Enhanced light trapping in GaAs / TiO₂ based
photocathodes for hydrogen production

Thomas Dursap¹, Mariam Fadel², Philippe Regreny¹, Cristina Tapia Garcia², Céline Chevalier¹,
Hai Son Nguyen¹, Emmanuel Drouard¹, Solène Brottet¹, Michel Gendry¹, Alexandru Danescu¹,
Matthieu Koepf²*, Vincent Artero², Matthieu Bugnet³*, José Penuelas¹,*

1 Univ. Lyon, CNRS, ECL, INSA Lyon, UCBL, CPE Lyon, INL, UMR 5270, 69130 Ecully,
France

2 Univ. Grenoble Alpes, CNRS, CEA, IRIG, Laboratoire de Chimie et Biologie des Métaux,
38000 Grenoble, France

3 Univ. Lyon, CNRS, INSA Lyon, UCBL, MATEIS, UMR 5510, 69621 Villeurbanne, France

ABSTRACT. Photoelectrochemical cells (PEC) are appealing devices for the production of
renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising
materials due to their tunable bandgaps, which can be appropriately adjusted for sunlight
harvesting. Because of the high cost of these semiconductors, the nanostructuration of the
photoactive layer can help to improve the device efficiency as well as drastically reduce the amount
of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect
ratio of nanowires, their enhanced ability to trap light as well as their improved charge separation
and collection abilities, and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO₂ shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin film counterpart. Morphological and electronic parameters, such as the aspect-ratio of the nanowires and their doping pattern were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co-nanoparticles used as hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities, comparable with previous noble-metal based systems. This study demonstrates the potential of GaAs nanostructured semiconductor and its reliable use for photo-driven hydrogen production.

KEYWORDS. hydrogen evolution reaction, photocathode, GaAs, nanowires, cobalt catalyst
Introduction

Concomitantly to the development of industrialized societies, the use of fossil fuels such as
natural gas, oil, and coal as primary energy sources has drastically increased since the beginning
of last century¹ leading to the release of ever-larger quantities of CO₂, which are at the origin of
the current global warming effect. The search for alternative strategies to provide energy at a large
scale with a lower carbon-footprint has become a critical issue for the scientific community.
Among all devices with high potential of applicability, intense efforts are carried out for the
development of photoelectrochemical cells (PEC). Following the basic principles of
photosynthesis, these devices are able to harvest and use solar energy to drive specific chemical
reactions, and convert abundant resources (H₂O, CO₂, N₂, etc.) into energy-rich compounds (H₂,
CH₃OH, NH₃, etc.) that are stable, can be stored, and are easily distributed²,³. In this context, solar-
driven water splitting is a major target as it involves rather simple chemical reactions and water,
which is a widespread and easy-to-handle reagent⁴. Electrochemical devices for water splitting
typically combine an anode material (or combination of materials) where water is oxidized to O₂
through the oxygen evolution reaction (OER), and a cathode material (or combination of materials)
where protons are reduced to H₂ through the hydrogen evolution reaction (HER). In the case of
PEC, at least one of the two electrode materials must be able to absorb sunlight to generate the
electrochemical potential required to drive the OER and HER reactions⁵. TiO₂ has been the first
effective photoanode material reported for water splitting, in 1972, by A. Fujishima and K. Honda⁶.
This pioneering study triggered intense efforts by the scientific community to improve the
efficiency of the solar-to-hydrogen (STH) conversion⁷. However, despite being a cheap and robust
material for the design of photoanodes, the large bandgap of TiO₂ (~3.2 eV) restricts its absorption
to a very small fraction of the solar spectrum (λ<sub>abs</sub> < 400 nm), strongly limiting the maximum STH
efficiency reachable with devices based only on this oxide⁸. In the quest for alternative solutions, III-V semiconductors (SCs) such as GaAs emerged as advantageous material constituents of photoelectrodes due to their narrower bandgap (1.4 eV in the case of GaAs) and adequately positioned valence band, allowing for the use of a substantially larger portion of the solar spectrum to drive the OER and HER reactions⁵,⁹,¹⁰. Thus, III-V SCs have attracted a fair amount of attention for solar fuels applications.

