Edge Facets Research Articles

Active facet determination of layered double hydroxide for oxygen evolution reaction

Correlation of edge area ratio and OER activity, cyanate inhibiting-probe and DFT calculation demonstrate that edge facet of LDHs is the real active facet in comparison with plane facet. Oxygen evolution reaction (OER) plays an indispensable role in developing renewable clean energy resources. One of the critical bottlenecks for the reaction is the development of highly efficient electrocatalyst to decrease the high overpotentials of four-electron transfer process of OER. Recently, layered double hydroxides (LDHs) have been widely investigated among the most promising electrocatalysts for OER due to their high intrinsic activity, excellent stability as well as low-cost. However, it remains unclear how the exposed facet of the LDHs affects their electrocatalytic activity. Here we elucidate the active edge facet of LDHs towards OER by combining the finely control of edge facet ratio coupled with molecular probe method and computational calculation. The LDHs with higher edge facet area ratio show superior activity with low onset potential as well as decreased Tafel slope. The active edge site is further proved by blocking the unsaturated edge sites with cyanate probe anion, of which the adsorption largely inhibits OER activity. Furthermore, based on density functional theory (DFT) calculation, two-dimensional map of theoretical overpotentials as a function of Gibbs free energy reveals that the edge (100) facet exhibits a much higher OER activity than basal plane (001) facet.

Lithium stripping: anisotropic evolution and faceting of pits revealed byoperando3-D microscopy

Operando3-D microscopy of individual pits during Li metal anode stripping demonstrates anisotropic expansion, edge faceting, and surface microstructure effects.

等离激元微腔耦合长波红外量子阱高消光比偏振探测器（特邀）

The long wavelength infrared polarimetric detector can greatly improve the recognition ability of thermal imaging. Owing to the physical limitation of the diffraction limit, the polarization extinction ratio of the current micro-grid polarizer-type long wavelength infrared polarimetric detectors can basically only be as high as about 10∶1. In this paper, a metal/dielectric/metal plasmonic microcavity structure has been fabricated, with the infrared detection active layer of the quantum wells being embedded inside the microcavity. Due to the near-field coupling between the upper grating and bottom reflector metals, a lateral Fabry-Perot resonance was established in the double-metal region, forming the plasmonic microcavity. Benefited from the mode selection characteristics of the microcavity and its resonant coupling with the quantum well intersubband transition, the normal incident light, which cannot be directly absorbed by the intersubband transition of the quantum wells, was coupled into the plasmonic microcavity, transforming its propagation direction into lateral and being absorbed by the quantum wells. The mechanism was confirmed by finite element simulation and the microcavity key parameters such as the grating width and the thicknesses were designed and optimized. Such a structure was applied to the detecting pixels sized at 27 × 27 μm, which was suitable for focal plane arrays. Resulting from the capture and confinement of the incident photons, the detectivity of the detecting pixels could be promoted by about one order of magnitude comparing to the un-structured 45o edge facet coupled detector fabricated from the same epitaxy wafer. The polarization extinction ratio greater than 100∶1 at about 13.5 μm of detecting peak wavelength in the long wavelength infrared waveband was achieved, while the peak intensity dependence on the polarizer azimuth angle fitted Malus law very well. Such a work provides a novel physical foundation and technical route for the development of high extinction ratio long wavelength infrared polarimetric imaging focal planes.

Facet-Dependent Photodegradation of Methylene Blue by Hematite Nanoplates in Visible Light.

The expression of specific crystal facets in different nanostructures is known to play a vital role in determining the sensitivity toward the photodegradation of organics, which can generally be ascribed to differences in surface structure and energy. Herein, we report the synthesis of hematite nanoplates with controlled relative exposure of basal (001) and edge (012) facets, enabling us to establish direct correlation between the surface structure and the photocatalytic degradation efficiency of methylene blue (MB) in the presence of hydrogen peroxide. MB adsorption experiments showed that the capacity on (001) is about three times larger than on (012). Density functional theory calculations suggest the adsorption energy on the (001) surface is 6.28 kcal/mol lower than that on the (012) surface. However, the MB photodegradation rate on the (001) surface is around 14.5 times faster than on the (012) surface. We attribute this to a higher availability of the photoelectron accepting surface Fe3+ sites on the (001) facet. This facilitates more efficient iron valence cycling and the heterogeneous photo-Fenton reaction yielding MB-oxidizing hydroxyl radicals at the surface. Our findings help establish a rational basis for the design and optimization of hematite nanostructures as photocatalysts for environmental remediation.

