Photophysical, Photoelectrochemical, and Photocatalytic Properties of Novel SnWO4 Oxide Semiconductors with Narrow Band Gaps

Novel SnWO4 visible-light active photocatalysts with two polymorphs (orthorhombic α and cubic β phases) were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic activities were investigated. It was found that the low-temperature phase, α-SnWO4 with corner-shared WO6 octahedra, exhibited a dark-red color and indirect band gap of 1.64 eV, whereas the high-temperature phase, β-SnWO4 with unshared WO4 tetrahedra, exhibited a light-yellow color and direct band gap of 2.68 eV. The Mott−Schottky plots obtained using a thick film electrode in 1 M NaCl electrolyte revealed the n-type semiconductive properties of the SnWO4 polymorphs; i.e., the flat-band potential values of α- and β-SnWO4 were −0.61 and −0.66 V (SCE), respectively. From the electronic band structure calculations performed using density functional theory, the Sn 5p and O 2p orbitals were hybridized to construct the valence band in both SnWO4 polymorphs. However, the constructions of the conduction band were quite different. β-SnWO4 with its shorter W−O bond lengths in the WO4 tetrahedra has a higher conduction-band potential than α-SnWO4 phase, which has larger W−O bond lengths in the WO6 octahedra and, thus, was able to produce H2 from an aqueous methanol solution under visible-light irradiation (>400 nm). Both SnWO4 polymorphs also exhibited good photocatalytic activity for the degradation of rhodamine B dye solution under visible-light irradiation (>420 nm). The photocatalytic activity of these SnWO4 polymorphs was higher than that of other visible-light active photocatalysts with much smaller particle sizes, such as nanosized WO3 (9.72 m2/g) and TiONx (112.13 m2/g). This higher photocatalytic activity of the SnWO4 polymorphs is mainly attributed to their smaller band gaps and unique band structures, resulting from their different bonding nature.