High Miller Index Research Articles

A topological screening heuristic for low-energy, high-index surfaces

Robust ab initio investigations of nanoparticle surface properties require a method to identify candidate low-energy surface facets a priori. By assuming that low-energy surfaces are planes with high atomic density, we devise an efficient algorithm to screen for low-energy surface orientations, even if they have high (hkl) miller indices. We successfully predict the observed low-energy, high-index {1012¯} and {101¯4} surfaces of hematite α-Fe2O3, the {311} surfaces of cuprite Cu2O, and the {112} surfaces of anatase TiO2. We further tabulate candidate low-energy surface orientations for nine of the most common binary oxide structures. Screened surfaces are found to be generally applicable across isostructural compounds with varying chemistries, although relative surface energies between facets may vary based on the preferred coordination of the surface atoms.

Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage via DFT Calculations Coupled with in Situ TEM and IR.

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by in situ TEM and quantitatively with sample-averaged measurements of surface atom configurations by in situ infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact.

Design and Development of Instrumentations for the Preparation of Platinum Single Crystals for Electrochemistry and Electrocatalysis Research. Part 1: Semi-Automated Crystal Growth

In this contribution, we report on a novel design of semi-automated system for the growth of spherical platinum (Pt) single crystals. The system is based on the flame fusion methodology, where the crystal melting/solidification process is controlled by the vertical movement of a hydrogen–oxygen flame by using an actuator. The paper systematically examines the impact of several process variables on the crystal growth. First, the hydrogen–oxygen flame is optimized through control of the gas flow rate and the mixing ratio to ensure the formation of high-quality platinum single crystals. The procedure of crystal growth is documented in a step-by-step manner, starting with cleaning of the Pt wire, followed by the initial formation of metallic sphere, and ending with repetitive zone refining combined with chemical etching. The equilibrium shape of the platinum single crystals is discussed, and the individual low and high Miller index facets identified and visualized. The critical step in the single-crystal growth, the repetitive zone refining, is semi-automated in order to ensure the highest quality crystals and reproducibility, and to minimize any human error.

Surface Structure Engineering of Cu Thin Films for Electrochemical CO2 Reduction

Global dependence on fossil fuels as energy sources and the alarming increase of greenhouse gas emissions has necessitated the development of carbon-free and carbon-neutral renewable energy sources for the future. The sequestration of CO2 emissions along with subsequent electrochemical reduction into fuel products, forms a carbon-neutral synthetic fuel cycle which could potentially be streamlined into existing fuel infrastructures. To date, only Cu has displayed any propensity as a catalyst to electrochemically reduce CO2 into longer chain hydrocarbons. Previous studies on Cu single crystals have shown large facet sensitivities for electrochemical CO2reduction product selectivity. The aim of our research is to utilize these surface structure motifs to design active and selective catalysts that are amenable to device integration. To this end, we use physical vapor deposition to synthesize Cu thin films in both low and high Miller index orientations for electrochemical CO2 reduction. X-ray pole figures demonstrate that Cu thin films can be epitaxially grown in the <100>, <111>, and <751> orientations by utilizing the orientation of the single crystal substrate. The different Cu thin film orientations were tested for their CO2 reduction activity and selectivity using electrochemical measurements in tandem with gas phase (gas chromatography) and liquid phase (nuclear magnetic resonance) product detection methods. Here, we will demonstrate how surface structure engineering can guide selectivity for C-C coupling and oxygenate formation during electrochemical CO2 reduction.

