The Research Of Tungsten Oxide Based Reducing Gas Sensor

There is still an unmet need for developing gas detection approaches with high sensitivity,rapid response recovery,low detection limit,good selectivity and long-term stability,which has always attracted great attention in both industrial and scientific fields and played an important role in safe guarding human health and promoting sustainable development in the human society.Some sophisticate instruments have already been developed to analyze and detect toxic and exhaust gases.However,their applications in on-site and real-time monitoring situation will be limited owing to the high cost,large size,difficulties in using and complicated sample processing together with the prolonged analysis time.Therefore,the development of fast response/recovery time,highly sensitive,low-cost gas sensors for real-time monitoring is greatly important during environmental protection.Oxide semiconductor based gas sensors have come into our sight for their low-cost,miniaturized and easy-to-use,which have presented their advantages in real-time detection.In this paper,we are devoted to fabricate tungsten oxide(WO₃)based gas sensors with high sensing performance toward reducing gases possessing sluggish response/recovery time(such as toluene,hydrogen sulfide or triethylamine).The receptor function,transducer function and utility factor of the sensors have been improved through modulating morphologies and microstructures,adjusting work function,carbon modification,functionalization with noble metal and construction of heterojunctions,thereby achieving real-time monitoring of toxic and harmful gases under extreme conditions such as low concentrations or high humidity.The specific research contents are as follows:1.By adjusting the molar ratio of the hexamethylenetetramine(HMT)to tungsten chloride(WCl₆)in the process of solvothermal synthesis,the morphologies of WO₃were controlled from microflowers,nanosheets to nanoparticles.After sintering at optimum temperature,the dispersed WO₃ nanoparticles with a size of about 50 nm were obtained.At a lower sintering temperature,the crystallinity of WO₃was poor.When the sintering temperature was too high,the nanoparticles tended to agglomerate,which would cause adverse effect on the performance of the sensor.By comparing the gas sensing results of the prepared samples,it can be seen that the dispersed WO₃ nanoparticle based sensor exhibits high sensitivity(132.0),good selectivity,anti-humidity and fast response(2 s)/recovery(6 s)time at 225°C.However,compared to other samples,the HMT₁-WO₃ sample has the lowest content of adsorbed oxygen.Therefore,the result indicates that the adsorbed oxygen content is not determining factor for its eminent gas sensing performance in this experiment.Hence,the improvement of sensing performance for the sensor based on dispersed WO₃ nanoparticles may be attributed to the porous structure of the sensitive layer formed by the accumulation of dispersed nanoparticles,the high resistance caused by good crystallinity and rich grain boundaries among the nanoparticles.2.Diverse morphology WO₃with different crystal facet ratios were prepared by one-step solvothermal method using alcohol solvents with different carbon chains accompanied by annealing treatment.When the number of C is 1,2,4 or 6,the morphologies of the obtained samples after sintering at 400°C is microspheres closely packed by nanoparticles,loose and porous microbial flowers assembled by nanoparticles,flake flowers assembled by nanoplates and rod flower structures consisted of nanorods,respectively.The testing results show that C2 sample has bimodal pore distribution,which is beneficial to the enhancement of the sensing performance.According to XRD analysis,the characteristic peak position of C6 has changed.Comparing the work functions of C1,C2 and C4 sample,it is found that there is a certain relationship between the crystal facet ratios and their work function.In addition,calcination temperature is also an important factor affecting the morphology.By changing the sintering temperature of C2,it can also be found that the crystallinity of the sample is slightly poor when the sintering temperature is 300°C.When the sintering temperature rises to 400°C,the nanofibers disappear and become nanoparticles.When the sintering temperature is too high,the crystals continue to grow and the grain boundaries become blurred.The results of the sensing test showed that the C2-400 sensor exhibited high response to triethylamine,excellent selectivity,fast response/recovery time(3 s/5 s)and low detection limit(200 ppb).3.The design of carbon modified WO₃ sensitive material to construct hydrogen sulfide sensor with high anti-humidity and long-term stability.The main problems existing in hydrogen sulfide detection are the slow response/recovery speed,poor stability and long-term stability at high concentrations,which can lead to irreversible poisoning of the device.We added an appropriate amount of glucose to tungsten chloride during the solvothermal process combined with the optimal calcination temperature to obtain carbon modified coral-like WO₃.The addition of glucose can alleviate the accumulation of WO₃ after high temperature treatment.Coral-like WO₃is about 1?m in diameter,and possesses rich mesopores and macropores.According to the experimental results,firstly,carbon modification avoids the agglomeration of small particles to a certain extent and thus maintains the unique morphology of coral-like WO₃.Secondly,carbon acts as electron acceptor and donor,promoting the transport of electrons between W and O,resulting in more W⁶⁺components.The heterojunctions between C and W at the interface can be formed,and electrons are transmitted from W to C.Thus,the presence of heterojunctions in WO₃-2@C will provide more adsorption and reaction sites,and the work function increase.Finally,owing to the good hydrophobicity of carbon and its strong adsorption to the target gas,the WO₃-2@C sensor exhibits good anti-humidity.In addition,the response/recovery time of the sensor is virtually unaffected whether at low or high concentrations of hydrogen sulfide.After a long-term stability test of 42 days,the response value of the WO₃-2@C sensor was reduced by 5.7%.Low-cost carbon modification combined with the bimodal pore structure of WO₃ will provide a new means for real-time monitoring of hydrogen sulfide.4.Carbon/precious metal co-modification,combined with heterojunction construction,endows C/Pd O@WO₃-W₁₈O₄₉ base sensor with ultra-low detection limit(50 ppb),high anti-humidity,long-term stability and fast detection to triethylamine(<2 s).This study optimized and improved the sensing characteristics of triethylamine in terms of sluggish desorption,detection limit and anti-humidity properties.We prepared urchin-like C/Pd O@WO₃-W₁₈O₄₉ with the size of 2?m by one-step solvothermal method and annealing treatment.The C/Pd O@WO₃-W₁₈O₄₉ urchin was self-assembled by nanofibers with a diameter of?15 nm.The self-assembly of these nanofibers will cause large pores,which provides abundant active sites and promotes the diffusion of the target gas.According to the testing results,the recovery time of the pure WO₃ based sensor to 100 ppm triethylamine at 325°C is 546 s,and the lowest detectable concentration of triethylamine is 3 ppm.It is worth noting that after carbon and Pd O modification,the WCP-2 based sensor exhibits an ultra-fast response and recovery speed to triethylamine at 325°C.And the response and recovery speed is nearly not affected by the concentration and humidity.The lowest detectable concentration of triethylamine is 50 ppb.After 23 transient repeatability cycle tests and a 19-day long-term stability test,the response values of the sensor fluctuated less and still had a fast response/recovery speed without sluggish.The improvement of the detection limit and anti-humidity properties of the WCP-2 based sensor will provide a new perspective for the performance optimization of the sensor under special conditions.