CuO/NiOx thin film–based photocathodes for photoelectrochemical water splitting

CuO/NiO_x thin film–based photocathodes for photoelectrochemical water splitting

Abstract: Copper oxides are considered to be very promising materials for promoting a hydrogen evolution reaction (HER). However, some CuO features, such as the recombination of charge carriers, electron diffusion length, and the chemical stability need to be improved. In this work, NiO_x was studied as co-catalyst to FTO/CuO, and the films were physically characterised by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The photo-electrochemical activity and stability toward the HER were evaluated by depositing films on fluorine-tin doped oxide (FTO) substrate. Although, the CuO (− 3.61 eV) and NiO_x (− 1.13 eV) conduction band positions, estimated by UV-Vis diffuse reflectance and the Mott–Schottky measurements, did not allow electronic transfer from CuO to NiO_x, the formation of a thin layer of NiO_x on CuO was beneficial for the activity of this material. The best CuO/NiO_x film exhibited a photocurrent density of − 1.02 mA cm⁻² at 0 V vs. RHE, which was higher than that for CuO (− 0.92 mA cm⁻²). A mechanism of electron transfer between CuO and NiO_x is proposed. The absorption of visible light by CuO leads to the generation of electron-hole pair. Part of the photogenerated electrons are trapped by the NiOOH present in the NiO_x layer, as demonstrated by XPS. The oxy-hydroxide is reduced to metallic Ni, and this species acts as a catalyst for the hydrogen evolution reaction. Due to the increase in interfacial pH caused by the evolution reaction of H₂, the formed metallic Ni can be regenerated to NiO_x.