Gas Sensing Properties Of SnO₂ Nanofibers Treated With Carbonization Method

Detection of harmful gases is playing an important role in the fields of environment and medical treatment with the improvement of people's living standard.Therefore,the research of gas sensors has become a hot topic in scientific knowledge nowadays.Sensors based on semiconductor oxides are widely used in the detection of flammable and explosive gases due to the following properties:high sensitivity,well selectivity and stability.The performances of the sensors are mainly determined by the characteristics of based materials.Receptor function,transducer function and utility factor,largely affected by the microstructure and morphology of the materials,were reported to be the main factors for gas sensing properties.Thus,it is of great significance to analyze and research the preparation of materials with certain microstructure and high surface activity to improve the performance of gas sensors.More attention has been focused on electrospinning method due to the excellent properties of nanofibers prepared by this technology:high specific surface area,well permeability and high carrier mobility.SnO₂,a typical n-type semiconductor,which has a wide band gap and well electrical and chemical properties,plays a significant role in catalysts,bioengineering and gas sensors.In spite of the long history of the research on gas sensors based on SnO₂ nanofibers,the further improvement of the performances of sensors is also a priority in the field,particularly the response to target gases of gas sensors.Some recent researches indicate that the increase of gas response often accompany with the increase of oxygen vacancy.Oxygen vacancy can be generated through the traditional high-speed ion bombardment method.However,such method can only treat the surface of the sensing material and leaves the internal of materials untouched..Therefore,the modification effect is not well.In addition,A great deal of studies about gas sensors based on oxide semiconductors have been reported.However,conventional encapsulation methods can only ensure one type of oxides to be exposed to the garget gas,and can not fully develop the synergistic effect of the two materials.Aiming at these problems above,carbonization followed by calcination in air is developed to increase the oxygen vacancy on the surface of pure and Pd-doped SnO₂nanofibers.Then,through using carbonization method to synthesize the Sn doped carbon nanofiber,followed by growing Fe₂O₃ nanoroads through hydrothermal method and calcining in air to remove the carbon material,SnO₂/Fe₂O₃ composite nanofibers with a hollow structure has been,and the sensing properties were investigated.The main contents are as follows:The porous SnO₂ nanofibers with hollow structure were prepared by carbonizing the precursors fabricated by electrospinning and sintering with different heating rates in air.It was proved that the carbonization method could effectively improve the response of the gas sensors according to the gas sensing investigation,and a better morphology and higher gas response could be obtained at a lower heating rate?2°C/min?.The response to 100 ppm ethanol at the optimum operating temperature is 20.4,which is significantly higher than that of the sensors without carbonization treatment?7.6?.The response of sensors with higher heating rate?5°C/min?to 100 ppm ethanol was about 14.5,which was higher than that of the sensors without carbonization treatment but lower than that of the sensor with the low heating rate.Accordingly,we prepared carbonized Pd-doped SnO₂ nanofibers at a low heating rate.The response to100 ppm toluene at the optimum operating temperature was increased from 10 to 24.6compared with the uncarbonized Pd-doped SnO₂ nanofibers.A lower detection limit?1.6 to 500 ppb toluene?and a fast response time?³ s?were also observed.In conclusion,carbonization treatment can significantly improve the response of the sensors,and the optimal operating temperature and selectivity were almost not changed greatly.The improvement of sensing performances with carbonization measure is mainly attributed to the increment of specific surface area and oxygen vacancy by analyzing the morphological structure,surface adsorption oxygen state and specific surface area of the prepared materials.Furthermore,the formation mechanism of this microstructure was studied by TGA.Different SnO₂/Fe₂O₃ composite nanofibers were successfully prepared by the following methods:The SnO₂ nanofibers and the carbonized precursors prepared by electrospinning were applied as templates for the growth of Fe₂O₃ nanoroad by hydrothermal method.According to the results of characterization,the composite nanofibers with carbonized template possess a hollow structure.The carbonization also had no effects on the optimal working temperature of the composite nanofibers,and the response to 100 ppm ethanol at the optimum working temperature was increased from21 to 44,and the detection limit was lower?1.6 to 20 ppb ethanol?.In summary,the improvement of gas sensitivities is mainly owing to the increment of specific surface area and the formation of heterojunction.