Photoelectrochemical Hydrogen Generation by
Ni-Coated Gallium Arsenide Epilayers on Silicon

Fatima Merhi, Bienvenu Boulingui Koumba, Rozenn Bernard, Antoine Létoublon, Paul Huillery, Yoan Léger, Nicolas Bertru, Charles Cornet, Patrick Garrigue, Bruno Fabre, et al.

► To cite this version:

Fatima Merhi, Bienvenu Boulingui Koumba, Rozenn Bernard, Antoine Létoublon, Paul Huillery, et al.. Photoelectrochemical Hydrogen Generation by Ni-Coated Gallium Arsenide Epilayers on Silicon. Journal of Physical Chemistry C, 2025, 129 (25), pp.11290. 10.1021/acs.jpcc.5c01840 . hal-05129283

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Photoelectrochemical Hydrogen Generation

by Ni-coated Gallium Arsenide Epilayers on Silicon

Fatima Merhi,ª,ᵇ Bienvenu Boulingui-Koumba,ᶜ Rozenn Bernard,ᶜ Antoine Létoublon,ᶜ Paul Huillery,ᶜ Yoan Léger,ᶜ Nicolas Bertru,ᶜ Charles Cornet,ᶜ,\* Patrick Garrigue,ª Bruno Fabre,ᵇ,\*

Gabriel Logetª,\*

a Univ. Bordeaux, CNRS, Bordeaux INP, ISM-UMR 5255, Pessac 33607, France.

b Univ Rennes, CNRS, ISCR-UMR6226 Rennes F-35000, France.

c Univ Rennes, INSA Rennes, CNRS, Institut FOTON-UMR 6082, F-35000, Rennes, France.
ABSTRACT

Green hydrogen ($H_2$) can play a pivotal role in reducing global carbon emissions. This renewable fuel can be produced by water-splitting photoelectrochemical (PEC) devices that require a photoanode and a photocathode serving as solar light absorbers and energy converters. So far, the use of III-V semiconductor materials as photoelectrodes resulted in the best performance for PEC water splitting. However, the cost of these materials still prohibits their spreading. Instead of employing bulk III-V wafers as photoelectrode substrates, using III-V thin film layers on inexpensive substrates to manufacture photoelectrodes appears, thus, a promising solution to solve this problem. Herein, we present the preparation and the study of photocathodes consisting of a thin film of GaAs grown by molecular beam epitaxy (MBE) on a p-doped Si (Si:p) substrate, that are subsequently modified by photoelectrodeposition of Ni catalyst. These Ni/GaAs/Si:p photocathodes are used for the solar-driven $H_2$ evolution reaction (HER) in alkaline medium. We show that these photocathodes are stable in operation for several hours under illumination with a 100% Faradaic efficiency for $H_2$. This constitutes the first example of the use of Ni as a catalyst onto GaAs photocathodes. While these results support the fact that MBE-grown III-V thin films can afford HER in alkaline media when modified with an inexpensive catalyst, the effect of the GaAs thickness on incident-photon-to-electron conversion efficiency (IPCE) data show that the III-V layer hinders the overall HER activity of the photoelectrode. Therefore, further material engineering strategies are required to improve HER performance of these promising photocathodes.

KEYWORDS: III-V Semiconductors; Silicon Photocathodes; Photoelectrochemical Cells; Hydrogen Evolution Reaction
INTRODUCTION

