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Plasma emission across metal nanoparticle surfaces and semiconductor -liquid interface characterizations
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Content
Plasma emission across metal nanoparticle surfaces and
semiconductor -liquid interface characterizations
By
Bofan Zhao
A Dissertation Presented to
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of
the Requirement for the Degree of
DOCTOR OF PHILOSOPHY
(ELECTRICAL ENGINEERING)
May 2023
ii
Dedication
This thesis is dedicated to my grandmother who have encouraged me to keep finding solutions
for all the problems I met since I was a kid. I can’t achieve this without your inspiration. Hope
you rest in peace.
谨 以 本 文 献 给 我 的 奶 奶 , 您 从 小 对 我 在 面 对 问 题 时 要 自 己 想 办 法 “ 的 教 育 使 我 受 益 终 生 ,
希望您在另一个世界 过的安详
iii
Acknowledgements
I want to first thank my advisor, Dr. Stephen B. Cronin, for offering me this great opportunity and
privilege as a Ph.D. student in this outstanding group, especially for his efforts together with
patients put on me in these 5 years. I was lucky enough to confirm your offer 5 years ago to start
this wonderful journey, which helped me learned a lot.
I also want to thank Dr. Wei Wu, Dr. Aiichiro Nakano, Dr. Pin Wang, Dr. Rehan Kapadia and Dr.
Chongwu Zhou for joining my dissertation committee or qualifying committee. Your advice is
treasured.
It is an honor for me to have worked with some previous and current Ph. D. Students, who are not
only my colleagues but also my friends in these years. Dr. Lang Shen, Dr. Nirakar Poudel, Dr.
Bingya Hou, Dr. Jihan Chen, Dr. Yu Wang, Dr. Haotian Shi, Dr. Bo Wang and Dr. Sisi Yang
joined this group before me, and they never hesitate to give me help in my projects or my life. Zhi
Cai joined this group with me at the same year, but we’ve known each other at the first day in
Master program. I’m glad to be your friend for al these 7 years. I am also grateful to have worked
with Indu Aravind, Ruoxi Li, Sizhe Weng, Boxin Zhang, Caleb Medchill, Haley Weinstein, Rifat
Shahriar and Yuyun Wang in this group. Our friendship will be remembered for my whole life.
I want to thank my parents and my family for supporting me these years via hundreds of video
calls. I can always feel I am loved and cared even we were separated by this giant Pacific Ocean.
It has been a torture that we haven’t meet each other for 5 years due to pandemic but your love
made this much less suffering.
I also feel grateful for my friends who are not in my research group who made my life colorful and
enjoyable these years. Mingrui Chen, Max Lien, Zerui Liu, Yunxiang Wang, Qingyang Zhou,
iv
Yuehui Wang, Run Xia, Zhuwei Xu, Xinglei Ren, Zhiyuan Song, Boguang Sun and Chuyuan
Zheng, thank you for being my friends in my life.
Last but not the least, I want to thank a special girl in my life, Jiahui Liu. Every chat with you has
cheered my life and my soul up. You helped me a lot at my darkest time when the pandemic hit
this world and trapped me overseas. It’s a fortune and privilege to have known you since middle
school.
v
Table of Contents
Dedication ....................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
List of Figures ............................................................................................................................... vii
Abstract ........................................................................................................................................... 1
Chapter 1: Introduction to Plasma, Methane Conversions and Hot Electrons ............................... 4
1.1 Introduction to plasma generation in air ............................................................................... 4
1.2 Plasma driven chemical reactions and plasma catalysis ....................................................... 5
1.3 Introductions to methane conversions ................................................................................... 7
1.4 Hot electrons in metal nanoparticles ..................................................................................... 8
Chapter 2: Nanoparticle-enhanced Plasma Discharge Using Nanosecond High Voltage Pulses . 10
2.1 Abstract ............................................................................................................................... 10
2.2 Introduction ......................................................................................................................... 11
2.3 Methods ............................................................................................................................... 12
2.4 Results & Discussion .......................................................................................................... 14
2.5 Conclusion ........................................................................................................................... 19
Chapter 3: Au Nanoparticle Enhancement of Plasma-Driven Methane Conversion into Higher
Order Hydrocarbons via Hot Electrons......................................................................................... 21
3.1 Abstract ............................................................................................................................... 21
3.2 Introduction ......................................................................................................................... 22
3.3 Methods: .............................................................................................................................. 24
3.4 Results and Discussion: ....................................................................................................... 26
3.5 Conclusion ........................................................................................................................... 33
Chapter 4: Enhanced Plasma Generation from Metal Nanostructures via Photoexcited Hot
Electrons ....................................................................................................................................... 35
4.1 Abstract ............................................................................................................................... 35
4.2 Introduction ......................................................................................................................... 36
4.3 Method ................................................................................................................................ 37
4.4 Results & Discussion .......................................................................................................... 40
4.5 Conclusions ......................................................................................................................... 45
4.6 Future Work Related to Plasma Discharge on Metal Nanoparticle Surfaces ..................... 46
Chapter 5 Introduction to Semiconductor Photocatalysis ............................................................. 48
vi
5.1 Introduction to semiconductor-liquid junction .................................................................... 48
5.2 Introduction to photocatalysis of electrochemical water splitting ...................................... 50
Chapter 6: Band Flattening and Photo-induced Modulation of Depletion Region in
Semiconductor Photoelectrodes – Key loss mechanism ............................................................... 53
6.1 Abstract: .............................................................................................................................. 53
6.2 Introduction: ........................................................................................................................ 54
6.3 Method ................................................................................................................................ 55
6.4 Results and Discussion ........................................................................................................ 57
6.5 Future work ......................................................................................................................... 65
Bibliography ................................................................................................................................. 70
Appendix ....................................................................................................................................... 92
vii
List of Figures
Figure 1.1 Schematic of plasma generation pathways in air .......................................................... 5
Figure 1.2 Schematics of the interactions between plasma and catalyst in plasma driven
catalysis system ............................................................................................................................... 7
Figure 1.3 Typical methane conversion methods and their reaction temperature ranges.50 .......... 8
Figure 1.4 Electron distribution in metal nanoparticles (a) right after photon illumination, (b)
when electrons reach their own thermal equilibrium and (c) after decay back to ambient
thermal equilibrium ......................................................................................................................... 9
Figure 2.1 (a) Photograph of the plasma discharge across a 5mm gap on a glass slide and (b)
typical output characteristics of nanosecond high voltage pulse generator. ................................. 13
Figure 2.2 (a) Plasma emission spectra and (b) log-linear plot of the peak area per pulse with
and without Au and Pt nanoislands for the 738nm emission line of Ar. ...................................... 15
Figure 2.3 (a) HRTEM image of Pt nanoparticles and (b) quasistatic simulations of the
relative electric field enhancement produced by the Pt nanoparticles. (c) Emission intensity
on samples with glass only at different applied voltage ............................................................... 17
Figure 2.4 Spectra and electric field intensity distributions of (a,b) Au and (c,d) Pt
nanoparticles calculated using the FDTD method. ....................................................................... 19
Figure 3.1 (a) Experimental setup of the coaxial reactor in which mass spectrometry data
was taken. and (b) schematic diagram of glass-slide reactor for plasma emission spectrum
collection. (c) Typical waveform of high voltage pulse. (d) Cross section and (e) real picture
of glass slide reactor for our spectrum observation experiment. .................................................. 25
Figure 3.2 (a) CH4 emission spectra (Swan band) on different substrates with and without
Au nanoparticles and (b) their voltage dependence. Here, both configurations utilize copper
electrodes. “Glass” refers to copper electrodes on a bare glass slide, whereas “AuNP” refers
to the addition of Au nanoparticles deposited on the glass slide in between the copper
electrodes. ..................................................................................................................................... 28
Figure 3.3 (a) HRTEM image of Au nanoparticles and (b) quasistatic simulations of the
relative electric field enhancement produced by the Au nanoparticles. ....................................... 30
Figure 3.4 (a) Mass spectrometry data of co-axial reactor (peaks near 40 and 44 are related
to contaminations in coaxial reactor and impurities from gas line) and (b) its frequency
dependence. ................................................................................................................................... 32
Figure. 4.1 (a) Typical output waveform from the high voltage nanosecond pulse generator
(b) A schematic diagram of the experimental setup for observation of optical emission (c)
HRTEM image of nanoparticles morphology deposited on glass-slides. ..................................... 39
Figure 4.2 (a) Plasma emission intensity plotted as a function of peak pulse voltage with
and without 532nm laser illumination observed from Pt nanoparticles. Plasma emission
spectra under (b) 4.5kV and (c) 5kV peak voltage with and without 532nm laser illumination
on Pt nanoparticles (PtNP). ........................................................................................................... 41
viii
Figure 4.3 (a) Plasma emission spectra observed from Au nanoparticles under different laser
illumination conditions. (b) Plasma emission intensity observed by discharging 2.4 kV
pulses across Au nanoparticles (AuNP) plotted as a function of laser power. The laser
wavelength here is 532 nm. .......................................................................................................... 43
Figure 4.4 Energy band diagrams of gold nanoparticles (a) without and (b) with 532nm
wavelength laser irradiation. Illustration of hot electron distributions at (c) ~50 fsec and (d)
~2psec. .......................................................................................................................................... 45
Figure 4.5 Comparison between electron field emission on metal nanoparticle surfaces
within (a) gaseous environment and (b) aqueous solutions. In aqueous solutions, we expect
to see the electric field enhancement effect can affect electron emission and bubble
formation processes. ..................................................................................................................... 47
Figure 5.1 A schematic of band diagram of a n-type semiconductor and a redox couple
(A/A-) (a) before thermal equilibrium, (b) after thermal equilibrium and (c) under
illuminations.166 .......................................................................................................................... 49
Figure 5.2 A band diagram of an ideal single junction semiconductor water splitting device. .... 51
Figure 5.3 Photo-I-V curves for TiO2-coated GaAs with and without Pt co-catalyst
deposited by sputter deposition ..................................................................................................... 52
Figure 6.1 Schematic of three terminal measurement setup with lock-in amplifier(a) and
sample structure (b)....................................................................................................................... 57
Figure 6.2 (a)Photocurrent measured on sample with and without Pt under different laser
illumination conditions and associated IPCE (b). Please note that all data points from figure
(a) are taken at lower illumination conditions, where sample with Pt posses lower IPCE. .......... 59
Figure 6.3 Mechanism of band flattening due to high illumination density and slow surface
reaction rate. Before photon illuminations, the semiconductor-liquid junction has a depletion
width of Wo. After illuminations, charges are excited and separated at this depletion region,
but accumulated charges will reduce the built-in field and the depletion width (W). .................. 61
Figure 6.4 (a) Mechanism of band flattening and depleting width narrowing under laser
illumination. (b) Calculated relationship effective depletion width and IPCE (c) Calculated
relationship Accumulated charge density and IPCE ..................................................................... 64
Figure 6.5 (a)A schematic of surface enhanced Raman spectroscopy (SERS) measurement
of 4-MBN and its Stark shift on glass substrate in 0.05M Na2SO4 solution. .............................. 66
Figure 6.6 (a) Experimental setup of pump-probe measurement for 4-MBN Raman peak. (b)
Photoluminescence background from Zn doped GaP substrate near 4-MBN C-N stretch
Raman peak. (c) Self cleaning effect of TiO2 leads to instability of 4-MBN molecules. ............ 67
Figure 6.7 Experimental set up of time gated Raman measurement with a synchronized
pump laser. .................................................................................................................................... 69
Figure A1. Enhancement of Ar plasma emission with Cu nanoparticles. (a) emission of Ar
on different substrate with 5 kV peak voltage (b) emission intensity changing with peak
voltage on different substrate. A 1000x enhancement can be observed near 5 kV. ..................... 92
ix
Figure A2. Photographs of (a) the plasma discharge across a 5mm gap on a glass slide and
(b,c) glass-slide flow cell. ............................................................................................................. 92
Figure A3. A schematic diagram of the structure of our simulated sample. ................................ 94
Figure A4. Distribution of particle sizes ....................................................................................... 94
Figure A5. Non-uniform tetrahedral mesh use in our COMSOL simulations. ............................. 95
Figure A6. Mesh convergence test for 5nm thick circular discs of radius 5nm. .......................... 96
Figure A7. Maximum electric field enhancement plotted as a function of particle size. ............. 97
Figure A8. Example waveforms (vs. time) of (a) voltage and current, (b) energy of a 30X
TPS high voltage pulse generator with the glass slide-based reactor at 400Hz and 13kV
power output. ................................................................................................................................ 97
Figure A9. Emission spectrum exhibiting Swan bands obtained during plasma discharge. ........ 98
Figure A10. Gas chromatography result showing H2 produced during plasma discharge in
the coaxial reactor. ........................................................................................................................ 98
Figure A11. Summary of potential reaction pathways of methane upconversion (black
molecules represent stable products and red molecules are unstable intermediates). .................. 99
Figure A12. Plasma emission intensity of argon 912.3 nm emission peak discharged across
(a) (c) Pt nanoparticles and (b) (d) Au nanoparticles with and without 633nm or 785nm laser
irradiation. No obvious enhancement can be observed with laser irradiation of both
wavelength for both metal nanoparticles. ................................................................................... 100
Figure A13. (a) Transient absorption spectra measured on a Au grating nanostructure plotted
as a function of wavelength and time. (b) Time dependence obtained with a 505 nm probe
wavelength. These results suggest that the lifetime of hot electrons inside gold
nanostructures can be extend several psec. ................................................................................. 101
Figure A14 TEM result of GaAs/5nm TiO2/2nm Pt .................................................................. 102
Figure A15 Photocurrent of (a) GaAs/5nm TiO2 without and (b)with 2nm Pt at different
potentials under various illumination power density of 532nm. These datasets are acquired
in three-terminal measurements with lock-in amplifier. ............................................................. 103
x
1
Abstract
Recent years have witnessed numerous consequences of greenhouse effect, climate change
and energy crisis, which lead to growing demands for improving the efficiency of combustion of
fossil fuels and alternative ways of acquiring energies other than fossil fuels. Numbers of previous
works have presented a list of potential approaches to satisfy these demands, among which plasma
driven chemical reactions and photoelectrochemical reactions have drawn recent attention. In this
thesis, a new approach of plasma generation across nanoparticle surfaces is first evaluated, which
is proved to enhance plasma generation and be further enhanced via photogenerated hot electrons.
