Preparation And Performance Research Of Low-temperature Gas Sensor Based On Novel Two-dimensional Materials

The rapid economic development has not only improved people's living standards,but also brought environmental pollution problems.In particular,the toxic and harmful gases in air pollution have always been the"killers"of people's health.Therefore,accurate detection of these toxic and harmful gases is important for safeguarding people's physical health.Among various types of gas detection devices,optical detection equipments have the advantages of high accuracy,robust reliability,and good repeatability.However,they are expensive,bulky,and costly,which are not conducive to real-time monitoring.Hence,it is an urgent task to develop and prepare gas-sensitive materials and related low-cost gas sensors that can detect toxic and harmful gases quickly,accurately,and temporarily.The current commercial metal oxides based resistive gas sensors always suffer from high working temperature(200-500?)that requires a heating device thereby accompanied by a lot of energy consumption and integration difficulty,particularly unsuitable for flammable and explosive environments;meanwhile,poor selectivity and susceptibility to humidity interference from ambient environment also severely limit further applications.The new two-dimensional(2D)materials such as reduced graphene oxide(rGO),tungsten disulfide(WS₂),and molybdenum disulfide(MoS₂)have completely changed the pattern of high temperature detection,and has been extensively researched and developed due to its ability to detect gases at low temperatures even room temperature.This work focuses on the influence of composition of composite film(Chapter 3 to Chapter 5)and the optimization of the film structure(Chapter 6)on the sensing performance.Using new two-dimensional materials mixed with metal oxides,conductive polymers or different two-dimensional materials to form a heterojunction composite film,reducing gas NH₃,oxidizing gas NO₂ and several VOC vapors(chloroform,water,ethanol,acetone and formaldehyde)were employed as target gases,and the sensing performance and the corresponding sensitization mechanisms were studied with a single sensitive material are studied.Achieve the aim of low temperature detection of resistive gas sensor.The research focuses are listed as follows:(1)The rGO-Cu₂O nanocomposite material was prepared via the hydrothermal method and spraying technology to serve as the sensing layer,and NH₃was detected at low temperature(?100?).A series of characterization techniques such as SEM,TEM,XRD,FTIR and XPS were used to study the morphological and compositional characteristics of the prepared nanocomposites.It was found that,the prepared sensor could quickly and completely recover within 26s when a heat pulse of70 ^oC for 5 seconds was applied during the NH₃ desorption process.Comparatively,77%recovery was attained even after 10 minutes without heat pulse.It indicated that the introduction of heat pulses during desorption process could greatly improve the recovery performance.The sensors exhibited good repeatability and negligible baseline drift on seven cyclic exposures to150ppm NH₃.Also,the sensor response at different operating temperatures was tested.The results showed that the response of rGO/Cu₂O sensors was much higher than that of pure rGO counterpart at each temperature(25,60and 100?).We also found the sensor response achieved the maximum value at 60?which was adopted as the optimum working temperature.When exposed to numerous interference gases,rGO/Cu₂O sensors also showed good selectivity to NH₃.The introduction of Cu₂O nanomaterials into rGO nanosheets was conducive to form a heterojunction film that could greatly improve the the sensor performance in terms of operating temperature,response,and selectivity.(2)A porous polyaniline/tungsten disulfide(PANI/WS₂)nanocomposite film was prepared to detect ppm-level NH₃ gas at room temperature(25?)under humid background atmosphere.The as-prepared PANI/WS₂ sensor with 1.5mg of WS₂loading exhibited a stronger response(1.25 vs.1.02),better stability and accelerated response/recovery speeds toward 10ppm NH₃ at 68%RH(the optimal RH level)in comparison with PANI counterpart.As the amount of WS₂ increased,the composite film structure was significantly enhanced,and the stability in a humid environment was also stronger.Noteworthy was that excess WS₂ loading adversely weakened the sensor response.This work adjusted the RH condition and WS₂ content from the perspectives of optimizing test conditions and compositional combination to improve the sensor performance.(3)rGO/MoS₂ composite thin film sensors were prepared for NO₂ detection at low temperature,wherein multiple characterization techniques containing TEM,XRD,XPS and Raman were employed.The experimental results indicated that rGO/MoS₂composite film possessed a larger exposure area,more sorption sites and more p-n heterojunctions.The sensing response reached 59.8%toward 2ppm NO₂ at 60?(optimal operation temperature),which was nearly 200%enhancement in comparison with bare rGO one.Furthermore,we also found that RH posed a small interference effect on sensor response towards NO₂.When exposed to 120ppb NO₂,the small response decay after a few weeks revealed its decent long-term stability.Moreover,an excellent selectivity toward NO₂ gas was exhibited against other interfering gas species.Through further exploration of ppb-level NO₂ detection,the detection limit was calculated to be 5.7ppb.Therefore,MoS₂ incorporation into rGO favored the production of 2D material heterojunction that could significantly improve the sensor's response,detection limit,and working stability.(4)rGO/ZnOdouble-layer film was synthesized by facile sol-airbrush technology at room temperature.Compared with pure rGO film,rGO/ZnO bilayer counterpart possessed higher surface roughness and larger spacing.Sensing performance measurements indicated that rGO/ZnO film had 30%response enhancement to chloroform vapor compared to pure rGO one due to the improvement of film structure.A slight response enhancement was also observed on exposure to other vapors(water,ethanol,acetone and formaldehyde)with both response and recovery times less than 10s.To explore the stability of rGO film,we studied the effect of different flow rates of background gas on the electric resistance variation.It was found that electric resistance fell when gas flow rate decreased,rose when the flow rate decreased,and the response became smaller after annealing.It was because the air flow exerted pressure on the surface of the rGO film.Adjusting the air flow would change the gap spacing within the rGO network,which then affected the amount of gas adsorption and response.The authors proposed a new method for calculating sensor response and verified that this method could eliminate its influence on the sensor performance analysis bearing baseline drift with more reliable results.