The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states:1. At very low exposures (0.05 L) three vibrational losses at 345 cm^?1, 1170 and 3310 cm^?1 are observed which are attributed to the symmetric Fe-N stretching-, N-H₃ deformation and N-H₃ stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C_3v).2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm^?1, 1640 cm^?1 and 3370 cm^?1. The latter two are interpreted as the degenerate NH₃ deformation and - stretching modes of molecularly adsorbed NH₃. The 1450 cm^?1 loss is a combination of the losses at 345 cm^?1 and 1105 cm^?1. The observation of 5 vibrational losses is consistent with an adsorption complex of C_s symmetry.3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm^?1 and by two additional very intense modes at 160 cm^?1 and 350 cm^?1 which are due to rotational and translational modes.4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm^?1, 415 cm^?1 and 520 cm^?1, and a symmetric deformation mode at 1090 cm^?1. A broad loss feature around 3300 cm^?1 is attributed to hydrogen bonding in the condensed layer.Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN₂ species were detected.     

The initial oxidation of Cr(100)

Electron energy loss spectroscopy (EELS), Auger spectroscopy, and low-energy electron diffraction (LEED) have been used to study oxygen chemisorption on and the initial oxidation of Cr(100). With O₂ exposures up to 5 L, CrO vibrational frequencies between 495 and 545 cm⁻¹ are observed. A CrO stretching frequency at 635 cm⁻¹, probably due to rhombohedral Cr₂O₃, is observed to emerge strongly by ≈ 60 L. Based on a sequence of O₂ exposures at 300 K and on a second sequence with 625 and 1175 K anneals, a model of the initial oxidation of Cr(100) is presented. Subsurface oxygen in interstitial sites with Cr atoms maintaining bulk positions is proposed to act as a nucleus for subsequent oxide growth. According to this model, oxide growth at 300 K occurs primarily through domain expansion, while frequent creation of new domains occurs at 625 K. At elevated temperatures, competition between domain growth and diffusion into the bulk is observed.     

The bonding of water molecules to platinum surfaces

Using high resolution electron energy loss spectroscopy in ultra-high vacuum we have studied the vibrational spectrum of submonolayer and multilayer quantities of water adsorbed on platinum (100) surfaces. For adsorbed multilayers the spectrum resembles the spectrum of ice I. For submonolayer quantities of H₂O we find three different OH stretching vibrations, 2850, 3380, and 3670 cm^?1. The highest frequency is attributed to free OH groups. The vibration around 3380 cm^?1 indicates H bonding between oxygen atoms. It is therefore concluded that the water molecules cluster even at low coverage. The lowest OH stretching frequency is attributed to H bonding to platinum. We find also evidence for additional oxygen lone pair orbital bonding to the surface which disappears however when the first monolayer is completed. The relation to currently considered models in electrochemistry of aqueous solutions is discussed.     

A vibrational characterisation of the O/A1(111) system: a reassignment of HREELS data

In light of recent STM measurements of the O/A1(111) system, we reassign the dipole active modes observed at low coverage to resolve discrepancies between the interpretation of Strong, Firey, deWette and Erskine [Phys. Rev. B 26 (1982) 3483], invoking subsurface oxygen, and a variety of other studies which find no evidence for surface oxygen. The STM results, which show that very small island sizes are stabilized over an exposure range up to ∼ 200 L with a total coverage ≤ 0.2 ML, are incompatible with the assumption of long range periodicity required for lattice dynamical modeling. The consequence is that vibrational modes polarized parallel to the surface may become dipole active. Within an Al₃O cluster model appropriate to exposures ≤ 3 L where most oxygen atoms are isolated species in three-fold hollow sites, the strong feature at 584 cm⁻¹ (72 meV) is still attributed to top-layer oxygen motion perpecdicular to the surface (the symmetric Al₃O stretch) but the second intense feature at 480 cm⁻¹ (60 meV) is assigned to the umbrella mode involving predominantly Al motion parallel to the surface rather than the motion of two AlO layers moving perpendicular to the surface out of phase with each other. The lowest frequency mode near 224 cm⁻¹ (28 meV) derives from the frustrated translation of the cluster perpendicular to the surface. At higher exposures (> 10 L) where multiple oxygen islands begin to appear, totally symmetric combinations of the E-derived asymmetric Al₃O stretching motion polarised nominally parallel to the surface become dipole allowed and can be assigned to the loss at 850 cm⁻¹ (105 meV), which was previously attributed to subsurface oxygen.     

