Saturation Oxygen Coverage Research Articles

Chemisorption of oxygen and subsequent reactions on low index surfaces of β–Mo2C: Insights from first-principles thermodynamics and kinetics

Oxygen chemisorption on β–Mo2C surfaces, the subsequent CO/CO2 desorption and oxygen diffusion to the carbon vacancy have been investigated by density-functional theory. The most stable structures together with the energetics of oxygen stepwise adsorption, CO/CO2 desorption and oxygen diffusion to the carbon vacancy were identified. We examined the effect of oxygen coverage on the morphology of β–Mo2C by plotting the equilibrium crystal shape. Thermodynamic effect of temperature and reactant or product pressure on the CO/CO2 desorption were investigated. The CO/CO2 desorption is more favorable at the saturated oxygen coverage than the low oxygen coverage thermodynamically. The subsequent oxygen diffusion to the carbon vacancy after CO/CO2 desorption may happen depending on the surfaces and oxygen coverage.

CO oxidation on fully oxygen covered Ru(0001): Role of step edges

We present a reaction mechanism for CO oxidation at the $\text{Ru}(0001)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. By means of density-functional theory studies, we compare the reactivity of flat and stepped regions of the Ru(0001). We find that at saturation oxygen coverage, where the flat parts are unreactive toward CO adsorption an intricate reaction pathway for CO adsorption and ${\text{CO}}_{2}$ formation exists at the monatomic Ru step. By suggesting an efficient mechanism for CO oxidation on nonoxidic Ru, our results offer an explanation for the controversy in the experimental literature regarding how nonoxidic Ru may be as active in CO oxidation as oxidic rutile ${\text{RuO}}_{2}$.

Oxidized GaN(0001) surfaces studied by scanning tunneling microscopy and spectroscopy and by first-principles theory

Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in situ oxidization at 550°C using molecular oxygen, Auger electron spectroscopy indicates a saturation oxygen coverage of 2.1±0.5 ML (monolayer). For these surfaces scanning tunneling microscopy reveals two surface phases, one with 33×33−R30° periodicity and the other with disordered two times periodicity. Scanning tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any states of the oxide lie predominantly outside of the GaN gap. From first-principles total energy calculations of surface formation energies two models of energetically favorable surfaces structures are developed, with oxygen coverages of 1.25 and 2 ML, respectively. Both structures satisfy electron counting and have surface band gaps close in size to that of GaN, in agreement with experiment.

Photon emission in ion beam sputtering of an Mg target

Studies of excitation processes in sputtering due to bombardment of an Mg target with saturated oxygen coverage by 3–56 keV ions of various projectiles, namely, krypton, argon, neon, helium, oxygen and nitrogen have been carried out in a low energy ion beam facility developed in the laboratory. It is found that the relative excitation probability of the sputtered atomic and ionic species depends on the projectile mass as well as the energy. The results are discussed.

Atomic scale characterization of oxidized 6H–SiC( [formula omitted]) surfaces

The initial oxidation of the 6H–SiC( 1 1 2 ̄ 0 ) surface was studied by high-resolution medium energy ion scattering (MEIS) combined with photoelectron spectroscopy induced by synchrotron-radiation-light (SR-PES). The oxygen coverage is saturated at 0.35 ± 0.04 ML (1 ML=1.49 × 10 15 atoms/cm 2) with O 2-exposure of 5000 L at room temperature. The oxidation rate and the saturated oxygen coverage are considerably low and small compared with those for the α-SiC(0 0 0 1)- 3 × 3 and α-SiC(0 0 0 1)-3 × 3 surfaces. The Si-2p core level analysis shows that the primary components come from the Si + and Si 2+ states and the ratio of the Si 2+ state relative to the Si + state is almost 1/2. The above MEIS and PES results suggest that an O atom is preferentially adsorbed to a corner bridge site of Si-adatoms and side by side another O atom is inserted into the backbond of the one of the corner adatoms connected to a second-layer-Si atom. In the thermal oxidation under O 2-pressure of 1.0 × 10 −4 Torr, the oxygen coverage is linearly increased with elevating oxidation temperature and an ultra-thin SiO 2 layer is formed at temperatures higher than 500 °C. The present MEIS analysis has also revealed the fact that a maximum compressive strain of 2% is induced by thermal oxidation (700 °C) at the SiC substrate just below the oxide layer and the strain decreases gradually down to a depth about 1.5 nm from the surface.

