Construction Of Metal Oxide Semiconductor Heterojunctions And Their Gas Sensing And Photoelectrochemical Performance And Mechanism

Energy crisis and environmental pollution are two important issues that human beings are currently facing.The development and research of high-performance gas sensors for environmental monitoring and efficient solar photoelectrochemical（PEC）water splitting system are two promising approaches to addressing the problems we face.Among many materials,metal oxide semiconductors（MOSs）have become the core materials for the development of gas sensors and PEC water splitting photoelectrodes with low cost,high efficiency,and stable structure,and MOSs have great development potential.In this thesis,a variety of metal oxide semiconductors were designed and synthesized by different simple,low-cost,and efficient methods,including wide bandgap semiconductors（SnO₂,ZnO）and narrow bandgap semiconductors（α-Fe₂O₃,Bi VO₄）.However,the single metal oxides cannot meet the demand actual demand for sensitive materials and photoanode materials.The use of energy band matching principle to construct heterojunctions（α-Fe₂O₃/Bi VO₄,ZnO/Bi VO₄ and SnO₂/Bi VO₄）is an effective method to improve the gas sensing and PEC properties of single metal oxides.In addition,the performance strengthening of heterojunction composites was realized by modifying the reduced graphene oxide（r GO）,carbon quantum dots（CQDs）and Ni-Fe Prussian blue（Ni Fe PB）co-catalyst.The morphologies and crystal structures of heterojunctions were characterized by electron microscopy and various spectrum techniques.The gas sensing and PEC properties of heterojunctions were tested,and the structure-effect relationship between the multicomponent structure and the gas sensing and PEC properties was revealed.The improved mechanism was also discussed in detail based on the thermodynamic energy band structure and surface reaction kinetics.The details are as follows:（1）In order to solve the problem of poor sensitivity and short carrier diffusion length ofα-Fe₂O₃,the metal-organic decomposition（MOD）method was developed.Bimetallic ternary oxide Bi VO₄ was introduced into the preparation of sensitive materials and photoanodes.Theα-Fe₂O₃/Bi VO₄heterojunctions were constructed by reasonable and orderly structural design.The inner electronic field at the heterojunction interface can modulate charge transfer.The effects of different Bi VO₄ modification amounts on the crystal structure and properties of heterojunctions were systematically studied,which expands the application fields of Bi VO₄.The results show that when the atomic ratio of Fe to Bi is 8:1,the response of theα-Fe₂O₃/Bi VO₄ is 4 times higher than that of pureα-Fe₂O₃ to 2 ppm NO₂ at 110℃,and shows excellent selectivity and stability;Theα-Fe₂O₃ photoanode with excellent PEC performance was obtained by adjusting the crystal plane ofα-Fe₂O₃,the photocurrent density and incident photon-to-electron conversion efficiency（IPCE）ofα-Fe₂O₃/Bi VO₄/CQDs are 2.2 and 1.92 times higher than those ofα-Fe₂O₃,respectively,the charge transfer resistance of ternary photoanode decreased from 1357Ω（α-Fe₂O₃）to 481Ω,and the onset potential is negatively shifted by～243 m V compared toα-Fe₂O₃.The successful construction of type II heterojunction effectively reduces the recombination of electrons and holes,CQDs not only increase the active sites,but also play an important role in promoting water oxidation kinetics and expanding the visible light absorption range.（2）Based on the energy band matching principle,a series of ZnO/BiVO₄heterojunction composites with different proportions were successfully constructed.Chemical reduction and electrodeposition-impregnation methods were developed to prepare r GO modified heterojunction sensitive materials for the detection of NO₂ and Ni Fe PB co-catalyst modified ZnO/Bi VO₄heterojunction photoanodes for improving the PEC water splitting efficiency.The results show that the ZnO/Bi VO₄/r GO（0.5 wt%）composite with the molar ratio of ZnO to Bi VO₄ of 8:1 exhibits competitive sensing performance,the response reaches 126.6 to 1 ppm NO₂ at 95℃,which is 5.7 times higher than that of pure ZnO,the composite also has a low detection limit,rapid response and recovery,good linearity,and excellent selectivity,the high conductivity,high electron mobility and a large number of active sites of r GO are conducive to the rapid electron migration and gas adsorption;the photocurrent density of the ZnO/Bi VO₄/Ni Fe PB photoanode reaches 1.66 m A cm^-2 at 1.23 V vs.RHE,which is 4.15 times higher than that of the ZnO photoanode(0.4 m A cm^-2).The onset potential exhibits a cathodic shift of～283 m V compared to ZnO.The IPCE and the ABPE are 3.1 times and 6.4 times higher than that of ZnO,respectively,the co-catalyst can timely consume the holes accumulated at the interface of the electrode/electrolyte and accelerate water evolution oxygen reaction kinetics under the applied potential.（3）Aiming at the poor selectivity of SnO₂ and the band gap is too wide to absorb ultraviolet light,the II-type heterojunctions of SnO₂/Bi VO₄ were successfully constructed by MOD method,which regulate the migration of interface carriers,increase the active sites of gas adsorption,and expand the absorption range of light.Using the excellent charge transfer ability and many active sites of r GO,the heterojunction performance is enhanced.The SnO₂/Bi VO₄/r GO composite with the best composition was obtained by reasonably regulating the multi-component structure.The results show that 0.5wt%r GO modified SnO₂/Bi VO₄ exhibits excellent selectivity to TEA,the response（11.4）to 20 ppm TEA at 132℃ is 3.3 times that of pure SnO₂,and the minimum detection concentration of TEA is 0.11 ppm;the photocurrent density(2.05 m A cm^-2)of the SnO₂/Bi VO₄/r GO photoanode is 3.73 times higher than that of the Bi VO₄ photoanode(0.55 m A cm^-2),IPCE reaches 44.4%at 400 nm,and the onset potential exhibits a cathodic shift of～300 m V compared to Bi VO₄.The synergistic effect of heterojunction and r GO is the key for the enhancement of performance.