Contribution of Dielectric Screening to the Total Capacitance of Few-Layer Graphene Electrodes

We apply joint density functional theory (JDFT), which treats the electrode/electrolyte interface self-consistently, to an electric double-layer capacitor (EDLC) based on few-layer graphene electrodes. The JDFT approach allows us to quantify a third contribution to the total capacitance beyond quantum capacitance (<i>C</i>_Q) and EDL capacitance (<i>C</i>_EDL). This contribution arises from the dielectric screening of the electric field by the surface of the few-layer graphene electrode, and we therefore term it the dielectric capacitance (<i>C</i>_Dielec). We find that <i>C</i>_Dielec becomes significant in affecting the total capacitance when the number of graphene layers in the electrode is more than three. Our investigation sheds new light on the significance of the electrode dielectric screening on the capacitance of few-layer graphene electrodes

Ligand-Conformation Energy Landscape of Thiolate-Protected Gold Nanoclusters

Although several thiolate-protected Au nanoclusters have yielded to total-structure determination, the ligand-conformation energy landscapes and how they affect the relative stability of the whole clusters are not well understood. In this work, we employ a force-field-based approach to perform the ligand-conformation search for isolated thiolate-protected Au nanoclusters using Au₂₅(SR)₁₈ (R = C₂H₄Ph) as an example. We find that the ligand-conformation energy landscape of Au₂₅(SC₂H₄Ph)₁₈ comprises multiple low-energy funnels of similar stability instead of a single global minimum. In fact, we find slightly more stable conformations of isolated Au₂₅(SC₂H₄Ph)₁₈ than those observed in the experiment from a crystalline state, indicating that specific environments such as crystal packing and solvents may all affect the ligand conformation. This work reveals the role of ligand conformation in the cluster energy landscape

Revisiting Structural Models for Au₁₈(SR)₁₄

Au₁₈(SR)₁₄ (−SR being a thiolate group) is a small, stable thiolated gold nanocluster experimentally identified in 2005, but its structure remains elusive. A previously proposed model for Au₁₈(SR)₁₄ based on density functional theory (DFT) structural optimization consists of a Au₈ core protected by two −RS–Au–SR–Au–SR– (dimer) and two −RS–Au–SR–Au–SR–Au–SR– (trimer) motifs. Here we revisit structure prediction for Au₁₈(SR)₁₄ from extensive exploration of the possible isomers for Au₁₈(SCH₃)₁₄ by applying structural hypotheses based on both “staple motifs” and “ring and staple motifs”. Three isomers based on the staple motifs are found to be more stable than the best previous model. The two lowest-energy Au₁₈(SCH₃)₁₄ isomers (I and II) also have a Au₈ core protected by two dimer and two trimer motifs, but the core geometry and electronic properties are different. The third lowest-energy isomer (III) consists of a Au₈ core protected by two −RS–Au–SR– (monomer) and two −RS–Au–SR–Au–SR–Au–SR–Au–SR– (tetramer) motifs. By changing R from CH₃ to the experimentally used cyclohexanyl group (C₆H₁₁), we found that isomer III is the most stable for Au₁₈(SC₆H₁₁)₁₄. The computed X-ray diffraction (XRD) pattern and optical spectrum of isomer III are in good agreement with the experimental data. This work suggests that Au₁₈(SR)₁₄ may have monomer and tetramer motifs in the protective layer

Mechanism of Hydrogen Evolution Reaction on 1T-MoS₂ from First Principles

The 1T phase of transition-metal dichalcogenides (TMDs) has been demonstrated in recent experiments to display excellent catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using 1T MoS₂ as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the nonreactive basal plane of 2H phase MoS₂, the catalytic activity of the basal plane of 1T phase MoS₂ mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (Δ<i>G</i>_H) of H as the descriptor, we found that the optimum evolution of H₂ will proceed at surface H coverage of 12.5% ∼ 25%. Within this coverage, we examined the reaction energy and barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, whereas the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of 1T MoS₂ via the Volmer–Heyrovsky mechanism. We further screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make 1T MoS₂ a better HER catalyst

Comprehensive View of the Ligand–Gold Interface from First Principles

A large number of ligands have been used to stabilize and functionalize the gold surfaces and nanoclusters, but there has been no systematic comparison about their binding strengths with gold. In this work, we studied the interaction between 27 ligands of six different types (thiolates, phosphines, amines, aryl radicals, alkynyls, and N-heterocyclic carbenes) with the model Au surfaces by first-principles density functional theory (DFT). On the perfect Au(111), we found the order of binding strengths to be bulky N-heterocyclic carbenes (NHCs) ≈ alkynyls > thiolates ≈ phosphines > aryls ≈ less sterically bulky NHCs > alkylamines. The much stronger interaction of bulky carbenes to Au than the less sterically bulky NHCs arises from the van der Waals (vdW) attraction of bulky side groups with gold surface via the short Au···HCH₂R contact. Further, we showed that the presence of a gold adatom on Au(111) leads to enhanced binding and a similar order for most of the ligands examined. Overall, bulky NHCs and alkynyl groups form the strongest interaction to both Au(111) and Au_ad–Au­(111). This suggests that NHCs can be employed as alternatives to the currently widely used thiolates and the emerging alkynyl ligands for the preparation of more stable self-assembled monolayer structures on metal surfaces. Further, this insight allowed us to design viable magic-number gold clusters with NHCs as the protecting ligands