And then, we probed into photocatalysis approach using semiconductors and evaluated the
interface between semiconductor and liquid, which is responsible of the reduction of incident
photon to current efficiency (IPCE).
Chapter 1 is a brief introduction of plasma, methane conversion and hot electrons in metal
nanoparticles. In this chapter, we will discuss plasma generation in air and transient plasma, the
application of plasma driven chemical reactions, methane conversion reactions, and some typical
parameters of hot electrons in metal nanoparticles.
In chapter 2, we presented a method of enhancing plasma generation triggered by
nanosecond high voltage pulses using metal nanoparticle surfaces, which is directly related to the
field enhancement near these nanoparticle surfaces. An up-to-1000X magnification of argon
plasma emission is observed with the presence of these nanoparticles and our finite difference time
domain (FDTD) simulation results suggest this should not be attributed to the plasmon resonance
phenomenon (an optical effect), which results in an enhanced coupling of light from the nearfield
to the far field.
2
Chapter 3 discussed the potential of applying this mentioned enhancement effect of plasma
generation brought up by metal nanoparticle surfaces in assisting chemical reactions. While
discharging in methane environments, an 50-fold enhancement of C2 Swan band emission, which
is proportional to the C2 radical densities, can be provided by the metal nanoparticle surfaces. Mass
spectrum confirmed that the driven chemical reactions should be methane upconvertion.
Chapter 4 probed into the electrical field emission process in the related metal particle
surface substrates. Due to the small size of metal nanoparticles, hot electrons can travel to the
surfaces of particles before quenched into thermal equilibrium. As a result, plasma emission on
metal nanoparticles can be further assisted via hot electron photoexcitation and a up to 200X
increase of argon plasma emission can be achieved under 532nm laser illumination, which opens
up an new potential approach of solar energy harvesting.
Future works related to plasma discharge on metal nanoparticle surfaces will be explored
in the chapter 5. In this chapter, the potential and challenges related to observing plasma discharge
on metal nanoparticle surfaces under liquid phase will be discussed.
A brief introduction of semiconductor-interface and their applications in
photoelectrochemical reactions are presented in chapter 6. We discussed the structure and
mechanism of semiconductor-liquid junction and water splitting reaction in solutions, after which,
the current limitations of applying semiconductors in photoelectrochemical reactions will be
probed into.
Chapter 7 focused on a drop in incident photon to current efficiency (IPCE) as photo
illumination power increases in driving hydrogen evolution reactions (HER) on a GaAs/TiO2
semiconductor photo cathode. Band flattening mechanism due to the charge accumulation at the
3
interface is proposed and evaluated via MATLAB calculations. To further verify this issue, we
applied catalytical layer on top of this structure and less reduction of IPCE with growing
illumination power is observed. Further evidence from semiconductor-liquid junction
characterizations (electric field, charge density and surface potential) during this process will be
needed. Additional works using surface reporters are evaluated and possible challenges will be
discussed.
4
Chapter 1: Introduction to Plasma, Methane Conversions and Hot Electrons
1.1 Introduction to plasma generation in air
Plasma, known as the fourth state of maters, has been involved in all sorts of human
activities since the discovery of fire. Plasma is typically considered as a neutral system composed
of electrons, ions and charge-neutral particles and radicals, processing some interesting properties
including electrical conduction, photon emission, and chemical reactivity.
1-7
Plasma generation in
air requires dumping energy into gaseous spices ignited by the process called electrical breakdown,
which is an electrical arcing due to the passage of electrical current through a gas.
8
Various
methods of energy dumping in air, including thermal heating and energetic beam injection, have
been explored to generate plasma and the most commonly used method is subjecting the interested
gas in electric field. Continuous DC/AC fields and pulsed discharges are the most common forms
while utilizing electric field to initiate plasma in air. Charged carriers, including free electrons in
air due to cosmic radiations or electrons emitted from electrodes as a result of field emission, can
be accelerated in electrical field, leading to collisions with ambient species. This can create more
charged particles that will result in further accelerations and collisions, ending up in an avalanche
reaction, which can balance the losses of free charges in air and develop a steady state plasma.
These collisions will create charged and neutral radicals with high chemical activity. However, it
will also inevitably cause heating of the gaseous environment in the time scale of 100ns to 1 µs,
which indicates energy drawing into thermal energy.
9
5
Figure 1.1 Schematic of plasma generation pathways in air
One possible solution for reducing this energy drawing related to heating of the gaseous
environment would be transient plasma, a type of non-thermal plasma generated by high voltage
pulses with a time scale of nanosecond or less. Considering the time scale of gas heating (100ns
to 1µs), electrons and background gases will equilibrium at different temperatures and the energy
source of plasma will be cut off before massive collisions and heating take place.
9, 10
The high
energy electron will then assist radical formations and improve the energy efficiency of plasma
driven chemical reactions.
11, 12
1.2 Plasma driven chemical reactions and plasma catalysis
Plasma has a promising application potential in driving chemical reactions due to its
energetic nature, where chemically reactive radicals are produced during massive collisions.
However, these collisions and highly active radicals are hard to follow a designed reaction pathway,
making plasma driven chemical reactions hard to control and low in selectivity. As a result, a
significant amount of supplied energy will be draw into unfavored reaction pathways. The
application potential of plasma driven chemical reactions has been explored in several groups of
chemical reactions, including carbon chemistries (conversion between CO2, VOCs, and
Gas
Thermal Energy
Energetic Beam Electrical Field
(DC, DC pulses, AC…… )
Heated charges
Chemical reactions
thermal heating
……
Plasma
6
hydrocarbons)
13-19
, nitrogen chemistries (NH3, NOx and HCN)
20-26
and waste treatment (water
treatment and particle precipitations from exhaust)
27-32
. Catalysis, known for its capability of
reducing reaction barriers and improve reaction rates, is introduced into plasma driven chemical
reactions, which makes the reaction system prefer several certain reaction pathways and thus,
improve energy efficiency. The first known work combining plasma driven chemical reaction and
catalysis is done by Ray and Anderegg in 1921, during which they observed the catalytical effect
of silver in enhancing plasma driven monoxide oxidation.
33
The most noteworthy difference
between plasma catalysis and conventional thermal catalysis is the existence of activated charged
or neutral species, further lowering the energy barrier and providing alternative reaction
pathways.
34-36
In the meantime, catalyst in plasma catalysis system might be affected by plasma as
well due to the collisions, which might change the physical or chemical properties of catalyst
surface including improved absorption possibility
37-39
, creation of local hot spots
40
, removal of
cokes
41
and activation due to photon excitations
42
. Despite these differences, plasma catalysis still
requires a catalytical surface as is required in conventional thermal catalysis, which indicates those
merits will be limited near the catalytical surface. However, plasma is typically generated in a
three-dimensional space instead of a two-dimensional surface. This suggests less control of
reaction pathways and a waste of energy at the space away from the catalytical surface.
7
Figure 1.2 Schematics of the interactions between plasma and catalyst in plasma driven catalysis system
1.3 Introductions to methane conversions
As the main component of natural gas, methane has drawn increasing attention as the result
of its low cost and abundance. However, as the global consumption of methane keeps exploding,
its greenhouse effect, 21 times higher than CO2,
43
makes it less ideal as a massively consumed
energy source. In the meantime, its low boiling point (109K) makes it hard to liquefy and thus its
transportation from producing area becomes inconvenient and expensive. Methane is also known
for its highest ignition threshold and lowest burn rate among all n-alkanes (i.e., CₙH₂ₙ₊₂). Due to
these listed shortcomings of methane, conversion into other useful chemicals has been pursued by
many researchers.
44-48
However, the high strength of C-H bonds and the molecular configuration
of methane inhabits such conversion and numerous methods, which can be roughly classified as
indirect and direct conversions, have been studied.
49, 50
Indirect conversion route is the
Plasma
Radicals
Charged
Species
UV
Photons
Catalyst
Lower Barrier
Absorption
New Pathways
Collisions
Absorption
New Pathways
Excitation
8
commercialized route to transfer methane and it refers to converting methane into syngas first via
steam reforming and then further conversions into methanol or hydrocarbons through Fischer-
Tropsch synthesis.
51, 52
Direct conversion does not require the first step of syngas formation and
converts methane directly into liquid oxygenates and hydrocarbons via various methods including
photocatalysis
53-55
, thermal catalysis
56-59
and others
60-62
.
Figure 1.3 Typical methane conversion methods and their reaction temperature ranges.
50
1.4 Hot electrons in metal nanoparticles
Hot electron can be excited via numerous ways, and optical pump is one of the most frequently
used methods in research field. The relaxation process has been widely studied with a widely
accepted hot electron relation mechanism described as shown in Figure 1.4. Light illumination on
metal nanoparticles will pump up electrons to reach a non-equilibrium distribution of electron gas
in the metal nanoparticles with hot electrons and hot holes excited (Figure 1.4 a). These hot carriers
will interact with each other due to Coulomb interactions and reach a thermal equilibrium state,
which has a higher Fermi level compared with ambient lattice temperature (Figure 1.4 b). The
relaxation time in this process can be in the range of 10 fs. Due to electron- phonon scattering, the
electron distribution will eventually decay back to reach a thermal equilibrium with ambient lattice
temperature (Figure 1.4 c) and this process will take place within several picoseconds.
63-67
Many
experiments, where transient absorption spectroscopy is applied as a probe for hot electron lifetime,
9
have been carried out to testify this proposed mechanism and showed that lifetime of hot electrons
in Au nanostructures is around 3 ps.
68-70
Considering the Fermi velocity of electron in Au (10
8
cm/s), the mean free path of electron can be at the range of 10 nm before scattered by electrons
and electrons can travel 2–3 microns before thermalized to ambient equilibrium.
71
This suggest a
considerable number of hot electrons can reach the metal nanoparticle surfaces before quenched
down to thermal equilibrium.
72
Figure 1.4 Electron distribution in metal nanoparticles (a) right after photon illumination, (b) when electrons reach their own
thermal equilibrium and (c) after decay back to ambient thermal equilibrium
10
Chapter 2: Nanoparticle-enhanced Plasma Discharge Using Nanosecond High
Voltage Pulses
This chapter is similar to Zhao et al., published in the Journal of Physical Chemistry C
2.1 Abstract
By discharging nanosecond high voltage (5kV) pulses across an insulating substrate containing
Au, Pt, or Cu nanoparticles, a three order of magnitude (1000X) enhancement in the generation of
plasma can be achieved through local field enhancement on the surface of the nanoparticles. The
low-temperature nature of this transient plasma is crucial to maintaining the structural integrity of
these delicate nanoparticles. These nanoparticles provide up to 1000-fold enhancement in the
generation of the plasma, which is localized to the surface of the nanoparticles where it is
potentially useful (e.g., for catalysis). We performed both time-domain and frequency domain
calculations of the electromagnetic response of the nanoparticles based on high resolution
transmission electron microscope (HRTEM) images, which show local field enhancement of the
nanosecond high voltage pulse on the order of 3X. Since the plasma initiation depends
exponentially on the peak electric field strength, this 3-fold increase in the local electric field can
result in a several order-of-magnitude increases in the generation of plasma at a given applied
external field strength. In order to rule out plasmon-resonant enhancement, which is often
associated with small metal nanoparticles, we performed finite difference time domain (FDTD)
simulations in the optical frequency domain, which show that the effect of plasmon resonance is
negligible for Pt nanoparticles. We, therefore, attribute the nanoparticle-based enhancement to the
generation of plasma (an electrostatic effect) rather than enhanced coupling of light from the
nearfield to the far field via the plasmon resonance phenomenon (an optical effect).
11
2.2 Introduction
Local electric field enhancement of electron emission (i.e., field emission) has been achieved
using nanostructures in which the DC fields are enhanced around a nanoscale sharp feature. For
example, carbon nanotubes have been used to produce low-field electron emission with an onset
field as low as 0.8 V/µm following the Fowler-Nordheim equation, which is estimated to be
enhanced by 8000 times with respect to the corresponding bulk material.
73-75
Semiconductor
nanowire structures have also been utilized to produce low-field electron emitters and/or enhance
field emission current densities.
76-78
Baby et al. reported the effect of metal nanoparticles
decoration on electron field emission property of graphene sheets, in which the nanoparticles
provide local field enhancement.
79
The generation of plasma involves a number of non-deterministic and stochastic processes
and is usually initiated by electron emission, followed by acceleration, and then ionization of gas
molecules.
80
. In 1962, Neugebauer and Webb reported that electron conduction on ultrathin metal
films deviates from ohmic behavior at high electric fields.
