Discovery of Graphene Growth Alloy Catalysts Using High-Throughput Machine Learning.

Despite today's commercial-scale graphene production using chemical vapor deposition (CVD), the growth of high-quality single-layer graphene with controlled morphology and crystallinity remains challenging. Considerable effort is still spent on designing improved CVD catalysts for producing high-quality graphene. Conventionally, however, catalyst design has been pursued using empirical intuition or trial-and-error approaches. Here, we combine high-throughput density functional theory and machine learning to identify new prospective transition metal alloy catalysts that exhibit performance comparable to that of established graphene catalysts, such as Ni(111) and Cu(111). The alloys identified through this process generally consist of combinations of early- and late-transition metals, and a majority are alloys of Ni or Cu. Nevertheless, in many cases, these conventional catalyst metals are combined with unconventional partners, such as Zr, Hf, and Nb. The approach presented here therefore highlights an important new approach for identifying novel catalyst materials for the CVD growth of low-dimensional nanomaterials.

Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles.

Employing neutral impact collision ion scattering spectroscopy (NICISS), we have directly measured the concentration depth profiles (CDPs) of various monovalent ions at the vapor-formamide interface. NICISS provides CDPs of individual ions by measuring the energy loss of neutral helium atoms backscattered from the solution interface. CDPs at the vapor-formamide interface of Cl-, Br-, I-, Na+, K+, and Cs+ are measured and compared to elucidate the interfacial specific ion trends. We report a reverse Hofmeister series in the presence of inorganic ions (anion and cation) at the vapor-formamide interface relative to the water-vapor interface, and the CDPs are found to be independent of the counterion for most ions studied. Thus, ions at the surface of formamide follow a "Hofmeister paradigm" where the counterion does not impact the ion series. These specific ion trends are complemented with surface tension and X-ray absorption near-edge structure (XANES) measurements on formamide electrolyte solutions.

Why Carbon Nanotubes Grow.

Despite three decades of intense research efforts, the most fundamental question "why do carbon nanotubes grow?" remains unanswered. In fact, carbon nanotubes (CNTs) should not grow since the encapsulation of a catalyst with graphitic carbon is energetically more favorable than CNT growth in every aspect. Here, we answer this question using a theoretical model based on extensive first-principles and molecular dynamics calculations. We reveal a historically overlooked yet fundamental aspect of the CNT-catalyst interface, viz., that the interfacial energy of the CNT-catalyst edge is contact angle-dependent. The contact angle increases via graphitic cap lift-off, drastically decreasing the interfacial formation energy by up to 6-9 eV/nm, overcoming van der Waals cap-catalyst adhesion, and driving CNT growth. Mapping this remarkable and simple interplay allows us to understand, for the first time, why CNTs grow.

Understanding specific ion effects and the Hofmeister series.

Specific ion effects (SIE), encompassing the Hofmeister Series, have been known for more than 130 years since Hofmeister and Lewith's foundational work. SIEs are ubiquitous and are observed across the medical, biological, chemical and industrial sciences. Nevertheless, no general predictive theory has yet been able to explain ion specificity across these fields; it remains impossible to predict when, how, and to what magnitude, a SIE will be observed. In part, this is due to the complexity of real systems in which ions, counterions, solvents and cosolutes all play varying roles, which give rise to anomalies and reversals in anticipated SIEs. Herein we review the historical explanations for SIE in water and the key ion properties that have been attributed to them. Systems where the Hofmeister series is perturbed or reversed are explored, as is the behaviour of ions at the liquid-vapour interface. We discuss SIEs in mixed electrolytes, nonaqueous solvents, and in highly concentrated electrolyte solutions - exciting frontiers in this field with particular relevance to biological and electrochemical applications. We conclude the perspective by summarising the challenges and opportunities facing this SIE research that highlight potential pathways towards a general predictive theory of SIE.

Thermochemical stabilities of giant fullerenes using density functional tight binding theory and isodesmic-type reactions.