Besides the choice of the SC material implemented within PEC devices, the control of the morphology of the electrodes was recently recognized as a critical factor controlling both the light absorption properties as well as the efficiency of the electrochemical reaction occurring at the interface with the electrolyte¹¹. III-V nanowires (NWs) present a high aspect-ratio improving their light-trapping efficiency and have a strong ability to be grown on low-cost silicon substrate, making them particularly attractive to design innovative photoelectrodes for light-driven water splitting¹².

While many studies have been focusing on the use of III-V NWs as photoanodes for the OER, only few reports are available on the use of these materials as photocathodes for the HER¹³⁻¹⁵. A recent work by Zhou *et al.* indicated a significant enhancement of the STH efficiency, reaching performances close to 10% using the GaP NW combined with a complex heterostructure and bandgap engineering¹⁵. If the corrosion of these materials has limited the viability of their use in actual devices thus far, Cui *et al.* recently demonstrated the possibility to overcome this issue by protecting the surface of the nanowires with a thin TiO₂ oxide layer¹⁴. While this latter study suggests that III-V NW-based photoelectrodes could still be used for solar fuel production, the current state-of-the-art also shows that several open questions remain to be answered. For instance, the influence of morphological parameters of the nanowires such as their aspect-ratio, and their crystallinity or verticality on the PEC performance is seldom investigated. Additionally, the influence of the doping profile of the semiconductor, such as a p-n junction, remains unexplored.
The use of non-noble metal catalysts, instead of commonly used Pt¹⁴, is also a meaningful approach for cost-reduction and use of abundant resources that should be privileged¹⁶. Furthermore, the TiO₂ protective layer quality could possibly introduce variations of the PEC efficiency. Therefore, it appears necessary to study these different pathways with the aim to further optimize the efficiency of PEC cathode components for HER.

In this work, the influence of the NW geometry on the photocathode efficiency is investigated by comparing the photocurrent generated by a 2D GaAs thin film, and by self-assisted GaAs NWs protected with a thin TiO₂ shell, both grown on low-cost silicon (111) substrates by molecular beam epitaxy. After confirming the benefit of using a nanostructured electrode compared to flat GaAs substrate, the performances of photoelectrodes featuring different NW morphologies are systematically evaluated. For the best architecture, the benefit of including a buried p-n junction within NWs is studied. Finally a Co-based catalyst is integrated at the surface of the NWs, which leads to photocathodes exhibiting comparable photocurrent densities as Pt-decorated TiO₂-protected electrodes previously reported in literature¹⁴. A schematic illustrating the different steps leading to the final nanostructures is shown in Figure 1. This study demonstrates that finely tuning the NW morphology and doping profile provides efficient photocathodes for HER at neutral pH without the need of using a noble metal catalyst.

# Results and discussion

## Geometry

In order to evaluate the influence of the geometry of the active material on the photocurrent density generated under irradiation, a thin 2D layer of B-doped GaAs and a sample of self-assisted B-doped GaAs NWs were deposited on separate p-doped silicon substrates. Details of the growth are available in Methods. The NWs were ~2.3 ± 0.3 µm long, for a mean diameter of 125 ± 20 nm,
reaching a density of around 1.1 NWs/µm². The total volume of GaAs deposited during the NWs growth hence corresponds to 3.1 x 10⁻⁶ cm³ for an effective surface, i. e. the surface in contact with the electrolyte, of around 1.01 cm², which is equivalent to the effective surface of the GaAs thin film counterpart.

