【作者】 张博；

【导师】 卢革宇；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2018， 博士

【摘要】 气体传感器在安全生产、食品加工、环境保护和医疗诊断等领域均具有广泛的应用。其中,基于半导体金属氧化物的气体传感器因其灵敏度高、成本低、测量对象广和便携性好等优点一直是该领域的热点研究方向。随着人们环保意识的不断增强,各项检测指标的提升对传统的半导体金属氧化物气体传感器提出了更新的挑战。除了常见的指标如响应值、响应恢复特性及选择性之外,气体传感器的最佳工作温度和长期稳定性开始受到普遍的重视。因此,开发低温甚至室温下长期稳定工作的气体传感器已经成为新的科研方向。我们梳理发现,提升半导体金属氧化物气体传感器性能的方法主要包括改善材料的微观形貌（多孔结构、分等级结构等）和材料的修饰改性（异价离子掺杂、不同金属氧化物复合和贵金属担载等）。然而,这类方法通常注重敏感材料对待测气体响应值这一单一指标的提升,忽视了这种提升有时甚至建立在传感器工作温度的升高上。在过高的温度下,半导体金属氧化物的晶粒会发生聚集,严重影响气体传感器的稳定性。石墨烯具有良好的电学特性,石墨烯的引入往往能够通过提升半导体金属氧化物的电学性能,从而在很大程度上降低传感器的工作温度,这为我们设计低温甚至室温下工作的气体传感器提供了巨大的支撑。通过氧化还原法得到的石墨烯（还原氧化石墨烯,r GO）在气体传感方面更具优势,r GO不仅具备良好的电学性能与大的比表面积,其表面缺陷、空位等也可以充当气体传感活性位点,这将很大程度上提升待测气体分子的吸附和反应。本文选取了两种有代表性的半导体金属氧化物材料,N型的α-Fe₂O₃和P型的Co₃O₄,通过水热和水浴的方法将其与石墨烯进行复合。我们以同时改善响应值和最佳工作温度两项指标为导向,构建了基于石墨烯修饰的半导体金属氧化物气体传感器。我们重点探究了氧化物形貌、尺寸及石墨烯含量对复合材料敏感性能的影响,并结合材料的结构表征和气敏性能提出了与之相匹配的敏感机理。具体的研究内容如下:1.我们通过一步水热法合成了石墨烯/圆角立方状α-Fe₂O₃的复合材料,其中石墨烯含量较低,质量分数为0.1-4.0 wt%。表征结果显示,复合材料中α-Fe₂O₃为具有微米级尺寸的圆角立方体,且尺寸均一、分散性良好。与相同合成条件下得到的纯α-Fe₂O₃相比,复合物中α-Fe₂O₃的尺寸约为纯α-Fe₂O₃颗粒尺寸的一半,这证明了石墨烯的存在抑制了α-Fe₂O₃的晶体生长。TEM表征显示石墨烯具有卷曲、折叠的形貌,其与α-Fe₂O₃颗粒紧密地结合在一起。气敏测试结果显示,含1.0 wt%石墨烯的复合物在225℃下对丙酮的响应最高,响应值为13.9,约为纯α-Fe₂O₃在237.5℃下对丙酮响应值的2.5倍。在响应值提升的同时,传感器的工作温度得到小幅降低。另外,我们发现在相同温度下复合物在空气中的电阻要远大于纯α-Fe₂O₃。我们认为,石墨烯在较低含量时分散在整个复合物,石墨烯与α-Fe₂O₃之间形成的异质接触及由α-Fe₂O₃向石墨烯的电子流动可能是其电阻增大的原因,也是复合物气敏响应提升的主要原因。2.我们通过改变合成条件并增加石墨烯的含量（6.5-12.2 wt%）合成了一种新颖的石墨烯封装的α-Fe₂O₃复合材料,并成功构建了基于这种复合材料的室温NO2传感器。表征结果显示,纯α-Fe₂O₃颗粒的尺寸为50 nm且分散性良好。复合物中α-Fe₂O₃颗粒的尺寸与纯α-Fe₂O₃近似,但具有更好的均一性。有趣的是,尽管在前驱物中加入了大量的石墨烯,我们通过SEM并没有直接观察到石墨烯的存在,因此推测有极小尺寸的石墨烯附着在α-Fe₂O₃颗粒表面。通过TEM和HRTEM测试,我们证实了具有特征弯曲晶格的石墨烯均匀地包覆在α-Fe₂O₃颗粒的外表面,形成了封装的结构。石墨烯的这种存在形式也导致了复合物拥有更大的比表面积。气敏测试结果显示,具有12.2 wt%石墨烯的复合物在室温下对5 ppm NO2的响应值可以达到8.2,其响应时间也只有2.1分钟。对应地,纯α-Fe₂O₃在125℃下对5 ppm NO2的响应值仅为2.1,响应时间为2.6分钟。这种石墨烯封装的α-Fe₂O₃复合材料表现出的性能在所有报道的基于石墨烯/α-Fe₂O₃的室温NO2气体传感器中是领先的。我们将其优异的气敏性能归结于复合物更大的比表面积、更多的表面吸附氧含量和石墨烯与α-Fe₂O₃之间大量P-N异质接触的形成。3.我们通过分步反应可控制备了r GO/多孔片状Co₃O₄复合材料。首先利用水热法合成了多孔的片状Co₃O₄,然后将其与氧化石墨烯（GO）充分混合,最后热还原得到r GO/多孔片状Co₃O₄复合材料。结果显示,r GO/Co₃O₄复合材料相较于纯Co₃O₄具有更大的比表面积和更高的缺陷氧和吸附氧含量。我们对所有制备的材料进行了气敏测试,结果显示,复合物对NO2的最佳工作温度随着石墨烯含量的增加逐渐下降至室温。其中,含有2.4 wt%石墨烯的复合物在室温下对NO2表现出最高的响应,其对5 ppm NO2的响应值为26.8%,且响应时间仅为1.5分钟。对应地,纯Co₃O₄在100℃下对5 ppm NO2的响应值仅为11.8%,其响应时间更是长达3.5分钟。我们将复合物提升的敏感性能归结为Co₃O₄的多孔结构、大的比表面积和石墨烯和Co₃O₄之间同型P-P异质接触的存在。更多还原

【Abstract】 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 α-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 α-Fe₂O₃,where the content of graphene is low（0.1-4.0 wt%）,through an one-step hydrothermal method.Characterization results indicated that α-Fe₂O₃ in the composite were micron-sized round-edged cubes,which had uniform size and good dispersion.Compared with pure α-Fe₂O₃ synthesized under the same condition,the size of α-Fe₂O₃ in the composite equalled one half of that of pure α-Fe₂O₃,which proved that the existence of graphene hindered the crystal growth of α-Fe₂O₃.TEM test showed that graphene in the composite had frizzy and folded morphology and combined closely with α-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 α-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 α-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 α-Fe₂O₃.The formation of P-N heterojunctions between graphene and α-Fe₂O₃ and the flow of the electrons from α-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 α-Fe₂O₃ composite through increasing the graphene content（6.5-12.2 wt%）and changing the synthesis condition.Characterization results indicated that particles in pure α-Fe₂O₃ had the size of 50 nm and a good dispersion.The size of α-Fe₂O₃ in the composite approximated that of pure α-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 α-Fe₂O₃.Through the tests of TEM and HRTEM,we found that graphene with characteristic curved crystal lattice wrapped the outside surface of α-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 α-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/α-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 α-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.更多还原