Surface diffusion of Pd and Au on W single crystal planes: I. Spreading behaviour of Pd and Au layers

In two subsequent publications we report on the diffusion behaviour of Pd and Au on W(110) and stepped W(110) vicinals. The first part deals with the concentration profiles obtained by Pd(Au) spreading from linear sources and recorded by a scanning Auger microscope (SAM). The diffusion experiments were carried out in the temperature range 600–900°C and revealed the formation of Pd(Au) layers of distinct coverages ($\text{1}\text{2}$,1,$\text{3}\text{2}$, 2 monolayers) during the spreading process. Whereas on the W(110) plane essentially a Pd monolayer phase proceeds with a sharp boundary and a square root of time dependence, the concentration profiles observed on the stepped W(10 9 0), W(650) and W(750) surfaces exhibit somewhat rounded shapes. The spreading behaviour can be qualitatively rationalized by considering the gain in binding energy upon the formation of Pd-W bonds and strong lateral Pd(Au)-Pd(Au) attractive interactions. The activation energy for Pd diffusion deduced from the temperature dependence of the spreading rate amounts to 40–45 kcal/mole. In the case of the stepped surfaces this energy applies to both spreading directions parallel and perpendicular to the steps. The preexponential factor however differs by one order of magnitude favouring diffusion along the steps. Combined diffusion experiments involving Pd and Au diffusion simultaneously reveal that Au is the faster moving species. The following paper reports the eminent role of step edges as diffusion paths and dwells on the anisotropic diffusion behaviour caused by regular step structures.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

A measurement of surface diffusion across steps on a curved surface (Pd on W)

The surface diffusion of palladium on the curved part of a tungsten crystal is studied by field electron microscopy. The variation of the local coverage distribution is measured by a probe-hole device on the stepped surface region around (001). The measured data allow a determination of the mass transport surface diffusion coefficient D of Pd on W across atomic steps as a function of temperature, coverage and step density. D has been found (1) to be constant for a given step density and for coverages lower than about 5 × 10¹⁴ Pd adatoms/cm², (2) to increase for higher coverages, and (3) to increase with increasing step density for a given coverage. The activation energy of the process is nearly constant (about 24 kcal/mol) for all coverages up to about 6 × 10¹⁴ adatoms/cm², while the pre-exponential factor of D increases with increasing step density. Interpretation of the results gives some information on the diffusion mechanism.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Dislocation diffusion mechanism in Pd/Ag crystals

In epitaxial Pd/Ag crystals, dislocations in the palladium film have a large influence on the effective diffusion coefficient. In silver the diffusion coefficient hardly depends on the dislocation density. With a dislocation density of 10¹⁰m⁻² the diffusion coefficient in silver is larger than in the palladium, and at 773 K it is 10⁻¹⁸ m²/s. At a dislocation density of 3·10¹² m⁻², the diffusion coefficient in the palladium becomes larger than in silver, and at 773 K it is 3·10⁻¹⁸ m²/s. It is most likely that diffusion in silver takes place via the lattice, while in palladium it occurs at mobile dislocation sites. State Technical University, Samarsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 116–118, August, 1996.     

Kinetics of adsorption on stepped surfaces and the determination of surface diffusion constants

Nitrogen adsorption on stepped W(110) surfaces is examined to illustrate a theory of surface kinetics. Experimental findings by Besockeet al. have shown that nitrogen chemisorbs dissociatively only at the step corner sites of a W(110) surface. Thus the rate of dissociation reveals the mobility of nitrogen and its interaction with the surface. Using continuous-time-random-walk theory, we obtain the probability that molecules reach the step corner sites as a function of time. A kinetic model of nitrogen dissociation is proposed to calculate a coverage function that is in good agreement with experiment. The surface diffusion constant of nitrogen molecules is obtained and is in accordance with previous observations that nitrogen molecules are first weakly physisorbed on the W(110) terrace. Finally, the coverage functions for different step densities are predicted.     

Chemically driven diffusion of Pd in the surface region of Zn(0001): An ultraviolet photoemission investigation

The diffusion of deposited Pd through the (0001) surface region of zinc has been studied with photoemission at hν = 21.2 eV by following the time evolution of the Zn 3d and Pd 4d peaks for a Pd initial coverage of 1, 3, 10 and 15 monolayers. The time decay of the Pd 4d signal is explained with a model where the diffusion coefficient D is not constant; it is (4.6 ± 0.5) × 10^?19 cm² s^?1 for t?7000s, then decreases to (5.5 ± 1) × 10^?20 cm² s^?1 for t ? 15,000 s. The D values correlate well with the spectroscopic results on the valence state evolution during diffusion. At short times (higher D) the spectra show an electron energy gain of Pd atoms during diffusion while at higher time (lower D) this gain is negligible. The initial diffusion is chemically driven while at longer times the diffusion becomes gradually entropic.     

