Hierarchical NiO And Their Supercapacitor Performance

Owing to variety of electronic structures,metal oxide nanomaterials have exhibited promising applications in catalysis,sensing,optics,magnetism and supercapacitors.The aim of the paper was the preparation of hierarchical and porous nanomaterials with well-defined size,dimensionality,diversity and large surface area.Designed novel synthesis technology and investigated the formation mechanism were the main research contents.It was important significance to reveal the relations between the morphology and performance,assembling the nanocrystals into superstructures and paving the way to the real applications.In this thesis,valuable explorations have been carried out with the targate for synthesis of hierarchical and porous nickel oxide nanostructures along with their formation mechanisms and novel properties,emphasizing on the relationship between crystal symmetry and morphology manipulation as well as the connection between properties and size/morphology.A novel synthesis approach of hydrothermal method fabricated to La3+ doping had been developed.The NiO porous and hollow spheres were fabricated using colloidal carbon spheres as hard template with different La proportion.The relationship between doping effect and morphology properties of NiO was investigated.The specific capacitance and reversibility of the electrode have been measured based on the porous characteristic of the electrodes.The results indicated that La3+-doped NiO microspheres has changed the crystals growth habits and exhibited smaller feature sizes,the specific capacitance of the La3+-doped NiO electrode was remarkably enhanced compared with the corresponding pure NiO,which might be caused by the crystal defect once La replaced nickel atom in the lattice.At the same time,the porous structure system of La3+-doped NiO material was the fibrous porous network which can shorten the diffusion path length of OH-ions for the reversible redox process by acting as an "ion buffering reservoir".This result provided the experimental reference of using rare earth element doping for the nanodevice design.Hierarchical NiO nanospheres composed of porous nanosheets are prepared by a facile trisodium citrate assisted precipitation route followed by a calcination process.Effects of the trisodium citrate on the micro structure and electrochemical performances of NiO nanospheres are systematically investigated.The studies on the crystal growth mechanism revealed that Ni(OH)2 layers intercalated with citrate should have much weaker hydrogen bonding,yielding thinner and more flexible sheets.By the effect of oriented attachment,the a-Ni(OH)2 nanospheres intercalated citrate were formed which had smaller size and transformed to NiO with almost porous nanostructure via calcination.During the calcination,the hierarchical morphologies of NiO did not changed,but the slit-shaped pores on the flakes are formed by the gas impulse arising from the calcination of the intercalated citrate,which effectively enhance their surface area.Taking advantage of these characteristics,NiO-Cit nanospheres improves the capacitance of the electrode,and showed good retention for more than 1000 cycles in a cycling test with lower charge-transfer resistance,suggesting its high promising prospective for SCs.In the trisodium citrate assisted synthesis systerm,a facile lamellar template method has been developed for the preparation of uniform porous hollow core-shell structure NiO(HCS-NiO)with a nanoarchitectured wall structure.The morphology of the products was thoroughly investigated by XRD,SEM and TEM measurements.The results indicated that ethanol interacted with trisodium citrate played an important role for the formation of hollow core-shell spheres.The different ion-dipole interaction attribute to vary longth of the alkyl chains,which led to the imperfection of the hollow core-shell structure.The results of the electrochemical measurement showed that NiO microspheres with hollow core-shell showed excellent capacitive properties,the exceptional cyclic,structural,and electrochemical stability with?95%coulombic efficiency.The flower like NiO with hierarchically structure was synthesized by in-situ growth on the nickel foam.The effects of the nickel sources,ammonia amount,deposite temperature and different substrates on the morphology and electrochemical properties have been investigated.The electrodes exhibited excellent capacitive characteristics and long cyclic stability after 3000 cycles,which attributed to the porous morphology and stability structure.When the systhesis process changed into the microwave systerm,the basic crstal structure of nanoprimes,nanofibers and the hollow flower like NiO assembled by the crystal were obtained though controlling the reaction time and the species of the nickel sources.Because of the high heating rate by the microwave,which enhancd the reaction efficiency and larged the heated point.Consequently,the microwave effect resulted in the formation of more Ni(OH)2 crystals,smaller dimension of the NiO spheres.The unique morphology and excellent capacitance properties may ba as a basic for the synthesis porous hierarchically structure with a facile approach.