The Synthetize And Photocatalytic Activity Of Double-Element Modified Nano-TiO₂

Nanocrystalline TiO₂ is an important oxide semiconductor material with directand wide bandgap, excellent optical, electrical and chemical properties. Recently, As an advanced oxidation technology, the nano-TiO₂ photocatalytic technology have attracted much attention. But the photoeatalytic performance of pure nano-TiO₂ is not good, So its application is limited. In order to improve the photocatalytic activity, modification of pure nano-TiO₂ is an effective way.Nonmetal-metal, metal-metal-codoped TiO₂ nanoparticle and semiconductor composite TiO₂ had been prepared by modified hydrothermal method. Their characteristics were investigated by XRD, TEM, SEM, DRS, XPS, BET and photocatalytic degradation for methyl orange (MO).(1) Bare TiO₂, N-doped and N, Cd-codoped TiO₂ has been synthesized by thermolysis and hydrothermal method, respectively. The products were characterized by various technique, such as X-ray diffraction (XRD), transmission electron microseope (TEM), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) specific surface area analysis and so on. Activities of samples were studied on the degradation of methyl orange (MO). The experimental results showed that N-Cd-codoped TiO₂ synthesized by thermolysis and calcined at 450℃shows the highest activity. The N, Cd-TiO₂ photacatalysts possessed an anatase crystalline framework, having particle sizes of 10-15 nm. N atom was incorporated into the TiO₂ crystal lattice, while Cd atoms existed on the crystal surface. Doping resulted in the observation of optical absorption edges shifting to the red light. The optical absorption edge of N, Cd-codoped TiO₂ was 435 nm, which illustrated that theproducts represented photoactivity in the visible region. It indicated a strong N-Cd synergistic interaction appeared to play a decisive role in driving the excellent photoactivity performance of N, Cd-codoped TiO₂. Simultaneously, first-principles density functional theory (DFT) calculations were performed to explore the effect of dopants on electronic and optical properties of N, Cd-TiO₂. There were impurities states between the valence band and the conduction band, which came from N2p and Cd5s, respectively. The electron on the valence band can transit to the impurity states, and then were migrated secondly to the conduction band in the light irradiation. So the electronic excitation in N, Cd-TiO₂ became easier compared to pure TiO₂, and the absorption edge shift to the visible light region. It could account for the experimentally observed optical absorption edge of N, Cd-TiO₂ shifting toward the lower energies.(2) Bare TiO₂, La-doped TiO₂, Zn-doped TiO₂, La-Zn co-doped TiO₂ photocatalysts were prepared by precipitation-hydrothermal method. It was found that absorption of La³⁺ and Zn2+ co-doped TiO₂ under ultraviolet and visible light region were significantly improved, and the absorption edge shifted to the low energies (visible light region). Compared to bare TiO₂, the co-doping of La³⁺ and Zn2+ into TiO₂ reduced combination rates of the electron-hole pairs, which resulted in the optical response enhancement.(3) La-CdS/TiO₂ photocatalysts were prepared using hydrothermal method by metal doping coupled with semiconductor composite method. The influences on morphology, the crystalline phase and photocatalystic activities of samples were studied with the experimental condition of various ratio of CdS/Ti in reaction and doping element. La doping can form shallow potential capture wells in the TiO₂ semiconductor, and reduce the rate of photo-carrier recombination effectively, CdS composite can significantly increase the absorption of the samples in the visible region.When n(TiO₂:CdS) = 100:5, La-CdS/TiO₂ showed the highest photocatalytic ativity. Doping of La and compositing of CdS possessed the synergistic effects on enhancement of TiO₂ photocatalytic activity.