Preparation And Gas Sensing Properties Of Micro-nano Structured Cobalt Titanate

The sensors based on nanocrystalline CoTiO₃ are suitable for the rapid,reliable ethanol detection.However,the sensors based on CoTiO₃ nanopowders show poor performance for the detection of low concentration of ethanol.The gas sensing performance is strongly dependent on the effective surface area,the number of active sites,as well as the pore structure characteristics.Thus,the micro-nano structure constructed from nanocrystalline CoTiO₃ is an ideal structure for high gas sensitivity due to its high specific surface area availability and abundant interconnected transport channels.Therefore,the synthesis method and formation mechanism of porous CoTiO₃ powders with micro-nano structure have been investigated.It is of great theoretical significance and practical application value to investigate high gas sensing performance of CoTiO₃ sensors over a wide concentration range in practical applications.In this dissertation,micro-nano structured CoTiO₃ porous powders with different morphological features were synthesized by sol-gel method and solvothermal method,respectively.The as-synthesized CoTiO₃ porous powders were characterized by field emission scanning electron microscope（FESEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,diffused reflectance spectrum,specific surface area and pore structure testers.The influence of the main process conditions on the microstructure and phase composition of the as-synthesized samples was investigated,the formation mechanism and the possible gas sensing mechanism of the corresponding micro-nano hierarchical structure were explored.Hierarchically porous CoTiO₃ powders were prepared by sol-gel method using tetrabutyl titanate as a titanium source,cobalt nitrate as a cobalt source,diethanolamine（DEA）as a templating agent,and acetic acid as a pH modifier.The prepared porous CoTiO₃ powders demonstrate a characteristic hierarchical distribution with mesopores from 2-10 nm and macropores approximately from 0.15 to 3.0μm agglomerated by nanoparticles,and its specific surface area achieves 47.04 m²/g.The hierarchically porous CoTiO₃ powders displayed the maximum response value（71.5）towards 100 ppm ethanol at 420°C,which was 3.8 times higher than that of CoTiO₃ nanopowders.The corresponding response and recovery time were about 15 and 22 s,respectively.The formation of macropores in the as-synthesized micro-nano structure powders was related to the phase separation caused by the difference in polarity between the gel phase and the solvent,and the formation of mesopores was related to the oxidation and decomposition of organic substances during the calcination process.The optimum preparation process conditions were as follows:the sol concentration is 0.1 mol/L,pH=5.0,the molar ratio of DEA:Co:Ti was 1.6:1:1,the molar ratio of H₂O:Co:Ti was 26:1:1,and the calcination was carried out at 700°C for 2 h.Porous CoTiO₃ microspheres with a diameter from 1.6 to 3.0μm were prepared by solvothermal method using CTAB as a surfactant,diethanolamine as a template,ammonia as a pH modifier.The specific surface area of the porous CoTiO₃ microspheres is 32.28 m²/g,and the most probable pore size range is 2-20 nm.The template agent DEA was a key factor in controlling the formation of microspheres.As the molar ratio of DEA:Co:Ti increased from0:1:1 to 2.9:1:1,the as-synthesized samples evolved into porous microspheres with the increase of DEA contents.The optimum process conditions were as follows:pH=8.2,the molar ratio of CTAB:Co:Ti was 0.3:1:1,the molar ratio of DEA:Co:Ti was 2.9:1:1,the molar ratio of H₂O:Co:Ti was 18:1:1 and solvothermal treatment at 200°C for 24 h,then the calcination was carried out at 700°C for 2 h.The sensor based on porous CoTiO₃microspheres achieved the maximum response value（70.5）towards 100 ppm ethanol at340°C and the response-recovery time were about 23 and 32 s,respectively.