Effect of Cu doping on the anatase-to-rutile phase transition in TiO₂ photocatalysts: Theory and experiments

This paper shows that incorporation of Cu inhibits the anatase to rutile phase transition at temperatures above 500 °C. The control sample, with 0% Cu contained 34.3% anatase at 600 °C and transitioned to 100% rutile by 650 °C. All copper doped samples maintained 100% anatase up to 600 °C. With 2% Cu doping, anatase fully transformed to rutile at 650 °C, at higher Cu contents of 4% & 8% mixed phased samples, with 27.3% anatase and 74.3% anatase respectively, are present at 650 °C. All samples had fully transformed to rutile by 700 °C. 0%, 4% and 8% Cu were evaluated for photocatalytic degradation of 1, 4 dioxane. Without any catalyst, 15.8% of the 1,4 dioxane degraded upon irradiation with light for 4 h. Cu doped TiO₂ shows poor photocatalytic degradation ability compared to the control samples. Density functional theory (DFT) studies of Cu-doped rutile and anatase show formation of charge compensating oxygen vacancies and a Cu²⁺ oxidation state. Reduction of Cu²⁺ to Cu⁺ and Ti⁴⁺ to Ti³⁺ was detected by XPS after being calcined to 650–700 °C. This reduction was also shown in DFT studies. Cu 3d states are present in the valence to conduction band energy gap upon doping. We suggest that the poor photocatalytic activity of Cu-doped TiO₂, despite the high anatase content, arises from the charge recombination at defect sites that result from incorporation of copper into TiO₂.