Separable magnetic MoS2@Fe3O4 nanocomposites with multi-exposed active edge facets toward enhanced adsorption and catalytic activities

We synthesized fluffy MoS2 material stacked by ultrathin nanosheets through adjusting different mass ratios of starting materials via a facile hydrothermal method. Surprisingly, XRD characterization results demonstrated that the intensity ratio of (100) to (002) of ultrathin MoS2 nanosheets was 0.90 with a 4:5 (CH3CSNH2: Na2MoO4·2H2O) mass ratio (M4-5). To the best of our knowledge, this value should be high, implying that the obtained MoS2 nanosheets exposed considerable (100) facets. Because the noncentrosymmetric Mo-2S on edges terminated by dangling bonds exhibits rich defects, high active MoS2 edge planes (100) prevail over inert MoS2 basal planes (002). To overcome its easy suspension and difficult recovery, the magnetic Fe3O4 was selected for in situ doping with ultrathin M4-5, and a unique structure of stable MoS2@Fe3O4 (M@F) nanocomposites-like randomly stacked rose petals-wrapped spherical chocolate beans was obtained for the first time. The introduction of Fe3O4 has to sacrifice some edge active sites, but after balancing the magnetic and active sites, (100)/(002) of MoS2 in M@F can also reach 0.56, showing that edges of MoS2 nanosheets are still multi-exposed. Abundant exposed edges of M@F provide numerous active centers, so M@F nanocomposites exhibited excellent performance for both the 4-nitrophenol (4-NP) reduction and dye adsorption. Therefore, it has great application prospects in wastewater treatment and reduction reaction and is also worth exploring in other applications.

Structural Features Associated with Twinning in the Growth of Gallium Arsenide Single Crystals by the Czochralski Method

The structural features induced by twinning in tin- and tellurium-doped gallium arsenide crystals during their growth by the Czochralski method with liquid melt sealing by boric anhydride have been considered. It is shown that growth twins and edge facets arise at the crystal periphery in the region of supercooled melt. The dopant concentration near a twin is always higher than in the region free of twins. A change is observed in the dislocation-growth direction in crystal during twinning.

In Situ Dynamics during Heating of Copper-Intercalated Bismuth Telluride

Summary The presence of a van der Waals gap within layered materials enables the intercalation of guest species, which can be used for tuning properties. The introduction of intercalant species may also lead to structural transformations across a range of temperatures and use conditions, but we have a limited understanding of the high-temperature evolution of intercalated layered materials. Here, we use in situ transmission electron microscopy to investigate temperature-dependent structural evolution of Bi2Te3, a layered material with well-known application as a thermoelectric, which contains intercalated Cu. As temperature is increased, the initially disordered Cu undergoes local ordering within the van der Waals gap, along with anisotropic sublimation behavior of the crystals. As the temperature is raised further, layered Cu2Te crystals nucleate and grow with a crystallographic relationship to the Bi2Te3 lattice. Notably, the presence of Cu decreases the thermal stability of the Bi2Te3 and substantially alters the transformation pathways during heating.

Selective Epitaxial Growth of Oriented Hierarchical Metal-Organic Framework Heterostructures.

Metal-organic framework (MOF) heterostructures have shown promising applications in gas adsorption, gas separation, catalysis, and energy, arising from the synergistic effect of each component. However, owing to the difficulty in controlling the size, shape, nucleation, and growth of MOFs, it remains a great challenge to construct MOF heterostructures with precisely controlled orientation, morphology, dimensionality, and spatial distribution of each component. Here, we report a seeded epitaxial growth method to prepare a series of hierarchical MOF heterostructures by engineering the structures, sizes, dimensionalities, morphologies, and lattice parameters of both MOF seeds and the secondary MOFs. In these heterostructures, PCN-222 (also known as MOF-545) nanorods selectively grow along the major axis of the ellipsoid-like PCN-608 nanoparticles, on the two end facets of the hexagonal prism-like NU-1000 nanorods, and on the two basal planes of the hexagonal PCN-134 nanoplates, while Zr-BTB nanosheets selectively grow on the six edge facets of PCN-134 nanoplates. The selective epitaxial growth of MOFs opens the way to synthesize different hierarchical heterostructures with tunable architectures and dimensionalities, which could process various promising applications.

