| With the continuous development of technology and industry,the air pollution problem is becoming more and more serious.As one of the most important gas pollutants,nitrogen dioxide(NO2)is one of the main causes of environmental problems such as acid rain and photochemical smog,which poses a serious threat to human health.Therefore,it is significant to efficiently detect the concentration of NO2 in the air.Metal oxide semiconductor nanomaterials are widely studied as gas sensing materials,but they often have shortcomings such as poor selectivity,low sensitivity,susceptible to ambient humidity.Therefore,it is of great significance to develop new material systems with better gas sensing properties for efficient detection of NO2 in the atmosphere.As an emerging semiconductor material in recent years,zinc hydroxyfluoride(ZnOHF)has been widely used in photocatalysis,optoelectronics and many other fields.It gradually becomes a potential functional material for its simple preparation method and unique energy band structure.ZnOHF is a wide band gap semiconductor,and its energy band gap is similar to that of zinc oxide.However,due to the existence of fluorine with high electronegativity in its lattice,the potentials of both conduction band and valence band increase to a certain extent,which will affect the gas adsorption and dissociation process on the surface of the material.In addition,the crystal of ZnOHF belongs to the orthorhombic system with low symmetry,which is easier to orientately grow to obtain low-dimensional structures such as nanorods and nanosheets,which increases the specific surface area of the material and is conducive to the contact and reaction between the gas and the material.Therefore,ZnOHF nanomaterials have good development potential in the field of gas sensors.In this paper,ZnOHF-derived semiconductor nanomaterials which were modified by ion doping and other methods were synthesized and its NO2 sensing properties were studied,and the role of fluorine in the gas sensing process of zinc-based semiconductors was further studied.We have obtained a variety of semiconductor nanomaterials with excellent NO2 gas sensing properties in this paper.The main research works of this paper are as follows:1)The ZnOHF material with a 3D flower-like structure was prepared via a hydrothermal method,and the morphology and size of the prepared material were adjusted by changing the feeding ratio in the preparation process.The effect of the materials’ morphology and structure on the gas sensing properties of the material was studied.By comparing with ZnO,the difference of energy band between these two materials was studied,and the effect of the energy band structure on the gas sensing properties of the material was analyzed.The gas sensing test to NO2 showed that the 3D flower-like ZnOHF prepared with the F/Zn feed ratio of 20 had the best gas sensing performance.This material exhibited high sensitivity(82.71),short response/recovery time(13 s/35 s),and excellent selectivity to 10 ppm NO2 at 200℃,greatly outperforming the ZnO raw material.In addition,the prepared ZnOHF can detect NO2 with a wide detection range from 100 ppb to 50 ppm.The mechanism analysis shows that the conduction band potential of ZnOHF is relatively high,which makes it difficult for oxygen molecules to dissociate on the surface of the material.Therefore,almost all conduction band electrons can be used for NO2 gas sensing.2)Al3+ doped ZnOHF was synthesized by a simple one-step hydrothermal method.On the basis of the first work,the properties of the material were further improved by introducing Al3+doping and ultraviolet light irradiation,and the influence mechanism of ion doping and photoexcitation on the gas sensing properties of the material was studied.The SEM images show that the Al3+ doped sample is a bundle-like structure composed of nanorods with a diameter of about 25 nm and a length of 1.5 μm.The gas sensing test results showed that 0.5 at.%Al-ZnOHF exhibited the highest sensitivity(110.83)to 10 ppm NO2 at 100℃ under UV assistance,which was 4.38 times that of pure ZnOHF(25.29).In addition,the sensor based on 0.5 at.%Al-ZnOHF also exhibited a short response/recovery time(35 s/96 s)and excellent selectivity to NO2.The analysis of the gas sensing mechanism shows that the modulation of the energy band structure and optical properties of the material by Al3+ doping are the main reasons for the improvement of the response;the assistance of ultraviolet light not only reduces the working temperature of the material,but also greatly improves the gas adsorption and desorption process.3)In order to further explore the role of fluorine in NO2 gas sensing of Zn-based semiconductor materials,ZnO nanobeams were prepared by a solvothermal method and modified by the NaBF4 solution impregnation method.The occurrence process of two modification reactions,F-doping and BF3 group grafting,during NaBF4 treatment was explored,and the effects and reasons of these two modification methods on the gas sensing properties of ZnO were analyzed.The morphology of the as-prepared ZnO consists of a bundle of nanorods.The diameter of the nanorods is between 100 nm and 200 nm and the length is about 1.5 μm.The NaBF4 treatment has no effect on the micromorphology and crystal structure of ZnO.The gas sensing performance test exhibits that the ZnO material treated with 0.1M NaBF4 solution has the best gas sensing performance to NO2.Its response value to 5 ppm NO2 at 160℃ reaches 38.33,which is higher than that of pure ZnO(10.62).With the increase of NaBF4 solution concentration,the response value of ZnO showed a trend of first decreasing,then increasing and then decreasing.The response and recovery time gradually reduced and the anti-humidity property was greatly enhanced.The gas sensing mechanism analysis shows that there are two modification methods in the NaBF4 solution treatment process,namely the doping of F-and grafting of BF3 groups.The combined effect of these two methods resulted in the significant improvement of its gas sensing performance. |
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