of 16
1
Molecular Additives
Improve
Selectivity of CO
2
Photoelectrochemical Reduction over Gold
Nanoparticles on Gallium Nitride
Aisulu Aitbekova
1
, Nicholas B. Watkins
2
, Matthias H. Richter
3
, Phillip R. Jahelka
1
, Jonas C.
Peters
2
, Theodor Agapie
2
, Harry A.
Atwater
1
1
Applied Physics and Materials Science, California Institute of Technology, Pasadena, 91125,
CA, USA
2
Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, 91125, CA,
USA
3
Division of Engineering and Applied Science,
California Institute of Technology, Pasadena,
91125, CA, USA
*Corresponding author: haa@caltech.edu
Table of Contents
Materials
................................
................................
................................
................................
..........
2
Experimental Methods
................................
................................
................................
.....................
2
Figure S1: Optical absorption measurements and Mott
-
Sch
ottky plot
................................
............
4
Figure S2: XPS characterization
................................
................................
................................
......
5
Figure S3: Device fabrication
................................
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..........
6
Figure S4: A Nyquist plot
................................
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................................
................
7
Figure S5: Chronopotentiometry
................................
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................................
.....
8
Figure S6: Chronoamperometry
................................
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................................
......
9
Figure S7: Faradaic Efficiency
................................
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......
10
Figure S8: SEM
................................
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11
Figure S9: SEM
................................
................................
................................
.............................
12
Figure S10:
XPS
................................
................................
................................
............................
13
Figure S11: Cross
-
sectional SEM
................................
................................
................................
..
14
Table S1: Hall measurements
................................
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........
15
Table S2: ICP
-
MS
................................
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................................
..........................
15
References
................................
................................
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......
1
6
2
Supporting Information
Materials
Pt foil (99.99
%
Pt, 25 mm × 25 mm × 0. 05 mm,
Sigma
-
Aldrich
), potassium carbonate (99.995
%,
Thermo
Scientific
), ammonium hydroxide
solution
(
ACS reagent, 28
30 % NH
3
basis,
Honeywell
), nitric acid (
GR ACS, Sigma
-
Aldrich
), and hydrochloric acid (
ACS reagent, ca. 37 %
solution in water, Acros Organics
) were used as received, unless stated otherwise.
Se
lemion AMV
anion
-
exchange membrane was purchased from AGC Engineering Co.
CO
2
(
research grade
), Ar
(
ultra
-
high purity
) were purchased from Airgas.
Silver colloidal suspension (SPI Flash
-
Dry) was
purchased from SPI Supplies. Loctite EA 9460 UV
-
resistant epo
xy was purchased from Henkel.
Water was purified
by a Milli
-
Q Advantage
A10 Water Purification System (Millipore) with
specific resistance of 18.2 MΩ·cm at 25 °C.
Experimental Methods
Synthesis of p
-
GaN
and Au/p
-
GaN
photocathodes
Commer
cial
p
-
GaN
(4.5 ± 0.5
m,
c
-
axis 0001 orientation
, Mg
doping concentration of (3
-
7) x
10
18
cm
-
3
)
on sapphire (4
30 ± 25
)
was purchased from Pam
-
Xiamen.
P
rior to depositing metals
via
electron
-
beam
(E
-
beam)
vapor deposition,
p
-
GaN wafers
were cleaned in ethanol and w
ater (30 s
sonication each) and dipped
into
a
30 vol.% NH
4
OH solution
for 30 s
.
Then
the wafers were rinsed
with ultrapure water, mounted onto an E
-
beam
substrate holder
, and dried with nitrogen gas. 20
nm Ni and 20 nm Au were deposited sequentially
at a base pressure of ca.
1 x 10
-
7
T
orr and a
deposition rate of
0.5
Å s
-
1
(Ni) and
1.0 Å s
-
1
(Au)
using a shadow mask. 1.5 nm Au was then
deposited under the same
conditions
with
another shadow mask to avoid co
-
depositing
Au
onto
Ni/Au.
For solid
-
state current
-
voltage measurements, Ni/Au and Au contacts
(d
= 1
mm
2
)
were
deposited using 20 nm/20
nm Ni/A
u and 200 nm Au.
Solid
-
state current
-
voltage
and Hall
measurements
Electrical measurements were conducted
under an optical microscope using piezoelectric
microcontact probes (Imina
Technologies, miBots
) to electrically address the contact pads on
the
p
-
GaN
substrate. The current
-
voltage (I
-
V) behavior was then collected through a
Keithley 236 source
-
meter unit and operated with custom
-
built software.
Hall measurements were
performed on
a Lakeshore Cryotronics M91 Fast Hall System
.