81
Electron emission from discontinuous
island metal films was believed to be the reason for this deviation and, typically, this emission is
localized in discrete tiny regions referred to as ‘emission centers’. Electron emission from these
emission centers has been thoroughly researched in last century with two main emission
mechanisms proposed. G. Dittmer attributed this type of electron emission to standard electric
field emission from small islands in 1972.
82
However, later works proposed that hot electrons,
generated in these mean free path-sized nanoparticles, further enhanced the field emission from
these nano-islands compared to their bulk counterpart materials.
83-86
In our proposed mechanism,
after electrons are emitted outside metal nano-islands, they will be accelerated by the strong
12
electric field and then ionize gaseous molecules in the ambient environment. Once ionized, these
gaseous ions are then accelerated in the applied field which, in turn, ionize other molecules in an
avalanche processes that produces a propagating plasma streamer. In our work, we exploit this
nanoparticle-based enhamcenet of electron emission to initiate plasma formation at lower applied
potentials. Because of the inherent incompatibilities of high voltages typically used to produce
plasma discharges (~10kV) and the small length scales of nanoparticles (~10nm), nanoparticle
enhancement of plasma formation has not yet been demonstrated. Also, physical bombardment by
highly energetic ions in the plasma often results in rapid sputtering and ablation of nanoparticle
material, thus making nanoparticles further incompatible with plasma-based processes.
87
2.3 Methods
In the work presented here, we utilize nanosecond, high voltage pulses to provide a cold
plasma, which is far gentler to the metal nanoparticles than a conventional RF plasma. In order to
explore the effects of local field enhancement, we monitor the onset of light emission produced by
an argon plasma discharge as a function of peak pulse voltage both with and without nanoparticles.
Electrostatic and electrodynamic and simulations are performed of the nanoparticles based on high
resolution transmission electron microscope (HRTEM) images in order to provide a detailed
microscopic picture of the field enhancement process.
We use a 5-10 nanosecond, high voltage pulse generator (SSPG-20X, Transient Plasma
Systems, Inc.) to produce a plasma discharge across two parallel copper electrodes on a glass slide
separated by approximately 5mm, as shown in Figure 2.1a. The plasma discharge can be seen here
in this image as purple light.
88, 89
90, 91
Further details of the experimental setup are given in the
Supporting Information document. A typical waveform of the pulse characteristics is plotted in
13
Figure 2.1b exhibiting a 5-10nsec rise time. Once the streamer is formed, the electric field collapses
before a significant amount of current can flow and, as a result, very little power is draw in the
creation of this plasma. Here, the electrons get accelerated to extremely high kinetic energies
initiating a plasma discharge, while the ions in the plasma remain close to room temperature.
Figure 2.1 (a) Photograph of the plasma discharge across a 5mm gap on a glass slide and (b) typical output characteristics of
nanosecond high voltage pulse generator.
The electric field distribution of these nanoparticle films at optical frequencies are calculated
using the finite difference time domain (FDTD) method using the Lumerical FDTD-solution
software package. Here, an HRTEM image was imported to define the geometry of these
nanoparticles in the simulations. Simulations were performed with a mesh size of 1nm. A plane
wave was incident normal to the nanoparticle film. Periodic boundary conditions were applied at
the in-plane boundaries and perfectly matched layer (PML) boundary conditions were applied at
the out-of-plane boundaries of the nanoparticle film. Reflected and transmitted powers were
measured using power monitors placed behind the source and after the structure, respectively. A
2D field monitor was placed at the plane of the film to record the electric field intensity profile.
14
2.4 Results & Discussion
Figure 2.2a shows plasma emission spectra taken in argon (738 nm line) with and without
Au nanoparticles with 4.9kV nsec pulses. While the center frequency and lineshape of these spectra
are nearly identical, the spectrum with nanoparticles is 1000X more intense than the spectrum
taken without nanoparticles. Figure 2.2b shows the integrated peak intensities plotted as a function
of pulse voltage on a log-linear scale with and without both Au or Pt nanoparticles. As mentioned
above, the low-temperature nature of this transient plasma is crucial to maintaining the structural
integrity of these delicate nanoparticles. We have also seen similar enhancement with Cu
nanoparticles, as shown in Figure A1 of the Appendix. The argon plasma exhibits many peaks in
the 700-800nm range, all of which show similar enhancement.
15
Figure 2.2 (a) Plasma emission spectra and (b) log-linear plot of the peak area per pulse with and without Au and Pt nanoislands
for the 738nm emission line of Ar.
We deposit nanoparticles using electron beam evaporation of metals (Pt, Au, Cu) with nominal
thicknesses of 5-10nm, which are not thick enough to form continuous films and instead create
island-like structures.
92-94
Figure 2.3a shows an HRTEM image of Pt nanoparticles with a 5nm
nominal thickness deposited in this way. In order to understand the role of the electrostatic
response of the nanoparticles in the enhancement of plasma generation, we performed quasistatic
electromagnetic calculations, in which the electric field is changing very slowly and there is no
16
magnetic induction, using the AC/DC module in the COMSOL Multiphysics solutions package.
Further details of the COMSOL simulation are given in the Supplemental Document. Here, we
assign floating boundary conditions for the nanoparticles and use an extremely fine physics-
optimized mesh to the system, which is essential for accurately treating the short length scales over
which fields decay at the nanoparticle surfaces. Figure 2.3b shows the electric field enhancement
observed on Pt nanoparticles in response to the applied DC voltage. Here, we observe local field
enhancement on the order of 3X on surfaces of the nanoparticles. Similar results are observed with
Au and Cu nanoparticles. Since the plasma is initiated by field emission of electrons, which
depends exponentially on electric-field, this 3-fold increase in the local electric field can produce
a several order of magnitude increase in the plasma generation, as observed in Figure 2.2.
17
Figure 2.3 (a) HRTEM image of Pt nanoparticles and (b) quasistatic simulations of the relative electric field enhancement produced
by the Pt nanoparticles. (c) Emission intensity on samples with glass only at different applied voltage
Metal nanoparticles have been used in the optical frequency domain to enhance Raman
scattering for several decades through the surface enhanced Raman scattering (SERS)
phenomenon, which can provide single molecule sensitivity and 8-10 orders of magnitude
enhancement.
95, 96
In order to rule out the possible effects of plasmon-resonant enhancement, often
18
associated with small metal nanoparticles, we performed extensive finite difference time domain
(FDTD) simulations in the optical frequency domain (see Figure 2.4).
94, 97-103
Figures 2.4a and 2.4b
show the absorption spectrum of an array of Au nanoparticles and a corresponding electric field
intensity (i.e., |E
2
|) distribution at the resonant wavelength of 733nm. Here, we see localize bright
spots (or “hot spots)” on the surface of the nanoparticles due to the plasmon resonant effect.
22, 23
Figures 2.4c and 2.4d show the absorption spectrum of an array of Pt nanoparticles and the
corresponding electric field intensity distribution at a wavelength of 733nm, which shows that the
plasmonic effect for Pt nanoparticles is negligible. That is, for non-plasmonic metals like Pt, there
is essentially no enhancement in the local fields at optical frequencies. Since both Pt and Au
nanoparticles produce comparable (~1000X) enhancement in the plasma emission spectra (Figure
2), we conclude that the nanoparticle-based enhancement is due to the generation of plasma rather
than a coupling of light from the near field to the far field via the plasmon resonance phenomenon.
19
Figure 2.4 Spectra and electric field intensity distributions of (a,b) Au and (c,d) Pt nanoparticles calculated using the FDTD method.
2.5 Conclusion
In conclusion, we demonstrate a 1000-fold enhancement in the generation of a transient
plasma discharged across Pt, Au, and Cu nanoparticles. The main mechanism of enhancement is
achieved through local field enhancement at the nanoparticles surface, where it is potentially most
useful for applications such as catalysis. This “cold” plasma is relatively gentle and maintains the
mechanical integrity of the nanoparticles. Based on high resolution transmission electron
microscope (HRTEM) images, we calculate the electrostatic response of the nanoparticles using
quasistatic simulations, which predicts local field enhancement of the nanosecond high voltage
pulse on the order of 3X. Because the field emission of electrons is responsible for the initiation
of the plasma and depends exponentially on electric field, this 3-fold increase in the local electric
Pt
(a)
(c)
Au
(b)
(d)
733nm
733nm
20
field can produce a several order-of-magnitude increase in plasma generation, as observed
experimentally. The effect of plasmon-resonant enhancement is ruled out by finite difference time
domain (FDTD) simulations performed in the optical frequency domain, which show negligible
enhancement for Pt nanoparticles at optical frequencies. The nanoparticle-based enhancement is,
therefore, attributed to the generation of plasma instead of plasmonic coupling of light from the to
the far field to the nearfield.
21
Chapter 3: Au Nanoparticle Enhancement of Plasma-Driven Methane
Conversion into Higher Order Hydrocarbons via Hot Electrons
This chapter is similar to Zhao et al., published in ACS Applied Nano Materials
3.1 Abstract
We demonstrate a more than 50-fold enhancement in the upconversion of methane to
higher order hydrocarbons by discharging a nanosecond pulsed plasma across Au nanoparticles.
Here, the enhancement occurs as a result of the local field enhancement provided by the
nanoparticles. The transient nature of the pulsed plasma enables the structure of these delicate
nanoparticles to be preserved during the plasma discharge by producing a low temperature plasma.
Plasma emission spectroscopy shows signatures of the C2 Swan bands with and without the
presence of these nanoparticles. Mass spectrometry demonstrates that methane is converted into
higher order hydrocarbons with different groups of peaks representing species with molecular
masses of 35-45, 45-60, and 60-70 amu, corresponding to C3, C4, and C5 species, respectively,
under plasma discharge conditions. Electrostatic simulations show that a 3-4X enhancement in the
field is produced at the nanoparticle surfaces. The exponential relation between electric field
strength and plasma formation gives rise to a 50X increase in highly-reactive, plasma-initiated
radical species that are responsible for driving the methane upconversion process. This
upconversion is important for several applications including mitigation of greenhouse emissions
and improving the combustion of natural gas.
22
3.2 Introduction
Due to the greenhouse effect and wide abundance of methane, conversion into other useful
chemicals like syngas (CO and H2) or higher order hydrocarbons has been pursued by many
researchers.
44-48
However, the high dissociation energy of C-H bonds makes such conversion
challenging and numerous methods, such as oxidative or non-oxidative catalytical reactions
104, 105
and thermal coupling
106
, have been studied. Plasma discharges contain high energy radical species
that provide another approach for methane upconversion and reforming, which has been
investigated using several different plasma generation techniques, including microwave plasma,
107
dielectric barrier discharge plasma,
108, 109
and spark discharge.
110
However, methods to improve
the efficiency and selectivity of this process are still very much needed in order to make this a
viable approach for practical applications.
In addition to mitigation of greenhouse gas emissions, the upconversion of methane to
higher order hydrocarbons presents an important mechanism for enhancing the combustion of
natural gas. While CH4 is cheap, abundant, and relatively clean to burn, it has the highest ignition
threshold and lowest burn rate among all n-alkanes (i.e., CₙH₂ₙ₊₂), thus, reducing its combustion
robustness; a well-known problem in natural gas-burning systems. Several groups (including our
own) have used transient plasma to enhance the ignition and burning of fuels that are present in
natural gas, with CH4 being the dominant one.
111-119
In contrast, C2H4 is one of the most readily
combustible hydrocarbons due to its highly energetic C=C bond and its high diffusivity (i.e.,
relatively low molecular weight), and a stoichiometric C2H4/air flame burns twice as fast compared
to a stoichiometric CH4/air flame. These prior investigations demonstrate that transient plasma can
be used to achieve combustion under very lean fuel: air ratios, and hence lower greenhouse gas
(GHG) emissions. However, the mechanism of this enhancement is not fully understood. The
23
plasma-driven upconversion reported here presents one potential mechanism underlying this
enhanced combustion of natural gas.
The formation of a plasma discharge consists of a series of several sequential processes,
starting from electron emission, followed by acceleration and gas ionization, and ultimately
leading to (and driving) chemical reactions.
80
Electron emission from discontinuous ultrathin metal
films (i.e., nano islands) has been studied as far back as 1962 by Neugebauer and Webb, who
reported a deviation of electron conduction from Ohmic behavior on this kind of substrate.
81
The
mechanism of electron emission from thin metal films was attributed to electric field emission at
first.
82
However, later works on these discontinuous ultrathin metal films revealed the importance
of the contribution of long-lifetime hot electrons, which have a relatively high probability to reach
the nanoparticle surfaces in this process.
83-86
In addition, nanoscale sharp features, such as carbon
nanotubes and semiconductor nanowires, have been known for their capability to enhance local
electrostatic fields and, hence, field emission of electrons.
73-78
Our group reported enhanced
plasma emission spectra in Ar on metal nanoparticles due to this local field enhancement, and we
have ruled out the possibility that such an enhancement originates from plasmon-resonant (i.e.,
optical) effects.
120
In the work presented here, we explore the upconversion of methane to higher order
hydrocarbons by discharging a nanosecond pulsed plasma across Au nanoparticles. This
upconversion process is monitored using mass spectrometry and plasma emission spectroscopy.
The local field enhancement provided by the nanoparticles underlying, which is responsible for
the 50-fold enhancement of this process, is quantified using electrostatic simulations based on high
resolution transmission electron microscopy (HRTEM) images of the Au nanoparticles.
