Step Decoration Research Articles

Probing water adsorption characteristics of Pt step-edge decorated Cu(211) surface

The surface structure and atomic composition can affect the adsorption characteristics of water on metal surfaces. In this study, we investigated the adsorption of water on Cu(211) stepped surfaces decorated with Pt by a combination of infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) studies. We have observed that step sites of Cu can increase the strength of the binding of water molecules to the surface and facilitate water partial dissociation and the formation of OH groups on the surface. Step decoration by Pt can change the water adsorption characteristics and eliminate the water dissociation. Water adsorbs molecularly on the fully Pt-decorated steps of the Cu(211) surface. Molecular water and OH adsorbed on Cu(211), which can make a chain structure, are disrupted with Pt atoms.

In situ decoration of Co3O4 on N-doped hollow carbon sphere as an effective bifunctional oxygen electrocatalyst for oxygen evolution and oxygen reduction reactions

Transition metal oxide based materials are believed as a sustainable and effective electrocatalysts for oxygen evolution and oxygen reduction reactions (OER/ORR). However, poor conductivity, specific surface area, and tunable crystallography are major bottlenecks. To alleviate such issues, we developed a highly active spinel Co3O4 decorated N-doped hollow carbon sphere (Co3O4/NHCS) by single step decoration using cobalt phthalocyanine (CoPc) precursor for the first time. The introduction of various extents of CoPc significantly affects the sphere wall thickness, defect sites, doping amount of cobalt content, and the concentration of Co3+ and Co2+ sites. The resulting optimum loading of 0.2 g of Co3O4/NHCS-0.2, exhibits a highly uniform hollow carbon sphere with a high wall thickness (61 nm), better surface area (445 m2 g−1), and moderate defect sites. Notably, an exterior concentration of octahedral Co3+ (35.50%) and tetrahedral Co2+ (26.54%) sites serve as the active centres for OER and ORR, respectively. Remarkably, the obtained material exhibits exceptional OER performance with a potential of 1.80 V vs. RHE in 0.1 M KOH medium for achieving a current density of 10 mA cm−2, outperforming the benchmark RuO2 material. In addition, Co3O4/NHCS-0.2 demonstrates an impressive ORR electrocatalytic activity with onset potential of 0.89 V vs. RHE, excellent current density of 5.0 mA cm−2, lower Tafel slope of 60 mV dec−1 and close to four electron transfer. Moreover, the moderate carrier concentration, flat band potential, higher tetrahedral Co2+ and pyridinic N sites significantly improved the ORR activity and methanol oxidation tolerance ability.

Substitutional Co-mediated island-edge decoration in Co/Cu(001): A kinetic Monte Carlo study

Film morphologies of heteroepitaxial systems are as complex as they are diverse and in some cases may involve step decoration of substrate islands by adsorbate atoms as well as the presence of intermixed, heterogeneously nucleated islands. Given that epitaxial growth takes place under far-from-equilibrium conditions, it is not totally unreasonable to assume that kinetics—differences in surface mobilities—is an important driver for island-edge decoration. Indeed, this is believed to be the case in our heteroepitaxial model system, Co/Cu(001), in which deposited Co in the submonolayer regime transforms into substitutional Co atoms through a thermally activated process known as atomic exchange. Here, a Co atom from the adlayer descends into a subsurface site, dislodging a substrate Cu atom, which then moves up to the surface—in the process creating a substitutional Co atom and a Cu atom. In this kinetic Monte Carlo simulation study, we propose that while sCo serves as pinning centers for both Cu and Co atoms alike, it is only very weak in the case of the former and does not have a stabilizing effect on Cu-dominated islands by itself. Thus, the classic picture of Cu islands nucleating around sCo sites and agglomerating with each additional fast diffusing Cu atom, and whose edges are subsequently decorated by the slower moving Co atoms, needs to be revisited. On the contrary, our analysis indicating the presence of both Co and sCo atoms at the rapidly shifting island periphery suggests that island-edge decoration is ephemeral and that sCo itself has a less intuitive, outsize role in Cu-dominated island's rapid growth. We also found that differences in surface mobilities alone lead to intermixing rather than island-edge decoration.

