In chemistry, size does matter

UC DAVIS (US) — Particle size has a far more dramatic impact on chemical reactivity than previously thought, according to a new study.

The findings may have implications for understanding a wide range of vital chemical reactions, from rusting iron to the origins of life, say researchers who studied the energy changes involved in oxidation and reduction reactions in oxides of transition metals.

Results were published Oct. 8 in the journal Science.

“Oxidation and reduction reactions are the energy source for most chemistry in nature,” says Alexandra Navrotsky, who directs the Nanomaterials in the Environment, Agriculture and Technology program at University of California at Davis.

Metals such as iron, manganese, cobalt, and nickel can combine with different numbers of oxygen atoms to form compounds with different “oxidation states” and properties.

Different crystal structures correspond to different oxidation states: metals in the lowest oxidation state; an intermediate structure called a spinel; and rock salt oxides at the highest levels of oxidation.

Using very accurate measurement of the energy changes involved in changing from one oxidation state to another, Navrotsky’s team made two major discoveries about the relationship between the size of particles and their behavior in oxidation and reduction reactions.

First, they found that the energy of oxidation varies dramatically with particle size.

Materials made of nanoparticles measured in billionths of an inch behaved quite differently from bulk materials, Navrotsky says. “This had not been predicted, nor been thought about before.”

The second major finding was that, in general, very small particles formed with a lower energy cost for a given metal in the spinel structure compared to other states.

Navrotsky says that’s because the surface energy of the crystal is lowest in the spinel form, allowing smaller particles to form.

Because metal oxides are so widespread, the discoveries have wide implications. For example, they explain why nanoparticles of wustite, an iron compound, oxidize in exactly the right way to make the heads that read hard disks.

They also could lead to new ways to make materials for energy storage or catalysis, and to new understandings of the chemical reactions that powered the first life on Earth.

The U.S. Department of Energy provided funding for the study.

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