Block copolymers for the synthesis of pure and Bi-promoted nano-TiO₂ as active photocatalysts

Nano-TiO₂ 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 TiO₂ 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-TiO₂. 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 Bi₂O₃–TiO₂ composites. X-ray diffraction, N₂ 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 TiO₂ composites with respect to traditional sol–gel TiO₂ 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 TiO₂). The top performing photocatalyst is the Bi₂O₃–mesoporous TiO₂, which shows the largest recombination time of photogenerated electrons and holes as determined by photocurrent measurements.