| With the rapid development of industry and the improvement of people’s living standards,environmental safety issues have become increasingly important.Volatile organic compounds(VOCs)derived from industrial production,engineering construction,home decoration,and other processes,can harm the respiratory system,seriously damage the kidneys and nervous system,and even cause cancer and death.Therefore,the research on gas sensors is of great significance.In the research of gas sensing technology,metal oxide semiconductor gas sensors have become the mainstream direction and introduction technology due to their advantages of high sensitivity,fast response/recovery time,low cost,and portability.The sensing mechanism is that the material surface adsorbs oxygen to form a space charge layer,and the target gas undergoes an oxidation-reduction reaction with the adsorbed oxygen ions,changing the thickness of the space charge layer and thereby changing the conductivity of the gas-sensitive material.Among them,material morphology and particle size have a significant impact on gas adsorption and desorption;In addition,the interface barrier of semiconductor heterojunctions causes band bending and forms an internal electric field.Doping oxide semiconductors can effectively change the barrier height and band structure,achieving effective regulation of the response value of gas sensors.The current oxide semiconductor gas sensors still have shortcomings,such as high operating temperature and high power consumption,which restrict their further development in practical applications.In response to the above issues,this paper is based on rare earth perovskite semiconductor materials,modified by doping and constructing heterojunctions,designed a series of gas sensors with low operating temperature and high response values combined with a 1.5 mm × 1 mm planar electrode.In addition,with the assistance of light,a significant increase in the gassensitive response value of the sensor was observed,and photogenerated carrers made a significant contribution to increasing the material conductivity and gas-sensitive response value.The main content is as follows:1.A rare earth perovskite YFeO3 planar electrode sensor capable of detecting VOCs gas at lower operating temperatures has been studied,which is of great significance for practical applications.The YFeO3 particles prepared by the sol-gel method are small in size and evenly distributed,which is conducive to gas adsorption.The interface conduction and transfer barrier of the charge is low and the speed is fast,which makes the optimal operating temperature of the sensor only 110℃.In addition,the YFeO3 material has a bandgap of 1.837 eV,which enables valence band electrons to transition to the conduction band under light excitation(470 nm,420 nm,and 370 nm),thereby improving conductivity.After oxygen adsorption,the number of electrons captured from the material surface increases,thereby increasing the thickness of the hole accumulation layer.After the target gas contacts and undergoes an oxidation-reduction reaction,the conductivity quickly recovers and a larger conductivity difference is obtained,resulting in a significant improvement in the gas sensing response value.The electronic structure of YFeO3 material was calculated and analyzed based on the first principles,and the gas adsorption of acetone on the YFeO3(010)crystal plane was explored through a partial adsorption model.2.NdFeO3,SmFeO3,and LaFeO3 nanomaterials with different annealing temperatures were prepared by the sol-gel method,and their gas-sensing properties were studied under light excitation.After analysis,the bandgap values of YFeO3(700℃),NdFeO3(800℃),SmFeO3(700℃),and LaFeO3(700℃)materials are 1.837 eV,1.965 eV,1.985 eV,and 2.156 eV,respectively,which are smaller than the photon energy of the light source.Therefore,under light excitation,the performance of the sensor can be improved.Under UV excitation,the response values of YFeO3,NdFeO3,SmFeO3,and LaFeO3 sensors to 30 ppm acetone gas were 128.1,60.6,29.38,and 54.53,respectively.Compared with dark conditions,the performance of the sensor improved by 65%,51%,42%,and 38%,respectively.Therefore,the smaller the material bandgap value,the stronger the effect of light excitation on the sensor.3.To further improve the gas sensing performance of the YFeO3 sensor,a SmFeO3/YFeO3 composite planar electrode sensor with a heterostructure was designed.The band gap value of SmFeO3 material is 1.985 eV,slightly higher than that of YFeO3.In the heterostructure pSmFeO3/p-YFeO3 composite,due to the difference of SmFeO3 and YFeO3 Fermi energy levels,electrons move at the interface until the Fermi energy level reaches equilibrium,and the existence of heterostructure leads to band bending and the formation of an internal electric field.The band gap of p-SmFeO3/p-YFeO3 composite nanomaterials is between 1.837 and 1.985 eV.When the target VOCs gas is introduced,a redox reaction occurs with the adsorbed oxygen,the recombination of electron-hole pairs reduces carrier concentration and rapidly reduces conductivity.The response value of the sensor to 30 ppm acetone gas under final UV excitation increased to 134.02,which further increased the gas sensing response based on YFeO3.4.To further confirm the role of heterojunction composite materials in improving sensor performance,p-p heterojunction composite NdFeO3/YFeO3 nanomaterials were prepared.The sensor prepared with this material has a lower operating temperature and high gas response,providing a foundation for future research on portable or wearable gas sensors.The preparation of small uniform grains and large specific surface area by the sol-gel method is conducive to the adsorption and desorption of gases.In NdFeO3/YFeO3 composites,the charge transfer and Fermi surface reconstruction of the heterojunction interface lead to the bending of the energy band,and the formation of an internal electric field at the interface,which is also conducive to the generation of hole carriers under light and increase the conductivity of the system.The band gap of heterojunction composite materials is between 1.837 and 1.965 eV,which is smaller than the energy of incident photons.After the introduction of VOCs gas and the occurrence of a redox reaction,the difference in conductivity of the system increased,ultimately leading to an increase in the response value of NdFeO3/YFeO3 to 30 ppm acetone gas under UV excitation to 159.43,which is consistent with theoretical predictions.5.Similarly,p-LaFeO3/p-YFeO3 composite material sensors were studied.Under UV excitation,their response to acetone gas can reach 156%,with an increase of 49%and 25%based on SmFeO3/YFeO3 and NdFeO3/YFeO3,respectively.The band gap of p-LaFeO3/pYFeO3 composite material is between 1.837 and 2.162 eV.After introducing VOCs gas and undergoing an oxidation-reduction reaction,the difference in conductivity between the front and rear will further increase when the hole accumulation layer recovers,ultimately leading to a further increase in the response value of the sensor.In addition,LaFeO3/YFeO3 sensors have a low detection limit of 0.8 ppm for acetone gas,which is very competitive among sensors of similar materials.6.Finally,the gas sensing performance of La1-xYxFeO3 planar electrode sensors doped with A-site was studied.After doping Y on the A-site of LaFeO3 material,the bandgap value of the material will be less than 2.162 eV.In the La1-xYxFeO3 sensor,doping forms a competitive relationship with the adsorption of oxygen molecules and the reduction of electronic energy bands.At x=0.8,the response value of La0.2Y0.8FeO3 to 30 ppm methanol gas reaches the maximum value of 101.53.Under light illumination,photo-generated charge carriers significantly enhance the conductivity of the material and the gas-sensing response value.With the increase of photon energy(470 nm,420 nm,and 370 nm),the gas sensing response value can increase by 66%,83%,and 109%.Therefore,the A-site doping of appropriate elements in perovskite nanomaterials under the assistance of light is also an effective method for preparing gas sensors with a low operating temperature and high response value. |
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