Layer-by-layer growth of polar MgO(111) ultrathin films

By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.     

Ammonia adsorption on and diffusion into thin ice films grown on Pt(111)

Ammonia adsorption on and diffusion into thin ice films grown on a Pt(111) surface were studied using Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectroscopy. After exposing the crystalline ice film to ammonia molecules at 45 K (ammonia/ice film), we have detected an intriguing feature at 1470 cm(-1) in the FTIR spectra, which is derived from the adsorption of ammonia on the ice with a characteristic structure which appears in thin film range. The peak intensity of this feature decreases gradually as the thickness of the substrate ice increases. In addition, we have detected a feature at 1260 cm(-1) which appears after annealing the ammonia/ice film. The feature corresponds to the ammonia molecules which reach the ice/Pt(111) interface through the ice film. Intriguingly, the intensity of this feature decreases with the ice thickness and there is a linear relation of the peak intensity of the features at 1470 and 1260 cm(-1). We propose a model in which the solubility of the ammonia molecules is much higher for the thin ice film than that for the ideal ice.     

The interfacial properties of MgCl(2) thin films grown on Si(111)7 x 7

Photoelectron spectroscopy with synchrotron radiation and low energy electron diffraction (LEED) were used in order to study the MgCl(2)Si(111) system. At submonolayer coverage of MgCl(2), a new LEED pattern was observed corresponding to a (sqr rt 3 x sqr rt 3)R30 degrees overlayer superimposed on the underlying reconstructed Si(111)7 x 7. The surface species at this stage are mainly molecular MgCl(2) and MgCl(x) (x<2) or MgO(x)Cl(y) attached to the Si substrate through Cl bridges coexisting with monodentate SiCl. The interfacial interaction becomes more pronounced when the submonolayer coverage is obtained by annealing thicker MgCl(2) layers, whereby desorption of molecular MgCl(2) is observed leaving on the nonreconstructed silicon surface an approximately 0.2 ML thick MgCl(x) layer which again forms the (sqr rt 3 x sqr rt 3 )R30 degrees superstructure.     

Auger and electron energy loss spectroscopies of small palladium particles supported on (111) oriented MgO films

The most common tool used to characterize supported metal clusters is the transmission electron microscope. The main advantage of TEM is the combination of high (lateral) resolution imaging with electron diffraction. However the TEM observations are usually made ex-situ i.e. UHV deposited clusters have to be exposed to the atmosphere during transfer to the TEM. This could be a severe limitation for very small reactive clusters. This paper demonstrates that electron spectroscopies can provide in-situ information on the cluster growth (AES), on the electronic structure (ELS) and on the local atomic order (SEELFS) of the clusters. These techniques were applied to Pd clusters of varying size (～10–200 Å) vapor-deposited on thin (111) MgO support films under UHV conditions. An expansion of the lattice and a shift of the loss peaks towards higher energies are observed with decreasing particle size.     

Electrocatalytic oxidation enhancement at the surface of InGaN films and nanostructures grown directly on Si(111)

Pronounced electrocatalytic oxidation enhancement at the surface of InGaN layers and nanostructures directly grown on Si by plasma-assisted molecular beam epitaxy is demonstrated. The oxidation enhancement, probed with the ferro/ferricyanide redox couple increases with In content and proximity of nanostructure surfaces and sidewalls to the c-plane. This is attributed to the corresponding increase of the density of intrinsic positively charged surface donors promoting electron transfer. Strongest enhancement is for c-plane InGaN layers functionalized with InN quantum dots (QDs). These results explain the excellent performance of our InN/InGaN QD biosensors and water splitting electrodes for further boosting efficiency.     

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.     

Molecular orientations and interfacial structure of C60 on Pt(111)

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13?×?√13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3?×?2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.     

Layer-by-layer growth of thin amorphous solid water films on Pt(111) and Pd(111)

The growth of amorphous solid water (ASW) films on Pt(111) is investigated using rare gas (e.g., Kr) physisorption. Temperature programmed desorption of Kr is sensitive to the structure of thin water films and can be used to assess the growth modes of these films. At all temperatures that are experimentally accessible (20-155 K), the first layer of water wets Pt(111). Over a wide temperature range (20-120 K), ASW films wet the substrate and grow approximately layer by layer for at least the first three layers. In contrast to the ASW films, crystalline ice films do not wet the water monolayer on Pt(111). Virtually identical results were obtained for ASW films on epitaxial Pd(111) films grown on Pt(111). The desorption rates of thin ASW and crystalline ice films suggest that the relative free energies of the films are responsible for the different growth modes. However, at low temperatures, surface relaxation or "transient mobility" is primarily responsible for the relative smoothness of the films. A simple model of the surface relaxation semiquantitatively accounts for the observations.     

