Role of the Sn-TiO₂/Ti-SnO₂ Heterojunction in Enhancing the Photocatalytic Oxidation of Arsenite (As^III) through the Promotion of Charge Carrier Lifetime

This study proposes the heterojunction photocatalyst, Sn-doped TiO₂/Ti-doped SnO₂ (herein named Sn₁Ti₁O₂), as a promising alternative to pure TiO₂. Sn₁Ti₁O₂ demonstrates improved light harvesting efficiency over TiO₂ by generating longer-lived electron-hole (e_CB^--h_VB⁺) pairs, while also displaying a smaller band gap compared to pure TiO₂. Consequently, we show that it is a promising candidate for the photocatalytic oxidation (PCO) of As^III to the less toxic and more readily removable form As^V. Transient absorption spectroscopy (TAS) shows increased e_CB^--h_VB⁺ recombination half-lives from ∼0.5 ms in TiO₂ to ∼1 ms in Sn₁Ti₁O₂. The initial transient absorption signal for Sn₁Ti₁O₂ is twice that of pure TiO₂, suggesting early time scale (pre-μs) suppression of (e_CB^--h_VB⁺) recombination. Moreover, TAS showed that Sn₁Ti₁O₂ possesses more reactive charge carriers than TiO₂ under reactions with chemical scavengers. For the first time, TAS experiments were conducted using both a colorimetric indicator (molybdate) and As^III to determine the PCO kinetics from As^III to As^V. The TAS molybdate─As^III 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^V─molybdate blue complex. PCO experiments showed that ^•OH radicals played the predominant role during PCO, followed by superoxide radicals (O₂^•─). ^•OH scavengers including isopropanol, rebamipide anhydrous, and dimethyl sulfoxide (DMSO) reduce the PCO yield of As^III to 21, 30, and 23%, respectively. While O₂^•─ scavengers including superoxide dismutase (SOD) and <i>p</i>-benzoquinone suppressed the PCO yield of As^III to a lesser degree, with yields of 35 and 49% seen, respectively. The effects of irradiance intensity, salinity, pH, As^III concentration, and photocatalyst mass on both the quantum efficiency (QE) and PCO kinetics were investigated. The Sn₁Ti₁O₂ catalyst exhibited effective recyclability, validating its economical reusability. Overall, the study demonstrates the potential of the Sn₁Ti₁O₂ heterojunction photocatalyst for the PCO of As^III to the less toxic As^V in water treatment, showing faster oxidation kinetics and improved charge separation compared to pure TiO₂ as proven by TAS.