Preparation And Research Of Gas Sensors Based On Nanostructured Nickel Oxide

Nickel oxide is a kind of metal oxide crystal with face centered cubic structure,and usually taken as a model for p-type semiconductors with a band gap at room temperature in the range of 3.6⁴.0 eV,extensively utilized in gas sensing,catalysis,light emitting diodes,battery cathodes,supercapacitors.Nanostructured materials exhibit more attractive properties than conventional materials,such as very small particle size,large exposed surface areas and high surface energy,these properties can significantly influence the material performances.Therefore,nanocrystalline nickel oxide have been conducted extensive researches and received much attention because of their excellent physical,chemical,optical and catalytic properties.Preparation and characterization of nano-structured nickel oxide are important and active research objectives in the materials science field for a long time.At present,the main methods to prepare nanostructured NiO are: magnetron sputtering,sol-gel,hydrothermal synthesis,vapor deposition,laser pulse deposition,electrodeposition,etc.The synthesized nanostructures include: nano-films,nano-sheets,nano tubes,nano particles,nano wires,nano flowers,nano spheres and nano blocks.There are many defects in existing preparation process of nanocrystalline nickel oxide,such as: too many kinds of raw materials,complex technological process,require expensive facilities and instruments,requires high temperature or vacuum environment,high-cost,difficult to large-scale industrial production and so on.Electrodeposition possesses have many advantages including low cost and industrial applicability,simple operation,versatility,few size and shape limitations,etc.,and has been widely recognized as a feasible and economic technique for the production of nanocrystals.In a traditional nickel electroplating process,organic additives are necessary to lower the grain growth rate.However,these organic additives usually result in inclusions of surface-active elements such as sulfur and carbon in the deposits,which segregate out to the grain boundaries and adversely affect some of the properties,also produce toxic and harmful products,very harmful to the environment and human health.In this study,nano-sheets with thickness of about 100 nm and rose-like with diameters of 400⁵00 nm NiO nanostructures were prepared on ITO glass substrates by simple direct current electrodeposition in NiSO₄·6H₂O solution at room temperature followed by oxidation in air.No catalysts,complexing agent or buffer,surfactant solution or other organic additives were used during the whole preparation process.The structure and morphology of the nickel oxide nanostructures were characterized using scanning electron microscopy?SEM?,X-ray diffraction?XRD?and a transmission electron microscope?TEM?.The results of SEM and TEM show that the NiO hierarchical architectures were assembled by uniform nanosheets with thicknesses 15²0 nm.The results of XRD test show that the deposited products after oxidation did not show any characteristic peaks of Ni phase,indicating that the metallic Ni nanostructures are fully oxidized.The influences of principal technological parameters such as bath composition,bath pH,electrodeposition time,bath concentrations,temperature and current density on the morphology and microstructure of the electrodeposited nickel nanostructures were investigated.Gas sensing properties of the as-synthesized NiO nanostructures gas sensors were evaluated by the detection of volatile gases including ethanol,methanol,acetone and formaldehyde.It was found that rose-like NiO nanostructure exhibited a highly response to the three gases at 230?C.Furthermore,the NiO nanorose-based sensor gave much higher response to the three gases than nanosheets-based sensor.The results indicated the NiO nanorose-based sensors displayed a rapid response and recovery as well as good reproducibility.The rose-like morphology has several important properties such as large surface area and stability,which are key factors related to gas sensing properties such as sensitivity and response and recovery times.Based on the theoretical model of oxide semiconductor reported in the literature,the process of carrier change properties in different gases environment was analyzed.Based on the comprehensive analysis of the theoretical model,we found 3 ways to improve the sensitivity of the gas sensors: to reduce the grain size,to control the surface morphology of the gas sensing materials and to dope the additives.Based on the theory analysis,different NiO nanostructures were doping with Au nanorods,Pt spherical nanoparticles and rose-like Ni nanoparticles,the different doped amount nano-structures NiO were prepared into different gas sensors.The results of gas sensitivity test proved that Au nanorods and Pt nanoparticles doping can significantly improve the sensitivity of rose-like NiO nanostructure based gas sensors?the sensitivity to ethanol gas were increased by 67% and 3.7 times,respectively?.According to the test results,the influence of three kinds of metal load on the performance of the sensor was analyzed.When using metal load to enhance the performance of oxide semiconductor gas sensor,the metal materials with high catalytic activity,high work function and small particle size should be selected.As the traditional experimental methods are costly and time-consuming,with the improvement of hardware and software,computer simulation has become a powerful tool to investigate complex systems.In fact,density functional theory?DFT?has become an attractive,theoretical method,because it gives exact,basic,and vital parameter values for even hugely complex molecules.Three kinds of surfaces,including the stoichiometric surface,the oxygen-deficient surface,and an O2 molecule absorbed oxygen-deficient surface,were constructed by Studio Materials 7.0 software.Absorption of CO and ethanol molecule on the perfect and defective NiO?111?surface has been studied by CASTEP module calculations at generalized gradient approximation?GGA?level with a periodic supercell approach,and the adsorption energy of NiO?111?surface and the changes of the local density of states in the adsorption process were analyzed...