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Structural and Photoelectrochemical Characterization of Gallium Phosphide
Semiconductors Modified with Molecular Cobalt Catalysts
by
ANNA MARY BEILER
A Dissertation Presented in Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Approved July 2018 by the
Graduate Supervisory Committee:
Gary F. Moore, Co-chair
Thomas A. Moore, Co-chair
Kevin E. Redding
James P. Allen
ARIZONA STATE UNIVERSITY
December 2018
i
ABSTRACT
The molecular modification of semiconductors has applications in energy
conversion and storage, including artificial photosynthesis. In nature, the active sites of
enzymes are typically earth-abundant metal centers and the protein provides a unique
three-dimensional environment for effecting catalytic transformations. Inspired by this
biological architecture, a synthetic methodology using surface-grafted polymers with
discrete chemical recognition sites for assembling human-engineered catalysts in three-
dimensional environments is presented. The use of polymeric coatings to interface cobalt-
containing catalysts with semiconductors for solar fuel production is introduced in
Chapter 1. The following three chapters demonstrate the versatility of this modular
approach to interface cobalt-containing catalysts with semiconductors for solar fuel
production. The catalyst-containing coatings are characterized through a suite of
spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection
Fourier transform infrared spectroscopy (GATR-FTIR) and x-ray photoelectron (XP)
spectroscopy. It is demonstrated that the polymeric interface can be varied to control the
surface chemistry and photoelectrochemical response of gallium phosphide (GaP) (100)
electrodes by using thin-film coatings comprising surface-immobilized pyridyl or
imidazole ligands to coordinate cobaloximes, known catalysts for hydrogen evolution.
The polymer grafting chemistry and subsequent cobaloxime attachment is applicable to
both the (111)A and (111)B crystal face of the gallium phosphide (GaP) semiconductor,
providing insights into the surface connectivity of the hard/soft matter interface and
demonstrating the applicability of the UV-induced immobilization of vinyl monomers to
ii
a range of GaP crystal indices. Finally, thin-film polypyridine surface coatings provide a
molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto
GaP. In all constructs, photoelectrochemical measurements confirm the hybrid
photocathode uses solar energy to power reductive fuel-forming transformations in
aqueous solutions without the use of organic acids, sacrificial chemical reductants, or
electrochemical forward biasing.
iii
DEDICATION
for my family
to Mom, for loving me unconditionally
to Dad, for imparting the joy of learning
and to Leon, who understands the significance of this journey
“As we are a doomed race, chained to a sinking ship, as the whole thing is a bad joke, let
us, at any rate, do our part; mitigate the suffering of our fellow-prisoners; decorate the
dungeon with flowers and air-cushions; be as decent as we possibly can.”
― Virginia Woolf, Mrs. Dalloway
“There's nothing left except to try.”
― Madeleine L'Engle, A Wrinkle in Time
iv
ACKNOWLEDGMENTS
This dissertation has been guided by Gary F. Moore, who introduced me to the field of
solar fuels and personally taught me many lab techniques and key concepts in
electrochemistry and solar energy conversion. Tom Moore’s work inspired me to pursue
research in the field of artificial photosynthesis, and he has been invaluable in supporting
and mentoring me throughout graduate school. My committee members, Kevin Redding
and Jim Allen, gave much-appreciated insight and encouragement along the way.
The NSF IGERT-SUN program provided funding and training in interdisciplinary aspects
of solar energy conversion, and Wim Vermaas and Anna Keilty were an indispensable
part of the experience. My IGERT-SUN peers challenged me intellectually and
introduced me to new scientific and engineering concepts (including fermentation studies
with Joe Laureanti). The ASU School of Molecular Sciences and the GPSA provided
grants, travel resources, and teaching appointments. Ellie and Michael Ziegler generously
sponsored me through the Phoenix ARCS Foundation, and the women of the
International Chapter of the P.E.O. Sisterhood graciously invited me into their lives.
The staff at ASU has been wonderfully responsive and helpful in my research. Special
thanks go to Tim Karcher for help with XPS experiments and to Diana Convey for
teaching me ellipsometry techniques. My colleagues in the G. F. Moore lab have been
invaluable in my day-to-day life. I cannot imagine my time at ASU without Diana
Khusnutdinova. For years we shared almost every detail of our working lives with each
other, as well as countless cups of coffee and tea. Brian Wadsworth is always down for
impromptu lab karaoke, and his recurring loan of my Bard electrochemistry textbook has
v
definitely paid off. Edgar Reyes has a quiet assurance and dedication to organic
synthesis. I was privileged to work with several undergraduates: Samuel Jacob, Avraham
Echeverri, and Sylvia Nanyangwe, as well as Ahlea Reyes, a high school student. It was a
pleasure collaborating with Minseob Koh at the Scripps Research Institute and Jim
Thorne at Boston College. Thanks to Jim for great discussions about impedance
spectroscopy and the general meaning of life. Natalia H. von Reitzenstein is an amazing
friend and all-around efficacious woman. Elise Pagel reminds me how fortunate I am to
be in graduate school. I have watched James Engeln go through medical school without
complaint, and his steadfast cheerfulness inspires optimism. Isaac Roll has shown me
Arizona through a local’s eyes and knows how to appreciate an old country tune. Susan
Fisher-Shetler still makes me laugh harder than anyone else.
My mom, Rachel Beiler, has taught me resilience, hospitality, and an enduring love of
coffee. Her warmth and love are all-encompassing. My dad, Ben Beiler, taught me to
love books and encouraged me to think critically. Both of my parents have a curiosity
that has led them to new and sometimes uncomfortable places, and their courage propels
me to keep pushing boundaries.
Leon Leid, my lion of a partner, has been my fierce advocate and strongest supporter
throughout graduate school. His love of the Arizona desert took me on overnight hikes
and motorcycle escapades to remote wilderness areas. He kept me fed and watered,
listened to me tirelessly, gave stellar advice, and encouraged me to keep going through
the hard times. His insatiable curiosity inspires me to never stop exploring. I look forward
to continuing our adventures together.
vi
TABLE OF CONTENTS
Page
LIST OF TABLES ................................................................................................................ viii
LIST OF FIGURES ................................................................................................................. ix
CHAPTER
1 INTRODUCTION .................................................................................................. 1
1.1 Biological Inspiration ....................................................................................... 2
1.2 Photoelectrochemical Approach ....................................................................... 4
1.3 Molecular Catalysts for Hydrogen Production ................................................. 5
1.4 Surface-Immobilized Catalysts at Semiconductors .......................................... 7
1.5 Examples of Molecular Modification of Photocathodes ................................ 10
1.6 Prospects in the Molecular Modification of Semiconductors ........................ 12
1.7 References ........................................................................................................ 14
2 POLYVINYLIMIDAZOLE AND POLYVINYLPYRIDINE GRAFTS ON
GALLIUM PHOSPHIDE FOR COBALOXIME IMMOBILIZATION ........... 19
2.1 Introduction ...................................................................................................... 20
2.2 Results and Discussion ................................................................................... 24
2.3 Conclusions ..................................................................................................... 39
2.4 Experimental Methods .................................................................................... 40
2.5 References ........................................................................................................ 45
vii
CHAPTER Page
3 SOLAR HYDROGEN PRODUCTION USING MOLECULAR CATALYSTS
IMMOBILIZED ON GALLIUM PHOSPHIDE (111)A AND (111)B
POLYMER-MODIFIED PHOTOCATHODES ................................................. 49
3.1 Introduction ...................................................................................................... 50
3.2 Results and Discussion ................................................................................... 52
3.3 Conclusions ..................................................................................................... 65
3.4 Experimental Methods .................................................................................... 66
3.5 References ........................................................................................................ 72
4 COBALT PORPHYRIN-POLYPYRIDYL SURFACE COATINGS FOR
PHOTOELECTROSYNTHETIC HYDROGEN PRODUCTION .................... 76
4.1 Introduction ...................................................................................................... 77
4.2 Results and Discussion ................................................................................... 79
4.3 Conclusions ..................................................................................................... 90
4.4 Experimental Methods .................................................................................... 91
4.5 References ........................................................................................................ 94
Complete Bibliography ................................................................................................. 98
viii
LIST OF TABLES
Table Page
Table 2.1 N 1s Binding Energies (eV) .............................................................................. 26
Table 2.2 PEC Characteristics .......................................................................................... 34
Table 3.3 PEC Characteristics .......................................................................................... 63
Table 4.4 PEC Characteristics .......................................................................................... 89
ix
LIST OF FIGURES
Figure Page
Figure 1.1. Schematic diagram representing the movement of electrons following
illumination in photosystem II during the process of oxygenic photosynthesis. ................ 2
Figure 1.2. Graph showing the reduction potential range of an array of iron enzymes.
Adapted from Ref. 11. ........................................................................................................ 3
Figure 1.3. (left) Photo of a semiconductor working electrode in operation. (right) Photo
of a customized photoelectrochemical cell, including a platinum counter electrode, a
Ag/AgCl reference electrode, a GaP working electrode, and a quartz window. ................ 5
Figure 1.4. Conduction band (top bar) and valence band (bottom bar) positions vs NHE
of GaP (Eg = 2.3 eV), GaAs (1.4 eV), InP (Eg = 1.3 eV) and Si (Eg = 1.1 eV) as a function
of maximum current density under AM 1.5 illumination. The dotted and dashed lines
indicate the reversible potential of the H+/H2 and O2/H2O half reactions, respectively.
Adapted from Krawicz et. al ............................................................................................... 6
Figure 1.5. Schematic of a cobaloxime catalyst immobilized onto GaP through a
polypyridine interface. ...................................................................................................... 11
Figure 2.6. The air mass 1.5 global tilt solar flux spectrum (solid) and the transmission
spectrum of GaP (dashed). The shaded grey area shows the integrated region of the solar
spectrum from 280 nm up to the band gap of GaP. .......................................................... 21
Figure 2.7. Schematic representation of the attachment method used to assemble
cobaloxime-modified photocathodes. ............................................................................... 23
Figure 2.8. XP survey spectrum of (left) PVIlGaP and (right) ColPVIlGaP. ................... 25
x
Figure Page
Figure 2.9. N 1s core level XP spectra of (top) PVIlGaP and (bottom) PVPlGaP. The
solid blue (b) and solid dark blue (c) lines represent the component fit of the imine
nitrogen, and the dashed blue (a) line the component fit of the amine nitrogen. The circles
represent the spectral data, the gray line is the linear background, and the black line the
overall fit. .......................................................................................................................... 26
Figure 2.10. N 1s core level spectra of (top) ColPVIlGaP and (bottom) ColPVPlGaP. The
solid blue (b) and solid dark blue (c) lines are the component fits for the unbound imine,
the dotted blue line (a) the component fit for the unbound amine, the solid red (f) and
solid dark red (h) lines the component fit for the bound imine, and the dotted red line (e)
the component fit for the bound amine. The dashed red (d) and dashed dark red (g) line
are the component fits for the glyoximate contributions. The circles represent the spectral
data, the gray line is the Shirley background, and the black line is the overall fit. .......... 28
Figure 2.11. XP spectrum of the Co 2p region of ColPVIlGaP. Circles are the spectral
data, and the solid lines represent the background (gray), and component fit (red). ........ 29
Figure 2.12. GATR-FTIR spectra of PVIlGaP (blue) and PVPlGaP (dotted dark blue)
grafted onto gallium phosphide, as well as unmodified GaP (dashed black). Spectra are
normalized for comparison. .............................................................................................. 30
Figure 2.13. (a) FTIR transmission spectra of Co(dmgH)2Cl2 in KBr (green),
Co(dmgH)2(Py)Cl in KBr (dashed dark red), and Co(dmgH)2(meIm)Cl in KBr (red), as
well as (b) GATR-FTIR absorbance spectra of ColPVIlGaP (red), ColPVPlGaP (dashed
xi
Figure Page
dark red), and GaP following a fresh HF etch (dashed black). Insets show the NO− region.
........................................................................................................................................... 31
Figure 2.14. Spectra obtained from ellipsometry measurement (red) of PVIlGaP at 69°
(top) and 74° (bottom) and model fit to data (black dash). ............................................... 32
Figure 2.15. Linear sweep voltammograms of PVPlGaP (dark blue), PVIlGaP (blue),
ColPVPlGaP (dark red) and ColPVIlGaP (red) working electrodes at pH 7 in the dark
(dotted lines) and with 100 mW cm-2 illumination (solid and dashed lines). An
unmodified GaP electrode under illumination is shown for comparison (black). ............ 34
Figure 2.16. Gas chromatogram of 5 mL of headspace before (black dash) and after (red)
bulk electrolysis of ColPVPlGaP working electrodes at pH 7 with 100 mW cm-2
illumination for 1 hour at 0 V vs. RHE. ............................................................................ 35
Figure 2.17. Cyclic voltammograms of Co(dmgH)2(meIm)Cl (green) and
Co(dmgH)2(Py)Cl recorded in 0.1 M tetrabutylammonium hexafluorophosphate
in acetonitrile at a scan rate of 100 mV s-1 with a glassy carbon working electrode at
room temperature and ferrocene as an internal standard with E1/2 taken as 0.40 V vs NHE.
Data are normalized for comparison. Data collected by Samuel Jacob. ........................... 36
Figure 3.18. Schematic of cobaloxime linked to either the GaP(111)A face or the GaP
(111)B face through surface-grafted polyvinylimidazole. ................................................ 51
Figure 3.19. FTIR transmission spectra of (a and c) N-vinylimidazole monomer in KBr
(blue) as well as a powder sample of the model cobaloxime catalyst,
Co(dmgH)2(meIm)Cl, in KBr (red). GATR-FTIR absorbance spectra of (b) the (111)A-
xii
Figure Page
face and (d) (111)B-face of unmodified GaP (black dash) polyvinylimidazole-grafted
GaP (blue) and cobaloxime-modified GaP (red). The spectra shown in parts a and c are
identical and included to facilitate comparisons. .............................................................. 54
Figure 3.20. XP spectra of the Ga 2p (left) and P 2p (right) regions of unmodified GaP
(111)A (top) and GaP(111)B (bottom). The circles are the spectral data, and the solid
lines represent the background (gray), component fit (green), and overall fit (black). .... 56
Figure 3.21. XP spectra of the Ga 2p (left) and P 2p (right) regions of polymer-modified
GaP (111)A (top) and GaP(111)B (bottom). The circles are the spectral data, and the
solid lines represent the background (gray), component fit (blue), and overall fit (black).