We present a systematic assessment of the density functional tight binding (DFTB) method for calculating heats of formation of fullerenes with isodesmic-type reaction schemes. We show that DFTB3-D/3ob can accurately predict Δf H values of the 1812 structural isomers of C60 , reproduce subtle trends in Δf H values for 24 isolated pentagon rule (IPR) isomers of C84 , and predict Δf H values of giant fullerenes that are in effectively exact agreement with benchmark DSD-PBEP86/def2-QZVPP calculations. For fullerenes up to C320 , DFTB Δf H values are within 1.0 kJ mol-1 of DSD-PBEP86/def2-QZVPP values per carbon atom, and on a per carbon atom basis DFTB3-D/3ob yields exactly the same numerical trend of (Δf H [per carbon] = 722n-0.72 + 5.2 kJ mol-1 ). DFTB3-D/3ob is therefore an accurate replacement for high-level DHDFT and composite thermochemical methods in predicting of thermochemical stabilities of giant fullerenes and analogous nanocarbon architectures.

Reaction pathways in the solid state and the Hubbard U correction.

We investigate how the Hubbard U correction influences vacancy defect migration barriers in transition metal oxide semiconductors. We show that, depending on the occupation of the transition metal d orbitals, the Hubbard U correction can cause severe instabilities in the migration barrier energies predicted using generalized gradient approximation density functional theory (GGA DFT). For the d0 oxide SrTiO3, applying a Hubbard correction to the Ti4+ 3d orbitals below 4-5 eV yields a migration barrier of ∼0.4 eV. However, above this threshold, the barrier increases suddenly to ∼2 eV. This sudden increase in the transition state barrier arises from the Hubbard U correction changing the Ti4+ t2g/eg orbital occupation, and hence electron density localization, along the migration pathway. Similar results are observed in the d10 oxide ZnO; however, significantly larger Hubbard U corrections must be applied to the Zn2+ 3d orbitals for the same instability to be observed. These results highlight important limitations to the application of the Hubbard U correction when modeling reactive pathways in solid state materials using GGA DFT.

Deep eutectic solvents-The vital link between ionic liquids and ionic solutions.

When selecting a solvent for a given solute, the strongly held idiom "like dissolves like", meaning that polar solvents are used for polar solutes, is often used. This idea has resulted from the concept that most molecular solvents are homogeneous. In a deep eutectic solvent (DES), however, both components can be ionic or non-ionic, polar or non-polar. By tuning the components, DESs can solubilize a wide variety of solutes, often mixing hydrophobic and hydrophilic components, and the mixture can be designed to control phase behavior. The liquids often contain significant short-length order, and preferential solvation of one component often occurs. The addition of small polar molecules such as water or alcohols results in non-homogeneous liquids, which have significantly decreased viscosity and increased ionic conductivity. Accordingly, the areas covered in this special issue focus on structure and dynamics, solvation, the mobility of charged species, and the ability to obtain controllable phase behavior by adding polar diluents or using hydrophobic DESs.

The Hubbard-U correction and optical properties of d0 metal oxide photocatalysts.

We report a systematic investigation of individual and multisite Hubbard-U corrections for the electronic, structural, and optical properties of the metal titanate oxide d0 photocatalysts SrTiO3 and rutile/anatase TiO2. Accurate bandgaps for these materials can be reproduced with local density approximation and generalized gradient approximation exchange-correlation density functionals via a continuous series of empirically derived Ud and Up combinations, which are relatively insensitive to the choice of functional. On the other hand, lattice parameters are much more sensitive to the choice of Ud and Up, but in a systematic way that enables the Ud and Up corrections to be used to qualitatively gauge the extent of self-interaction error in the electron density. Modest Ud corrections (e.g., 4 eV-5 eV) yield the most reliable dielectric response functions for SrTiO3 and are comparable to the range of Ud values derived via linear response approaches. For r-TiO2 and a-TiO2, however, the Ud,p corrections that yield accurate bandgaps fail to accurately describe both the parallel and perpendicular components of the dielectric response function. Analysis of individual Ud and Up corrections on the optical properties of SrTiO3 suggests that the most consequential of the two individual corrections is Ud, as it predominately determines the accuracy of the dominant excitation from O-2p to the Ti-3d t2g/eg orbitals. Up, on the other hand, can be used to shift the entire optical response uniformly to higher frequencies. These results will assist high-throughput and machine learning approaches to screening photoactive materials based on d0 photocatalysts.

