| Recently,the environmental pollution and energy crisis are becoming more and more serious caused by industrial development.As a green technology of environmental protection,photocatalytic water splitting and photocatalytic degradation of organic dyes assisted by a semiconductor have been considered to be one of the most promising approaches for solving both energy and environmental issues worldwide.However,there are still significant problems associated with the development of photocatalysts for photocatalytic reaction,such as poor visible light adsorption and low quantum efficiency in the visible light region.Therefore,it is still a challenge to increase the quantum efficiency,stability,photocatalytic activity for the broad spectrum responsive photocatalysts for further practical application.Among them,tungsten trioxide?WO3?,a transition metal oxide semiconductor with special tunnel structure,appropriate band gap energy,excellent stability against photocorrosion and non-toxic property,has received great attention because of its potential applications in the field of photocatalysis.However,the conduction band of WO3 is about 0.4 eV below the hydrogen redox potential,too low for hydrogen production,resulted in weak photocatalytic activity.The studies in this thesis have been focused on the fabrication of WO3 heterojunction photocatalysts with hierarchical structure by compositing with other oxides,for instance TiO2,Bi2O3,BiOCl and AgCl,based on the modulation of band-gap and photocatalytic properties of semiconductors.To explore their tremendous optical and photocatalytic properties,advanced techniques including X-ray diffraction?XRD?,field emission scanning electron microscopy?FESEM?,transmission electron microscopy?TEM?,high resolution transmission electron microscopy?HRTEM?,X-ray photoelectron spectroscopy?XPS?,UV-Vis absorption,photoluminescence?PL?,photocurrent,Mott-Schottky plots and electrochemical impedance spectroscopy?EIS?,were employed to characterize the as-synthesized heterojunction photocatalysts.Based on the above-mentioned research contents,the mainly research works have been carried out as follows:?1?Monoclinic tungsten trioxide with miro-nano porous sphere-like structure has been synthesized via hydrothermal method.Then,the miro-nano TiO2/WO3 series heterojunction photocatalysts with porous hierarchical sphere-like structure have been synthesized via the second hydrothermal method.The structural characterizations revealed that heterojunctions have been successfully constructed between the TiO2and WO3.The molar ratio of TiO2 and WO3 has a significant influence on the activity of the photocatalysts.The TiO2/WO3?WTO3?sample performed the best photocatalytic properties among all of the tested samples,which exhibited great photocatalytic ability for the degradation of malachite green?MG?under visible light???420 nm?,the MG dye was completely degraded on the WTO3 nanocomposite within 60 minutes.Moreover,the efficient visible light activated photocatalyst remained stable after four irradiation cycles.The enhanced photocatalytic activity could be attributed to three-dimensional hierarchical structure and the easier transition and separation of photogenerated electron–hole pairs in Ti O2 and WO3heterojunctions,due to their matching band positions.?2?A series Bi2O3/WO3 hierarchitecture p-n heterojunction photocatalysts were fabricated by a two-step hydrothermal method.With different dosage of Bi?NO3?3·5H2O,varies morphology Bi2O3 nanoparticles were grown on the micrometer-scale flower-like hierarchical WO3.The three-dimension?3D?Bi2O3/WO3 photocatalysts exhibited higher photocatalytic activity with high stability under visible light illumination,which was much higher than that of pure Bi2O3 and WO3.The optimized WBO4 composite achieved highest photocatalytic degradation activity.It could almost completely degrade the MG dye reaching up to 99.9%within60 min under visible light irradiation.In addition,the catalyst was recoverable and reusable after four cycling runs,demonstrating the good stability of Bi2O3/WO3photocatalysts.The enhanced activity was mainly attributed to their p-n heterojunction structure.The charge transfer between Bi2O3 and WO3 restricted the recombination of photogenerated electrons and holes,which resulted in higher photocatalytic activity than that of single phase photocatalysts.Furthermore the uniquely hierarchitecture structure and good visible light absorption also played an important role.Taken them together,a photocatalytic enhancement mechanism of the p-n heterojunctions has been proposed based on the relative band gap position of these two semiconductors and the integrated heterostructure.?3?A series 2D BiOCl nano-plates and 1D WO3 nano-rods hierarchitecture p-n heterojunction photocatalysts were fabricated by a two-step hydrothermal method.With different dosage of Bi?NO3?3·5H2O and KCl,varies contents BiOCl nanoparticles were grown on the WO3 rods.The 3D BiOCl/WO3 photocatalysts exhibited higher photocatalytic activity with high stability under visible light illumination.Bismuth oxychloride?BiOCl?showed high efficient photocatalytic performance due to its layered structure,[Cl–Bi–O–Bi–Cl]sheets stacked through nonbonding interactions.The large spaces between these sheets help the separation of the photo-induced electron–hole pairs,which was the critical factor in photocatalysis.The optimized WBOC3 composite achieved highest photocatalytic degradation activity.It could almost completely degrade the MG dye reaching up to 99.9%within40 min under visible light irradiation.In addition,the catalyst was recoverable and reusable after four cycling runs,demonstrating the good stability of BiOCl/WO3photocatalysts.The enhanced activity was mainly attributed to their p-n heterojunction structure.The charge transfer between BiOCl and WO3 restricted the recombination of photogenerated electrons and holes,which resulted in higher photocatalytic activity than that of single phase photocatalysts.Furthermore,the good visible light absorption also played an important role which could be attributed to the oxygen vacancies on the surface of samples.Taken them together,a photocatalytic enhancement mechanism of the p-n heterojunctions has been proposed based on the relative band gap position of these two semiconductors and the integrated heterostructure.?4?The Ag/AgCl nanoparticles decorated on the hexagonal phase WO3nanorods as a hierarchical Z-scheme photocatalytic system has been successfully constructed by means of solvothermal reactions and precipitation route,followed by photoinduced reduction technique.The enhanced photocatalytic efficiency might arise from the Surface Plasmon Resonance?SPR?effect from metallic Ag and photosensitization of AgCl.The interaction between Ag/AgCl and WO3 has been investigated by PL,M-S,and EIS analysis,and which was facile to the electron transfer between Ag/AgCl and WO3 and as well as leaving more charge carriers to form reactive species,resulted in the enhancing photocatalytic activity of Ag/AgCl/WO3 composite Z-scheme photocatalytic system.The WOAC4 sample showed the highest photocatalytic activity,it could almost completely degrade the MG dye reaching up to 99.9%within 50 min under visible light irradiation..A possible catalytic mechanism for Ag/AgCl/WO3 was proposed,based on the relative band gap position of these two semiconductors and the integrated heterostructure. |
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