Nontypical Wulff-Shape Silicon Nanosheets with High Catalytic Activity.

Nanostructured silicon with an equilibrium shape has exhibited hydrogen evolution reaction activity mainly owing to its high surface area, which is distinct from that of bulk silicon. Such a Wulff shape of silicon favors low-surface-energy planes, resulting in silicon being an anisotropic and predictably faceted solid in which certain planes are favored, but this limits further improvement of the catalytic activity. Here, we introduce nanoporous silicon nanosheets that possess high-surface-energy crystal planes, leading to an unconventional Wulff shape that bolsters the catalytic activity. The high-index plane, uncommonly seen in the Wulff shape of bulk Si, has a band structure optimally aligned with the redox potential necessary for hydrogen generation, resulting in an apparent quantum yield (AQY) of 12.1% at a 400 nm wavelength. The enhanced light absorption in nanoporous silicon nanosheets also contributes to the high photocatalytic activity. Collectively, the strategy of making crystals with nontypical Wulff shapes can provide a route toward various classes of photocatalysts for hydrogen production.

Vertical Conductivity and Topography in Electrostrictive Germanium Sulfide Microribbon via Conductive Atomic Force Microscopy.

Layered group IV monochalcogenides are two-dimensional (2D) semiconducting materials with unique crystal structures and novel physical properties. Here, we report the growth of single crystalline GeS microribbons using the chemical vapor transport process. By using conductive atomic force microscopy, we demonstrated that the conductive behavior in the vertical direction was mainly affected by the Schottky barriers between GeS and both electrodes. Furthermore, we found that the topographic and current heterogeneities were significantly different with and without illumination. The topographic deformation and current enhancement were also predicted by our density functional theory (DFT)-based calculations. Their local spatial correlation between the topographic height and current was established. By virtue of 2D fast Fourier transform power spectra, we constructed the holistic spatial correlation between the topographic and current heterogeneity that indicated the diminished correlation with illumination. These findings on layered GeS microribbons provide insights into the conductive and topographic behaviors in 2D materials.

Hydrogen-doped viscoplastic liquid metal microparticles for stretchable printed metal lines.

Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn extensive attention for wearable electronics and electronic skins. Printable inks that contain liquid metal are strong candidates for these applications, but the insulating oxide skin that forms around the liquid metal particles limits their conductivity. This study reveals that hydrogen doping introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirmed the hydrogen doping, and first-principles calculations were used to rationalize the obtained conductivity. The printed circuit lines show a metallic conductivity (25,000 S cm-1), excellent electromechanical decoupling at a 500% uniaxial stretching, mechanical resistance to scratches and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows the direct printing of three-dimensional circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001).

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.

Atomic order, electronic structure and thermodynamic stability of nickel aluminate.

The atomic order, electronic structure and thermodynamic stability of nickel aluminate, NiAl2O4, have been analyzed using periodic density functional theory and cluster expansion. NiAl2O4 forms a tetragonal structure with P4122 space group. At temperatures below 800 K, it is an inverse spinel, with Ni occupying the octahedral sites and Al occupying both the octahedral and the tetrahedral sites. Some Niocta + Altetra ⇌ Nitetra + Alocta exchange occurs above 800 K, but the structure remains largely inverse at high temperatures, with about 95% Niocta at 1500 K. Various functionals, with and without van der Waals corrections, were used to predict the experimental formation energy, lattice parameters and electronic structure. In all cases, the NiAl2O4 is found to be ferromagnetic and a semiconductor with an indirect band gap along the Γ â†’ M symmetry points. NiAl2O4 is found to be thermodynamically stable at operating conditions of 900-1000 K and 1 atm relative to its competing oxide phases, NiO and Al2O3.

Control over Electron-Phonon Interaction by Dirac Plasmon Engineering in the Bi2Se3 Topological Insulator.

