Study On The Design And Preparation Of Semiconductor Heterojunction Composites And Their Efficient Photocatalytic H₂ Generation Properties From H₂O Splitting

Materials,energy and information are considered to be the three pillar industries for the development of human society now and in the future.Semiconductor composites have a wide applications range in modern electronic technology,energy,photocatalysis,sensing,lighting,communications and biomedicine due to their unique physical and chemical properties,and will be developed rapidly in these fields in the future.Since the Honda-Fujishima effect was discovered in 1972,semiconductor photocatalytic technology that uses semiconductor materials to convert solar energy into chemical energy has developed rapidly.Among them,the semiconductor photocatalytic water splitting technology for hydrogen production can convert solar energy into carbon-free clean energy(hydrogen energy),which is considered to be one of the ideal technologies to deal with the fossil energy crisis and environmental pollution problems.However,after 50 years of development,the problem of low energy conversion efficiency from solar energy to hydrogen energy has always plagued researchers and hindered further industrial production.To solve this problem,researchers have made active attempts in two aspects:(1)Regulating the energy band structures of semiconductors to make them have the performance of photocatalytic hydrogen production from water splitting(meeting thermodynamic requirements);(2)Constructing heterojunction composites to accelerate the separation and transfer of photogenerated electron-hole pairs(photogenerated carrier pairs),and increase the activity of photocatalytic hydrogen production(breaking through the kinetics limitations).In many attempts,the construction of semiconductor heterojunction composites can effectively enhance the separation and migration rates of photogenerated carrier pairs,and improve the photocatalytic hydrogen production rate from water splitting.The main purpose of this thesis is to design and prepare semiconductor heterojunction composite materials,and several kind of composites for high-efficiency photocatalytic hydrogen production from water splitting have been successfully prepared.The performance and mechanism of the photocatalytic hydrogen production from water splitting with the obtained composites are systematically studied.At the same time,the relationship between the sample's microstructure and the hydrogen evolution performance is analyzed,and the constructions of different types of heterostructures and their influences on the separation and recombination of photogenerated electron-hole pairs are also discussed by the photoelectric performance tests.The specific research content is as follows:1.The preparation and efficient photocatalytic hydrogen production performance from water splitting research of Cu₂O/MTiO₃(M=Ca,Sr,Ba)p-n heterojunction composites.Firstly,CaTiO₃,Sr TiO₃and Ba TiO₃with different morphology are synthesized by hydrothermal method,which are hollow cuboid shape,pebble shape and coral shape,respectively.Then,the Cu₂O/MTiO₃p-n heterojunction composites are successfully prepared by loading a layer of Cu₂O on the surface of MTiO₃via Na BH₄reduction method.Through comparison experiments,it is found that when ethylene glycol and polyvinylpyrrolidone(PVP)are added to water at the same time,Cu₂O nanoparticles can be uniformly dispersed on the surface of CaTiO₃,indicating that ethylene glycol and PVP are beneficial to the dispersion of Cu₂O nanoparticles.The photocatalytic hydrogen production experiment shows that the Cu₂O/CaTiO₃composite photocatalyst has the best hydrogen production performance among Cu₂O/MTiO₃samples under?>300 nm light irradiation.Among the Cu₂O/CaTiO₃series of samples,when the reactants calcium titanate and copper acetate are 50 and 10 mg,respectively(50Ca10Cu),the sample has the highest hydrogen evolution rate(HER 8.268 mmol g^-1h^-1),which is about 344.5 times as high as that of CaTiO₃.And the 50Ca10Cu sample showed good stability in the photocatalytic cycle repeat experiment.The ultraviolet-visible(UV-Vis)diffuse reflectance spectrum test shows that after loading Cu₂O,the light absorption range of the Cu₂O/CaTiO₃composite sample can be extended to the visible light region.The photoelectric performance tests results showed that the recombination efficiencies of the photogenerated electron-hole pairs in Cu₂O/CaTiO₃composite are significantly reduced and the separation and migration rates are signally increased as compared to pure CaTiO₃sample,which is helpful to enhance the photocatalytic hydrogen generation performance.In addition,the formation process of the p-n heterojunction is analyzed in the Cu₂O/CaTiO₃samples.2.The preparation and efficient photocatalytic hydrogen production performance from water splitting research of the CaTiO₃/Cu/TiO₂vector Z-type heterojunction composite.Using the synthesized hollow CaTiO₃as carrier in the first part of the work,the all-solid CaTiO₃/Cu/TiO₂vector Z-type heterojunction composite is successfully synthesized via the two-step hydrothermal method for the first time.Through comparison experiments,it is found that isopropanol in the solution can control the in-situ hydrolysis of CaTiO₃to form TiO₂nanosheets in the second hydrothermal process.Under?