........................................................................................................................................... 57
Figure 3.22. N 1s core level spectra of (a) PVIlGaP(111)A, (b) ColPVIlGaP(111)A, (c)
PVIlGaP(111)B, and (d) ColPVIlGaP(111)B. Circles represent the spectral data. Solid
and dash lines are background (gray), component (red or blue), and overall (black) fits to
the experimental data. ....................................................................................................... 58
Figure 3.23. Linear sweep voltammograms of working electrodes using GaP substrates
with the varied properties and illumination conditions..................................................... 61
Figure 3.24. Linear sweep voltammograms of (a) GaP(111)A and (b) GaP(111)B
working electrodes following polyvinylimidazole grafting (blue) and cobaloxime
attachment (red). All measurements were performed at pH 7 in the dark (dashed-dotted
lines) or under 100 mW cm−2 illumination (solid lines). .................................................. 62
xiii
Figure Page
Figure 3.25. Three-electrode electrolysis measurements using cobaloxime-modified
GaP(111)A (dashed) and cobaloxime-modified GaP(111)B (solid) working electrodes
polarized at 0 V vs RHE. All measurements were performed at pH 7 under 100 mW cm−2
illumination. ...................................................................................................................... 64
Figure 4.26. Voltammograms of 5,10,15,20-tetra-p-tolylporphyrin cobalt(II) (CoTTP)
recorded in 0.1 M tetrabutylammonium hexafluorophosphate in dimethylformamide and
0 mM tosic acid (green solid line), 10 mM tosic acid (blue dashed line) or CO2 (black
dashed line) at a scan rate of 250 mV s-1 using a glassy carbon working electrode at room
temperature and ferrocene as an internal standard. Data collected by Diana
Khusnutdinova. ................................................................................................................. 79
Figure 4.27. (left) Schematic representation of the synthetic method used to prepare the
PPy|GaP and CoTTP|PPy|GaP samples. (right) GATR-FTIR absorbance spectra of the (a)
1700-950 cm-1 region of unfunctionalized GaP (black), PPy|GaP (blue), and
CoTTP|PPy|GaP (purple), including (b) an expanded plot of the 1022-987 cm-1 region in
spectra of CoTTP|PPy|GaP. .............................................................................................. 81
Figure 4.28. N 1s core level XP spectra of unfunctionalized GaP (black) and PPy|GaP
(blue). ................................................................................................................................ 84
Figure 4.29. (a) Molecular structure of a CoTTP unit coordinated to polypyridine. (b)
High-energy resolution XP spectrum of the N 1s region of CoTTP|PPy|GaP. Solid lines
represent the background (gray), component (blue, green, and red), and overall (black)
fits to the experimental data (circles). Coloring indicates the component area assigned to
xiv
Figure Page
remnant pyridinic nitrogens (blue), porphyrin pyrrolic nitrogens bound to cobalt centers
(green), and pyridinic nitrogens bound to cobalt centers (red). (c) High-energy resolution
XP spectrum of the Co 2p3/2 region of CoTTP|PPy|GaP................................................... 85
Figure 4.30. (a) Chronoamperogram of a CoTTP|PPy|GaP working electrode polarized at
0 V vs RHE under alternating (200 mHz) non-illuminated and illuminated conditions. (b)
Linear sweep voltammograms of CoTTP|PPy|GaP working electrodes recorded in the
dark (dashed line) and under simulated 1-sun illumination (solid line). The dotted vertical
line at 0 V vs RHE represents the equilibrium potential of the H+/H2 couple. The
calculated HER activity per cobalt center is included on the right ordinate axis. (c) Gas
chromatograms of a 5 mL aliquot of the headspace drawn from a sealed PEC cell using a
CoTTP|PPy|GaP working electrode polarized at 0 V vs RHE before (dashed line) and
after (solid line) 30 min of AM1.5 illumination. All measurements were performed using
a three-electrode configuration in 0.1 M phosphate buffer (pH 7). .................................. 87
Figure 4.31. GATR-FTIR absorbance spectra showing the νCo-N region of the
CoTTP|PPy|GaP before photoelectrochemical characterization (top, purple), following
exposure to buffer (middle, dark red), and after photoelectrochemical characterization
(bottom, green). ................................................................................................................. 89
Figure 4.32. Linear sweep voltammograms of unfunctionalized GaP (black), PPy|GaP
(blue), and CoTTP|PPy|GaP (purple) working electrodes. Circles represent the maximum
power point. Dashed horizontal and dashed vertical lines represent the current density
and potential, respectively, at the maximum power points. .............................................. 90
1
CHAPTER 1 INTRODUCTION
Portions of this work are excerpted with permission from:
Beiler, A. M.; Moore, G. F. Nat. Chem. 2017, 10, 3–4.
Copyright 2017 Springer Nature
Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. Ind. Eng. Chem. Res. 2016,
55, 5306–5314.
Copyright 2016 American Chemical Society
Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. ACS Appl. Mater. Interfaces
2016, 8, 10038–10047.
Copyright 2016 American Chemical Society
Beiler, A. M.; Khusnutdinova, D.; Wadsworth, B. L.; Moore, G. F. Inorg. Chem. 2017,
56, 12178–12185.
Copyright 2017 American Chemical Society
2
1.1 Biological Inspiration
Biology provides inspiration for developing human-engineered systems to capture,
convert, and store solar energy in chemical bonds.1–6 In nature, photo-initiated redox
reactions drive the process of photosynthesis (Figure 1.1). Plants and other
photosynthetic organisms absorb sunlight and use the energy to carry out a series of
complex redox reactions and synthetic transformations, creating the foods we eat and
ultimately the fossil fuels that power our modern economies. In this context, sunlight acts
as a reagent in chemical reactions, and the products of the overall reaction store the sun’s
energy in the form of molecular bonds. Catalytic sites, providing relatively low-energy
pathways for carrying out the overall chemical transformations, facilitate the formation of
these bonds and must be reduced or oxidized at least twice for each successful
completion of a reaction cycle.
Figure 1.1. Schematic diagram representing the movement of electrons following
illumination in photosystem II during the process of oxygenic photosynthesis.
3
In biological catalysis, enzymes use soft material interactions, including coordination to
amino acid residues along a protein scaffold, to provide appropriate three-dimensional
environments for controlling the catalytic activity and selectivity of metal centers
composed of earth-abundant elements.7,8 This, in accordance with the Sabatier principle,
results in examples of enzymes that have exceptionally high activities and exquisite
selectivity. The reduction potentials of the active sites of iron-based enzymes span a volt,
modulated by the identity and number of coordinating ligands, charges on the
surrounding protein residues, and/or solvent accessibility (Figure 1.2).9,10 In a similar
vein, a focus on the role of secondary coordination sphere interactions has emerged as a
bioinspired design theme in synthetic molecular catalysis, providing improved routes for
binding substrates, lowering transition state energies along a reaction coordinate, and
releasing products.11,12
Figure 1.2. Graph showing the reduction potential range of an array of iron enzymes.
Adapted from Ref. 11.
4
1.2 Photoelectrochemical Approach
Future clean energy scenarios require effective methods for converting and storing
intermittent energy sources.13,14 Taking inspiration from the solar-to-fuels paradigm in
biology, scientists have set out to re-design photosynthesis for industrial applications
using top-down as well as bottom-up strategies. In this context, photoelectrochemical
(PEC) fuel production using sunlight as an energy input has emerged as a promising
storage option that could be integrated with our existing petroleum-based infrastructure.15
In this approach, semiconductors are used as the light-absorbing components to drive a
thermodynamically uphill fuel-forming reaction. In one PEC application, the overall
water-splitting reaction uses solar energy to provide the standard change in Gibbs free
energy of 4.92 eV (475 kJ mol-1) (Equation 1.1).
2𝐻2𝑂 (𝑙) → 2𝐻2 (𝑔) + 𝑂2(𝑔) (1.1)
As a four-electron process, the corresponding standard potential is Ε0 = -1.23 V.
Although a PEC water-splitting device is composed of 2 electrodes, a cathode at which
the hydrogen evolution reaction (HER) takes place and an anode at which the oxygen
evolution reaction (OER) occurs, component electrodes may be studied individually in a
three-electrode setup for optimization of the individual reactions (Figure 1.3). The work
presented in this dissertation focuses on homogeneous-heterogeneous catalysis at a
molecular-modified cathode.
Semiconductors used for reductive processes are typically p-type semiconductors, in
5
Figure 1.3. (left) Photo of a semiconductor working electrode in operation. (right) Photo
of a customized photoelectrochemical cell, including a platinum counter electrode, a
Ag/AgCl reference electrode, a GaP working electrode, and a quartz window.
which addition of a dopant shifts the energy of the Fermi level closer to the valence band
energy, resulting in a material in which electrons are the minority carrier. The optical
band gap, Eg, of the semiconductor is one of the factors that determine the maximum
photocurrent density, with smaller band gap materials capable of generating greater
photocurrents (Figure 1.4).17 Commonly used semiconductors for photocathode materials
include silicon, indium phosphide, gallium arsenide, and gallium phosphide, as they have
energy band gaps suitable for visible light absorption. In addition, the conduction bands
of these materials are thermodynamically poised to drive H2 evolution or CO2 reduction.
1.3 Molecular Catalysts for Hydrogen Production
At the photocathode of a water splitting cell, the hydrogen evolution half-reaction occurs
(Equation 1.2):
2𝐻+ + 2𝑒− → 𝐻2 (1.2)
6
Figure 1.4. Conduction band (top bar) and valence band (bottom bar) positions vs NHE
of GaP (Eg = 2.3 eV), GaAs (1.4 eV), InP (Eg = 1.3 eV) and Si (Eg = 1.1 eV) as a function
of maximum current density under AM 1.5 illumination. The dotted and dashed lines
indicate the reversible potential of the H+/H2 and O2/H2O half reactions, respectively.
Adapted from Krawicz et. al.16
The formation or dissociation of hydrogen at an electrode surface can present relatively
large energy barriers. If an applied electrochemical potential greater than the
thermodynamic potential of the reaction is required to overcome energetic barriers, the
reaction operates at an overpotential. Therefore, an active area of research in the field of
the photoelectrochemical generation of solar fuels focuses on strategies to interface
semiconductors with appropriate electrocatalysts to reduce the overpotential requirements
for generating fuels and industrially relevant chemical feedstocks.17–21 While the precious
metal platinum is the state-of-the-art catalyst for HER, there has been increased interest
in employing earth-abundant metals.22–26 Advances in this field include the design and
7
study of bio-inspired catalysts, and there is ongoing interest in controlling reaction
networks in complex environments.27
Unlike traditional surface electrocatalysts such as platinum, which are an integral part of
the electrode and contribute to the Fermi level, molecular catalysts are discrete entities
with distinct chemical and electronic properties.5 Molecular catalysts provide local three-
dimensional environments for binding substrates, lowering transition-state energies along
a reaction coordinate, and releasing products. Their unique selectivity and ability to lower
energy requirements for achieving chemical transformations make them attractive targets
for integration with solid-state substrates for technological applications, including
electrochemical and photoelectrochemical activation for generating fuel and other
valuable chemical products.
1.4 Surface-Immobilized Molecular Catalysts at Semiconductors
Molecular catalysts powered by visible-light-absorbing semiconductors represent one
approach in artificial photosynthesis to developing an integrated system for generating
solar fuels.20,28–31 However, finding new ways to immobilize catalysts onto
semiconductor surfaces and characterize the resultant hard-to-soft matter interfaces
remains a major challenge.20 Addressing this obstacle provides an improved fundamental
understanding of catalysis in complex environments and enables technological
advancements that depend on the precise control and selectivity of molecular
components.
Molecular catalysis of the HER reaction can proceed via a homolytic or heterolytic
pathway. In a homolytic pathway, the product is formed through a symmetrical
8
bimolecular chemical step. Conversely, a heterolytic pathway involves the reaction of
chemically distinct species. For example, the mechanism of HER catalysis by
cobaloximes has been proposed to occur via several pathways.32–34 After reduction of the
cobalt center from Co(III) to Co(I), the Co(I) species is protonated to generate a cobalt
hydride, HCo(III). In the homolytic pathway, two cobalt hydride species react with one
another to release hydrogen. Alternately, the HCo(III) generated in the initial protonation
step can react with an acid in solution to generate hydrogen through a heterolytic
pathway. It has also been proposed that the HCo(III) can be further reduced to HCo(II),
which can then proceed to generate H2 through a homolytic or heterolytic pathway.
Plotting the exchange current density of various metals for HER versus the metal-hydride
bond strength yields a volcano plot, in which a maximum rate of exchange current
density is reached at intermediate metal-hydride bond strengths, which stabilize the bond
but also allow the product to be released. The mechanism through with HER occurs has
important implications for surface-immobilized catalysts. If the optimal catalytic pathway
is homolytic, the surface-attached catalysts must be close enough to each other and
oriented in such a way as to favor the interaction of two protonated metal centers.
Addition of a catalyst to the electrode surface may provide a lower energy pathway for a
reaction to occur, and/or increase the efficiency of charge transfer across the interface by
capturing minority carriers at the surface. However, it is often unclear whether the role of
surface-immobilized catalysts is confined to improving the rate of interfacial electron
transfer, as it can also serve to alter the surface energetics that in turn affect surface
recombination rates.35-37 Since electrochemical measurements give information on both
9
the thermodynamics and kinetics of a reaction, it can be difficult to clearly assign
whether the role of the catalyst in improved photoelectrochemical performance is to
change the surface energetics or to provide a lower energy pathway for the reaction.38-41
Nonetheless, thin catalyst layers have been reported to form adaptive
semiconductor/electrocatalyst junctions where the catalysts serve as an independently
tunable entity whose redox states changes the effective (non-equilibrium) barrier height,
resulting in higher photovoltages and efficiencies relative to impermeable
electrocatalysts. 42,43
In integrating molecular catalysts to semiconductors, several requirements must be
considered. A prevailing paradigm in surface catalysis suggests a higher loading of active
sites is crucial for optimizing reaction conditions. This parameter can be addressed by
using nano- or meso-structured substrates, or by employing three-dimensional
environments for the catalyst.44,45 Conversely, an emerging strategy is that employing
low catalyst coverage for achieving accumulative charge separation could increase the
yields of accumulated redox equivalents.46 In either case, the surface coating should be
optimized so that the catalyst loading does not hinder charge transport or block light
absorption.47 Colored materials, including catalysts, can screen an underpinning
semiconductor by “parasitically” absorbing photons that would otherwise contribute to
production of photocurrent. Conversely, colored materials are also capable of extending
the light harvesting capabilities of a semiconductor through dye sensitization
mechanisms.47,48 Finally, typical benchmarking parameters such as turnover number
(TON) and turnover frequency (TOF) used for homogeneous catalysts are not as easily
10
applied to heterogeneous catalysis since it can be difficult to determine the number of
catalytically active sites.
Immobilization of catalysts at solid supports to form heterogeneous architectures
provides several advantages. Surface immobilization allows catalysts that are normally
not soluble in aqueous conditions to be employed in aqueous environments. By
integrating a catalyst with a surface, less material is needed as all the catalyst is in close
proximity to the electrode and can be activated more readily. In addition, immobilization
may improve the stability of a molecular catalyst. For example, a cobalt diamine-dioxime
cobalt catalyst grafted onto carbon nanotubes showed improved stability for hydrogen
evolution compared to the solution counterpart, which may result from the disfavoring of
decomposition pathways involving the interaction of two catalysts.23
1.5 Examples of Molecular Modification of Photocathodes
Molecular modification of surfaces promises enhanced selectivity and tunability for
catalyzing chemical transformations at solid supports, leading researchers to investigate
strategies to effectively immobilize molecular catalysts onto semiconductor surfaces and
characterize the resultant modified surfaces. Reports describing strategies to directly
integrate molecular catalysts for hydrogen production to visible-light-absorbing
semiconductors are limited, but include: [Fe2S2 (CO)6] adsorbed onto indium phosphide
nanocrystals,49 trinuclear molybdenum cluster salts drop-cast on Si(100)50
ferrocenophane-containing polymers deposited on Si(100) and (111),51 modified Dubois-
type nickel bisdiphosphine catalysts covalently attached to amine-modified Si(100) and
GaP(100),52 Negishi coupling of phosphine ligands onto Si(111) followed by
11
metalation,53 a metal-organic surface composed of a cobalt dithiolene polymer dropcast
onto Si(100),54 and cobaloxime catalysts adsorbed onto a TiO2-coated GaInP2
semiconductor.55 Some of the cathode assemblies reported to date require an
electrochemical bias poised negative of 0 V vs the reversible hydrogen electrode (0 V vs
RHE) (i.e., an applied cathodic polarization beyond the thermodynamic potential of the
H+/H2 half-reaction under the conditions tested to produce hydrogen). In addition, some
require extreme pH operating conditions or nonaqueous conditions using relatively strong
organic acids as a proton source, features that may be undesirable in the context of large-
scale deployment, environmental impact, and biocompatibility. Although immobilization
via drop-casting methods and other electrostatic-based deposition methods offer ease of
assembly, the development of more precise surface attachment chemistries could offer
additional control over diffusion, migration and convection dynamics, orientation of
attached components, surface loading, passivation of the substrate, and interfacial
energetics.9,23,56-65
UV-induced grafting has been used to polymerize 4-vinylpyridine onto p-type GaP to
provide surface-attached pyridine ligands for subsequent assembly of cobaloxime
catalysts (Figure 1.5).66 In this construct, the polymer coating of the GaP surface likely
provides protection against oxide layer passivation by restricting access to surface
GaP(100) sites while providing attachment sites for assembly of the catalysts. The Co-
functionalized photocathode produced 2.4 mA cm-2 of current density at 310 mV
underpotential under 100 mW cm-2 illumination at pH 7, an improved photoperformance
compared to the polyvinylpyridine-functionalized GaP photocathode, which required an
12
Figure 1.5. Schematic of a cobaloxime catalyst immobilized onto GaP through a
polypyridine interface.
overpotential of 270 mV to achieve the same current density. This work was extended to
fluorinated cobaloximes, where a similar attachment strategy was used to
immobilize difluoroborylcobaloxime catalyst.67 The pH dependence of the
photoelectrochemical responses of fluorinated and non-fluorinated cobaloximes tested at
pH 7 or pH 4.5 was consistent with the electrochemical properties of the analogous non-
surface-attached catalysts.
