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

• Samples of different mol% of copper doped titania were studied. • At 650 °C, samples with 0% and 2% Cu were comprised of 100% rutile. • 4% and 8% Cu doping the samples showed 27.3% and 74.3% anatase respectively. • DFT studies of show formation of oxygen vacancies and a 2 + oxidation state for Cu. 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 TiO 2 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 Cu 2+ oxidation state. Reduction of Cu 2+ to Cu + and Ti 4+ to Ti 3+ 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 TiO 2 , despite the high anatase content, arises from the charge recombination at defect sites that result from incorporation of copper into TiO 2 .