The Research On The Gas-sensing Properties Of Graphene-modified Semiconductor Metal Oxide Composites

Gas sensors have been widely used in various areas such as safety production,food processing,environmental protection and medical diagnosis.Among all kinds of gas sensors,semiconductor metal oxide?SMO?-based gas sensors are always the hot research direction in this area due to their high response,low cost,wide range of measurement objects and good portability.As the enhancement of people's environmental awareness,the improvement of many technical indicators has put forward newer challenges for traditional SMO-based gas sensors.Except common parameters such as response,response-recovery properties and selectivity,the optimal working temperature and long-term stability of gas sensors have received universal attention.Therefore,the development of gas sensors working under low or even room temperature has become the newest research direction.We combed and found that the usual ways of enhancing the performences of SMO-based gas sensors includes the improvement of material's microstructure?porosity,hierarchical structure,etc.?or material's modification?the aliovalent ion doping,the recombination of different SMOs and the noble metal loading?.Nevertheless,these methods commonly focus more on the enhancement of the responses of sensing materials toward target gases,neglecting that this enhancement were sometimes based on the elevation of the working temperature of gas sensors.At higher temperature,the grains of SMOs materials may aggregate together,which will seriously affect the stability of gas sensors.Graphene possesses good electrical characteristic.The addition of graphene will always lower the working temperature of SMO-based gas sensors to a large extent through improving their electrical properties,which will provide huge support to us when we design gas sensors working at low or even room temperature.Reduced graphene oxide?r GO?,obtained oxidation-reduction method,has more advantage in gas-sensing area.It has not only good electrical properties and big specific surface area,but also surface defects and vacancies which can serve as the gas-sensing active sites.This will promote the adsorption and reactions of gas molecules a lot.This text selects two kinds of representative SMO materials,N-type ?-Fe₂O₃ and P-type Co₃O₄.We combined them with graphene through hydrothermal and water-bath methods.We built gas sensors based on graphene-modified SMOs,aiming at improving the response and optimal working temperature at the same time.We explored more about the effect of SMO morphology,size and graphene content on the gas-sensing properties of the composites.In addition,we put forward the corresponding sensing mechanism combining the structure characterizations and gas-sensing properties of the materials.Concrete study content as follows:1.We synthesized the composite of graphene/round-edged cubic ?-Fe₂O₃,where the content of graphene is low?0.1-4.0 wt%?,through an one-step hydrothermal method.Characterization results indicated that ?-Fe₂O₃ in the composite were micron-sized round-edged cubes,which had uniform size and good dispersion.Compared with pure ?-Fe₂O₃ synthesized under the same condition,the size of ?-Fe₂O₃ in the composite equalled one half of that of pure ?-Fe₂O₃,which proved that the existence of graphene hindered the crystal growth of ?-Fe₂O₃.TEM test showed that graphene in the composite had frizzy and folded morphology and combined closely with ?-Fe₂O₃ particles.Gas-sensing tests showed that 1.0 wt% composite exhibited the highest response toward acetone at 225?,about 13.9,which was 2.5-fold as high as pure ?-Fe₂O₃ at 237.5?.The optimal working temperature of the composite decreased a bit while its response was enhanced.We found that the resistance of composite in air was much larger than pure ?-Fe₂O₃ under the same temperature.We think that graphene with a low content was isolatedly dispered among the composite and the whole resistance of the composite still depended on ?-Fe₂O₃.The formation of P-N heterojunctions between graphene and ?-Fe₂O₃ and the flow of the electrons from ?-Fe₂O₃ to graphene might account for the increase of the composite resistance and also the higher sensing response.2.We synthesized a kind of novel graphene-encapsulated ?-Fe₂O₃ composite through increasing the graphene content?6.5-12.2 wt%?and changing the synthesis condition.Characterization results indicated that particles in pure ?-Fe₂O₃ had the size of 50 nm and a good dispersion.The size of ?-Fe₂O₃ in the composite approximated that of pure ?-Fe₂O₃ but had a better homogeneity.Interestingly,though much graphene in the precursors,we didn't directly observe the existence of graphene through SEM.We speculated that graphene with an extremely small size anchored on the surface of ?-Fe₂O₃.Through the tests of TEM and HRTEM,we found that graphene with characteristic curved crystal lattice wrapped the outside surface of ?-Fe₂O₃ surface,forming an encapsulated configuration.This existence form of graphene also resulted in the higher BET surface area of the composite.Gas-sensing tests showed that 12.2 wt% composite exhibited a high response of 8.2 to 5 ppm NO2 at room temperature and its response time was 2.1 min ?Correspondingly,the sensor based on pure ?-Fe₂O₃ could only show a low response of 2.1 to 5 ppm NO2 at 125? and its response was 2.6 min.This sensing performance the composite exhibited was in the lead among all the graphene/?-Fe₂O₃-based room-temperature NO2 gas sensors.We attributed the good sensing properties of the composite to its larger specific surface area,more adsorbed oxygen content and the formation of many P-N heterojunctions between graphene and ?-Fe₂O₃.3.We controllably synthesized the composite of r GO/porous Co₃O₄ slice through a stepwise reaction.We first prepared porous Co₃O₄ slices through a hydrothermal method,then mixed the as-prepared Co₃O₄ with GO fully,and at last obtained the composite through reducing GO to r GO.Results showed that r GO/Co₃O₄ composite possessed a larger specific surface area and more vacancy and chemisorbed oxygens compared with pure Co₃O₄.The gas-sensing tests of all the prepared samples showed that the optimal working temperature of the composites decreased gradually to room temperature along with the increase of graphene content in them.Among them,the 2.4 wt% composite exhibited the highest response to NO2 at room temperature,the response of which to 5 ppm NO2 was 26.8% and its response time was only 1.5 min.Correspondingly,pure Co₃O₄ only showed a low response of 11.8% to 5 ppm NO2 at 100? and its response time was as long as 3.5 min.We attributed the enhanced sensing performance of the composite to Co₃O₄'s porosity,its larger specific surface area and the isotype P-P heterojunctions between graphene and Co₃O₄.