Highly enhanced photocatalytic reduction of Cr(VI) on AgI/TiO₂ under visible light irradiation: Influence of calcination temperature

Highlights: • β-AgI riched AgI/TiO2 was obtained after simple heat pretreatment at 350 °C. • Greatly enhanced visible light response was observed on AgI/TiO2-350. • AgI/TiO2-350 exhibited superior photocatalytic activity and stability. • 5 times rate constant for Cr(VI) reduction was achieved. • More efficient separation and easier transfer of e⁻–h⁺ pairs were facilitated. - Abstract: AgI/TiO₂ was prepared using a dissolution-precipitation method, followed by calcination at different temperatures (100–700 °C). The as-prepared AgI/TiO₂ powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis-DRS) and electrochemical impedance spectroscopy (EIS). The results revealed that calcination temperature significantly impacted the visible light absorption of AgI/TiO₂ along with a shift from metastable γ-AgI to relatively stable β-AgI. We found that highest photocatalytic reduction rate of Cr(VI) and β-AgI content were obtained for a calcination temperature of 350 °C. Furthermore, the pseudo-first order rate constant was five times that for a photocatalyst calcined at 100 °C. The dramatically enhanced reduction rate of Cr(VI) was attributed to enhanced visible light absorption and greatly reduced charge transfer resistance, which eventually facilitates more efficient separation and easier transfer of photogenerated electron–hole pairs to the catalyst surface. Other experimental conditions were also carefully investigated and optimized with initial AgI loading percentage (5%), catalyst dosage (1.0 g/L), coexisting organics (1.0 mmol/L EDTA) and pH (1–2). The optimal AgI/TiO₂ exhibited good stability with little change in activity after 5 cycles.