1.6 Prospects in the Molecular Modification of Semiconductors
Controlling matter and information at the nanoscale presents a challenge for science and
the imagination.68 In this vein, integrated materials have been developed using a range of
surface functionalization chemistries,60,69,70 and recent advances in polymer chemistry
offer opportunities to tailor their structure and macromolecular properties.71,72 This
dissertation presents the characterization and photoelectrochemical characterization of
H O
NO
N
O
HO
N
N Cl
Co
N
m
N
n
13
photocathodes with thin-film coatings. Surface chemistry utilizing molecular components
is used to build photoactive materials that include applications to artificial
photosynthesis as well as emerging solar-to-fuels technologies.73
By integrating gallium phosphide with molecules that are well known for efficient
catalysis, hybrid materials for photoelectrochemical catalytic transformations can be
made. The research findings presented in the following chapters extend the work
presented above, in which a polymeric brush serves as a structural interface between the
soft molecular components and the semiconductor surface. It is shown that exchange of
the polymeric interface from polyvinylpyridine to polyvinylimidazole results in distinct
spectroscopic signatures associated with the unique molecular surface environments of
the polymer films, and that the macroscopic photoelectrochemical characteristics are also
influenced by the choice of polymer. Polyvinylpyridine can be UV-grafted to both the A
(Ga-rich) and B (P-rich) sides of GaP(111), providing an immobilization method for
cobaloximes at multiple crystal faces and yielding insights on the molecular connectivity
of the polymer to GaP. The polymeric interface can also serve to coordinate cobalt
porphyrin hydrogen evolution catalysts, and post-photoelectrochemical analysis shows
that the catalysts retain their molecular integrity. Throughout the dissertation,
structure/function relationships at interfaces are explored through surface spectroscopies
and photoelectrochemical techniques, advancing knowledge of relationships between the
molecular structures at surfaces and the resultant macroscopic properties. Thus, this work
presents novel constructs not only for generating solar fuels, such as hydrogen, but also
for studying catalysis in complex environments at hard/soft matter interfaces.
14
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19
CHAPTER 2 POLYVINYLIMIDAZOLE OR POLYVINYLPYRIDINE GRAFTS
ON GALLIUM PHOSPHIDE FOR COBALOXIME IMMOBILIZATION
Portions of this work are excerpted with permission from:
Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. Ind. Eng. Chem. Res. 2016,
55 (18), 5306–5314.
Copyright 2016 American Chemical Society
20
2.1 Introduction
Chapters 2-4 illustrate how the modular approach to semiconductor functionalization
allows for versatility in the choice of component parts- polymer, semiconductor, or
catalyst- and focus on surface characterization of the surface assemblies. Distinct
structural details of the film composition and connectivity in the catalytic coating are
revealed, illustrating the ability to effect molecular-level changes at the surface, and the
photoelectrochemical (PEC) performance of the constructs is evaluated. In this chapter, it
is shown that cobaloxime catalysts for hydrogen evolution can be immobilized at gallium
phosphide through a polyvinylpyridine (PVP) or polyvinylimidazole (PVI) interface.
Structural characterization of the modified wafers by ellipsometry as well as X-ray
photoelectron (XP) and grazing angle attenuated total reflection Fourier transform
infrared (GATR-FTIR) spectroscopies is used to evaluate the success of the surface
attachment strategy and gain insights into the chemical composition of the surface
coatings. PEC characterization, including gas chromatography (GC) analysis of the
photochemical products, verifies the functionality of the hybrid constructs.
2.1.1 Gallium Phosphide
GaP is a III-V semiconductor used in optoelectronic applications with an indirect band
gap of 2.26 eV and a direct band gap of 2.78 eV (Figure 2.6). Additionally, GaP has
promising features as a candidate material for applications in solar fuels generation,
including a capacity for achieving relatively large photovoltages. The energetic
positioning of the conduction band (~ -1 V vs NHE) more than satisfies the
thermodynamic requirement for driving the hydrogen evolution reaction (HER),
21
Figure 2.6. The air mass 1.5 global tilt solar flux spectrum (solid) and the transmission
spectrum of GaP (dashed). The shaded grey area shows the integrated region of the solar
spectrum from 280 nm up to the band gap of GaP.
prompting researchers to investigate this material for PEC applications since the 1970s.1–9
However, its performance is limited by photocorrosion in aqueous conditions and surface
defect sites that hinder efficient transfer of photogenerated minority carriers at the
semiconductor|liquid junction.4 In this chapter, p-type GaP(100) substrates serve as
commercially available materials for developing a catalyst attachment chemistry for
integrating cobaloximes to the semiconductor via a polymeric interface.
2.1.2 Polymeric Interface
The constructs exploit the properties of olefins to undergo self-initiated photografting and
photopolymerization at semiconductor surfaces as well as a variety of other hydrogen- or
hydroxyl-terminated materials.10–19 The UV-induced surface grafting reaction has been
used to graft monolayers or polymer brushes onto conductive and semiconductive
400 600 800 1000 1200 14000
1x1014
2x1014
3x1014
4x1014
5x1014
Flu
x (
Ph
oto
ns s
-1 c
m-2 n
m-1)
Wavelength (nm)
Tra
nsm
issio
n
22
substrates, and several mechanisms have been proposed for the reaction.20 In the self-
initiated photografting and photopolymerization (SIPGP) mechanism, the neat monomer
acts as a photosensitizer, absorbs a photon and is excited to a triplet state, which is in
equilibrium with a biradical species. The biradical species can react with a monomer in
solution, propagating radical polymerization. Alternately, the biradical species can
abstract a surface hydrogen atom, creating a radical surface site for surface-initiated
polymerization. In addition, a surface-initiated radical mechanism has been proposed for
the photografting reaction at silicon, in which incoming UV light cleaves the Si-H bond,
which has a bond strength of ~ 3.5 eV, therefore requiring UV light of < 350 nm.10,19,21–24
The Si radical site can then react with the β-carbon of an alkene to form a Si-C bond.
Finally, the α-carbon radical abstracts a hydrogen atom from a neighboring Si-H group,
and the surface reaction is propagated across the surface in a random walk to form an
alkylated monolayer.
Surface-initiated grafting can also occur under white light.25–29 Under these conditions,
the proposed mechanism occurs via nucleophilic attack by the alkene to a valence band
hole generated by photogenerated electron-hole separation. In a similar manner, UV
excitation can result in photoemission of an electron from the semiconductor to
solution,18 resulting in a persistent hole in the valence band subject to nucleophilic attack
by the alkene. The predominant mechanism contributing to surface grafting of alkenes
under UV light is ascribed to a photoemission process, as the lifetime of the hole is much
longer in the photoemission mechanism compared to the exciton mechanism.
In this work, UV-initiated surface grafting and polymerization are used to prepare PVI or
23
Figure 2.7. Schematic representation of the attachment method used to assemble
cobaloxime-modified photocathodes.
PVP grafts (Figure 2.7). Post-polymerization modification is then used to direct and
assemble molecular catalysts onto built-in recognition sites– ligands– on the surface-
attached polymer. Thus, the polymeric brush serves as a structural interface between the
soft molecular catalysts and the hard semiconductor surface. To facilitate direct
comparisons between the PVI- and PVP-functionalized materials in the work reported
here, the grafts were assembled on GaP (100) wafers with identical physical properties
including doping conditions, resistivity, mobility, carrier concentration and etch pit
density (i.e., the wafers are cut from the same ingot).
2.1.3 Cobaloximes
Cobaloximes are a class of cobalt-containing catalysts known for catalyzing hydrogen
evolution in organic and aqueous conditions and have shown promise in electrocatalysis
N
GaP
PVP|GaP Co|PVP|GaP
Co|PVI|GaP
H O
NO
N
O
HO
N
N Cl
Co
N
m
N
n
N
N
H O
NO
N
O
HO
N
NCl
Co
m
n
N
N
N
n
n
N
N
H O
NO
N
O
HO
N
N Cl
Co
N
m
N
n
N
N
H O
NO
N
O
HO
N
NCl
Co
m
n
N
N
N
n
n
N
N
H O
NO
N
O
HO
N
N Cl
Co
N
m
N
n
N
N
H O
NO
N
O
HO
N
NCl
Co
m
n
N
N
N
n
n
N
N
N
N
H O
NO
N
O
HO
N
NCl
Co
m
n
N
N
PVI|GaP
24
and solar-to-fuel applications. Since the first report of electrochemical hydrogen
evolution catalyzed by a fluorinated cobaloxime by Connolly and Espenson, cobaloximes
and their mechanism of catalysis have been extensively studied by chemical,
electrochemical, and photochemical means.30–42 Previous reports show that cobaloximes
can be assembled onto GaP via a polyvinylpyridine interface, resulting in a cathode
material that uses only photonic energy, and no external electrochemical forward biasing
or sacrificial electron donors, to generate a relatively stable current and associated rate of
hydrogen production in pH neutral aqueous solutions.43 In addition, treatment of the
PVP-functionalized GaP using cobaloxime catalyst precursors with a ligand macrocycle
that is modified at the molecular level has been shown to affect the PEC response to
illumination and pH observed at the construct level.44 Thus, the photoactivity of the
modified semiconductor can be influenced by changes to the ligand environment of the
attached cobalt complexes.
2.2 Results and Discussion
2.2.1 Wafer Preparation and Characterization
A two-step method is used to assemble catalysts at the semiconductor surface. First, UV-
induced grafting of the appropriate polymer to the GaP (100) surface yields
polyvinylimidazole-grafted GaP (PVIlGaP) or polyvinylpyridine-grafted GaP (PVPlGaP)
constructs. In the second step, wet chemical treatment of the polymer-modified surface is
used to assemble cobaloximes on the polymer graft. This chemical transformation
exploits the base-promoted ligand exchange of axial X-type chloride ligands on cobalt
catalyst precursors with pendent N-type ligands on the surface-grafted polymers, yielding
25
cobaloxime-modified PVPlGaP (ColPVPlGaP) or cobaloxime-modified PVIlGaP
(ColPVIlGaP) (Figure 2.8). Analysis of the chemically modified surfaces using XP
spectroscopy provides evidence of successful functionalization with the desired
molecular connectivity. Survey spectra of the PVPlGaP and PVIlGaP samples show C, N
and O elements and cobaloxime-functionalized samples show additional C, O, Cl and Co
elements associated with attached cobaloximes (Figure 2.8). N 1s core level XP spectra
of polymer-functionalized GaP surfaces (Figure 2.9) present a single nitrogen feature
centered at 398.7 for PVPlGaP corresponding to the pyridyl nitrogens of the graft.
Conversely, N 1s core level spectra of PVIlGaP surfaces exhibit two distinct nitrogen
features centered at 398.3 and 400.2 and having a 1:1 spectral intensity ratio arising from
the two types of nitrogen environments in PVI,45–47 with the amine nitrogen feature at
higher binding energies than those assigned to imine nitrogen (Table 2.1).
Figure 2.8. XP survey spectrum of (left) PVIlGaP and (right) ColPVIlGaP.
1300 650 0
N1s
C1s
Binding Energy (eV)
Inte
nsity (
a.u
.)
Wide
Ga 3pP2p
Ga 2p
Ga LMM
1300 650 0
Ga 2p
Co 2p
O1s
N1sC1s
Cl 2p
Binding Energy (eV)
Inte
nsity (
a.u
.)
Wide
26
Figure 2.9. N 1s core level XP spectra of (top) PVIlGaP and (bottom) PVPlGaP. The
solid blue (b) and solid dark blue (c) lines represent the component fit of the imine
nitrogen, and the dashed blue (a) line the component fit of the amine nitrogen. The circles
represent the spectral data, the gray line is the linear background, and the black line the
overall fit.
Table 2.1 N 1s Binding Energies (eV)
Construct Imine
nitrogen
Amine
nitrogen
Co-imine
nitrogen
Co-amine
nitrogen
Glyoximate
nitrogen
PVPlGaP 398.7 -- -- -- --
ColPVPlGaP 398.6 -- 400.6 -- 400.0
PVIlGaP 398.3 400.2 -- -- --
ColPVIlGaP 398.4 400.0 399.0 400.6 400.0
a
b
c
27
The assembly of cobaloximes on the PVP or PVI grafts gives rise to additional nitrogen
features at higher binding energies in the N 1s core level spectra. Spectral fitting of the
XP data facilitates assignment of the individual contributions (Figure 2.10). For
ColPVIlGaP, a spectral contribution at 400.6 eV is assigned to pyridyl nitrogens on the
graft that are coordinated to cobalt centers of attached cobaloximes and the contribution
centered at 400.0 eV is indicative of the N 1s features of the cobaloxime glyoximate
ligands. The remnant contribution at 398.6 eV is assigned to pyridine units on the
polymer chains that are not coordinated to cobalt centers. For ColPVIlGaP surfaces,
assembly of Co centers to imidazole units on the polymer graft contribute to the peaks at
399.0 eV and 400.6 eV, while the contributions at 398.4 eV and 400.6 eV are assigned to
remnant imidazole nitrogens on the polymer that are not coordinated to cobalt centers.
The prominent peak at 400.0 eV is assigned to glyoximate nitrogens of the cobaloximes.
For both the ColPVIlGaP and ColPVPlGaP surfaces, the Co:Cl spectral intensity ratio is
1:1, corroborating the proposed synthetic mechanism in which one of the axial chloride
ligands on the Co-precursor complex is replaced with a nitrogen-containing ligand on the
polymer graft. In this mechanism, charge balance of the cobalt complex when replacing
chloride, an anionic X-type ligand, with a neutral L-type ligand is accounted for by the
base-promoted conversion of the dimethylglyoxime ligand to the dimethylglyoximate
monoanion. As anticipated, the Co 2p core level spectra signal shows the expected 15 eV
splitting and 2:1 ratio of the 2p3/2 and 2p1/2 peaks (Figure 2.11), consistent with the Co3+
oxidation state of the grafted complex.48,49 The collective information regarding the
28
Figure 2.10. N 1s core level spectra of (top) ColPVIlGaP and (bottom) ColPVPlGaP. The
solid blue (b) and solid dark blue (c) lines are the component fits for the unbound imine,
the dotted blue line (a) the component fit for the unbound amine, the solid red (f) and
solid dark red (h) lines the component fit for the bound imine, and the dotted red line (e)
the component fit for the bound amine. The dashed red (d) and dashed dark red (g) line
are the component fits for the glyoximate contributions. The circles represent the spectral
data, the gray line is the Shirley background, and the black line is the overall fit.
a
b
c
e f
d
h g
29
Figure 2.11. XP spectrum of the Co 2p region of ColPVIlGaP. Circles are the spectral
data, and the solid lines represent the background (gray), and component fit (red).
elemental identity and chemical composition obtained from XPS analysis provides
confirmation of the synthetic efforts.
