Effective Functionalization of Disordered Oxide Lattices on Iron Particle Surfaces Using Mechanochemical Reactions

The mechanochemical surface functionalization of iron oxides with disordered lattices on bare iron (Fe) particles was investigated using simple milling processes to clarify the formation mechanism of the oxide layer and investigate the near-surface models with different states. The homogeneous α-Fe particles at the milling equilibrium were first prepared under an argon atmosphere. After the subsequent milling reaction of the particles with oxygen molecules, the surface analyses by X-ray diffraction and Raman and X-ray photoelectron spectroscopies revealed that the near-surface layers consisted of two iron oxide phases (α-Fe₂O₃ and Fe₃O₄) through oxygen atom diffusion, and the α-Fe₂O₃ was dominantly grown on the near surface. During the initial reaction, the signals from an electron spin resonance suggested the dangling bond formation on α-Fe₂O₃. The oxygen atoms effectively induce disordered lattices in the local area to form oxidized Fe³⁺ clusters, and the geometric distortion formed the dangling bonds, which were theoretically supported by a molecular orbital calculation to elucidate the increase in the unpaired electron sites on the α-Fe₂O₃. Therefore, the defective Fe³⁺ ions induced by the lattice mismatching between the clusters and bare α-Fe are found to form the disordered lattice that contains the oxygen atoms with unpaired electrons, which are successfully induced by the near-surface strain based on the simple mechanochemical reactions. The patterns of surface activation of the Fe particle surfaces by oxidization will be capable of novel chemical reactions by selective oxygen insertion as well as deep oxidation.