Effect of atomicity on the oxidation of cationic copper clusters studied using thermal desorption spectrometry

Abstract

The resistivity to oxidation of small copper clusters, Cu_n⁺ (n ≤ 5), in the gas phase with a precise atomicity at the molecular level was investigated using a combination of thermal desorption spectrometry and mass spectrometry. Oxide clusters, Cu_nO_m⁺, with more O atoms than those present with a stoichiometry of n : m = 1 : 1 were produced at room temperature in the presence of O₂, and the weakly bound excess oxygen atoms involved in the clusters were removed by post heating. Non-oxidized Cu₂⁺ and Cu₃⁺ clusters were formed in the range of 323–923 K, whereas partially oxidized clusters, Cu₄O₂⁺ and Cu₅O₂⁺, were generated for n = 4 and 5. Considering the fact that Cu_nO_m⁺ (m = n/2 + 1) tends to be generated for n ≥ 6, the small copper clusters were concluded to be resistive to oxidation. The possible reaction paths for the oxidation of Cu₂⁺ and Cu₄⁺ clusters were obtained by density functional calculations, which were consistent with the experimental findings. The oxidation states of the Cu atoms in the clusters were discussed based on the natural charges of the atoms.