The samples were placed in 0.1 M potassium phosphate buffer (pH 7) and linear sweep voltammograms (LSV) were recorded under chopped-light irradiation using a Xe lamp equipped with an AM 1.5 filter (simulated 1 sun irradiation, 100 mW/cm²; see Methods for details). As shown in Figure 2.a, photocurrents were observed for both NW and thin film samples, although with distinct onset potentials and current densities. The NWs led to photocurrents with an onset potential of + 0.35 V vs RHE, while a lower onset potential of + 0.25 V vs RHE was observed in the case of the thin film indicating that the NWs are able to drive more efficiently (ie with higher photopotential) reductive processes under irradiation. Moreover, the NWs delivered significantly higher photocurrent than the thin film, with almost two to three times higher current recorded over the full range of the explored potentials, although the volume of the photoactive material (GaAs) deposited is ten times higher for the thin film. Since the geometric surface area for light collection of the two samples were both equal to ~1.0 cm², it is likely that the enhancement of the photocurrent is largely associated to the improved light-trapping efficiency of the NWs compared to the thin film. Indeed, as shown in Figure 2.b, the NW geometry induces a reduction of the reflectivity down to 2%, compared to around 30% for the thin film, indicating an enhancement of the sample light absorption efficiency, and thus demonstrating the strong potential of the nanostructuration of GaAs into NWs to design improved photoelectrodes for PEC applications. The NWs grown for this comparison are shown in Figure 3.a. Besides the light trapping effect, several other factors can influence the overall efficiency of charge carrier generation and collection upon nanostructuration of the absorbing material¹⁷. Additional experiments that are outside the scope of this study would,
however, be necessary to assess the contribution of each individual factors. In any case, our
observations are in agreement with the observations performed by A. Javay's group¹⁸ or E. P. A.
M. Bakkers's group¹⁹ on InP or GaP NWs, respectively, and C. Cornet's group²⁰ for GaAs/Si
photoelectrodes.

Morphology

The impact of the morphology of these objects on the photocurrent density were further
investigated under simulated 1 sun irradiation. Three NW samples were grown with different
morphologies as illustrated in Figure 3.a-c, and described in Table 1. The different morphologies
were obtained for different radial growth times (more details in Method section). Sample 1, shown
in Figure 3.a, presents the largest aspect ratio (length/diameter) but the smallest effective surface
area. Samples 2 and 3, corresponding to NWs after 1 hour and 2 hours of radial p-doped GaAs
growth, respectively, are shown in Figure 3.b and Figure 3.c. By increasing the length and the
diameter of the NWs (values reported in Table 1), the total effective surface area of GaAs materials
exposed to the electrolyte increases, for a comparable NW density.

The sample bearing the shortest and thinnest NWs (Figure 3.a) delivers a photocurrent of
around 0.10 mA/cm² at 0.0 V vs RHE, while a value close to 0.31 mA/cm² was obtained at 0.0 V
vs RHE for the thickest NWs (Figure 3.c). The improvement of the photocurrent density can be
attributed to the increase of the effective surface area of GaAs material in contact with the
electrolyte (Figure 3.e), and to enhanced light trapping efficiency by the largest NWs as shown in
Figure 3.f. The reflected light in the 350-850 nm spectral range drops from about 2% for the shortest
and thinnest NWs (blue lines) to about 0.5% for the thickest ones (green lines). While the
reflectance of the different samples was experimentally measured, it was not possible to obtain
information on the absorptance of those samples. Finite difference time domain (FDTD)
simulations were used to further investigate on the optical properties of our samples, using ANSYS Lumerical FDTD solutions.

The simulated reflectance thus confirms the tendencies observed in the experiment (see Supporting Information for details). Simulations also show the contribution of the GaAs material and of the Si substrate on the light absorptance (see Supporting Information for details), confirming the optimal sample morphology.

