| Resource shortages and environmental pollution are two maj or problems restricting the development of human society in the 21st century,which have attracted worldwide attention.However,the main resources for maintaining the world economic development are primary energy sources-mainly fossil energy,and the consumption is increasing year by year.Facing the increasingly severe and crisis situation,people begin to focus on the development and utilization of renewable and clean energy,and inexhaustible solar energy is the most ideal energy which attracts the most attention.Photocatalysis technology can convert solar energy into storable chemical energy through semiconductor photocatalysts,which provides a very promising and effective way for humans to deal with energy and environmental problems.Also,hydrogen(H2)is a very ideal clean energy source,semiconductor photocatalytic hydrogen production has been attracting worldwide attention since its appearance,and the investigation on photocatalysis has last for decades.Photocatalytic hydriodic acid(HI)splitting is a more facile way to convert solar energy into chemical energy than H2O splitting.Compared to the fo ur-electron process of H2O splitting,the HI splitting is a two-electron process,and it is easier to catalytically split HI because the oxidation overpotential of I-is lower.In addition,the Iodine-Sulfur Cycle proposed by General Atomics of the United States is recognized as the most efficient reaction to thermally catalyze water splitting for hydrogen production.A key step in this reaction is thermal decomposition of HI to produce hydrogen,thus studying on the photocatalytic HI splitting to replace thermal energy with solar energy is very important and valuable in industry.On the other hand,the working environment of photocatalytic HI splitting is a strong acidic solution,most of the traditional photocatalysts cannot be stably therein,let alone play a role in photocatalysis.Therefore,it is particularly important to find new and suitable high-efficiency photocatalysts for photocatalytic HI splitting.The organic-metal hybrid halide perovskite materials are rising star materials that recently emerged with the rapid development of solar cells.These perovskite materials have shown many advantages,such as wide light absorption range,long carrier diffusion distance,long carrier life,and bipolar charge transport,which are exactly the necessary physical properties required by semiconductor photocatalysts and show an attractive prospect of photocatalytic application.Surprisingly,lead-iodine perovskite materials represented by MAPbI3(MA=CH3NH3)can exist stably in the aqueous HI solution saturated with MAPbI3,and working as photocatalysts for photocatalytic HI splitting.However,the efficiency for the phtocatalytic HI splitting using MAPbI3 as a photocatalyst is still low according to the only report about halides perovskite photocatalysis in the aqueous solution.According to the mechanism of phtocatalysis,the process of a photocatalytic reaction mainly includes the light absorption and carriers generation in the semiconductor,transport and separation of photogenerated carriers,and redox reaction on the surface of the catalyst.Since the halide perovskite material is a direct bandgap semiconductor,and it has a wide-range light absorption extending to near infrared region,so the most possible factor for limiting its photocatalytic performance is the low transport and separation efficiency of photogenerated carriers.However,there are few reports on the modification of perovskite photocatalysts.This is because the photocatalysis environment of perovskite materials is strong acid,the common methods for phtocatalyasts modification,such as semiconductor compositing to construct heterojunctions,dye modification,surface organic molecular modification,are infeasible.Therefore,it is very necessary to find out suitable and feasible methods for perovskite photocatalysts modification in the current strong acid system.In this thesis,the organic-metal hybrid halide perovskite MAPbX3(X = I,Br)materials,which show outstanding performances in the field of solar cells,are applied to the field of photocatalysis in aqueous solutions.To improve their phtocatalytic HX(X = I,Br)splitting activities,measures are taken from the aspects of material properties and photocatalysis mechanism.The crystallinity and morphology of the material are improved by optimizing the conditions of material synthesis and post-treatment,the interfacial separation efficiency between photogenerated electrons and holes is enhanced by surface modification and material compositing,the migration and transport of photogenerated carriers from the interior to the surface of the perovskite particle are accelerated by controlling the concentration of material components,and the recombination of photogenerated electrons and holes is restrained by adjusting the ion proportion of material composition.The details are as follows:In chapter one,the development history,the characteristics of crystal structure and electronic structure,physical and chemical properties and their main applications in photocatalysis of perovskite materials,were introduced.Then,the halide perovskite,as a class of perovskite materials,was introduced in detail,including