On the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone

We present novel microstructural analyses of zircon from a variety of strained rocks. For the first time, multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones. Our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation, based on careful in situ observations. For sampling, we selected zircon-bearing rocks that were deformed by simple shear. Samples covered a range of P–T conditions and lithologies, including various meta-igneous and meta-sedimentary gneisses.Microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of: (i) the long axes of plastically deformed zircon crystals, (ii) the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and (iii) the foliation and lineation directions of the respective samples. We assume that zircon crystals did not experience post-deformation rigid body rotation, and thus the true geometric link can be observed. The relationships are the following: (a) plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane; (b) crystals with <c> axes oriented at an angle>15° to the foliation plane are undeformed or fractured.Zircon crystals that have <c> axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the [001] crystallographic direction. Zircon grains with the <c> axis aligned at 30–60° to the lineation within the foliation plane often develop either two low Miller indices misorientation axes or one high Miller indices misorientation axis. Host phases have a significant influence on deformation mechanisms. In a relatively soft matrix, zircon is more likely to develop low Miller indices misorientation axes than in a relatively strong matrix.These relationships are independent of zircon's grain size and shape, and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal-plastic deformation behavior of zircon and on the geometry of its slip systems. We describe previously unknown rheological and crystallographic properties of zircon, which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation. Additionally, relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology. If deformation events result in zircon distortion and, as a consequence, partial or complete resetting of the zircon isotopic system, the age of deformation can be accurately dated.

Formation of three-dimensional nano-trees with perpendicular branches by electrodeposition of CuSn alloy

Abstract Three-dimensional nano-tree-like structures consisting of single-crystalline CuSn alloy nanowires branching perpendicularly from their parent branches were observed. The nano-trees were formed by constant potential co-electrodeposition of Cu and Sn. The geometry of the branches had a four-fold symmetry around the parent branch, which originated from the crystalline structure of the mother CuSn crystal. XRD and TEM analysis revealed that the nanowire branches consisted of single-crystalline γ-CuSn with 〈100〉 preferred growth directions. The addition of polyethylene glycol (PEG) with a molecular weight of 1000 in the plating bath is essential for the formation of the nano-tree structure. It is speculated that adsorbed PEG molecules on CuSn crystal planes with high Miller indices suppress CuSn alloy deposition, enhancing anisotropic growth of the CuSn crystal to form the nano-tree structure.

Activity of Fe2O3 with a high index facet for bituminous coal chemical looping combustion: a theoretical and experimental study

Iron-based oxide with high miller index facet Fe2O3(104) effectively promotes oxygen transfer at different oxidation states for oxidizing coal molecules into CO2 and H2O during chemical looping combustion of solid fuel.

High Miller-index germanium crystals for high-energy x-ray imaging applications.

Near-normal-incidence bent crystals are widely used for x-ray imaging applications. Advantages include high collection solid angle and potentially high efficiency for narrow-band sources, while disadvantages include relatively large (several Å) interatomic spacings and a limited number of suitable matches between a crystal 2d value and an integral multiple of useful emission line wavelengths. The disadvantages become more significant at x-ray energies >10 keV. The former disadvantage can be mitigated by using high-order reflections from crystal planes having low Miller indices, but both disadvantages can be mitigated by using low-order reflections from crystal planes having high Miller indices. We report here on integrated reflectivity measurements we performed of Ge (15,7,7) (2d=0.6296 Å), a candidate for imaging Ru He-α (θ(B)=87°). We find good agreement with calculations, and the data show a multitude of closely spaced reflections with slightly different Bragg angles including a fifth-order reflection of Ge (3,1,1) that has comparable reflectivity. This demonstrates that arbitrary choices of Miller indices in Ge crystals can be used to fine-tune Bragg angles for near-normal-incidence x-ray imaging at tens of kiloelectron volt x-ray energies with minimal lower-energy contamination from lower-order reflections, and that existing calculational tools can be used to reliably estimate integrated reflectivity.

Investigation of faceted void morphologies in UO2 by phase field modelling

In the present study a phase field model for high surface energy anisotropy is developed to model the morphologies of voids in UO2. In order to precisely account for the high anisotropy, an alternative forward-backward strategy based on a staggered grid with an averaged interface normal scheme is used in the numerical procedure. A variety of equilibrium void shapes are reproduced with respect to a constant volume condition. The facet areas and facet energies are calculated. The simulations show excellent agreement with the analytic predictions obtained through Wulff constructions. For the void shapes with high Miller index facets, it is discovered that a slight decrease in total surface area will result in a substantial increase in the total surface energy.

