Synthesis And Photocatalytic Activities Of CNTs-CdS-TiO₂ Composites

With the rapid development of our country’s economy, the environmental pollution has become more and more serious. Using TiO₂ for environmental purification are increasingly arousing people’s concern. The forbidden band width of TiO₂ is 3.2 eV, which is responsive to less than 387 nm wavelength ultraviolet light. UV light accounts for only 4% of the sunlight. Because of low photocatalytic efficiency and excitation only under ultraviolet light, TiO₂ has not been applied in large scale. The forbidden band width of CdS is 3.2 e V,which is responsive to visible-light. When CdS-TiO₂ composite is irradiated under light, photogenerated electrons are excited from the VB of CdS and then transferred to the CB of TiO₂, whereas the photogenerated holes are left in the VB of CdS. Thus the efficiency of electron-hole separation is increased. With the introduction of CNTs into TiO₂, light adsorption ability, is improved. It can also absorb light to create photo-induced electons, which are transferred into the conduction band of the CdS and TiO₂ particles, thus increasing the amount of electrons. Based on the present problems of pure TiO₂, in order to improve the photocatalytic performance, TiO₂ should be mixed with CdS and TiO₂.TiO₂, CdS-TiO₂, CNTs-TiO₂, CNTs-Cd S-TiO₂ photocatalysts were prepared by sol gel method. The molecular structure, morphology andcrystallinity were characterized by scanning electron microscopy（SEM）, X-ray diffractometey（XRD） and transmission electron microscopy（TEM）. The specimens were analyzed with ultraviolet visible absorption spectrophotometry.The visible-light catalytic activity of composites was evaluated by degradation of methylene blue under simulated visible-light. The conclusions are as follows.（1） TiO₂ was prepared by sol-gel method, for which 5 mL of TBOT, 4 mL of water, 40 mL of ethyl alcohol, 0.3 g of polyethylene and 1.5 mL of acetylacetone are the optimal parameters. As for TiO₂ photocatalytic material,the degree of crystallization is increased and particle size is small and with uniform narrowly distributied after 500℃ heat treatment. The visible photocalalytic activity of the photocatalytic materials was characterized by degradation rate of methylene blue under visible light. The experiment results show that the highest degradation ratio was obtained when the heat treatment temperature was 500 ℃ and the content was 0.15 g in 25 mL of 5 mg/L methylene blue.（2） CdS-doped CdS-TiO₂ photocatalysts were prepared by sol-gel method.XRD results show that TiO₂ is anatase phase, CdS is hexagonal phase, the doping of CdS promotes the growth of TiO₂ crystal. SEM results show that CdS-TiO₂ photocatalyst had small particle with narrow distribution when the amount of doping CdS was 5%. The doping of CdS expanded the TiO₂ light response range. Cd S-TiO₂ photocatalyst had the largest photocurrent when the CdS doping amount was 5%, which reached 0.16 mA/cm2. The visiblephotocatalytic activity of the CdS-TiO₂ was characterized by degradation rate of methylene blue under visible light. The results show that CdS-TiO₂ photocatalyst has the largest degradation rate when the doping content of CdS was 5%, which reached 50%.（3） CNTs-doped TiO₂ photocatalyst was prepared by sol-gel method. The results show that CNTs-TiO₂ photocatalytic materials had the largest photocurrent when the CNTs doping amount is 0.3%,which reached 0.20mA/cm2. The visible photocatalytic activity of the CNTs-TiO₂ materials was characterized by degradation rate of methylene blue under visible light. The results show that CNTs-TiO₂ photocatalytic materials had the largest degradation rate when the doping content of the CNTs was 0.3%, which reached 40%.（4） The CNTs-CdS-TiO₂ was prepared by sol-gel method. SEM and TEM results show the CdS and TiO₂ coating on the surface of CNTs. That the photocurrent of 0.3%CNTs-5%CdS-TiO₂ had obvious enhancement compared with that of 5%CdS-TiO₂ and 0.3%CNTs-TiO₂, which reached 0.28 mA/cm2.The visible photocatalytic activity of the CNTs-CdS-TiO₂ materials was characterized by degradation rate of methylene blue under visible light. The largest degradation rate reached 60%.