Oxidation of galena in acetate buffer investigated by atomic force microscopy and photoelectron spectroscopy

Galena oxidation was investigated by AFM in acetate buffer under potentiostatic control and by photoelectron spectroscopy on potentiostatically pretreated specimens. At +236 mV (SHE) formation of sulfur protrusions could be observed with AFM. XPS showed the formation of elemental sulfur to start at potentials more anodic than +161 mV (SHE). Elemental sulfur could only be retained on the galena surface if sample cooling was started before the beginning of the evacuation in the spectrometer entry chamber. Sulfur-oxygen species could not be detected on galena samples oxidized in acetate buffer even when investigated with synchrotron-excited X-ray photoelectron spectroscopy. AFM images showed two important features: Oxidation starts with a roughening of the sample surface. At slightly more anodic potentials oxidation products are present on the samples as protrusions of 10-200 nm in height and with mutual distances of several hundred nanometers. Two types of sulfur deposits are formed differing in the emergence potential, size, and mutual distance. The formation of such protrusions can only be understood if the reactants for the depositions reach the growing protrusion by diffusion in the liquid phase. Therefore, it is proposed that the process causing the surface roughening is a dissolution of PbS to lead(II) ions and hydrosulfide ions while the deposition reaction is the electrochemical oxidation of hydrosulfide ions to elemental sulfur. By removal of the hydrosulfide ion from the aqueous solution, further dissolution becomes possible at other sample regions. The sulfur formation occurs at distinct points which are not preferentially located at steps. It is likely that the sulfur formation starts at impurity locations. Different impurities may be responsible for different rates of deposit formation, leading to protrusions of different size which however cannot be distinguished by XPS.