>300 nm light irradiation,the highest photocatalytic HER of the CaTiO₃/Cu/TiO₂composite samples can reach 23.550 mmol g^-1h^-1,which is about 981.3 times and 93.1 times as high as that of the CaTiO₃and CaTiO₃/TiO₂samples,respectively.And the CaTiO₃/Cu/TiO₂sample showed good stability in the photocatalytic cycle repeat experiment.The Density Functional Theory calculation(DFT)was uesd to prove that the vector Z-type heterojunction is formed in the CaTiO₃/Cu/TiO₂sample.The broad and weak absorption peak at about 400-595 nm in the UV-Vis diffuse reflectance spectrum is the characteristic absorption peak of the surface plasmon resonance effect(SPR)of metal Cu.The SPR effect of Cu is conducive to the separation of photogenerated carrier pairs and can enhance the photocatalytic performance of sample.The photoelectric performance test results show that the construction of vector Z-type heterojunctions in CaTiO₃/Cu/TiO₂composite and the electronic mediator(Cu nanoparticles)in the material can effectively enhance the separation and migration rates of photogenerated electron-hole pairs,reduce their recombination efficiencies,and improve the photocatalytic hydrogen production activity.3.The preparation and photocatalytic hydrogen production performance from water splitting research of Bi@H-TiO₂/B-C₃N₄type II heterojunction composite with visible light response.Visible light accounts for about 43%of the energy of sunlight.In order to make full use of visible light in sunlight and break through the limitation that TiO₂cannot use visible light,the dark brown B-C₃N₄and black TiO₂are prepared by the Na BH₄thermal reduction method firstly,respectively.Then,a simple hydrothermal method is used to successfully prepare a type II heterojunction Bi@H-TiO₂/B-C₃N₄composite photocatalyst with strong visible light absorption.Under?>300 nm light and visible light(?>400 nm)irradiation,the highest photocatalytic HER of the Bi@H-TiO₂/B-C₃N₄composites can be up to 223.08 and18.84?mol g^-1h^-1,respectively.And the Bi@Ti-BCN-2 sample also shows good stability in repeated experiments.The UV photoelectron spectroscopy(UPS)test is used to prove that there are two kind of photo-generated electron transfer pathways in Bi@H-TiO₂/B-C₃N₄type II heterojunction,namely B-C₃N₄?H-TiO₂?Biand B-C₃N₄?Bi,respectively.In the UV-Vis diffuse reflectance spectrum,the metal Bihas a strong absorption peak at 279 nm and a broad and weak absorption peak at about 350-600 nm.They are the characteristic absorption peaks of the SPR effect of metal Bi,which can enhance visible light capture,promote photogenerated charge separation and improve the hydrogen production activity of the photocatalyst.In addition,the photoelectric performance test results show that when Bimetal spheres are loaded on the surface of H-TiO₂/B-C₃N₄,the separation and migration rates of photogenerated electron-hole pairs increase and their recombination efficiencies decrease,which are beneficial to improve the photocatalytic hydrogen evolution performance.4.The preparation and efficient photocatalytic hydrogen production performance from water splitting research of kermesinus BiOI with oxygen vacancy defects.Firstly,the orange O-BiOI is synthesized by hydrothermal method;and then,the kermesinus K-BiOI with many oxygen vacancy defects on the surface is prepared by the light driving method.Under UV-Vis light irradiation,oxygen vacancy defects are introduced on the K-BiOI surface,and the color of the sample changes from orange to kermesinus.In the case of no precious metal cocatalysts loading,the HER of the K-BiOI sample reached 6.51 mmol g^-1h^-1.It is proved that oxygen vacancy defects can be used as active sites for photocatalytic hydrogen production from water splitting because the HER is reduced after adding H₂O₂to the reaction solution.Electron paramagnetic resonance spectroscopy and surface photovoltage tests confirmed that the oxygen vacancy defects are introduced on the K-BiOI surface.By testing the Mott-Schottky line of the sample,it is proved that the energy band position of K-BiOI sample meets the requirements of the photocatalytic hydrogen production from water splitting.The UV-Vis diffuse reflectance spectrum test shows that the light absorption range of K-BiOI extends to the entire visible light region,which enhances light absorption and promotes the improvement of photocatalytic hydrogen production activity.Moreover,the photoluminescence spectrum test result shows that the photogenerated electron-hole pair on the surface of K-BiOI has a low recombination efficiency,which is helpful to enhance the photocatalytic hydrogen generation performance.In addition,the possible formation process of oxygen vacancy defects and the excitation-recombination process of photogenerated carrier pairs on the surface of K-BiOI are proposed.