Structural Insight on Defect-Rich Tin Oxide for Smart Band Alignment Engineering and Tunable Visible-Light-Driven Hydrogen Evolution.

Abstract

Herein, a series of defect-rich tin oxides, SnxOy, were synthesized with tunable Sn2+/Sn4+ composition ratio and defect chemistry, aiming to explore the impact of local structural modulation, non-stoichiometric chemistry, and defective center on the modulation of band gap values, band edge potential positions, and photocatalytic hydrogen evolution performance. The phase structure, morphology, surface component, and photoelectric properties were analyzed by multiple testing methods. The modulation of the Sn2+/Sn4+ molar ratio was analyzed by X-ray photoelectron spectroscopy and the spectra of Mossbauer and electron spin resonance, which indicated the existence of interstitial tin and oxygen vacancy, predicting a highly disordered local structure. In addition, the photocatalytic activity was evaluated by water splitting for hydrogen production under visible light. The optimal photocatalytic activity toward H2 production rate reached 58.6 μmol·g-1·h-1 under visible light illumination. However, the photocatalytic activity gradually decreased with an increase of synthetic temperature. Much higher Sn2+/Sn4+ molar ratio in the present defective tin oxide gave rise to more negative band edge potentials for hydrogen production. Meanwhile, the driving force was decreased with the diminished Sn2+. Large amounts of hydroxyl groups, Sn2+, and relatively negative potential of conduction band in non-stoichiometric SnxOy play critical roles in visible light harvesting and photocatalytic water splitting. Furthermore, the relationships among crystal structure, electronic properties, and photocatalytic activities were clarified by theoretical calculation. This work provides a novel strategy for the development of highly efficient photocatalytst by regulating the internal electronic structure and surface defects.