The doping of carbon nanotubes with nitrogen and their potential applications
Introduction
Carbon (C) nanotubes (NT) nowadays represent one of the most active research fields. The literature regarding the study of properties and applications of pristine C-NT has progressed enormously in the recent years and a great interest has been lately observed towards improving and controlling their properties through different functionalization methods. A modification of the crystalline nanotube properties by controllably placing defects or foreign atoms (heteroatoms) brings along tremendous technological implications [1], [2], [3], [4], which justifies the number of experimental and theoretical investigations focused on this topic. Although some work has been done in different ways regarding endohedral doping and intercalation, the case of substitutional doping has still some difficulties to overcome.
As in other crystalline solid structures, the position of heteroatoms in the C-NT network must not exclusively be substitutional to affect the properties of the structures. In fact, the properties of nanoscopic objects depend crucially on the position of each atom [5], [6], [7], [8], [9], [10], [11], [12]. Bearing in mind that N contains one additional electron as compared to C, novel electronic properties can be expected if N atoms directly substitute C atoms in the graphitic lattice and one could anticipate that they would generate an n-type material [13]. However, a direct substitution of the C atoms is not the only possibility to incorporate N in the nanotube assembly. Due to its size, N can also generate a defect in the tube structure keeping the heteroatoms on the walls, requiring a rearrangement of the neighboring C atoms. In this case the n-type behavior is not immediately inferred. The electronic behavior depends on the new geometry generated which involves a different wall structure, and one cannot discard that this new arrangement makes a p-type doping also feasible. Defects, direct substitution of heteroatoms and atom rearrangements are the so-called on-wall dopants. However, some atoms or molecules can also be encapsulated in the hollow core of the tubes (endohedral doping) or trapped within bundles intercalated between the outer shells of the tubes (exohedral doping), as illustrated in Fig. 1. Intercalation and endohedral doping in C-NT with atoms or molecules have exhibited peculiar changes. In fact, In all these cases, the controlled modification of the C-NT properties introducing has recently received considerable attention.
Nitrogen atoms incorporated in the C-NT structure represent for several reasons a practical and illustrative case. If a carbon single-walled nanotube (C-SWNT) is doped with a foreign N atom, its outstanding electronic properties differ drastically from an undoped C-SWNT. Even the cases of multi-walled (MW) and C-SWNTs doped with N are radically different. As compared to bulk doped carbons and also against other doped carbon layered structures, (i.e. bulk doped graphite), the electronic properties of C-NTs differ due to quantum confinement effects and the curvature of the cylinders.
This document deals extensively with the effects related to the incorporation of N in C-NTs. However, we devote part of the following sections to give a fundamental insight into the facts related of N incorporation in other carbon systems that are certainly covered in a broader context elsewhere [14], [15], [16]. The properties of doped C-NT and heteronanotubes formed of stable carbon and nitrogen-containing stoichiometries are described separately. Also a brief summary of the state-of-the-art synthesis of these materials is given, to finally conclude with the potential and already achieved applications.
Here it is important to remark that in most of the literature related to nitrogen-doped C-NTs the word doping is still not used in a standard way. This is related to a conceptual matter to which we will dedicate great part of Sections 2 Nitrogen-containing carbon systems, 3 Nitrogen-containing nanotubes, 4 C. To avoid confusion “SWNT” and “MWNT” will be used in this document to abbreviate single- and multi-walled nanotubes, regardless their atomic structure. The atomic composition will be identified with the atomic symbols as prefixes (i.e. C-SWNT stands for a pristine single-walled carbon nanotube).
Section snippets
Nitrogen-containing carbon systems
The properties of carbon based structures are intrinsically linked to the type of hybridization in which the carbon atoms bond with neighboring atoms (i.e. sp1, sp2, sp3). There are several review features that provide clear descriptions how different carbon materials form according to these types of hybridizations [15], [17]. Historically, the studies on C systems have developed in a way that nitrogen has always been taken into account. This is clearly due to the size proximity of N in
Nitrogen-containing nanotubes
The first studies analyzing the doping effects on C-NTs appeared in 1993 [32], not much later than Iijima’s reports on transmission electron microscopy observations were first published [33], [34]. This is just one of the many examples that inspired the investigation of nitrogen doping in C-NT.
Starting from a conceptual viewpoint: a carbon tubular nanostructure, which contains N atoms may not always be called a N-doped carbon nanotube. This is undoubtedly far from being a triviality.
CxNy heteronanotubes
As discussed above, individual N heteroatoms can substitute C atoms in the lattice or probably create defects in the tube walls. As mentioned before, from semiconductor physics the doping concept is related to a doping level in the order of parts per million. However, the cases in which higher heteroatom incorporation occurs must also be considered and in this context the concept of a CxNy heteronanotube must be taken into account. Keeping in mind that a C-SWNT can be pictured as a rolled-up
Synthesis of nitrogen-containing nanotubes
Progress in the synthesis of CNx-NT with the conventional methods used to produce C-NT is being made and it is now possible to produce multi-walled structures in a very controlled manner. Before any further discussion, although it might appear as a triviality, it is worth mentioning that many of the efforts devoted to the synthesis of N-containing MWNT aim at obtaining the highest record on N incorporation. However, the final objective should not only be to reach the maximum possible amount N
Applications
From the beginning of this manuscript the importance of doping amounts, bonding environments, nanotube stoichiometry and doping levels have been emphasized. The type of N-containing nanotube and the amount of N incorporation should be pursued bearing in mind their later application. This section summarizes the directions to which this research is oriented focuses nowadays.
First regarding C-MWNTs with N wall doping, the defects and rugosity of the outer walls are particularly appealing. This is
Conclusions
If doped C-NT are to be used as building blocks in applications, it is imperative to fine tune their physical properties such as wall reactivity, mechanical performance and electronic behavior by controlling the amount of foreign atoms inserted into the tube lattices. This manuscript reviews specifically the effects of nitrogen doping in C-NT starting with an overview of the changes in the physical and chemical properties upon nitrogen incorporation. Single and multi-walled NT have been treated
Acknowledgements
R.A. acknowledges the financial support of the European Commission under the 6 Framework Programme (STREP Project BNC Tubes) Contract (Number NMp4-CT-2006-033350). M.H.R., P.A. and T.P acknowledge the financial support of the following project DFG PI 440/4-6. P.A. thanks fruitful discussions with M. Terrones as well as the assistance of S. Daothong and J. Parjanne during the preparation of this manuscript. A.R. acknowledges funding by the Spanish MEC (FIS2007-65702-C02-01), “Grupos Consolidados
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