Selective chemistry of single-walled nanotubes

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Selective chemistry of single-walled nanotubes is a field in Carbon nanotube chemistry devoted specifically to the study of functionalization of single-walled carbon nanotubes.

Carbon nanotube chemistry

Carbon nanotube chemistry involves chemical reactions, which are used to modify the properties of carbon nanotubes (CNTs). CNTs can be functionalized to attain desired properties that can be used in a wide variety of applications. The two main methods of CNT functionalization are covalent and non-covalent modifications.

Contents

Structure and reactivity

Reactivity of fullerene molecules with respect to addition chemistries is strongly dependent on the curvature of the carbon framework. Their outer surface (exohedral) reactivity increases with increase in curvature. In comparison with fullerene molecules single-walled nanotubes (SWNTs) are moderately curved. Consequently, nanotubes are expected to be less reactive than most fullerene molecules due to their smaller curvature, but more reactive than a graphene sheet due to pyramidalization and misalignment of pi-orbitals. The strain of a carbon framework is also reflected in the pyramidalization angle (Өp) of the carbon constituents. Trigonal carbon atoms (sp2 hybridized) prefer a planar orientation with Өp=0° (i.e. graphene) and fullerene molecules have Өp= 11.6°. The (5,5) SWNT has Өp~6° for the sidewall. Values for other (n,n) nanotubes show a trend of increasing Өp (sidewall) with decrease in n. Therefore, generally the chemical reactivity of SWNT increases with decrease in diameter (or n, diameter increases with n). Apart from the curvature SWNT reactivity is also highly sensitive to chiral wrapping (n,m) which determine its electronic structure. Nanotubes with n - m = 3i (i is an integer) are all metals and rest are all semiconducting (SC). [1]

Fullerene allotrope of carbon

A fullerene is an allotrope of carbon in the form of a hollow sphere, ellipsoid, tube, and many other shapes and sizes. Spherical fullerenes, also referred to as Buckminsterfullerenes or buckyballs, resemble the balls used in association football. Cylindrical fullerenes are also called carbon nanotubes (buckytubes). Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings. Unless they are cylindrical, they must also contain pentagonal rings.

Graphene bi-dimensional crystalline structure of carbon

Graphene is an allotrope (form) of carbon consisting of a single layer of carbon atoms arranged in a hexagonal lattice. It is a semimetal with small overlap between the valence and the conduction bands. It is the basic structural element of many other allotropes of carbon, such as graphite, charcoal, carbon nanotubes and fullerenes.

Pi bond

In chemistry, pi bonds are covalent chemical bonds where two lobes of an orbital on one atom overlap two lobes of an orbital on another atom and this overlap occurs laterally. Each of these atomic orbitals has zero electron density at a shared nodal plane, passing through the two bonded nuclei. The same plane is also a nodal plane for the molecular orbital of the pi bond.

Sidewall functionalization

Carbon nanotubes are metallic or semiconducting, based upon delocalized electrons occupying a 1-D density of states. However, any covalent bond on SWNT sidewall causes localization of these electrons. In the vicinity of localized electrons, the SWNT can no longer be described using a band model that assumes delocalized electrons moving in a periodic potential.

A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor used in laser diodes, solar cells, microwave frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

Density of states a system describes the number of states per interval of energy at each energy level available to be occupied

In solid state physics and condensed matter physics, the density of states (DOS) of a system describes the number of states per an interval of energy at each energy level available to be occupied. It is mathematically represented by a density distribution and it is generally an average over the space and time domains of the various states occupied by the system. A high 'DOS' at a specific energy level means that there are many states available for occupation. A DOS of zero means that no states can be occupied at that energy level. The DOS is usually represented by one of the symbols g, ρ, D, n, or N.

Covalent bond chemical bond that involves the sharing of electron pairs between atoms

A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration.

Two important addition reactions of SWNT sidewall are: (1) Fluorination, and (2) Aryl diazonium salt addition. These functional groups on SWNT improve solubility and processibility. Moreover, these reactions allow for combining unique properties of SWNTs with those of other compounds. Above all, the selective diazonium chemistry can be used to separate the semiconducting and metallic nanotubes.

