Study On The Surface And Interface Regulation Of Klectrocatalytic And Photocatalytic Materials And Their Application In Water Splitting

With the rapid development of economy and the continuous improvement of people's living standards,the energy demand has continued to rise.At present,non-renewable fossil fuels such as oil,coal and natural gas are still used as the main energy sources today.However,these fossil fuels not only have limited reserves,but also cause a large number of environmental pollution problems during the process of consumption.Therefore,in order to solve the current energy crisis and environmental pollution problems,it is very important to explore new types of clean energy.Among many new energy sources,hydrogen is considered as one of the most ideal secondary energy sources because of its high energy density,clean,pollution-free and renewable.Therefore,the development of hydrogen energy is crucial to solve the energy crisis and environmental pollution problems that human beings are currently facing.Among many hydrogen production methods,electrocatalytic and photocatalytic water splitting to produce hydrogen are the two important methods.The electrocatalytic water splitting technology is mature,clean and pollution-free,the purity of hydrogen product is high.It is the most promising hydrogen production technology that can be developed on a large scale,which is of great significance for solving the large-scale hydrogen demand.Although it still needs to consume electyric energy,with the development of renewable resources such as wind energy and solar energy,the effective use of these natural energy sources can greatly reduce the cost of hydrogen production from electrocatalytic water splitting.Photocatalytic water splitting to produce hydrogen is a promising technology,photo-generated electrons and holes can be produced by light excitation of the semiconductor photocatalysts,photo-generated electrons and holes with high enough energy can decompose water into hydrogen and oxygen.This method can continuously use the inexhaustible sunlight,which is a very clean way of hydrogen production.Therefore,it is important to explore and find the efficient and stable electrocatalytic and photocatalytic materials.At present,in the process of electrocatalytic water splitting,it is necessary to overcome the overpotential required for water splitting,which can cause excessive energy waste,this is the major problem in the process of electrocatalytic water splitting.In the process of photocatalytic water splitting for hydrogen production,the light absorption capacity of the photocatalyst and the separation efficiency of photo-generated carriers can restrict the photocatalytic H2 production activity and solar conversion efficiency.The two factors are the major problems in the process of photocatalytic water splitting.It is well known that almost all catalytic reactions are carried out at the surface and interface of the catalyst,therefore,the surface states of the catalyst(component,structure,morphology,defects,number of reactive sites,etc.)will strongly affect the catalytic activity.When the designed catalyst material contains multiple phase components,the interface between the two components is also an important parameter which can affect the performance of the catalyst.On the one hand,the interface is a necessary position for carriers transfer between the two components,and the transfer efficiency of carriers depends largely on the interface structure.On the other hand,some interfacial effects such as surface polarization which can play an important role in coordinating the activation of surface molecules.Therefore,the regulation and design of the surface and interface is very important for optimizing the water splitting performance of the photocatalytic and electrocatalytic materials.In this thesis,our research mainly focus on the surface and interface regulation of the electrocatalytic and photocatalytic materials,the hydrogen production performances of the electrocatalytic and photocatalytic materials can be significantly improved by the strategies of doping,morphology control,heterojunction and cocatalyst modification.The main contents and conclusions are as follows:In chapter 1,the research backgroumd and significance,basic principle and activity evaluation index of photocatalytic and electrocatalytic water splitting for hydrogen production are briefly introduced.Then,a systematic summary of the materials of photocatalytic and electrocatalytic water splitting was made.All the catalytic reactions are carried out at the surface and interface of the material,according to current research,the surface and interface control strategies in improving the performance of photocatalytic and electrocatalytic water splitting are summarized in detail.Finally,the research significance and research content of this thesis were summarized.In chapter 2,we mainly focused on the effects of morphology control on the H2 production performance of electrocatalytic water splitting.In the process of synthesizing of MoS2/Ni3S2 heterjunction on nickel foam,thiourea,L-cysteine and thioacetamide(TAA)were used as sulfur sources,respectively.The characterization test found that the different sulfur source selection can cause a large difference in the morphology and microstructure of MoS2/Ni3S2 heterjunction.Through the comparison of the electrocatalytic HER performance,NF-MoS2/Ni3S2-TAA has the smallest overpotential at the same current density and Tafel slope of 48.62 mV/dec compared to NF-MoS2/Ni3S2-L-cysteine and NF-MoS2/Ni3S2-thiourea electrodes.The morphology of NF-MoS2/Ni3S2-TAA is composed of closely contacted Ni3S2 nanorods and MoS2 nanosheets,this morphology makes NF-MoS2/Ni3S2-TAA has large electrochemical active area,more HER reactive sites and favorable carrier transport.In chapter 3,we mainly studied the method of constructing heterjunction to improve the performance of electrocatalytic water splitting for H2 production.MoS2 nanosheets were firstly synthesized on carbon cloth by hydrothermal method,and Co(OH)2 nanosheets were loaded on MoS2 nanosheets by electrodeposition method to form Co(OH)2/MoS2 heterojunction,this heterojunction catalyst can realize efficient alkaline electrocatalytic HER under light irradiation.After electrochemical HER test,the HER activity of Co(OH)2/MOS2 heterojunction is much better than that of Co(OH)2 or MoS2 alone,and the HER activity of Co(OH)2/MoS2 heterojunction can be further improved under light irradiation.In order to study the mechanism of the improved HER performance,we carried out DFT calculation on the key reaction process