Fossil fuels are the major contributors to the global energy crisis as well as environmental problems such as planet warming. These concerns urge the world to find an alternative energy source that could reduce worldwide dependence on non-renewable and expensive resources.¹,²
One promising candidate to substitute fossil fuels is hydrogen (H₂).³,⁴ Among the various ways to produce H₂, a green method could be achieved through photoelectrochemical (PEC) cells. PEC devices allow to split water under solar light into O₂ and H₂ by the oxygen evolution reaction (OER) at the photoanode and the hydrogen evolution reaction (HER) at the photocathode.⁴ PEC technology utilizes light absorbing semiconductors (SCs) of suitable bandgap, allowing the conversion of light energy into chemical energy stored within the bond of the H₂ molecules.⁵,⁶ Thus, the choice of the SC material, whether to be used as a photocathode or as a photoanode, is important for the effectiveness and the cost of the PEC process. III-V SCs are materials composed of elements from group III (Ga, In, ...) and group V (As, P, ...). This family of materials has been investigated by researchers for its outstanding optoelectronic properties.⁷ Particularly, GaAs is considered a suitable candidate for solar water splitting. It exhibits a direct band gap of 1.42 eV which enables wide range solar light absorption.⁸ These characteristics, in addition to its high electron mobility, make the choice of GaAs advantageous in PEC systems.⁹ However, the high cost of III-V materials is a limitation for their incorporation in PEC systems for water splitting process. One strategy to overcome this problem is the use of epitaxially grown III-V thin films on cost-effective Si substrates.¹⁰⁻¹² In addition to the light absorbing semiconductor, an effective catalyst is also required to reach a moderate overpotential.¹³ Pt and noble metal analogous are well-known as the most efficient catalysts for HER.¹⁴ Despite their excellent catalytic activity and stability, the high cost and low abundance of Pt limits its
application on large scales. On the other hand, the use of Ni metal is appealing due to its
abundance and its electrochemical efficiency in high pH electrolytes, especially for HER and
OER.¹⁵ Ni catalyst can be deposited on SCs through different methods such as
electrodeposition¹⁶ and sputtering.¹⁷

In this work, we epitaxially grow GaAs thin films on Si:p substrates to produce GaAs/Si:p photocathodes. These systems were modified by photoelectrochemical deposition of Ni catalyst to prepare Ni/GaAs/Si:p and tested for HER in 1 M KOH electrolyte. Our results show that Ni/GaAs/Si:p can promote sunlight-assisted HER with high performance and allow to elucidate the unexpected evolution of GaAs/Si:p photocathodes performance with the GaAs epitaxial layer thickness.

# EXPERIMENTAL SECTION

## Chemicals

Acetone (electronic (MOS) grade, Fisher Scientific) and anhydrous ethanol (RSE for electronic use, Carlo Erba) were used without further purification. The ultra-pure water had a resistivity of 18.2 MΩ cm (Purelab Classic UV from Veolia Water STI). Hydrochloric acid HCl (VWR chemicals) was used after dilution with ultra-pure water (HCl:H₂O ratio 1:1). Buffer oxide etchant (BOE, Transene Inc) was used without any further dilution. Electrolysis solutions were prepared from KOH pellets (Fisher Scientific).

## Electrode fabrication
Photoelectrodes were fabricated either from commercial p-type GaAs (100) (Zn-doped, $10^{17}$ cm⁻³) and p-type Si (100) (B-doped, $10^{14}-10^{16}$ cm⁻³) wafers or from epitaxial GaAs/Si layers. In the latter case, solid source molecular beam epitaxy (MBE) was used for the epitaxial growth of GaAs thin films with different thicknesses on Si:p substrates, in the apparatus previously described.¹⁸ The growth conditions were described in the following reference.¹² The GaAs layers were not doped intentionally. First, all GaAs, Si, and GaAs/Si wafers were cut into 0.5 cm² pieces and degreased by sonication in acetone, ethanol and ultra-pure water for 10 min each. Before the PEC experiments, all photoelectrodes were fabricated by applying an indium-gallium eutectic (Alfa Aesar) droplet to the backside of the SC piece. To ensure the electrical contact, the samples were attached by a copper wire using silver paste (Electron Microscopy Sciences) and covered by an insulating epoxy-based resin (Loctite 9460, Henkel). Then, the electrodes were cured at 90°C for 1 h. The exact surface area of all the electrodes was precisely measured using ImageJ freeware. Before conducting any photoelectrochemical experiment, the unmodified GaAs and GaAs/Si photoelectrodes were immersed in a mixed HCl/water 1:1 v/v for 2 min, and stirred for 5 min to remove native oxides.¹⁹ For Si photoelectrodes used for IPCE comparison, the Si substrates were immersed in a 6:1 BOE for 30 s to remove $SiO_x$ and generate a H-terminated Si surface before Ni deposition.²⁰

