A new route to oxygen-rich nanomaterials

Molybdenum oxides are widely used in applications ranging from catalysis and gas sensing to electronic devices. In bulk materials, molybdenum trioxide (MoO₃) represents the most oxygen-rich stable oxide, with three oxygen atoms per molybdenum atom. But what happens when matter is reduced to a cluster containing just a few metal atoms? Can such tiny objects accommodate chemical compositions and structures that are impossible in extended solids?

An international team involving the University of Trieste, Elettra-Sincrotrone Trieste and King's College London has addressed this question by investigating the oxidation of molybdenum nanoclusters containing exactly six or thirteen atoms. The results reveal an unexpected ability of these clusters to incorporate oxygen well beyond the limits of bulk molybdenum oxides.

The experiments were performed at the SuperESCA beamline of Elettra. Using the ENAC (Exact Number of Atoms in Each Cluster), the cluster source developed at the Nanoscale Materials Laboratory, the researchers deposited size-selected molybdenum clusters onto a graphene-supported surface, keeping the sample at just 40 K. The clusters were then exposed to molecular oxygen while being irradiated with soft X-rays, as shown in Fig. 1. High-resolution X-ray photoelectron spectroscopy reposted in Fig. 2a, combined with advanced theoretical calculations, allowed the team to follow the chemical changes occurring in the clusters and determine their oxygen content.

Figure 1: Schematic representation of the experiment: from the deposition of size-selected molybdenum clusters using ENAC to oxygen exposure, X-ray irradiation and formation/evolution of oxygen-rich clusters.

The results were striking. To reproduce the experimental spectra, the calculations required oxygen-to-molybdenum ratios corresponding to Mo:O = 1:4 for the six-atom clusters and 1:5 for the thirteen-atom clusters (see Fig. 2b). In addition atomic valence, rather than formal oxidation state, provides a physically meaningful and transferable descriptor for interpreting the main trend in core electron binding energy shifts in nanoscale disordered systems (see Fig. 2c).

These findings show that the chemical behaviour of matter can change dramatically when its dimensions approach the atomic scale. Unlike extended crystals, whose structures are constrained by the regular arrangement of their atoms, nanoclusters can reorganize their internal structure to accommodate unusual chemical environments.

The most surprising aspect of the study is that these transformations occur at a temperature where ordinary thermal energy is far too small to drive substantial atomic rearrangements. The explanation lies in the interaction between radiation and matter. When X-rays eject electrons from the sample, they also generate secondary electrons. These energetic electrons can transfer energy to the clusters, helping to activate the dissociation of physisorbed oxygen molecules and trigger structural rearrangements that would otherwise be inaccessible at such low temperatures.

The calculations show that the molybdenum framework can expand and reorganize, allowing oxygen atoms to penetrate the cluster and form new bonding arrangements. In particular, tetrahedral units in which a molybdenum atom is surrounded by four oxygen atoms become possible. Such structures are familiar in molecular chemistry but are strongly constrained in extended molybdenum oxides.

This phenomenon is described as photon-driven fluxionality: the ability of a nanocluster to change its atomic structure through electronic excitation, even under cryogenic conditions. In this process, the X-rays do not simply provide a probe of the material; through the electrons they generate, they also actively influence its chemical transformation.

Figure 2: Comparison between experimental and theoretical calculations. (a) Mo 3d spectra for Mo₆ and Mo₁₃ at different oxidation levels. (b) Calculated core-electron binding energies for clusters with different Mo:O ratios. (c) Relationship between DFT-calculated core-electron binding energies and atomic valences.

Beyond the specific case of molybdenum, this work highlights how the structural flexibility of very small clusters can open chemical pathways that are unavailable to bulk materials. It also demonstrates the power of combining high-resolution synchrotron spectroscopy with first-principles calculations to investigate complex, disordered nanosystems. The findings suggest that controlled electronic excitation could become a tool for modifying the structure and reactivity of nanomaterials. In the future, similar mechanisms may be explored using other sources of energetic photons, potentially extending these approaches beyond large-scale synchrotron facilities.

At the atomic scale, light can therefore do more than reveal the structure of matter: it can help create structures that would otherwise remain inaccessible.
 

This research was conducted by the following research team:

Yao Wei¹, Deborah Perco², Alejandro Santana-Bonilla¹, Federico Loi^2,3, Paolo Lacovig⁴, Silvano Lizzit⁴, Lev Kantorovich¹, and Alessandro Baraldi^2,4
1 King’s College London, London
² Dip. di Fisica, Università di Trieste, Trieste, Italy
³ Heyrovský Institute of the Czech Academy of Sciences, Prag, Czech Republic
⁴ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy

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