Although graphitic materials were thought to be hydrophobic, recent experimental results based on contact angle measurements show that the hydrophobicity of graphitic surfaces stems from airborne contamination of hydrocarbons. This leads us to question whether a pristine graphitic surface is indeed hydrophobic. To investigate the water wettability of graphitic surfaces, we use molecular dynamics simulations of water molecules on the surface of a single graphene layer at room temperature. The results indicate that a water droplet spreads over the entire surface and that a double-layer structure of water molecules forms on the surface, which means that wetting of graphitic surfaces is possible, but only by two layers of water molecules. No further water layers can cohere to the double-layer structure, but the formation of three-dimensional clusters of liquid water is confirmed. The surface of the double-layer structure acts as a hydrophobic surface. Such peculiar behavior of water molecules can be reasonably explained by the formation of hydrogen bonds: The hydrogen bonds of the interfacial water molecules form between the first two layers and also within each layer. This hydrogen-bond network is confined within the double layer, which means that no “dangling hydrogen bonds” appear on the surface of the double-layer structure. This formation of hydrogen bonds stabilizes the double-layer structure and makes its surface hydrophobic. Thus, the numerical simulations indicate that a graphene surface is perfectly wettable on the atomic scale and becomes hydrophobic once it is covered by this double layer of water molecules.

We investigated the selectivity of N-doped graphene nanoclusters (N-GNCs) toward the oxygen reduction reaction (ORR) using first-principles calculations within the density functional theory. The results show that the maximum electrode potentials ( U Max ) for the four-electron (4e – ) pathway are higher than those for the two-electron (2e – ) pathway at almost all of the reaction sites. Thus, the N-GNCs exhibit high selectivity for the 4e – pathway, that is, the 4e – reduction proceeds preferentially over the 2e – reduction. Such high selectivity results in high durability of the catalyst because H 2 O 2 , which corrodes the electrocatalyst, is not generated. For the doping sites near the edge of the cluster, the value of U Max greatly depends on the reaction sites. However, for the doping sites around the center of the cluster, the reaction-site dependence is hardly observed. The GNC with a nitrogen atom around the center of the cluster exhibits higher ORR catalytic capability compared with the GNC with a nitrogen atom in the vicinity of the edge. The results also reveal that the water molecule generated by the ORR enhances the selectivity toward the 4e – pathway because the reaction intermediates are significantly stabilized by water.

We investigate the stickiness of the two-dimensional piecewise linear map with a family of marginal unstable periodic orbits (FMUPOs), and show that a series of unstable periodic orbits accumulating to FMUPOs plays a significant role to give rise to the power law correlation of trajectories. We can explicitly specify the sticky zone in which unstable periodic orbits whose stability increases algebraically exist, and find that there exists a hierarchy in accumulating periodic orbits. In particular, the periodic orbits with linearly increasing stability play the role of fundamental cycles as in the hyperbolic systems, which allows us to apply the method of cycle expansion. We also study the recurrence time distribution, especially discussing the position and size of the recurrence region. Following the definition adopted in one-dimensional maps, we show that the recurrence time distribution has an exponential part in the short time regime and an asymptotic power law part. The analysis on the crossover time T(c)(*) between these two regimes implies T(c)(*) approximately -log[micro(R)] where micro(R) denotes the area of the recurrence region.

Doping is an efficient way to modify the electronic structure of graphene. Although there have been a considerable number of studies on the electronic structure of impurity-doped graphene, every study has suggested a different interpretation of the appearance of impurity levels of dopants located near the so-called zigzag edge of graphene nanoribbons (GNRs). Here, we propose a charge transfer model that satisfactorily explains the change in electronic structure upon N(B) doping of zigzag GNR (ZGNR). The structural stability and electronic structure of the doped ZGNR have been investigated using first-principles calculations based on the density functional theory. The formation energy of doping increases as a function of the distance between the N(B) atom and the zigzag edge, and two tendencies are observed depending on whether the dopant is an odd or even number of sites away from the zigzag edge. Such peculiar behavior of the formation energy can be successfully explained by charge transfer between the so-called edge state localized at the edge and the 2p-state of the dopant. Such an electron (hole) transfer leads to the compensation (disappearance) of the local spin-magnetic moment at one side of the ZGNR, manifesting in the ferromagnetic ground state of ZGNR.