| Triethylamine（TEA）is an important industrial raw material often used in biology,medicine,dyes,and aerospace.However,the volatility,toxicity,irritation,corrosiveness,flammability,and explosiveness pose a serious threat to human health and industrial safety,so there is an urgent requirement for effective means of real-time and accurate detection.Metal oxide semiconductor gas sensors have the advantages of long life,small size,low cost,and high stability,which are the current research hotspots in the field of gas sensors.Nevertheless,in order to further meet the requirements of TEA detection in industrial environments,it is necessary to solve issues such as high operating temperature,low sensitivity,and poor selectivity of the sensors.In this dissertation,spinel-structured cobalt（II,III）oxide（Co3O4）with good low-temperature catalytic activity was selected as a sensitive material to improve the sensitivity and selectivity of the sensors to TEA by combining crystal facet engineering strategy,nanosheet spatial structure optimization,cation valence modulation,and metal ion doping on the basis of exploiting the large specific surface area of nanosheet structure.Meanwhile,the internal relationship between the surface states of sensitive materials and the gas-sensing performance of sensors was deeply analyzed,the sensing mechanism of gas sensors was explored,and the sensing enhancement mechanisms of different modification strategies were systematically summarized.The following is the main research of this dissertation:（1）Co3O4 mesoporous nanosheets exposed with highly active crystal facets were prepared using a crystal facet engineering strategy,which improved the sensitivity of the Co3O4 gas sensor to TEA and explored the effects of crystal facet on gas-sensing performance.The Co3O4 mesoporous nanosheets with consistent basic morphology but mainly exposed{111}or{112}crystal facets were prepared by controlling the synthesis conditions,where the{111}crystal facets are composed entirely of Co2+,while the{112}crystal facets consist of both Co2+and Co3+.The results of gas sensing tests showed that the Co3O4-60 sensor with mainly exposed{111}crystal facets exhibited higher sensitivity to TEA,with a response of 26.6 to 100 ppm TEA at 150°C,which was 2.9 times higher than the Co3O4-100 sensor which mainly exposed{112}crystal facets.Meanwhile,the Co3O4-60 sensor also showed bimodal detection of TEA and toluene by varying the operating temperature,thus expanding its scope.Upon comprehensive analysis,it was concluded that the higher proportion of Co2+in the Co3O4 material with mainly exposed crystal facets of{111}and its induced increase in the content of surface chemisorbed oxygen,as well as the decrease in the apparent activation energy,were the main reasons for the improved performance of the gas sensor.（2）Co2+-rich Co3O4 nanosheet-assembled hierarchical microspheres were prepared by a combination of cation valence modulation and nanosheet self-assembly,which further improved the gas response of the sensor to TEA,and explored the effects of cation valence and micro-assembly structure on gas sensing performance.To address the problem that nanosheet-structured materials were highly susceptible to stacking in device preparation and affecting gas diffusion,Co3O4 nanosheets（Co3O4 sheet-T）were assembled into hierarchical microspheres（Co3O4 sphere-T）enriched with Co2+using polyvinyl pyrrolidone（PVP）as a structure-directing agent and a reducing agent in combination with different calcination temperatures（T:300,400,or 500°C）.A comprehensive comparison was made between the physical properties and gas sensing characteristics of Co3O4 sheet-T as the basic structural unit and its assembled Co3O4sphere-T.Among them,the Co3O4 sphere-300 sensor exhibited the best performance,with a gas response of 34.1 to 100 ppm TEA,which was 3.3 times higher than that of the Co3O4 sheet-300 sensor.And the baseline resistance was reduced from 45.9 to 4.2kΩ;the detection limit was lowered from 2 to 0.5 ppm;the selectivity index（SI）for the typical interfering gas,toluene,was increased from 1.9 to 2.4.The sensing enhancement mechanism of the Co3O4 sphere-300 sensor could be attributed to the nanosheet assembly hierarchical structure promoting gas diffusion,the Co2+-rich surface catalyzing gas-sensing reactions,and the higher content of oxygen vacancies at lower calcination temperatures favoring gas adsorption.（3）The selectivity of the sensor was improved by a combination of multivalent cation doping and nanosheet assembly hierarchical structure,and the effects of doping concentration and doping sites on the gas-sensing performance were explored.Co3O4nanosheet-assembled hollow microtubes doped with different levels of iron ions(Fe2+/Fe3+)（CF0,CF1,CF2,and CF3）,which could up to 31.5 wt%,were prepared using layered metal hydroxide（LMH）as precursors.The systematic study revealed that the grain size,lattice parameter,and morphology of the sensitive materials were related to ion doping concentration,while the energy band structure and cation valence state were affected by both ion doping concentration and doping sites.The CF2 sensor with20.5 wt%Fe ion doping showed the best gas-sensing performance,with a gas response of 4.2 times greater than that of the undoped CF0 sensor for 100 ppm TEA,and a significantly higher selectivity index（SI=3.0）for the typical interfering gas（toluene）.In addition,the sensor still had excellent sensing characteristics under high humidity（90%RH）.The performance enhancement of the CF2 sensor could be attributed to the synergistic effect of suitable dopant ion concentration and tetrahedral doping sites.（4）The role of doping high-valent cations into octahedral sites of Co3O4 sensitive materials in improving the gas-sensing performance of the sensor was further investigated.Co3O4 nanosheet-assembled dodecahedra doped with different contents of tungsten ions(W6+)（Co3O4,1W-Co3O4,2W-Co3O4,and 3W-Co3O4）were prepared by combining the porous properties of metal-organic-framework（MOF）material with the advantages of the basic structural units of nanosheets.The test results showed that the2W-Co3O4 sensor with a doping content of 0.86 at%exhibited excellent TEA detection performance,with a gas response of 35.8 to 100 ppm TEA,which was a 4.3-fold enhancement compared to the undoped Co3O4 sensor,and the selectivity index for the typical interfering gas,toluene,reached 4.2.The lattice defects generation,surface cation valence modulation,and grain size alteration induced by the doping of W ions into the octahedral sites of the Co3O4 spinel structure were the main reasons affecting the gas-sensing performance of the sensor.In addition,a humidity compensation strategy was adopted to improve the resistance to humidity interference of the sensor,further enhancing its practicality. |
