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

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 (FemOnq, q = 0, ±1) have been extensively reported in literature, new and very interesting distribution of FemOn+ clusters is observed in this study. Under saturated O2 growth conditions, the smallest (leading) cluster in m = 2k + 1 (k = 2−14) cluster series is with stoichiometry of Fe2kO3kFeO+, which is perfect (iron atoms are perfectly oxidized) in terms of average oxidation states of iron (Fe3+) and oxygen (O2−) atoms. For m = 2k (k = 2–15) cluster series, the leading cluster is either Fe2kO3k+ (the least over-oxidized) or Fe2kO3k−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 Fe2O3 nano-materials. The distribution of the cobalt oxide clusters (ComOn+) under saturated O2 growth conditions is complex and very different from that of FemOn+. A very interesting result for cobalt species is that two clusters Co11O13+ and Co12O13+ are missing in the cluster distribution although their oxygen-neighbor clusters Co11O12,14+ and Co12O12,14+ are generated. This suggests relatively high stability for Co11O12+ and Co12O12+ clusters. The DFT calculations predict that Co12O12 cluster are with tower or cage structure rather than the compact NaCl-like arrangement that is found for bulk CoO.