Synthesis and Characterization of Carboxylate−FeOOH Nanoparticles (Ferroxanes) and Ferroxane-Derived Ceramics

Abstract

A new environmentally benign process for producing Fe-based ceramics of variable properties is presented. We have demonstrated that the reaction between lepidocrocite and acetic acid (=AA) in water results in the formation of carboxylate−FeOOH nanoparticles called ferroxane−AA analogous to aluminum-based alumoxanes. The structure of the ferroxane particles consists of an FeOOH core part with the structure of the lepidocrocite (γ-FeOOH), coated with AA. FTIR and EXAFS at the Fe K edge indicates that AA is chemically adsorbed onto the FeOOH core part. The size of these ferroxanes is 0.3 μm composed of nanodomains of 20−30 Å with a γ-FeOOH structure. Thermolysis of the ferroxane−AA yields Fe oxide ceramics. The specific surface area of 134 m2/g does not increase from the initial mineral to the ferroxane while the pore size distribution becomes monodisperse and the diameter of the pore size decreases from 55 to 12 and 13 nm after the firing of the ferroxanes. Moreover, the ferroxane−AA has been successfully doped with metal to form a precursor for Fe mixed-metal oxides. Upon thermolysis the products evolve to homogeneous Fe−metal oxides. A low firing temperature for conversion to ceramic as well as the use of environmentally benign feedstock suggest that the process for creating ferroxane-derived ceramics should have minimal environmental impacts.

Conductivity at Low Humidity of Materials Derived from Ferroxane Particles

“…The powders have the crystallographic structure of lepidocrocite (γ-FeOOH) both before and after the treatment with acetic acid, which is consistent with previous literature reports [6]. After sintering at 300…”

“…The dispersion is again stirred at room temperature for 12-20 hours and centrifugated for 1-2 hours at 4000 rpm. The reactions taking place have been described by Rose et al [6].…”

“…The lepidocrocite (γ-FeOOH) and ferroxane powders are prepared by a synthesis procedure based on what is reported in the literature [4,6,7]. The synthesis procedure is shown in Figure 1 NaOH] = 0.6, which is reported to favor the formation of pure lepidocrocite [6,9].…”

“…The synthesis procedure is shown in Figure 1 NaOH] = 0.6, which is reported to favor the formation of pure lepidocrocite [6,9]. This produces a suspension having a dark green colour, which is stirred at room temperature for 12-20 hours, until it turns orange in colour.…”

“…During the last decade materials derived from carboxylicacid-stabilized γ-FeOOH nanoparticles (ferroxanes) have been investigated as proton conductors by Tsui et al [4] Tsui and Wiesner [5], Rose et al [6]. These studies report conductivities up to 2.65 · 10 −2 S cm −1 at room temperature [4] and for some of the materials investigated the conductivity is reported to be almost independent of the relative humidity in the range 30-100% at room temperature (equivalent to p H2O = 0.0096-0.032 atm).…”