Direct water splitting by surface-conditioned n-GaP (100) photoanodes

12.06.2019 von 13:00 bis 14:00

Abstract

III-V semiconductors are candidates for photoelectrochemical (PEC) water splitting but they are prone to corrosion and suffer from corrosion-related decrease of efficiency. 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 principle, GaP can be used as photocathode and photoanode. We have studied the structural and chemical surface modifications of n-GaP(100) photoanodes before and after extended PEC treatment by scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). A 2±0.5 nm thin stable oxide film is produced at the surface of n-GaP(100) photoanode via oxidizing the surface at 0.8 V vs RHE (reversible hydrogen electrode) and subsequent hydrogenation to passivate the electrically active defects in the oxide film. After appropriate surface-conditioning, direct water splitting is demonstrated without any applied potential.

Indirect charge transfer from photoanode to electrolyte via interface states in oxide.

Direct water splitting by surface-conditioned n-GaP (100) photoanodes

During direct water splitting, charge transfer across n-GaP(100) photoanode-electrolyte interface is analyzed with electrochemical impedance spectroscopy (EIS). The Nyquist plots, measured with EIS, are reproduced with simulation of an equivalent electrical circuit. It is determined that during water splitting, the charge transfer from V.B of the photoanode to the electrolyte via interface states in the oxide. It is also determined that the direct water splitting is a 6-step charge transfer process, including 4-step water oxidation reaction.

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