Role Of Surface Steps Research Articles

Role of surface steps in activation of surface oxygen sites on Ir nanocrystals for oxygen evolution reaction in acidic media

Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrOx formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts.

(Invited) Facets of Nanocrystal: A Knob to Tune Electrocatalytic Activity

The electrocatalytic reactions strongly depend on the surface atomic structures of the electrocatalysts. Thus, tuning the surface facets of the nanoparticles can be a powerful tool to control the activity for a specific electrocatalytic reaction. In previous studies, we employed high-resolution transmission electron microscopy to reveal the surface atomic structure of Pt nanoparticles, where the amount of high-index surface facets on Pt nanoparticles was quantified.1,2 Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly showed the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation.1 On the other hand, the oxygen reduction reaction (ORR) activity of Pt nanoparticles is not influenced by surface steps in contrast to methanol oxidation reaction (MOR) activity.2 More recently, we demonstrate the synthesis of Pd@Ir core−shell concave nanocubes as an advanced electrocatalyst for acidic oxygen evolution reaction (OER). The Pd@Ir core−shell concave nanocubes were successfully prepared by atomic layer-by-layer deposition of Ir atoms on the surface of free-standing Pd nanocubes in a liquid phase. Using X-ray photoelectron spectroscopy (XPS), the right composition and oxidation states of these nanocubes are confirmed. In the 0.1 M HClO4 electrolyte, the synthesized Pd@Ir concave nanocubes showed significantly increased activity compared to the IrO2 nanoparticles. The improved activity is mainly due to well-controlled surface structures based on high-index facets. We also observed that the specific activity of Pd@Ir concave nanocubes increased after the chronoamperometry test for 10 hours. S. W. Lee, S. Chen, W. Sheng, N. Yabuuchi, Y.-T. Kim, T. Mitani, E. Vescovo, and Y. Shao-Horn, Roles of surface steps on Pt nanoparticles in electro-oxidation of carbon monoxide and methanol, Journal of the American Chemical Society, 2009; 131(43): p. 15669-15677.S. W. Lee, S. Chen, J. Suntivich, K. Sasaki, R. R. Adzic, and Y. Shao-Horn, Role of surface steps of Pt nanoparticles on the electrochemical activity for oxygen reduction, The Journal of Physical Chemistry Letters, 2010; 1(9): p. 1316-1320.

Oxygen reduction on nanostructured platinum surfaces in acidic media: Promoting effect of surface steps and ideal response of Pt(1 1 1)

In this paper, the role of surface steps on the electrocatalytic activity of platinum nanostructured surfaces for the oxygen reduction reaction (ORR) in acidic medium is evaluated by using stepped surfaces of structure Pt(S)[n(111)×(111)] with large terraces (20≤n≤50). It is realized that the inclusion of an even low amount of surface steps enhances the ORR, and linear activity trends are measured when currents at constant potential are plotted vs. the step density. As a consequence, an ideal ORR curve for a defect-free Pt(111) surface is extrapolated at zero defect density from experimental data and can be compared to that of a quasi-perfect Pt(111) electrode. It is clearly shown that surface steps promote the electrode activity toward ORR in acid medium. Results are discussed in light of available theoretical and experimental data.

Electron Microscopy Investigation of the Role of Surface Steps in the Generation of Dislocations during MOCVD Growth of GaN on 4H-SiC

Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.

Dislocation nucleation from surface steps: Atomistic simulation in aluminium

Abstract The possible role of surface steps in the nucleation of dislocations from a free surface has been studied by means of a static atomistic simulation using a many-body potential for aluminium. The fcc crystal with a {100} free surface containing a monatomic step lying along a (110) dense direction is submitted to an increasing uniaxial stress along a direction belonging to the {100} plane. For a sufficiently high applied stress, well below the theoretical strength. dislocations are nucleated at the step and glide in the {111} planes emerging at the step. The effect of a stress orientation is examined. The type of dislocation formed. that is Shockley partials of 90° and 30° character or perfect dislocations, is rationalized by considering the resolved shear stress in the {111} planes. The plane where glide will occur is favoured well before nucleation; a shear of increasing amplitude and extension is progressively localized on this plane. The role of the stress field due to the step, in the formation of the localized shear, is discussed.

Antiphase disorder in GaAs/Ge heterostructures for solar cells

Antiphase disorder in metal organic vapour phase epitaxy grown GaAs/(100)Ge heterostructures has been studied both in as-grown materials and in GaAs solar cells by chemical etching, transmission electron microscopy, and cathodoluminescence. All the samples are single domains at the surface due to the self-annihilation of antiphase domains whose size decreases as the misorientation angle increases. Completely antiphase domain-free epitaxy has been achieved for substrate miscuts greater than 3° off towards [111]. A reversal in sublattice location has been found in the GaAs layers varying the misorientation angle and the growth temperature. A model to explain this result has been proposed based on the role of surface steps in the nucleation process. Strong interaction between antiphase boundaries and misfit dislocations has been found in all the heterostructures. In solar cells antiphase domains have been observed in high densities in the initial layer of GaAs deposited on Ge. The successful realisation of high efficiency solar cells is due to the overgrowth of these domains by single phase material over most of the wafer area.

