Synthesis and Characterization of Chemically Pure Nanometer-Thin Zero-Valent Iron Films and Their Surfaces

Abstract

The synthesis and characterization of 15, 25, 50, and 70 nm thin iron films having chemical impurities below the detection limit of various analytical techniques is reported. As established herein, the films are chemically pure and formed by electron beam deposition from inexpensive and readily available iron sources of 3N5 purity. Chemical purity of the thin films was achieved using mean deposition rates of 0.3 nm/s or higher, at which point the melting point of iron is reached at the iron source surface and a shutter is opened, from which point on the rate of transfer of impurities present in the source to the target is low enough that they are not observed in the film as confirmed via X-ray photoelectron spectroscopy (XPS), reported here for energies between 0 and 1200 eV. Nanoindentation measurements indicate the iron films to be 14 times harder than bulk iron. The iron films are shown by XPS to be coated with a 3 nm thin overlayer of Fe 3+ , which is possibly present in the form of Fe 3 O 4 , even though other forms of iron oxide are likely to be present as well, as indicated by Raman and XPS spectroscopy. Grazing incidence angle X-ray diffraction experiments indicate the presence of crystalline Fe 0 with low index faces exposed but no crystallinity of the iron oxide overlayer. Atomic force microscopy of the iron film surfaces indicates narrowing and shifts to lower heights in the height distribution of nanoscale features formed during the film deposition process as the film thickness decreases. Second harmonic generation is then used to determine that the interfacial charge density of the thinnest iron film is −0.007(3) C/m 2 at pH 7.

Energy conversion via metal nanolayers

“…In the experiments, we form single-and dual-element nanolayers from low-cost 99.95% purity iron, 99.98% Ni, 99.7% V, 99.9995% aluminum, and 99.994% chromium sources. X-ray photoelectron spectroscopy (XPS) reveals a lack of common low-boiling point contaminants like calcium, magnesium, sodium, or zinc in the iron nanolayers and shows the presence of an ~3 nm thin oxidized iron nano-overlayer (9). Grazing incidence angle X-Ray diffraction experiments indicate the presence of crystalline Fe 0 with low index faces exposed but no crystallinity of the iron oxide overlayer (9).…”

“…X-ray photoelectron spectroscopy (XPS) reveals a lack of common low-boiling point contaminants like calcium, magnesium, sodium, or zinc in the iron nanolayers and shows the presence of an ~3 nm thin oxidized iron nano-overlayer (9). Grazing incidence angle X-Ray diffraction experiments indicate the presence of crystalline Fe 0 with low index faces exposed but no crystallinity of the iron oxide overlayer (9). Control experiments show that this nano-overlayer forms spontaneously when the iron nanolayer is exposed to air and remains stable over prolonged (years) periods of time.…”

“…4 offers the following phenomenological interpretation of our findings, followed by a detailed microscopic investigation below. At the pH values used here (5.8 for low and 8.0 for high-salinity water), the water:oxide interfaces we Page 10 investigate are charged (9,(12)(13)(14)(15)(16)(17). The electrostatic potential reaches not only into the aqueous solution but also into the oxide (18,19), depending on the local dielectric properties (20).…”

“…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).…”

“…"Instant Ocean" was used as well. Second harmonic generation c(3) measurements(9,11) of the iron nanolayer indicate a negative interfacial charge density of -0.007 (3) C m -2 at pH7, consistent with a considerable number density of deprotonated Fe-OH groups at the oxide/water interface near neutral pH. The change in interfacial electrostatic potential, F (0), or "gate" voltage, estimated from Gouy-Chapman theory, would then be in the -100 mV range when changing the salt concentration from 0.1 mM to 1 M. When flowing water of alternating salinity at 20 mL min -1 across a ~10 nm thin Fe:FeOx nanolayer in the small cell, currents of ~0.2 µA (Fig.…”