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