24
3.3 Methods:
Plasma is generated by a nanosecond high voltage pulse (Figure 3.1c), which consumes far
less energy in the creation of the plasma than conventional RF sources. The transient nature of the
plasma necessitates that very little current is drawn in creating the plasma. Here, once the streamer
is formed, the applied electric field drops to zero before a significant amount of current (or electric
power) can flow. Furthermore, this transient plasma is more gentle to the delicate nanoparticles,
as compared with conventional RF plasmas. The high voltage pulses are produced by a pulse
generator (SSPG-20X, Transient Plasma Systems, Inc.) with a rise time of 5-10 nanoseconds, as
shown in Figure 3.1c. After gaseous molecules are ionized, the output voltage drops quickly before
a considerably large current flow can be formed, limiting the power that is drawn in the creation
of this plasma.
25
Figure 3.1 (a) Experimental setup of the coaxial reactor in which mass spectrometry data was taken. and (b) schematic diagram of
glass-slide reactor for plasma emission spectrum collection. (c) Typical waveform of high voltage pulse. (d) Cross section and (e)
real picture of glass slide reactor for our spectrum observation experiment.
These high voltages nanosecond pulses are applied to a coaxial reactor with an inner
electrode wire diameter of 0.025” diameter and outer diameter of 1.5 inches diameter, 30 inches
in length, as illustrated in Figure 3.1a. Products from the reactor are then analyzed by mass
spectrometry (Pieffer, GSD 301) to characterize products formed by discharging plasma in flowing
CH4. Figure 3.1b shows a photograph of a smaller reactor setup, referred to as glass slide flow
reactor, that enables observation of plasma emission spectra, which consists of two copper
electrodes separated by an approximately 5mm gap inside a glass-slide flow reactor. We deposit
26
nanoparticles using electron beam evaporation of metal films with nominal thicknesses of 5-10nm
in between Cu electrodes, which are not thick enough to form continuous films and instead create
island-like structures
92-94
The gas flow rate during these measurements was held constant at 100
sccm, while the plasma power spanned a range from 4.6W to 35W for these measurements. Glass
slide flow reactors were also fabricated without Au nanoparticles as a comparison in order to
evaluate the effect of nanoparticles. The plasma discharge observed in air (see Figure 3.1e)
produces purple light that corresponds to the C-B transition in nitrogen.
88, 89
The glass-slide based
reactor has a low profile and fits easily underneath our high numerical aperture microscope
objective lens, enabling plasma emission spectra in the visible wavelength range to be obtained
with high collection efficiency, as illustrated in Figure 3.1b. Based on the linear relationship
between the C2 Swan band emission and C2 radical species concentration, as established by
Goyette et al.,
121
we can use the Swan band spectral intensity as a proxy for CH4 conversion.
3.4 Results and Discussion:
Figure 3.2 shows a comparison of the plasma emission spectra obtained in a CH4 gas
environment by discharging the nanosecond pulsed plasma both with and without nanoparticles.
Here, copper electrodes are used both with and without nanoparticles. We observe several sets of
Swan bands, which correspond to C2 (diatomic carbon) species
122
123
indicating that, in addition to
reducing CH4 (i.e., a high barrier reaction), C2-species are formed, presenting the exciting
possibility of converting an abundant greenhouse gas into an energy dense hydrocarbon fuel.
Figure 3.2b shows the integrated area (i.e., plasma emission intensity) plotted as a function the
nanosecond peak pulse voltage both with and without nanoparticles. At 9kV, the plasma emission
intensity obtained with nanoparticles is 50X larger than that obtained without nanoparticles. Below
27
9kV, we only observe plasma emission with Au nanoparticles, indicating that enhancement factors
much higher than this are created by the local field enhancement of the nanoparticles. In a previous
study carried out in argon, enhancement factors of more than 1000-fold were observed with the
presence of the nanoparticles. Above 9kV, the plasma emission is larger for the electrodes without
nanoparticles, where the applied field is strong enough to initiate a plasma from the bulk electrodes.
Here, the Au nanoparticle film creates a shunt of the externally applied field applied between the
copper electrodes. Effectively, this film behaves as a conducting film at high voltages and an
insulating film at low voltages.
28
Figure 3.2 (a) CH4 emission spectra (Swan band) on different substrates with and without Au nanoparticles and (b) their voltage
dependence. Here, both configurations utilize copper electrodes. “Glass” refers to copper electrodes on a bare glass slide, whereas
“AuNP” refers to the addition of Au nanoparticles deposited on the glass slide in between the copper electrodes.
Figure 3.3b shows the electric field enhancement observed on discontinuous Au
nanoparticles in response to the nsec high voltage pulse. Here, we performed quasistatic
electromagnetic simulations using the COMSOL Multiphysics solutions package. The TEM image
in Figure 3.3a is used to define the spatial extent of the Au nanoparticles. Floating boundary
29
conditions were used, in which the electric potential of each nanoparticle is freely varying. An
extremely fine mesh (physics-optimized) was used to accurately model the short lengths over
which electric fields decay near the nanoparticle surfaces. The electric field enhancement created
by the Au nanoparticles is plotted Figure 3.3b in response to an externally-applied DC voltage. It
should be noted that these are full 3D simulations, and we are plotting 2D cross-section in the
plane of the glass surface. These simulations exhibit a 3-4 fold enhancement in the local electric
fields on the nanoparticles surfaces. Since plasmas are generally initiated by field emission of
electrons that depends exponentially on the local E-field, this 3-4 fold increase in the local field
can easy increase the plasma density by 50X under the same externally-applied voltages, as
observed in Figure 3.2b. Here, a quasi-static simulation was used to treat these nanosecond high
voltage pulses, which is justified because the timescale over which charge is redistributed in these
metal nanoparticles is on the order of 50 fsec. Therefore, the pulse rise time (i.e., 5 nsec) is 5 orders
of magnitude slower than this, justifying the use of the quasi-static simulations. Raman
spectroscopy of the nanoparticles performed after plasma-driven methane conversion shows no
peaks corresponding to carbon deposits (i.e., D-band or G-band). This indicates that there is no
formation of coke, soot, or other carbonaceous deposits on the nanoparticles during the methane
upconversion process.
30
Figure 3.3 (a) HRTEM image of Au nanoparticles and (b) quasistatic simulations of the relative electric field enhancement produced
by the Au nanoparticles.
Figure 3.4 shows the mass spectrometry data demonstrating the formation C3 (e.g., C3H8
(44 amu)), C4 (e.g., C4H10 (58 amu)), and C5 (e.g., C5H12 (70 amu)) hydrocarbon species under
31
plasma excitation of CH4. Here, we believe the formation of C2-species, as observed by the Swan
bands plotted in Figure 3.4a plays, an important role in the upconversion of these single C atom
species to higher order hydrocarbons. Hydrogen is also detected in the product gases, as shown in
Figure S4. The data shown in Figure 3.4 illustrates our ability to convert two abundant greenhouse
gases CH4 to higher order hydrocarbons that have a much higher energy density than methane.
The reaction pathways of several related reactions (including the upconversion of methane) have
been studied, proposing the initial formation of single carbon radicals, like CH3 and CH2, by the
deprotonation of methane. Various combination pathways have been discussed to generate C2
molecules and radicals, such as CH3 + CH3 → C2H6 and CH3 + CH2 → C2H4 + H2. These C2 radicals
and molecules will further combine to produce longer hydrocarbon chains.
124-127
A summary of
the potential reaction pathways can be found in Figure A11.
32
Figure 3.4 (a) Mass spectrometry data of co-axial reactor (peaks near 40 and 44 are related to contaminations in coaxial reactor and
impurities from gas line) and (b) its frequency dependence.
Figure 3.4b shows the integrated area in the mass spectra plotted as a function of pulse repetition
rate (i.e., frequency). Here, a linear relationship can be seen for all products within the tested
33
frequency range. This result is not surprising since the plasma density is generally proportional to
the pulse repetition rate.
Many difficult and interesting reactions have been driven by plasma (e.g., N2 reduction with water
to ammonia and CO2 reduction to hydrocarbons). However, the main factor limiting the practical
application/adoption of this approach is the lack of energy efficiency and prohibitively high
voltages needed to initiate the plasma discharge. This nanoparticle-based approach enables these
plasma-driven reactions to take place at lower voltages and potentially lower power (i.e., higher
efficacies). Integral to this approach, are the nanosecond pulses, which preserve the integrity of
the delicate nanoparticle nanostructures. Lastly, this general approach ensures that the reaction
takes place near the surface of the metal nanoparticles, which many open up a new avenue for
catalyst-assisted plasma chemistry.
3.5 Conclusion
In conclusion, we demonstrate a more than 50-fold enhancement in methane upconversion
to higher-order hydrocarbons using a transient plasma discharged across an array of metal
nanoparticles. Here, local field enhancement at the nanoparticle surfaces provide a mechanism for
increasing plasma discharge. Through this mechanism, plasma-driven chemical reactions can be
enhanced or initiated under lower field conditions. Quasistatic electric field simulations are
performed based on high resolution transmission electron microscope (HRTEM) images and
predict local field enhancement factors of 3 to 4-fold near the nanoparticle surfaces. Since the
plasma initiation depends exponentially on the peak electric field strength, this 3-4 fold increase
in the local electric field strength can result in a more than order-of-magnitude increase in the
generation of plasma at a given applied external field strength. Due to the linear relationship
34
between the C2 Swan Band emission intensity and C2 radical productivity, we can conclude similar
enhancement in methane conversion into higher order hydrocarbons. This general approach can
potentially be used to intentionally limit (or confine) plasma driven reactions to catalytic surfaces
to improve catalytic selectivity.
35
Chapter 4: Enhanced Plasma Generation from Metal Nanostructures via
Photoexcited Hot Electrons
This chapter is similar to Zhao et al., published in the Journal of Physical Chemistry.
4.1 Abstract
We report hot electron-enhanced plasma generation by irradiating metal nanostructures
with laser light. Here, a high voltage nanosecond pulse is discharged across two electrodes
interspersed with metal nanoparticles (e.g., Au and Pt) both with and without laser excitation. With
laser excitation (532 nm in wavelength), we observe a 200-fold increase in the plasma emission
intensity (i.e., plasma density) and a lower threshold for the onset of plasma discharge (i.e., lower
voltage). This enhancement of plasma emission/discharge occurs for two reasons. First, the hot
electrons photoexcited in these metals lower the effective work function needed to be overcome
for thermionic emission thus, initiating the plasma. Second, the metal nanostructures minimize the
average distance photoexcited carriers (i.e., hot electrons) have to travel to reach the surface. As
such, the photoexcited hot carriers within the metal nanostructures can easily reach the surface
before relaxing back to equilibrium. While these metal nanostructures have been shown to be
strongly plasmonic (e.g., Au nanoparticles), we believe that the plasmon resonance is not playing
an important role in this plasma emission process. Plasma emission under 633nm and 785 nm laser
wavelength irradiation was also tested but no enhancement in plasma emission was observed. We
attribute this to the low photon energy (i.e., 1.9 eV), which lies below the threshold for inberband
transitions in Au and Pt.
36
4.2 Introduction
Hot electrons photoexcited in metals have been extensively discussed over the past decade
as a novel mechanism for driving different chemical reactions
128-131
as well as solid-state electronic
devices.
132-135
For example Guo et al. used Au/Pd superstructures to improve hot electron
utilization and further enhance catalytical performance in the activation of molecular oxygen as
well as C-C coupling reactions.
136
Hot electron-doped MoS2 was also shown to provide a better
catalytical surface compared with its undoped counterpart in driving hydrogen evolution
reactions.
137
In solid state devices, tunneling of hot electrons was utilized in hot electron transistors
(HET), and HETs fabricated with semiconductor materials with bipolar
138
and unipolar
139
designs
were reported. 2D materials, such as graphene, was also used in HETs and later works
demonstrated both in-plane and vertical HET structures with graphene.
138-141
High performance IR
detectors were also realized by using hot electron generation in Schottky junction-based
semiconductor devices .
142, 143
In addition, plasmon resonant absorption was also utilized and
plasmonic hot-electron IR detectors were fabricated with a detectivity of 4.38 ×10
11
cm Hz
1/2
/W.
142, 144
As far back as 1961, electron emission from discontinuous metal ultrathin films was
reported to explain the electron conduction behavior of metal island films and its deviation from
Ohm’s law.
81
This deviation was first considered to be the result of field emission of electrons
from the small metal islands by Dittmer in 1972.
82
However, it was revealed that hot-electrons,
generated in nanoparticles within the length scale of the hot electron mean free-path, play a
predominant role in this process.
83-86
Previous work by Zhao et al. reported that plasma discharge
across metal nanoparticle substrates can be enhanced up to 1000-fold as a result of the locally-
enhanced electrostatic fields at the surface of these nanoparticles in the absence of laser light, and
this work indicated the importance of electron emission from metal nanoparticles in the plasma
37
generation mechanism.
145
Previous work using a similar approach also demonstrated an up to 50-
fold enhancement in C2 radicals in the plasma-driven upconversion of methane. While these
previous studies indicate the great promise for discharging nanosecond high voltage pulses across
Au nanoparticles, no previous measurements have been done using laser excitation in
conjunction.
60
Lasers have been used to generate plasma for several decades, in a process referred to as
laser-induced plasma (LIP). Here, high energy laser pulses deliver a significant amount of energy
onto the surface of target materials, leading to vaporization, excitation, and ionization of materials
on these surfaces.
146-148
As a result, materials on the surface can be excited into a plasma state and
this process has been used in material characterization (e.g, laser-induced breakdown spectroscopy)
and Pulsed Laser Deposition (PLD).
