Contamination of TiO2 thin films spin coated on borosilicate and rutile substrates

The present work reports data for TiO2 thin films on borosilicate glass and (001) single-crystal TiO2, annealed at 200–550 °C for 8 h. Characterization included GAXRD, laser Raman microspectroscopy, AFM, UV–Vis, XPS, SIMS, TEM, ellipsometry, and methylene blue (MB) dye degradation. The substrate determined the TiO2 polymorph that formed, while the annealing temperature and boron contamination from the substrate determined most of the associated properties. The films on glass substrates were amorphous following annealing at 200 °C but were anatase at higher temperatures. The films on rutile exhibited epitaxial growth at all annealing temperatures. Annealing caused diffusion of glass component elements into the films and counterdiffusion of Ti into the glass substrates. Since aqueous MB testing caused decreased glass ion concentrations, the diffusion mechanism is via the grain boundaries. Volatilization of boron occurred during annealing at 550 °C. The morphological features dominated the optical properties; the anatase films exhibited high transmissions and low reflectances, while the rutile films exhibited the converse. The band gap decreased slightly with increasing annealing temperatures, reflecting increasing crystallinity. The refractive indices showed an anomalous trend of decrease with increasing annealing temperature and associated crystallinity; this is attributed to the effects of boron volatilization and associated air-filled pore formation. Although the anatase films outperformed the rutile films, the effect of annealing temperature is likely to have been dominant in that it determined the relative extents of crystallinity, grain size, RMS roughness, optical indirect band gap, and oxygen vacancy concentration.