Boron Nitride Nanotube Nucleation via Network Fusion during Catalytic Chemical Vapor Deposition.

Despite boron nitride nanotubes (BNNTs) first being synthesized in the 1990s, their nucleation mechanism remains unknown. Here we report nonequilibrium molecular dynamics simulations showing how BNNT cap structures form during Ni-catalyzed chemical vapor deposition (CVD) of ammonia borane. BN hexagonal ring networks are produced following the catalytic evolution of H2 from the CVD feedstock, the formation and polymerization of B-N chain structures, and the repeated cleavage of homoelemental B-B/N-N bonds by the catalyst surface. Defect-free BNNT cap structures then form perpendicular to the catalyst surface via direct fusion of adjacent BN networks. This BNNT network fusion mechanism is a marked deviation from the established mechanism for carbon nanotube nucleation during CVD and potentially explains why CVD-synthesized BNNTs are frequently observed having sharper tips and wider diameters compared to CVD-synthesized carbon nanotubes.

Chiral-selective etching effects on carbon nanotube growth at edge carbon atoms.

Chemical vapor deposition (CVD) utilizing metal cluster nanoparticle catalysts is commonly used to synthesize carbon nanotubes (CNT), with oxygen-containing species such as water or alcohol included in the feedstock for enhanced yield. However, the etching effect of these additives on the growth mechanism has rarely been investigated, despite evidence suggesting that etching potentially affects the chirality distribution of product CNTs. We used quantum chemical methods to study how water-based etchant radicals (OH and H) may enhance the chiral selectivity during CVD growth using CNT cap models. Chemical reactivities of the caps with the etchant radicals were evaluated using density functional theory (DFT). It was found that the reactivities on the cap edges correlate with the chirality of the caps. These results suggest that proper selection of etchant species can provide opportunities for selective chirality control of the product CNTs. © 2018 Wiley Periodicals, Inc.

Electronic structure and high-temperature thermochemistry of BaZrO3-δ perovskite from first-principles calculations.

ABO3-δ perovskites are attractive candidates for high-temperature mixed ionic electronic conduction processes, due to their ability to produce mixed oxidation states and accommodate oxygen vacancies. Here, we examine the electronic structure and high-temperature thermochemistry of stoichiometric and non-stoichiometric cubic BaZrO3-δ perovskites for high defect concentration (δ = 0-0.5) using first-principles density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. Our results show that the electronic structures of these perovskites under increasing oxygen deficiency are characterized by highly localized reduction of Zr4+ t2g orbitals in the vicinity of the oxygen defects, irrespective of the value of δ. Temperature dependent thermodynamic properties of pristine- and defective-BaZrO3-δ show consistency with oxygen vacancy concentration. A comparison of predicted thermochemical properties with and without explicit vibrational corrections demonstrates their relative stability and implications at high-temperatures, as reduction Gibbs free energies in BaZrO3-δ exhibit large deviations above 1000 K. We elucidate the physical origins of these deviations via a phonon mode analysis.

Performance of DFT for C60 Isomerization Energies: A Noticeable Exception to Jacob's Ladder.

The ability to accurately calculate relative energies of fullerenes is important in many areas of computational nanotechnology. Because of the large size of fullerenes, their relative energies cannot normally be calculated by means of high-level ab initio procedures, and therefore, density functional theory (DFT) represents a cost-effective alternative. In an extensive benchmark study, we calculate the electronic energies of eight C60 isomers by means of the high-level G4(MP2) composite procedure. G4(MP2) isomerization energies span a wide range between 307.5 and 1074.0 kJ mol-1. We use these benchmark data to assess the performance of DFT, double-hybrid DFT (DHDFT), and MP2-based ab initio methods. Surprisingly, functionals from the second and third rungs of Jacob's Ladder (i.e., GGA and meta-GGA functionals) significantly and systematically outperform hybrid and hybrid-meta-GGA functionals, which occupy higher rungs of Jacob's Ladder. In addition, DHDFT functionals do not offer a substantial improvement over meta-GGA functionals, with respect to isomerization energies. Overall, the best performing functionals with mean absolute deviations (MADs) below 15.0 kJ mol-1 are (MADs given in parentheses) the GGA N12 (14.7); meta-GGAs M06-L (10.6), M11-L (10.8), MN15-L (11.9), and TPSS-D3BJ (12.8); and the DHDFT functionals B2T-PLYP (9.3), mPW2-PLYP (9.8), B2K-PLYP (12.1), and B2GP-PLYP (12.3 kJ mol-1). In light of these results, we recommend the use of meta-GGA functionals for the calculation of fullerene isomerization energies. Finally, we show that inclusion of very small percentages of exact Hartree-Fock exchange (3-5%) slightly improves the performance of the GGA and meta-GGA functionals. However, their performance rapidly deteriorates with the inclusion of larger percentages of exact Hartree-Fock exchange.

