Hierarchical meso- and nanostructures of functional materials such as metal oxides have attracted considerable interests due to their unique properties and potential applications in advanced devices and systems. The performance of materials depends not only on their chemical composition, phase, size and morphology, but their assemblies as well. The controlled growths and structural characterizations of materials are of considerable significant.At present, the research on the construction of heterostructures is mainly concentrated on chemical vapor deposition and thermal evaporation. Nevertheless, complicated experimental procedures and critical conditions, e.g. high temperature or high pressure, were usually required to obtain these fantastic hierarchical materials. Compared with other approaches, hydrothermal method is regarded as an attractive candidate for synthesis of homogeneous architectures due to the advantages of environmentally-friendly, simplicity, easy-controllability and low cost. In this thesis, an effective two-step route was reported to synthesize hierarchical structures by combination of electrospinning and hydrothermal method. The main contents of the research were listed as below:1. ZnO/TiO2 hierarchical material with tunable structures were synthesized by combination of electrospinning and hydrothermal method, the influence of preparation conditions on the epitaxial ZnO structures have been investigated. With precursor concentration ranging from 0.015mol/L～0.045mol/L , ZnO/TiO2 hierarchical structures can be obtained with high dendity ZnO nanorods growth on the TiO2 electrospinning fibers. By adding trisodium citrate to the hydrothermal system, a layer of uniform ZnO nanoplates grown on TiO2 fibers were generated and the formation mechanism of the secondary growth ZnO structures have been investigated.2. SnO2/TiO2 hierarchical structures were successfully synthesized by changing the hydrothermal growth conditions. The influence of preparation conditions on the epitaxial SnO2 structures has been investigated. The result shows that, with concentration ratio of 20:1（NaOH to SnCl4）, SnO2/TiO2 hierarchical structures with a layer of uniform SnO2 nanorods grown on the suface of TiO2 fibers can be obtained. The possible formation mechanism of the secondary growth of SnO2 nanorods is also proposed.3. The wettability of as-prepared TiO2 fibers, ZnO/TiO2 and SnO2/TiO2 hierarchical structures were investigated in detail. The initial water contact angel was super-hydrophilic with a cntact angle of 0°. The TiO2 fibers and ZnO/TiO2 hetrojunctions could keep super-hydrophilicty even stored in the dark fou several months. However, the surface of SnO2/TiO2 heterostructures displayes hydrophilic, super-hydrophobic. The result shows that the formation of the dual switchable surface wettablity of SnO2/TiO2 was due to the unique surface structure of the films induced by the epitaxial growth of SnO2 nanorods.
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Construction of Special Polypyrrole Micro/nanostructures Based on One Dimensional Nanostructures of Polypyrrole
One-dimensional (1D) nanostructures, such as nanowires and nanotubes, are considered to be building blocks as well as interconnect for fabricating nanoscale devices. Therefore, the assembly of 1D nanostructures in the fabrication of nanoelectronic and nanophotonic devices is currently attracting significant attention. In this regard, controlled fabrication of self-assembled 1D nanostructure, such as hierarchical structures with junctions and branches, nanowires with well order or with ultra high length, is important from the fundamental as well as technological point of view. In this research, several hierarchical structures with novel morphologies as well as the ordered nanowire arrays and ultra long nanowires of polypyrrole (PPy) were sythesized easily by using biomolecules as morphology directing agents. These synthesis methods offered some appealing characteristics such as simple, cheap, rapid, mild and environmentally benign. The morphology, structure, and electrochemical performance of the samples were systematically studied by scanning electron microscopy (SEM), fourier-transform infrared spectroscopy (FTIR), energy dispersive spectrometry (EDS), and cyclic voltammetry (CV). The influences of the experimental conditions on the morphologies of the products and the surface performance have been investigated, and the formation mechanism of the hierarchical structures was also discussed. The main contents are shown as follows:The controlled synthesis of hierarchical structures made of short nanowires/ micron-sized “hills and valleys” or nanowire networks/ micron-sized “cauliflower” has been successfully realized via a simple two step electrochemical method by using ITO as work electrode. In the first step, microstructured PPy films were prepared by electropolymerization in an electrochemical system. Then PPy nanowires were electro-deposited on the surface of the as-synthesized microstructured PPy films in another electrochemical system by employing starch as morphology directing agent. As a result, two typical dual-scale hierarchical structures of PPy were obtained. The surface wettabilities of the resulting nanowire-based PPy hierarchical structures were examined. It was found that although having almost the same chemical composition, the two PPy hierarchical structures exhibited entirely different surface wettability. The surface of the short nanowire-based hierarchical structure was hydrophobic, with water contact angle 95～129°; while the surface of the nanowire network-based hierarchical structure exhibited a superhydrophilic property, the contact angle of which was close to 0°.The controlled synthesis of hierarchical structures made of nanowires/ microspheres or nanowires/ nanorods has been successfully realized via another simple two step electrochemical method by using Ni as work electrode. The whole process was conducting in only one electrochemical system involving gelatin as morphology directing agent. In the first step, a high potential was applied to prepare PPy microspheres or nanorods. Then a low potential was used to synthesize nanowires on the as-prepared PPy microspheres or nanorods. As a result, another two novel hierarchical structures of PPy were gained.Ordered nanowire arrays and nanowires with ultra high length (up to 10μm) have been successfully synthesized on Ni electrodes by one step electrochemical method under potentiostatic or galvanostatic technique with the assistance of gelatin. It is demonstrated that the morphology of the final products is significantly affected by the applied potential, current and the reaction time. The morphology evolvement and the growth mechanism have been studied carefully, and the gelatin-induced oriented growth mechanism was proposed for the formation mechanism of the nanowires.
