Electronic Structure Tuning Of Gallium-based Semiconductor Nanomaterials For Performance Optimization

Gallium-based semiconducting oxides that contain metal ion with d¹⁰ electronic configuration are a class of important compound semiconductors,widely used as photocatalysts and sensing materials.Given that the intrinsic property of semiconductor oxide is closely linked with their electronic structure,photocatalysis or sensing reaction occurred on semiconductor surface can be promoted by tuning their electronic structure which results in optimized energy band,carrier concentration and charge migration.In addition,along with the vigorous development of nanotechnology and intensive study of nanomaterials,researchers realize that micro-/nano-structures have a significant impact on material functional properties.As an important aspect of nanoscale materials,one-dimensional nanomaterials have attracted researcher’s great attention due to the advantages including directional electron transfer channel and large specific surface area.In this work,gallium-based oxide semiconductors are chosen as our research models,and electrospinning techniques are adopted as synthetic method,to prepare a series of porous gallium-based oxide semiconductor nanofibers.The photocatalytic and gas sensing performances of gallium-based oxides are remarkably enhanced through analyzing key influencing factors of these reactions and subsequent optimization of electronic structures of these oxides,and we further develop the relationship between the electronic structures and functional properties of these materials.Specifically,the thesis mainly includes the following three parts:1.Electronic structure optimization on atomic scale is of significant importance for the synthesis of ideal photocatalyst by promoting photon excitation to produce electron-hole pairs,and effective separation of electron-hole pairs and migration to the catalyst surface to participate in redox reaction.We selectβ-Ga₂O₃ as the research model,and a In-doping route is presented to produce a porous Ga-In bimetallic oxide(Ga_1.7In_0.3O₃)nanophotocatalyst with atomically thin pore walls.First-principle calculations reveal that Ga_1.7In_0.3O₃ has a unique electronic structure arising from its ultrathin walls.The bottom of the conduction band and the top of the valence band of Ga_1.7In_0.3O₃ are distributed on two opposite surfaces of the ultrathin nanosheets.Hence,photogenerated electron-hole pairs upon photoexcitation can reside at the two outermost surfaces of the material,shortening the distance by which the photogenerated charges travel from the sites where they are generated to the sites where they catalyze the reactions.The upper surface of Ga_1.7In_0.3O₃ has a higher electrostatic potential than the lower surface,indicating the existence of an internal electric field between the upper and lower surfaces which can suppress photoexcited charge carrier recombination.Additionally,the porous structure within the walls results in a large density of exposed surface catalytic sites.In the photocatalytic water splitting test,Ga_1.7In_0.3O₃ displays a stable hydrogen evolution at a rate of 2295 mmolh^-1g^-1,which is about three times higher than the value obtained for Ga₂O₃(850 mmolh^-1g^-1),while In₂O₃ does not show photocatalytic activity.These results comfirm that optimized electronic structure is responsible for the superior photocatalytic activity,and the findings provide new insights into photocatalast design on atomic scale.2.Solid solution strategy is an effective method to optimize the sensing properties of semiconductor oxides.We report a serize of Ga-In based bimetallic oxide(Ga_x In_2-xO₃)solid solution nanofibers to act as sensing materials for formaldehyde detection.Ga_xIn_2-xO₃ solid solutions show more flexible and superior compared with their individual components.Along with Ga/In atom ratio varies,crystallite phase,band structures and nanostructures of Ga_xIn2?xO₃ nanofibers can be rationally altered,and simultaneously offer opportunity for optimizing sensing properties.In particular,Ga_0.6In_1.4O₃ nanofibers assembled by small nanoparticles（～4.6 nm）possess the advantages including optimized band structure,abundant surface adsorbate oxygen and porous nanostructures,and thus exhibit best sensing performances.Toward 100 ppm formaldehyde,its highest response（R_a/R_g=52.4,at 150°C）is～4 times higher than that of the pure In₂O₃（R_a/R_g=13.0,at 200°C）,while pure Ga₂O₃ cannot be used for formaldehyde detection due to its wide band gap and thereby too large base resistance.3.Surface oxygen chemistry of semiconductor metal oxides is the basis for sensing reaction between the pre-adsorbed surface oxygen and the target gases that are to be detected,and thus fundamentally determines their sensing performances.The specific reaction between the absorbed oxygen with targeted gas is of paramount importance but still a challenge for selective detection purpose.For instance,n-type semiconductors,such as SnO₂,ZnO and In₂O₃,are some of the most prominent sensing materials studied previously,but they generally suffer from cross-response to other interfering gases.The major cause of poor selectivity can be attributed to the fact that the surface adsorbed oxygen on them has the too strong ability to oxidize a wide range of reducing gases/vapors including both targeted gas/vapor and interfering gases.We research a series of spinel gallate,and recognise that CdGa₂O₄ may be a promising candidate due to its suitable bandgap and carrier type.More importantly,comparative study on electronic structures of CdGa₂O₄,SnO₂,ZnO and In₂O₃ reveal that CdGa₂O₄ can maintain lower oxidizing ability of adsorbed oxygen and thus achieve good formaldehyde selectivity.Based on these conclusions,we design a cation off-stoichiometric CdGa₂O₄ spinel oxide decorated with a small amount of CdO nanocrystals,which can further improve the formaldehyde response,without losing the selectivity.This is based on the advantages of increased amount of adsorbed oxygen in the Ga-rich environment,as well as heterojunctions between CdO nanoparticles and Ga-rich spinel oxide.Our findings propose a general approach for achieving highly selective and sensitive detection of target gases by optimizing surface oxygen property of semiconductor oxides.