Functionalization of Nanotube Surfaces Part 1 (Nanotechnology)

INTRODUCTION

Carbon is one of the most prevalent elements in nature, and is critical to a number of important biological and geological structures. Of importance to chemistry, it can form sp-, sp2-, and sp3-hybridized bonds permitting the production of a wide range of geometries and hence, compounds. Graphite, albeit a key component of pencils, has one of the largest in-plane elastic moduli of any material, because of the extended sheets of sp2-hybridized carbon arranged in a honeycomb mesh of hexagons. Diamond is the gem of choice, although it is simply a ther-modynamically metastable form of carbon with the largest hardness of any material; it has the highest thermal conductivity and melting point of any solid. Fullerenes,[1-3] discovered by Kroto et al. in 1985, were produced by using laser vaporization techniques in the gas phase and were initially serendipitously detected using a molecular beam apparatus. They now have been reported to occur naturally in some forms of carbon soot[4] and have been produced by resistive heating of carbon rods in a va-cuum,[5] in plasma discharges between carbon electrodes in He, and by oxidative combustion of gasoline/benzene/ argon gas mixtures.[6] These molecules, essentially truncated icosahedrons, can be viewed as graphite sheets wrapped into closed carbon cages, consisting of 60 atoms shaped into a soccer ball with pentagons that can then accommodate curvature in the system.[2]

Another potentially more useful class of materials has arisen from the generation of fullerenes, that is essentially a nanometer-scale self-assembled structure of carbon. This time, however, we are not referring to cake and sandwich structures but rather to graphitic carbon, forming seamless cylindrical shells,[7] the carbon nanotubes. Of an intrinsic structural beauty,[8,9] these tubes consist of shells of sp2-hybridized (trivalent) carbon atoms forming a hexagonal network that is itself arranged helically within the tubular motif (Fig. 1). Nanotubes are observed in two distinct structural forms (which can be controlled by growth conditions): multiwall nanotubes (MWNTs), which are made by coaxially nesting successively larger nanotubes[10] separated roughly by the interplanar graphite spacing of ~ 0.34 A, and single-wall nanotubes (SWNTs), which consist of a single graphene sheet rolled into a seamless tube.[11-14]


OVERVIEW

The diameter and helicity of a defect-free SWNT can be uniquely characterized by the roll-up vector, ch = na+mb = (n,m), connecting crystallographically equivalent sites on a two-dimensional (2-D) graphene sheet, where a and b are the graphene lattice unit vectors, and n and m are integers. Each pair of integers (n,m) defines a different way of rolling up the graphene sheet to form a carbon nanotube. Electronic band structure calculations predict that the (n,m) indices will determine whether a SWNT will be a metal or a semiconductor.

To first order, (n,0) or zigzag SWNTs will exhibit two distinct types of behavior. The nanotubes will be metallic when n/3 is an integer, but otherwise semiconducting. So as ch rotates away from (n,0), chiral (n,m) SWNTs are possible with similar electronic properties to zigzag tubes. When (2n+m)/3 is an integer, the tubes are metallic; otherwise, they are semiconducting. If ch is rotated 30° relative to (n,0), m=n. These (n,n) or armchair tubes are metallic. Zigzag and armchair tubes with wrapping angles of 0° or 30° are called achiral tubes. Single-walled nanotubes with intermediate wrapping angles are referred to as chiral tubes. Both (n,0) and (n,n) nanotubes have especially high symmetry and exhibit a mirror symmetry plane normal to the tubule axis.[9] In other words, their structure determines their electronic behavior.