Quasiparticle interference of spin momentum locked surface states at step edges on Re(0001)

Quasiparticle interference patterns formed by a surface state on the Re(0001) surface were investigated using scanning tunneling spectroscopy. The energy dispersion is inferred from Fourier-transformed differential conductivity maps for occupied and unoccupied states. The band dispersion for occupied states agrees with earlier published results obtained by angle-resolved photoemission spectroscopy. An analysis of the phase of interference patterns at step edges reveals a drastic change in the effective energy barrier for backscattering above and below the Fermi level. The attenuation of the interference pattern with increasing distance indicates interband scattering is the dominant scattering mechanism. Step decoration by Ni has a negligible influence on the pattern, excluding spin-flip scattering as a dominant contribution. The one-dimensional Re/Au line interface, however, reverses the scattering barrier behavior, indicating a coupling of surface states.

Surface step decoration of isolated atom as electron pumping: Atomic-level insights into visible-light hydrogen evolution

Here we realize the decoration of isolated platinum atoms onto the surface step of CdS nanowires. The single-atom co-catalyst with high stability can realize maximum atom efficiency and significantly boost electron-hole separation efficiency in chromophore units, generating a greatly enhanced photocatalytic hydrogen evolution performance, 7.69 times as high as Pt nanoparticles do and 63.77 times higher than that of bare CdS nanowires. Directional migrations of photogenerated excitons from the conduction band of CdS to catalytic platinum centers have been witnessed by transient absorption spectroscopy, leading to the supply of long-lived electrons for highly efficient photocatalytic hydrogen evolution. Density functional theory calculations further confirm that the excellent catalytic performance is associated with positively charged platinum sites with partially vacant 5d orbitals, which change distribution of charge density and facilitate higher excited carrier density.

Low Pt Content Proton Exchange Membrane Fuel Cell Stack

Pt doped cerium oxide thin films with a nano-porous structure was prepared by reactive rf magnetron deposition on a fuel cell gas diffusion layer [1] coated with nitrogenated amorphous carbon films (CNx) as an interlayer. We show that the structure and morphology of the cerium oxide-CNx composites depend on the oxygen and nitrogen concentration in working gas used for catalyst deposition. Considerable plasma erosion of the carbonaceous interlayer accompanied by the formation of highly porous carbon/cerium oxide bilayer systems is reported. Oxygen plasma etching of the carbon interlayer occurring simultaneously with cerium oxide film growth is considered to be the driving force for the formation of cerium oxide film nanoporosity with large active surface. Thin film techniques permits to grow the catalyst film characterized by highly dispersed platinum [2] showing high hydrogen oxidation activity and stability. These new materials may help to substantially reduce the demand for expensive noble-metals in proton exchange membrane fuel cell (PEMFC) catalytic applications. We prepared 50 cm2 MEAs with 6 μg Pt/cm2 anode and 300 μg Pt/cm2 cathode catalysts. The MEAs activity were measured in 10 cell hydrogen/air water cooled PEMFC stack, developed in our laboratory. Efficiency measurements performed at different pressures, temperatures, and hydrogen flow conditions were compared with performance of standard reference MEAs. We showed that MEAs made by assembling thin film Pt-CeOx/CNx anodes, containing 6 μg Pt per 1 cm2 only, and reference cathodes can be used for fabrication of state of the art low Pt content PEMFC stacks delivering high specific power. It is shown that the almost Pt free anode technology represents promising solution for future fabrication of PEMFC stacks using Pt free cathodes under development. Reference [1] Fiala, R; Figueroba, A; Bruix, A; Vaclavu, M; Rednyk, A; Khalakhan, I; Vorokhta, M; Lavkova, J; Illas, F; Potin, V; Matolinova, I; Neyman, KM; Matolin, V, High efficiency of Pt2+- CeO2 novel thin film catalyst as anode for proton exchange membrane fuel cells, Appl. Catal. B-Environ., 197 : 262–270, 2016. [2] Dvorak, F; Camellone, MF; Tovt, A; Tran, N-D; Negreiros, FR; Vorokhta, M; Skala, T; Matolinova, I; Myslivecek, J; Matolin, V; Fabris, S, Creating single-atom Pt-ceria catalysts by surface step decoration, Nat. Commun., 7 (Feb): Art. No. 10801, 2016.