Lewis Strength Determines Specific-Ion Effects in Aqueous and Nonaqueous Solvents.

An analysis of specific-ion effects in aqueous and nonaqueous solvents using energy decomposition analysis is presented. Specific-ion effects induce or influence physicochemical phenomena in a way that is determined by the identity of the ions present, and not merely by their charge or concentration. Such effects have been known since the seminal work of Hofmeister and are often categorized according to the well-known Hofmeister series. Examples of specific-ion effects are ubiquitous throughout chemistry and biology and are traditionally explained in terms of the influence ions have on the structure of water. However, this explanation is unsatisfactory because it is unable to adequately explain and predict frequently observed series reversals and anomalies. Further, recent experiments have shown that specific-ion effects are observed in nonaqueous solvents. By modeling solvated ion-N-isopropylacrylamide (NIPAM) complexes, we show here that specific-ion effects on ion-NIPAM interaction free energies are observed not only in water, but also in several nonaqueous solvents (methanol, acetonitrile, DMSO) in correspondence with the ions' Lewis Strengths. Interestingly, the same trends are observed in the absence of a solvent environment altogether. Counterion effects on ion-NIPAM interaction free energies are negligible for dissociated ion pairs but are evident in associated ion pairs because of the modulation of repulsive ion-NIPAM interactions. We propose a mechanism for explaining reversals in specific-ion effects, based on the competing strengths of the ion-solvent and ion-NIPAM interaction and their relative Lewis strengths. This extends existing theories regarding specific-ion effect reversals in aqueous solutions, as we show that solvent properties must also be taken in to account for specific-ion effects to be predicted in arbitrary solvent environments.

o-Vanillin Derived Schiff Bases and Their Organotin(IV) Compounds: Synthesis, Structural Characterisation, In-Silico Studies and Cytotoxicity.

Six new organotin(IV) compounds of Schiff bases derived from S-R-dithiocarbazate [R = benzyl (B), 2- or 4-methylbenzyl (2M and 4M, respectively)] condensed with 2-hydroxy-3-methoxybenzaldehyde (oVa) were synthesised and characterised by elemental analysis, various spectroscopic techniques including infrared, UV-vis, multinuclear (¹H, 13C, 119Sn) NMR and mass spectrometry, and single crystal X-ray diffraction. The organotin(IV) compounds were synthesised from the reaction of Ph2SnCl2 or Me2SnCl2 with the Schiff bases (S2MoVaH/S4MoVaH/SBoVaH) to form a total of six new organotin(IV) compounds that had a general formula of [R2Sn(L)] (where L = Schiff base; R = Ph or Me). The molecular geometries of Me2Sn(S2MoVa), Me2Sn(S4MoVa) and Me2Sn(SBoVa) were established by X-ray crystallography and verified using density functional theory calculations. Interestingly, each experimental structure contained two independent but chemically similar molecules in the crystallographic asymmetric unit. The coordination geometry for each molecule was defined by thiolate-sulphur, phenoxide-oxygen and imine-nitrogen atoms derived from a dinegative, tridentate dithiocarbazate ligand with the remaining positions occupied by the methyl-carbon atoms of the organo groups. In each case, the resulting five-coordinate C2NOS geometry was almost exactly intermediate between ideal trigonal-bipyramidal and square-pyramidal geometries. The cytotoxic activities of the Schiff bases and organotin(IV) compounds were investigated against EJ-28 and RT-112 (bladder), HT29 (colon), U87 and SJ-G2 (glioblastoma), MCF-7 (breast) A2780 (ovarian), H460 (lung), A431 (skin), DU145 (prostate), BE2-C (neuroblastoma) and MIA (pancreatic) cancer cell lines and one normal breast cell line (MCF-10A). Diphenyltin(IV) compounds exhibited greater potency than either the Schiff bases or the respective dimethyltin(IV) compounds. Mechanistic studies on the action of these compounds against bladder cancer cells revealed that they induced the production of reactive oxygen species (ROS). The bladder cancer cells were apoptotic after 24 h post-treatment with the diphenyltin(IV) compounds. The interactions of the organotin(IV) compounds with calf thymus DNA (CT-DNA) were experimentally explored using UV-vis absorption spectroscopy. This study revealed that the organotin(IV) compounds have strong DNA binding affinity, verified via molecular docking simulations, which suggests that these organotin(IV) compounds interact with DNA via groove-binding interactions.

