Periodic Trends in Adsorption and Activation Energies for Heterometallic Diffusion on (100) Transition Metal Surfaces

A first-principles analysis of trends in metal-on-metal hopping diffusion for 64 admetal/substrate systems is presented. Focusing on the (100) facets of various transition metal substrates, we demonstrate that the calculated hopping diffusion barriers may be interpreted in terms of the cohesive energies of the admetals and substrates, as well as the lattice constants of the substrates. We further show that general linear relationships exist between the diffusion barriers and the corresponding adsorption energies on each transition metal substrate. The slopes in these Brønsted–Evans–Polanyi relationships are related to the degree of resemblance between the initial states and the transition states for hopping diffusion, and the slopes are found to depend sensitively on the nature of the transition metal substrate. Substrates with higher cohesive energies and smaller lattice constants generally exhibit smaller slopes and, therefore, a closer correspondence between the transition states and the initial states. These relationships, in addition to providing fundamental insights into trends in diffusion across different transition metal surfaces, give a powerful and convenient means of predicting diffusional kinetics from purely thermodynamic quantities. The results may ultimately provide a useful input to kinetic Monte Carlo (kMC)-type simulations, enabling efficient and accurate studies of heteroepitaxial metal-on-metal growth

First Principles Simulations of the Electrochemical Lithiation and Delithiation of Faceted Crystalline Silicon

Silicon is of significant interest as a next-generation anode material for lithium-ion batteries due to its extremely high capacity. The reaction of lithium with crystalline silicon is known to present a rich range of phenomena, including electrochemical solid state amorphization, crystallization at full lithiation of a Li₁₅Si₄ phase, hysteresis in the first lithiation–delithiation cycle, and highly anisotropic lithiation in crystalline samples. Very little is known about these processes at an atomistic level, however. To provide fundamental insights into these issues, we develop and apply a first principles, history-dependent, lithium insertion and removal algorithm to model the process of lithiation and subsequent delithiation of crystalline Si. The simulations give a realistic atomistic picture of lithiation demonstrating, for the first time, the amorphization process and hinting at the formation of the Li₁₅Si₄ phase. Voltages obtained from the simulations show that lithiation of the (110) surface is thermodynamically more favorable than lithiation of the (100) or (111) surfaces, providing an explanation for the drastic lithiation anisotropy seen in experiments on Si micro- and nanostructures. Analysis of the delithiation and relithiation processes also provides insights into the underlying physics of the lithiation–delithiation hysteresis, thus providing firm conceptual foundations for future design of improved Si-based anodes for Li ion battery applications

Concentration-Dependent Ordering of Lithiated Amorphous TiO₂

We present the results of molecular dynamics simulations on the disorder–order transition of highly lithiated amorphous TiO₂. Our simulations suggest the presence of a threshold Li concentration above which long-range order gradually sets in for the fully lithiated amorphous TiO₂ at high temperatures. Our results indicate a clear correlation between the diffusional characteristics of Li, Ti, and O and the extent of ordering, both of which depend on Li concentration. Analyses of the changes in the system’s configurational energy, the pair correlation entropy, and various orientational bond-order parameters as a function of simulation time suggest a structural evolution from an amorphous to an ordered cubic TiO₂ structure, providing molecular-level explanation of the recent experimental observations on this unique lithium-induced phase transitions. The structural stability under extreme pressure conditions and the Li diffusivity in the ordered structure are also reported for assessing its potential to be used as a metal oxide anode for Li-ion batteries

Localized Order–Disorder Transitions Induced by Li Segregation in Amorphous TiO₂ Nanoparticles

Li segregation and transport characteristics in amorphous TiO₂ nanoparticles (NPs) are studied using molecular dynamics (MD) simulations. A strong intraparticle segregation of Li is observed, and the degree of segregation is found to correlate with Li concentration. With increasing Li concentration, Li diffusivity and segregation are enhanced, and this behavior is tied to the structural response of the NPs with increasing lithiation. The atoms in the amorphous NPs undergo rearrangement in the regions of high Li concentration, introducing new pathways for Li transport and segregation. These localized atomic rearrangements, in turn, induce preferential crystallization near the surfaces of the NPs. Such rich, dynamical responses are not expected for crystalline NPs, where the presence of well-defined lattice sites leads to limited segregation and transport at high Li concentrations. The preferential crystallization in the near-surface region in amorphous NPs may offer enhanced stability and fast Li transport for Li-ion battery applications, in addition to having potentially useful properties for other materials science applications

