The density functional theory method (M05-2X/6-31G(d)) was used to investigate reaction mechanisms for deoxygenation of graphene oxides (GOs) with hydrazine or heat treatment. Three mechanisms were identified as reducing epoxide groups of GO with hydrazine as a reducing agent. No reaction path was found for the hydrazine-mediated reductions of the hydroxyl, carbonyl, and carboxyl groups of GO. We instead discovered the mechanisms for dehydroxylation, decarbonylation, and decarboxylation using heat treatment. The hydrazine de-epoxidation and thermal dehydroxylation of GO have opposite dependencies on the reaction temperature. In both reduction types, the oxygen functionalities attached to the interior of an aromatic domain in GO are removed more easily, both kinetically and thermodynamically, than those attached at the edges of an aromatic domain. The hydrazine-mediated reductions of epoxide groups at the edges are suspended by forming hydrazino alcohols. We provide atomic-level elucidation for the deoxygenation of GO, characterize the product structures, and suggest how to optimize the reaction conditions further.

The self-assembly of ink molecules deposited using dip-pen nanolithography (DPN) is modeled as a two-dimensional diffusion with a source (tip). A random walk simulation and simple analytic theory are used to study how the diffusion dynamics affects patterns generated in DPN. For a tip generating a constant flux of ink molecules, circles, lines, and letters are studied by varying the deposition rate of ink molecules and the tip scan speed. Even under the most favorable condition studied here, peripheries of patterns fluctuate from perfect circles or lines, due to the random, diffusional nature of self-assembly. The degree of fluctuation is quantified for circles and lines. Circles generated by fixing the tip position do not depend on the deposition rate if the same amount of ink is deposited. For a moving tip, patterns change drastically depending on tip speed and deposition rate. Overall, fast scan or slow deposition relative to the diffusion time scale makes lines narrower. When the tip deposits ink too slowly or scans too fast, patterns become incoherent, making molecules in patterns separated from each other. Therefore, there seems to be an optimal choice of the deposition rate and tip speed that gives both narrow and coherent patterns. We also explore the consequences of varying the relative rates of diffusion of ink molecules on bare surface and on previously deposited molecules.

It has been speculated that the catechol (1,2-dihydroxybenzene) functionality of marine mussels is responsible for its strong and versatile adhesion on various wet surfaces. To elucidate features of this adhesion, we performed a periodic density functional theory calculation for catechol adsorption on silica surfaces. We obtained its binding energy and geometry on two representative hydroxylated surfaces of cristobalite, which mimic amorphous silica. Catechol strongly adhered to both surfaces by making three or four hydrogen bonds. Catechol achieved versatility in adhesion via torsion of its hydroxyls. The binding energy of catechol, which amounts to 14 kcal/mol, was larger than that of water, irrespective of the surface. With the inclusion of dispersion interaction, the binding energy of catechol further increased up to 33 kcal/mol, and its preferential adsorption over water became evident. Both the hydroxyls and phenylene ring of catechol contribute to its strong adhesion due to hydrogen bonds and dispersion.