Design Of Heterostructures Of Metal Oxides And Research On Gas-sensing Properties

Metal oxide semiconductors（MOS）are the most commonly used gas sensitive materials due to its diverse structure and stable chemical properties,its has become a hot topic in the filed of gas sensors.However,the MOS gas sensor suffers from high operating temperature,high energy consumption and low detection limit.A plenty of studies have shown that the composite metal oxides can effectively improve the selectivity and other performance of MOS sensors.The heterojunction formed between two different materials can cause the charge transfer,the formation of depletion layer and potential barrier on the material surface,which enhances the gas sensing response of the sensor.However,most of the current studies on metal oxide heterojunction sensors are experimental,and the mechanism of heterojunction enhancing gas sensing response is also controversial,which is mainly caused by the variety of sensitive materials.Therefore,it is of great significance to clarify the mechanism of heterogeneous gas sensitization enhancement.Once the mechanism of enhancing gas sensitivity of heterojunction is identified,we can selectively select sensitive materials for specific design and achieve the purpose of saving time and cost.In view of the above problems,the basic idea of this paper is to design and construct nanostructures with large specific surface area,and further improve the gas sensing performance of pure metal oxide semiconductor materials by loading precious metals or synthesizing multi-metal oxides.Using the catalytic spill-over effect of precious metals,the electron depletion layer model and the hole accumulation layer model as well as the Schottky barrier to try to explain the gas sensing mechanism.The main research results are as follows:（1）By introducing Au/In₂O₃（M/S）junction,the important role of the catalytic spill-over effect of precious metals in the metal oxide semiconductor gas sensor modification was revealed.In addition,the influence of precious metal load on gas sensing was studied.Firstly,the In₂O₃ microspheres were directly synthesized by simple hydrothermal method,and then the HAuCl₄ was mixed with the products to obtain the Au/In₂O₃ microspheres by redox reaction.The gas sensitivity test results show that Au/In₂O₃ microsphere composite material has a very high response to triethylamine（648.2-100 ppm）,a fast response and recovery time,a low limit of detection（108 ppb）and good stability under an appropriate Au load.Using the catalytic sensitization effect of gold,the sensing mechanism was explained in detail.（2）Through the fabrication of ZnO/PtO/Pt nanoarray,the study revealed the important role of Schottky contact and electron depletion layer in the metal oxide semiconductor gas sensor modified by such precious metals.First,the ZnO seed layer was grown on the glass substrate by spin coating method,and then the ZnO nanoarray was grown on the seed layer by hydrothermal method.Finally,the ZnO/PtO/Pt nanoarray was obtained by dropping H₂PtCl₆.The gas sensitivity test results show that the ZnO/PtO/Pt sensor has a very high response,that is,the response to 50 ppm triethylamine reaches 3513,and the response of original ZnO is 6.15.By comparing their responses,it indicates a 500-fold enhancement is obtained by ZnO/PtO/Pt.The ZnO/PtO/Pt sensor also provides a very high sensitivity of 1.32 ppm^-1,as well as a low limit of detection limit of8.3 ppb,which explains the increased gas sensitivity by the variation of Schottky barrier height（SBH）.（3）By preparing ternary metal oxide CoSnO₃,the role of multiple metal ions in semiconductor gas sensors is revealed.First,a simple solution synthesis method was used to prepare the CoSnO₃ nanoboxes.The gas sensing test shows that the CoSnO₃ nanoboxes sensor has a better performance to triethylamineat a low temperature of 100 ^oC.The mechanism of gas sensitivity was explained by hole accumulation layer.In order to explore the influence of the addition of Co and Sn on the gas sensitivity of the material,a number of control tests were also prepared,and the sensing mechanism of the materials were interpreted by p-n and p-p heterojunctions.