The presence of defects in graphene has an essential influence on its physical and chemical properties. The formation, behaviour and healing of defects are determined by energetic characteristics of ...atomic scale structure changes. In this article, we review recent studies devoted to atomic scale reactions during thermally activated and irradiation-induced processes in graphene. The formation energies of vacancies, adatoms and topological defects are discussed. Defect formation, healing and migration are quantified in terms of activation energies (barriers) for thermally activated processes and by threshold energies for processes occurring under electron irradiation. The energetics of defects in the graphene interior and at the edge is analysed. The effects of applied strain and a close proximity of the edge on the energetics of atomic scale reactions are overviewed. Particular attention is given to problems where further studies are required.
Although fullerenes can be efficiently generated from graphite in high yield, the route to the formation of these symmetrical and aesthetically pleasing carbon cages from a flat graphene sheet ...remains a mystery. The most widely accepted mechanisms postulate that the graphene structure dissociates to very small clusters of carbon atoms such as C(2), which subsequently coalesce to form fullerene cages through a series of intermediates. In this Article, aberration-corrected transmission electron microscopy directly visualizes, in real time, a process of fullerene formation from a graphene sheet. Quantum chemical modelling explains four critical steps in a top-down mechanism of fullerene formation: (i) loss of carbon atoms at the edge of graphene, leading to (ii) the formation of pentagons, which (iii) triggers the curving of graphene into a bowl-shaped structure and which (iv) subsequently zips up its open edges to form a closed fullerene structure.
The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often ...insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.
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► Flakes with vacancy at the edge are more stable than those with vacancy in the middle. ► Thermally activated motion of vacancy towards the edge occurs at room temperature. ► The ...probability of return motion back to the middle is negligible. ► Mechanisms driving structural transformations in graphene are discussed.
Density functional theory calculations show that graphene flakes with monovacancy at the edge are energetically more stable than the flakes with vacancy in the middle. The energies of metastable and transition states for one step of vacancy motion towards the edge are calculated. We show that thermally activated motion of vacancy towards the edge occurs even at room temperature whereas the probability of return motion back to the middle is negligible. Molecular dynamics simulations of the vacancy motion in graphene flakes confirm this conclusion. The obtained results explain the mechanisms driving structural transformations in graphene.
The porous framework Cu2(H2O)2L⋅4 H2O⋅2 DMA (H4L=oxalylbis(azanediyl)diisophthalic acid; DMA=N,N‐dimethylacetamide), denoted NOTT‐125, is formed by connection of {Cu2(RCOO)4} paddlewheels with the ...isophthalate linkers in L4−. A single crystal structure determination reveals that NOTT‐125 crystallises in monoclinic unit cell with a=27.9161(6), b=18.6627(4) and c=32.3643(8) Å, β=112.655(3)°, space group P21/c. The structure of this material shows fof topology, which can be viewed as the packing of two types of cages (cage A and cage B) in three‐dimensional space. Cage A is constructed from twelve {Cu2(OOCR)4} paddlewheels and six linkers to form an ellipsoid‐shaped cavity approximately 24.0 Å along its long axis and 9.6 Å across its central diameter. Cage B consists of six {Cu2(OOCR)4} units and twelve linkers and has a spherical diameter of 12.7 Å taking into account the van der Waals radii of the atoms. NOTT‐125 incorporates oxamide functionality within the pore walls, and this, combined with high porosity in desolvated NOTT‐125a, is responsible for excellent CO2 uptake (40.1 wt % at 273 K and 1 bar) and selectivity for CO2 over CH4 or N2. Grand canonical Monte Carlo (GCMC) simulations show excellent agreement with the experimental gas isotherm data, and a computational study of the specific interactions and binding energies of both CO2 and CH4 with the linkers in NOTT‐125 reveals a set of strong interactions between CO2 and the oxamide motif that are not possible with a single amide.
The rich in pore: A highly porous fof‐type CuII paddlewheel porous material, NOTT‐125, the first reported example of a MOF containing the oxamide functional group, is reported. Two types of cage exist within NOTT‐125 (see figure), both of which are bounded by the oxamide functionality. The large pore volume and functionalised pores are responsible for NOTT‐125a exhibiting high CO2 gas storage capacity and selectivity of CO2 uptake over N2 and CH4.