Interaction of oxygen with Zn(0001) surface

EELS and XPS studies show the presence of both adsorbed atomic and molecular oxygen at low temperatures. The nature of the oxide layer formed on the surface has been characterized by angular dependent and variable temperature EELS. A loss peak around 550 cm^?1 is assigned to an electronic transition.     

A model for the oxidation of Mg(0001) based upon LEED,AES, ELS and work function measurements

The Mg(0001) face is subjected to oxygen adsorption from 0 to 10³ L. Three characteristic stages of oxygen adsorption are detected from 0 to 10 L. The AES signal of clean Mg decays exponentially against exposure with slopes α _ai such that α_A2 (0.75 → 3 L) >α_A1 (0 → 0.75 L)>α_A3 (3 → 10 L). For increasing exposures, they correspond to: (1) a clear (1 × 1)-Mg(0001), (2) a diffuse (1 × 1)-Mg(0001) and (3) a (1 × 1) with a weaker (1 × 1)-R30°-MgO(111) LEED patterns, respectively. At the end of the third stage, a supplementary ($\text{7}\text{×}\text{7}\text{2}\text{)?R19°?MgO(111)}$ pattern is observed. In ELS, a very fast intensity decrease of energy loss peaks due to surface and bulk plasmon excitations of the clean metal is recorded during the first stage. The energy loss peak due to the oxidized surface plasmon excitation reaches a maximum intensity at the end of the second stage. Energy loss peaks to be attributed to excitations in bulk MgO appear during the third stage. The work function of the sample decreases and shows a minimum around 6 L, and then slowly increases. Beyond 10 L, a logarithmic relation between oxide thickness and exposure seems to exist. These results are interpreted by the following sequential processes: stage 1: random oxygen chemisorption followed by oxygen incorporation (α_A1); stage 2: assembling into islands and lateral island growth (α_A2); stage 3: oxide formation (α_A3) and stage 4: oxide thickening. Lattice models describing these processes are proposed and discussed. The influence of surface roughness on the results is emphasized.     

High resolution electron energy loss spectroscopic study of the interaction of oxygen with magnesium single crystal surfaces.

From the beginning of the interaction of oxygen with the clean Mg(0001) and Mg(1$\text{1}$00) surfaces, two vibrational bands were measured by HREELS. The first one at 480 cm^?1 is attributed to atomic oxygen adsorbed at the surface. The second band at 620 cm^?1 is related to the stretching vibration of incorporated oxygen precursor to oxide formation. No specific difference was observed in the position and evolution of the two bands for both surfaces, but the electronic intensity reflected in the specular peak exhibited a totally different behaviour. The widths of the vibrational bands suggest a strong coupling with the continuum of electron-hole pairs of the metal.     

Propene-V2O2 interactions studied by infrared emission spectroscopy

Infrared emission spectroscopy was used to investigate the interactions between propene and vanadium hemipentoxide. The spectrum obtained for V₂O₅ alone was very similar to the spectrum given by KBr disc transmission method. Reaction of propene on V₂O₅ was performed between 110 and 250°C; IR spectra were recorded in situ. Reduction of the oxide occurred and its bands were strongly altered. The bands attributed to the terminal oxygen (1018 cm^?1) and to the doubly bridged oxygen (820 cm^?1) were mainly affected by the reaction with propene. For a reaction temperature of 250°C, the recorded spectrum was close to that given by hydrogen reduction at 230°C. It can be concluded that V₂O₅ was reduced by propene with the formation of a superficial suboxide. By oxygen treatment, the reduced form was restored to the initial V₂O₅ sample. Infrared emission spectroscopy appears as a very suitable method for studying the interactions of the reactants with the catalysts.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Oxidation of Ni(Pt)Si by molecular vs. atomic oxygen

