Role\nof the Sn-TiO<sub>2</sub>/Ti-SnO<sub>2</sub> Heterojunction\nin Enhancing the Photocatalytic Oxidation of Arsenite (As<sup>III</sup>) through the Promotion of Charge Carrier Lifetime

This study proposes\nthe heterojunction photocatalyst, Sn-doped\nTiO<sub>2</sub>/Ti-doped SnO<sub>2</sub> (herein named Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub>), as a promising alternative to pure\nTiO<sub>2</sub>. Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub> demonstrates\nimproved light harvesting efficiency over TiO<sub>2</sub> by generating\nlonger-lived electron–hole (e<sub>CB</sub><sup>–</sup>―h<sub>VB</sub><sup>+</sup>) pairs, while also displaying\na smaller band gap compared to pure TiO<sub>2</sub>. Consequently,\nwe show that it is a promising candidate for the photocatalytic oxidation\n(PCO) of As<sup>III</sup> to the less toxic and more readily removable\nform As<sup>V</sup>. Transient absorption spectroscopy (TAS) shows\nincreased e<sub>CB</sub><sup>–</sup>―h<sub>VB</sub><sup>+</sup> recombination half-lives from ∼0.5 ms in TiO<sub>2</sub> to ∼1 ms in Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub>. The\ninitial transient absorption signal for Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub> is twice that of pure TiO<sub>2</sub>, suggesting early\ntime scale (pre-μs) suppression of (e<sub>CB</sub><sup>–</sup>―h<sub>VB</sub><sup>+</sup>) recombination. Moreover, TAS\nshowed that Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub> possesses more\nreactive charge carriers than TiO<sub>2</sub> under reactions with\nchemical scavengers. For the first time, TAS experiments were conducted\nusing both a colorimetric indicator (molybdate) and As<sup>III</sup> to determine the PCO kinetics from As<sup>III</sup> to As<sup>V</sup>. The TAS molybdateAs<sup>III</sup> experiment results indicate\nthat the oxidation process occurs on the sub-microsecond time scale,\nwith a notable increase in absorption at ∼700 nm, providing\nevidence of the formation of the As<sup>V</sup>molybdate blue\ncomplex. PCO experiments showed that <sup>•</sup>OH radicals\nplayed the predominant role during PCO, followed by superoxide radicals\n(O<sub>2</sub><sup>•</sup>). <sup>•</sup>OH\nscavengers including isopropanol, rebamipide anhydrous, and dimethyl\nsulfoxide (DMSO) reduce the PCO yield of As<sup>III</sup> to 21, 30,\nand 23%, respectively. While O<sub>2</sub><sup>•</sup> scavengers including superoxide dismutase (SOD) and <i>p</i>-benzoquinone suppressed the PCO yield of As<sup>III</sup> to a lesser\ndegree, with yields of 35 and 49% seen, respectively. The effects\nof irradiance intensity, salinity, pH, As<sup>III</sup> concentration,\nand photocatalyst mass on both the quantum efficiency (QE) and PCO\nkinetics were investigated. The Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub> catalyst exhibited effective recyclability, validating its\neconomical reusability. Overall, the study demonstrates the potential\nof the Sn<sub>1</sub>Ti<sub>1</sub>O<sub>2</sub> heterojunction photocatalyst\nfor the PCO of As<sup>III</sup> to the less toxic As<sup>V</sup> in\nwater treatment, showing faster oxidation kinetics and improved charge\nseparation compared to pure TiO<sub>2</sub> as proven by TAS.