Alister J. Page

The ONIOM Method and Its Applications

Lung Wa Chung,† W. M. C. Sameera,‡ Romain Ramozzi,‡ Alister J. Page, Miho Hatanaka,‡ Galina P. Petrova, Travis V. Harris,‡,⊥ Xin Li, Zhuofeng Ke, Fengyi Liu, Hai-Bei Li, Lina Ding, and Keiji Morokuma*,‡ †Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China ‡Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihiraki-cho, Sakyo, Kyoto 606-8103, Japan Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan 2308, Australia Faculty of Chemistry and Pharmacy, University of Sofia, Bulgaria Boulevard James Bourchier 1, 1164 Sofia, Bulgaria Department of Chemistry, State University of New York at Oswego, Oswego, New York 13126, United States State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710119, China School of Ocean, Shandong University, Weihai 264209, China School of Pharmaceutical Sciences, Zhengzhou University, 100 Kexue Avenue, Zhengzhou, Henan 450001, China

SWNT Nucleation from Carbon-Coated SiO2 Nanoparticles via a Vapor−Solid−Solid Mechanism

Since the discovery of single-walled carbon nanotubes (SWNTs) in the early 1990s, the most commonly accepted model of SWNT growth on traditional catalysts (i.e., transition metals including Fe, Co, Ni, etc.) is the vapor-liquid-solid (VLS) mechanism. In more recent years, the synthesis of SWNTs on nontraditional catalysts, such as SiO(2), has also been reported. The precise atomistic mechanism explaining SWNT growth on nontraditional catalysts, however, remains unknown. In this work, CH(4) chemical vapor deposition (CVD) and single-walled carbon nanotube (SWNT) nucleation on SiO(2) nanoparticles have been investigated using quantum-chemical molecular dynamics (QM/MD) methods. Upon supply of CH(x) species to the surface of a model SiO(2) nanoparticle, CO was produced as the main chemical product of the CH(4) CVD process, in agreement with a recent experimental investigation [Bachmatiuk et al., ACS Nano 2009, 3, 4098]. The production of CO occurred simultaneously with the carbothermal reduction of the SiO(2) nanoparticle. However, this reduction, and the formation of amorphous SiC, was restricted to the nanoparticle surface, with the core of the SiO(2) nanoparticle remaining oxygen-rich. In cases of high carbon concentration, SWNT nucleation then followed, and was driven by the formation of isolated sp(2)-carbon networks via the gradual coalescence of adjacent polyyne chains. These simulations indicate that the carbon saturation of the SiO(2) surface was a necessary prerequisite for SWNT nucleation. These simulations also indicate that a vapor-solid-solid mechanism, rather than a VLS mechanism, is responsible for SWNT nucleation on SiO(2). Fundamental differences between SWNT nucleation on nontraditional and traditional catalysts are therefore observed.

Nearly Exclusive Growth of Small Diameter Semiconducting Single-Wall Carbon Nanotubes from Organic Chemistry Synthetic End-Cap Molecules

The inability to synthesize single-wall carbon nanotubes (SWCNTs) possessing uniform electronic properties and chirality represents the major impediment to their widespread applications. Recently, there is growing interest to explore and synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable advantage of such processes is that no metal catalysts are used, and the produced nanotubes will be free of metal contamination. Many of these methods, however, suffer shortcomings of either low yield or poor controllability of nanotube uniformity. Here, we report a brand new approach to achieve high-efficiency metal-free growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic characterization, electrical transport measurements, and density functional theory calculations. Our strategy combines bottom-up organic chemistry synthesis with vapor phase epitaxy elongation. We identify a strong correlation between the electronic properties of SWCNTs and their diameters in nanotube growth. This study not only provides material platforms for electronic applications of semiconducting SWCNTs but also contributes to fundamental understanding of the growth mechanism and controlled synthesis of SWCNTs.

Dynamics of Local Chirality during SWCNT Growth: Armchair versus Zigzag Nanotubes

We present an analysis of the dynamics of single-walled carbon nanotube (SWCNT) chirality during growth, using the recently developed local chirality index (LOCI) method [ Kim et al. Phys. Rev. Lett. 2011 , 107 , 175505 ] in conjunction with quantum chemical molecular dynamics (QM/MD) simulations. Using (5,5) and (8,0) SWCNT fragments attached to an Fe(38) catalyst nanoparticle, growth was induced by periodically placing carbon atoms at the edge of the SWCNT. For both armchair and zigzag SWCNTs, QM/MD simulations indicate that defect healing-the process of defect removal during growth-is a necessary, but not sufficient, condition for chirality-controlled SWCNT growth. Time-evolution LOCI analysis shows that healing, while restoring the pristine hexagon structure of the growing SWCNT, also leads to changes in the local chirality of the SWCNT edge region and thus of the entire SWCNT itself. In this respect, we show that zigzag SWCNTs are significantly inferior in maintaining their chirality during growth compared to armchair SWCNTs.

3-Dimensional atomic scale structure of the ionic liquid-graphite interface elucidated by AM-AFM and quantum chemical simulations.