## Surface, optical and structural characterization

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed using JSM 7100 F (JEOL) microscope. Atomic Force Microscopy (AFM) was performed using Bruker multimode 8 microscope, ScanAsyst mode with silicon tip on nitride lever. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a VG ESCALAB 250 spectrometer (ThermoFisher Scientific). XPS data processing was performed
using the CasaXPS analysis software. The component peaks were corrected with reference to the
C 1s peak for –CH₂–CH₂– bonds set at 285 eV. Photoluminescence (PL) was performed using a
200 mW cw frequency doubled Nd:YAG (532 nm). Samples were mounted in an ARS closed-
cycle dry He 4K cryostat. PL signal was detected using a Horiba IHR 320 spectrometer equipped
with a sincerity CCD camera. PL spectra were normalized with great care so that normalized PL
intensities are strictly comparable from one sample to the other. X-ray diffraction (XRD)
measurements were carried out by using a four circle Bruker D8 Discover diffractometer. It was
equipped with a Cu sealed tube as X-ray source, a linear hyperbolic multilayer mirror that
produces a quasi-parallel beam of 1.2mm in the vertical direction. We also employed a 2bounce
Ge220 monochromator for selection of Kₐ₁ wavelength (0.15406nm) with a vertical divergence
of 0.01°. 2.5° Sollers slits were also employed to limit the divergence in the other direction
(horizontal) as well as reduce the background. Finally, a Eiger-250K Dectris detector was
employed in 1D mode for an efficient scattering collection.

PEC measurements

Cyclic voltammetry (CV) and chronoamperometry (CA) measurements were performed in a
standard PEC cell with a quartz window into which a GaAs, Si, GaAs/Si, Ni/GaAs, Ni/Si, or
Ni/GaAs/Si working electrode (WE), a Hg/HgO (1 M KOH) reference electrode (RE), and a
graphite counter electrode (CE) were introduced. The cell was filled with 1 M KOH aqueous
solution and degassed with argon. The stability test was done for 10 h at a potential of -2 V vs
Hg/HgO. All CVs, CAs, and photoelectrodeposition of Ni were performed with a Zennium Pro
potentiostat (Zahner). The light was provided by a solar simulator with an irradiance of 100
mW/cm² and equipped with AM 1.5G filter (Pico, G2V Optics). The CVs reported in this work
were recorded at 50 mV s⁻¹ without iR compensation. All potentials measured vs. Hg/HgO were
converted versus the reversible hydrogen electrode (RHE) according to the equation given below:

$$E_{\text{RHE}} = E_{\text{Hg/HgO}} + 0.059 \text{ pH} + E^{0}_{\text{Hg/HgO}} \text{ with } E^{0}_{\text{Hg/HgO}} = +0.102 \text{ V} \quad (1)$$

where $E_{\text{Hg/HgO}}$ is applied potential versus the used reference electrode. Incident photon-to-current efficiency (IPCE) measurements were performed with a CIMPS-QE IPCE 3 workstation (Zahner) comprising a TLS03 tunable light source. The measurements were carried out using a three-electrode PEC cell, consisting of a WE, a Hg/HgO (1 M KOH) RE, and a graphite rod as a CE, all connected to an electrochemical potentiostat (Zahner-Zennium). The applied potential was -2 V vs Hg/HgO. The light modulation frequency was 5 Hz, the settling time was 5 s, and the number of counts was 10.

## Ni photoelectrodeposition

For the photoelectrodeposition of Ni catalyst, GaAs, Si, and GaAs/Si p-type photocathodes were immersed in an aqueous solution of 20 mM $NiCl_2$ (Alfa Aesar) and 100 mM boric acid (Acros, 99.5%). A three-electrode system was used where the RE was Ag/AgCl (3 M NaCl), the CE was Ni foil, and the GaAs, Si, or GaAs/Si as WE. A fixed potential of -1.5 V vs Ag/AgCl under illumination (100 mW/cm²) was applied for all the deposition experiments. After deposition, the surface was rinsed with ultra-pure water and dried under Ar.

## Gas chromatography

Gas chromatography (GC) was conducted in a PEC cell with a quartz window, under illumination. 1 M KOH was used as the electrolyte solution for this experiment. A potential of -2 V vs Hg/HgO was applied for 3.3 h while measuring the current and monitoring the amount of
produced H₂ gas. The experiment was carried out using a micro-GC Agilent 990 instrument equipped with a thermal conductivity detector.

