Particle Size and Structural
Control of ZnWO<sub>4</sub> Nanocrystals via Sn<sup>2+</sup> 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 Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> doping in ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> ions
were homogeneously incorporated in the ZnWO<sub>4</sub> host lattice,
leading to a monotonous lattice expansion, and part of Sn<sup>2+</sup> ions were expelled at surface sites for decreased crystallinity
and particle size reduction. By Sn<sup>2+</sup> doping, ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> and Zn<sup>2+</sup>, lattice variation, and
particle size reduction. Meanwhile, the BET surface areas were also
greatly enlarged from 40.1 to ∼110 m<sup>2</sup>·g<sup>–1</sup>. Contrary to the theoretical predictions of the quantum
size effect, Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> doping, the photocatalytic performance
of Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> nanocrystals was optimized
at Sn<sup>2+</sup> doping level of 0.451