Totally found 250 items.

  • [期刊] Effects of thermodynamic inhibitors on the dissociation of methane hydrate: a molecular dynamics study
    We investigate the effects of methanol and NaCl, which are known as thermodynamic hydrate inhibitors, on the dissociation kinetics of methane hydrate in aqueous solutions by using molecular dynamics simulations. It is shown that the dissociation rate is not constant but changes with time. The dissociation rate in the initial stage is increased by methanol whereas it is decreased by NaCl. This difference arises from the opposite effects of the two thermodynamic inhibitors on the hydration free energy of methane. The dissociation rate of methane hydrate is increased by the formation of methane bubbles in the aqueous phase because the bubbles absorb surrounding methane molecules. It is found that both methanol and NaCl facilitate the bubble formation. However, their mechanisms are completely different from each other. The presence of ions enhances the hydrophobic interactions between methane molecules. In addition, the ions in the solution cause a highly non-uniform distribution of dissolved methane molecules. These two effects result in the easy formation of bubbles in the NaCl solution. In contrast, methanol assists the bubble formation because of its amphiphilic character. References:
  • [期刊] Theoretical insights into nucleation of CO2 and CH4 hydrates for CO2 capture and storage
    We present a hybrid three-dimensional (3D) theoretical approach, the density functional theory (DFT) integrated with the reference interaction site model (RISM), to investigate the nucleation of CO2 and CH4 hydrates. Within the theoretical framework, the 3D-RISM is applied to describe gas density distributions in hydrate cages, and the 3D-DFT is used to describe the interfacial structure and properties of the two hydrates, as well as their nucleation. The crystal-liquid phase equilibria of CO2 and CH4 hydrates are predicted by the hybrid 3D-DFT-RISM, and compared with the available experimental data to examine the theoretical model. In particular, the local and interfacial structure and properties, the critical nucleus radii and free-energy barriers at moderate concentration supersaturation are presented to analyze their nucleation. The formation enthalpies for the two hydrates are calculated to evaluate the possibility of CO2 storage by CH4-CO2 replacement in hydrate. References:
  • [期刊] Line defects and induced doping effects in graphene, hexagonal boron nitride and hybrid BNC
    Effects on the atomic structure and electronic properties of two-dimensional graphene (G) and h-BN sheets related to the coexistence of dopants and defects are investigated by using density functional theory based methods. Two types of extended line defects are considered for pristine G and h-BN sheets. In these sheets, the presence of individual doping increases the charge transport character. The coexistence of dopants and defects tunes the band gap towards lower values and causes the direct-indirect band gap change. The relative stability and the electronic properties of various BxNyCz systems are analyzed in detail. We find that the structural properties of these types of systems strongly depend on the orientation of grain boundaries and whether these are parallel or perpendicular to the extended line defects. The electronic structure analysis of the different systems evidences the shift of absorption to the visible region.
  • [期刊] Facile synthesis of SnO2 nanocrystals anchored onto graphene nanosheets as anode materials for lithium-ion batteries
    A SnO2/graphene nanocomposite was prepared via a facile solvothermal process using stannous octoate as a Sn source. The as-prepared SnO2/graphene nanocomposite exhibited excellent electrochemical behavior with a high reversible capacity, a long cycle life and a good rate capability when used as an anode material for lithiumion batteries.
  • [期刊] Graphene nanopores: electronic transport properties and design methodology
    Graphene nanopores (GNPs) hold great promise as building blocks for electronic circuitry and sensors for biological and chemical sensing applications. Methods to design graphene nanopores that achieve desirable conduction performance and sensing characteristics have not been previously described. Here we present a study of the quantum transport properties of GNPs created by drilling pores in armchair and zigzag graphene ribbons. For the first time, our study reveals that the quantum transmission spectra of GNPs are highly tunable and GNPs with specific transport properties can be produced by properly designing pore shapes. Our investigation shows that the biological sensing capabilities of GNPs are transmission spectrum dependent, can vary dramatically, and are critically dependent on pore geometry. Our study provides design guidelines for creating graphene nanopores with specific transport properties to meet the needs of diverse applications and for developing sensitive biological/chemical sensors with required performance characteristics.
  • [期刊] Molecular mobility on graphene nanoribbons
    Graphene nanoribbons (GNRs) have been proposed to be used as nanoscale mass-conveyor highways. However, how factors such as edge confinement, edge rippling, ribbon twisting, and thermal fluctuations affect the mobility of admolecules on GNRs for such application remains unknown. Using molecular dynamics (MD) simulations, we address these issues by investigating the surface mobility of a physisorbed C60 admolecule on pristine GNRs. We show that the absorption energy and edge energy barrier of a GNR are able to keep and confine the admolecule motion only on one side of the GNR. The twisting of narrow GNRs in combination with thermal fluctuations causes the admolecule to move in a helical trajectory, and thus markedly affects its mobility. As the GNR width increases, its twisting is gradually inhibited, and the motion of the admolecule becomes planar. A comparison between the results from our MD simulations and those from the confined Langevin model indicates that the distinct behavior of molecular mobility on narrow GNRs is due to their twisting rather than geometrical confinement. These findings shed light on the important factors that control the characteristics of the GNR-based mass transport highways.
