Strong Interaction between Gold and Anatase TiO₂(001) Predicted by First Principle Studies

The adsorption of gold clusters (Au_<i>n</i>, <i>n</i> = 1–10) on the minority surface, (001), of anatase titanium dioxide (TiO₂) has been studied in the framework of density functional theory. Various adsorption geometries of gold (Au) clusters on clean, unreconstructed TiO₂(001) have been investigated. It is found the adsorption of gold on TiO₂(001) is much stronger than that on the majority surface, (101). Due to the strong interfacial bonding, the valence electrons of gold have been highly delocalized and dominate the highest occupied frontier orbitals of Au/TiO₂(001). Consequently, it is predicted that the support of TiO₂(001) may offer better catalysis performance than conventionally used TiO₂(101)

p‑Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unraveling Charge Transfer Mechanisms for Enhanced Hydrogen Evolution Reaction Performance

Recently, hybrid electrocatalyst systems involving an active layer of <i>g</i>-C₃N₄ on a conductive substrate of N-doped graphene (<i>g</i>-C₃N₄@NG) have been shown to achieve excellent efficiency for the hydrogen evolution reaction (HER) [e.g., Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Nat. Commun. 2014, 5, 3783]. We demonstrate here through first principle calculations examining various hybrid <i>g</i>-C₃N₄@MG (M = B, N, O, F, P. and S) electrocatalysts that the N-doped case may be regarded as an example of a more general modulation doping strategy – by which either electron donating or electron withdrawing features induced in the substrate can be exploited to promote the HER. Despite the intrinsically cathodic nature of the HER, our study reveals that <i>all</i> of the graphene substrates have an increasingly electron withdrawing influence on the <i>g</i>-C₃N₄ active layer as H atom coverage increases, modulating binding of the H atom intermediates, the overpotential, and the likely operational coverage. In this context, it is not surprising that p-doping of the substrate can further enhance the effect. Our calculations show that B is the most promising doping element for <i>g</i>-C₃N₄@MG (M = B, N, O, F, P, and S) electrocatalysts due to the predicted overpotential of 0.06 eV at full coverage and a large interfacial adhesion energy of −1.30 eV, offering prospects for significant improvement over the n-dopant systems such as <i>g</i>-C₃N₄@NG that have appeared in the literature to date. These theoretical results reveal a more general principle for rational design of hybrid electrocatalysts, via manipulation of the Fermi level of the underlying conductive substrate

Borophene as a Promising Material for Charge-Modulated Switchable CO₂ Capture

Ideal carbon dioxide (CO₂) capture materials for practical applications should bind CO₂ molecules neither too weakly to limit good loading kinetics nor too strongly to limit facile release. Although charge-modulated switchable CO₂ capture has been proposed to be a controllable, highly selective, and reversible CO₂ capture strategy, the development of a practical gas-adsorbent material remains a great challenge. In this study, by means of density functional theory (DFT) calculations, we have examined the possibility of conductive borophene nanosheets as promising sorbent materials for charge-modulated switchable CO₂ capture. Our results reveal that the binding strength of CO₂ molecules on negatively charged borophene can be significantly enhanced by injecting extra electrons into the adsorbent. At saturation CO₂ capture coverage, the negatively charged borophene achieves CO₂ capture capacities up to 6.73 × 10¹⁴ cm^–2. In contrast to the other CO₂ capture methods, the CO₂ capture/release processes on negatively charged borophene are reversible with fast kinetics and can be easily controlled via switching on/off the charges carried by borophene nanosheets. Moreover, these negatively charged borophene nanosheets are highly selective for separating CO₂ from mixtures with CH₄, H₂, and/or N₂. This theoretical exploration will provide helpful guidance for identifying experimentally feasible, controllable, highly selective, and high-capacity CO₂ capture materials with ideal thermodynamics and reversibility

Conductive Boron-Doped Graphene as an Ideal Material for Electrocatalytically Switchable and High-Capacity Hydrogen Storage

