1.Department of Basic Science, Air Force Engineering University, Xi’an 710051, China 2.Tsinghua National Laboratory for Information Science and Technology, Institute of Microelectronics, Tsinghua University, Beijing 100084, China 3.The First Aeronautic Institute, Air Force Engineering University, Xinyang 464000, China 4.Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
Abstract:In recent years, flexible gas sensors have aroused wide interest of researchers due to their enormous potential applications in wearable electronic devices. In this paper, a flexible gas sensor is prepared. We use silver nanowires as flexible interdigital electrodes for gas sensors and reduced graphene oxide as gas-sensing materials. We also study its gas sensitivity and flexibility properties such as responsiveness, recovery, and repeatability to nitrogen dioxide. The experimental results show that the silver nanowire flexible electrode and the reduced graphene oxide gas sensor prepared can detect the NO2 gas with a concentration of 5—50 ppm at room temperature. The response (Ra/Rg) of the sensor to 50 ppm NO2 is 1.19. It demonstrates high response ability and repeatability. The recovery rate can be kept above 76%. The sensitivity of the sensor is 0.00281 ppm-1. The response time and recovery time of the prepared AgNWs IDE-rGO sensor for 5 ppm NO2 gas are 990 s and 1566 s, respectively. At the same time, the sensor still exhibits excellent gas sensing performance at a bending angle in range from 0° to 45°. The device has relatively stable conductivity and good bending tolerance. The sensing mechanism of the sensor can be attributed to the direct charge transfer between the reduced graphene oxide material and NO2 gas molecules. In addition, the high catalytic activity and excellent conductivity of Ag that is a common catalyst material, may also play an important role in improving the gas sensitivity of reduced graphene oxide materials. Silver nanowires, as a material for interdigital electrodes, provide excellent conductivity for device as well as support for the flexibility of device. It provides the fabricated sensor for good mechanical flexibility. And the gas-sensing performance of the AgNWs IDE-rGO sensor is mainly achieved by the use of reduced oxidized graphene material reduced by hydrazine hydrate. In summary, the silver nanowire flexible electrode and the graphene gas sensor prepared in this work are helpful in realizing the flexibility of the gas sensor. It lays a foundation for the further application of flexible gas sensors and has great application prospects in wearable electronic equipments. Keywords:silver nanowires/ flexible electrodes/ reduced graphene oxide/ gas sensing
还原氧化石墨烯是通过使用Li的方法化学还原氧化石墨烯分散液获得[18]. 具体来说, 首先取1.25 mL的GO悬浮液(2 mg/mL), 加入去离子水将GO悬浮液稀释至10 mL, 并将稀释的GO水溶液通过超声处理10 min; 然后加入38 μL的氨水和2.3 μL的水合肼作为化学还原剂; 最后, 将混合溶液在油浴(95 ℃)中加热1 h以将GO还原为rGO, 即可得到还原氧化石墨烯溶液. AgNWs IDE-rGO传感器的制备是采用喷涂的方法实现, 其制备过程示意图如图1所示. 首先, 将AgNWs溶液超声处理10 min, 以形成用于遮蔽式掩模喷涂的均匀分散体. 然后将制备好的不锈钢电极掩模紧密地固定在基板(便签纸)上, 通过自制喷涂系统对AgNWs IDE进行图案化. 之后, 通过在AgNWs IDE上滴下2 μL的rGO传感层来沉积传感膜. 最后用银浆把制备的传感器的引脚和银线连接起来, 用于之后的传感器气敏性能测试. 图 1 AgNWs IDE-rGO器件的制备过程示意图 Figure1. Schematic diagram of the fabrication process of the AgNWs IDE-rGO device.
图2(a)是制备的银纳米线叉指电极的光学图片, 图2(b)是单个电极的结构尺寸示意图, 制备的银纳米线叉指宽度和间距均是200 μm. 采用喷涂的方法来制备传感器有很多好处, 比如成本低、制备方法简单等, 并且还能够避免复杂的微图案化过程. 最重要的是, 这项工艺可用于未来的柔性电子器件的制备, 实现柔性传感器阵列的大规模制备. 图 2 (a)银纳米线叉指电极的光学图片; (b)单个电极结构尺寸示意图 Figure2. (a) Optical image of the AgNWs IDE array on portable stickers; (b) dimensions of the single electrode structure.
22.3.测 量 -->
2.3.测 量
传感器的气敏性能测量为室温下在动态配气测试系统中进行, 如图3所示. 将AgNWs IDE-rGO传感器放入密封性良好的测试腔内, 测试前, 先通入干燥空气, 等待传感器电阻值稳定. 测试时, 数字流量控制器的一个通道通入干燥空气作为载气, 另一个通道通入NO2气体, 通过改变两种气体的配比来得到实验中所需的不同NO2气体浓度. 将传感器交替地暴露在干燥空气和给定浓度的NO2中, 将集成测试系统(Keithley 2700)的采集信号输入端与传感器连接起来, 测量得到气体传感器响应和恢复阶段的实时电阻变化. 图 3 气敏测试装置示意图 Figure3. Schematic illustration of the gas sensing test setup.
图 1 AgNWs IDE-rGO器件的制备过程示意图
图 2 (a)银纳米线叉指电极的光学图片; (b)单个电极结构尺寸示意图
图 3 气敏测试装置示意图
图 4 对5—50 ppm NO2的实时响应曲线
图 5 响应与浓度的关系
图 6 恢复率与浓度的关系
图 7 传感器对15 ppm二氧化氮气体响应的重复性测试
图 8 传感器对5 ppm二氧化氮气体的响应和恢复时间
图 9 传感器对不同目标气体的选择性
图 10 AgNWs IDE-rGO传感器的柔韧性能测试 (a)器件电阻随弯曲次数的变化; (b)在不同弯曲角度下的响应与浓度的关系