How size affects particles in the nanoworld

For ultrasmall particles, size is as important as chemical composition. The mobility of atoms on a particle's surface can make a brittle material ductile (malleable). Particles of identical composition but that are slightly different in size can have distinctly different physical and chemical properties. Scientists exploring the size-properties connection are finding that familiar materials they think they understand can behave in unexpected ways.

Such is the nanoworld, where things are measured by the nanometer. That's one billionth of a meter. It's about the length of six carbon atoms bonded together. Strange things can happen with particles that measure between a few to a couple of dozen nanometers in at least one dimension.

Researchers with the National Institute of Standards and Technology (NIST) found this out as they studied silica, a brittle material. As a NIST announcement explained last month, experiments had shown that silica became "as ductile as gold at the nanoscale." Subsequent computer simulations of nanoparticle aggregates now illustrate how this happens. They also suggest that the smaller the particles in the material aggregate, the more ductile the material becomes. Crystalline structures, in particular, should withstand stress well beyond the critical point seen in macroscopic samples of the material.

A material is better able to withstand stress without cracking when its atoms can move around and maintain cohesion. Brittle materials have structural flaws that limit that mobility and act as failure points. Atoms on a nanoparticle's surface are less constrained than they are in bulk material. This dominates the particles' physical properties and makes a normally brittle material ductile. Also the nanoparticles don't have the bulk materials' structural flaws.

NIST ductility researcher Doo-In Kim notes: "The terms 'brittle' and 'ductile' are macroscopic terminology. It seems that these terms don't apply at the nanoscale."

Meanwhile, earth scientists have realized they need to learn more about nanoparticle behavior, especially in mineralogy, to fully understand what's happening on our planet. Reviewing this challenge last month in Science, Michael Hochella at Virginia Tech in Blacksburg and several coauthors point out that minerals influence most of Earth's physical, chemical, and biological processes. Minerals exert that influence through a wide range of compositions and properties. That's well known on the macro scale.

What's new now, the authors say, is that "we are gaining a much better appreciation for another aspect of mineral diversity – that which is expressed in the nanoscale size range." They add that an important aspect of that diversity is the ability of nanoparticles to "express a range of physical and chemical properties depending on their size and shape."

Very little is known about the influence of this nano diversity in nature. There may be specific size and shape configurations of specific mineral particles that favor or inhibit important phenomena. The best-known example is ozone-layer destruction in polar regions. There, atmospheric pollutants are transformed into ozone-destroying chlorine compounds on the surfaces of tiny particles suspended in the stratosphere.

NIST nanoparticle scientists hope their work eventually leads to new materials for applications in our macroscopic world. Dr. Hochella and his coauthors feel a greater sense of urgency. Given the possibility of serious environmental impacts, they call the need to understand the various geophysical effects of natural and manmade nanoparticles "critical challenges" for our time.