Wafer-scale development, characterization, and high temperature stabilization of epitaxial Cr2O3 films grown on Ru(0001).

This work outlines conditions suitable for the heteroepitaxial growth of Cr2O3(0001) films (1.5-20 nm thick) on a Ru(0001)-terminated substrate. Optimized growth is achieved by sputter deposition of Cr within a 4 mTorr Ar/O2 20% ambient at Ru temperatures ranging from 450 to 600 °C. The Cr2O3 film adopts a 30° rotated honeycomb configuration with respect to the underlying Ru(0001) substrate and exhibits a hexagonal lattice parameter consistent with that for bulk Cr2O3(0001). Heating to 700 °C within the same environment during film preparation leads to Ru oxidation. Exposure to temperatures at or above 400 °C in a vacuum, Ar, or Ar/H2 3% leads to chromia film degradation characterized by increased Ru 3d XPS intensity coupled with concomitant Cr 2p and O 1s peak attenuations when compared to data collected from unannealed films. An ill-defined but hexagonally well-ordered RuxCryOz surface structure is noted after heating the film in this manner. Heating within a wet Ar/H2 3% environment preserves the Cr2O3(0001)/Ru(0001) heterolayer structure to temperatures of at least 950 °C. Heating an Ru-Cr2O3-Ru heterostacked film to 950 °C within this environment is shown by cross-sectional scanning/transmission electron microscopy (S/TEM) to provide clear evidence of retained epitaxial bicrystalline oxide interlayer structure, interlayer immiscibility, and epitaxial registry between the top and bottom Ru layers. Subtle effects marked by O enrichment and O 1s and Cr 2p shifts to increased binding energies are noted by XPS in the near-Ru regions of Cr2O3(0001)/Ru(0001) and Ru(0001)/Cr2O3(0001)/Ru(0001) films after annealing to different temperatures in different sets of environmental conditions.

Electron stimulated hydroxylation of a metal supported silicate film.

Water adsorption on a double-layer silicate film was studied by using infrared reflection-absorption spectroscopy, thermal desorption spectroscopy and scanning tunneling microscopy. Under vacuum conditions, small amounts of silanols (Si-OH) could only be formed upon deposition of an ice-like (amorphous solid water, ASW) film and subsequent heating to room temperature. Silanol coverage is considerably enhanced by low-energy electron irradiation of an ASW pre-covered silicate film. The degree of hydroxylation can be tuned by the irradiation parameters (beam energy, exposure) and the ASW film thickness. The results are consistent with a generally accepted picture that hydroxylation occurs through hydrolysis of siloxane (Si-O-Si) bonds in the silica network. Calculations using density functional theory show that this may happen on Si-O-Si bonds, which are either parallel (i.e., in the topmost silicate layer) or vertical to the film surface (i.e., connecting two silicate layers). In the latter case, the mechanism may additionally involve the reaction with a metal support underneath. The observed vibrational spectra are dominated by terminal silanol groups (ν(OD) band at 2763 cm(-1)) formed by hydrolysis of vertical Si-O-Si linkages. Film dehydroxylation fully occurs only upon heating to very high temperatures (∼ 1200 K) and is accompanied by substantial film restructuring, and even film dewetting upon cycling hydroxylation/dehydroxylation treatment.

Ultrathin silica films: the atomic structure of two-dimensional crystals and glasses.

For the last 15 years, we have been studying the preparation and characterization of ordered silica films on metal supports. We review the efforts so far, and then discuss the specific case of a silica bilayer, which exists in a crystalline and a vitreous variety, and puts us into a position to investigate, for the first time, the real space structure (AFM/STM) of a two-dimensional glass and its properties. We show that pair correlation functions determined from the images of this two-dimensional glass are similar to those determined by X-ray and neutron scattering from three-dimensional glasses, if the appropriate sensitivity factors are taken into account. We are in a position, to verify, for the first time, a model of the vitreous silica structure proposed by William Zachariasen in 1932. Beyond this, the possibility to prepare the crystalline and the glassy structure on the same support allows us to study the crystal-glass phase transition in real space. We, finally, discuss possibilities to use silica films to start investigating related systems such as zeolites and clay films. We also mention hydroxylation of the silica films in order to adsorb metal atoms modeling heterogenized homogeneous catalysts.

Model oxide-supported metal catalysts--comparison of ultrahigh vacuum and solution based preparation of Pd nanoparticles on a single-crystalline oxide substrate.

Using single-crystalline Fe(3)O(4)(111) films grown over Pt(111) in UHV as a model-support, we have characterized the nucleation behaviour and chemical properties of Pd particles grown over the film using different deposition techniques with scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Comparison of Pd/Fe(3)O(4) samples created via Pd evaporation under UHV conditions and those resulting from the solution deposition of Pd-hydroxo complexes reveals that changes in the interfacial functionalization of such samples (i.e. roughening and hydroxylation) govern the differences in Pd nucleation behavior observed over pristine oxides relative to those exposed to alkaline solutions. Furthermore, it appears that other differences in the nature of the Pd precursor state (i.e. gas-phase Pd in UHV vs. [Pd(OH)(2)](n) aqueous complexes) play a negligible role in Pd nucleation and growth behaviour at elevated temperatures in UHV, suggesting facile decomposition of the Pd complexes deposited from the liquid phase. Applying temperature programmed desorption and infrared spectroscopy to probe the CO chemisorption properties of such samples after reduction in different reagents (CO, H(2)) shows the formation of bimetallic PdFe alloys following reduction in H(2), but monometallic Pd particles after CO reduction.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra.

