Control Growth And Application Of Nickel And Nickel Oxide Nanocrystals

Sensor technology is one of the most rapidly developing high-tech, which is animportant symbol of the development of modern science and technology. Conductancenano-sensors are being widely applied in aerospace, national defense, informationtechnology, mining machinery, food security, homeland security, biological detection,monitoring of agriculture, medical, environments, and so on. NiO has stable physical andchemical properties, which are considered to be one of the ideal materials for buildingminiature sensors, and the conductance nano-sensor is the most important application ofNiO. In our work, we synthesized the Ni and NiO nanocrystals （nanowires, dendriticcrystals and nanocones）, and exploited the NiO nanowire and Zn doped NiO dendriticcrystal as the sensing materials, to fabricate the gas sensor based on the networks of NiOnanocrystals. NiO nanometer effect makes the fast adsorption of gas molecules, and Zndoping makes the fast desorption of gas molecules in gas sensors. Meanwhile, we greatlyimproved the field electron emission properties of Ni nanocone arrays via the removal ofgas molecules on the nanocones’ surface, which provide a new idea of field electronemitters. Specific research progresses are as follows:（1） Ni nanowires with large aspect ratio have been prepared via a hydrazine hydratereduction method under assistance of magnetic fields, which have the high stability in air,and the coercivity （Hc,257.6） of nanowires is higher than that of the bulk materials.Meanwhile, we find that the surface morphology, microstructure and aspect ratio of Ninanowires can be controlled through adjusting the Ni²⁺concentration, reaction temperature, NaOH concentration, solution species, intensity of the applied magnetic field, reaction timeand N₂H₄·H₂O concentration. The growth mechanism of Ni nanowires was proposedaccording to lots of experiments, which illustrate that the different morphology ofnanocrystals can be assembled with Ni nuclei via external magnetic field and optimalreaction parameters. In addition, we developed a batch Ni nanowires preparation device,which mainly comprises five large institutions （the reaction chamber, the compensationloop part, thermostatic temperature control part, a magnetic field generator, gas protectingpart）.（2） Transformation from Ni nanowire arrays to NiO semiconducting nanowire arrayscan achieved via in situ high temperature oxidation method. It was found that the lower theoxidation temperature is, the smaller the average grain size of the NiO nanowries is, thelarger the bad gap energy is. The forming mechanism of NiO nanowires was providedaccording to the different temperature and time experiments. And then, we assembled theas-synthesized NiO nanowires into the gas sensor using the lift-off technology, and findthat this sensor have the fast response, rapid recovery, outstanding reproducibility andexcellent selectivity. In addition, we find that the sensitivity of sensor gradually increaseswith the decrease of average grain size of NiO nanowires due to the smaller grains havingmore grain boundaries, which are considered as the active sites to adsorb more gasmolecules.（3） Symmetric Ni dendritic crystals, which consist of a long main trunk and highlyordered secondary and tertiary branches, have been successfully synthesized via a simpleand inexpensive electrolytic process in ethylene glycol solution. The length of the maintrunk is about10μm, and that of each branch is about1.5μm with a width of210nm.Microstructure characterization indicates that the angles between the central trunk and allthe secondary branches in dendritic structure are almost the same at about70o, and theangles between the secondary branches and tertiary branches are about60o. A possible growth mechanism of magnetic pure metal dendritic crystals was proposed afterinvestigating four different stages during the electrolytic process. Meanwhile, we obtainedthe optimal parameters of preparation of Ni dendritic crystals: NiCl2concentration is0.010mol/L, electrolytic voltage is50V, electrolytic temperature is60oC, and electrolytic timeis more than5h.（4） Zn-doped NiO dendritic crystals have been successfully synthesized via anelectrolytic approach combined with subsequent high temperature oxidation. The trunkshave lengths in the range6–10μm with diameters varying from190nm to200nm, and thebranches have lengths in the range1–3μm with diameters varying from150nm to180nm.Microstructure characterization indicated that the solubility limit of zinc ions in the NiOlattice sites was lower than7mol%. We systematically investigated the gas sensingproperties of the Zn-doped NiO sensors for NH₃gas detection at room temperature. Thesensor with doped NiO dendritic crystals gave5–8times faster responses and30–50timesfaster recovery speeds than the sensor with pristine NiO dendritic crystals, which isimportant for the practical application of this NiO sensor. Lastly, a possible gas sensingmechanism of NiO dendritic crystal sensors was discussed.（5） We present the fabrication of Ni nanocone arrays on Ni foil substrate as well asgas exposure field electron emission experiments using them as cold electron cathodes.The Ni nanocones have base diameters ranging from50to450nm and heights rangingfrom50to200nm. The tip diameter of the nanocones is about10nm and the apex angle ofnanocone is about40o. Field electron emission measurements indicated that the as-grownNi nanocone array is an excellent field emitter exhibiting low turn-on field, high currentdensity, and large field enhancement factor due to sharper tips and better contact with theNi substrate. Meanwhile, we find that adsorbed gas molecules greatly hindered the fieldelectron emission performance of the Ni nanocone array. Repeated applying voltage or‘vacuum J–E annealing’ could significantly improve field emission properties and stability, which is attributed to desorption of the adsorbed gas molecules through Joule heating.