Structural and optical properties of highly hydroxylated fullerenes: Stability of molecular domains on the C60 surface

The excitation spectra and the structural properties of highly hydroxylated C(60)(OH)(x) fullerenes (so-called fullerenols) are analyzed by comparing optical absorption experiments on dilute fullerenol-water solutions with semiempirical and density functional theory electronic structure calculations. The optical spectrum of fullerenol molecules with 24-28 OH attached to the carbon surface is characterized by the existence of broad bands with reduced intensities near the ultraviolet region (below approximately 500 nm) together with a complete absence of optical transitions in the visible part of the spectra, contrasting with the intense absorption observed in C(60) solutions. Our theoretical calculations of the absorption spectra, performed within the framework of the semiempirical Zerner intermediate neglect of diatomic differential overlap method [Reviews in Computational Chemistry II, edited by K. B. Lipkowitz and D. B. Boyd (VCH, Weinheim, 1991), Chap. 8, pp. 313-316] for various gas-phase-like C(60)(OH)(26) isomers, reveal that the excitation spectra of fullerenol molecules strongly depend on the degree of surface functionalization, the precise distribution of the OH groups on the carbon structure, and the presence of impurities in the samples. Interestingly, we have surprisingly found that low energy atomic configurations are obtained when the OH groups segregate on the C(60) surface forming molecular domains of different sizes. This patchy behavior for the hydroxyl molecules on the carbon surface leads in general to the formation of fullerene compounds with closed electronic shells, large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, and existence of an excitation spectrum that accounts for the main qualitative features observed in the experimental data.

Stability of highly OH-covered C60 fullerenes: role of coadsorbed O impurities and of the charge state of the cage in the formation of carbon-opened structures

We have performed both semiempirical as well as ab initio density functional theory calculations in order to investigate the structural stability of highly hydroxylated C60(OH)32 fullerenes, so-called fullerenols. Interestingly, we have found that low-energy atomic configurations are obtained when the OH groups are covering the C60 in the form of small hydroxyl islands. The previous formation of OH molecular domains on the carbon surface, stabilized by hydrogen bonds between neighboring OH groups, defines the existence of C60(OH)32 fullerene structures with some elongated C-C bonds, closed electronic shells, and large highest occupied-lowest unoccupied molecular orbital energy gaps, with the latter two being well-known indicators of high chemical stability in these kind of carbon compounds. The calculated optical absorption spectra show that the location of the first single dipole-allowed excitation strongly depends on the precise distribution of the OH groups on the surface, a result that, combined with optical spectroscopy experiments, might provide an efficient way to identify the structure of these kinds of fullerene derivatives. We found that the presence of a few coadsorbed oxygen species on the fullerene surface leads in general to the existence of C60(OH)32O(x) (x = 1-4) compounds in which some of the C-C bonds just below the O impurities are replaced by C-O-C bridge bonds, leading to the formation of stable carbon-opened structures in agreement with the recent experimental work of Xing et al. (J. Phys. Chem. B 2004, 108, 11473). Actually, a more dramatic cage destruction is obtained when considering multiply charged C60(OH)32O(x)(+/-m) (m = 2, 4, 6) species (that can exist in both gas-phase and aqueous environments), where now sizable holes made of 9- and 10-membered rings can exist in the carbon network. We believe that our results are important if the controlled opening of carbon cages is needed and it should be taken into account also in several technological applications where the permanent encapsulation of atomic or molecular species in these types of fullerene derivatives is required.

Orbital magnetism in transition-metal clusters: from Hund's rules to bulk quenching

The local and average orbital moments <L> of transition-metal (TM) clusters are determined bridging the gap between atomic Hund's rules and solid-state quenching. A remarkable enhancement of <L> is revealed in agreement with recent measurements. In small Ni(N) (N< or =10), <L> represents (20-40)% of the total magnetization and is therefore crucial for the comparison between theory and experiment. Larger clusters (N> or =150) show nearly bulklike quenching at the interior but retain a considerable surface enhancement. Trends for different TM's are discussed.

