Sol-gel synthesis of TeO₂-based materials using citric acid as hydrolysis modifier

Sol-gel processing of tellurium oxide has been investigated in the tellurium isopropoxide/citric acid/isopropanol/water system. As evidenced by Fourier transformed infrared spectroscopy (FTIR), citric acid has been found to be a relevant chemical modifier to control hydrolysis-condensation reactions of highly reactive tellurium isopropoxide Te(OCH(CH3)(2))(4). Thus, depending on the main synthesis chemical parameters such as alkoxide concentration, water and modifier ratios, colloidal sols and gels have been successfully synthesised. The thermal behaviour of the dried gels has been investigated by X-ray diffraction, differential scanning calorimetry coupled with thermogravimetry and also FTIR spectroscopy. On the one hand, the crystallisation of the non-centrosymmetric gamma-TeO2 polymorph as well as the alpha-TeO2 phase which the crystallite size ranges from a few ten nanometers (similar to 50 nm) to a few microns as a function of heat treatment, and, on the other hand, the synthesis of homogeneous sols which can be handled in air and so particularly suitable for the elaboration of thin films provide new opportunities for making tellurite based materials and thin film devices for practical applications.