Atomistic simulation of Fe deposition and alloy formation on Pt substrates

Fe-Pt alloys are of significant importance toward future applications of high-density magnetic recording media. In this work, we apply the BFS method for alloys to study the energetic pathway for subsurface Fe-Pt alloy formation upon deposition of Fe atoms on Pt(1 0 0), Pt(1 1 1), and vicinal Pt(9 9 7) substrates. The simulation results indicate preference for Fe atoms to occupy sites in the Pt subsurface layers and form an ordered alloy phase upon deposition on a low-index Pt surface. This behavior results in Pt surface segregation leading to nucleation of 3D Pt islands. However, the energetics behind deposition of Fe on Pt(9 9 7) indicate that Fe atoms prefer decoration of Pt step edges prior to formation of the ordered Fe-Pt surface alloy, where the ordered alloy is observed to form at the edges of the monoatomic surface steps. In each case presented here, the results are in agreement with experiment, and the formation of a Fe-Pt subsurface alloy is explained by a simple analysis emerging from the competition between BFS strain and chemical energy contributions.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

Thermo-stimulated surface segregation in the ordering alloy Pt80Co20(1 1 1): Experiment and modeling

In this paper, a method of Ionization Spectroscopy (IS) is proposed for the non-destructive layer-by-layer analysis of the elemental composition of a solid surface. Using ionization energy loss spectra, a layer-by-layer concentration profile of the Pt₈₀Co₂₀(1 1 1) alloy surface is obtained for different annealing temperatures. For the disordered Pt₈₀Co₂₀(1 1 1) at room temperature, the first atomic layer consists of pure Pt with damped oscillations in the deeper layers. Heating the sample reduces the oscillations. However, at a temperature of 823 K, a sandwich-like structure of the type Pt/Co/Pt was found in the first three atomic layers. For the ordered state the first atomic layer also consists of pure Pt with bulk concentration in other layers. LEED analysis shows a p(2 × 2) superstructure for the surface of the ordered Pt₈₀Co₂₀(1 1 1) alloy. The segregation behavior in this alloy is further studied by Monte Carlo (MC) simulations combined with the Constant Bond Energy (CBE) model. The results of the MC simulations agree well with the experiments at the higher temperatures, both for the surface composition and the concentration depth profile. At lower temperatures, some discrepancies exist between the MC results and the measured concentration profile.     

Effects of layering and magnetic annealing on the texture of CoPt films

The effect of magnetic field annealing of magnetron sputtered CoPt alloy films and Co/Pt bilayers on the crystallographic texture of the obtained chemically ordered (L1₀) CoPt films is presented. In CoPt alloy films the main effect of the magnetic field is to suppress (1 1 1) growth in the early stages of L1₀ formation whereas the development of (0 0 1) versus (1 0 0) texture is related to chemical ordering strain. A higher degree of (0 0 1) texture is obtained by magnetically annealing Co/Pt bilayers since the initial (1 1 1) texture in the as-sputtered films is avoided and Co-Pt alloying occurs in the presence of the magnetic field.     

Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM

The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θ_Pt, growth temperature T_G and post-growth annealing temperature T_A. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10⁻⁴ ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer.     

Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

Ceria nanoformations in CO oxidation on Pt(1 1 1): Promotional effects and reversible redox behaviour

A well-defined CeO_x/Pt(1 1 1) model catalytic system has been fabricated using the self-assembling of Ce adatoms on a Pt(1 1 1) surface with a subsequent oxidation of the nucleating Ce submonolayer (0.3 ML). The resulting system of the “inverse supported catalyst” type consists of CeO_x nanoformations (2D islands of 5-15 nm size and ∼0.3 nm in height) more or less uniformly distributed over the Pt(1 1 1) surface. This CeO_x/Pt(1 1 1) system has been tested in the CO oxidation reaction where both the CO₂ production rate and the Ce oxidation state were monitored in situ. An enhanced reactivity and a remarkable shift of the bistable region of the reaction towards higher CO pressures were observed when compared to a clean Pt(1 1 1) surface. The CeO_x islands exhibit a pronounced redox behaviour that follows the hysteresis cycle of the reaction. The usefulness of such a type of the “inverse model catalyst” for studying the oxygen diffusion supply and the redox behaviour of ceria in the ceria-platinum catalysts is demonstrated.     

Interaction of CO with atomically well-defined PtxRuy/Ru(0 0 0 1) surface alloys

The interaction of CO with structurally well-defined Pt_xRu_y surface alloys supported on Ru(0 0 0 1) was investigated by thermal desorption spectroscopy and infrared reflection-absorption spectroscopy. The surface composition and the distribution of the surface atoms were controlled by high resolution scanning tunneling microscopy. On these surfaces, which have a nearly random distribution of the two surface species, the adsorption (and desorption) of CO is strongly modified compared to the pure elemental surfaces, by strain effects and electronic ligand effects. CO adsorbs exclusively in a linear configuration on Pt and Ru atoms for all surfaces investigated. The adsorption energy of CO is lowered on the alloy surfaces with respect to both Pt(1 1 1) and Ru(0 0 0 1), similar as for pseudomorphic monolayer Pt films. For both Pt and Ru sites the adsorption strength decreases with increasing Pt concentration.     

