Performance Regulation Of Nanostructured Nickel Oxide Gas Sensors

Among various new gas-detection technologies,gas sensor has become one of the best alternatives to expensive and heavy analytical instruments for detecting gases,because of its advantages of small size,light weight,low power consumption,easy integration,high sensitivity,good selectivity,high reliability,real-time rapid detection and convenient construction of the sensor network.Oxide semiconductor gas sensor is the most important branch in the field of chemical sensor and indispensable in environment detection and safety monitoring.It has always been the frontier and hot spot of the international research.In general,p-type oxide semiconductors exhibit excellent catalytic oxidation activity in the oxidation of volatile organic compounds(VOCs)gases.However,due to the limitation of its conduction mechanism,gas sensor based on p-type metal oxide semiconductor(MOS)shows low-sensitivity and poor-selectivity performance,which makes it difficult to achieve selective detection of trace gases.Aiming at above problems,it is imperative to modify the functions of p-type oxide semiconductors.At present,there are three main functional modification methods including noble metal loading,aliovalent metal ion doping and micro-nano heterogeneous contact.Owing to the limited noble metals(Au,Ag,Pd,Pt,etc.)available for loading and their obviously increased sensor costs,in this thesis,we mainly modified the representative p-type nickel oxide(Ni O)nanostructures via ion doping and micro-nano heterogeneous contact to achieve performance regulation of the sensors.The main details are as follows:Combining the advantages of the catalytic activity of Ni O and its hierarchical nanostructure,two-dimensional nanosheet assembled flower-like Ni O microspheres were prepared.On this basis,modifications of metal cation aliovalent substitutional doping and isovalent interstitial doping were used to improve their sensing properties,thus different atom ratios of Sn4+ and Sn2+ doped hierarchical flower-like Ni O microspheres were obtained,respectively.Such two kinds of doping modified sensing materials were well characterized for morphology,micro-nano structure and composition analyses then tested for gas sensing properties.As for the aliovalent doping,the results showed that the sensor based on 3.0 at% Sn4+-doped Ni O exhibited the best sensing performance to xylene gas with a lower detection limit(300 ppb)and a 12 times higher sensitivity of 20.2-100 ppm,compared with the undoped Ni O based sensor;For the isovalent doping besides,the 1.5 at% Sn2+-doped Ni O based sensor also showed greatly improved xylene sensitivities with a low detection limit of 500 ppb,and this sensor still exhibited better selectivity and stability under the high humidity conditions.The mechanism research shows that their promoted sensing properties mainly result from the doping regulated changes of the chemisorbed and deficient oxygen contents and the hole concentration.According to the regulatory roles of nanoscale p-n and p-p heterojunctions in modulating carrier concentration and transport,intimate Sn O2-Ni O,Ni O/Ni Cr2O4 and WO3-Ni O heterogeneous composites with micro-nano contact were respectively synthesized through one-step hydrothermal method,then well characterized and tested.Among them,the Sn O2-Ni O nanoparticle based gas sensor exhibited excellent toluene performance with good anti-humidity properties and a very low detection limit;The gas sensor fabricated from Ni O/Ni Cr2O4(Cr/Ni=25 at%)nanoparticles showed a higher sensitivity(66.2-100 ppm)to xylene with a lower detection limit(1.2-50 ppb)and better selectivity(Sxylene/Sethanol=11.8,Sxylene/Sacetone=10.2);And the sensor based on 10 at% WO3-Ni O hollow porous flowers showed an ultrasensitive(354.7-50 ppm)and fast(<1 min)response to xylene,giving a lower detection limit(1.5-50 ppb)and better selectivity(Sxylene/Sethanol=10.3,Sxylene/Sacetone=8.1).Compared with the sensing performance of a single oxide,the gas sensing properties of above heterogeneous composites are greatly improved,which results from the optimized structural parameters,the synergistic catalytic promotion,the reduced carrier concentration and the narrowed conduction channel caused by nanoscale heterojunctions.