Carbon nanotubes (CNTs) act as efficient nanoreactors, templating the assembly of sulfur-terminated graphene nanoribbons (S-GNRs) with different sizes, structures, and conformations. Spontaneous ...formation of nanoribbons from small sulfur-containing molecules is efficiently triggered by heat treatment or by an 80 keV electron beam. S-GNRs form readily in CNTs with internal diameters between 1 and 2 nm. Outside of this optimum range, nanotubes narrower than 1 nm do not have sufficient space to accommodate the 2D structure of S-GNRs, while nanotubes wider than 2 nm do not provide efficient confinement for unidirectional S-GNR growth, thus neither can support nanoribbon formation. Theoretical calculations show that the thermodynamic stability of nanoribbons is dependent on the S-GNR edge structure and, to a lesser extent, the width of the nanoribbon. For nanoribbons of similar widths, the polythiaperipolycene-type edges of zigzag S-GNRs are more stable than the polythiophene-type edges of armchair S-GNRs. Both the edge structure and the width define the electronic properties of S-GNRs which can vary widely from metallic to semiconductor to insulator. The encapsulated S-GNRs exhibit diverse dynamic behavior, including rotation, translation, and helical twisting inside the nanotube, which offers a mechanism for control of the electronic properties of the graphene nanoribbon via confinement at the nanoscale.
Structural characterisation of individual molecules by high‐resolution transmission electron microscopy (HRTEM) is fundamentally limited by the element and electron energy‐specific interactions of ...the material with the high energy electron beam. Here, the key mechanisms controlling the interactions between the e‐beam and C–H bonds, present in all organic molecules, are examined, and the low atomic weight of hydrogen—resulting in its facile atomic displacement by the e‐beam—is identified as the principal cause of the instability of individual organic molecules. It is demonstrated theoretically and proven experimentally that exchanging all hydrogen atoms within molecules with the deuterium isotope, and therefore doubling the atomic weight of the lightest atoms in the structure, leads to a more than two‐fold increase in the stability of organic molecules in the e‐beam. Substitution of H for D significantly reduces the amount of kinetic energy transferred from the e‐beam to the atom (main factor contributing to stability) and also increases the barrier for bond dissociation, primarily due to the changes in the zero‐point energy of the C–D vibration (minor factor). The extended lifetime of coronene‐d12, used as a model molecule, enables more precise analysis of the inter‐molecular spacing and more accurate measurement of the molecular orientations.
Due to the instability of C–H bonds in the electron beam, atomic‐resolution imaging of individual organic molecules in transmission electron microscopy is effectively prohibited. Isotope exchange of protium for deuterium significantly enhances the stability of organic molecules as the higher atomic weight of deuterium simultaneously decreases the amount of energy transferred from the electron beam, and increases the barriers for atom ejection.
Carbon nanotubes are the most commonly used 'building blocks' of modern nanotechnology. Their unique mechanical and electronic properties, stability and functionality show great promise in creating ...functional devices on the nanometre scale. One of the great challenges in using this scale is the ability of physical manipulation of the components, such as their positioning and assembling. Strong correlation between the structure and mechanical interactions of the walls of carbon nanotubes provides self-regulation of their relative motion. This can be further exploited in low-friction and high-stiffness devices. In this paper, we present a condensed overview of the recent progress in fundamental understanding of nanomechanical and nanoelectromechanical behaviour of carbon nanotubes and their applications in nanodevices.
Although the outer surface of single-walled carbon nanotubes (atomically thin cylinders of carbon) can be involved in a wide range of chemical reactions, it is generally thought that the interior ...surface of nanotubes is unreactive. In this study, we show that in the presence of catalytically active atoms of rhenium inserted into nanotubes, the nanotube sidewall can be engaged in chemical reactions from the inside. Aberration-corrected high-resolution transmission electron microscopy operated at 80 keV allows visualization of the formation of nanometre-sized hollow protrusions on the nanotube sidewall at the atomic level in real time at ambient temperature. Our direct observations and theoretical modelling demonstrate that the nanoprotrusions are formed in three stages: (i) metal-assisted deformation and rupture of the nanotube sidewall, (ii) the fast formation of a metastable asymmetric nanoprotrusion with an open edge and (iii) a slow symmetrization process that leads to a stable closed nanoprotrusion.
Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to ...triple carbon–carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free‐standing and surface‐adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non‐covalent interactions between reactant molecules and the nanofibre channel. The specific π–π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the β‐addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis‐orientation with the internal graphitic step‐edges of GNF. The importance of π–π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors.
Nanoscale reaction vessels: Carbon nanoreactors are prepared by encapsulating catalytic Rh or RhPt nanoparticles in hollow graphitised nanofibres. Inside the nanoreactors, the pathways of the hydrosilylation reactions differ from those on the surface of nanofibres or in the bulk phase (see scheme).
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