When cooling does not freeze the structure: nanocluster fluxionality at low temperature
Even at temperatures where structural dynamics are expected to be largely suppressed, nanoclusters retain a remarkable ability to rearrange.
Y. Wei, D. Perco et al.,
ACS Nano 20, 25203 (2026)
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At the sub-nanometric scale, matter can behave in ways that have no direct counterpart in bulk materials. One of the most intriguing manifestations of this behavior is fluxionality, i.e., the ability of a cluster to explore different atomic configurations in response to relatively small energetic perturbations. Such structural flexibility is increasingly recognized as a key ingredient in the chemical properties of nanoclusters, particularly in catalysis. Yet, one would naturally expect this dynamic behavior to become strongly suppressed at very low temperature, where thermal energy is insufficient to overcome the barriers separating different configurations.
Our work shows that this intuitive picture can break down dramatically for size-selected molybdenum nanoclusters supported on graphene. We investigate clusters containing only a few Mo atoms, up to Mo₁₃, deposited on epitaxial graphene and exposed to molecular oxygen at temperatures as low as 40 K. Their oxidation is followed experimentally by high resolution X-ray photoelectron spectroscopy (HR-XPS) and interpreted through extensive first-principles calculations of the complete supported systems. At such low temperature, conventional thermally activated restructuring would be expected to be strongly inhibited. Nevertheless, the clusters undergo profound structural transformations as their interaction with oxygen proceeds. The key to this unexpected behavior is the coupling between oxygen dissociation, electronic excitation, and the exceptionally soft structural energy landscape of the clusters. The interaction with oxygen does not simply decorate an essentially rigid metallic nanoparticle. Instead, increasing oxygen incorporation progressively reorganizes the entire cluster, producing substantial changes in Mo–Mo distances, coordination, and overall cluster geometry. This structural flexibility allows the clusters to accommodate extraordinarily large amounts of oxygen. Oxygen contents far exceeding those expected from the stoichiometry of stable bulk molybdenum oxides can be reached, demonstrating that bulk stoichiometric rules cease to provide an adequate description at the sub-nanometric limit. The calculations reveal that oxygen incorporation progressively expands and restructures the clusters and promotes local coordination environments that do not have a simple bulk counterpart. In particular, highly oxygenated configurations contain MoO₄-like tetrahedral motifs, which provide an efficient way of stabilizing oxygen-rich structures. An important consequence is that the conventional assignment of a unique formal oxidation state to each Mo atom becomes increasingly inadequate. The measured Mo 3d core-level shifts are better understood in terms of the continuously evolving local atomic valence and coordination environment, together with final-state screening effects. The remarkable aspect of these observations is their occurrence at temperatures at which the available thermal energy alone should be far too small to sustain extensive structural rearrangements. |
The energy released or transferred during oxygen activation provides access to otherwise inaccessible regions of the potential-energy landscape. The cluster can therefore reorganize while the chemical process is occurring, rather than having to overcome the relevant barriers through thermal activation alone.
Fluxionality in these systems should consequently not be regarded simply as the thermal motion of a small particle between nearly degenerate structures. Instead, the chemical reaction itself can activate the structural degrees of freedom of the cluster.The resulting picture is therefore one of a strongly coupled process: oxygen activation modifies the electronic structure, the cluster responds structurally, and this structural response in turn enables additional oxygen incorporation and stabilization. Our results demonstrate that even at cryogenic temperatures, sub-nanometric clusters cannot necessarily be described as static atomic structures. They are better viewed as adaptive nanoscale entities whose structure evolves together with their chemical environment. This behavior has important implications for understanding heterogeneous catalysis and other chemical processes involving clusters containing only a few atoms. The structure responsible for reactivity may not correspond to the initially deposited cluster, nor even to a single well-defined equilibrium geometry. More generally, the results illustrate how reducing matter to the sub-nanometric scale can fundamentally alter the relationship between temperature, structure, stoichiometry, and chemical reactivity. In this regime, cooling does not necessarily freeze the structure: chemical energy can keep the atomic landscape accessible even when thermal energy cannot. Retrieve article Photon-Driven Cluster Fluxionality: Breaking the Bulk Stoichiometry Ceiling in Subnanometric Molybdenum Oxides Yao Wei, Deborah Perco, Alejandro Santana-Bonilla, Fderico Loi, Paolo Lacovig, Silvano Lizzit, Lev Kantorovich, Alessandro Baraldi |
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