The Study On Doping And Defects Construction In Non-noble Photo-and Electro-catalytic Materials

With the increasing of energy crisis and environmental issues,the development and utilization of clean and renewable energy has attracted people’s keen attention.Hydrogen,of which the only product is water when combustion,is pollution-free to the environment.Photo-or-electrochemical water splitting as a potential way to obtain high-efficiency hydrogen energy,and the broad applications of hydrogen energy itself have aroused great enthusiasm of the world-wide scientists.Noble metals and their oxides are highly efficient catalysts for water electrolysis,but their high price limits their wide application.The photocatalysts can greatly reduce the activation energy and accelerate the reaction kinetics in water splitting.In view of this,it is important to develop low-cost and high-efficiency photocatalysts.In recent years,a large amount of non-noble metal-based materials have been widely studied for water splitting,such as transition metal phosphides,sulfides,two-dimensional layered materials（such as graphene and MoS₂）,as well as the non-metallic semiconductor materials such as g-C₃N₄,but their performance needs to be further improved.Therefore,much effort has been adopted to enhance their catalytic efficiency.Among them,elemental-doping and defect-construction are effective methods to improve the catalytic performance of materials.This method can regulate electronic properties of catalysts in a certain extent so that the carriers can be better migrated.Defects and impurity atoms can also be active sites,which is conducive to adsorption of the reactants.In this dissertation,we focused on the effects of doping and vacancy defects on achieveing efficient photocatasysts.Meanwhile the explaination of some important experimental results,the photoelectrochemical properties of catalysts were analyzed theoretically.The results obtained in the thesis provide some important scientific basis of doping and defect effects for improving the conversion efficiency of solar to hydrogen production using semiconductors.The main contents of the thesis are summaried as follows:1.Due to the suitable visible light absorption,Iron oxide（α-Fe₂O₃）is often used as the photoanode material for solar water splitting.However,the lower electron mobility and shorter hole diffusion distance in intrinsic ferric oxide make its energy conversion efficiency very poor.In Chapter 2,Ti doped Fe₂O₃ films were synthesized by a simple wet-chemical loading of amorphous layer of titanium dioxide on the surface of Fe₂O₃films followed by high temperature solid-state reaction.Then,the photoelectrochemical properties of Ti doped Fe₂O₃ films were systematically studied by optimizing the Ti-doping concentrations during synthesis.The results indicated that the Ti-doping not only can increase the donor concentration of the Fe₂O₃ films,but also reduces the recombination rate in it.As an amorphous co-catalyst,Co-Pi was further used to enhance the hole transfer on the electrode/electrolyte interface of Ti doped iron oxide electrode.As expected,the Ti-Fe₂O₃/Co-Pi photoanode produced a further improved photocurrent density of 0.76 mA/cm² at 1.23 V vs RHE,which was much higher than the photocurrent density of individual Fe₂O₃（0.25 mA/cm²）and Ti-Fe₂O₃ photoanode（0.51 mA/cm²）.This work provides useful insights for understanding the mechanism of element doping on improving the photochemical properties of hematite photoanode.2.In chapter 3,graphite carbon nitride(g-C₃N_4-x)with controllable nitrogen vacancy defects（V_N）was prepared by a simple selenium vapor treatment process.The as-prepared CN samples have 2D graphene-like structure.By using UV-vis spectra,photo-response measurements and time-resolved spectroscopy,it was found that the nitrogen vacancies can effectively improve the visible light absorption of g-C₃N₄.At the same time,the formation of vacancy defects g-C₃N₄ is conducive to separate the photogenerated electrons and holes more effectively.The photocatalytic hydrogen production measurements showed that the photocatalytic activity of V_N containing g-C₃N_4-x photocatalyst was significantly better than that of the intrinsic CN sample.The hydrogen production rate of g-C₃N_4-x was 1.16 mmolg^-1h^-1 under the visible light irradiation of>420 nm,which was 3.4 times of the intrinsic g-C₃N₄.The effect of V_N on band structure and physical properties in g-C₃N₄ catalysts was studied by DFT calculations.It was found that vacancy defects could reduce the band gap of g-C₃N₄ to improve the absorption ability of CN catalyst in visible region.In addition to reducing the bandgap,V_N forms a deep donor level in the band gap of carbon nitride,which can convert the photons with energy lower than the band gap in to photocurrent.This work provides a new and effective strategy for the synthesis of g-C₃N₄ with suitable V_N defects and other g-C₃N₄ based photocatalysts for hydrogen production.3.Hydrogen evolution reaction（HER）of cathode and oxygen evolution reaction（OER）of anode are of two important half-reactions in water splitting.Due to the four-electron transfer process involved in OER,the overpotential is much higher than that of the HER,which is the main bottleneck that restricting the efficiency for water electrolysis.In the chapter 4,one-dimensional nitrogen-doped carbon nanofibers with embedded FeNiP nanoparticles were synthesized by a facile electrospinning and subsequent phosphating treatment.In the composites,the nano-sized FeNiP particles were coated by several layers of high crystalline graphene and evenly implanted into a porous N-doped carbon fiber skeleton.The as-prepared FeNiP@N-CFs composites exhibited excellent OER electrocatalytic activity.In 1.0 M KOH,the overpotential corresponding to the anode current of 10 mAcm^-22 is only 300 mV,and the slope of Tafel is only 47 mv dec^-1.Its performance is better than that of commercial RuO₂ and other oxygen evolution catalysts.The excellent OER stability of FeNiP@N-CFs was also proved by long-time cycling test.The ideal OER performance of FeNiP@N-CFs catalysts can be attributed to the high conductivity network provided by the one-dimensional N-doped carbon fibers coated with graphene and the high active hydroxyl oxide species that in situ formed during test.This work provides a general method for preparation of other bimetallic compounds and N-doped carbon fiber composite materials with high catalystic properties.