Hydro/Solvothermal Synthesis And Properties Characterization Of Hierarchical Nanostructures

Hierarchical nanomaterials is a new research object which is constructed of primary building blocks according to certain rules, concluding1D,2D and3D system, at least, one dimension of those systems should be in nanoscale scope. Thus, the hierarchical nanomaterials not only have the intrinsic properties of nanoparticles, but also the couple effects and synergies should be exist derive from nano-structured aggregation,which can endow the hierarchical nanomaterials a series of novel physical and chemical properties. It can promote the potential application of hierarchical nanomaterials in many areas, such as magnetic, optoelectronic device, energy storage, sensor and catalysis. So hierarchical nanotructured materials has attracted great interest of chemists and materials scientists. In this dissertation, hydrothermal/solvothermal method was used to prepare coral-like Fe3O4, a series of core-shell metal sulfides and flower-like β-Ni(OH)2, the formation mechism was proposed and the relationship between materials structure and properties was elaborated.1. Preparation of coral-like magnetite through a glucose-assisted solvothermal synthesisIn the ethylene glycol/water mixed solvent system, FeSO4·7H2O and KOH were used as the starting materials, glucose and its derivatives can play the role of stabilizing agent of iron ions, coral-like hierarchical Fe3O4was prepared under200℃solvothermal condition. The hierarchical Fe3O4was an aggregate of nanoparticles with the size about10nm, where the root of the nanostructure is composed of random-aggregated particles, and the twigs were formed from the oriented-aggregation of nanoparticles. With the extened reaction time, the hierarchical structure dissociated into discrete Fe3O4nanoparticles. Our study show that under KOH alkaline condition, some glucose was oxidized into its derivatives like pentose, glucuronic acid, glucoside and so on. Both the glucose and its derivatives have a wealth of hydroxyl group and carboxyl group which can coordinate with iron ions to form a stable chelate compound. Then, as the reation proceeded, the glucose and its derivatives would gradually decompose by oxidative degradation, and so it can release iron ions slowly to form Fe3O4nanocrystaline. This process provide a rate-determining step for the formation of hierarchical structure, and then it can form a concentration gradient in the raction solution. Further, the concentration gradient oscillation will induce the Fe3O4nanopartiles aggregated into coral-like structure. Under the effect of heat flow, the hierarchical structure would grow in fan-shaped appearance. The magnetic measurement showed that the coupling effects among the interconnected particles make the hierarchical Fe3O4have strong coercive force.2. Synthesis of metal sulfide nanoboxes based on Kirkendall effect and Pearson hardnessIn this chapter, firstly, under the assist of Fe3+, hydrochloric acid was used to etch silver nanoclusters for the formation of AgCl nanocubes which would be used as sacrificial templates. Then based on the solubility product effect, the AgCl nanocubes were converted to core-shell Ag2S nanoboxes, anion diffusion accompanied by the nanoscale Kirkendall effect would generate hollow structures inside the resulting nanoarchitecture. Finally, according to Pearson hard-soft acids and bases theory, in the methanol solvent, tributylphosphine was used as phase transfer agent for silver ion to carry out cation exchange reaction, this process showed a topotaxial reaction, and the shape of the obtained CdS, PbS, ZnS and AgInS2was completely preserved. The Uv-vis diffuse reflectance spectra show the obtained CdS nanoboxes have a preferable utilization in the visible region, this is attributed to the novel core-shell structure of CdS nanoboxes and coupling effect among the interconnected CdS naoparticles which can produce self-narrowed band gap. Additionally, the present synthetic strategy can be extended to prepare other chalcogenide nanostructures with different morphologies.3. Hydrothermal synthesis of hierarchial β-Ni(OH)2with ultrathin nanosheets as electrochemical pseudocapacitor and gas sensor In the reaction system, NiCl2·6H2O and hexamethylenetetramine were used in hydrothermal reflux condition, and hierarchial β-Ni(OH)2with ultrathin nanosheets were prepared. Electron microscopy showed the every flower-like sample was composed of several nanosheets, the nanosheets were lagerer than500nm, and they were connected to each other through the center to form flower-like structures. XRD and HRTEM showed the thickness direction of the nanosheets is [001], and the thickness was about9.5nm. The ultrathin thickness can provide a very short diffusion pathway for ions as well as rapid response to the adsorption of target gas molecules, which can improve the electrochemical activity and gas sensitive property. Electrochemical measurements showed the specific capacitance of the electrodes made from the flower-like β-Ni(OH)2can achieve1727F/g when the current density is1A/g. Increasing the current density to20A/g, the specific capacitance can still maintained to1235F/g. After1000charge-discharge cycles, it showed that at1A/g, the loss of the specific capacitance is only1.6%, at20A/g, the loss is27.9%. In addition, gas sensitivity tests showed that the device made of the β-Ni(OH)2can respond to ethanol and acetone at a concentration of1ppm.