## p-n junction

In order to drive the targeted chemical reactions at the solid-liquid interface, efficient charge migration must occur from the core of the NWs to its surface. This phenomenon requires the establishment of an adequate gradient of potential between the bulk SC material, here GaAs, and the electrolyte. Ensuring that electrons reach the edge of the GaAs NWs should improve the overall efficiency of the photocathodes. For this purpose, an n-doped shell was grown using Si as dopant around the p-doped core NW, as illustrated in Figure 4.a, aiming for the same dimensions as the NWs of sample 3. The implementation of such p-n junction leads to band edge bending at the interface between the p-core and n-shell, as shown in Figure 4.b, which should favor the separation of the electron-hole pairs and reduce their recombination rate within the NWs. On the bare NWs (sample 3), the LSV measurements performed on samples featuring the buried p-n junction reveal an increase of the photocurrent density of about 30%, up to around 0.4 mA/cm² at 0 V vs RHE, compared to 0.3 mA/cm² for the monolithic p-doped NWs, as shown in Figure 4.c. Furthermore, another significant enhancement of the current density delivered by the samples featuring the NWs containing the p-n junction is observed after the integration of Co-based nanoparticles on the electrodes, a well-known catalyst for the hydrogen evolution reaction (HER)²¹. Indeed, after the photoassisted deposition of the nanoparticles (see the Methods for the experimental details), the
current density reaches up to 1.15 mA/cm² at 0 V vs RHE in neutral electrolyte (Figure 5.b and c).
This contrasts with the samples based on monolithic p-doped NWs, which only deliver 0.38
mA/cm² (Figure 5.a and c) even after Co deposition, under the same conditions. Noteworthy, the
Co-decorated NW samples featuring the buried p-n junction appear to outperform the recently
reported monolithic GaAs NW photocathodes decorated with Pt nanoparticles tested under acidic
conditions (pH 1, simulated 1 sun irradiation, 0.87 mA/cm² at 0 V vs RHE)¹⁴. These results
demonstrate that the careful tuning of the NW dimensions and structure, combined with non-noble-
metal catalysts can provide viable alternatives for the design of efficient photocathodes for HER in
neutral electrolyte.

## Enhancement of the stability through TiO₂ protection

Despite their promising performances for the photoelectrocatalytic water splitting, III-V SCs
suffer from degradation through photocorrosion when placed in aqueous electrolytes. To overcome
this issue, an amorphous TiO₂ protective layer of about 5-7 nm thickness was deposited by atomic
layer deposition (ALD) on the p-core / n-shell NWs. This deposition technique was chosen to
ensure a conformal deposition²²⁻²⁵. Scanning transmission electron microscopy (STEM) and
electron energy-loss spectroscopy (EELS) were performed on the cross section of a typical NW, as
shown in Figure 6. The TiO₂ protective shell is visible in Figure 6.e, which is a higher magnification
of the Z-contrast high-angle annular dark field (HAADF) image in Figure 6.a. Ga and Ti EELS
elemental maps shown in Figure 6.b (Ga core), Figure 6.c (Ti shell) and Figure 6.d (Ga core + Ti
shell) confirm the core-shell geometry of the NWs. Figure 6.e-h, provide a closer view of the 5-7
nm thick protective TiO₂ shell.

LSV measurements performed on p-core/n-shell sample, with and without the protection of the
TiO₂ layer and after deposition of the Co nanoparticles are shown in Figure 7.a. Despite the slight
decrease of the current density from about 1.15 mA/cm² down to 0.8 mA/cm² at 0 V vs RHE, which is attributed to the induced resistivity of the oxide layer covering the NWs, the stability of the measured photocurrent over time was enhanced with TiO₂. Indeed, Figure 7.b shows the chronoamperometric (CA) measurements recorded under simulated 1 sun illumination over 13 h. During the first 30 minutes, the sample without the protection layer (purple line) delivered a higher current density compared to the sample with a protective layer (dark blue line). However, after less than one hour, the photocurrent of the sample without TiO₂ shell dropped and stabilized at around 0.06 mA/cm². Meanwhile, the sample with the protective shell stabilized after few hours at around 0.15 mA/cm². These results show that the addition of this protective oxide shell induces a significant enhancement of the stability of the photocathode over time likely due to the reduced photocorrosion of the GaAs NWs. It is worth noting that the transient photocurrents recorded at the start of the chronoamperometry is a common feature observed for photoelectrodes²⁶⁻²⁸. The rapid rise and decay of photocurrent when irradiation is switched on, is mostly representative of non-faradaic processes, like charge accumulation, occurring mostly at the liquid-semiconductor and TiO₂-GaAs interfaces. The distinct interfaces of the samples w/o TiO₂ shell with the electrolyte explain the various transient photocurrents measured.

