The surface activity of ultrafine particles

Within the last 20 years, advances in characterization methods, particularly in the field of high-resolution electron microscopy, have made it possible to probe the surface and internal structure of sub-100 nm particles, or nanoparticles. Such studies have indicated conclusively that surface-energy considerations in metal nanoparticles cause these particles to adopt structures which only approximate to close packing but are terminated by close-packed faces. In oxides, where stoichiometry must be maintained, the adoption of low-index crystallographic faces almost invariably necessitates the introduction of cation or anion vacancies, and both have been observed. In such cases, the structure at the edges of the particles differs greatly from that of bulk phases; and it seems highly probable that the physical and chemical properties of these particles are also different. In certain cases it appears that new structural types, found only in nanoparticulate form, may exist. The significance of these findings, particularly as regards their relevance to particulate pollutants in the atmosphere, may be of great interest.