Uranyl–Glutardiamidoxime Binding from First-Principles Molecular Dynamics, Classical Molecular Dynamics, and Free-Energy Simulations

Exploring the structural interplay of ligands with uranyl can provide important knowledge for technology advances in uranium extraction from seawater. However, obtaining such chemical information is not an easy endeavor experimentally. From a plethora of computational methods, this work provides both microscopic insights and free-energy profiles of the binding between uranyl and deprotonated glutardiamidoxime (H₂B) for which experimental structural information is not available, despite H₂B being an important model ligand with an open-chain conformation for understanding aqueous uranium extraction chemistry. In our molecular dynamics (MD) simulations, we explicitly accounted for the water solvent as well as the Na⁺ and Cl^– ions. We found that hydrogen bonding plays a critical role in dictating the binding configurations of B^2– and HB^– with uranyl. Simulated free energies of sequential ligand binding to form UO₂B, [UO₂B₂]^2–, and [UO₂(HB)­B]⁻ show very good agreement with the experimental values, lending support to our structural insights. The potential of mean force simulations showed the common feature of an important intermediate state where one end of the ligand binds to uranyl while the other end is solvated in water. Bringing the loose end of the ligand to bind with uranyl has a free-energy barrier of 15–25 kJ/mol. The present work shows that the combined simulation approach can reveal key structural and thermodynamic insights toward a better understanding of aqueous complexation chemistry for uranium extraction from the sea

Site Partition: Turning One Site into Two for Adsorbing CO₂

We propose the concept of site partition to explain the role of guest molecules in increasing CO₂ uptake in metal–organic frameworks and to design new covalent porous materials for CO₂ capture. From grand canonical Monte Carlo simulations of CO₂ sorption in the recently synthesized CPM-33 MOFs, we show that guest insertion divides one open metal site into two relatively strong binding sites, hence dramatically increasing CO₂ uptake. Further, we extend the site partition concept to covalent organic frameworks with large free volume. After insertion of a designed geometry-matching guest, we show that the volumetric uptake of CO₂ doubles. Therefore, the concept of site partition can be used to engineer the pore space of nanoporous materials for higher gas uptake

Uranyl–Glutardiamidoxime Binding from First-Principles Molecular Dynamics, Classical Molecular Dynamics, and Free-Energy Simulations

Exploring the structural interplay of ligands with uranyl can provide important knowledge for technology advances in uranium extraction from seawater. However, obtaining such chemical information is not an easy endeavor experimentally. From a plethora of computational methods, this work provides both microscopic insights and free-energy profiles of the binding between uranyl and deprotonated glutardiamidoxime (H₂B) for which experimental structural information is not available, despite H₂B being an important model ligand with an open-chain conformation for understanding aqueous uranium extraction chemistry. In our molecular dynamics (MD) simulations, we explicitly accounted for the water solvent as well as the Na⁺ and Cl^– ions. We found that hydrogen bonding plays a critical role in dictating the binding configurations of B^2– and HB^– with uranyl. Simulated free energies of sequential ligand binding to form UO₂B, [UO₂B₂]^2–, and [UO₂(HB)­B]⁻ show very good agreement with the experimental values, lending support to our structural insights. The potential of mean force simulations showed the common feature of an important intermediate state where one end of the ligand binds to uranyl while the other end is solvated in water. Bringing the loose end of the ligand to bind with uranyl has a free-energy barrier of 15–25 kJ/mol. The present work shows that the combined simulation approach can reveal key structural and thermodynamic insights toward a better understanding of aqueous complexation chemistry for uranium extraction from the sea

Nitrogen-Doped Mesoporous Carbon for Carbon Capture – A Molecular Simulation Study

Using molecular simulation, we investigate the effect of nitrogen doping on adsorption capacity and selectivity of CO₂ versus N₂ in model mesoporous carbon. We show that nitrogen doping greatly enhances CO₂ adsorption capacity; with a 7 wt % dopant concentration, the adsorption capacity at 1 bar and 298 K increases from 3 to 12 mmol/g (or 48% uptake by weight). This great enhancement is due to the preferred interaction between CO₂ and the electronegative nitrogen. The nitrogen doping coupled with the mesoporosity also leads to a much higher working capacity for adsorption of the CO₂/N₂ mixture in nitrogen-doped mesoporous carbon. In addition, the CO₂/N₂ selectivity is almost 5 times greater than in nondoped carbon at ambient conditions. This work indicates that nitrogen doping is a promising strategy to create mesoporous carbons for high-capacity, selective carbon capture

Solubility of Gases in a Common Ionic Liquid from Molecular Dynamics Based Free Energy Calculations

Solubility of eight common gases in the 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide, [emim]­[Tf₂N], ionic liquid was systematically investigated based on alchemical free energy calculations from molecular dynamics simulations. The simulated solubilities and trend in terms of Henry’s law constants agree qualitatively with the experiment. Polar gases such as H₂S and nonpolar gases with a large quadrupole moment such as CO₂ show the highest solubility, while nonpolar gases of small quadrupole moments (such as N₂ and H₂) are least soluble. The solute–ionic liquid interaction correlates with the observed solubility order. We also examined the temperature dependence of solubility for CO₂ and N₂ and found that the CO₂ solvation in IL is exothermic with a negative solvation enthalpy, while the N₂ solvation is endothermic, in agreement with the experiment