Photolytic Water Splitting By Surface-Conditioned n-Gallium Phosphide(100) Photoanodes

Abstract

The production of hydrogen via photoelectrochemical (PEC) water splitting using III-V semiconductors as photoelectrodes is a field of current research and challenges in materials science. Under PEC conditions relevant to water splitting, III-V semiconductors are prone to corrosion and suffer from corrosion-related decrease of efficiency, which so far impedes long-term usage of III-V semiconductor-based photoelectrodes. Gallium phosphide (GaP) has an indirect band gap of 2.26 eV which covers both the hydrogen evolution potential (HEP) and the oxygen evolution potential (OEP). Thus, in principal, GaP can be used both as photocathode and photoanode. Notwithstanding the favorable band gap energy, the use of GaP photoelectrodes for the photolysis of water has so far not been successful without applying an additional bias potential. In this contribution we demonstrate that, by specific surface conditioning, we are able to control and optimize the photoelectrochemical surface properties of photoanodes prepared from n-doped GaP(100) for PEC water splitting. We have studied the structural and chemical surface modifications of such photoanodes before and after extended PEC processes by scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). An approximately (2±1) nm thin oxide film is produced at the surface of n-GaP(100) via oxidizing the surface at 0.8 V vs RHE (reversible hydrogen electrode) and subsequent hydrogen evolution at very cathodic potentials. The latter step is crucial for a substantial reduction of the potential for water splitting, presumably because the evolved hydrogen passivates electrically active defects in the oxide film. After appropriate surface conditioning, photolytic water splitting was observed without applied voltage or addition of catalysts, as confirmed by detection of hydrogen gas evolving at the Pt counter cathode with a gas spectrometer. The n-GaP(100) photoanode was exposed to high-intensity illumination (120 mW/cm2) at 0 V vs RHE in 0.02 M HCl electrolyte for a duration of 3 days while yielding stable photocurrents. Ex-situ surface characterization of the n-GaP(100) photoanodes was carried out after extended PEC water splitting. AFM revealed a very flat surface, and no signs of corrosion were observed even after extended periods of water splitting. AES depth profiles showed that phosphorous was depleted near the surface of the n-GaP(100) photoanode. Using AES spectra, the thickness of the oxide film was estimated. XPS measurements revealed that the surface oxide consists mainly of Ga2O3, while small concentrations of GaPO4, P2O5 and also metal-like Ga are also present at the topmost surface layers of the n-GaP(100) photoanode. Electrochemical impedance spectroscopy (EIS) was carried out to understand the charge transfer processes across the semiconductor/electrolyte interface. Our EIS results suggest that defect states in the surface oxide and metal-like Ga at the very surface in contact with the electrolyte play crucial roles in the effective charge transfer across the semiconductor-electrolyte interface. Our surface conditioning processes lead to the formation of a nonporous, thin and stable Ga surface oxide which inhibits PEC surface corrosion and, at the same time, subserves the water splitting process.