Adsorption and reactivity of SO2 on Ir(1 1 1) and Rh(1 1 1)

The adsorption and reactivity of SO₂ on the Ir(1 1 1) and Rh(1 1 1) surfaces were studied by surface science techniques. X-ray photoelectron spectroscopy measurements showed that SO₂ was molecularly adsorbed on both the Ir(1 1 1) surface and the Rh(1 1 1) surface at 200 K. Adsorbed SO₂ on the Ir(1 1 1) surface disproportionated to atomic sulfur and SO₃ at 300 K, whereas adsorbed SO₂ on the Rh(1 1 1) surface dissociated to atomic sulfur and oxygen above 250 K. Only atomic sulfur was present on both surfaces above 500 K, but the formation process and structure of the adsorbed atomic sulfur on Ir(1 1 1) were different from those on Rh(1 1 1). On Ir(1 1 1), atomic sulfur reacted with surface oxygen and was completely removed from the surface, whereas on Rh(1 1 1), sulfur did not react with oxygen.     

Proline adsorption on TiO2(1 1 0) single crystal surface: A study by high resolution photoelectron spectroscopy

The surface chemistry and binding of dl-proline were investigated on the oxidised (stoichiometric) and reduced (sub-stoichiometric) TiO₂(1 1 0) single crystal surfaces. TiO₂ was chosen as the substrate as it best represents the surface of a biomedical implant, which bio-molecules interact with during the healing of bone/teeth fractures (molecular recognition). High resolution X-ray photoelectron spectroscopy (HR-XPS) studies of the C1s and N1s regions revealed that dl-proline is present in two forms (dissociated and zwitterionic) on the oxidised TiO₂ surface. On TiO₂(1 1 0) surfaces reduced by Ar⁺ sputtering, a significant increase in the amount of zwitterionic proline at the surface was detected when compared with the oxidised surface. Study of the temperature effect showed that in both cases the zwitterionic structure was the less stable structure. The reason for its relative instability appears to be thermodynamic.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Room-temperature-adsorption behavior of acetic anhydride on a TiO2(1 1 0) surface

We have studied the adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface using XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction) and HREELS (high resolution electron energy loss spectroscopy) to determine the origins of the unique adsorption properties of carboxylic acids on a TiO₂(1 1 0) surface. The C 1s XPS data indicated that the saturation carbon amount of adsorbed acetic anhydride was 12 ± 3% larger than that of the adsorbed acetic acid. LEED showed p(2 × 1) weak spots for the acetic anhydride adsorbed surface. The HREELS spectra revealed the dissociative adsorption of acetic anhydride. Based on these findings, we concluded that the neutralization of the bridging oxygen atoms associated with the dissociative adsorption is necessary for the stable adsorption of carboxylates on the 5-fold Ti sites.     

Configuration of pentacene (C22H14) films on Si(1 0 0)-2 × 1 studied by NEXAFS

Pentacene films on Si(1 0 0)-(2 × 1) surface at 300 K were investigated using near edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. NEXAFS spectra show that pentacene molecules are chemisorbed on the Si(1 0 0)-(2 × 1) surface for monolayer with flat-laying and predominantly physisorbed on the Si(1 0 0)-(2 × 1) surface for multilayer films with an upright molecular orientation. Absorption angle of pentacene molecules were measured through π^∗ transition. The angles between the double bond and the silicon surface were 35-55°, 65° and 76° at monolayer, 24 and 48 nm pentacene deposited on the Si(1 0 0) surface, respectively. We observed that the intermediate flat-laying phase is favored for monolayer coverage, while the films of molecules standing perpendicular to the Si(1 0 0) surface are favored for multilayer coverage.     

Adsorption of water on thin V2O3(0 0 0 1) films

V₂O₃(0 0 0 1) films have been grown epitaxially on Au(1 1 1) and W(1 1 0). Under typical UHV conditions these films are terminated by a layer of vanadyl groups as has been shown previously [A.-C. Dupuis, M. Abu Haija, B. Richter, H. Kuhlenbeck, H.-J. Freund, V₂O₃(0 0 0 1) on Au(1 1 1) and W(1 1 0): growth, termination and electronic structure, Surf. Sci. 539 (2003) 99]. Electron irradiation may remove the oxygen atoms of this layer. H₂O adsorption on the vanadyl terminated surface and on the reduced surface has been studied with thermal desorption spectroscopy (TDS), vibrational spectroscopy (IRAS) and electron spectroscopy (XPS) using light from the BESSY II electron storage ring in Berlin. It is shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature. On the reduced surface water partially dissociates and forms a layer of hydroxyl groups which may be detected on the surface up to T ∼ 600 K. Below ∼330 K also co-adsorbed molecular water is detected. The water dissociation products desorb as molecular water which means that they recombine before desorption. No sign of surface re-oxidation could be detected after desorption, indicating that the dissociation products desorb completely.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Thermal reactions of 2-iodoethanol on Cu(1 0 0)

