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.

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

“…Interestingly, this overall shifting improves the agreement with recent experimental measurements on fractions of copper oxidation states at low concentrations of copper clusters. 13 Under ambient conditions (1 atm and RT), the experiment indicates a dominance of the Cu(II) oxidation state. This is consistent with the appearance of complexes with 7 O 2 molecules per cluster in the phase diagrams at 1 atm and RT (phases shown in green in the upper panels of Fig.…”

“…The main reason for this selection is that the 3D Cu 5 structure has been the one identified in recent experimental measurements. 13 A carbon-based support has been modeled with the polycyclic aromatic hydrocarbon (PAH) circumpyrene (C 42 H 16 ). 29 Using the 3 Â 3 graphene supercell model reported in ref.…”

“…For the sake of comparison, they are also shown for the Cu 5 monomer in the lowest-energy doublet spin state (see also ref. 13). The bottom panels of Fig.…”

“…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. In contrast to 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 evidence has shown a reversible oxidation of surfacesupported Cu 5 clusters at thermodynamic equilibrium using in situ X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. 13 Highly oriented pyrolytic graphite (HOPG) was considered as a chemically inert support, at different oxygen gas pressures, and in the 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 few 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 firstprinciples studies, [20][21][22][23][24] molecular oxygen can be adsorbed onto TiO 2 rutile (110) as either a superoxo O 2 À radical or a peroxo O 2 2À species, both in molecular and dissociative forms, existing in several (photo-)desorption channels as well.…”