Finite-size effect on magnetic properties in iron sulfide nanowire arrays

Fe(ii) and Fe(iii) dithiocarbamate complexes as single source precursors to nanoscale iron sulfides: a combined synthetic and in situ XAS approach

Nanoparticulate iron sulfides have many potential applications and are also proposed to be prebiotic catalysts for the reduction of CO₂ to biologically important molecules, thus the development of reliable routes to specific phases with controlled sizes and morphologies is important. Here we focus on the use of iron dithiocarbamate complexes as single source precursors (SSPs) to generate greigite and pyrrhotite nanoparticles. Since these minerals contain both iron(iii) and iron(ii) centres, SSPs in both oxidation states, [Fe(S₂CNR₂)₃] and cis-[Fe(CO)₂(S₂CNR₂)₂] respectively, have been utilised. Use of this Fe(ii) precursor is novel and it readily loses both carbonyls in a single step (as shown by TGA measurements) providing an in situ source of the extremely air-sensitive Fe(ii) dithiocarbamate complexes [Fe(S₂CNR₂)₂]. Decomposition of [Fe(S₂CNR₂)₃] alone in oleylamine affords primarily pyrrhotite, although by careful control of reaction conditions (ca. 230 °C, 40-50 nM SSP) a window exists in which pure greigite nanoparticles can be isolated. With cis-[Fe(CO)₂(S₂CNR₂)₂] we were unable to produce pure greigite, with pyrrhotite formation dominating, a similar situation being found with mixtures of Fe(ii) and Fe(iii) precursors. In situ X-ray absorption spectroscopy (XAS) studies showed that heating [Fe(S₂CNⁱBu₂)₃] in oleylamine resulted in amine coordination and, at ca. 60 °C, reduction of Fe(iii) to Fe(ii) with (proposed) elimination of thiuram disulfide (S₂CNR₂)₂. We thus carried out a series of decomposition studies with added thiuram disulfide (R = ⁱBu) and found that addition of 1-2 equivalents led to the formation of pure greigite nanoparticles between 230 and 280 °C with low SSP concentrations. Average particle size does not vary significantly with increasing concentration, thus providing a convenient route to ca. 40 nm greigite nanoparticles. In situ XAS studies have been carried out and allow a decomposition pathway for [Fe(S₂CNⁱBu₂)₃] in oleylamine to be established; reduction of Fe(iii) to Fe(ii) reduction triggers substitution of the secondary amide backbone by oleylamine (RNH₂) resulting in the in situ formation of a primary dithiocarbamate derivative [Fe(RNH₂)₂(S₂CNHR)₂]. This in turn extrudes RNCS to afford molecular precursors of the observed FeS nanomaterials. The precise role of thiuram disulfide in the decomposition process is unknown, but it likely plays a part in controlling the Fe(iii)-Fe(ii) equilibrium and may also act as a source of sulfur allowing control over the Fe : S ratio in the mineral products.

Characterization and Optical Properties of the Single Crystalline SnS Nanowire Arrays

The SnS nanowire arrays have been successfully synthesized by the template-assisted pulsed electrochemical deposition in the porous anodized aluminum oxide template. The investigation results showed that the as-synthesized nanowires are single crystalline structures and they have a highly preferential orientation. The ordered SnS nanowire arrays are uniform with a diameter of 50 nm and a length up to several tens of micrometers. The synthesized SnS nanowires exhibit strong absorption in visible and near-infrared spectral region and the direct energy gap E(g) of SnS nanowires is 1.59 eV.