Effect of surface impurities on the Cu/Ta interface

Auger electron spectroscopy and temperature programmed desorption studies under ultra-high vacuum conditions demonstrate that even sub-monolayer coverages of oxygen or carbide on polycrystalline Ta significantly degrade the strength of Cu/Ta chemical interactions, and affect the kinetics of Cu diffusion into bulk Ta. On clean Ta, monolayer coverages of Cu will de-wet only above 600 K. A partial monolayer of adsorbed oxygen (3 L O 2 at 300 K) results in a lowering of the de-wetting temperature to 500 K, while saturation oxygen coverage (10 L O 2, 300 K) results in de-wetting at 300 K. Carbide formation also lowers the de-wetting temperature to 300 K. Diffusion of Cu into the Ta substrate at 1100 K occurs only after a 5-min induction period at this temperature. This induction period increases to 10 min for partially oxidized Ta, 15 min for carbidic Ta and 20 min for fully oxidized Ta.

Simulation of the Kinetics of Oxidation of Saturated Hydrocarbons on Pt

The steady-state kinetics of C3H8 oxidation on Pt are calculated assuming the process to be limited by concerted dissociative adsorption of C3H8 on vacant sites near adsorbed oxygen atoms (with breaking of a C–H bond and formation of OH). The results, obtained by using (1) the ideal model implying Langmuir adsorption, (2) a model based on the assumption that the saturation oxygen coverage is lower than one monolayer, and (3) a model taking into account adsorbate–adsorbate lateral interactions, indicate that the latter two models can reproduce the apparent reaction orders with respect to C3H8 and O2, observed experimentally under the lean-burn conditions. Model 3 is preferable because its basis is physically more sound.

The Effect Of Surface Oxides On Cu/Ta Interfacial Interactions

AbstractWe report results of Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) studies under ultra high vacuum (UHV) conditions which demonstrate that even submonolayer coverages of oxygen on Ta significantly degrade the strength of Cu/Ta chemical interactions, and affect the kinetics of Cu diffusion into bulk Ta. On clean Ta, monolayer coverages of Cu will de-wet only above 600 K. A partial monolayer of adsorbed oxygen (3L O2 at 300 K) results in a reduction of the de-wetting temperature to 500 K, while saturation oxygen coverage (10 L O2, 300 K) results in de-wetting at 400 K. Diffusion of Cu into the Ta substrate at 1100 K occurs only after a 300-second induction period at this temperature. The induction period increases to 600 sec for partially oxidized Ta and to 1200 sec for saturation oxygen coverage. TPD studies indicate no desorption of Cu for temperatures below 1300 K. The higher desorption temperature of Cu (compared to the 1150 K sublimation temperature) indicates that all the Cu originally deposited is now chemically bound to Ta.

Oxygen desorption from α-Al 2O 3 (0001) supported Rh, Pt and Pd particles

Temperature-programmed desorption (TPD) was used to study the interaction of oxygen with both large (∼ 10 nm) and small (<2 nm) Rh, Pt and Pd particles supported on α-Al 2O 3(0001). The results of this study indicate that O 2 desorption from large particles of Rh, Pt and Pd is similar to that from low-index planes of the respective metal single crystals. Additionally, O 2 desorption from small Rh particles is similar to that from the close-packed planes of bulk Rh. In contrast, O 2 desorption from small particles of Pt and Pd occurs at a significantly higher temperature than that from the respective, low-index single crystal surfaces despite a much higher saturation oxygen coverage on the small particles. Possible explanations for the differences in O 2 desorption as a function of particle size for the various metals is discussed.

