Three Distinctive\nSteps for Heterogeneous Nucleation\nof Tunnel-Structured Mn Oxide on Quartz under Light Exposure

Natural manganese (Mn) oxide coatings, resulting from\nthe heterogeneous\nnucleation on foreign substances, have garnered interest based on\ntheir importance in the reaction with organic substances and in environmental\nsystems. However, the heterogeneous nucleation of the natural Mn oxide\ncoatings still remains elusive. Here, <i>via</i> fast photochemical\noxidation of Mn<sup>2+</sup>(aq), we show that Mn(IV) oxide nuclei\nform and aggregate on quartz in three distinct successive stages:\n(i) a nanocrystalline film of unaligned grain forms, (ii) nanoislands\ndevelop on the film, and (iii) nanorods form on the nanoislands. Each\nstage has different crystalline structures and forms by aligned attachment\nof nanoscale precursors on the preceding surface. Crystal lattice\nanalyses confirm the crystalline development, from the short-range\norder of the Mn oxide film to the long-range order of the nanorods.\nAlso, the heterogeneous nucleation observed in this work produced\ngroutellite-like tunnel structures of Mn oxide on quartz. This revealed\npathway of the heterogeneous nucleation can offer a new perspective\non the variety of poorly crystalline structures of natural Mn oxides\nfound in the environment, which can affect elemental redox cycles,\ncontaminant sequestration and removal, and soil carbon storage.