulating an isolated ferrocene
molecule in a cubic water box (304 atoms) of edge length 1
nm, as seen in Figure 3.6. Born-Oppenheimer molecular
dynamics (BOMD)
98
in the Quickstep
99
scheme was performed in NVE ensemble for 5.7 ps and
NVT ensemble (300 K) for 9.6 ps. In this case we used GTH-PBE basis set with DZVP-
MOLOPT-SR-GTH pseudopotential for all elements. Periodic boundary condition was used in
all three dimensions.
Figure 3.6: The cubic water of 1 nm
edge dimensions with the ferrocene
inside for BOMD calculations.
30
The behavior of 4-MBN molecule on gold layer was studied both in vacuum and in water
(Figure 3.7). Both systems had dimensions of 1.2 nm x 1.2 nm x 2.4 nm. The system containing
Au and 4-MBN in vacuum had 67 atoms, whereas
the solvated system contained 304 atoms. For the
first system, BOMD was performed for 10 ps in
NVE ensemble followed by 18 ps in NVT
ensemble. For the latter system, we ran 5.7 ps of
BOMD in NVE ensemble followed by 9.6 ps of
NVT ensemble. For all BOMD simulations,
temperature was set at 300 K, and a plane wave
cutoff of 260 Ry was used.
These geometry optimized snapshots were given to William Lake and Professor Sharon
Hammes-Schiffer at Yale University to perform quantum mechanical (QM) frequency
calculations. We are still in the process of simulating this last part but in short, the calculation
overview is the following. The orientations of 4-MBN in the MD snapshots are used to calculate
the molecular normal modes of the 4-MBN nitrile stretch. At the relaxed near-surface geometry,
the 4-MBN is propagated along the normal mode, and potential energy curve for the nitrile
stretch is built. From there, 1-D vibrational Schrödinger equation is solved to perform frequency
calculations.
3.3 Results and Discussion
Figure 3.7: The systems of 4-MBN
on gold studied in both vacuum
(left) and in water (right).
31
We begin by presenting the electrochemical characterization of the ferrocene mixed monolayers.
The cyclic voltammogram (CV) of the mixed monolayer
samples show qualitatively similar features, and a
representative CV of the short chain ferrocene mixed
monolayer is shown in Figure 3.8. The E
1/2
appears at +
0.47 V vs. Ag/AgCl and does not change significantly
between samples. The observed separation of ~ 50 mV
between the oxidative and reductive peaks is typical and
on the smaller end for such systems. The oxidative peak
has a width of 100 mV, which is only slightly larger than
the ideal width of 90 mV expected for non-interacting redox probes on a surface. This indicates
that the mixed monolayer and the electrolyte allows for adequate separation between ferrocenes
to minimize the nearest neighboring interactions. We also showed the linear dependence of the
oxidative and reductive currents on scan rate (Figure E.1). This is considered a signature of
monolayer electrochemistry and demonstrates that the redox processes occur on the surface and
are not limited by diffusion.
39
The coverage of ferrocene is estimated from the net charge of the
oxidative and reductive peaks. The density of ferrocene is estimated to be around 1 molecule nm
-
2
.
Figure 3.8: A representative cyclic
voltammogram of the 6-FCHT:4-
MBN mixed monolayer.
32
Spectra of the mixed monolayers in the nitrile frequency range at open circuit potential
(OCP) is similar to previously reported SAMs
5
of tethered benzonitrile and shown in Figure 3.9
(left). An overall red shift of ~ 3-5 cm
-1
is observed for the mixed monolayers. Red shift of the
nitrile probe is expected when it is immersed in a dielectric medium. Therefore, the observed
shifts can be interpreted as the modification of the effective dielectric surrounding of the tethered
nitrile in the mixed monolayer environment.
The center frequencies of the nitrile probe for all samples as a function of the applied
potential in the forward scan is shown in Figure 3.9 (right). Actual spectra of the nitrile stretch as
a function of potential for the short chain 6-FCHT:4-MBN mixed monolayer is shown in Figure
E.2. The data is reversible when the potential is ramped in the opposite direction (from + 0.6 V
to -1 V vs Ag/AgCl). Data for the reverse scan of the mixed monolayer systems are shown in
Figure E.3.
The Stark behavior of 4-MBN monolayer is comparable to previous reports.
5
A slight
flattening of the Stark response at high positive potentials is expected for 4-MBN
5
and is
Figure 3.9. (Left) The spectra for the different mixed monolayer systems at open circuit
potential and (right) the center nitrile frequencies plotted as a function of voltage.
33
reproduced here as well. The mixed monolayer samples show different behavior. First the
ferrocene mixed monolayers show an overall red shift relative to neat 4-MBN. The origin of this
effect may be due to the ferrocene head blocking the electrolyte ions from approaching the
electrode. The effect is more pronounced for the longer chain ferrocene, which is much longer
than the 4-MBN molecule and can have a more disruptive effect on the ionic structure near the
surface. Second, the slope of the frequency as a function of potential is smaller for the ferrocene
mixed monolayers. Particularly, in the range where charge transfer to ferrocene is possible (~ 0.4
– 0.5 V as seen in the CV in the inset), the slopes are quite small, although from the density of
data in that region making a quantitative comparison is difficult.
Given the uncertainties about the geometries of the mixed monolayers, it is difficult to
infer a direct conclusion about the influence of charge transfer on the Stark effect from
experimental data alone. For that reason, we resorted to theoretical and computational
investigation of the systems. Our computational work has two components. First, MD
simulations are used to understand the molecular geometries, with emphasis on the relative
orientation of ferrocene and 4-MBN. Then, electronic structure theory is used on sample
geometries for calculating frequencies and Stark shifts for the 4-MBN.
Figure 3.10 shows atomic partial number density and charge density profiles of key
atoms in the systems. Three setups with random ligand placements have been overlaid. The
bottom row corresponds to the control setups, whereas the top two rows contain the ferrocene
containing ligands. Partial density of the surface layer of Au (atom name AUS) has been shown
in light green.
34
At relatively low surface density of ligands (< 2 nm
-2
) 4-MBN molecules prefer to have a
high tilt with respect to the Au surface normal. This observation has been verified using AIMD
simulations. The end-to-end distance of a 4-MBN molecule parallel to the Au surface normal is
approximately 0.72 nm. In contrast, during MD the most probable distance of the nitrile nitrogen
(averaged over at least ~ 6 x 10
5
conformations) is approximately 0.3 nm for all systems. This is
shown in the first blue peak from the left in the first column of Figure 3.10. Most probable
location for the redox center (Fe atom) was seen to be under 0.5 nm for both short-chain and
long chain alkyl-ferrocene ligands (end-to end distance of alkyl-ferrocene molecules are 1.06 nm
and 1.67 nm for short and long chain respectively). However, the number density of Fe atoms at
Figure 3.10. The densities of different atoms relative to the surface of the gold electrode.
This is shown as atomic partial number densities and charge density profiles of key
atoms (water, nitrogen of 4-MBN, iron of 6-FCHT, potassium of KCl, chloride of KCl,
and gold of the electrode).
35
this first peak is comparatively higher for the system with short-chain alkyl-ferrocene ligands.
Beyond 5 Å, we observe another population of nitrile nitrogen (NZ) atoms (~0.6 – 0.75 nm)
followed by yet another population of Fe atoms (> 0.75 nm). The end carbon atom (denoted C12
in Figure 3.10) for the alkanethiol control (end-to-end distance 1.57 nm) shows that the highest
partial density peak is at almost same distance from Au surface as nitrile nitrogen (~0.3 nm),
implying that without the highly charged ferrocene moiety, the alkanethiol molecules mostly like
to lie closer to the Au surface. It also highlights the role of ferrocene in the layering positively
and negatively charged atoms, as seen in the partial charge density plot (Figure 3.10 middle
panel).
These structural observations indicate that the most probable distance between the
vibrational probe molecule and the redox center is approximately 0.12 – 0.2 nm. We selected two
representative redox centers (one with short, another with long alkyl chain) based on the most
probable Fe–Au
surface
distance by scanning the production MD trajectory from 10–100 ns. These
structures were used for QM calculations of vibrational frequencies under applied external
potential. The QM frequency calculation results are still in working progress, and tentative
results are shown in Appendix E. Frequency shift calculations using the multipole based model
adapted from Lee et al.
100
are shown in Figure E.4. The QM frequency calculations for only 4-
MBN and mixed monolayers are shown in Figure. E.5.
From the combined computational and ongoing experimental work presented above, we
arrive at the following scenario for the interfacial field as sensed by the probe while the potential
is scanned. To understand the process, we identify three windows of potential. The first window
is before the onset of electron transfer from ferrocene. The second region spans the potential
range where charge transfer to/from ferrocene/ferrocenium is possible. The third region is the
36
range of potentials at which all surface ferrocenes have been all converted to ferrocenium and no
further charge transfer is possible. Figure 3.11 shows a cartoon representation of the interfacial
field, polarization, and the charge state of ferrocene in these three zones. In the first zone (Figure
Figure 3.11. The three zones of potential where we hypothesize explain the Stark shift
behavior of 4-MBN in different environments. The zones are: before the onset current
of 6-FCHT oxidation, during the redox activity region of 6-FCHT, and after the full
oxidation of 6-FCHT where all the ferrocenes have been oxidized to ferrocenium.
37
3.11(i)), charge transfer from ferrocene is not possible as this is before the redox activity
potential of ferrocene. Any applied potential will lead to a corresponding interfacial electric field
arising from accumulation of ions from the solution. The field will polarize the probe molecule
and will cause a frequency shift. In this zone, the interface is polarizable and a linear Stark shift
of the probe with respect to applied potential is expected. In the second zone (Figure 3.11(ii)),
charge transfer from ferrocene becomes possible. As mentioned previously, for an ideal and non-
interacting surface-bound redox probe, the width of this zone is expected to be around 90 mV.
The width observed by us is 100mV and is reasonably close to the ideal limit. Within this zone,
any increase in potential will drive charge transfer from ferrocene. Therefore, a change in
potential will not polarize the interface, but rather drive a current across the monolayer. The
interface is nominally conductive (i.e. not capacitive) in this zone. Furthermore, even though
there are fields emanating from the positively charged ferroceniums, counter chloride ions in the
solution will likely ion pair with the ferroceniums that will reduce this field felt by the 4-MBN.
Therefore, the applied potential does not lead to a further built up of polarization of the 4-MBN
Stark probe, resulting in a flattening/plateauing Stark shift effect observed in this region. In the
third zone (Figure 3.11(iii)), all of the ferrocenes have been oxidized to ferroceniums and no
further charge transfer can occur. Therefore, once again, the interface will polarize in response to
the applied potential. In this zone, the probe molecule is expected to frequency shift in response
to potential. We emphasize that the experiment limits exploring the very high positive potentials
within the third zone because the monolayers are not stable. However, computational results are
consistent with the picture presented above. Another complication in the experimental
observations is that benzonitrile has an inherent nonlinear response to field as reported by us
38
before. For that reason, from the experimental data alone it is hard to isolate the effect described
above.
3.4 Conclusions and Future Work
In conclusion, we have assembled a mixed monolayer system consisting of a molecular
Stark probe (4-MBN) and surface-tethered redox active molecule (6-FCHT and 11-FCHT) to
probe the interfacial electric field as a function of charge transfer. To our surprise, charging the
surface from ferrocene to ferrocenium only mildly affected the Stark shift of the 4-MBN.
Theoretical calculations provide hints that the 4-MBN nitrile vibrational frequency depends on
the ability of the electrode to polarize the 4-MBN instead of the electric fields emanating from
the ferrocenium. We identify three zones of potential to hypothesize the experimental results. In
the two zones of potential outside of ferrocene redox activity, the 4-MBN approximately linearly
Stark shifts as expected from previous work. This is due to the electrode polarizing the molecule
at oxidizing potentials. However, in the ~ 100 mV window where the ferrocene is redox active
and charge transfer is possible, the electrons are “busy” oxidizing the ferrocene, and therefore
cannot further polarize the 4-MBN. This results in a plateauing effect of the linear Stark
behavior. Further theoretical calculations are required to fully explain our experimental
observations, but this is our hypothesis so far.
39
Chapter 4
Quinone Proton-Coupled Electron Transfer in Bulk and Surfaces
4.1 Introduction
The increasing need for renewable energy demands continuous studies of environmentally
friendly, cost efficient, and sustainable batteries. With some of the problems that current alkaline
batteries have such as the flammability of the electrolytes or dendrite formation, there is a favorable
draw towards other cathode and anode materials, specifically organic batteries. Recent studies
show that different derivatives of quinone possess high specific capacities as a suitable
replacement for the anode of batteries. Quinone undergo proton coupled electron transfer (PCET)
process in protic solvents, in which each quinone molecule can store two electrons and protons.
This process can be utilized in battery applications to store and shuttle charges. Inspired by this,
we collaborated with Professor Yan Yao from the University of Houston and Professor Puja Goyal
from Binghamton University to study quinones in organic battery applications. Quinones are good
candidates for battery materials for several reasons. First, they exist abundantly in nature such as
L-dopaquinone in melanin, ubiquinone in vitamins, and anthraquinones in dyes, just to name a
few. Since they are ubiquitous, they can be relatively easy and inexpensive to obtain. Second, since
they are organic compounds, they are reasonably safe and possess low toxicity compared to other
battery materials. Third, they undergo multiple proton-coupled electron transfers (PCET) upon
oxidation/reduction, making them charge-dense molecules that can be charged and discharged for
multiple cycles. Lastly, their electrochemical properties can be changed by adding or removing
organic substituents to the quinone backbone, opening the ability to tune their properties to better
40
suit their applications. All of these qualities make quinones excellent candidates for shuttling
charges (i.e. electrons) used in batteries.
