Gas Sensors Based On One-Dimensional Oxide Semiconductors

In recent years,gas sensors based on oxide semiconductors have important applications in the fields of safety monitoring,medical diagnosis,environmental protection,Internet of Things and artificial intelligence.And they are always the research hotspot in the field of chemical sensors.How to improve the sensitivity of this kind of sensor is one of the most concerned issues for researchers.In general,methods for increasing the sensitivity of oxide semiconductors sensors include the incorporationof the second component,exposure of high energy crystal faces,regulation of particle sizes,and selection of operating temperatures.Among them,the incorporationof the second component is a common method for improving the sensitivity of the sensors,and the sensitizing effect is generally attributed to the electronic sensitization and chemical sensitization function of the loadedsensitizer.However,whether it is electronic sensitization or chemical sensitization,the sensitivity is closely related to the distribution of the second component on the surface of the sensitive materials and the size of the particlesof the second component.When the second component is loaded by the conventional impregnation method,the sensitizer particles loaded at the time of achieving the optimum sensitivity are large,which makes the amount of sensitizer increase significantly.In addition,the sensitivity of the sensor is also closely related to the microstructures of the materials.The loose porous structures can promote the target gas to diffusesufficiently in the sensitive body,thereby ensuring the utilization efficiency of the sensitive body and improving the sensitivity of the sensor.Therefore,in this paper,the electrospinning method is applied to synthesize sensitive materials of?-Fe2O₃ and In₂O₃,utilizing the unique one-dimensional packing structure of the materials to ensure the utilization efficiency of the materials;the second component in situ incorporationmethod is used to improve the sensitivity of the materials;the particle size of the loaded noble metal is controlled by metal organic framework compounds?MOFs?to achieve an optimum catalytic effect at a small loading.The specific research content is as follows:1.Using the noble metal Pt as the second component,four groupsof different molar ratios of Pt-modified?-Fe₂O₃ nanowires were synthesized by electrospinning.The percentages of Pt were 0,0.5 mol%,1 mol%and 3 mol%,respectively.The characterization results of SEM and TEM showed that all the nanowires had a diameter of about 100 nm,the morphology was uniform and the dispersion was good.It could also be observed that as the incorporation amount of Pt increased the particles on the nanowires became tight and the pores between the particles decreased.The characterization results of XRD showed that Pt elemental peaks appeared when the incorporation amount of Pt was 3 mol%.The results of TEM and XPS furtherproved that the noble metal Pt was mostly present mostly in the form of elemental.The test results of gas sensing showed that 1 mol%Pt modified?-Fe2O₃ nanowire has the highest sensitivity to the target gas.The response to 10 ppm hydrogen sulfide was 157at an optimum operating temperature of 175?.The response and recovery times were 30 s and 18 min,respectively.Also,the sensor could detect hydrogen sulfide gas as low as 1 ppm with a corresponding response value of 1.7.Compared with pure?-Fe₂O₃ nanowires,the response of 1 mol%Pt modified?-Fe₂O₃ nanowires to 10ppm hydrogen sulfide gas was 52 times higher.The main reason for the improvement of gas sensing properties is the chemical sensitization of noble metal Pt.2.The improvement of gas sensing properties of?-Fe₂O₃ nanofibers was further studied with Al₂O₃ as the second component.The?-Fe₂O₃ nanofibers incorporated with four groups of different mass ratios of Al₂O₃ were prepared by electrospinning.The mass ratiosof Al source?aluminum isopropoxide?in precursor solution were 0,12.5 wt%,25 wt%and 37.5 wt%,respectively.The results of SEM and TEM characterization showed that the diameter of the four groups of nanofibers did not change significantly?¹00 nm?.However,the size of the nanoparticles constituting the nanofibers significantly decreased significantly with the increase of the mass ratio of Al sourcein the precursor.And the morphology of the nanowires evolved from rough nanotubes to porous nanofiber and then to smooth nanofibers.The XRD results of the samples confirmed that the aluminum in the materials was mainly present in the amorphous state of Al₂O₃.The gas sensing test results showed that the sample with 25wt%Al had the best response to triethylamine.The response value to 100 ppm triethylamine was 15.2 at 250?.The response and recovery times were 1 s and 17 s,respectively.The low detection limit was 0.5 ppm.The enhancement in response of the sensor is mainly due to the increase in the amount of defective oxygen in the sample,that is,the gas-sensitive property of the materials is also optimal when the content of defective oxygen in the sample is the highest.3.Pure In₂O₃ nanofibers,ZnO-In₂O₃ nanofibers and Pt-ZnO-In₂O₃ nanofibers were prepared by electrospinning.ZIF-8?Zeolite imidazole framework-8?was used to control the particle size of the noble metal Pt.The latter two samples were prepared by adding ZIF-8 or Pt@ZIF-8 to the precursor solution,followed by electrospinning and calcination to obtain the final products.In the preparation of the electrospun precursor of Pt-ZnO-In₂O₃ nanofibers,the In³⁺released from indium nitratehydrate would hydrolyze and the acidic environment provided by the hydrogen ions of the hydrolyzate would etchZIF-8.Then the Zn²⁺which constituted for ZIF-8and Pt nanoparticles would be released.The loading of Pt in the materialswas approximately0.4 mol%?Pt:In₂O₃=0.4:100?.The TEM results confirmed that the size of the Pt nanoparticles prepared by this method was 3 nm.The results of gas sensitivity test showed that Pt-ZnO-In₂O₃ nanofibers responded best to acetone.The response to 100ppm acetone was 57.1 at 300?,and the response and recovery time were 1 s and 44s,respectively.The low limit of detection was 0.5 ppm.The improvement of gas sensitivity is mainly attributed to the good catalytic effect of small particles of Pt.However,the operating temperature and the low detection limit are both high,further improvement is needed.4.Pure In₂O₃ nanofibers,ZnO-In₂O₃ nanofibers and series of PdO-ZnO-In₂O₃nanofibers were prepared by electrospinning.ZIF-8?Zeolite imidazole framework-8?was used to control the particle size of the noble metal Pd.ZnO and PdO were incorporated by adding ZIF-8 or Pd@ZIF-8 in the electrospinning precursor.The TEM results confirmed that the size of the Pd nanoparticles prepared by this method was 2.5 nm.The results of the gas sensing test showed that the sensors based on PdO-ZnO-In₂O₃ nanofibers whose addition amount of Pd@ZIF-8 in the precursor solution was 10 mg had a response value of 50 to 100 ppm acetone at 250?.The response time and recovery time were 3 s and 310 s,respectively,and the detection limit was 0.3 ppm.The PdO loading in the materials was calculated to be approximately 0.2 mol%?Pd:In₂O₃=0.2:100?.This experiment further confirmed the decrease in the size of the sensitizer,which not only achieved high sensitivity,but also significantly reduced the loading amount of the sensitizer.