Solution-processed highly efficient Au nanoparticles and their reduced graphene oxide nanocomposites as charge trapping media for ZnO thin film transistor nonvolatile memory

Solution-processed, non-volatile undoped and Hf-doped ZnO thin film transistor (TFT) memory was demonstrated by inserting gold nanoparticles (AuNPs) and AuNPs reduced graphene oxide (AuNPs-rGO) nanocomposite charge trapping media between undoped and Hf-doped ZnO and SiO2 dielectrics. The one step reduction and decoration of graphene oxide with AuNPs to create the nanocomposites used thermostable Nisin peptides. Undoped and Hf-doped ZnO TFT memory devices made with AuNPs and AuNPs-rGO exhibited large gate bias-dependent clockwise hysteresis windows. The memory devices exhibited excellent programmable windows of up to 40 V during program/erase operations. This is attributed to trapping and de-trapping of charge carriers by the AuNPs and the AuNPs-rGO nanocomposites. The resulting memory devices exhibited remarkable long-term retention characteristics. Hf-doped ZnO TFT memory devices made with AuNPs or AuNPs-rGO nanocomposites exhibited higher ON/OFF ratios than those made with undoped ZnO. The memory devices made with AuNPs-rGO nanocomposites exhibited ON/OFF ratios greater than 104.

Creating single-atom Pt-ceria catalysts by surface step decoration.

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

In-plane spin reorientation transition in Co/Py bilayers grown epitaxially on vicinal Cu(001)

Co/Py bilayers were grown epitaxially onto a vicinal Cu(001) substrate with the atomic steps parallel to the [110] crystalline axis. We show that the magnetization in vicinal Cu/Co/Py/Cu(001) undergoes an in-plane spin reorientation transition (SRT) from perpendicular to parallel direction of the steps as the Co film thickness increases. By performing Rotation Magneto-Optic Kerr Effect (ROTMOKE) measurement as a function of both the Py and the Co film thicknesses, we show that the observed in-plane SRT results from a competition between the step-induced uniaxial anisotropies in the Py and Co films that favor the Py and Co magnetizations perpendicular and parallel to the atomic steps, respectively. Step decoration experiment further shows that the uniaxial anisotropy originates from the step edges of the vicinal surface.

Reversible thermally induced phase transition in ordered domains of Co(II)-5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)-porphyrin on Cu(111)

We investigated the adsorption behavior of Co(II)-5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)-porphyrin (CoTTBPP) on Cu(111) by scanning tunneling microscopy (STM). At room temperature (RT), the coverage dependent adsorption behavior follows an expected scheme: at low coverage step decoration is found, which evolves into supramolecular domains with a hexagonal order at higher coverage. Interestingly, upon cooling the sample to 180K the occurrence of a clearly distinguishable coexisting herringbone phase is observed. Upon heating to RT again, the herringbone phase vanishes. Thus a temperature dependent, fully reversible phase transition was observed. High resolution STM micrographs allow for the determination of the intramolecular conformations which are different for the two supramolecular arrangements. In addition, we studied the bias voltage dependent appearance of the molecule in STM and assigned a dominant contribution of the central Co at negative bias voltages close to the Fermi edge to the occupied dz2 orbital. Interestingly, the herringbone phase, which dominates at 180K, exhibits a significantly higher molecular density than the monomodal hexagonal arrangement at RT, which is in line with the “normal” behavior of freezing substances.

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...

Electrocatalytic oxidation and adsorption rate of methanol at Pt stepped single-crystal electrodes and effect of Ru step decoration: a DEMS study.

The adsorption and oxidation of methanol at Pt(331) and Ru-step-decorated Pt(331) electrodes are studied recording currents and ion currents by online differential electrochemical mass spectrometry. The CO(2) current efficiencies and the degree of surface poisoning with CO(ad) formed during methanol oxidation are independent of the flow rate, confirming the parallel pathway mechanism. The CO(2) current efficiencies decrease with increasing methanol concentration and increase with increasing potential, whereas those of methyl formate show a reverse trend. At potentials higher than 0.6 V, neither the CO(2) current efficiencies nor the methanol oxidation currents increase with increasing Ru coverage. Instead, methanol oxidation is inhibited due to blocking of the most active platinum step sites. At potentials lower than 0.6 V, however, not only the onset of methanol oxidation shifts negatively, by about 0.1 V, but also the methanol oxidation current and the CO(2) current efficiencies increase. Crucial for the use in fuel cells is the complete oxidation to CO(2), which can be achieved if the reactants first adsorb at the electrode surface along the reaction path with adsorbed CO as an intermediate. Therefore, we directly determine the methanol adsorption rates at Pt(331) as well as at Ru-step-decorated Pt(331), Pt(332), Pt(100), and Pt(11,1,1) electrodes. The methanol adsorption rate is doubled by a double step density in the case of the Pt(331) and Pt(332) electrodes, higher at higher Ru coverages, and increases by a factor of three upon increasing the potential by 0.1 V (corresponding to a Tafel slope of approximately 200 mV dec(-1)). At Pt(331) electrodes with partial step decoration, stripping of adsorbed CO (from CO gas) reveals two adsorbate states, which are also discernable when the adsorbate formed from methanol dehydrogenation is stripped.

