Particle Size and Structural Control of ZnWO₄ Nanocrystals via Sn2+ Doping for Tunable Optical and Visible Photocatalytic Properties

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Abstract

In this work, we report on the microwave-assisted hydrothermal synthesis of Sn2+-doped ZnWO4 nanocrystals with controlled particle sizes and lattice structures for tunable optical and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV–vis diffuse reflectance spectroscopy, and Barrett–Emmett–Teller technique. The effects of Sn2+ doping in ZnWO4 lattice on the crystal structure, electronic structure, and photodegradation of methylene orange dye solution were investigated both experimentally and theoretically. It is found that part of the Sn2+ ions were homogeneously incorporated in the ZnWO4 host lattice, leading to a monotonous lattice expansion, and part of Sn2+ ions were expelled at surface sites for decreased crystallinity and particle size reduction. By Sn2+ doping, ZnWO4 nanocrystals showed a significant XPS binding energy shift of Zn 2p, W 4f, and O 1s, which is attributed to the combination of electronegativity between Sn2+ and Zn2+, lattice variation, and particle size reduction. Meanwhile, the BET surface areas were also greatly enlarged from 40.1 to ∼110 m2·g–1. Contrary to the theoretical predictions of the quantum size effect, Sn2+-doped ZnWO4 nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of bulk and surface doping effects as well as lattice variations. With well-controlled particle size, crystallinity, and electronic structure via Sn2+ doping, the photocatalytic performance of Sn2+-doped ZnWO4 nanocrystals was optimized at Sn2+ doping level of 0.451.

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