Diazonium compound

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group R−N+
2X−
 where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halogen. Alkyldiazonium compounds are generally unstable and nonisolable, due to the extreme leaving group ability of N2 in SN1/E1 (secondary and tertiary alkyldiazonium salts) or SN2 (methyl and primary alkyldiazonium salts) substitution and elimination reactions. They have been used as substrates in physical organic chemistry studies, but their uncontrolled reactivity generally renders them synthetically unimportant. (As an exception, a methyldiazonium carboxylate ion pair is believed to be a fleeting intermediate in the methylation of carboxylic acids by diazomethane). On the other hand, aryldiazonium salts are more stable (though still dangerously explosive under certain conditions) and are highly versatile reagents for chemical synthesis and important intermediates in the organic synthesis of azo dyes.

Solubility Capacity of a designated solvent to hold a designated solute in homogeneous solution under specified conditions

Solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.

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Fluorination

The first extensive SWNT sidewall reaction was fluorination in 1998 by Mickleson et al. These fluorine moieties can be removed from the nanotube by treatment in hydrazine and the spectroscopic properties of the SWNT can be restored completely. [2]

Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation.

Diazonium chemistry

One of the most important SWNT sidewall reaction is that with diazonium reagent which if done under controlled conditions can be used to do selective covalent chemistry.

Water-soluble diazonium salts react with carbon nanotubes via charge transfer in which they extract electrons from SWNT and form a stable covalent aryl bond. This covalent aryl bond forms with extremely high affinity for electrons with energies near the Fermi level, Ef of the nanotube. Metallic SWNT have a greater electron density near Ef resulting in their higher reactivity over semiconducting nanotubes. The reactant forms a charge-transfer complex at the nanotube surface, where electron donation from the latter stabilizes the transition state and accelerates the forward rate. Once the bond symmetry of the nanotube is disrupted by the formation of this defect, adjacent carbons increase in reactivity and initial selectivity for metallic SWNT is amplified. Under carefully controlled conditions this behavior can be exploited to obtain highly selective functionalization of metallic nanotubes to the near exclusion of the semiconductors. [3] [4]

Aryl

In the context of organic molecules, aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams.

The Fermi level chemical potential for electrons of a body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by µ or EF for brevity. The Fermi level does not include the work required to remove the electron from wherever it came from. A precise understanding of the Fermi level—how it relates to electronic band structure in determining electronic properties, how it relates to the voltage and flow of charge in an electronic circuit—is essential to an understanding of solid-state physics.

A charge-transfer complex or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor and the receiving species is called the electron acceptor.

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Selective reaction conditions

Primary condition is addition of reactant molecules at a very small rate to SWNT solution for a sufficient long time. This ensures reaction with only metallic SWNTs and with no semiconducting SWNTs as all the reactant molecules are taken up by the metallic SWNTs. Long time injection ensures that all metallic tubes are reacted. For example, one highly selective condition is: addition of 500 µL of 4-hydroxybenzene diazonium tetrafluoroborate solution in water (0.245 mM) at an injection rate of 20.83 µL/h into 5 mL of SWNT solution (1 wt % sodium dodecyl sulfate (SDS)) over 24 hrs. However, if the entire diazonium solution is added all instantaneously then semiconducting SWNTs will also react due to presence of excess reactant. [4]

Spectroscopy and functionalization

SWNTs have unique optical and spectroscopic properties largely due to one-dimensional confinement of electronic and phonon states, resulting in so-called van Hove singularities in the nanotube density of states (DOS).

Probing selective chemistry via optical absorption

Optical absorption monitors the valence (v) to conduction (c) electronic transitions denoted Enn where n is the band index. The E11 transitions for the metallic nanotubes occur from ~440 to 645 nm. The E11 and E22 transitions for the semiconducting nanotubes are found from 830 to 1600 nm and 600 to 800 nm, respectively. These separated absorption features allow for the monitoring of valence electrons in each distinct nanotube. Reaction at the surface result in localization of valence electrons makes them no longer free to participate in photoabsorption which results in decay of the spectrum features. [3] [5]

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Selective diazonium chemistry abruptly decreases the peak intensities that represent the first Van Hove transition of metallic species (E11, metal), while the peak intensities representing the second (E22, semiconducting) and first (E11, semiconducting) Van Hove transition of the semiconducting species show little or no change. A relative decrease in metallic SWNT absorption features over semiconducting features represents a highly preferential functionalization of the metallic nanotubes. [3] [5]

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Raman spectroscopy

Raman spectroscopy is a powerful technique with wide-ranging applications in carbon nanotube studies. Some important Raman features are radial breathing mode (RBM), tangential mode (G-band), and disorder-related mode.

