Experimentally Constrained Molecular Relaxation: The Case of Glassy GeSe2

An ideal atomistic model of a disordered material should contradict no experiments,and should also be consistent with accurate force fields (either {\it ab initio}or empirical). We make significant progress toward jointly satisfying {\it both} of these criteria using a hybrid reverse Monte Carlo approach in conjunction with approximate first principles molecular dynamics. We illustrate the method by studying the complex binary glassy material g-GeSe$_2$. By constraining the model to agree with partial structure factors and {\it ab initio} simulation, we obtain a 647-atom model in close agreement with experiment, including the first sharp diffraction peak in the static structure factor. We compute the electronic state densities and compare to photoelectron spectroscopies. The approach is general and flexible.Comment: 6 pages, 4 figure

Inclusion of Experimental Information in First Principles Modeling of Materials

We propose a novel approach to model amorphous materials using a first principles density functional method while simultaneously enforcing agreement with selected experimental data. We illustrate our method with applications to amorphous silicon and glassy GeSe$_2$. The structural, vibrational and electronic properties of the models are found to be in agreement with experimental results. The method is general and can be extended to other complex materials.Comment: 11 pages, 8 PostScript figures, submitted to J. Phys.: Condens. Matter in honor of Mike Thorpe's 60th birthda

Simulation of Atomic Diffusion in the Fcc NiAl System: A Kinetic Monte Carlo Study

The atomic diffusion in fcc NiAl binary alloys was studied by kinetic Monte Carlo simulation. The environment dependent hopping barriers were computed using a pair interaction model whose parameters were fitted to relevant data derived from electronic structure calculations. Long time diffusivities were calculated and the effect of composition change on the tracer diffusion coefficients was analyzed. Our results indicate that this variation has noticeable impact on the atomic diffusivities. A clear reduction in the mobility of both Ni and Al is demonstrated with increasing Al content. Examination of the pair interaction between atoms was carried out for the purpose of understanding the predicted trends

First-Principles Modeling in Heterogeneous Electrocatalysis

The last decade has witnessed tremendous progress in the development of computer simulation based on quantum mechanical description of the interactions between electrons and between electrons and atomic nuclei with electrode potentials taken into account–promoting the possibility to model electrocatalytic reactions. The cornerstone of this development was laid by the widely used computational hydrogen electrode method which involves a posteriori correction of standard constant charge first principles studies in solvent environment. The description of this technique and its contribution to our effort to understand electrocatalytic reactions on the active sites of metal-based nanoparticles are reviewed. The pathways and energetics of the relevant elementary reactions are presented. We also discussed a recent attempt in the literature to account for the inflow and outflow of electrons from the electrode as electrochemical reactions proceed, which has been greatly assisted by the development of density functional theory within the grand canonical framework. Going beyond the computational hydrogen electrode method by explicit incorporation of electrode potential within the calculations permits access to more detailed insights without requiring extra computational burden

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

The inclusion of experimental information in firs

Dimensionality of Nanoscale TiO₂ Determines the Mechanism of Photoinduced Electron Injection from a CdSe Nanoparticle

Assumptions about electron transfer (ET) mechanisms guide design of catalytic, photovoltaic, and electronic systems. We demonstrate that the mechanism of ET from a CdSe quantum dot (QD) into nanoscale TiO₂ depends on TiO₂ dimensionality. The injection into a TiO₂ QD is adiabatic due to strong donor–acceptor coupling, arising from unsaturated chemical bonds on the QD surface, and low density of acceptor states. In contrast, the injection into a TiO₂ nanobelt (NB) is nonadiabatic, because the state density is high, the donor–acceptor coupling is weak, and multiple phonons accommodate changes in the electronic energy. The CdSe adsorbant breaks symmetry of delocalized TiO₂ NB states, relaxing coupling selection rules, and generating more ET channels. Both mechanisms can give efficient ultrafast injection. However, the dependence on system properties is very different for the two mechanisms, demonstrating that the fundamental principles leading to efficient charge separation depend strongly on the type of nanoscale material

Selective Electrocatalytic Reduction of CO2 into CO at Small, Thiol-Capped Au/Cu Nanoparticles

The electrochemical CO reduction reaction (CO RR) is a promising approach for converting fossil fuel emissions into environmentally sustainable chemicals and fuels. The ability to control the surface structure of CO RR nanocatalysts provides an opportunity to tune product selectivity. Bimetallic gold-copper catalysts have been identified as emerging electrocatalyst candidates, but Cu incorporation typically lowers product selectivity compared with pure Au. Here we show sustained CO selectivity and activity up to 49% Cu content in small (\u3c2 \u3enm), thiol-capped Au/Cu nanoparticles (NPs). Bimetallic NPs containing 49% Cu selectivity converted CO into CO with 100 ± 6% CO Faradaic efficiency and average mass activity of ∼500 mA/mg during a 12 h electrolysis experiment at -0.8 V vs RHE. Au/Cu NPs synthesized without thiol ligands selectively produced H , whereas larger (\u3e10 nm), thermally dethiolated Au/Cu NPs produced a wider product distribution including H , CO, and C H . Density functional theory (DFT) modeling of CO RR and H evolution at realistic, thiol-capped Au/Cu NP structures indicated that copper-thiol surface structures sustained CO selectivity by stabilizing key∗CO intermediates while making∗H binding less favorable. Calculations also predicted that removing a significant fraction of the thiol ligands would increase∗CO binding strength such that desorption of CO product molecules could become the most thermodynamically challenging step. This result, coupled with increased∗H stability on dethiolated nanoclusters, points to decreased CO RR selectivity for small, ligand-free catalysts, which is in line with experimental observations from our group and others. Our results demonstrate that thiol-ligand surface structures can sustain the CO selectivity of bimetallic Au/Cu NPs and reduce precious metal requirements for CO RR. 2 2 2 2 2 2 2 4 2 2 2

Electrocatalytic Oxygen Evolution with an Atomically Precise Nickel Catalyst

The electrochemical oxygen evolution reaction (OER) is an important anodic process in water splitting and CO₂ reduction applications. Precious metals including Ir, Ru. and Pt are traditional OER catalysts, but recent emphasis has been placed on finding less expensive, earth-abundant materials with high OER activity. Ni-based materials are promising next-generation OER catalysts because they show high reaction rates and good long-term stability. Unfortunately, most catalyst samples contain heterogeneous particle sizes and surface structures that produce a range of reaction rates and rate-determining steps. Here we use a combination of experimental and computational techniques to study the OER at a supported organometallic nickel complex with a precisely known crystal structure. The Ni₆(PET)₁₂ (PET = phenylethyl thiol) complex out performed bulk NiO and Pt and showed OER activity comparable to Ir. Density functional theory (DFT) analysis of electrochemical OER at a realistic Ni₆(SCH₃)₁₂ model determined the Gibbs free energy change (Δ<i>G</i>) associated with each mechanistic step. This allowed computational prediction of potential determining steps and OER onset potentials that were in excellent agreement with experimentally determined values. Moreover, DFT found that small changes in adsorbate binding configuration can shift the potential determining step within the OER mechanism and drastically change onset potentials. Our work shows that atomically precise nanocatalysts like Ni₆(PET)₁₂ facilitate joint experimental and computational studies because experimentalists and theorists can study nearly identical systems. These types of efforts can identify atomic-level structure–property relationships that would be difficult to obtain with traditional heterogeneous catalyst samples