Dissertation
Dissertation > Industrial Technology > General industrial technology > Materials science and engineering > Special structural materials

Synthesis and characterization of amorphous and dendritic crystal nano-materials

Author XieSongHai
Tutor FanKangNian
School Fudan University
Course Physical and chemical
Keywords amorphous alloy nano Ni-P controllable NiP@PS hollow sphere electroless plate dendritic crystal in situ HRTEM
CLC TB383.1
Type PhD thesis
Year 2005
Downloads 659
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Part IAmorphous alloys have been known for their short-range ordering and long-range disordering structure. As compared with the normal crystalline metals, they are found special properties in many research fields. The controllable synthesis of nanocrystals is hot research project recently, however, few works focus on the controllable synthesis of nano-noncrystalline materials. Even some works were published but no systematical experiments were performed.In the first part, chemical reduction method was employed to synthesis Ni-P amorphous nano particles. Uniform particle sizes and controllable compositions Ni-P nanoparticles are obtained by adjusting the reaction conditions systematically. Consequently, the Ni-P amorphous nanoparticles are measured by XRD, SEM and HRTEM to learn more about the materials structure, morphology and composition variation. Detailed analysis and interpretation are presented after correlating to the synthesis conditions. Some conclusions drawn on the preparation and characterization are followed:1. It was convenient to synthesis Ni-P samples with different particle sizes and compositions by simply adjusting the molar ratio of H2PO2-/Ni2+, pH value and reaction temperature. For example, nanoparticles with different particle size and compositions could be obtained by changing molar ratio of H2PO2-/Ni2+, while at much higher molar ratio of H2PO2-/Ni2+ will result a higher probability of NiHPO3. In the pH range of 12.6-13.1, Ni-P amorphous nanoparticles could be produced with different compositions but with same particle size, however, cotton-like poly-hydroxyl species Ni(OH)x would be favored at higher pH value. Some residual KCl and the byproduct by self-decomposing of H2PO2- are also adsorbed on that. Whatever Ni(OH)x or NiHPO3 is porous morphology and amorphous structure. The particle size could be varied in the range of 77.4-99.8 nm but with unchanged compositions when the reaction temperatures are adjusted from 333 to 373 K. All these series specimens are found with uniform particle size.2. The compositions and particle sizes are not seriously affected by applying differentnickel salts as raw materials. The solvent will affect the result by the homogeneous status after introducing to water as co-solvent. The less solvable in water, the smaller particle size will be produced mainly because the valid concentrations of reaction materials and reaction rates. Similar tendencies are found for phosphorus content which could be interpreted by the adsorption status of H2PO2-.3. The introducing of AOT to reaction solutions presents a complex result. A reverse volcano type tendency is observed for particle size and phosphorus content. All the results could be explained by microemulsions formed with AOT plus solution and the effect on the adsorption of reactant on Ni-P micronucleus by AOT adsorption. With too many AOT introduced, core-shell and hollow spheres morphologies could be found which is attributed to NiHPO3 coverage and emulsion formed by AOT. The Ni(OH)x and NiHPO3 morphologies and loosed porous characters in Ni-P result in larger surface areas and the later one is considered as the minor contributor.Part IIMeanwhile, in situ technique was considered as the powerful tool to detect the amorphous structure transformation as compared normal characterizing tools, but for the amorphous alloy nanoparticles, few research groups worked on the process in order to shed light on the amorphous structure transfer to crystalline phase.Based on the results in part I, DSC and in situ technique combined with XRD and EM were used to understand the effect on the crystalline behavior by P-content. Different Ni-P amorphous nanoparticles were applied to catalyze carbon nanofibres (CNF). Some conclusions could be drawn as following:1. In the studies on thermal stability of amorphous Ni-P nanoparticles and phase transformation, the DSC data showed that lower P-content samples only have one thermal peak or even no peak are observed, but two peaks for higher P-content samples. It indicates that the lower P-content specimens are existed in