Study of the early stages of growth of Co oxides on oxide substrates (original) (raw)
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The growth of cobalt oxides on HOPG and SiO2 surfaces: A comparative study
Surface Science, 2014
The growth of cobalt oxides by reactive thermal evaporation of metallic cobalt on highly oriented pyrolytic graphite (HOPG) and SiO 2 (X cut quartz surface), in an oxygen atmosphere at room temperature, has been chemically and morphologically studied by means of X-ray photoelectron spectroscopy and atomic force microscopy. The chemical analysis, which also includes cluster calculations, reveals that for the early deposition stages on both substrates, Co 2+ species are stabilized at the surface up to a coverage which depends on the substrate. Further coverages lead to the formation of the spinel oxide Co 3 O 4 . The results are discussed in terms of the dependence of the surface energy on the size of the CoO deposited moieties. On the other hand, it has been found that the initial way of growth of cobalt oxides on HOPG is of Stranski-Krastanov mode whereas on SiO 2 the growth is of Volmer-Weber mode. The differences in the growth morphology have been discussed in terms of the surface diffusivity of the CoO deposits on the substrates.
Surface Science, 2008
The influence of deposition and annealing temperature on the morphology of ultra-thin cobalt layers on the native SiO 2 surfaces has been investigated using AFM and XPS. To provide well defined conditions, the SiO 2 layer was cleaned by thermal annealing at 560-580°C which caused desorption of the carbonaceous compounds. The deposition of Co on the native SiO 2 at room temperature leads to the formation of smooth uniform layers. Upon annealing of these layers at temperatures above 260-320°C Co islands are formed. Further annealing at temperatures higher than 500°C causes desorption of Co atoms from the oxide surface. No diffusion of Co atoms through the native SiO 2 layer during the annealing has been observed up to the detection limit of XPS. The deposition at elevated temperatures in the range of 360-430°C leads to the formation of separate cobalt islands randomly arranged on the surface.
The state of the oxygen at the surface of polycrystalline cobalt oxide
Journal of Electron Spectroscopy and Related Phenomena, 1995
XPS and factor analysis (FA) have been applied to characterize the surface state of three polycrystalline cobalt oxide samples with different crystallographic bulk structure (CO304 and CoO) and surface characteristics. The study of the thermal behaviour of the O 1 s and Co2p spectra and of their changes as an effect of Ar + bombardment and exposure to O2 have permitted verification of the existence of three components at the O 1 s spectra with binding energies at 529.6 (OI), 531.1 (Oi1) and 532.1 eV (Onl) and three components at the Co2p level. The shape of these components is similar to the Co2p spectra of CO304 (COlil), CoO (COil) and Co O (COl) compounds. In the three samples component On yields component Om by heating at 473 K < T < 673 K. Then, the intensity of species Om and OI decreases to a minimum at 923 K.
ACS Catalysis, 2019
In the search for precious-metal free electrode materials for electrochemical water splitting, transition metal oxides have been receiving much recent interest as active and stable electrocatalysts for the anodic oxygen evolution reaction (OER). We present operando surface X-ray diffraction studies of two structurally well-defined epitaxial cobalt oxide thin films-Co3O4(111) and CoOOH(001) electrodeposited on Au(111). The potential-dependent structural changes during cyclic voltammograms were monitored with high time resolution up to OER current densities as high as 150 mA cm-2. The CoOOH(001) film is found to be smooth and perfectly stable over a wide potential range. In the case of Co3O4(111), fast and fully reversible structural changes are observed. Specifically, the surface region of Co3O4(111) starts restructuring at potentials 300 mV negative of the onset of the OER, indicating that the process is related to the thermodynamically predicted Co3O4 / CoOOH(001) transition rather than to the catalytic reaction. The formed skin layer is of defined thickness, which changes linearly with applied potential, and is the OER active phase. Surprisingly, the catalytic activity of the skin layer covered Co3O4 film and that of the smooth CoOOH(001) are almost identical, if the true microscopic surface area is taken into account. This indicates that the number of OER active sites on the two oxides is similar, despite the very different defect density, and is at variance with previous suggestions that di-µ-oxo bridged Co cations are exclusively responsible for the OER activity of Co oxides. For the smooth CoOOH(001) a turnover frequency of 4.2 s-1 per surface atom is determined at an overpotential of 400 mV. Furthermore, our studies demonstrate that the pseudo-capacitive charging current in the pre-OER potential range must be assigned to a bulk process that is accompanied by potential-dependent changes of the unit cell volume in the Co3O4 bulk.
We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O 2 , with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O 2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co 3 O 4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
Adsorption and Activation of CO on Co3O4(111) Thin Films
The Journal of Physical Chemistry C, 2015
To explore the catalytic properties of cobalt oxide at the atomic level, we have studied the interaction of CO and O 2 with wellordered Co 3 O 4 (111) thin films using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HR-XPS), infrared reflection absorption spectroscopy (IRAS), and temperatureprogrammed desorption spectroscopy (TPD) under ultrahigh vacuum (UHV) conditions. At low coverage and temperature, CO binds to surface Co 2+ ions on the (111) facets. At larger exposure, a compressed phase is formed in which additional CO is located at sites in between the Co 2+ ions. In addition, a bridging carbonate species forms that is associated with defects such as step edges of Co 3 O 4 (111) terraces or the side facets of the (111) oriented grains. Preadsorbed oxygen neither affects CO adsorption at low coverage nor the formation of the surface carbonate, but it blocks formation of the high coverage CO phase. Desorption of the molecularly bound CO occurs up to 180 K, whereas the surface carbonate decomposes in a broad temperature range up to 400 K under the release of CO and, to a lesser extent, of CO 2. Upon strong loss of crystalline oxygen, the Co 3 O 4 grains eventually switch to the CoO rocksalt structure.
Thin films of cobalt oxides obtained by vacuum co-deposition of Co and TeO2
Vacuum, 2002
The composition and structure of thin films obtained by vacuum co-deposition of Co and TeO 2 in an atomic ratio R Co=Te ¼ 1:5 are studied. It is shown that during the co-deposition a chemical reaction between both substances takes place resulting in the formation of amorphous CoO phase and elemental Te, very finely dispersed in the amorphous matrix. The reaction in as-deposited films proceeds until the complete oxidation of Co to Co 2+ . It is found that the crystalline phases of CoO, Co 3 O 4 and/or Te are formed upon thermal heating of the films above 3001C. In this case, the reaction proceeds until complete consumption of TeO 2 via the oxidation of Co 2+ to higher valence and the formation of mixed oxides. r
Chemistry of Materials, 2001
The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) -diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350-500°C at different oxygen pressures, to tailor film composition from CoO to Co 3 O 4 . The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co-O thin films are presented and discussed.
IR studies of CO and NO adsorbed on well characterized oxide single microcrystals
Catalysis Today, 1996
A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, a-Cr20 3, ot-A1203, MgAI204 and other spinels, TiO 2, ZrO 2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions.