Microstructure and crystal structure development in porous titania coatings prepared from anhydrous titanium ethoxide solutions

Porous titania coatings were prepared by spin coating anhydrous titanium ethoxide-ethanol solutions in a controlled humidity atmosphere. Ti ethoxide reacted with atmospheric moisture during deposition, to form amorphous particles (approximately 200 nm), a dense layer or a combination of the two, depending on the processing conditions. Relatively humid atmospheres, low concentrations of Ti ethoxide in the coating solution and slow spinning rates favoured particle formation. These particulate coatings were typically composed of agglomerated particle clusters. Agglomeration could be prevented by adding hydroxypropyl cellulose to alkoxide solution to act as a steric stabilizer for newly formed particles. During thermal treatment, the coatings crystallized into the anatase phase and then transformed into the rutile structure at higher temperatures. The anatase-rutile transformation in porous coatings occurred over a range of 850-1150°C and strongly depended on microstructural features. More porous coatings with larger particle clusters transformed to rutile at lower temperatures. Tensile stress in the coating caused by constrained shrinkage inhibited the phase transformation. The substrate constraint slowed the transformation rate in coatings relative to free powder. Stress relief through rupture of particle cluster connections allowed transformation to occur at lower temperatures.