Importantly, a substantial increase of the Faradaic efficiency (FE) measured for H₂ production is observed for the samples bearing the TiO₂ protective layer, with a yield reaching 59% when unprotected samples yield about 27% FE only. These measurements are limited by the use of single compartmented cell that was necessary to accommodate the samples, allowing for the diffusion of traces of O₂ produced at the anode to the photocathode where O₂ is reduced competitively against protons. The FE values reported here, are thus likely underestimated.
After the reaction, a cross section lamella was extracted from the electrode and characterized using STEM-EELS, as shown in Figure 8.a-d. As expected the TiO₂ shell successfully protected the GaAs core, and remained intact with no distinct degradation, confirming the efficiency of the protection. The improved FE observed for TiO₂-coated samples is, thus, attributed to the reduced photocorrosion of the GaAs semiconductor otherwise leading to an evolution of the chemical composition of the NWs surface likely promoting competitive catalytic or non-catalytic side reactions (O₂ reduction, electrochemical surface etching...).

## Conclusions

In this study, the influence of the geometry of GaAs semiconductor as photocathodes for the water reduction was investigated. A strong improvement of the cell efficiency was demonstrated for the 1-dimensional NW geometry, which is mainly due to a significant reduction of light reflected by the electrode. The photoelectrochemical activity was strongly enhanced using the combination of a p-n junction buried within the GaAs NW and of the deposition of Co nanoparticles acting as catalysts for the HER. Furthermore, the stability of the NW-based photocathode over time was significantly improved with the deposition of a thin TiO₂ protective layer, leading to an increase of the efficiency of the cells by a factor 3 after 13 hours. The fabricated photocathodes display photocurrent density up to 1.15 mA/cm² at 0 V vs RHE under neutral conditions. Such performance is at the state of the art for similar GaAs photoelectrodes but would lead to tandem photoelectrochemical cells with solar-to-hydrogen performances below 1%. This calls for future tenfold improvement of the performances of such scalable materials to allow for a viable solar hydrogen technology.
## Methods

All the GaAs NWs samples were grown on 2 inches epi-ready p++ Si(111) substrates (Sil'tronix silicon technologies) using a solid-source molecular beam epitaxy (MBE) reactor. The SiO₂ native oxide on the Si substrates was preserved in order to enable the self-assisted growth²⁹. The wafers were cleaned in acetone and ethanol during 10 minutes in ultrasonic bath, and were degassed at 200°C under ultra-high vacuum before their introduction inside the MBE chamber. The thin film was synthetized for 20 minutes at a growth rate of about 0.25 nm/s, leading to a thickness of around 300 nm on a 1 x 1 cm² Si(111) substrate, leading to a total GaAs volume of around 3.0 × 10⁻⁵ cm³, and an effective surface of 1 cm². For the self-assisted GaAs NWs, the substrate was heated to 440°C and 1 monolayer (ML)³⁰,³¹ of Ga was pre-deposited to form the Ga catalyst droplet, at a deposition rate of 0.08 ML/s, measured in units of equivalent growth rates of GaAs 2D layers by RHEED oscillations on a GaAs substrate³². The sample was then heated to 570°C and the Ga and As₄ fluxes were opened simultaneously to initiate the growth of the self-assisted GaAs NWs. A precise control of the incoming fluxes, valves and shutters was operated via a homemade software.

The GaAs NWs were grown with a Ga and As₄ flux of 0.5 ML/s and 1.2 ML/s, respectively, corresponding to a V/III flux ratio of 2.4. The NWs were doped with Be, ensuring the p-doping of the objects. In order to investigate the influence of the NW morphology on the PEC efficiency, the GaAs NWs were radially grown during 1 and 2 hours with a V/III flux ratio of 2.4, at a thermocouple temperature of 400°C. The p-n junction was elaborated with a 1 hour growth of p-doped (Be) core, followed by the growth of the n-doped (Si) shell during 1 hour. All the growths were monitored by RHEED at 20 keV, to obtain information on the crystal structure evolution of the NWs. The samples were rotating all along the growth to ensure the homogeneity of the NWs.

