| Energy,as an important element in promoting the progress of human civilization,has always attracted great attention.With the rapid development of modern industry and the ever-increasing of world population,fossil fuels,an essential part in the energy supply,are increasingly depleted.Meanwhile,the environmental problems caused by burning of fossil fuels are becoming increasingly serious,too.Therefore,exploring a clean and renewable alternative energy source has been on the research agenda.Among them,photocatalytic water splitting for hydrogen evolution has gained extensiveattention among scientific researchers all over the world for decades.Solar energy,as a renewable resource,has its inexhaustible feature that ensures a constant supply of energy.Photocatalytic water splitting,utilizing sunlight energy as the only energy source,is an effective means to convert solar energy into chemical energy(hydrogen energy).The hydrogen energy has the advantages of high energy density,ease of storage and transportation.Moreover,the hydrogen combustion product is water,clean and pollution free,which is an environmentally friendly fuel to the sustainable development.Therefore,photocatalytic hydrogen production,theoretically,can simutaneously solve the two major world problems:energy shortage and environmental pollution.Although some breakthroughs have been made in the field of photocatalysis,there is still a long way to go before industrialization.As for the photocatalysis,the key matter is the development of efficient and stable photocatalysts.In order to maximize the conversion of solar energy,two prerequisites are in consideration for photocatalysts:a wide range of sunlight absorption and high photo-generated carrier separation as well as transfer efficiency.The heterostructures,in which the valence band and conduction band arrangements are able to satisfy type-II or Z-scheme heterojunctions,can perfectly combine the advantages of two kinds of individual semiconductor,greatly improving the carrier separation efficiency.Among numerous semiconductor photocatalysts,CdS has been specifically interesting due to its narrow band gap and proper band edge position for water splitting in the visible light area.However,its photocatalytic activity and stability are largely hindered by high recombination rate of photo-generated electron-hole pairs and severe photocorrosion.Here,we systematically develop the effective strategies to design a series of CdS-based composite nanomaterials with the aim at improving the efficiency of hydrogen production.The specific research details are in the following five chapters:In the first chapter,we firstly introduced the research background of photocatalytic hydrogen production.And then,we systematically summarized the basic principle of photocatalytic hydrogen production from semiconductor photocatalysts,the research progress of visible-light-driven photocatalysts,and several effective strategies to construct high-efficiency photocatalysts.Based on research results and theoretical knowledge,the research contents and significance of this thesis are presented.In the second chapter,a series of size-controlled CdS core-shell and CdS@ZnxCd1-XS core-double shell submicrospheres were synthesized by a simple reflux method.Polyvinylpyrrolidone worked as a soft template combined with the intense thermal decomposition of thiourea.Monodispersed CdS submicrospheres with core-shell structure were obtained by quenching the reactions at 185 ℃.Alternatively,Zn source was injected into the above reaction system when the CdS core-shell structure was basically formed.The large concentration difference between Zn2+ and Cd2+ was adopted to overcome the solubility constant difference between ZnS and CdS,to enable the cation exchange reaction.A layer of solid solution shell of ZnxCd1-xS was coated outside of CdS core-shell structure to form core-double shell submicrospheres.The corresponding structure and growth mechanism were demonstrated by a series of characterizations.In addition,a comparative investigation of their photocatalytic properties showed that the hydrogen production activity of CdS@ZnxCdi-xS was 5.17 mmol h-1 g-1,which was 12.3 times higher than that of the pure CdS core-shell structure.The steady-state PL and EIS test have proved that the composite material has enhanced carrier separation efficiency in photocatalytic hydrogen production.In the third chapter,multi-component Au-CdS/ZnS-RGO heterojunction nanomaterials were designed.First,CdS/ZnS-RGO heterostructure was synthesized by a simple one-step solvent method.The molar ratio of CdS to ZnS can be well controlled by feeding.During the reaction,diethylenetriamine and graphene played a decisive role in regulation of sample morphology and structure.In addition,the variation in solubility product constants of two kinds of semiconductors made ZnS and CdS precipitate nonsynchronously,leading to the formation of heterojunctions.Then,Au nanoparticles were loaded on CdS/ZnS-RGO by one-step reflow.These nanocrystals were selectively precipitated on the surface of semiconductors because of the negatively charged surface of graphene repulsive to AuCl4-.Au nanoparticles maintained the size of around 5 nm with oleylamine acting as a reducing agent and surfactant.In the structure,there were three photogenerated carrier separation paths synergistically,which greatly improved the photocatalytic activity.CdS and ZnS formed a quasi-type-Ⅱ,coupling with excellent transport and capture capabilities of graphene and noble metal Au for photo-generated electrons.The hydrogen production rate of the optimum photocatalyst was as high as 9.96 mmol h-1 g-1.There was no visible decay in hydrogen evolution activity even up to 30 hours.In the fourth chapter,we reported the preparation of Cd1-xZnxS and Cd1-xZnxS-RGO with similar morphology but distinct internal crystal structure by a facile one-pot solvothermal method.Both nanorods obtained in our work with a length of around 30 nm and a diameter of around 10 nm.Cd1-xZnxS with nano-twin structure composed of alternative ZB/WZ,while Cd1-xZnxS-RGO with homogenous WZ phase,due to the excellent thermal conductivity of graphene,which relieved the thermal fluctuation as a buffer during the solvothermal process.The differences in internal structure were closely related to their catalytic properties.The internal electrostatic field formed by the homojunctions can effectively strengthen the carrier mobility during photocatalysis.The dense type-II structure with alternate ZB/WZ alignment not only shortened the carrier migration path but also greatly improved the carrier separation efficiency.Therefore,Cd1-xZnxS exhibited better photocatalytic performance,while Cd1-xZnxS-RGO showed superior photoelectrochemical(PEC)performance due to the perfect single crystalline phase of Cd1-xZnxS combined with the graphene exhibiting excellent conductivity.Such a hybrid structure enabled the photo-generated carriers to move almost freely in the external electric field created by the bias added.The resultant Cd1-XZnxS and Cd1-xZnxS-RGO hold preponderance in photocatalytic and PEC performance,respectively.In the fifth chapter,we designed a redox-mediator-free Z-scheme CdS/Co9S8-RGO semiconductor heterojunction.The growth mechanism of the precursor CdS/CoS-RGO and the photocatalytic mechanism of Z-scheme CdS/Co9S8-RGO were studied.First,we took advantage of graphene to control the morphology of semiconductor materials and synthesized CdS/CoS-RGO via a simple one-step hydrothermal method.Followed by calcination,CdS/Co9S8-RGO was obtained under a reducing atmosphere.Both CdS/Co9S8 heterostructure with core-shell structure and graphene showed prominent ability to capture photons.Moreover,the outstanding electrical conductivity of graphene and the Z-scheme heterojunction of the two semiconductors in close contact with each other make a great contribution to the efficient transfer and separation of photo-generated carriers.Relying on the above excellent light absorption and the effective carrier transfer,the hydrogen evolution rate of CdS/Co9S8-RGO reached up to 4.58 mmol h-1 g-1,nearly twice as high as that of its precursor CdS/CoS-RGO,and the photocatalytic stability was also excellent. |
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