A 44-element mesh of Schneiders’ pyramid: Bounding the difficulty of hex-meshing problems

This paper shows that constraint programming techniques can successfully be used to solve challenging hex-meshing problems. Schneiders’ pyramid is a square-based pyramid whose facets are subdivided into three or four quadrangles by adding vertices at edge midpoints and facet centroids. In this paper, we prove that Schneiders’ pyramid has no hexahedral meshes with fewer than 18 interior vertices and 17 hexahedra, and introduce a valid mesh with 44 hexahedra. We also construct the smallest known mesh of the octagonal spindle, with 40 hexahedra and 42 interior vertices. These results were obtained through a general purpose algorithm that computes the hexahedral meshes conformal to a given quadrilateral surface boundary. The lower bound for Schneiders’ pyramid is obtained by exhaustively listing the hexahedral meshes with up to 17 interior vertices and which have the same boundary as the pyramid. Our 44-element mesh is obtained by modifying a prior solution with 88 hexahedra. The number of elements was reduced using an algorithm which locally simplifies groups of hexahedra. Given the boundary of such a group, our algorithm is used to find a mesh of its interior that has fewer elements than the initial subdivision. The resulting mesh is untangled to obtain a valid hexahedral mesh.

3D modeling of growth ridge and edge facet formation in [formula omitted] floating zone silicon crystal growth process

A 3D quasi-stationary model for crystal ridge formation in FZ crystal growth systems for silicon is presented. Heat transfer equations for the melt and crystal are solved, and an anisotropic crystal growth model together with a free surface shape solver is used to model the facet growth and ridge formation. The simulation results for 4″ and 5″ crystals are presented and compared to experimental ridge shape data.

Grating couplers fabricated by e-beam lithography for long-range surface plasmon waveguides embedded in a fluoropolymer.

Long-range surface plasmon polariton waveguides consisting of Au stripes integrated with input and output grating couplers embedded in thick Cytop claddings are proposed and demonstrated experimentally. Under the right conditions, grating couplers enable broadside (top) coupling with good efficiency while producing a low level of background light. The scheme does not require high-quality input and output edge facets, and it simplifies optical alignments. We demonstrate coupling using a cleaved bow-tie fiber and a lensed fiber, and we determine the grating coupling efficiencies in both cases over a broad operating wavelength range. The lensed fiber produces a better overlap with the long-range surface plasmon mode of interest and thus results in a better coupling efficiency with essentially no background light as observed on an infrared camera. The measurements are compared with theoretical results obtained using a realistic model of the structures, including out-of-plane curvature in the grating profile resulting from our fabrication process. The coupling scheme along with the surface plasmon waveguides hold strong potential for biosensing applications.

In-Plane Anisotropic Faceting of Ultralarge and Thin Single-Crystalline Colloidal SnS Nanosheets.

The colloidal synthesis of large, thin two-dimensional (2D) nanosheets is fascinating but challenging, since the growth along the lateral and vertical dimensions needs to be controlled independently. In-plane anisotropy in 2D nanosheets is attracting more attention as well. We present a new synthesis for large colloidal single-crystalline SnS nanosheets with the thicknesses down to 7 nm and lateral sizes up to 8 μm. The synthesis uses trioctylphosphine-S (TOP-S) as sulfur source and oleic acid (with or without trioctylphosphine, TOP) as ligands. Upon adjusting the capping ligand amount, the growth direction can be switched between anisotropic directions (armchair and zigzag) and isotropic directions ("ladder" directions), leading to an edge-morphology anisotropy. This is the first report on solution-phase synthesis of large thin tin(II) sulfide (SnS) nanosheets (NSs) with tunable edge faceting. Furthermore, electronic transport measurements show strong dependency on the crystallographic directions confirming structural anisotropy.

From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni-Cr(100) Alloys.

The interaction of oxygen with Ni-Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy-oxide interface. The progression of oxidation was observed for Ni(100) and Ni-Cr(100) thin films including Ni-8 wt % Cr(100) and Ni-12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O2 at 500 °C, and additional annealing steps were performed at 500 and 600 °C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni-Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni(100) is absent on Ni-Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying. Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moiré patterns in the STM images. The surface oxide can be characterized by NiONi(6 × 7) and NiO-Ni(7 × 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera-Mott regime, and the restructuring of the alloy-oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni-Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera-Mott regime.

Edge facet dynamics during the growth of heavily doped n-type silicon by the Czochralski-method

During the growth of [0 0 1]-oriented, heavily n-type doped silicon crystals by the Czochralski (CZ) method dislocation formation occurs frequently which leads to a reduction of the crystal yield. In this publication the evolution of the solid-liquid interface and the formation of the {1 1 1} edge facets are analyzed on a microscopic scale as possible reason for dislocation formation in heavily n-type doped [0 0 1]-oriented CZ crystals. A correlation between the length of the {1 1 1} edge facets and the curvature of the interface is found. They ultimately promote supercooled areas and interrupted growth kinetics, which increase the probability for dislocation formation at the boundary between the {1 1 1} edge facets and the atomically rough interface.

