LaCoO₃-based CO Sensors

Carbon monoxide(CO)is a colorless,odorless,flammable,explosive and yet highly toxic gas,therefore,early detection of CO is critical.CO can be reliably detected only at relatively high temperatures(>250°C),the high operating temperature may trigger flame or even explosion.Therefore,CO sensors that can work at low temperatures even room temperature is strongly needed.In this work,LaCoO3 was chosen as the sensing material system.In order to obtain higher sensitivity and lower operating temperature,the solid state reaction synthesis,the co-precipitation method and the electrospinning method were applied to prepare the LaCoO3 material.The LaCoO3-based materials were doped with Fe,Nd and functionalized with Pd,Ag nanodots.The sensing properties of these sensors were measured.We studied the effect of material morphology,element doping and noble metal modification on the improvement of CO sensing properties of LaCoO3-based materials,especially for the reduction of operating temperature.Furthermore,the CO sensing mechanism of LaCoO3 was also studied.In Chapter 1,the significance of this work,application background and the types of the CO sensors were discussed.The gas sensing mechanism and research status of metal-oxide semiconductor(MOS)based CO sensors were described.And the development trend and the evaluation criteria for the CO sensor were elaborated.In addition,we summarized the basic properties,synthesis methods and doping studies of LaCoO3.In Chapter 2,the chemical reagents,instruments and equipment used in this work were listed,and the characterization methods were explained.In Chapter 3,the LaCoO3 precursor was synthesized by solid state reaction and the screen printing technology was applied to prepare the thick film sensors.The good CO performance and the low operating temperature potential of LaCoO3 were verified.The CO gas sensing mechanism of LaCoO3 was also studied.O2 adsorbed on the LaCoO3 surface and dissociated into Oads,COads oxidized by Oads to produce CO2 and CO32-.The released electrons during these reactions decrease the hole concentration,and thus the sensor resistance increases.In Chapters 4,LaCoO3-based sensors were prepared by LaCoO3 nanopowder with particle size of-82 nm prepared by co-precipitation.Co-precipitated LaCoO3 sensors demonstrate excellent CO sensing properties and the operating temperature of the LaCoO3-based CO sensor was reduced to 100 ?.The high response to CO could be ascribed to the high content of O22-/O-species in co-precipitated LaCoO3.In Chapters 5,mesoporous LaCoO3 nanowires with diameters of 200-800 nm and grain sizes of 30-60 nm were prepared by electrospinning,and functionalized with Pd nanodots by dropping of PdCl2.The influences of electrospinning and calcination parameters on the morphology of nanowires were explored.Pd-LaCoO3 mesoporous nanowires demonstrated excellent CO sensing characteristics at low temperatures and the operating temperature of the LaCoO3-based CO sensor was reduced to 60 °C.The excellent CO sensing properties of Pd-LaCoO3 nanowires at low temperatures can be ascribed to the mesoporous nanostructure and the catalytic sensitization of Pd nanodots.In Chapters 6,to further improve the CO sensing properties of LaCoO3-based CO sensors,we doped LaCoO3 with Fe.LaCo1-xFexO3(x ranging from 0 to 1)nanoparticles were prepared using the co-precipitation method.The CO sensing properties of LaCO1-xFexO3 nanoparticles with the Fe doping content x was investigated.The LaCo0.3Fe0.7O3 sensor(x = 0.7)showed the highest response at low operating temperature,and reliable dynamic response-and-recovery to different CO concentrations at a temperature as low as 50 °C was obtained.In Chapters 7,A-site doping and noble metal functionalization were investigated based on LaCo0.3Fe0.7O3.La1-xNdxCo0.3Fe0.7O3(x ranging from 0 to 1)nanoparticles were prepared using the co-precipitation method.The CO sensing properties of La1-xNdxCo0.3Fe0.7O3 nanoparticles with Nd doping content x was investigated.The La0.7Nd0.3Co0.3Fe0.7O3 sensor(x = 0.3)showed the best sensing properties at low operating temperature,and the operating temperature of the LaCoO3-based CO sensor was reduced to room temperature(no heating).The CO gas sensing mechanism of LaCoO3-based material at room temperature was studied.In contrast to the high temperature case,CO get electron and produce O-and C.Thus the hole concentration increases,the sensor resistance decreases.Finally,the Ag nanodots were prepared to functionalize the La0.7Nd0.3Co0.3Fe0.7O3 sensor.The CO sensing performance of the La0.7Nd0.3Co0.3Fe0.7O3 sensor was improved by the functionalization with Ag nanodots.Chapter 8 summarizes the full text.In this thesis,LaCoO3 materials with different morphologies were synthesized by different synthesis methods.LaCoO3 was doped with Fe and Nd,and functionalized by Pd and Ag nanodots.The LaCoO3-based CO sensor demonstrated excellent CO sensing properties and the operating temperature was decreased to room temperature.The CO sensing mechanism of LaCoO3-based material was explained as well.