Raman spectra of single walled carbon nanotubes at high temperatures: pretreating samples in a nitrogen atmosphere improves their thermal stability in air

We present a combined experimental and theoretical study dedicated to analyzing the structural stability and chemical reactivity of single walled carbon nanotubes (SWCNTs) in the presence of air and nitrogen atmospheres in the temperature interval of 300-1000 K. The temperature dependence of the radial breathing mode (RBM) region of the Raman spectra is irreversible in the presence of air, but it is reversible up to 1000 K in a nitrogen atmosphere. Our density functional theory (DFT) calculations reveal that irreversibility is due to partial degradation of SWCNTs produced by dissociative chemical adsorption of molecular oxygen on intrinsic defects of the nanotube surface. Oxygen partially opens the nanotubes forming semi-tubes with a non-uniform diameter distribution observed by Raman scattering. In contrast, heating CNTs in a nitrogen atmosphere seems to lead to the formation of nitrogen-doped SWCNTs. Our DFT calculations indicate that in general the most common types of nitrogen doping (e.g., pyridinic, pyrrolic, and substitutional) modify the location of the RBM frequency, leading also to frequency shifts and intensity changes of the surrounding modes. However, by performing a systematic comparison between calculated and measured spectra we have been able to infer the possible adsorbed configurations adopted by N species on the nanotube surface. Interestingly, by allowing previously nitrogen-exposed SWCNTs to interact with air at different temperatures (up to 1000 K) we note that the RBM region remains nearly unperturbed, defining thus our nitrogen-pretreated SWCNTs as more appropriate carbon nanostructures for high temperature applications in realistic environments. We believe that we have implemented a post-growth heat-treatment process that improves the stability of carbon nanotubes preserving their diameter and inducing a defect-healing process of the carbon wall.

Structure, electronic properties, and aggregation behavior of hydroxylated carbon nanotubes

We present a combined experimental and theoretical study to analyze the structure, electronic properties, and aggregation behavior of hydroxylated multiwalled carbon nanotubes (OH-MWCNT). Our MWCNTs have average diameters of ~2 nm, lengths of approximately 100-300 nm, and a hydroxyl surface coverage θ~0.1. When deposited on the air/water interface the OH-MWCNTs are partially soluble and the floating units interact and link with each other forming extended foam-like carbon networks. Surface pressure-area isotherms of the nanotube films are performed using the Langmuir balance method at different equilibration times. The films are transferred into a mica substrate and atomic force microscopy images show that the foam like structure is preserved and reveals fine details of their microstructure. Density functional theory calculations performed on model hydroxylated carbon nanotubes show that low energy atomic configurations are found when the OH groups form molecular islands on the nanotube's surface. This patchy behavior for the OH species is expected to produce nanotubes having reduced wettabilities, in line with experimental observations. OH doping yields nanotubes having small HOMO-LUMO energy gaps and generates a nanotube → OH direction for the charge transfer leading to the existence of more hole carriers in the structures. Our synthesized OH-MWCNTs might have promising applications.

Nanotubols under H2O2 exposure: is it possible to poly-hydroxylate carbon nanotubes?

We present a combined experimental and theoretical study dedicated to analyze the variations in the surface chemistry of hydroxylated multiwalled carbon nanotubes (MWCNTs), so called nanotubols, when exposed to H₂O₂ at high temperatures.