Interactions between alkali metals and oxygen on a reconstructed surface: An STM study of oxygen adsorption on the alkali-metal-covered Cu(110) surface

Room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces was studied by scanning tunneling microscopy. Depending on the alkali-metal precoverage, two different scenarios exist for the structural evolution of the surfaces. For alkali-metal coverages ${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$\ensuremath{\le}0.13 ML [${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$=0.13 corresponds to the (1\ifmmode\times\else\texttimes\fi{}3) missing-row reconstructed Cu(110) surface], oxygen adsorption leads first to a transient contraction of the missing rows into islands of a (1\ifmmode\times\else\texttimes\fi{}2) structure. After longer exposures it causes the local removal of the alkali-metal-induced reconstruction, and the (2\ifmmode\times\else\texttimes\fi{}1) Cu-O ``added-row'' structure with ${\mathrm{\ensuremath{\theta}}}_{\mathrm{O}}$=0.5 is formed. In this structure the alkali-metal atoms are incorporated in the Cu-O chains. For higher alkali-metal precoverages, in the range of the (1\ifmmode\times\else\texttimes\fi{}2) reconstruction (${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$\ensuremath{\approxeq}0.2), more than one-half a monolayer of oxygen can be incorporated into the (1\ifmmode\times\else\texttimes\fi{}2) phase with only a minor structural effect before, at higher oxygen coverages, complex oxygen--alkali-metal--Cu structures with oxygen coverages well above 0.5 ML are formed. The saturation oxygen coverage is drastically enhanced beyond ${\mathrm{\ensuremath{\theta}}}_{\mathrm{O}}$=0.5, the quasisaturation value of the clean surface. Based on mass-transport arguments the substrate is reconstructed for all ratios of oxygen and alkali metal investigated here. Hence, adsorbate-substrate interactions are essential for these structures; they are not dominated by interactions between alkali metals and oxygen, i.e., by adsorbate-adsorbate interactions.

Adsorption of H 2O on oxidized Fe(100) surfaces: comparison between the oxidation of iron by H 2O and O 2

The adsorption and reaction of water on the preoxidized Fe(100) surface is studied using LEED, AES, thermal desorption and high resolution EELS. Oxidation of the Fe(100) surface by water adsorption results in a disordered c(2 × 2) surface, with a saturation oxygen coverage of 0.39 ML. This surface cannot be further oxidized by exposure to water, but can be further oxidized by molecular oxygen. When water is adsorbed on the submonolayer oxygen pre-exposed surface, hydrogen transfer from water to adsorbed atomic oxygen occurs, forming a hydroxyl group. This group migrates from the fourfold hollow site, where it is formed, to the bridge site on this surface. Adsorbed oxygen facilitates water decomposition via this hydrogen transfer process. On the oxygen precovered surface with an oxygen coverage of 1 ML (p(1 × 1)O surface), water adsorbs molecularly and desorbs at 175 K without decomposition.

Interaction of cesium and barium on partially oxygen covered Nb(110)

We present results of investigations of the adsorption and desorption of Cs and Ba on clean and oxygenated Nb(110) surfaces. The experiments were conducted under ultrahigh vacuum conditions using the techniques of line-of-sight thermal desorption mass spectrometry, Auger electron spectroscopy, and retarding potential work function measurement. The room temperature work function was found to increase from 4.62 eV for clean Nb(110) to 5.63 eV for Nb(110) with a saturated chemisorbed layer of oxygen. For a 4.8 L (1 L=1 Langmuir=1×10−6 Torr s) oxygen exposure, corresponding to about 70% of saturated oxygen coverage, minimum work functions of 2.21 and 1.42 eV were found with the adsorption of Ba and Cs, respectively. The minimum work function tends to increase with further adsorption of oxygen. The adsorption of Cs on the Ba/Nb(110) surface yielded a minimum work function of 1.47 eV. The results of the thermal desorption studies showed an initial (low coverages) binding energy of 3.72 eV for Ba on Nb(110) and 2.42 eV for Cs on Nb(110). We also present desorption energy data for Cs on the Ba/Nb(110) surface.