Electrocatalytic, switchable hydrogen storage promises both tunable kinetics and facile reversibility without the need for specific catalysts. The feasibility of this approach relies on having materials that are easy to synthesize, possessing good electrical conductivities. Graphitic carbon nitride (g-C₄N₃) has been predicted to display charge-responsive binding with molecular hydrogenthe only such conductive sorbent material that has been discovered to date. As yet, however, this conductive variant of graphitic carbon nitride is not readily synthesized by scalable methods. Here, we examine the possibility of conductive and easily synthesized boron-doped graphene nanosheets (B-doped graphene) as sorbent materials for practical applications of electrocatalytically switchable hydrogen storage. Using first-principle calculations, we find that the adsorption energy of H₂ molecules on B-doped graphene can be dramatically enhanced by removing electrons from and thereby positively charging the adsorbent. Thus, by controlling charge injected or depleted from the adsorbent, one can effectively tune the storage/release processes which occur spontaneously without any energy barriers. At full hydrogen coverage, the positively charged BC₅ achieves high storage capacities up to 5.3 wt %. Importantly, B-doped graphene, such as BC₄₉, BC₇, and BC₅, have good electrical conductivity and can be easily synthesized by scalable methods, which positions this class of material as a very good candidate for charge injection/release. These predictions pave the route for practical implementation of electrocatalytic systems with switchable storage/release capacities that offer high capacity for hydrogen storage

Charge-modulated permeability and selectivity in graphdiyne for hydrogen purification

<p>Using first-principle calculations, we show that injecting positive charges into graphdiyne can substantially improve its hydrogen purification capability. When positive charges are introduced, the H₂ penetration barrier decreases while the penetration barriers of CO and CH₄ are significantly increased, hence leading to enhanced permeability and selectivity for hydrogen purification from CO and CH₄. These predictions show that application of positive charge provides a unique pathway, which avoids complicated synthesis routes, to enhance hydrogen purification performance, and may prove to be instrumental in searching for a new class of high-permeability and high-selectivity molecular-sieving membranes.</p

Formation and Migration of Oxygen Vacancies in SrCoO₃ and Their Effect on Oxygen Evolution Reactions

Perovskite SrCoO₃ is a potentially useful material for promoting the electrocatalytic oxygen evolution reaction, with high activities predicted theoretically and observed experimentally for closely related doped perovskite materials. However, complete stoichiometric oxidation is very difficult to realize experimentallyin almost all cases there are significant fractions of oxygen vacancies present. Here, using first-principles calculations we study oxygen vacancies in perovskite SrCoO₃ from thermodynamic, electronic, and kinetic points of view. We find that an oxygen vacancy donates two electrons to neighboring Co sites in the form of localized charge. The formation energy of a single vacancy is very low and is estimated to be 1.26 eV in the dilute limit. We find that a vacancy is quite mobile with a migration energy of ∼0.5 eV. Moreover, we predict that oxygen vacancies exhibit a tendency toward clustering, which is in accordance with the material’s ability to form a variety of oxygen-deficient structures. These vacancies have a profound effect on the material’s ability to facilitate OER, increasing the overpotential from ∼0.3 V for the perfect material to ∼0.7 V for defective surfaces. A moderate compressive biaxial strain (2%) is predicted here to increase the surface oxygen vacancy formation energy by ca. 30%, thus reducing the concentration of surface vacancies and thereby preserving the OER activity of the material

Asymmetrically Decorated, Doped Porous Graphene As an Effective Membrane for Hydrogen Isotope Separation

We propose a new route to hydrogen isotope separation which exploits the quantum sieving effect in the context of transmission through asymmetrically decorated, doped porous graphenes. Selectivities of D₂ over H₂ as well as rate constants are calculated based on ab initio interaction potentials for passage through pure and nitrogen functionalized porous graphene. One-sided dressing of the membrane with metal provides the critical asymmetry needed for an energetically favorable pathway

Binding and Release between Polymeric Carrier and Protein Drug: pH-Mediated Interplay of Coulomb Forces, Hydrogen Bonding, van der Waals Interactions, and Entropy

