Ozone oxidation of surface-adsorbed polycyclic aromatic hydrocarbons: role of PAH-surface interaction.

The heterogeneous chemistry of surface-adsorbed polycyclic aromatic hydrocarbons (PAHs) plays key roles in nanoscience, environmental science, and public health. Experimental evidence shows that the substrate can influence the heterogeneous oxidation of surface-bound PAHs, however, a mechanistic understanding of the role of the surface is still lacking. We examine the effects of the PAH-substrate interaction on the oxidation of surface-adsorbed anthracene, pyrene, and benzo[a]pyrene by ozone (O(3)) using density functional theory. We find that some O(3) oxidation mechanisms for these planar PAH molecules lead to nonplanar intermediates or products, the formation of which may necessitate partial desorption or "lift-off" from a solid substrate. The energy penalty for partial desorption of each PAH from the surface is estimated for four different substrate types on the basis of literature data and accounted for in the thermodynamic analysis of the reaction pathways. We find that the attractive PAH-substrate interaction may render oxidation pathways involving nonplanar intermediates or products thermodynamically unfavorable. The influence of the PAH-substrate interaction could contribute in part to the variations in PAH oxidation kinetics and product distributions that have been observed experimentally. Our choice of test molecules enabled us to identify trends in reactivity and product formation for four types of potentially reactive site (zigzag, armchair, bridge, and internal), allowing us to infer products and mechanisms of O(3) oxidation for PAHs of larger sizes. Implications for atmospheric chemistry and the stability of graphene in the presence of O(3) are discussed.

Aggregation behavior and chromonic liquid crystal phase of a dye derived from naphthalenecarboxylic acid.

Polarizing microscopy, X-ray scattering, and absorption spectroscopy are used to investigate the aggregation process and chromonic liquid crystal of the anionic compound Bordeaux dye, a product of the sulfonation of the dibenzimidazole derivative of naphthalenetetracarboxylic acid. Polarizing microscopy reveals that the liquid crystal phase forms at room temperature when the concentration is only about 6 wt%, a value lower than what is found in many aggregating systems. The X-ray results indicate that the aggregation is via columns, with a cross-sectional area about 2.5 times larger than the individual molecule. Absorption spectroscopy shows a significant change in the absorption spectrum due to aggregation, which is nicely explained by a simple theory of isodesmic aggregation and excitonic coupling between the molecules in an aggregate. The "stacking free energy change" for a molecule in an aggregate relative to a molecule in solution is estimated to be about 9 kBT, a larger value than that found in the one other system where it has been estimated.

Graphene oxidation: thickness-dependent etching and strong chemical doping.

Patterned graphene shows substantial potential for applications in future molecular-scale integrated electronics. Environmental effects are a critical issue in a single-layer material where every atom is on the surface. Especially intriguing is the variety of rich chemical interactions shown by molecular oxygen with aromatic molecules. We find that O 2 etching kinetics vary strongly with the number of graphene layers in the sample. Three-layer-thick samples show etching similar to bulk natural graphite. Single-layer graphene reacts faster and shows random etch pits in contrast to natural graphite where nucleation occurs at point defects. In addition, basal plane oxygen species strongly hole dope graphene, with a Fermi level shift of approximately 0.5 eV. These oxygen species desorb partially in an Ar gas flow, or under irradiation by far UV light, and readsorb again in an O 2 atmosphere at room temperature. This strongly doped graphene is very different from "graphene oxide" made by mineral acid attack.