| NO2 and HCHO are common gaseous pollutants.NO2 and HCHO cause many major environmental problems,as well as serious health problems such as respiratory diseases,leukaemia,and cancer.Therefore,the efficient detection of HCHO and NO2 is of great significance.In recent years,metal oxide semiconductor(MOS)-based chemical resistance sensors have become a research hotspot due to their high sensitivity,small size,low cost,and easy maintenance.As a wide-gap n-type semiconductor material,In2O3 has been widely studied because of its high surface reactivity and high carrier conductivity.However,single-phase In2O3 sensing materials has high operating temperature,poor selectivity,and other problems in the process of gas detection.Noble metal modification and heterostructure construction are effective ways to improve the gas sensing properties of In2O3.However,noble metal nanoparticles synthesized by traditional methods often have some problems,such as uneven size distribution,low atomic utilization rate,and easy agglomeration of noble metal nanoparticles at high temperatures.And the heterojunction constructed by the conventional method also has some disadvantages,such as limited interface contact between two phases and low carrier transfer efficiency.In the view of above drawbacks,this study firstly prepared Au25 nanoclusters(NCs)and loaded them on three-dimensional ordered macroporous(3DOM)In2O3 to improve the Au atomic utilization rate and prevent the agglomeration of nanoparticles.In addition,In2O3@ZnO core-shell heterojunction was constructed by the MOFs conversion method,and the carrier migration efficiency was improved by increasing the contact area of the heterojunction,to improve the gas sensing response.The specific research conclusions are as follows:(1)3DOM In2O3 was prepared by the polystyrene template method,Au25 NCs were prepared by the NaBH4 reduction method,and a series of Au/3DOM In2O3 composites with different Au25 NCs loading amounts were prepared by electrostatic self-assembly method.In addition,3DOM In2O3 loaded with Au144(2.3 nm)and Au NPs(2.9 nm)was further synthesized to explore the effect of Au particle size on the gas sensing properties.SEM results showed that In2O3 material prepared by polystyrene template has a three-dimensional ordered macroporous structure with a pore size of 175 nm.UV-Vis results showed that two characteristic peaks of Au25 NCs appeared at 440 nm and 670 nm,and the particle size of Au25 NCs measured by TEM was 1.2 nm.The results were consistent with the previous report,indicating the successful synthesis of Au25 NCs.Similarly,the UV-Vis spectral characteristic peaks(517 nm and 700 nm)and MALDI-TOF mass spectrometry(m/z=33k)of Au144 NCs were also consistent with the literature,indicating the successful synthesis of Au144 NCs.Through the gas sensing response of formaldehyde,the loading amount of Au25 NCs was optimized.The results showed that Au/3DOM In2O3 samples(2Au25/In)with Au25 NCs loading of 2 wt%show the best formaldehyde response.Further optimization of working temperature showed that the optimum working temperature of the sample was 260℃,the response to 10 ppm formaldehyde of 2Au25/In was 12,about 7 times the response value of the original 3DOM In2O3 sample.In addition,benzene,ethanol,methanol,acetone,ammonia,CO,and CO2 were selected for selectivity tests.The results showed that the response of 2Au25/In to formaldehyde was 2.8～14.2 times that of other interfering gases.Moreover,under the condition of 90%relative humidity,the response of 2Au25/In to 10 ppm formaldehyde can still reach 4.5.More importantly,the response of the sample to formaldehyde depends on the size of Au NCs.When the Au particle size increased from 1.3 nm to 2.3 nm,the response of the sample to formaldehyde decreased from 26 to 17.When the Au particle size further increased to 2.9 nm,the formaldehyde response value was only 6.The 3DOM structure prepared by this work was beneficial to gas diffusion and electron conduction.In addition,the electronic and chemical sensitization effects of Au25 NCs are the main reason for improving the gas sensitivity of formaldehyde.(2)A series of In2O3@ZnO core-core hollow nanotubes were prepared using MIL68(In)@ZIF-8 as a sacrificial template.SEM results showed that the In2O3 nanotubes after heat treatment have hollow hexagonal prism morphology with the tube wall thickness of 32 nm,inherit the morphology of MIL-68(In)precursor.TEM results showed that the composite has a good core-shell structure.The mass ratio of In2O3 to ZnO was optimized by the gas sensing response of NO2.The results showed that when the mass ratio of In2O3 to ZnO was 2:1,the sample(In@Zn-2)has the best NO2 response.Further optimization of working temperature showed that the optimal operating temperature of the sample was 160℃,and the response of In@Zn-2 to 1 ppm NO2 was 73.The response and recovery times of In@Zn-2 were only 3 s and 6 s,which is significantly lower than that of single-phase ZnO(37 s/21 s)and In2O3(2 s/62 s).In addition,CO,NO,NH3,ethanol,toluene,and acetone were selected for the selectivity test.The results showed that In@Zn-2 has a response of less than 10 to 50 ppm CO,NH3,ethanol,toluene,acetone,and 5 ppm NO,indicating that In@Zn-2 has good NO2 selectivity.Stability test results show that the response value and response/recovery time of In@Zn-2 to 1 ppm NO2 did not significantly decrease within 60 days.The improvement of NO2 gas sensitivity was mainly attributed to the formation of n-n heterojunction between In2O3/ZnO,which makes oxygen molecules capture more electrons at the In2O3/ZnO interface and generate more adsorbed oxygen species. |
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