Atomic-Scale Mechanism of Crack-Tip Plasticity: Dislocation Nucleation and Crack-Tip Shielding

By isolating the process of dislocation emission from a crack tip under an applied tensile stress, we extract from a molecular dynamics simulation the atomic-level displacement and stress fields on the activated slip plane before and after the nucleation event. The stress-displacement relations so obtained provide a direct link with recent continuum descriptions of brittle versus ductile behavior in crack propagation. Crack-tip shielding by the emitted dislocations is demonstrated, as is the role of surface steps in dislocation nucleation and crack-tip blunting. {copyright} {ital 1997} {ital The American Physical Society}

New phenomena in the adsorption of alkali metals on Al surfaces

Recent studies of the adsorption and co-adsorption of alkali metals on Al surfaces have revealed a wealth of hitherto unexpected phenomena, including: order-preserving phase transformations between metastable and stable structures; reconstruction of the substrate by adsorption at room temperature; and formation of binary and ternary surface alloys. The results of experimental structural determinations are reviewed, with the major emphasis on Low-Energy Electron Diffraction (LEED) studies of the ordered structures formed by adsorption of the alkalis on Al(111). The experimental results are compared where possible with corresponding results of ab initio calculations of total energy by Scheffler and co-workers. The experiment-theory comparisons provide valuable insight into the adsorption mechanisms, including the possible role of surface steps as both sources and sinks.

Step Instabilities: A New Kinetic Route to 3D Growth

Atomic force microscopy studies of Ge/Si(001) molecular beam epitaxy growth reveal a crucial new role of surface steps in the 2D to 3D transition. At or near step flow we show that {ital S}{sub {ital A}} steps undergo a stress-driven triangular step instability. The resulting spatial variation of surface strain, although small, can dramatically influence the activation barrier for 3D island nucleation. This provides a surprising kinetic route for the onset of 3D growth associated with the apex regions of triangular steps.

Role of Surface Steps in Thin Film Growth and Properties Studied by Leem

Atomic steps are a common defect at surfaces. Observations made with low energy electron microscopy are presented which demonstrate the impact of steps in growth, phase transitions, magnetic and orientational properties of thin films and clusters at surfaces. Most of these observations can be explained if steps modify the overlayer strain or interface energy.

The role of surface steps in the asymmetric surface dislocation nucleation in vicinal heterostructures with compressive and tensile stresses

The surface nucleation of misfit dislocations in vicinal (001)-oriented heterostructures is discussed. It is shown that beside the asymmetrical stressing of opposite dislocation slip planes due to the vicinal substrate, the surface steps have a similar effect. The effect of the steps has the same-sign asymmetry for a compressive stressed epilayer, but is opposite in the tensile case. The effect on dislocation nucleation energy is calculated. For miscut angles used normally, the step energy contribution exceeds that due to the vicinal substrate. The effect on epilayer tilting is also discussed.

The role of surface steps in the asymmetric surface dislocation nucleation in vicinal heterostructures with compressive and tensile stresses

The surface nucleation of misfit dislocations in vicinal (001)-oriented heterostructures is discussed. It is shown that beside the asymmetrical stressing of opposite dislocation slip planes due to the vicinal substrate, the surface steps have a similar effect. The effect of the steps has the same-sign asymmetry for a compressive stressed epilayer, but is opposite in the tensile case. The effect on dislocation nucleation energy is calculated. For miscut angles used normally, the step energy contribution exceeds that due to the vicinal substrate. The effect on epilayer tilting is also discussed.

Role of surface steps on the charge state of MeV He ions specularly reflected from a crystal surface

Charge state distributions of scattered ions were measured at glancing angles of incidence for MeV He ions incident on a clean (100) surface of a single crystal of SnTe under UHV conditions. Most of the features of the scattering-angle dependence of the charge state distribution of the He ions which are reflected from the topmost atomic layer of the crystal are explained in terms of the position-dependent charge exchange probabilities which are calculated from Bohr and Bohr-Lindhard models. The increase of the He + fraction of the ions scattered at angles smaller than the specular reflection angle is attributed to scattering at surface steps.

Self-organizing epitaxial growth for the deposition of novel superlattices

We report a novel self-organizing epitaxial deposition mode for the AlGaAs alloy during MEE deposition. The role of surface steps appears to be essential as a catalyst in the self-ordering of the alloy. Further we have shown that the attachment kinetics favor the segregation of the binary alloy with the largest formation enthalpy at step edges. The formation of novel “coherent tilted superlattices” is a direct consequence of this form of epitaxy.

On the role of surface steps as epitaxy catalyst

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