Crystallization and phase-transition characteristics of sol-gel-synthesized zinc titanates

The synthesis of a ZnO-TiO₂ nanocomposite usually results in the formation of one or more of three compounds - Zn₂TiO₄, Zn₂Ti₃O₈, and ZnTiO₃ - along with other secondary impurity phases, such as rutile-TiO₂ or ZnO. Obtaining a phase-pure composite of either of these materials is one of the challenges in materials chemistry. For example, pure ZnTiO₃ cannot be synthesized under normal conditions, because the ZnTiO₃ phase readily transforms to Zn₂TiO₄ and rutile. Zn ₂Ti₃O₈ is reported as a metastable form of ZnTiO₃. The Zn₂TiO₄ form is usually synthesized via solid-state reaction at high temperatures (typically above 1000 °C). In the current study, the crystalline and phase-transformation behaviors of sol-gel-synthesized zinc titanates has been investigated systematically, with regard to various Zn:Ti precursor molar ratios. X-ray diffraction (XRD) has shown that, with excess titanium precursor, zinc metatitanate (ZnTiO ₃) is the preferred zinc titanate phase formed at temperatures of 600-900 °C, with a direct transformation of zinc metatitanate to zinc orthotitanate (Zn₂TiO₄) occurring at 1000 °C. However, synthesis involving titanium precursor in the presence of excess zinc precursor forms Zn₂Ti₃O₈ with a cubic defect spinel structure at temperatures of 700-900 °C, with a direct transformation to zinc orthotitanate occurring at 1000 °C. The current study also indicated that the percentage of zinc titanate phases can be controlled by varying the initial sol-gel reaction conditions. Spectroscopic investigations have been carried out to understand the effect of precursor (Zn(CH₃COO) ₂•2H₂O and H₂C₂O₄) formulations on the crystallization and phase-transition behaviors of these composites. Fourier transform infrared and Raman spectroscopy have shown the presence of the oxalate form of zinc and titanium. However, neither of these techniques could reveal if the titanium oxalate chains exist separately from zinc oxalate chains or if they combine during the initial stages of preparation. Further powder XRD investigation proposed that two separate metal oxalate chains were formed during the synthesis. Previous reports indicate that the formation of Zn₂TiO₄ and Zn₂Ti ₃O₈ is limited by the presence of anatase and ZnTiO ₃ only forms in the presence of rutile. Results from the present study confirm that, in a sol-gel synthesis, the formation of various forms of zinc titanates is influenced by the zinc oxide/titanium dioxide composition.