Study on the influence of heat treatment on the crystallographic phases of nanostructured TiO2 films

Modified sol–gel method was applied for the synthesis of single-layer and multilayers TiO2 films from titanium tetraisopropoxide (TTIP) as precursor, acetyl acetone (AcAc) as a complexing agent, and polyethylene glycol (PEG) as an additive. The films were deposited by spin- and dip-coating and characterized by SEM and XRD. Powdery samples were prepared from the same solutions after annealing at different temperatures and studied by IR spectroscopy. XRD analysis revealed that phase composition of the obtained films depended on the used way of thermal treatment. If the multilayer spin-coated film was heated up to 773 K after the deposition of each layer then a mixture of anatase and brookite was obtained with crystallites size of 20 nm. On the other hand if the film was dried at 393 K after the deposition of each layer and heated up to 773 K only after the deposition of the last layer then the obtained sample contained pure anatase phase with crystallites size of 24.7 nm. A theoretical model was proposed to explain the observed results. SEM images showed that spin-coated films were nanostructured and contained spherical nanometric grains and pores. It was shown that higher PEG concentration provided higher porosity of the films. Dip-coated films were composed of nanograins and geometric particles on entire crack free surface. The effect of several parameters on the films morphology such as PEG concentration, withdrawal speed, and method of heat treatment was investigated.