Random Phase Approximation in Surface Chemistry: Water Splitting on Iron

SBH17: Benchmark Database of Barrier Heights for Dissociative Chemisorption on Transition Metal Surfaces

Accurate barriers for rate controlling elementary reactions on metal surfaces are key to understanding, controlling, and predicting the rate of heterogeneously catalyzed processes. While barrier heights for gas phase reactions have been extensively benchmarked, dissociative chemisorption barriers for the reactions of molecules on metal surfaces have received much less attention. The first database called SBH10 and containing 10 entries was recently constructed based on the specific reaction parameter approach to density functional theory (SRP-DFT) and experimental results. We have now constructed a new and improved database (SBH17) containing 17 entries based on SRP-DFT and experiments. For this new SBH17 benchmark study, we have tested three algorithms (high, medium, and light) for calculating barrier heights for dissociative chemisorption on metals, which we have named for the amount of computational effort involved in their use. We test the performance of 14 density functionals at the GGA, GGA+vdW-DF, and meta-GGA rungs. Our results show that, in contrast with the previous SBH10 study where the BEEF-vdW-DF2 functional seemed to be most accurate, the workhorse functional PBE and the MS2 density functional are the most accurate of the GGA and meta-GGA functionals tested. Of the GGA+vdW functionals tested, the SRP32-vdW-DF1 functional is the most accurate. Additionally, we found that the medium algorithm is accurate enough for assessing the performance of the density functionals tested, while it avoids geometry optimizations of minimum barrier geometries for each density functional tested. The medium algorithm does require metal lattice constants and interlayer distances that are optimized separately for each functional. While these are avoided in the light algorithm, this algorithm is found not to give a reliable description of functional performance. The combination of relative ease of use and demonstrated reliability of the medium algorithm will likely pave the way for incorporation of the SBH17 database in larger databases used for testing new density functionals and electronic structure methods.

A theoretical study of adsorption on iron sulfides towards nanoparticle modeling

Surface modification of zero-valent iron (nZVI) nanoparticles, which are frequently used in the removal of chlorinated hydrocarbons from contaminated groundwater, can increase their surface stability without significant loss of reactivity. Sulfidation is a process during which thin iron sulfide phases are formed on nZVI particles. In this work, the adsorption capability of two iron sulfide minerals (mackinawite and pyrite) and ZVI with respect to two small polar molecules (H2O and H2S) and trichloroethylene (TCE) was modeled by using the quantum mechanics (QM) approach. High-level QM methods used on cluster models helped in benchmarking and validation of density functional theory methods used on periodic slab models of the (001) surfaces of iron sulfides and the (111) surface of ZVI. This careful computational treatment was necessary for achieving reliable results because modeled iron containing compounds represent computationally demanding systems. The results showed that adsorption was strongly affected by surface topology, accessibility of surface sites, and the shape of adsorbed molecular species. The mackinawite surface is practically hydrophobic having weak interactions with polar molecules (about -5/-6 kcal mol-1), in contrast to the surfaces of pyrite and ZVI (adsorption energies are about three times larger). On the other hand, the adsorption of weakly polar planar TCE molecule is relatively strong and similar for all three surfaces (in the range of -11 to -17 kcal mol-1). Moreover, it was shown that the dominant component of the adsorption energy of TCE had originated from dispersion interactions, which were less important for small polar molecules.

Directly patching high-level exchange-correlation potential based on fully determined optimized effective potentials

The key element in Kohn-Sham (KS) density functional theory is the exchange-correlation (XC) potential. We recently proposed the exchange-correlation potential patching (XCPP) method with the aim of directly constructing high-level XC potential in a large system by patching the locally computed, high-level XC potentials throughout the system. In this work, we investigate the patching of the exact exchange (EXX) and the random phase approximation (RPA) correlation potentials. A major challenge of XCPP is that a cluster's XC potential, obtained by solving the optimized effective potential equation, is only determined up to an unknown constant. Without fully determining the clusters' XC potentials, the patched system's XC potential is "uneven" in the real space and may cause non-physical results. Here, we developed a simple method to determine this unknown constant. The performance of XCPP-RPA is investigated on three one-dimensional systems: H₂₀, H₁₀Li₈, and the stretching of the H₁₉-H bond. We investigated two definitions of EXX: (i) the definition based on the adiabatic connection and fluctuation dissipation theorem (ACFDT) and (ii) the Hartree-Fock (HF) definition. With ACFDT-type EXX, effective error cancellations were observed between the patched EXX and the patched RPA correlation potentials. Such error cancellations were absent for the HF-type EXX, which was attributed to the fact that for systems with fractional occupation numbers, the integral of the HF-type EXX hole is not -1. The KS spectra and band gaps from XCPP agree reasonably well with the benchmarks as we make the clusters large.

