We report the light-induced modification of catalytic selectivity for photoelectrochemical CO₂ reduction in aqueous media using copper (Cu) nanoparticles dispersed onto p-type nickel oxide (p-NiO) photocathodes. Optical excitation of Cu nanoparticles generates hot electrons available for driving CO₂ reduction on the Cu surface, while charge separation is accomplished by hot-hole injection from the Cu nanoparticles into the underlying p-NiO support. Photoelectrochemical studies demonstrate that optical excitation of plasmonic Cu/p-NiO photocathodes imparts increased selectivity for CO₂ reduction over hydrogen evolution in aqueous electrolytes. Specifically, we observed that plasmon-driven CO₂ reduction increased the production of carbon monoxide and formate, while simultaneously reducing the evolution of hydrogen. Our results demonstrate an optical route toward steering the selectivity of artificial photosynthetic systems with plasmon-driven photocathodes for photoelectrochemical CO₂ reduction in aqueous media

Atwater, Harry A.

Cheng, Wen-Hui

DuChene, Joseph S.

Li, Xueqian

Tagliabue, Giulia

Welch, Alex J.

English

Caltech Authors - Main

Optical Excitation of a Nanoparticle Cu/p-NiO Photocathode Improves Reaction Selectivity for CO₂ Reduction in Aqueous Electrolytes

	 1	
Supporting Information for: Optical Excitation of a Nanoparticle Cu/p-NiO 
Photocathode Improves Reaction Selectivity for CO2 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 Cu/p-NiO photocathodes 
 
Plasmonic Cu/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 O2 gas at 6 sccm. After deposition of a 60 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 was then deposited onto the p-NiO 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 the 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 nanoparticles were reduced by performing successive cyclic 
voltammetry scans from 0.8 VRHE to −0.6 VRHE (V vs. RHE) at a scan rate of 50 
mV s-1 until the reductive wave around 0.7 VRHE due to copper oxide reduction 
was no longer observable. The sample was then removed from the electrolyte, 
blown dry with a stream of N2, and then immediately loaded into the 
	 2	
spectrophotometer to obtain the optical properties of the reduced form of Cu 
presumably present upon actual electrochemical CO2 reduction conditions.  
 
Photoelectrochemical characterization of bare p-NiO and plasmonic Cu/p-
NiO photocathodes 
 
All glassware was cleaned with aqua regia (3:1 HCl:HNO3) before use to remove 
any trace metal ions. All electrochemical experiments were performed in a three-
electrode configuration with the p-NiO or Cu/p-NiO photocathode as the working 
electrode, a Pt wire mesh counter electrode, and a saturated calomel reference 
(SCE) electrode all immersed in 50 mM K2CO3 electrolyte. The electrolyte was 
sparged with CO2 gas prior to measurements to remove dissolved O2 from the 
solution and experiments were conducted under a CO2 blanket. All electrode 
potentials 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 poised at an applied 
potential of Eappl = −0.2 VRHE. The IPCE was calculated according to the following 
expression: 
 𝐼𝑃𝐶𝐸 𝜆 =
'ph A cm2
./ W cm2
× 1240
5 nm
×100% 
Open-circuit photovoltage measurements were performed in a quiescent 
solution after allowing 2-3 hours 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 CO2 reduction  
 