In situ amplitude modulated atomic force microscopy (AM-AFM) and quantum chemical simulations are used to resolve the structure of the highly ordered pyrolytic graphite (HOPG)-bulk propylammonium nitrate (PAN) interface with resolution comparable with that achieved for frozen ionic liquid (IL) monolayers using STM. This is the first time that (a) molecular resolution images of bulk IL-solid interfaces have been achieved, (b) the lateral structure of the IL graphite interface has been imaged for any IL, (c) AM-AFM has elucidated molecular level structure immersed in a viscous liquid and (d) it has been demonstrated that the IL structure at solid surfaces is a consequence of both thermodynamic and kinetic effects. The lateral structure of the PAN-graphite interface is highly ordered and consists of remarkably well-defined domains of a rhomboidal superstructure composed of propylammonium cations preferentially aligned along two of the three directions in the underlying graphite lattice. The nanostructure is primarily determined by the cation. Van der Waals interactions between the propylammonium chains and the surface mean that the cation is enriched in the surface layer, and is much less mobile than the anion. The presence of a heterogeneous lateral structure at an ionic liquid-solid interface has wide ranging ramifications for ionic liquid applications, including lubrication, capacitive charge storage and electrodeposition.

Sub-surface nucleation of graphene precursors near a Ni(111) step-edge

Graphene nucleation on Ni(111) has been modeled using QM/MD simulations. We demonstrate that graphene precursor nucleation can occur underneath the catalyst surface. In addition, a Ni(111) step-edge is not a static structure, as is often assumed; it is instead highly malleable, being deformed and subsequently healed during graphene nucleation.

Quantum chemical investigation of epoxide and ether groups in graphene oxide and their vibrational spectra.

We present a detailed analysis of the factors influencing the formation of epoxide and ether groups in graphene nanoflakes using conventional density functional theory (DFT), the density-functional tight-binding (DFTB) method, π-Hückel theory, and graph theoretical invariants. The relative thermodynamic stability associated with the chemisorption of oxygen atoms at various positions on hexagonal graphene flakes (HGFs) of D(6h)-symmetry is determined by two factors - viz. the disruption of the π-conjugation of the HGF and the geometrical deformation of the HGF structure. The thermodynamically most stable structure is achieved when the former factor is minimized, and the latter factor is simultaneously maximized. Infrared (IR) spectra computed using DFT and DFTB reveal a close correlation between the relative thermodynamic stabilities of the oxidized HGF structures and their IR spectral activities. The most stable oxidized structures exhibit significant IR activity between 600 and 1800 cm(-1), whereas less stable oxidized structures exhibit little to no activity in this region. In contrast, Raman spectra are found to be less informative in this respect.

Molecular Dynamics Simulation of the Low-Temperature Partial Oxidation of CH4

Low-temperature partial oxidation of methane was investigated using reactive molecular dynamics (MD) and quantum mechanical (QM) methods. In particular, the ReaxFF hydrocarbon force field [Chenoweth, K.; et al. J. Phys. Chem. A 2008, 112, 1040] was employed to simulate a [20 CH(4) + 10 O(2)] model system at 500 degrees C. The chemical mechanism of the partial oxidation of methane was proposed on the basis of analysis of the computed trajectory of this model system. The partial oxidation of methane was observed to be initiated by the abstraction of hydrogen from CH(4) by O(2) and the atomization of CH(4) itself. Subsequent radical recombination between hydrogen atoms and the dehydrogenation of CH(4) were the primary pathways by which H(2) was formed. In agreement with current models of low-temperature combustion, radicals including H(3)C-OO and H(2)C-OO were also observed during the MD simulation. The observed reaction mechanism was subsequently analyzed using QM methods. For instance, structural features of prominent radical species observed during the MD simulation were analyzed using density functional theory (DFT) and coupled-cluster (CCSD(T)) methods. Enthalpies of reaction of all observed chemical processes were calculated using DFT and the W1 composite method. Where possible, comparisons with experimental data were made.

Stochastic structure determination for conformationally flexible heterogenous molecular clusters: application to ionic liquids

We present a novel method that enables accurate and efficient computational determination of conformationally flexible clusters, “Kick3” This method uses stochastically generated structures in combination with fast quantum mechanical methods. We demonstrate the power of this method by elucidating the structure of ionic liquid (IL) ([xMIM+][ NO3−])n clusters (x = E, B, D, n = 1–10,15). Dispersion‐corrected, third‐order self‐consistent‐charge density‐functional tight‐binding (DFTB3) is shown to be a computationally efficient, yet reliable approximation to density functional theory for predicting and understanding IL structure and stability. The presented approach, therefore, enables the accurate and efficient screening of ILs with high potential toward practical applications, without recourse to more expensive quantum chemical methods.

Theoretical Insights into Chirality-Controlled SWCNT Growth from a Cycloparaphenylene Template

A self-assembly mechanism for low-temperature SWCNT growth from a [6]cycloparaphenylene ([6]CPP) precursor via ethynyl (C(2)H) radical addition is presented, based on non-equilibrium quantum chemical molecular dynamics (QM/MD) simulations and density functional theory (DFT) calculations. This mechanism, which maintains the (6,6) armchair chirality of a SWCNT fragment throughout the growth process, is energetically more favorable than a previously proposed Diels-Alder-based growth mechanisms [E. H. Fort, et al., J. Mater. Chem. 2011, 21, 1373]. QM/MD simulations and DFT calculations show that C(2)H radicals play dual roles during SWCNT growth, by abstracting hydrogen from the SWCNT fragment and providing the carbon source necessary for growth itself. Simulations demonstrate that chirality-controlled SWCNT growth from macrocyclic hydrocarbon seed molecules with pre-selected edge structure can be accomplished when the reaction conditions are carefully selected for hydrogen abstraction by radical species during the growth process.