Study On Structure Design And Gas Sensing Performance Of Indium Oxide Based Gas Sensor

With the rapid development of industrialization and urbanization,people are increasingly aware of environmental protection and their own health.In recent years,news about the greenhouse effect,acid rain and the ozone layer have been seen frequently,which reveal the impact of air pollution on the natural environment.Air pollutants mainly come from factory exhaust,automobile exhaust and household heating using coal as a raw material.If the post-treatment of these exhaust gases is not adequate,it will further aggravate air pollution and threaten human property safety and life and health.Therefore,scientists have invented gas sensors to detect whether the concentration of toxic gases threatens human safety.Although gas sensors have been developed for decades,oxide semiconductor gas sensors have been favored by scientists since their discovery due to their low cost and high sensitivity.Among these,In₂O₃is a specialⅢ-Ⅵgroup n-type oxide semiconductor,with suitable bandgap width（2.8 e V）,stability and low toxicity and showed high sensitivity toward most gases.Base of these,In₂O₃ is often used as a gas-sensing material.However,the pure In₂O₃ showed unsatisfactory for gas sensing performance.To improve the gas sensing performance,several strategies have been developed,including doping,morphology control and composite.In this paper,doping and morphology control are used to improve the gas sensitivity of In₂O₃.The detailed description as following:（1）Cd doped In₂O₃ was synthesized by facile solvothermal,template-free preparation method.The phase structure and properties were characterized by various methods.The surface of Cd doped In₂O₃ became coarser compared to pure In₂O₃ from scan electron microscope（SEM）,which enhanced the ability to absorb the target gas.The electron paramagnetic resonance（EPR）showed that the Cd doped In₂O₃ can generate more defects（oxygen vacancies）.By measuring the gas sensitivity,the response of Cd doped In₂O₃ was 1.6 times than that of pure In₂O₃ toward 100ppm acetone,and the recovery time is further shortened from 117 s for pure In₂O₃ to 87 s.This is because the surface oxygen defect acts as the donor level in the system,which increases the electron density of In₂O₃ and increases the potential difference between the Fermi level and the chemical potential of acetone.Thereby,the Cd-In₂O₃ can improve the gas sensitivity.（2）In this paper,with surfactant assistance,different morphology of precursors was synthesized by a facile solvothermal method.The flower-like precursor was obtained with the addition of sodium dodecyl sulfate（SDS）.The morphology of the precursor transferred from flower-like to ellipsoid-like through dissolution-recrystallization process when ethylenediaminetetraacetic acid（EDTA）was added.The phase analysis confirmed that both flower-like and ellipsoid-like precursors contained the mixture phase of In OOH and In（OH）_3.After calcining,the flower-like precursor transformed into the mixture phase of rhombohedral and cubic phases（rh-c-In₂O₃）.In contrast,the ellipsoid-like precursor singly converted to cubic In₂O₃（c-In₂O₃）.TG-DSC confirms that the key to the difference is whether the In OOH phase is transformed into rh-In₂O₃ during calcination.Moreover,we also investigated the gas sensing performance.Compared with the c-In₂O₃ sensor,the rh-c-In₂O₃ sensor showed～4.4 times higher response（R_g/R_a=6.89）toward 3 ppm NO₂ at 250℃.Meanwhile,the fast response time（ca.8.2 s）and recovery time（ca.7.6 s）of rh-c-In₂O₃ toward 3 ppm NO₂.The extraordinary sensitivity of rh-c-In₂O₃ was mainly owned to the size which compatible of Debye length and electron transfer between rh-In₂O₃ and c-In₂O₃ by constructing the n-n homojunction.