crystalline structure or only one phase transformation evolved, but two transformation process for higher P-content samples. The difference of crystallization behavior such as crystallization thermal for R-series samples could be correlated to their different microstructures.2. In situ XRD data proves that the lower P-content samples only process one phasetransformation, that is, transferring from amorphous to crystalline Ni+Ni3P at 573 K. As compared to that, two phase transformations are observed on higher P-content, the one from amorphous to metastable phase Ni5P2 and Ni12P5 at 573 K and the other one is from metastable phase to Ni3P at 723 K. The ICP-AES results confirm that a P-rich amorphous phase might be formed during the P loss after crystallization based on XRD spectra. The observation by HRTEM presents core-shell structure in particles after heating treatment with P-rich amorphous shell ca. Ni:P = 1.3 :1. It also could be found in the HRTEM images that the Ni and Ni3P are exist together. The nanodomain of Ni are considered to disperse in Ni3P phase.3. The amorphous Ni-P nanoparticles will sinter together to form single crystalline clusters, which is observed by in situ SEM technique. The in situ TEM technique show that the lower P-content samples (R-2.2) will not change obviously in shape but many Ni nanodomains emerged during the heating process and no core-shell structures are observed. For the higher P-content sample R-11.0, core-shell particles are observed and sintered together to single crystalline. In R-2.2, only Ni3P and Ni are found by HRTEM and SAED. The phase of crystalline Ni5P2 in R-11.0 sample is recorded by HRTEM and SAED, which confirmed that the existence of metastable phase and the two phase transformation. In the same sample, a superstructure projected along the zone axis of [001] of Ni3P which is indexed as a 2×1×1 superlattice and confirmed by SAED pattern.4. The application of Ni-P to carbon nanofibres (CNF) production, catalysis and electrochemistry have been explored. Different Ni-P samples are successfully used to produce platelet type CNF with double size of Ni-P in diameter. It is the first time to get this kind of CNF catalyzed with Ni-P amorphous nanoparticles. The high selectivity to platelet CNF is correlated to the final shape of Ni-P after the reaction. In the catalytic hydrogenation and Li ion battery preparation, the performance is not satisfied.Part IIIRecently, hollow spheres have attracted increasing interest due to their unique structures exhibiting great potential for many technological applications ranging from drug delivery, optical and magnetic micro-devices, and catalysis to contaminated wasteremoval. A variety of hollow spheres including polymers, metals, oxides, carbons and semiconductors has been synthesized. However, no report on the preparation of core-shell materials with the shell made of amorphous alloy has been prepared to the best of our knowledge. It makes the amorphous shell more meaningful because the amorphous alloys have distinct characteristics as compared with their crystalline counterparts as described in Part I.It has been a long history in the synthesis of amorphous metal borides and phosphides materials by using the electroless deposition method. This method is efficient, low cost and convenient for the deposition of the amorphous as well as crystalline metal-metalloid materials. However, in order to deposit the desired material on a substrate, it is necessary that the surface should be pretreated with a homogeneously covered catalyst.In this paper, a surface seeding combined with electroless plating strategy was employed to deposit amorphous nickel phosphide (Ni-P) selectively on the PS microspheres to form core-shell NiP@PS structures. The activation of the inert PS microspheres was achieved by immersing in SnCl2 and PdCl2 aqueous solutions sequentially. The formed metallic Pd and also the growing Ni-P pilot the reduction of Ni2+ ions by NaH2PO2 on PS microspheres to form Ni-P amorphous alloy shell. The amorphous Ni-P hollow spheres were obtained by refluxing the NiP@PS spheres in tetrahydrofuran (THF). Some conclusions could be drawn and listed as following:1. For the first time, the core-shell NiP@PS colloids and hollow Ni-P spheres with an average diameter of about 2.2μm (core -2.0 μm, shell -100 nm) have been prepared via the electroless plating Ni-P on the pretreated PS spheres. The key strategy is the activation of the inert PS microspheres, which is achieved by pretreatment with colloidal Pd. Or the inert surface of PS microspheres will result the loosely combination of Ni-P shell and PS. The NiP@PS composite structure will not be produced successfully.2. The pH value affected the NiP@PS surface morphology, composition of Ni-P core-shell structure seriously, which was proved by the analysis result of the NiP@PS prepared under different conditions. In the pH range of 5.5-8.5, the surfaces are compact and smooth which suggested as the better condition for plating. The higher P-content about 13 wt.