Research On Iron-based Oxide Acetone Sensors

With the rapid development of China’s economy and society,people’s living standard has been greatly improved.But along with the economic development and environmental problems,especially the air pollution problem,have also seriously endangered the health of people and animals.Therefore,people are more and more aware of the prevention and treatment of environmental pollution.In order to achieve effective monitoring and detection of flammable,explosive and toxic gases in industrial production and daily life,various practical gas sensors with good sensing performances are gradually developed and used.Acetone,a toxic volatile organic compound（VOCs）,is an important raw material for organic synthesis in industrial production and can also be used as a solvent,extractant and cleaning agent.It can enter the interior of the human body through respiration and skin contact,causing varying degrees of damage to health.Moreover,automobile exhaust also contains a small amount of acetone,which can cause pollution to the atmosphere.In addition,human exhaled gas contains a certain amount of acetone,and these endogenous acetone can be used as a biomarker for the diagnosis of diabetic ketosis.Therefore,acetone sensors play an essential role in ensuring safe production,automobile exhaust monitoring and diabetes diagnosis.Especially for diabetes diagnosis,it has the advantages of being non-invasive,painless and portable compared to traditional diagnostic methods.Applications in specific environments require prepared acetone sensors with high sensitivity,low optimal operating temperature,good selectivity,high moisture resistance,fast response and recovery rates,well repeatability,and good long-term stability.In the field of gas sensors,semiconductor metal oxides（SMO）have unique advantages,such as stable physical and chemical properties,low cost,simple preparation process,and easily tunable morphology.When SMO-based gas sensors contact with the target gas,a series of reactions of gas molecules occur on the surface of the sensing material,which will lead to changes in the concentration of carriers in the main body of the sensing material,thus causing changes in the resistance value.Therefore,it is a very effective means to improve the gas sensing performances of gas sensors by changing the experimental conditions,transition metal doping and noble metal loading to achieve the regulation of the components,morphology,size and surface state of the sensing material.The abundant reserves of iron make it inexpensive and easy to obtain.Among them,zinc ferrate（Zn Fe₂O₄）and iron trioxide（Fe₂O₃）have been widely used as gas sensing materials for the preparation of various gas sensors,including acetone sensors,due to their chemical stability,low cost,corrosion resistance and environmental friendliness.These iron-based oxide based acetone sensors have some shortcomings in practical applications,such as poor selectivity and moisture resistance.In this paper,several types of typical Zn Fe₂O₄and Fe₂O₃sensors are prepared by different methods,and the response values,selectivity,moisture resistance and other gas sensing properties of the prepared materials are improved by using oxide composite,heterogeneous ion doping and noble metal loading.The morphology,dimensions,composition,crystal structure,and chemical state of various sensing materials were investigated by various characterization tools.The gas sensing properties of the prepared acetone sensors were investigated using a static gas sensing test system,and the mechanism of sensitivity to acetone gas was analyzed.The details of this paper are as follows.（1）Zn Fe₂O₄yolk-shell microspheres with hierarchical structure assembled from nanosheets with diameters of about 1-1.5μm were successfully prepared by a simple combination of self-templating solvothermal method and calcination.Further,Zn O/Zn Fe₂O₄composite was prepared by growing a layer of Zn O flocs on the surface of Zn Fe₂O₄microspheres by using hydrothermal method,and then Au nanoparticles were loaded on the surface of Zn O/Zn Fe₂O₄microspheres by using co-precipitation method.The pure Zn Fe₂O₄exhibited low response to acetone（8.4 to 100 ppm at279℃）,while Au-Zn O/Zn Fe₂O₄exhibited a high response value to acetone（18.2 for100 ppm at 206℃）,fast response time（2 s）,good selectivity(S_acetone/S_xylene=2.53)and stability（11%decrease in 30 days）.Moreover,the lower limit of detection of acetone by Au-Zn O/Zn Fe₂O₄was 0.7 ppm in a high humidity environment（85%RH）,which completely satisfied the requirement of clinical detection of diabetes.It indicates that the prepared Au-Zn O/Zn Fe₂O₄yolk-shell microspheres can be used as an ideal gas-sensitive material for the preparation of acetone sensors.On the one hand,the excellent gas sensing properties are attributed to its large specific surface are.On the other hand,the synergistic effect of Zn O-Zn Fe₂O₄n-n heterojunction and Au loading.（2）Hierarchical porous Fe₂O₃microtubules were prepared using acid-base treated willow branch slices as biotemplates.Among them,Fe₂O₃-D2 microtubules retained the layered microtubule-like structure of willow branch slices well without layer stacking.The Fe₂O₃-D2 material exhibited excellent selectivity(S_acetone/S_benzene=7.3)and high response（16.3 to 20 ppm acetone）.In addition,the Fe₂O₃-D2 material has a fast response/recovery time for acetone（2/22 s for 100 ppm acetone）and a low detection limit（response value of 3.2 to 1 ppm acetone）.This high sensitivity of the Fe₂O₃-D2 material is attributed to its loose and porous structure and large specific surface area(22.19 m²g^-1),which facilitate the adsorption and diffusion of acetone molecules on the surface of the Fe₂O₃-D2 material,resulting in a better gas sensing performances.（3）The n-type Fe₂O₃pure material and p-type Cu&Zn-doped Fe₂O₃composites were prepared by hydrothermal method.The gas sensing performance tests showed that Cu and Zn doping not only converted the conductivity type of Fe₂O₃,but also significantly improved the gas sensing performances to acetone.5 at%Cu and Zn doped Fe₂O₃achieved response values of 35.8 and 59.7 to 100 ppm acetone,which were much higher than that of pure Fe₂O₃at 5.4.Meanwhile,the doping also improved the Fe₂O₃sensor’s selectivity,response recovery rate,moisture resistance and long-term stability.On the one hand,the doping of transition metals（Cu and Zn）not only makes a large number of oxygen defects in Fe₂O₃,which enhances the ability to adsorb oxygen and acetone gas molecules and therefore improves the sensing performances;On the other hand,Cu and Zn doping makes the band gap of Fe₂O₃becomes smaller and the Fermi energy level decreases,which causes the conductive type of Fe₂O₃to change from n-type to p-type.（4）Ag-loaded and Zn-doped peanut-like Fe₂O₃nanomaterials（Ag@Zn-Fe₂O₃）were prepared by hydrothermal and co-precipitation methods.The response of Ag@Zn-Fe₂O₃100 ppm acetone is 5.6 times more than the pure Fe₂O₃.The optimum operating temperature was also reduced from 172 to 150℃.The selectivity,moisture resistance and long-term stability of acetone were optimized.The mechanism of this sensitization is attributed to the synergistic effect of Zn doping and Ag particle loading.