Study On Synthesis Of Titania Nanofilms Via In Situ Chemical Oxidation And Their Photocatalytic Degradation Performance

Development of the effective methods aimed at removal of pollutants from water and air by using the renewable energy sources e.g. solar energy becomes a subject of the intensified research over the last decades. One of the most safe and environmentally friendly chemical methods is photocatalysis. The key to the photocatalysis purifying technology is the photocatalysis material. Titanium dioxide nanofilms, as photocatalysts, possess advantages such as low cost, innocuity safety, well stability and easily recovery. Therefore they are promising materials for applications in pollution control. It is well known that the morphologies of nanofilms are demonstrated to have great effects on their widely varying properties and corresponding potential applications. Therefore, the fabrication of TiO₂ nanofilms with special morphologies could effectively improve their photocatalytic activity. In the present work, we synthesized various TiO₂ nanofilms by in situ chemical oxidation method. Meanwhile, the properties and photocatalytic degradation activities were investigated. In this dissertation, our investigations are carried out as follows:(i) When Ti sheets were treated with an aqueous mix-solution of sodium hydroxide and hydrogen peroxide in a reaction kettle for 24 h at 80℃and then with HCl aqueous solution and heat treatment, low-dimensional titania nanowires network films were prepared. The formation process is that the oxidative solution causes partial dissolution of the Ti sheets to form an amorphous sodium titanate sol, which gradually form sodium titanate nanowires through dissolution precipitation mechanism. TiO₂ nanowires were finally obtained after the treatment by HCl aqueous solution and the heat treatment. The TiO₂ nanofilms can be applied to photodegrade phenol solution. The film prepared shows excellent photocatalytic performance and stability and the phenol removal is as high as 75.9% after 2 h irradiation. The excellent photocatalytic activity must be attributed to the synergetic effect of the net-like anatase structure.(ii) Ti sheets were directly treated with hydrogen peroxide and further hydrothermal treatment was conducted. Nanoflower films on Ti substrates were obtained by these steps. The oxidation step ensured the formation of flower-like amorphous TiO₂ on titanium surface. The hydrothermal step facilitated the crystallization of amorphous titanium oxide and the formed films possess both anatase and rutile phases. The photocatalytic activity was evaluated by decomposing methyl orange in water. Compared with the traditional annealed sample without hydrothermal treatment, the rate of degradation tripled. The effects of hydrothermal temperature and time are also investigated. The photocatalytic activitivies of the flower-like films are improved with the increasing hydrothermal temperature or prolonged hydrothermal time for the higher crystallinity.Further study focuses on the modification of the nanoflower films through roasting the films with urea. The study showed that the band-gap of the modified TiO₂ thin films is reduced in varying degrees. The photocatalytic activity was also investigated by photocatalytic degradation of methyl orange. Photocatalytic results showed that the photocatalytic efficiency under visible light or full spectrum light illumination was enhanced after modification. The modified TiO₂ film which roasted at 350℃showed the best activity under the visible light irradiation. 72.8% methyl orange was degraded in 3 hours. DRS test showed that the band-gap was down to 2.4eV. XPS results indicated that the visible light photocatalytic activity is due to the effect of the polymer formed during urea pyrolysis process.(iii) Different amount of peroxotungstic acid sol was added in peroxide dioxide and Ti sheets were reacted with these mixed oxidation solution. W-doped TiO₂ nanostructured films were synthesized in situ on Ti substrate by this one-pot process. According to our data analysis, the films showed typical nanoflower, nanoweb or nanopore morphologies. Tungsten content in the films changes with the amount of additional sol Tungsten is doped into the TiO₂ lattice to form Ti-O-W bond, which is obtained by the condensation of peroxo titanic acid and peroxotungstic acid. The absorption edge of the films, measured by a UV-Vis spectrophotometer, was observed to shift toward longer wavelengths when tungsten was incorporated to the TiO₂ films. The band gap energy was less than 3.1 eV for W-doped TiO₂ systems and the band gap of grown TiO₂ is 3.1eV. Furthermore, photocatalytic activity of the films was also examined by measuring the decomposition rate of methyl orange under both visible and full-septrum irradiations. It was observed that the W-doped layers were more photoactive as compared to as-synthesized pure TiO₂ layers. Under the visible light irradiation, the degradation rate could reach up to 19.3% in 3 hours. For the film prepared under optimum condition, the methyl orange removal is as high as 91.3% after 3 h full spectrum light illumination and the speed of degradation is 4.3 times higher than undoped TiO₂ films.