149-152
Essentially, LIP involves exciting solid materials into
the plasma phase, which is quite distinct from our work presented here, in which gaseous species
are excited into the plasma phase using a combination of applied electrical and optical fields.
4.3 Method
Here, a cold (i.e. non-thermal) plasma is generated from nanosecond, high voltage pulses
(SSPG-20X, Transient Plasma Systems, Inc.). Figure 4.1a shows a typical waveform of the pulse
characteristics, which exhibit a 5-10 nsec rise time. These nanosecond high voltage pulse
discharges create a relatively cold (non-thermal) plasma, which protects the integrity of
nanoparticles. With these high voltage pulses, the electric field collapses before a significant
amount of current flows in the system, and plasma ignition occurs without producing a substantial
amount of heat. In order to evaluate the enhancement produced by laser-induced hot electrons, we
monitor the emission spectra of an argon plasma discharge (912.3 nm line) at different peak pulse
38
voltages both with and without laser irradiation. The 912.3 nm line corresponds to an optical
transition in neutral argon from 4p state to 4s state.
153
Argon emission spectra are collected from
a region between two parallel copper electrodes, which are made by depositing Cu tape on a
standard glass slide, interspersed with metal nanoparticles in a flowing Ar environment, as
illustrated in Figures 1b and 1c. We flow Ar at a rate of 100sccm, and the plasma discharge is
created by high voltage pulses at a repetition rate of 1 kHz repetition rate. The emission spectra
were collected in Renishaw InVia micro-Raman spectrometer and the objective lens used in this
work was an Olympus LUCPlanFLN 40x cover-glass corrected lens. Photographs of our
experimental setup are presented in Figure A2. Figure 4.1c shows a typical high resolution
transmission electron microscope (HRTEM) image of the metal nanoparticles used in this work,
deposited via electron beam evaporation with a nominal thickness of 5-10 nm.
154-156
As is shown
in the Figure 4.1c, this thickness is not enough to form a continuous metal thin film and, instead,
creates thin island-like structures or “nanoparticles”.
39
Figure. 4.1 (a) Typical output waveform from the high voltage nanosecond pulse generator (b) A schematic diagram of the
experimental setup for observation of optical emission (c) HRTEM image of nanoparticles morphology deposited on glass-slides.
40
4.4 Results & Discussion
Figure 4.2 shows the plasma emission intensity plotted as a function of peak pulse voltage
for an electrode gap containing platinum nanoparticles. This plot shows two datasets, one with the
laser on and the other with the laser off, showing an up to 200-fold increase in the plasma emission
intensity. The plasma emission intensity plotted in Figure 4.2a corresponds to the integrated areal
intensity after baseline-subtraction and fitting these peaks with a Gaussian lineshape. Here, the
plasma emission intensity serves as a proxy for the relative plasma density. While we do not have
a way to quantify the absolute plasma density, these measurements provide a relative measure of
the plasma density. Figures 4.2b and 4.2c show the raw spectra taken at peak voltages of 4.5 kV
and 5 kV, respectively. All spectra shown here are baseline-subtracted. The data shown in Figure
4.2 was taken with a constant laser power density of 2.4 W/cm
2
focused through a cover-glass
corrected, high numerical aperture objective lens. As illustrated in the Figure 4.2, it should be
noted that plasma can be initiated at a lower pulse voltage threshold (i.e., 4.0 kV) with laser
irradiation than is possible without irradiation (i.e., 4.5kV). However, the enhancement of plasma
emission intensity with laser irradiation decreases at higher voltages. In the range of lower applied
pulse voltages (i.e., 4-5 kV), the limiting factor for plasma emission is the hot electron population,
which can be significantly modified under laser irradiation. However, at high peak voltages (i.e.,
5.5kV) photoexcited hot electrons are not needed in order to initiate the plasma. Here, the limiting
factor becomes field emission of electrons from the nanoparticles (and copper electrodes) and,
thus, laser irradiation has a limited effect on the plasma emission intensity.
41
Figure 4.2 (a) Plasma emission intensity plotted as a function of peak pulse voltage with and without 532nm laser illumination
observed from Pt nanoparticles. Plasma emission spectra under (b) 4.5kV and (c) 5kV peak voltage with and without 532nm laser
illumination on Pt nanoparticles (PtNP).
42
While Figure 4.2a shows the peak pulse voltage dependence of plasma emission, Figure
4.3 shows the laser power dependence of the plasma emission intensity, which shows a linear
dependence on the laser power. These data were taken with gold nanoparticles instead of platinum
nanoparticles. This data was taken at a fixed pulse peak voltage of 2.4 kV. Here, the plasma is
initiated at a much lower peak pulse voltage of 2.4 kV, likely because of the lower work function
of gold compared with platinum and gold’s strong interband transitions around 532nm wavelength
(i.e., 2.4 eV) that originate from the d-band electrons in gold.
157-159
Interestingly, no enhancement
in the plasma emission was observed with 633nm photons (i.e., 1.96 eV) or 785 nm photons (i.e.,
1.58 eV), since these photons lie below the threshold for interband transitions in Au. Another
possible explanation is sample-to-sample variation. That is, small variations in the size, shape, and
separation of the nanoparticles can lead to large changes in the local fields and, hence, plasma
initiation voltage.
145
Nevertheless, it should be noted that all quantitative comparisons in plasma
emission intensity were done on the same sample. Therefore, this sample-to-sample variation will
not affect the main conclusions of this paper.
43
Figure 4.3 (a) Plasma emission spectra observed from Au nanoparticles under different laser illumination conditions. (b) Plasma
emission intensity observed by discharging 2.4 kV pulses across Au nanoparticles (AuNP) plotted as a function of laser power.
The laser wavelength here is 532 nm.
Figure 4.4 shows the energy band diagrams illustrating the hot electron-driven process by which
plasma emission is enhanced in these metal nanoparticle geometries. Figure 4.4a shows the energy
44
band diagram for a typical metal with no photoexcitation. Here, the workfunction, which needs to
be overcome in order for thermionic emission to occur, thus initiating plasma discharge, is
approximately 5.1 eV. Figure 4.4b illustrates the case with photoexcited hot carriers in the metal,
where a photon excites an electron at the Fermi energy to 2.4 eV above the Fermi energy, thus,
lowering the effective work function by as much as 2.4 eV. In gold, there are a large number of
states corresponding to d-band electrons, which lie roughly 2 eV below the Fermi level.
157, 158
As
such, the hot electrons that are actually formed by a 2.4 eV photon are centered around an energy
of approximately 0.4 eV above the Fermi energy, as illustrated in Figure 4c. This results in a lower
effective work function (Φ≈4.7 eV). However, it should be noted that the lowering of the effective
work function results in an exponential increase in thermionic emission and, hence, plasma
discharge intensity. The hot electron distribution illustrated in Figure 4.4c, decays over very short
time scales, less than 50 femtoseconds. Based on our previous work using ultrafast pump-probe
spectroscopy (i.e., transient absorption spectroscopy), this narrow distribution of hot electrons
decays into a hot Fermi distribution, as illustrated in Figure 4.4d. This hot Fermi distribution
decays back to equilibrium with the lattice temperature over a timescale of 2 psec,
70, 98, 160, 161
as
shown in the Figure A13. Here, we believe it is electrons in the tail of this hot Fermi distribution
that contribute most significantly to the effect observed in this paper.
45
Figure 4.4 Energy band diagrams of gold nanoparticles (a) without and (b) with 532nm wavelength laser irradiation. Illustration of
hot electron distributions at (c) ~50 fsec and (d) ~2psec.
4.5 Conclusions
In conclusion, we demonstrate that transient plasma discharge across metal nanoparticles can be
enhanced up to 200X under 532nm wavelength laser illumination. This enhancement is achieved
by laser-induced hot-electron excitation, which can generate large hot-electron populations and
reduce the effective work function of the material, enabling increased electron emission from the
metal nanoparticle surfaces. Due to the small size of these nanoparticles, which are on the order of
the mean free path of the electrons, these hot electrons have a high probability of traveling to the
nanoparticle surfaces and, thus, being emitted by the applied electric field. No enhancement is
observed under 633nm wavelength illumination, which lies below the threshold for interband
transitions in these metals. Because of the exponential dependence of electron field emission, and
hence plasma initiation, this general scheme provides a sensitive method for studying these
46
relatively short-lived hot electrons. This reported enhancement in plasma generation is induced by
photoexcitation, thus, opens up a new potential pathway for photon energy utilization and
harvesting by assisting plasma generation and potentially driving chemical reactions near the metal
nanoparticle surface.
4.6 Future Work Related to Plasma Discharge on Metal Nanoparticle Surfaces
In Chapter 2 to Chapter 4, we focused on the plasma discharge on metal nanoparticle surfaces in
gaseous environment. Outstanding enhancement effect is observed, and we manage to further
enhance plasma discharge with laser illuminations. It will be interesting if further work can be
extended into plasma discharge in aqueous environment, which has a lot of potential applications
in water treatment and purification. During this process, highly oxidative radicals, known for their
potentials in decomposing organic pollutant and bacteria, can be generated, including hydroxyl
radical (
•
OH), superoxide anion (O2
–
), and hydrogen peroxide (H2O2).
27, 162, 163
The most important
difference between discharging in aqueous and gaseous environments lies on the molecular density.
As is introduced in the Chapter 1, the plasma ignition is caused by an avalanche reaction where
massive collisions take place. However, aqueous environment makes such avalanche reaction
unlikely to occur due to the high molecular density, low charge mobility and short mean free path.
In that case, the formation of bubbles, which can be a result of joule heating or electrolysis,
becomes a significant process for plasma formation in liquid. These bubbles can support avalanche
reactions inside, which opens up a plasma channel.
164, 165
The electric field enhancement effect
brought up by metal nanoparticle surfaces can potentially enhance this bubbling formation process
as well, together with electron field emission. In that case, it might be interesting to probe how
metal nanoparticles can affect plasma generation in aqueous environment.
47
Figure 4.5 Comparison between electron field emission on metal nanoparticle surfaces within (a) gaseous environment and (b)
aqueous solutions. In aqueous solutions, we expect to see the electric field enhancement effect can affect electron emission and
bubble formation processes.
Substrate
E
Hot e
-
migration
Joule heating
e
-
field emission
Substrate
E
Hot e
-
migration
Joule heating
e
-
field emission
Bubbles
(a)
(b)
48
Chapter 5 Introduction to Semiconductor Photocatalysis
Solar energy harvesting has been the most frequently discussed topic ever since the energy
crisis, when human beings started to look for alternative energy sources other than fossil fuels.
Solar cells with various semiconductor materials and different structures have been explored and
even commercialized to help us convert solar energy into electricity. However, some important
progress will be needed to let solar power plants fulfill our energy demands. First, the materials
for harvesting and storage of energy should be environmental-friendly and safe. Second, stable
and constant energy flux will be needed to make solar energy a reliable energy source. Another
way of converting solar energy would be photocatalysis. This can also be considered as storing
solar energy in chemical bonds of chemical fuels, which can give us stable energy flux after
centuries of work in combustion engines. Considering the global warming issue, which restricting
the carbon emission, semiconductor photocatalysis of water splitting becomes an ideal solution for
all these problems mentioned above.
5.1 Introduction to semiconductor-liquid junction
Semiconductor can absorb photon energy via charge photoexcitation from valance band to
conduction band, following by recombination of electron-hole pairs. In order to harvest energy
from these photogenerated hot carriers, separation of electron-hole pair will be needed, which is
typically achieved via a built-in electrical field of a junction structure. Thermal equilibrium
requires electron transfer between semiconductor and redox couples in liquid, resulting in an
electric field which can balance the potential difference between semiconductor Fermi-level and
Nernst potential of redox couple. After the establishment of this new equilibrium, a semiconductor-
liquid junction is formed, containing a built-in field that can assist charge separation process in
49
after photoexcitation. Figure 5.1 shows an example of n-type semiconductor-liquid junction.
Electrons in semiconductor electrode, which has higher Fermi-level, will transfer into the solution,
leaving excess positive charge centers in semiconductor and excess negative charges in solutions.
This built-in will direct minority carriers into solutions, which suggest n- and p- type
semiconductors will be typically used as photoanode and photocathode, respectively. Positive
charges will distribute near semiconductor-liquid interface across a depth called depletion width
and photogenerated electron-hole pairs inside this region can be separated. Negative charges will
distribute in the solutions and vast majority of them will be located in a thin layer called Helmholtz
layer. This type of charge distribution makes semiconductor-liquid junction work as single side
junction.
Figure 5.1 A schematic of band diagram of a n-type semiconductor and a redox couple (A/A-) (a) before thermal equilibrium, (b)
after thermal equilibrium and (c) under illuminations.
166
Under low illumination conditions, minority carriers will be excited into higher energy
levels and using Fermi-Dirac distribution, minority carriers will possess a different Fermi-level
called quasi-Fermi level. The splitting between quasi-Fermi level of minority carriers and Fermi
level of majority carriers determines the photovoltage that can be generated from semiconductor-
liquid junctions under illuminations. When there is no net current, we can get the highest
(a) Before Equilibrium (b) After Equilibrium (c) Illuminated
E
c
E
F
E
v
E
c
E
F
E
v
-qE(A/A
-
)
-qE(A/A
-
)
-qE(A/A
-
)
E
c
E
F,n
E
v
W
E
F,p
V
oc
N-type Liquid N-type Liquid N-type Liquid
50
photovoltage called open-circuit voltage (Voc), which can be measured via different methods
experimentally.
