Band gap optimization of tin tungstate thin films for solar water oxidation

Abstract

Semiconducting ternary metal oxide thin films exhibit a promising application for solar energy conversion. However, the efficiency of the conversion is still limited by a band gap of a semiconductor, which determines an obtainable internal photovoltage for solar water splitting. In this report the tunability of the tin tungstate band gap by O2 partial pressure control in the magnetron co-sputtering process is shown. A deficiency in the Sn concentration increases the optical band gap of tin tungstate thin films. The optimum band gap of 1.7 eV for tin tungstate films is achieved for a Sn to W ratio at unity, which establishes the highest photoelectrochemical activity. In particular, a maximum photocurrent density of 0.375 mA cm−2 at 1.23 VRHE and the lowest reported onset potential of −0.24 VRHE for SnWO4 thin films without any co-catalyst are achieved. Finally, we demonstrate that a Ni protection layer on the SnWO4 thin film enhances the photoelectrochemical stability, which is of paramount importance for application.