Block copolymers for the synthesis of pure and Bi-promoted nano-TiO _2 as active photocatalysts

摘要

Nano-TiO _2 is certainly the most studied semiconductor for environmental purposes. Here, a template synthesis using block copolymers is combined to Bi promotion to the purpose of enhancing the TiO _2 photocatalytic activity by modulating the oxide surface area and porosity as well as by slowing down the electron-hole recombination. Three block copolymers of the Pluronic family, characterized by different micelle sizes in water as determined by light scattering analysis, are employed to induce mesoporosity in nano-TiO _2. The surfactants are removed by combining UV and thermal treatments to avoid pore collapse while obtaining a good oxide crystallinity. A fine modulation of pore size and total volume is obtained by changing polymer type and concentration, effectively enhancing the ability of the oxide to promote the mineralization of methylene blue stains. The mesoporous oxides are then used as scaffolds to obtain Bi _2O _3-TiO _2 composites. X-ray diffraction, N _2 adsorption at subcritical temperatures, high-resolution transmission microscopy, Fourier transform infrared spectroscopy, and zeta potential determinations give insight on the composite structure and on the specificity of the Bi-mesoporous TiO _2 composites with respect to traditional sol-gel TiO _2 nanomaterials. All samples are tested for the photodegradation of methylene blue stains and of formic acid under dry and wet conditions, respectively. The presence of Bi promotes the photocatalytic activity of the final samples in both tested reactions (about 30 % mineralization enhancement with respect to unpromoted TiO _2). The top performing photocatalyst is the Bi _2O _3-mesoporous TiO _2, which shows the largest recombination time of photogenerated electrons and holes as determined by photocurrent measurements.