Birnessite, a layered type of Mn oxide, has potential applications in pollution control, photovoltaics and catalysis. Birnessite properties are controlled by its structure, chemical composition, particle sizes and morphologies. These parameters can be tuned during synthesis to obtain birnessite with desired properties. Here we have synthesized birnessite nanoparticles of various sizes by adding oxyanions (phosphate, silicate and sulfate) during reductive precipitation of KMnO4 by HCl. The obtained products were characterized using multiple analyses including X-ray diffraction, X-ray atomic pair distribution function analysis, X-ray absorption spectroscopy, and electron microscopy. Results indicate that in addition to decreasing stacking order of the MnO6 sheets, all three anions decrease the sizes of the sheets in the lateral dimension. This can be ascribed to oxyanion adsorption on the edge sites of each MnO6 sheet, inhibiting its lateral growth. Moreover, Mn average oxidation state decreases and the Mn(III) content increases with increasing oxyanion/Mn ratio. Among the three anions, phosphate shows the strongest effect, followed by silicate and sulfate, which is attributed to their decreasing adsorption affinity on the edge sites. To remove the oxyanions to obtain oxyanion-free birnessites, selected birnessite products were washed with 1 M KOH. Upon washing, the MnO6 sheets become larger and more MnO6 sheets are stacked, particularly for the birnessite products of the phosphate system that have very small MnO6 sheets. As birnessite solubility in 1 M KOH solution is low, the particle growth probably occur via the “oriented attachment” mechanism, by which nanoparticles with common crystallographic orientations directly combine together to form larger ones. Diffuse reflectance UV-vis spectral analysis shows that the sizes of the birnessite nanoparticles greatly affect their optical absorption spectra and the direct bandgap increases from 1.48 to 1.82 eV with decreasing particle size. The results have important implications for fabricating birnessite nanoparticles with desired properties.

Single approach to synthesize birnessite of various sizes

251st National Meeting (2016)

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