Because of their seamless cylindrical graphitic shells, carbon nanotubes[16] might be stiffer and stronger than potentially any other known material with implications for the design of composite materials, as well as nanometer-scale devices.[17] In fact, mechanical measurements have shown that multiwalled nanotubes have a very high stiffness and, in fact, a high Young’s modulus in the TPa range, at least as large if not higher than the in-plane modulus of graphite.[18] By measuring the amplitude of the intrinsic thermal vibrations of MWNTs, one group has shown that the Young’s modulus of elasticity (the ratio of the force per unit area applied to the nanotube to the deformation thereby produced) is about 1 x 1012 Pa or 1 x 1012 N/m2, which is among the highest of any known material. Furthermore, bending and buckling experiments1-19-1 indicate that MWNTs can be repeatedly bent through large angles without undergoing catastrophic failure, and that they can sustain forces that will deform a local area of the nanotube by as much as 16% without irreversible damage. Thus not only do these tubes possess extremely desirable mechanical properties of strength and flexibility,1-20-1 but also, as has been noted, at least some are metallic in character.[21]

 A single-wall carbon nanotube (SWNT) may be visualized as a sheet of graphite rolled up to form a seamless cylinder that is 1 atom in thickness. The roll-up vector or direction determines whether the tube is metallic or semiconducting, and tubes are accordingly classified as either zigzag, armchair, or chiral.

Fig. 1 A single-wall carbon nanotube (SWNT) may be visualized as a sheet of graphite rolled up to form a seamless cylinder that is 1 atom in thickness. The roll-up vector or direction determines whether the tube is metallic or semiconducting, and tubes are accordingly classified as either zigzag, armchair, or chiral.

Indeed, the fundamental importance and uniqueness of SWNTs, relative to other types of fibrous materials such as Kevlar, arises from their molecularly ”perfect” structure; there are no or relatively few imperfections within the length of the material that would cause weakness or instability. Furthermore, no surface passivation is required of these seamless cylinders. They are thought to have a host of wide-ranging, potential applications including catalyst supports in heterogeneous catalysis, field emitters, high-strength engineering fibers, sensors, actuators, tips for scanning probe microscopy, gas storage media, and as molecular wires for the next generation of electronics devices.[22-30] Understanding the chemistry of SWNTs thus becomes critical to rational manipulation of their properties. In fact, the ability to disperse and solubilize carbon nanotubes would open up new prospects in aligning and forming molecular devices and allow for the development of a reproducible chemistry of these materials to take place. Nonetheless, this objective necessitates controlled chemical functionalization of tubes, a relatively unexplored area of research.

In this article, we will explore existing strategies of nanotube functionalization, ending with recent efforts to treating nanotubes as chemical reagents (be it inorganic or organic) in their own right or in understanding chemical reactivity involving tubes from a structural perspective, which would expand the breadth and types of reactions SWNTs can undergo in solution phase. Indeed, from their inherent structure, one can view nanotubes as sterically bulky, p-conjugated ligands or conversely, as electron-deficient alkenes. Moreover, controllable chemical func-tionalization suggests that these unique electronic and mechanical properties can be tailored in a determinable manner. For instance, it has been predicted[31] that covalent chemical attachments can decrease the maximum buckling force by about 15% regardless of tubular helical structure or radius. Prevailing themes in nanotube func-tionalization have been involved with dissolution of tubes, enabling more precise manipulation and fraction-ation of these materials, as well as predictable functiona-lization of nanotube surfaces, at the ends (Fig. 2) as well as the sidewalls (Fig. 3). Indeed, controllable derivatiza-tion is essential for advanced functionalized material applications, as well as for the efficient generation of SWNT nanocomposites.

