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 (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.