Floating Silicon Method single crystal ribbon – observations and proposed limit cycle theory

In the Floating Silicon Method (FSM), a single-crystal Si ribbon is grown while floating on the surface of a Si melt. In this paper, we describe the phenomenology of FSM, including the observation of approximately regularly spaced “facet lines” on the ribbon surface whose orientation aligns with (111) crystal planes. Sb demarcation experiments sectioned through the thickness of the ribbon reveal that the solid/melt interface consists of dual (111) planes and that the leading edge facet growth is saccadic in nature, rather than steady-state.To explain this behavior, we propose a heuristic solidification limit cycle theory, using a continuum level of description with anisotropic kinetics as developed by others, and generalizing the interface kinetics to include a roughening transition as well as a re-faceting mechanism that involves curvature and the Gibbs–Thomson effect.

Facile Surfactant‐Free Synthesis of p‐Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors

AbstractA surfactant‐free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single‐phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot‐pressed nanostructured compacts (Eg≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S2σ) at 550 K. S2σ values are 8‐fold higher than equivalent materials prepared using citric acid as a structure‐directing agent, and electrical properties are comparable to the best‐performing, extrinsically doped p‐type polycrystalline tin selenides. The method offers an energy‐efficient, rapid route to p‐type SnSe nanostructures.

Facile Surfactant-Free Synthesis of p-Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors.

A surfactant‐free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single‐phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot‐pressed nanostructured compacts (E g≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S 2 σ) at 550 K. S 2 σ values are 8‐fold higher than equivalent materials prepared using citric acid as a structure‐directing agent, and electrical properties are comparable to the best‐performing, extrinsically doped p‐type polycrystalline tin selenides. The method offers an energy‐efficient, rapid route to p‐type SnSe nanostructures.

Zigzag faceting and width refinement of graphene nanoribbons and nanoperforations via catalyzed edge-annealing on Cu(111)

Abstract Top-down subtractive lithography has previously been used to pattern graphene nanostructures which lack ideal properties due to (1) limited resolution and (2) disordered edges. Here, we introduce a method to convert such disordered edges into relatively smooth zigzag edges via annealing on a Cu(111) substrate at ~950 °C. The Cu catalyzes the re-arrangement of graphene edge atoms to energetically favorable sites, inducing zigzag edge faceting. The dimensions of the graphene nanostructures can be increased, decreased, or held constant during the annealing by tuning the relative balance between growth and etching reactions, described by a fundamental growth rate equation. To demonstrate the flexibility of this method, we lithographically pattern graphene nanoribbons with zigzag or armchair orientations, or alternatively perforate graphene with circular holes, and then anneal these nanostructures to realize zigzag edge termination in each case, with nanostructure feature size tailored from 8 to 80 nm. The annealed nanostructures have smoother zigzag edges (~40% reduction in 1σ line edge roughness), and Raman spectroscopy confirms that they have lower edge disorder than top-down patterned samples.

Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange.

CuInS2 nanocrystals are prepared by ion exchange with template Cu2-xS nanoplatelets and InX3 [X = chloride, iodide, acetate (OAc), or acetylacetonate (acac)]. The morphologies of the resultant nanocrystals depend on the InX3 precursor and the reaction temperature. Exchange with InCl3 at 150 °C produces CuInS2 nanoplatelets having central holes and thickness variations, whereas the exchange at 200 °C produces intact CuInS2 nanoplatelets in which the initial morphology is preserved. Exchange with InI3 at 150 °C produces CuInS2 nanoplatelets in which the central hollowing is more extreme, whereas exchange with In(OAc)3 or In(acac)3 at 150 °C produces intact CuInS2 nanoplatelets. The results establish that the ion exchange occurs through the thin nanoplatelet edge facets. The hollowing and hole formation are due to a nanoscale Kirkendall Effect operating in the reaction-limited regime for displacement of X(-) at the edges, to allow insertion of In(3+) into the template nanoplatelets.

Role of edge facets on stability and electronic properties of III–V nanowires

Results of our ab initio calculations of 〈111〉-oriented GaP, GaAs, GaSb, InP, InAs and InSb nanowires with the zinc-blende structure indicate morphology to crucially affect their electronic properties. For these nanowires, where {011} facets characterize their hexagonal cross section, the formation of small {112} facets between the adjacent {011} ones provides a more stable structure and removes surface states from the gap region even without hydrogen passivation. Our new structural model also predicts a crossover between the indirect and direct band gap in GaP, GaAs and GaSb nanowires when increasing diameters starting from 4 nm, while InP, InAs and InSb nanowires display the direct band gap at diameters of 1.5 nm and larger. Analysis of charge distribution between atoms suggests that {011} facets are positively charged even though a (011) surface of these materials is considered to be non-polar.