Defect-Rich Brown TiO_2–x Porous Flower Aggregates: Selective Photocatalytic Reversibility for Organic Dye Degradation

A low-temperature, Mn(II)-assisted sol-solvothermal strategy has been developed for the synthesis of positively surface-charged defective brown TiO2–x flower aggregates with porous nature. The porous structure possessed enormous surface defect states such as trivalent titanium ion (Ti3+) and oxygen vacancy (Vo) sites. The defect states present in the brown TiO2–x facilitated enhanced absorption even in the NIR region of the solar spectrum, whereas for the negatively surface-charged TiO2 sample, synthesized in the absence of Mn(II), the absorption was limited to the visible region. Obviously, band-gap narrowing occurred for brown TiO2–x as compared to the yellow TiO2 synthesized in the absence of Mn(II). Interestingly, studying the photocatalytic efficiency of these materials using a methyl orange–methylene blue (MO-MB) dye mixture model system under solar illumination revealed selective photocatalytic reversibility, with MB and MO photodegradation performed by yellow TiO2 and brown TiO2–x, respectively. This is the first report on the use of surface-charged brown TiO2–x with porous flower aggregate morphology for selective photocatalysis.