Microwave-assisted Synthesis Of Co₃O₄/C Multilevel Nanocomposites And Their Gas-sensing Properties

At present,420 million people are diagnosed with diabetes,and the number of undiagnosed cases is 210 million,effective diagnosis of diabetes is particularly important.The concentration of acetone of diabetic patients in exhaled gas（≥1.8ppm）is higher than healthy person（<0.9 ppm）,which can be used as a key indicator of diabetes screening.Blood glucose detection,gas chromatography and mass spectrometry are common methods for the diagnosis of diabetes.These methods often have disadvantages such as long detection time,complex process,high price,low response and large equipment volume,which are not conducive to large-scale application.Semiconductor oxide gas sensor has the advantages of small volume,low price and high response,and has also been studied for the detection of exhaled gas,but there are widespread problems such as high working temperature,poor selectivity and low response.Therefore,it is of great significance to design and prepare high-performance acetone sensitive materials.Due to the high specific surface area,superior conductivity properties and microwave absorption properties of carbon materials（such as graphene and carbon nanotubes）,this paper introduced the method of microwave assisted to prepare the Co₃O₄/C nanocomposite,and concrolled its interface structure by optimizing the process,thus enhanced its acetone-sensing at low temperature;Through the characterization of the chemical composition,microstructure and interface structure of the typical samples,we compared and analyzed the acetone-sensing of the samples,and discussed the relationship between structure and performance and the gas sensing mechanism in depth.Specific research contents are as follows:（1）Microwave-assisted liquid phase preparation method was used to rapidly grow Co（OH）₂ nanoparticles in situ on the surface of two-dimensional layered GO substrate.After calcination,Co₃O₄/rGO nanocomposites with multiple channels were prepared.The composites have a wide pore diameter distribution（5～200 nm）,forming a very characteristic multi-stage pore structure including mesoporous and macroporous,which enhances the diffusion and transmission of the target gas.The gas-sensing test results showed that the response of Co₃O₄/rGO-1 to high concentration acetone（100 ppm）reached 7.9 and to low concentration acetone（0.5ppm）reached 1.9 at low temperature when the GO content was 1wt%,and the response/recovery times to 50 ppm acetone were 52 s and 9 s at 160℃,respectively.（2）MWCNTs were functionalized with concentrated sulfuric acid and concentrated nitric acid to obtain oxygen-containing functional groups such as carboxyl group and hydroxyl group,and mixed with Co₃O₄ precursor solution.After microwave irradiation and subsequent calcination,three-dimensional porous Co₃O₄/MWCNTs nanocomposites were prepared.The experimental results showed that when the content of MWCNTs was 5wt%,the response value of Co₃O₄/MWCNTs-5 to 0.1 ppm acetone was 2.2 at 160℃,and the response value to100 ppm acetone was 43.5,which was 19 times that of pure Co₃O₄ under the same condition.The response/recovery times to 50 ppm acetone were 23 s and 66 s at160℃,respectively.Stability tests found that the response value of sensor to 50 ppm acetone decreased by only 1.7%after 43 days.This is because the introduction of MWCNTs reduces the size of Co₃O₄ particles,increases the specific surface area and porosity of the material by forming a three-dimensional porous structure,thus improving the gas-sensing of the material.（3）In order to further improve the response-recovery time and long-term stability of the above Co₃O₄/rGO,the original microwave-assisted preparation technology was adopted.Functionalized MWCNTs were used as support interluded between GO layers to load Co₃O₄ particles,and then constructed a multilevel nanocomposite with tube/sheet/particle structure,increasing the transmission channels of gas.The gas-sensing test results showed that when 2wt%carbon material was introduced（MWCNTs and GO mass ratio is 6:1）,the response value of Co₃O₄/（MWCNTs/rGO）to 100 ppm acetone was increased to 11 at 160℃,and the response/recovery times to 50 ppm acetone were shortened to 33 and 23 s,and the sensor had good long-term stability.