X
-
ray
photoelectron m
easurements
XPS data were collected using a Kratos Axis Nova system with a base pressure of 1 × 10
9
Torr. The X
-
ray source was a monochromatic Al Kα line at 1486.6 eV. Data were analyzed using
CasaXPS. Spectral energy for all
spectra were calibrated using the
C1s peak (284.8
eV).
Optical absorption measurements
3
Optical absorption measurements were collected in transmission and reflection modes on
Cary
5000
spectrometer in the
300
800 nm range
with 0.1 s time per data point and 1 nm data interval.
Prior to performing measurements, beam was aligned using reference samples. Baseline correction
was applied using 0 % and 100 % transmission and reflection standards
.
Scanning electron microscopy me
asurements
SEM measurements were
performed
on Nova NanoSEM 450 using 10 kV high voltage and a spot
size of 3.
Photoelectrochemical characterization of p
-
Ga
N
and Au/p
-
GaN
and CO
2
PEC measurements
All electrochemical experiments were performed in a
three
-
electrode configuration with the p
GaN
or Au/p
-
GaN photocathode as the
working electrode, a Pt
metal
counter electrode, and
a
leakless
Ag/AgCl
reference electrode
(
eDAQ, 2 mm OD
)
in 0.1 M KHCO
3
(pH 6.8) electrolyte. The
electrolyte was made by
sparging 50 mM K
2
CO
3
with CO
2
for 12 h.
P
rior
to
each measurement
the CO
2
-
saturated
0.1 M KHCO
3
solution
was additionally sparged with CO
2
for at least 1 h.
All
electrode potentials were converted to the reversible hydrogen electrode (RHE)
scale through th
e
following equation: E vs. RHE = E vs.
Ag/AgCl
+ (0.059 V pH
-
1
x
pH) +
Ref
V.
Ref V was
obtained for each experiment by calibrating the Ag/AgCl electrode with respect to
the HydroFelx
hydrogen reference electrode.
Linear sweep voltammograms were recorded with 20 mV/s steps.
Details of a two
-
compartment compression cell can be found somewhere else
.
1
Prior
to each use,
the cell was soaked in a 10 vol.% HNO
3
solution, thoroughly rinsed with ultrapure water, boiled
in water for 1
5
min, and dried in an oven at 80
-
100 °C. The light source is provided by a mercury
-
xenon lamp (
6291 Hg(Xe) Arc Lamp is 200 watt and has a 0.5 x 1.5 mm effective arc size
)
operating with an IR filter
.
Potentiostatic electrochemical impedance spectroscopy
(PEIS)
measurements were
conducted in the frequency range
200 kHz
500 mHz with a 10
-
m
V
sinusoidal amplitude
.
Photoelectrochemical CO
2
reduction measurements were conducted in the
same cell using 5 sccm of CO
2
or Ar
with Au/p
-
GaN or bare p
-
GaN as
a
working electrode, Pt
foil
as a
counter electrode, and a
leakless Ag/AgCl reference electrode. Gas
-
phase products from the
cell
passed through a liquid trap and then to an Agilent GC (
7890A
) with
Molsieve 5A and
Hayesep
columns. P
roducts were detected usin
g a thermal conductivity
detector (TCD) and flame
ionization detector (F
ID)
equipped with a methanizer
(
Jetanizer by Activated Research Company
)
.
Quantitative analysis of gaseous products was based on calibration with
five different gas
standards.
Synthe
sis
and deposition
of molecular additives
Diphenyliodonium triflate (
Ad
d
)
was
synthesized according to the previously reported
procedures.
2
For
in
-
situ
deposition experiments, the additiv
e
w
as
add
ed to 0.1 M KHCO
3
solution
and PEC CO
2
RR
CA
experiments were conducted
in the 10 mM additive solutions
under
full
-
spectrum
light illumination at
-
0.2 V vs RHE.
Add
was additionally pre
-
deposited by exposing
photocathodes to the conditions
described above for 1, 5, and 10 min, rinsing the photocathodes
with ultrapure water, and replacing the additive solution with the fresh 0.1 M KHCO
3
electrolyte.
Each pre
-
deposition experiment was done with a new photocathode sample.
4
Figures
Figure
S
1
. a) Optical absorption of p
-
GaN substrate, demonstrating absorption in the UV region.
The less than 100 % above
-
the
-
bandgap absorption is due to the surface reflectivity.
Fringes in
the visible regime are due to Fabry
-
Pérot interferences within the GaN/sapphire layer; b) Mott
-
Schottky plot of electrochemical impedance data of bare p
-
GaN ph
otocathodes obtained at 2
kHz. The negative slope confirms the
p
-
type character of the GaN substrate. Fitting the data
using a Mott
-
Schottky equation yields a flat
-
band potential of 2.7 V vs RHE
.