RBM features correspond to the coherent vibration of the C atoms in the radial direction of the nanotube. These features are unique in carbon nanotubes and occur with the frequencies ωRBM between 120 and 350 cm−1 for SWNT in the diameter range (0.7 nm-2 nm). They can be used to probe the SWNT diameter, electronic structure through their frequency and intensity (IRBM) respectively and hence perform an (n,m) assignment to their peaks. The addition of the moiety to the sidewall of the nanotube disrupts the oscillator strength that gives rise to RBM feature and hence causes decay of these features. These features are distinct for species of a particular nanotube (n,m) and hence enables to probe which SWNTs are functionalized and to what extent.

Two main components of the tangential mode include G+ at 1590 cm−1 and G− at 1570 cm−1. G+ feature is associated with carbon atom vibrations along the nanotube axis. The G− feature is associated with vibrations of carbon atoms along the circumferential direction. The G-band frequency can be used (1) to distinguish between metallic and semiconducting SWNTs, and (2) to probe charge transfer arising from doping a SWNT. Frequency of G+ is sensitive to charge transfer. It upshifts for acceptors and downshifts for donors. Lineshape of G− is highly sensitive to whether SWNT is metallic (Breit-Wigner-Fano lineshape) or semiconducting (Lorentzian lineshape).

The disorder-related mode (D peak) is a phonon mode at 1300 cm−1 and involves the resonantly enhanced scattering of an electron via phonon emission by a defect that breaks the basic symmetry of the graphene plane. This mode corresponds to the conversion of a sp2-hybridized carbon to a sp3-hybridized on the surface. Intensity of D peak measures covalent bond made with the nanotube surface. This feature does not increase as a result of surfactant or hydronium ion adsorption on the nanotube surface.

Selective reaction and Raman features

Selective functionalization increases the intensity of the D peak due to formation of aryl-nanotube bond and decreases the tangential mode due loss of electronic resonance. These two effects are generally summed together as increase in their peaks ratio (D/G). RBM peaks of metallic nanotubes decay and the peaks corresponding to those of semiconducting nanotubes remain almost unchanged. [3] [4]

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Reaction mechanism

Diazonium reagent and SWNT reaction has a two step mechanism. First, the diazonium reagent adsorbs noncovalently to an empty site on the nanotube surface, forming a charge-transfer complex. This is a fast, selective noncovalent adsorption and diazonium group in this complex partially dopes the nanotube, diminishing the tangential mode in the Raman spectrum. Desorption of A from nanotube is negligible (k−1 ~ 0). In second step complex B then decomposes to form a covalent bond with the nanotube surface. This is a slower step that need not be selective and is represented by the restoration of the G peak and increase in the D band. [6]

Reversibility of diazonium chemistry

Nanotubes reacted with the diazonium reagent can be converted back into pristine nanotubes when thermally treated at 300 °C in an atmosphere of inert gas. This cleaves the aryl hydroxyl moieties from the nanotube sidewall and restores the spectroscopic feature (Raman and absorption spectra) of pristine nanotube. [4]

Chemical separation of metallic and semiconducting SWNTs

Metallic and semiconducting carbon nanotubes generally coexist in as-grown materials. To get only semiconducting or only metallic nanotubes selective functionalization of metallic SWNTs via 4-hydroxybenzene diazonium can be used. Separation can be done in solution by deprotonation of the p-hydroxybenzene group on the reacted nanotubes (metallic) in alkaline solution followed by electrophoretic separation of these charged species from the neutral species (semiconducting nanotubes). This followed by annealing would give separated pristine semiconducting and metallic SWNT. [4]

Related Research Articles

Metallic bonding is a type of chemical bonding that rises from the electrostatic attractive force between conduction electrons and positively charged metal ions. It may be described as the sharing of free electrons among a structure of positively charged ions (cations). Metallic bonding accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and luster.

Buckminsterfullerene chemical compound

Buckminsterfullerene is a type of fullerene with the formula C60. It has a cage-like fused-ring structure (truncated icosahedron) that resembles a soccer ball (football), made of twenty hexagons and twelve pentagons, with a carbon atom at each vertex of each polygon and a bond along each polygon edge.

Sigma bond strongest type of covalent chemical bond; formed by head-on overlapping between atomic orbitals. Sigma bonding for diatomic molecules (using the language and tools of symmetry groups):σ-bond is symmetrical with respect to rotation about the bond axis

In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 . Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix or hybridize. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this mixing depends on the relative energies of the MOs of like symmetry.

