The Construction Of Metal Oxide Composite Film For Water Oxidation

In order to reduce China’s dependence on fossil energy and reduce carbon emissions,efficient development and utilization of new energy has become an important research direction.The conversion of electrical（photoelectric）to chemical energy by electrolytic cell is one of the strategies for effective use of intermittent electric energy.However,the kinetics of water oxidation on the electric（photoelectric）anode is relatively slow,and the unsatisfactory charge transfer and separation path will limit the further improvement of energy conversion efficiency of the electrolytic cell.Metal oxide films have been used in the development of anodic materials due to their unique advantages,such as better electrical contact with the substrate,clearer surface interface structure suitable for research models,and easier to realize multistage series structure to meet the design requirements of electrolytic cell,etc.In order to further optimize the charge transfer and separation path of anode materials,reasonable material structure regulation and synthesis methods are necessary.The composite structure is widely used in the performance optimization of catalysts because of the wide selection of materials and simple construction ideas.By means of composite structure building,in addition,the existanve of the synergistic effect could increase the capability of charge transfer and separation of catalyst,increase surface catalytic sites and improve catalytic activity of the catalyst,extend the light absorption range of the catalyst,which can effectively improve the single electric（photoelectric）water oxidation kinetics of the anode materials and optimization of the charge transfer and separation path,Thus,the energy conversion efficiency of electrolytic cell can be improved.Although composites can optimize the charge transfer and separation paths,the mechanism of synergistic optimization of charge transfer and separation paths is not completely the same in different composite structures.In addition,the mechanism of the formation of synergistic effects in different composite structures has not been clearly understood.So far,the construction and regulation strategies of composite metal oxide films are limited.In view of the above problems,this paper uses pulsed laser deposition system to form multilayer composite structure by introducing epitaxial insertion layer,using metal plasma to induce oxide surface reconstruction to form composite structure,and growing metal oxide on functional substrate to form composite structure.Three effective methods of composite film construction are presented and three kinds of composite metal oxide films with different surface and interface structures were constructed.Then,from the perspective of the charge transfer effect between two phases and the co-optimization phenomenon of orbital energy levels,the mechanism of the formation of the synergistic effect in different composite film structures is discussed,and the influence mechanism of different synergistic effects on the charge transfer and separation path of electrochemical（photoelectric）water oxidation is studied in detail.Therefore,a reasonable construction strategy of composite metal oxide thin film electric（photoelectric）anode is proposed.The main research contents are as follows:1.Form LaCoO₃multilayer composite structure by introducing epitaxial insertion layer and La Co O₃surface charge transfer path optimization:In electrochemical water oxidation reaction,the distance between oxygen 2p center and Fermi level of transition metal oxide is a key factor affecting the adsorption and desorption of oxygen intermediates.However,the optimization method of this orbital level remains to be explored.A multilayer composite film（LCO/LSMO/STO）was formed with La Co O₃（LCO）in the form of an epitaxial La_0.67Sr_0.33Mn O₃（LSMO）insertion layer on the niobium doped strontium titanate（STO）（100）crystal plane by pulsed laser deposition system.The charge transfer effect between LCO/LSMO was found by studying the structure and energy change of valence band tops.The e_gelectrons in LSMO can be transferred to the LCO film by electron exchange through Mn-O-Co hybrid orbitals at the composite film interface,thus optimizing the oxygen2p center position of LCO closer to the Fermi level,which is a synergistic effect.Based on the study of charge transfer resistance of LCO and the electrolyte interface,the oxygen 2p center close to the Fermi level,can effectively shorten the charge transfer gap between the O 2p orbit of the reactive intermediates and metal 3d orbit,improve the adsorption and desorption ability of oxygen intermediates.And it will lead to the rapidly reducing of charge transfer resistance with the increases of voltage.Further,its electrochemical water oxidation activity was increased.This