The specific quinone that we chose to study was 1,4-tetrachlorobenzoquinone (TCBQ). It
undergoes two proton two electron transfer reaction and becomes the protonated form, H
2
TCB, as
shown in Figure 4.1. We wanted to test the batteries under acidic conditions, and TCBQ is stable
and resilient in these conditions. Our collaborators also tested with X-ray Powder Diffraction
(XRD) the charging cycles of TCBQ, and we have reason to believe the TCBQ battery redox
reaction is at least somewhat
reversible, meaning the battery
can be recharged for multiple
cycles. Quinones have carbonyls
that are Raman active, which we
can probe spectroscopically. In
this study, we investigate the
PCET pathway of TCBQ battery
via operando Raman
spectroscopy. Specifically, we wanted to answer the following questions: (1) What is the
mechanism of the proton-coupled electron transfer? Do the protons insert into the quinone crystal,
or do they become solvated first prior to insertion into the crystal? (2) Do the quinone crystals
change their structures upon proton and electron transfer or do they preserve their physical
properties? Do the lattice structures deform? (3) How do the protons tunnel within the crystals
once they enter the materials? Here, I will discuss the results of the coin cell batteries I made
Figure 4.1. The molecular structures for TCBQ and
H
2
TCBQ, showing the two PCET processes that they
undergo when they are in their charged/discharged
forms.
41
incorporating TCBQ, as well as the attempts I made to tether quinones at the surface of electrodes
in order to have a further fundamental understanding of the PCET process.
4.2 Experimental Methods
The active anode material was prepared by grinding and mixing the TCBQ crystals, carbon
black powder to increase conductivity, and polytetrafluoroethylene as the binder to glue everything
together. I used a hand milling technique with mortar and pestle so I could control the quinone
crystal size. The size of the TCBQ crystals had to be small enough to immerse with the other
materials and for a deep penetration of electrons/protons, but large enough for the Raman laser to
probe the surface activity. With a microscope to observe the material, I found that the sweet spot
of the TCBQ size was in the order of ~ 100-200 𝜇𝑚. After grinding all the active materials
together, I brought it to a hydraulic press machine, where all the materials are pressed with high
pressure into a circular pellet. This ensures all the materials are bound together and in good contact.
Figure 4.2. A cartoon of the active material preparation and diagram of the assembly of the
coin cell.
42
Once the battery pellet was made, I assembled the coin cell as shown in Figure 4.2. The top and
bottom metal cap of the coin cell are the positive and negative ends of the battery, respectively.
The steel mesh increases the conductivity of the active material (the anode with the TCBQ) to the
positive electrode. The separator is used to separate the working electrode and the counter
electrode, which are the active material and activated carbon, respectively. I also added a spring
to increase the tension force to ensure good contact of all the components of the material. Two
drops of 4.4 M H
2
SO
4
was pipetted onto the spacer as a source of protons.
All spectra were taken with a 532 nm laser from a Raman instrument (Horiba Scientific)
with a 100 x focusing objective, as shown in Figure 4.3. A hole was drilled on the positive cap so
the laser can probe the active material and collect the backscattered Raman signal. A spectrum was
Figure 4.3. A picture of the in-situ Raman setup. The coin cell battery is hooked up to the
potentiostat with red copper wires and taped to a microscope slide for stability on the
microscope stage. The 532 nm Raman laser is hooked up to a microscope where the light
can be focused onto the top of the hole drilled on the coin cell battery.
43
taken every 5 minutes with a 1200 gr/mm grating. Copper wires were glued with epoxy to the
positive and negative cap for better connection as the working (active material + steel mesh +
positive cap) and counter/reference (activated carbon + spacer + spring + negative cap) electrodes
to the potentiostat. The battery was discharged by holding the potential at – 247 𝜇𝐴 for 150 minutes
and charged by reversing the sign of the voltage and holding the potential at + 247 𝜇𝐴 for another
150 minutes. Or, in other words, the battery was discharged/charged at a rate of 0.4 C. The coin
cell was held in place with tape and unperturbed during the whole experiment.
For the surface studies, self-assembled monolayers were prepared on wafer substrates.
Silicon wafers with a 10 nm Ti adhesion layer and 100 nm of deposited Au (LGA Thin Films, Inc.)
were used for preparing the self-assembled monolayers. The wafers were sonicated in acetone,
ethanol, and water for 5 minutes each time for cleaning. Then, they were soaked in a solution of
20 mM 11-mercaptoundecylhydroquinone (11-MUHQ) in ethanol overnight to ensure a saturated
coverage. Electrochemical measurements were made in a three-electrode system, with the wafer
as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode.
Cyclic voltammograms were scanned from - 0.4 V vs Ag/AgCl to + 0.8 V vs Ag/AgCl in a 1 M
H
2
SO
4
aqueous solution.
4.3 Results and Discussion
The Raman spectra of the powder TCBQ and H
2
TCBQ are shown in Figure 4.4. These
spectra were used as references for comparison of the starting material and the active material in
the coin cell battery as it was discharged and charged. Note the two stretches for TCBQ at 1612
cm
-1
and 1697 cm
-1
, which represent the stretching frequencies of the carbonyls. These are the
44
peaks that I tracked during the spectroelectrochemical in-situ experiment of the coin cell battery
as probes for the identity of the state of the quinones.
Figure 4.5 shows the electrochemical results of the TCBQ battery. Battery performance is
often measured and plotted by its voltage and specific capacity. The voltage of a battery is the
amount of potential a battery can deliver for a certain amount of total charge. If a battery is a water
tower, the voltage can be thought of as the pressure difference from the top of the tower to the
bottom. In an ideal world, a perfect battery should be able to maintain its voltage while it is being
discharged until the end of its cycle. Figure 4.5 shows the working voltage of the TCBQ battery
as it was discharged and charged. While there was a potential that was measured, it decreased over
time, indicating that while the battery had power to do work, it was unable to maintain the same
Figure 4.4. Raman spectra of the controls (TCBQ and H
2
TCBQ).
45
amount over time. This could be due to many reasons such as material degradation, some sort of
barrier for charge transfer that builds up over time, or instability of the material.
If the voltage is the power of the battery, the specific capacity is a measure of how long the
battery can sustain its voltage. The units of battery capacity are typically in ampere-hour/gram
(Ahg
-1
), which provides information about the amount of time that a battery can deliver charge per
mass of its active material. So, for example, if a battery has a specific capacity of 250 mAhg
-1
, then
in theory, it should be able to deliver 2 mA of average current for a total of 125 hours before it is
completely discharged. Studies have shown that quinone batteries can have specific capacities as
high as 395 mAhg
-1
101
, a suitable replacement for anodes of current batteries. As seen in Figure
4.5, the specific capacity of the TCBQ coin batteries in this study is closer to ~ 10 mAhg
-1
,
much
lower than the theoretical ideal value. I believe this is because the TCBQ crystals could not be
grinded too small and had to be large enough for the Raman laser to probe them spectroscopically.
Because of the larger surface area to volume ratio, it is more difficult for the protons and electrons
to penetrate into and out of the crystal during PCET. This was a mandatory tradeoff for us to
Figure 4.5. The battery performance shown as the specific capacity plotted as a
function of measured voltage for the discharging and charging cycle.
46
measure both the electrochemical performance of the battery as well as probe the spectroscopic
features of the material.
The spectroelectrochemical results are shown in Figure 4.6. The black spectra are the
references TCBQ and H
2
TCBQ taken from Figure 4.4 for comparison with the spectra of the coin
cell battery. From the discharge cycle, we see that the initial spectrum has the two carbonyl peaks
that belong to TCBQ, but within the first 15 minutes of discharging at a rate of 0.4 C, those peaks
disappear. It isn’t until more than halfway through the cycle that the peaks around 350 cm
-1
that
match those of H
2
TCBQ grow in and show up. When we hold the potential in the opposite direction
at + 247 𝜇𝐴 to charge the battery back to its benzoquinone form, we see that the low wavenumber
peaks of H
2
TCBQ disappear within the first 25 minutes and the carbonyl peaks of TCBQ grow in
even before halfway through the charging cycle. Another, and perhaps the most, interesting
observation is that not only do the carbonyl peaks of TCBQ show up as we charge the battery, but
Figure 4.6. The spectroelectrochemical data for the discharging cycle.
47
their signal intensities also grow larger than what we started with at the beginning of the discharge
cycle! We were very excited when we saw this but didn’t, and still don’t, have a very clear
explanation for why this happened. My best hypothesis so far is that in the process of TCBQ
evolving to H
2
TCBQ, the crystals underwent some form of melting or physical breakdown, shown
by the disappearance of the carbonyl peaks and the absence of any H
2
TCBQ features in the middle
of the discharging cycle. It isn’t until the end of the discharge cycle that the peaks around 350 cm
-
1
of H
2
TCBQ appear, a hint that the material recrystallized to H
2
TCBQ. Another evidence of this
phase change hypothesis is when we charge the battery back, shown in Figure 4.7. Here, evidence
of the reappearance of the carbonyl peaks show that H
2
TCBQ was converted back to TCBQ. In
fact, the carbonyl peaks after the charging cycle grow larger than the initial signal-to-noise ratio
of the carbonyl peaks we started with in Figure 4.6. We conjecture from this that when we charged
the battery, the H
2
TCBQ re-crystallized into TCBQ crystals with larger surface-to-volume ratio so
that more of the surface of TCBQ was available for the PCET process, resulting in enhanced
Figure 4.7. The spectroelectrochemical data for the charging cycle.
48
carbonyl signals by the end of the charging cycle. We have some evidence of this from X-ray
diffraction and theoretical calculations from our collaborators that show the possibility of a phase
change of the samples during these processes. As far as I know, this is the first time a TCBQ
battery has been probed operando spectroelectrochemically, where charge transfer was successful
and we were able to show spectral evolution of TCBQ to the doubly protonated reduced form upon
discharging, and a reformation of the deprotonated and oxidized form upon charging. While this
was exciting and promising, we encountered many difficulties. For one, these results were not
reversible. Once the battery was discharged and charged, we could not discharge the battery again.
The TCBQ seemed to undergo some sort of changes where it could not be converted to H
2
TCBQ
again. Another issue we encountered was the difficulty in seeing Raman features of the sample.
When preparing the active material pellet, I noticed that while having smaller quinone crystals, by
grinding them longer with the mortar and pestle, increased the specific capacity of the battery, I
could not see the carbonyl stretches of the TCBQ. The crystals were too small and had to be larger
(~100-200 𝜇𝑚 in diameter) in order to be probed by the Raman laser. Additionally, even when we
could see spectral features of the TCBQ at the start of the discharge cycle, it did not always undergo
PCET and convert to H
2
TCBQ. Sometimes the spectral features would disappear, and we would
lose signal. Other times we would not observe any changes at all. The requirement of all the
parameters to match made the experiment quite tedious and difficult. We decided to take a step
back and approach this problem from a more fundamental standpoint, which will be discussed in
the next section. I did not continue this study during the rest of my PhD, but if someone wants to
pick up the project again or extend this study, I recommend looking at the end potential data points
of the spectroelectrochemical data before performing a full discharge/charge cycle to save time,
49
studying TCBQ single crystals before utilizing them in a battery application to simplify the
problem, and/or looking at other types of quinones to see if they perform better as battery materials.
4.4 A Fundamental Study of Quinones and Future Directions
In addition to studying quinones in battery applications, I also tried to study quinones in a
more fundamental approach. Even though quinones have received a lot of research attention, it is
surprising that there is a significant part of their fundamental chemistry that remains unclear. One
of the major avenues of study on quinones is understanding the pathway of PCET that they
undergo. As shown in Figure 4.8, the “nine-membered square scheme” of quinones is often the
conventional diagram scheme used to understand all the possible pathways that electrons
(horizontal) and protons (vertical) can be
transferred. Because quinone PCET can involve
multiple electrons and protons, they can walk
through the square scheme in different pathways
depending on the different conditions. To study
this, I wanted to anchor quinones at the surface
of electrodes and study their properties at the
surface. With surface tethered species, you can
control the electrostatic environment of the
molecules by tuning the potential of the
electrode and play different solvation games by
switching the solvent and observing the
molecular effects. The latter can be done
electrochemically and spectroscopically.
Figure 4.8. The “nine-membered square
scheme” of benzoquinone/hydroquinone.
50
I found a molecule, 11-mercaptoundecylhydroquinone (11-MUHQ), that has a
hydroquinone on one end and a thiol group on the other end that could anchor to a gold electrode
for surface electrochemistry. Figure 4.9 shows the cyclic voltammogram (CV) of the redox activity
of 11-MUHQ and the scan rate dependence of the CV behavior, showing a linear dependence on
scan rate, a characteristic of surface-adsorbed species. To understand the different pathways
through the nine-squared scheme, you need to first have a handle of controlling the CV behavior
of 11-MUHQ. One method is by observing the electrochemical effects as a function of changing
the solvation surrounding the monolayer. I was able to replicate some of the results by Quan et al.
102
where the CV of 11-MUHQ was measured under a water titration in acetonitrile study. This is
shown in Figure 4.10. In summary, as water concentration is increased in bulk acetonitrile, the
peak-to-peak separation of 11-MUHQ decreases, with both the anodic and cathodic peaks shifting
closer towards each other. Even at 0.5 % water concentration, the anodic wave already shifts by ~
10 mV, indicating that water has an affinity towards solvating the surface 11-MUHQ upon
oxidation. This effect of water suggests that the water solvation around the oxidized form of 11-
Figure 4.9. (Left) The CV of 11-MUHQ on the gold wafer substrate at 200 mV/s
and (right) the CV at various scan rates.
51
MUHQ aids in stabilizing the benzoquinone form (perhaps through hydrogen bonding), resulting
in a shift of anodic peak potential to a smaller value. Appendix F shows the CV of a surface-
tethered ferrocene, a much simpler system in which the adsorbed ferrocene undergoes one electron
transfer. Solvation effects on the oxidation/reduction power were explored as well, in this case
using surfactants as solvating entities to shift the anodic and cathodic peaks of the ferrocene. This
can be useful if anyone wants to further study the square scheme of quinones and build from a
simpler system of one electron transfer (i.e. ferrocene). It would be very insightful if one can have
a spectroscopic handle on these surface-tethered quinones and perform spectroelectrochemical
experiments to probe the species generated at various potentials (i.e. SEIRAS, SERS, Sum-
frequency generation).
Figure 4.10. Various CVs of 11-MUHQ as a function of water titration
in bulk acetonitrile.
