Heterostructure Enhanced Gas Sensing Properties Of Oxide Semiconductors And Their Working Mechanism

The core of gas sensor is gas sensitive material.Zinc oxide?ZnO?and iron oxide??-Fe₂O₃?,as two typical metal oxide semiconductor materials,are also the most commonly used gas-sensitive materials.They have rich structure and morphology,high chemical stability and can detect a variety of gases.However,these pure phase commercial gas sensors still have many problems.Firstly,most of the materials are powder,which will destroy the intrinsic nanostructure physical properties by the traditional slurry-coating process.Secondly,these gas-sensitive materials have low sensitivity,high working temperature,long response-recovery time and poor selectivity.Thirdly,the research on the mechanism of gas sensitivity is not deep enough.It is still in the phase of phenomenological theory,lacking of direct evidence and systematic theoretical support.For the above problems,the basic idea of this paper is to further improve the gas-sensitive performance of pure phase oxide semiconductor materials by designing and constructing nanowires heterostructure with large specific surface area.By means of theoretical analysis such as semiconductor band theory and density functional theory?DFT?simulation,this paper attempts to systematically explain the gas-sensitive mechanism of heterogeneous junction enhanced oxide semiconductor.In this paper,ZnO and?-Fe₂O₃ porors nanorods were studied.The pure phase gas-sensitive materials were directly constructed on the plate electrode by seeding layer induction hydrothermal method.Metal/semiconductor junction?M/S?or semiconductor N/N heterostructure was constructed on the surface by the sputtering,spin coating,and pulsed laser deposition?PLD?.The mechanism of Schottky contact,noble metal catalysis,depletion layer of NN heterostructure,oxygen content of chemical adsorption and other factors to improve gas sensitivity were explored.The main research results are as follows:1.Through the introduction of Au/ZnO?M/S?junction,the important roles of Schottky contact and noble metal catalysis in this kind of metal modified oxide semiconductor gas sensor were revealed.Firstly,ZnO nanorod arrays were grown directly on Al₂O₃ plate electrode by seeding layer induction hydrothermal method.Secondly,the Au nanoparticles were randomly loaded on the surface of ZnO nanorods by sputtering method to construct the Au/ZnO heterogeneous structure gas sensor.The results of gas sensitivity test show that the composite structure has good selectivity to the triethylamine gas at nearly room temperature?40??,and the sensitivity of the best Au/ZnO device to the 50 ppm triethylamine gas could reach 22,which is about 8⁹ times of the pure phase ZnO nanorod device.Based on the semiconductor band theory,this paper explains in detail the electron transfer process at the interface between Au and ZnO after the contact of Au and ZnO.The mechanism of noble metal catalysis and metal"overflow"phenomenological model in enhancing gas sensitive performance of semiconductor was deepened.2.By introducing Au/?-Fe₂O₃?M/S?junction,the enhanced gas sensitivity mechanism of M/S junction was extended to porous oxide semiconductor gas sensitive material with large specific surface area,which further improved the performance of the device.Firstly,porous?-Fe₂O₃ nanorod were prepared by hydrothermal method induced by?-Fe₂O₃ seed crystal layer on Al₂O₃ flat electrode.Secondly,the heterostructure of Au/?-Fe₂O₃ porous nanorod was prepared by spin coating method.Then the schottky contact between Au and?-Fe₂O₃ was confirmed.The gas sensitivity test results once again prove that the M/S junction is helpful to improve the gas sensitivity performance.The best Au/?-Fe₂O₃ device has better performance such as shorter recovery time?8 s?and lower detection limit?1 ppm?at 40?.In addition,the influence of humidity on gas sensitivity and its mechanism were further studied.3.The effects of the semiconductor NN depletion layer model and the surface adsorption oxygen model on the gas sensor performance were compared.The?-Fe₂O₃/ZnO nanorods were designed by hydrothermal and PLD methods to improve the gas sensitivity of ZnO nanorods.The results of gas sensitivity test showed that the best operating temperature of?-Fe₂O₃/ZnO composite heterostructure was 300?,and the sensitivity of?-Fe₂O₃/ZnO to 50ppm triethylamine gas was significantly higher than that of ZnO material.The sensitivity of the best?-Fe₂O₃/ZnO device was up to 63.Furthermore,the energy band theory is applied to explain the mechanism of complex structure of such NN heterostructure to improve gas sensitivity performance.Combined with XPS experimental evidence,the idea that the mechanism of surface adsorbed oxygen content plays an important role in improving gas sensitivity performance is put forward,and the reliability of this idea is explained and verified by density functional theory?DFT?.