Biogenic and Synthetic MnO₂ Nanoparticles: Size and Growth Probed with Absorption and Raman Spectroscopies and Dynamic Light Scattering

Abstract

MnO2 nanoparticles, similar to those found in soils and sediments, have been characterized via their UV–visible and Raman spectra, combined with dynamic light scattering and reactivity measurements. Synthetic colloids were prepared by thiosulfate reduction of permanganate, their sizes controlled with adsorbates acting as capping agents: bicarbonate, phosphate, and pyrophosphate. Biogenic colloids, products of the manganese oxidase, Mnx, were similarly characterized. The band-gap energies of the colloids were found to increase with decreasing hydrodynamic diameter, D h, and were proportional to 1/D h 2, as predicted from quantum confinement theory. The intensity ratio of the two prominent Mn–O stretching Raman bands also varied with particle size, consistent with the ratio of edge to bulk Mn atoms. Reactivity of the synthetic colloids toward reduction by Mn2+, in the presence of pyrophosphate to trap the Mn3+ product, was proportional to the surface to volume ratio, but showed surprising complexity. There was also a remnant unreactive fraction, likely attributable to Mn­(III)-induced surface passivation. The band gap was similar for biogenic and synthetic colloids of similar size, but decreased when the enzyme solution contained pyrophosphate, which traps the intermediate Mn­(III) and slows MnO2 growth. The band gap/size correlation was used to analyze the growth of the enzymatically produced MnO2 oxides.

Nanoparticle classification, physicochemical properties, characterization, and applications: a comprehensive review for biologists

“…While the average size of Ag NPs produced by potato ( Solanum tuberosum ) was found to be around 10–12 nm with a wider distribution profile between 3–65 nm [ 143 ]. In a different application, DLS was employed to study the size increase of biogenic MnO 2 NPs overtime, demonstrating that their size is 7.5 nm after 3 min of the initiation of the reaction, then their size grows overtime until it become 54 nm after 31 min [ 144 ].…”

Photocatalytic Oxidation of Dissolved Mn2+ by TiO2 and the Formation of Tunnel Structured Manganese Oxides

“…Mn(III,IV) species were not detected by the LBB method in the filtrate of the reaction suspensions at all times (Figure S3). Considering that homogeneously nucleated Mn oxides are 6−30 nm in diameter, 73 which can readily pass through 0.22 μm filters, the absence of Mn oxides in the filtrate indicates the high likelihood of heterogeneous nucleation of Mn oxides on the surface of TiO 2 instead of homogeneous nucleation of Mn oxide nanoparticles in solution.…”

Structural characterization and magnetic response of poly(p-xylylene)–MnSb and MnSb films deposited at cryogenic temperature

“…Lastly, the peaks at around 550 and 650 cm can be related to the formation of surface Mn-oxide phases, e.g. MnO or Mn O , which should yield Raman peaks at 570 and 650 cm , correspondingly, or their superposition 63 , 64 . It should be noted that the peak at 650 cm was observed in GaMnSb composites with MnSb and Mn Sb inclusions 58 , and it appears to be more intensive in the sample with higher crystallinity.…”