## RESULTS AND DISCUSSION

<figure><img src="image_2.png" /><figcaption>**Figure 1** a) Structure of the Ni/GaAs/Si:p photocathode. b) Top-view SEM image of a Ni(10s)/GaAs(1µm)/Si:p photocathode. Inset: cross-section SEM image of bare GaAs(1µm)/Si:p. c) EDS spectrum of Ni(10s)/GaAs(1µm)/Si:p. d) XPS spectra of Ni(10s)/GaAs(1µm)/Si:p and bare GaAs(1µm)/Si:p.</figcaption></figure>
The Ni/GaAs/Si:p photocathode, depicted in **Figure 1a**, was designed following a two-step procedure. First, a GaAs thin film was epitaxially grown by MBE on the surface of a p-doped Si (Si:p) substrate, controlling the thickness of the epilayer (0.02, 0.2, 0.5, or 1 µm). Subsequently, XRD, PL and SEM were used to characterize the produced samples (see Section 1 in SI). A representative cross-section SEM for GaAs(1µm)/Si:p is shown in the inset of **Figure 1b**, XRD patterns are shown in **Figures S1 and S2** and PL spectra are shown in **Figures S3 and S4**. Afterwards, Ni, known as a highly active catalyst for alkaline HER, was photoelectrochemically deposited on the top of GaAs/Si:p by applying a constant potential ($E_d$ = -1.5 V vs Ag/AgCl) under illumination with a solar simulator for different times (between 10 s and 5 min). The potentiostatic deposition was performed in an aqueous solution containing 20 mM $NiCl_2 \cdot 6H_2O$ and 100 mM boric acid. Under these conditions, the deposition occurred throughout the reduction of $Ni^{2+}$ into $Ni^0$ by photogenerated electrons onto the surface of the SC. After the deposition of Ni catalyst, the surface was characterized by SEM, AFM, EDS and XPS.

After Ni photoelectrodeposition, SEM (**Figure 1b**) and EDS (**Figure 1c**) confirmed the presence of the Ni film that was composed of coalescent nanoparticles covering most of the GaAs surface, even if pinholes and Ni-free areas were also present (see **Figures S5 and S6**). AFM measurements shown in **Figure S7**, revealed that a 10 s photoelectrodeposition led to a ≈90 nm-thick Ni film. Moreover, XPS measurements were carried out on bare and Ni-modified GaAs(1µm)/Si:p photoelectrodes in order to gain further insights on the chemical composition of the photoelectrodes (**Figure 1d**). While the survey spectrum of GaAs(1µm)/Si:p (black curve) displayed all the peaks expected for GaAs²¹ and adventitious carbon, the main peaks of the Ni(10s)/GaAs(1µm)/Si:p spectrum (green curve) were attributed to Ni, including $Ni_{2p_{3/2}}$ (at 852.5 eV), Ni LMM Auger (at 641.6, 711, and 777.3 eV) and Ni 3p (at 66 eV) components. No
peaks belonging to the underlying GaAs layer were observed on Ni(10s)/GaAs(1µm)/Si:p, suggesting a globally uniform coverage of Ni catalyst, and confirming the SEM observations. To sum up, characterization data suggests that the cathodic photoelectrodeposition of Ni onto GaAs(1µm)/Si:p is a simple and efficient method to prepare Ni/GaAs/Si:p substrates.

Next, the cathodic behavior of various substrates was investigated in 1 M KOH (pH 14) aqueous solution. **Figure 2a** compares the cathodic response of bare GaAs(1µm)/Si:p with that of coated Ni(10s)/GaAs(1µm)/Si:p. In the dark, the cathodes expectedly produced low current density (< 0.15 mA/cm²). However, under illumination with simulated sunlight (AM 1.5G, 100 mW/cm²), the photocurrent density considerably increased. In this case, the absorption of light by the modified semiconductor immersed in the electrolyte solution initiates the generation of minority carriers, promoting HER through the reduction of water and the formation of H₂ gas according to the reaction:

$$2 \text{H}_2\text{O(l)} + 2 \text{e}^- \rightarrow \text{H}_2\text{(g)} + 2 \text{OH}^-(\text{aq}) \text{ (@ pH 14)}$$

