Hydrothermal Synthesis Of NiO And Corresponding Composite Semiconductors For Electrochemical And Gas Sensing Performance

Nickel oxide(NiO) is a significant chemically and thermally stable p-type semiconductor with a band gap energy in the range of 3.6-4.0 eV, which has received considerable attention in two research fields: the structural and functional characterizations. Hydrothermal method as a convenient, low-lost, and large-scale synthetic approach can be controllably used to synthesize various morphologies such as nanosheets, nanowires, nanoflowers, and other 3D nanostructures. Due to its flexibility in morphology, NiO is widely used in the application of electrochemical and gas sensing. However, the general nanostructures of pure NiO exhibit small specific surface area and quite low conductivity, which limits both electrochemical and gas sensing performance. In order to improve NiO semiconductor material, it is necessary to design and obtain unique nanostructures with large specific surface area. Moreover, based on NiO, its composite oxides(NiCo2O4, NiMo O4 and so on) which ones have a better conductivity can be chosen as special functional materials to enhance the electrochemical and gas sensing performance.In this dissertation, systematic and planned hydrothermal synthesis of various NiO and corresponding composite oxides with interesting and special nanostructures has been discussed in detail. All kinds of advanced electron microscopes are used to characterize the structure. Then, the products of high quality are taken to test electrochemical or gas sensing properties according to the previous proposal. It can be seen that works on these topics have made great achievements as listed below.The research of NiO for the gas sensing application is shown below.It is successful to synthesize low dimensional NiO nanostructures including nanowires(1D) and nanosheets(2D). For the nanowires, NiC2O4·2H2O is chosen as the precursor of NiO. It is found that the existence of ethylene glycol(EG) and polyethylene glycol(PEG) play an important role in synthesis of long and disperse NiO nanowires. For the nanosheets, regular NiO hexagonal nanosheets are successfully prepared by controlling the concentration of ammonia solution with a simple hydrothermal method. The formation mechanisms of these regular hexagonal nanosheets are elaborated according to the crystal structure and characterization. Based on above two different nanostructures, the gas-sensing performances of the as-prepared NiO nanostructures were investigated towards ethanol, CO, toluene, and ammonia on the sensitivity, stability, response-recovery time and selectivity.Three dimensional(3D) NiO nanostructures, particularly the nanoflowers, are also successfully obtained by adding different surfactants. For the pure NiO, multifarious hierarchical flower-like nickel oxides are synthesized by hydrothermal method. Most interestingly, when added appropriate PVP, glycerol and CTAB respectively, three different NiO 3D nanostructures can be obtained, which were just like camellia, rose and microspheres. Based on the nucleation and crystal growth, two main processes, the influences of surfactants, reaction time and temperature are also discussed in detail. Furthermore, the gas-sensing performances of the as-prepared NiO nanostructures were investigated towards ethanol for comparison. For the tungsten doped NiO chrysanthemum-like nanostructure, it is the first time to demonstrate its potential gas sensing performance. WO3·0.33H2O-NiO, NiO, and WO3·0.33H2 O samples are fabricated into three gas sensors to detect their responses to the ethanol. It is found that the tungsten doped NiO chrysanthemum-like nanostructure exhibit quite good gas sensing property toward the ethanol. At the meantime, the doping model and gas-sensing mechanism were proposed.The research of NiO and corresponding composites for the electrochemical application is shown below.Carbon fibers(CFs, the diameter of ~5 μm and dozens microns in length) that obtained by cotton wool after vacuum calcination are chosen as one kind of natural template for the growth of nickel oxide nanosheets(NiO-NSs). CF@NiO-NSs core-shell and NiO-NSs shell structure are severally prepared by heat treatment in vacuum and air. Then, both of them are fabricated into electrodes for the measurement of electrochemical performance. Benefiting from the structural features, the CF@NiO-NSs electrode exhibits better electrochemical performance with a high specific capacitance(891.1 F g-1 at the current density of 1 A g-1), good rate capability(still 83.3% retention even at a high current density of 20 A g-1) and excellent cycling stability(retaining 95.43% after 1000 cycles). These results make the CF@NiO-NSs a promising electrode material for high-performance supercapacitors.NiCo2O4 and its sulfurizing product NiCo2S4 nanosheet arrays with self-decorated nanoneedles on nickel foams are synthesized by a facile and efficient two-step hydrothermal approach. It can be demonstrated that morphologies of various three-dimensional nanostructures such as nanoneedles arrays, nanosheet arrays, and nanoneedles-decorated nanosheet arrays can be manipulated by tuning the amount of the additive ammonium fluoride alone in the reaction. The unusual nanostructure of nanoneedles grown on surface of NiCo2S4 nanosheet arrays shows a large electroactive surface area and superior electrochemical properties. The electrode made of the NiCo2S4 nanosheet arrays with self-decorated nanoneedles shows greatly improved electrochemical performance with an ultrahigh specific capacitance of 2617.6 Fg-1 at a current density of 15 mAcm-2 and of 2051.0 Fg-1 even at a current density of 30 mAcm-2. The electrode also exhibits an excellent cycling stability by retaining 93.2% after 5000 cycles which renders the NiCo2S4 nanosheet arrays with the self-decorated nanoneedles a potential electrode material for high-performance supercapacitors.Groove-like NiMo O4 hollow nanorods(NiMo O4-GHRs) have been prepared via a facile one-pot hydrothermal approach, characterized with FE-SEM containing EDS, X-ray diffraction pattern, advanced TEM techniques and further investigated as a positive electrode material for pseudocapacitors. Such hollow nanorods of high aspect ratio(nearly about 104) lead to a large specific surface area, which increases the electroactive surface area. Benefiting from the structural features, the NiMo O4-GHRs electrode exhibits good electrochemical performance with a high specific capacitance(1102.2 F g-1 at the current density of 1 A g-1), good rate capability(still 64.5% retention even at a high current density of 20 A g-1) and excellent cycling stability(retaining 90% after 1000 cycles). These results make the NiMo O4-GHRs a promising electrode material for high-performance supercapacitors.