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

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