X-ray photoelectron spectroscopy (XPS) has been used to characterize the oxidation of a clean Ni(Pt)Si surface under two distinct conditions: exposure to a mixed flux of atomic and molecular oxygen (O + O₂; PO+O₂ = 5 × 10⁻⁶ Torr) and pure molecular oxygen (O₂; PO₂ = 10⁻⁵ Torr) at ambient temperatures. Formation of the clean, stoichiometric (nickel monosilicide) phase under vacuum conditions results in the formation of a surface layer enriched in PtSi. Oxidation of this surface in the presence of atomic oxygen initially results in formation of a silicon oxide overlayer. At higher exposures, kinetically limited oxidation of Pt results in Pt silicate formation. No passivation of oxygen uptake of the sample is observed for total O + O₂ exposure <8 × 10⁴ L, at which point the average oxide/silicate overlayer thickness is 23 (3) Å (uncertainty in the last digit in parentheses). In contrast, exposure of the clean Ni(Pt)Si surface to molecular oxygen only (maximum exposure: 5 × 10⁵ L) results in slow growth of a silicon oxide overlayer, without silicate formation, and eventual passivation at a total average oxide thickness of 8(1) Å, compared to a oxide average thickness of 17(2) Å (no silicate formation) for the as-received sample (i.e., exposed to ambient.) The aggressive silicon oxidation by atomic oxygen, results in Ni-rich silicide formation in the substrate and the kinetically limited oxidation of the Pt.     

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10¹⁴ S atoms cm^?2 ($\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ structure) oxygen exposures of 5 × 10^?5 Torr s do not result in the adsorption of oxygen nor in the formation of SO₂. At concentrations lower than 3.8 × 10¹⁴ S stoms cm^?2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO₂ and O₂. With decreasing sulfur concentration the amount of desorbing O₂ increases and that of SO₂ passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO₂ is adsorbed with high sticking probability and can be desorbed nearly completely as SO₂ with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO₂ is highly sensitive to hydrogen. Small H₂ doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO₂ on an oxygen predosed surface small amounts of SO₃ were desorbed in addition to SO₂ and O₂ during heating. Preadsorbed oxygen produces variations of the SO₂ peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.     

Increasing the quantum efficiency of nickel phthalocyanine films by oxygen adsorption

An examination of the surface photovoltage indicates that when oxygen adsorbs on a nickel phthalocyanine polycrystalline film, one form adsorbs irreversibly with a sticking probability of 9 × 10^?3 and a second form adsorbs reversibly with a sticking probability >0.1. The reversibly adsorbed oxygen can be removed by evacuating the ambient oxygen, while the irreversible form can only be removed by heating the sample to 433 K. The irreversibly adsorbed oxygen causes an order of magnitude increase in the photovoltage, even though a comparison of the photovoltaic relaxation times indicates that this oxygen has actually slightly lowered the energy band bending at the surface depletion layer. This increase in the photovoltage is therefore attributed to an increased quantum efficiency of minority carrier injection in a process which is analogous to that observed for oxygen in the bulk.     

Transition of synthetic chromium oxide gel to crystalline chromium oxide: a hot‐stage Raman spectroscopic study

Chromium oxide gel material was synthesised and appeared to be amorphous in X‐ray diffraction study. The changes in the structure of the synthetic chromium oxide gel were investigated using hot‐stage Raman spectroscopy based upon the results of thermogravimetric analysis. The thermally decomposed product of the synthetic chromium oxide gel in nitrogen atmosphere was confirmed to be crystalline Cr₂O₃ as determined by the hot‐stage Raman spectra. Two bands were observed at 849 and 735 cm⁻¹ in the Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes of O Cr^III OH and O Cr^III O. With temperature increase, the intensity of the band at 849 cm⁻¹ decreased, while that of the band at 735 cm⁻¹ increased. These changes in intensity are attributed to the loss of OH groups and formation of O Cr^III O units in the structure. A strongly hydrogen‐bonded water H O H bending band was found at 1704 cm⁻¹ in the Raman spectrum of the chromium oxide gel; however, this band shifted to around 1590 cm⁻¹ due to destruction of the hydrogen bonds upon thermal treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303 cm⁻¹ attributed to the thermal decomposed product Cr₂O₃. The use of the hot‐stage Raman spectroscopy enabled low‐temperature phase changes brought about through dehydration and dehydroxylation to be studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Initial oxidation of the Mg(0001) surface,an electron spectroscopy study

The initial oxidation of Mg(0001) has been studied using AES (Auger electron spectroscopy), LEED (low energy electron diffraction), and EELS (electron energy loss spectroscopy). The oxidation proceeds through different stages; first oxygen atoms are incorporated to chemisorption sites below the top layer magnesium. This chemisorption phase is followed by the formation of an oxide layer. The oxide layer covers the Mg surface after an oxygen exposure of ～ 10 L O₂. After this exposure the bulk-like MgO formation slowly increases the oxide thickness. The oxide layer formed for exposures up to ≤ 10 L O₂ gives rise to a diffuse LEED pattern of the same symmetry as the original “clean” LEED pattern; the possibility of an epitaxial oxide formation at this stage is discussed.     

