| The rapid growth of the global economy as well as mankind population has triggered energy shortage and environmental deterioration due to the limit less consumption of fossil fuels.The exploitation and employment of new clean energy has become an urgent challenge for mankind in this century.As a kind of clean energy,solar energy is almost inexhaustible,but its high environmental instability limits the direct use.Solar water splitting by semiconductor materials represents an attractive method to convert sustainable solar energy into hydrogen energy,which can realize a clean and efficient hydrogen cycle.However,the conventional photocatalytic water splitting mainly consists of' absorption of solar light,the separation and migration of photoinduced carriers,and surface chemical reactions.Among them,the narrow light absorption region and severe photo-generated carrier recombination of semiconductor materials are the key factors hindering the conversion of solar energy.Therefore,how-to promote the charge separation and transport within the semiconductor photocatalytic material and improve the effective use of long-wavelength sunlight is the fundamental issue for improving the photocatalytic efficiency.Based on that,this thesis takes traditional TiO2 as the research object,and explores the effects of bulk defects,heteroatom doping and built-in electric field on its charge carriers' separation and photocatalytic performance.The specific research content as follows:An effective method has been developed to control the formation of oxygen vacancies in monoatomic layer porous TiO2 through atomic layer deposition.X-ray photoelectron spectroscopy confirmed the different concentrations of oxygen vacancies formed by adjusting the stoichiometric ratio of TiO2.Time-resolved fluorescence decay spectroscopy and photocurrent measurements show that oxygen vacancies accelerate the separation of charge carriers and extend the lifetime of electrons.As a result,the porous TiO2 with a large number of oxygen vacancies showed a much higher photocatalytic H2 generation rate(3.41 mmol g-1 h-1),which was about 20 times than that of the porous TiO2 with feweroxygen vacancies.Combined with density functional theory calculations with experimental results,the introduced oxygen vacancies can bring in defect energy levels and delocalize the surrounding charges,thereby expanding the light absorption region and promoting the separation and migration of photo-induced carriers.This defect engineering not only opens up a way to adjust the stoichiometric ratio at the atomic level,but also control the formation of defects in metal oxides based semiconductors.In this work,a new type of TiO2/Cu/TiO2 sandwich structure is designed and constructed.A large number of vacancies are introduced into the Ti02 crystal lattice through the metallic copper reduction method under heat treatment.At the same time,Cu atoms can diffuse into TiO2lattice to form Cu-doped TiO2 The synergistic effect between oxygen vacancies and Cu atoms achieves an increase in photocurrent of the TiO2/Cu/TiO2 sandwich structure,which is about 2.4 times than that of bare TiO2 films.Through experimental results and density functional theory calculations,the presence of oxygen vacancies and copper dopants can attribute the increase in photoactivity to the electronic structure adjustment of TiO2.The oxygen vacancies and Cu dopants in TiO2 formed by the reduction of copper metal can introduce impurity levels and narrow the band gap of pure TiO2,thereby improving the visible light response.More importantly,the introduced Cu2+and oxygen vacancies in TiO2 lattice can significantly increase the charge density near the conduction band and promote the separation of photogenerated carriers.In addition,oxygen vacancies on the surface can be used as active sites for sufficient chemical reactions.This work represents a novel method to prepare doped metal oxide catalysts with a large number of vacancies to improve photocatalytic activity.Based on the limited space charge region of a single p-n junction,a multilayer TiO2-CoTiO3composite structure was designed and prepared.The atomic layer deposition method is used to alternately deposit TiO2 and CoOx with different thicknesses.After high temperature post-treatment,CoOx and TiO2 react to form a TiO2-CoTiO3 multilayer film.The space charge region formed at the junction of the multilayer n-type TiO2 and p-type CoTiO3 in the film extends the range of the space electric field,which greatly promotes the separation and migration of bulk carriers,and the carrier concentration in this film is nearly ten times higher than pure n-type TiO2.At the same time,the photo-generated charge separation efficiency shows a significant increase in relative to pureTiO2.In addition,because CoTiO3 has a lower band gap,the composite material has a stronger absorption in visible light.Therefore,compared with TiO2 and single TiO2-CoOx,the PEC performance of the prepared TiO2-CoTiO3 increased by 2.5 and 1.9 times respectively.This study proves that reducing the thickness of semiconductors and building a multilayer p-n junction structure can significantly increase the separation of photo-generated charges. |
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