Carbon monoxide adsorption on Co deposited Pt(1 0 0)-hex: IRRAS and LEED investigations

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm⁻¹, respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co_0.3 nm/Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm⁻¹. For Co_0.3 nm/Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co_0.3 nm/Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm⁻¹, which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co_0.3 nm/Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt-Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co_x/Pt(1 0 0)-hex are discussed.     

Initial stages of hydroxide formation and its reduction on Co(0 0 0 1) studied by in situ STM and XPS in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E<−0.55 V (SHE) the formation of a Co(OH)₂(0 0 0 1) superstructure is observed. It shows a hexagonal symmetry with an average periodicity of P=1.25±0.20 nm. The coincidence cell of the observed structure consists of 16 unit cells Co(OH)₂(0 0 0 1) showing an average lattice parameter of a=0.33±0.05 nm and thus the Co(OH)₂ monolayer forms a 5 × 5 superstructure with respect to the underlying metallic Co(0 0 0 1) substrate. XPS results clearly prove the presence of hydroxide and exclude the formation of CoO in the subpotential range.At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH⁻-Co²⁺- OH⁻ trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)₂ layer is found to be not completed which is confirmed by XPS results.     

Pt interactions with annealed and chemically-etched Nb-doped SrTiO3(0 0 1) surfaces: Metal/oxide surface chemical effects on band bending behavior

XPS and LEED have been used to characterize the interaction of sputter-deposited Pt (maximum coverage <5 ML) with Nb-doped SrTiO₃(0 0 1) surfaces prepared either by annealing in O₂ and then UHV, or by chemical-etching in aqua regia. The annealed surface exhibits an ordered (1 × 1) LEED pattern, with additional diffraction spots and streaks indicating the presence of oxygen vacancies. Increasing Pt coverage results in the decrease of the observed Pt(4f_7/2) binding energy and the uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to smaller binding energies, as expected for Pt cluster growth and surface-to-Pt charge donation on an n-type semiconductor. The etched surface is disordered, and exhibits a hydroxylated surface with a contaminant C film of ∼23 ? average thickness. Pt deposition on the etched surface results in an immediate decrease in the intensity of the OH feature in the O(1s) spectrum, and a uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to larger binding energies with increasing Pt coverage. The observed Pt(4f_7/2) binding energy on the etched surface (∼72 eV) is independent of Pt coverage, and indicates substantial electronic charge donation from the Pt to surface hydroxyl species. The observation of band bending towards higher binding energies upon Pt deposition (behavior normally associated with p-type semiconductors) demonstrates that sub-monolayer quantities of adsorbates can alter metal/oxide interfacial charge transfer and reverse the direction of band bending, with important consequences for Schottky barrier heights and device applications.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Surface roughening by anisotropic adatom kinetics in epitaxial growth of La0.67Ca0.33MnO3

The growth mechanisms and surface morphology of colossal magnetoresistance (CMR) La_0.67Ca_0.33MnO₃ films deposited by rf magnetron sputtering on SrTiO₃(0 0 1) substrates are investigated. The films are epitaxial, coherently strained and ferromagnetic. It is found that at early growth stages, in nanometric films, a layer-by-layer mechanism dominates, which results in step and terrace surface morphology. Upon further growth, the flat surface becomes unstable when large two-dimensional (2D) islands form. The Erlich-Schwoebel step-edge energy barrier induces an anisotropic adatom kinetics that favors 2D nucleation on top of the islands as it reduces downhill adatom current. As a result, there is an evolution with growth to mound-like structures of increasing height. Critical thickness for mound formation and average mound separation can be tuned by substrate miscut angle and growth temperature. These results provide a detailed understanding of the roughening process in manganites and are relevant for the controlled fabrication of CMR films, in particular for its use in epitaxial heterostructures.     

Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts

Pt, Ru and Pt/Ru nano-particles, synthesized in ethylene glycol solutions, are studied using infrared (IR) spectroscopy and high resolution transmission electron microscopy (HRTEM). The synthesis method allows the control of the mono- and bi-metallic catalyst particle sizes between 1 and 5.5 nm. The IR spectra of CO adsorbed (CO_ads) on the Pt, Ru and bi-metallic Pt/Ru colloids are recorded as a function of the particle size. The stretching frequency of CO_ads depends on the particle size and composition. Strong IR bands due to the stretching vibration of CO_ads are observed between 2010 and 2050 cm⁻¹ for the Pt nano-particles, while two IR bands between 2030 and 2060 cm⁻¹ for linear bonded CO_ads, and at lower wavenumbers between 1950 and 1980 cm⁻¹ for bridged bonded CO_ads, are found for the Ru particles. The IR spectra for the Pt/Ru nano-sized catalyst particles show complex behaviour. For the larger particles (>2 ± 0.5 nm), two IR bands representative of CO_ads on Ru and Pt-Ru alloy phases, are observed in the range of 1970-2050 cm⁻¹. A decrease in the particle size results in the appearance of a third band at ∼2020 cm⁻¹, indicative of CO_ads on Pt. The relative intensity of the band for CO_ads on the Pt-Ru alloy vs. the Pt phase decreases with decreasing particle size. These results suggest that Ru is partially dissolved in the Pt lattice for the larger Pt/Ru nano-particles and that a separate Ru phase is also present. A Pt-Ru alloy and Ru phase is observed for all Pt/Ru particles prepared in this work. However, a decrease in particle size results in a decrease of the number of Pt and Ru atoms in the Pt-Ru alloy phase, as they are increasingly present as single Pt and Ru phases.     

Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)

The electrodeposition of Au on Pt(1 1 1) from electrolytes containing μM concentrations of was studied by in situ scanning tunneling microscopy. Under these conditions the Au flux is limited by diffusion in the electrolyte over a wide potential range, which allows to assess the effect of the electrochemical environment on the growth kinetics. Similar to gas phase metal deposition Au film growth proceeds via nucleation and lateral growth of Au monolayer islands, with the saturation island density strongly depending on the deposition potential and on the anion species in the electrolyte. For deposition in H₂SO₄ solution the saturation island density continuously increases with increasing potential between −0.2 and 0.5 V (SCE), whereas in Cl-containing H₂SO₄ it first decreases and then increases again. Following nucleation and growth theories this behavior can be attributed to potential-induced changes of the Au surface mobility, caused by changes in the density and structure of coadsorbed sulfate/bisulfate and chloride adlayers. Under conditions of high Au surface mobility multilayer growth proceeds via a typical Stranski-Krastanov growth mode, with layer-by-layer growth of a pseudomorphic Au film up to 2 ML and 3D growth of structurally relaxed islands at higher coverage, indicating thermodynamic control under these conditions.     

Adsorption and reaction of bicyclic hydrocarbons at Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys: trans-decahydronaphthalene (C10H18) and bicyclohexane (C12H22)

Adsorption and desorption of trans-decahydronaphthalene (C₁₀H₁₈) and bicyclohexane (C₁₂H₂₂) can be used to probe important aspects of non-specific dehydrogenation leading to surface carbon accumulation and establish better estimates of activation energies for C-H bond cleavage at Pt-Sn alloys. This chemistry was studied on Pt(1 1 1) and the (2 × 2)-Sn/Pt(1 1 1) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys by using temperature programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). These hydrocarbons are reactive on Pt(1 1 1) surfaces and fully dehydrogenate at low coverages to produce H₂ and surface carbon during TPD. At monolayer coverage, 87% of adsorbed C₁₀H₁₈ and 75% C₁₂H₂₂ on Pt(1 1 1) desorb with activation energies of 70 and 75 kJ/mol, respectively. Decomposition of C₁₀H₁₈ is totally inhibited during TPD on these Sn/Pt(1 1 1) alloys and decomposition of C₁₂H₂₂ is reduced to 10% of the monolayer coverage on the (2 × 2)-Sn/Pt(1 1 1) alloy and totally inhibited on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy. C₁₀H₁₈ and C₁₂H₂₂ are more weakly chemsorbed on these two alloys compared to Pt(1 1 1) and these molecules desorb in narrow peaks characteristic of each surface with activation energies of 65 and 73 kJ/mol on the (2 × 2) alloy and 60 and 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy, respectively. Alloyed Sn has little influence on the monolayer saturation coverage of these two molecules, and this is decreased only slightly on these two Sn/Pt(1 1 1) alloys. The use of these two probe molecules enables an improved estimate of the activation energy barriers E* to break aliphatic C-H bonds in alkanes on Sn/Pt alloys; E* = 65-73 kJ/mol on the (2 × 2)-Sn/Pt(1 1 1) alloy and E* ? 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy.     

Predicting the metal growth mode and wetting of noble metals supported on c-ZrO2

Employing periodic density-functional calculations, we predict the metal growth mode for ideal and defective interfaces of noble metals (Pd and Pt) deposited on top of cubic-ZrO₂. We discuss various energetic contributions and surface textures. We propose that isolated noble metal atoms are supported on both steps and terraces on the surfaces. Pt, on the other hand, shows a higher probability to decorate steps and other low oxygen coordinated sites, while Pd forms larger metal aggregates on both high oxygen coordinated sites (terraces) as well as low oxygen coordinated sites (steps). Oxygen defects on the support act as nucleation sites for the metal cluster growth. We also investigate metal wetting of Pd and Pt on the {1 1 1} surface of cubic-zirconia. Our calculations predict that the wetting angles are ca. 110-130° for 1 and 2 mono-layers of Pd and Pt, respectively, suggesting that metal wetting decreases as the metal loading increases.     

Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

Ethylene dissociation on flat and stepped Ni(1 1 1): A combined STM and DFT study

The dissociative adsorption of ethylene (C₂H₄) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges.DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces.The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms.Finally a high surface area NiAg alloy catalyst supported on MgAl₂O₄ was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results.