On the electrochemical properties of platinum stepped surfaces vicinal to the (100) pole. A computational study

DFT studies on platinum stepped surfaces have been carried out in order to understand the differences in the electrochemical behavior between the surfaces with (111) and (100) terraces. Thus, adsorption energies of different species on selected surfaces have been computed. For the adsorption of Bi and Cu on the Pt(553) and Pt(711) surfaces, it has been found that the adsorption energy on the site corresponding to the step decoration for the Pt(553) surface is ca. 0.5eV higher than that calculated on the (111) terrace sites. On the other hand, there is no preferential adsorption site for Cu or Bi on the Pt(711) surface, since the energy differences between the different sites on this stepped surface with (100) terraces are very small. CO and OH adsorption on the surfaces with (100) terraces, namely the Pt(100), the Pt(711) and the Pt(510) surfaces, has been also investigated. The energy differences between step sites and terrace sites for the surfaces are very small, ca. 0.2eV for OH adsorption and <0.1eV for CO adsorption. For OH, the preferred adsorption mode is a bridge mode, whereas the adsorption energy for the on top and bridge configurations of CO are similar on those surfaces. The comparison with previous DFT calculations indicates that the perturbation created by the step on the (100) terrace is significantly smaller than that created on the (111) terraces. Thus, the modification of the electrochemical properties produced by the presence of a step in the (100) terrace is minor, in agreement with the experimental results.

Structure and Electronic Properties of CoO Nanostructures on a Vicinal Pd(100) Surface

Quasi-one-dimensional CoO nanostructures have been fabricated by step decoration growth on a Pd(1 1 23) surface, a vicinal of Pd(100). The step decoration is dominated by Co–Pd atomic exchange processes, which lead to the formation of 2–4 atom-row wide CoO nanostripes attached to and partially embedded into the outer terrace areas; the CoO stripes grow in a pseudomorphically strained hexagonal phase, as evidenced by atom-resolved scanning tunneling microscopy images and density functional theory simulations. A fraction of the Pd atoms ejected from the steps and outer terraces in the Co–Pd exchange assembles into short monatomic chains, which are periodically attached to the CoO steps. The local electronic structure of the CoO rows at the steps as measured by scanning tunneling spectroscopy is distinctly different from those of embedded CoO rows and of two-dimensional CoO monolayer phases at the terraces, with a significantly higher density of O 2p and Co 3d states around the Fermi energy as confirmed by d...

Decoration of atomic steps on (001)NaCl cleavage face during deposition of preformed Ag clusters

Ag clusters obtained by the gas-aggregation technique were deposited on the cleavage face (001)NaCl. The average size of the nanoparticles was varied in the range from 0.2 to 10 nm and the temperature was varied from room temperature to 470 K. Owing to the mobility of the clusters on the substrate, they coalesced with formation of larger particles, decorating atomic cleavage steps on (001)NaCl already at room temperature. It was shown that the step decoration is possible only when the average size of the deposited clusters is commensurable with the height of the cleavage steps.

Scanning Tunneling Microscopy Evidence for the Dissociation of Carbon Monoxide on Ruthenium Steps

In heterogeneous catalysis, identifying the active site for key reaction steps is an important contribution for the optimization of industrial synthesis. The structure sensitivity of CO dissociation on a metal catalyst, which is the rate-limiting step for the methanation and the Fischer–Tropsch processes under certain conditions, has been debated for years. Here, scanning tunneling microscopy (STM) and density functional theory (DFT) are used to clarify the role of monatomic steps in the splitting of CO on a stepped Ru(0 1 54) crystal, which displays alternating steps with either 4-fold or 3-fold symmetry. After CO doses at elevated temperatures, the STM images reveal step decorations characteristic of atomic oxygen resulting from CO dissociation on every second step. The comparison of the STM images with the results of DFT calculations shows that the step decoration occurs on the steps displaying the 4-fold symmetry. We conclude that the active sites for CO dissociation on ruthenium are located on the 4-fold symmetry monatomic steps.

