Study On The Design Synthesis And Photocatalytic Performances Of Bi-based Photocatalysts

Semiconductor photocatalytic technology that can transform the inexhaustible low-density solar energy into high-density electrochemical energy has been widely used in the removal of organic pollutants from water or air,reduction of CO₂ for methane production,photocatalytic water splitting for hydrogen generation,etc.This technology provides a clean and efficient way to solve the two major problems,namely environmental pollution and energy crisis.As is well known,the photocatalysts play the most important role in photocatalytic technology.However,the traditional semiconductor photocatalysts?such as TiO₂,ZnO,etc.?are restricted in large-scale industrial applications due to their low quantum efficiency and inefficient utilization of solar energy.To make full use of the advantages of the photocatalytic technology in environmental protection and new energy utilization,it should commercialize the photocatalytic technology and improve its practical application.Therefore,design and development of other efficient photocatalytic materials become inevitable.In recent years,bismuth-based semiconductor materials have attracted more attention due to their unique band structure,excellent photocatalytic performance,low cost,stable physicochemical and environment-friendly properties.Although the preparation and modification of bismuth-based photocatalytic materials have been comprehensively investigated by many scholars,the rapid recombination of photogenerated charge carriers still seriously hinders the improvement of the catalytic performance.Based on this,bismuth tungstate?Bi₂WO₆?and bismuth phosphate?BiPO₄?,two common bismuth-based photocatalytic materials,were selected as research targets in the present study.By adjusting the morphology,construction of heterojunction structure and surface modification,the separation efficiency of photogenerated charge carriers were significantly improved and hence resulted in the enhanced photocatalytic performances.Using rhodamine B?RhB?solution as simulated dye wastewater,the photocatalytic activity of the obtained catalyst was evaluated.The effects of the morphology,grain size,specific surface area,band structure and crystallinity of the photocatalysts on the performances were investigated.In addition,the relationship between the structural characteristics and their photocatalytic performances was also discussed.The main results are as follows:?1?Using L-lysine as soft template,Bi₂WO₆ with three different types of morphologies were selectively synthesized by adjusting the pH value of the hydrothermal solution.According to the experimental results,the hierarchical flower-like Bi₂WO₆,clew-like Bi₂WO₆ constructed by perpendicularly interwoven nanosheets and nanoplates-like Bi₂WO₆ could be selectively synthesized when the pH values of the solutions were adjusted to be 1,6 and 11,respectively.According to the effects of hydrothermal time on the crystal structure and morphology,the formation mechanisms of different Bi₂WO₆structures were investigated.The results indicated that the flower-like and clew-like Bi₂WO₆superstructures might be formed through self-aggregation,Ostwald ripening and self-assembly processes,while the plate-like Bi₂WO₆ was formed through the anisotropic growth of the formed nanoparticles paralleled to the layed structure.In addition,the relationship between the structural characteristics and their photocatalytic performances was also discussed.It was found that the flower-like Bi₂WO₆ exhibited the highest photoactivity in RhB degradation under visible light irradiation.The enhanced photocatalytic activity was mainly ascribed to the unique flower-like structure that could provide more reactive sites and enhance the light absorption,thereby leading to the improved photocatalytic performances.?2?Nest-like bismuth tungstate?Bi₂WO₆?was incorporated with reduced graphene oxide?RGO?via an ionic liquid-assisted hydrothermal process.The[Bmim][BF₄]?1-butyl-3-methylimidazolium tetrafluoroborate?ionic liquid,as a kind of green solvent,acted as a soft template during the synthetic process.The synergistic effects of RGO and ionic liquids on the improved performances of Bi₂WO₆ were systematically studied.The results indicated that the ionic liquid not only played a role in morphological modulation,but also acted as a dispersant to promote the uniform distribution of the nest-like Bi₂WO₆ onto the RGO surface.This was beneficial to the interface contact between the organic pollutants and the catalyst,thus leading to the improved photocatalytic performance.In addition,the introduction of RGO could significantly enhance the photocatalytic performances of Bi₂WO₆.The improved performances were correlated with the large surface area,intense light absorption and the efficient separation of photoinduced electron-hole pairs.Noticeably,when the RGO content was beyond 5.0 wt%,the photocatalytic performance of Bi₂WO₆/RGO composites deteriorated.The excessive RGO could serve as a kind of recombination center and improve the recombination rate of photoinduced electron-hole pairs,thus decreasing the photocatalytic performance.?3?Using[Bmim][PF₆]?1-butyl-3-methylimidazolium hexafluorophosphate?as reactant and template,hierarchical flower-like BiPO₄ microspheres were successfully synthesized by a microwave-assisted hydrothermal route.The effects of heating mode and ionic liquid type?cation,carbon chain length?on the morphological modulation of BiPO₄ were investigated in detail.On the basis of a series of time-dependent experiments,a possible mechanism was proposed for the formation of the hierarchical flower-like Bi PO₄microspheres.The composition of the as-prepared samples was analyzed by Fourier transform infrared spectroscopy?FT-IR?and X-ray photoelectron spectroscopy?XPS?,which confirmed that the[Bmim]⁺cation was modified onto the surface of the BiPO₄ photocatalyst.Based on the experimental results,it was suggested that the significantly improved photocatalytic activity of BiPO₄ could be mainly attributed to the ionic liquid modification,which not only enhanced the light absorption but also effectively improved the separation efficiency of photogenerated electrons and holes,thereby prolonging the lifetime of carriers.This facilitated the oxidation-reduction reaction of photocatalytic process and thus resulted in the improvement of photocatalytic activity.?4?BiPO₄-MoS₂/graphene ternary heterojunction photocatalyst was prepared through hydrothermal route using Mo S₂/graphene as cocatalyst.Through optimizing the loading content of each component,the BiPO₄-MoS₂/graphene nanocomposite exhibited the highest photocatalytic activity for the degradation of Rhodamine?RhB?when the content of MoS₂ and graphene was 2 wt%and 7 wt%,respectively.The enhanced photocatalytic activity of the new composite photocatalyst was attributed to the positive synergetic effect of the layered graphene and MoS₂ as cocatalyst,which provided more reactive sites available to participate in the oxidation-reduction process and significantly enhanced the transfer and separation efficiency of the photogenerated charge carriers.Based on the theoretical calculations and trapping experiments,the charge transfer and reaction path in BiPO₄-MoS₂/graphene ternary composite were elucidated,which might provide theoretical basis for design and construction of other semiconductor/graphene/MoS₂ composite photocatalysts.