A computational model for a molecular chemical sensor.

In this study, we propose that a molecular junction with a sharp Negative Differential Resistance (NDR) current peak could improve the selectivity, thereby functioning as a potential molecular sensor for molecule recognition. Using DFT-NEGF simulations, we investigate the connection between molecule-molecule coupling, molecule-electrode coupling and the corresponding NDR peak shape. Based on this analysis we propose three design rules to control the sensitivity of a sensor and determine that one mechanism for NDR is for a localised molecular orbital involved in resonant tunneling to enter and leave the bias window. Our findings provide useful insight into the development of single molecule sensors for molecule recognition.

Enabling Ab Initio Molecular Dynamics under Bias: The CP2K+SMEAGOL Interface for Integrating Density Functional Theory and Non-Equilibrium Green Functions.

Density functional theory (DFT) combined with non-equilibrium Green's functions (NEGF) is a powerful approach to model quantum transport under external bias potentials at reasonable computational cost. In this work, we present a new interface between the popular mixed Gaussian/plane waves electronic structure package, CP2K, and the NEGF, code SMEAGOL, the most feature-rich implementation of DFT-NEGF available for CP2K to date. The CP2K+SMEAGOL interface includes the implementation of current induced forces. We verify this implementation for a variety of systems: an infinite 1D Au wire, a parallel-plate capacitor, and a Au-H2-Au junction. We find good agreement with SMEAGOL calculations performed with SIESTA for the same systems and with the example of a solvated Au wire demonstrating for the first time that DFT-NEGF can be used to perform molecular dynamics simulations under bias of large-scale condensed phase systems under realistic operating conditions.

Revealing Interface Polarization Effects on the Electrical Double Layer with Efficient Open Boundary Simulations under Potential Control.

A major challenge in modeling interfacial processes in electrochemical (EC) devices is performing simulations at constant potential. This requires an open-boundary description of the electrons, so that they can enter and leave the computational cell. To enable realistic modeling of EC processes under potential control we have interfaced density functional theory with the hairy probe method in the weak coupling limit (Zauchner et al. Phys. Rev. B 2018, 97, 045116). Our implementation was systematically tested using simple parallel-plate capacitor models with pristine surfaces and a single layer of adsorbed water molecules. Remarkably, our code's efficiency is comparable with a standard DFT calculation. We reveal that local field effects at the electrical double layer induced by the change of applied potential can significantly affect the energies of chemical steps in heterogeneous electrocatalysis. Our results demonstrate the importance of an explicit modeling of the applied potential in a simulation and provide an efficient tool to control this critical parameter.

Efficient and Selective Electrochemical CO2 to Formic Acid Conversion: A First-Principles Study of Single-Atom and Dual-Atom Catalysts on Tin Disulfide Monolayers.

Electrochemical CO2 reduction reaction (CO2RR) is a sustainable approach to recycle CO2 and address climate issues but needs selective catalysts that operate at low electrode potentials. Single-atom catalysts (SACs) and dual-atom catalysts (DACs) have become increasingly popular due to their versatility, unique properties, and outstanding performances in electrocatalytic reactions. In this study, we used Density Functional Theory along with the computational hydrogen electrode methodology to study the stability and activity of SACs and DACs by adsorbing metal atoms onto SnS2 monolayers. With a focus on optimizing the selective conversion of CO2 to formic acid, our analysis of the thermodynamics of CO2RR reveals that the Sn-SAC catalyst can efficiently and selectively catalyze formic acid production, being characterized by the low theoretical limiting potentials of -0.29 V. The investigation of the catalysts stability suggests that structures with low metal coverage and isolated metal centers can be synthesized. Bader analysis of charge redistribution during CO2RR demonstrates that the SnS2 substrate primarily provides the electronic charges for the reduction of CO2, highlighting the substrate's essential role in the catalysis, which is also confirmed by further electronic structure calculations.

Hydrogen oxidation reaction at the Ni/YSZ anode of solid oxide fuel cells from first principles.

By means of ab initio simulations we here provide a comprehensive scenario for hydrogen oxidation reactions at the Ni/zirconia anode of solid oxide fuel cells. The simulations have also revealed that in the presence of water chemisorbed at the oxide surface, the active region for H oxidation actually extends beyond the metal/zirconia interface unraveling the role of water partial pressure in the decrease of the polarization resistance observed experimentally.