Chiral “Pinwheel” Heterojunctions Self-Assembled from C₆₀ and Pentacene

We demonstrate the self-assembly of C₆₀ and pentacene (Pn) molecules into acceptor–donor heterostructures which are well-ordered anddespite the high degree of symmetry of the constituent molecules<i>chiral</i>. Pn was deposited on Cu(111) to monolayer coverage, producing the random-tiling (<i>R</i>) phase as previously described. Atop <i>R</i>-phase Pn, postdeposited C₆₀ molecules cause rearrangement of the Pn molecules into domains based on chiral supramolecular “pinwheels”. These two molecules are the highest-symmetry achiral molecules so far observed to coalesce into chiral heterostructures. Also, the chiral pinwheels (composed of 1 C₆₀ and 6 Pn each) may share Pn molecules in different ways to produce structures with different lattice parameters and degree of chirality. High-resolution scanning tunneling microscopy results and knowledge of adsorption sites allow the determination of these structures to a high degree of confidence. The measurement of chiral angles identical to those predicted is a further demonstration of the accuracy of the models. van der Waals density functional theory calculations reveal that the Pn molecules around each C₆₀ are torsionally flexed around their long molecular axes and that there is charge transfer from C₆₀ to Pn in each pinwheel

Understanding Polyol Decomposition on Bimetallic Pt–Mo CatalystsA DFT Study of Glycerol

Catalytic dehydrogenation and C–C and C–O bond cleavage for glycerol decomposition on bimetallic Pt–Mo alloy model catalysts are studied using periodic density functional theory. The scaling relationship developed for monometallic systems for fast binding energy prediction has been tested and validated on both Pt-skin and Pt₃Mo-skin bimetallic surfaces. Using only the binding energies of atomic C and O for corresponding alloy surfaces, this simple relationship is shown to be an extremely efficient approach to speeding up the catalytic trend analysis for bimetallic alloy catalysts. Similar to Pt(111), it is found that the Pt-skin surface also favors dehydrogenation via C–H bond cleavage and faster C–C bond cleavage over C–O bond cleavage, but the overall activity decreases compared with pure Pt. On Pt₃Mo-skin surfaces, the overall reaction becomes much more exothermic, but Mo species significantly affect the selectivity by favoring the C–O bond cleavage. Thermodynamic analyses also predict that surface Mo species can be easily oxidized under typical reforming conditions, forming molybdate clusters and severely altering surface structures and potentially catalytic properties. Guided by experimental observations, this study also explores possible bifunctional characteristics for Pt–Mo bimetallic catalysts responsible for improved reforming activity and hydrogen production rates

First-Principles Predictions and <i>in Situ</i> Experimental Validation of Alumina Atomic Layer Deposition on Metal Surfaces

The atomic layer deposition (ALD) of metal oxides on metal surfaces is of great importance in applications such as microelectronics, corrosion resistance, and catalysis. In this work, Al₂O₃ ALD using trimethylaluminum (TMA) and water was investigated on Pd, Pt, Ir, and Cu surfaces by combining <i>in situ</i> quartz crystal microbalance (QCM), quadrupole mass spectroscopy (QMS), and scanning tunneling microscopy (STM) measurements with density functional theory (DFT) calculations. These studies revealed that TMA undergoes dissociative chemisorption to form monomethyl aluminum (AlCH₃*, the asterisk designates a surface species) on both Pd and Pt, which transform into Al­(OH)₃* during the subsequent water exposure. Furthermore, the AlCH₃* can further dissociate into Al* and CH₃* on stepped Pt(211). Additional DFT calculations predicted that Al₂O₃ ALD should proceed on Ir following a similar mechanism but not on Cu due to the endothermicity for TMA dissociation. These predictions were confirmed by <i>in situ</i> QCM, QMS, and STM measurements. Our combined theoretical and experimental study also found that the preferential decoration of low-coordination metal sites, especially after high temperature treatment, correlates with the differences in free energy between Al₂O₃ ALD on the (111) and stepped (211) surfaces. These insights into Al₂O₃ growth on metal surfaces can guide the future design of advanced metal/metal oxide catalysts with greater durability by protecting the metal against sintering and dissolution and enhanced selectivity by blocking low-coordination metal sites while leaving (111) facets available for catalysis

