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1
Supporting Information for
:
Optical Excitation of
a Nanoparticle Cu/p
-
NiO
Photocathode
Improves
Reaction Selectivity for CO
2
Reduction in Aqueous
Electrolytes
Joseph S. DuChene
†‡
, Giulia Tagliabue
†‡
, Alex J. Welch
§
,
Xueqian Li
§
,
Wen
-
Hui Cheng
§
,
and
Harry A. Atwater
§
*
Thomas J. Watson Laboratory of Applied Physics and
§
Joint Center for Artificial
Photosynthesis, California Institute of Technology, Pasadena, California 91125 United
States.
Methods
Synthesis of
p
-
NiO films and
plasmonic C
u/p
-
NiO
photocathodes
Plasmonic C
u/p
-
NiO photocathodes were
constructed via electron beam physical
vapor deposition
. The
p
-
type NiO (p
-
NiO)
films were
first
synthesized
on fluorine
-
doped tin oxide (FTO) glass substrates
by depositing
Ni metal at a rate of 0.25
Å
s
-
1
under flowing O
2
gas at
6 sccm. After deposition of a
6
0 nm
-
thick NiO film on
the FTO substrate,
the film was annealed in ambient air at 300 °C for 1 h to
ensure complete conversion to the desired p
-
NiO phase. After the heat
treatment, 3 nm of Cu
wa
s then deposited onto the p
-
N
iO
surface using electron
-
beam physical vapor deposition at a base pressure of ca. 1 x 10
-
7
torr and a
deposition rate of 1.0 Å s
-
1
.
Optical characterization of
materials
The optical properties of the Cu nanoparticles and th
e bare p
-
NiO films were
characterized via UV
-
Vis absorption spectroscopy (Varian Cary UV
-
50) both “as
-
deposited”
(
prior to any electrochemical treatment
)
and after electrochemical
reduction with cyclic voltammetry.
The Cu nanoparticles were also deposited
onto bare FTO glass substrates in the absence of the underlying p
-
NiO film.
The
background absorption of bare FTO glass was used as the reference standard
for all samples. The films were first characterized in their as
-
deposited state and
then the Cu nanop
articles were reduced
by performing successive
cyclic
voltammetry
scans
from
0.8 V
RHE
to
0.6 V
RHE
(V vs. RHE) at
a scan rate of 50
mV s
-
1
until
the reductive wave around 0.7
V
RHE
due to copper oxide reduction
was no longer observable. The sample was then removed from the electrolyte,
blown dry with a stream of N
2
, and then immediately loaded into the
2
spectrophotometer to obtain the optical properties of the reduced form of Cu
presumably present upon actual ele
ctrochemical CO
2
reduction conditions.
Photoelectrochemical characterization of
bare
p
-
N
iO and
plasmonic
Cu/p
-
N
iO
photocathodes
A
ll glassware was cleaned with aqua regia (3:1 HCl:HNO
3
) before use to remove
any trace metal ions.
All electrochemical experiments were performed in a
three
-
electrode
configuration
with the p
-
N
iO or Cu/p
-
N
iO
photocathode as the working
electrode, a Pt wire mesh counter electrode, and a saturated calomel reference
(SCE)
electrode all immersed in 50 mM K
2
CO
3
electrolyte. The electrolyte was
sparged with
CO
2
gas prior to measurements to remove dissolved O
2
from the
solution and experiments were conducted under a
CO
2
blanket.
All electrode
po
t
ent
ials were converted to the reversible hydrogen electrode (RHE) scale
through the following equation:
E
vs. RHE =
E
vs. SCE + (0.059
V
pH
-
1
x pH) +
0.2401 V
.
Electrochemical impedance spectroscopy was performed
under dark
conditions
with a 20 mV sinusoidal
amplitude across a range of frequencies (0.1
50 kHz).
At higher frequencies (
1
-
10 kHz
)
used for analysis
, the data can be
reasonably represented by a resistor in series with a capacitor.
1
Incident photon
-
to
-
charge conversion efficiency [IPCE(
λ
)] measurements
of the plasmonic Cu/p
-
NiO and bare p
-
NiO photocathodes was acquired using a
series of high
-
power LEDs (Thor Labs, Inc.) with central wavelengths of 415 nm
(FWHM 14 nm), 450 nm (FWHM 18 nm), 505 nm (FWHM 30 nm), 565 nm
(FWHM 104 nm), and 780
nm (FWHM 30 nm).
The LED
power was
adjusted
to
ensure
that the same photon flux was incident on the sample for all wavelengths
during the IPCE measurement.
The IPCE was determined by collecting the
photocurrent from each device under illumination while po
ised at an applied
potential of
E
appl
= −0.2 V
RHE
.
The IPCE was calculated according to the
following
expression:
퐼푃퐶퐸
=
'
ph
A
cm
2
.
/
W
cm
2
×
1240
5
nm
×
100%
Open
-
circuit photovoltage measurements were performed in a quiescent
solution after allowing 2
-
3 hou
rs for Fermi level equilibration between the working
electrode and the electrolyte to achieve a steady baseline open
-
circuit voltage
prior to illumination with a 565 nm LED at full power (160 mW cm
-
2
)
.
Photoelectrochemical experiments for plasmon
-
driven
CO
2
reduction
The CO
2
reduction reaction
(CO
2
RR) was conducted in a thre
e
-
electrode
configuration with Cu/p
-
N
iO
or
bare
p
-
N
iO cathode
as the working electrode, Pt
wire
gauze
as the counter electrode, and a
leakless Ag/AgCl electrode
as the
reference electrode.
All
photoelectrochemical
experiments
were
conducted within
a custom
-
built
, air
tight
cell equipped with a quartz window
, as described
previously
.
1
The photoelectrochemical experiments were performed in
50 mM
K
2
CO
3
electrolyte
(pH
7) that was fully saturated with CO
2
by vigorous bubbling
of the cathode and anode compartments for 1 h before commencing with the