Surface analysis using GATR-FTIR spectroscopy provides further structural
characterization of the modified GaP substrates, including direct observation of
vibrational modes associated with the grafted polymers and assembly of intact
cobaloxime complexes. FTIR spectra following the photochemical treatment of freshly
cleaned and etched GaP surfaces with 4-vinylpyridine or N-vinylimidazole show distinct
C-H and C-N modes in the 1400 to 1600 cm-1 range indicative of successful
polymerization, and unique from the C-H and C-N vibrations associated with monomeric
4-vinylpyridine or N-vinylimidazole (Figure 2.12). Additionally, there is a lack of the
pronounced feature at 1371 cm-1 associated with the C=C bond of the vinyl group
monomer50 and instead a peak at 1454 cm-1, indicative of the planar deformation
vibration of the CH2 groups along the polymer chain backbone, is observed.51
800 790 780 770 760
Inte
nsity (
a.u
.)
Co 2p
Binding Energy (eV)
30
Figure 2.12. GATR-FTIR spectra of PVIlGaP (blue) and PVPlGaP (dotted dark blue)
grafted onto gallium phosphide, as well as unmodified GaP (dashed black). Spectra are
normalized for comparison.
Treatment of the polymer-grafted surfaces with a solution of the catalyst precursor,
Co(dmgH2)(dmgH)Cl2, gives rise to new vibrational modes on the surface that are
ascribed to the formation of intact cobaloxime catalyst. These modes are distinct from
those observed in the FTIR spectrum of the Co(dmgH2)(dmgH)Cl2 precursor but are
nearly identical to the vibrational modes observed in spectra of the model molecular
catalysts Co(dmgH)2(Py)Cl or Co(dmgH)2(meIm)Cl (Figure 2.13a). In particular,
measurements of the NO- stretching frequency of the cobaloxime-functionalized surfaces
provide direct spectroscopic evidence supporting assembly of the cobaloxime units at the
pendent pyridyl or imidazole sites of the polymer graft. For the Co(dmgH2)(dmgH)Cl2
precursor complex the NO- stretch occurs at 1225 cm-1, however for the model
cobaloximes Co(dmgH)2(Py)Cl and Co(dmgH)2(meIm)Cl, this mode is centered at 1240
or 1237 cm-1, respectively. FTIR spectra of the polymer-functionalized GaP surfaces
1700 1450 1200
Wavenumbers (cm-1)
Ab
so
rba
nce
31
Figure 2.13. (a) FTIR transmission spectra of Co(dmgH)2Cl2 in KBr (green),
Co(dmgH)2(Py)Cl in KBr (dashed dark red), and Co(dmgH)2(meIm)Cl in KBr (red), as
well as (b) GATR-FTIR absorbance spectra of ColPVIlGaP (red), ColPVPlGaP (dashed
dark red), and GaP following a fresh HF etch (dashed black). Insets show the NO− region.
1275 1225
1700 1450 1200
Wavenumbers (cm-1)
% T
ran
sm
itta
nce
(a)
1700 1450 1200
Wavenumbers (cm-1)
Ab
so
rba
nce
(b)
1275 1225
32
following wet chemical treatment with a solution of Co(dmgH2)(dmgH)Cl2 show a strong
NO- vibration at 1241 cm-1 for PVP-functionalized GaP and at 1240 cm-1 for PVI-
functionalized GaP (Figure 2.13b), values that closely match those of the model catalysts
within the 4 cm-1 resolution of the spectroscopic scans. Additionally, there is no
pronounced peak at 1225 cm-1, indicating that minimal to no precursor complex remains
in or on the cobaloxime-modified GaP following ultrasonic cleaning of the samples.
Thus, FTIR analysis indicates effective assembly of cobaloxime units using the built-in
recognition sites of the polymer grafts, in agreement with our surface analysis using XP
spectroscopy.
Information on the length of the PVP or PVI polymer grafts is provided by spectroscopic
ellipsometry measurements, yielding a PVP thickness of 10 nm and a PVI thickness of 6
nm on the GaP (100) substrates (Figure 2.14). Using these film thickness values, the
corresponding pyridyl and imidazole site density can be estimated given the bulk polymer
Figure 2.14. Spectra obtained from ellipsometry measurement (red) of PVIlGaP at 69°
(top) and 74° (bottom) and model fit to data (black dash).
500 750
10
20
Wavelength (nm)
y in
de
gre
es
33
density (1.15 g cm-3 for PVP and 1.25 g cm-3 for PVI) is representative of the solvent-free
layer density. For a 10 nm thick solvent-free PVP layer the corresponding maximum site
density is 6.6 x 1015 cm-2 or 11 nmol cm-2. In the case of a 6 nm thick solvent-free PVI
layer the maximum site density is 4.8 x 1015 cm-2 or 8 nmol cm-2. The larger film
thickness of PVP grafts with respect to those obtained for the PVI grafts is consistent
with previous reports of lower grafting concentrations reported for the UV
polymerization of PVI relative to PVP.45,52 The identity of the polymer graft thus can be
used to control the physical characteristics at the mesoscale in addition to the chemical
identity of the ligand sites at the nanoscale.
2.2.2 Photoelectrochemical Performance
PEC performance of the assembled constructs is evaluated by fabricating photocathodes
from the functionalized and unmodified constructs (Figure 2.15). All measurements are
performed in aqueous solutions buffered at pH 7 (0.1 M phosphate buffer) in the dark and
at AM1.5 illumination using a 100 W solar simulator with a sample size consisting of at
least 4 individually tested electrodes. The polymer-grafted GaP electrodes (PVPlGaP and
PVIlGaP) produce a photocurrent density of 0.68 ± 0.18 mA cm-2 and 0.43 ± 0.02 mA
cm-2, respectively, when polarized at 0 V vs RHE. For the cobaloxime-modified GaP
electrodes, polarized at 0 V vs RHE, the current density is 1.3 ± 0.05 mA cm-2 for
PVPlGaP and 1.2 ± 0.08 mA cm-2 for PVIlGaP. For the cobaloxime-modified samples, a
positive shift in the potential needed to reach a selected current density (with Von defined
as the potential required to achieve a current density of 1 mA cm-2) is observed as
compared to the potential required using electrodes containing only the polymer graft
34
Figure 2.15. Linear sweep voltammograms of PVPlGaP (dark blue), PVIlGaP (blue),
ColPVPlGaP (dark red) and ColPVIlGaP (red) working electrodes at pH 7 in the dark
(dotted lines) and with 100 mW cm-2 illumination (solid and dashed lines). An
unmodified GaP electrode under illumination is shown for comparison (black).
Table 2.2 PEC Characteristics
Construct Voc (V vs RHE) Von* (V vs RHE) J at 0 V vs RHE (mA cm-2)
GaP(100)
PVPlGaP
0.57 ± 0.03
0.57 ± 0.01
-0.037 ± 0.06
-0.068 ± 0.09
0.86 ± 0.01
0.68 ± 0.18
ColPVPlGaP 0.61 ± 0.05 +0.24 ± 0.04 1.3 ± 0.05
PVIlGaP 0.53 ± 0.05 -0.14 ± 0.02 0.43 ± 0.02
ColPVIlGaP 0.58 ± 0.02 +0.073 ± 0.05 1.2 ± 0.08
*where Von is defined here as the potential required to achieve a 1 mA cm-2 current
density.
-0.8 -0.6 -0.4 -0.2
-1.2
-0.8
-0.4
0.0
E (V vs Ag/AgCl)
J (
mA
cm
-2)
-0.2 0.0 0.2 0.4
E (V vs RHE)
35
and without cobaloxime modification. For the PVP-grafted electrodes, cobaloxime
modification yields a +310 ± 100 mV shift in Von while for PVI-grafted electrodes,
cobaloxime modification yields a +210 ± 50 mV shift in Von. Likewise, Von for
ColPVPlGaP electrodes wafers that are tested in a three-electrode configuration is 170
mV positive of those obtained using ColPVIlGaP electrodes (Figure 2.15 and Table 2.2).
The measured open-circuit photovoltages (Voc) of the electrodes are also consistent with
the increased photoactivity of the electrodes following cobaloxime addition.
Analysis of the headspace of the PEC cell by gas chromatography (Figure 2.16) after 60
minutes of chronoamperometry at a voltage slightly positive of 0 V vs RHE confirms the
formation of H2 gas with a near-unity faradaic efficiency for the cobaloxime-modified
electrodes. For comparison, electrochemical measurements using solutions of the
Figure 2.16. Gas chromatogram of 5 mL of headspace before (black dash) and after (red)
bulk electrolysis of ColPVPlGaP working electrodes at pH 7 with 100 mW cm-2
illumination for 1 hour at 0 V vs RHE.
De
tecto
r R
esp
on
se
(a
.u.)
H2
26 28 30 32 34
Retention Time (sec)
36
cobaloxime model compounds Co(dmgH)2(meIm)Cl and Co(dmgH)2(Py)Cl recorded in
acetonitrile solutions show that structural modifications to the ligand environment (axial
coordination to imidazole vs. pyridine) result in a 60 mV shift of the CoII/CoI redox
couple to more negative potentials (Figure 2.17), illustrative of the ligand environment’s
effect on the redox properties of the cobaloxime catalysts. For the cobaloxime-modified
GaP substrates reported here, the differences in PEC performance in aqueous
environments may in part be due to the inherent difference in activity of the cobaloximes
in the grafted polymer environment (PVI vs. PVP).
Figure 2.17. Cyclic voltammograms of Co(dmgH)2(meIm)Cl (green) and
Co(dmgH)2(Py)Cl (red) recorded in 0.1 M tetrabutylammonium hexafluorophosphate
in acetonitrile at a scan rate of 100 mV s-1 with a glassy carbon working electrode at
room temperature and ferrocene as an internal standard with E1/2 taken as 0.40 V vs NHE.
Data are normalized for comparison. Data collected by Samuel Jacob.
37
2.2.3 Turnover Frequency
An estimate of the per cobalt center turnover frequency (TOF) of the cobaloxime-
modified electrodes can be arrived at using the following assumptions: 1) there is
uniform surface coverage of the graft and uniform distribution of cobaloximes along the
depth of the polymer graft, 2) the film thickness determined by ellipsometry is
representative of the solvent-free thickness of the film, and 3) all cobalt centers are active
and are the only sites of hydrogen production. Given the first and second set of
conditions, cobalt loading of the films can be estimated from the associated XP
spectroscopy and ellipsometry data. In the case of the ColPVPlGaP construct, the spectral
intensity ratio of the nitrogen contribution at 400.6 eV, assigned to pyridyl nitrogen
coordinated to cobalt centers, and the remnant peak at 398.6 equate to a 30% loading of
cobalt centers to pyridyl groups on the PVP graft. An analysis of spectral intensity ratio
of the nitrogen contributions using ColPVIlGaP samples indicates that 35% of the
available imidazole sites on the PVI graft are associated with an attached cobaloxime. An
analysis of the total Co:N ratios yields identical loadings. If the cobaloximes are,
however, not distributed evenly along the nitrogen sites of the grafted brushes but are
instead concentrated toward the upper portions of the polymer (i.e. away from the
underpinning semiconductor surface), then our analysis results in an overestimate of the
loading due to signal attenuation of nitrogen sites located at lower depths along the
polymer. Likewise, if solvent remains in the brush following cleaning and drying, then
the calculated FT represents an upper limit on the site density.
38
Using cobaloxime loadings obtained from the analysis described above, the current
densities measured in three-electrode polarization experiments using cobaloxime-
modified GaP electrodes give information on the activity of the electrodes per number of
cobaloximes assembled on the polymer grafts. For example, the current density of 1.3
mA cm-2, measured at 0 V vs. RHE and under 1 sun illumination, for the ColPVPlGaP
electrode yields a TOF of 2.1 s-1, and the current density of 1.2 mA/cm2 for the
cobaloxime-PVI functionalized electrode, measured under similar conditions, gives a
TOF of 2.4 s-1. This analysis represents the activity of the electrode per amount of cobalt
present, regardless of the activity of individual centers. If not all cobaloxime centers are
active or equally active, as could likely be the case for the inherently heterogenized
molecular components reported here, our analysis could underestimate the TOF of
individual cobaloxime sites. Conversely, if there are other sites of hydrogen production
on the surface the TOF could be overestimated for individual cobaloxime sites. For
comparison, a 1.0 mM solution Co(dmgH)2(Py)Cl at a graphite electrode poised at -0.90
V vs. Ag/AgCl in 1,2-dichlorethane in the presence of 0.2 M Et3NH(BF4) (a sacrificial
reductant) yielded ≈100 turnovers in 2.5 hours with no visible degradation of the catalyst.
Previous reports, however, indicate that immobilized catalysts can have significantly
higher turnover ability compared to their solution counterpart. In a report by Artero and
coworkers using cobalt diimine-dioxime catalysts grafted onto carbon nanotubes
polarized at -0.59 V vs. RHE in acetate buffer (pH 4.5) a TOF of ≈2.2 s-1 is reported. The
authors speculate that immobilization of the catalyst prevents inactivation through
dimerization and enhances electron transfer dynamics from the conductive electrode to
39
the catalytic site.53 Similarly, a more recent report by Turner and coworkers assigns a
TOF of 1.9 s-1 to 3.4 s-1 using an isonicotinic acid-modified cobaloxime catalysts
adsorbed on a TiO2-coated GaInP2 electrode operating at 0 V vs. RHE in an basic
aqueous solution (pH 13).54
2.3 Conclusions
Cobaloxime catalysts for hydrogen production have been assembled on the surface of
GaP(100) using both PVP and PVI grafts. The variation of the polymeric interface affects
the photoelectrochemical performance of the constructs. Surface-sensitive spectroscopic
analyses verify the successful synthesis of cobaloximes on the polyvinylpyridine or
polyvinylimidazole grafts using molecular assembly at recognition sites on the surface-
attached polymers. PEC characterization of the cobaloxime-modified assemblies shows a
marked increase in per geometric area hydrogen production rates as compared to results
obtained using electrodes without cobaloxime functionalization. This work illustrates the
versatility of the polymer graft as an interface for controlling structure at the nano- and
mesoscales as well as functional performance at the macroscale as evidenced by PEC
characterization of the photocathodes assembled from the modified wafers. The
imidazole- and pyridyl-grafted cobaloxime constructs are both capable of achieving a
current density greater than 1 mA cm-2 when polarized at potentials positive of the
reversible hydrogen electrode (RHE) and thus under reverse biased conditions, achieving
1 mA cm-2 at +0.24 V vs. RHE for the pyridyl-based construct and +0.073 V vs. RHE for
the imidazole-based assembly. The cobaloxime-PVP architecture shows improved per
geometric area photocurrent production over the cobaloxime-PVI construct, achieving a
40
13.4 ± 1.0% higher photocurrent density for electrodes polarized at the potential of the
reversible hydrogen electrode. Yet the imidazole graft is 60% the length and more
conservative in use of cobaloxime complexes, containing 80% the number of cobalt
centers per geometric area. The more effective use of cobalt in the PVI-assembly is
evidenced in the per-cobalt TOF analysis of the ColPVPlGaP and ColPVIlGaP
photocathodes, estimated at 2.1 s-1 and 2.4 s-1 respectively. Future work will focus on an
improved understanding of surface coverage and distribution of cobaloximes along the
polymer graft using angle-resolved XP spectroscopy measurements, which should enable
a refined model and assessments of the loadings and per cobalt TOFs achieved using
these hybrid constructs. This work illustrates the potential to control and optimize the
properties of visible-light-absorbing semiconductors using polymeric overlay techniques
coupled with the principles of synthetic molecular design. Key features of the construct
include the use of polymer-functionalized semiconductors, the assembly of molecular
components at the polymeric interface, a modular architecture, and PEC operation in
aqueous conditions (an imperative for the photoelectrochemical reduction of water).