Tuning the Surface Chemistry of Chiral Cu(531)S for Enhanced Enantiospecific Adsorption of Amino Acids

Amino acids are important bioorganic compounds composed of amine and carboxylic acid because they are the main building blocks of many biomolecules. All of them are chiral except glycine. Thus, they have two enantiomers which provide dramatically different biological effects, thereby requiring their separation. High Miller index metal surfaces often define intrinsically chiral structures. A number of previous studies have proved the enantiospecific adsorption difference of chiral molecules on those surfaces. To further enhance the enantiospecificity, step decoration, which is doping the kink site of chiral metal surface with a second metal, can be one route. It may induce one enantiomer adsorbed on the surface to become more stable than the other, inducing the larger enantiospecific energy difference. In this study, we performed density functional theory (DFT) calculations to systemically examine the adsorption geometries and energetics of each enantiomer of alanine, serine, and cysteine, and their enanti...

TurboEELS—A code for the simulation of the electron energy loss and inelastic X-ray scattering spectra using the Liouville–Lanczos approach to time-dependent density-functional perturbation theory

We introduce turboEELS, an implementation of the Liouville–Lanczos approach to linearized time-dependent density-functional theory, designed to simulate electron energy loss and inelastic X-ray scattering spectra in periodic solids. turboEELS is open-source software distributed under the terms of the GPL as a component of Quantum ESPRESSO. As with other components, turboEELS is optimized to run on a variety of different platforms, from laptops to massively parallel architectures, using native mathematical libraries (LAPACK and FFTW) and a hierarchy of custom parallelization layers built on top of MPI. Program summaryProgram title: turboEELSCatalogue identifier: AEXB_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEXB_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License V 2.0No. of lines in distributed program, including test data, etc.: 1371515No. of bytes in distributed program, including test data, etc.: 37355712Distribution format: tar.gzProgramming language: Fortran 95.Computer: Any computer architecture.Operating system: GNU/Linux, AIX, IRIX, Mac OS X, and other UNIX-like OS’s.Classification: 7.2.External routines: turboEELS is a tightly integrated component of the Quantum ESPRESSO distribution and requires the standard libraries linked by it: BLAS, LAPACK, FFTW, MPI.Nature of problem: Calculation of the electron energy loss and inelastic X-ray scattering spectra of periodic solids.Solution method: The charge-density susceptibility of a periodic system is expressed in terms of the resolvent of its Liouvillian superoperator within time-dependent density functional perturbation theory. It is calculated using non-Hermitian or pseudo-Hermitian variants of the Lanczos recursion scheme, whose implementation does not require the calculation of any virtual states. Pseudopotentials (both norm-conserving and ultrasoft) are used in conjunction with plane-wave basis sets and periodic boundary conditions. Relativistic effects (spin–orbit coupling) can be included in calculations.Restrictions: Linear-response regime. Adiabatic exchange–correlation kernels only. No hybrid functionals. Collinear spin-polarized formalism is not supported, only non-collinear spin-polarized case can be used. Spin–orbit coupling cannot be used with ultrasoft pseudopotentials. No magnetism. No Hubbard U formalism. No PAW pseudopotentials.Unusual features: No virtual orbitals are used, nor even calculated. A single Lanczos recursion gives access to the whole spectrum at fixed transferred momentum.Additional comments: The distribution file of this program can be downloaded from the Quantum ESPRESSO website: http://www.quantum-espresso.org/, and the development version of this program can be downloaded via SVN from the QE-forge website: http://qe-forge.org/gf/project/q-e/.!!!!! The distribution file for this program is over 37 Mbytes and therefore is not delivered directly when download or Email is requested. Instead a html file giving details of how the program can be obtained is sent. !!!!!Running time: From a few minutes for elemental bulk systems with a few atoms in the primitive unit cell on serial machines up to many hours on multiple processors for complex systems (e.g., surfaces with high Miller indices) with dozens or hundreds of atoms.

