X-ray photoelectron spectroscopy, otherwise known as XPS, is a powerful spectroscopic technique for measuring the binding energies of core electrons. X-ray photoelectron spectroscopy as a technique is compatible with gases, liquids and solids and has become a staple analytical technique in a number of fields, including nanotechnology and nanoscience.1
Image Credit: Alexander Gatsenko/Shutterstock.com
X-ray photoelectron spectroscopy is based on the same principle as all photoelectron spectroscopy methods. If a molecule or material is irradiated with light of a known energy above the ionization threshold, a photoelectron will be ejected. If the kinetic energy of the released photoelectron is measured and the energy of the incident radiation is well-characterized, the binding energy of the orbital from which the electron was ejected can be calculated.
The highly localized nature of the core orbitals probed in X-ray photoelectron spectroscopy means that each element tends to have a unique binding energy for its core electrons. Therefore, X-ray photoelectron spectroscopy is an excellent tool for the analysis of the elemental compositions of samples and surfaces.2
A similar analysis with lower photon energies to probe electrons from the valence orbitals that are often delocalized over several atomic sites in the molecule can be much more challenging to interpret, particularly in terms of the elemental composition or stoichiometry of the sample.
Applications of X-Ray Photoelectron Spectroscopy
In materials and the analytical sciences, X-ray photoelectron spectroscopy is a routinely used technique for elemental and stoichiometric analysis of materials and surfaces. For many materials experiments, cathode-ray lamps can be used to generate sufficiently bright X-rays to record X-ray photoelectron spectroscopy in a laboratory setting. Advanced light sources are tunable in terms of photon energy and enable a much greater range of X-ray experiments.
For nanotechnology, X-ray photoelectron spectroscopy has become a standard characterization method for many materials. The element sensitivity can be used to characterize the composition of nanoparticles as well as ensure the particles are the right size and shape for an application. Many nanoparticles make use of heavy elements, like titanium or gold which can be selectively detected using sufficiently high energy X-rays.
X-ray photoelectron spectroscopy can also be used for process improvement and quality control, such as for evaluating how nanocomposite thin film parameters vary by changing the conditions used for the film deposition.
The selectivity of X-ray photoelectron spectroscopy makes it ideal for confirming the change in ratios of specific chemical bonds, as the X-ray photoelectron spectroscopy peak positions are sensitive to the chemical environment. In this way, X-ray photoelectron spectroscopy can be used to evaluate which chemical structures or bonds contribute to the macroscopic materials' properties, such as the overall hardness of the material.
Morphological studies of nanomaterials are another application of X-ray photoelectron spectroscopy. While many materials can have the same elemental composition, the morphology of the material can play an important role in the types of interactions it has with the environment.
For metal-organic-frameworks (MOFs) and other nanostructured materials that make use of their physical shapes to act as filters or traps, X-ray photoelectron spectroscopy can be useful in evaluating what morphologies are the most beneficial for a given application. With high resolution measurements, even subtle changes in the bonding environment for chemically equivalent species can be differentiated and X-ray energies can be selected not just to look at specific elements, but also to probe to different depths in the nanomaterials.
Many battery studies are making use of morphological X-ray photoelectron spectroscopy analysis to better understand how the nanoscale structures of the electrodes affect the overall material and battery performance.
The Future of X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy is now a mature spectroscopy with routine use in a range of different scientific areas. A range of electron spectrometer types is available, from time-of-flight designs, magnetic bottles and hemispherical analyzers and measurements can be performed on a variety of sample types, including in operando measurements on catalysts.4
Another large development is the advent of free-electron lasers that can generate ultrashort X-ray pulses with high peak intensities. There are now over five operational X-ray free-electron lasers worldwide, with SHINE in Shanghai due to come online in the coming years. The UK is currently considering the conceptual design case for another such facility there.6
X-ray free-electron lasers have made it possible to perform time-resolved X-ray photoelectron spectroscopy experiments that can be used to recover dynamical processes in molecules and materials that unfold in response to some kind of external stimulus, like a light pulse.
For many nanomaterials, being able to explore their dynamic response as well as their static characteristics is very important for their use in light-driven applications. Nanoparticles have been one type of material under particular investigation with such time-resolved methods.
References and Further Reading
Fadley, C. S. (2010). X-ray photoelectron spectroscopy : Progress and perspectives. Journal of Electron Spectroscopy and Related Phenomena, 178–179, pp. 2–32. https://doi.org/10.1016/j.elspec.2010.01.006
Watts, J. F., & Wolstenholme, J. (2019). An introduction to surface analysis by XPS and AES. John Wiley & Sons. doi.org/10.1002/0470867930.
Alagia, M., Richter, R., Stranges, S., Agåker, M., Ström, M., Söderström, J., ... & Rubensson, J. E. (2005). Core level ionization dynamics in small molecules studied by x-ray-emission threshold-electron coincidence spectroscopy. Physical Review A, 71(1), p. 012506. https://doi.org/10.1103/PhysRevA.71.012506
Benayad, A., Santini, C. C., & Bouchet, R. (2021). Operando XPS : A Novel Approach for Probing the Lithium / Electrolyte Interphase Dynamic Evolution. The Journal of Physical Chemistry A, 125, pp. 1069–1081. https://doi.org/10.1021/acs.jpca.0c09047
STFC (2022) £3.2 million in funding announced. Available at: https://www.clf.stfc.ac.uk/Pages/%C2%A33.2m-in-Funding-Announced-for-UK-XFEL-Research-and-Development.aspx,
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.