SIDEWALL FUNCTIONALIZATION OF SWNTS

The sidewalls of purified SWNTs have been noncova-lently functionalized[32- with a bifunctional molecule, namely 1-pyrenebutanoic acid, succinimidyl ester, which p stacks onto the graphene sidewalls; amide groups on proteins then react with the anchored ester to form amide bonds for protein immobilization. In an analogous fashion, ferritin, streptavidin, and biotin have been similarly localized on nanotube surfaces with high specificity and efficiency. In another approach to immobilizing proteins on the nanotube surface, SWNTs have been func-tionalized with bovine serum albumin (BSA) proteins via a diimide-activated amidation under ambient conditions. Based on data such as atomic force microscopy (AFM), thermal gravimetric analysis, Raman spectroscopy, and gel electrophoresis, the resultant nanotube-BSA conjugates are structurally interlinked, highly water-soluble, and bioactive, forming dark-colored aqueous solutions.[33] It turns out that Raman spectroscopy, in particular, is notably effective at denoting the presence and extent of functionalization as well as for understanding vibrational and electronic properties of nanotubes and fullerenes. Whereas the radial breathing modes (RBM)[34,35] near 200 cm-1 depend sensitively on tube diameter, the high-frequency tangential displacement G modes[36] near ~ 1590 cm-1 and the second-order G’ bands[37] near 2600 cm-1 are sensitive to the charge exchanged between nanotubes and guest atoms that have intercalated into the interstitial channels in the tube bundles. Indeed, the line shapes of the G band allow for differentiation between semiconducting and metallic tubes. Semiconducting tubes are characterized by sharp Lorentzians while metallic tubes show Brit-Wagner-Fano line broadening. Importantly, in the context of this review, the shape and intensity of a weak disorder mode peak at 1290-1320 cm- 1 has been correlated with the extent of nanotube sidewall functionalization.[38]

 (a) Schematic of a common functionalization route, used to derivatize SWNTs at ends and defect sites. SWNTs are acid-treated to generate carboxylic acid groups at the ends and defect sites. These acidic functionalities are then linked to amines either by an initial reaction with SOCl2 and subsequently with an amine, or though carbodiimide-mediated amide bond formation with amines. An alternative route to functionalization is through zwitterionic interactions between carboxylic acid and amine groups. Panel (b) shows an example where oxidized SWNTs were linked to an amino-derivatized crown ether moiety.

Fig. 2 (a) Schematic of a common functionalization route, used to derivatize SWNTs at ends and defect sites. SWNTs are acid-treated to generate carboxylic acid groups at the ends and defect sites. These acidic functionalities are then linked to amines either by an initial reaction with SOCl2 and subsequently with an amine, or though carbodiimide-mediated amide bond formation with amines. An alternative route to functionalization is through zwitterionic interactions between carboxylic acid and amine groups. Panel (b) shows an example where oxidized SWNTs were linked to an amino-derivatized crown ether moiety.

Much effort has been involved with fluorinating tubes. High-temperature (over 500°C for several hours) reactions using pure fluorine gas destroyed nanotube structure, whereas at room temperature, transmission electron microscopy (TEM), X-ray diffraction, and Fourier transform-infrared (FT-IR) spectroscopy showed that fluorine-SWNT intercalation compounds could be formed.[39] Similarly, MWNTs[40] can be fluorinated at room temperature by using a gaseous mixture of BrF3 and Br2. SWNTs fluorinated at temperatures ranging from 150 to 325°C survived the fluoridation process. Two major diagnostic features[41] in the Raman spectra of the fluorinated tubes are 1) a decrease in intensity of the radial breathing mode near 200 cm- 1 and 2) an increase in the intensity of the disorder D band at ~ 1320 cm-1, which are attributable to the destruction of the intrinsic sp2-hybridized structure as C—F bonds are created and sidewall carbons become sp3-hybridized. These fluori-nated tubes can also be solubilized in various alcohol solvents via ultrasonication. Scanning tunneling microscopy (STM) images (Fig. 4) of fluorinated tubes reveals a dramatic banded structure indicating broad continuous regions of fluorination terminating abruptly in bands orthogonal to the tube axis. These results suggest that the fluorine adds more favorably around the circumference of the tube, backed up by semiempirical AM1 calcula-tions.[42] Further calculations show that the F atoms tend to bond next to each other.[43]

Schematic demonstrating various sidewall functionalization reactions of SWNTs.

Fig. 3 Schematic demonstrating various sidewall functionalization reactions of SWNTs.

Sidewall-fluorinated nanotubes can be further modified by reacting them with alkyl magnesium bromides in a Grignard synthesis or by reaction with alkyllithium precursors to yield sidewall-alkylated nanotubes, via a concerted, allylic displacement mechanism.[44] These routes render SWNTs soluble in various organic solvents such as chloroform, methylene chloride, and tetrahydro-furan. Covalent attachment to the sidewalls was confirmed by ultraviolet (UV)-visible (vis)-near IR spec-troscopy, in which optical features originating from transitions between the van Hove singularities in the 1-D electron density of states of the tube disappear as the electronic structure becomes perturbed.