167-170
An expression for open-circuit voltage is given by:
𝑉 𝑜𝑐
= ( 𝑛 𝑘 𝐵 𝑇 𝑞 ⁄ ) ln ( 𝐽 𝑝 ℎ
/ 𝛾 𝐽 𝑠 ) ) (5.1)
Where 𝑛 is the diode quality factor, 𝑘 𝐵 is the Boltzmann’s constant, 𝑇 is the temperature (in K), 𝑞
is an electron charge, 𝛾 is the ratio of the actual junction area to the geometrical surface of the
electrode, 𝐽 𝑝 ℎ
and 𝐽 𝑠 are photocurrent density and saturation current density.
166
5.2 Introduction to photocatalysis of electrochemical water splitting
Electrochemical water splitting can be divided into two sub reactions, hydrogen evolution
reaction (HER) and oxygen evolution reaction (OER):
OER: 𝐻 2
𝑂 + 2 ℎ
+
=
1
2
𝑂 2
+ 2 𝐻 +
(5.2)
HER: 2 𝐻 +
+ 2 𝑒 −
= 𝐻 2
(5.3)
Total reaction: 𝐻 2
𝑂 =
1
2
𝑂 2
+ 𝐻 2
𝛥𝐺 = 2 3 7 2 . 2 𝑘𝐽 /𝑚𝑜𝑙
Under standard chemical reaction pathways, the above chemical reaction requires 237.2kJ/mol of
energy, which correspond to 𝛥 𝐸 0
= 1 . 23 𝑉 for each electron transferred, based on the Nernst
equation. This suggest the band gap of a semiconductor must be larger than 1.23eV to drive the
above reactions in a single junction cell. However, surface kinetics at the semiconductor-liquid
interface could introduce losses and thus requires for overpotentials to drive HER and OER
reactions.
171-173
In practical cases, the band gap of semiconductors should be 1.6-2.4eV to split
water molecules. In order to maintain the thermal dynamic stability of reductive and oxidative
reactions while driving HER and OER with a single junction, semiconductor should possess a
conduction band higher than the reductive potentials and a valence band lower than oxidative
potentials, which makes few semiconductor materials compatible. This can be solved by adding
51
an extra bias via a power supply and the semiconductor photoelectrode will only serve as
photocathode or photoanode. The energy efficiency ( 𝜂 ) of an ideal photocatalysis cell with extra
bias can be measured in a J-V measurements and expressed as:
𝜂 = 𝐽 𝑚𝑝
( 1 . 23 𝑉 − 𝑉 𝑎 𝑝 𝑝 ) /𝑃 𝑖𝑛
(5.4)
Where 𝐽 𝑚𝑝
is the experimentally measured current density, 𝑉 𝑎 𝑝 𝑝 is the potential applied between
cathode and anode, 𝑃 𝑖𝑛
is the power density of an illumination source.
Figure 5.2 A band diagram of an ideal single junction semiconductor water splitting device.
As is mentioned above, surface kinetics will lead to overpotentials, which can drive rate
limited kinetics, for electrolysis reaction to take place. The absorption, formation and desorption
rate on most bare semiconductor surfaces are low. This indicates a considerable overpotential will
be required and thus, the energy efficiency will be unideal. One common solution to this issue is
adding catalyst on semiconductor surfaces (e.g., via adding a layer of catalytical metal
E
c
E
v
E
g
-qE
0
(H
+
/H
2
)
-qE
0
(O
2
/H
2
O)
e
-
h
+
1.23eV
52
nanoparticles). This layer of discontinuous catalytical metal will not only improve the rate of slow
surface kinetics, but also change the energetics of the electron transportation at the semiconductor-
liquid interface. The electrons that injected into the redox couples are from catalytical metals
instead of semiconductors. The influence of catalyst layer can be shown in Figure 5.3. After adding
a thin layer of Pt, we can notice an earlier current take of due to the reduced overpotential and a
larger photocurrent as a result of larger surface reaction rate, Some most frequently used catalyst
include, Pt
174, 175
, Ni
176
, metal alloys
177, 178
and some mixture of metal with nonmetal
components
179, 180
.
Figure 5.3 Photo-I-V curves for TiO2-coated GaAs with and without Pt co-catalyst deposited by sputter deposition
-1.6 -1.2 -0.8 -0.4 0.0
-2.0
-1.5
-1.0
-0.5
0.0
With Pt
Without Pt
Current (mA)
Potential vs Ag/AgCl
Dark
1
53
Chapter 6: Band Flattening and Photo-induced Modulation of Depletion
Region in Semiconductor Photoelectrodes – Key loss mechanism
6.1 Abstract:
The phenomenon of band flattering has been conventionally though to only occur at
extremely high light intensities, when the photoexcited charge density becomes comparable to the
doping concentration (~10
16
cm
-3
). Under these conditions, the band bending and associated built-
in fields are removed and photoexcited charge is no longer separated efficiently, thus, eliminating
the key driving force underlying the photoelectrochemical process and ruining device
performance. However, the work presented in this paper shows that this phenomenon of band
flattening actually occurs at much lower light intensities (~µW) for poor catalytic surfaces with
slow charge transfer rates (GaAs/TiO2). The phenomenon of band flattening can be seen in the
non-linearity of the incident photon to current efficiency (IPCE) vs laser power and photo-I-V
curves. Interestingly, for 1-10µW, the IPCE of GaAs/TiO2 is nearly 100% and drops down to 60%
at intensities as low as 1mW/cm
2
(i.e., 100X less intense than sunlight). This demonstrates that
this is indeed a key loss mechanism. A physical model is built up that can correlate measured
incident photon to current efficiency (IPCE) to accumulated charge density near the surface. To
further understand and testify this key loss mechanism, we compare IPCE of samples with and
without a co-catalyst metal, Pt, which significantly reduced the IPCE dropping with illumination
power. Through a systematic study as a function of several variables and a simple analytic model,
we are able to identify and separate the effects of band flattening as a key loss mechanism.
54
6.2 Introduction:
Global warming has urged human beings finding an alternative energy source that is carbon
free. Photocatalysis of water splitting based on various materials becomes an ideal solution to this
issue since combustion of hydrogen is clean and storage of hydrogen is easier than storing
electricity.
181-188
Band bending at the semiconductor/liquid interface gives rise to large electric
fields within the semiconductor that provide the main driving force underlying all
photoelectrochemical processes (i.e., photocatalytic) processes. Upon photoexcitation, these large
built-in electric fields within the depletion region of the semiconductor separate electrons and
holes, sweeping the photoexcited minority carriers towards the semiconductor surfaces where this
charge is ultimately transferred to the ions in solution. This band bending arises from the energy
difference between semiconductor Fermi level and redox potential of redox couples in the adjacent
solutions. While bringing semiconductor and liquid into contact, charge transfer will take place
until a thermal equilibrium can be reached between these two phases. As a result, an electric field
will be built up in-between the excess ionized charge centers in the semiconductor after charge
transfer and excess charges from the solutions accumulated at the Helmholtz layer. Band bending
will also affect the adsorption and desorption of reactants and products of the electrochemically
driven reaction, indicating a huge influence in reaction kinetics.
166, 188
Thorough understanding of
band bending in semiconductor liquid junction is crucial for manufacturing a stable photo-
catalytical device.
Photocatalysis involves many important kinetics, which can be roughly classified into three
regions which are solid state, interface and solutions. Photon absorption, electron-hole pair
separation, free carrier transportation (diffusion and drifting due to the electrical field) and charge
recombination are mainly taking place in solid state region, especially in the space charge region
55
or depletion region. Interface kinetics are mainly charge transfer into surface active sites, charge
transfer from active site to the redox reactions, adsorption and desorption of reactants and products.
In solution phase, the most important kinetics will be diffusion of reactants and products. It should
be noted that the final reaction rate will be determined by the slowest process on cathode, anode
and solutions with related interfaces. In most semiconductor materials, electron-hole pairs will be
generated with a time scale of hundred fs with a carrier lifetime of tens of nanoseconds.
189-192
However, Surface reaction rates can be much slower. Electrons might need 10-1000 µs to reduce
water molecule and holes require 2 µs to drive OER on TiO2 surfaces.
193
These difference of time
scales between photoexcitation, carrier lifetime and reaction rates suggest that a considerable
number of hot carriers might exist in the semiconductor before recombination or chemical reaction
consume these carriers. The influence of these accumulated charges to the semiconductor liquid
junction worth probing into.
In the work presented here, we explored the reduction of incident photon to current efficiency
(IPCE) with an increasing illumination density on GaAs/TiO2 electrode. We suspect this reduction
to the charge accumulation at the space charge region and correlate IPCE with charge accumulation
density through calculations.
6.3 Method
Here, P-type Zn-doped (10
16
~10
17
/cm
3
) GaAs (111) wafers (from UniversityWafer, Inc.) are
deposited with 5nm/50nm Ti/Au as back contact after etched in 1:1 HCl solutions for 15min. GaAs
can be electro-corroded under negative potentials and thus, a protective metal oxide layer will be
needed.
194-196
5nm of TiO2 is deposited at the front surface of GaAs via atomic layer deposition
(ALD) method using Tetrakis(dimethylamino)titanium (TDMAT) and H2O precurors at 250 °C.
TiO2 with thicknesses of ~5nm to provide both highly conductive and highly stable passivating
56
layers.
155, 197-203
In our previous work, we have demonstrated that these TiO2 films provide
catalytically active sites (i.e., O-vacancies) that improve the catalytic processes of water splitting
and CO2 reduction.
204-207
However, these TiO2 surfaces have a much lower performance than metal
co-catalysts, such as Pt, Ru, and Cu, most likely because the density of active sites and turn over
frequencies are much higher on these metal surfaces than TiO2.Sample geometry is shown in the
Figure 6.1b and a detailed TEM result is provide in Fig. A14 in appendix. 2nm Pt deposition is
performed via sputtering deposition on TiO2 layer if needed. Wafer pieces will then be wired with
silver paints and stabilized on glass slides using epoxy as sealing to avoid metal contact exposed
to the testing solutions. IPCE measurements are performed in H2SO4 solution with pH=1. Chopped
532nm laser (Laserglow, Inc., LCS-0532) is used as light source and tuned with neutral density
filter. Gamry is used for three-terminal potential static measurements, where Ag/AgCl and Pt is
used as reference electrode and counter electrode, respectively. To measure the small current under
low illumination conditions, a lock-in amplifier is connected to the Gamry. 532 nm laser is
chopped at 200Hz and the lock-in amplifier filters out signals modulated at this frequency only.
For modeling of band flattening and depletion width modulation due to accumulated charges, we
performed calculations on MATLAB.
57
Figure 6.1 Schematic of three terminal measurement setup with lock-in amplifier(a) and sample structure (b).
6.4 Results and Discussion
Figures 6.2a show photocurrent of TiO2-coated GaAs with and without Pt co-catalyst plotted
as a function of laser illumination power. Here, photocurrents, which is in linear relationship with
IPCE, only saturate at large negative potentials (as is shown in Figure A14). We believe is limited
by diffusion, or Tafel, or depletion region size, and therefore saturated photocurrents (by
eliminating/minimizing these other loss mechanisms via adding overpotentials) enables us to study
the phenomenon of band flattening. We can clearly notice that photocurrent deviate from linear
1E-6 1E-5 1E-4 0.001
50
60
70
80
90
100
IPCE (%)
Laser Power (W/cm
2
)
GaAs 5nm TiO2 2nm Pt
GaAs 5nm TiO2
Counter
Electrode
TiO2
Gamry
Lock-in
Amplifier
Chopper
Beam
Expander
Collimator
Ag/AgCl
532nm
laser
p-GaAs
Contact
Schematic illustration of GaAs/TiO
2
photocathode (left
panel), and IPCE performance under CW illumination (right
panel).
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0
20
40
60
80
100
IPCE ( % )
Potenti al ( V ) vs Ag / AgCl
1 mW
2 mW
4 mW
5 mW
8 mW
10 mW
15 mW
20 mW
25 mW
30 mW
Potential vs. Ag/AgCl (V)
Schematic illustration of GaAs/TiO
2
photocathode (left
panel), and IPCE performance under CW illumination (right
panel).
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0
20
40
60
80
100
IPCE ( % )
Potenti al ( V ) vs Ag / AgCl
1 mW
2 mW
4 mW
5 mW
8 mW
10 mW
15 mW
20 mW
25 mW
30 mW
Potential vs. Ag/AgCl (V)
IPCE (%)
Schematic illustration of GaAs/TiO
2
photocathode (left
panel), and IPCE performance under CW illumination (right
panel).
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0
20
40
60
80
100
IPCE ( % )
Potenti al ( V ) vs Ag / AgCl
1 mW
2 mW
4 mW
5 mW
8 mW
10 mW
15 mW
20 mW
25 mW
30 mW
Schematic illustration of GaAs/TiO
2
photocathode (left
panel), and IPCE performance under CW illumination (right
panel).
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0
20
40
60
80
100
IPCE ( % )
Potenti al ( V ) vs Ag / AgCl
1 mW
2 mW
4 mW
5 mW
8 mW
10 mW
15 mW
20 mW
25 mW
30 mW
Schematic illustration of GaAs/TiO
2
photocathode (left
panel), and IPCE performance under CW illumination (right
panel).