2.4 Experimental Methods
2.4.1 Materials
Single crystalline p-type gallium phosphide wafers were purchased from University
Wafers. The material is single side polished to an epi-ready finish. The p-type Zn-doped
GaP(100) wafers have a resistivity of 0.2 Ω cm-2, a mobility of 66 cm2 V-1 s-1, and a
carrier concentration of 4.7 x 1017 cm-3, with an etch pit density of less than 8 x 104 cm-2.
Preparation of all wafers began with initial degreasing with an acetone-soaked cotton
41
swab followed by consecutive ultrasonic cleaning in acetone and isopropanol for 5
minutes each followed by drying under a nitrogen stream. The wafers were then exposed
to an air-generated plasma (Harrick Plasma, USA) at 30 W for 2 minutes. Finally the
wafers were etched in buffered hydrofluoric acid (6:1 HF:NH4F in H2O) for 5 minutes
followed by rinsing with distilled methanol and drying under a nitrogen stream then
storing under vacuum (-750 Torr). All reagents were purchased from Aldrich.
Isopropanol was cleanroom grade and used as received. Dichloromethane, methanol and
vinylimidazole were freshly distilled before use. All solvents were stored over the
appropriate molecular sieves prior to use. Milli-Q water (18.2 MΩ·cm) was used to
prepare all aqueous solutions.
2.4.2 Cobaloxime Synthesis
Synthesis of Co(dmgH2)(dmgH)Cl2 and Co(dmgH)2(py)Cl. These complexes were
prepared by Samuel Jacob according to a previously reported procedure.55
Synthesis of Co(dmgH)2(meIm)Cl. These complexes were prepared by Samuel Jacob. A
suspension of Co(dmgH2)(dmgH)Cl2 (1.1 g, 3.0 mmol) and 1-methylimidazole (0.58 g ,
7.0 mmol) was vigorously agitated in 60 mL of chloroform. After 10 minutes, water (30
mL) was added and the mixture was stirred for 2.5 h. The mixture was filtered and
washed with 60 mL portions of water using a separatory funnel until the aqueous layer
was clear. The organic layer was concentrated under reduced pressure and precipitated
with the addition of ethanol. The crude product was recrystallized from dichloromethane
and ethanol yielding a brown crystalline solid (45% yield). 1H NMR (400 MHz, CDCl3):
42
2.39 (12H, s, CH3), 3.60 (3H, s, ArCH3), 6.66-6.67 (1H, m, ArH), 6.73-6.74 (1H, m,
ArH), 7.27 (1H, s, ArH), 18.43-18.51 (2H, brs, OH). UV-vis (CH2Cl2) 252, 297, 355 nm.
2.4.3 Wafer Functionalization
The freshly etched wafers were placed into a quartz flask containing the appropriate
argon-sparged monomer (N-vinylimidazole or 4-vinylpyridine) and purged with argon
for 15 minutes, after which they were placed into a UV chamber and irradiated with
shortwave (254 nm) light for 2 hours.11,18,56 The resultant graft serves a dual purpose,
offering a protection layer for the underpinning GaP surface47,57 and providing multiple
sites for subsequent assembly of cobaloxime catalysts at the polymeric interface.
Following the UV-induced attachment, the wafers were ultrasonically cleaned in freshly
distilled methanol and dried under a nitrogen stream then stored under vacuum (-750
Torr). Cobaloxime functionalization was achieved using wet-chemical treatment of the
polymer-modified substrate with a methanolic solution of Co(dmgH2)(dmgH)Cl2 (1 mM)
and 1 mM triethylamine (TEA). After 12 hours, the wafers were removed from the
reaction mixture, ultrasonically cleaned in freshly distilled methanol, dried under a
nitrogen stream and stored under vacuum (-750 Torr).
2.4.4 Wafer Characterization
XPS was performed using a monochromatized Al Kα source (hν = 1486.6 eV) operated
at 63 W with a takeoff angle of 0° relative to the surface normal and a pass energy for
narrow scan spectra of 20 eV at an instrument resolution of 700 meV. Survey spectra are
collected with a pass energy of 150 eV. Spectral fit was analyzed using CASA software
and all spectra were calibrated by adjusting adventitious carbon to 284.8 eV. Curves were
43
fitted using a linear background for polymers and Shirley background for cobaloxime-
functionalized films. N 1s components were constrained to a fwhm of 1.3 eV.
Grazing angle attenuated total reflection Fourier transform infrared spectroscopy (GATR-
FTIR) was performed using a VariGATR accessory (Harrick Scientific) with a Ge crystal
plate installed in a Bruker Vertex 70. Samples were pressed against the Ge crystal to
ensure effective optical coupling. Spectra were collected using 256 scans under a dry
nitrogen purge at 4 cm−1 resolution with a GloBar MIR source, a broadband KBr
beamsplitter and a liquid nitrogen cooled MCT detector. Background measurements were
obtained from the bare Ge crystal and the data processed using OPUS software. GATR
measurements were baseline corrected for rubberband scattering.
Film thickness (FT) was determined using a J.A. Woollam Variable Angle Spectroscopic
ellipsometer with a spectral range of 200 nm-1200 nm. Measurements were taken at 69⁰
and 74⁰ incidence angles and analysis performed using Complete EASE software. The
model used to determine the FT of surface-grafted polyvinylimidazole was composed of
a GaP substrate layer (Palik), a GaP oxide layer (Zollner) with a fixed thickness of 1.01
nm and a Cauchy layer for the polymer film of each sample.
2.4.5 Electrode fabrication
GaP working electrodes were fabricated by applying an indium gallium eutectic (Aldrich)
to the backside of a wafer, then fixing a copper wire to the back of the wafer using a
conductive silver epoxy (Circuit Works). The copper wire was passed through a glass
tube, and the wafer was insulated and attached to the glass tube with Loctite 615 Hysol
Epoxi-patch adhesive. The epoxy was allowed to fully cure before testing the electrodes.
44
2.4.6 Photoelectrochemical Characterization
PEC characterization was performed using 100 mW cm−2 illumination from a 100 W
Oriel Solar Simulator equipped with an AM1.5 filter. Three-electrode linear sweep
voltammetry and bulk electrolysis experiments (chronoamperometry) were performed
with a Biologic potentiostat using a platinum coil counter electrode, a Ag/AgCl, NaCl (3
M) reference electrode (0.21 V vs. NHE) and GaP working electrodes (including GaP
following acid etch, polymer-grafted GaP and cobaloxime-modified GaP) in a PEC cell
containing a quartz window. The supporting electrolyte was 0.1 M phosphate buffer (pH
7). Linear sweep voltammograms were recorded at sweep rates of 100 mV s−1 under a
continuous flow of 5% hydrogen in nitrogen. Chronoamperometry was performed with
the working electrode and at -0.61 V vs. Ag/AgCl, positive of 0 V vs. RHE, where E vs.
RHE = E vs. NHE + 0.05916 x pH (V) = E vs. Ag/AgCl + 0.05916 x pH +0.21 (V).
2.4.7 Hydrogen Detection
Gas analysis was performed via gas chromatography (GC) using an Agilent 490 Micro
GC equipped with a 5 Å MolSieve column at a temperature of 80°C and using argon as
the carrier gas. Gas samples were syringe-injected using 5 mL aliquots of headspace gas
collected with a gas-tight Hamilton syringe from a sealed PEC cell both prior to and
following 60 min of three-electrode photoelectrolysis using a cobaloxime-modified
working electrode polarized at +0.013 V vs. RHE. Prior to the experiment the cell was
purged for 30 min with argon before sealing. The retention time of hydrogen was
confirmed using a known calibrated source obtained from a standard lecture bottle
containing a hydrogen and argon mixture.
45
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49
CHAPTER 3 SOLAR HYDROGEN PRODUCTION USING MOLECULAR
CATALYSTS IMMOBILIZED ON GALLIUM PHOSPHIDE (111)A OR (111)B
POLYMER-MODIFIED PHOTOCATHODES
Portions of this work are excerpted with permission from:
Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. ACS Appl. Mater. Interfaces
2016, 8, 10038–10047.
Copyright 2016 American Chemical Society
50
3.1 Introduction
The crystal face of GaP(100) terminates with a mixed phase of Ga and P sites, while the
(111A) and (111B) faces consist of surfaces with predominantly atop Ga or atop P,
respectively. Both faces are subject to oxidative and corrosive processes; however
methods have been developed to chemically protect and passivate these otherwise
structurally unstable surfaces, including treatment with (NH4)2Sx,1 alkylation or
allylation with subsequent secondary functionalization of the A-side,2 and alkyl halide
reactions with the P sites via a Williamson ether synthesis on the B-side.3 An attachment
strategy that can be utilized across multiple crystal face orientations may be useful in
interfacing molecular catalysts to nanostructured materials that terminate with a range of
indices, thereby permitting a relatively dense loading of catalysts and relieving the
turnover frequency (TOF) of individual active sites required to achieve a selected current
density.
This chapter presents the functionalization of GaP(111) with cobaloxime 4–19 for
hydrogen production using a chemical attachment strategy that can be deployed on both
the A and B crystal faces (Figure 3.18). It has been shown that polyvinylimidazole (PVI)
confers stability to metal surfaces20,21 and in these efforts, it serves the dual role of
providing a linkage to molecular catalysts as well as a protective layer for the
underpinning GaP. As compared to unmodified electrodes, cobaloxime modification
yields a greater than four-fold increase in the photocurrent density measured for
electrodes polarized at zero volts versus the reversible hydrogen electrode (0 V vs. RHE).
Chronoamperometry coupled with gas analysis measurements show the photocurrent
51
densities are relatively stable, with a drop off of less than 1% over 55 minutes following
an initial decrease of up to 12% during the first 5 minutes of photoelectrochemical (PEC)
characterization, and that the current produced is associated with a nearly quantitative
production of hydrogen gas. This work illustrates the promise of these constructs to
produce a fuel using only light as an energy input with no requirement of forward
electrochemical biasing, sacrificial reagents or extreme pH conditions. Additionally, the
modular assembly method allows modification of components as new materials and
discoveries emerge. In particular, and as illustrated in this chapter, the ability to export
the grafting chemistry across a selection of crystal face orientations is a desirable feature
Figure 3.18. Schematic of cobaloxime linked to either the GaP(111)A face or the GaP
(111)B face through surface-grafted polyvinylimidazole.
52
in the context of functionalizing the inherently multi-faceted surfaces of nanostructured
light-harvesting materials which have shown great promise in emerging light capture and
conversion technologies due to their relatively large aspect ratios and short minority-
carrier diffusion lengths.22–26
3.2 Results and Discussion
3.2.1 Wafer Preparation and Characterization
Cobaloxime-modified GaP(111) wafers are assembled via a two-step process. First, PVI
is grafted onto gallium phosphide wafers using surface-initiated polymerization of the
vinylimidazole monomer under shortwave UV light.27–30 Second, surface-attached
cobaloxime complexes are formed using a wet chemical treatment that exchanges one of
two chloride ligands from the complex Co(dmgH2)(dmgH)Cl2 with an imidazole ligand
on the PVI graft. Sample preparation starts with placing clean, freshly etched GaP wafers
into a quartz flask containing the monomer under argon. The flask is then illuminated
with UV light (254 nm) for 2 hours before removing and cleaning the polymer-
functionalized wafers with successive solvent washes followed by drying under a stream
of nitrogen. The polymer-modified wafers are then placed in a sealed flask containing an
argon-sparged methanolic solution that is 1 mM in triethylamine and 1 mM
Co(dmgH2)(dmgH)Cl2. After 12 hours, the catalyst-modified wafers are removed from
the flask and cleaned with successive solvent washes followed by drying under a stream
of nitrogen then under vacuum. Following the PVI-grafting procedure, spectroscopic
ellipsometry measurements, performed in air, yield a polymer thickness of 6 nm. An
upper limit on the imidazole unit site density can be provided given the bulk PVI density
53
(1.25 g cm-3)31 is representative of the solvent-free layer density. Thus, for a 6 nm thick
PVI layer, the maximum site density is 5 x 1015 cm-2 = 8 nmol cm-2.
Surface analysis using grazing angle attenuated total reflection (GATR-FTIR)
spectroscopy provides compelling evidence for successful chemical functionalization of
the GaP(111) surfaces. GATR-FTIR spectra of unmodified GaP(111A) and GaP(111B)
as well as spectra collected following photochemical grafting of vinylimidazole, yielding
PVIlGaP (111A) or PVIlGaP (111B), and spectra collected following assembly of
cobaloximes, yielding ColPVIlGaP (111A) or ColPVIlGaP (111B), are shown in Figure
3.19. For the GaP samples functionalized only with PVI, and prior to attachment of
catalysts, distinct IR absorbance bands associated with multimode in-ring C=C as well as
in-ring C=N stretches of PVI are observed at 1500 cm-1. In addition, the distinct 1373
cm-1 peak of the C=C stretch associated with the vinyl group of the monomer is not
pronounced on the PVI-functionalized wafers. Instead, a new peak indicative of the
planar deformation vibration of the CH2 groups in the polymer chain32 and centered at
1454 cm-1, is present.
Following treatment of PVIlGaP surfaces with the Co(dmgH2)(dmgH)Cl2 solution,
unique vibrational modes characteristic of imidazole units coordinated to cobaloxime
complexes are detected, including a peak at 1570 cm-1 characteristic of the C=N stretches
on the glyoximate ligands of cobaloximes coordinated to an axial imidazole.33
Furthermore, unlike the C=N and C=C vibrations of the imidazole ring observed at 1500
cm-1 on PVIlGaP samples, additional vibronic features appear at 1517 cm-1 on samples of
and ColPVIlGaP(111B) associated with the C=N and C=C vibrations of imidazole units
54
Figure 3.19. FTIR transmission spectra of (a and c) N-vinylimidazole monomer in KBr
(blue) as well as a powder sample of the model cobaloxime catalyst,
Co(dmgH)2(meIm)Cl, in KBr (red). GATR-FTIR absorbance spectra of (b) the (111)A-
face and (d) (111)B-face of unmodified GaP (black dash) polyvinylimidazole-grafted
GaP (blue) and cobaloxime-modified GaP (red). The spectra shown in parts a and c are
identical and included to facilitate comparisons.
coordinated to cobalt centers. This 17-wavenumber difference is consistent with
previously reported IR spectra of metals coordinating to PVI.34 Finally, and as described
in the following sentences, comparison of the NO- stretching region in spectra of
ColPVIlGaP with spectra of Co(dmgH2)(dmgH)Cl2 and Co(dmgH)2(meIm)Cl, a model
cobaloxime compound bearing an axial N- methylimidazole unit, affords a direct
1700 1450 1200
Wavenumber (cm-1)
Abso
rbance
111A(c)
% T
ransm
ittance
(a)
1700 1450 1200
Wavenumber (cm-1)
111B(d)
(b)
ν C-C & C=N
(ring)
ν N-O
(cobaloxime)ν C-C & C=N
(ring)ν C=C
(ring)
ν N-O
(cobaloxime)
ν C=N
(dmg)ν C=N
(dmg)
ν C=C
(ring)
55
spectroscopic method of confirming successful synthesis of intact cobaloxime complexes
on the polymeric interface. The FTIR spectrum of the precursor material,
Co(dmgH2)(dmgH)Cl2, used to assemble the attached cobalt complexes to the PVI-
functionalized electrode includes a strong NO- stretch at 1225 cm-1. However, this feature
is not pronounced in the spectrum of PVI-GaP samples following treatment with
Co(dmgH2)(dmgH)Cl2. Instead the strong NO- stretching frequency is observed at 1239
cm-1 on the surface of the A and B wafers, a value more consistent with that observed for
the NO- stretch at 1237 cm-1 measured for the cobaloxime model compound,
Co(dmgH)2(meIm)Cl. All spectra are collected at 4 cm-1 resolution and the lack of
apparent peak features centered at 1225 cm-1 in spectra of ColPVIlGaP surfaces indicate
that minimal to no residual Co(dmgH2)(dmgH)Cl2 remain following ultrasonic cleaning
of the samples. In addition, the close alignment of the spectral features of ColPVIlGaP
samples with those obtained for powders of the model cobaloxime compound indicates
that that surface-assembled cobaloxime species have a vibrational structure similar to that
of the non-surface-attached model compound.
