The nature of graphite- and pyridinelike nitrogen configurations in carbon nitride nanotubes: dependence on diameter and helicity.
The nature of graphite- and pyridinelike nitrogen configurations in carbon nitride nanotubes: dependence on diameter and helicity.
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DOI：
10.1002/smll.200700543
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发表时间：
2008-04
期刊：
影响因子：
13.3
通讯作者：
S. Yang;W. Shin;J. Kang
中科院分区：
文献类型：
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作者：
S. Yang;W. Shin;J. Kang
自1991年发现碳纳米管（CNTs）以来，[1]由于其显着的电子特性，它们一直是深入研究的主题。[2，3]碳纳米管的电子性质强烈依赖于它们的螺旋度，直径和缺陷。[4]然而，在生长过程中精确控制这些参数仍然是非常困难的。CNT结构中的氮掺杂是定制和优化其电子特性的最容易的方法之一，因为氮可以在费米能级附近引入电子态。[5，6]以前的研究已经确定了氮键合构型的三种主要类型：（i）类石墨氮（GN），其中氮原子取代石墨碳原子;（ii）类吡啶氮（PN），其中氮原子与两个碳原子键合;和（iii）分子N2。此外，Robertson和Davis报道只有GN可以产生施主态：[7] GN使用三个价电子形成三个s键，第四个电子填充ap态，第五个电子形成p状态，这给出了p掺杂效应。相比之下，PN使用两个电子形成两个s键，其第三个电子形成一个p键，两个电子形成两个类似p的非键合状态，这使得sp2网络具有非掺杂特性。另一方面，Czerw et al. [8]提出，空氮配合物的PN键配置与碳空位也可能是负责显着的施主功能密切高于费米能级。此外，氮显着改变了碳纳米管的形态，导致竹状结构。[9]这种分隔的形状归因于在N掺杂的CNT（氮化碳纳米管）内引入氮，这意味着分隔部分和内部部分的氮浓度高于外部部分的氮浓度。[10]这表明，氮是不均匀地分布在N-掺杂的碳纳米管，因为氮的键合配置，取决于碳纳米管的直径。我们最近的两个实验结果和其他研究人员清楚地表明，理解氮原子在GN或PN构型中的排列是非常重要的。[11]例如，我们最近通过选择性掺杂sp2 N（GN）来制造N掺杂纳米管，导致场发射性能显著改善，而掺杂sp3 N（PN）导致性能恶化。[11]另一方面，我们前期的研究表明，在制备了sp3 N掺杂纳米管后，可以通过表面掺杂sp3 N原子直接制备出尺寸为几纳米的金属量子点。这些大大提高了全电池的化学活性和直接从水中产生氢气。[11]在这方面，理解具有不同直径和螺旋度的N掺杂CNT中的氮的性质在不同领域具有实际意义。在这里，我们阐明的性质的GN或PN配置的N-掺杂的碳纳米管具有不同的直径和螺旋使用第一性原理密度泛函理论（DFT）方法。氮的性质也有望能够解释氮掺杂碳纳米管的不寻常的竹状结构，这是必要的各种应用，如储氢。在这篇通讯中，考虑了两组：（n，n）扶手椅管，n = 4-8，和（n，0）锯齿形管，n = 7-11。对于这些管，我们使用具有以氢原子终止的开放端的簇模型来避免悬挂键的影响来表示CNT。对于所有的管，在B3 LYP/6- 31 G水平上进行了全几何优化，[12]并且N原子在...
Since the discovery of carbon nanotubes (CNTs) in 1991,[1] they have been the subject of intense studies because of their remarkable electronic properties.[2, 3] The electronic properties of CNTs are strongly dependent on their helicities, diameters, and defects.[4] However, to precisely control these parameters during growth is still very difficult. Nitrogen doping in CNT structures is one of the most accessible means to tailor and optimize their electronic properties, as nitrogen can introduce electronic states around the Fermi level.[5, 6] Previous investigations have identified three primary types for the nitrogen bonding configurations:(i) graphitelike nitrogen (GN), in which the nitrogen atom replaces a graphitic carbon atom;(ii) pyridinelike nitrogen (PN), in which the nitrogen atom bonds with two carbon atoms; and (iii) molecular N2. In addition, Robertson and Davis reported that only GN can generate a donor state:[7] the GN uses three of its valence electrons to form three s bonds, its fourth electron to fill ap state, and its fifth electron to form a pà state, which gives a p-doping effect. In contrast, the PN uses two electrons to form two s bonds, its third electron to make one p bond, and two electrons to form two p-like nonbonding states, which gives nondoping characteristics in sp2 networks. On the other hand, Czerw et al.[8] suggested that vacancy–nitrogen complexes in the PN bond configuration with a carbon vacancy could also be responsible for prominent donorlike features closely above the Fermi level. In addition, nitrogen significantly alters the morphology of CNTs, leading to bamboolike structures.[9] This compartmentalized shape is attributed to nitrogen introducing corrugation within N-doped CNTs (carbon nitride nanotubes), reffecting that the nitrogen concentrations of the compartment part and inner part are higher than that of the outer part.[10] This indicates that nitrogen is nonuniformly distributed in N-doped CNTs because of the bonding configuration of nitrogen, depending on the diameters of the CNTs. Our recent two experimental results and other researchers clearly indicate that understanding the arrangement of nitrogen atoms in GN or PN configurations is very important.[11] For example, our recent fabrication of N-doped nanotubes by selective doping with sp2 N (GN) resulted in significantly improved field-emission performance, while doping with sp3 N (PN) led to deteriorated performance.[11] On the other hand, our previous study indicated that, after we prepared sp3 N-doped nanotubes, metal quantum dots of a few nanometers in size could be directly fabricated via sp3 N atoms doped on the surface. These dramatically enhanced the chemical activity for a full cell and direct hydrogen generation from water.[11] In this respect, understanding the nature of nitrogen in N-doped CNTs with different diameters and helicities is of practical relevance in diverse areas. Here, we elucidate the nature of the GN or PN configurations for N-doped CNTs with different diameters and helicities using the first-principles density functional theory (DFT) method. The nature of nitrogen is also expected to be capable of explaining the unusual bamboolike structure of N-doped CNTs, which are necessary for various applications such as hydrogen storage. In this Communication, two groups are considered:(n, n) armchair tubes, with n ¼ 4–8, and (n, 0) zigzag tubes, with n ¼ 7–11. For these tubes, we used cluster models with open ends terminating in hydrogen atoms to avoid the effects of dangling bonds to represent the CNTs. For all of the tubes, full geometry optimizations were carried out at the B3LYP/6-31G level,[12] and the substitution energies of the N atoms in the …