The influence of Al doping on the photocatalytic activity of nanostructured ZnO: The role of adsorbed water

Al doped ZnO nanoparticles (labeled as x% A-ZnO, x = 0.0, 0.5, 1.0, and 1.5) were synthesized by a simple sol-gel method, characterized and tested as photocatalysts for the degradation of a model compound under UV-light irradiation. Diffuse reflectance spectroscopy (DRS) and photoelectrochemical analysis have been performed to evidence their optical and electronic behavior. The structural features of the powders were highlighted by means of specific surface area (SSA) analysis, X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopy. Al doping results in enhanced photocatalytic activity of the modified powders with respect to pure ZnO. In particular, the highest activity was achieved with the 1.0% A-ZnO sample. The presence of Al did not significantly change neither the optical absorption nor the electronic structure of the powders. In fact, the observed blue shift of the band gap and cathodic shift of the quasi Fermi level of the modified powders occurred only at a virtually negligible extent. On the other hand, the structural analysis evidenced that the enhanced photocatalytic activity may be ascribed to the important surface defectivity induced mainly by substitution of Zn2+by Al3+ions and to the deriving higher hydrophilicity of the Al doped samples.