Structure and energetics of polyhydroxylated carbon fullerenes

Abstract

We have performed semiempirical modified-neglect-of-diatomic-overlap (to determine the most stable geometrical arrangements) as well as ab initio density-functional theory calculations (to obtain the electronic structure and total energies) at $T=0$ to analyze the energetics and structural properties of OH species adsorbed on the external surfaces of spheroidal ${\mathrm{C}}_{60}$ as well as on finite length open-ended armchair (6,6) and (8,8) carbon nanotubes. Interestingly we have found, for the low coverage regime, that the adsorbed OH groups prefer to organize as a hydroxyl cluster (having up to seven OH molecules) in only one side of the ${\mathrm{C}}_{60}$ surface. The observed clustering leads to the formation of a new amphiphilic molecule that naturally explains the stability of ${\mathrm{C}}_{60}(\mathrm{OH}{)}_{n}$ $(n\ensuremath{\sim}9\ensuremath{-}12)$ Langmuir monolayers at the air-water interface observed by several authors, where it is thus only the highly hydroxylated part of the carbon cage the one that dissolves in water slightly. For 8 to 14 adsorbed OH groups, a second hydroxyl island is gradually stabilized on the opposite side of the carbon structure, and finally, with increasing coverage the coexistence of ringlike and cluster arrays of OH groups seems to lead to the complete solubility of the carbon compound. In all cases, the OH molecules have been found to occupy on-top sites with a C-O-H bond tilted away from the surface normal and no hydrogen bond formation between the adparticles is obtained, in contrast to what is normally observed in the compact OH hexagonal phases stabilized on extended metal substrates. The calculated vibrational frequencies of our adsorbates have a good correspondence with the experimental measurements and provide a clear signature of the clustering of the OH molecules at low coverages. Finally, OH adsorption on the external surface of cylindrical carbon structures leads to the formation of quasi-one-dimensional molecular arrays, consistent with the synthesis of metal nanowires on the surface of carbon nanotubes, their length being determined by the length of the nanotube.