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0
20
40
60
80
100
IPCE ( % )
Potenti al ( V ) vs Ag / AgCl
1 mW
2 mW
4 mW
5 mW
8 mW
10 mW
15 mW
20 mW
25 mW
30 mW
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
0.0000 0.0005 0.0010 0.0015
0.0
50.0µ
100.0µ
150.0µ
200.0µ
250.0µ
Photocurrent (A)
Laser Power (W/cm
2
)
GaAs 5nm TiO2 2nm Pt
GaAs 5nm TiO2
GaAs 5nm TiO2 2nm Pt linear fitting
GaAs 5nm TiO2 linear fitting
58
growth with increasing illumination intensity, which suggests a dropping of IPCE, and samples
with Pt coating showed smaller deviation. Figure 6.2b shows the maximum (i.e., large negative
potentials) IPCE plotted as a function of the incident optical power density. At low negative
potentials, we observed IPCE values of nearly 100% for the bare TiO2 surface. We also observe a
large laser power dependence at low light intensities and only 60% of IPCE can be measured for
higher light intensities (~1mW). For the Pt-coated surface/sample, the maximum IPCE curve
shows less dependence on of the optical power density, and it variates from 70% to 80%. While a
40% drop of IPCE is shown in the sample without Pt, samples with Pt gives us only 10% of IPCE
loss at the sample power density range.
59
Figure 6.2 (a)Photocurrent measured on sample with and without Pt under different laser illumination conditions and associated
IPCE (b). Please note that all data points from figure (a) are taken at lower illumination conditions, where sample with Pt posses
lower IPCE.
1E-6 1E-5 1E-4 0.001
50
60
70
80
90
100
IPCE (%)
Laser Power (W/cm
2
)
GaAs 5nm TiO2 2nm Pt
GaAs 5nm TiO2
(a)
(b)
0 10µ 20µ 30µ 40µ 50µ
0.0
1.0µ
2.0µ
3.0µ
4.0µ
5.0µ
Photocurrent (A)
Laser Power (W/cm
2
)
GaAs 5nm TiO2 2nm Pt
GaAs 5nm TiO2
GaAs 5nm TiO2 2nm Pt linear fitting
GaAs 5nm TiO2 linear fitting
1.0µ 2.0µ 3.0µ 4.0µ 5.0µ
100n
200n
300n
400n
500n
600n
700n
60
We hypothesis that the dropping of IPCE is related to the band flattening and the difference
brough up by Pt, a well-known catalyst for HER, is related to the fast surface kinetics. Surface
chemical reaction is typically the rate-limiting process in photocatalysis. The phenomenon of band
flattening is illustrated in Figure 6.3. In this scenario, we believe the photoexcited charge is first
separated (Figure 6.3b), and then the positive and negative charge accumulates on either side of
the depletion region (Figure 6.3c), resulting in a long-lived cancelation of the Ɛ-field (i.e., 𝜀 t o t
=
0), a modification of depletion width and associated flattening of the bands (i.e., ∆ 𝜙 𝑏 𝑎 𝑛𝑑 𝑠 =
− ∫ 𝜀 ( 𝑥 ) 𝑑𝑥 = 0). We believe that this band flattening effect is largely due to the slow kinetics of
the relatively poor catalytic surface (i.e., TiO2) and can potentially be improved by increasing the
kinetics associated with charge transfer at the semiconductor-liquid interface (e.g., by adding a
metal co-catalyst). It is for this reason that we see a deviation from the linear relation in Figure
6.2A, thus, indicating that some form of band flattening is occurring in this system at relatively
low optical power densities (~0.05mW/cm
2
). Considering the fact that only charges excited within
and near the depletion width can be harvest be the photocatalysis system, this modification of
depletion width will lead to lower IPCE. This is a deleterious effect that we would like to avoid.
Pt, a well-known catalyst, can accelerate charge transfer from solid states to HER reactions and
thus, less charge accumulation will take place, give us a higher IPCE under high illumination
conditions. It should be noted that, under low illumination conditions, lower IPCE is observed with
the presence of Pt. This might be due to the difference in rate-limiting process under different
illuminations. Under low illumination conditions, photogeneration rate becomes the rate-control
process and Pt, the coverage of which is will reduce carrier generation rate, could further limit the
number of free charges reaching the reaction surface.
61
Figure 6.3 Mechanism of band flattening due to high illumination density and slow surface reaction rate. Before photon
illuminations, the semiconductor-liquid junction has a depletion width of Wo. After illuminations, charges are excited and separated
at this depletion region, but accumulated charges will reduce the built-in field and the depletion width (W).
In order the better understand the photoelectrochemical process, we use a simple analytic
model to estimate the potential and light intensity dependence of the charge separation process, as
Before Photoexcitation
During Photoexcitation
After Photoexcitation
p-Gap
Depletion Region
TiO
2 + -
+
+
+
-
-
-
Charge
recombines
Charge
separated
+ -
+ -
(a)
(b)
(c)
Figure 8. Diagrams
illustrating the
phenomenon of band
flattening at a
photoelectrode surface
(a) before, (b) during, and
(c) after photoexcitation.
Before Photoexcitation
During Photoexcitation
After Photoexcitation
p-Gap
Depletion Region
TiO
2 + -
+
+
+
-
-
-
Charge
recombines
Charge
separated
+ -
+ -
(a)
(b)
(c)
Figure 8. Diagrams
illustrating the
phenomenon of band
flattening at a
photoelectrode surface
(a) before, (b) during, and
(c) after photoexcitation.
Before Photoexcitation
During Photoexcitation
After Photoexcitation
p-Gap
Depletion Region
TiO
2 + -
+
+
+
-
-
-
Charge
recombines
Charge
separated
+ -
+ -
(a)
(b)
(c)
Figure 8. Diagrams
illustrating the
phenomenon of band
flattening at a
photoelectrode surface
(a) before, (b) during, and
(c) after photoexcitation.
W
o
After Charge Accumulation
(c)
W
During Photoexcitation
(b)
Before Photoexcitation
(a)
62
is shown in Figure 6.4. Here, we assume that there is 100% charge separation efficiencies within
the depletion region, that is, only light within the depletion region is separated, and there is no
recombination. As a result, separated charge number ( 𝑄 𝑠 𝑒 𝑝 , in units of #/cm
-3
s
-1
) and the IPCE are
only a function of depletion width for a given material and we can calculate IPCE based on
equation (6.1) and (6.2) with different depletion width, as shown in Figure 6.4a. With equation
(6.3) and (6.4), we can calculate depletion width and electric field without illumination. After
illumination and charge accumulation, electric field generated from accumulated charge will
reduce the total electric field and narrow the effective depletion width, leading to band flattening
and reduced IPCE. By comparing the difference in IPCE before and after illumination, narrowed
depletion width and reduced total electric field can be calculated with equation (6.3) and (6.4).
This difference of electric is caused by accumulated charges induced electric field. Assuming
accumulated charges can be simplified with plate capacitor model, accumulated charge density
can be calculated from this induced electric field with equation (6.5). Related equations are shown
as the following:
𝑄 𝑠 𝑒 𝑝 = ( 𝐼 0
− 𝐼 ( 𝑥 ) ) / ℎ 𝑤 (6.1)
𝐼𝑃𝐶 𝐸 = 𝑄 𝑠 𝑒 𝑝 / 𝐼 𝑜 (6.2)
𝑊 = √
2 ɛ
𝑠 | 𝑉 − 𝑉 𝑓𝑏
|
𝑒 𝑁 𝑎 (6.3)
𝐸 𝑚𝑎𝑥
= √
2 𝑒 𝑁 𝑎 | 𝑉 − 𝑉 𝑓𝑏
|
ɛ
𝑠 (6.4)
𝐸 =
𝜎 ɛ
𝑠 ⁄
(6.5)
where a is the absorption coefficient depends strongly on wavelength and I(x) decays exponentially
within the semiconductor (i.e., I(x)= Io exp(-ax). Here, the integrand is the electron-hole pair
63
generation rate, given by: aI(x)/hw in units of #/cm
-3
s
-1
. Based on the experimentally-measured
flat band potential (VFB), this model gives the # of photoexcited carriers available for
photocatalysis per cm
-2
s
-1
as a function of reference potential, which can be related back to
measured photocurrent density and IPCE. Accumulated charge density is 𝜎 in equation (6.5). The
calculation results clearly show how charge accumulations will affect IPCE in a semiconductor-
liquid junction.
64
Figure 6.4 (a) Mechanism of band flattening and depleting width narrowing under laser illumination. (b) Calculated relationship
effective depletion width and IPCE (c) Calculated relationship Accumulated charge density and IPCE
(a)
(b)
(c)
0 100 200 300 400
0.0
0.2
0.4
0.6
0.8
1.0
IPCE
Effective Depletion Width (nm)
0 1 2 3 4
0.0
0.2
0.4
0.6
0.8
1.0
IPCE
Accumulated Charge Density 10
11
/cm
2
65
6.5 Future work
To further evaluation this hypothesis, we need to probe the interface between
semiconductor and liquid junction. An important physical parameter in the proposed process could
be surface electrical field. If the proposed mechanism can describe the real reason behind the drop
of IPCE, we would anticipate a change in surface electric field as a result of depletion width
modification and we need some surface reporter to characterize this parameter.
Our group has previously worked 4-mercaptobenzonitrile (4-MBN) Stark shift effect in
Raman spectroscopy, which is a vibrational frequency shifting due to applied electric field. Stark
Shift of 4-MBN originates from its C-N stretch (2225cm
-1
under 633nm laser)
208
, which contains
a dipole moment and thus will interact will an applied electric field. Numerous previous works
from different groups have proved the linear relationship between electric field and Stark shift
𝐸 =
𝛥 𝜔 𝑐 − 𝑛 0 . 36
( 𝑀𝑉 / 𝑐𝑚 ) (6.6)
where 𝛥 𝜔 𝑐 − 𝑛 is the shifting of Raman peak from the frequency when no electric field is applied.
208-212
This linear relationship enables us to use 4-MBN as a surface electric field reporter. A thin
layer of discontinuous metal nanoislands will be needed in 4-MBN measurement due to its ability
of enhancing Raman scattering and bonding thiolated molecules, resulting a structure as shown in
Figure 6.5a. If the proposed mechanism is true, we will expect to see a smaller surface electric
field due to the band flattening effect and this can be reflected by the Stark shift of 4-MBN
molecule bonded at this surface. Our group has also reported another molecule, copper (II)
phthalocyanine (CuPc) contains similar Stark shift behavior under electric field. However, this
molecule is 5 orders of magnitude less sensitive to electric field compare to 4-MBN so we don’t
think it is applicable in this work.
213
66
Figure 6.5 (a)A schematic of surface enhanced Raman spectroscopy (SERS) measurement of 4-MBN and its Stark shift on glass
substrate in 0.05M Na2SO4 solution.
In order to character the semiconductor-liquid surface under illumination, we need to
deposit the same structure on top of semiconductor surface and upgrade our current Renishaw
InVia micro-Raman system with the capability of doing pump-probe measurement, which can be
achieved by adding a 405nm pump laser and a long path filter as is shown in Figure 6.6. In this
pump-probe measurement, the electric field will be probed by 633nm (1.96eV) laser for 4-MBN
Raman signal collection and the semiconductor should be pumped by the 405nm (3.06eV) laser
only. This requires us to change our semiconductor substrate into those materials with wider band
gap, such as GaP (2.24eV)
214
or TiO2 (3.1eV, but Ti3+ impurities and O vacancies will narrow the
band width to 2.5 to 2.8 eV)
215
. However, some issues may be brought up by semiconductor
substrates as well. The first issue will be photoluminescence (PL) of semiconductor substrates.
Figure 6.6b shows during the dual wavelength measurement, even 0.1µW of 405nm laser will
cause a PL background which is strong enough to make the Raman peak of MBN unable to be
fitted. Another issue will be the stability of detector molecules on semiconductor surfaces. Intrinsic
TiO2 will be n-type due to the O vacancies.
216, 217
The junction between TiO2 and H2O will sweep
holes in TiO2 into the semiconductor surface and oxidize the organic molecules on top of it.
218
As
Glass
(a) (b)
Au Au Au
TiO2
ITO
Au Au Au
TiO2
GaP
(a) (b)
vs.
Au Au Au
TiO2
ITO
Au Au Au
TiO2
GaP
(a) (b)
vs.
2552
5
5
2552
5
5
Au Au Au
TiO2
ITO
Au Au Au
TiO2
GaP
(a) (b)
vs.
Au Au Au
TiO2
ITO
Au Au Au
TiO2
GaP
(a) (b)
vs.
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
2223
2224
2225
2226
2227
2228
2229
2230
Center (cm
-1
)
Potential vs Ag/AgCl (V)
67
shown in the Figure 6.6c, we can hardly detect any MBN signal after 2 rounds of Raman
measurements (1 min). Considering all these factors, the substrate should be a p-type wide bandgap
semiconductor without PL peak near 731nm.
Figure 6.6 (a) Experimental setup of pump-probe measurement for 4-MBN Raman peak. (b) Photoluminescence background from
Zn doped GaP substrate near 4-MBN C-N stretch Raman peak. (c) Self cleaning effect of TiO2 leads to instability of 4-MBN
molecules.
Graphene is another surface reporter that is potentially worth probing into. Our group has
previously reported that using graphene G-band shift, which is related to its Fermi level and
graphene doping concentration, we can calculate the local ion concentration following the charge
neutrality requirement, as described by equation (6.7).
