Electron energy loss spectroscopic investigation of Co metal, CoO, and Co 3O 4 before and after Ar + bombardment

Abstract

X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (ELS) have been used to study a sputter-cleaned polycrystalline Co metal foil and CoO plus Co 3O 4 powders before and after ion bombardment. The ELS data have been collected using primary beam energies ranging from 100 to 1150 eV in order to distinguish between surface and bulk features and to facilitate determination of peak locations. It is shown that ELS spectra from these reference materials are considerably different and should, consequently, allow species identification in future catalyst characterization studies. The main surface features obtained from Co metal are observed at 3.7 and 7.0 eV, while the major bulk energy losses are located at 8.2, 9.8, 23.1, 25.7, and 28.1 eV. Despite the collected data on Co ELS and the electronic structure of Co metal, unambiguous peak assignments are difficult. Interband transitions in the energy loss region under investigation originate from the relatively localized Co 3d filled states, but the limited information about the empty states in the conduction band make specific assignments of the energy loss features in the Co ELS spectra challenging. XPS analysis of as-entered CoO and Co 3O 4 reveals that both samples consist mainly of Co 3O 4, in agreement with a Co 3O 4-terminated CoO surface. After prolonged Ar + sputtering both oxides are CoO-like according to high-resolution spectra of the Co 2p binding energy region. In agreement with the XPS data the ELS spectra obtained from as-entered CoO and Co 3O 4 are very similar. In contrast, ELS spectra obtained from CoO and Co 3O 4 after prolonged Ar + sputtering are considerably different. The ELS spectra obtained from CoO are not altered significantly with sputtering suggesting that the Co 3O 4 layer cannot be removed using Ar + bombardment. The sputtered Co 3O 4 instead gives rise to ELS spectra that appear CoO-like. The major ELS features in the spectra obtained from Co 3O 4 are located at 2.5, 6.7, 22.1, and 25.0 eV, while those obtained from Ar +-sputtered Co 3O 4 (CoO) are observed at 7.0, 8.6, 18.3, 21.9, and 36.1 eV. The relative intensities of the features at 6–8 eV and 22–25 eV are considerably different in the two sets of spectra. The intensity ratio of the peaks at 6.7 and 25.0 eV in Co 3O 4 is significantly higher than the 7.0 to 21.9 eV peak intensity ratio of CoO. Although several of the energy losses in the two spectra overlap and make identification difficult despite the large intensity difference, there are peaks that are specific for each compound, i.e. energy losses at 2.5 and 28.5 eV in Co 3O 4 and at 36.1 eV in CoO. Consequently, ELS can be used to differentiate the two cobalt oxides and information complementary to XPS data can be obtained from the ELS data.