Volume 44: Nanoparticles and the Environment

Jillian F. Banfield and Alexandria Navrotsky, editors

2001 i-xiv + 349 pages. ISBN 0-939950-56-1; ISBN13 978-0-939950-56-0

Description

This volume was prepared in conjunction with a short course, “Nanoparticles in the Environment and Technology,” convened on the campus of the University of California, Davis, CA on December 8 and 9, 2001.

Over the years, volumes in this series have taken a variety of forms. Many have focused on mature fields of investigation to draw together a comprehensive body of work and provide a definitive, up to date reference. A few, however, have sought to provide enough coverage of an emerging or re-emerging field to allow the reader to identify important and exciting gaps in current knowledge and opportunities for new research. This volume falls into the later category. Our primary goal in convening the short course and assembling this text is to invigorate future research.

Early Reviews in Mineralogy dealt with specific groups of minerals, one (or two) volumes at a time. In contrast, this volume deals explicitly with the topic of crystal size in many different systems. Until recently, the special and complicated nature of the very smallest particles rendered them nearly impossible to study by conventional methods. Even today, the challenges associated with evaluating the size-dependence of a mineral’s bulk and surface structures, properties, and reactivity are significant. However, ongoing improvements in sophisticated characterization, theory, and data analysis make particles previously described (often inaccurately) as “amorphous” (or even more mysteriously as “X-ray amorphous”) amenable to quantitative evaluation. Thermochemical, crystal chemical, and computational chemical approaches must be combined to understand particles with diameters of 1 to 100 nanometers. Determination of the variation of structure, properties, and reaction kinetics with crystal size requires careful synthesis of size- and perhaps morphology-specific samples. These problems demand integration of mineralogical and geochemical approaches. Thus, it is appropriate that the current issue belongs to the era of Reviews in Mineralogy and Geochemistry.

Nanoparticles and the Environment targets naturally occurring, finely particulate minerals, many of which form at low temperature. Thus, many of the compounds of interest are those of the “clay fraction”. Of course, there have been decades of critical work on the structures, microstructures, and reactivity of finely crystalline or amorphous minerals, especially oxides, oxyhydroxides, hydroxides, and clays. We will not summarize what is known in general about these (for this, the reader is referred to earlier Reviews in Mineralogy volumes). Rather, our goal is to focus on the features of these materials that stem directly or indirectly from their size.

The term “nanoparticles” is much more than a re-labeling designed to align “clay” (sized) minerals with nanotechnology and its goals. The term signifies that the substance has physical dimensions that are small enough to ensure that the structure and/or properties and/or reactivity are measurably particle size dependent, yet the particle is large enough to warrant its distinction from aqueous ions, complexes, or clusters. The chemistry, physics, and geology of particles at this intermediate scale are unique, fascinating, and important. Of particular interest are those properties that emerge only after a cluster of atoms has grown beyond some specific size, and disappear once the particle passes out of the “nanoparticle” size regime.

There are some compelling examples of size-dependent phenomena. It is well known that the melting temperature of nanocrystals (defined as crystals having properties intermediate between molecular and crystalline) decreases dramatically as the radius of the cluster decreases. Absorption and luminescence spectra for small crystals are determined by the quantum-size effect. Decreasing nanocrystal size correlates with increased total energy of band edge optical transitions. As a consequence, the color of some nanocrystals correlates strongly with their particle size.

Current world-wide interest in “nanotechnology” and “nanomaterials” offers a unique opportunity for the Earth sciences. Both the level of visibility and the explosion of synthesis and characterization techniques in physics, chemistry, and materials science provide mineralogy and geochemistry with new opportunities. It is important for us to show that the “nano” field consists of more than micromachines and electronic devices, and that nanoscale phenomena permeate and often control natural processes.

Why all the fuss about nanoparticles now? As increasing attention in engineering is focused on making smaller and smaller machines, questions about the fundamental processes that govern nanoparticle form, stability, and reactivity emerge. The geoscience community is well equipped to tackle the basic science concepts associated with these questions. However, we have our own reasons to study size-dependent phenomena.

Size-dependent structure and properties of Earth materials impact the geological processes they participate in. This topic has not been fully explored to date. Chapters in this volume contain descriptions of the inorganic and biological processes by which nanoparticles form, information about the distribution of nanoparticles in the atmosphere, aqueous environments, and soils, discussion of the impact of size on nanoparticle structure, thermodynamics, and reaction kinetics, consideration of the nature of the smallest nanoparticles and molecular clusters, pathways for crystal growth and colloid formation, analysis of the size-dependence of phase stability and magnetic properties, and descriptions of methods for the study of nanoparticles. These questions are explored through both theoretical and experimental approaches.

Nanoparticles participate in every crystallization reaction and they constitute a major source of surface area in environments where virtually every important reaction takes place on a surface. They are components of enzymes and key biomolecules and their presence may record the early existence of life. How can we not be fascinated by these remarkable, and special, forms of matter?

Jillian F. Banfield, Madison, Wisconsin, USA
Alexandria Navrotsky, Davis, California, USA
September 1, 2001

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