Dendritic Oxide Growth in Zerovalent Iron Nanofilms Revealed by Atom Probe Tomography

Abstract

Atom probe tomography (APT) analysis of chemically pure nanofilms of zero-valent iron (Fe(0), or ZVI) and their thermal oxide nano-overlayers reveals the presence of dendritic iron oxide features that extend from the oxide nano-overlayer surface into the ZVI bulk. The dendrites are observed by APT to be in the 5 nm x 10 nm size range and form quickly under natural atmospheric conditions. Their growth into the ZVI lalyer is, within the limit of our threemonth long study, self-limiting (i.e. their initial growth appears to quickly discontinue). The atomistic views presented here shed first light on the atmospheric corrosion process of Fe(0)bearing engineered nanostructures and their surfaces in the limit of low bulk impurities. Possible roles of the newly identified oxidized iron dendrites are also discussed in the context of passivation processes limiting technological applications of Fe(0).

Energy conversion via metal nanolayers

“…The sharper the gradient, the larger the current density, j. Dendritic iron oxide features of ~5 nm x ~10 nm size (Fig. 5A) that extend from the surface into the bulk of the iron metal nanolayer, as revealed by APT (10), may open possibilities for an electron and/or hole ratchet, similar to what has been proposed for low-light energy-driven transducers (25), or pose limits due to tunneling losses. Structures whose oxide nano-overlayers contain only a single oxidation state, such as those formed from Al or Cr metal, should still produce currents due to contact electrification, but the lack of intra-oxide electron transfer would diminish their current output.…”

“…Raman and XPS spectroscopy of the iron nanolayers indicate that the oxide nanooverlayer is composed of some Fe (III), Fe3O4, and other forms of iron oxide (9). Given nm-scale spatial variation of the oxide nano-overlayer thickness revealed by atom probe tomography (APT) reconstruction (10) described in more detail below, we expect corresponding heterogeneities in the electrostatic potentials -and charge distributionsin the metal below as well (see simulation results below). We prepared Fe:FeOx nanolayers having 5, 10, 20, and 50 nm thickness, which differ in their transparency (Fig.…”

“…Methods. The nm-thin iron layers and their oxide nano-overlayers were prepared on glass microscope slides (VWR) and characterized as described in our previous work (9,28). We used computer controlled multi-channel Ismatec peristaltic pumps (ISM4408).…”