% was found when the pH value is adjusted to more alkalinity. The integrated hollow spheres could be easier obtained when the NiP@PS in a loosely state. On the contrary, the compact one prefers to be broken. During the refluxing treatment,the loosely structure will favor the PS gel leaking out the shell, on the other hand, the compact one will be compressed and broken.3. At different plating time, the surface morphologies will vary with different reaction period when the pH value was fixed at 8.5. The phenomenon could be attributed to the different plating rate in vertical and horizontal resulted by different catalytic performance of Pd colloids and Ni-P self-catalytic.4. At different pH value, the magnetic properties of the amorphous NiP@PS core-shell composites and Ni-P hollow spheres have been studied. All these samples show better coercivity and squareness than that in the literature. The magnetic properties change obviously with different the P-content, which are correlated to their microstructures.6. Since these anticorrosive magnetic amorphous alloys consist of a cavum of diameter -2.0 μm, it is possible to be applied as the magnetic micron capsule to catalysis, pharmaceutical, etc.Part IVAs a large family of materials, dendrite crystals, representing fractal structures have been studied extensively for their interesting crystallizing behavior in past decades. Most of the work focused on the novel morphologies, the elemental compositions and the orientations of crystal growth. However, the mechanisms for the formation of these fractal crystals were hardly investigated apart from some simulated models proposed on the basis of theoretical considerations. In a previous report, Co-catalyzed growth of three-dimensional fractal fishbone-like Mg-Si-0 crystals was presented and both vapor-liquid-solid (V-L-S) and nucleation-aggregation (N-A) mechanisms were applied in order to interpret the crystal growth with such an elegant structure. Some questions on the formation of the fractal structure still cannot be fully understood even by using these two mechanisms.In this paper, investigated by HRTEM images and EDX microanalysis, the fishbone-like crystal are observed and recorded in detail and the formation mechanism of this material is newly proposed. According to the data analysis, the conclusions could be drawn as the following:1. Co-catalyzed growth of fishbone-like fractal nanostructures has been studied. Thenanostructures are indexed as Mg2SiO4, the orthorhombic unit cell of Mg2SiO4 is with the cell parameters of a = 0.59817 nm, b = 1.01978 nm, and c = 0.47553 nm, space group Pmnb (62). Theoretical simulations are performed and a well agreed result is obtained.2. According to the HRTEM images and EDX microanalysis, the formation mechanism of this material is far more complicated than normally expected. The formation mechanisms are proposed as the following:a. We have demonstrated that Co and Mg2SiO4 crystals grow simultaneously during the reaction, and a dual-catalytic process has been proposed.b. The experimental evidence has shown that the polygonal Co particles are the final form. We found that Co formed nanocrystallites of 2-5 nm in diameter in the surface coating layer of the Mg2SiO4 crystalline stems and then aggregate into large clusters. These clusters further grow and recrystallize into polygonal single crystals. Instead of the single crystals, it is the polycrystalline clusters that played the most important role in Mg2SiO4 growth.c. The growth of Mg2SiO4 stems is in three-dimensions. Mg, Si, and O atoms deposited on the surface of the stems to form an amorphous coating layer preliminary. Some Mg2SiO4 nanocrystallites of in diameter 5 nm or less are developed in this coating layer, probably catalyzed by the individual Co nanoparticles that were also presented in this layer. The Mg2SiO4 nanocrystallites are then recrystallized into the core crystal. Using this mechanism, we can explain why the shape of the Mg2SiO4 stems is almost perfectly cylindrical and the angles between the main stem and the secondary branches and between secondary branches and subsecondary branches are independent on the crystal orientation.

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