Chemical reactivity of Ni-Rh nanowires

Stationary stripe patterns and chemical waves on the bimetallic Rh(110)/Ni surface during the H2 + O2 reaction

Chemical wave patterns that develop in the O₂ + H₂ reaction on a bimetallic Rh(110)/Ni surface have been studied with photoelectron emission microscopy (PEEM) in the 10^-6 to 10^-4 mbar range. The bifurcation diagram for Ni coverages up to 3 monolayers (ML) was mapped out for T = 770 K. Stationary concentration patterns of macroscopic stripes as well as target patterns and irregular chemical waves were observed.

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study

The possibility of depositing precisely mass-selected Ag clusters (Ag₁, Ag₃, and Ag₇) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d_5/2 core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag₇ deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. II. Photoelectron spectroscopy and microscopy

Chemical waves in the H₂ + O₂ reaction on a Rh(111) surface alloyed with Ni [Θ_Ni < 1.5 monolayers (ML)] have been investigated in the 10^-7 and 10^-6 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior. The coincidence of a high oxygen signal with a low Ni 2p intensity, which seemingly contradicts the chemical attraction between O and Ni, has been explained with a phase separation of the oxygen covered Rh(111)/Ni surface into a 3D-Ni oxide and into a Ni poor metallic phase. Macroscopic NiO islands (≈1 μm size) formed under reaction conditions have been identified as 2D-Ni oxide. Titration experiments of the oxygen covered Rh(111)/Ni surface with H₂ demonstrated that the reactivity of oxygen is decreased by an order of magnitude through the addition of 0.6 ML Ni. An excitation mechanism is proposed in which the periodic formation and reduction of NiO modulate the catalytic activity.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. I. Photoelectron emission microscopy

Chemical waves that arise in the H₂ + O₂ reaction on a bimetallic Rh(111)/Ni surface have been studied in the 10^-6 and 10^-5 mbar range at T = 773 K with photoelectron emission microscopy (PEEM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Nickel coverages of 0.3, 0.6, and 1.0 monolayers were investigated. Coadsorbed with some oxygen, Ni starts to penetrate the Rh bulk region substantially only beyond 900 K. In the 10^-5 mbar range, chemical waves are characterized by three distinct gray levels in PEEM. This number reduces to only two levels in the 10^-6 mbar range. In situ LEED showed the periodic appearance of a (n × 1) (n = 8, 10) pattern during chemical waves which was assigned to a 2D-Ni oxide. With in situ AES, one observes that the bright phase in PEEM correlates with a high Ni coverage and the dark phase with a low Ni coverage.

Self-Organized Growth, Structure, and Magnetism of Monatomic Transition-Metal Oxide Chains

We report on the self-organized growth of monatomic transition-metal oxide chains of (3×1) periodicity and unusual MO_{2} stoichiometry (M=Ni, Co, Fe, Mn) on Ir(100). We analyze their structural and magnetic properties by means of quantitative LEED, STM, and density functional theory (DFT) calculations. LEED analyses reveal a fascinating common atomic structure in which the transition-metal atoms sit above a missing-row structure of the surface and are coupled to the substrate only via oxygen atoms. This structure is confirmed by DFT calculations with structural parameters deviating by less than 1.7 pm. The DFT calculations predict that the NiO_{2} chains are nonmagnetic, CoO_{2} chains are ferromagnetic, while FeO_{2} and MnO_{2} are antiferromagnetic. All structures show only weak magnetic interchain coupling. Further, we demonstrate the growth of oxide chains of binary alloys of Co and Ni or Fe on Ir(100), which allows us to produce well-controlled ensembles of ferromagnetic chains of different lengths separated by nonmagnetic or antiferromagnetic segments.

Tuning excitability by alloying: the Rh(111)/Ni/H2 + O2 system

The dynamic behavior of the O2 + H2 reaction on a Rh(111) surface alloyed with Ni has been studied in the 10(-5) mbar range using photoemission electron microscopy (PEEM) as a spatial resolving method. For T = 773 K and p(O2) = 5 × 10(-5) mbar the bifurcation diagram has been mapped out as a function of the Ni coverage in a range of 0 ML ≤ Θ(Ni) ≥ 1.3 ML. A critical Ni coverage of Θ(Ni,crit) = 0.13 monolayers (ML) is required for excitability. In the excitable parameter range pulse trains and irregular chemical wave patterns are found. Whereas the propagation speed of the pulses exhibits no clear-cut dependence on the Ni coverage, the frequency of the local PEEM intensity oscillations increases linearly with Ni coverage in the range from Θ(Ni) = 0.13 ML to Θ(Ni) = 1.3 ML.

Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of 'magic width' stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries.

ATOMISTIC MODELING OF THE FORMATION AND STABILITY OFNiANDVNANOWIRES ON A STEPPEDRh(553)SUBSTRATE

The formation and stability of Ni and V nanowires on a stepped Rh(553) substrate is investigated using the BFS method for alloys via an atom-by-atom description of the formation process. In agreement with experiment, Ni is found to form one-dimensional wires attached to the surface steps, while V does not. However, a detailed analysis of the energetics reveals that there the apparent differences leave room for the possibility of alloying in the case of Ni and the formation of structures along the step in the case of V .

Adsorption and desorption of CO on Ni decorated stepped Rh(553)

Vicinal surfaces are important in surface science, as they show interesting electronic structures and reactivities due to the steps. In this paper the adsorption and desorption of carbon monoxide on the stepped Rh(553) surface decorated with Ni is reported. With 0.1 to 0.3 monolayer Ni on Rh(553) one and two atoms broad Ni wires along the Rh steps are formed. The adsorption and desorption of carbon monoxide on these surfaces is investigated using thermal desorption spectroscopy (TDS) and reflection absorption infra red spectroscopy (RAIRS). TDS shows a marked change from just one broad TDS peak on pure Rh(553) to 4 distinct peaks with increasing Ni decoration. RAIRS shows that already at 0.1 monolayer Ni the CO adsorption states on bridge sites on the Rh step atoms are completely quenched. In addition it is shown that with Ni films up to 3 monolayer the on top adsorption sites for CO on Ni are preferred over the bridge and hollow adsorption sites in contrast to what is known from the Ni(111) surface.

Oxide-metal nanowires by oxidation of a one-dimensional Mn-Pd alloy: stability and reactivity

Distinct one-dimensional (1D) oxide nanowires decorating the step edges of a stepped Pd(1 1 9) surface are formed by partial and complete oxidation of a 1D Mn-Pd alloy. Under full postoxidation treatment at 470-570 K, 1D MnO(2) nanowires coupled pseudomorphically to the Pd steps are obtained. Oxidized nanowires, which maintain the basic structural pattern of the 1D Mn-Pd alloy, are instead prepared by exposure of the Mn-Pd alloy to O(2) at 90 K and subsequent short heating to 400 K. A relatively weak Mn-O bonding characterizes these oxidized alloy wires, which are readily reduced by reaction with CO at moderate temperature (350 K). The here reported system emphasizes the influence of kinetic constraints in the formation of oxide nanostructures.

Low-dimensional surface oxides in the oxidation of Rh particles

The oxidation of rhodium particles leads to the formation of low-dimensional nanostructures, namely ultrathin oxide films and stripes adsorbed on the metallic surface. These structures display unique electronic and structural properties, which have been studied in detail experimentally and theoretically in recent years. In this review, the state of research on low-dimensional surface oxides formed on Rh surfaces will be discussed with a special focus on the contributions derived from computational approaches. Several points elucidating the novel properties of the surface oxides will be addressed: (i) the structural relation between the surface oxides and their bulk counterparts, (ii) the electronic properties of the low-dimensional oxide films and (iii) potential catalytic and electronic applications of the surface oxides.

Step-orientation-dependent oxidation: from 1D to 2D oxides

Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.

One-dimensional 3d electronic bands of monatomic Cu chains

The electronic structure of an array of monatomic Cu chains grown on the Pt(997) surface has been examined by angle-resolved photoemission. The monatomic wires exhibit properties associated with 3d electron confinement in one dimension. Along the wire direction, the 3d bands states display a dispersive character, with periodicity in reciprocal space defined by the wire array geometry. These observations are compared and analyzed with ab initio calculations based on the full-potential linearized augmented plane-wave method.