TiO₂–Cu photocatalysts: a study on the long- and short-range chemical environment of the dopant

Abstract

In this study, the short- and long-range chemical environments of Cu dopant in TiO₂ photocatalyst have been investigated. The Cu-doped and undoped TiO₂ specimens were prepared by the sol–gel approach employing CuSO₄·5H₂O and Ti(O-iPr)₄ precursors and subjecting the dried gels to thermal treatment at 400 and 500 °C. The photocatalytic activity, investigated by methylene blue degradation under sunlight irradiation, showed a significantly higher efficiency of Cu-doped samples than that of pure TiO₂. The X-ray diffraction results showed the presence of anatase phase for samples prepared at 400 and 500 °C. No crystalline CuSO₄ phase was detected below 500 °C. It was also found that doping decreases the crystallite size in the (004) and (101) directions. Infrared spectroscopy results indicated that the chemical environment of sulfate changes as a function of thermal treatment, and UV–vis spectra showed that the band gap decreases with thermal treatment and Cu doping, showing the lowest value for the 400 °C sample. X-ray absorption fine structure measurements and analysis refinements revealed that even after thermal treatment and photocatalytic assays, the Cu²⁺ local order is similar to that of CuSO₄, containing, however, oxygen vacancies. X-ray photoelectron spectroscopy data, limited to the near surface region of the catalyst, evidenced, besides CuSO₄, the presence of Cu¹⁺ and CuO phases, indicating the active role of Cu in the TiO₂ lattice.