52
Chapter 5
Long-Lived Transient Responses from Photoexcitation of Prussian Blue and its Analogs
5.1 Introduction
One of the last projects that I was involved in during my PhD was studying Prussian Blue
(PB) and its analogs. Prussian Blue is a vibrantly, dark blue pigment that belongs to a class of
family called mixed valence polynuclear transition metal cyanide complexes. It is oftentimes
considered the first coordination compound that was synthesized. The structure consists of
alternating Fe(II) and Fe(III) bridged by CN ligands in between the irons in an octahedral
coordination system giving an overall cubic lattice structure, as shown in Figure 5.1. In general,
the metalorganic framework is made up of Fe
II
-C≡N-
Fe
III
units. It exists as various combinations of different
oxidation states depending on how it is made. In its
mixed valence form of Fe(III) and Fe(II), it is called
Prussian Blue and it appears very dark and vibrant blue.
The color comes from the charge transfer of electrons
from Fe(II) to Fe(III), where the absorption 𝜆
-:;
is
around 680 nm, giving it its blue color. The net charge of
the Fe(II) Fe(III) complex is -1, so to charge balance, it
encapsulates a positive cation, oftentimes K
+
or Na
+
,
depending on its starting material. The ability to encapsulate charges opened up a research field
of utilizing PB and its analogues for battery materials.
103
You can make PB with both the irons
in their +3 oxidized states, which in this case will result in a neutral complex that doesn’t have
Figure 5.1. A cartoon diagram of
the lattice structure of PB.
53
any cations to charge balance it and the solution becomes vibrantly green. Hence the name that’s
given to it, the Berlin Green. You can also go the other direction where you have two of the irons
in their most reduced state of +2, which then you get an overall -2 charge that is compensated
with two cations. In this case, the complex you form is white and called Prussian White.
PB was discovered in 1706 by accident by paint maker Johann Diesbach in Berlin,
Germany at the time who wanted to make a red pigment called Florentine Red. Florentine Red
was conventionally made with a mixture of cochineal, iron sulfate, and potash (which is a
potassium rich salt). He ran out of potash, so he went to his nearby friend who was working in a
lab at that time who was making animal oil that required addition of potash to animal blood.
However, as is true for many lab chemicals, the potash he borrowed was contaminated.
Specifically, with some potassium hexacyanoferrate, which is actually a starting material to
make PB. So ironically, what was initially expected to be a synthesized paint that was supposed
to be bright red, turned out to be vibrant blue and what is known today as Prussian Blue.
Apart from its interesting origin, PB has many significant applications. An example is
art- it is often used as a blue pigment for paintings. Some examples include “Under the Wave Off
Kanagawa” by Katsushika Hokusai and “The Starry Night” by Vincent Van Gogh. Medicinal
applications of PB include detection of toxic heavy metals, since its cage like structure can host
monovalent metallic cations, and staining iron deposits in bone marrow samples for bioimaging
purposes. Another application is the ability for PB and their analogs to host cations, as
mentioned earlier, making them suitable materials for reusable battery cathode materials. Lastly,
another recent finding is that cobalt PB analogue can be used as an electrocatalyst for water
oxidation.
5.2 Questions We Wanted to Answer
54
Although PB has been around for over 300 years, there are still many questions that
remain unsolved. One of the challenges with handling PB and their analogues is the variability in
the types of films you get. Depending on the synthesis procedure, you will alter the properties of
the material such as the oxidation states of the metal centers, the stoichiometry of different
metals in the material, and the lattice structure, which will all affect how it performs as
electrocatalysts. It is quite a tedious task to control and sustain the type of material you have, and
extensive characterizing methods are oftentimes required to analyze the type of film you have
each time you make it. It is therefore a nontrivial task to have consistent samples that can be
studied. However, with the very attractive properties that PB and its analogues have, it is still
imperative to study these materials to better utilize them in their plethora of applications. My
project on PB focused on a fundamental understanding of the intervalence charge transfer
between metal states. Specifically, I wanted to measure the lifetime of photoinduced charge
separated species in PB films. I wanted to know upon photoexcitation of metal to metal charge
transfer, how long do the excited electrons live for? Do we indeed have long-lived separated
charge entities? And if so, can we probe this with pump probe spectroscopy? These are important
because when you want to use a material to drive reactions such as the catalysis of water
oxidation, it is crucial for the charge separated species to exist for as long as possible because
once electron-hole recombination occurs, the reaction ceases. In the following sections, I will
describe the procedure I followed to synthesize PB films, the setup for the transient Visible-
pump IR-probe experiment, the results of the data, and conclusions followed by potential future
directions.
5.3 Experimental Methods
55
PB films were synthesized following a successive ionic layer adsorption and reaction
(SILAR) method. This is discussed and referenced further in section C.4, where I discuss the
method of using SILAR to deposit hematite on Ag substrates. In summary, the SILAR method is
thin film deposition procedure where the substrate is immersed in cation and anion starting
materials. By sequentially dipping, the film is slowly and uniformly grown layer by layer. The
substrates used were silicon wafers with a 10 nm Ti adhesion layer and 100 nm Au deposited on
top (LGA Thin Films, Inc.). Prior to deposition, the wafers were cleaned in an ultrasonication
bath in ethanol for 5 min, followed by ultrapure water for another 5 min. For PB film syntheses, I
combined the SILAR method with the co-precipitation reaction by Stefaans et al.
104
to make
films of PB and its analogues on gold wafer substrates. The solutions for the deposition were 100
mM of (K
3
[FeCl3(CN)
6
]) in water and 100 mM of FeCl
3
(or CoCl
2
) in water. The wafers were
sequentially dipped in three solutions for 1 min each in this series: a beaker of 100 mM of
(K
3
[FeCl3(CN)
6
]), a beaker of water for washing, and a beaker of 100 mM of MCl
3
. This was
done for a total of 15 rounds to ensure a thick layer of film was deposited on the gold layer. After
the 15
th
cycle, there should be a visibly clear change of color once the deposition has been
complete. A picture of the starting materials and the deposited films of various analogs of PB are
shown in Figure G.1. The FeFe PB film was dark blue while the CoFe PB film was dark brown.
The films were then left to dry in air for an hour before the pump probe experiment. At this
point, the films are labeled using the metals of their starting materials only. The oxidation states
of the metals are not included in their naming system because they are known to undergo charge
transfer and exists as mixed valence states, as shown in the FTIR data.
Prior to the pump probe experiment, FTIR spectra were taken of the films after they were
dried. This was done in the reflectance geometry, where the films were laid face down on top of
56
a metal mask that fits the VeeMAX III. The spectra were backgrounded with respect to the bare
gold before film deposition and averaged over 128 scans at a resolution of 4 cm
-1
. The VeeMAX
III was set at an incident angle ~ 65
°
.
Figure 5.1 shows a cartoon diagram of the experimental setup for the pump probe
experiment. The pump wavelengths used for this experiment was 800 nm and 400 nm. A 1 kHz
regeneratively amplified Ti:sapphire laser (Coherent) was used to generate the 800 nm pulse.
This was attenuated by a half-wave plate (HWP) and polarizer and redirected by gold covered
mirrors to reach the sample, where the films are placed for reflectance measurements. The 400
nm pump pulse was generated by frequency doubling to 800 nm with a BBO. The pump power
used was ~ 40 mW, corresponding to ~ 40 𝜇𝐽 of power. The samples were the two PB films
prepared via the procedure described above. One was a Fe(III)Fe(III) film and the other was a
Co(II)Fe(III) film. The IR source was a mid-infrared quantum cascade laser (MIRcat-QCL) from
DRS Daylight Solutions. One advantage of the QCL is that it provides high intensity IR laser
powers that far exceed the limit of the FTIR glowbar. The IR source from the QCL can also be
Figure 5.2. A cartoon schematic of the Visible pump IR probe experimental setup.
57
tuned to an extremely fine resolution of 0.1 cm
-1
. Each laser provides a bandwidth of ~ 200 cm
-1
and the one in the Dawlaty lab consists of four lasers that give us the ability to access
wavelengths from 1500 to 2300 cm
-1
. For this experiment, we focused on the nitrile frequency
region from 2200 to 2300 cm
-1
. The power of the IR was attenuated with a polarizer to match the
power of the pumps (~ 40 mW). We collected a data point every 1 cm
-1
. The IR signal was
detected with a nitrogen-cooled MCT detector (InfraRed Associates Inc. MCT-13-1.00) with an
operational amplifier (InfraRed Associates Inc. MCT-1000). The response time of the MCT
detector was measured and determined to be 1.6 𝜇𝑠, shown in the Figure G.2, dictating the limit
of detection. The MCT detector used in this experiment was a photoconductive (PC) detector
that can only detect AC signals or changes in the IR light. This is an important point to keep in
mind and will be discussed in the following section. The signal was measured with an
oscilloscope (Keysight Technology, 54855 A Infiniium Oscilloscope) that was averaged over
1000 scans and a lock-in amplifier SR830 (Stanford Research Systems). All the data was post
58
processed and plotted via MATLAB. LabView was used to interface the QCL, the lock-in, and
the oscilloscope.
5.4 Results and Discussion
Figure 5.2 shows the FTIR spectra of the two films that were prepared in Section 5.2. The
blue spectrum belongs to that of the CoFe film and has three distinctive peaks at 2091 cm
-1
, 2121
cm
-1
, and 2159 cm
-1
. The red spectrum is that of the FeFe film and shows three distinctive peaks
at 2077 cm
-1
, 2116 cm
-1
, and 2164 cm
-1
. These peaks correspond to the different cyano-group
stretching frequencies that have different neighboring metals and combinations of oxidation
states. As mentioned before, the composition of these PB films is highly variable, which makes it
Figure 5.3. The FTIR spectrum of (red) CoFe PB film and (blue) FeFe PB film in the
reflectance geometry.
59
a difficult task to accurately assign the stretching frequencies. I have observed that even
switching the substrates changes the vibrational features, as shown in Figure G.3.
Although difficult to assign, there are general trends and supporting experiments from the
literature that can provide hints on what these stretches are. It is known that the stretching
frequencies of the CN ligands of PB are governed by their neighboring environments, which
depend on the local electronegativity and the oxidation states of the metals. The CN ligand is
oriented in a way that the carbon binds to the iron while the nitrogen binds to the metal (M) from
the metal chloride. In a Fe-C≡N-M coordination, the carbon forms a bond with the iron through
𝜎 bond donation from its bonding 𝜎 orbital. In the other direction, the carbon accepts electrons
through its antibonding 𝜋 orbital. In general, anything that pulls electron density away from the
antibonding orbital will result in a higher frequency (blue) shift of CN stretching frequency while
anything that increases the electron density to the antibonding orbital will result in a lower
frequency (red) shift. This trend is manifested in the different forms of Prussian Blue, where
Berlin green (Fe
III
-C≡N-Fe
III
), which has the most electron withdrawing effects on the CN
antibonding orbital because the irons are in their most oxidized forms, displays CN frequencies
at 2090 cm
-1 105
while Prussian white (Fe
II
-C≡N-Fe
II
), which has the least electron withdrawing
effects, shows a red shifted CN frequency at 2067 cm
-1
.
106
This idea was also tested with a
spectroelectrochemical Raman experiment where I tracked the nitrile stretches of a CoFe film
while I applied different ranges of potential. At open circuit potential (OCP), the CoFe shows
two nitrile vibrations at ~ 2090 cm
-1
and ~ 2120 cm
-1
, and upon the oxidation of the film, a third
peak that is blue shifted appears ~ 2190 cm
-1
. The data is shown in Figure G.4. – Figure G.7.
With these in mind, I have made the following assignments for the following peaks of the data in
Figure 5.2. For the FeFe film: 2077 cm
-1
as the Fe
II
-C≡N-Fe
II
stretch, 2116 cm
-1
as the Fe
II
-C≡N-
60
Fe
III
stretch, and 2164 cm
-1
as the Fe
III
-C≡N-Fe
III
stretch. For the CoFe film: 2091 cm
-1
as the
Fe
II
-C≡N-Co
II
stretch, 2121 cm
-1
as the Fe
II
-C≡N-Co
III
stretch, and 2159 cm
-1
as the Fe
III
-C≡N-
Co
III
stretch.
Figure 5.4 shows the 800 nm pump probe data for FeFe film. The data is shown as a
contour plot, with the corresponding steady state FTIR from Figure 5.3 plotted next to the
transient data where the wavenumbers are matched. Prussian Blue is reported to have a bandgap
~ 1.1 – 2.1 eV,
107
which corresponds to 1127 nm – 590 nm. Figure G.8 shows the absorption
spectrum for the FeFe film used in this experiment, and an absorption 𝜆
-:;
~ 700 nm is
observed, with a tail that extends past 800 nm. Pumping the system at 800 nm, therefore, should
photoinduce charge transferred species that will change the local environments of the CN
Figure 5.4. The pump probe data for FeFe film pumped at 800 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same film
is plotted next to the data with matching wavenumbers accordingly.
61
ligands, and should manifest as changes in the CN nitrile frequencies that we probe
spectroscopically.
There are a few takeaways from this data. First, we will focus on the signs of the signals.
The dotted lines show wavenumbers where the signals flip signs, an indication of a blue/red shift
of peak, as described in detail in Appendix B. There are four locations where the signs of the
signal flips: 2065 cm
-1
, 2137 cm
-1
, 2158 cm
-1
, and 2172 cm
-1
. If we interpret the flip sign at 2137
cm
-1
as a frequency shifted peak, this signal could arise from the steady state Fe
II
-C≡N-Fe
III
peak
at 2116 cm
-1
that underwent charge transfer between the iron centers and became Fe
III
-C≡N-Fe
III
,
resulting in a blue shifted peak at that resembles more of that of the steady state Fe
III
-C≡N-Fe
III
stretch at 2164 cm
-1
. Or, it could be the Fe
III
-C≡N-Fe
III
stretch that underwent charge transfer,
where one of the Fe
III
accepted an electron from a neighboring unit and became Fe
II
-C≡N-Fe
III
,
resulting in a red shifted peak that resembles that of the steady state Fe
II
-C≡N-Fe
III
stretch at
2116 cm
-1
. The signal flip at 2065 cm
-1
could be interpreted as the Fe
II
-C≡N-Fe
III
that became
reduced to Fe
II
-C≡N-Fe
II
, resulting in a red shift. The other two locations where the signals flip
signs are more difficult to assign to a specific transition as they do not match cleanly to a
blue/red shift of the steady state peaks. However, we can conjecture a few hypotheses. For
example, the two signal flips at 2158 cm
-1
and 2172 cm
-1
could be a result of the nitrile frequency
of the Fe
III
-C≡N-Fe
III
experiencing different local environments. If the units adjacent to Fe
III
-
C≡N-Fe
III
become reduced, this effect could be coupled to the Fe
III
-C≡N-Fe
III
stretch and push
electron density towards the antibonding orbital of the CN ligand, resulting in a slight red shift.
