Fabrication And Characteristics Of MXene-Based Composite Film Gas/Humidity Sensors

With the rapid development of the Internet of Things（Io Ts）,gas and humidity sensors are widely used in the fields of smart homes,wearable electronics,and smart mobile terminals.However,there are still various issues to develop high-sensitivity,high-selectivity,low-power,intelligent,and self-powered gas/humidity sensors.Sensing materials are key elements of the sensors.Therefore,innovative research on the sensing materials is an important way for the gas/humidity sensors to solve existing challenges and meet diverse needs.In recent years,transition metal carbides/nitrides/carbonitrides（MXene）,such as titanium carbide（Ti₃C₂T_x）,vanadium carbide（V₂CT_x）,and niobium carbide（Nb₂CT_x）,have shown broad application prospects in the field of the sensors due to their unique conductivity,two-dimensional（2D）layered structure,abundant active sites and functional groups.Therefore,focusing on the MXene materials,this dissertation studies the design and preparation,performance research,mechanism analysis,and application demonstration of MXene-based composite film gas/humidity sensors.The main research contents are summarized as follows.1.In view of the issues of Ti₃C₂T_x material such as small response,slow recovery,and poor reversibility when detecting NO₂,a resistive-type Ti₃C₂T_x/γ-poly（l-glutamic acid）（γ-PGA）-based NO₂ gas sensor is designed and prepared by usingγ-PGA modification.Owing to the enhancement of effective adsorption and blocking effect assisted by water molecules,the positive response behavior of the Ti₃C₂T_x/γ-PGA-based gas sensor to NO₂ gas is improved effectively,and its response to 50 ppm NO₂（～1127.3%）is 85 times higher than that of pristine Ti₃C₂T_x-based gas sensors.Moreover,the as-fabricated Ti₃C₂T_x/γ-PGA-based sensor not only exhibits a short response/recovery time（43.4 s/3 s）,but also shows a good reversibility and repeatability at room temperature,where the relative standard deviation（RSD）of the response values is less than 1%.Furthermore,the gas sensing response of the Ti₃C₂T_x/γ-PGA sensor is studied at different relative humidity（RH）,and the humidity compensation of the sensor is carried out by a multiple regression method to eliminate the influence of humidity on the gas sensing response.Finally,based on the synergistic effect of the composite under humid environment and the competitive adsorption between NO₂ gas molecules and water molecules,a gas sensing mechanism model with enhanced blocking effect is proposed.2.To address the problems of cross-sensitivity and high detection limit of the Ti₃C₂T_x-based gas sensors,a double transition-metal（DTM）titanium molybdenum carbide（Mo₂TiC₂T_x）is proposed with superstrong surface adsorption for NO₂ gas molecule,and it is further coupled with molybdenum disulfide（MoS₂）by in-situ assemble method to construct an edge-enriched heterostructure.Based on that,a resistive-type Mo₂TiC₂T_x/MoS₂-based NO₂ gas sensor is designed and fabricated.Due to the synergistic effect of strong adsorption,rich adsorption sites,and heterointerface of the Mo₂TiC₂T_x/MoS₂ composite,the as-fabricated Mo₂TiC₂T_x/MoS₂ gas sensor exhibits a sensitivity of 7.36%ppm^–1,detection limit of 2.5 ppb,and high selectivity at room temperature.In addition,combining characterization and experiment results with DFT theoretical calculations（including adsorption energy,density of states,and charge density difference）,a gas sensing mechanism model based on the Mo₂TiC₂T_x/MoS₂ composite is established.Finally,by integrating the NO₂ gas sensor with a signal acquisition terminal and receiving terminal,a portable and wireless warning system is demonstrated for excessive NO₂ gas concentration.3.In view of the dependence of most electronics on external power sources,a dual-function device with humidity sensing and power generation is designed.A neuron-like Nb₂CT_x/SA composite film is fabricated by electrospinning,and used as an active layer to develop a power-generating humidity sensor（PGHS）.The neuron-like network structure of the Nb₂CT_x/SA composite film could provide numerous adsorption sites and nanochannels for water molecules to diffusion and ionization,improving the response of the PGHS at low humidity.The as-fabricated Nb₂CT_x/SA-based PGHS exhibits a sensitivity of 7.8 m V RH^–1 at 28.8%-91.5%RH,response/recovery time of 27.0 s/20.6 s at 91.5%RH,and hysteresis of 6.0%RH.In addition,the Nb₂CT_x/SA-based device can be used as a generator,which shows a highest voltage output of 0.53 V and power of123 n W with the optimal load resistance.Based on the ion diffusion effect,a mechanism model for coupling humidity sensing with power generation is established.In addition,the portable PGHS is compatible with human respiration assessment,non-contact monitoring,and smart alarm applications.Finally,by simply integrating the Nb₂CT_x/SA-based PGHS with a capacitor and light-emitting diode（LED）,a prototype of self-powered sensing system capable of indicating different RH（i.e.,low,medium,or high）is demonstrated without external power supplies.4.To simplify the device preparation process and improve the output of the PGHS,a hyaluronic acid（HA）-induced crumpling of Nb₂CT_x nanosheet is designed to construct hydrogen bond conduction network,and a Nb₂CT_x/HA-based PGHS is developed based on the working mechanism of primary battery.Compared with pristine Nb₂CT_x and HA films,the humidity sensing and power generation performances of the Nb₂CT_x/HA composite film are greatly improved owing to the abundant adsorption sites and accelerated proton transport.Driven by the water molecules,the Nb₂CT_x/HA composite-based PGHS shows a wide detection range（10.9%-91.5%RH）,fast response/recovery speed（15.1 s/3.4 s）,high response of 0.7 V at high humidity,and low hysteresis（3.6%RH）.Moreover,a power output of 0.96μW with the optimal load resistance and sustainable voltage output of 12 h of the Nb₂CT_x/HA-based PGHS are manifested.Furthermore,a redox reaction-based mechanism model of Nb₂CT_x/HA composite PGHS is established at dry environment,low and high humidity.Finally,multifunctional applications of the PBHS are demonstrated in breath detection,diaper alarm,non-contact intelligent switch,and driving multiple LEDs.