An experimental correlation between points of zero charge and X-ray photoelectron spectroscopy chemical shifts of oxides

Measurements are reported of X-ray photoelectron spectroscopy (XPS) investigations, performed under conditions of ultrahigh vacuum, of several metal oxides either produced commercially (high purity) or prepared in the laboratory. The same oxides were also subjected to electrochemical measurements in aqueous solutions for the determination of the point of zero charge (PZC) values and/or the isoelectric points (IEPs). The PZC (IEP) values appear to be linearly correlated with the sum of the metal and oxygen XPS chemical shifts (DM + DO). The higher (more alkaline) the PZC (IEP), the lower the XPS (DM + DO) term and vice versa. The weighting of the different components in the accuracy of the correlation is discussed. It is shown how a cross comparison between experimental XPS and electrochemical data obtained for the same samples can be used as a diagnostic criterion for the occurrence of secondary phenomena at the solid/liquid boundary.