Partial oxidation of propene on oxygen-covered Au(111)

Partial oxidation of propene is promoted by Au following deposition of atomic oxygen (0.3 ML) via O3 decomposition on Au(111) at 200 K. Several partial oxidation products--acrolein, acrylic acid, and carbon suboxide (O=C=C=C=O)-are produced in competition with combustion to CO2 and H2O. Acrolein is the primary partial oxidation product, and it is further oxidized to the other products by excess oxygen. We propose that acrolein is derived from allyloxy intermediate that is formed via insertion of oxygen into the allylic C-H bond. While no propene epoxide formation is detected from oxidation of C3H6, a small amount of epoxidation is observed during reaction of C3D6 and CD3CH=CH2. These results are strong indications that small changes in the energy required for allylic C-H activation, in this case due to a kinetic isotope effect, may dramatically change the selectivity; thus, small modifications of the properties of oxygen on Au may lead to the more desirable epoxidation process. Our results are discussed in the context of the origin of activity of Au-based catalysts.     

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate''s orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.     

Initial growth of lutetium(III) bis-phthalocyanine on Ag(111) surface

The adsorption of lutetium(III) bis-phthalocyanine (LuPc(2)) on Ag(111) was investigated using scanning tunneling microscopy and spectroscopy (STM/STS). A comprehensive study was carried out toward understanding the driving mechanism responsible for the formation of the first and second monolayers (MLs). In both MLs, the adsorbed molecules are found to exhibit different in-plane orientations arranged according to a "chess-board" like pattern. Highly resolved STM images allowed an exact determination of the corresponding angle mismatch, which differs for the first and second MLs. The tunneling transport through individual molecules reveals a negative differential resistance (NDR) effect detectable within the current-voltage curves. The corresponding density of states (DOS) representation is consistent with a resonant tunneling mechanism sustained by the valence band (VB) states close to the Fermi energy (E(F)) recorded via highly resolved ultraviolet photoemission spectroscopy (UPS).     

Electrodeposition of PbSe epitaxial films on (111) InP

Epitaxial growth of PbSe is addressed on (111) InP single crystals by electrodeposition from aqueous solutions containing Se(IV), Pb(II) and Cd(II). The structure of the deposits is studied by reflection high-energy electron diffraction and X-ray diffraction. It is shown that an epitaxial growth is only observed in the presence of cadmium in the electrolyte. The best epitaxy is obtained for an optimal 0.5 M cadmium concentration.     

Similarities and differences on the molecular mechanism of CO oxidation on Rh(111) and bimetallic RhCu(111) surfaces

The reaction between adsorbed CO and atomic O on various sites of Rh(111) and on the bimetallic RhCu(111) surface has been investigated by first principles density functional theory using slab models. The most likely reaction pathway for CO oxidation on Rh(111) involves probably migration of atomic oxygen from fcc to hcp sites. On the bimetallic surface the mechanism is similar, although depending on the type of bimetallic site a reduction of the energy barrier is predicted. Consequences for the NO reduction by CO reaction are analyzed.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.     

Synthesis and orientational assemblies of MgO nanosheets with exposed (111) facets

Two different orientational assemblies of MgO nanosheets were obtained by tuning the polarity of solvents during the drop deposition method. Different UVvisible diffuse reflectance spectra were recorded and are related to the orientations of the exposed facets.     

Lattice strain effects on CO oxidation on Pt(111)

Surface strain plays a major role in determining the rate limiting step and catalytic activity of platinum for CO oxidation.     

Effect of Zn on the adsorption of CO on Pd(111)

Introduction of a second metal can greatly modify the surface reactivity of a host metal. Recently Jeroro and Vohs found that Pd(111) deposited with 0.03-0.06 monolayer of Zn might possess unique activity to methanol steam reforming reaction. To investigate the distribution of the deposited Zn, we examined the adsorption of CO on two types of model systems. In the first model, Zn is in the top-layer of Pd(111) only, while in the second model Zn is placed in the subsurface exclusively. It is found that Zn atoms in the topmost layer show negligible effect on CO adsorption especially at hollow sites, whereas the second layer Zn atoms affect significantly the interaction of CO with the substrate. It is revealed that the negligible influence of the first layer Zn on CO adsorption is due to the offsetting of the ligand effect by the strain effect. On the other hand, the ligand effect dominates the CO adsorption in the second model where the strain effect is insignificant. It is demonstrated that the d-band centers correlate well with the binding energies of the second model, whereas no such good correlation exists for the first model. Our results show that the subsurface plays a more important role and the observed dramatic modification of surface reactivity of Pd(111) deposited with 0.03-0.06 ML Zn is most likely originated from the subsurface Zn atoms, if the coverage is not underestimated and the deposited Zn atoms are distributed uniformly within a layer.