Understanding the mutual interaction between electronic excitations and lattice vibrations is key for understanding electronic transport and optoelectronic phenomena. Dynamic manipulation of such interaction is elusive because it requires varying the material composition on the atomic level. In turn, recent studies on topological insulators (TIs) have revealed the coexistence of a strong phonon resonance and topologically protected Dirac plasmon, both in the terahertz (THz) frequency range. Here, using these intrinsic characteristics of TIs, we demonstrate a new methodology for controlling electron-phonon interaction by lithographically engineered Dirac surface plasmons in the Bi2Se3 TI. Through a series of time-domain and time-resolved ultrafast THz measurements, we show that, when the Dirac plasmon energy is less than the TI phonon energy, the electron-phonon coupling is trivial, exhibiting phonon broadening associated with Landau damping. In contrast, when the Dirac plasmon energy exceeds that of the phonon resonance, we observe suppressed electron-phonon interaction leading to unexpected phonon stiffening. Time-dependent analysis of the Dirac plasmon behavior, phonon broadening, and phonon stiffening reveals a transition between the distinct dynamics corresponding to the two regimes as the Dirac plasmon resonance moves across the TI phonon resonance, which demonstrates the capability of Dirac plasmon control. Our results suggest that the engineering of Dirac plasmons provides a new alternative for controlling the dynamic interaction between Dirac carriers and phonons.

Over-Stoichiometry in Heavy Metal Oxides: The Case of Iono-Covalent Tantalum Trioxides.

Oxides of tantalum (common examples including TaO, TaO2, and Ta2O5) are key oxide materials for modern electronic devices, such as dynamic random-access memory and field effect transistors. Of late, new forms of stable tantalum oxides have been proposed as two-dimensional nanosheet structures with a nonconventional stoichiometry of TaO3 via soft-chemical delamination of RbTaO3. However, not much is known about the elusive nanosheet-structured TaO3, unlike other closely related common trioxides of W and Mo. In this work, using first-principles density functional theory calculations, we have studied various TaO3 structures as inspired from previous theoretical and experimental studies and discuss their properties with respect to the more conventional oxide of tantalum, Ta2O5. We have calculated their thermodynamics and lattice properties and have found a new stable-layered ß-TaO3 and its exfoliated monolayer phase (ß'). By further analyzing their electronic structures, we discuss the mixed iono-covalent bonding characteristics in the TaO3 phases, challenging the conventional formal oxidation state model for metal oxides. Finally, we propose how these new TaO3 oxide materials may be potentially useful in photodevice applications.

Anisotropic vacancy-mediated phonon mode softening in Sm and Gd doped ceria.

Ceria doped with Sm and Gd (SDC and GDC) has been suggested as a promising candidate for the electrolyte used in solid oxide fuel cells (SOFCs), since it has relatively high oxygen ion conductivity at intermediate temperature. There have been many previous experimental and computational studies to investigate the properties, structure, and effect of vacancies, etc. for SDC and GDC. However, in these previous studies, it is commonly assumed that the interaction between oxygen vacancies is negligible and many focus only on the mono-vacancy system. In addition, the possibility of anisotropic vibrational motion of the oxygen ions around vacancies is often neglected. In this paper, using both first-principle density-functional theory and classical molecular dynamics calculations, we investigate the structural and vibrational properties of the optimized SDC and GDC structures, such as bonding analysis, phonon density-of-state and mean-square-displacement of the oxygen ions. Also, we report the direction-dependent vibrations at the specific frequency of the oxygen ions near the vacancies, activation energies, and diffusion coefficients of SDC and GDC which can extend our understanding of diffusion dynamics in doped ceria-based electrolytes for SOFC applications.

Uncovering the Thermo-Kinetic Origins of Phase Ordering in Mixed-Valence Antimony Tetroxide by First-Principles Modeling.

Phase ordering in the mixed-valence oxide Sb2O4 has been examined by density functional theory (DFT) calculations. We find that the ground-state total energies of the two phases (α and ß) are almost degenerate and are highly sensitive to the choice of the approximation to the exchange correlation (xc) functional used in our calculations. Interestingly, with the inclusion of the zero-point energy corrections, the α phase is predicted to be the ground state polymorph for most xc functionals used. We also illustrate the pronounced stereochemical activity of Sb in these polymorphs of Sb2O4, setting an exception to the Keve and Skapski rule. Here, we find that the actual bonding in the α phase is more asymmetric, while the anomalous stability of the ß phase could be rationalized from kinetic considerations. We find a non-negligible activation barrier for this α-ß phase transition, and the presence of a saddle point (ß phase) supports the separation of Sb(III) over a continuous phase transition, as observed in experiments.

Anharmonicity in the High-Temperature Cmcm Phase of SnSe: Soft Modes and Three-Phonon Interactions.

The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalization scheme to quantify the effect of the soft modes on the calculated properties, and confirm that the anharmonicity is an inherent feature of the Cmcm phase. These results suggest a design concept for thermal insulators and thermoelectric materials, based on displacive instabilities, and highlight the power of lattice-dynamics calculations for materials characterization.