Pd diffusion on MgO(1 0 0): The role of defects and small cluster mobility

Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D-3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 × 10¹⁴ s⁻¹ for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at ?600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks II. Surface diffusion

Surface diffusion of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum was studied by density functional theory using the PW91 approximation to the energy functional. Monomer diffusion on the terrace is facile, with an activation barrier of 0.20 eV, while dimer and trimer diffusions are restricted due to their high activation barriers of 0.43 and 0.48 eV, respectively. During monomer diffusion on the terrace the O–Pt distance increases by 0.54 Å, about 23% of the initial distance of 2.34 Å. The calculated rate of monomer diffusion hops is in good agreement with the onset temperature of diffusion measurements of Daschbach et al., J. Chem. Phys., 120 (2004) 1516. Alternative monomer diffusion pathways, in which the molecule rolls or flips, were also found. These pathways have diffusion barriers of 0.22 eV. During dimer diffusion on the terrace, the donor molecule rises 0.4 Å at the saddle point, while the acceptor rises by only 0.03 Å. Monomer diffusion up to steps and kinks, with activation barriers of 0.11–0.13 eV, facilitate chain formation on top of step edges. The energy landscape of monomer diffusion from terrace to step to kink sites is downhill with a maximum activation barrier of 0.26 eV. A model for water adsorption is presented in which monomer diffusion leads to concurrent formation of terrace clusters and population of steps/kinks, the latter consistent with the STM measurements of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

Thermodynamics of xenon adsorption on Pd(s)[8(100) × (110)]: From steps to multilayers

The thermodynamic properties of the adsorption of xenon on the stepped Pd(s)[8(100)×(110)] surface have been studied over a wide range of pressure (5×10^?11 to 1×10^?4 Torr) and temperature (40–140 K). We have measured adsorption isobars using AES in order to evaluate the surface coverage. By choosing pressure and temperature we have studied under equilibrium conditions, the successive adsorption of xenon on the steps and on the terraces until the first layer is formed, the condensation of the second layer as well as the formation of xenon multilayers. For a small range of pressure and temperature, adsorption takes place only on the atomic steps. The LEED pattern shows that only every other site along the steps is occupied. The extrapolated initial heat of adsorption for steps is E_ad^S = 10.2 kcal/mol, decreasing monotonically by about 2 kcal/mol as the relative coverage of the step sites increases. The dipole moment of the Xe atoms adsorbed on steps is 1.12 D. During adsorption on the terraces the LEED observations suggest that the xenon adlayer is non-localized up to completion of the hexagonally close packed monolayer. The initial heat of adsorption on the terraces, E_ad^T is 8.2 kcal/mol and decreases continuously to a value of 6.9 kcal/mol for a complete monolayer due to lateral repulsive interactions between the adsorbed xenon atoms. The induced dipole moment of Xe on terraces is reduced to 0.49 D. The 5p_{$\text{1}\text{2}$} binding energy of Xe adsorbed on terrace sites is 0.3 eV smaller than that of Xe occuping step sites. The differential molar entropy of the adsorbed layer on the terraces as a function of coverage compares fairly well with the calculated value for an ideally mobile two-dimensional gas. No indication of the growth of two-dimensional xenon islands has been found under these conditions. The isosteric heat of adsorption for the second layer is E_ad^sec = 5.8 kcal/mol independently of the coverage. The condensation of the second layer is a first order two-dimensional gas ? two-dimensional solid phase transition in opposition to the continuous nature of the adsorption of the first layer (extending over a wide range of temperature for a given pressure). The induced dipole moment is further reduced for the Xe second layer to a value of 0.11 D. Finally, the condensation of multilayers proceeds with a latent heat of transformation of E_cond = 3.8 kcal/mol in excellent agreement with the known bulk value for the heat of sublimation of xenon. The line shape of the NVV low energy Auger transitions of xenon or the UPS binding energies of the Xe 5p_{$\text{3}\text{2}$,$\text{1}\text{2}$} spectra allow a clear distinction between first, second and higher layer Xe atoms. We have also established the temperature/pressure conditions for equilibrium between first, second and bulk xenon layers, i.e. a so-called “roughening point”.     

Palladium isotope separation under high mechanical stresses induced in Pd foils upon loading with deuterium

The effects of a significant decrease in the sputtering rate and of the symmetrical separation of the isotope pairs ¹⁰⁸Pd-¹⁰⁵Pd and ¹¹⁰Pd-¹⁰⁴Pd at depths up to 500 Å are experimentally detected in Pd specimens saturated with deuterium during electrolysis (i.e., having a high concentration of internal stresses). These effects are shown to be qualitatively explained using the concepts of isotope separation by centrifugation and diffusion with allowance for defects and mechanical stresses that appear in the near-surface Pd layer during deuterium penetration.     

