Unprecedented porosity transformation of hierarchically porous TiO₂ derived from Ti-Oxo clusters

Although attaining a high porosity in TiO₂ is critical to enhancing its photocatalytic and photoelectric activities, its synthesis has been challenging owing to the high reactivity of conventional Ti precursors and the laborious template removal process. Thus, we herein report a versatile method for preparing hierarchically porous organic-functionalized TiO₂ (HiPOTs) using Ti-oxo clusters consisting of a rigid reactive ligand, para-aminobenzoate (p-ABA). The presence of p-ABA as a structure-directing template is crucial to obtain microporous structures with sufficiently high yields. The HiPOTs gradually transform from hierarchically micro/mesoporous structures into mesoporous structures during a sol–gel process. The Brunauer–Emmett–Teller surface areas of the HiPOTs range from 242 to 739 m²/g, which are among the highest reported for porous TiO₂ materials. The presence of p-ABA on the HiPOT surface decreases the band gap of TiO₂ to 2.7 eV, and prolonging the sol–gel process releases greater quantities of p-ABA, thereby increasing the band gap and the crystallinity of the anatase phase. Interestingly, unlike conventional TiO₂, which experiences rapid charge recombination, the Ti³⁺ oxidation states of HiPOTs are successfully isolated during UV irradiation and can be applied as a proof of concept to generate reactive oxygen species such as ¹O₂ and ^•O₂⁻.