Nitrogen (N)-doped carbon materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is essential for several renewable energy systems. However, the ORR active site (or sites) is unclear, which retards further developments of high-performance catalysts. Here, we characterized the ORR active site by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species. The ORR active site is created by pyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts are comparable with those of N-doped graphene powder catalysts. Thus, the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N.

Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet (''raining'' simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of k BT0 (where kB is the Boltzmann constant, and T 0 is the ambient temperature) for a rugged surface at the critical pillar height to Ϸ8 kBT0 for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars. free-energy barrier ͉ molecular dynamics simulation ͉ nanodrop raining experiment ͉ Wenzel-to-Cassie state transition I t is well known that microtextured or nanotextured hydrophobic surfaces can become superhydrophobic (1-39). In fact, nature provides first examples of superhydrophobic surfaces, such as lotus leaves and water striders' nonwetting legs (40-42). Synthetic microtextured surface structures like the lotus leaves have been fabricated to achieve high water repellency such that on these surfaces, water droplets are typically in the Cassie state (43) rather than the Wenzel state (44). In general, water droplets adhere more strongly to the textured surface in the Wenzel state than in the Cassie state, causing stronger contact-angle hysteresis. Hence, in many practical applications such as self-cleaning surfaces (6, 17), the Cassie state is preferred over the Wenzel state. It is also known that as the degree of surface roughness increases, the Cassie state becomes increasingly favorable compared with the Wenzel state. Hence, at certain degree of roughness, the Wenzel state and Cassie state can become more or less equally favorable and may even coexist on the same surface. From a statistical-mechanics point of view, the 2 states can coexist when they are separated by a high free-energy barrier by which one state is still metastable (free-energy local minimum), and the other is thermodynamically stable (free-energy global minimum). In this article, we present computer simulation evidence of coexisting Wenzel/Cassie state (or the bistable state) for water droplets on pillared hydrophobic surface. We have studied 4 conditions that affect the transition between the Wenzel and Cassie state: (i) The height of nanopillars, (ii) the spacing between pillars, (iii) the impinging velocity of w...

Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of HB, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB) with an average yield of 42.3% at room temperature. The sheets feature an sp-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, infrared absorption, electron energy loss spectroscopy, and density functional theory calculations. The established cation-exchange method for metal diboride opens new avenues for the mass production of several types of boron-based 2D materials by countercation selection and functionalization.

We report on the local atomic and electronic structures of a nitrogen-doped graphite surface by scanning tunneling microscopy, scanning tunneling spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbors) and graphitic-N (N bonded to three C nearest neighbors). Distinct electronic states of localized π states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on experimental results and theoretical simulations.