Photocatalytic Performance of Hydrothermal Temperature Dependent Dip Coated TiO2 Thin Films

Anatase titania thin films were prepared by hydrothermal assisted sol-gel dip coating at two different hydrothermal temperatures: 90 °C and 180 °C for 12 h each. Some of the as-deposited films were annealed at 500 °C for 6 h. Both as-deposited and annealed films consisted of tiny spherical particles. Crystallite size and particle size increased with increased hydrothermal temperature and annealing. Atomic force microscopy showed that root mean square and average surface roughness increased with increased hydrothermal temperature and annealing. The prepared films exhibited almost zero transmittance in the violet-ultraviolet transition region with a gradual rise in the visible region up to a maximum value of ~40% at the near infrared. The transmission decreased for annealed films and as hydrothermal temperature was increased. Band gap values did not show any significant difference before and after annealing, although they decreased with increased hydrothermal temperature. Improved crystallinity and greater packing density at higher hydrothermal temperature and annealing led to a corresponding increase in the refractive index. The intensity of photoluminescence peaks was quenched when samples were annealed and as the hydrothermal temperature increased, because of annihilation of oxygen vacant states by the ambient oxygen. Improved crystallinity diminished the number of defect sites in the films, thus reducing the amount of radiative recombination of the e ‒ /h + pair. Annealed samples and those prepared from sols processed at higher hydrothermal temperature showed better photocatalytic activity. The maximum degradation efficiency of 62.8% was demonstrated by annealed thin films prepared from sols hydrothermally processed at180 °C after 90 minutes of UV irradiation. Graphical abstract Highlights Anatase TiO 2 was obtained in an autoclave at low temperature (90 °C). The phase remained unaltered after annealing at 500 °C and also at elevated hydrothermal synthesis temperature of 180 °C. Agglomerated spherical particles were produced with Gaussian type size distribution. Higher preparation temperature and annealing quenched the photoluminescence (PL) intensity that eventually increased efficiency of photo-degradation process. Efficient photodegradation was achieved only after 90 minutes of UV irradiation with a very small area thin film photocatalyst (1.5 cm ⨯ 1 cm).