The Synthesis Of Nano-and Micro-materials By Precursor Pyrolysis Methods And Solution-based Chemical Methods

In this dissertation, new chemical methods were explored to synthesize nanomaterials and meganetic materials. Precursor pyrolysis methods and solution-based chemical methods were developed. Nanometer-scale Fe₃O₄/C composites, plate-like Cu, carbon-encapsulated Fe₃C nanoparticles, dendritic Co were synthesized by designing possible chemical reactions and controlling the experimental parameters. At the same time, their possible growth mechanisms and magnetic properties were also studied. The experimental details are summarized as follows.1. An organic/inorganic mixture precursor pyrolysis method was developed to synthesize nanometer-scale Fe_O4/C composites. A novel structure (monodispersed Fe_O4 nanoplates between two carbon films) was synthesized by pyrolyzing the mixture of ferrocene (Fe-(Cp)₂, Cp=C₅H₅) and sodium oxalate in a stainless steel autoclave of 64 mL capacity at 600°C. To the best of our knowledge, this is the first report about the synthesis of ordered monodispersed Fe_O4 triangular nanoplates and the 2D layered nanometer-scale composites formed by Fe_O4 nanoplates and carbon films. To further study the effect of the reaction system pressure on the morphology of the final products, the experiment was carried out at relatively high/low pressure by increasing/decreasing the amount of sodium oxalate while keeping other conditions unchanged, and the results indicate that relatively high pressure is favorable for the formation of carbon-encapsulated Fe_O4 nanorods and the relatively low pressure tend to obtain irregular Fe_O4 nanoparticles and carbon fibres. The Fe_O4 nanoplates are triangular single crystals, sometimes truncated, with edge lengths ranging from 50 to 100 nm. The carbon-encapsulated Fe_O4 nanorods are also single crystal with a diameter ca. 100 nm and lengths up to 1μm. Many TEM observations were carried out to further investigate the possible growth process and formation mechanism. With increasing temperature, the Fe-(Cp)2 decomposes into Fe atoms and carbon atoms, these freshly formed carbon atoms grow into small carbon films, and at the same time the Fe atoms will react with the CO or CO₂ decomposed from Na₂C₂O₄ to form Fe_O4 particles, and then these Fe_O4 particles accreted on the carbon films grow bigger. The formation of the layered structure can be regarded as a self-assembly process, maybe because Fe-(Cp)₂ is a sandwich organometallic compound. In addition, the room-temperature hysteresis loop shows that the layered nanoplates display ferromagnetic properties, which might be used as information storage materials. Furthermore, this kind of layered structure may offer an opportunity to investigate dimensionally confined systems, and the triangular vacancies on the carbon films may be used as template to synthesize other kinds of triangular plate-like nanomaterials.In our experiment, if Na₂C₂O₄ was substituted by oxalates, carbonates or carboxylate which can release CO or CO₂ at high temperature, such as C₂H₂O₄·2H₂O, NaHCO₃, Na₂CO₃, citrate or a mixture of these, other nanometer-scale Fe₃O₄/C composites might also be produced. To further execute the researches on this aspect, more experiments were carried out. When NaHCO₃ used as materials, Fe₃O₄ nanoplates were obtained including much more hexagonal plates than triangular plates and they overlapped each other. The nanoplates are regular with the edge lengths of ca. 250-700 ran. The HTREM image indicates that the fringes of nanoplates are encapsulated by a layer carbon film with the thickness of ca. 30 run. The Fe₃O₄ nanoplates are well crystallized, but the carbon film shows poor crystallinity. When sodium citrate used as materials, the products mainly consists of Fe₃O₄ particles or nanoplates encapsulated in carbon. The size of the particle is about 300-400 nn, the edge length of nanoplates is about 90-140 nm and the thickness of carbon is ca. 100 nm. The relatively thicker carbon coated on the particles and nanoplates can be clearly observed, which maybe because the content of carbon is higher than oxalates or carbonates. Why this reaction system is favorable for the formation of plate-like structure is discussed.2. Fe₃C nanoparticles encapsulated in carbon shell with size range of 20-50 nm were obtained in large scale by reacting anhydrous FeCl₃, hexamethylenetetramine and metal Na in an autoclave at 650℃. The preparation methods of Fe₃C are developed, and in our experiment the operation is easier and feasible. To investigate the influences of the reaction temperature and the amount of HMT used, a series of