of electrocatalytic HER.Due to the formation of Co(OH)2/MoS2 heterojunction,the surface and interface of the heterojunction has reduced adsorption free energy of H2O and H intermediate compared to those of MoS2 alone,which is beneficial to the water splitting and aggregation of H intermediate.We propose that in the process of HER,Co(OH)2 promotes the initial water splitting step(Volmer)of HER and simultaneously provides the active site for OH-adsorption,the produced H*intermediate will adsorb to the nearby MoS2 sites,which is beneficial to the aggregation of H intermediate to produce H2.At the same time,introducing the light irradiation can enhance the conductivity of the MoS2 semiconductor and the exchange current density,which is beneficial to further enhance its electrocatalytic HER activity.The experimental results show that the construction of effective heterojunction in electrocatalytic HER can optimize the surface and interface properties of the catalyst,give full play to the advantages of heterojunction,and achieve the efficient electrocatalytic water splitting activity ultimately.In chapter 4,we mainly studied the doping strategy to improve the performances of photocatalytic and electrocatalytic water splitting for H2 production.(1)In the electrocatalytic water splitting,Mo-doped Ni3FeN was synthesized by hydrothermal-nitriding treatment in two steps,and its application as a bifunctional material in electrocatalytic water splitting was studied in detail.It was found that the doping of Mo significantly improved the negative HER activity of Ni3FeN,its excellent OER activity was retained and even slightly improved.We have found that the doping of Mo increased the electrochemically active area(ECSA)of Ni3FeN,indicating that the number of active sites for the HER can be increased.Further,by DFT caiculation we can get that Mo-doped Ni3FeN has more favorable adsorption and desorption free energy of hydrogen intermediates(AGH*)and increased density of slates near the F'ermi level(enhanced conductivity,which facilitates charge transport),these factors are responsible for the increased HER activity of Mo-doped Ni3FeN.When the Mo-doped Ni3FeN is used as the bifunctional electrodes for water splitting in a two electrode system,only 1.554 V is required to achieve a current density of 10 mA/cm2 and maintain long-term stability.Mo doping can effectively improve the electrocatalytic water splitting performance of Ni3FeN.(2)The P-doped and cyano-modified g-C3N4 photocatalyst was synthesized by high temperature treatment of the mixed NaH2PO2 and g-C3N4 materials under argon.The presence of P element and cyano group in g-C3N4 was confirmed by FTIR,XPS and EDS tests,wherein the cyano group was derived from the deprotonation of the amino group at the end of the g-C3N4 triazine ring system.After the photocatalytic H2 production tests,we found that the treated g-C3N4 showed the significantly enhanced photocatalytic H2 production activity than that of the original g-C3N4.The optimal activity of g-C3N4-425 exhibited 6.7 times higher H2 performance than the original g-C3N4.PL,TRPL and transient photocurrent tests have demonstrated better separation of photogenerated carriers in the treated g-C3N4.By analysis,the generated cyano group acts as a strong electron-withdrawing group to promote the separation of photo-generated carriers.While P-doping can reduce the energy band width of g-C3N4,enhance the light absorption,and suppress the recombination of photo-generated carriers.Under the synergistic effect,the photocatalytic hydrogen production activity can be significantly improved.In chapter 5,we mainly studied the cocatalyst modified strategy to improve the performances of photocatalytic water splitting for H2 production.(1)A series of MnxCd1-xS solid solutions were synthesized by hydrothermal method,and experiment results have shown that Mn0.5Cd0.5S has the best photocatalytic H2 production activity.Subsequently,the NiS cocatalyst was loaded on Mn0.5Cd0.5S by simple in-situ deposition method,the photocatalytic H2 production activity was significantly improved.0.3 wt%NiS/Mn0.5Cd0.5S has the best photocatalytic H2 production activity,which is 18.6 and 3.1 times than Mn0.5Cd0.5S and 1.0 wt%Pt/Mn0.5Cd0.5S,respectively.It also exhibited excellent photocatalytic H2 production stability.It can be obtained by PL and TRPL test that the separation efficiency of photo-generated carrier was improved after loading the NiS cocatalyst,therefore,we obtained that in the process of photocatalytic H2 production under visible light irradiation,NiS as the capture centers and high reactive sites of photo-generated electrons could effectively avoid the photo-generated carrier recombination and improve the photocatalytic H2 production activity of Mn0.5Cd0.5S.(2)Cu2-xS was deposited on Mn0.5Cd0.5S by chemical deposition method,and the built-in electric field formed by the p-n junction of Cu2-xS/Mn0.5Cd0.5S was skillfully used to promote the transfer of photogenerated holes to Cu2-xS,photogenerated electrons were used to photoreduce(NH4)2MOS4 to produce MoS2,this method can realize the synthesis of spatially separated Cu2-xS and MoS2 cocatalyst on Mn0.5Cd0.5S photocatalyst.During the photocatalytic H2 production,the photocatalytic activity of the dual-cocatalyst modified Mn0.5Cd0.5S is higher than that of the single Cu2_xS or MoS2 modified Mno.5Cdo.5S,which can fully reflect the synergistic effect between the dual cocatalysts.Therefore,during the photocatalytic H2 production,we conclude that photogenerated electrons and holes are transferred to MoS2 and Cu2-xS,respectively.Photogenerated holes are rapidly consumed by the sacrificial agent in the solution and photogenerated electrons are used to participate in photocatalytic water splitting for H2 production.Thereby,it can achieve efficient separation of photogenerated carriers and efficient photocatalytic H2 production activity.In chapter 6,we firstly summarized the research contents and conclusions of our thesis,and the main innovations and shortcomings of this thesis were put forward.Finally,we made a prospect and expectation for the next work.In summary,the surface and interface properties of the catalysts have an important influence on its catalytic activity,the surface and interface regulation strategy can effectively improve the H2 production activity of the electrocatalyst and photocatalyst.This thesis has certain guiding significance for further exploring and developing the surface and interface modification method and promoting the development and application of electrocatalytic and photocatalytic water splitting technology.