Mesoporous metal oxide films composed of nanocrystal assemblies with an aligned crystallographic orientation are key nanostructures for efficient interfacial reactions; however, the development of a simple and versatile method for their formation on substrates still constitutes a challenge. Here we report the template-free centimetre-scale formation of novel cobalt oxide films of Co3O4 and CoO with a [111]-oriented mesoporous structure starting from stacking cobalt hydroxide continuous films. The cobalt hydroxide precursor is formed electrochemically on conductive substrates from a Co(NO3)2 aqueous solution at room temperature. A thorough characterization by means of scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis-NIR spectroscopy, IR spectroscopy and Raman spectroscopy analyses reveals that the precursor film is an α-type layered cobalt hydroxide salt (α-Co-LHS) containing interlayer nitrate and hydrated water, i.e., α-Co(OH) x (NO3) y ·nH2O, with a [001]-oriented stacking film structure. Heat treatment of the [001]-α-Co-LHS films using different conditions, i.e., under air at 550 °C or under vacuum at 500 °C, results in the selective formation of Co3O4 or CoO mesoporous films, respectively. A plausible explanation for the observed centimetre-scale topotactic-like transformation from α-Co-LHS[001] to Co3O4[111] or CoO[111] is given according to the atomic framework similarity between the hydroxide precursor and the final oxides.
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