Study of the decomposition of aqueous citratoperoxo-Ti(IV)-gel precursors for titania by means of TGA-MS and FTIR

An aqueous solution-gel route is developed for the preparation of TiO₂. In this report, we study an aqueous citratoperoxo-Ti(IV)-precursor at pH 2.0, which is compatible with polyvinyl alcohol (PVA) and therefore can be applied for the preparation of a thick mesoporous TiO₂ film.With regard to deposition of films, it is important to gain insight in the behaviour of the precursor during thermal treatment. Therefore, the thermal decomposition mechanism of a citratoperoxo-Ti(IV)-gel and a PVA modified citratoperoxo-Ti(IV)-gel is studied. Weight losses and evolved gasses are characterized by TGA-MS (5 °C min⁻¹), while gel structure and changes in the solid upon heating are studied by means of FTIR. For both gels, decomposition in dry air can be divided into five regions. After drying of the samples in the first region (∼100 °C), decomposition of the organic matter not coordinated to the metal ions occurs (∼200 °C). The third region (∼310 °C) involves the decomposition of citrato ligands. Finally, the residual organic matter is combusted in the last two regions. Only in dry air it is possible to fully remove the organic matrix in both gels at temperatures below 600 °C.It is also proven that the citratoperoxo-Ti(IV)-complexes, seen at pH 7.0, exist in the precursor gel already at pH 2.0.     

An aqueous solution–gel citratoperoxo–Ti(IV) precursor: synthesis,gelation, thermo-oxidative decomposition and oxide crystallization

A watersoluble citratoperoxo–Ti(IV) complex was synthesized and the chemical reactions that occur during the different synthesis steps were studied. A crystalline complex could be obtained by slow evaporation of highly concentrated precursor solutions, while fast evaporation led to the preparation of a homogeneous, amorphous glassy solid, the gel. The chemical structure of the crystallized complex ion and of the gel were shown to be related, but differed due to the presence of a crosslinked ammonium citrate network in the gel, which prevents crystallization. The thermo-oxidative decomposition pathway of the crystalline complex and the gel to the metal oxide is described based on in-situ thermal analysis techniques combined with Fourier transform infrared and mass spectrometry, diffuse reflection infrared by Fourier transform as well as high-temperature X-ray diffraction. The study offers a fundamental understanding of the behavior of the aqueous solution gel Ti(IV) precursor which is applicable in the preparation of all kinds of multimetal oxide systems containing Ti(IV) ions.