Adsorption of carbon monoxide on small aluminum oxide clusters: Role of the local atomic environment and charge state on the oxidation of the CO molecule

We present extensive density functional theory (DFT) calculations dedicated to analyze the adsorption behavior of CO molecules on small AlxOy (±) clusters. Following the experimental results of Johnson et al. [J. Phys. Chem. A 112, 4732 (2008)], we consider structures having the bulk composition Al2O3, as well as smaller Al2O2 and Al2O units. Our electron affinity and total energy calculations are consistent with aluminum oxide clusters having two-dimensional rhombus-like structures. In addition, interconversion energy barriers between two- and one-dimensional atomic arrays are of the order of 1 eV, thus clearly defining the preferred isomers. Single CO adsorption on our charged AlxOy (±) clusters exhibits, in general, spontaneous oxygen transfer events leading to the production of CO2 in line with the experimental data. However, CO can also bind to both Al and O atoms of the clusters forming aluminum oxide complexes with a CO2 subunit. The vibrational spectra of AlxOy + CO2 provides well defined finger prints that may allow the identification of specific isomers. The AlxOy (+) clusters are more reactive than the anionic species and the final Al2O(+) + CO reaction can result in the production of atomic Al and carbon dioxide as observed from experiments. We underline the crucial role played by the local atomic environment, charge density distribution, and spin-multiplicity on the oxidation behavior of CO molecules. Finally, we analyze the importance of coadsorption and finite temperature effects by performing DFT Born-Oppenheimer molecular dynamics. Our calculations show that CO oxidation on AlxOy (+) clusters can be also promoted by the binding of additional CO species at 300 K, revealing the existence of fragmentation processes in line with the ones experimentally inferred.

Thiol-based molecular overlayers adsorbed on C60: role of the end-group and charge state on the stability of the complexes

We present pseudo-potential density functional theory calculations dedicated to analyze the stability and electronic properties of thiol-based molecular overlayers adsorbed on C60. We consider short molecules having a S atom as a headgroup, alkyl chains containing one to three C atoms, and a CH3 species as a terminal group. The thiol molecules are bonded to the carbon surface (through the S atom) with adsorption energies that vary in the range of ~1-2 eV and with S-C bond lengths of ~1.8 Å. For neutral C60(SCH3)n complexes, low energy atomic configurations are obtained when the thiol groups are distributed on the surface forming small molecular domains (e.g., pairs, trimers, or tetramer configurations of neighboring thiol molecules). In contrast, less stable random distributions are defined by orientationally disordered overlayers with highly distorted underlying carbon networks. The inclusion of London dispersion interaction slightly affects the structure of the molecular coating but increases the adsorption energies by values as large as 0.3 eV. Interestingly, the relative stability of the previous adsorbed phases differ from the one obtained when considering single sulfur adsorption on C60, a result that reveals the crucial role played by the terminal CH3 groups on the structure of the molecular coating. The positive (negative) charging of the [C60(SCH3)n](±q) complexes, with q as large as 8e, changes the geometrical structure and the chemical nature of the ligand shell inducing lateral molecular displacements, S-S bonding between neighboring thiols, as well as the partial degradation of the molecular coating. Finally, we consider the stability of two-component mixed overlayers formed by the coadsorption of CH3-, OH-, and NH2-terminated alkanethiols of the same length. In agreement with the results found on Au surfaces, we obtain lowest energy atomic configurations when molecular domains of a single component are stabilized on C60, a result that could be of fundamental importance in biomedical applications.

Magnetic properties of Co nanoparticles: role of the coexistence of different geometrical phases

Following the experimental results of Respaud et al. [Phys. Rev. B 57, 2925 (1998)] we report self-consistent electronic structure calculations in order to analyze the magnetic properties of Co nanoparticles in which a coexistence of bcc and compact (fcc) phases are present within the particles. In all cases, the local spin moments S(i) are found to be saturated (approximately 1.7 microB) while, in contrast, the local orbital moments L(i) and the magnetic anisotropy energy (MAE) are found to be very sensitive to the size and structure of the systems. Interestingly, we obtain considerably enhanced values for L(i) at the internal bcc/fcc interfaces which can be even larger than at surfaces sites and, in addition, we found that by varying the fraction of bcc and fcc phases within the particles, several reorientations of the magnetization can be induced, a result that could open new possibilities to tune the MAE of magnetic nanoparticles.

