Template effect in the competition between Haeckelite and graphene growth on Ni(111): quantum chemical molecular dynamics simulations

Quantum chemical molecular dynamics (QM/MD) simulations of ensembles of C(2) molecules on the Ni(111) terrace show that, in the absence of a hexagonal template or step edge, Haeckelite is preferentially nucleated over graphene as a metastable intermediate. The nucleation process is dominated by the swift transition of long carbon chains toward a fully connected sp(2) carbon network. Starting from a pentagon as nucleus, pentagons and heptagons condense during ring collapse reactions, which results in zero overall curvature. To the contrary, in the presence of a coronene-like C(24) template, hexagonal ring formation is clearly promoted, in agreement with recent suggestions from experiments. In the absence of step edges or molecular templates, graphene nucleation follows Ostwald's "rule of stages" cascade of metastable states, from linear carbon chains, via Haeckelite islands that finally anneal to graphene.     

Interaction of Pt clusters with the anatase TiO(2)(101) surface: a first principles study

The adsorption of Pt(n)() (n = 1-3) clusters on the defect-free anatase TiO(2)(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO(2)(101) surface has a stepped structure with a step width of two O-Ti bond distances in the (100) plane along the [10] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt-O bonds formed upon Pt adsorption are covalent. Among six stable Pt(2) adsorption configurations examined, Pt(2) was found to energetically favor the O-O bridge sites on the step edge along [010] with the Pt-Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O-O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt(3) structures were determined as minima for Pt(3) adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt(3) occupies the edge O-O site with the Pt(3) plane being upright and almost perpendicular to the [001] terrace. The preference of Pt(n)() to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.     

金属衬底在石墨烯化学气相沉积生长中的作用

以过渡金属为催化衬底的化学气相沉积法(Chemical Vapor Deposition，CVD)已经可以制备与机械剥离样品相媲美的石墨烯，是实现石墨烯工业应用的关键技术之一。原子尺度理论研究能够帮助我们深刻理解石墨烯生长机理，为实验现象提供合理的解释，并有可能成为将来实验设计的理论指导。本文从理论计算的角度，总结了各种金属衬底在石墨烯CVD生长过程中的各种作用与相应的机理，包括在催化碳源裂解、降低石墨烯成核密度等，催化加快石墨烯快速生长，修复石墨烯生长过程中产生的缺陷，控制外延生长石墨烯的晶格取向，以及在降温过程中石墨烯褶皱与金属表面台阶束的形成过程等。在本文最后，我们对当前石墨烯生长领域中亟需解决的理论问题进行了深入探讨与展望。     

Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite‐Free Lithium Metal Anodes

Lithium (Li) metal is the most promising electrode for next‐generation rechargeable batteries. However, the challenges induced by Li dendrites on a working Li metal anode hinder the practical applications of Li metal batteries. Herein, nitrogen (N) doped graphene was adopted as the Li plating matrix to regulate Li metal nucleation and suppress dendrite growth. The N‐containing functional groups, such as pyridinic and pyrrolic nitrogen in the N‐doped graphene, are lithiophilic, which guide the metallic Li nucleation causing the metal to distribute uniformly on the anode surface. As a result, the N‐doped graphene modified Li metal anode exhibits a dendrite‐free morphology during repeated Li plating and demonstrates a high Coulombic efficiency of 98 % for near 200 cycles.     

Transition metal surface passivation induced graphene edge reconstruction

In vacuum, the bare zigzag (zz) edge of graphene is reconstructed into a line of pentagon-heptagon pairs, while the pristine armchair (ac) edge is retained. Our first-principle explorations of graphene edges on three metal surfaces [Cu(111), Co(111), and Ni(111)] indicate an opposite tendency, that is, the pristine zz edge is energetically favorable and the reconstructed ac edge with dangling C atoms is highly stable on Co(111) and Ni(111) surfaces. Insightful analysis shows that passivation of the graphene edge by metal surfaces is responsible for the dramatic differences. Beyond this, the unique edge configuration has a significant impact on the graphene CVD growth behavior.     

