Study On The Design And Synthesis Of Semiconductor Based Composites And Their Performance For Photocatalytic Water Splitting

In current society,the rapid development will undoubtedly threatens and brings two major problems:energy crisis and environment pollution.Hydrogen?H₂?is now regard as one clean and renewable energysource dues to its mearts including high energy density,easy to store,etc.,attracting much more attention.Thus,how to produce H₂ becomes one hot topic to many researcheres.The essence of photocatalytic H₂ generation from water splitting is the conversion of solar energy to chemical fuel,in which the energy is stored in the form of chemical bond and similar to the natural photosynthesis.During the process of artificial photosynthesis,the secomidoctor-based materials are selected as the medium to solve the increasing global environment pollution and energy shortage.During a typical photocatalytic reaction,it includs three basic processes:absorption of photons,photogenerated charge carriers'separation and transportation,and the redox reaction occurs on the surfaceof the photocatalyst.Generally,the second one plays the decisive role because that it directly affects the number of photogenerated electrons and holes arriving in the surface of a photocatalyst.On this concept,how to maximize the separation of photogenerated charge carriers in time and space is the primal problem.Among so many semiconductors,g-C₃N₄ is found to be one promising catalystfor photocatalytic H₂ generation form water splitting dueto its high stability,easy obtainable,and appropriate conduction band position(E_CB=-1.13 V vs.NHE).Compared with g-C₃N₄,CdS is a more traditional one which usually applied for visible-light-induced H₂generation due to its narrow band gap?ca.2.4 eV?,appropriate CB potential for proton reduction,and high electronic mobility.The third material that we mentioned below is Fe₂O₃,which is an important photoanode material for photoelectrochemical water oxidation owning to its mearts including good sunlight-harvesting capability,excellent stability,non-toxic and natural abundance.These three included materials all suffer from poor charge separation resulting in their low quantum efficiency.Thus,our research foucses on how to improve the separation of photogenerated electron-hole pairs and their transportation.Therefore,various methods have been applied including loading co-catalysts to provide active sites as well as facilitate the charge carriers'separation;constructing heterojunctions with the build-in electric field,and so on.Thus,we chose purposefully the materials of g-C₃N₄,CdS and Fe₂O₃ to study their internal relationship between compound structure and charge behaviour.In these works,g-C₃N₄ and CdS are employed as the catalysts for photocatalytic H₂ generation from water splitting,while Fe₂O₃ acts as the photoanode for water oxidation.1)Noble-metal-free CoP modified g-C₃N₄ acts as an efficient visible-light-induced photocatalyst for H₂ generation:we have adopted an easy way to successfully synthesize the CoP/g-C₃N₄ composite,in which CoP is a cocatalyst to trap the electrons in CB of g-C₃N₄ results in the efficient separation of charge carriers in g-C₃N₄.Further,CoP can also enhance the light harvesting efficiency of CoP/g-C₃N₄.These results reveal that CoP/g-C₃N₄ shows the high photocatalystic H₂ generation rate of 474.4mmol h^-1 g^-1 under visible light irradiation,which is 131-fold to that of pure g-C₃N₄ and even higher than Pt/g-C₃N₄under same condition.To study on the mechanism and the charge carrier behavior in photocatalytic reaction,the energy band positions both of CoP and g-C₃N₄ are analyzed.As results,the matched energy band positions between them provides the specific direction for fast transfer of electrons and holes,and prolongs their lifetimes for highly efficient photocatalytic reaction.This work guide the researcher how to introduce the appropriate cocatalyst in reaction of photocatalytic H₂ generation.2)Efficient visible-light-driven H₂ generation from water splitting catalyzed by highly stable CdS@Mo₂C-C core-shell nanorods:the target core-shell photocatalyst of CdS@Mo₂C-C is synthesized through a sequential solvothermal-hydrothermal-carbonization process.In this system,Mo₂C-C plays the vital roles:?1?acts as a cocatalyst to draw the electrons in CB of CdS and decrease the surface potential barriers for photocatalytic reaction;?2?processes high electrical conductivity to fast transport the charges to the surface of the photocatalyst;?3?protects the inner CdS from photocorrosion.The photocatalytic results show that,as compared with pure CdS(0.41 mmol h^-1 g^-1),CdS@Mo₂C-C processes the photocatalytic H₂ generation rate as high as 17.24 mmol h^-1 g^-1 with high stability.Furthermore,the mechanism of photocatalytic reaction is discussed in detail.Additionally,the though of this present work may bring new insights into finding more stable,fficient,and sustainable photocatalysts to solar energyconversion.3)Highly efficient photoelectrochemical?PEC?water splitting by surface modification of Co-Pi loaded Co₃O₄/Fe₂O₃ p-n heterojunction nanorod arrays:the Ti-doped Fe₂O₃?Ti:Fe₂O₃?nanorod arrays is first obtained by a simple hydrothermical method with the help of calcination.Then,the Co₃O₄ and Co-Pi are introduced on the surface of Ti:Fe₂O₃ through the methods of galvanostatic electrodeposition and photo-assisted electrodeposition orderly.Finally,the target photoanode of Co-Pi/Co₃O₄/Ti:Fe₂O₃ is achieved.The PEC results reveal that it shows the high photocurrent density of 2.7 mA cm^-2 at 1.23 V vs.RHE?V versus reversible hydrogen electrode?,which is 18-and 2.2-fold than that of Fe₂O₃ and Ti:Fe₂O₃,respectively.Further,the modifiying of Co-Pi/Co₃O₄ shift negatively the onset potential of Ti:Fe₂O₃,that is to say they decrease the overpotential of the PEC reaction.The reason for improvement of the activity over Co-Pi/Co₃O₄/Ti:Fe₂O₃ are listed as follows:the build-in electric field formed at the interface of Co₃O₄/Ti:Fe₂O₃ p-n junction can facilitate the charge carriers'separation and transportation;the cocatalyst of Co-Pi can strongly remove the holes stored in VB of Co₃O₄ and fastly inject them into the electrolyte for PEC water oxidation,which process further boosts the separation of charge carriers.4)Carbon quantum dots?CQDs?sensitized integrated Fe₂O₃@g-C₃N₄ core-shell nanoarrays photoanode towards highly efficient water oxidation:the Ti:Fe₂O₃@g-C₃N₄-CQDs nanorod arrays are prepared by a sequential hydrothermal-vapor deposition-spin coating process.In this structure,we find that Ti:Fe₂O₃ is coated by g-C₃N₄ nanosheets with CQDs highly dispersion on the surface of these nanosheets.The experimental results show that the photocurrent density achieved on Ti:Fe₂O₃@g-C₃N₄-CQDs up to 3.4 mA cm^-2 at 1.23 V vs.RHE,which is 2-fold larger than that of Ti:Fe₂O₃.The analysis based on a series measurements suggests that:?1?the VB-holes of Ti:Fe₂O₃ transfer in the specific direction,that is Ti:Fe₂O₃?g-C₃N₄?CQDs;?2?the good catalytic effect for H₂O₂ decomposition of CQDs that catalytic the H₂O by the stepwise two-electron/two-electron process;?3?the favorable electron transport behavior of CQDs that boosts charge separation.The thought in this work affords insightful for developing photoanodes with high photocurrent density.