**Reaction 1**

<figure><img src="image_3.png" /><figcaption>**Figure 2** a) CVs of GaAs(1µm)/Si:p and Ni(10s)/GaAs(1µm)/Si:p in the dark (grey and black curves) and under illumination (purple and blue curves) and of a Ni electrode (red curve). b) LSVs (under illumination) of Ni-modified GaAs(1µm)/Si:p with different deposition times (5 s to 5 min) and in the dark (black curve). c) LSVs (under illumination) of Ni(10s)/GaAs(xµm)/Si:p photocathodes with different GaAs epilayer thicknesses (x = 0.02, 0.2, 0.5, and 1 µm) and in the dark (black curves). Conditions: 1 M KOH electrolyte, 50 mV/s scan rate, and simulated sunlight illumination (AM 1.5G, 100 mW/cm²).</figcaption></figure>
Considering the blue (Ni(10s)/GaAs(1µm)/Si:p) and the purple (GaAs(1µm)/Si:p) CVs (Figure 2a), it is seen that the deposition of the Ni catalyst on GaAs(1µm)/Si:p produces a 300 mV less cathodic shift in the onset potential of HER. The difference in the onset potential between the blue (Ni(10s)/GaAs(1µm)/Si:p) and the red (Ni) CVs allows to estimate a ca. 330 mV photovoltage. However, the presence of Ni on the surface of the photocathode led to a decrease in the maximum photocurrent to almost half of that observed with bare GaAs(1µm)/Si:p. This is not surprising as the relatively thick Ni layer on GaAs/Si:p reduced the transmission of a significant portion of light towards the photoelectrode thus leading to a drop in the maximum photocurrent density.

Several photoelectrodeposition durations were tested on GaAs(1µm)/Si:p to produce Ni/GaAs(1µm)/Si:p with different Ni surface coverages (Figure 2b). In all cases, the presence of Ni was confirmed by SEM and EDS analyses. As the deposition time increased, more $Ni^{2+}$ ions were reduced to $Ni^0$ particles on the surface of the photocathode, producing thicker metal layers. All surfaces exhibited negligible current ($< -0.2 \text{ mA/cm}^2$) in the dark but generated photocurrent density under illumination. An important loss in the HER photocurrent density was noticed as a function of deposition time, with a duration of 5 min deposition that produced a photocurrent density lower than -1 mA/cm$^2$. This is in good agreement with the $j$ decay due to light shading by Ni films effect, already observed between GaAs(1µm)/Si:p and Ni(10s)/GaAs(0.2µm)/Si:p in Figure 2a. The choice of 10 s photoelectrodeposition time for further study was based on the results in Figure 2b where the 10 s-coated substrate showed the best compromise in terms of onset potential and photocurrent density. Then, we investigated the effect of the thickness of the GaAs epilayer. Four different Ni(10s)/GaAs/Si:p substrates, with GaAs thicknesses of 0.02, 0.2,
0.5 and 1 µm were tested for HER, as shown in **Figure 2c**. Again, all surfaces exhibited negligible current (< -0.15 mA/cm²) in the dark and photogenerated current density under illumination. The cathodic peak, observed sometimes before HER (i.e., at ca. 0.3 V for Ni(10s)/GaAs(0.2µm)/Si:p) is attributed to the reduction of solid state Ni¹¹ species to Ni⁰. While having a comparable HER onset potential, the four photocathodes showed a clear trend regarding the maximum photocurrent density. As the thickness of the GaAs epitaxial layer decreased, the photocurrent density increased (**Figure S8**) with values that range from -3 to -9 mA/cm², for Ni(10s)/GaAs(1µm)/Si:p to Ni(10s)/GaAs(0.02µm)/Si:p respectively. This trend is unexpected. Indeed, due to the excellent transport and optical properties of GaAs, better photoelectrode performance with the GaAs content was anticipated. Then, complementary experiments were performed to understand this effect.

