Influence of ligand groups in Ti precursors on phase transformation and microstructural evolution of TiO₂ thin films prepared by the wet chemical process

TiO₂ thin films prepared by metalorganic decomposition (MOD-TiO₂) and sol-gel processes (SG-TiO₂) were investigated in terms of the anatase-to-rutile phase transformation and microstructural evolution. It was found that the chemical reactivity of the ligand groups initially coordinated on the titanium precursor plays a decisive role in the structure development of as-deposited SG-and MOD-TiO₂ films. MOD-TiO₂ films consist of small aggregated particles and therefore, tend to coalesce together to form an inhomogeneous microstructure during the anatase-to-rutile phase transformation. On the other hand, SG-TiO₂ films consist of uniform large particles that tend to grow homogeneously. MOD-TiO₂ films showed a higher crystallization temperature than the SG-TiO₂ films but the temperature of the anatase-to-rutile phase transformation is much lower in MOD-(approximately 775 °C) as compared to SG-TiO₂ films (approximately 930 °C). The activation energy (Q) was estimated as 524 and 882 kJ/mol for the MOD-and SG-TiO₂ films, respectively. The lower transformation temperature and activation energy in MOD-TiO₂ films were due to smaller grain size and more potential nucleation sites existing in the un-transformed MOD-TiO₂ film structure, which can accelerate the rate of anatase-to-rutile transformation.