Visible-light photocatalytic activity of rutile (TiO₂) annealed with melamine under an argon atmosphere

Visible-light active photocatalysts were obtained by rutile (TiO₂) high-temperature treatment in an inert argon atmosphere with addition of different amounts of melamine. The samples were characterized using X-ray diffraction, transmission and scanning electron microscopy, Fourier-transform infrared and Raman spectroscopy, UV-Vis diffuse reflectance, photoluminescence and X-ray photoelectron spectroscopy, temperature-programmed desorption of NH₃ and CO₂, and Brunauer-Emmett-Teller and thermogravimetric analysis. The effect of varying melamine amount on the crystal phase, crystallite size, and morphological and optical properties of the prepared materials was investigated. The photocatalytic activity was tested in the two types of reactions: photooxidation of dyes and photoreduction of Cr₂O₇^-2 under UV and visible-light irradiation. Rutile with the addition of melamine, annealed in an argon atmosphere, exhibited photocatalytic activity when irradiated with visible light in all the reactions studied. We explain this by a change in its morphological and optical properties, such as the formation of new surface-active centers and increased absorption in the visible range of light.