As a multifunctional material with strong redox ability, stable chemical properties, wide source and environmental friendliness, nano-TiO2 is considered as one of the most promising semiconductor photocatalytic materials. Among various morphologies of nano-TiO2, hollow-structured TiO2 microspheres have aroused increasing interests owing to their low density, large specific surface area, favorable permeability and high stability. Undoubtedly, it is of great significance to seek an ideal approach with simple process, favorable repeatability and controllable morphology for hollow nano-TiO2 microsphere preparation. According to the preparation principle, preparation methods of hollow nano-TiO2 microspheres can be divided into sol-gel method, hydrothermal method, solvothermal method, spray drying method and layer self-assembly method. According to whether the template is added in the preparation process, these methods can also be classified into hard template method, soft template method and template-free method. In this article, preparation approaches of hollow nano-TiO2 microspheres with and without template are reviewed. Among them, the hard template is regarded as the earliest method applied for the preparation of hollow TiO2 microspheres. Concerning the hard template method, the morphology, cavity size and surface charge of the obtained hollow nano-TiO2 are closely related to the types of adopting templates. Currently, there are mainly three kinds of hard template agents commonly used by researchers, including polymers, carbon spheres and inorganic oxides. Howe-ver, it is time consuming and a waste of organic solvent to prepare template agent, which resulted in an increase in production cost and environmental pollution. The soft template method is considered as one of the most efficient approach to prepare hollow nano-TiO2, which shares similar preparation principle with hard template method. The major distinction between hard template method and soft template method lies in the selection of template agents. Specifically speaking, the former usually employ rigid inorganic particles as template agents, and the latter commonly adopt low-intensity gas or liquid like emulsion droplets, supramolecular micelles, polymer aggregates or vesicles as template agents. Particularly, soft template method is superior to hard template method because of its convenience for removing the template agent in the later stage (no high temperature treatment is required). Therefore, soft template method features high efficiency and simple process. The template-free method is one of the most promising preparation methods for hollow nano-TiO2 microspheres. Unfortunately, the one-step reaction of template-free method leads to poor controllability for its products, which blocks the practical application, not to mention mass production. Nevertheless, thanks to the simple preparation steps, low cost and high yield of template-free method, it still exhibits great potential in batch production and large-scale preparation of hollow TiO2 microspheres. At present, in addition to the effective and mature preparation process, numerous efforts have been made by researchers in pursuit of high-efficiency photocatalytic performance of hollow nano-TiO2 microspheres. It is believed that the photocatalytic properties of hollow nano-TiO2 can be further improved through the following three attempts. Firstly, the combination of a plurality of semiconductor materials will contribute to improving the response area under visible light. Secondly, non-metal anions (nitrogen, carbon) or metal (iron, copper) ion doping is beneficial to the separation efficiency of photoinduced titanium dioxide electron-hole pairs. Thirdly, surface modification of metal oxide and in-situ polymerization modification are conducive to reduce the recombination of electron-hole pairs. It is worth pointing out that prolonging the lifetime of photogenerated car-riers and increasing photocatalytic activity will become the research focus of hollow TiO2 microspheres in the future. © 2019, Materials Review Magazine. All right reserved.
