Organic-Inorganic Hybrid Nanowires: Synthesis, Characterization And Design Of Nanostructured Metal-Oxide/Carbide

In this decade, the synthesis and application of nanomateials have attracted much attention in the fields of physics, chemistry, material science, biology and etc., with the development of nanoscience and nanotechnology. The superior performance of various nanomaterials than bulky counterparts has been demonstrated in many fields, indicating the potential application in the future. Among them, organic-inorganic hybrid nanowires have become a new highlight because of the uniform 1D morphology, anisotropic transport, easy assembly, tunable structures and functional properties.However, there are still challenges in the urgent research on organic-inorganic hybrid nanowires. Firstly, the kind of organic-inorganic hybrid nanowires is limited; secondly, the growth mechanism of such materials is obscure; and thirdly, the employment of organic-inorganic nanowires to fabricate other nanostructures has been ignored in the previous work. These factors limit seriously the development of such materials. Therefore, the research dealing with synthesis and growth mechanism of novel organic-inorganic hybrid nanowires, as well as designing other nanostructures based on the hybrids, has been expected to promote the development and application of such nanomaterials.This dissertation focus on organic-inorganic hybrid nanowires, with the deep investigation into three parts including "synthesis and characterization", "fabricating 1D metal-oxides from organic-inorganic nanowires" and "designing metal-carbide catalysts based on organic-inorganic hybrids". To explore more hybrid nanowires, a series of GeOx/ethylenediamine and MoOx/amine nanowires were originally developed and well characterized. New growth mechanisms were proposed on the basis of systematic study about their synthesis. Meanwhile,the subnanometer contact between organic and inorganic components in the hybrids was found significant for preparing the nanostructures of metal oxides and carbides, and thus several 1D metal oxides (e.g. GeO2 and MoO2/C nanowires) and carbide catalysts (e.g. Mo2C nanowires, Mo2C/CNT and Co-Mo2C/CNT) were successfully achieved, which exhibit high performance in electrochemistry and catalysis, respectively. Besides, due to the facile process, low cost and novel product structures, the synthetic strategies proposed in this work will open opportunities for the synthesis and design of functional materials. The details of each chapter are list below. In chapter 3, GeOx/ethylenediamine nanowires were firstly synthesized via Fe2O3-assited hydrothermal method, which is possible to large-scale preparation for the low cost and easy control. Through the systematic characterization, the novel crystalline structure and hybrid components are well confirmed, in which the GeOx units are connected by ethylenediamine through hydrogen bonding, forming the subnanometer structure. Strong quantum confinement effect can be considerably leaded by this subnanometer structure, reflected by the blue shift in UV spectra. Furthermore, the bright blue light can be emitted by such as-obtained nanowire, suggesting the potential application in future integrated optical nanodevices.In chapter 4, the growth of GeOx/ethylenediamine nanowires was discussed in detail. Several factors in the synthesis process, such as reacting time, temperature, amines and metal-oxide assisting agents, are systematically analyzed, and thus the growth mechanism of Fe2O3-assisted Surface-Solution-Solid was proposed. Herein, Fe2O3 leads to the formation of GeOx/ethylenediamine hybrid units on the surface, which is followed by an anisotropic growth induced by the solid-liquid interface reactions. Meanwhile, another novel structure of MoO2/ethylenediamine nanosheets was obtained through the similar growth process, indicating the potential of this mechanism as a new route to fabricating organic-inorganic hybrid nanomaterials.In chapter 5, a series of MoOx/amine nanowires were fabricate based on the anisotropic growth of molybdate crystals, including Mo3O10(C6H8N)2·2H2O,MoOx/1,6-hexanediamine and MoOx/imidazole. Detailed study was carried out on the formation of such nanowires, and the synthesis was well optimized. Furthermore, employing Mo3O10(C6H8N)2·2H2O as precursors, the controllable synthesis of MoOx/polyaniline nanowires/tubes can be realized through in-situ polymerization. The work in this chapter, not only furthers the research into organic-inorganic hybrid nanowires of MoOx, but also proposes easy strategies to enrich the system of such hybrid nanostructures, aiming the following design of functional Mo-based oxides and carbides.In chapter 6, an easy strategy was proposed to fabricate 1D oxides from organic-inorganic hybrid nanowires, regarding that the 1D nanostructures of oxides without anisotropic growth were difficult to obtain. After the calcination in air or inert flow, nanowires of oxides or carbon-hybrid metal-oxides can be easily achieved, respectively. Especially, the carbon-hybrid structures in such nanowires may enhance and improve some application in electrochemistry. For example,α-GeO2 nanowires with photoluminescence properties were obtained through calcining GeOx/ethylenediamine in air flow. Meanwhile, novel MoO2/C nanowires were successfully fabricated via the calcination of MoOx/amine nanowires under inert flow, which exhibited the high rate capability even under high current density as anode materials for Li ion battery. The potential of the synthetic strategy in design functional nanomaterials is well revealed by the universality for various nanostructures and high performance for energy-storage.In chapter 7, we proposed a novel strategy to synthesize nanostructures of carbides based on organic-inorganic hybrid composites with subnanometer periodic structures. By employing the uniform reactions between subnanometer-contacting organic and inorganic components throughout the whole hybrids, this strategy successfully avoids the disadvantages in traditional temperature-program-reduction (TPRe) method using solid-gas interface reactions. Nanoporous Mo2C nanowires can be well obtained via calcining MoOx/amine nanowires under Ar flow, which are composed of Mo2C nanoparticles and possesses large surface with little depositing carbon. These nanowires display higher H2 yield and longer lifetime than bulky Mo2C obtained by TPRe method in the probe reaction of producing H2 from methanol decomposition. Thus, the significance of synthetic strategy for fabricating carbides from organic-inorganic nanocomposites is well demonstrated.In chapter 8, the strategy of fabricating carbides from organic-inorganic nanocomposites was further extended to design of supported Mo2C catalysts, such as Mo2C/CNT, Co-Mo2C/CNT and Co-Mo2C/AC. The physical characterization shows that the active composites present individually as highly dispersive nanoparticles on the surface of support, and no alloy phase can be observed. The formation of Mo2C can be attributed to the organic-inorganic hybrid precursors, and thus this process is significant for the advantages discussed in chapter 7,such as easy control, safety and little depositing carbon on surface. In the catalytic reaction of producing H2 from methanol decomposition, supported Mo2C catalysts show the higher performance than unsupported catalysts. Significantly, the Co-Mo2C/CNT displays ultrahigh activity, selectivity and stability in such reaction, indicating the potential as efficient substitutes for the high-price noble-metal catalysts. Furthermore, the research into the effect of metallic Co during catalytic process shows that Co transfers the coke to format carbon nanotubes, improving remarkably the stability. This mechanism may benefit the design other carbide catalysts for relevant reactions. To sum up, this dissertation focus on organic-inorganic hybrid nanowires, and the organic-inorganic hybrid nanowires system of GeOx and MoOx have been established through facile hydrothermal and co-precipitation methods. Furthermore, various nanostructures of molybdenum oxides and carbides have been also realized based on the organic-inorganic hybrid nanowires, whose performance in Li ion battery and producing H2 from methanol decomposition are discussed. Besides the novel nanostructures with functional properties, several mechanism and synthetic strategies have been proposed in this work, which will open up opportunities for the synthesis and design of new nanomaterials.