Structure of single sheet iron oxides produced from surfactant interlayered green rusts

Abstract

Two-dimensional (2D) single sheets formed via delamination of Layered Double Hydroxides (LDHs) are of great interest because of their high reactive surface area and unique chemical and physical properties. Here, we focus on formation and properties of single sheet iron oxides (SSI) that may be used as sorbents, electro-and photocatalysts. We have developed a method for synthesis of SSIs from the Fe II -Fe III LDH (green rust, GR) using surfactant intercalation of preformed GR, followed by aerial oxidation and delamination. The average height of the SSIs from AFM is 1 nm and the lateral dimensions vary from 20 to 100 nm. Selected area electron diffraction of SSI shows a hexagonal pattern with d ≈ 0.28 nm consistent with diffraction along [001] in metal hydroxide layers in GR. Chemical analysis, thermogravimetric analysis and X-ray photoelectron spectra analysis lead to SSI formula of FeO 0.82 (OH) 1.38 • 0.7H 2 O. For the oxidized, surfactantintercalated GR and SSI, pair distribution functions from high energy X-ray scattering data demonstrate that two distinct interatomic distances exist for first neighbor Fe-Fe pairs, somewhat similar to the patterns for ferrihydrite and goethite, but in contrast to the single Fe-Fe distance observed for the unoxidized GR. A modeled structural rearrangement with vertical displacements of Fe atoms from the original hydroxide layer to form a trilayer with Fe polyhedra that are linked via both corner-and edge-sharing produced a PDF in agreement with the main features of the measured pattern. Mössbauer spectroscopy shows that the SSI orders magnetically at 130 K and that the hyperfine parameters are distinct for SSI and different from other iron oxides such as ferrihydrite and feroxyhite. The synthesis pathway via GR C12 is critical for formation of SSI as the "insolating" C12 interlayer hinders bonding and polymerization between Fe III -O/OH octahedra across interlayers during oxidation and thus only structural reorganisations of single brucitic Fe II -Fe III hydroxide layers can take place during oxidation with no possibilities for nucleation of iron oxides with higher crystallinity like goethite. The strong increase of single-and double coordinated O/OH groups in the SSI compared with the parent GRs predicts a high reactivity of SSI as surface complexation sorbents.

Effects of metal cation substitution on hexavalent chromium reduction by green rust

“…The reduced pair distribution functions G(r) calculated for all reaction products produced in this study (Fig. 6) are nearly identical to those observed by Yin et al [56], who characterized the oxidation products of 3:1 (i.e. [Fe(III)]/[Fe(II)] = 3) oxidized green rust chloride single sheets separated by dodecanoate intercalation (singlesheet iron oxide, SSI) and green rust sulfate oxidized by an excess of Cr(VI), respectively.…”

“…Based on the XRD, PDF and XAS results, all oxidized samples appear to have maintained a layered structure composed of stacked SSI (Fig. 9) similar to those characterized by Yin et al [56], with variable coherency and partial transformation to feroxyhyte in the case of oxMg-GR. Following oxidation (regardless of mechanism), the resulting strain causes displacement of Fe(III) octahedra into an interlayer region labelled in Fig.…”

“…5) patterns indicate that there is some coherence between the stacked, oxidized layers, but the associated intersheet correlations most likely have amplitudes too low to be visible in the PDF patterns, as random Fe dislocations may remove atomic correlations while leaving the stacking in the [001] direction coherent enough to generate identifiable XRD and SAED reflections. However, there are significant differences in peak intensity from the Yin et al [56] PDFs at low R-values, particularly between 3.0 and 3.5 Å. The amplitudes of the peaks at 3.05 Å and 3.45 Å are very similar in the present study's PDF patterns while the intensity at 3.41 Å in Yin et al [56] is greater.…”

“…However, there are significant differences in peak intensity from the Yin et al [56] PDFs at low R-values, particularly between 3.0 and 3.5 Å. The amplitudes of the peaks at 3.05 Å and 3.45 Å are very similar in the present study's PDF patterns while the intensity at 3.41 Å in Yin et al [56] is greater. Oxidation of 2:1 green rust sulfate (i.e.…”

Order and Disorder in Layered Double Hydroxides: Lessons Learned from the Green Rust Sulfate─Nikischerite Series

“…The first 10 Å of the PDF for GR chloride, for example, largely resembles that of GR(Na,SO 4 ). 52 Similarly, refinement using structures for sulfate bearing GR, with 41 or without 53 interlayer Na, results in practically the same fit and high atomic displacement factors for interlayer atoms. This means that the structural analysis of the PDFs presented here is independent of the choice of model for the interlayer structure.…”