ClO-driven degradation of graphene oxide: new insights from DFT calculations

We present an extensive investigation using density functional theory (DFT) calculations on various model graphene oxide (GO) nanostructures interacting with chlorine monoxide ClO, aiming to understand the role of this highly oxidizing species in C-C bond breakage and the formation of significant holes on GO sheets. During its function, the myeloperoxidase (MPO) enzyme abundantly generates chlorine-oxygen-containing species and their presence has been identified as the cause of degradation in carbon nanotubes of diverse sizes, morphologies, and chemical compositions, both in in vivo and in vitro samples. Notably, Kurapati et al. (Small, 2015, 11, 3985-3994) demonstrated efficient degradation of single GO monolayers through MPO catalysis, though the exact degradation mechanism remains unclear. In our study, we discover that breaking C-C bonds in a single graphene oxide sheet is achievable through a simple mechanism involving the dissociation of two ClO molecules that are chemically attached as nearest neighbor species but bonded to opposite sides of the GO layer (up/down configuration). Two new carbonyl oxygens appear on the surface and the Cl atoms can be transferred to the carbon layer or as physisorbed species near the GO surface. Relatively small energy barriers are associated with these molecular events. Continuing this process on neighboring sites leads to the presence of larger holes on the GO surface, accompanied by an increase in carbonyl species on the carbon network, consistent with X-ray photoelectron spectroscopy measurements. Indeed, the distribution of oxygen functionalities is found to be crucial in defining the damage pattern induced in the carbon layer. We emphasize the important role played by the local charge distribution in the stability or instability of chemical bonds, as well as in the energy barriers and reaction pathways. Finally, we explore the possibility of achieving chlorination of GO following MPO exposure. The here-reported predictions could be the root cause of the experimentally observed low stability of individual GO sheets during the MPO catalytic cycle.

N 1s core-level binding energies in nitrogen-doped carbon nanotubes: a combined experimental and theoretical study

We report a combined experimental and theoretical study dedicated to analyze the N 1s core-level binding energies (CLBE) in N-doped carbon nanotubes (N-CNTs). X-ray photoelectron spectroscopy (XPS) data are obtained from N-CNT samples synthesized using the chemical vapor deposition technique. Extensive density functional theory (DFT) calculations are performed on various model single- and double-walled N-CNTs where N 1s CLBEs are determined using Koopman's theorem. However, we also present additional calculations within the (Z + 1) approximation to analyze the role of final-state effects. From XPS data up to 2 at% of N content was found in our samples and the high resolution analysis of the N 1s line shows, according to previous experimental results, that N species exist in CNTs as graphitic, pyrrolic, pyridinic, and molecular configurations. However, peak decomposition is characterized by five broad Gaussian curves that overlap considerably among them, having different widths and heights, implying a more complex distribution of N atoms within the structures. DFT calculations performed on model N-CNTs reveal a strong dependence of N 1s CLBE values and their shifts on the local atomic environment. Different types of graphitic N cover an energy range of 3 eV, while various configurations for pyridinic, pyrrolic, and molecular species reveal a dispersion in their energy values of 5.7, 2.7, and 5.2 eV, respectively. The previous distributions of theoretical CLBEs also strongly overlap, implying that some peaks in the XPS spectra must be understood as composite signals where the signals of different N defects coexist. We find, in agreement with the experimental data, that freestanding molecular nitrogen and (weakly interacting) encapsulated N₂ within the hollow core of model CNTs have very similar CLBEs. Furthermore, we predict that chemisorbed N₂ on defective regions of the nanotube walls has N 1s binding energy values that are considerably larger when compared to encapsulated N₂, thus making possible their identification. In contrast to previous reports, we find a nontrivial dependence between CLBEs and the local electronic occupation at N sites. The assignment of spectral details in the XPS data to well-defined N-defects on CNTs is not straightforward and needs to be more deeply analyzed.