Temperature effects on the oxidation kinetics of clean and cesiated Si(111) and Si(100) surfaces

We present new experimental results from a comparative study of the oxidation kinetics of the clean, and modified with 0.2 ML Cs, Si(111) and Si(100) surfaces at substrate temperatures ranging from 300 to 900 K. The observed difference in the temperature dependence of the oxygen sticking coefficient and the oxygen saturation coverage for clean and cesiated Si surfaces illustrates that the presence of small amounts of Cs affects in a diverse manner the main stages of the oxidation process — the initial sticking coefficient characterizing the relatively fast initial uptake is enhanced and becomes temperature independent whereas the following slow oxide growth characterized by the achieved saturation oxygen coverage remains unaffected and becomes less temperature dependent.

Observation and characterization of a strained lateral superlattice in the oxidation of Ni(001).

The saturated oxide formed on Ni(001) under UHV conditions has been studied by x-ray photoelectron diffraction (XPD) and low-energy electron diffraction. At ambient temperature and with a 1200-L exposure to oxygen, the saturation oxygen coverage is found to be 4.3\ifmmode\pm\else\textpm\fi{}0.4 monolayers (ML) as measured relative to the Ni(001) surface-atom density. [1 langmuir (L)${==10}^{\mathrm{\ensuremath{-}}6}$ Torr sec.] The oxide is furthermore found to grow primarily as NiO(001) in a highly strained superlattice for which the horizontal lattice constant is expanded by (1/6 compared to the underlying Ni(001). The XPD results are found to be very sensitive to the degree of short-range order in the oxide before and after light annealing, particularly when obtained with high angular resolutions of approximately \ifmmode\pm\else\textpm\fi{}1.0\ifmmode^\circ\else\textdegree\fi{}. Single-scattering calculations with spherical-wave scattering are found to give an excellent description of the XPD from the annealed and more-ordered overlayer. These results also suggest the general utility of high-resolution XPD for studying the degree of short-range positional order present in epitaxial overlayers.

Formation of a potassium-dioxygen surface complex on Ru(001)

A new dioxygen species with the O 1s binding energy at 532.3 eV is observed in XPS spectra of a potassium (√3 × √3)R30° layer saturated with oxygen on a Ru(001) surface at 80 K. Desorption states of oxygen and potassium at 950 K are coupled to the new dioxygen species, and dioxygen in this state undergoes complete isotopic scrambling. Presence of potassium leads to a twofold increase in saturated oxygen coverage as compared with clean Ru(001). These data imply the formation of a thermally stable K-O complex with stoichiometry near to that of potassium peroxide K 2O 2.

Oxidation Kinetics of Silicon Surfaces: Reactive Sticking Coefficient, Apparent Saturation Coverage and Effect of Surface Hydrogen

ABSTRACTThe kinetics of the initial oxidation of silicon surfaces by O2 were studied using laser-induced thermal desorption (LITD), temperature programmed desorption (TPD) and Fourier Transform Infrared (FTIR) spectroscopy. The LITD results showed that the oxidation of Si(111)7×7 by O2 was characterized by two kinetic processes: an initial rapid oxygen uptake followed by a slower growth that asymptotically approached an apparent saturation oxygen coverage. The initial reactive sticking coefficient of O2 on Si(111)7×7 decreased with surface temperature. In contrast, TPD experiments on Si(111)7×7 and FTIR studies on porous silicon demonstrated that the apparent saturation oxygen coverage increased as a function of surface temperature. Experiments with preadsorbed hydrogen also revealed that silicon oxidation was inhibited as a function of increasing hydrogen coverage on the Si(111)7×7 surface.

Adsorption of HCOOH on Rh(111) and its reaction with preadsorbed oxygen

The interaction of formic acid with clean and oxygen-dosed Rh(111) surfaces has been investigated by electron energy loss (in the electronic range), thermal desorption and photoelec-tron spectroscopy. Formic acid adsorbs readily on the Rh(111) surface at 100 K with a high sticking probability. Three adsorbed states have been distinguished: a condensed layer ( T p =170 K), a chemisorbed layer ( T p = 202 K) and the formation of formate species. The latter is stable up to 200 K, but decomposes completely at 200–250 K. The major products are: H 2 ( T p = 323 K) and CO 2 ( T p = 255–290 K), but H 2 O ( T p = 263 K) and CO ( T p = 530 K) are also formed. Preadsorbed oxygen exerted a readily observable influence on the interaction of HCOOH with the Rh(111) surface. It increased the extent of dissociation of HCOOH and extended the region of stability of surface formate by at least 80–100 K. This was demonstrated by higher stability of photoemission peaks due to formate and by simultaneous production of CO 2 and H 2 O with T p = 377–385 K at saturation oxygen coverage.

