Role of Hydrogen Bonding on Transport of Coadsorbed Gases in Metal-Organic Frameworks Materials.

Coadsorption of multicomponents in metal-organic framework (MOF) materials can lead to a number of cooperative effects, such as modification of adsorption sites or during transport. In this work, we explore the incorporation of NH and HO into MOFs preloaded with small molecules such as CO, CO, and SO. We find that NH (or HO) first displaces a certain amount of preadsorbed molecules in the outer portion of MOF crystallites, and then substantially hinders diffusion.

Anisotropic Ordering in 1T' Molybdenum and Tungsten Ditelluride Layers Alloyed with Sulfur and Selenium.

Alloying is an effective way to engineer the band-gap structure of two-dimensional transition-metal dichalcogenide materials. Molybdenum and tungsten ditelluride alloyed with sulfur or selenium layers (MXTe, M = Mo, W and X = S, Se) have a large band-gap tunability from metallic to semiconducting due to the 2H-to-1T' phase transition as controlled by the alloy concentrations, whereas the alloy atom distribution in these two phases remains elusive. Here, combining atomic resolution Z-contrast scanning transmission electron microscopy imaging and density functional theory (DFT), we discovered that anisotropic ordering occurs in the 1T' phase, in sharp contrast to the isotropic alloy behavior in the 2H phase under similar alloy concentration.

Design of Optimally Stable Molecular Coatings for Fe-Based Nanoparticles in Aqueous Environments.

Magnetic nanoparticles are widely used in biomedical and oil-well applications in aqueous, often harsh environments. The pursuit for high-saturation magnetization together with high stability of the molecular coating that prevents agglomeration and oxidation remains an active research area. Here, we report a detailed analysis of the criteria for the stability of molecular coatings in aqueous environments along with extensive first-principles calculations for magnetite, which has been widely used, and cementite, a promising emerging candidate.

Novel Pd_{2}Se_{3} Two-Dimensional Phase Driven by Interlayer Fusion in Layered PdSe_{2}.

Two-dimensional (2D) materials are easily fabricated when their bulk form has a layered structure. The monolayer form in layered transition-metal dichalcogenides is typically the same as a single layer of the bulk material. However, PdSe_{2} presents a puzzle.

Trapping gases in metal-organic frameworks with a selective surface molecular barrier layer.

The main challenge for gas storage and separation in nanoporous materials is that many molecules of interest adsorb too weakly to be effectively retained. Instead of synthetically modifying the internal surface structure of the entire bulk-as is typically done to enhance adsorption-here we show that post exposure of a prototypical porous metal-organic framework to ethylenediamine can effectively retain a variety of weakly adsorbing molecules (for example, CO, CO, SO, CH, NO) inside the materials by forming a monolayer-thick cap at the external surface of microcrystals. Furthermore, this capping mechanism, based on hydrogen bonding as explained by ab initio modelling, opens the door for potential selectivity.

Structural band-gap tuning in g-C3N4.

g-C3N4 is a promising material for hydrogen production from water via photo-catalysis, if we can tune its band gap to desirable levels. Using a combined experimental and ab initio approach, we uncover an almost perfectly linear relationship between the band gap and structural aspects of g-C3N4, which we show to originate in a changing overlap of wave functions associated with the lattice constants. This changing overlap, in turn, causes the unoccupied pz states to experience a significantly larger energy shift than any other occupied state (s, px, or py), resulting in this peculiar relationship.

Study of van der Waals bonding and interactions in metal organic framework materials.

Metal organic framework (MOF) materials have attracted a lot of attention due to their numerous applications in fields such as hydrogen storage, carbon capture and gas sequestration. In all these applications, van der Waals forces dominate the interaction between the small guest molecules and the walls of the MOFs. In this review article, we describe how a combined theoretical and experimental approach can successfully be used to study those weak interactions and elucidate the adsorption mechanisms important for various applications.

Factors Controlling the Reactivity of Catalytically Active Monolayers on Metal Substrates.

The focus of this work is on the Pt/MS structures (MS = Au, Ir, Ru, or Pt substrate), as promising electrocatalysts and a prototype for more general systems: (active element monolayer)/(metal substrate) (AE/MS). We evaluate from first principles the effects of AE monolayer strain and the interlayer AE-MS electronic state hybridization on surface reactivity and reveal rationale for the interlayer hybridization to dominate over the strain effect in determining the AE/MS surface reactivity. We find, however, that, if AE is weakly bound to MS, the surface electronic structure does not suffice to characterize the surface reactivity, because of involvement of other factors related to lattice response to adsorption of a reaction intermediate.

Sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride photocatalyst: insights from first principles.

We present here results of our first-principles studies of the sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride (g-C(3)N(4)). Using the ab initio thermodynamics approach combined with some kinetic analysis, we reveal the favorable S-doping configurations. By analyzing the valence charge densities of the doped and undoped systems, we find that sulfur partially donates its p(x)- and p(y)- electrons to the system with some back donation to the S p(z)-states.

First principles study of oxygen adsorption on Se-modified Ru nanoparticles.

We present here the results of our density-functional-theory-based calculations of the electronic and geometric structures and energetics of Se and O adsorption on Ru 93- and 105-atom nanoparticles. These studies have been inspired by the fact that Se/Ru nanoparticles are considered promising electrocatalysts for the oxygen reduction reaction (ORR) on direct methanol fuel cell cathodes and the oxygen binding energy is a descriptor for the catalyst activity toward this reaction. We find the character of chemical bonding of Se on a flat nanoparticle facet to be ionic, similar to that obtained earlier for the Se/Ru(0001) surface, while in the case of a low-coordinated Ru configuration there is an indication of some covalent contribution to the bonding leading to an increase in Se binding energy.

Factors controlling the energetics of the oxygen reduction reaction on the Pd-Co electro-catalysts: insight from first principles.

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd(0.75)Co(0.