  • [期刊] High-coverage stable structures of 3d transition metal intercalated bilayer graphene
    Alkali-metal intercalated graphite and graphene have been intensively studied for decades, where alkalimetal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. Using first-principles calculations, we have predicted various stable structures of high-coverage 3d transition metal (TM) intercalated bilayer graphene (BLG) stabilized by the strain. Specifically, with reference to the bulk metal, Sc and Ti can form stable TM-intercalated BLG without strain, while the stabilization of Fe, Co, and Ni intercalated BLG requires the biaxial strain of over 7%. Under the biaxial strain ranging from 0% to 10%, there are four ordered sandwich structures for Sc with the coverage of 0.25, 0.571, 0.684, and 0.75, in which the Sc atoms are all distributed homogenously instead of locating at the hollow sites. According to the phase diagram, a homogenous configuration of C8Ti3C8 with the coverage of 0.75 and another inhomogeneous structure with the coverage of 0.692 were found. The electronic and magnetic properties as a function of strain were also analyzed to indicate that the strain was important for the stabilities of the high-coverage TM-intercalated BLG.
  • [期刊] Graphene networks and their influence on free-volume properties of graphene-epoxidized natural rubber composites with a segregated structure: rheological and positron annihilation studies
    Epoxidized natural rubber-graphene (ENR-GE) composites with segregated GE networks were successfully fabricated using the latex mixing combined in situ reduced technology. The rheological behavior and electrical conductivity of ENR-GE composites were investigated. At low frequencies, the storage modulus (G0) became frequency-independent suggesting a solid-like rheological behavior and the formation of GE networks. According to the percolation theory, the rheological threshold of ENR-GE composites was calculated to be 0.17 vol%, which was lower than the electrical threshold of 0.23 vol%. Both percolation thresholds depended on the evolution of the GE networks in the composites. At low GE concentrations (<0.17 vol%), GE existed as individual units, while a “polymer-bridged GE network”was constructed in the composites when GE concentrations exceeded 0.17 vol%. Finally, a “three-dimensional GE network”with percolation conductive paths was formed with a GE concentration of 0.23 vol%, where a remarkable increase in the conductivity of ENR-GE composites was observed. The effect of GE on the atom scale free-volume properties of composites was further studied by positron annihilation lifetime spectroscopy and positron age momentum correlation measurements. The motion of ENR chains was retarded by the geometric confinement of “GE networks”, producing a high-density interfacial region in the vicinity of GE nanoplatelets, which led to a lower ortho-positronium lifetime intensity and smaller free-volume hole size.
  • [期刊] Morphology controllable nano-sheet polypyrrole-graphene composites for high-rate supercapacitor
    Polypyrrole is a promising candidate for supercapacitor electrode materials due to its high capacitance and low cost. However, the major bottlenecks restricting its application are its poor rate capability and cycling stability. Herein, we control the morphology of polypyrrole-graphene composites by adjusting the graphene content, causing the typical “cauliflower”morphology of polypyrrole to gradually turn into the homogeneous nano-sheet morphology of these composites. The composites consequently exhibit good thermal stability, high protonation level (37.4%), high electronic conductivity (625.3 S m−1), and fast relaxation time (0.22 s). These remarkable characteristics afford a high capacitance of 255.7 F g−1 at 0.2 A g−1, still retaining a capacitance of 199.6 F g−1 at 25.6 A g−1. In addition, high capacitance retention of up to 93% is observed after 1000 cycles testing at different current densities of 0.2, 1.6, 6.4, 12.8 and 25.6 A g−1, indicating high stability. The composite’s excellent electrochemical performance is mainly attributed to its nano-sheet structure and high electronic conductivity, providing unobstructed pathways for the fast diffusion and exchange of ions/electrons.
  • [期刊] Modelling of graphene functionalization
    Graphene has attracted great interest because of its remarkable properties and numerous potential applications. A comprehensive understanding of its structural and dynamic properties and those of its derivatives will be required to enable the design and optimization of sophisticated new nanodevices. While it is challenging to perform experimental studies on nanoscale systems at the atomistic level, this is the ‘native’ scale of computational chemistry. Consequently, computational methods are increasingly being used to complement experimental research in many areas of chemistry and nanotechnology. However, it is difficult for non-experts to get to grips with the plethora of computational tools that are available and their areas of application. This perspective briefly describes the available theoretical methods and models for simulating graphene functionalization based on quantum and classical mechanics. The benefits and drawbacks of the individual methods are discussed, and we provide numerous examples showing how computational methods have provided new insights into the physical and chemical features of complex systems including graphene and graphene derivatives. We believe that this overview will help non-expert readers to understand this field and its great potential.
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