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Optical properties of ZnO loaded in mesoporous silica

The optical absorption and photoluminescence (PL) of ZnO loaded in mesoporous silica have been investigated. These ZnO/SiO2 composites are synthesized by impregnation of mesoporous silica with an aqueous solution of zinc acetate, followed by firing at 550 °C. A significant blue shift of the absorption edge compared to that of bulk ZnO in the absorption spectra is observed, and the band gap luminescence of ZnO is predominant in the emission spectrum, while the defect induced green luminescence, typical of ZnO, does not appear. However, the above sample, after additional annealing at 350 °C in air for three hours, exhibits an absorption edge (365 nm) near to that of the bulk ZnO; the colour of the sample also turns to yellowish from colourless, and green luminescence also appears. Combining the analysis of x-ray photoelectron spectroscopy (XPS), we propose that Zn-O-Si cross linking bonds formed in the interface between ZnO and the pore walls of silica have a great influence on the optical properties of ZnO/SiO2 before the additional annealing at 350 °C and the corresponding optical properties reflect both those of SiO2 and ZnO. After annealing, the Zn-O-Si cross linking bonds mentioned above are destroyed, and the optical properties of ZnO are exhibited alone.