219
𝑛 𝐺 = ∫ 𝐴 𝑜 exp (−
𝑧 𝜆 𝐷 ) 𝑑𝑧 = 𝐴 𝑜 ∞
0
𝜆 𝐷 (6.7)
where 𝑛 𝐺 is the charge density in the graphene electrode, 𝐴 𝑜 is the ion density in solution at the
interface and 𝜆 𝐷 is the Debye length. In semiconductor electrode with graphene on it, charges from
the space charge region should be considered and equation (6.7) should be modified into
Counter
Electrode
Working
Electrode
Raman
Laser
(633)
Gamry
Water
Immersion
Lens
Spectrometer
CCD
Reference
Electrode
Pump
Laser
(405nm)
405nm Long Pass
Filter
633nm Long Pass
Filter
2000 2100 2200 2300 2400 2500
10.0k
15.0k
20.0k
25.0k
30.0k
35.0k
Counts
Raman Shift (cm
-1
)
MBN on p-GaP/5nm Al2O3/7nm Au 633nm only
MBN on p-GaP/5nm Al2O3/7nm Au 633nm and 405nm
2200 2220 2240 2260
48k
49k
50k
51k
52k
53k
54k
55k
56k
57k
58k
59k
60k
Counts
Raman Shift (cm-1)
633nm only
633nm +405nm
633nm only after 405nm
(a)
(b)
(c)
68
𝑄 𝑠𝑐
+ 𝑛 𝐺 + 𝐴 𝑜 𝜆 𝐷 = 0 (6.8)
where 𝑄 𝑠𝑐
is the charges in semicondutor. 𝑄 𝑠𝑐
, 𝑛 𝐺 and 𝐴 𝑜 , will change if the band flattening effect
take place. However, the mobility of holes in semiconductors is considerably higher than ionic in
aqueous solutions. For example, the hole mobility in GaP 140 to 180 cm
2
V
-1
S
-1
but the H
+
ion
mobility is only 36.23x10
-8
m
2
V
-1
S
-1
.
220, 221
According to the previous work, water splitting
happens at the electric field at the order of MV/cm.
208, 213
Debye length in pure water is 948nm
and the absorption depth of 405nm photons in GaP is in the range of 100nm.
222
This suggest that
ion migration out of Debye length requires 30ns while photoexcited carriers will reach the
semiconductor-liquid interface within 100fs. The slow kinetics in aqueous solution and fast carrier
migration in solid states enables us to neglect the changes of 𝐴 𝑜 at the first several nanoseconds
and we can get the changes of 𝑄 𝑠𝑐
with the changes of 𝑛 𝐺 . This measurement requires further
upgrade to the above experimental setup with the ability of time-gated Raman measurements and
synchronized pulsed laser. This can be achieved via a nanosecond level time resolved iCCD
camera (PI-Max 3), which can also serve as an delay generator, and a pulsed laser synchronized
with it, as is shown in Figure 6.7. This experiment design would be more applicable if we can find
a better solution/ion pair that would provide us longer Debye length and lower ion mobility.
69
Figure 6.7 Experimental set up of time gated Raman measurement with a synchronized pump laser.
Counter
Electrode
Working
Electrode
Raman
Laser
(CW)
Gamry
Water
Immersion
Lens
Spectrometer
iCCD
Reference
Electrode
Pump
Laser
(Pulsed)
405nm Long Pass
Filter
532nm Long Pass
Filter
Sync
70
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Appendix
Figure A1. Enhancement of Ar plasma emission with Cu nanoparticles. (a) emission of Ar on different substrate with 5 kV peak
voltage (b) emission intensity changing with peak voltage on different substrate. A 1000x enhancement can be observed near 5 kV.
Figure A2 shows a photograph of our experimental setup in which the high voltage cable from our
nanosecond high voltage pulse generator is attached to a glass slide-based reactor, which consists
of two parallel copper electrodes separated by an approximately 5mm gap. The gas flow cell is
constructed by sandwiching four glass slides together with 3D-printed manifold as shown in Figure
A2b. This flow cell has a low profile and fits easily underneath our high numerical aperture
microscope objective lens, enabling plasma emission spectra in the visible wavelength range to be
obtained with high collection efficiency, as illustrated in Figure A2c.
Figure A2. Photographs of (a) the plasma discharge across a 5mm gap on a glass slide and (b,c) glass-slide flow cell.
93
In our electrostatic simulations, we extracted the nanoparticle geometries (i.e., size and shape)
from an HRTEM image (Figure 2.2a) and imported these into the COMSOL Multiphysics
simulation package. The nanoparticle film had an area of 550nm x 550nm and thickness of 5nm.
A built-in material model for Au was used for the nanoparticles. Two metal electrodes are placed
in-plane with the nanoparticle film with a separation of 725nm. The total simulation volume was
set to be 1250 x 1000 x 800nm with air surrounding the particle film and electrodes. A schematic
diagram illustrating the simulation cross section is given in Figure A3. We performed a quasi-
static simulation to determine the electric field profile using the electric current physics AC/DC
module in COMSOL. Here, a 100MHz sinusoidal signal of unit amplitude was applied across the
nanoparticles, and the electric field was monitored. In the low-frequency region, ranging from
radiofrequency waves to infrared wavelengths, the amount of metallic losses is very low, and
metals thus tend to behave like perfect reflectors. As a result, no field penetration was assumed
inside the nanoparticles. Floating potential boundaries were used at the nanoparticle surfaces and
the electric field was calculated in the surrounding air medium. The field enhancement at the
surface of the nanoparticle is normalized to the electric field at the same position without the
nanoparticle film.
94
Figure A3. A schematic diagram of the structure of our simulated sample.
Figure A4 shows the nanoparticle size distributions of Au and Pt nanoisland films measured by
atomic force microscopy (AFM).
Figure A4. Distribution of particle sizes
95
For COMSOL simulations, we use a non-uniform tetrahedral mesh with a minimum element size
of 0.25nm is used. The mesh grid for the particles is shown in the figure below.
Figure A5. Non-uniform tetrahedral mesh use in our COMSOL simulations.
In order to verify that our mesh size is sufficient to accurately predict the underlying physics
associated with these metal nanoparticles, simulations have been performed on a 5nm radius disk
with 5nm thickness by varying the minimum element size in the non-uniform tetrahedral mesh
from 0.1nm to 5nm to verify the convergence below 1nm.
96
Figure A6. Mesh convergence test for 5nm thick circular discs of radius 5nm.
We define electric field enhancement as the ratio of the absolute values of simulated local electric
field with nanoislands to the electric field without nanoislands (on bare glass slide). For COMSOL
simulations,
𝐸 𝑒𝑛 ℎ 𝑎 𝑛𝑐𝑒 𝑚 𝑒 𝑛𝑡 ( 𝑥 , 𝑦 ) =
| 𝐸 |
𝑤𝑖𝑡 ℎ 𝑝 𝑎𝑟𝑡 𝑖 𝑐𝑙 𝑒
( 𝑥 , 𝑦 )
| 𝐸 |
𝑤𝑖𝑡 ℎ 𝑜 𝑢𝑡 𝑝 𝑎𝑟𝑡 𝑖 𝑐𝑙 𝑒 ( 𝑥 , 𝑦 )
For FDTD simulations,
𝐸 𝑒𝑛 ℎ 𝑎 𝑛𝑐𝑒 𝑚 𝑒 𝑛𝑡 ( 𝑥 , 𝑦 ) =
| 𝐸 |
2
| 𝐸 0
|
2
where Eo is the electric field of incident wave and E is the electric field with the nanoislands.
97
Figure A7. Maximum electric field enhancement plotted as a function of particle size.
Figure A8. Example waveforms (vs. time) of (a) voltage and current, (b) energy of a 30X TPS high voltage pulse generator with
the glass slide-based reactor at 400Hz and 13kV power output.
0 100 200 300 400 500
-15
-10
-5
0
5
10
15
0 100 200 300 400 500
-100
-50
0
50
100
Voltage
Current
Time (ns)
Voltage (kV)
Current (A)
0 500 1000 1500 2000 2500
0
5
10
15
20
Energy (mJ)
Time (ns)
Energy
(a) (b)
98
Figure A9. Emission spectrum exhibiting Swan bands obtained during plasma discharge.
Figure A10. Gas chromatography result showing H2 produced during plasma discharge in the coaxial reactor.
99
Figure A11. Summary of potential reaction pathways of methane upconversion (black molecules represent stable products and red
molecules are unstable intermediates).
100
Figure A12. Plasma emission intensity of argon 912.3 nm emission peak discharged across (a) (c) Pt nanoparticles and (b) (d) Au
nanoparticles with and without 633nm or 785nm laser irradiation. No obvious enhancement can be observed with laser irradiation
of both wavelength for both metal nanoparticles.
101
Figure A13. (a) Transient absorption spectra measured on a Au grating nanostructure plotted as a function of wavelength and time.
(b) Time dependence obtained with a 505 nm probe wavelength. These results suggest that the lifetime of hot electrons inside gold
nanostructures can be extend several psec.
102
Figure A14 TEM result of GaAs/5nm TiO2/2nm Pt
103
Figure A15 Photocurrent of (a) GaAs/5nm TiO2 without and (b)with 2nm Pt at different potentials under various illumination
power density of 532nm. These datasets are acquired in three-terminal measurements with lock-in amplifier.
-0.8 -0.6 -0.4 -0.2 0.0
0.0
2.0µ
4.0µ
6.0µ
8.0µ
10.0µ
12.0µ
GaAs 5nm TiO2
Photocurrent (A)
Potential vs. Ag/AgCl (V)
1.21 μW/cm²
6.91 μW/cm²
38.41 μW/cm²
140.4 uW/cm²
-0.8 -0.6 -0.4 -0.2 0.0
0.0
1.0µ
2.0µ
3.0µ
4.0µ
5.0µ
6.0µ
7.0µ
8.0µ
9.0µ
10.0µ
Photocurrent (A)
Potential vs. Ag/AgCl (V)
1.252 µW/cm²
7.82 µW/cm²
42.28 µW/cm²
152.6 µW/cm²
GaAs 5nm TiO2 2nm Pt
(a)
(b)
Abstract (if available)
Abstract
Recent years have witnessed numerous consequences of greenhouse effect, climate change and energy crisis, which lead to growing demands for improving the efficiency of combustion of fossil fuels and alternative ways of acquiring energies other than fossil fuels. Numbers of previous works have presented a list of potential approaches to satisfy these demands, among which plasma driven chemical reactions and photoelectrochemical reactions have drawn recent attention. In this thesis, a new approach of plasma generation across nanoparticle surfaces is first evaluated, which is proved to enhance plasma generation and be further enhanced via photogenerated hot electrons. And then, we probed into photocatalysis approach using semiconductors and evaluated the interface between semiconductor and liquid, which is responsible of the reduction of incident photon to current efficiency (IPCE).
Chapter 1 is a brief introduction of plasma, methane conversion and hot electrons in metal nanoparticles. In this chapter, we will discuss plasma generation in air and transient plasma, the application of plasma driven chemical reactions, methane conversion reactions, and some typical parameters of hot electrons in metal nanoparticles.
In chapter 2, we presented a method of enhancing plasma generation triggered by nanosecond high voltage pulses using metal nanoparticle surfaces, which is directly related to the field enhancement near these nanoparticle surfaces. An up-to-1000X magnification of argon plasma emission is observed with the presence of these nanoparticles and our finite difference time domain (FDTD) simulation results suggest this should not be attributed to the plasmon resonance phenomenon (an optical effect), which results in an enhanced coupling of light from the nearfield to the far field.
Chapter 3 discussed the potential of applying this mentioned enhancement effect of plasma generation brought up by metal nanoparticle surfaces in assisting chemical reactions. While discharging in methane environments, an 50-fold enhancement of C2 Swan band emission, which is proportional to the C2 radical densities, can be provided by the metal nanoparticle surfaces. Mass spectrum confirmed that the driven chemical reactions should be methane upconvertion.
Chapter 4 probed into the electrical field emission process in the related metal particle surface substrates. Due to the small size of metal nanoparticles, hot electrons can travel to the surfaces of particles before quenched into thermal equilibrium. As a result, plasma emission on metal nanoparticles can be further assisted via hot electron photoexcitation and a up to 200X increase of argon plasma emission can be achieved under 532nm laser illumination, which opens up an new potential approach of solar energy harvesting.
Future works related to plasma discharge on metal nanoparticle surfaces will be explored in the chapter 5. In this chapter, the potential and challenges related to observing plasma discharge on metal nanoparticle surfaces under liquid phase will be discussed.
A brief introduction of semiconductor-interface and their applications in photoelectrochemical reactions are presented in chapter 6. We discussed the structure and mechanism of semiconductor-liquid junction and water splitting reaction in solutions, after which, the current limitations of applying semiconductors in photoelectrochemical reactions will be probed into.
Chapter 7 focused on a drop in incident photon to current efficiency (IPCE) as photo illumination power increases in driving hydrogen evolution reactions (HER) on a GaAs/TiO2 semiconductor photo cathode. Band flattening mechanism due to the charge accumulation at the interface is proposed and evaluated via MATLAB calculations. To further verify this issue, we applied catalytical layer on top of this structure and less reduction of IPCE with growing illumination power is observed. Further evidence from semiconductor-liquid junction characterizations (electric field, charge density and surface potential) during this process will be needed. Additional works using surface reporters are evaluated and possible challenges will be discussed.
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Zhao, Bofan
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Plasma emission across metal nanoparticle surfaces and semiconductor -liquid interface characterizations
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Doctor of Philosophy
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Electrical Engineering
Degree Conferral Date
2023-05
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