On the other hand, if the environment becomes more oxidized, this could result in a blue shifted
CN stretch of Fe
III
-C≡N-Fe
III
. Another way to interpret the data would be to look at the signs of
the signals one at a time. Negative signals (more absorption) mean a growth of the species that
62
existed prior to photoexcitation and positive signals (less absorption) mean a decrease in
population. This is difficult to interpret because many of the signals are close in wavenumbers
and likely bleed into each other, so the magnitude and signs of the signals shouldn’t be treated as
the result of only one process. Additional experiments are required to fully understand the origin
of these signals such as X-ray absorption spectroscopy and X-ray photoelectron spectroscopy to
determine the stoichiometries of the metal oxidation states, spectroelectrochemistry to control the
oxidation processes to be probed spectroscopically, and time-resolved absorption spectroscopy to
probe the species that are changing with time as a function of photoexcitation.
Another takeaway from the pump probe data is the duration of the photoinduced signals.
Looking at the Figure 5.4, the longest transient signals last up until ~ 100 𝜇s. Figure 5.5 shows
some representative transient slices for three different wavenumbers: 2030 cm
-1
, 2120 cm
-1
, and
2220 cm
-1
. The x axis was scaled accordingly so time 0 matches the start of the signal. The figure
shows transient signals that flip signs in both directions- the origin of this signal flip sign in the
time domain is still unknown and to be further investigated. Many of the applications that utilize
PB and their analogues require the existence of their charge separated species, and therefore the
longer that these species live, the better. But how “long” is considered good enough? For
reference, water oxidation kinetics occurs in the milliseconds to seconds regime, so we need the
photoinduced species to live for at least that long if we want to use them to catalyze processes
63
like water oxidation. Although the transient signals of FeFe does not last that long, remember
that this is only with the excitation of light (the pump). Oftentimes, another driving force such as
external bias is used in addition to photoexcitation in order to separate the charges even further
and generate species that last long enough to drive reactions.
Figure 5.6 shows the 400 nm pump probe data for the FeFe film. As mentioned before,
800 nm and 400 nm lasers were used to pump the system. The intensities of the two pump lasers
were adjusted so their average powers matched (~ 40 mW) so any differences in the pump probe
data should be a result of the difference in the pump laser wavelengths and not laser power. An
interesting observation here is that even though each photon in the 400 nm pump has more
energy, there are fewer features that show up here compared to the 800 nm pumped system.
Figure 5.5. A few representative time slices for three different wavenumbers of
the FeFe sample pumped at 800 nm.
64
There are two overlapping locations where the signals flip signs: 2065 cm
-1
and 2137 cm
-1
. This
suggests that while the 400 nm pump induces some similar changes to the FeFe films as those of
800 nm, it cannot access as many electronic manifolds as the 800 nm pump can. Perhaps there
exists some trap or defect states that the 800 nm pump can directly excite the electrons to while
the 400 nm pump cannot. Figure 5.7 shows three representative time traces at the same
wavenumbers (2030 cm
-1
, 2120 cm
-1
, and 2220 cm
-1
) as those shown for the 800 nm pump.
Qualitatively, the time traces at these wavenumbers resemble those pumped at 800 nm. However,
if you zoom in carefully, you will see that the signals pumped at 400 nm decay faster than those
pumped at 800 nm. The longest transient signal from the contour plot is in the order of ~ 50 𝜇s,
almost half the time as the longest signal pumped at 800 nm. Again, this perhaps seems
Figure 5.6. The pump probe data for FeFe film pumped at 400 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same film
is plotted next to the data with matching wavenumbers accordingly.
65
counterintuitive since the 400 nm pump laser has more energy per photon, and one might think
the 400 nm could access more electronic manifolds. This is interesting and shows that the
dynamics of these transient signals are pump wavelength dependent.
We now look at the pump probe data for CoFe film. The absorption spectrum of this film
in Figure G.8 shows two peaks with 𝜆
-:;
around 400 nm and 550 nm that has a tail that extends
past 800 nm. There have also been hybrid density functional theory (DFT) calculations that have
estimated the density of states for different energy levels of the components of CoFe film,
108
where they calculated the band gap of CoFe-PB to be ~ 2 eV (~ 620 nm) corresponding to a
transition from Fe (t
2g
) to Co (e
g
) state. With these in mind, we should anticipate both the 400 nm
Figure 5.7. A few representative time slices for three different wavenumbers
of the FeFe sample pumped at 400 nm.
66
and 800 nm pump to be capable of inducing charge transfer species that can be probed with the
CN ligand nitrile IR frequencies. Figure 5.8 and Figure 5.9 show the pump probe data for the 800
nm pump and representative time traces for four wavenumbers (2030 cm
-1
, 2100 cm
-1
, 2130 cm
-1
,
and 2220 cm
-1
). More time traces were picked because there are overall less features in the CoFe
pump probe data, so I incorporated an additional time trace for better comparison between the
400 nm and 800 nm pump data. There is only one location where the signal flips signs, and this
occurs at 2107 cm
-1
. If interpreted as a differential signal from a vibrational peak that was blue
shifted, this signal could originate from the stretch Fe
II
-C≡N-Co
II
at 2091 cm
-1
that underwent
charge transfer and became Fe
II
-C≡N-Co
III
, resulting in a higher frequency shift. Or, it could be
Fe
III
-C≡N-Co
III
that reduced to Fe
II
-C≡N-Co
III
and underwent a red shift. The negative signals
from 2057 cm
-1
to 2105 cm
-1
dissipates within 10 𝜇s while the positive signals from 2109 cm
-1
to
Figure 5.8. The pump probe data for CoFe film pumped at 800 nm. The color bar shows
the signs and intensities of the signals. The steady state FTIR spectrum for the same film
is plotted next to the data with matching wavenumbers accordingly.
67
2159 cm
-1
takes almost 80 𝜇s to return to its original state. This suggests that even if the location
of the signal sign flip is a result of a blue/red shifted CN stretch, there are additional lingering
effects that haven’t dissipated yet in the regions from 2109 cm
-1
to 2159 cm
-1
. The negative
signal lives for another additional 60 𝜇s, hinting the continuous growth of a species. Another
observation to note is that there are overall less transient signals compared to the FeFe film. This
suggests that there are fewer electronic states that the 800 nm pump can access in the CoFe films
compared to the FeFe system.
Figure 5.10 and Figure 5.11 show the transient signals of the CoFe film pumped at 400
nm and representative time traces at the same wavenumbers as those shown in Figure 5.9. From
the representative time traces, it is clear that the transient signals are not the same at certain
Figure 5.9. A few representative time slices for four different wavenumbers of the
CoFe sample pumped at 800 nm.
68
wavelengths and are pump wavelength dependent. For example, the CoFe film showed a
transient response at 2030 cm
-1
when it was photoexcited with 400 nm, but no response with the
800 nm pump. On the other hand, the CoFe sample did not show any transient responses at
wavelengths 2130 cm
-1
or 2220 cm
-1
when it was pumped with 400 nm but did when it was
pumped with 800 nm. Like the FeFe data, this also hints that different pump wavelengths can
access different electronic manifolds in the CoFe system.
There are overall less transient features compared to the 800 nm pumped CoFe, and the
signals decay faster. A very interesting result is that there is also one location where the transient
signal signs flip, at 2107 cm
-1
that matches the results from the 800 nm pumped system. And this
sign flip lives for about the same amount of time as the other system, ~ 20 𝜇s. However, all the
Figure 5.10. The pump probe data for CoFe film pumped at 400 nm. The color bar shows the
signs and intensities of the signals. The steady state FTIR spectrum for the same film is
plotted next to the data with matching wavenumbers accordingly.
69
signals die after this point. It does not show the negative feature that grows and lingers on. This
suggests while both the 400 nm and 800 nm pump can generate the same species upon
excitation, the photoinduced species might relax through different pathways, resulting in the
differences in the longer time scale transient responses.
Alternatively, we could completely scrap out the idea of charge transferred species and
assign the pump probe signals to lattice distortions that arise from heating the films. With this
hypothesis, the photoinduced species have already recombined, and what is left are the remnants
of thermal effects from the pump. The film lattice structures could expand and/or contract,
resulting in elongated CN bonds that would result in a red shift or shortened CN bonds that
would result in a blue shift. The data probably consists of frequency shifted peaks, changes in
populations, and lattice distortions. To test this theory, I prepared another CoFe film sample and
Figure 5.11. A few representative time slices for four different wavenumbers of the
CoFe sample pumped at 800 nm.
70
performed a temperature study. I repeated the pump probe experiment with the film at 800 nm
and 400 nm pump lasers. The contour plot for this set of data and a few representative transients
at a few wavenumbers are shown in the Figure G.9 and G.10. Additionally, I heated the film at
various temperatures from room temperature to 140 ℃ and took the spectrum of the film with the
lock-in to compare that with the spectrum of the pump probe experiment. If the two spectra
match, this would show that the transient signals from the pump probe data are indeed
temperature effects and not charge separated entities. Figure 5.12 shows the spectra of the CoFe
film heated at different temperatures. The left panel shows the raw data read from the lock in as a
function of various temperatures and the right panel shows the differential spectra of each heated
Figure 5.12. The spectra of CoFe film as a function of heating at different
temperatures. The left panel shows the raw voltage data read from the lock-in and the
right panel shows differential spectra subtracted from the room temperature spectrum.
71
spectra subtracted by the room temperature spectrum. This is for easier comparison with the
pump probe spectrum that shows pump induced differential spectra. From the differential
spectra, you can see that all the signals are negative, meaning that heating the film increased the
absorption in this region. The negative signal grows more negative as the temperature is
increased until it hits a saturation point at 140 ℃ where the differential signal doesn’t change
much anymore upon further heating.
Figure 5.13 shows a differential spectrum at a time slice t = 25 𝜇s of the CoFe film
pumped at both 800 nm (red) and 400 nm (blue). Another set of data was collected at another
spot on the same CoFe film to ensure the spectra consistently matched. This is shown in the
Figure G.11. The first observation is that not all the signals are negative, as is the case in the
heated sample. In this pump probe data, the signal sign also flips at 2107 cm
-1
, consistent with
Figure 5.13. The differential spectrum of CoFe film at t = 25 𝜇s pumped at 800
nm (red) and 400 nm (blue).
72
the previous pump probe data on CoFe. Although the origin of the transient signals is still
ambiguous, this experiment hints that they do not originate from thermal lattice expansion and
indeed are from photoinduced charge separated species. This is important because charge
separated entities are required to drive reactions like water splitting and electrocatalysis.
5.5 Conclusions and Future Direction
This work, as far as I know, was the first study that probed the transients of FeFe and
CoFe films prepared in the specific conditions described in this Chapter. Although a lot of
questions remain, it opened up many avenues of study that can be further expanded upon. I
showed that CoFe and FeFe films can be deposited on Au substrates via a SILAR method and
that upon photoexcitation at 800 nm and 400 nm, they show transient signals at different
wavelengths in the nitrile region that respond differently. There are hints that show these
transient signals belong to long-lived charge separated entities as the differential pump probe
signals didn’t match those of heated film spectra. Spectroelectrochemical Raman data also
provides some insight on the species that are formed upon photoexcitation. Further experiments
and analysis are required to fully elucidate the unknowns, but nevertheless, it is a very interesting
study in my opinion. I hope that data provided in this Chapter will aid anyone who wants to
study this system!
73
Chapter 6
Conclusions and Future Work
In conclusion, I took part in a series of projects that involved probing chemical species at
the surface and in the bulk during charge transfer using spectroelectrochemical techniques. I
found a technique to measure the enhancement factor of SEIRAS using a monolayer system and
Beer’s Law analysis. The SEIRAS enhancement factor for the symmetric and asymmetric CH
2
stretches were reported to be in excess of 1,000 at the surface. I also extended this study and
measured the SEIRAS penetration depth of a Prussian Blue film by electrodepositing the film a
layer at a time while tracking its spectroscopic features. Next, I used SEIRAS and vibrational
Stark Shift spectroscopy to measure the interfacial electric field as a function of charge transfer
by incorporating a mixed monolayer system of 4-MBN and surface-tethered ferrocene. To our
surprise, interfacial charge transfer only perturbs the local electric field mildly, sensed by our 4-
MBN molecular Stark probe. Following the analysis of interfacial one electron transfer, I studied
a two electron two proton transfer of benzoquinone/hydroquinone by probing the molecule’s
PCET reaction while undergoing charge transfer utilized in batteries. I was able to probe the
transformation of benzoquinone and hydroquinone (and vice versa) upon PCET using Raman
spectroscopy. Lastly, I extended these studies to probe intervalence charge transfer between
metal centers of a Prussian Blue film. Specifically, I measured the lifetime of photoinduced
charge separated species of 400 nm and 800 nm pumped CoFe and FeFe films. I showed
evidence that photoinduced transient signals are charge separated entities that are pump
wavelength dependent.
Over the course of my PhD, I learned several lessons, but one of the most important ones
was that “molecules don’t care about you” (quote from Jahan). Before entering graduate school, I
74
had a very specific, textbook way of thinking about scientific problems. I expected all my
experiments to work perfectly if I could plan, draw, and solve everything on pen and paper
beforehand. I quickly learned that this was not the correct and reasonable way to approach
scientific problems. In the real world, molecules will do whatever they do, and it is our job as
researchers to figure out the truth. Although frustrating, this became one of the most humbling
and valuable lessons I learned. I learned to be patient with myself and others and to always be
willing to pivot directions when experiments didn’t work out. I learned the benefits of
collaborative work that forced me to think outside the box. I learned to be excited when
experimental results turned out to be different than what I initially expected because it meant the
molecules are trying to tell me a story and I now have a puzzle to solve. And I wouldn’t be able
to learn any of this if it wasn’t for the constant, positive guidance and encouragement from Jahan
and my fellow lab members.