Vibrational spectroscopy of oxygen on the (100) and (111) surfaces of lanthanum hexaboride

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O₂ produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ～ 734 cm^?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm^?1 with full widths at half maxima of only 12 cm^?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB₆. On the boron-terminated LaB₆(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O₂.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

Interaction of O2 with Pd single crystals in the range 1-150 Torr: Oxygen dissolution and reaction

The interaction of O₂ with Pd(1 1 1), Pd(1 1 0) and Pd(1 0 0) was studied in the pressure range 1-150 Torr by the techniques of temperature programmed decomposition (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The oxidation of Pd was rate-determined by oxygen diffusion into Pd metal followed by the diffusion into PdO once the bulk oxide layer was formed. The dissolution of oxygen atoms into Pd metal followed the Mott-Cabrera model with diffusion coefficient 10⁻¹⁶ cm² s⁻¹ at 600 K and activation energy of 60-85 kJ mol⁻¹. The bulk oxide phase was formed when a critical oxygen concentration was reached in the near-surface region. The formation of PdO was characterized by a decrease in the oxygen uptake rate, the complete fading of the metallic Pd LEED pattern and an atomic ratio O/Pd of 0.15-0.7 as measured by AES. The diffusion of oxygen through the bulk oxide layer again conformed to the Mott-Cabrera parabolic diffusion law with diffusion coefficient 10⁻¹⁸ cm² s⁻¹ at 600 K and activation energy of 111-116 kJ mol⁻¹. The values for the diffusion coefficient and apparent activation energy increased as the surface atom density of the single crystals increased.     

Vibrational study of the initial stages of the oxidation of Si(111) and Si(100) surfaces

Using high-resolution electron energy loss spectroscopy (EELS) the vibrations of Si(111) and Si(100) surfaces in the early stages of oxidation have been investigated. Three different stages of oxidation, the last being the formation of a thin layer of vitreous SiO₂ are identified when the surfaces are held at a temperature of 700K during the exposure with molecular oxygen. We show that also the first two stages involve atomic oxygen in bridging positions between silicon atoms. Small exposures at low temperatures (100 K) produce vibrational features of a different, possibly molecular, species. For higher exposures at the same temperature the spectrum again develops the characteristics of atomic oxygen and the molecular species eventually disappears. Exposure at room temperature leads to a mixture of atomic and molecular oxygen for smaller exposures and to purely atomic oxygen for exposures greater than 10² L. At room temperature even exposures as high as 10¹¹ L do not produce the spectrum of vitreous SiO₂. The same is found for the natural, room temperature grown, oxide layer on silicon wafers which we have studied by introducing the sample into the spectrometer through an air-lock. Annealing of the wafer to 700 K produced the characteristic spectrum of vitreous SiO₂. The results are discussed in comparison with previous work.     

Oxidation of cleaved InAs(1 1 0) surfaces at room temperature: Surface band-bending and ionization energy

The interaction of unexcited, molecular oxygen with cleaved InAs(1 1 0) surfaces was investigated at room temperature by using a Kelvin probe and photoemission spectroscopy excited by HeI radiation. Exposures up to 10⁵ L of O₂, which result in an oxygen uptake up to only a few percent of a monolayer, cause the formation of an inversion layer on the specimen doped p-type and of an accumulation layer on those doped n-type. Between 10⁵ and 10⁸ L of O₂ the Fermi level is found to be pinned at 0.13 eV above the bottom of the conduction band on samples doped p- as well as n-type. This energy position of the Fermi level agrees with the charge- neutrality level of the virtual gap states as calculated by Tersoff. Since the ionization energy remains unchanged up to exposures of 10⁸ L of O₂ it is concluded that the oxygen is incorporated into rather than adsorbed on cleaved InAs surfaces.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.