Sn2+-based pyrochlore oxysulfides with narrow band gaps for photocatalytic water splitting

Abstract

Efficient solar-driven production of hydrogen requires the development of semiconductors with narrow band gaps that can absorb a wider range of visible light, while still maintaining the appropriate band levels for both water reduction and oxidation processes. A few Sn2+-based oxides (e.g., SnNb2O6 and Sn2Nb2O7) have attracted attention as photocatalysts due to their ability to respond to visible light, attributed to the strong interaction between the 5s orbital of Sn2+ and the O-2p orbital. However, their band gaps are limited around 2.3 eV (∼540 nm). Here, we focused on Sn2+-based oxysulfides Sn2Nb2O7-xSx, which are sulfur-substituted Sn2Nb2O7 pyrochlore compounds. These compounds exhibit a wider range of visible light absorption due to their smaller band gap (∼2.1 eV). However, their band levels and photocatalytic activity have not been explored yet. In this study, Sn2Nb2O7-xSx samples were synthesized by a solid-state reaction between SnO, Nb2O5, and SnS in which the band gaps shift toward longer wavelength (from 525 to 600 nm) almost linearly with increasing sulfur content. This can be attributed to a negative shift of the valence band maxima due to the increased contribution of S-3p orbitals, which have higher energy compared to O-2p orbitals. The obtained oxysulfides possess appropriate band levels for both water reduction and oxidation. Thus, the photocatalysts were observed to function for both water reduction and oxidation in the presence of sacrificial reagents under visible light, without noticeable decrease in activity. The H2 and O2 evolution rates of these samples increased as x increased up to x = 0.4, which corresponds to a sulfur concentration of approximately 0.26 (i.e., the measured values of 2 × S/Sn).