Sample analysis using X-ray photoelectron (XP) spectroscopy provides further evidence
of successful surface functionalization. Survey XP spectra of PVIlGaP samples show the
presence of additional O, N, and C elements as compared to spectra of unfunctionalized
GaP. In contrast to the Ga 2p and P 2p core level XP spectra of wafers with a native
oxide layer (Figure 3.20), the spectral features associated with gallium oxide and
phosphate are significantly reduced following the H2SO4 etching and subsequent polymer
grafting procedure, despite the samples being exposed to air following the polymer-
56
Figure 3.20. XP spectra of the Ga 2p (left) and P 2p (right) regions of unmodified GaP
(111)A (top) and GaP(111)B (bottom). The circles are the spectral data, and the solid
lines represent the background (gray), component fit (green), and overall fit (black).
grafting step (Figure 3.21). For the untreated GaP (111A) and GaP (111B) wafers, the Ga
2p3/2 spectral intensity ratios, AGa−O/AGa−P, are 0.08 and 0.49, and the P 2p spectral
intensity ratios, AP-O/AP-Ga, are 0.04 and 0.12, respectively. By contrast, inspection of the
XP core level spectra for PVIlGaP surfaces show a significant attenuation of the substrate
1125 1115
Binding Energy (eV)
Inte
nsity (
a.u
.)Ga 2p
111A
(b)
140 135 130 125
Binding Energy (eV)
Inte
nsity (
a.u
.)
P 2p
111A
(c)
1125 1115
Binding Energy (eV)
Inte
nsity (
a.u
.)
Ga 2p
111B
(b)
140 135 130 125
Binding Energy (eV)
Inte
nsity (
a.u
.)P 2p
111B
(c)
57
Figure 3.21. XP spectra of the Ga 2p (left) and P 2p (right) regions of polymer-modified
GaP (111)A (top) and GaP(111)B (bottom). The circles are the spectral data, and the
solid lines represent the background (gray), component fit (blue), and overall fit (black).
signals due to the presence of the grafted molecular layer. On the PVIlGaP A face, the
AGa-O/AGa-P ratio is 0.06 and there is no detectable phosphate feature. On the PVIlGaP B
face, both the gallium signal and phosphate feature are unapparent. In the C 1s core level
XP spectra of the polymer-functionalized wafers, a shakeup satellite appearing 6 eV
higher in binding energy than the C 1s feature centered at 284.8 eV is indicative of pi/pi*
1120 1115
Binding Energy (eV)
Inte
nsity (
a.u
.)Ga 2p
111A
(b)
135 130 125
Binding Energy (eV)
Inte
nsity (
a.u
.)
P 2p
111A
(c)
1120 1115
Binding Energy (eV)
Inte
nsity (
a.u
.)
Ga 2p
111B
(b)
135 130 125
Binding Energy (eV)
Inte
nsity (
a.u
.)
P 2p
111B
(c)
58
bonding of aromatic polymers. Lastly, the N 1s core level XP spectra of PVIlGaP
samples show two features centered at ~400 and 398 eV with near-equal spectral
intensity as anticipated for the 1:1 ratio of amine and imine nitrogens associated with
imidazole units on the PVI graft (Figure 3.22a and 3.22c). This is in sharp contrast to
Figure 3.22. N 1s core level spectra of (a) PVIlGaP(111)A, (b) ColPVIlGaP(111)A, (c)
PVIlGaP(111)B, and (d) ColPVIlGaP(111)B. Circles represent the spectral data. Solid
and dash lines are background (gray), component (red or blue), and overall (black) fits to
the experimental data.
(a
.u.)
N 1s(a)
Inte
nsity
Binding Energy (eV)
111A(b)
N 1s(c)
Binding Energy (eV)
111B(d)
59
spectra reported on GaP samples following grafting of PVP where only a single nitrogen
feature is observed at 398.7 eV.35,36
Survey XP spectra of ColPVIlGaP samples confirm the presence of additional Co and Cl
elements associated with attached cobaloximes. Analysis of the Co and Cl spectral
intensities obtained from core level XP spectra, following the application of appropriate
sensitivity factors, yields a measured Co:Cl ratio of 1:1. However, the measured Co:Cl
ratio using a Co(dmgH2)(dmgH)Cl2 powder sample is 1:2. These ratios are consistent
with the proposed attachment mechanism invoking a base-promoted conversion of the
dimethylglyoxime ligand of Co(dmgH2)(dmgH)Cl2 to a dimethylglyoximate monoanion
and the associated replacement of one of the axial X-type chloride ligands with an L-type
nitrogen ligand on the PVI-polymer brush.
Additional structural confirmation is provided by comparison of the N 1s and Co 2p core
level spectra of ColPVIlGaP samples which yields a Co:N spectral intensity ratio of 1:11
and 1:10 on the 111A and 111B modified surfaces, indicating that 28%-32% of the
imidazole units on the polymer graft are coordinated to a Co center, provided there is an
evenly distributed loading of Co sites along the ~6 nm depth polymer coat. Further,
unlike the N 1s core level spectra obtained for PVIlGaP surfaces which show only two
distinct nitrogen features with approximately equal spectral intensity, N 1s core level
spectra of ColPVIlGaP samples can be fit with three additional nitrogen contributions
assigned to imine, amine and glyoximate nitrogens coordinated to the Co centers of the
cobaloximes (Figure 3.22b and 3.22d). As measured in previous reports, coordination of
cobalt centers to a surface-grafted PVP-polymer brush is associated with displacement of
60
the N 1s feature assigned to attached pyridyl nitrogens to higher binding ratios estimated
from the Co:N spectral intensities. Finally, the Co 2p core level spectra of ColPVIlGaP
samples show samples show peaks centered at 780.0 eV (2p3/2) and 795.0 eV (2p1/2), a
2:1 branching and no apparent shakeup satellite structures, all features indicative of the
Co3+ species.37,38
3.2.2 Photoelectrochemical Performance
Photoactivity of the electrode assemblies is assessed via electrochemical characterization
in aqueous solutions buffered at pH 7 (0.1 M phosphate buffer) in the dark and upon
AM1.5 illumination using a 100 mW cm−2 solar simulator. For the 111 samples reported
here, all A and B face constructs are prepared using wafers cut from the same ingot
resulting in samples that achieve a similar saturating photocurrent when under equal
illumination conditions, thus simplifying the comparison of results obtained using the
photoelectrodes. The doping conditions of these wafers as well as the light source used
for illumination differ from previous reports from, and both of these factors can affect the
maximum attainable current density (Figure 3.23). Figures 3.24a and 3.24b show the
three-electrode polarization curves obtained using surface-functionalized GaP working
electrodes. For the cobaloxime-modified electrodes, the photocurrent measured at 0 V vs.
RHE is significantly higher than those achieved testing unmodified or polymer-only
treated electrodes, with the PVIlGaP electrodes having a current density of 0.32 ± 0.04
and 0.30 ± 0.08 mA cm-2 and the ColPVIlGaP electrodes having a current density of 0.89
± 0.02 and 0.89 ± 0.03 mA cm-2 for the A and B face, respectively (Table 3.3). Consistent
with the increased photoactivity of the molecular catalyst-modified electrodes, the
61
Figure 3.23. Linear sweep voltammograms of working electrodes using GaP substrates
with the varied properties and illumination conditions.
1 (solid line) illuminated with a Newport Oriel Apex illuminator (100 mW cm‐2) using
Co‐PVP-modified Zn doped p‐GaP(100) with a resistivity of 1.17 x 10‐1 Ω.cm, a mobility
of 77 cm2 V‐1 s‐1, and a carrier concentration of 6.95 x 1017 cm‐3.36 2 (dotted line)
illuminated with a Solar Light PV cell test simulator (100 mW cm‐2) using the same
electrode described in 1.39 3 (dashed-dotted line) illuminated with a Solar Light PV cell
test simulator (100 mW cm‐2) using Co‐PVP-modified Zn doped p‐GaP(100) with a
resistivity of 5.5 x 10‐2 Ω.cm, a mobility of 54 cm2 V‐1 s‐1, and a carrier concentration of
2.2 x 1018 cm‐3.35 4 (dashed-dotted line) illuminated with a LSC-100 Series Oriel Solar
Simulator equipped with an AM1.5 filter (100 mW cm‐2) using Co‐PVI-modified Zn
doped p‐GaP(100) with a resistivity of 4.6 x 10‐2 Ω.cm, a mobility of 50 cm2 V‐1 s‐1, and a
carrier concentration of 2.7 x 1018 cm‐3 (chapter 2).
62
measured open-circuit photovoltages (as determined by the zero-current value in the
linear sweep voltammograms) of the PVIlGaP electrodes are 0.56 ± 0.02 V vs. RHE and
0.58 ± 0.04 V vs. RHE for the A and B faces, respectively; with an increase to 0.64 ±
0.02 following cobaloxime-modification of A-face electrodes and to 0.65 ± 0.02
following cobaloxime-modification of B-face electrodes (Table 3.3). The minimum
sample size of each photocathode type reported (GaP, PVIlGaP and ColPVIlGaP)
included testing of three distinct electrode assemblies. The polymer-only functionalized
electrodes show a slightly greater deviation in their current- voltage responses as
compared to results obtained using ColPVIlGaP assemblies, especially in the B-face
Figure 3.24. Linear sweep voltammograms of (a) GaP(111)A and (b) GaP(111)B
working electrodes following polyvinylimidazole grafting (blue) and cobaloxime
attachment (red). All measurements were performed at pH 7 in the dark (dashed-dotted
lines) or under 100 mW cm−2 illumination (solid lines).
-1.0 -0.5 0.0
-1.0
-0.5
0.0
J (
mA
cm
-2)
E (V vs Ag/AgCl)
(a)
0.0 0.5
E (V vs RHE)
111A
-1.0 -0.5 0.0
-1.0
-0.5
0.0
E (V vs Ag/AgCl)
111B
(b)
0.0 0.5
E (V vs RHE)
63
samples. It has previously been shown that the B face of GaP(111) is more prone to
residual oxide coverage following initial etching and cleaning,40 which may provide an
explanation for the slightly increased variability in the data reported here for the
PVIlGaP(111B) electrodes. However, following cobaloxime attachment the electrodes
prepared on either face show nearly identical current and voltage responses with minimal
variability of results between individually fabricated electrodes. For all catalyst-modified
photocathodes reported here, hydrogen production is confirmed via gas chromatography
analysis of the headspace, showing near-unity faradaic efficiency. The photocurrent
response and associated rate of hydrogen production for both the A and B faces
functionalized GaP(111) wafers are nearly identical. The measurements also show that
the photocurrent over an hour is relatively stable, with a decrease of less than 1% over 55
minutes following an initial drop off of up to 12% during the first 5 minutes of
photoelectrochemical (PEC) characterization (Figure 3.25). Previous computational
Table 3.3 PEC Characteristics
Construct Voc (V vs RHE) J at 0 V vs RHE (mA cm-2)
GaP(111)A
PVIlGaP(111)A
0.02 ± 0.02
0.56 ± 0.02
0.22 ± 0.06
0.32 ± 0.04
ColPVIlGaP(111)A 0.64 ± ± 0.02 0.89 ± 0.02
GaP(111)B
PVIlGaP(111)B
0.53 ± 0.06
0.02± 0.04
0.22 ± 0.06
0.30 ± 0.08
ColPVIlGaP(111)B 0.58 ± 0.02 0.89 ± 0.03
64
and experimental studies show that redox cycling of cobaloxime solutions can result in
the dissociation of N-containing ligands from cobalt centers, and the loss of current
during the initial 5 minutes of PEC characterization of the cobaloxime-modified
assemblies reported here may in part be due to loss of loosely bound catalysts located at
electrolyte-exposed edges of the polymer that are not protected by the encapsulating
environment of the polymer.36
3.2.3 Turnover Frequency
The data obtained from ellipsometry, XP spectroscopy, and PEC measurements allow an
estimate of the TOF of the ColPVIlGaP construct per cobalt center. For example, an
electrode operating at a 0.9 mA cm-2 current density (a value representative of that
measured using ColPVIlGaP electrodes polarized at 0 V vs. RHE) with a 6 nm PVI graft
Figure 3.25. Three-electrode electrolysis measurements using cobaloxime-modified
GaP(111)A (dashed) and cobaloxime-modified GaP(111)B (solid) working electrodes
polarized at 0 V vs RHE. All measurements were performed at pH 7 under 100 mW cm−2
illumination.
65
and 30% attachment to cobaloxime catalysts yields a TOF of 7,000 per hour per cobalt
center, assuming that all the cobalt centers are photoelectrochemically active. For
comparison, a TOF of 8,000 per hour was reported by Artero and coworkers using an
analogous cobaloxime species surface grafted to an electroactive carbon support
polarized at -0.59 V vs. RHE41 and a TOF of 1.9 per second was reported by Turner et
al.42 using a cobaloxime catalyst assembled on a TiO2-coated GaInP2 semiconductor
poised at 0 V vs RHE.
3.3 Conclusions
In conclusion, cobaloxime catalysts are successfully assembled on both crystal faces of
gallium phosphide 111 by employing a vinylimidazole polymer brush interface.
Structural analysis of the reported assemblies is performed using spectroscopic
ellipsometry, GATR-FTIR, and XP spectroscopies. PEC characterization of the modified
cathodes shows significant improvement of the photoperformance for hydrogen
production of GaP electrodes following polymeric cobaloxime modification, obtaining a
current density of ≈1 mA cm-2 when polarized at 0 V vs. RHE. Our measurements
indicate that a slightly higher loading of catalysts can be achieved via functionalization of
the B face with respect to results obtained on A face modified samples. Yet, the
photoelectrochemical characterization of samples using A or B face modification show
nearly identical photoperformance under the conditions tested, illustrating that factors
other than catalyst loading can limit photoactivity. Previous light intensity dependence
measurements obtained using ColPVPlGaP samples show a near-linear response of the
current measured at 0 V vs. RHE, indicating that in such hybrid constructs, photocarrier
66
transport to the interface may in part limit the performance of the Co-modified GaP
photocathodes. Although the mechanistic details of vinyl group attachment chemistry are
not settled,43–49 molecular binding appears to occur over bridging oxygen atoms on GaP
(100) surfaces.50 A similar attachment chemistry may occur for the functionalized GaP
(111A) and GaP (111B) constructs reported in this work, consistent with the similarity in
catalysts-polymer loadings and photoelectrochemical performance achieved using these
substrates as well as the analysis of surface oxygen content performed prior to and
following surface functionalization. Efforts to further analyze the loading, mesoscale
architecture, surface energetics (thermodynamics) and activity of catalysts (kinetics) are
underway. Nonetheless, attachment chemistry reported in this chapter is not limited to a
specific crystal face of gallium phosphide, a promising feature regarding the applicability
of this attachment strategy across varying crystal face orientations.
