Increased photocurrent of CuWO4 photoanodes by modification with the oxide carbodiimide Sn2O(NCN)

Abstract

Tin(II) oxide carbodiimide is a novel prospective semiconductor material with a band gap of 2.1 eV and lies chemically between metal oxides and metal carbodiimides. We report on the photochemical properties of this oxide carbodiimide and apply the material to form a heterojunction with CuWO₄ thin films for photoelectrochemical (PEC) water oxidation. Mott–Schottky experiments reveal that the title compound is an n-type semiconductor with a flat-band potential of −0.03 V and, as such, the position of the valence band edge would be suitable for photochemical water oxidation. Sn₂O(NCN) increases the photocurrent of CuWO₄ thin films from 32 μA cm⁻² to 59 μA cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE) in 0.1 M phosphate buffer (pH 7.0) under backlight AM 1.5G illumination. This upsurge in photocurrent originates in a synergistic effect between the oxide and oxide carbodiimide, because the heterojunction photoanode displays a higher current density than the sum of its individual components. Structural analysis by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveals that Sn₂O(NCN) forms a core–shell structure Sn₂O(NCN)@SnPO_x during the PEC water oxidation in phosphate buffer. The electrochemical activation is similar to the behavior of Mn(NCN) but different from Co(NCN).