Oxidizing these alkylated nanotubes in air allows for recovery of pristine nanotubes. As for fluorinated tubes, it was found that hydrazine could be used to regenerate the unfluorinated starting material;[45] a procedure was also established to ”tune” the fluorine content of the tubes by first fluorinating heterogeneously, solvating in alcohol, and then defluorinating with substoichiometric quantities of hydrazine to generate tubes with different fluorine contents.[46] Heating at 100°C in noble gases of fluori-nated tubes can also lead to recovery of pristine tubes, as determined by a decrease in the IR intensity of C—F stretch bands at 1207 cm-1, reappearance of bands for the A2u mode at 867 cm-1 and E1u mode at 1579 cm-1, as well as the reduced resistance of nanotubes samples.[47]

Scanning tunneling microscopy (STM) images. (a) An image of a SWNT fluorinated at 250°C for 12 hr. The approximate carbon-to-fluorine ratio by microprobe analysis is 2:1. (b) An image of a SWNT fluorinated at 150°C for 5 hr. The corresponding carbon-to-fluorine ratio, in this case, by micro-probe analysis is 3.7:1. The darker area on the left side of the image appears to be a less-fluorinated area. (c) A high-resolution image of a carbon nanotube fluorinated at 250°C for 1 hr, obtained by using a C60 functionalized STM tip.

Fig. 4 Scanning tunneling microscopy (STM) images. (a) An image of a SWNT fluorinated at 250°C for 12 hr. The approximate carbon-to-fluorine ratio by microprobe analysis is 2:1. (b) An image of a SWNT fluorinated at 150°C for 5 hr. The corresponding carbon-to-fluorine ratio, in this case, by micro-probe analysis is 3.7:1. The darker area on the left side of the image appears to be a less-fluorinated area. (c) A high-resolution image of a carbon nanotube fluorinated at 250°C for 1 hr, obtained by using a C60 functionalized STM tip.

There are a number of other sidewall functionalization strategies. Single-wall nanotubes have been chemically functionalized in situ by a thermally induced reaction with diazonium compounds, generated by action of isoamyl nitrite on aniline derivatives[48] (Fig. 3). Tetra-tert-butylphthalocyanines have been immobilized on SWNTs through a p-p interaction.[49] Direct addition to the unsaturated p-electron systems of SWNTs can occur through the following: 1) a [2 + 1] cycloaddition of nitrenes; 2) the addition of nucleophilic carbenes; and 3) the addition of radicals (through the photoinduced addition of perfluorinated alkyl radicals).[50] Tubes func-tionalized in these ways enable a large variety of functional groups to be introduced onto nanotube surfaces with implications for nanocomposite formation (Fig. 3).

Recently, our group was involved with developing[51] an ozonolysis protocol (Fig. 5) involving treatment of tubes at – 78°C, followed by reaction with various reagents, in independent runs, to generate a higher proportion of carboxylic acid/ester, ketone/aldehyde, and alcohol groups, respectively, on the nanotube surface. This ”one-pot” oxidative methodology has three major consequences: first, the purification of as-prepared SWNTs to obtain a high-quality product; second, the chemical functionalization of nanotube sidewalls; and third, a systematic procedure to select for particular distributions of oxygenated functional groups in the resultant purified SWNTs.

Significantly, this work bears out the theoretical prediction that 1,3 dipolar cycloaddition to the SWNT sidewalls is experimentally achievable. In fact, these experiments indicate that a certain degree of control over the presence of particular functional groups on the surfaces of these SWNTs can be obtained by judicious choice of the appropriate cleaving agent, all of which can be achieved with minimal sample loss. More importantly, this protocol presents a nondestructive, low-temperature method of introducing oxygenated functionalities directly onto the sidewalls and not simply at the end caps of SWNTs. Thus molecular species, which can be currently tethered to the ends of the nanotubes, can now be chemically dispersed along the sidewalls as well, with implications for the design of nanotube-based optoelectronic devices and well-dispersed nanocomposites.

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