Adsorption structure of acetic anhydride on a TiO2(1 1 0) surface observed by scanning tunneling microscopy

The carboxylic acids are stably adsorbed on TiO₂(1 1 0) surfaces at room temperature. To demonstrate the neutralization mechanism proposed by Ashima et al. [H. Ashima, W.-J. Chun, K. Asakura, Surf. Sci. 601 (2007) 1822.] that explains the stable adsorption of carboxylic acids, we studied the full-coverage adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface by STM (scanning tunneling microscopy). We directly observed three postulated species on the TiO₂(1 1 0) surface; normal acetates (termed acetate A) forming a (2 × 1) ordered structure, a minor acetate species (termed acetate B) which was present between the bridging oxygen and the 5-fold Ti, and the oxygen vacancies. We determined the ratio of these three species. This ratio was in good agreement with the postulated conversion reaction of acetate B to A.     

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.     

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.     

Formamide reactions on rutile TiO2(0 1 1) surface

The reaction of formamide over the (0 1 1) faceted TiO₂(0 0 1) surface has been studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). Two main reactions were observed: dehydration to HCN and H₂O and decomposition to NH₃ and CO. The dehydration reaction was found to be three to four times larger than the decomposition at all coverages. Each of these reactions is found to occur in two temperature domains which are dependent upon surface coverage. The low temperature pathway (at about 400 K) is largely insensitive to surface coverage while the high temperature pathway (at about 500 K) shifts to lower temperatures with increasing surface coverage. These two temperature pathways may indicate two adsorption modes of formamide: molecular (via an η¹(O) mode of adsorption) and dissociative (via an η²(O,N) mode of adsorption). C1s and N1s XPS scans indicated the presence of multiple species after formamide absorption at 300 K. These occurred at ca. 288.5 eV (-CONH-) and 285 eV (sp³/sp² C) for the C1s and 400 eV-(NH₂), 398 eV (-NH) and 396 eV (N) for the N1s and result from further reaction of formamide with the surface.     

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.     

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.     

The effect of preadsorbed K on the size distribution of Au nanoparticles on TiO2(1 1 0) surface

The effect of K on the morphology of Au nanoparticles deposited on TiO₂(1 1 0) surface is investigated by STM-STS and AES methods. For comparison, the enhanced concentration of oxygen defect sites generated by Ar⁺ bombardment was also studied. It was found that both the K additive and the oxygen defect sites induce a pronounced decrease in the average size of the Au nanoparticles evolved at 320 K. On the clean TiO₂(1 1 0) the average size of Au particles is 4.3 nm at approximately monolayer coverage of gold, while in the presence of K or oxygen vacancies this value decreased to 2.5 nm. In spite of the reduced average diameter detected at room temperature, the mean size of the Au nanoparticles increased significantly from 2.5 nm up to 7 nm on the effect of annealing at 500-700 K for K precoverages of 0.3-1 ML. For the clean and the Ar⁺ pretreated TiO₂(1 1 0) surfaces the mean size of the Au particles changed only slightly on the effect of the same thermal treatments.     

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.     

A study of the interaction between perylene and the TiO2(1 1 0)-(1 × 1) surface-based on XPS, UPS and NEXAFS measurements

The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO₂(1 1 0)-(1 × 1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO₂(1 1 0)-(1 × 1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO₂(1 1 0)-(1 × 1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO₂ substrate due to weak van der Waals interactions.     

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.     

Comparative investigation of the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloys using DFT

The electronic structure of the c(2 × 2)-Si/Cu(0 1 1) surface alloy has been investigated and compared to the structures seen in the three phases of the (√3 × √3)R30°Cu₂Si/Cu(1 1 1) system, using LCAO-DFT. The weighted surface energy increase between the alloyed Cu(0 1 1) and Cu(1 1 1) surfaces is 126.7 meV/Si atom. This increase in energy for the (0 1 1) system when compared to the (1 1 1) system is assigned to the transition from a hexagonal to a rectangular local bonding environment for the Si ion cores, with the hexagonal environment being energetically more favorable. The Si 3s state is shown to interact covalently with the Cu 4s and 4p states whereas the Si 3p state, and to a lesser extent the Si 3d state, forms a mixture of covalent and metallic bonds with the Cu states. The Cu 4s and 4p states are shown to be altered by approximately the same amount by both the removal of Cu ion cores and the inclusion of Si ion cores during the alloying of the Cu(0 1 1) surface. However, the Cu 3d states in the surface and second layers of the alloy are shown to be more significantly altered during the alloying process by the removal of Cu ion cores from the surface layer rather than by the addition of Si ion cores. This is compared to the behavior of the Cu 3d states in the surface and second layers of the each phase of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloy and consequently the loss of Cu-Cu periodicity during alloying of the Cu(0 1 1) surface is conjectured as the driving force for changes to the Cu 3d states. The accompanying changes to the Cu 4s and 4p states in both the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloys are quantified and compared. The study concludes with a brief quantitative study of changes in the bond order of the Cu-Cu bonds during alloying of both Cu(0 1 1) and Cu(1 1 1) surfaces.     