Each sample was imaged and characterized with a JEOL SEM using a 10 kV acceleration voltage.
The effective surface of the samples was determined as follows: The surface of a single NW was calculated with an average length of 2.35 µm (2.35 × 10⁻⁴ cm) and an average diameter of 125 nm (1.25 × 10⁻⁵ cm) giving S<sub>NW</sub> = 9.22 × 10⁻⁹ cm². The NW density (1.1 NW/µm² = 1.1 × 10⁸ NW/cm²) was finally used to calculate the total effective area S<sub>eff</sub> = 1.01 cm².

The reflectivity measurements were performed with a home-built reflectivity setup. The excitation is a white light (Halogen lamp) that is collimated and focused onto a 200 µm-diameter spot on the sample. The reflected light is collimated and is either i) directly imaged by a CMOS camera for a quick visualization (inset in Figure 3.e), or is ii) projected onto the entrance slit of the spectrometer then collected by a CCD camera at the output of the spectrometer to obtain the reflectivity spectra.

The TiO₂ protective shell was deposited by PE-ALD at 250°C and around 200 mTorr using tetrakis(dimethylamido)titanium (TDMAT) as metal precursor and gaseous O₂ as oxygen precursor.

The degradation of the NWs after the PEC and the homogeneity of the TiO₂ shell deposition were investigated in high-resolution by HAADF-STEM. The chemical study of the GaAs/TiO₂ interface was highlighted by elemental maps using EELS, in STEM mode. The HAADF-STEM and STEM-EELS analyses were performed in a JEOL JEM-ARM200F NeoARM, equipped with a cold-FEG, a last generation aberration-corrector (CEOS ASCOR) of the probe-forming lenses, a Gatan GIF Quantum, and operated at 200 kV. The cross-section lamellae were extracted from the electrode using focused ion beam (FIB) lift-out. STEM-EELS datasets were acquired using the spectrum imaging technique implemented in Gatan microscopy suite. Elemental maps of 158 × 242 pixels were obtained from the Ga-L₂₃ and Ti-L₂₃ edges, with an exposure times of 5.0 ms and
a dispersion of 1.0 eV/channel. The noise level was minimized by post-processing using principal component analysis implemented in Gatan microscopy suite³³.

All chemicals used for the (photo)electrochemical measurements were obtained from Merck and used as received. The measurements were conducted in the three-electrode configuration using a BioLogic SP300 potentiostat. They were run in a single compartment gas-tight cell in N₂-purged 0.1 M potassium phosphate buffer (pH 7). GaAs samples were used as working electrodes (WE) and mounted on FTO-coated glass (3.1 mm thickness; 7 Ω/cm⁻², SOLEMS S.A.) using silver paste to ensure the electrical contact between the back of the GaAs samples and the FTO coating. An O-ring was used to define the active surface area of the electrode material in contact with the electrolyte (geometrical surface area of 0.384 cm²). A titanium wire was used as counter electrode (CE) and a home-made Ag/AgCl (3 M KCl) electrode as reference electrode (RE). The [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ couple (E = 0.425 V vs NHE in 0.1 M potassium phosphate buffer at pH = 7) was used to standardize the measurements in aqueous solution³⁴. Potentials are referred to RHE by using the equation E<sub>RHE</sub> = E<sub>Ag/AgCl</sub> + (E<sup>0</sup><sub>Ag/AgCl</sub> + 0.059 x pH) where E<sub>RHE</sub> is the reported potential vs. RHE, E<sub>Ag/AgCl</sub> the potential measured vs the Ag/AgCl RE, and E<sup>0</sup><sub>Ag/AgCl</sub> the standard potential of the RE (0.223 V vs NHE)³⁵. Co nanoparticles were deposited from a solution of 0.5 mM Co(NO₃)₂·6H₂O in 0.5 M phosphate buffer at pH 7, by applying a potential of -0.65 V vs Ag/AgCl (-0.01 V vs RHE), for 45 minutes under 1 sun simulated irradiation (see below).