Moving forward, I have accepted a postdoctoral position at Imperial College and will be
joint mentored by Professor James Durrant and Professor Ifan Stephens. I will be utilizing the
SEIRAS technique to characterize metal oxide electrocatalysts at electrode surfaces operando. I
am very excited to continue my research journey and learn along other experts of this field.
75
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87
Appendix A
Further Explanation of SEIRAS
A.1 What is Surface Plasmons?
So what are surface plasmons? During my PhD, I would hear people say “plasmon” this
“plasmon” that, but it was always jargon to me, and for the longest time, the concept of surface
plasmons was a mystery to me. Until one day, Jahan and I decided to get to the bottom of it and
with pen and paper, understand surface plasmons from the grounds up. In this section, I provide
a tutorial on surface plasmons, and hope that this will be useful for people who want to
understand this concept.
To understand surface plasmons, we will first start with the definition of plasma. Plasma
is defined as a state of matter that consists of ionized particles. One can think of it as a mobile
“soup” of charged materials. Plasma oscillations is rapid oscillations of electrons in a conductive
material. Plasmons are a collection of plasma oscillations. And lastly, surface plasmons are
plasmons that are bound at an interface. They are electromagnetic radiations that exist at
interfaces between a dielectric and a metal. To understand surface plasmons and know the
88
conditions in which they will be launched, we must break the problem down into small,
digestible pieces. Figure A.1 shows the steps we must take to do so.
We start with the idea of traveling waves and recall that all electromagnetic waves
traveling in one direction can be expressed by the equation
<
!
2(;,?)
<;
!
=
#
A
!
<
!
2(;,?)
5 mO.D. in signal-to-noise ratio,
then the SEIRAS activity of the substrate is good enough to detect monolayer absorption signals.
Detailed instructions on how to set up an ATR-SEIRAS experiment and typical calibration
spectra are discussed in the following section.
A.3 Setting Up an Electrochemical ATR-SEIRAS Experiment
In this section, I will describe the general guidelines to setting up and collecting data for a
spectroelectrochemical experiments using ATR-SEIRAS. As described in Section A.2, once gold
is deposited on the ATR prisms (i.e. Si and ZnSe crystals), a resistance test is performed to
ensure the substrate is conductive enough for electrochemical measurements. Following the
electrochemical test, we perform a SEIRAS test to ensure the spectroscopic signal is enhanced
enough that can be detected over the noise of the spectra. From looking at the numerous SEIRAS
spectra and the average noise of the FTIR spectra, we have set the calibration of a sample
absorption spectra that is larger than 5 mO.D. to be good enough. This is done by taking an
absorption spectrum of ethanol backgrounded to the spectrum of gold on ZnSe in air, where A
=−𝑙𝑜𝑔
#0
X:-.7T X.TG?S6-
5:G8YSZ6H& X.TG?S6-
. Figure A.12 shows a typical spectrum of ethanol on gold
deposited on ZnSe crystal that has been backgrounded by the same substrate in air. The ethanol
peaks shown from 2850 cm
-1
to 3000 cm
-1
are the CH
2
stretches of ethanol. The red bar shows an
105
absorption depth of 5 mO.D. for
calibration. If the sample vibrational
peaks are larger than this, we proceed
forward with the experiment. If the
signal is lower than the bar, the gold
should be polished off from the
crystals and the gold should be
redeposited as described in Section
A.2. At this point, the substrate can be
further treated if necessary. For
example, if a monolayer is desired,
the crystal should be soaked in the appropriate solution.
Figure A.12. A SEIRAS spectrum of ethanol
and the calibration bar of 5 mO.D.
106
Once the substrate is ready, it can be inserted in the FTIR. A VeeMAX III (Pike
Technologies) accessory is used as shown in Figure A.13. The accessory is compatible with the
FTIR in the Dawlaty lab and can be inserted in the sample chamber of the FTIR so the IR beam
path can be directed to the crystal, which fits in a Teflon electrochemical holder as shown in
Figure A.13. The Teflon cup can be attached to the holder plate via four screws that provides a
seal for electrolyte that can be poured into for electrochemical measurements. A leak test is
performed before inserting the Teflon cell into the VeeMAX III accessory so there is no damage
to the optical mirrors in the
VeeMAX III. This is done by
adding ethanol or water into the cell
and checking for leaks from the
bottom of the Teflon plate once the
cell is screwed onto the plate. Once
the leak test is passed, the Teflon
accessory can be placed on top of
the VeeMAX III. Although the
Teflon cell is symmetrical, there is a
direction to the way it sits on top of
the VeeMAX III. If you look at the
bottom of the Teflon plate, you should see two holes on one side of the plate. These holes should
match two protrusions on the VeeMAX III if you look at it from the top. If incorrectly placed,
the cell will sit at an angle, and you will not get a working interference pattern from the IR beam
(this will manifest in the OMNIC software if you try to take a spectrum). There is a lid that can
Figure A.13. A picture of the VeeMAX III accessory
inserted into our FTIR sample chamber, with the
Teflon electrochemical cell accessory attached on
top.
107
be attached on the top of the Teflon cup. I’ve drilled three holes where the working, counter, and
reference electrode can be inserted for a three-electrode cell system, as shown in a cartoon in
Figure A.14. The counter and reference electrodes should be suspended over the crystal in
solution while the working electrode should be in good contact with the gold deposited ATR
crystal for continuous conductivity. This is usually done with a gold rod that I insert in the
smallest hole on the Teflon lid and push from the top so it touches the gold deposited on the
ATR prism. To ensure good contact from the rod to
the prism, I perform a conductivity test where I
complete the circuit by touching another part of the
crystal with a long metal piece (i.e. a spatula) and
connect the metal piece to one end of a voltmeter
and the gold rod to the other end. I then either
check the resistance across this circuit (which
should be nominally 0) or do a beep test, where the
voltmeter will beep if the path is conductive. Once
this passes, the three electrodes can be connected to
a potentiostat to apply external bias to the system. At this point, you should be careful not to
move the gold rod too much or you might lose connection with the crystal. You can do another
conductivity test once you hook everything up to the potentiostat or trust that the rod is still in
place. I will warn that it is worth taking the time to make sure all these tests pass, and that the
setup is sturdy before applying external bias, because if the circuit is shorted or electrodes not
hooked up properly, the potentiostat will show an “overload” error, which oftentimes strips the
deposited gold from the prism and you have to start all over again! If you want to purge the
Figure A.14. A cartoon representation
of the three electrode system setup in
the ATR geometry.
108
chamber, we have also asked the machine shop to make a plexiglass cover that seals the chamber
and has inserts where you can hook them up to things like nitrogen gas or the lab’s purge gas
generator (which is free of hydrocarbons, CO
2
, and water vapor). Additional Teflon
electrochemical cells can also be replicated in the machine shop.
At this point, you are ready to collect data! There are two different detectors in the FTIR:
the deuterated triglycine sulfate (DTGS) detector and mercury cadmium telluride (MCT)
detector that is liquid nitrogen cooled. A DTGS is a crystal that is sensitive for room temperature
detection of mid-infrared measurements. In a DTGS detector, a change in temperature from the
IR radiation will cause a dielectric constant change in the crystal. This will change the
capacitance that is picked up by the detector and is converted to voltage so it can be digitized and
Fourier transformed. The MCT detector is a semiconductor that upon IR radiation, the electrons
in the material will be excited from the valence band to the conduction band. These electrons that
have been excited to the conduction band will generate an electrical current that can be picked up
and then converted to a voltage response, and then Fourier transformed. A DTGS detector
operates at room temperature and is more convenient to use, but it is slower in response and its
sensitivity. The MCT detector, on the other hand, is more sensitive and digitizes faster than the
DTGS detector. It is more sensitive because its smaller bandgap allows for the detection of
smaller signals. However, because of this, even room temperature thermal fluctuations will
excite some electrons to the conduction band and result in a noisy signal. Because of this, the
MCT detector always needs to be cooled with liquid nitrogen if used. For experiments involving
monolayers, the MCT detector is always advised because the signals are too small to be picked
up with the DTGS detector. You can switch between the detectors under the “Bench” tab in the
“Experimental Setup” option in OMNIC.
109
Figure A.15 shows a screenshot of the typical parameters that are set on the OMNIC
software for collecting a spectrum. The box in red is the number of scans that you want to
average over, and above it is the estimated time it will take for those number of scans. I typically
average over 128 scans, which takes about 1 minute for the MCT detector. The green box is the
resolution of your spectrum. The resolution of the FTIR will dictate how far the moving mirror
in the Michelson interferometer moves. The higher the resolution, the farther the mirror moves,
and the longer it will take to collect a spectrum. You always want to set the resolution smaller
than the smallest change you are trying to pick up. For example, if you are trying to look at Stark
shifts in the order of 1 cm
-1
, you should set the resolution to at least be 1 cm
-1
or smaller. The
black box is the format that you want OMNNIC to save and present your data as on the computer
screen. For example, you can set it as “Transmission” and the instrument will plot and save the
data as % Transmission as the y axis. I almost always prefer “SingleBeam,” which is the raw
detector response that is Fourier transformed versus wavenumber. This allows me to manipulate
and post process the data however I want. Keep in mind that with the raw data at hand, you can
110
plot any data in all the other formats such as % Absorbance or % Transmission. The blue box is
the automatic atmospheric suppression, which is the OMNIC software’s internal method of
attempting to suppress the effects of atmospheric water vapor and carbon dioxide. It is generally
advised not to use this feature, and instead suppress the atmospheric conditions by taking a
background spectrum and subtracting or dividing it off. However, it can be useful if your signal
is not in the regions of water vapor or carbon dioxide, and they are a nuisance to look at or if the
atmospheric conditions are constantly changing. The purple box is the tab that controls the
background spectrum. If the first two options are clicked, the software will ask you to take a
Figure A.15. The typical parameters that are set for collecting a SEIRAS spectrum.
111
background spectrum before/after every sample spectrum. For a typical spectroelectrochemical
experiment, you only need one background spectrum so I will either take it and save it, and then
click the last option where I can specify a file to use as background or click the third option
where you manually tell the software to only ask you to take a new background spectrum after
every N number of spectra. It is by default set to 1000 minutes, which is typically much longer
than a full day’s of experiment, so you essentially only need to take the background spectrum
once. Lastly, I advise to always have the autosave option clicked so even if you accidentally
delete a spectrum, it will always be saved in the OMNIC Autosave folder.
112
Appendix B
Vis Pump IR Probe Setup: Reading Signals with a PC Detector
The MCT detector used in the Visible pump IR probe setup is a photoconductive (PC)
detector, which is usually made of semiconductors such as silicon or mercury cadmium telluride
in this case. Incoming light that hits the detector produces excited electrons that are promoted
from the valence band to the conduction band and generates a current that changes the electrical
conductivity of the material. The signal is digitized to a voltage signal that can be read by other
electronics. An operational amplifier that is connected to the detector amplifies the signal and is
connected to an oscilloscope to measure the kinetics of the signal. A PC detector can only sense
alternating currents (AC), or signals that are alternating or changing. Therefore, if you have a
light source that is continuous wave (CW) such as the IR probe light in the experiment described
in Chapter 5 without any perturbation (before the pump is introduced), the detector will not be
able to pick up any signal and the oscilloscope will show a flat line, as seen in Figure B.1. If you
were to suddenly block the IR light with a card or hand, the oscilloscope will be able to pick up a
signal because now there is a change in the light that is hitting the detector. There will only be a
signal at the moment you induce a change (i.e. block the IR light, photoinduced changes). A
positive sign in the oscilloscope indicates more light while a negative sign indicates less light.
113
If you want to study the photoinduced transient signals of a sample, you can place the
sample in front of the detector as shown in Figure B.1. The CW IR light is still going through the
sample to the detector, but now you introduce a laser that will pump the sample. The pump is a
laser source pulsing at a certain frequency. A copy of this source is hooked up to the
oscilloscope’s trigger, so the oscilloscope will pick up any changes that are occurring at the same
Figure B.1. A representative diagram of the signal picked up by the oscilloscope if the
MCT detector sees a CW IR light source and if there is a change in the light.
Figure B.2. A representative diagram of the signals that can be picked up by the
oscilloscope if there is transient response to a pulsed pump source.
114
frequency as the pump laser frequency. Now, the idea is not dissimilar to the concept described
above. The PC detector will still only pick up any AC signals, so there needs to be a change to
the IR light for detection. And because you hooked up a copy of the pump to the oscilloscope,
the only signals that you will see are the changes that occur at the same frequency as that of the
pump. If a signal is picked up by the oscilloscope, it means that the pump induced a change to
the IR probe light, and either introduced more light (positive signal) to the detector or less light
(negative signal). The time trace shows how the signal is changing with time and provides
further information on the decay rate of the signal. Figure B.2 shows a cartoon representative
plot of a transient response from the oscilloscope, in this case a negative signal. The IR probe
source is a QCL, which emits one wavelength at a time. Therefore, if you want to collect the
transient responses for a range of wavelengths, you must command the QCL emit over a range of
wavenumbers. It needs to park at one wavelength, give the oscilloscope (and lock in if you want
to read from the lock in as well) enough
time to average scans, save the data, and
then move to the next wavelength. This is
all interfaced with LabView and can be
found in the QCL laptop in the MIRcat
folder called “LabView programs.”
Figure B.3 shows a representative data set
for the photothermal modulation
transients of a silicon optical cavity. This
was taken from the SI of the paper Anuj
Pennathur and I wrote called “Short Pulse
Figure B.3. A representative set of transient
data of a silicon optical cavity that was pumped
with 17 𝜇𝐽 of energy from an 800 nm pulsed
laser and probed with CW IR light from a
QCL.
115
Photothermal Modulation of Silicon Optical Cavities Measured by Continuous Mid-IR Probe.”
More information can be found in the paper if there is interest.
