Oxygen-saturated Surface Research Articles

Interaction of O 2 with Pt(100) : I. Equilibrium measurements

Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10 15 O atoms cm −2), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (⩾ 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.

Interaction of O 2 with Pt(100) I. Equilibrium measurements

Abstract Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10 15 O atoms cm −2 ), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (⩾ 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.

The chemisorption of O 2 and NO on reduced and sulfided molybdena-alumina catalysts

The chemisorptions of O 2 and NO were investigated at −78 °C and at room temperature using pulse-flow and volumetric techniques. The oxygen chemisorption was fast and irreversible while the NO chemisorption, after a fast initial process, increased slowly with time. This latter portion could be removed by evacuation and readsorbed or replaced with O 2. The strong NO chemisorption was approximately four times larger than that of O 2. Infrared spectra showed that the strongly chemisorbed NO was present in pairs, suggesting that the oxygen chemisorption was as atoms. The interference of one chemisorption by the other was studied. The introduction of O 2 did not affect the strongly chemisorbed NO whereas approximately one NO molecule was chemisorbed per oxygen atom on the oxygen saturated surface. This process was also observable in the infrared spectra. A characteristic pair of bands at 1817 and 1713 cm −1 which appeared on chemisorption of NO on the reduced catalyst (and at 1795 and 1704 cm −1 on the sulfided catalyst) have been assigned to a dinitrosyl species or NO dimer. These bands were unaffected when O 2 was admitted to the system. On the other hand, when the NO was admitted to an oxygen-covered surface the same bands appeared but with much lower intensities. As previously reported, both chemisorptions were correlatable with the anion vacancy concentration. The results could be rationalized by assuming that these chemisorptions took place on Mo 2+ centers (two anion vacancies on the same Mo). Sulfided catalysts produced nearly identical results.

Positive secondary ion yields under In +- and O 2+-bombardement

Positive secondary ion yields of 20 elements under In +- and O 2 + bombardement are compared for oxygen-saturated surfaces. In +-ions were produced in a newly developed electrohydrodynamic (EHD) ion source, while O 2 +-ions were generated in a duoplasmatron. Practical sensitivities for In +2-bombardement exceed those for O 2 +-bombardement by a factor correlated to the respective sputtering yield ratios. These results indicate that indium ions from a liquid metal ion source may be successfully applied as primary ions in secondary ion mass spectrometry.

The use of secondary ion mass spectrometry for studies of oxygen adsorption and oxidation

Thin oxide layers on silicon and tantalum as well as oxygen saturated silicon surfaces have been investigated by means of SIMS. Sputter depth profiling was achieved using a raster scanned 2 keV argon ion beam. The results clearly indicate that oxygen adsorption does not lead to on oxide-like structure. Moreover it is found that air-grown room temperature oxides on silicon are depleted of oxygen as compared to the composition of thick silicon dioxides. The results are discussed with reference to recently reported SIMS studies on oxygen covered surfaces.

An XPS and UPS study of the interaction of oxygen with sodium-dosed Ag(110)

The adsorption of Na and the coadsorption of Na and O 2 on Ag(110) have been studied by XPS and UPS. Adsorption of Na results in a rapid decrease in the work function. Δφ reaching a limiting value of −2.0 eV at θ Na = 0.5 and thereafter remaining constant. In the coverage range 0 < θ Na <1 adsorption of O 2 onto the Na dosed surface always results in an increase in the work function to an almost constant value of Δφ ~ −1 eV. At the same time the XPS data show that the Na/O stoichiometry of the oxygen saturated surface remains essentially constant and independent of the initial Na dose. Calibration experiments using sodium formate as a standard compound indicate that this surface phase has the stoichiometry Na 2O. For θ Na > 1 there is a sharp change in behaviour; the work function of the oxygen saturated surface begins to decrease rapidly, and eventually falls below the value for the Na covered surface itself at θ Na ~ 1.5. The XP spectrum now shows the appearance of a new oxygen peak which increases in intensity as θ Na increases beyond unity. The UP spectra indicate that the binding energy of the surface orbital derived from Na (3s) is increased by ~6.7 eV as compared with the free atom value, and the emission at ~3 eV below E F, which is associated with surface oxygen is not greatly affected by the presence of Na. These results are discussed against the background of information already available from LEED, Auger, and thermal desorption studies, and we attempt to give a consistent interpretation of the properties of the system at coverages both below and above one monolayer.

Chemisorption, photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO 2 is the only photodesorbed species from both the single crystal and powder samples studied. The CO 2 photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO 2 photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO 2 − “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO 2 and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO 2 − “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.

Secondary ion emission from silicon and silicon oxide

The emission of Si +, Si 2+, Si 3+, Si 2 +, SiO + and B + from boron doped silicon has been studied at oxygen partial pressures between 2 × 10 −10 and 2 × 10 −5 Torr. Sputtering was done with 2 to 15 keV argon ions at current densities between 3 and 40 μ A cm 2 . The relative importance of the different ionization processes could be deduced from a detailed study of the yield variation at varying bombardment conditions. Comparison with secondary ion emission from silicon dioxide allows a rough determination of the composition of oxygen saturated silicon surfaces.

Interaction of oxygen with polycrystalline tungsten. Part 3.—Electron stimulated desorption

This paper is a sequel to a detailed study of the kinetics of adsorption and desorption in the interaction of O2 with polycrystalline tungsten, in which a number of oxide and O atom states were identified by thermal desorption. In electron stimulated desorption (ESD) studies two states of adsorbed oxygen were distinguished, β1 and β2; the former has a high ESD cross section and the latter a low cross section. In this paper an attempt is made to correlate the states observed in the thermal desorption study with these β1 and β2 states. The ESD properties of the β1 state are determined, and the sticking probability for readsorption onto an oxygen-saturated surface depleted of β1 by ESD is determined as a function of the incremental oxygen coverage. The steady state between electronic desorption and readsorption from the gas phase is examined, and isotopic exchange between 16O and 18O in the β2 and β1 states is observed and characterized. The appearance of the β1 state is related in a complex way to the formation of tungsten oxides which can be thermally desorbed in the temperature range 1200–1600 K.