Chemisorption of bromine on cleaved silicon (111) surfaces: An X-ray standing wave interference spectrometric analysis

The coverage and location of bromine adsorbed from dilute methanol solution on cleaved silicon (111) surfaces have been determined by X-ray standing wave interference spectrometry. Two different locations of bromine atoms have been observed. One position, similar to that observed previously for chemically cleaned silicon (111) crystals, is directly over the first monolayer silicon atoms (atop position). The second position is much more loosely bound and may be in the open surface interstitial hollow over-the-fourth-layer atoms. In contrast to the chemically cleaned surface, where surface silicon atoms are bulk-like, we conclude that for a cleaved crystal, in the presence of bonded broine atoms, the surface silicon atoms are relaxed outward by 0.13±0.06 A ̊ . It is suggested that this relaxation may open access to the interstitial site. Adsorption on chemically cleaned surfaces gave correlated coverage in the atop position of 0.3 monolayer (ML) or less. For cleaved surfaces, coverage of bromine in the strongly bound atop position never exceeded 0.24 ML which implies possible steric hindrance to bromine adsorption on adjacent sites The weak bonding site was observed with coverage up to 1 ML and could be obtained even for samples with saturation oxygen coverage before bromine adsorption. The cases of coadsorption of bromine were analyzed with a general model which allows difference of population at individual sites.

Reaction of methanol with Cu(111) and Cu(111) + O(ads)

The reactive chemistry of methanol on the Cu(111) surface, both with and without preadsorbed oxygen atoms, is investigated between 190 and 700 K. The clean Cu(111) surface is inert, and molecularly absorbed methanol, the only stable surface species identified on this surface, desorbs at about 210 K. Various trends are examined as a function of oxygen coverage, from the clean surface to the saturation oxygen coverage (approximately 0.45 O atom/Cu atom). The capacity of the surface to adsorb methanol (190 K), and the formaldehyde yield (∼ 400–450 K) are both maximized when the oxygen coverage is about 0.26 O atom/Cu atom. Trends in the yield of other products, and the temperature for decomposition of the stable methoxy intermediate are examined. Also, the rate of methoxy decomposition is limited by CH bond breaking as evidenced by a deuterium kinetic isotope effect (CH versus CD). A minor decomposition path for methanol on O + Cu(111) involves CO 2 formation, probably via a formate surface intermediate. Preadsorbed oxygen serves as an acceptor of the methanol hydroxyl hydrogen, enabling facile methanol conversion to methoxy at low oxygen coverage for T ⩾ 190K. However. at high oxygen coverage ( θ ⪆ 0.26 O atom/Cu atom) oxygen inhibits surface reactivitv. A two-dimensional model which defines three types of surface sites is used to explain the general trend of methanol reactivity as a function of oxygen coverage.

Initial oxidation of Zn(0001) surfaces: H2 adsorption and XPS studies

H2 adsorption capacity measurements have been combined with XPS intensity measurements to study the kinetics of O2 uptake on Zn(0001) surfaces. On clean Zn(0001) the temperature programmed desorption spectrum of H2 shows half-order desorption kinetics and an apparent desorption energy of 11 kcal/mole. As the surface is oxidized by progressive dosing with O2, two features are noted: The H2 desorption spectrum characteristic of clean Zn(0001) is attenuated in a nonlinear fashion and a new H2 desorption state with an activation energy of 18 kcal/mole is observed for surfaces with less than saturation oxygen coverage. XPS results indicate that H2 adsorption is not completely blocked until an oxygen coverage corresponding to approximately 3 monolayers is achieved. Both H2 uptake and XPS intensity measurements also indicate that the oxygen sticking coefficient increases with oxygen coverage up to about one half of the O2 saturation coverage. These results indicate that the initial oxidation of Zn surfaces involves the formation and growth of multilayer oxide islands, where oxygen incorporation at the perimeter of the islands is the rate limiting step.