Further characterization methods were performed on the photocathodes of 1 µm-thick GaAs. Although thinner epilayers gave higher photocurrent, a dual bandgap-induced photocurrent is more likely to be present as we decrease GaAs thickness.¹² For this reason, the choice of 1 µm epilayer thickness was made to better understand the catalytic effect of Ni on these photoelectrodes while minimizing the contribution of the dual bandgap operation. To investigate the stability of the studied photocathodes, a 10 h-long electrolysis was performed under illumination for Ni(10s)/GaAs(1µm)/Si:p (**Figure 3a**) at -1 V vs RHE. The photocathode was stable over the test duration, producing a constant photocurrent density around -4 mA/cm².

**Figure 3c** shows the photoelectrochemical behavior of Ni(10s)/GaAs(1µm)/Si:p before and after the stability test in 1 M KOH electrolyte. No change in the HER onset potential was noticed. However, the maximum photocurrent at 1.5 V vs RHE varied from -4 to -4.5 mA/cm² after the stability test. This change in current could be attributed to a loss of a small amount of Ni
particles during the stability test. To assess the efficiency of Ni(10s)/GaAs(1µm)/Si:p for HER, the amount of produced H₂ was measured by GC during the electrolysis. **Figure 3b** displays the evolution of the Faradaic efficiency for H₂, η<sub>H₂</sub>, as a function of the electrolysis time. This curve shows that η<sub>H₂</sub> varied from 80 to 100% from 1 to 3.3 h, confirming the efficiency and selectivity of Ni-modified GaAs/Si:p photocathodes for producing H₂ through PEC water splitting. XPS analysis of the Ni 2p spectrum (**Figure S9**) after 3 CV cycles revealed that the Ni coating maintained a strong Ni<sup>0</sup> character, covered by a Ni(OH)<sub>2</sub> layer.

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
**Figure 3** a) CA recorded during a 10 h stability test with Ni(10s)/GaAs(1µm)/Si:p at a fixed potential of -1 V vs RHE under simulated sunlight illumination. b) Plot of η<sub>H₂</sub> as a function of time at a fixed potential of -1 V vs RHE under simulated sunlight illumination. c) CVs recorded before and after the 10 h stability test, at Ni(10s)/GaAs(1µm)/Si:p in the dark and under simulated sunlight illumination. Scan rate = 50 mV/s. d) Normalized IPCE spectra recorded at -1 V vs RHE for Ni(10s)/GaAs(1µm)/Si:p (blue), Ni(10s)/GaAs:p (green) and Ni(10s)/Si:p (pink). Electrolyte: 1 M KOH.

To better understand the effect of the activity of the GaAs film on the overall PEC performance, photocurrent spectroscopy measurements were performed at -1 V vs RHE on Ni(10s)/GaAs(1µm)/Si:p, as shown in **Figure S10**. To investigate the effect of the GaAs layer, the normalized IPCE spectrum is plotted in **Figure 3d** and compared with that of other p-type GaAs and Si photocathodes (based on commercial wafers) modified by Ni photoelectrodeposition (10 s), Ni(10s)/GaAs:p and Ni(10s)/Si:p, respectively. Surprisingly, data reveal that the photoconversion efficiency of Ni(10s)/GaAs(1µm)/Si:p was very small at low wavelengths and that its main activity was located in the 700 nm to near-infrared (IR) region. In contrast, the commercial wafer-based photocathodes presented the expected behaviour.<sup>22,23</sup> Specifically, Ni(10s)/GaAs:p was active in all the investigated wavelength range, with an IPCE that decreased between 800 and 900 nm, in good agreement with its bandgap value (E<sub>g</sub> = 1.42 eV). The same statement was true for Ni(10s)/Si:p which had a broadband activity over all visible spectrum before decreasing at higher wavelengths, in good agreement with Si bandgap (E<sub>g</sub> = 1.1 eV). The IPCE spectrum of Ni(10s)/GaAs(1µm)/Si:p strongly differed and presented a somehow "opposite" behavior when compared to Ni(10s)/GaAs:p. Indeed, this photocathode was poorly active in the GaAs-absorbing region (from 300 to 700 nm) and its maximum response was located in the wavelength window where only Si was absorbing. These results show clearly that GaAs light absorption is detrimental to the overall Ni(10s)/GaAs(1µm)/Si:p photocathode. They reveal that only the electrons photogenerated in Si are effectively employed for the HER
reaction while those generated in the GaAs epilayer are not participating to the photoconversion
process. This performance is further supported by the CV of Ni(10 s)/Si:p (Figure S11), which is
similar to that of Ni(10 s)/GaAs/Si:p, but with a higher photocurrent, indicating that the GaAs
epilayer does not significantly change the photoelectrochemical behavior but rather alter the
overall photoresponse depending on its thickness. Both XRD experiments (Section 2 of SI) and
temperature/power-dependent PL experiments (Section 3 of SI) point towards the very high
density of crystal defects, acting as traps for charge carriers in the GaAs/Si samples, to explain
these results; especially the overall lower photoelectrochemical performance of the GaAs/Si
photoelectrodes, as compared to the GaAs ones. However, they are not sufficient for explaining
the shape of the IPCE spectrum recorded for Ni(10s)/GaAs(1µm)/Si:p. Indeed, the lower
photoactivity of the GaAs/Si photocathode at short wavelength is unexpected as it strongly
differs with what was observed on GaAs/Si:n photoanodes, that exhibited broadband
photoconversion, despite a comparable volume density of crystal defects.<sup>12,24</sup> The precise
understanding of the physics of GaAs(1µm)/Si:p and GaAs(1µm)/Si:n photoelectrodes requires a
broader set of experiments which are currently ongoing in our team.