The accelerating search for new types of drugs and delivery strategies poses challenge to understanding the mechanism of delivery. To this end, a detailed atomistic picture of binding between the drug and carrier is quintessential. Although many studies focus on the electrostatics of drug–vector interactions, it has also been pointed out that entropic factors relating to water and counterions can play an important role. By carrying out extensive molecular dynamics simulations and subsequently validating with experiments, we shed light herein on the binding in aqueous solution between a protein drug and polymeric carrier. We examined the complexation between the polymer poly­(ethylene glycol) methyl ether acrylate-<i>b</i>-poly­(carboxyethyl acrylate (PEGMEA-<i>b</i>-PCEA) and the protein egg white lysozyme, a system that acts as a model for polymer–vector/protein–drug delivery systems. The complexation has been visualized and characterized using contact maps and hydrogen bonding analyses for five independent simulations of the complex, each running over 100 ns. Binding at physiological pH is, as expected, mediated by Coulombic attraction between the positively charged protein and negatively charged carboxylate groups on the polymer. However, we find that consideration of electrostatics alone is insufficient to explain the complexation behavior at low pH. Intracomplex hydrogen bonds, van der Waals interactions, as well as water–water interactions dictate that the polymer does not release the protein at pH 4.8 or indeed at pH 3.2 even though the Coulombic attractions are largely removed as carboxylate groups on the polymer become titrated. Experiments in aqueous solution carried out at pH 7.0, 4.5, and 3.0 confirm the veracity of the computed binding behavior. Overall, these combined simulation and experimental results illustrate that coulomb interactions need to be complemented with consideration of other entropic forces, mediated by van der Waals interactions and hydrogen bonding, to search for adequate descriptors to predict binding and release properties of polymer–protein complexes. Advances in computational power over the past decade make atomistic molecular dynamics simulations such as implemented here one of the few avenues currently available to elucidate the complexity of these interactions and provide insights toward finding adequate descriptors. Thus, there remains much room for improvement of design principles for efficient capture and release delivery systems

Structures, Energetics, and Electronic Properties of Layered Materials and Nanotubes of Cadmium Chalcogenides

Geometric structures, energetics, and electronic properties of single-layer sheets, multilayer stacks, and single-walled nanotubes (SWNTs) of cadmium chalcogenides CdX (X = S, Se, Te) have been studied using ab initio density functional theory, along with spin–orbit coupling, van der Waals (vdW) interactions, and the GW approximation. Methodologies applied to the rationally designed materials have been validated through the experimental structural parameters and band gaps of 3D bulk zinc blende and wurtzite phases of CdX. The 2D single-layer sheet of CdS is found to be completely planar, while those of CdSe and CdTe are slightly corrugated, all showing a honeycomb lattice. The 2D sheets are destabilized with respect to the bulk zinc blende and wurtzite phases, but can be significantly stabilized by forming 3D multilayer stacks as a result of interlayer interactions. 1D (5,5) armchair and (9,0) zigzag SWNTs are also stabilized from their single-layer sheet counterparts. Both SWNTs consist of two concentric cylinders, with the Cd and X atoms in the inner and the outer cylinders, respectively, and with the intercylinder separations showing the same trend as the degree of nonplanarity in the single-layer sheets. By analogy to quantum dots of CdX, we suggest quantum flakes as interesting targets for experimental synthesis due to the diverse band gaps complementary to those of the bulk phases, allowing a much wider wavelength range, from infrared, visible, to ultraviolet, to be utilized

Dynamical Interactions of 5‑Fluorouracil Drug with Dendritic Peptide Vectors: The Impact of Dendrimer Generation, Charge, Counterions, and Structured Water

Molecular dynamics simulations are utilized to investigate the interactions between the skin cancer drug 5-fluorouracil (5FU) and peptide-based dendritic carrier systems. We find that these drug–carrier interactions do not conform to the traditional picture of long-time retention of the drug within a hydrophobic core of the dendrimer carrier. Rather, 5FU, which is moderately soluble in its own right, experiences weak, transient chattering interactions all over the dendrimer, mediated through multiple short-lived hydrogen bonding and close contact events. We find that charge on the periphery of the dendrimer actually has a negative effect on the frequency of drug–carrier interactions due to a counterion screening effect that has not previously been observed. However, charge is nevertheless an important feature since neutral dendrimers are shown to have a significant mutual attraction that can lead to clustering or agglomeration. This clustering is prevented due to charge repulsion for the titrated dendrimers, such that they remain independent in solution