Effect Of Morphology, Structure And Modifying On The Photocatalytic Performance Of TiO₂

Nano materials have attracted much attention since they have excellent performance in information, biology, energy, and environment. Photocatalysis is also a hot research area in recent years, because it can explode many reactions which can not take place in normal environment. The development of photocatalyst is one of the key factors to realize the photocatalytic reactions. Among the various semiconductor photocatalysts, titania is widely used because of its strong oxidizing power, absence of toxicity and long-term photostability.In this thesis, we first synthesized and investigated the photocatalytic performance of the TiO₂ with different morphology and structure prepared by chemical vapor deposition (CVD), sol-gel, MgO deposition, ultrasonic and hydrothermal methods. And then, we studied the effect of modifying with carbon nanotubes (CNTs) and Cu²⁺ on the photocatalytic performance of TiO₂. The detailed contents are as follows:(1) Five methods were used to prepare nano TiO₂ photocatalyst and the catalysts were tested in the photocatalytic degradation of X-3B. Firstly, we prepared TiO₂ microspheres using the CVD method. At lower temperature, the spheres were composed of small rods grown from the center. At higher temperature, the rods melt together to form more smooth spheres. Since the spheres are in micrometers, the photocatalytic efficiency of them was very low. The optimal flux is 200sccm considering the quality of the product. Secondly, we had prepared TiO₂ nano particles by sol-gel method and an improve sol-gel method. The particles prepared by the sol-gel methods are very small but they agglomerated badly. The improved sol-gel method could some extent improve the photocatalytic efficiency of TiO₂ but the agglomeration still exist. Thirdly, TiO₂ nanorod and arrays were successfully prepared by the ultrasonic method, the TiO₂ arrays exhibited better photocatalytic efficiency than the TiO₂ rods since there are some small holes on the surface of the arrays which results in bigger surface area. The increase of the surface area is beneficial to promote the photocatalytic degradation of the dyes. Fourthly, we have successfully prepared TiO₂ particles, rods, flowers and sheets by hydrothermal method under acidic condition. The morphology of the samples could be controlled by adjusting the temperature, pressure and the concentration of TiCl₄. Pressure is helpful for the formation of the nanorods at 80℃and the nanoflowers at 160℃. It is also favorable for the crystallization of the as-prepared TiO₂ nanostructures. The flower-like structure has the best and the sheet-like structure shows the worst photocatalytic activity. With the increase of TiCl₄ concentration, the particle size increases and the surface area as well as the photocatalytic efficiency of the samples decrease except for the flower-like structure. In conclusion, TiO₂ with different morphology and structures were prepared by the five methods. The differences in the morphology and structure result in the differences in the photocatalytic efficiency of the products.(2) CNTs have attracted much attention since its discovery. Through the previous research results, CNTs are suitable for supporting TiO₂. So we had prepared CNTs in our lab and studied the factors affecting the preparing of CNTs.As for acetylene, iron catalyst can effectively catalyze the growth of CNTs. 640℃is the best temperature and 200sccm should be the most suitable gas flux, 30 minutes is enough for 500mg catalyst to prepare CNTs. When CO₂ was used as the carbon source, it broke to C atoms at 930℃, and the products are carbon nanorods rather than CNTs on the Fe/Cr/MgO catalyst.Rod-like Fe₂O₃ was prepared by hydrothermal method and it was used as the catalyst to prepared CNTs. XRD result shows that the product isα-Fe₂O₃ crystal and the SEM images show that they are rod-like which are about 60-80nm in diameter and 200nm in length. CNTs with diameter of about 30nm and micrometers in length were successfully produced by chemical vapor deposition method using the rod-like Fe₂O₃ catalyst. Formation mechanism of rod-like nano Fe₂O₃ was also discussed. The CNTs prepared by the rod like catalyst is easier to be purified than the particle like catalyst.The purified CNTs were then used as support for TiO₂ by sol-gel method. The CNT supported TiO₂ composites have been used as photocatalyst in the reduction of CO₂ with H₂O. The results show that the addition of CNTs could obviously improve the photocatalytic efficiency of TiO₂ and the CNTs have better effect than active carbon. C₂H₅OH can be selectively produced by the sol-gel method prepared CNT supported TiO₂, while HCOOH is the main product catalyzed by the hydrothermal treated one and P25. The CNT supported TiO₂ prepared by the sol-gel method exhibits higher efficiency than pure TiO₂ in the reusing cycles. Taking into account of the semiconducting properties of TiO₂, these CNT supported TiO₂ nanocomposites can also be applied to sensors, high-density electronic devices and lithium ion batteries as well as photocatalytic decomposition of pollutants. Moreover, the photocatalytic products CH4, HCOOH, C₂H₅OH can be reused as energy sources.(3) Cu²⁺ doped TiO₂ were prepared via a simple aqueous-phase stirring for 24h at a low temperature of 85℃, employing only TiCl₄, HCl and CuCl₂ as the starting materials. The results show that addition of copper influences the structure of TiO₂. The crystallite size decreased sharply with the increase of the copper content up to 10%. While there was no significant change in the crystallite size of titania particles as the copper content exceeded 20%. It confirms that the embedding of some portion of copper into titania particle inhibits the growth of rutile crystal of titania particles. This inhibition is correlated with the segregation of copper oxide at the boundaries of TiO₂. The embedded copper influences the photocatalytic activity of TiO₂ as well as the structure. The 2% copper-doped rutile TiO₂ exhibited relatively the highest photocatalytic activity which is about 3 times of the undoped TiO₂ in the photodegradation of aqueous brilliant red X-3B solution. The imbedding of the 2% Cu²⁺ also caused the red-shift of the absorption of TiO₂. Excessive of Cu²⁺ imbedding could not promote the photocatalytic efficiency of TiO₂, which also should be due to the morphology and structure of the TiO₂ photocatalyst.(4) Nano CuO was prepared by hydrothermal method using Cu(NO₃)₂ and NaOH as the source materials. Particle like CuO prepared by sol-gel method was also used for compare. The morphology of the sheet like CuO kept stable after 500℃sintering, while the particle like CuO agglomerated even after 200℃sintering. And the sheet like CuO has larger surface area and dispersed better than the particle like CuO. The two kinds of CuO were used in the negative electrode of Li-ion battery. The Li-ion battery using sheet like CuO as the negative electrode has higher specific energy and worked more stable than using the particle like CuO.