Outstanding Nobility Observed in Cu5 Clusters Reveals the Key Role of Collective Quantum Effects

Abstract

Subnanometer-sized metal clusters often feature a molecule-like electronic structure, which makes their physical and chemical properties significantly different from those of nanoparticles and bulk material. Considering potential applications, there is a major concern about their thermal stability and susceptibility towards oxidation. Cu clusters of only 5 atoms (Cu<sub>5</sub> clusters) are first synthesized in high concentration using a new-generation wet chemical method. Next, it is shown that, contrary to what is currently assumed, Cu<sub>5</sub> clusters display nobility, beyond resistance to irreversible oxidation, at a broad range of temperatures and oxygen pressures. The outstanding nobility arises from an unusual reversible oxidation which is observed by <i>in situ</i> X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy on Cu<sub>5</sub> clusters deposited onto highly oriented pyrolitic graphite at different oxygen pressures and up to 773 K. This atypical property is explained by a theoretical approach combining different state-of-the-art first principles theories. It reveals the essential role of collective quantum effects in the physical mechanism responsible for the nobility of Cu<sub>5</sub> clusters, encompassing a structural ‘breathing’ through concerted Cu–Cu elongations/contractions upon O<sub>2</sub> uptake/release, and collective charge transfer as well. A predictive phase diagram of their reversible oxidation states is also delivered, agreeing with the experimental observations. The collective quantum effects responsible of the observed nobility are expected to be general in subnanometer-sized metal clusters, pushing this new generation of materials to an upper level.

Computational Characterization of the Intermixing of Iron Triade (Fe, Co, and Ni) Surfaces and Sub-nanometric Clusters with Atomic Gold

“…Our selection is based on previous experimental measurements on the catalytic activity of Au n clusters (with n in between 3 and 10) for the oxidation of thiophenol, showing that the Au 5 cluster holds the best performance. 49 30 when the cluster size is reduced to a very small number of atoms, a sub-nanometer network of discrete molecule-like d orbitals centered on the metal atoms is formed. The special structures of these molecular orbitals favor that the metal atoms become cooperatively active in the given (e.g., substitution) process.…”

“…Taken together, our findings reveal the major structural fluxionality of triade clusters compared with extended surfaces. As shown on a study of the oxidation quenching of Cu 5 clusters, when the cluster size is reduced to a very small number of atoms, a sub-nanometer network of discrete molecule-like d orbitals centered on the metal atoms is formed. The special structures of these molecular orbitals favor that the metal atoms become cooperatively active in the given (e.g., substitution) process.…”

“…On the other hand, sub-nanometric metal clusters have recently emerged as a new generation of quantum materials with exceptional properties. For instance, it has been recently demonstrated that the smallest metal clusters are highly stable − and possess special chemical and physical properties, making them innovative materials for numerous applications in, for example, energy conversion, catalysis, ,− and photocatalysis. , For instance, it has been shown that the deposition of Cu 5 and Ag 5 clusters onto TiO 2 surfaces can be useful for the creation of novel visible-light photoactive materials, , potential photocatalysts for CO 2 reduction and elimination from the atmosphere, and two-dimensional (2D) polaronic materials. , …”

Aggregation and support effects in the oxidation of fluxional atomic metal clusters. The paradigmatic Cu5 case

“…One major concern has been on the stability of Cu 5 clusters in an O 2 -rich atmosphere as it is generally assumed that nanoparticles suffer from an increasing susceptibility to oxidation with decreasing size. Opposing that assumption, recent works have indicated that Cu 5 clusters experience reversible oxidation, 7,[9][10][11][12][13] making them paradigmatic.…”

“…The most recent experimental evidences have shown the reversible oxidation of surface-supported Cu 5 clusters at thermodynamical equilibrium by in situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy. 13 Highly Oriented Pyrolitic Graphite (HOPG) was considered as a chemically inert support, different oxygen gas pressures, and a temperature range going from roomtemperature (RT) up to 773 K. 13 In general, the interaction of molecular oxygen with metal and metal oxides such as TiO 2 rutile (110) has been a longstanding problem over the last decades as extensively reviewed by Yates's [15][16][17] and Henderson's 18,19 groups from an experimental perspective. As shown in the earliest first-principles studies, [20][21][22][23][24] molecular oxygen can be adsorbed onto TiO 2 rutile (110) as either a superoxo O − 2 radical or a peroxo O 2− 2 species, both in molecular and dissociative forms, existing several (photo-)desorption channels as well.…”