Organics on oxidic metal surfaces: a first-principles DFT study of PMDA and ODA fragments on the pristine and mildly oxidized surfaces of Cu(111).

Metal-organic hybrid materials are ubiquitous and a fundamental understanding of the hybrid-interface is key for the development of these hybrid material systems. In this work, using first-principles density-functional theory (including van der Waals (vdW) corrections), we study the fundamental physico-chemical properties of the molecular fragments of pyromellitic dianhydride oxydianiline (PMDA-ODA) on pristine Cu(111), as well as oxidic p4:O/Cu(111) in order to investigate the effect of mild oxidation of the metal substrate on PMDA-ODA adsorption. Firstly, we report the most favorable adsorption geometries amongst the various surface models and correlate the adsorption behavior with the electronic structure of the molecular fragments and the substrate layer. PMDA adsorbs weakly on both the clean and mildly oxidized copper surface via vdW forces while ODA adsorbs much stronger with a significant charge transfer between the substrates. Here, the oxidic layer is found to reduce the adsorption strength of both fragments and in particular, the ODA molecule interacts with the substrate via additional hydrogen bonding. Finally, our simulated scanning tunneling microscopy (STM) images suggest possible orientations of PMDA and ODA on clean and oxidic Cu surfaces to guide future experiments.

Unraveling the origins of conduction band valley degeneracies in Mg2Si(1-x)Sn(x) thermoelectrics.

To better understand the thermoelectric efficiency of the Mg-based thermoelectrics, using hybrid density-functional theory, we study the microscopic origins of valley degeneracies in the conduction band of the solid solution Mg2Si(1-x)Sn(x) and its constituent components--namely, Mg2Si and Mg2Sn. In the solid solution of Mg2Si(1-x)Sn(x), the sublattices are expected to undergo either tensile or compressive strain in the light of Vegard's law. Interestingly, we find both tensile strain of Mg2Si and compressive strain of Mg2Sn enhance the conduction band valley degeneracy. We suggest that the optimal sublattice strain as one of the origins of the enhanced Seebeck coefficient in the Mg2Si(1-x)Sn(x) system. In order to visualize the enhanced band valley degeneracy at elevated temperatures, the ground state eigenvalues and weights are projected by convolution functions that account for high temperature effects. Our results provide theoretical evidences for the role of sublattice strain in the band valley degeneracy observed in Mg2Si(1-x)Sn(x).

In search of non-conventional surface oxidic motifs of Cu on Au(111).

Growing ultrathin oxide layers on metal surfaces presents a new class of low-dimensional nanomaterials with exceptional chemical and physical properties. These "new oxides" can be used in many niche technologies and applications such as nanoscale electronics and heterogeneous nanocatalysis. In this work, we study the formation of surface oxidic structures and motifs of Cu, supported on the Au(111) substrate, using first-principles density-functional theory calculations in conjunction with an ab initio atomistic thermodynamics model. In particular, we systematically examine and analyze the detailed atomic structure and surface energetics of various oxidic motifs of Cu on Au(111), in particular, p2, p2s, p2(6q6) and the newly suggested metastable p2(6q6) + O3, in comparison to both the binary O/Cu(111) and O/Au(111) systems. Depending on the oxygen atmosphere and the type of surface defects introduced in the oxidic layer, various non-conventional, non-hexagonal surface oxidic motifs of Cu could be obtained. Our theoretical results agree with recent scanning tunneling microscopy (STM) experiments and we propose that metastable non-hexagonal surface motifs may pave a way to pursue further studies of these interesting complex surface oxidic layers on various metal supports.

Single-Atom Catalyst of Platinum Supported on Titanium Nitride for Selective Electrochemical Reactions.

As a catalyst, single-atom platinum may provide an ideal structure for platinum minimization. Herein, a single-atom catalyst of platinum supported on titanium nitride nanoparticles were successfully prepared with the aid of chlorine ligands. Unlike platinum nanoparticles, the single-atom active sites predominantly produced hydrogen peroxide in the electrochemical oxygen reduction with the highest mass activity reported so far. The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, also exhibited unique selectivity on the single-atom platinum catalyst. A lack of platinum ensemble sites changed the reaction pathway for the oxygen-reduction reaction toward a two-electron pathway and formic acid oxidation toward direct dehydrogenation, and also induced no activity for the methanol oxidation. This work demonstrates that single-atom platinum can be an efficient electrocatalyst with high mass activity and unique selectivity.

Nonstoichiometric nucleation and growth of multicomponent nanocrystals in solution.

