A Study On The Modification And Photocatalytic Performance Of TiO₂-Based Photocatalysts

Photocatalysis has become a research hotspot in both the utilization of solar energy to generat H2 from water and the degradation of dye pollutants. During the photocatalytic process, the efficiencies for the separation of photogenerated charges and the utilization of incident light are two determining factors on the photocatlystic activities and the conversion efficiency of solar energy. In this work, the TiO₂ based photocatalysts including hydrogenated TiO₂ and TiO₂-supported Agx Au1-x-TiO₂ alloy SPR catalysts are studied and used for water spillting to produce H2 and the photocatalytic degradation of dye pollutants, respectively.Hydrogenation is an efficient strategy to improve the photocatalytic activity of TiO₂. By using traditional impregnation-reduction method, the Pt-P25 catalyst was synthesized. The results of H2-TPR suggest that the hydrogenation extent of Pt-P25 is greatly enhanced by introducing H2 spillover into the hydrogenation process. The Pt-P25 hydrogenated at 400 oC displays the best photocatalytic activity for H2 generation（7.7 mmol·h·g-1）, which is 38 times to that of pure P25; meanwhile, the sample Pt-P25-400 also shows high stability under simulated sunlight irradiation. Based on the results of multiple characterizations, it is found that the H2 spillover involved hydrogenation process could produce oxygen vacancies by disordering the surface lattice of TiO₂, thus enhancing the separation of photogenerated charges. Based upon the catalytic performance and catalyst characterization results, a plausible H2-spillover enhanced hydrogenation mechanism is proposed.It is known that 1D TiO₂ nanobelts often exhibit faster charge transfer dynamics during photocatalytic reactions, however, the crystallized TiO₂ nanobelts is hard to be hydrogenated. So, in this part, we aim to employ the H2-spillover effect to hydrogenate the 1-D TiO₂ nanobelts so as to increase their photocatalytic activity for water splitting. Based upon the results of H2-TPR, the hydrogenation temperature of pristine Pt-Belt was optimized, which is 700 oC. The activity evaluation indicates that the sample Pt-Belt-700 shows the highest H2 production rate（16.6 mmol·h·g-1）, which is 10 times to that of pure TiO₂ nanobelts. In situ Raman, EPR and positron annihilation results imply that the H2 spilloverhas enhanced hydrogenation, generateing more defects on the surface or subsurface of TiO₂ but reducing the bulk defects, and thus leading to high charge separation efficiency and high photocatalytic activity.The generality of as-proposed H2 spillover stragety was also investigated from two aspects. On one hand, by changing the kinds of noble metals such as Pd, Rh and Ru, we studied the effect of H2 spillover enhanced hydrogenation; on other hand, by using the different semiconductors such as ZnO and WO3, the hydrogenation effect was investigated. It is found that all the H2 spillover enhanced hydrogenated samples exhibited higher catalytic activity, which demonstrates the generality of the proposed strategy.To improve the visible light utilization efficiency, a series of TiO₂ supported Agx-Au1-x-TiO₂ alloy photocatalysts with SPR effect were prepared by a facile photoreduction combined with a simple replacement reaction between Ag and Au3+ ions. It is revealed that this series of catalysts possess tunable surface plasmon resonance（SPR） frequencies. EDS Line-scanning results suggest the formation of Au-Ag alloy in the nanoparticles. The results of activity evaluation show that the catalysts containing Ag-Au alloy show much higher visible light-responsive photocatalytic activity than the commercial P25 and the single supported Ag or Au catalysts. By mixing the catalysts Agx-Au1-x-TiO₂ with different SPR frequencies, the photocatalytic activity can be further improved due to the widened visible light absorption range.