Ultrasonic Preparation Of TiO₂ And Nitrogen-doped TiO₂ Catalyst And Their Photocatalytic Activities

Heterogeneous photocatalytic oxidation technology with semiconductor has been a newly developed pollution treatment process in recent decades. Its application is very extensive, such as wastewater treatment, air decontamination, anti-virus, etc.. Metal oxide semiconductors such as TiO₂, Fe₂O₃, CdS, WO₃, SnO₂, etc., have been largely used as photocatalysts. In particular, TiO₂ was regarded as the most promising green photocatalyst due to its higher photodegradation of the organic contaminants as well as its many advantages ( i.e., safety, low cost and no pollution). However, there still exist two major defects for TiO₂ itself as follows: both of the quantum production and the utilization of solar energy are low, which clearly limits its efficient application. This thesis aims at increasing the charge separation and extending the energy range of photoexcitation by synthesizing pure TiO₂ and nitrogen doped TiO₂ under ultrasonic irradiation. The photocatalytic decomposition of methyl orange in aqueous solution was used as a probe reaction to evaluate their photocatalytic activities. The light aborption property, crystal structure, etc., were characterized by various techniques. The structure-activity relationship between the physicochemical properties and the photocatalytic activities of TiO₂ samples was investigated.First, this topic study the effect of each preparation condition on photocatalytic activity. The result show that calcination temperature is one of the key factors that influence the catalyst activity, because it defines the crystalline phase and partical size of TiO₂. It was found that 500℃were the optimal calcination temperature of both TiO₂ and nitrogen doped TiO₂.Then, TiO₂ nanocrystallites were synthesized by sol-gel process under different frequency, including 28KHz, 45KHz and 100KHz.The characterization analysis results show that ultrasonication not only reduce the crystallite size and aggregation but also enhances the hydrolysis of titanium alkoxide for faster and better crystallization of anatase nanoparticles, which results in significant improvement to the photocatalytic activity of TiO₂. The experimental results indicate that the TiO₂ prepared under ultrasonic of 28KHz achieve the highest photocatalytic activity at ultraviolet lamp. Its apparent decomposition rate is 1.15times higher than that of TiO₂ which is prepared without ultrasonic irradiation. The crystallite size of TiO₂ prepared under ultrasonic is 18.94nm and its surface area is 72.25 m²Â·g^-1, which are respectively better than that of TiO₂ prepared without ultrasonic.At last, nitrogen-doped TiO₂ in anatase form were prepared by sol-gel method using triethylamine as nitrogen raw material with and without ultrasonic irradiation, they are respectively called UN-TiO₂ and N-TiO₂.The experimental results indicate that the nonmetal nitrogen doping makes the TiO₂ absorption wave-length occurrence red shift. Compared to N-TiO₂, the visible light absorption ability of UN-TiO₂ is enhanced further and its crystallite size is improved due to that ultrasonic irradiation accelerate nitrogen doping and enhances TiO₂ better crystallization. The apparent decomposition rate of methyl orange on UN-TiO₂ is up to about 100% after 60 minutes under visible light. The crystallite size of UN-TiO₂ is 20.85nm and its surface area is 66.89 m²Â·g^-1, which are better than that of N-TiO₂, whose are respectively 22.85nm and 62.88 m²Â·g(-1).The ratio of N to Ti in UN-TiO₂ is 0.0244:1.