Levels in 110Pd, 112Pd, 114Pd and 116Pd from the beta decays of the on-line mass separated Rh isotopes

The energy levels of even ^110–116pd nuclei have been studied from β-decays of odd-odd ^110–116Rh isotopes produced at the on-line isotope separator IGISOL. Two β-decaying states with I^π = 1⁺ and I ⩾4 have been identified in all the cases. New level schemes with preliminary spin assignments have been constructed for ¹¹²Pd, ¹¹⁴Pd and ¹¹⁶Pd. Similarities with the level structures of their Xe isotones have been pointed out.     

Surface atom displacement processes

Progress in field ion microscope studies of adatom displacements on metal surfaces is reviewed. It is concluded that of the displacement processes that contribute to surface diffusion only displacements between low-coordination (terrace) sites are well characterised. Procedures and preliminary results of FIM studies of adatom displacement over steps are described. Activation energies measured for passage of Ta, W, Re, Ir and Pt adatoms across (110) W steps are found to equal activation energies for diffusion over (110) W, despite the highly reflecting character of the step for all the adsorbates except Pt. Displacements of adatoms interacting with other adatoms are discussed. Results presented show that interaction of transition metal adatoms forming close-packed dimers on (110) W is rather weak, with a minimum interaction energy [?U(r) < 4kJ/mol] for Re₂ corresponding to a very weak attraction for Re adatoms 0.27 nm apart.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Measurement of the mass of110Pd and114Pd using the (14C,16O) reaction

A measurement of the mass excess of¹¹⁰Pd and¹¹⁴Pd has been made by detecting emergent¹⁶O ions in aQ3D magnetic spectrometer using the¹¹²Cd(¹⁴C,¹⁶O)¹¹⁰Pd and¹¹⁶Cd(¹⁴C,¹⁶O)¹¹⁴Pd reactions at 60 MeV. The results are compared with theoretical predictions.     

Surface diffusion versus atomic addition: using temperature to maneuver the morphology of Pd nanostructures by seeded-growth

We report herein the synthesis of Pd nanostructures ~64–95-nm range in size displaying controlled surface morphologies by a seeded-growth method employing Pd nanoparticles as seeds. Interestingly, we found that the surface texture and thus the surface area of the produced Pd nanomaterials could be tuned by varying the seeded-growth temperature. Pd nanostructures displaying increasingly higher surface textures were obtained as the seeded-growth temperature was decreased from 95 to 30 °C. These results could be explained based on the variations in the relative rates of atom deposition (V _deposition) and surface diffusion (V _diffusion) during the Pd growth. The catalytic activities of the Pd nanostructures toward the reduction of 4-nitrophenol augmented with the increase in the surface texture of the produced nanostructures. The results presented herein can have important implications for designing facile approaches to the synthesis of Pd nanostructures with desired features and optimized catalytic performances that can be highly accessible and attractive for large scale production.     

Silver grain boundary diffusion in Pd

Two to ten nanometer thick polycrystalline Pd films were prepared on the (1 1 1) surface of Ag single crystal and investigations of the Ag diffusion along Pd grain boundaries were carried out using the Hwang-Balluffi method. The samples were monitored by Auger electron spectroscopy (AES) during isothermal heat treatments in the 438-563 K temperature range. Using plausible simplifying assumptions, the activation energy of the product of the grain boundary (GB) diffusion coefficient and k′ (k′ = c_s/c_gb; c_s and c_gb are the surface and GB concentration, respectively) was calculated (0.99 ± 0.08 eV) from the evaluated saturation coefficients of the surface accumulation. This energy, for weak temperature dependence of k′, is approximately equal to the activation energy of the GB diffusion.     

Binding of In and Pb surfactants on Cu{1 1 1} surfaces

Surfactants generally affect both diffusion and nucleation on surfaces, and their function depends on their spatial distribution. Using density-functional-theory based ab initio calculations, the authors have determined the binding energies of In and Pb surfactants at various surface sites of Cu{1 1 1}. The calculation results show that In surfactant prefers incorporation inside/near surface steps or inside top terrace layer, in contrast to at adatom positions. The relative preference among step and terrace sites depends on the availability of surface vacancies, which form either during deposition without sufficient diffusion or by thermal activation. The preference of In surfactant inside top terrace layer makes it effective in the generation of strain on terrace and the slow-down of Cu adatom diffusion. In contrast, incorporation of Pb surfactant inside top terrace is energetically less preferable. As a result, Pb surfactant is less effective in the generation of strain on terrace.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

Absolute coverages for the various surface phases of CO and O adsorbed on Pd(110)

The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the ¹²C(d, p)¹³C and ¹⁶O(d, p₁)¹⁷O¹ reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 10¹⁴ CO molecules cm⁻²) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML.