Role of the Sn-TiO/Ti-SnO Heterojunction in Enhancing the Photocatalytic Oxidation of Arsenite (As) through the Promotion of Charge Carrier Lifetime.

This study proposes the heterojunction photocatalyst, Sn-doped TiO/Ti-doped SnO (herein named SnTiO), as a promising alternative to pure TiO. SnTiO demonstrates improved light harvesting efficiency over TiO by generating longer-lived electron-hole (e-h) pairs, while also displaying a smaller band gap compared to pure TiO. Consequently, we show that it is a promising candidate for the photocatalytic oxidation (PCO) of As to the less toxic and more readily removable form As. Transient absorption spectroscopy (TAS) shows increased e-h recombination half-lives from ∼0.5 ms in TiO to ∼1 ms in SnTiO. The initial transient absorption signal for SnTiO is twice that of pure TiO, suggesting early time scale (pre-μs) suppression of (e-h) recombination. Moreover, TAS showed that SnTiO possesses more reactive charge carriers than TiO under reactions with chemical scavengers. For the first time, TAS experiments were conducted using both a colorimetric indicator (molybdate) and As to determine the PCO kinetics from As to As. The TAS molybdate─As experiment results indicate that the oxidation process occurs on the sub-microsecond time scale, with a notable increase in absorption at ∼700 nm, providing evidence of the formation of the As─molybdate blue complex. PCO experiments showed that OH radicals played the predominant role during PCO, followed by superoxide radicals (O). OH scavengers including isopropanol, rebamipide anhydrous, and dimethyl sulfoxide (DMSO) reduce the PCO yield of As to 21, 30, and 23%, respectively. While O scavengers including superoxide dismutase (SOD) and -benzoquinone suppressed the PCO yield of As to a lesser degree, with yields of 35 and 49% seen, respectively. The effects of irradiance intensity, salinity, pH, As concentration, and photocatalyst mass on both the quantum efficiency (QE) and PCO kinetics were investigated. The SnTiO catalyst exhibited effective recyclability, validating its economical reusability. Overall, the study demonstrates the potential of the SnTiO heterojunction photocatalyst for the PCO of As to the less toxic As in water treatment, showing faster oxidation kinetics and improved charge separation compared to pure TiO as proven by TAS.