Challanges in the oxidation at the sub-nanoscale
At the sub-nanometric scale, determining oxidation states becomes increasingly complex, as the properties of matter are strongly influenced by the discrete number of atoms involved.D. Perco et al., J. Am. Chem. Soc. 147, 215012 (2025)
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The concept of oxidation state, first introduced by Lavoisier in the 18th century, remains a fundamental principle across numerous scientific disciplines today. For example, the abundance of carbon on Earth is intricately linked to the oxidation state of iron under the extreme pressures of the Earth's upper mantle, where specific oxidation conditions significantly influence seismic waves. In medicine, Pt4+ complexes have been found to be more effective as anticancer agents compared to Pt2+, while copper's redox properties are essential for a variety of biological processes. It takes a fundamental role especially in material science where the oxidation state of an element is most critical, as it governs its chemical and physical properties. However, even in recent decades, the concept of oxidation state has been the subject of debate and critical assessments within the scientific community, culminating in the establishment of a new definition by IUPAC and alternative frameworks. From an experimental standpoint, it is widely accepted that X-ray Photoelectron Spectroscopy (XPS) provides fingerprints for determining atomic oxidation states. Since its widespread adoption through the pioneering work of Kai Siegbahn, XPS has become an essential analytical tool in various fields of materials science. Its elemental and surface sensitivity make it especially suited for determining oxidation states in both bulk and surface environments. In this study, we explore whether this approach can be successfully applied at the sub-nanometric scale, where atomic aggregates consist of only few atoms. To address the challenge, we conducted high-resolution XPS experiments at the SuperESCA beamline on size-selected tungsten clusters composed of only a few atoms and produced by ENAC (Exact Number of Atoms in each Cluster), the source developed at the Nanoscale Materials Laboratory. We deposited W13 and W25 tungsten clusters at 40 K onto graphene and with an extremely low atomic density (0.06% of MonoLayer) to suppress surface diffusion and avoid any sintering. We than exploited the intense photon flux available at the beamline, enabling real-time acquisition of W 4f core-level spectra during O2 exposure.. The spin–orbit splitted W 4f peaks progressively shift to higher binding energies with increasing exposure, stabilizing after a few Langmuir, indicative of saturation in oxygen. However, the resulting spectral complexity, much greater than that observed for on solid surfaces, required a theoretical framework for interpretation. We therefore employed DFT to support the analysis. In the initial structural investigations we systematically introduced increasing amounts of O atoms into the clusters, that induces pronounced structural distortion, notably increasing the average W–W distance. For all the several investigated configurations, W 4f core levels were calculated via DFT. To understand the correlation between core levels and oxidation states, we than applied Pauling’s bond valence formalism. Using this method, we computed valences for all W atoms across oxygen coverages. |
The graph of calculated core levels plotted against valences shows a strong linear correlation between core levels and valence with mean core levels, valences, and standard deviations for each W-nO family indicated by colored rectangles (n=number of bond between W and O). Two key observations emerge: (i) for each atomic coordination, core level dispersion is significant and (ii) notable overlap occurs between different coordination families. The distribution of calculated core electron binding energy where than used to fit the experimental spectra with excellent agreement. This clearly indicates that even high-resolution XPS measurements cannot precisely determine in a straightforward way atomic oxidation state in small clusters.
This variability stems from the fluctuating interatomic distances in nanoclusters composed of few atoms. In bulk material, despite identical oxygen coordination, W–O bond lengths are fixed and vary between compounds; this is not the case for nanostructures, thus challenging conventional oxidation state assignments. Limitations in Determining Oxidation States in Condensed Matter at the Subnanometric Scale Deborah Perco, Monica Pozzo, Andrea Berti, Federico Loi, Paolo Lacovig, Silvano Lizzit, Dario Alfè and Alessandro Baraldi, J. Am. Chem. Soc. 147, 21501 (2025). |
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