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.

CO adsorption and desorption on size-selected Pd(n)/TiO2(110) model catalysts: size dependence of binding sites and energies, and support-mediated adsorption.

The nature of CO adsorption on Pd(n)/TiO(2)(110) (n = 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters ("on-top" sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd ("peripheral" sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pd(n) clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pd(n) clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO(2) to binding sites associated with the Pd clusters.

DFT Investigation of Ammonia Formation via a Langmuir-Hinshelwood Mechanism on Mo-Terminated δ-MoN(0001).

In this work, we employed density functional theory to elucidate the energetics associated with elementary steps along a Langmuir-Hinshelwood mechanism for the Haber-Bosch synthesis of ammonia from N2 and H2 on a hexagonal, Mo-terminated molybdenum nitride surface. Using nudged elastic band calculations, we determined the energy barriers involved in the reaction processes. An active site consisting of four nearest-neighbor Mo atoms, previously identified as an active site on similar surfaces, was chosen to investigate the reaction processes. Using this approach, we calculate a barrier of ∼0.5 eV for the dissociation of N2. The superior activity of the dissociation of the strong N2 bonds is rationalized based on the unique geometric and electronic configurations present at these active sites. Despite the favorable energetics for nitrogen dissociation, the energy cost for hydrogenation of NH x (0 ≤ x ≤ 2) species is shown to be energetically limiting for the formation of ammonia through the Langmuir-Hinshelwood mechanism at these sites, with elementary step activation barriers calculated to be as large as ∼2 eV. A comparison to Haber-Bosch results derived from a similar γ-Mo2N model system suggests the relative independence of surface chemistry and bulk stoichiometry for rhombic Mo4 active sites present on molybdenum nitrides.

Size-dependent oxygen activation efficiency over Pd(n)/TiO2(110) for the CO oxidation reaction.

The dissociative binding efficiency of oxygen over Pd(n)/TiO(2)(110) (n = 4, 7, 10, 20) has been measured using temperature programmed reaction (TPR) mass spectrometry and X-ray photoemission spectroscopy (XPS) following exposure to O(2) with varying doses and dose temperatures. Experiments were carried out following two different O(2) exposures at 400 K (10 L and 50 L) and for 10 L of O(2) exposure at varying temperatures (T(surf) = 200, 300, and 400 K). During TPR taken after sequential O(2) and CO (5 L at 180 K) exposures, unreacted CO is found to desorb in three features at T(desorb) ≈ 150, 200, and 430 K, while CO(2) is observed to desorb between 170 and 450 K. We show that Pd(20) has exceptionally high efficiency for oxygen activation, compared to other cluster sizes. As a consequence, its activity becomes limited by competitive CO binding at low O(2) exposures, while other Pd(n) sizes are still limited by inefficient O(2) activation. This difference in mechanism can ultimately be related back to differences in electronic properties, thus making this question one that is interesting from the theoretical perspective. We also demonstrate a correlation between one of the two CO binding sites and CO(2) production, suggesting that only CO in that site is reactive.

Insights into Silica Bilayer Hydroxylation and Dissolution.

Hydroxylation and dissolution of well-structured silica bilayer films grown on a ruthenium single-crystal support (SiO2/Ru(0001)) was studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). Water desorption signals from SiO2/Ru(0001) hydroxylated by electron-bombardment of adsorbed ice at 100 K were found to be comparable to those of hydroxylated bulk silica samples and attributed to adsorbed molecular water and silanol groups (vicinal and terminal). Isotopic exchange between 18O-labeled SiO2 and 16O-labeled water suggests the occurrence of dynamic siloxane bond cleavage and re-formation during electron bombardment. Together with the observed strong dependence of hydroxylation activity on ice coverage, which is found to increase with increasing thickness of the ice layer, a hydroxylation mechanism based on the activation of siloxane bonds by water radiolysis products (e.g. hydroxyls) and subsequent water dissociation is proposed. Dissolution rates obtained from the attenuation of Si 2p and O 1s XPS signal intensities upon exposure of bilayer SiO2/Ru(0001) to alkaline conditions at various temperatures are in agreement with the proposed rate model for bulk silica dissolution by OH- attack and provide further corroboration of the proposed hydroxylation mechanism.

Electronic structure controls reactivity of size-selected Pd clusters adsorbed on TiO2 surfaces.

The catalytic activity of metal clusters of different sizes adsorbed on oxide surfaces can be explored systematically by using model catalysts. We studied the temperature-programmed reaction of CO with O2 catalyzed by Pd clusters (Pd(n), for n = 1, 2, 4, 7, 10, 16, 20, and 25) that were size-selected in the gas phase and deposited on rutile TiO2(110). X-ray photoemission spectroscopy revealed that the Pd 3d binding energy varied nonmonotonically with cluster size and that the changes correlated with strong size variations in CO oxidation activity. Taking final-state effects into account, low activity was correlated with higher-than-expected Pd 3d binding energy, which is attributed to a particularly stable valence electronic structure; electron transfer from the TiO2 support to the Pd clusters also occurs. Ion scattering shows that small clusters form single-layer islands on the surface and that formation of a second layer begins to occur for clusters larger than Pd10.