Recent Research Developments

Nov 27, 2002
 

Growth and chemical reactivity of goethite (FeOOH) nanoparticles.

Nanoparticle growth and phase transformations are critically important to the geochemical cycling of both natural and anthropogenic chemical species. Goethite (a-FeOOH) is a common and important mineral in the biogeochemical cycle of iron and other metals at the Earth's surface. Furthermore, this mineral commonly occurs as nanoparticles in the 3-10 nm size range (e.g., Penn et al., Geology, 2000). High-resolution transmission electron microscopy (HRTEM) and low-temperature magnetometry of synthetic goethite nanocrystals show that when aqueous suspensions of goethite nanocrystals are aged at 90°C, the nanocrystals grow almost exclusively by oriented aggregation of superparamagnetic 3-4 nm primary nanocrystals (see TEM images in the figure below). Furthermore, recent work suggests that the rate of redox using hydroquinone as the reducing agent and goethite nanoparticles as the reductant is strongly particle size dependent. For example, redox reactions using either ~3.5 nm or ~30x350 nm goethite particles show that the rate of redox is 2-3 times faster (normalized to total surface area of the particles) in experiments using the smaller goethite nanoparticles. Finally, the solid product of these redox reactions is a magnetic black solid that appears to have some similarities to magnetite. Thus, redox of goethite nanoparticles in aqueous suspensions may serve as a novel route for the production of magnetic nanoparticles. Work involving the use of Mössbauer spectroscopy and HRTEM is currently underway in order to determine the structure of this product.

Figure caption: Four TEM images of goethite nanoparticles that were aged in aqueous suspension for 24 hours (top left and bottom two images) or 144 hours (top right). Note the bimodal particle size distribution. The average particle size of the smaller particles is ~3.5 nm, and our results show that this size does not increase or decrease with longer aging times. The lower two images are high-resolution TEM images. The transparent circles drawn in the right-hand image serve to highlight the organization of the primary nanoparticles in the elongated geothite nanocrystal. The fact that the lattice fringes are parallel along the entire length of the crystal indicates that the crystal orientations of the primary particles are all the same.

This work involves an interdisciplinary team of individuals:
Prof. Lee Penn from the Chemistry Department,
Graduate student Amy Anschutz from the Chemistry Department,
Dr. David Burleson from the Chemistry Department,
Post-doc Yohan Guyodo at the Institute for Rock Magnetism, and
Prof. Subir Banerjee, who is the director of the Institute for Rock Magnetism.

We have recently submitted some of these results for publication:

Yohan Guyodo, Alison Mostrom, R. Lee Penn, and Subir K. Banerjee. From Nanodots to Nanorods: Oriented Aggregation and Magnetic Evolution of Nanocrystalline Goethite. Submitted to Geology, Nov. 2002.