113
If you want the photoinduced spectrum (wavelength vs intensity instead of time vs
intensity), there are two ways to obtain this data. One method would be to collapse the transient
data from the oscilloscope in the section above. Adding all the y axis intensities for each
wavenumber will result in a spectrum. The second method would be to connect the detector to a
lock-in amplifier. The lock-in picks up and amplifies the average signal detected from the
detector. Instead of providing time information, the lock-in reads the average intensity of any
signal that is responding at the frequency of the pump. The lock-in does this for each IR
wavelength, and you slowly build the spectrum from this, one wavelength at a time. Again,
because the detector is photoconductive and can only pick up changes, the resulting spectra is a
differential spectrum and shouldn’t be confused with the raw spectrum. Figure B.4 shows the
possible differential spectra that can be observed. The solid line represents the absorption
Figure B.4. A representative chart of all the different types of differential signals that can
be picked up from the lock-in.
116
spectrum before the sample sees the pump, and the dotted line represents the absorption
spectrum after the sample is pumped. The plot after the red arrow indicates what the resulting
differential spectra would look like once it is read from the lock-in. For example, in scenario 1,
the pump induces an increase in absorption, which means more light is absorbed by the sample,
and less light is delivered to the detector. Therefore, the change that the detector picks up is an
overall decrease in photons, resulting in an average negative signal picked up by the lock-in. The
opposite would hold true, and result in a positive signal (scenario 2). In the case of a red or blue
shifted peak (scenario 3 and 4), some parts of the spectrum experiences more light while less
parts experience more, and the those sections of the spectrum will result in a negative and
positive signal, respectively. Note that if there are no photoinduced changes, the resulting
differential spectrum would be a flat line hovering 0. These are all important notes to keep in
mind when analyzing differential spectra. Chapter 5 shows real data that was collected with this
experimental setup.
117
Appendix C
Stark Shift Spectroscopy on Functionalized Metal Oxides
C.1 Motivation
Apart from surface-tethered monolayers and PB, another material I was interested in
studying was metal oxides. This was partially motivated by previous metal oxide work that a
former lab member, Dr. Shima Haghighat, had studied. Shima was particularly interested in
hematite films and characterizing the kinetics of charge transfer.
114, 115
Additionally, I started a
collaboration with Professor James Durrant’s group at Imperial College that was interested in
characterizing the surface electric field at the metal oxide-electrolyte interface. To achieve the
latter, I wanted to attach a nitrile Stark probe onto the surface of hematite films and probe the
nitrile frequency in-situ while the film oxidized water. The task turned out to be much more
complex and difficult than anticipated, and while I didn’t complete the project, there were a few
milestones that were overcome. Here, I will discuss the achievements we made and future
directions if anyone wants to continue this project.
C.2 Introduction
Water oxidation is an essential chemical reaction that is central to the production of
energy carriers such as hydrogen. With the high demand for more efficient and low-cost
technologies to generate green, renewable fuels, there is a constant need to expand fundamental
research in this field. Specifically, there is a large push for better and cheaper ways to convert
sunlight into chemical fuels. One example of this is using semiconductors to harness energy from
the sunlight to split water and produce hydrogen as green fuel. Metal oxides have been shown to
118
exhibit state-of-the-art performance for catalyzing photoelectrochemical oxygen evolution
reaction during the splitting of water. Amongst all the metal oxides, hematite is one of the most
promising semiconductor materials as a catalyst to drive water oxidation for several reasons.
One, iron is one of Earth’s most abundant elements, so there is a plethora of resources available
and therefore synthesis of hematite is very cost efficient. Second, hematite is environmentally
benign and
chemically stable. If
you leave a hematite
film in the lab and
come back after 100
years, it will still be
the same performing
and stable film as it
was the day you left
it. Lastly, hematite
has a bandgap of
around 2-2.2 eV, which is around 550 – 600 nm, that allows it to absorb a good portion of the
sunlight and use that energy to split charges and oxidize water. As shown in Figure C.1,
continuous wave illumination of light (400 nm) on hematite generates charges that can oxidize
water more easily, seen by the earlier onset of current (dashed line). The nanostructured iron
oxide samples were prepared by our collaborators Kay et al.
116
and made by atmospheric
pressure chemical vapor deposition (APCVD). In a nutshell, metal oxides have oxidizing power
to split water because upon photoexcitation, electrons in the valence band get excited and
Figure C.1 The linear sweep voltammogram of hematite film for non-
illuminated (solid)and illuminated sample (dashed).
119
promoted to the valence band, resulting in charge separated electron-hole pairs. The “holes,” or
empty orbitals, in the conduction band now have oxidizing power, or “suction power” to grab
electrons that are closest to them in energy. At an interface between a metal oxide film and water
electrolyte, the water orbitals are often lower in energy than the orbitals of the electrons (that are
now holes) that had been photoexcited to the conduction band, which will allow electrons from
water orbitals to transfer to the hematite orbitals. Of course, this is a simplified picture, and the
reality is that there are more complications to this process. One barrier is that there is constant
competition between water oxidation and the electron hole recombination from the electrons in
the hematite. In fact, with sunlight as the only source of excitation, most of the excited electrons
will actually recombine with the holes, resulting in minimal to no water oxidation. The trick to
overcoming this is to apply an external potential bias that will drive the electron-hole pair farther
from each other, accumulating holes in the valence band at the interface, which will give them
more time to favor water oxidation. Electron-hole recombination is just one of many challenges.
Because the splitting of water involves multielectron and multiholes, the mechanism of the
reaction is rather intricate, ambiguous, and dependent on many parameters such as the film
synthesis procedure and local environments.
117, 118
A major question that remains unanswered is
the origin of the rate law analysis of water oxidation. As will be discussed soon, there is evidence
of high order (orders of ~ 3+) dependence of water oxidation on surface hole density of hematite
films, and the explanation for this remains ambiguous. During my visiting time at the Durrant
group at Imperial College in the Fall of 2021, I repeated some of the rate law analysis
experiments on hematite films made from our collaborators and attempted to elucidate some of
the questions regarding the interfacial electric field via Stark Shift spectroscopy. I will show the
data I collected at Imperial and the attempts I made to attach a Stark probe on the samples.
120
C.3 Rate Law Analysis on Hematite Films
The hematite samples, as mentioned earlier, were cauliflower hematite films made from
our collaborators. The first step of the rate law characterization of these films was to take the
absorption spectrum. The absorption spectrum is shown in Figure C.2. Two dashed lines mark
the theoretical hematite bandgap (~ 560 nm)
and the wavelength of light used as the pump
for the transient absorption experiment, which
will be discussed next.
Transient absorption (TA) experiments on
these samples were performed to understand
how the absorption spectrum changed as a
function of time as well as to identify different
charge induced species. Figure C.3 shows the
experimental setup for the TA. The pump was a 355 nm source from the third harmonic of an
Figure C.2 The absorption spectrum of
hematite films prepared by our
collaborators.
Figure C.3 The experimental setup for the TA of hematite films.
121
Nd:YAG laser at a repetition rate of 0.33 Hz (6 ns pulse width). The pump wavelength was
chosen so that the energy is well above the bandgap of hematite, so we could ensure all, if not
most, of the electrons in the valence band were excited to the conduction band. The probe used
was a tungsten lamp that had a range of wavelengths available in the visible range (550 nm – 900
nm), and a monochromator was placed right after to pick out specific wavelengths to probe. The
pump remains the 355 nm laser source while the monochromator parks at one wavelength of the
probe at a time to build a full spectrum covering the range you are interested in probing. At each
probe wavelength, a transient decay of the signal was collected. The hematite film is assembled
in a three-electrode cell, where the working electrode is the hematite film on ITO, the counter
electrode is a platinum gauze mesh, and the reference electrode is a Ag/AgCl/0.3 M NaCl
solution. The electrolyte solution in the cell was maintained at 0.1 M NaOH. All TA signals were
collected by a Si PIN photodiode (Hamamatsu), which was connected to an oscilloscope and
DAQ card to digitize the signal at various time scales, the oscilloscope providing shorter times
(µs to ms) resolution and the
DAQ card providing longer
(ms to seconds) time signals.
Figure C.4 shows the
signs of all the possible
features that can be observed
from TA. Excited state
absorption (ESA) is when the
probe interacts with electrons
that have been excited and promoted to the conduction band and further excites them to a higher
Figure C.4 A concept figure of the energy diagram of
the TA experiment and the signs of the TA signals for
all the different possible processes that can be probed.
122
energy level. Hole absorption (HA) is when the probe interacts with empty orbitals in the valence
band and excites them to the conduction band. Both processes will result in more absorption, and
therefore the TA signal plotted as a change in absorption, ∆𝐴, will be positive. Ground state
bleach (GSB) corresponds to the reduced absorption due to photogenerated holes from the pump
and stimulated emission (SE) comes from the downward transition of electrons from the
conduction to the valence band. Both GSB and SE therefore result in a negative TA signal. It has
been previously shown in other references the occurrence of these processes
118
, but for the
purpose of this project, I will only focus on one TA feature, which is the feature corresponding to
HA of hematite. Figure C.5 shows the TA data represented in two forms. The left panel shows
the spectrum of the TA as a function of different time responses. There are both negative and
positive signals, but we will focus only on the positive feature from ~ 650 nm to ~ 750 nm. This
positive feature has been previously assigned to photogenerated hole absorption at 700 nm where
it was shown that this signal died in the presence of methanol as hole scavengers, and its lifetime
Figure C.5. The TA data shown as a function of (left) wavelength and time response
held at 1.5 V vs RHE and (right) time and external bias applied at probe 𝜆 =
700 𝑛𝑚.
123
decreased by fivefold.
119
120
Knowing this, we can observe the dynamics of the photogenerated
hole absorption if we plot the transient signal at 700 nm (Figure C.5 (right)). This was also
repeated as a function of varying external bias, from 0.5 V vs RHE to 1.5 V vs RHE, labeled as
different colors. There seems to be two phases detected from this plot. The signal up to 10 ms is
often called the “fast phase” and is assigned to a fast electron-hole recombination process.
119
The
signal after 10 ms is called the “slow phase” and is assigned to longer living holes. Both signals
increase in lifetime and amplitude as the applied bias is increased, most likely due to a larger
drive that separates the holes and excited electrons, and therefore increasing the lifetime of the
holes.
With the spectral assignment of photogenerated holes by the optical excitation of a
hematite photoanode, we then transitioned to quantifying the accumulation of these holes during
the splitting of water. This was done by performing photoinduced absorption (PIA) spectroscopy
experiments on the hematite films in longer time scales. We tracked the changes of the signal of
photogenerated holes assigned at 700 nm as a function of pumping the system with a 5 s on/off
(total of 10 seconds) light pulse from a 365 nm LED light source while holding an external bias
Figure C.6. The experimental setup for PIA.
124
of 1.5V vs RHE on the hematite working electrode. A large bias was used to ensure no electron-
hole recombination occurred, and that the only PIA signals we observed were due to the transfer
of the photogenerated holes to the electrode. The LED (LIZ1-10U600, LedEngin Inc.) intensity
was varied by tuning the current (from ~ 0.1 to 0.7 A), which was measured and recorded prior
to taking any PIA measurements. Photocurrents were measured by recording the change in
potential across a 100 Ω resistor that was set by the potentiostat, and the signal was collected by
an oscilloscope. The signal was then converted to units of photocurrent by Ohm’s law and
reported as photocurrent density by dividing the values by the surface area of the photoanode (~
0.5 cm
2
). The experimental setup is shown in Figure C.6. It is very similar to the TA experiment
setup, with the exception of the pump source (now an LED light), the removal of the
oscilloscope (because we are now only interested in long time scale transient signals), and the
probe wavelength set at a constant of 700 nm (because we are only interesting in probing the
dynamics of the holes). The PIA signal that was probed at 700 nm and the photocurrent densities
of the hematite photoanode are shown in Figure C.7. The PIA signals have a slight lag in its rise
125
and decay following the switching on (t = 0 s) and off (t = 5 s) of the LED pump light, although
the responses increase as the LED light intensity is also increased. On the other hand, the
photocurrent signals seem to show less of a lag response. The photocurrent appears to increase
and decrease quite rapidly following the switching on and off of the LED light. The rising and
decaying time constants seem to be independent of the LED light intensity. There is still some
ambiguity to the reasoning behind these observations, but the lagging response of the PIA
kinetics compared to the photocurrent kinetics are tentatively explained by the fact that the holes
need longer time to accumulate (as well as decay) at the hematite surfaces compared to the
electrons.
121
As shown in the SI Figure 5 of a paper by Le Formal et al.
121
, the PIA signal can be
converted to surface hole density with a calibration conversion. The PIA signal is collected at 0.9
V vs RHE, a bias position that is before the onset of any photocurrent signals. At this potential,
there will be a transient sharp positive (negative) photocurrent peak when the LED light is
Figure C.7. (Left) The PI absorption signal that was probed at 700 nm as a function of
increasing pump intensity. (Right) The long time scale transient photocurrent also as a
function of increasing pump intensity.
126
switched on (off). These transient peaks represent the accumulation of surface holes at the
positive peak and a rapid
recombination of electrons
and the surface holes at the
negative peaks. By
integrating over the cathodic
transient peak after the LED
is switched off, one can
calculate the total number of
electrons, and hence surface
holes. This is repeated for a
series of different LED light
intensities, which will result
in a range of surface hole
concentrations. These values are then plotted against the PIA signals with the respective LED
intensities, and you now have a calibration curve of accumulated hole densities vs PIA signal.
This analysis results in a calibration of 1 ∆ 𝑚𝑂.𝐷. that equates to 9.36 x 10
14
holes cm
-2
.
Assuming the linearity of this plot is sustained even at large photocurrent densities, you can use
this calibration to convert any PIA signal to surface hole density, and from there generate a rate
law analysis relationship between photocurrent and density of accumulated surface holes. Figure
C.8 shows this relationship for the set of PIA and photocurrent data I retrieved from the hematite
samples. There are three regimes of rate orders. The first is a first order kinetics (𝛼 = 1) that is
observed at low surface hole densities. The hypothesized mechanism for this region is water
Figure C.8. The rate law analysis plotted as a
relationship between the photocurrent and density of
accumulated surface holes (extrapolated from the
calibration curve) for the hematite photoanodes. The
sample was held at 1.5 V vs RHE.
127
oxidation that occurs via single-hole oxidation steps.
120-123
The second regime of rate order 3
(𝛼 = 3) is consistent with what has been observed.