Structural Evolution of Boron Clusters on Ag(111) Surfaces – From Atomic Chains to Triangular Sheets with Hexagonal Holes

Unlike graphene and other 2D materials, borophene is 2D polymorphic with diverse hexagonal holes (HHs)-triangles ratios and the concentrations of HHs are highly substrate dependent. Here, we systematically explored the evolution of boron cluster on Ag(111) surface, B_N@Ag(111) (N=1∼36), to understand the nucleation of 2D boron sheet on metal surface. Our calculation showed that, with the size increasing, the structures of most stable B_N clusters undergo an evolution from compact triangular lattice, such as double-chains or triple-chains, to the ones with mixed triangular-hexagonal lattices. The first single-HH appears at N=12 and the first double-HH appears at N=27. The stability of large B_N clusters with mixed structures is derived from the charge transfer between triangular lattice and the HHs, as well as between the substrates and the B_N clusters. Our results provide a deep understanding on the formation of small boron clusters in the initial nucleation stage of borophene growth.     

Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes

Lithium (Li) metal is the most promising electrode for next-generation rechargeable batteries. However, the challenges induced by Li dendrites on a working Li metal anode hinder the practical applications of Li metal batteries. Herein, nitrogen (N) doped graphene was adopted as the Li plating matrix to regulate Li metal nucleation and suppress dendrite growth. The N-containing functional groups, such as pyridinic and pyrrolic nitrogen in the N-doped graphene, are lithiophilic, which guide the metallic Li nucleation causing the metal to distribute uniformly on the anode surface. As a result, the N-doped graphene modified Li metal anode exhibits a dendrite-free morphology during repeated Li plating and demonstrates a high Coulombic efficiency of 98 % for near 200 cycles.     

A Generalized Strategy for the Synthesis of Large‐Size Ultrathin Two‐Dimensional Metal Oxide Nanosheets

Two‐dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface‐to‐volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large‐size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet‐chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main‐group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state.     

A Generalized Strategy for the Synthesis of Large-Size Ultrathin Two-Dimensional Metal Oxide Nanosheets

Two-dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface-to-volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large-size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet-chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main-group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state.     

Energetics and heterodiffusion of Cu on Ag(110) stepped surfaces

A molecular‐dynamics simulation study has been performed to investigate the Cu adatom diffusion on the (110) stepped surfaces of Ag, using interatomic potentials described by the embedded atom method. We have systematically calculated the energy barriers for different possible diffusion mechanisms, which occur on the terrace and near the step edge. Our findings show that the predominant atomistic diffusion process at step edge and on the terrace is the exchange mechanism with anES barrier about 220 meV lower than that via jumping (290 meV), indicating that the incorporation of Cu adatom into Ag(110) is ‘easier’ than making a jump on the surface. On the other hand, the calculation of the Ehrlich–Schwoebel (ES) barrier demonstrates that this quantity is equal to 0 meV for the exchange process near the step edge and about 60 meV for channel–channel migration. Thus, the mass transport across steps may be important due to the lack of the ES barriers for exchange mechanism, revealing the possible layer‐by‐layer growth mode for our heterogeneous system. Copyright © 2015 John Wiley & Sons, Ltd.     

A Molecular Pillar Approach To Grow Vertical Covalent Organic Framework Nanosheets on Graphene: Hybrid Materials for Energy Storage

Hybrid 2D–2D materials composed of perpendicularly oriented covalent organic frameworks (COFs) and graphene were prepared and tested for energy storage applications. Diboronic acid molecules covalently attached to graphene oxide (GO) were used as nucleation sites for directing vertical growth of COF‐1 nanosheets (v‐COF‐GO). The hybrid material has a forest of COF‐1 nanosheets with a thickness of 3 to 15 nm in edge‐on orientation relative to GO. The reaction performed without molecular pillars resulted in uncontrollable growth of thick COF‐1 platelets parallel to the surface of GO. The v‐COF‐GO was converted into a conductive carbon material preserving the nanostructure of precursor with ultrathin porous carbon nanosheets grafted to graphene in edge‐on orientation. It was demonstrated as a high‐performance electrode material for supercapacitors. The molecular pillar approach can be used for preparation of many other 2D‐2D materials with control of their relative orientation.     

