Effect of HCl and H_2SO_4 Treatment of TiO_2 Powder on the Photosensitized Degradation of Aqueous Rhodamine B Under Visible Light

摘要

The acid treatments of TiO_2 nanopowder with HCl or H_2SO_4 solution increase the concentration of the hydroxyl group on TiO_2 surfaces compared to bare TiO_2, which acts as a Bronsted acid site. For the case of the HCl-treated TiO_2, the dissociation of Bronsted acid (proton donor) sites on TiO_2 leads to a drop in the pH levels of rhodamine B (RhB) dye solutions (leading to the protonation of the RhB molecule), which allows the physisorption of the uncharged carboxyl acid group on the positively charged TiO_2 surface. The carboxyl acid group is believed to afford a more efficient charge injection from the Visible-light-excited RhB to the conduction band of TiO_2 compared to the N-ethyl group, yielding a significantly enhanced photodegradation of RhB mainly via the N-de-ethylation pathway. For the case of the H_2SO_4-treated TiO_2, although the dissociation of Bronsted acid sites on TiO_2 is also achieved, its photoactivity is much lower than that of the HCl-treated TiO_2. It seems that the presence of SO_4~(2-) on the H_2SO_4-treated TiO_2 behaves as an ~·OH scavenger to prevent the photodegradation of the dye.