Surface Oxygen-containing Species Research Articles

Electron impact effects on the oxidation of Si(111) at 90 K

The Si(111)-7×7 surface has been subjected to oxidation by molecular O2 at 90 K and the kinetics of this process have been studied by x-ray photoelectron spectroscopy (XPS). In the midst of the oxidation process, the thin oxide layer was electronically excited in ultrahigh vacuum using 100 eV electron bombardment. No charging of the oxide layer was observed. It was found that excitation of the oxide layer by electron bombardment led to almost no change in the oxidation kinetics, measured following bombardment. XPS studies showed that two oxygen-containing surface species are produced by oxidation (in the absence of electrons) with O(1s) binding energies of 533.1 and 535.1 eV. Upon electron bombardment, the higher binding energy species is converted to the lower binding energy species. Continued oxidation after electron bombardment showed that the higher binding energy species was replenished again. This result suggests that adsorption at 90 K leads to highly strained Si–O–Si species and that electron bombardment of these species produces the stable oxidized structure. The results are compared to similar experiments on Al2O3 where, in contrast to a SiO2 film, it was found that surface charging of a thin Al2O3 film on Al(111) leads to a greatly enhanced oxidation rate.

Regeneration of gated carbon nanotube field emission

We report large increases in field emission current when operating carbon nanotubes in substantial pressures of hydrogen, especially when the nanotubes were contaminated. We have previously demonstrated two different configurations of integrally gated carbon nanotube field emitter arrays (cNTFEAs), CNTs grown inside microfabricated gate apertures with and without silicon posts. Salient features of these in situ grown microgated cNTFEAs include the absence of electrical arcing, low operating voltage, and enhancing effect of some residual ambient gases. Operating both configurations of cNTFEAs without special precleaning in greater than 10−5Torr hydrogen produced orders of magnitude enhancement in emission. For a cNTFEA intentionally degraded by oxygen, the operation in hydrogen resulted in a 340-fold increase in emission current at constant gate voltage. The results suggested a dependence on atomic hydrogen produced from the interaction between emission electrons and molecular hydrogen. The observed emission enhancement could be due to a surface dipole formation, hydrogen doping, or removal of oxygen-containing surface species.

Direct ethanol fuel cells based on PtSn anodes: the effect of Sn content on the fuel cell performance

In the present work, several carbon supported PtSn catalysts with different Pt/Sn atomic ratios were synthesized and characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Both the results of TEM and XRD showed that all in-house prepared carbon supported Pt and PtSn catalysts had nanosized particles with narrow size distribution. According to the primary analysis of XPS results, it was confirmed that the main part of Pt of the as-prepared catalysts is in metallic state while the main part of Sn is in oxidized state. The performances of single direct ethanol fuel cells were different from each other with different anode catalysts and at different temperatures. It was found that, the single DEFC employing Pt 3Sn 2/C showed better performance at 60 °C while the direct ethanol fuel cells with Pt 2Sn 1/C and Pt 3Sn 2/C exhibited similar performances at 75 °C. Furthermore, at 90 °C, Pt 2Sn 1/C was identified as a more suitable anode catalyst for direct ethanol fuel cells in terms of the fuel cell maximum power density. Surface oxygen-containing species, lattice parameters and ohmic effects, which are related to the Sn content, are thought as the main factors influencing the catalyst activity and consequently the performance of single direct ethanol fuel cells.

H 2O trapping on various materials studied by AFM and XPS

Adsorption and desorption of water on thin films of iron oxide formed on the iron surface were studied by AFM and X-ray photoemission spectroscopy (XPS). The morphological change of iron oxide or water droplets on the surface of iron oxide were observed by AFM. Electronic state of O 1s was observed by XPS after a sample was exposed to distilled water. Several peaks appeared and assignment to oxygen-containing surface species was discussed.

Surface diffusion and the spillover of H-adatoms and oxygen-containing surface species on the surface of carbon black and Pt/C porous electrodes

Abstract The cyclic voltammetric behavior of Vulcan XC-72 ∣ Nafion® and carbon-supported Pt ∣ Nafion® interfaces was studied by using the powder microelectrode technique. It was concluded that catalyst particles not in direct contact with Nafion® also contributed to the cyclic voltammograms although the electrode ∣ Nafion® interfaces formed only at the end face of the electrodes. The phenomena were interpreted in terms of surface conductivity and spillover of adsorbed species over the surface of catalyst particles. The effects of electrochemical pretreatment of the electrodes and the prolonged potential cycling on the cyclic voltammograms have been observed, which indicated that surface conductivity, and surface diffusion as well as spillover were sensitive to the surface status of the catalyst particles.

Study of the powder/membrane interface by using the powder microelectrode technique I. The Pt-black/Nafion® interfaces

The polarization behavior of a Pt microdisk electrode, a platinized Pt microdisk electrode and a Pt-black powder microelectrode immersed in sulfuric acid solution, in contact with a moist Nafion® membrane were studied by cyclic voltammetry. The experimental results reveal that the surface diffusion of the adsorbed species plays an important role in determining the kinetics of interfacial reactions taking place at the powder/Nafion® membrane interface. The weakly adsorbed hydrogen atoms diffuse significantly faster than the strongly adsorbed hydrogen and the oxygen-containing surface species. Surface active sites in the range of tens of microns from the powder/Nafion® interface can be involved in interfacial reactions. The apparent coefficients of diffusion of various surface species were estimated from the variation of peak current with scan rate of potential. The apparently higher irreversibility of electrode reaction observed at the powder/membrane composite electrode is explained by considering mechanism of current generation of this type of electrode.

An in Situ FT-IR Study of CO Hydrogenation over Cerium Oxide

The CO hydrogenation over cerium oxide was studied by using a closed circulating reaction system and in situ Fourier transform infrared spectroscopy (FT-IR) at 300-673 K. The major products of the CO-H2 reaction over cerium oxide were found to be olefins (C=2-C=4), CH4, CO2, and H2O. IR bands due to surface oxygen-containing species such as formate, dioxymethylene, methoxy, and formyl were always detected during the CO-H2 reaction at 300-573 K over cerium oxide. No direct relationship between the surface oxygen-containing species and the production of hydrocarbons were found. The mechanism of CO hydrogenation on cerium oxide involving the surface redox was discussed.