The CO2 reduction reaction (CO2RR) was conducted in a three-electrode 
configuration with Cu/p-NiO or bare p-NiO 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, airtight cell equipped with a quartz window, as described 
previously.1 The photoelectrochemical experiments were performed in 50 mM 
K2CO3 electrolyte (pH 7) that was fully saturated with CO2 by vigorous bubbling 
of the cathode and anode compartments for 1 h before commencing with the 
	 3	
experiment. The internal solution resistance was estimated from an impedance 
measurement, which routinely indicated a solution resistance of around 50 Ohm. 
No iR correction was applied to any of the electrochemical experiments. Prior to 
electrocatalysis, a Cu/p-NiO film was prepared by performing five successive 
linear sweep voltammograms from 0.8 VRHE to −0.6 VRHE (V vs. RHE) at a scan 
rate of 50 mV s-1 until no reductive wave was observable in the voltammogram, 
indicating complete reduction of any residual copper oxides.  
After this initial electrochemical preparation of the Cu/p-NiO photocathode, 
the device was then held potentiostatically at the indicated applied potential 
(starting from −0.7 VRHE) under dark conditions for 2 h. The experiments were 
performed under continuous flow conditions at a CO2 flow rate of 5 sccm in a 
flow-cell device with periodic sampling of the reactor headspace every 15 min 
over the course of the 2 h reaction with a gas chromatograph. The gas 
chromatograph (SRI-8610) was equipped with a Hayesep D column and a 
Molsieve 5A column using N2 as carrier gas. The gaseous products were 
detected using a thermal conductivity detector (TCD) and flame ionization 
detector (FID) equipped with a methanizer. The average of gas-phase products 
was determined from seven data points taken over 2 h, discarding the first GC 
measurement that was acquired prior to steady-state conditions. Quantitative 
analysis of gaseous products was based on calibration with several gas 
standards over many orders of magnitude in concentration. After each 
electrolysis experiment at a given applied potential, liquid products were 
collected from the cathode and anode compartments at the end of the 2 h 
electrolysis experiment and analyzed by high-pressure liquid chromatography 
(Thermo Fischer Dionex UltiMate 3000). 
After electrolyte sampling, the electrochemical cell was flushed with H2O 
three times and then re-filled with fresh 50 mM K2CO3 electrolyte and scanned 
from 0.8 VRHE to −0.6 VRHE at a scan rate of 50 mV s-1 to ensure no copper 
oxides were formed on the device upon brief exposure to ambient air. The new 
electrolyte was then purged with CO2 at a flow rate of 15 sccm for 5-10 min to 
saturate the solution with CO2, as evidenced by a similar CO2 flow rate measured 
before and after the electrochemical cell with mass flow controllers (Alicat 
Scientific). Then the CO2 flow rate was returned to 5 sccm before the light was 
switched on to repeat electrolysis at the same applied potential as before. 
Visible-light irradiation (λ = 565 ± 52 nm FWHM) with a high-power LED was 
incident on the sample through the quartz window at an incident power of I0 = 
160 mW cm-2 (measured at the quartz window). Photoelectrolysis experiments 
were conducted with a thermocouple inserted into the cell to monitor the solution 
temperature during the 2 h experiment. We observed a modest increase in 
solution temperature from around 25 °C at the start of the experiment to 
eventually stabilize around 29 °C after 2 h of photoelectrolysis (Figure S9). 
This electrolysis procedure was repeated (dark/light) at each applied 
potential from −0.7 VRHE to −0.9 VRHE and was performed starting from −0.7 VRHE 
before moving to more negative potentials. All catalytic experiments at a given 
applied potential were repeated in triplicate on fresh Cu/p-NiO photocathodes.  
 
	 4	
 
 
 
Figure S1. Materials characterization of p-type NiO films. (a) X-ray diffraction 
pattern from NiO film on FTO glass showing the characteristic (200) peak of NiO. 
All other peaks can be attributed to the underlying FTO substrate. (b) X-ray 
photoelectron spectroscopy high-resolution scan of the Ni 2p region, showing the 
characteristic binding energies and satellite features of NiO. (c) Mott-Schottky 
plot obtained from NiO films on FTO glass substrate, which shows a negative 
slope indicative of p-type conductivity. From a linear fit of these data, the flat-
band potential (Efb) is estimated to be ca. 0.75 VRHE (Volts vs. RHE) with an 
acceptor concentration of ca. 1 x 1019 cm-3. (d) Tauc plot of the NiO film showing 
a band gap of around 3.7 eV. All these data indicate material properties 
consistent with previous literature reports of p-type NiO thin films.1  
 
	 5	
 
 
 
Figure S2. XPS of Cu LMM region from Cu/p-NiO photocathodes prior to 
electrochemical reduction. The spectral profile and the peak in kinetic energy at 
917.5 eV is consistent with the CuO phase.   
 
	 6	
 
 
Figure S3. Absorption spectra of Cu nanoparticles on bare FTO substrates 
without the underlying p-NiO film. The absorption spectra of the as-deposited Cu 
nanoparticles (yellow curve) show spectral features consistent with the CuO 
phase. After electrochemical reduction via cyclic voltammetry (red curve), the 
spectrum changes to reflect metallic Cu nanoparticles with a small peak around 
650 nm attributable to the surface plasmon resonance of metallic Cu 
nanoparticles. 
 
