Quantitative X-ray photoelectron spectroscopy study of enargite (Cu3AsS4) surface

ROSSI, ANTONELLA;ATZEI, DAVIDE;DA PELO, STEFANIA;FRAU, FRANCO;LATTANZI P;ENGLAND K. E. R;VAUGHAN D. J.

2001-01-01

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Abstract

The surface chemical state and composition of natural and synthetic enargite samples were characterized with X-ray photoelectron spectroscopy (XPS). Freshly cleaved, powdered and 'as received' samples were analysed at liquid nitrogen temperature. For the natural 'as received' samples as well as for the natural and synthetic powdered samples the binding energies of Cu 2p3/2, As 3d5/2 and S 2p are consistently found at 932.5 ± 0.2, 43.9 ± 0.2 and 162.2 ± 0.2 eV, respectively. A second component of S 2p at 163.9 ± 0.2 eV is present and may be assigned to elemental sulphur. Freshly cleaved enargite samples showed binding energies of 43.2 and 161.3 eV, respectively, for As 3d5/2 and S 2p3/2. At the surface of crystals 'as received', an additional signal at ∼169 eV, referable to sulphate sulphur, is also present, as well as additional signals for Cu 2p3/2 (∼934 eV), and As 3d5/2 (∼45.5 eV). The thickness and the composition of this outer oxidized layer together with the composition of the interface beneath the film are calculated on the basis of a model developed for this purpose, taking into account the attenuation of the photoelectrons due to the contamination and the oxidized layer. An oxidized film of ∼0.5 nm is present at the surface of the 'as received' crystals whose composition was found to be ∼45% copper, 24% arsenic and 31% sulphur; hence, the oxidized layer appears to be enriched in arsenic compared with the bulk composition. The composition of the material beneath the oxidized layer was found to be enriched in copper and depleted in sulphur with respect to the bulk analysis. Any further study of the interaction of natural enargite samples with the environment has to take into account the existence of this thin outer layer.