Transition Metal Catalyst Aided Growth of Novel Carbon Nanostructures

一维碳纳米结构以其独特的形貌和新颖的性质，引起了人们的广泛关注。它们独特的原子结构不仅赋予了它们在电子输运、热导、荧光、场致发射、储氢等方面的广泛应用能力，而且还具有用作制备其它类型的1D和2D纳米结构的模板的能力。作为一维碳纳米管家族的重要成员之一，碳纳米管具有圆柱形和中空的特点，其合成策略、结构特征、生长机理以及性能和应用等方面都得到了广泛的研究。碳纳米管的合成通常采用过渡金属（Fe、Co、Ni及其合金）作为催化剂，通过催化化学气相沉积（CVD）技术进行。催化CVD技术的优点在于其在低温下以及以低成本大规模生产CNT的能力。除了CNT之外，许多其他类型的一维碳纳米结构，即纳米锥、纳米球、纳米线圈、纳米螺旋等，也通过使用不同的反应条件经由过渡金属催化的CVD工艺制造。值得注意的是，在这些氯化萘中，新型的聚合物纳米结构（PNs），其中大量的氢原子仍然存在，可以在473 K的低温下与催化铜纳米颗粒合成。与碳纳米管相比，由于石墨烯片的堆积方式和尺寸的不同，PNs呈现出许多新颖的形貌，而且，PNs还表现出与其高比表面积相关的有趣性质，如铁磁性和碳化后潜在的储氢能力。此外，PNs的形态以及性质强烈地依赖于相应催化剂的几何形状（形状和尺寸）。这些新的性质表明PNs在未来的纳米技术中具有重要的潜在应用。然而，聚合物纳米纤维的生长机理，特别是催化剂纳米颗粒的尺寸和形状对聚合物纳米纤维结构特性的影响还不清楚。另一方面，在比CNT低得多的温度下制备PNF的事实表明与在1000 K下合成的CNT的气-液-固（VLS）机制不同的生长机制。由于在Cu催化剂纳米颗粒上生长的PNF的反应温度低，并且在所有温度下碳在Cu中的溶解度低，因此PNF的生长机制被认为是表面扩散机制，这是基于低聚物在催化剂颗粒表面形成并扩散而不是碳原子在整个催化剂颗粒中溶解和传输的概念（VLS理论）。随后，发现除了Cu纳米颗粒之外，Fe在施加类似的反应条件时也具有产生PNF的活性。因此，有理由期待更多的过渡金属，如Co、Ni纳米颗粒也能够产生PNF。因此，迫切需要详细的研究形成PNF的生长热力学和动力学过程。除此之外，本提案的目的不仅是在低温下合成采用过渡金属，特别是Fe，Co和Ni，纳米颗粒作为催化剂的新型形态PNF，而且还系统地表征PNF的微观结构，并了解催化剂结构（尺寸，形状，表面电子结构）和相应的PNF结构之间的关系。在实际应用中，我们将以热化学气相沉积法为生长技术，制备不同尺寸和形状的铁、钴和镍纳米粒子作为催化剂，合成不同类型的PNF。将详细研究所得新型PNF的微观结构与负责催化剂的几何形状（形状和尺寸）之间的相关性，以建立对低温下表面扩散机制的普遍理解。该项目的成功不仅具有科学意义，而且还将为设计纳米器件提供技术推动。

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.