Interfacial study of Cu2O/Ga2O3/AZO/TiO2 photocathode for water splitting fabricated by pulsed laser deposition

Abstract

Cuprous oxide (Cu₂O) is expected to be one of the most promising materials for utilizing solar energy. One strategy comprises using Cu₂O as a photocathode to obtain hydrogen by the reduction of water. However, the instability of Cu₂O in solution hinders its employment to a great extent. Here, we fabricated a multilayer photocathode with the structure of Cu₂O/Ga₂O₃/AZO/TiO₂ by the pulsed laser deposition (PLD) technique. The highest photocurrent density was increased to 6.5 mA cm⁻² in comparison to 4.5 mA cm⁻² for bare Cu₂O at 0 V versus a reversible hydrogen electrode (RHE), and the stability was also improved to some extent. The properties of Ga₂O₃ were affected to a great extent by the oxygen pressure, which thereby changed the photovoltage and stability of the photocathode. Alignment of band energies is introduced to explain this phenomenon. In a sample containing Ga₂O₃ under a low oxygen pressure, the relatively lower Fermi level and conduction band edge in the Ga₂O₃ layer formed a region of electron accumulation near the interface, which impaired its performance. In addition, the interfacial chemical state of copper element was investigated by X-ray photoelectron spectroscopy (XPS) in combination with Ar ion etching. It was revealed that for both samples Cu⁺ was reduced to Cu⁰ during the fabrication process near the Cu₂O/Ga₂O₃ interface, which is harmful for stability. These factors resulted in the relatively unstable performance of the photocathode. This work reveals the performance of a multilayer photocathode grown by PLD and points out possible ways of improving it.