Hydrothermal Synthesis of Single-crystalline α-Fe2O3 Hierarchical Structures and Photocatalytic Properties Researches
Nanomaterials with a hierarchical structure have attracted intensive research attention. Their complicated structures are usually concomitant with diverse characteristics, thus having various applications. Recently, nanomaterials with various hierarchical morphologies have been synthesized and successfully used in catalysis and environmental improvement, and as sensors in biological systems.In this paper, a dual iron precursors system in a hydrothermal process was developed for controllable fabrication ofα-Fe2O3 hierarchical structures with different morphologies. Micro-pines, snowflakes and bundles were successfully synthesized simply by tuning the total concentration of the two iron precursors K4[Fe（CN）6] and K3[Fe（CN）6] and their molar ratio. The obtainedα-Fe2O3 hierarchical structures were characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and Selected Area Electron Diffraction. The effect of experimental conditions on the morphologies of theα-Fe2O3 crystals was systematically investigated. A possible formation mechanism of differentα-Fe2O3 hierarchical structures was proposed. Good photocatalytic properties were observed for all the hierarchical structures.On the other hand, Prussian blue （PB） and its analogs have attracted intensive research attention due to their unique properties of molecular magnets, electrochemistry, optics, hydrogen storage and biosensors. Prussian blue, the first synthetic coordination compound, is a mixed-valence iron-（III） hexacyanoferrate（II） compound of composition Fe4[Fe（CN）6]3·XH2O with a face-centered-cubic structure, in which Fe3+ in the N-coordinated sites is in the high-spin state and Fe2+ in the C-coordinated sites is in the low-spin state. According to previous reports, two redox processes can take place as to Prussian Blue: chemical reduction of PB produces Prussian white (Fe2II [FeII（CN）6], PW); chemical oxidation of PB yields Prussian yellow (FeIII[FeIII（CN）6], PY). However, to fabricate Prussian blue analog Prussian yellow nanoparticles with different shapes remains a challenge. Here, we have successfully prepared Prussian yellow Fe[Fe（CN）6] colloidal nanospheres （hollow nanospheres and solid nanospheres） by a simple hydrothermal method with the assistance of different phosphate. The obtained Prussian yellow Fe[Fe（CN）6] colloidal nanospheres were characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. A possible formation mechanism of different Prussian yellow Fe[Fe（CN）6] nanostructures was proposed.
In this paper, cuboid-shaped ZnO with hierarchical structures, sphere-shaped ZnO microstructures, hexagon-likeβ-Ni(OH)2 nanoflakes and flower-likeβ-Ni(OH)2 microspheres were successfully prepared through a facile hydrothermal approach. The influence of fabrication conditions on the structure and morphology of ordered nanostuctures were characterized by TEM, SEM, XRD,PL and UV-Vis spectrometer. The synthesis mechanism and experiment conditions were also studied. The results and conclusions are as followed:1. Cuboid-shaped ZnO with hierarchical structures were prepared by rection of Zn(NO3)2 and NaOH through a mixed surfactants mediated hydrothermal method at 100℃for 24h. The morphologies and the structures were characterized by SEM and TEM. It has been found that the product consist of a large quantity of nearly monodisperse cuboid-shaped microparticles of 2.7μm in length and 2.1μm in width, and almost all of them have uniform size and present one kind of structure. The as-prepared samples were constructed with many interleaving nanosheets in a perfectly aligned manner. The nanosheet building blocks are about 20 nm in thickness and several hundred nanometers in width. Room temperature photoluminescence spectrum of the as-obtained ZnO hierarchitectures was recorded showing multiple peaks of visible emissions in bluegreen region. The influence of various experimental conditions on the reaction has been investigated in detail. 2. ZnO microspheres has been successfully synthesized in glycol mixed solvent via a facile solvothermal process with SDBS and OP-10 surfactants in the presence of Trisodium citrate at 170℃for 24h. The results were characterized using scanning electron microscopy (SEM) and transmission electron microscope(TEM). It has been found that the products were consisted of a large quantity of nearly monodisperse microspheres of 3.5μm in diameter. The microspheres were constructed with ZnO nanoparticles and the surface of the microspheres is rough. The influence of various experimental conditions on the reaction has been investigated in detail.3.β-Ni(OH)2 nanoflakes and flower-likeβ-Ni(OH)2 microspheres were synthesized by rection of NiSO4 and NaOH via a simple hydrothermal process with additives of ammonium oxalate or 1,2-ethylenediamine at 170℃for 24h. The SEM observation results displayed that the samples were uniform in size. The electrochemical property of the sample was obtained through cyclic voltammetry method.