Templating of Metal Oxide and Oxhydroxides Nanoparticles in Surfactant Media

Abstract

For industrial objectives, synthesis of akaganéite ffl-FeOOH) and goethite ((X-FeOOH) iron oxyhydroxides with nanometre particle size and acicular morphology was targeted. Both P & a-FeOOH particles were generated via forced hydrolysis of Fe" ions in acidic media. Templated syntheses of these particles were carried out in surfactant lyotropic liquid crystal media and in aqueous glyme solutions. Partial phase diagrams for different commercial surfactantIH20 systems (Neodol (1-5E), Brij 97 and CTAB) have been examined for stability to the iron oxyhydroxide synthesis conditions (pH, ionic media and iron cations), with only CTAB/water system losing lyotropic phase integrity in the presence of iron cations.
The Neodol (I-SE) maintained an ordered lamellar (L4 phase for a surfactant molar ratio between 0.035 and 0.1, and up to 50°C. The Brij 97 gave a stable hexagonal phase from 0.014 to 0.025 mol/mol (up to about 45°C) and a lamellar phase from 0.047 to 0.1mol/mol (up to about 65°C). The stability of these polyethylene oxide non-ionic surfactant materials was examined against the different conditions that can be used for the synthesis of 13- & a-FeOOH such as pH, ionic species, Fe" and Fe" ions. The addition of Cr, NO3, S042, and P043 anions and Nat, C2t Mg2 and Cr3 cations induced a noticeable destabilisation effect at the high concentration end of the lyotropic liquid crystal phases.
Small angle x-ray diffraction was used to determine the spacing available for templated reaction. The d-spacing of the L. phase of Neodol (I-SE) varied between = 4.8 and 7 nm with concentration, which was found to be equivalent to water layer thickness of =3.3 - 5.5 nm. The Brij 97 provided a minimum templating distance of about 1.4 nm in the hexagonal phase and 1.95 nm in the lamellar.
In the Neodol (l-SE)/water surfactant system, f3- & a-FeOOH particles were characterised using FT-IR, UV-VIS, PCS, TEM and AFM techniques. The -FeOOH formed cigar-shaped acicular particles both in free solution and in the lamellar phase.
The size of particles from free solution ranged between 300 - 500 nm in length and 30 to 60 nm in width. The size of particles grown in Neodol (I-SE) / water system ranged from 25 to 40 nm in length and 4 to 7 nm in width and showed narrower size distribution, using PCS, compared to their counterparts prepared in aqueous solution. The a-FeOOH particles produced in aqueous and Neodol (1-5E) surfactant medium appeared as aggregates. The aggregates produced in aqueous solution were not uniform in shape with sizes ranging from 100 to 200 nm. The aggregates formed from the ordered surfactant
phase were elongated in shape with sizes ranging from SO to IOU nm. PCS also showed smaller size distribution of u-FeOOH from templated synthesis compared to ct-FeOOH prepared in aqueous solution. Also, the 13-FeOOH iron oxide prepared in the Brij 97 / water system appeared as aggregates under the TEM.
The effect of the ethylene oxide species, the surfactant head group, on the hydrolysis of Fe" ions and the consequent production of 13-FeOOH particles has been examined using, a series of glymes, organic molecules containing ethylene oxide species. The glymes were observed to induce faster precipitation and templating effects on 3-FeOOH particles
produced. Glymes also showed evidence of possible chelating effects influencing the hydrolysis mechanistic processes.