Hydrothermal reduction of commercial P25 photocatalysts to expand their visible-light response and enhance their performance for photodegrading phenol in high-salinity wastewater

Abstract This study aimed to expand the visible-light response of commercial titanium dioxide (TiO2, P25) photocatalysts using surface modification by hydrothermal reduction with various reductants to improve their photocatalytic performance for phenol degradation in high-salinity wastewater under room visible-light irradiation. Structure characterization results for different reduced P25 photocatalysts show that hydrothermal reduction cannot only generate disordered crystalline shell–core structures on the catalyst surface but also introduce several Ti3+ into the photocatalysts, facilitating the visible-light response of reduced P25. The moderate conditions of hydrothermal reduction not necessarily destroyed the inner crystalline structure of P25 to form amorphous TiO2 as deep trapping sites for photogenerated charges, different from the results for the reduced P25 obtained by hydrogeneration reduction (P25–H2). Therefore, the reduced P25 obtained by hydrothermal reduction with Zn as reductant featured a slightly higher photocatalytic activity for phenol than P25–H2 under visible-light irradiation, although P25–H2 exhibited a smaller Eg and stronger visible-light absorbance. The highest removal rate for phenol of reduced P25 within 5 h reached up to 87.5% and considerably exceeded that of P25–H2 (60.0% removal rate of phenol). Apart from the reduction capability of metals, the solubility of the corresponding metal hydroxide plays a major role in P25 reduction in the hydrothermal process. The water-insoluble metal hydroxide precipitates on the metal surface and inhibits the reduction between metal and P25, thereby decreasing the photocatalytic performance of reduced P25 for phenol excited by room visible-light.