Atomic Layer Deposition of Hafnium(IV) Oxide on Graphene Oxide: Probing Interfacial Chemistry and Nucleation by using X‐ray Absorption and Photoelectron Spectroscopies

Interfacing graphene with metal oxides is of considerable technological importance for modulating carrier density through electrostatic gating as well as for the design of earth‐abundant electrocatalysts. Herein, we probe the early stages of the atomic layer deposition (ALD) of HfO₂ on graphene oxide using a combination of C and O K‐edge near‐edge X‐ray absorption fine structure spectroscopies and X‐ray photoelectron spectroscopy. Dosing with water is observed to promote defunctionalization of graphene oxide as a result of the reaction between water and hydroxyl/epoxide species, which yields carbonyl groups that further react with migratory epoxide species to release CO₂. The carboxylates formed by the reaction of carbonyl and epoxide species facilitate binding of Hf precursors to graphene oxide surfaces. The ALD process is accompanied by recovery of the π‐conjugated framework of graphene. The delineation of binding modes provides a means to rationally assemble 2D heterostructures.     

Importance of electronegativity differences and surface structure in molecular dissociation reactions at transition metal surfaces

The dissociative adsorption of N2 has been studied at both monatomic steps and flat regions on the surfaces of the 4d transition metals from Zr to Pd. Using density functional theory (DFT) calculations, we have determined and analyzed the trends in both straight reactivity and structure sensitivity across the periodic table. With regards to reactivity, we find that the trend in activation energy (Ea) is determined mainly by a charge transfer from the surface metal atoms to the N atoms during transition state formation, namely, the degree of ionicity of the N-surface bond at the transition state. Indeed, we find that the strength of the metal-N bond at the transition state (and therefore the trend in Ea) can be predicted by the difference in Mulliken electronegativity between the metal and N. Structure sensitivity is analyzed in terms of geometric and electronic effects. We find that the lowering of Ea due to steps is more pronounced on the right-hand side of the periodic table. It is found that for the early transition metals the geometric and electronic effects work in opposition when going from terrace to step active site. In the case of the late 4d metals, however, these effects work in combination, producing a more marked reduction in Ea.     

Impedance of metal hydride electrodes. Rate of phase transformation limited by nucleation and growth mechanisms

A mathematical model for the electrochemical impedance spectroscopy of a metal hydride electrode was developed. Ac impedance data of phase transformation were derived by considering a nucleation and growth mechanism based on the theory developed by Johnson–Mehl–Avramy. Different mechanisms such as grain edge and grain boundary preferential nucleation sites are discussed. Global Nyquist plots of the metal hydride electrode are obtained by adding a surface charge transfer reaction and a double-layer capacitance to the model.     

Bulk and surface oxygen vacancy formation and diffusion in single crystals, ultrathin films, and metal grown oxide structures

The neutral oxygen vacancy (OV) energy formation for bulk, subsurface sites at different depths from the surface and various surface sites has been estimated for single crystals, unsupported ultrathin films of MgO, CaO, and BaO, and MgO ultrathin films supported on Ag(001). From the calculated energy barriers for diffusion through the surface and from the surface to the bulk it is found that diffusion is a hindered event, especially for MgO. Nevertheless, diffusion from the terrace to step edges is largely favored while diffusion through terrace sites is less likely and surface to bulk has a very low probability. It is argued that this explains recent scanning tunneling microscopy images for MgO thin films supported on Ag(001) showing OV populating preferentially the step edge sites.     

Lattice mismatch induced nonlinear growth of graphene

As a two-dimensional material, graphene can be obtained via epitaxial growth on a suitable substrate. Recently, an interesting nonlinear behavior of graphene growth has been observed on some metal surfaces, but the underlying mechanism is still elusive. Taking the Ir(111) surface as an example, we perform a mechanistic study on graphene growth using a combined approach of first-principles calculations and kinetic Monte Carlo (kMC) simulations. Small carbon clusters on the terrace or at step sites are studied first. Then, we investigate how these small carbon species are attached to graphene edges. Generally, attachment of carbon atoms is thermodynamically favorable. However, due to substrate effect, there are also some edge sites where graphene growth must proceed via cluster attachment. The overall growth rate is determined by these cluster attachment processes, which have a much lower chance of happening compared to the monomer attachment. On the basis of such an inhomogeneous growth picture, kMC simulations are performed by separating different time scales, and the experimentally found quintic-like behavior is well reproduced. Different nonlinear growth behaviors are predicted for different graphene orientations, which is consistent with previous experiments. Inhomogeneity induced by lattice mismatch revealed in this study is expected to be a universal phenomenon and will play an important role in the growth of many other heteroepitaxial systems.     