Enantiospecific Chemisorption of Amino Acids on Step Decorated Chiral Cu Surfaces

Highly stepped metal surfaces can define intrinsically chiral structures, and can potentially be used to separate chiral molecules. The decoration of steps on these surfaces with additional metal atoms may be one potential avenue for improving the enantiospecificity of those surfaces. Density functional theory (DFT) calculations have been performed to study the enantiospecific chemisorption of amino acids adsorbed on the pure, Pd-decorated, and Au-decorated Cu(643)S surfaces. Negligible differences in adsorption energies for the most stable minima of enantiomers of alanine were found on these surfaces. There are, however, measureable energy differences between the two enantiomers of both serine and cysteine in their most stable states for all surfaces. For serine and cysteine in μ3 adsorption geometries, no enhancement in enantiospecificity upon the step decoration is observed, while for those in μ4 geometries, it is improved, especially on Au-decorated surface. Our results provide initial information on how tuning the chemistry of intrinsically chiral surfaces can affect the enantiospecific adsorption of amino acids on these surfaces.

Role of surface defect sites: from Pt model surfaces to shape-controlled nanoparticles

In the present paper, preferentially oriented (111) Pt nanoparticles (mostly octahedral and tetrahedral, namely {111}Pt nanoparticles) have been characterized and compared with a Pt(554) single-crystal electrode as their voltammetric features are quite similar in 0.5 M H2SO4. The anion and Bi adsorption behaviours suggest that the {111}Pt nanoparticles contain relatively wide hexagonal domains and also isolated sites which could adsorb solely hydrogen. Bi step decoration has been successfully extended to modify the defects of {111}Pt nanoparticles without blocking terrace sites. CO charge displacement has been applied to determine the potential of zero total charge (pztc) of non-decorated and Bi decorated surfaces. It has found that the positive shift of pztc on defect-decorated {111}Pt nanoparticles is not so significant in comparison with that of Pt(554) due to the relative short mean length of (111) domains on the {111}Pt nanoparticles. CO stripping demonstrates that {111}Pt nanoparticles exhibit higher reactivity toward CO oxidation. This reflects the role of the defect sites in nanoparticles, evidenced by the disappearance of the “pre-wave” in the stripping voltammogram once the defects were blocked by Bi. The stripping peaks shift to higher potential on Bi decorated surfaces, indicating the active role of both steps and defects for CO oxidation. By comparing the CO stripping charge and the change in hydrogen adsorption charge of surfaces with and without Bi decoration, including reasonable deconvolution, the local CO coverage on defect and terrace sites were obtained for the first time for the {111}Pt nanoparticles, and the results are in good agreement with those obtained on Pt(554). Chronoamperometry studies show tailing in all current–time transients of CO oxidation on all surfaces studied. The kinetics of CO oxidation can be satisfactorily simulated by a modified Langmuir–Hinshelwood model, demonstrating that CO oxidation on all studied surfaces follows the same mechanism.

Morphology and thermal stability of AlF3 thin films grown on Cu(100)

The growth of ultrathin epitaxial layers of aluminum fluoride on Cu(100) has been studied by a combination of surface science techniques. Deposition at room temperature results in step decoration followed by the formation of dendritic two-dimensional islands that coalesce to form porous films. Ultrathin layers (up to 2 monolayers in thickness) are morphologically unstable upon annealing; de-wetting takes place around 430K with the formation of three-dimensional islands and leaving a large fraction of the Cu surface uncovered. Films several nanometers thick, on the contrary, are stable up to ca. 730K where desorption in molecular form sets on. The effect of electron irradiation on the AlF3 has also been characterized by different spectroscopic techniques; we find that even small quantities of stray electrons from rear electron beam heating can provoke significant decomposition of the aluminum fluoride, resulting in the release of molecular fluorine and the formation of deposits of metallic aluminum. These features make AlF3 an interesting material for spintronic applications.

Coverage-dependent self-organized ordering of Co- and Ti-silicide nanoislands along step-bunch edges of vicinal Si(111)

We report on self-organized solid-phase epitaxially grown Ti- and Co-silicide nanoislands ordered along step-bunch edges of a vicinal Si(111) surface, as a function of the initial metal (Ti,Co) coverage. While the degree of self-ordering (in terms of step decoration) for Co-silicide islands was higher than for Ti-silicide islands, both types of island had an inverse dependence of the order parameter on coverage. In addition, the positions of the island sites were found to be correlated, and affected by the step-island and island-island interactions, leading to their alignment along the step-bunch $\ensuremath{\langle}110\ensuremath{\rangle}$ directions, even for the terrace islands. Aberration-corrected high-resolution transmission electron microscopy was used to characterize the islands' crystal structure and growth orientation. The above findings can be understood by the use of coarse-grain modeling.