Editorial Feature

What is X-Ray Photoelectron Spectroscopy?

X-ray photoelectron spectroscopy (XPS) is a spectroscopic technique for measuring the elemental composition of a sample. XPS can be performed on materials or molecular samples and, for materials, is usually a surface-sensitive technique. There are variations of the technique that use higher energy incident photons (hard X-ray photoelectron spectroscopy) to probe further into the bulk of materials.1

What is X-Ray Photoelectron Spectroscopy?

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XPS Measurements

An XPS measurement relies on the photoelectric effect and detection of the kinetic energies of the photoelectrons produced on ionization. In an XPS experiment, the sample of interest is irradiated with light with a photon energy that is greater than the binding energy of the core electrons of the element of interest.

The core electrons in a material are tightly bound, and often, the binding energies of these electrons are relatively similar between the isolated element and the element in its full material environment. The similarity in energy is because the introduction of the material environment to an isolated element has a significant impact on the binding energies of the valence electrons but less so on the core electrons that experience greater shielding from the environment.

XPS measurements can be performed using a range of X-ray sources. For laboratory-based measurements, cathode discharge tubes are often used which can achieve photon energies in the tens of keV. However, the photon fluxes that are achievable with cathode-ray sources are relatively limited compared to what can be generated at advanced light sources such as synchrotrons.

Where more advanced polarization control is required, energy tunability, high spatial resolution and better photon doses, synchrotrons and other advanced X-ray infrastructures are ideal for performing XPS measurements.

In an XPS measurement, the incident photon ionizes a core electron from the material and the kinetic energy of this emitted photoelectron is then detected. If the incident photon energy and kinetic energy of the photoelectron are both known accurately and precisely, the binding energy of the orbital from which the photoelectron originated can be calculated. In materials, the shifts of the binding energies from the isolated element arise from changes in the chemical and physical environment and processes such as electronic interactions and charge transfer with the environment.3

Materials Characterization

Information on the binding energies from XPS measurements can be used for elemental analysis on a sample and to identify the nature of the local binding environment.2 XPS can provide information on coordination numbers in materials as well as the electronic state of the element being probed. The sensitivity of XPS to chemical environments means that it is sometimes known as electron spectroscopy for chemical analysis (ESCA) – a reference to its use in the quantitative and qualitative analysis of samples.

The use of XPS is not restricted to standard materials. XPS can also be used for the analysis and characterization of nanomaterials.4 ­XPS can be used to profile the surfaces of nanoparticles for their consistency and provide insights into the surface chemistry there as well as elemental information on the chemical composition of the nanoparticle.

Performing XPS on nanomaterials is not significantly different from standard XPS measurements on materials. Often the nanoparticles are prepared by cleaning them and placing them on a surface, but it is possible to make measurements on nanoparticles in solution using technologies such as liquid microjets.4 As XPS measurements rely on conservation of momentum principles, there are particular challenges in dealing with solution-phase experiments and the shorter mean free path of the emitted photoelectrons so the detected kinetic energies are truly meaningful and relative to the real binding energies.

Nanotechnology

New generation advanced light sources and source upgrades are continually in progress to provide better quality photons for XPS measurements. One way is not just moving to higher photon fluxes to reduce total measurement times but also to improve the spatial coherence of beams.

XPS is already widely used for many types of nanomaterials and nanotechnology for both biological and material samples.5 Recent developments also include the use of ambient pressure XPS to characterize the subsurface hydrogen absorption on palladium nanoparticles that could be used for fuel cells.6

Most XPS measurements are performed under high vacuum conditions to avoid contamination of the surface layers of the material and ensure there is no kinetic energy loss of the measured photoelectrons due to collisions with other species present. With ambient measurements, it is possible to probe chemical processes happening, such as carbon dioxide reduction on surfaces and evaluate the potential of new nanomaterials as potential catalysts.

Continue reading: Femtosecond X-Ray Spectroscopy.

References and Further Reading

Gorgoi, M., Mårtensson, N., & Svensson, S. (2015). HAXPES studies of solid materials for applications in energy and information technology using the HIKE facility at HZB-BESSY II. Journal of Electron Spectroscopy and Related Phenomena, 200, 40–48. https://doi.org/10.1016/j.elspec.2015.05.005

Bagus, P. S., Ilton, E. S., & Nelin, C. J. (2013). The interpretation of XPS spectra : Insights into materials properties. Surface Science Reports, 68(2), 273–304. https://doi.org/10.1016/j.surfrep.2013.03.001

Isaacs, M. A., Davies-jones, J., Davies, P. R., Guan, S., Lee, R., Morgan, D. J., & Palgrave, R. (2021). Advanced XPS characterization : XPS-based multi-technique analyses for comprehensive understanding of functional materials. Materials Chemistry Frontiers, 5, 7931–7963. https://doi.org/10.1039/d1qm00969a

Olivieri, G., & Brown, M. A. (2016). Structure of a Core – Shell Type Colloid Nanoparticle in Aqueous Solution Studied by XPS from a Liquid Microjet. Topics in Catalysis, 59(5), 621–627. https://doi.org/10.1007/s11244-015-0517-3

Giuliani, A., Fiori, F., Manescu, A., Komlev, V. S. , Renghini, C., & Rustichelli, F. (2011). Synchrotron Radiation and Nanotechnology for Stem Cell Research. In (Ed.), Stem Cells in Clinic and Research. IntechOpen. https://doi.org/10.5772/19959

Tang, J., Seo, O., Rivera, D. S., Koitaya, T., Yamamoto, S., Nanba, Y., Song, C., Kim, J., & Yoshigoe, A. (2022). Hydrogen absorption and diffusion behaviors in cube-shaped palladium nanoparticles revealed by ambient-pressure X-ray photoelectron spectroscopy. Applied Surface Science, 587, 152797. https://doi.org/10.1016/j.apsusc.2022.152797

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Rebecca Ingle, Ph.D

Written by

Rebecca Ingle, Ph.D

Dr. Rebecca Ingle is a researcher in the field of ultrafast spectroscopy, where she specializes in using X-ray and optical spectroscopies to track precisely what happens during light-triggered chemical reactions.

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