Study On Improving The Charge Separation Efficiency During Photoelectrochemical Water Splitting And Design And Synthesis Of High Efficient Photoelectrodes

With the continuous growth of China's economy and consumption of energy,China's energy shortage has become increasingly prominent.The resulting environmental pollution caused by the large amount of greenhouse gas emissions such as carbon dioxide is also becoming increasingly serious.Therefore,the problem of energy shortage and environmental pollution has become one of the two most important problems restricting the further development of China's economy.Therefore,China has more urgent problems to be solved such as the development of new clean energy,reduce carbon dioxide emissions,China's energy shortage and environmental pollution.To solve energy shortage and environmental pollution,hydrogen is the one of the most potential due to it has a high energy density?120 J/g,about 3 times the gasoline?,and the combustion process will not produce any pollutants.Photoelectrochemical?PEC?water splitting has attracted increasing interests owing to its potential applications on solving energy crisis by producing hydrogen from water and sunlight with high theoretical conversion efficiencies?>30%?It is well known that the conversion efficiency of photoelectrochemical water splitting is mainly determined by the light absorption,the internal carrier separation efficiency and the interfacial carrier separation efficiency.At present,the spectral response range of photoelectrochemical materials has been extended to the entire visible region.Therefore,the spectral response range of photoelectrocatalytic materials is no longer the most important problem to restrict the STH conversion efficiency.Compared to the light absorption,the internal and interfacial carried separation efficiency of the photocathode are still low,which become the key issues of the PEC conversion efficiency and further impact the conversion efficiencies of PEC photoelectrodes.Therefore,it becomes a research emphasis to improve the internal photogenerated carriers separation efficiency and the interfacial carriers separation efficiency during the process of photoelectrochemical water splitting,which promote the development and practical application of photoelectrochemical water splitting technology.Combined with the principle of photoelectrochemical water splitting,the main factors that affect the low of internal photogenerated carriers separation efficiency are as follows:poor conductivity,high defect concentration,the lack of effective driving force for carrier separation and the long diffusion distance of carriers,et al.For the interface photogenerated carriers separation efficiency,the influencing factor is that the low rate constant of the water oxidation reaction and the higher barrier of the water oxidation reaction which leads to the high resistance of the hole from the photoanode to the electrolyte.In this thesis,we research mainly focus on the various strategies have been developed to further improve the internal and interfacial carried separation efficiency of the photocathode with high photoelectric conversion efficiency,such as doping method,constructing homogeneous heterojunction,epitaxial growth of epitaxial growth and the anisotropy of polar single crystal.The specific research contents are as follows:In chapter 1,we briefly introduced the development,main applications,principal mechanism and the influencing factors of semiconductors photoelectrochemical water splitting technology.Then we summarize the current research status and problems of photoelectrochemical water splitting technology.Finally,the significance,research ideas and the outline of this thesis were summarizedIn chapter 2,we mainly use the doping method to improve the internal and interface photogenerated carriers separation efficiency.M:BiVO₄ photocathode?M=Co,Fe,Ni,Mn?was prepared by incorporating 3d transition metal element into BiVO₄,and then also studied the interfacial carrier separation efficiency.The experimental results show that Co doped BiVO₄ photocathode can greatly improve the photocurrent density than that of Fe,Ni and Mn doping.Further study of the Co-doped BiVO₄ photocathode found that Co doping can not only reduce the BiVO₄ onset potential but also increase the interfacial separation efficiency.This indacted the surface exposed Co2+ ions in Co-doped BiVO₄ could act as OECs,which can lower the barriers for water oxidation and promote the interfacial charge separation during PEC water splittingIn chapter 3,we mainly study constructing homojuction to improve the photogenerated carriers separation efficiency.We incorporated the electron acceptor Mo6+ into BiVO₄ to form an n-type semiconductor,incorporate the electron acceptor Co2+ into BiVO₄ to form a p-type semiconductor.We fabricated Mo:BiVO₄/Co:BiVO₄ photoanodes and investigated their PEC performances in this work.As expected,the PEC performances of Mo:BiVO₄/Co:BiVO₄ photoanodes were greatly enhanced without the loading of any OECs,which were attributed to the improved charge separation both in the bulk and at the interface.Co:BiVO₄ layers were found to be important for the enhanced charge separation efficiencies,which can not only act as reactive sites for water oxidation to promote the interfacial charge separation,but also be able to tune the built in electric fields in the and the charge transfer in Co:BiVO₄ layers to promote the bulk charge separation.The effects of Co doping concentrations and the number of Co:BiVO₄ layers on the PEC performances of Mo:BiVO₄/Co:BiVO₄ photoanodes were systematically investigated and optimized in this work.More importantly,the absence of any OECs and the perfect lattice matching between Mo:BiVO₄ and Co:BiVO₄ can greatly reduce the defect numbers in the electrode,which could minimize the charge recombination during PEC water splitting.In chapter 4,we mainly study constructing epitaxial heterojunctions to improve the photogenerated carriers separation efficiency.We fabricated In2O3/ZnO hetero epitaxial-junction photoanodes containing In2O3 nanoparticles with {001} facets exposed and investigated their PEC performances.With the assistance of In2O3,the visible light photoresponses can be observed in In2O3/ZnO heterojunctions.Furthermore,due to the epitaxial relationship between?10-10?ZnO and {-211} In203 planes,In2O3 nanoparticles can be epitaxially grown on the lateral {100} surface of ZnO NRs to form a high quality heterojunction interface,which can lower the charge recombination and facilitate the charge separation during PEC water splitting.Moreover,due the surface energy conservation,low index crystal facets of In203 with lower surface energies,such as {001} and {111} crystal facets,would be formedspontaneously,which can effectively lower the onset potential and facilitate the water oxidation.With the combination of the effects mentioned above,the PEC performance of as prepared In2O3/ZnO hetero-epitaxial-junction photoanodes can be greatly improved compared with either In2O3 or ZnOIn chapter 5,we mainly study the using the built-in electric field in crystal engineering to improve the photogenerated carriers separation efficiency.ZnO is a semiconductor with asymmetric polar crystal structures?P63mc?,where high quality bulk materials can be easily obtained at reasonable cost.ZnO has a strong spontaneous polarization along the[0001]direction?0.047 C/m2?and a high electron mobility?ca.400 cm2V-1s-1 at 300 K?.All these attributes make it a good candidate to fabricate ZnO SC polar photoanodes.In this work,we fabricated ZnO SC PEC photoanode and systematically investigated the influence of the internal electrostatic fields on the separation of photogenerated charge carriers and the anisotropic performances during PEC water splitting.The experimental results indicated that the internal electrostatic field in ZnO SC photoanodes can greatly influence the bulk and interfacial charge separation efficiencies,which is closely dependent on the orientations of the electrostatic fields.As the internal electrostatic field is accordance with the direction of photogenerated holes' transportation,both the bulk and the interfacial charge separation efficiencies can be greatly improved,which lead to an enhanced PEC performance.This provided a new way to fabricate high efficient PEC photoanodes with higher photoconversion efficiencies,which is important for the development and practical applications of PEC water splitting.In chapter 6,the content of this paper is summarized,and the innovation of this paper is put forward.The existing problems and the plan of the next work are raised according to the current research situationIn summary,based on the research work of this paper,it is proved that the composition and electronic structure of the semiconductor photoelectrode material can be control by using doping,semiconductor recombination,crystal built-in electric field construction and new material development,Thereby improving the separation efficiency of the internal and interfacial of the photogenerated carriers and improving the photoelectrochemical performance of hydrogen decomposition.This is great significant to promote the development of photoelectrochemical water splitting.