Is Single Layer MoS2 Stable in the Air?

Molybdenum disulfide (MoS₂ ) is extensively studied because of its potential applications in catalysis, electronic and optoelectronic devices, and composite nanostructures. However, a recent experimental study indicated that, contrary to current beliefs, MoS₂ monolayers lack long-term stability in air. Here, a study is presented on the oxidation of MoS₂ monolayers based on density functional theory (DFT) calculations. The results suggest that single-layer MoS₂ samples with exposed edge sites are indeed unstable to oxidation, which occurs because of the low energetic barrier to dissociation of oxygen molecules at the Mo-edges of MoS₂ . After an oxygen molecule dissociates, oxygen atoms replace sulfur atoms, and further oxidation causes the formation of a one-dimensional chain-like structure resembling that of bulk MoO₃ . This MoO₃ structure facilitates the spread of oxidation onto the surface, and the stress associated with the misfit between the MoS₂ and MoO₃ lattices may cause the experimentally observed cracking of MoS₂ flakes.

Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite

Understanding strength and nature of noncovalent binding to surfaces imposes significant challenge both for computations and experiments. We explored the adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas chromatography and theoretical calculations, providing new insights into the strength and nature of adsorption of small organic molecules on these surfaces. The measured adsorption enthalpies on fluorographite range from -7 to -13 kcal/mol and are by 1-2 kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity of organic adsorbates to fluorographene than to graphene. The dispersion-corrected functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86b-vdW) achieved the best agreement with the experimental data. Computations show that the adsorption enthalpies are controlled by the interaction energy, which is dominated by London dispersion forces (∼70%). The calculations also show that bonding to structural features, like edges and steps, as well as defects does not significantly increase the adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.

First-principles study of the mechanism of wettability transition of defective graphene

Hydrophobicity of graphene limits its application potential in polar media, therefore modifications of graphene wettability have been in an area of active research for many years. Recently, a reversible wettability transition of graphene has been reported (Xu et al 2014 Sci. Rep. 4 6450). The presence of undercoordinated carbon atoms in otherwise hydrophobic graphene is believed to trigger the hydrophobic to hydrophilic transition, but the underlying mechanism, especially of the reverse process, remained unclear. Using density functional theory with range-separated hybrid functional HSE06, we investigate the dissociative adsorption of up to two water molecules on the defective graphene layer containing odd number of missing lattice atoms. We show, that depending on the defect type either a full dissociation of the water molecule or a partial splitting of H₂O to OH and H takes place leading to the saturation of graphene dangling bonds due to the formation of oxiranes or by hydroxyls, respectively. The dissociation barriers are significantly lower for the water dimer than for the single molecule. Our findings providing detailed insights into the remarkable differences between the reactivity of vacancy defects with water shed new light on the wettability-transition mechanism of defective graphene.

Modelling of graphene functionalization

This perspective describes the available theoretical methods and models for simulating graphene functionalization based on quantum and classical mechanics.

Study of the interplay between N-graphene defects and small Pd clusters for enhanced hydrogen storage via a spill-over mechanism

The hydrogen spill-over mechanism was studied by applying Density Functional Theory.

Density Functional Investigation of the Adsorption of Isooctane, Ethanol, and Acetic Acid on a Water-Covered Fe(100) Surface

The presence of water in biofuels poses the question of how it affects the frictional performance of additives in fuels containing organic substances. To investigate the effect of water on the adsorption of molecules present in fuel and its additives we simulated within the framework of density functional theory the adsorption of ethanol, isooctane (2,2,4-trimethylpentane), and acetic acid on a bare and a water-covered Fe(100) surface. Van der Waals interactions are taken into account in our computations. In those molecules, where dispersion forces contribute significantly to the binding mechanism, the water layer has a stronger screening effect. Additionally, this effect can be enhanced by the presence of polar functional groups in the molecule. Thus, with the introduction of a water layer, the adsorption energy of isooctane and ethanol is reduced but it is increased in the case of the acetic acid. The adsorption configuration of ethanol is changed, while the one of acetic acid is moderately, and for isooctane only very slightly altered. Therefore, the effect of a water layer in the adsorption of organic molecules on an Fe(100) surface strongly depends on the type of bond and consequently, so do the tribological properties.

Chemical nature of boron and nitrogen dopant atoms in graphene strongly influences its electronic properties

The electronic properties of the N- and B-doped graphenes depend, besides the doping element, on the nature of doping.

Adjusting the electronic properties of silicon carbide nanoribbons by introducing edge functionalization

The electronic properties of silicon carbide nanoribbons can be adjusted by introducing edge functionalization.