| Gas sensors based on the oxide semiconductor have been widely used in the fields of atmospheric environment monitoring,food safety,and medical diagnosis due to their advantages of fast response,simple preparation,and easy integration.Gas sensors based on oxide semiconductor usually require higher operating temperatures to provide chemical reaction activation energy to make the active group of sensing materials react with target gases.However,higher operating temperature of gas sensors not only leads to increased power consumption,but also poses safety hazards.Therefore,the development of gas sensors based on oxide semiconductor which could work at room temperature is a great significance in the field of practical applications.In recent years,the light-excited method has become an effective way for realizing room temperature operation of gas sensors based on oxide semiconductor.Among many oxide semiconductors,indium oxide(In2O3)not only has strong light absorption ability and high electron mobility,but also has abundant free electrons on its surface that can participate in gas-sensing reactions,so it has become an ideal sensing materials of light-excited gas sensor.However,the light-excited gas sensor based on In2O3 still faces the following problems at room temperature: First,the photo-generated carriers of single-component In2O3 will recombine quickly,resulting in a low photon quantum yield,which making a lower sensitivity of the gas sensor.Second,In2O3 sensing materials with nano-sized are beneficial for gas sensing reaction,but the large potential barriers between nano-powders lead to slow charge transfer and longer response/recovery time of the gas sensor.In addition,with the rapid development of wearable devices,the development of light-excited In2O3-based gas sensors with excellent mechanical flexibility and sensing performance still faces great challenges.In order to solve the above problems,a series of room temperature NO2 gas sensors with onedimensional structure of In2O3-based heterojunction nanofibers were designs and fabricates.The built-in electric field at the heterojunction interface is used to improve the separation efficiency of photo-generated carriers.And the ultra-long one-dimensional structure of the nanofibers is used to speed up the transport and transfer of charges.The specific research contents are as follows:1.In2O3/Zn O heterojunction nanofibers with yolk-shell structure were successfully prepared by the combination of electrospinning,hydrothermal and atomic layer deposition method.The study shows that the sensitivity of gas sensor based on In2O3/Zn O heterojunction nanofibers to 1 ppm NO2 under UV excitation(S = 6.0)is about 5,5.4 and 3.8 times higher than that sensors based on pure In2O3 nanofibers,Zn O hollow nanofibers and In2O3/Zn O coreshell nanofibers.The detection limit of the In2O3/Zn O heterojunction nanofiber sensor under UV excitation is about 50 ppb.This excellent sensing property is attributed to the ordered builtin electric field in the In2O3/Zn O heterostructure that enhanced the separation efficiency of photo-generated electron-hole pairs.In addition,the In2O3/Zn O heterojunction nanofibers with yolk-shell nanostructures have a large specific surface area,which provides many active sites,which is conducive to the adsorption of gas molecules,and its ultra-long one-dimensional structure accelerates the transport and transfer of charges.2.In2O3/In2S3 heterojunction nanofibers with hierarchical structure were successfully prepared by the combination of electrospinning and hydrothermal method.The study shows that the sensitivity of the In2O3/In2S3 heterojunction nanofiber sensor to 1 ppm NO2 under visible light excitation(S = 6.5)is about 4.6 times higher than the In2O3 nanofiber sensor.The lower detection limit of the In2O3/In2S3 nanofiber sensor under visible-light excitation is about50 ppb.This excellent gas-sensing performance is attributed to the built-in electric field at the In2O3/In2S3 heterojunction interface,which is beneficial to the separation of photo-generated carriers,allowing more photo-generated electrons and holes to reach the surface of the materials to participate in the gas-sensing reaction,and because In2S3 has strong visible-light absorption capacity,which could improve the gas sensing performance of sensing materials.In addition,the In2O3/In2S3 heterojunction nanofibers with multi-level nanostructures have a large specific surface area,which can provide more active sites,that is conducive to the adsorption of NO2 gas molecules on the surface of the materials.3.In2O3/g-C3N4/Au ternary heterojunction nanofibers were successfully prepared by the combination of electrospinning,gas-solid reaction and in situ growth methods.The research shows that the sensitivity of the In2O3/g-C3N4/Au ternary heterojunction nanofiber sensor to 1ppm NO2 under visible light excitation(S = 17.2)is 6.8 and 2.2 times higher than In2O3 and In2O3/g-C3N4 nanofibers,respectively.The detection limit of the In2O3/g-C3N4/Au ternary heterojunction nanofiber sensor under visible-light excitation is about 20 ppb.This excellent gas sensing performance is attributed to(1)the strong visible-light absorption of g-C3N4;(2)the ordered built-in electric field at the interface of the In2O3/g-C3N4 heterojunction promotes the efficient photogenerated carriers;(3)The localized surface plasmon resonance effect of Au nanoparticles loaded on the surface of In2O3/g-C3N4 heterostructures under visible light excitation further promotes the generation of highly active photogenerated carriers and expands sensitive materials;(4)The Schottky junction at the interface of g-C3N4 and Au further improves the separation efficiency of photogenerated carriers,so that more photogenerated electrons-holes can participate in the surface-sensitive reaction and improve the sensitivity gas sensing properties of materials.4.Yttria-stabilized zirconia(YSZ)nanofiber substrates were prepared by electrospinning,and then an In2O3/g-C3N4 sensing layer was grown on the surface of the substrate by atomic layer deposition and gas-solid reaction method.All-inorganic flexible YSZ/In2O3/g-C3N4 heterojunction nanofibers were prepared.Studies have shown that the all-inorganic flexible YSZ/In2O3/g-C3N4 nanofibers sensor exhibits fast response/recovery speed and extremely low detection limit(50 ppb)for NO2 gas.This excellent gas sensing performance is mainly attributed to the built-in electric field formed at the interface of the g-C3N4 heterojunction improves the separation efficiency of photogenerated carriers,allowing more photo-generated charges to participate in the surface reactions of sensing materials.In addition,the sensor can ensure stable detection in any bending state,which is attributed to:(1)The dangling bonds generated by the fracture of the surface lattice of YSZ nanofibers can have strong bonds with the semiconductor sensing materials,so that the sensing materials are difficult to fall off,and its high heat resistance also has good compatibility with the heat treatment of semiconductors;(2)The YSZ substrate has ultra-small grain size and ultra-fine fiber diameter,which can reduce grain boundaries and effectively reduce the flexural modulus of the section,thus ensuring the mechanical flexibility of the sensor;(3)The In2O3/g-C3N4 sensing layer with ultra-thin thickness(～7 nm)can withstand large deformations together with the flexible YSZ substrate. |
PDF Full Text Request