121
This is counterintuitive because it means
that at this region, the rate of water oxidation is sensitive to the surface hole density to a power of
3 (i.e. if you double the [hole], the reaction will increase by an eightfold)! Additionally, one
would probably consider a higher rate law order of four instead of three since water oxidation
requires four holes and four electrons. There are some hypotheses for the mechanism of this
higher rate order, which is discussed by Le Formal et al.
121
, although still debated. The last
regime of rate orders of 7+ is completely nonphysical and lacks a fundamental explanation.
These higher order rate laws are not particular to hematite; they have been observed in other
metal oxides as well from our collaborators at Imperial College. We are currently working on
combining all the data and writing up the results.
The surprising and interesting findings from the rate law analysis of hematite is what
inspired the collaboration with Professor James Durrant’s group. As far as we know, the
reasoning behind the higher rate orders remains ambiguous. What we do know confidently from
the data is that at higher light intensities, the reaction of water oxidation grows more sensitive to
the concentration of photogenerated surface holes. We conjecture from this that not all holes
have the same oxidizing power, and perhaps deeper holes, as they accumulate at the surface, may
have a stronger “pull” for electrons. It would be useful, therefore, if we can measure this
oxidizing power. We resorted to vibrational Stark Shift spectroscopy, hoping that if we could
measure the interfacial electric field while repeating the PIA experiments mentioned earlier, we
would be able to assign the potentials and make comments about the varying oxidizing power of
the accumulated surface holes. This required a Stark probe that could be attached to the hematite
and would remain stably anchored while water oxidation reaction progressed. Our frequently
128
used Stark probe, 4-MBN, could not be used because we no longer have a Au surface that the 4-
MBN can form self-assembled monolayers anymore. This was no trivial task, and while we
never successfully functionalized hematite so we could perform in-situ experiments, we had a
few promising results. This is discussed in the following section.
C.4 Attempts to Functionalize Metal Oxides
There are a few methods I attempted to functionalize hematite (and other metal oxides)
with a Stark probe. The potential Stark probe I chose was 3,4 dihydroxybenzonitrile (3,4
DHBN). The molecule 3,4 DHBN belongs to a family of molecules called catechols, which are
organic compounds with the formula C
6
H
4
(OH)
2
. Catechols are known to bind to metal oxides
spontaneously in certain conditions.
124
125
They also have a nitrile stretch that would be
vibrationally bright to probe, similar to 4-MBN. Inspired by this knowledge, I found procedures
in literature that showed successful functionalization of different metal oxides with different
types of catechols. Instead of directly moving to metal oxide films, I attempted to first
functionalize metal oxide nanoparticles following a procedure by Zeininger et al.
126
In summary,
nanoparticles of varying metal oxides were dispersed in a 0.15 wt % in isopropanol, followed by
ultrasonication with a 10 mM solution of 3,4 DHBN in isopropanol. Functionalized nanoparticles
were then extracted via vacuum filtration and set aside to dry, with multiple isopropanol washes
to ensure all physiosorbed 3,4 DHBN were removed. Characterization was performed using
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), with the presence of a
nitrile peak used as an indication of successful functionalization. Figure C.9 shows the spectra
for various samples (left) and the general schematic of how the nanoparticles are functionalized
and can be detected with DRIFTS (right). The procedure was straight forward and consistently
129
resulted in functionalized nanoparticles. The next step was to try metal oxide films, which were
the ultimate samples I wanted to test for water oxidation.
To probe water oxidation on hematite surface, not only did I need a suitable Stark probe
that would be stable during water oxidation, I also needed the hematite film to be on a
roughened, conductive surface so it would be spectroscopically feasible to pick up very small
signals (essentially monolayer scale of signal-to-noise). With this in mind, I made hematite films
via the (SILAR) method,
127
where a roughened Ag foil strip is successively dipped in FeCl
3
and
NaOH to deposit hematite one layer at a time, followed by an annealing process in the oven at
300
o
C for 30 min, as shown in Figure C.10. The Ag substrate was a silver foil with thickness of
0.1 mm and 99.9% purity (Sigma-Aldrich), which was electrochemically etched prior to the
SILAR deposition of hematite. Again, metal substrates were roughened to enhance the
spectroscopy signal. After metal oxide deposition, the film was characterized by surface
Figure C.9 The DRIFTS spectra of 3,4 dihydroxybenzonitrile functionalized on
different metal oxides (left) and a general schematic of the starting materials
and the basic mechanism of DRIFTS.
130
enhanced Raman spectroscopy (SERS) to detect the presence of hematite vibrational stretches.
Figure C.11 shows a picture of the etched Ag strip with hematite deposited on top and the SERS
data for the sample. The band positions at 227 cm
-1
, 412 cm
-1
, and 1322 cm
-1
match those of
hematite found in literature
128
and are evidence of the presence of hematite. Following the
deposition of hematite on etched Ag
foil strips via the SILAR method, the
substrates were soaked in 10 mM 3,4
DHBN in ethanol solution overnight
for functionalization of the hematite
and then ultrasonicated in ethanol for
5 minutes to remove any
physiosorbed 3,4 DHBN. Figure
C.12 shows a cartoon diagram of the
functionalized substrate and the
SERS spectrum of the actual sample.
The spectrum is zoomed in on the
nitrile region and shows the presence of a nitrile stretch that belongs to the 3,4 DHBN. Although
Figure C.11 A picture of the hematite
deposited on etched Ag substrate via the
SILAR method and the SERS spectrum of
the sample.
Figure C.10 A cartoon schematic of the SILAR method for preparing hematite films.
131
this particular run was successful, the results were inconsistent and oftentimes the nitrile
stretches after functionalization were not detectable. Additionally, even when the nitrile stretch
was present, the signal-to-noise ratio was frequently very small and would disappear once the
substrate was soaked in water. I also could not draw large catalytic currents when attempting to
drive water oxidation, so these samples were poor photoanodes. Because of these inconsistent
results, I could not proceed forward with the in-situ experiments for water oxidation.
Another attempt I made to functionalize the hematite films was to directly soak the
APCVD hematite films in 10 mM 3,4 DHBN in ethanol solution. Although these hematite films
are on smooth ITO substrates, the hematite films themselves exhibit “cauliflower-like”
nanostructures in the order of 5-10 nm that I was hoping would serve as a roughened surface that
had a large enough surface area to increase the signal-to-noise ratio to pick up the nitrile
vibrational stretches. Figure C.13 shows a picture of the APCVD hematite photoanodes on ITO
and the SERS spectra, zoomed in on the nitrile region, for unfunctionalized samples and
functionalized samples. While there is a slight hint of the nitrile stretch at ~ 2225 cm
-1
for the
Figure C.12. The SERS spectrum (zoomed in at the nitrile region) of the hematite film on
Ag substrate that is functionalized with 3, 4 DHBN and a cartoon depiction of what the
sample looks like once prepared.
132
functionalized spectrum, the
signal is very weak and
oftentimes could not be
detected, even with long time
averages. Similar to the samples
prepared with the SILAR
method, the nitrile peaks would
also disappear once immersed in
water for in-situ experiments.
C.5 Conclusion and
Future Work
In conclusion, the rate
law analysis of hematite
photoanodes showed very
counterintuitive but interesting results. Further analysis is required to understand the underlying
mechanism behind higher order rate orders observed at high light intensities. A potential tool that
will help elucidate this is via vibrational Stark Shift spectroscopy. This might allow us to assign
the potentials of photogenerated holes at the surface of these hematite films. In order to do so, we
needed a Stark probe that adsorbs on the surface of the photoanodes and is stable during the
process of water oxidation. Additionally, we needed substrate that was roughened to give us
enhanced spectroscopic signal. I attempted to functionalize hematite with 3,4 DHBN, a molecule
that belongs to a family of catechols that has been shown to adsorb on metal oxides. The nitrile
on 3,4 DHBN could act as a potential Stark probe that could measure the interfacial electric field
Figure C.13 SERS spectra of the hematite film that was
functionalized with 3,4 DHBN (blue) and not
functionalized with 3,4 DHBN (red). There is a small
peak at 2225 cm
-1
for the functionalized spectrum that
corresponds to the nitrile group of 3,4 DHBN.
133
at the boundary of hematite and water. Although I tried various functionalization procedures, and
some of the samples showed the nitrile stretches in SERS, the signals were weak and/or the
catechols could not stably adsorb on the hematite upon the application of external bias.
Nevertheless, these efforts show some promising results, and I believe they have potential to be
expanded. A possible direction would be to try attaching a Stark probe via a different anchoring
group, specifically phosphate groups instead of hydroxy groups. It has been previously shown
that metal oxides can be surface-coated with different types of phosphate groups via self-
assembled monolayers.
129, 130
A phosphate group could potentially form a stronger anchor to
hematite that would allow an in-situ experiment for water oxidation. Another alternative would
be to try more surface specific spectroscopic techniques such as sum frequency generation that
might be able to pick up the small nitrile signals from 3, 4 DHBN. These are just a couple
examples of alternative experiments that one can try.
134
Appendix D
Supporting Information for A Direct Determination of SEIRAS Enhancement Factor and
Penetration Depths in Surface Enhanced IR Absorption Spectroscopy
D.1 Linear Dependence of Peak Currents to Scan Rate for 6-FCHT
The CV shown here (left) is the baseline corrected version of the CV shown in the main
text Figure 2.2. This was achieved by fitting the data to a polynomial and then subtracting it from
the original data. The baseline subtraction excluded the regions of oxidation and reduction, 0.43
V – 0.53 V for the anodic wave and 0.37 V – 0.47 V for the cathodic wave. This procedure was
utilized for all CVs corresponding to different scan rates as well. The peak current of the
Figure D.1. (Left) The CV from Figure 2.2 that is baseline corrected. (Right) The peak currents of the
reductive (black) and oxidative (red) peaks as a function of scan rate. The data points were each fitted to a
linear line, and the R
2
is shown for each fit.
135
oxidative and reductive peaks are plotted as a function of scan rates (right). Each set of data was
fit to a linear line, and the R
2
is shown.
D.2 The Interference Fringe Pattern of the FTIR Spacer
To calculate the real thickness of the spacer in the experiment compared to the nominal
one (100 𝜇m ), we took the IR spectrum of the FTIR cell with the spacer filled with air. From the
fringing effect seen in Figure D.2, we can calculate the actual path length from this equation
1,2
:
b = 10 N / 2 (𝜈
#
-𝜈
$
)
where b = actual path length of the cell in mm
Figure D.2. The interference fringe pattern obtained with the FTIR transmission cell with
the spacer filled with air.
1900 1950 2000 2050 2100 2150 2200 2250
Wavenumber (cm
-1
)
18
19
20
21
22
23
Single Beam Intensity
X 2237.02
Y 19.9484
X 1876.4
Y 21.5361
136
N = number of fringes in a spectral region
𝜈
#
, 𝜈
$
= the extreme points of the spectrum in cm
-1
For the data in Figure D.2:
b = 10 * (7) / 2 (2237 cm
-1
– 1876 cm
-1
)
= 0.097 mm = 97 𝜇m
D.3 The Linear Fits for Bulk Absorption of 6-FCHT
Figure D.3. The linear fits for the bulk adsorption of 6-FCHT showing the (0,0) point
and best fit R
2
values.
137
D.4 The Baseline Corrected 6-FCHT SEIRAS Spectrum
D.5 Estimation of Penetration Depth Based on a Homogeneous f
Figure D.4. The SEIRAS spectrum shown here is the baseline corrected version of the spectrum in the
main text Figure 2.4. The spectrum, excluding the region of interest (2811 cm
-1
– 2975 cm
-1
), were fit to a
polynomial, and then subtracted from the polynomial to extract the absorption peak heights that are
baseline subtracted.
Figure D.5. Left panel: dilution of acetonitrile (ACN) with ethanol on the SEIRAS substrate. The
molar absorptivity of bulk ACN is 45 M
-1
cm
-1
. The SEIRAS absorption of pure acetonitrile was
measured to be 16 mOD. Assuming the molar absorptivity decays exponentially from the SEIRAS
substrate into the bulk (middle panel), we can use Aacn with the measured enhancement factor from 6-
FCHT to calculate the penetration depth of the IR. This yields unphysical numbers (right panel) due to
treating every surface site as having the same spatial molar absorptivity decay profile.
138
The value of 0.2 nm was arrived at by assuming that the enhancement factor is uniform
everywhere on the metal. Since we know that absorption features beyond this length scale is
observable, the enhancement factor cannot be uniformly distributed. The reason for arriving at
such a short penetration depth is not the random orientation of the monolayer, but rather the fact
that a very large amount of the ACN will benefit from the enhancement if the enhancement is
uniformly distributed.
D.6 The Calculation for PB Electrodeposition Rate
Figure D.6. A cartoon depiction of the unit cell of the Prussian Blue film and the calculation
for the estimation of ~ 14 nm deposition increments for electrodepositing the film at - 40 𝜇𝐴
for 145 seconds. One electron transfer was estimated to correspond to a unit cell of volume
125 Å
!
.
4
The total charge in Coulombs was calculated for holding the potential at - 40 𝜇𝐴 for
145 seconds and then converted to the volume it would take up, assuming one electron
transfer occurs per unit cell. This volume was then divided by the surface area of the
substrate to estimate the depth (length) of the deposition.
139
D.7 Representative Chronocoulometry Data for PB Film
Figure D.7. A representative set of chronopotentiometry data for the deposition of
a ~ 14 nm film of Prussian Blue. This was repeated multiple times with SEIRAS
spectra being collected between consecutive chronopotentiometry scans.
140
D.8 SEIRAS Spectra and Penetration Depth for Another Gold ZnSe
Figure D.8. (Left) The SEIRAS spectra for PB film as a function of thickness and (right) The
integrated peak area of an electrodeposited Prussian Blue film on top of the SEIRAS substrate as a
function of film thickness. This is data corresponding to a different SEIRAS substrate than the one
in the main text (Figure 2.5). We provide a qualitative measure of the gold thickness by measuring
the resistance across a 1 cm distance on the gold deposited substrates. This substrate had a
resistance of ~ 5 Ω 𝑐𝑚
"#
while the substrate in Figure 2.5 had a resistance of ~ 15 Ω 𝑐𝑚
"#
. This
SEIRAS substrate was thicker, and more conductive than the substrate in the main text. This
substrate had a penetration depth of ~ 42 nm.