experiments were carried out. At a relatively low tempature 550℃, Fe₃C with poor crystallinity can also be obtained. When the amount of HMT was reduced, the peaks indexed to bcc-Fe become stronger, which indicate that the input FeCl₃ can not be absolutely evolved to Fe₃C and the content of iron increases in the final products while decreasing the amount of carbon source. Magnetization measurements show that the as-obtained materials display superparamagnetic properties at room temperature, which might be used in the soft magnetic material fields. It is expected that this simple but rational synthetic method could be adapted for fabrication other carbide materials more economically.3. A capping agent-assisted solution-phase approach was developed to prepare uniform plate-like copper crystallites. These plate-like crystallites were synthesized by a facile hydrothermal method using formaldehyde (HCHO) as the reducing agent in the NaOH aqueous solution at 180℃. FESEM and TEM images reveal that the final plate-like products are single crystal and consists of much more hexagonal plates than triangular ones with the edge length ranging from 0.5 to 4μm and with the thickness ranging from 250 to 350 nm. Many TEM observations were carried out on the products obtained at different reaction stages to further investigate the possible growth process. It can be observed that the plate-like crystallites were gradually evolved from small particles and some irregular plates. At the same time, it is believed that PEG tends to attach to the lowest energy {111} plane and suppresses the growth rate of this facet. Thus, anisotropic polygonal plates covered by {111} facets are preferentially produced. When the volume of NaOH solution was increased at the absence of PEG the shrub-like structures were obtained because the relatively high OH concentration accelerates the velocity of the reaction and leads to the aeolotropic growth of copper along several directions. To research the growth process of the shrub-like structures, we have systematically studied the experimental results at different reaction stages. A possible formation model of shrub-like structures is proposed. The effect of temperature, reaction time, NaOH concentration, the amount of capping agent and formaldehyde on the morphologies of the final products has been investigated. From the experimental data, we can conclude that high NaOH concentration favors the formation of shrub-like copper structures. If NaOH was absent, no copper products can be obtained. Therefore, we can confirm that NaOH is absolutely necessarily for the copper particle' s formation, and its concentration will influence the final morphology of the product. When PEG was absent or the amount of PEG was increased from 0.15 g to 0.3 g, the experimental results show that only few regular plate-like copper were obtained in the products. Reaction temperature also has some influence on the product Pure Cu was obtained when the reaction temperature was higher than 140°C. But when the temperature was lower than 160°C, the products were all in irregular shape. When the volume of HCHO solution was reduced to 1.5 mL, the final product was a mixture of Cu and Cu₂O. The as-obtained products are well crystallized, and these as-obtained plate-like copper maybe have potential electrochemical applications in microelectronic devices4. A solvothermal reduction method using the mixed solvents was developed to prepare hierarchical cobalt dendrites. These dendrites were generated by using cobalt carbonate together with cobalt oxalate as precursor and using hypophosphite (H₂PO₂^-) as the reducing agent in basic solution at 160°C. To further investigate the possible growth process and formation mechanism, a series of XRD and SEM characterizations were carried out on the products obtained at different reaction stages. The XRD analysis results prove that the Co²⁺ reacts with sodium oxalate/sodium carbonate to form indissoluble cobalt carbonate and cobalt oxalate, men gradually reduced to form Co. The SEM images clearly show the evolution of cobalt structures from prisms and irregular particles into micro-scale dendritic crystallites. It is believed that the formation of cobalt dendrites can be ascribed to the intrinsical anisotropy of the hexagonal structure of cobalt, and controlled by preferential tropistic growth. The effects of the viscosity of solvents and the NaOH concentration on the morphologies evolution of the final products have also been studied. The as-prepared cobalt dendrites exhibit ferromagnetism with a greater coercivity than the bulk materials and might find possible application on information storage.