Controlled Synthesis of Vertically Aligned SnO₂ Nanograss-Structured Thin Films for SnO₂/BiVO₄ Core–Shell Heterostructures with Highly Enhanced Photoelectrochemical Properties

Fabrication of semiconductor thin films with uniform and vertically aligned one-dimensional nanostructures is an active area of research. We report the synthesis of vertically aligned nanograss (NG)-structured SnO2 thin films on a wide range of substrates with a vapor–solid deposition process. In this process, some chemical and physical parameters, such as chemical composition, deposition height from the precursor mixture, deposition temperature, and substrate roughness, are found to play key roles during the growth of SnO2 nanograsses (SNGs). The effects of density change and cross-sectional dimension (width) of the nanograsses (NGs) on surface area improvement of the thin films have been examined by varying the respective parameters. BiVO4 (BV) solution layers were coated onto SNG, forming core–shell type-II heterojunction thin films (SNG-BV). The thickness of the drop-cast BiVO4 solution layers onto the NGs was controlled by the number density of the NGs per unit area. Light absorption efficiency (ηabs) of the core–shell SNG-BV films has been optimized by controlling quasi-arranged periodicity of the core NGs and accessible shell thickness of BiVO4 layers. The charge separation efficiency (ηsep) of SNG-BV films strongly depends on the thickness of the BiVO4 layers onto NGs. Thin layers of BiVO4 coating along the axial direction of thinner SnO2 NGs (25–50 nm) shows enhanced ηsep but lower ηabs due to poor light absorption. On the other hand, the thicker core NGs (40–200 nm) with low surface area provide thick layers of BiVO4, which drives strong light absorption but suffers from efficient ηsep. However, intermediate layers of BiVO4 onto uniformly arranged SnO2 NGs with 30–70 nm width shows enhanced ηabs as well as efficient ηsep compared to other SNG-BV samples. This result demonstrates that control over the horizontal dimension of the core materials in the core–shell heterojunction (keeping vertical restriction) is a viable approach for optimizing the photoelectrochemical efficiency.