its origin and development,structure and basic properties,and particularly its rise and development in the field of solar cells.In addition,we summarized the application and development of halide perovskite materials in the field of photocatalysis in recent years,detailed its application prospect and significance in the field of aqueous photocatalysis,and explored the current problems in photocatalysis of halide perovskite materials.Finally,the content and significance of the thesis were summarized.In chapter two,powder samples of organic-metal halide perovskites MAPbI3 and MAPbBr3 were synthesized by saturated solution precipitation method,and the saturated solutions using corresponding hydrohalic acids as solvents were prepared,where the photocatalytic HX splitting were carried out using perovskite photocatalysts.The morphology and crystallinity of perovskite materials were modified by controlling the synthesis temperature,improving the photocatalytic performance of perovskite materials to some degree.Then,the morphology and crystallinity of perovskite materials were further optimized by post-treatment in polar solvent atmosphere,which further improved the dispersion of the materials in saturated solution.Together with optimizing the amount of noble metal loaded on the surface of perovskite particles as cocatalyst,the photocatalytic hydrogen evolution activity of perovskite powder samples was enhanced by more than 20 times.In chapter three,graphene oxide(GO)with good acid resistance was selected to modify the MAPbI3 perovskite in strong acidic solution by photoreduction method,successfully synthesizing the MAPbI3/rGO(rGO= reduced GO)composite.When used in aqueous HI solution saturated with MAPbI3 for photocatalytic hydrogen production,the hydrogen evolution activity of MAPbI3/rGO is 67 times that of pure MAPbI3 under the same measuring conditions,and the former exhibits excellent photocatalytic hydrogen evolution stability.Through a series of characterization and theoretical analysis,we proposed that rGO was a good electron conductor and receiver,after its compositing with MAPbI3,the photogenerated carriers generated in MAPbI3 could be rapidly transferred to rGO through the Pb-O-C bond formed between them,getting separation effectively with photogenerated holes,reducing protons to produce hydrogen at the rGO sites.Herein,the great enhancement of photocatalytic H2 evolution of perovskite material was obtained through improving the separation efficiency of interfacial photogenerated carriers.In chapter four,the mixed halide perovskite material MAPbBr3-xIx was synthesized through ion exchange method by introducing I-to MAPbBr3,and light irradiation was innovatively introduced to the materials synthesis process,which is proved to accelerate the ion exchange and affect the distribution of halogen ions.It was found that the bandgap of MAPbBr3-xIx particles tended to be narrowest on the surface and became wider on going into the interior resulting from the iodine-concentration gradient,so that they had a correct bandgap funnel structure that was needed for transferring photogenerated charge carriers from the interior to the surface,which was verified by a series of characterization methods.In addition,cocatalysts were loaded on the surface of MAPbBr3-xIx particles to improve the superficial separation of photogenerated carriers,as a result,a photocatalytic solar-to-chemical conversion efficiency of 1.05%was achieved.Herein,the efficiency of photocatalytic H2 evolution using perovskite was greatly improved achieved by promoting the carrier transf-er from the internal to the surface of photocatalyst particles.In chapter five,mixed A-site perovskite MAxFA1-xPbI3(0.2 ≤ x ≤ 1)was synthesized by saturated solution precipitation method,and the effects of MA/FA ratio on the crystal structure,morphology and band gap of perovskite materials were studied.Through the diffuse reflection and fluorescence spectra,we found that the band gap of MAxFAx-xPbI3 gradually narrowed with the increase of FA+ ratio,while the photocatalytic activities of the materials did not increase with the expanding light absorption range.Interestingly,when x = 0.5,namely,MA0.5FA0.5PbI3 shew the best photocatalytic activity,which corresponded to the best crystallinity and better morphology.This was because,on the one hand,when MA/FA = 1:1,mixed A-site MAxFA1-xPbI3 perovskite had the lowest energy and the most stable crystal structure;on the other hand,MAxFA1-xPbI3 was proved to have a slower carrier recombination rate than thoses of other FA/MA ratios of MAxFA1-xPbI3 perovskites according to the fluoresce lifetime measurement,which made photogenerated carriers in MAxFA1-xPbI3 more likely to get separation in the photocatalytic process.Herein,a better photocatalysis performance of perovskite was obtained by improving the physicochemical properties and reducing the recombination rate of photogenerated carriers.In chapter six,the main research contents and conclusions of this thesis were comprehensively summarized,the main innovation points were given,and the shortcomings in the current work were pointed out and discussed.Finally,a prospect and an outlook were provided for the future of perovskite photocatalysis. |
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