0.1 M KHCO
3
2 kHz
E
fb
= 2.7 V
b
a
5
Figure
S2
. XPS characterization of Au/p
-
GaN
confirming metallic Au (Au 4f5/2 and Au 4f7/2
of 87.6 and 84 eV, respectively).
6
Figure
S3
.
Nickel
-
gold (Ni
/Au
) and Au
are
deposited via electron
-
beam physical vapor
deposition
and annealed at 300
°
C in air. Working
photoelectrodes were made by
connecting the
Ni/Au
ohmic contact to metallic wires
using a silver paste and UV
-
resistant epoxy.
7
Figure S
4
. A Nyquist plot demonstrating the solution resistance of Au/p
-
GaN without the ohmic
contact (1 k
W
) and with (30
W
).
8
Figure
S
5
. Chronopotentiometry of the open
-
circuit voltage (V
oc
) from p
-
GaN photocathodes
under Hg
-
Xe lamp full
-
spectrum illumination. The positive shift in V
oc
upon UV light exposure
confirms the p
-
type character of GaN
.
9
Figure S
6
.
a)
CA at
-
0.2 V with Au/p
-
GaN under
dark,
visible light and
the
full
-
spectrum
illumination
; b) CA at
-
0.2 V with Au/p
-
GaN
under dark and visible light illumination. Note the
difference in the y
-
axis units.
10
Figure S
7
. CO FE (bar graphs) and current density (black squares) of Au/p
-
GaN with
in
-
situ
deposited
Add
under CO
2
and Ar
-
0.2 V vs RHE under full
-
spectrum light illumination.
11
Figure S8
. SEM
(a)
and particle size distribution
(b)
of Au/p
-
GaN after PEC CO
2
RR at
-
0.2 V
RHE under
full
-
spectrum
light illumination.
12
Figure S
9
.
a
-
b)
Representative SEM images of
the
40
-
Add/Au/pGaN sample
.
Electrolysis
conditions:
full
-
spectrum
light illumination,
-
0.2 V vs RHE.
470 nm
3
00 nm
200 nm
a
b
13
Figure S1
0
. XPS characterization of
the
p
-
GaN
sample with
in
-
situ
deposited
Add
after PEC
CO
2
RR at
-
0.2 V RHE under
full
-
spectrum
light illumination: a) Ga 2p; b) N 1s. The absence of
Ga 2p and N 1s peaks suggests the additive thickness of at least 10 nm based on the electron
escape depth
.
3
14
Figure S
11
.
Cross
-
sectional
SEM images of
(a) 1
0
-
Add/Au/pGaN
and (b)
40
-
Add/Au/pGaN
.
Electrolysis conditions: full
-
spectrum light illumination,
-
0.2 V vs RHE.
470 nm
b
10
0
nm
a
3
00 nm
8
5
nm
15
Tables
Table S1.
Hall measurements of p
-
GaN. The obtained carrier concentration
is consistent
with the
manufacturer’s specification (> 10
19
cm
-
3
Mg doping).
Mobility, cm
2
V
-
1
s
-
1
Sheet resistance, k
W
Carrier concentration, cm
-
3
4.2
9
8*10
17
Table S2. ICP
-
MS results of blank 0.1 M KHCO3 electrolyte and post
-
electrolysis p
-
GaN and
Au/p
-
GaN samples.
Sample
Au, ppb
Ga, ppb
Electrolyte (0.1 M KHCO
3
)
0.00
0.01
p
-
GaN
0.01
0.02
Au/p
-
GaN
0.02
0.02
16
References
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temperature
-
controlled photoelectrochemical cell for quantitative product analysis.
Review of
Scientific Instruments
2018
,
89
(5), 055112. DOI: 10.1063/1.5024802 (acccessed 7/11/2023).
(2) Bielawski, M.; Olofsson, B. High
-
yielding one
-
pot synthesis of diaryliodonium triflates from
arenes and iodine or aryl iodides.
Chemical Communications
2007
, (24), 2521
-
2523,
10.1039/B701864A
. DOI: 10.1039/B701864A. Li, F.; Thevenon, A.; Rosas
-
Hernández, A.;
Wang, Z.; Li, Y.; Gabardo, C. M.; Ozden, A.; Dinh, C. T.; Li, J.; Wang, Y.; et al. Molecular
tuning of CO2
-
to
-
ethylene conversion.
Nature
2020
,
577
(7791), 509
-
513. DOI:
10.1038/s41586
-
019
-
1782
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2.
(3) Shinotsuka, H.; Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of electron inelastic
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relativistic full Penn algorithm.
Surface and Interface Analysis
2015
,
4
7
(9), 871
-
888. DOI:
https://doi.org/10.1002/sia.5789
.