Residual stresses determination in textured substrates for plasma sprayed coatings

In this contribution, we have striven to respond to the desire of obtaining the residual stress tensor in the both cold-rolled and hot-rolled substrates designated for deposition of thermal coatings by plasma spraying. Residual stresses play an important role in the coating adhesion to the substrate and, as such, it is a good practice to analyse them. Prior to spraying, the substrate is often being grit blasted. Residual stresses and texture were quantitatively assessed in both virgin and grit blasted sample employing three attitudes. Firstly without taking preferred orientation into account, secondly from measurements of interplanar lattice spacings of planes with high Miller indices using MoKα radiation. And eventually, by calculating anisotropic elastic constants as a weighted average between single-crystal and X-ray elastic constants with weighting being done according to the amount of textured and isotropic material in the irradiated volume. In the ensuing verification analyses, it was established that the latter approach is suitable for materials with either very strong or very weak presence of texture.

Theoretical study of propene oxidation on Bi2O3 surfaces

The role of bismuth in the selective oxidation of propene has long been debated. We performed density functional calculations to study the dehydrogenation reaction of propene on Bi2O3 surfaces. Our calculated thermodynamic data reveal that the first dehydrogenation of propene on the most stable (010) surface and the (100) surface are difficult. Our calculations indicate that the barrier of the first hydrogen abstraction on the high Miller index surface (211) is much lower than those on the (100) and (010) surfaces, and is close to the experimental one. Further dehydrogenation is shown to be difficult and production of 1,5-hexadiene through dimerization of allyl is likely, in agreement with the experimental observations.

Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

The effects of two major electrodeposition process conditions, electrolyte bath temperature and current density, on the microstructure and crystallographic texture of pure tin coatings on brass and, ultimately, on the extent of whisker formation have been examined. The grain size of the deposited coatings increased with increasing electrolyte bath temperature and current density, which significantly affected the dominant texture: (211) or (420) was the dominant texture at low current densities whereas, depending on deposition temperature, (200) or (220) became the dominant texture at high current densities. After deposition, coatings were subjected to different environmental conditions, for example isothermal aging (room temperature, 50°C, or 150°C) for up to 90 days and thermal cycling between −25°C and 85°C for 100 cycles, and whisker growth was studied. The Sn coatings with low Miller index planes, for example (200) and (220), and with moderate aging temperature were more prone to whiskering than coating with high Miller index planes, for example (420), and high aging temperature. A processing route involving the optimum combination of current density and deposition temperature is proposed for suppressing whisker growth.

Electrochemical and Physicochemical Characterizations of Gold-Based Nanomaterials: Correlation between Surface Composition and Electrocatalytic Activity

Seminal papers have highlighted the promoting effect of gold when associated with other metals. There is still a lack of information about the origin of the exceptional electrochemical performances of gold-based nanostructures, known so far as one of the most active electrocatalysts for glucose-based energy conversion devices. In this report, we examined the correlation between the electrocatalytic properties and the surface composition on a line-up of Au-based nanostructures: gold-palladium, gold-platinum binaries, and gold-palladium-platinum ternaries from the so-called Bromide Anion Exchange (BAE) method. BAE enables to obtain carbon supported nanocrystals having high Miller indices. These truncated octahedron nanocrystals present twins as well as (110) facets. Both X-ray photoelectron spectroscopy (XPS) and electrochemical characterizations have shown that the surface of the multimetallic nanomaterials contains less atoms of gold. The most exposed platinum and palladium enable improving glucose electrooxidation reaction kinetics at low electrode potentials. Additionally, XPS measurements have shown that the binding energy (BE) of either Pd or Pt shifts towards low values when associated with Au, indicating strong electronic interactions between the different elements in the multimetallic nanomaterials. These properties have led to improved catalytic performances when tested for CO stripping experiments and glucose (potential fuel) electrooxidation reaction in alkaline medium.

