Modification And Photocatalytic Performance Under Visible Light Of Nanocrystalline TiO₂

Photocatalytic technology, as a modern green environmental protection technology, is novel, highly effective and energy-efficient. It takes semiconductor as the catalysts and decomposes organic pollutant in water and the air into low toxicity or non-toxic substance under the radiation of the light. TiO2, as one of the most popular photocatalysts for decomposing various environmental pollutants, has received much attention due to its non-toxicity, low-cost, photostability, and chemical inertness. Although it has great potential, some problems still hinder practical application of TiO2, such as the low photo-quantum efficiency and low photocatalytic activity under visible light. Therefore, visible-light-active TiO2has been developed by reducing the band gap; meanwhile, to decrease the recombination of photogenerated electrons and holes has been another key point in modification of TiO2. In this paper, the modified TiO2photocatalysts have been synthesized by different procedures to improve their photocatalytic efficiency under visible light.Noble metal, serving as a Schottky-barrier to trap the charge carriers, is usually used to modify the TiO2surface to improve the photocatalytic efficiency. As a result, the second chapter of the paper illuminated modification of TiO2by loading platinum (Pt) and palladium (Pd) via H-plasma reduction (PR) method, which is easier and faster than conditional temperature programmed reduction (TPR). The catalysts were characterized by Nitrogen adsorption-desorpotion, ICP and UV-Vis. The photodecomposition of methylene blue in aqueous solution by all the samples above was also investigated respectively. The results showed that photoactivity of all the noble metal loaded TiO2(denoted as Pt/TiO2, Pd/TiO2, PtPd/TiO2) prepared by PR method were much enhanced, compared to the catalysts prepared by TPR method and Degussa P-25.Yellowish S-doped Degussa P-25(S-TiO2) powders were prepared via a H2S/H2plasma method in a dielectric barrier discharge (DBD) reactor. The other catalyst synthesized by thermal treatment of P-25with the H2S/H2mixture (S/TiO2) was used for comparison. The photocatalytic performances of the catalysts were evaluated by the degradation of a20ppm aqueous solution of a reactive dyestuff, Methylene Blue, under the visible light. The catalysts were characterized by XRD, XPS and UV-Vis. The results indicated that the activity of S-TiO2was much higher than that of P-25, while that of S/TiO2was not significantly improved. The XRD revealed that neither the structure nor the crystal phase ration of P-25 was influenced by the treatments with H2S/H2. The UV-Vis showed that the visible-light absorption of S-TiO2was higher than that of P-25. The band-gap energy of P-25was calculated to be3.0eV, while that of S-TiO2decreased to2.7eV. As evidenced XPS, the negative-valence S was incorporated on the surface of TiO2after the H2S/H2plasma treatment, leading to a decrease in the surface of OH groups and increase in the oxygen vacancies.On the basis of the above two kinds of modification, this paper took a combination of them to get a novel kind of photocatalyst. First, TiO2loaded metallic oxide were made with deposition-precipitation (DP) method, and then, they were treated by H2S (10%)/H2-plasma in order to dope S in the TiO2lattice and reduce metallic oxide on TiO2. In the end, a novel kind of catalyst were obtained and denoted as S-Pt/TiO2and S-Pd/TiO2. The photodecomposition of methylene blue in aqueous solution by all the samples above was also investigated respectively. In contrast to those catalysts only loaded noble metal or doped S, these novel catalysts showed worse photocatalytic activity under visible light.