Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO has been widely investigated and considered as the most promising alternative photocatalyst to TiO₂. Herein, we present a review on the fabrication methods, growth mechanisms and photocatalytic applications of hierarchical ZnO nanostructures. Various synthetic strategies and growth mechanisms, including multistep sequential growth routes, template-based synthesis, template-free self-organization and precursor or self-templating strategies, are highlighted. In addition, the fabrication of multicomponent ZnO-based nanocomposites with hierarchical structures is also included. Finally, the application of hierarchical ZnO nanostructures and nanocomposites in typical photocatalytic reactions, such as pollutant degradation and H₂ evolution, is reviewed.

Graphene/metal oxide-based materials have been demonstrated as promising candidates for gas sensing applications due to the enhanced sensing performance and synergetic effects of the two components. Plenty of metal oxides such as SnO₂, ZnO, WO₃, etc. have been hybridized with graphene to improve the gas sensing properties. However, graphene/metal oxide nanohybrid- based gas sensors still have several limitations in practical application such as the insufficient sensitivity and response rate, and long recovery time in some cases. To achieve higher sensing performances of graphene/metal oxides nanocomposites, many recent efforts have been devoted to the controllable synthesis of 3D graphene/metal oxides architectures owing to their large surface area and well-organized structure for the enhanced gas adsorption/diffusion on sensing films. This review summarizes recent advances in the synthesis, assembly, and applications of 3D architectured graphene/metal oxide hybrids for gas sensing.

Boehmite (γ-AlOOH) nanocrystals with varying morphologies were synthesized by hydrothermal treatment of the freshly precipitated Al(OH)₃ gel at 240 °C for 16 h in different solutions. Under acidic conditions (initial pH 4.0), boehmite nanorods with an aspect ratio of about 2 and 4, respectively, were formed in the presence of nitrate and chloride, and boehmite nanowires with an aspect ratio of about 80 were formed in the case of sulfate. Under alkaline conditions (initial pH 10.5), boehmite nanoplates with a size of about 100 nm were produced, irrespective of the anion types. Further analyses indicated that the adsorption of the anions on a boehmite surface may be responsible for the shape formation of boehmite. Under acidic conditions the adsorption order was as follows: nitrate < chloride < sulfate, in agreement with the preferential growth tendency order: nitrate < chloride < sulfate, while little anions can be adsorbed under alkaline conditions. The selective adsorption of anions on the (010) and (001) facets of boehmite is suggested to be responsible for the preferential growth of boehmite along [100] direction.