Junjie Guo

Catalytically active single-atom niobium in graphitic layers

Carbides of groups IV through VI (Ti, V and Cr groups) have long been proposed as substitutes for noble metal-based electrocatalysts in polymer electrolyte fuel cells. However, their catalytic activity has been extremely limited because of the low density and stability of catalytically active sites. Here we report the excellent performance of a niobium-carbon structure for catalysing the cathodic oxygen reduction reaction. A large number of single niobium atoms and ultra small clusters trapped in graphitic layers are directly identified using state-of-the-art aberration-corrected scanning transmission electron microscopy. This structure not only enhances the overall conductivity for accelerating the exchange of ions and electrons, but it suppresses the chemical/thermal coarsening of the active particles. Experimental results coupled with theory calculations reveal that the single niobium atoms incorporated within the graphitic layers produce a redistribution of d-band electrons and become surprisingly active for O2 adsorption and dissociation, and also exhibit high stability.

Topological Defects: Origin of Nanopores and Enhanced Adsorption Performance in Nanoporous Carbon

A scanning transmission electron microscopy investigation of two nanoporous carbon materials, wood-based ultramicroporous carbon and poly(furfuryl alcohol)-derived carbon, is reported. Atomic-resolution images demonstrate they comprise isotropic, three-dimensional networks of wrinkled one-atom-thick graphene sheets. In each graphene plane, nonhexagonal defects are frequently observed as connected five- and seven-atom rings. Atomic-level modeling shows that these topological defects induce localized rippling of graphene sheets, which interferes with their graphitic stacking and induces nanopores that lead to enhanced adsorption of H(2) molecules. The poly(furfuryl alcohol)-derived carbon contains larger regions of stacked layers, and shows significantly smaller surface area and pore volume than the ultramicroporous carbon.

Crown ethers in graphene

Crown ethers are at their most basic level rings constructed of oxygen atoms linked by two- or three-carbon chains. They have attracted attention for their ability to selectively incorporate various atoms or molecules within the cavity formed by the ring. However, crown ethers are typically highly flexible, frustrating efforts to rigidify them for many uses that demand higher binding affinity and selectivity. Here we present atomic-resolution images of the same basic structures of the original crown ethers embedded in graphene. This arrangement constrains the crown ethers to be rigid and planar. First-principles calculations show that the close similarity of the structures should also extend to their selectivity towards specific metal cations. Crown ethers in graphene offer a simple environment that can be systematically tested and modelled. Thus, we expect that our finding will introduce a new wave of investigations and applications of chemically functionalized graphene.

Modern approaches to studying gas adsorption in nanoporous carbons

Conventional approaches to understanding the gas adsorption capacity of nanoporous carbons have emphasized the relationship with the effective surface area, but more recent work has demonstrated the importance of local structures and pore-size-dependent adsorption. These developments provide new insights into local structures in nanoporous carbon and their effect on gas adsorption and uptake characteristics. Experiments and theory show that appropriately tuned pores can strongly enhance local adsorption, and that pore sizes can be used to tune adsorption characteristics. In the case of H2 adsorbed on nanostructured carbon, quasielastic and inelastic neutron scattering probes demonstrate novel quantum effects in the motion of adsorbed molecules.

Unusual role of epilayer–substrate interactions in determining orientational relations in van der Waals epitaxy

Significance van der Waals (vdW) heterostructures of dissimilar 2D material sheets held together by vdW interactions promise new physics and applications not necessarily associated with the constituent 2D materials. Only epitaxial growth can achieve the orientational alignment in vdW heterostructures needed to obtain certain novel phenomena. As a case study of vdW epitaxy, we experimentally find that hexagonal boron nitride strictly aligns to Cu(100), whereas crystallographically similar graphene is known to exhibit a wide spread of in-plane rotations. Theoretical investigation reveals that this stark difference occurs because the C–Cu interactions are stronger than the B–Cu and N–Cu interactions. This counterintuitive discovery sheds light on orientational relationships in vdW epitaxy and their case specificity. Using selected-area low-energy electron diffraction analysis, we showed strict orientational alignment of monolayer hexagonal boron nitride (h-BN) crystallites with Cu(100) surface lattices of Cu foil substrates during atmospheric pressure chemical vapor deposition. In sharp contrast, the graphene–Cu(100) system is well-known to assume a wide range of rotations despite graphene’s crystallographic similarity to h-BN. Our density functional theory calculations uncovered the origin of this surprising difference: The crystallite orientation is determined during nucleation by interactions between the cluster’s edges and the substrate. Unlike the weaker B– and N–Cu interactions, strong C–Cu interactions rearrange surface Cu atoms, resulting in the aligned geometry not being a distinct minimum in total energy. The discovery made in this specific case runs counter to the conventional wisdom that strong epilayer–substrate interactions enhance orientational alignment in epitaxy and sheds light on the factors that determine orientational relation in van der Waals epitaxy of 2D materials.

Gigahertz Dielectric Polarization of Substitutional Single Niobium Atoms in Defective Graphitic Layers

We synthesize two Nb/C composites with an order of magnitude difference in the density of single niobium atoms substituted into defective graphitic layers. The concentration and sites of single Nb atoms are identified using aberration-corrected scanning transmission electron microscopy and density functional theory. Comparing the experimental complex permittivity spectra reveals that a representative dielectric resonance at ∼16  GHz originates from the intrinsic polarization of single Nb atom sites, which is confirmed by theoretical simulations. The single-atom dielectric resonance represents the physical limit of the electromagnetic response of condensed matter, and thus might open up a new avenue for designing electromagnetic wave absorption materials. Single-atom resonance also has important implications in understanding the correlation between the macroscopic dielectric behaviors and the atomic-scale structural origin.

Direct Observation of Defects in Hexagonal Boron Nitride Monolayers

1. Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN 37996, United States 2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States 3. Department of Electrical Engineering and Computer Science, the University of Tennessee, Knoxville, TN 37996, United States 4. Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210, United States