Defect Regulation And Photocatalytic Activity Of Titanium Dioxide And Zinc Oxide

In recent years,water and air pollution have become highly serious,therefore increasing focus has been paid on these issues.The photocatalytic technology can utilize the solar energy to water-splitting for hygrogen production,reduction of carbon dioxide and formation of organics,and degradation of organic pollutants,which could provide an important way to solve the energy shortage and the environmental pollution.Zinc oxide nanosheets,self-assemblied pompon-like zinc oxide mesocrystals,titanium dioxide nanosheets,and titanium dioxide mesocrystals catalysts were prepared by a hydrothermal method.The surface and bulk oxygen defects of the catalysts were regulated by annealing the catalysts in different atmospheres and the effect of defects on the photocatalytic activity was revealed.In addition,the interaction between oxygen defect and precious metal alloy on photocatalytic activity was investigated as well.The overall results result to the following conclusions.1.The ZnO-S was used as a model photocatalyst to uncover the correlation between the photocatalytic activity and surface species via studying their effects on the generation of reactive oxygen species?ROS?.Our results indicates that the surface defects and absorbed H₂O and O₂ play a highly pivotal role in the generation of ROS.The hydroxyl radical is the key species,and electrons and hole contribute equally to the ROS generation upon the ZnO-S sample.More importantly,the water and hydroxyl groups adsorbed on the surface defects are beneficial to the improvement of the surface acidic sites,which could contribute to the adsorption of the substrate.The results show that the catalyst with the optimum oxygen defect concentration has the best photocatalytic activity.Additionally,the ZnO-S samples exhibit higher photocatalytic activities than that of the rod-like ones,due to the high charge carrier separation efficiency in the samples with polar?001?and?00-1?facets.Our study reveals that the improved photocatalytic performance of ZnO-S photocatalysts can not be attributed to the enhanced visible absorption from the defect introduction,but the synergetic effect of defects and acid sites on particle surface.2.The microstructure and defects of the diverse ZnO photocatalysts were examined with various techniques.The results indicated that the P-ZnO assemblies were composed of mesocrystal nanosheets exposed high energy?002?facet with high crystallinity.Moreover,the results also showed that the P-ZnO exhibited higher photocatalytic activity than that of the nanosized ZnO?N-ZnO?,which could be attributed to not only the unique mesocrystal structure and high energy?002?facet exposed,but also the defects located at interfaces among nanocrystals in ZnO mesocrystals.In addition,the formation mechanism and the change of oxygen defect concentration at interface of the P-ZnO were investigated via a time-dependent method.It was found that the formation of P-ZnO hierarchical architecture assembled with ZnO mesocrystals involved a nonclassical crystallization growth and Ostwald Ripening process,interfacial oxygen defect concentration and crystallinity increase first and then decrease.3.The relationship between the concentration of defects and photocatalytic activity of S-TiO₂ was studied.Compared with the catalyst calcined in oxygen atmosphere at 500oC,the bulk and surface defects of the catalysts treated in hydrogen at 550oC for different periods were obviously increased.The results show that the separation efficiency of electrons and holes and charge transfer rate in the catalyst with suitable surface oxygen defects are significantly higher than those of the catalyst with more defects or the TO500 with less defects.At the same time,the appropriate concentration of surface oxygen defects is also beneficial to the surface acidic sites,and the synergetic effect of defects and surface acidic sites contributes to the improvement of the photocatalytic activity.4.The results of the TiO₂ mesocrystals proved that photocatalytic activity largely depends on defects,such as lattice distortion and oxygen vacancies,which are located at highly aligned interfaces of intergrains within TMC.Moreover,the mesocrystalline TiO₂ photocatalysts exhibited higher photocatalytic performance in organic degradation and hydrogen evolution,compared with single crystals and polycrystals;this can be ascribed to an appropriate amount of interfacial defects at the intergrains and improved carrier separation efficiency through the highly oriented interfaces.In addition,the photocatalytic performance of the TMC could be further improved through the regulation of defects by undergoing an annealing process under a redox atmosphere.5.Noble metal alloys were uniformly dispersed on the defective TiO₂-S with?001?facet exposed by an impregnation combined with reduction method.The effects of different alloy constituents,alloy loading,alloy ratios,and defect concentration of support on the photocatalytic properties were investigated.The catalysts with the best photocatalytic performance were characterized and analyzed in detail.The results show that the interaction of surface oxygen defect and the loaded precious metal alloys can synergistically improve the separation and transfer efficiency of electrons and holes,leading to the high photocatalytic activity.