Bifunctional catalysis by natural cinchona alkaloids: a mechanism explained.

The use of bifunctional chiral catalysts, which are able to simultaneously bind and activate two reacting partners, currently represents an efficient and reliable strategy for the stereoselective preparation of valuable chiral compounds. Cinchona alkaloids such as quinine and quinidine, simple organic molecules generously provided by Nature, were the first compounds to be proposed to operate through a cooperative catalysis. To date, a full mechanistic characterization of the dual catalysis mode of cinchona alkaloids has proven a challenging objective, due to the transient, non-covalent nature of the involved catalyst-substrate interactions. Here, we propose a mechanistic rationale on how natural cinchona alkaloids act as efficient bifunctional catalysts by using a broad range of computational methods, including classical molecular dynamics, a mixed quantum mechanical/molecular mechanics (QM/MM) approach, and correlated ab-initio calculations. We also unravel the origin of enantio- and diastereoselectivity, which is due to a specific network of hydrogen bonds that stabilize the transition state of the rate-determining step. The results are validated by experimental evidence.

Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts.

Although transition metal dichalcogenides such as MoS2 have been recognized as highly potent two-dimensional nanomaterials, general methods to chemically functionalize them are scarce. Herein, we demonstrate a functionalization route that results in organic groups bonded to the MoS2 surface via covalent C-S bonds. This is based on lithium intercalation, chemical exfoliation and subsequent quenching of the negative charges residing on the MoS2 by electrophiles such as diazonium salts. Typical degrees of functionalization are 10-20 atom % and are potentially tunable by the choice of intercalation conditions. Significantly, no further defects are introduced, and annealing at 350 °C restores the pristine 2H-MoS2. We show that, unlike both chemically exfoliated and pristine MoS2, the functionalized MoS2 is very well dispersible in anisole, confirming a significant modification of the surface properties by functionalization. DFT calculations show that the grafting of the functional group to the sulfur atoms of (charged) MoS2 is energetically favorable and that S-C bonds are formed.

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Solvent exfoliation of transition metal dichalcogenides: dispersibility of exfoliated nanosheets varies only weakly between compounds.

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS(2), MoS(2), MoSe(2), and MoTe(2), in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m(2), implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS(2) and MoSe(2) to have surface energies of ∼75 mJ/m(2), in good agreement with dispersibility measurements. However, this method suggested MoTe(2) to have a considerably larger surface energy (∼120 mJ/m(2)). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen's dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of δ(D) ∼ 18 MPa(1/2), δ(P) ∼ 8.5 MPa(1/2), and δ(H) ∼ 7 MPa(1/2) with all four types of layered material. Knowledge of these properties allows the estimation of the Flory-Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h.

Superionic conduction in substoichiometric LiAl alloy: an ab initio study.

Based on the new ab initio molecular dynamics method by Kühne et al. [Phys. Rev. Lett. 98, 066401 (2007)10.1103/PhysRevLett.98.066401], we studied the mechanism of superionic conduction in substoichiometric Li-poor Li_{1+x}Al alloys by performing simulations at different temperatures for an overall simulation time of about 1 ns. The dynamical simulations revealed the microscopic path for the diffusion of Li vacancies. The calculated activation energy (0.11 eV) and the prefactor (D_{0} = 6.9 x 10;{-4} cm;{2}/s) for Li diffusivity via a vacancy-mediated mechanism are in good agreement with experimental NMR data. The calculation of the formation energies of different defects-Li and Al Frenkel pair and Li antisites-revealed that only Li;{+} vacancies and Li_{Al} antisites are present in the stability range of the Zintl phase -0.1 < x < 0.2.

Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution.

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.

Ab initio exploration of rearrangement reactions: intramolecular hydrogen scrambling processes in acetone.

The recently developed metadynamics method is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. Comparison of different sets of collective coordinates allows efficient description of the underlying free energy surface. The simulations yielded numerous reactions: the enol-oxo tautomerism, the decomposition of acetone to various products, and rearrangement reactions. On the basis of the calculated activation barriers it is concluded that the enol-oxo tautomerism is the most frequent intramolecular proton-exchange process the acetone undergoes in the gas phase.