3.4 Experimental Methods
3.4.1 Materials
Single crystalline p-type gallium phosphide wafers were purchased from University
Wafers. The material was single side polished to an epi-ready finish. The p-type
GaP(111)A wafers have a resistivity of 0.046 Ω cm, a mobility of 50 cm2 V−1 s−1, and a
carrier concentration of 2.7 x 1018 cm−3. The etch pit density was less than 9.5 x 104 cm−2.
The p-type Zn-doped GaP(111)B(±0.5°) wafers have a resistivity of 0.054 Ω cm, a
mobility of 46 cm2 V−1 s−1, and a carrier concentration of 2.5 x 1018 cm−3. The etch pit
density was less than 2.0 x 104 cm−2.
67
3.4.2 Synthesis
All syntheses were carried out under an argon atmosphere using Schlenk techniques
unless otherwise stated. All reagents were purchased from Aldrich. Isopropanol was
cleanroom grade and used as received. Dichloromethane, methanol, and vinylimidazole
were freshly distilled before use. All solvents were stored over the appropriate molecular
sieves prior to use. Milli-Q water (18.2 MΩ cm) was used to prepare all aqueous
solutions. The cobaloxime syntheses was carried out as described in Section 2.4.2.
3.4.3 Wafer Preparation
All wafers were initially degreased with an acetone-soaked cotton swab. GaP(111) wafers
were further cleaned using consecutive ultrasonic treatments in a series of solvents
(acetone, 1 min; methanol, 1 min; dichloromethane, 30 s; methanol, 1 min; and water, 1
min), followed by a 30 s etch in concentrated sulfuric acid.40 Unmodified wafers used for
XPS analysis were thoroughly degreased but not acid-etched.
3.4.4 Wafer Functionalization
The freshly etched wafers were put into an argon-sparged solution of N-vinylimidazole
and exposed to 254 nm UV light for 2 h. After thoroughly rinsing with methanol, the
wafers were dried under N2 and stored under vacuum. Cobaloxime functionalization was
achieved by covering the PVIlGaP wafers with an argon-degassed solution of
Co(dmgH)2(dmgH)Cl2 and triethylamine (1 mM in each) in methanol and allowing them
to react overnight. The wafers were rinsed with methanol, then ultrasonically treated in
68
methanol for 1 min, followed by rinsing with isopropanol, and drying under N2 then
vacuum.
3.4.5 Electrode Fabrication
GaP working electrodes were fabricated by applying an indium−gallium eutectic
(Aldrich) to the backside of a wafer and then fixing a copper wire to the back of the wafer
using a conductive silver epoxy (Circuit Works). The copper wire was passed through a
glass tube, and the wafer was insulated and attached to the glass tube with Loctite 615
Hysol Epoxi-patch adhesive. The epoxy was allowed to fully cure before testing the
electrodes.
3.4.6 Instrumentation
UV−Vis Spectroscopy. Ultraviolet−visible (UV−vis) optical spectra were recorded on a
Shimadzu SolidSpec-3700 spectrometer with a D2 (deuterium) lamp for the ultraviolet
range and a WI (halogen) lamp for the visible and near-infrared regions. Transmission
and reflectance measurements were performed with an integrating sphere.
NMR Spectroscopy. Nuclear magnetic resonance (NMR) spectra were recorded on a
Varian NMR spectrometer operating at 400 MHz. Unless otherwise stated, all spectra
were collected at room temperature.
FTIR Spectroscopy. Grazing angle attenuated total reflection Fourier transform infrared
spectroscopy (GATR-FTIR) was performed using a VariGATR accessory (Harrick
Scientific) with a Ge crystal plate installed in a Bruker Vertex 70. Samples were pressed
against the Ge crystal to ensure effective optical coupling. Spectra were collected using
69
256 scans under a dry nitrogen purge with a 4 cm−1 resolution, GloBar MIR source, a
broadband KBr beamsplitter, and a liquid nitrogen cooled MCT detector. Background
measurements were obtained from the bare Ge crystal, and the data were processed using
OPUS software. GATR measurements were baseline-corrected using an ATR correction
for a germanium crystal and a sample with a refractive index of 1.43. Spectra from model
compounds in pressed KBr pellets were acquired with the same settings but using
transmission mode.
X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was
performed using a monochromatized Al Kα source (hν = 1486.6 eV), operated at 63 W,
on a Kratos system at a takeoff angle of 0° relative to the surface normal and a pass
energy for narrow scan spectra of 20 eV, at an instrument resolution of approximately
700 meV. Survey spectra were collected with a pass energy of 150 eV. Spectral fitting
was performed using Casa XPS analysis software. Spectral positions were corrected by
shifting the primary C 1s core level position to 284.8 eV, and curves were fit with quasi
Voigt lines following Shirley background subtraction. All N 1s fits were constrained to a
full width at half-maximum (fwhm) value of less than 1.3 eV.
Electrochemistry. Cyclic voltammetry was performed with a Biologic potentiostat using a
glassy carbon (3 mm diameter) disk, a platinum counter electrode, and a silver wire
pseudoreference electrode in a conventional three-electrode cell. Anhydrous acetonitrile
(Aldrich) was used as the solvent for electrochemical measurements. The supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate. The solution was
deoxygenated by bubbling with argon. The working electrode was cleaned between
70
experiments by polishing with alumina (50 nm diameter) slurry, followed by solvent
rinses. The potential of the pseudoreference electrode was determined using the
ferrocenium/ferrocene redox couple as an internal standard and adjusting to the normal
hydrogen electrode (NHE) scale (with E1/2 taken as 0.40 V vs NHE).
Photoelectrochemistry. Photoelectrochemical (PEC) characterization was performed
using 100 mW cm−2 illumination from a 100 W Oriel Solar Simulator equipped with an
AM 1.5 filter. Linear sweep voltammetry and three-electrode electrolysis
(chronoamperometry) were performed with a Biologic potentiostat using a platinum coil
counter electrode, a Ag/AgCl, NaCl (3 M) reference electrode (0.21 V vs NHE), and GaP
working electrodes (including GaP following acid etch, PVI-grafted GaP, and
cobaloxime-modified GaP) in a modified cell containing a quartz window. The
supporting electrolyte was 0.1 M phosphate buffer (pH 7). Linear sweep voltammograms
were recorded at sweep rates of 100 mV s−1 under a continuous flow of 5% hydrogen in
nitrogen. Open-circuit photovoltages were determined by the zero current value in the
linear sweep voltammograms. Chronoamperometry was performed with the working
electrode polarized at −0.610 V vs Ag/AgCl, slightly positive of 0 V vs RHE, where E vs
RHE = E vs NHE + (0.05916 V) (pH) (V) = E vs Ag/AgCl + (0.05916 V) (pH) + 0.21 V.
GC Analysis. Gas analysis was performed via gas chromatography (GC) using an Agilent
490 Micro GC equipped with a 5 Å MolSieve column at a temperature of 80 °C and
argon as the carrier gas. Gas samples were syringe-injected using 5 mL aliquots of
headspace gas collected with a gastight Hamilton syringe from a sealed PEC cell both
prior to and following 10 min of three-electrode photoelectrolysis using a cobaloxime-
71
modified working electrode polarized at 0 V vs RHE. Prior to the experiment the cell was
purged for 30 min with argon before sealing. The retention time of hydrogen was
confirmed using a known source of hydrogen obtained from a standard lecture bottle
containing a hydrogen and argon mixture.
Ellipsometry. Film thickness (FT) was determined using a J. A. Woollam variable angle
spectroscopic ellipsometer with a spectral range of 200−1200 nm. Measurements were at
70°, 75°, and 80° incidence angles, and analysis was done using VASE software. The
model used to determine the FT of surface-grafted polyvinylimidazole was composed of
a GaP substrate layer (Aspnes), a GaP oxide layer (Zollner) with a fixed thickness of 1.01
nm, and a Cauchy layer for the polyvinylimidazole film of each sample. The Cauchy
coefficients for GaP(111)A were A = 1.414 and B = −0.034, and the FT of the Cauchy
layer was determined to be 6.2 ± .2 nm with an MSE of 8.356.
The Cauchy coefficients for GaP(111)B were A = 1.371 and B = 0.008, and the FT of the
Cauchy layer was determined to be 6.5 ± 0.2 nm with an MSE of 3.745. In this analysis,
the thickness of the oxide layers is based on ellipsometry measurements performed on
unfunctionalized GaP(111)A and GaP(111)B surfaces. However, the vast majority of the
oxide is likely formed during transportation and handling of the sample in air (up to 10
min) following the etching process and subsequent ellipsometry measurement. The
process of photografting and polymerizing N-vinylimidazole onto freshly etched GaP,
under inert conditions, should give an oxide layer much thinner than the average oxide
FT of 1.01 nm; thus these values provide an upper limit of any oxide formation on
polyvinylimidazole-grafted GaP.
72
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76
CHAPTER 4 COBALT PORPHYRIN-POLYPYRIDYL SURFACE COATINGS
FOR PHOTOELECTROSYNTHETIC HYDROGEN PRODUCTION
Portions of this work are excerpted with permission from:
Beiler, A. M.; Khusnutdinova, D.; Wadsworth, B. L.; Moore, G. F. Inorg. Chem.
2017, 56, 12178–12185.
Copyright 2017 American Chemical Society
77
4.1 Introduction
The use of a surface-grafted organic coating to assemble molecular catalysts for hydrogen
evolution to a visible-light-absorbing semiconductor is reported in this chapter. This
approach combines features of solid-state light capture and conversion materials with
molecular components for enhancing photoelectrochemical fuel production. The
materials used to assemble the hybrid photocathode include GaP as the underpinning
semiconductor, 4-vinylpyridine as the monomeric unit used to form a polypyridine
surface coating (PPy), and 5,10,15,20-tetra-p-tolylporphyrin cobalt(II) (CoTTP) as a
molecular component for enhancing photoelectrochemical hydrogen production.
Metalloporphyrins serve important roles in biology as reactive sites for driving enzymatic
reactions and as components in emerging molecular-based materials with applications to
energy transduction.1–20 As electrocatalysts, they are capable of chemically transforming
protons into hydrogen as well as converting carbon dioxide into carbon monoxide and/or
hydrocarbons such as methane and ethylene when electrochemically activated in solution
or immobilized at a conductive substrate polarized at an appropriate potential (Figure
4.26). A synthetic methodology for directly grafting metalloporphyrins onto GaP using
precursor complexes bearing a 4-vinylphenyl surface attachment group at the beta
position of the porphyrin macrocycle has been previously reported.21 The cobalt
porphyrin-modified photocathodes are active for hydrogen production, evolving the
product gas at a rate of ≈10 μL min-1 cm-2. Comparison of the total surface cobalt
porphyrin concentrations with the amount of hydrogen produced indicate the HER
activity per metal site is among the highest reported for a molecular-modified
78
Figure 4.26. Voltammograms of 5,10,15,20-tetra-p-tolylporphyrin cobalt(II) (CoTTP)
recorded in 0.1 M tetrabutylammonium hexafluorophosphate in dimethylformamide and
0 mM tosic acid (green solid line), 10 mM tosic acid (green dashed line) or CO2 (black
dashed line) at a scan rate of 250 mV s-1 using a glassy carbon working electrode at room
temperature and ferrocene as an internal standard. Data collected by Diana
Khusnutdinova.
semiconductor photocathode operating at the H+/H2 equilibrium potential under 1-sun
illumination.
This work shows that an initially deposited organic interface consisting of repeating
pyridyl units can provide molecular recognition sites that self-assemble cobalt porphyrins
onto the polypyridine-modified GaP surface. This provides an alternate and streamlined
method for interfacing cobalt porphyrins to a semiconductor that does not require
synthetic modifications to the porphyrin macrocycle to install a surface-grafting
functional group. Thus, the attachment strategy also enables application of porphyrins
bearing substituents that are chemically incompatible or complicated by installment of
the vinylphenyl functionality.
79
4.2 Results and Discussion
4.2.1 Wafer Preparation and Characterization
The cobalt porphyrin-polypyridine-modified GaP semiconductors (CoTTP|PPy|GaP)
described in this report are prepared using a two-step method (Figure 4.27) leveraging the
UV-induced surface grafting chemistry of alkenes22–29 to apply an initial thin-film
polypyridine surface coating, followed by wet-chemical treatment to assemble the cobalt
porphyrin units. During this process, freshly cleaned and etched GaP(100) wafers are
placed in an argon-sparged solution of 4-vinylpyridine under shortwave UV illumination
for 2 hours before removing and cleaning the functionalized wafers with successive
solvent washes followed by drying under a stream of nitrogen. The resulting
polypyridine-modified substrates (PPy|GaP) are then exposed to a 1 mM solution of
CoTTP in toluene under an argon atmosphere for 18 hours, forming the cobalt porphyrin-
modified wafers, CoTTP|PPy|GaP.
Analysis of CoTTP|PPy|GaP surfaces using grazing angle attenuated total reflection
Fourier transform infrared (GATR-FTIR) spectroscopy confirms that cobalt centers of the
porphyrin macrocycles coordinate to pyridyl nitrogen sites of the PPy graft (Figure 4.27).
In addition to characteristic vibrational features associated with C-H and C-N vibrations
of surface-grafted pyridyl units as well as the Cβ-H, Cα-N, and C=C vibrations of the
porphyrin macrocycle, a prominent absorption band is measured at 1009 cm-1.
Absorption bands appearing in the region from 1019-1004 cm-1 in the IR spectra of
metalloporphyrins are ascribed to in-plane porphyrin deformation. This region is
sensitive to both the elemental nature of the metal center as well as local coordination
80
Figure 4.27. (left) Schematic representation of the synthetic method used to prepare the
PPy|GaP and CoTTP|PPy|GaP samples. (right) GATR-FTIR absorbance spectra of the (a)
1700-950 cm-1 region of unfunctionalized GaP (black), PPy|GaP (blue), and
CoTTP|PPy|GaP (purple), including (b) an expanded plot of the 1022-987 cm-1 region in
spectra of CoTTP|PPy|GaP.
81
environment due to inductive and mesomeric effects.30,31 The feature at 1009 cm-1 on
CoTTP|PPy|GaP surfaces, labeled as νCo-N (Figure 4.27b), is particularly diagnostic of
cobalt porphyrin complexes coordinated to an axial pyridyl unit32 and thus provides
structural information on the bonding motif of the surface-immobilized cobalt
porphyrins.
Further information on the composition and structure of the modified surface, including
film thickness, pyridyl site density, loading efficiency (defined here as the fraction of
pyridyl sites coordinated to a cobalt center), and associated cobalt concentration per
geometric area is gained using complementary methods of ellipsometry, inductively
coupled plasma mass spectrometry (ICP-MS) and X-ray photoelectron (XP)
spectroscopy. Spectroscopic ellipsometry measurements of PPy|GaP surfaces are used to
determine the film thickness and provide an estimate of the pyridyl site density. For the
samples in this report, the obtained film thickness is 1.45 0.17 nm. Given the bulk
polyvinylpyridine density (1.15 g cm-3) is representative of the solvent-free film, the
calculated pyridyl site density is 1.57 0.18 nmol cm-2.
