Engineered Sn-TiO2@SnO2 and SnO2@Sn-TiO2 heterophotocatalysts for enhanced As(III) remediation: a comprehensive bulk and surface characterization and precise photocatalytic oxidation rates determination

Abstract

Arsenite, As(III), is a highly toxic form of arsenic that poses a significant risk to human health if present in drinking water. Oxidation of As(III) to the less toxic As(V) using TiO2 as photocatalyst is an attractive solution in water treatment applications but challenged the high bandgap energy. In this study, we investigate the potential of doping TiO2 with Sn to reduce the bandgap and hence to improve the photocatalytic oxidation (PCO). To this end, we studied first the effect of varying Sn:TiO2 molar doping on the structure of the newly synthesized SnO2@TiO2 and Sn-TiO2@SnO2 hetero photocatalysts. We found that at low Sn:TiO2 doping ratios (0.1Sn:1TiO2), SnO2 tends to float on the surface and form a coat around the TiO2 (SnO2@Sn-TiO2), whereas at higher doping ratio (1Sn:1TiO2) a Sn-TiO2 coat forms alongside SnO2 clusters in the core of the catalyst (Sn-TiO2@SnO2). We assessed the PCO and observed significant shifts to lower conduction and valence band edge energies and a reduction of the bandgap at higher doping ratios. The smallest bandgap was 2.87 eV with a doping ratio of 1Sn:1TiO2. Sn-TiO2@SnO2 and as SnO2@Sn-TiO2 improved the PCO of TiO2 by ∼30 and 46 %, respectively. We finally determined the rate constant (k) for the As(III) oxidation using a combination of spectrochemical and surface sensitive techniques and determined for a 1Sn:1TiO2 (i.e. Sn-TiO2@SnO2) catalyst a value of 0.055 ±0.002 min−1, i.e., 78 folds faster than using only TiO2. We conclude that Sn doping of TiO2 is a very promising approach for improving the PCO of As(III) in water treatment.