Tunable ferromagnetism in assembled two dimensional triangular graphene nanoflakes

Triangular graphene nanoflakes (TGFs), due to their novel magnetic configurations, can serve as building blocks to design new magnetic materials. Based on spin polarized density functional theory, we show that the two dimensional (2D) structures composed of zigzag-edged TGFs linked by 1,3,5-benzenetriyl units (TGF(N)-C(6)H(3)) are ferromagnetic. Their magnetic moments can be tuned by changing the size and edge termination of TGFs, namely magnetic moments increase linearly with the size of TGFs, and double hydrogenation of the edge carbon atoms can significantly enhance stability of the ferromagnetic states. The dynamic stability of the assembled 2D structures is further confirmed by frequency calculations. The characteristic breathing mode is identified where the frequency changes with the inverse square root of the TGFs width, which can be used to identify the size of TGF(N)-C(6)H(3) in Raman experiments. This study provides new pathways to assemble 2D ferromagnetic carbon materials.     

Anisotropic diffusion of n-butane and n-decane on a stepped metal surface

The diffusion of single n-butane and n-decane molecules on a model stepped surface, Pt655, and on a corresponding flat surface, Pt111, is investigated using molecular-dynamics simulations and anisotropic united atom model. The surface step on Pt655 causes the alkane molecules to adsorb on the lower terrace in all-trans conformations with their long molecular axes adjacent and parallel to the step edge, and to diffuse anisotropically along the surface step via a constant wiggly motion without rotation or marked deviation from the parallel adsorption configuration. At relatively high temperatures, the alkane molecules can temporarily break away from the step edge but cannot migrate across the step edge in either the downstair or upstair direction. In comparison with the diffusion on Pt111, the diffusivity of n-decane is reduced by the surface step but its diffusion barrier is hardly affected. In the case of the shorter n-butane, however, the surface step significantly reduces the diffusion energy barrier and gives rise to higher diffusion coefficients at lower temperatures. Important implications of the simulation results are discussed.     

Large entropy difference between terrace and step sites on surfaces

Using atomic beam/surface scattering measurements to investigate the desorption kinetics of low-coverage Pb from Mo(100), we uncover a large entropy difference between Pb atoms at terrace and step sites, which should be general for adsorbates on surfaces at high temperatures. A line shape analysis of the transient desorption signal reveals the presence of two species with different lifetimes on the surface. An Arrhenius analysis of these lifetimes from 1150 to 1320 K provides the prefactors and desorption activation energies (332 and 411 kJ/mol) of these two states. A comparison of these energies to those measured directly via adsorption calorimetry strongly suggests that one state is a terrace-bound species. The other, more strongly bound species is attributed to steps. The more strongly bound step species has the higher rate constant for desorption because of its much larger desorption prefactor (9 x 10 (19) vs 5 x 10 (15) s (-1)). Within transition state theory, the ratio of these prefactors corresponds to 82 J/(mol K) higher entropy for the terrace species than for the step species. This large entropy difference is quantitatively reproduced by a simple model which assumes the terrace species is a 2D ideal gas parallel to the surface and the step species is a 1D ideal gas along the step edges. Such a difference will generally exist for adsorbed species when k B T exceeds the barrier height for adsorbate diffusion across terraces. A consequence of this large entropy difference is that the defect sites are much less populated relative to terrace sites than would be expected based on enthalpy alone. The measured prefactor for Pb desorption was used to analyze earlier surface lifetime measurements for Pb on MgO(100) to extract adsorption energies for that system, as well.     

Real‐Space Observation of Atomic Site‐Specific Electronic Properties of a Pt Nanoisland/Au(111) Bimetallic Surface by Tip‐Enhanced Raman Spectroscopy

Resolving atomic site‐specific electronic properties and correlated substrate–molecule interactions is challenging in real space. Now, mapping of sub‐10 nm sized Pt nanoislands on a Au(111) surface was achieved by tip‐enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4‐chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low‐coordinate atomic sites have a higher d‐band electronic profile and thus stronger metal–molecule interactions, leading to the observed blue‐shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono‐ and bi‐atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.