Engineered Sn doped TiO2 bifunctional catalyst for improved hetero-photocatalytic oxidation of arsenite in water

Abstract

Millions of people worldwide are exposed to arsenic contaminated water. TiO2 photocatalytic oxidation (PCO) of As(III) to the less toxic As(V) which can be easily removed is a challenging process due to several reasons particularly the bandgap limitation and the poor quantum efficiency. Here, a procedure was developed to accurately determine the oxidation rates of As(III) which is challenging. The calculated photon and energy absorption by TiO2 were 36.7 and 44.9 %, respectively. Finally, the quantum efficiency (QE) as a potential measure of the PCO was determined. SnO2@Sn-TiO2 and Sn-TiO2@SnO2 photocatalysts were synthesised and characterised using XRD, Raman, XPS, DRS, LEIS, EDX, HR-TEM, SEM, S-TEM and FTIR. These analyses revealed that at low doping ratios, SnO2 formed the shell of the catalyst (SnO2@Sn-TiO2), whereas higher Sn doping ratio developed a Sn-TiO2 shell alongside a core of SnO2 (Sn-TiO2@SnO2). Increasing Sn-doping resulted in a shift in the conduction and valence band edge energies accompanied with a reduction in the bandgap. The smallest bandgap in 1Sn:1TiO2 (2.87 eV) is linked to the improved electron-hole separation. Sn-TiO2@SnO2 and SnO2@Sn-TiO2 improved the PCO of TiO2 by ∼94 % to 145 %. For the first time, the PCO rate was accurately determined by mounting both adsorbed As(V) (using XPS) and aqueous As(V) (spectrophotometry). TAS experiments showed a long-lived charge carriers in 1Sn:1TiO2. The initial transient absorption signal was ~2 times higher than anatase TiO2, suggesting early timescale (pre-μs) suppression in the electron-hole recombination in 1Sn:1TiO2. The scavenger studies show that the hydroxyl radical (OH.) is the main oxidant. Lastly, the controls of experimental parameters on the PCO of As(III) over 1Sn:1TiO2 were studied. These controls include the distance between the light source and the solution surface which shows a high effect on the photon flux reaching the solution, the QE and PCO rate.