(81d) Worm-Like Micelle Assisted Synthesis of Iron Oxide Nanorods

For synthesis of nanoparticles, when assisted with a soft template, the morphologies of the micelles play an important role in evolution of shape of the nanoparticles. Depending on the concentrations of surfactant and counter-ions as well as that of the solvent, micelles can exhibit range of morphologies. Here we discuss about anisotropic iron oxide nanoparticles being synthesized by the direct reaction of iron chloride salt (FeCl₃.6H₂O) with Ammonium Hydroxide (NH₄OH) in a cetyl trimethyl ammonium chloride (CTAC) – sodium salicylate (NaSal) worm-like micellar system. It has been observed that the presence of a worm-like micellar phase is critical in the formation of these anisotropic nanoparticles. When NaSal is below the critical concentration required for the formation of worm-like micellar phase, only spherical particles were obtained. Addition of precursors to the surfactant system leads to formation of spherical nanoparticles at short times, which then possibly coagulate and consolidate along a surfactant backbone to form nanorods. Interestingly, when pre-formed spherical nanoparticles are added to a worm-like micellar system, similar nanorods are observed.

Even when the precursors are made to react in the worm-like micellar phase, the system needs to be sheared continuously as it ages. Spherical particles are obtained if the system does not experience any shear as it ages. The lack of continuous shear during the aging process may be hindering with the particle - particle interactions in order for the spherical nanoparticle to grow into nanorods. The initial study of the formation process using transmission electron microscopy indicates that the nanorods are formed by anisotropic coagulation and consolidation of spherical nanoparticles. We are now conducting transient studies of the system and analysing the same with a cryo-TEM in order to understand as well as capture the growth mechanism.

The system has also been subjected to controlled shear (0 – 32 s^-1) in a Linkam shear cell in order to examine the effect of shear on particle morphology. We observe a change in number yield of anisotropic particles as well as their morphology with a shift from lower to higher shear rate. The cross over frequency seems to plays a significant parameter with respect to effect of shear on the morphology. The controlled shear based experiments provide us with data that can help us explore scale-up for synthesis of anisotropic particles using soft templates. 

We have further analysed the system with varying concentration of precursor in the system and notice there is no drastic change in the size. Our group has attempted simulation to observe the effect on particle size with increase in initial loading but experimentally above a critical concentration, the nanoparticles precipitate out of the micellar system. We are exploring the effect of WLM concentration in the system in order to increase the particle loading capacity of the system as well.