The ability to assemble nanoscale functional building blocks is a useful and modular way for scientists to design valuable materials with specific physical and chemical properties. Chemists expect multicomponent, heterostructured nanocrystals to show unique electrical, thermal, and optical properties not seen in homogeneous, single-phase nanocrystals. Although researchers have made remarkable advances in heterogeneous nucleation and growth, design of synthetic conditions for obtaining nanocrystals with a target composition and shape is still a big challenge. There are several outstanding issues that chemists need to address before they can successfully carry out the design-based synthesis of multicomponent nanocrystals. For instance, small changes in the reaction parameters, such as the precursor, solvent, surfactant, reducing agent, and the reaction temperature, often result in changes in the structure and chemical composition of the final product. Although scientists do not fully understand the mechanisms underlying the nucleation and growth processes involved in the synthesis of these multicomponent nanocrystals, recent progress in understanding of the thermodynamic and kinetic factors have improved our control over their final structure and chemical composition. In this Account, we summarize our recent advances in understanding of the nucleation and growth mechanisms involved in the solution-based synthesis of multicomponent nanocrystals. We also discuss the various challenges encountered in their synthesis, emphasizing what still needs special consideration. We first discuss the three different nucleation paths from a thermodynamics perspective: amorphous nucleation, crystalline nucleation, and two-step nucleation. Amorphous nucleation and two-step nucleation involve the generation of nonstoichiometric nuclei. We initiate this process mainly by introducing an imbalance in the concentrations of the reduced elements. When the nonstoichiometric nuclei grow, we can add secondary elements to the growing nonstoichiometric nuclei. This leads to either the physical deposition or atomic mixture formation through the diffusion and rearrangement of constituents. The processes of mixture formation and the physical deposition of the secondary constituent element also compete and determine the shape and chemical composition of the final product. If the free energy change by mixture formation is positive (ΔGAB ≥ 0), physical deposition takes place predominantly, and the spreading coefficient (S) determines the structure of the nanocrystals. However, when mixture formation is highly spontaneous (ΔGAB < -ξ), the chemical composition of the final product is usually stoichiometric, and its shape then depends on the size of the primary nanocrystals. When the mixture formation and physical deposition are in competition (-ξ ≤ ΔGAB < 0), as commonly seen for many nanoalloy systems, both the chemical composition and the structure are determined by the size of the primary nanocrystals as well as the degree of mixture formation at the interface of the constituent components. Finally, we discuss the challenges and caveats that one needs to take into account when synthesizing multicomponent nanocrystals.

A rational computational study of surface defect-mediated stabilization of low-dimensional Pt nanostructures on TiN(100).

Platinum is known as a catalyst with exceptional reactivity for many important reactions, e.g. the oxygen reduction reaction. To reduce the high cost of pure platinum catalysts, platinum on a carbon support is widely used in industrial fuel cell applications. However, these Pt/C systems suffer from poor stability. As a cost-efficient and more durable alternative, Pt single-atom catalysts on a TiN support have recently been suggested, and it has been shown that the single-atom catalysts are stable when anchored at a nitrogen vacancy site on the TiN surface in a nitrogen-lean environment. To further explore the perspective of Pt/TiN catalytic systems, we provide insights into the stability and morphology of Pt nanostructures at the TiN(100) surface, using a density-functional theory approach in combination with ab initio atomistic thermodynamics. Our results show that the formation of two-dimensional Pt nano-layers is preferred over the formation of three-dimensional Pt nano-clusters on the TiN substrate. Similar to the single-atom catalysts, nano-layers of Pt can be stabilized on the TiN(100) surface by surface nitrogen vacancies under nitrogen-lean conditions. By analyzing the electronic metal-support interaction (EMSI) between the Pt nano-layer and the TiN surface with surface defects, we demonstrate that a strong EMSI between the surrounding Ti and Pt atoms is important for stabilizing the catalyst nano-layer at the TiN surface, and that N vacancies lead to stronger Pt-Ti interaction. This work provides a rational computational platform for the design of new generation high-performance Pt-based fuel cells.

Effect of gold subsurface layer on the surface activity and segregation in Pt/Au/Pt3M (where M = 3d transition metals) alloy catalyst from first-principles.

