(234d) Structure Characteristics and Photocatalytic Activity of Tio2 Thin Films Synthesized from Refluxed Sols (Rs)

Abstract: The refluxed sols (RS) containing nanocrystalline anatase TiO2 particles were synthesized under mild condition (100°C and ambient pressure) by hydrolysis and peptization of titanyl sulfate (TiOSO4) in abundant aqueous peroxide (H2O2) and subsequent reflux to enhance crystallization. Nanocrystalline anatase films were deposited on glass substrates by the dip-coating method from the aqueous refluxed sols and calcined at 100°C, 300°C and 500°C. Several testing techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Ultraviolet-visible spectrophotometer (UV-Vis) and X-ray photoelectron spectrum (XPS) were used to characterize the microstructure, phase evolution and chemical composition of the TiO2 thin films. The photocatalytic activities of the TiO2 films were evaluated by photocatalytic degradation of aqueous methyl orange solutions. The results indicate that the RS sols contain needle-like anatase particles with 30-80nm in length and 10-20nm in diameters, and the length and diameter increase with extending reflux time. The obtained films consist of spherical particles, and the diameters of anatase particles increase with increasing calcination temperatures. The TiO2 thin films are anatase phase and contain not only Ti and O elements, but also a small amount of N and Na elements. The TiO2 films are transparent and their maximal light transmittance exceeded 80% under visible light region. The UV-Vis spectra are processed to calculate the energy band gap , the band gap of the TiO2 films calcined at 100, 300 and 500°C are estimated to be 3.56, 3.47 and 3.39eV, respectively. It is noted that the bandgap energies of the films decrease and red shift in the absorption edge appears with increasing calcination temperatures. The TiO2 film heated at 100°C contains little anatase TiO2 crystals and most of TiO2 are amorphous, thus it has low photoactivity. At 300°C, the TiO2 thin film has better crystallinity, smaller crystal size and higher hydroxyl content on the surface, these factors are beneficial to the enhancement of photocatalytic activity, therefore the film calcined at 300°C shows the highest photocatalytic activity. When the calcination temperature reaches 500°C, the decrease in photocatalytic activity is due to sintering and growth of TiO2 crystallites resulting in the decrease of surface area. The results suggested that RS sols are promising novel precursors for synthesizing TiO2 films, and refluxing technique is quite efficient in preparing titania nanocrystalline films.

Keywords: Refluxed sols; Needle-like anatase crystals; Nano titania thin films; Photocatalysis; Band gap;