Density functional tight binding-based free energy simulations in the DFTB+ program.

The timescale problem-in which high barriers on the free energy surface trap molecular dynamics simulations in local energy wells-is a key limitation of current reactive MD simulations based on the density functional tight binding (DFTB) potential. Here, we report a new interface between the DFTB+ software package and the PLUMED library for performing DFTB-based free energy calculations. We demonstrate the performance of this interface for 3 archetypal rare-event chemical reactions, (i) intramolecular proton transfer in malonaldehyde, (ii) bowl inversion in corannulene, and (iii) oxygen diffusion on graphene. Using third-order DFTB in conjunction with metadynamics (with/without multiple walkers) and well-tempered metadynamics, we report here free energies of activation (ΔG‡ ) of 13.1 ± 0.4, 48.2 ± 1.7, and 52.0 ± 6.2 kJ mol-1 , respectively, for these processes. In each case, our DFTB free energy barriers and local minima compare favorably with previous literature results, demonstrating the utility of the DFTB+ - PLUMED interface. © 2018 Wiley Periodicals, Inc.

Inducing regioselective chemical reactivity in graphene with alkali metal intercalation.

First principles calculations demonstrate that alkali metal atoms, intercalated between metal substrates and adsorbed graphene monolayers, induce localised regions of increased reactivity. The extent of this localisation is proportional to the size of the alkali atom and the strength of the graphene-substrate interaction. Thus, larger alkali atoms are more effective (e.g. K > Na > Li), as are stronger-interacting substrates (e.g. Ni > Cu). Despite the electropositivity of these alkali metal adsorbates, analysis of charge transfer between the alkali metal, the substrate and the adsorbed graphene layer indicates that charge transfer does not give rise to the observed regioselective reactivity. Instead, the increased reactivity induced in the graphene structure is shown to arise from the geometrical distortion of the graphene layer imposed by the intercalated adsorbed atom. We show that this strategy can be used with arbitrary adsorbates and substrate defects, provided such structures are stable, towards controlling the mesoscale patterning and chemical functionalisation of graphene structures.

Accurate Thermochemical and Kinetic Stabilities of C84 Isomers.

Accurate double-hybrid density functional theory and isodesmic-type reaction schemes are utilized to report accurate estimates of the heats of formation (Δf H) for all 24 isolated-pentagon-rule isomers of the third most abundant fullerene, C84. Kinetic stabilities of these C84 isomers are also considered via C-C bond cleavage rates ( Pcleav) calculated using density functional theory. Our results show that the relative abundance of C84 fullerene isomers observed in arc discharge synthesis is the result of both thermochemical and kinetic factors. This provides timely insight regarding the characterization of several C84 isomers that have been obtained experimentally to date. For instance, the established assignments of C84 isomers of (using the Fowler-Manolopoulos numbering scheme) 22 [ D2(IV)], 23 [ D2 d(II)], 19 [ D3 d], 24 [ D6 h], 11 [ C2(IV)], and 4 [ D2 d(I)] are consistent with the relative Δf H and Pcleav values for these structures. However, our thermochemical and kinetic stabilities of C s isomers 14, 15, and 16 indicate that the two experimentally isolated C s isomers are 15 and 16, contrary to some previous assignments. Of the remaining isolated isomers of symmetry C2 and D2, definitive assignment was not possible with consideration of only Δf H and Pcleav.

