Enhanced photocatalytic degradation of dye pollutants under visible irradiation on Al(III)-modified TiO₂: Structure, interaction, and interfacial electron transfer

Aluminum(III)-modified TiO₂ was prepared by sol-gel process via a sudden gelating method. The structure of the modified material and the local environment of aluminum were investigated using X-ray diffraction, HRTEM, XPS, ²⁷Al MAS NMR, and ξ-potential measurements. The effect of the aluminum modification on interaction between the dye and photocatalyst, the interfacial electron transfer process, and thereby the degradation of dye pollutants under visible irradiation were also examined by FTIR spectra and UV-vis diffuse reflectance spectra. It was found that, rather than incorporating into the crystal lattice of TiO₂, the aluminum forms an overlayer of Al₂O₃ on the surface of TiO₂, interfaced with Ti-O-Al bonds. It is interesting that the carboxylate-containing dyes such as Rhodamine B (RhB) adsorb preferentially on the Al₂O₃, rather than the Ti(IV) sites on the surface of TiO₂. The photodegradation rate observed for RhB is nearly 5-fold faster than that obtained in the pristine TiO₂ system. The photodegradation of dyes on the aluminum(III)-modified photocatalyst is of great dependence on the structure and anchoring group of the dyes. Structure with carboxylate as anchoring group and amino group as electron donor is favorable for degradation. The mechanistic details are discussed on the basis of experimental results.