Role of the Sn-TiO<sub>2</sub>/Ti-SnO<sub>2</sub> Heterojunction in Enhancing the Photocatalytic Oxidation of Arsenite (As<sup>III</sup>) through the Promotion of Charge Carrier Lifetime
Hany Fathy Heiba, Jay C. Bullen, Andreas Kafizas, Camille Petit, Daqian Jiang, Dominik Weiß
Abstract
High Resolution Image Download MS PowerPoint Slide This study proposes the heterojunction photocatalyst, Sn-doped TiO 2 /Ti-doped SnO 2 (herein named Sn 1 Ti 1 O 2 ), as a promising alternative to pure TiO 2 . Sn 1 Ti 1 O 2 demonstrates improved light harvesting efficiency over TiO 2 by generating longer-lived electron–hole (e CB – ―h VB + ) pairs, while also displaying a smaller band gap compared to pure TiO 2 . 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 TiO 2 to ∼1 ms in Sn 1 Ti 1 O 2 . The initial transient absorption signal for Sn 1 Ti 1 O 2 is twice that of pure TiO 2, suggesting early time scale (pre-μs) suppression of (e CB – ―h VB + ) recombination. Moreover, TAS showed that Sn 1 Ti 1 O 2 possesses more reactive charge carriers than TiO 2 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 (O 2 •─ ). • OH scavengers including isopropanol, rebamipide anhydrous, and dimethyl sulfoxide (DMSO) reduce the PCO yield of As III to 21, 30, and 23%, respectively. While O 2 •─ scavengers including superoxide dismutase (SOD) and p -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 Sn 1 Ti 1 O 2 catalyst exhibited effective recyclability, validating its economical reusability. Overall, the study demonstrates the potential of the Sn 1 Ti 1 O 2 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 TiO 2 as proven by TAS.