Construction Of Quaternary Nanomaterials Based On TiO₂ NTAs And The Research Of Visible Light Photocatalysis Mechanism

Due to low energy consumption, nontoxicity, low cost and well chemical stability, photocatalytic technology has a potential application in the fields of hydrogen photoproduction and degradation of pollutants. Highly ordered TiO₂ nanotube arrays have excellent properties, due to its unique nanotube structure and higher photocatalytic activity, which makes them a basis material for photocatalyst. However, the wide band gap （3.2 eV） of anatase TiO₂ makes it only response to UV light. The photogenerated electron-hole pairs also tend to recombine quickly, which leads to low photocatalytic activity and photocatalytic efficiency.TiO₂ NTAs films are prepared by anodization method on Ti substrate. In order to extend the spectral response to visible light region, narrower band gap semiconductors CdSe and CdS were used to modify TiO₂ NTAs. Noble metal Pt was also used to depress the recombination of photogenerated electron-hole pairs. The progressive cascade structures were constructed with CdSe, CdS, Pt and TiO₂. Pt/TiO₂ NTAs binary nanosystems, CdSe/Pt/TiO₂ NTAs ternary nanosystems and CdSeCdS/Pt/TiO₂ NTAs quaternary nanosystems were built respectively. XRD, FESEM, FETEM, EDS, XPS, UV-Vis methods are employed to characterize the samples. The characterizations of different systems were conducted. And the photo-generated charge transfer path, visible light photocatalytic mechanism, photocatalytic dynamics and photostability of the photocatalyst were studied systematically.The main results are shown as follows:（1） The microstructure of Pt/TiO₂ NTAs, CdSe/Pt/TiO₂ NTAs and CdSe/CdS/Pt/TiO₂ NTAs are related to the amount of deposited particles, such as CdS, CdSe or Pt. And the uniform deposition of Pt, CdS and CdSe NPs are realized by controlling the concentration of H₂PtCl₆, deposition cycles of CdS and deposition time of CdSe. On the one hand, low amount of deposition particles leads to the weak visible light response of nanosystems. On the other hand, excessive nanoparticles may cause the light shielding effect and reduce the light-harvesting efficiency, which may lead to the decrease of photocatalytic activity. And some particles even act as recombination sites which limit the effective separation of the photogenerated electron-hole pairs. Furthermore, excessive nanoparticles will block the nanotubes, which may reduce the reactive contact area and limit the flow of medium.（2） The visible light photodegradation rate of Pt/TiO₂ NTAs to Methyl Orange solution reach 84.27% after being irradiated for 2 h, while that of CdSe/Pt/TiO₂ NTAs and CdSe/CdS/Pt/TiO₂ NTAs are 98.42% and 94.98% respectively under the same condition. Kinetic study shows that the reaction rate constant of CdSe/CdS/Pt/TiO₂ NTAs is 0.O₂479 min-1, which is 3.4 times that of Pt/TiO₂ NTAs and 1.2 times that of CdS/Pt/TiO₂ NTAs. Compared with the photodegradation rate of 85.6% for CdSe/Pt/TiO₂ NTAs after being used for 15 times and that of 84.3% for CdSe/CdS/TiO₂ NTAs, CdSe/CdS/Pt/TiO₂NTAs is 88.82%. The active oxidizing species （·OH, h⁺, ·O₂^-） could react with organic compounds during the photocatalytic process. Selective inhibition test indicates that·O₂^- radicals play a dominant role, and h⁺and·OH radicals play the secondary role to the photodegradation process.In this study, quaternary nanosystems were built with narrow band gap semiconductors CdS and CdSe, noble metal Pt and TiO₂ NTAs. CdS and CdSe nanoparticles act as visible light response component and Pt nanoparticles act as electron traps component. At the same time, the localized surface plasmon resonance （LSPR） of Pt nanoparticles make contributions to visible light response. The modification of CdSe, CdS and Pt nanoparticles can enhance the efficiency of photon harvesting and reduce the electron-hole recombination ratio by transferring the electrons from conduction band of CdSe to metal Pt via CdS through TiO₂. The synergy effect makes CdSe/CdS/Pt/TiO₂ NTAs photocatalyst better visible light photocatalysis properties, which provide a theoretical basis for the research of novel multi-nanophotocatalysts.