Inherent point defects at the thermal higher-Miller index (211)Si/SiO2 interface

Electron spin resonance (ESR) studies were carried out on the higher-Miller index (211)Si/SiO2 interface thermally grown in the temperature range Tox = 400–1066 °C to assess interface quality in terms of inherently incorporated point defects. This reveals the presence predominantly of two species of a Pb-type interface defect (interfacial Si dangling bond), which, based on pertinent ESR parameters, is typified as Pb0(211) variant, close to the Pb0 center observed in standard (100)Si/SiO2—known as utmost detrimental interface trap. Tox ≳ 750 °C is required to minimize the Pb0(211) defect density (∼4.2 × 1012 cm−2; optimized interface). The data clearly reflect the non-elemental nature of the (211)Si face as an average of (100) and (111) surfaces. It is found that in oxidizing (211)Si at Tox ≳ 750 °C, the optimum Si/SiO2 interface quality is retained for the two constituent low-index (100) and (111) faces separately, indicating firm anticipating power for higher-index Si/SiO2 interfaces in general. It implies that, as a whole, the quality of a thermal higher-index Si/SiO2 interface can never surmount that of the low-index (100)Si/SiO2 structure.

Contributions of dispersion forces to R-3-methylcyclohexanone physisorption on low and high Miller index Cu surfaces

The physisorption of R-3-methylcyclohexanone on low and high Miller index Cu surfaces is studied with temperature programmed desorption (TPD) and density functional theory (DFT). The DFT calculations are performed with D2, vdW-optB86b, and vdW-optB88 dispersion corrected methods. The adsorption energies calculated by the dispersion corrected methods are more comparable to the TPD results than those calculated without dispersion corrections, although, the former methods have a tendency to overbind the surface adsorbates. The implementation of dispersion corrected methods also indicates a possible adsorbate induced surface reconstruction on Cu(110).

The templated growth of a chiral transition metal chalcogenide

We demonstrate that an intrinsically chiral, high Miller index surface of an achiral metal can be used to template the enantioselective growth of chiral transition metal chalcogenide films. Specifically, Cu(643)R can be used as a template for the enantioselective growth of a chiral copper telluride alloy surface. Beyond a critical alloy thickness the chiral influence of the Cu(643)R surface diminishes and an achiral surface forms. Our work demonstrates a new method of producing chiral transition metal chalcogenide surfaces, with potential applications in the study of structurally chiral topological insulators.

Magnetism of MgO nanoparticles

MgO polycrystal is found to be weakly magnetic experimentally, although its single crystal is non magnetic. In this work, the magnetic properties of surfaces of crystal and nano-particles of MgO are studied by the first-principles density functional theory. The obtained results show that there are the oxygen-rich regions in all the magnetic surfaces discussed in this work, especially in the (111) surface with pure oxygen layer and the (114) surface with pure oxygen chains. Other surfaces with high Miller indices generally have the oxygen-rich regions. For MgO nano-particles, the facets with high Miller indices and the edges and vertexes formed by different orientation surfaces are oxygen-rich possibly and have strong magnetism. The itinerant magnetism is indentified for the magnetism on the surfaces of MgO crystal and the surfaces of MgO nano-particles. That the special MgO ∑ 7[111] grain boundary is not magnetic means that the magnetism of MgO grain boundary is weak if the chemical composition in grain-boundary region is slightly different from that in the crystal. It can be inferred that the magnetism of MgO polycrystal is mainly contributed by the polycrystal surface, the micro-pores, micro-voids and micro-cracks.

Abandoned Functionality of Thin‐Film Opal Photonic Crystals

Angle‐resolved polarized transmission spectra of thin‐film opals are studied experimentally and theoretically as a function of the azimuthal rotation of the plane of incidence at different angles of light incidence. In such 3‐dimensional lattices, the refraction acquires the form of diffraction orders, each with a distinct spectrum, that propagate simultaneously along different directions. The corresponding continuous and patchy stop‐bands in the photonic energy band structure of the opal photonic crystal are determined numerically. For diffraction at high Miller index planes, the diffraction pattern is often distorted due to multiple‐wave diffraction. The finite stacking of hexagonally close‐packed layers leads to an intrinsic 3‐fold rotation symmetry of these spectra, which is particularly pronounced in transmission for frequencies that correspond to wave vectors within the 1st Brillouin zone. Despite the cubic symmetry of the opal lattice, cross‐polarization coupling effects occur in the volume of the opal crystal. These effects are associated with the fact that, the opal eigenmodes are Bloch waves whose electromagnetic field distributions and dispersion relations are markedly different from plane waves.