Synthesis, characterization and photocatalytic activity evaluation of TiO2 – ZnO nanocomposites: Elucidating effect of varying Ti:Zn molar ratio

Abstract

The effect of ZnO content in TiO2-ZnO nanocomposites synthesized via sol-gel route was studied by varying the Ti:Zn molar ratios from 1:0.1 (TZ-1:0.1) to 1:1 (TZ-1:1). Phase transformation of anatase TiO2 to rutile and the ultimate formation of mixed oxides at the expense of rutile TiO2 and ZnO (ZnTiO3 and Zn2TiO4) were observed to occur in a Zn concentration dependent manner at a constant calcination temperature of 600 °C for 1 h. These phase transformations were concomitent with alterations in morphology and bandgap. Zn addition into the nanocomposites facilitated visible light activity at all the Ti:Zn molar ratios studied. Superior visible light photocatalytic decolorization of methyl orange dye, was observed in nanocomposites TZ-1:0.3 (Ti:Zn of 1:0.3) and TZ-1:0.4 (Ti:Zn of 1:0.4) that contained a blend of phases, predominated by rutile TiO2 and ZnTiO3 and not by the anatase phase. Analysis of reaction rate by fitting decolorization kinetics into the Langmuir – Hinshelwood model revealed that rate of decolorization under visible irradiation was significantly higher in TZ-1:0.4 (Langmuir-Hinshelwood rate constant, kLH of 6.8 × 10−2 min-1) than in TZ-1:0.3 (kLH of 5.3 × 10−2 min-1). However, the nanocomposite TZ-1:0.3 promoted greater extent of mineralization compared to TZ-1:0.4. Addition of Zn not only helped in tailoring the bandgap of the nanocomposites for facilitating visible light activity, it also affected photocatalytic decolorization and/or mineralization of methyl orange dye under both UV and visible irradiation by controlling the unique blend of phases existing in the nanocomposite.