Universitat Rovira i Virgili

X-ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy, XPS, or ESCA, Electron Spectroscopy for Chemical Analysis) is a non-destructive and quantitative technique for the chemical, physics and electronics properties analysis of elements (Z>2) present in the shallowest layers (20-30Å), making it a unique technique for the chemical analysis of surfaces. The sample is excited with an X-ray source (usually AlKα, 1486.7 eV). When the x-rays imping on the sample, photoelectrons with different kinetic energies (KE) are emitted. This kinetic energy is related to the binding energy (BE, eV) and the incident radiation energy (hv, eV) by equation 1. BE is characteristic of an electron from a given atomic orbital and element (tabulated values).

hv (eV) = KE (eV) + BE (eV)    (eq. 1)

Depending on the chemical environment of the atom, the binding energy (BE) of the emitted electrons changes slightly (called "chemical shift") so that both the element and its chemical environment can be identified (for example, in the carbon C1s region, C-C, C≡N, C-O-C, C-F3 and O-C=O species can be differentiated).

XPS technique is therefore a powerful surface analysis technique that can identify (and quantify) the elements present in the sample and provide information about the oxidation/reduction state of an element and its chemical environment.

Conventional XPS machines usually work under UHV (Ultra-high Vacuum) conditions. However, thanks to scientific advances in recent decades, systems have been developed that are capable of operating in the analysis chamber under NAP (Near Ambient Pressure) conditions. Different gases can be introduced into the analysis chamber at a pressure of around 1-25 mbar, thanks to several differential pumping stages between the analysis chamber and the analyzer.

Figure 1. Panoramic view of the XPS machine "ProvenX-NAP System".

SPECS Provenx NAP System

ProvenX-NAP System (Figure 1), designed by SPECS and acquired by the SRCiT thanks to the grant EQC2021-007785-P awarded by the Spanish Ministry of Science and Innovation), is divided in different parts: (1) sample introduction chamber "Load Lock", (2) preparation chamber "Pre-chamber", (3) coupled reactor "High Pressure Cell - HPC" and (4) analysis chamber which have been designed for a specific use, detailed below:

Figure 2. a) "Load Lock" chamber, b) sample preparation chamber, c) HPC coupled reactor, d) analysis chamber and e) different types of sample holders.


Summary of Technical Characteristics ProvenX-NAP System




Some books d ivide the applications of the XPS technique into three main topics, in materials science, organic materials and corrosion science and electrochemistry.

Within the first block, there are different fields in which the use of XPS is very useful, as information can be obtained about the composition and chemical state of surfaces and interfaces (surface distribution, thickness and structure of surface layers, molecules adsorbed to the surface, coating composition, among others). XPS, in materials science, is used to: (i) characterizing semiconductors; (ii) for the understanding of complex surface processes that affect various minerals during metal extraction and ore processing, e.g. met al readsorption; (iii) the study in nanoscience of compounds that have combined properties of both the organic and inorganic part, due to their potential application in medicine and biotechnology; (iv) for the understanding of different phenomena that take place at the surface level in metallurgical materials, e.g. alloy formation; (v) coatings, (vi) to understand the behavior of inorganic materials, e.g. catalysts, during a chemical reaction, by obtaining information on the species adsorbed on the catalyst surface, the chemical environment and the oxidation/reduction state of the active centers, whether or not catalyst restructuring occurs under reaction conditions; (vii) to understand the friction and wear mechanisms leading to structural failure in tribology and to analyze the chemical composition of worn surfaces, adsorption and the reaction of lubricating oil additives and wear resistant coatings.

XPS provides a similar kind of information in the field of biology and organic materials as in the case of material science. However, due to the complex nature of biological systems, XPS is commonly used in combination with other relevant techniques for obtaining reliable information. Further, there are challenges related to sample preparation, sample damage and interpretation of the data. XPS is used to study the surface chemistry of microbes, formation of biofilms, biocompatible materials and also pharmaceutical materials. The surface chemistry of microbes is important because they can form biofilms on material surfaces. Similarly in medicine, the interaction of biomaterials and pharmaceutical drugs within the host body is also dependent on their surface properties.

In the field of corrosion studies, the XPS provides quantitative insights into the chemical composition, the nature of valence states, elemental distribution within the surface films (including multi-layer structure), the thickness of the films and the composition of alloy surface under the films. Surface films are usually very complex with multilayered structures and reflect the electrochemical properties of metals and components of alloys. XPS can detect the changes in the composition and chemical structure of surface layers with potential, time and electrochemical conditions. The XPS analysis is helpful in understanding the general and localized corrosion mechanism and is usually used in the study of the passivity of metals and alloys, surface treatments and coatings.


  • This project (EQC2021-007785-P) financed by MCIN/AEI/10.13039/501100011033 and by the European Union “Next GenerationEU”/PRTR within the framework of the State Plan for Scientific and Technical Research and Innovation 2017-2020, with an eligible expense of 961.900€.
  • MCIN acronym of the Ministry of Science and Innovation; AEI acronym of the State Investigation Agency; 10.13039/501100011033 the DOI (Digital Object Identifier) of the Agency; PRTR acrnym of the Recovery, Transformation and Resilience Plan.

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