ICP-MS measurements following acid digestion of PPy|GaP samples indicate no
significant cobalt is present. However, following attachment of cobalt porphyrin
complexes, the Co surface concentration is estimated to be 0.39 0.08 nmol cm-2. The
reported cobalt surface concentration takes into account that the synthetic method (UV-
induced grafting of 4-vinylpyridine followed by wet chemical treatment with a CoTTP
solution) and the reaction vessel geometry used in this report yields samples with cobalt
porphyrin assembled on both sides of the single-side polished GaP wafers. The total
82
cobalt, including contributions from the front and back side of the GaP wafer and as
measured by ICP-MS following acid digestion of CoTTP|PPy|GaP samples, is 0.59
0.12 nmol cm-2. Thus, the total cobalt contribution of CoTTP|PPy|GaP samples is nearly
identical to that estimated and previously reported8c for CoP-GaP samples (0.59 nmol
cm-2) in which no attempt was made to distinguish contributions from the front and back
side of the GaP wafer and thus represents an upper limit. However, XP spectroscopic
analysis of the polished and unpolished faces of CoTTP|PPy|GaP samples prepared via
UV illumination from the polished side of the wafer indicate the surface cobalt
concentration on the unpolished (non-directly illuminated) side of the wafer is 33% lower
than that of the polished (directly illuminated) side of the wafer, yielding the reported
cobalt concentration of 0.39 0.08 nmol cm-2 on the polished face. These measurements
indicate 25 5% of the pyridyl units on the CoTTP|PPy|GaP surface are coordinated to a
porphyrin cobalt center, equating to a total N/Co ratio of 4.1 0.8. This ratio, as
measured by ICP-MS and ellipsometry, is used to establish the N 1s background in XP
spectroscopy and assists in guiding the component fitting. This method of combining
information obtained from ICP-MS, ellipsometry and XP spectroscopy is particularly
useful in analyzing thin-film surface coatings on GaP as the N 1s spectrum contains
background components arising from Ga 3d (Auger) LMM lines (Figure 4.28).33
As compared to XP spectra of unfunctionalized GaP or PPy|GaP samples, spectra of the
CoTTP|PPy|GaP samples show additional Co and N contributions. After applying
83
Figure 4.28. N 1s core level XP spectra of unfunctionalized GaP (black) and PPy|GaP
(blue).
the appropriate relative sensitivity factors, the N 1s to Co 2p spectral intensity ratio of 8.0
0.8 yields a loading efficiency of 26 6%, a value consistent with that determined by
the ellipsometry and ICP-MS analysis. In addition, unlike the single component observed
in the high-energy resolution N 1s spectra of PPy|GaP samples (Figure 4.28), spectra
obtained using CoTTP|PPy|GaP samples can be deconvoluted into three components
(Figure 4.29b). Based on the relative signal intensities and binding energies associated
with these components, the contributions centered at 398.5, 399.0, and 399.7 eV are
assigned to remnant pyridinic nitrogens of the surface graft, porphyrin pyrrolic nitrogens
bound to cobalt centers, and pyridinic nitrogens bound to cobalt centers, respectively.
Further, the ≈3:4:1 spectral intensity ratios associated with the three components are
consistent with both the 4:1 ratio of cobalt-pyrollic to cobalt-pyridinic nitrogen centers
anticipated for a cobalt porphyrin coordinated to a pyridyl unit, as well as the 26 6%
84
Figure 4.29. (a) Molecular structure of a CoTTP unit coordinated to polypyridine. (b)
High-energy resolution XP spectrum of the N 1s region of CoTTP|PPy|GaP. Solid lines
represent the background (gray), component (blue, green, and red), and overall (black)
fits to the experimental data (circles). Shading indicates the component area assigned to
remnant pyridinic nitrogens (blue), porphyrin pyrrolic nitrogens bound to cobalt centers
(green), and pyridinic nitrogens bound to cobalt centers (red). (c) High-energy resolution
XP spectrum of the Co 2p3/2 region of CoTTP|PPy|GaP.
402 400 398
pyridinicCoTTP
399.7
pyrrolicCoTTP
399.0
remnant
pyridinicPPy
398.5
N 1s
Binding Energy (eV)
Inte
nsity (
a.u
.)
(b)
790 785 780 775 770
In
ten
sity (
a.u
.)
780.1
Co 2p3/2
Binding Energy (eV)
(c)
NN
N
N
NN
Co
(a)
85
loading efficiency determined by comparison of the total N 1s and Co 2p3/2 spectral
intensity contributions. Finally, high-energy resolution XP spectra of the Co 2p region
(Figure 4.29c) show peaks centered at 780.1 eV (2p3/2) and 795.3 eV (2p½) with the
anticipated 2:1 branching ratio and 15.2 eV splitting.34 The Co 2p3/2 signal also shows
satellite peaks at higher binding energies, consistent with the presence of cobalt species in
the +2 oxidation state and the unpaired spin of the porphyrin cobalt center resting state.35
4.2.2 Photoelectrochemical Performance
The PEC performance of custom-built working electrodes composed of CoTTP|PPy|GaP
is assessed using three-electrode electrochemical techniques, including
chronoamperometry (Figure 4.30a) and linear sweep voltammetry (Figure 4.30b),
performed in buffered pH neutral aqueous conditions in the dark and under air mass 1.5
simulated solar illumination. Modification of GaP photocathodes with a cobalt porphyrin-
polypyridyl surface coating results in a significant anodic shift of the open-circuit
photovoltage (Voc), moving from +0.57 0.02 V vs RHE for the unmodified GaP
electrodes to +0.65 0.02 V vs RHE. Within the experimental error of the measurements,
the current density of -1.27 0.04 mA cm-2 achieved for CoTTP|PPy|GaP samples
polarized at 0 V vs RHE (i.e. at the H+/H2 equilibrium potential) is identical to that
reported using GaP samples modified with a directly attached cobalt porphyrin.21 Thus
the intervening polypyridyl surface coating does not diminish the performance gains
afforded by cobalt porphyrin surface modification yet reduces the synthetic efforts
required for assembly. In addition, the saturating photocurrent shows a linear increase
upon increasing illumination intensity, indicating that the PEC activity at 0 V vs RHE
86
Figure 4.30. (a) Chronoamperogram of a CoTTP|PPy|GaP working electrode polarized at
0 V vs RHE under alternating (200 mHz) non-illuminated and illuminated conditions. (b)
Linear sweep voltammograms of CoTTP|PPy|GaP working electrodes recorded in the
dark (dashed line) and under simulated 1-sun illumination (solid line). The dotted vertical
line at 0 V vs RHE represents the equilibrium potential of the H+/H2 couple. The
calculated HER activity per cobalt center is included on the right ordinate axis. (c) Gas
chromatograms of a 5 mL aliquot of the headspace drawn from a sealed PEC cell using a
CoTTP|PPy|GaP working electrode polarized at 0 V vs RHE before (dashed line) and
after (solid line) 30 min of AM1.5 illumination. All measurements were performed using
a three-electrode configuration in 0.1 M phosphate buffer (pH 7).
-0.8 -0.6 -0.4 -0.2 0.0
-1.2
-0.8
-0.4
0.0
E (V vs Ag/AgCl)
J (
mA
cm
-2)
(b)
-0.2 0.0 0.2 0.4 0.6
E (V vs RHE)
15
10
5
0
HE
R a
ctiv
ity (H
2 s-1 C
o-1)
100 200 300 400 500
-1.2
-0.8
-0.4
0.0
100 200 300 400 500
J (
mA
cm
-2)
Time (s)
(a)
29 30 31 32
(c)
De
tecto
r R
esp
on
se
(a
.u.)
Time (s)
AM 1.5
illumination
H2
FE ≈ 93%
dark
87
is in part limited by the photon flux. Characteristic of a photosynthetic assembly,36 these
results confirm that light, and no electrochemical forward biasing or use of sacrificial
redox reagents, is the energy input required to generate the photocurrent. Production of
hydrogen is confirmed via gas chromatography analysis of headspace samples taken from
a sealed PEC cell containing a CoTTP|PPy|GaP working electrode polarized at 0 V vs
RHE under simulated solar illumination (Figure 4.30c). Under these conditions, hydrogen
is produced at a rate of ≈10 μL min-1 cm-2 with a faradaic efficiency of 93 3%. Given
the cobalt porphyrin surface concentration (0.39 0.08 nmol cm-2) determined by XP
spectroscopy, ellipsometry, and ICP-MS, this equates to a HER activity per cobalt center
of 17.6 H2 molecules s-1 Co-1. During bulk electrolysis measurements, hydrogen bubbles
accumulate on the electrode surface and diminish the surface area of the working
electrode exposed to electrolyte, resulting in a decrease of current density over time.
Removal of bubbles during or following PEC characterization restores the current density
to a value nearly identical to that achieved for a freshly prepared electrode.
In addition, samples were prepared for post-PEC analysis using a customized three-
electrode cell equipped with a quartz window and spring-loaded copper support to secure
the GaP wafer, thus negating the need to encase the sample in epoxy. The electrodes were
polarized at 0 V vs RHE for a minimum of 3 min under 1-sun illumination. Samples were
rinsed with Mill-Q water and dried under vacuum before GATR-FTIR measurements.
The observation of a prominent νCo-N in-plane porphyrin deformation mode in GATR-
FTIR spectra of samples following bulk electrolysis indicates cobalt porphyrins maintain
their molecular integrity and coordination to pyridyl sites (Figure 4.31).
88
Figure 4.31. GATR-FTIR absorbance spectra showing the νCo-N region of the
CoTTP|PPy|GaP before photoelectrochemical characterization (top, purple), following
exposure to buffer (middle, dark red), and after photoelectrochemical characterization
(bottom, green).
Additional insights regarding the performance gains afforded by cobalt porphyrin-
polypyridyl surface modification come from analysis of the photovoltaic performance
parameters derived from the voltammetry measurements, including those performed on
control samples of working electrodes composed of unfunctionalized GaP and PPy|GaP
(Figure 4.32 and Table 4.4). Although CoTTP|PPy|GaP shows an enhanced Voc compared
to unmodified GaP, similar Voc gains are obtained following only application of the thin-
1020 1010 1000
A
bso
rba
nce
Wavenumber (cm-1)
pre-PEC
O2/H20
pre-PEC
buffer
exposure
post-PEC
42
Proteus Gamma I
Pine Instrument Company
buffer
exposure
post-PEC
Does the catalyst maintain molecular integrity during
photoelectrochemical operation?
89
Figure 4.32. Linear sweep voltammograms of unfunctionalized GaP (black), PPy|GaP
(blue), and CoTTP|PPy|GaP (purple) working electrodes. Circles represent the maximum
power point. Dashed horizontal and dashed vertical lines represent the current density
and potential, respectively, at the maximum power points.
Table 4.4 PEC Characteristics
Construct
Open Circuit
Voltage (V vs RHE)
Short Circuit
Current (mA cm-2)
Maximum Power
Point (mW cm-2)
Fill Factor
GaP(100)
PPylGaP
0.57 ± 0.02
0.64 ± 0.04
0.22 ± 0.06
0.32 ± 0.04
0.11 ± 0.05
0.15 ± 0.04
0.21 ± 0.08
0.23 ± 0.07
CoTTPlPPylGaP 0.65 ± 0.02 0.89 ± 0.02 0.32 ± 0.06 0.39 ± 0.07
E (V vs RHE)
-1.2
-0.8
-0.4
0.00.2 0.4 0.6
J (
mA
cm
-2)
90
film polypyridyl surface coating (Figure 4.32). These results contrast to those reported
using GaP semiconductors with thicker polymer coatings (on the order of ≈10 nm) 37–40
where the Voc instead decreases following pyridyl group modification of the GaP surface.
It is speculated that this is in part due to a larger resistance and poorer ion conductivity
associated with the thicker organic coatings.41–43 Nonetheless, although the Voc of the
PPy|GaP and CoTTP|PPy|GaP electrodes is similar, the fill factor (ff) of the JV response
under illumination nearly doubles following treatment with CoTTP, moving from 0.21
0.08 for unfunctionalized GaP to 0.23 0.07 for PPy|GaP, and further increasing to 0.39
0.07 for CoTTP|PPy|GaP (Figure 4.32 and Table 4.4). Due to the steep increase in
operating photocurrent densities that are produced by changes in ff, this can have a
significant effect on performance in a photoelectrosynthetic cell.44
4.3 Conclusions
In summary, surface-sensitive spectroscopic methods verify our synthetic efforts
dedicated to construction of a hybrid composite material that structurally interfaces a
cobalt porphyrin hydrogen evolution catalyst to a visible light-absorbing semiconductor
using surface-attached pyridyl groups as molecular recognition sites. Complementary
methods of ellipsometry, ICP-MS, and XP spectroscopy are used to quantify the surface
cobalt concentrations and the fraction of pyridyl sites coordinated to cobalt centers. The
synthetic methods used to obtain these assemblies demonstrate design principles for
furthering hard-to-soft matter interface chemistry and set the stage to better understand
the structure and function relationships of molecular-modified semiconductors. Key
features of the construct reported here include the relative ease of synthetic preparation
91
made possible by application of a thin-film organic coating that uses pyridyl groups to
assemble cobalt porphyrins on a GaP semiconductor surface, yielding a hybrid cathode
that uses solar energy to drive reductive fuel-forming reactions in aqueous solutions
without the use of organic acids or sacrificial chemical reductants. Thus, this work
introduces a highly useful, yet easily accessible, motif for preparing and studying
molecular-modified semiconductors. In order to elucidate the effect of the polymeric
interface, studies using different bases and polymeric architectures to assemble CoTTP as
well as other catalysts are underway.
4.4 Experimental Methods
4.4.1 Materials
All reagents were purchased from Sigma-Aldrich. Dichloromethane, hexanes, methanol,
and toluene were freshly distilled before use. Milli-Q water (18.2 MΩ·cm) was used to
prepare all aqueous solutions. Single crystalline p-type Zn-doped gallium phosphide (100)
wafers (University Wafers) were single side polished to an epi-ready finish. The GaP(100)
wafers have a resistivity of 0.16 Ω·cm, a mobility of 69 cm2 V-1 s-1, and a carrier
concentration of 4.5 x 1017 cm-3, with an etch pit density of less than 5 x 104 cm-2.
4.4.2 Synthesis
The compounds 5,10,15,20-tetra-p-tolylporphyrin (TTP) and 5,10,15,20-tetra-p-
tolylporphyrin cobalt(II) (CoTTP) were synthesized by Diana Khusnutdinova and Sylvia
Nanyangwe using modified versions of previously reported procedures.45,46
92
4.4.3 Wafer preparation
The use of 4-vinylpyridine as a precursor for surface functionalization of GaP has been
previously reported.21,38,40,47,48 Briefly, diced semiconductor samples were etched with
buffered hydrofluoric acid before exposure to an argon-sparged solution of the neat
monomer 4-vinylpyridine under 254 nm UV light for 2 h. The PPy-functionalized samples
were then soaked for 18 h in a CoTTP solution (1 mM) in toluene.
4.4.4 Wafer characterization
Surface characterization of the wafers was carried out using ellipsometry, grazing angle
attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR), X-ray
photoelectron (XP) spectroscopy, and inductively coupled plasma mass spectrometry (ICP-
MS).
4.4.5 Photoelectrochemistry
Photoelectrochemical (PEC) experiments were performed in 0.1 M phosphate buffer (pH
7) under AM 1.5 irradiation. A three-electrode configuration with GaP working electrodes
(including GaP following buffered hydrofluoric acid treatment, polypyridine-modified
GaP, and cobalt porphyrin-modified GaP) was used in a cell containing a quartz window.
HER activity was calculated assuming unity faradaic efficiency, although this was
confirmed at only one point (0 V vs RHE). Samples for post-PEC analysis were prepared
using a customized three-electrode cell equipped with a quartz window and spring-loaded
copper support to secure the GaP wafer, thus negating the need to encase the sample in
epoxy. An indium-gallium eutectic was applied between the copper support and GaP
sample. The electrodes were polarized at 0 V vs RHE for a minimum of 3 min under 1-sun
93
illumination. Samples were rinsed with Mill-Q water and dried under vacuum before
GATR-FTIR measurements. All PEC experiments were conducted under a continuous flow
of 5% hydrogen in nitrogen.
4.4.7 Gas chromatography
Gas chromatography (GC) was used to analyze aliquots of headspace gas taken from a
sealed PEC cell both prior to and following 30 min of bulk electrolysis at 0 V vs RHE. The
93% faradaic efficiency reported is likely a lower limit and does not correct for loss of
hydrogen from the cell or during the sampling procedure.
94
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