Temperature-programmed reaction/desorption, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy have been employed to investigate the reactions of ICH₂CH₂OH on Cu(1 0 0) under ultrahigh-vacuum conditions. ICH₂CH₂OH can dissociate on Cu(1 0 0) at 100 K, forming a -CH₂CH₂OH surface intermediate. Density functional theory calculations predict that the -CH₂CH₂OH is most probably adsorbed on atop site. -CH₂CH₂OH on Cu(1 0 0) further decomposes to yield C₂H₄ below 270 K. No evidence shows the formation of -CH₂CH₂O- intermediate in the reactions of ICH₂CH₂OH on Cu(1 0 0) in contrast to the decomposition of BrCH₂CH₂OH on Cu(1 0 0) and ICH₂CH₂OH on Ag(1 1 1) and Ag(1 1 0), exhibiting the effects of carbon-halogen bonds and metal surfaces.     

Sexithiophene films on ordered and disordered TiO2(1 1 0) surfaces: Electronic, structural and morphological properties

Growth of sexithiophene films on both ordered and disordered TiO₂(1 1 0) surfaces has been investigated by angle-resolved ultraviolet photoemission spectroscopy, atomic force microscopy, and X-ray diffraction including grazing-incidence characterization. The order (or disorder) of the TiO₂(1 1 0)-1 × 1 surface has been observed to profoundly influence the electronic, morphological, and structural properties of the 6T films: the band alignment, which determines the injection efficiency of contacts, has been considerably modified by 0.6 eV, and a morphology with either needle-like or dendritic-like islands has been obtained. The changes in the 6T film properties are associated with the orientational modifications of sexithiophene molecules within the films, either flat-lying or upright standing, 6T(0 1 0) or 6T(1 0 0) crystallites, respectively. The growth of different crystallite orientations is argued to be controlled by the kinetics mediated by the (dis)order of the TiO₂(1 1 0) surface rather than exclusively by chemical interaction between the molecule and the substrate.     

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO2

RuO₂(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O₂ and high reaction temperatures. Subsequently the RuO₂ was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO₂ gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO₂ displays high reactivity at relatively low temperatures (?450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O₂ species. RuO₂ deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.     

Simplified method to prepare atomically-ordered TiO2(1 1 0)-(1 × 1) surfaces with steps and terraces

An effective way to prepare atomically-ordered rutile TiO₂(1 1 0) surfaces that have distinct step and terrace structures suitable for oxide thin film deposition is demonstrated. Only a two-step procedure, consisting of 20% HF etching and UHV-annealing at 1100 °C, was required to yield a clean (1 × 1) structure with step and terrace structures. Investigation of the surface using scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy reveals that carbon contamination is removed at around 800 °C, and straight steps with clear terraces appear at around 1000 °C.     

Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

A density functional theory study of ethylene adsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models and Ni13 nanocluster

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian’03 software. It was found that ethylene has adsorbed molecularly on all clusters with π adsorption mode. Relative energy values were calculated to be −50.86 kcal/mol, −20.48 kcal/mol, −32.44 kcal/mol and −39.27 kcal/mol for Ni₁₃ nanocluster, Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) surface cluster models, respectively. Ethylene adsorption energy is inversely proportional to Ni coordination number when Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) cluster models and Ni₁₃ nanocluster are compared with each other.     

(2 × 1)-Na surface reconstruction induced by NaCl dissociation on Ag(1 1 0) during LEED analysis

This study first reports the initial growth stages of sodium chloride (NaCl) on Ag(1 1 0) at room temperature. NaCl grows in bi-layer mode along its [1 0 0] axis and gives rise to (4 × 1) and (1 × 2) reconstructed domains for coverages lower than two monolayers (ML), a minimal thickness inducing a bi-dimensional closed film. In addition, a 10 ML NaCl film has been examined by low energy electron diffraction (LEED). LEED analysis leads to the dissociation of the NaCl deposit in a few minutes. The NaCl dissociation implies Cl desorption from the surface and Na remaining on it. The residual Na is arranged in the form of a (2 × 1) surface reconstruction and is found to be strongly bounded to the Ag substrate. These findings have been established by using the X-ray photoelectron spectroscopy technique.     

Molecular adsorption and growth of naphthalene films on Ag(1 0 0)

The adsorption of naphthalene, vacuum deposited on a Ag(1 0 0) surface, was comprehensively investigated by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD) spectroscopy, X-ray photoelectron spectroscopy (XPS), and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the mono- and multilayer regime. A growth of long-range ordered monolayer at 140 K is observed with LEED. The polarization-dependent C 1s NEXAFS shows that the naphthalene molecules in the monolayer lie almost parallel to the Ag(1 0 0) surface. With increasing film thickness, the molecular orientation turns to upright position. Furthermore, NEXAFS measurements show that in the multilayer regime the molecular orientation depends on the substrate temperature during deposition.     

Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.