Research in Mineralogy and Materials Science:

Nanocrystals and the environment

Finely crystalline materials

The Banfield research group studies materials with particle diameters in the few nanometer size range.  Small particles are characterized by a large surface area to volume ratio and higher total energy due to excess energy associated with ions at the surfaces.

Why do we care about this?  Nanophase materials are extremely common products of microbial mineralization and chemical weathering reactions. These nanocrystalline oxides and sulfides (etc.) account for much of the reactive surface area in the Earth's near surface environment.  They are probably also the predominant component of Martian Dust.  Thus, nanocrystalline materials have the potential to dramatically impact the chemistry and physics of planetary surfaces.

How are small crystals different from big crystals?

The surface energy of a macroscopic crystal is an insignificant fraction of its total energy. However, this is not so for very small crystals. In fact, the phase stability of two polymorphs may be reversed because their surface energies are different. In the Banfield group, Amy Gribb and Dr. Hengzhong Zhang have worked on the size dependence of phase stability in nanocrystalline titania (TiO2).
 


Size has many other consequences for minerals and materials reactivity.  We are continuing or work to investigate how particle size affects phase transformation kinetics.  The following papers by Dr. Hengzhong Zhang and from the Ph.D. research of R. Lee Penn illustrate some of the concepts we have pursued.
 

  • Size has other implications. Small crystals grow by different pathways than big crystals. The crystal growth pathway that may predominate under some conditions has been described as "oriented attachment" or "oriented aggregation".  In this pathway, crystals no more than a few nanometers in diameter aggregate and rotate so that adjacent surfaces share the same crystallographic orientation. The pair of adjacent interfaces are eliminated and the pair of nanoparticles are converted to a larger single crystal.


    • Crystal growth in nanocrystalline materials via novel pathways has other implications.  Nanocrystal aggregation-based growth leads to formation of defects.  These include point (impurity, vacancy), line (dislocation), and planar (twins, stacking faults) defects.

    Small crystals may have crystal chemically different sites on their surfaces. Furthermore, for thermodynamic reasons, they nanocrystals may have a stronger tendency to adsorb ions onto their surfaces. We have conducted most of our work to date on synthetic TiO2 compounds.  However, recently, we have demonstrated that the concepts developed through study of this model system apply well to phenomena that occur in natural FeOOH and ZnS. Only a subset of this work is complete, and most is not yet published.

    We continue to explore size dependent properties and behavior of nanocrystals, and will focus future efforts on environmentally important compounds and processes, and on the interactions between nanocrystals and polymers (e.g., biomineralization).


    The researchers currently continuing this work include  Hengzhong Zhang Michael Finnigan, Masha Nesterova, and Forrest Huang

    Structure and microstructure analysis

    Considerable effort within our group has been devoted to identification of microstructures in minerals, and determination of their consequences for mineral reactivity and properties.


    Check out some recent publications from our group!

    e-mail me if you would like more information.

    Check out our work on Geomicrobiology and Geochemistry