First-Principles Analysis of Defect-Mediated Li Adsorption on Graphene

To evaluate the possible utility of single layer graphene for applications in Li ion batteries, an extensive series of periodic density functional theory (DFT) calculations are performed on graphene sheets with both point and extended defects for a wide range of lithium coverages. Consistent with recent reports, it is found that Li adsorption on defect-free single layer graphene is not thermodynamically favorable compared to bulk metallic Li. However, graphene surfaces activated by defects are generally found to bind Li more strongly, and the interaction strength is sensitive to both the nature of the defects and their densities. Double vacancy defects are found to have much stronger interactions with Li as compared to Stone–Wales defects, and increasing defect density also enhances the interaction of the Stone–Wales defects with Li. Li interaction with one-dimensional extended defects on graphene is additionally found to be strong and leads to increased Li adsorption. A rigorous thermodynamic analysis of these data establishes the theoretical Li storage capacities of the defected graphene structures. In some cases, these capacities are found to approach, although not exceed, those of graphite. The results provide new insights into the fundamental physics of adsorbate interactions with graphene defects and suggest that careful defect engineering of graphene might, ultimately, provide anode electrodes of suitable capacity for lithium ion battery applications

Ab Initio Thermodynamic Modeling of Electrified Metal–Oxide Interfaces: Consistent Treatment of Electronic and Ionic Chemical Potentials

Solid oxide fuel cells are attractive devices in a sustainable energy context because of their fuel flexibility and potentially highly efficient conversion of chemical to electrical energy. The performance of the device is to a large extent determined by the atomic structure of the electrode–electrolyte interface. Lack of atomic-level information about the interface has limited the fundamental understanding, which further limits the opportunity for optimization. The atomic structure of the interface is affected by electrode potential, chemical potential of oxygen ions, temperature, and gas pressures. In this paper we present a scheme to determine the metal–oxide interface structure at a given set of these environmental parameters based on quantum chemical calculations. As an illustration we determine the structure of a Ni-YSZ anode as a function of electrode potential at 0 and 1000 K. We further describe how the structural information can be used as a starting point for accurate calculations of the kinetics of fuel oxidation reactions, in particular the hydrogen oxidation reaction. More generally, we anticipate that the scheme will be a valuable theoretical tool to describe solid–solid electrochemical interfaces

Imaging Catalytic Activation of CO₂ on Cu₂O (110): A First-Principles Study

Balancing global energy needs against increasing greenhouse gas emissions requires new methods for efficient CO₂ reduction. While photoreduction of CO₂ is  a viable approach for fuel generation, the rational design of photocatalysts hinges on precise characterization of the surface catalytic reactions. Cu₂O is a promising next-generation photocatalyst, but the atomic-scale description of the interaction between CO₂ and the Cu₂O surface is largely unknown, and detailed experimental measurements are lacking. In this study, density-functional-theory (DFT) calculations have been performed to identify the Cu₂O (110) surface stoichiometry that favors CO₂ reduction. To facilitate interpretation of scanning tunneling microscopy (STM) and X-ray absorption near-edge structures (XANES) measurements, which are useful for characterizing catalytic reactions, we present simulations based on DFT-derived surface morphologies with various adsorbate types. STM and XANES simulations were performed using the Tersoff–Hamann approximation and Bethe–Salpeter equation (BSE) approach, respectively. The results provide guidance for observation of CO₂ reduction reaction on, and rational surface engineering of, Cu₂O (110). They also demonstrate the effectiveness of computational image and spectroscopy modeling as a predictive tool for surface catalysis characterization