Cluster Origin of Solvation Features of C-Nanostructures in Organic Solvents

Introduction

Interest in nanoparticles (NPs) arises from the shape-dependent physical properties of materials at the nanoscale (Faraday, 1857; Murphy et al., 2010). Occurrence of single-wall carbon nanocones (SWNCs) was used to investigate nucleation and growth of curved C-nanostructures (NSs) suggesting pentagon role. When a pentagon is introduced into a graphitic sheet nanographene (GR) (Figure 1d) via extraction of a 60º sector from the sheet one forms a cone leaf. Pentagons presence in an SWNC apex is analogue of their occurrence in single-wall C-nanotube (NT) (CNT) (SWNT) tip topology (cf. Figure 1b). Terminations of SWNTs attracted interest once Tamura & Tsukada (1995) theoretically predicted peculiar electronic states related to GR topological defects. Kim et al. (1999) observed resonant peaks in density of states (DOS) in SWNTs and Carroll et al. (1997), in multiple-wall (MNTs) C-nanotubes (MWNTs).

Figure 1.

Arrangement of C-nanostructures: (a) C₆₀; (b) SWNT; (c) NT-BUD; (d) GR; (e) GR-BUD

The SWNCs with discrete opening angles (apices, θ) of 19º, 39º, 60º, 85º and 113º of cone (cf. Figure 2) were observed in a C-sample generated by hydrocarbon (HC) pyrolysis (Krishnan et al., 1997), which was explained by a cone-wall model composed of wrapped GR sheets where geometrical requirement for seamless connection accounted for semidiscrete character and absolute values of cone angle. Total disclinations of all conic GR microstructures are multiples of 60º corresponding to the presence of a given number (P ≥ 0) of pentagons in SWNC apices. Considering GR sheet symmetry and Euler theorem, five types of SWNCs (corresponding to angles) are obtained from a continuous GR sheet matching to P = 1–5. Cone angle (θ) is given by sin(θ/2) = 1 – P/6 leading to SWNC angles where flat discs and caped SWNTs correspond to P = 0 and 6, respectively. The SWNC with P = 5 pentagons (θ = 19º) is named nanohorn (SWNH). Several configurations exist for an SWNC angle depending on the form in which pentagons are arranged in conic tips. According to isolated pentagon rule (IPR) derived from fullerenes (Figure 1a) (Kroto, 1987) configurations containing isolated pentagons lead to isomers that are more stable than those with grouped pentagons for NSs. Additional rules were derived from ab initio calculations (Jan & Jaffe, 1998) performed to evaluate stability of NSs containing isolated and grouped pentagons. Consideration of a curvature-producing pentagon as a defect, in a planar net of hexagons, results in that two-pentagon arrangement in a hexagonal lattice is specified by a hexagonal co-ordinate (a,b) with a pentagon in (a,b) and another in (0,0). Nearest-neighbouring pentagons are (1,1)-co-ordinated in C₆₀ (all pentagons connected by a C–C bond) and are (1,1)/(2,0)-co-ordinated in C₇₀ (the latter corresponds to two pentagons separated by one hexagon). In accordance with density-functional-theory (DFT) calculations pentagons (1,1) lead to more stable SWNC tip structures than those of pentagons (2,0), which is attributed to lower stress induced by each pair (1,1) in relation to pairs (2,0). The SWNCs present geometry asymmetry and are semiconductors. Insolubility of NSs in all solvents and tendency to agglomerate should be overcome before applications. Tagmatarchis et al. (2006) reported SWNC covalent functionalization with NH₄⁺ to improve solubility but still leaving aggregation behind. Progress was made to SWNC solubilization, which was achieved by covalent functionalization of its skeleton (Cioffi et al., 2006, 2007; Pagona et al., 2007a)/highly strained cone-ends (Pagona et al., 2006a) and supramolecular π–π stacking interactions (Pagona et al., 2006b, 2007b; Zhu et al., 2003) with pyrenes (Py’s)/porphyrins.