X-ray photoelectron spectroscopy (XPS)

Information obtained

  • Quantitative element composition of the top 10 nm of a solid.
  • Information about the atomic bonding environments.
  • Oxidation states and functional groups present.
  • Thickness of adsorbed layers.

Principle

When X-rays interact with the atoms in a sample, electrons are emitted. Their kinetic energy as they come out of the sample depends on the element and redox state. Only photoelectrons ejected from the top few molecular layers of the solid can escape without loosing energy solid, meaning that XPS provides chemical information about the top 1-10 nm only.

Special features

Angle resolved XPS: by changing the angle of the sample with respect to the photoelectron pathway to the detector, the analysis depth can be changed, which provides a means to make the analysis more or less surface sensitive.

Fast freezing or vitrification: samples that contain water (e.g. mineral powders with adsorbed or structural water or biological samples) can be rapidly cooled to liquid nitrogen temperature and analysed in the frozen state.

Heater and mass spectrometer: identification of changes in elemental composition and emitted gases from the samples during heating (from liquid nitrogen temperature to 500 °C).

Gas dosing and mass spectrometer: for determining adsorption affinity for molecules on surfaces.

XPS survey spectrum revealing the identity and atomic concentration of all the elements present in the near surface of the sample, in this case chalk. Calcite contains Ca, C and O; Na and Cl come from the salty water that was in the pores and F contamination comes from the vessel used the solvent treatment for removing the oil, by the oil company.
Picture of a drop of NaCl solution analyzed by XPS. The drop was deposited on a stub, rapidly cooled to -160°C using liquid nitrogen (fast freezing) and is kept at this temperature throughout the entire measurement.
High resolution spectrum of the C 1s binding energy region obtained by dosing ethanol on a calcite crystal inside the XPS analysis chamber. The 290.1 eV signal comes from the carbonate ions in the calcite whereas the 2 peaks in the 285-288 eV range come from ethanol adorbed on the crystal surface. (from N. Bovet, M.S. Javadi, S.L.S. Stipp, GCA, 2009, 73, (13), A150).