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

Abstract

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.