Photophysical, photoelectrochemical, and photocatalytic properties of novel SnWO₄ oxide semiconductors with narrow band gaps

Novel SnWO₄ visible-light active photocatalysts with two polymorphs (orthorhombic a 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, a-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 a- 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 orbitais 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 a-SnW04 phase, which has larger W-O bond lengths in the WO ₆ 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 TiON_x (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.