Defect-Rich Brown TiO2-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 (TO3+) and oxygen vacancy (V-o) 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.