Wet-chemical Preparation And NO₂ Gas-sensing Properties Of In₂O₃-based Nanomaterials

Despite years of progress in clean air technology,air pollution remains to be one of the main problems for the environment and human beings.Hazardous air pollutants in the environment are needed to be detected and controlled effectively to avoid human health risks.Research on nanomaterials has attracted huge attention because of their unique physical,chemical,catalytic,and optical properties regarding their bulk counterparts.These offer great significance in environmental nanotechnology to detect hazardous environmental gases even at very low concentrations.The synthesis of advanced nanomaterials for gas-sensing performances has been the main research interest during the last few years.Among the various types of nanostructured materials,Metal oxide semiconductors（MOS）have been widely employed in gas-sensing applications due to their low cost,simple fabrication procedure,and strong sensing capability against hazardous gases.Among different types of MOS,Indium oxide（In₂O₃）is a distinguished material with indirect wide band gap energy（2.85 e V）at room temperature.It has unique characteristics such as high thermal conductivity,good transparency,high electron mobility,and the ability to fabricate different types of nanostructures.It has attracted great consideration in gas-sensing applications owing to its low-cost,reduced size,and high sensitivity towards several reducing and oxidizing gases,high surface area,high gas diffusion,and ease in sensor fabrication.Many attempts have been made to progress the gas sensing of In₂O₃ by using various composite nanostructures.Generally,In₂O₃ provides a stacked and multilayered structure with micro/nano-structures dimension built up by low-dimensional nano-building blocks to synthesize versatile nanocomposites.In this dissertation,we describe a facile and tunable synthesis of In₂O₃ structure with a variety of hexagonal,micro/nano-spheres,and multilayered sheets via simple hydrothermal and co-precipitation processes.In addition,the crystal structure,morphologies,growth mechanism,and their sensing properties on the selectivity,sensitivity,response-recovery time,and stability are also studied.The main contents of this thesis are summarized as follows:Initially,In₂O₃ spheres were synthesized via a one-step solvothermal method.Two different types of In₂O₃spheres are prepared to detect NO₂ gas at low concentrations（1-10 ppm）at an operating temperature of 75-250°C.The novel In₂O₃ spheres（IOS）show outstanding gas sensing performance as compared with the In₂O₃ microspheres（IOMS）.They also exhibited a superiority response（29）of 10 ppm at 200°C.The sodium dodecyl sulfonate is an important factor to increase the gas sensing performance of the In₂O₃ spheres due to its unique structure and morphology.The high sensitivity（29 at 10 ppm）and low detection limit（1 ppm）is realized at the optimum temperature of 200°C,which possesses a fast response/recovery time of 44/56 s.Furthermore,the mechanism of IOS was discussed and proposed according to progressive experiment which will be promising to be applied for practiced NO₂ detection.After the successful synthesis of novel In₂O₃nanospheres and their outstanding application for NO₂ gas detection,we designated the controlled growth of In₂O₃ nanoparticles embedded on GO nanosheets by a facile precipitation method.The In₂O₃@GO nanocomposites exhibited outstanding gas sensing performance as compared with pure In₂O₃ nanoparticles towards NO₂.At 225°C,the sensor displayed high selectivity,best response（78）to 40 ppm NO₂,quick response,and recovery times of 106/42 s.The improved sensing performances of the nanocomposite had attributed to the large surface area,high gas adsorption-desorption capability,and the formation of p-n heterojunctions between In₂O₃ nanoparticles and GO nanosheets.The excellent gas-detecting activities of In₂O₃@GO nanocomposites are a promising candidate for the NO₂ gas sensor in the field of industry.Following the above research work,the spherical In₂O₃ nanoparticles were successfully decorated onto ZnO nanosheets to make ZnO@In₂O₃ nanocomposites via a simple co-precipitation method.The as-prepared nanocomposites are exposed to gas-sensing applications.The fabricated gas sensors exhibit high sensing response and selectivity towards NO₂ gas than other gases,which is tested at various working temperatures and different ranges of gas concentrations.The gas sensing property of ZnO nanosheets and ZnO@In₂O₃ nanocomposites has been investigated,which reveals that 10 wt%ZnO@In₂O₃ nanocomposites show the outstanding response of 68 at 70 ppm,possess fast response/recovery time 43/65 s,and demonstrate excellent stability and repeatability correspondingly.A plausible nanostructure formation and gas sensing mechanism are also discussed in detail.The fabricated sensors are considered potential applicants in the development of the NO₂-based gas sensor in the industry.In₂O₃ gas sensors are regarded as one of the potential materials for detecting and quantifying hazardous gases for monitoring systems and public safety assurance to address the severe environmental problems plaguing our civilization.In this work,In₂O₃/polyaniline composites were synthesized using the hydrothermal technique to explore nitrogen dioxide（NO₂）sensing capabilities.The scanning electron micrographs（SEM）,x-ray diffraction（XRD）,energy dispersive X-ray analysis（EDX）,and the gas sensing properties were investigated to ascertain the morphology,purity,response/recovery time and optimal temperature.The samples displayed a gas-accessible structure with clusters of several porous nanosheets.The as-synthesized In₂O₃/CP sensors have extremely high responses（189.5@100 ppm,11.8@3 ppm）to NO₂ gas at a working temperature（250°C）,quick response times（25 s@100 ppm,63 s@3ppm）and recovery times（105 s@100 ppm,144 s@3 ppm）,and a low detection limit（1 ppm）.Furthermore,the In₂O₃/CP sensor’s fundamental detecting mechanism for NO₂ gas was completely discussed.This work offers an intriguing perspective as well as a useful strategy for fabricating long-lifetime In₂O₃-based nanosheets using the hydrothermal process.