Linear sweep voltammetry (LSV) measurements were performed at 10 mV/s scan rate under chopped light irradiation. Photoelectrocatalytic experiments were performed under continuous light irradiation, by running chronoamperometry (CA) measurement at an applied potential of -0.8 V vs Ag/AgCl (-0.18V vs RHE). During the LSV and CA measurements the samples were front-illuminated using a class 3A solar simulator (Newport) equipped with an ozone-free 300 W Xe Arc lamp operated at 280 W and an AM 1.5 G filter. A calibrated reference cell (model 91150V,
Newport) was used to adjust the irradiance at the surface of the sample to 100 mW/cm² (simulated 1 sun irradiation). The H₂ accumulated in the headspace during the experiments was quantified by gas chromatography (GC) on a Clarus® 580 gas-chromatograph (PerkinElmer) by direct injection of 50 µL of a sample of the headspace gas, at the end of the CA. A calibration curve was previously established using N₂-H₂ mixtures of known ratio. Before the photoelectrocatalytic measurement, the electrolyte was thoroughly purged with nitrogen gas, until no traces of oxygen could be detected by GC measurements (ca 20 mins). The % Faradaic efficiency was calculated according to the equation: η<sub>F</sub>(%) = [(mols of H₂ × 2F)/Q]×100, where F is the Faraday constant (96485 C·mol<sup>-1</sup>) and Q is the charge passed during the experiment.
# Figures
<figure><img src="image_1.png" /></figure>

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Synthesis steps of the GaAs nanowires. Molecular beam epitaxy growth of p-doped GaAs core (a), n-doped GaAs shell (b), Atomic layer deposition of TiO₂ shell (c), and photoassisted deposition of Co nanoparticles (d).</figcaption></figure>

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

<figure><img src="image_6.png" /><figcaption>**Figure 2.** (a) Linear sweep voltammograms (10 mV/s) recorded in nitrogen-saturated 0.1 M potassium phosphate buffer (pH 7), under chopped-light irradiation (simulated 1 sun, 1.5 AM</figcaption></figure>
filter). (b) Reflectivity measured on a 400-870 nm range spectrum. The y-axis is in log scale. Pink and blue curves in (a) and (b) correspond to the B-doped GaAs thin film and to the self-assisted B-doped GaAs NWs, respectively.

<figure><img src="image_7.png" /></figure>

**Figure 3.** (a) - (c) SEM images of the samples showing the different morphologies obtained for various growth times. Scale bars are 1 µm. (d) PEC characterization of the 3 samples recorded at 10 mV/s in nitrogen saturated 0.1 M potassium phosphate buffer (pH 7), under chopped-light irradiation (1 sun, 1.5 AM filter). (e) Reflectivity of the samples measured on a 400-870 nm range spectrum. Blue, red and green curves displayed in (d) and (e) correspond to the NWs samples showed in (a), (b) and (c), respectively. Inset in (d) represent the current density obtained at 0 V vs RHE as a function of the effective surface of B-doped GaAs materials on 1 cm² of silicon substrate.
Inserts in (e) show pictures of the reference mirror (circled in black) and of the samples (a), (b) and (c), circled in blue, red and green, respectively.