Nanostructure of propylammonium nitrate in the presence of poly(ethylene oxide) and halide salts.

Nanoscale structure of protic ionic liquids is critical to their utility as molecular electrochemical solvents since it determines the capacity to dissolve salts and polymers such as poly(ethylene oxide) (PEO). Here we use quantum chemical molecular dynamics simulations to investigate the impact of dissolved halide anions on the nanostructure of an archetypal nanostructured protic ionic liquid, propylammonium nitrate (PAN), and how this impacts the solvation of a model PEO polymer. At the molecular level, PAN is nanostructured, consisting of charged/polar and uncharged/nonpolar domains. The charged domain consists of the cation/anion charge groups, and is formed by their electrostatic interaction. This domain solvophobically excludes the propyl chains on the cation, which form a distinct, self-assembled nonpolar domain within the liquid. Our simulations demonstrate that the addition of Cl- and Br- anions to PAN disrupts the structure within the PAN charged domain due to competition between nitrate and halide anions for the ammonium charge centre. This disruption increases with halide concentration (up to 10 mol. %). However, at these concentrations, halide addition has little effect on the structure of the PAN nonpolar domain. Addition of PEO to pure PAN also disrupts the structure within the charged domain of the liquid due to hydrogen bonding between the charge groups and the terminal PEO hydroxyl groups. There is little other association between the PEO structure and the surrounding ionic liquid solvent, with strong PEO self-interaction yielding a compact, coiled polymer morphology. Halide addition results in greater association between the ionic liquid charge centres and the ethylene oxide components of the PEO structure, resulting in reduced conformational flexibility, compared to that observed in pure PAN. Similarly, PEO self-interactions increase in the presence of Cl- and Br- anions, compared to PAN, indicating that the addition of halide salts to PAN decreases its utility as a molecular solvent for polymers such as PEO.

Catalytic CVD synthesis of boron nitride and carbon nanomaterials - synergies between experiment and theory.

Low-dimensional carbon and boron nitride nanomaterials - hexagonal boron nitride, graphene, boron nitride nanotubes and carbon nanotubes - remain at the forefront of advanced materials research. Catalytic chemical vapour deposition has become an invaluable technique for reliably and cost-effectively synthesising these materials. In this review, we will emphasise how a synergy between experimental and theoretical methods has enhanced the understanding and optimisation of this synthetic technique. This review examines recent advances in the application of CVD to synthesising boron nitride and carbon nanomaterials and highlights where, in many cases, molecular simulations and quantum chemistry have provided key insights complementary to experimental investigation. This synergy is particularly prominent in the field of carbon nanotube and graphene CVD synthesis, and we propose here it will be the key to future advances in optimisation of CVD synthesis of boron nitride nanomaterials, boron nitride - carbon composite materials, and other nanomaterials generally.

Nanostructure, hydrogen bonding and rheology in choline chloride deep eutectic solvents as a function of the hydrogen bond donor.

Deep eutectic solvents (DESs) are a mixture of a salt and a molecular hydrogen bond donor, which form a eutectic liquid with a depressed melting point. Quantum mechanical molecular dynamics (QM/MD) simulations have been used to probe the 1 : 2 choline chloride-urea (ChCl : U), choline chloride-ethylene glycol (ChCl : EG) and choline chloride-glycerol (ChCl : Gly) DESs. DES nanostructure and interactions between the ions is used to rationalise differences in DES eutectic point temperatures and viscosity. Simulations show that the structure of the bulk hydrogen bond donor is largely preserved for hydroxyl based hydrogen bond donors (ChCl:Gly and ChCl:EG), resulting in a smaller melting point depression. By contrast, ChCl:U exhibits a well-established hydrogen bond network between the salt and hydrogen bond donor, leading to a larger melting point depression. This extensive hydrogen bond network in ChCl:U also leads to substantially higher viscosity, compared to ChCl:EG and ChCl:Gly. Of the two hydroxyl based DESs, ChCl:Gly also exhibits a higher viscosity than ChCl:EG. This is attributed to the over-saturation of hydrogen bond donor groups in the ChCl:Gly bulk, which leads to more extensive hydrogen bond donor self-interaction and hence higher cohesive forces within the bulk liquid.