Transition Metal Catalyst Aided Growth of Novel Carbon Nanostructures

一维碳纳米结构(CNS)以其特殊的形貌和新颖的性质，多年来引起了人们的极大关注。它们独特的原子结构不仅赋予了它们在电子传输、热导、荧光、场发射、储氢等方面的广泛应用能力，而且还可以作为模板用于制备其他类型的一维和二维纳米结构。作为一维碳纳米管家族的重要成员之一，具有圆柱形和空心形特征的碳纳米管在合成策略、结构特征、生长机理以及性质和应用等方面受到了广泛的研究。碳纳米管的合成通常采用催化化学气相沉积(CVD)技术，以过渡金属(Fe、Co、Ni及其合金)为催化剂。催化化学气相沉积技术的优点是能够在低温下大规模生产碳纳米管，而且成本低。除了碳纳米管，许多其他类型的一维碳纳米结构，如纳米锥体、纳米晶盒、纳米线圈、纳米螺旋等，也通过不同的反应条件通过过渡金属催化的CVD工艺来制备。值得注意的是，在这些CNS中，有一种新型的聚合物纳米结构(PNS)可以在473K的低温下用催化的铜纳米粒子合成，其中有大量的氢原子残留在其中。由于石墨烯的堆积方式和尺寸不同，PNS表现出比碳纳米管更新颖的形貌，而且还表现出与其高比表面相关的有趣的性质，如铁磁性和碳化后的潜在储氢能力。此外，PNS的形态和性质强烈依赖于相应催化剂的几何形状(形状和大小)。这些新颖的性质表明，PNS在未来的纳米技术中具有重要的潜在应用前景。然而，聚合物纳米纤维的生长机理，尤其是催化剂纳米颗粒的尺寸和形状对其结构特征的影响尚不清楚。另一方面，纳米碳纳米管的生长温度远低于碳纳米管，这与1000K合成的碳纳米管的气-液-固(VLS)生长机理不同。由于纳米铜催化剂上生长的碳纳米管的反应温度较低，碳在铜中的溶解度较低，因此碳纳米管的生长机理应为表面扩散机理，该机理基于低聚物在催化剂颗粒表面形成和扩散的概念，而不是碳原子在催化剂颗粒中的溶解和传输(VLS理论)。随后发现，在相似的反应条件下，除铜纳米颗粒外，铁也具有生成PNFS的活性。因此，有理由期待更多的过渡金属，如Co，Ni纳米粒子也能够产生PNF。因此，迫切需要对PNF的生长热力学和动力学过程进行详细的研究。此外，本研究的目的不仅在于利用过渡金属，特别是Fe、Co、Ni纳米粒子作为催化剂，在低温下合成新型形态的PNFS，而且系统地表征PNF的微观结构，了解催化剂结构(尺寸、形状、表面电子结构)与相应PNFS结构之间的关系。在实际应用中，利用热化学气相沉积技术，制备了不同尺寸和形状的Fe、Co、Ni纳米粒子作为催化剂，合成了不同类型的PNF。为了建立对低温下表面扩散机理的普遍理解，我们将详细研究合成的新型PNF的微观结构与相应催化剂的几何形状(形状和大小)之间的关系。该项目的成功不仅具有科学意义，还将在设计纳米设备方面提供技术推动。

With their special morphologies and novel properties, the one-dimensional (1D) carbon nanostructures (CNs) have attracted a great attention over many years. Their unusual atomic architecture endows not only their extensive application capability in electronic transport, thermal conductivity, fluorescence，field emission, hydrogen storage etc., but also their ability to be used as templates for preparing other types of 1D and 2D nanostructures. As one significant member of the 1D CN’s family, carbon nanotubes (CNTs), with cylindrical and hollow characteristics, are extensively investigated with regards to their synthesis strategies, structural characteristics, growth mechanism and moreover with regards to their properties and applications. The synthesis of CNTs is usually carried out by catalytic chemical vapor deposition (CVD) techniques by employing transition metals (Fe, Co, Ni and their alloys) as catalysts. The advantage of the catalytic CVD techniques is in their large-scale production ability of the CNTs at low temperatures as well as at low costs. Besides CNTs, many other types of one dimensional carbon nanostructures namely nanocones, nanobells, nanocoils, nanohelixes etc., are also fabricated via transition metal catalyzed CVD processes by using different reaction conditions. It is interesting to note that, among these CNs, new types of polymer nanostructures (PNs), in which remarkable amount of hydrogen atoms remain, can be synthesized at a low temperature of 473 K with catalytic Cu nanoparticles. Because of different stacking modes and sizes of graphene sheets, PNs show much novel morphology compared with those of CNTs, and furthermore, they show interesting properties related to their high specific surface, such as ferromagnetic properties and potential hydrogen storage capability after carbonization. Additionally, the morphologies as well as properties of PNs strongly depend on geometries of corresponding catalysts (shape and size). These novel properties indicate that the PNs have significant potential application in future nanotechnology. However, the growth mechanism of the polymer nanofibers (PNFs), especially the size and shape effect of catalyst nanoparticles on the structure characteristic of the PNFs is still unclear. On the other hand, the fact that the PNFs were produced at much lower temperature than that of CNTs indicates a distinct growth mechanism from the vapor-liquid-solid (VLS) mechanism of CNTs synthesized at 1000 K. Due to the low reaction temperature of PNFs grown on Cu catalyst nanoparticles and the low solubility of carbon in Cu at all temperature, the growth mechanism of the PNFs is supposed to be surface diffusion mechanism, which is based on the concept that oligomers form and diffuse on catalyst particles surface instead of dissolving and transporting of carbon atoms throughout catalyst particles (VLS theory). Subsequently, Fe besides Cu nanoparticles were found to be also active to produce PNFs when similar reaction conditions were applied. Accordingly, it is reasonable to expect more transition metals, such as Co, Ni nanoparticles are also able to produce PNFs. Therefore detailed investigations of growth thermodynamic and kinetic processes of forming PNFs are urgently required. Beyond that, the aim of this proposal is not only to synthesize novel morphological PNFs employing transition metals, especially Fe, Co and Ni, nanoparticles as catalysts at low temperature, but also to systematically characterize the microstructures of PNFs and to understand the relationships between catalyst structures (size, shape, surface electronic structure) and corresponding PNFs structures. Practically, Fe, Co and Ni, nanoparticles with various size and shape will be prepared as the catalysts to synthesize different types of PNFs by using thermal CVD as the growth technique. The correlation between the microstructures of the resultant novel PNFs and the geometry (shapeand size) of the responsible catalysts will be researched in detail in order to establish a generalized understanding on the surface diffusion mechanism at low temperature. The success of this project will have not only a scientific significance but will also provide a technological boost in designing nanodevices.