<table><caption>**Table 1.** Measured length and diameter for the samples shown in **Error! Reference source not found..a-c**, here named as sample 1-3, respectively.</caption><thead><tr><th></th><th>Sample 1</th><th>Sample 2</th><th>Sample 3</th></tr></thead><tbody><tr><td>Length (μm)</td><td>2.3 ± 0.3</td><td>5.2 ± 0.4</td><td>9.4 ± 0.3</td></tr><tr><td>Diameter (nm)</td><td>125 ± 20</td><td>380 ± 20</td><td>740 ± 0.4</td></tr><tr><td>Density (NWs/μm²)</td><td>1.1</td><td>1.0</td><td>0.9</td></tr><tr><td>Aspect ratio (L/D)</td><td>18.4</td><td>13.7</td><td>12.7</td></tr><tr><td>Surface area (cm²)</td><td>1.0</td><td>7.2</td><td>17.8</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** (a) Schematic representation of the core-shell p-n (B-doped GaAs/ Si-doped GaAs) NW structure, showing the p-doped core (orange) and the n-doped shell (blue). (b) Simulation of the</figcaption></figure>
energy band diagram at the interface between the n-doped and the p-doped GaAs. The conduction and the valence bands are represented by the black and blue curves, respectively. (c) LSV measurements performed at 10 mV/s in 0.1 M potassium phosphate buffer (pH 7) under nitrogen, irradiated with chopped light (1 sun, 1.5 AM filter), showing the current densities of a sample without p-n junction (in green) and with p-n junction (in orange).

<figure><img src="image_9.png" /></figure>

**Figure 5.** (a)-(c) LSV measurements performed at 10 mV/s in 0.1 M potassium phosphate buffer (pH 7) under nitrogen, irradiated with chopped light (1 sun, 1.5 AM filter), showing the current density of samples with and without p-n junction in orange and green, respectively. The purple and black curves respectively report on the current density obtained for samples with and without p-n
junction, after the deposition of cobalt-catalyst nanoparticles. The cobalt nanoparticles were
deposited at -0.65 V vs Ag/AgCl during 45 minutes, under simulated 1 sun illumination.

<figure><img src="image_10.png" /><figcaption>**Figure 6.** (a) and (e) STEM-HAADF image of the cross-section of a core/shell GaAs NW with a TiO₂ protective shell. (b) and (f) Ga core, (c) and (g) Ti shell. (d) and (h) superimposed EELS maps of the Ga core (green) and Ti shell (red). (e) – (h) maps were obtained from the yellow rectangle in (a). Scale bars in (a)-(d) are 100 nm. Scale bars in (e)-(h) are 5 nm.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** (a) LSV measurements performed at 10 mV/s in 0.1 M potassium phosphate buffer (pH 7) under nitrogen, irradiated with chopped light (1 sun, 1.5 AM filter), showing the current density</figcaption></figure>
of the p-n junction samples, with and without the 10 nm thick TiO₂ protective shell. (b) Chronoamperometric measurements recorded for 13 hours at -0.18 V vs RHE in 0.1 M potassium phosphate buffer (pH 7) under simulated 1 sun illumination for sample with and without TiO₂ protective shell. The dark blue and purple curves on (a) and (b) correspond to the sample with and without the TiO₂ protective shell, respectively.

<figure><img src="image_12.png" /></figure>

**Figure 8.** (a) STEM-HAADF image of the cross-section of a GaAs NW with a TiO₂ protective shell after photoelectrocatalysis measurements. Photoelectrocatalytic HER experiments were conducted at -0.18V vs RHE under simulated 1 sun irradiation. (b) Ga core, (c) Ti shell and (d) superimposed EELS maps of the Ga core (green) and Ti shell (red). Scale bars are 100 nm.

**Associated content**

Supporting information available: Details of the FDTD simulation of the optical properties of the different samples studied, including Figures S1-S3. (DOCX)

**Author information**

**Corresponding Author**

*Address correspondence to matthieu.koepf@cea.fr (MK), matthieu.bugnet@insa-lyon.fr (MB), jose.penuelas@ec-lyon.fr (JP)

**Acknowledgments**
The authors acknowledge access to equipments at the NanoLyon platform and thank J. B. Goure for technical assistance. The authors acknowledge funding from the French Agence Nationale de la Recherche (ANR) for project BEEP 18-CE05-0017-01, Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003). The (S)TEM work was performed at the consortium Lyon-St-Etienne de microscopie. The authors are grateful to Y. Lefkir and S. Reynaud for technical assistance using the Jeol NeoARM instrument.

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<figure><img src="image_13.png" /></figure>