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 micmporous 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 m(2)/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 O-1(2) and O-center dot(2)-.