	 7	
 
 
Figure S4. Cyclic voltammetry of bare p-NiO cathodes under dark (black curve) 
and visible light (orange curve) showing that bare p-NiO film exhibits no 
measureable light response across the potential sweep. This observation is 
consistent with the large band gap of p-NiO (see Figure S1d). Therefore, all 
visible-light responses observed from plasmonic Cu/p-NiO photocathodes can be 
unambiguously assigned to hot-hole injection from Cu to the valence band of p-
NiO upon optical excitation of the Cu nanoparticles.  
 
 
 
 
 
 
 
	 8	
 
 
Figure S5. Power-dependence of the photocurrent Jph(I0) obtained from the 
plasmonic Cu/p-NiO photocathode under visible-light irradiation (λ = 565 ± 52 
nm) while potentiostatically poised at an applied potential of Eappl = −0.2 VRHE.  
	 9	
 
 
Figure S6. CO2 reduction with bare p-NiO cathodes under dark conditions. (a-b) 
Faradaic efficiency for H2 (squares), CO (circles), and HCOO− (triangles) with (c-
d) corresponding partial current density J for the hydrogen evolution reaction 
(JHER, squares), carbon monoxide (JCO, circles), and formate (JHCOO-, triangles). 
Electrolysis was performed under dark conditions in a CO2-saturated 50 mM 
K2CO3 electrolyte. The device was held potentiostatically at each applied 
potential for 2 h while the gas products were sampled every 15 min and analyzed 
by gas chromatography. Liquid products were collected and analyzed by HPLC 
at the end of each run. Each data point represents the average of three 
independent trials and the error bar indicates the standard deviation.  
 
	 10	
 
 
Figure S7. Comparison of the partial current densities (J) obtained from bare p-
NiO (open data points) and Cu/p-NiO cathodes (filled data points) under dark 
conditions. (a) Partial current density for the hydrogen evolution reaction (JHER) 
from p-NiO (open squares) relative to Cu/p-NiO (filled squares). (b) Partial 
current density for carbon monoxide (JCO) from p-NiO (open circles) relative to 
Cu/p-NiO (filled circles). (c) Partial current density for formate (JHCOO-) from p-NiO 
(open triangles) relative to Cu/p-NiO (filled triangles). Each data point represents 
the average of three independent trials and the error bar indicates the standard 
deviation. We observed a significant increase in the partial current densities for 
CO2 reduction products CO and HCOO− with the addition of Cu nanoparticles, 
while almost no change in the amount of H2 that was evolved. We therefore 
attribute the significant amount of H2 that is evolved from the plasmonic Cu/p-NiO 
device to the activity of the underlying p-NiO film, which almost exclusively 
produces H2 under CO2 reduction conditions (Figure S6).  
 
 
 
 
 
 
 
	 11	
 
 
Figure S8. X-ray photoelectron spectroscopy high-resolution scans of (a) Ni 2p 
region and (b) Cu 2p region from the Cu/p-NiO photocathodes before (solid 
curves) and after (dashed curves) electrocatalysis. Despite some slight changes 
in relative peak ratio after electrocatalysis, these spectra show very similar 
characteristic binding energies and satellite features indicative of NiO and CuO, 
respectively.  
 
 
 
 
 
 
 
 
	 12	
 
 
Figure S9. Plot of solution temperature over time during optical excitation of 
Cu/p-NiO photocathodes with a high-power LED. The solution temperature was 
recorded with a thermocouple immersed in the electrolyte of the specially-
designed2 photoelectrochemical cell, situated near the working electrode in the 
cell.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
	 13	
 
 
 
Supporting Information References 
(1) Thimsen, E.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J. J. Phys. Chem. C 
2012, 116, 16830-16840. 
(2) Corson, E. R.; Creel, E. B.; Kim, Y.; Urban, J. J.; Kostecki, R.; McCloskey, B. 
D. Rev. Sci. Instrum. 2018, 89, 055112.  
 
 
 


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Optical Excitation of a Nanoparticle Cu/p-NiO Photocathode Improves Reaction Selectivity for CO₂ Reduction in Aqueous Electrolytes

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