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.     

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.     

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) surfaces

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) stepped surfaces has been investigated by the extended London-Eyring-Polyani-Sato (LEPS) method constructed using a 5-parameter Morse potential. The calculated results show that there exist common characteristics of CO adsorption on the two surfaces. At low coverage, CO occupies threefold hollow site of the (1 1 1) terrace and is tilted with respect to the surface normal. Among the threefold hollow sites on the (1 1 1) terrace, the nearer the site is to the step, the greater is the influence of the step. The twofold bridge site on the (1 0 0) step is also a stable adsorption site at high coverage. Because of the different lengths of the (1 1 1) terraces, the (3 1 1) and (2 1 1) stepped surfaces have different characteristics. A number of new sites are exposed on the boundary regions, including the fourfold hollow site (H₄) of the (3 1 1) surface and the fivefold hollow site (H₅) of the (2 1 1) surface. At high coverage, CO resides in the H₅ site of the (2 1 1) surface, but the H₄ site of the (3 1 1) surface is not a stable adsorption site. This study further shows that the on-top site on the (1 0 0) step of Pd(3 1 1) is a stable adsorption site, but the same type of site on Pd(2 1 1) is not.     

First-principles calculations on electronic structures of Fe-vacancy-codoped TiO2 anatase (1 0 1) surface

The electronic structures of Fe-doped TiO₂ anatase (1 0 1) surfaces have been investigated by all spin-polarized density functional theory (DFT) plane-wave pseudopotential method. The general gradient approximation (GGA)+U (Hubbard coefficient) method has been adopted to describe the exchange-correlation effects. Through the density functional calculations for the formation energies of various configurations, the complex of a substitutional Fe plus an O vacancy was found to form easily in the most range of O chemical potential. The calculated density of the states of the system of Fe-doped surface with a surface oxygen vacancy shows a band gap narrowing from 2.8 to 1.9 eV comparing with the pure surface due to the synergistic effects of surface Fe impurities with O vacancies. The system processes high visible light sensitivity and photocatalytic ability by decreasing extrinsic absorption energy. By comparing the partial DOS of some O and Ti atoms lying in the outermost and bottom layers of Fe-doped surfaces, it was found that the influence of Fe impurities on the electronic structure of the system is localized.     

The dissociative adsorption of borane on the Ge(1 0 0)-2 × 1 surface: A density functional theory study

By means of cluster models coupled with density functional theory, we have studied the hydroboration of the Ge(1 0 0)-2 × 1 surface with BH₃. It was found that the Ge(1 0 0) surface exhibits rather different surface reactivity toward the dissociative adsorption of BH₃ compared to the C(1 0 0) and Si(1 0 0) surfaces. The strong interaction still exists between the as-formed BH₂ and H adspeices although the dissociative adsorption of BH₃ on the Ge(1 0 0) surface occurs readily, which is in distinct contrast to that on the C(1 0 0) and Si(1 0 0) surfaces. This can be understood by the electrophilic nature of the down Ge atom, which makes it unfavourable to form a GeH bond with the dissociating proton-like hydrogen. Alternatively, it can be attributed to the weak proton affinity of the Ge(1 0 0) surface. Nevertheless, the overall dissociative adsorption of BH₃ on group IV semiconductor surfaces is favourable both thermodynamically and kinetically, suggesting the interesting analogy and similar diversity chemistry of solid surface in the same group.     

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.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

Adsorption of organic molecules by photochemical reaction on Cl:Si(1 1 1) and H:Si(1 1 1) evaluated by HREELS

The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH₄ or CH₂CH₂ was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C₂H₅Cl, C₂H₅ groups were chemically bonded to the surface Si atoms through single covalent bonds. The C₂H₅ groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.     

Intramolecular structures of C60 and C84 molecules on Si(1 1 1)-7 × 7 surfaces by scanning tunneling microscopy

The intramolecular features of carbon 60 and carbon 84 molecules on Si(1 1 1)-7 × 7 surfaces were studied under a UHV-scanning tunneling microscope. Carbon molecules preferentially appear in faulted halves, rather than in unfaulted halves and corner holes; they are embedded in silicon substrates. The orientation and details of the structure of carbon molecules are determined by applying various sample biases to the silicon substrate. As compared with other fullerenes, a bright pentagonal ring with nebulous clusters which represents the cage structure is clearly observed on top of carbon 60 molecules. The bright stripes associated with partitioned curves which depict eight features of asymmetrical C₈₄ molecules are also investigated on Si(1 1 1)-7 × 7 surfaces. The orientations and possible configurations of C₆₀ and C₈₄ are considered in this work. The energy differences for various features of C₆₀ and C₈₄ molecules are estimated and discussed. The corresponding models with respect to each intramolecular feature are proposed and compared with recent theoretical calculation.