Tetragonal tungsten bronze type Sn(II)-based quaternary oxides: a new class of visible-light-absorbing semiconductors for photoelectrochemical water oxidation

The attractive photoelectrochemical (PEC) water splitting for hydrogen fuels always requires new semiconductors, which provide stronger visible light absorption with suitable band positions. Sn(II) complex oxides are expected to offer such new possibilities. In this work, a quaternary (Sn–Ta–W–O) Tetragonal Tungsten Bronze type Sn(II) oxide (Sn-TTB) and its photoanodes are reported for the first time. Results indicate that Sn-TTB has a bandgap of 1.85 eV with intense absorption up to 600 nm and the flat-band potential at −0.10 V vs. RHE. The IPCE of its photoanode is 10 times higher than that of the pyrochlore polymorph, owing to its better light absorption and higher carrier density. Experiments and calculations suggest strong interaction between Sn(II) ions and the W/TaO6 skeleton, facilitating the delocalization of charge carriers. Such an exploration indicates that the structural engineering of Sn(II) oxide may provide more exciting opportunities for solar energy conversion materials.