1950 2000 2050 2100 2150 2200 2250 2300
Wavenumber (cm
-1
)
-0.01
-0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
Absorbance (OD)
14 nm
28 nm
42 nm
56 nm
84 nm
112 nm
140 nm
141
Appendix E
Supporting Information for Interplay Between Charge Transfer and Interfacial Electric Fields
E.1 Cyclic Voltammogram of Mixed Monolayer of 6-FCHT:4-MBN
E.2 Representative Spectra of Mixed Monolayer of 6-FCHT:4-MBN
Figure E.1. (Left) the cyclic voltammogram of the mixed monolayer 6-FCHT:4-
MBN in 1 M KCl in aqueous solution as a function of scan rate. The half wave
potential remains approximately around + 0.47 V vs Ag/AgCl. (Right) The peak
currents for the anodic and cathodic waves plotted as a function of scan rate to show
the linearity dependence on scan rate, a characteristic of adsorbed species.
Figure E.2. Representative spectra zoomed in on the nitrile region of the mixed
monolayer of 6-FCHT:4-MBN as a function of potential.
142
E.3 The Reversibility of the Spectroelectrochemical Scans
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
Nitrile frequency (cm
-1
)
6-FCUT : 4-MBN Forward Scan
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
Nitrile frequency (cm
-1
)
6-FCUT : 4-MBN Backward Scan
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2218
2219
2220
2221
2222
2223
2224
2225
Nitrile frequency (cm
-1
)
11-FCUT : 4-MBN Forward Scan
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2218
2219
2220
2221
2222
2223
2224
2225
Nitrile frequency (cm
-1
)
11-FCUT : 4-MBN Backward Scan
143
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
Nitrile frequency (cm
-1
)
Mixed mono Alkanethiol Forward Scan
-1 -0.5 0 0.5
Voltage (V vs Ag/AgCl)
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
Nitrile frequency (cm
-1
)
Mixed mono Alkanethiol Backward Scan
Figure E.3. The forward and backward scan of (top) the short chain ferrocene and 4-
MBN mixed monolayer (middle) the long chain ferrocene and 4-MBN mixed
monolayer and (bottom) the alkanethiol and 4-MBN mixed monolayer. The nitrile
stretches were each fit to a Gaussian and the center frequency with the fitting
standard deviation were plotted as a function of applied potential.
144
E.4 Frequency Calculations Based on the Multipole Based Model
E.5 QM Frequency Calculations Based on MD Snapshots
Figure E.4. Frequency calculations at 0V vs Ag/AgCl that were based on the multipole
based model for four different geometry setups (one uniformly distributed ligands and
three random distributions). Qualitatively, the trends match experimental results.
Figure E.5. QM Frequency calculations using the snapshots generated with MD
simulations (4-MBN with and without ferrocene moiety nearby). The 4-MBN vibrational
frequency depends mostly on the surface charge of the electrode and not the charge of the
ferrocene, resulting in a minimal effect near the redox potential of ferrocene/ferrocenium.
145
Appendix F
Supporting Information for Quinone Proton-Coupled Electron Transfer in Bulk and Surfaces
F.1 Electrochemical Characteristic of 11-(ferrocenyl)undecanethiol
F.2 Surfactant Solvation Effects on the CV of 11-FCUT
Figure F.2 shows solvation effects of surfactants on the electrochemical redox behavior
of 11-FCUT, a surface-tethered ferrocene with eleven hydrocarbon chains. Specifically, I looked
at the effects of cationic and anionic surfactants on the formal potential of the redox activity of
ferrocene was studied. As seen in Figure F.2 (left), the addition of CTAB anodically shifted the
formal potential of 11-FCUT. This could be explained by the nature of surfactant head groups
that like to accumulate near the surface. The positively charged head groups of CTAB like to
come near the surface of the electrode and therefore would repel the positively charged
ferrocenium and increase the activation barrier required to oxidize ferrocene to ferrocenium. This
Figure F.1. (Left) The CV of surface tethered ferrocene, 11-(ferrocenyl)undecanethiol tethered
on a gold wafer electrode at various scan rates and (right) the linear dependence of
anodic/cathodic peak currents on scan rate.
146
would manifest in the CV as a positive shift of the oxidative wave, and is observed
experimentally, shown by the increase in potential of the anodic wave of the dotted line in Figure
F.2 (left) compared to the solid line (without the addition of CTAB). The same holds true for the
reductive wave once all the ferrocenes have been oxidized to ferrocenium and the potential is
cycled back. Because the solvation effects of CTAB changes the energetics of 11-FCUT so that
it is more difficult to oxidize ferrocene, it should be easier to reduce the ferroceniums when you
are walking the potential back in the reverse direction to reduce the ferroceniums. Therefore, you
should hit an earlier point of potential to observe the onset of reductive current. This is also seen
in the data in Figure F.2 (left) as well. The opposite story should hold true when you add an
anionic surfactant in the electrolyte, and an overall anodic shift of the CV peaks should be
expected. This is seen in the experimental data, shown in Figure F.2 (right). These results show
the importance of solvation effects on the charge transfer of redox active species and how the
energetics of the redox potential of species on the surface can be tuned.
Figure F.2. The solvation effects of surfactants on 11-FCUT that manifests as shifts in the CV.
The solid lines represent the 11-FCUT CV in aqueous 100 mM KCl solution and the dotted lines
represent the same sample in the same electrolyte with the addition of 1 mM surfactants. (Left)
A cationic surfactant, CTAB, that shifts the CV of 11-FCUT in the oxidative direction and
(right) an anionic surfactant, SDS, that shifts the CV of 11-FCUT in the reductive direction.
147
Appendix G
Supporting Information for Long-Lived Transient Responses from Photoexcitation of Prussian
Blue and its Analogs
G.1 The Synthesis of Other PB Analog Films Using SILAR Method
Figure G.1 A picture of the starting materials in aqueous solutions to make Prussian Blue and
its analogs and the types of films that were deposited on ITO via the SILAR method.
148
G.2 The Detector Response of the MCT Detector
G.3 FTIR Spectra of CoFe and FeFe Films on Different Substrates
PB and its analogue films were deposited on different substrates. The general reaction is
K
3
[Fe
III
(CN)
6
] + M
x
Cl
x
. The metal chlorides used were FeCl
3
, FeCl
2
, and CoCl
2
and the substrates
used were ITO and gold wafer. The films were deposited on these substrates using the SILAR
method described in Chapter 5. The bulk samples were synthesized simply by mixing the two
starting materials together, which resulted in PB nanoparticles precipitating. The solid that
crashed out was collected using vacuum filtration. The FTIR spectra shown were plotted in units
Figure G.2 The response of the MCT detector that was used in the Visible pump IR
probe experiment. This was done by directing the 800 nm pump pulse (~ 50 fs pulse
width) light onto the detector and then fitting the decay to an exponential. The time
extracted time was ~ 2 𝜇𝑠.
149
of % Transmission. The background spectra for the ITO samples, gold wafer samples, and bulk
samples were ITO, gold wafer, and air, respectively.
G.4 Spectroelectrochemical Raman Data for a CoFe film
Figure G.4 shows the cuvette setup for the spectroelectrochemical experiment of a CoFe
film. CoFe films were acquired from our collaborators at Prof. James Durrant’s group at Imperial
College. In short, the films were electrodeposited on ITO substrates following a hydrothermal
deposition method.
131
The working electrode was the CoFe film deposited on ITO, counter
electrode a Pt wire, and reference electrode Ag/AgCl. The potentials applied were adjusted
accordingly and plotted against RHE. The electrolyte was 50 mM potassium phosphate (KPi)
and 1 M potassium nitrate (KNO
3
) in aqueous solution. Potentials were applied in three different
zones, as shown in Figures G.5, G.6, and G.7, which represent three different regions of interest:
before CoFe oxidation, right after CoFe oxidation, and after water oxidation. A 532 nm laser was
Figure G.3. FTIR reflectance spectra, zoomed in on the nitrile region, of different
PB films on varying substrates (ITO, gold, and bulk). The spectra vary depending on
the type of substrate the films are deposited on.
150
used to collect Raman scatter. A spectrum was taken every 100 mV and at each data point, 3
scans were averaged with a 45 second acquisition time per scan.
Figure G.4. (Left) A cartoon diagram of the three-electrode setup in a cuvette and
(right) a CV of the CoFe film at 200 mV/s.
151
Figure G.5 shows the spectroelectrochemical data for the first window of applied
potential to the system, from 0.6 to 1 V vs RHE (forward) and then from 1 V to 0.6 V vs RHE
(backward).
The small CV on the bottom right corner shows the actual CV of the CoFe film from G.4 (right)
and the red box indicates the region of potential on the CV that is being swept. There are two
peaks ~ 2090 cm
-1
and ~ 2120 cm
-1
that remain constant throughout the application of bias in this
region. This makes sense since 1 V
RHE
is before the onset of any metal center oxidation, so there
should not be a significant change in the local environments surrounding the CN ligands in the
CoFe lattices.
Figure G.5 The forward and backward scan for the spectroelectrochemical experiment of
the CoFe film from 0.6 V to 1 V vs RHE.
152
At the second potential window where the bias is pushed to 1.5 V
RHE
, a new peak around
2200 cm
-1
emerges at 1.3 V
RHE
as seen in Figure G.6. This corresponds to the onset of current for
the first small peak in the CV. An onset of current measured electrochemically in the CV means
that there is current being drawn by the potentiostat, so charges are being transferred from one
place to another. This matches observations by other groups in the literature, where this redox
wave is assigned to the oxidation of CoFe film. Because CoFe PB films are nonstoichiometric
and exists as a mixture of varying metal redox states, it is difficult to assign this oxidation to a
specific transition. However, it is most likely that a species was oxidized at these potentials
because the appearance of the CN stretch at 2200 cm
-1
hints a change of the local environment of
the CN ligand to a more electron withdrawing surrounding (more oxidizing). The reasoning
behind this is discussed in Chapter 5.4. Potential dependent absorption spectra experiments of
this sample also hints an increase in the oxidation of the CoFe film.
131
Although I cannot assign
Figure G.6. The forward and backward scan for the spectroelectrochemical experiment
of the CoFe film from 0.6 V to 1.5 V vs RHE.
153
a specific transition, I will call the species that is generated at these positive potentials “oxidized
CoFe-PB
+
,” as is also used in another reference.
131
What is interesting in the Raman data is that
once the peak at 2200 cm
-1
is generated in the forward scan, it seems to persist even when CoFe-
PB
+
is converted back to the reduced state (as evidenced by the anodic current in the CV) in the
backward scan. This suggests a hysteresis effect where some of the oxidized CoFe-PB
+
does not
return to its reduced form even when you walk back in potential.
Figure G.7 shows the third potential window where the bias is applied to 2 V
RHE
. The
electrocatalytic water oxidation onset potential is at 1.6 V
RHE
, which is indicated by the large
onset of current observed from the CV. The Raman data in the forward scan shows the
appearance of the 2200 cm
-1
peak, this time a little delayed at 1.8 V
RHE
. Once again, it persists
even in the backward scan where the potentials are applied in the reverse direction. There is an
additional peak that appears in this set of data. On the backward scan, there is an emergence of a
shoulder peak that appears just slightly higher in frequency from the peak at 2120 cm
-1
. This
doesn’t appear prominently in the forward scan, hinting that it only grows towards the end of the
forward scan, and persists throughout the whole backward scan in the reductive potential
direction. A growth of another blue peak hints that another oxidizing species is born, and I will
call it “CoFe-PB
2+
,” meaning that the film is now doubly oxidized. Once again, the origin of
these peaks remain unknown, and further analytical experiments would be necessary to properly
assign these features.
154
Figure G.7. The forward and backward scan for the spectroelectrochemical experiment of the
CoFe film from 0.6 V to 2 V vs RHE.
155
G.5 UV-Vis of CoFe and FeFe PB Films
G.6 Additional Pump Probe Data for CoFe Film
Figure G.8. A picture of the CoFe and FeFe films deposited by the SILAR method on
gold (right) and their UV-Vis absorption spectra.
Figure G.9. (Left) Another set of 400 pump IR probe data for another CoFe film and (right)
representative time slices from five different wavelengths.
156
G.7 Additional Pump Probe Spectrum of CoFe Film
Figure G.10. (Left) Another set of 800 pump IR probe data for another CoFe film and (right)
representative time slices from five different wavelengths.
Figure G.11. Another set of spectra for the CoFe film pumped at 400 nm and 800 nm.
Abstract (if available)
Abstract
If I had to categorize my PhD projects into big themes and questions I wanted to answer, it would be the following two. First is probing chemical reactions and measuring electric fields at the interface. Surface chemistry is significant because a lot of the important chemistry occurs at the electrode-electrolyte boundaries. However, it is often difficult to probe reactions at the surface. Complications arise because at the surface, models that are used to describe phenomena in the bulk no longer apply. This can be overcome by using various spectroscopic tools and models, which is discussed throughout this thesis. The second theme is probing molecular species in the bulk while they undergo operando chemical transformations. My PhD work focuses on investigating these two umbrella themes by utilization of vibrational spectroscopy. This thesis is arranged as follows. I first explain the main concepts that relate to the systems of interest that I study. Then, I discuss a technique that I heavily used to study surface spectroelectrochemistry, Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS), and a quantitative method I used to measure the enhancement factor and penetration depth for different substrates. Next, I use the application of SEIRAS to discuss the interplay between surface electrostatics and a one electron charge transfer process by incorporating a mixed monolayer system of a molecular Stark probe and a surface tethered ferrocene. I then study a two-electron two-proton transfer reaction by probing the PCET process of benzoquinone/hydroquinone crystals utilized in batteries. Lastly, I extend these studies to a thin film system where I probed the lifetime of photoinduced charge-separated species of a Prussian Blue film by tracking the vibrational features of the CN ligands.
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Tseng, Cindy
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Probing charge transfer and electric fields at the bulk and interface using vibrational spectroscopy
School
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Degree
Doctor of Philosophy
Degree Program
Chemistry
Degree Conferral Date
2022-12
Publication Date
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