study provides a new strategy to accurately adjust the position of oxygen 2p center by using the charge transfer effect of perovskite complex phases,and provides a new direction for improving the catalytic activity of water oxidation reaction.2.Surface charge transfer path optimization of cobalt plasma-induced surface reconstruction of Bismuth vanadate composite films:Compared with electrochemical water oxidation,the performance of photochemical water oxidation is more dependent on the surface charge transfer capability of semiconductor materials.Cobalt plasma plume was obtained by irradiation of metal cobalt target with pulsed laser.Co O_x/Bi VO₄ composite films were obtained by using cobalt plasma to induce surface reconstruction of bismuth vanadate films under substrate temperature control in situ.Through the L-edge absorption spectrum of Co,it can be found that there is charge transfer effect between the[CoO₄]tetrahedron and its surrounding structure.In addition,the main peak of V 2p_3/2 is also found to shift in the samples with the best treatment temperature.These charge transfer effects indicate that the surface charge of the composite material has been redistributed,which is the embodiment of a synergistic effect.Its photocurrent density(J_ph)at 1.23 V v.s.RHE reached at 2.4 m A·cm^-2,which was three times higher than that of the untreated bismuth vanadate.The surface photogenerated charge transfer capacity of the sample with the best treatment temperature was enhanced（about 3.6 times）due to the most obvious surface charge redistribution,which optimized the charge transfer path of the bismuth vanadate surface,which was the most critical factor affecting its photochemical water oxidation.Compared with the treated samples,the improvement of the surface photogenerated charge transfer efficiency of the composite films is also due to the accumulation effect of Co²⁺/Co³⁺on the surface charge and the provision of electrochemical water oxidation catalytic center.Through theoretical calculation and simulation,it can be found that a new Co3d-O2p hybrid orbit appears at the top of valence band of Co O_x/Bi VO₄ composite film,and photogenerated holes are generated on the surface of bismuth vanadate,which can be transferred to water through this hybrid orbit,which is the reflection of the charge transfer path optimized by synergistic effect on the surface.The metal plasma treatment method proposed in this study can be considered as a new method for constructing composite films with charge transfer effect.At the same time,this study provides a new perspective to explain how synergy affects charge transfer pathways.3.Construction of silicon-based Bismuth vanadate composite film and optimization of surface charge transfer and bulk phase charge separation path:In addition to surface charge transfer ability,bulk phase charge separation also affects photochemical water oxidation of Bismuth vanadate.Bismuth vanadate composite films（BVO/n-Si）were grown on monocrystalline silicon（100）by pulsed laser deposition system.Compared with bismuth vanadate（BVO/FTO）grown on FTO glass,BVO/n-Si shows better photochemical water oxidation ability due to its higher capacity of photogenerated charge separation and transfer on bulk phase and surface.Under light,the photogenerated holes in silicon can be combined with photogenerated electrons generated by bismuth vanadate in the photochemical water oxidation process.This synergistic effect can effectively improve the bulk phase separation efficiency of photogenerated carriers of bismuth vanadate.In addition,the thickness of the bismuth vanadate film is close to the carrier migration length,so part of the photogenerated carriers of silicon can be transferred to water through bismuth vanadate,which makes the composite photoanode exhibit obvious incident photocurrent conversion efficiency at wavelengths greater than 500 nm.This work clearly explains the working principle of bismuth vanadate and silicon in the composite photoanode.At the same time,this work illustrates the effect of orbital energy level optimization on surface charge transfer and bulk phase charge separation path optimization,and provides an effective means to optimize the performance of bismuth vanadate photoanode.In this paper,from the perspective of charge transfer effect and orbital energy level optimization,the mechanism of synergistic effect optimization of charge transfer and separation path in composite films with different structures is discussed,which provides a new perspective for the study of structure-activity relationship of composite materials.By optimizing the surface and interface structure of the composite film,the electrical（photoelectric）chemical water oxidation performance of the composite film is improved,which provides a new strategy for the construction of the composite electric（photoelectric）anode.