Formation, distribution, and structures of oxygen-rich iron and cobalt oxide clusters

Abstract

A time of flight mass spectrometer coupled with a laser ablation/supersonic expansion cluster source is used to study the formation and distribution of cationic iron and cobalt oxide clusters. Although the distributions of iron oxide clusters ( F e m O n q , q = 0, ±1) have been extensively reported in literature, new and very interesting distribution of Fe m O n + clusters is observed in this study. Under saturated O 2 growth conditions, the smallest (leading) cluster in m = 2 k + 1 ( k = 2−14) cluster series is with stoichiometry of Fe 2 k O 3 k FeO +, which is perfect (iron atoms are perfectly oxidized) in terms of average oxidation states of iron (Fe 3+) and oxygen (O 2−) atoms. For m = 2 k ( k = 2–15) cluster series, the leading cluster is either Fe 2 k O 3 k + (the least over-oxidized) or Fe 2 k O 3 k−1 + (the least under-oxidized). Density functional theory (DFT) calculations indicate that these leading clusters are with unexpected structures although their appearance in the mass spectra is predictable. These clusters may serve as good models for predicting or interpreting novel properties of Fe 2O 3 nano-materials. The distribution of the cobalt oxide clusters (Co m O n +) under saturated O 2 growth conditions is complex and very different from that of Fe m O n +. A very interesting result for cobalt species is that two clusters Co 11O 13 + and Co 12O 13 + are missing in the cluster distribution although their oxygen-neighbor clusters Co 11O 12,14 + and Co 12O 12,14 + are generated. This suggests relatively high stability for Co 11O 12 + and Co 12O 12 + clusters. The DFT calculations predict that Co 12O 12 cluster are with tower or cage structure rather than the compact NaCl-like arrangement that is found for bulk CoO.