The effect of a subsurface hetero layer (thin gold) on the activity and stability of Pt skin surface in Pt3M system (M = 3d transition metals) is investigated using the spin-polarized density functional theory calculation. First, we find that the heterometallic interaction between the Pt skin surface and the gold subsurface in Pt/Au/Pt3M system can significantly modify the electronic structure of the Pt skin surface. In particular, the local density of states projected onto the d states of Pt skin surface near the Fermi level is drastically decreased compared to the Pt/Pt/Pt3M case, leading to the reduction of the oxygen binding strength of the Pt skin surface. This modification is related to the increase of surface charge polarization of outmost Pt skin atoms by the electron transfer from the gold subsurface atoms. Furthermore, a subsurface gold layer is found to cast the energetic barrier to the segregation loss of metal atoms from the bulk (inside) region, which can enhance the durability of Pt3M based catalytic system in oxygen reduction condition at fuel cell devices. This study highlights that a gold subsurface hetero layer can provide an additional mean to tune the surface activity toward oxygen species and in turn the oxygen reduction reaction, where the utilization of geometric strain already reaches its practical limit.

Re-visiting the O/Cu(111) system--when metastable surface oxides could become an issue!

Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for methanol synthesis, partial oxidation of methanol or the water-gas shift reaction. We assess the stability and population of the "8"-structure, a [formula, see text:] oxide phase, on the Cu(111) surface. This structure has been observed in X-ray photoelectron spectroscopy and low-energy electron diffraction experiments as a Cu(111) surface reconstruction that can be induced by a hyperthermal oxygen molecular beam. Using density-functional theory calculations in combination with ab initio atomistic thermodynamics and Boltzmann statistical mechanics, we find that the proposed oxide superstructure is indeed metastable and that the population of the "8"-structure is competitive with the known "29" and "44" oxide film structures on Cu(111). We show that the configuration of O and Cu atoms in the first and second layers of the "8"-structure closely resembles the arrangement of atoms in the first two layers of Cu2O(110), where the atoms in the "8"-structure are more constricted. Cu2O(110) has been suggested in the literature as the most active low index facet for reactions such as water splitting under light illumination. If the "8"-structure were to form during a catalytic process, it is therefore likely to be one of the reactive phases.

Why does bromine square palladium off? An ab initio study of brominated palladium and its nanomorphology.

A first-principles description and prediction of brominated nanocrystals of Pd is presented. In particular, we conducted an extensive study of the adsorption behaviour of Br on various Pd surfaces (including both low and high Miller-index surfaces) as a function of its surface coverage. By coupling our calculated surface energies with ab initio (electrochemical) thermodynamics and the Gibbs-Wulff shape model, we find that the relative stability of the Pd surfaces is strongly modified by Br, allowing high Miller-index surfaces of Pd (namely the (210) surface) to become competitively favourable at moderate concentrations of Br. We also show that Pd nanoparticles assume a cube-like crystal shape at high concentrations of Br, exposing mainly the (100) facets with a Br surface coverage of 0.5 ML. This not only confirms and explains recent solution synthesis results, but also provides a quantitative atomic picture of the exposed surface facets, which is crucial in understanding the local surface chemistry of shape-controlled nanoparticles for better nanocatalyst design.

Ammonolysis-Driven Exsolution of Ru Nanoparticle Embedded in Conductive Metal Nitride Matrix to Boost Electrocatalyst Activity.

Exsolution is an effective method for synthesizing robust nanostructured metal-based functional materials. However, no studies have investigated the exsolution of metal nanoparticles into metal nitride substrates. In this study, a versatile nitridation-driven exsolution method is developed for embedding catalytically active metal nanoparticles in conductive metal nitride substrates via the ammonolysis of multimetallic oxides. Using this approach, Ti1-xRuxO2 nanowires are phase-transformed into holey TiN nanotubes embedded with exsolved Ru nanoparticles. These Ru-exsolved holey TiN nanotubes exhibit outstanding electrocatalytic activity for the hydrogen evolution reaction with excellent durability, which is significantly higher than that of Ru-deposited TiN nanotubes. The enhanced stability of the Ru-exsolved TiN nanotubes can be attributed to the Ru nanoparticles embedded in the robust metal nitride matrix and the formation of interfacial Ti3+─N─Ru4+ bonds. Density functional theory calculations reveal that the exsolved Ru nanoparticles have a lower d-band center position and optimized hydrogen affinity than deposited Ru nanoparticles, indicating the superior electrocatalyst performance of the former. In situ Raman spectroscopic analysis reveals that the electron transfer from TiN to Ru nanoparticles is enhanced during the electrocatalytic process. The proposed approach opens a new avenue for stabilizing diverse metal nanostructures in many conductive matrices like metal phosphides and chalcogenides.