In mesoscopic physics, a quantum wire is an electrically conducting wire in which quantum effects influence the transport properties. Usually such effects appear in the dimension of nanometers, so they are also referred to as nanowires.

Nucleophilic aromatic substitution

A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems:

Fullerene chemistry

Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example, fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.

Carbon nanobud synthetic allotrope of carbon combining carbon nanotube and a fullerene

In nanotechnology, a carbon nanobud is a material that combines carbon nanotubes and spheroidal fullerenes, both allotropes of carbon, in the same structure, forming "buds" attached to the tubes. Carbon nanobuds were discovered and synthesized in 2006.

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Chemical force microscopy

Chemical force microscopy (CFM) is a variation of atomic force microscopy (AFM) which has become a versatile tool for characterization of materials surfaces. With AFM, structural morphology is probed using simple tapping or contact modes that utilize van der Waals interactions between tip and sample to maintain a constant probe deflection amplitude (constant force mode) or maintain height while measuring tip deflection (constant height mode). CFM, on the other hand, uses chemical interactions between functionalized probe tip and sample. Choice chemistry is typically gold-coated tip and surface with R-SH thiols attached, R being the functional groups of interest. CFM enables the ability to determine the chemical nature of surfaces, irrespective of their specific morphology, and facilitates studies of basic chemical bonding enthalpy and surface energy. Typically, CFM is limited by thermal vibrations within the cantilever holding the probe. This limits force measurement resolution to ~1 pN which is still very suitable considering weak COOH/CH3 interactions are ~20 pN per pair. Hydrophobicity is used as the primary example throughout this consideration of CFM, but certainly any type of bonding can be probed with this method.

Carbon nanotubes (CNTs) are very prevalent in today’s world of medical research and are being highly researched in the fields of efficient drug delivery and biosensing methods for disease treatment and health monitoring. Carbon nanotube technology has shown to have the potential to alter drug delivery and biosensing methods for the better, and thus, carbon nanotubes have recently garnered interest in the field of medicine.

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Carbon peapod

Carbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. It is named due to their resemblance to the seedpod of the pea plant. Since the properties of carbon peapods differ from those of nanotubes and fullerenes, the carbon peapod can be recognized as a new type of a self-assembled graphitic structure. Possible applications of nano-peapods include nanoscale lasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices thanks to the memory effects and superconductivity of nano-peapods.

Synthesis of carbon nanotubes

Techniques have been developed to produce carbon nanotubes in sizable quantities, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD). Most of these processes take place in a vacuum or with process gases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.

References

  1. ↑ Chen, Z; Thiel, W; Hirsch, A (2003). "Reactivity of the convex and concave surfaces of single-walled carbon nanotubes (SWCNTs) towards addition reactions: dependence on the carbon-atom pyramidalization". Chemphyschem : a European journal of chemical physics and physical chemistry. 4 (1): 93–7. doi:10.1002/cphc.200390015. PMID   12596472.
  2. ↑ Mickelson, E (1998). "Fluorination of single-wall carbon nanotubes". Chemical Physics Letters. 296: 188. Bibcode:1998CPL...296..188M. doi:10.1016/S0009-2614(98)01026-4.
  3. 1 2 3 4 Strano, Ms; Dyke, Ca; Usrey, Ml; Barone, Pw; Allen, Mj; Shan, H; Kittrell, C; Hauge, Rh; Tour, Jm; Smalley, Re (2003). "Electronic structure control of single-walled carbon nanotube functionalization". Science. 301 (5639): 1519–22. Bibcode:2003Sci...301.1519S. doi:10.1126/science.1087691. PMID   12970561.
  4. 1 2 3 4 5 Kim, Woo-Jae; Usrey, Monica L.; Strano, Michael S. (2007). "Selective Functionalization and Free Solution Electrophoresis of Single-Walled Carbon Nanotubes: Separate Enrichment of Metallic and Semiconducting SWNT". Chemistry of Materials. 19 (7): 1571. doi:10.1021/cm061862n.
  5. 1 2 M.J. Connell (2006). Carbon Nanotubes Properties and Applications. Taylor & Francis.
  6. ↑ Usrey, Ml; Lippmann, Es; Strano, Ms (2005). "Evidence for a two-step mechanism in electronically selective single-walled carbon nanotube reactions". Journal of the American Chemical Society. 127 (46): 16129–35. doi:10.1021/ja0537530. PMID   16287300.
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