CONCLUSIONS

To conclude, we have reported the preparation and study of photocathodes consisting of a GaAs
thin film grown by MBE on a Si:p substrate and coated with a catalytic Ni layer. The low-cost Ni
catalyst was deposited using a cheap aqueous-based method. Several photocathode stacks
comprising different thicknesses of GaAs and various Ni loadings were tested for solar-assisted
H₂ production in an alkaline electrolyte. The photocathodes exhibited stable operation for 10 h
with high Faradaic efficiency for H₂. This report constitutes the first example of incorporating a
Earth-abundant Ni cocatalyst on GaAs-based photocathodes and reveals Ni as a strong candidate
to improve the performance of III-V epilayers employed for photoelectrochemical applications.

This is very important as III-V-based photoelectrodes present the best performance for solar water splitting to date²⁵⁻²⁷ and the combined use of epilayers deposited on Si and cheap catalysts may lead to a considerable decrease of the overall cost of III-V-based PEC systems. Besides, this report also points out that further in-depth investigations are required to understand the optical activities of these systems, which could lead to considerable improvements of their photoconversion efficiency in the near future.

## ACKNOWLEDGEMENTS

The authors acknowledge RENATECH (French Network of Major Technology Centers) within Nanorennes for technological support. This research was supported by the "France 2030" program of the French National Research Agency, NAUTILUS Project (Grant no. ANR-22-PEHY-0013), the EQUIPEX NANOFUTUR (ANR-21-ESRE-0012), the PIANIST (Grant No.ANR-21-CE09-0020) project, Rennes Métropole and Région Bretagne. The authors acknowledge Dr. C. Lagrost for the XPS measurements. The XPS measurements have been performed on the ASPHERYX platform (ScanMAT, UAR 2025 University of Rennes-CNRS; CPER-FEDER 2015-2020).

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## ASSOCIATED CONTENT

**Supporting Information.** Supplementary XRD, PL, SEM, AFM and IPCE data as well as supplemental description of the XRD and PL characterizations are provided in supporting information. The following files are available free of charge.

## AUTHOR INFORMATION

Corresponding Authors
## AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## FUNDING SOURCES

This research was supported by the “France 2030” program of the French National Research Agency, NAUTILUS Project (Grant no. ANR-22-PEHY-0013), the EQUIPEX NANOFUTUR (ANR-21-ESRE-0012), the PIANIST (Grant No.ANR-21-CE09-0020) project, Rennes Métropole and Région Bretagne.

<footnote>*charles.cornet@insa-rennes.fr</footnote>

<footnote>*bruno.fabre@univ-rennes1.fr</footnote>

<footnote>*gabriel.loget@cnrs.fr</footnote>