STRUCTURES SMALL ENOUGH TO BE DIFFERENT (AND USEFUL) What do we know about materials at this point? We know how they are held together in

molecular form and in larger, more organized solid crystals. We know about van der Waals forces and their drastically heightened importance at the atomic and molecular scale. We know about the various forms crystals can assume. We have spent the time to gain an understanding of these fundamental properties of materials for one reason: to understand how these properties change as the amount of material diminishes, even all the way down to individual atoms, and how very small pieces of material acquire unique properties and, therefore, unique uses.

Our discussion will focus on stable structures with features so small they give materials unique and useful physical properties.

A lot of information is contained within this statement. Let us examine it piece by piece. Stable structures are not gases or liquids, lacking consistent organization. A stable structure is organized, typically as a solid or as a specialized type of molecule. Or even as an ordered array of just a few atoms. A stable structure can be synthetic or naturally occurring. The important thing is that the features of the structure are small (typically ranging from micrometers to picometers), and the very size of these features endows the structure with unique and useful physical properties—properties that do not exist at larger (or smaller) sizes.

A basketball-, a marble-, and a pollen-sized ball of gold vary only in size. They all melt at 1064°C. They are all of the color gold. However, a ball of gold just a few nanometers in diameter melts at about 750°C. And a liquid filled with these nanometer-sized balls turns the liquid red, not gold. Gold particles are just one of the many structures having unique and usable properties owing solely to their small size.

No matter how small a structure is, some of the things we have learned about materials are still the same. Covalent, ionic, metallic, and van der Waals bonds still hold everything together. (In both a small and a large piece of graphite, covalently bonded layers of carbon are held in a stack by weak van der Waals forces. See Figure 4.8.) Crystals still form with the usual 14 lattice structures, even when there are limited numbers of atoms; however, the lattice structure of a small piece of material can be different from the lattice structure of the bulk material.

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4.4.1 Particles

Tiny particles of matter with diameters typically ranging from a few nanometers to a few hundred nanometers have distinctive properties. Such a particle can be made from a single crystal, an aggregate of crystals, or be completely noncrystalline; it can be a metal, a semiconductor, or an insulator. Particles with sizes larger than typical molecules but too small to be considered bulk solids can exhibit hybrid physical and chemical properties. The smaller a particle, the higher is its surface-to-volume ratio. In Chapter 2 we learned that the surface-to-volume ratio scales as 1/D, where D is the characteristic dimension of the object (see Equation 2.3). This physical characteristic of particles can affect the way they behave with one another and with other substances.

In chemical and physical processes, only the surface of an object is exposed to the reac-tion and participates in the process. In a lump of sugar, a small percentage of the sugar molecules are on the surface, whereas powdered sugar has a high percentage. This is why a 1-g lump of sugar takes a long time to dissolve in water compared to the same mass of powdered sugar, which has more molecules in direct contact with the water.

While a bulk solid material will typically have less than 1% of its atoms on the surface, a particle can be so small as to have over 90% of its atoms on the surface. As more atoms are

FIGURE 4.8 The structure of graphite. Weak van der Waals forces, represented by dashed lines, hold covalently bonded layers of carbon together.

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added, this percentage drops: a particle with a 10-nm diameter has about 15% surface atoms;

a 50-nm particle has about 6% surface atoms. A few examples of particles with high surface-to-volume ratios are shown in Figure 4.9. The close packing of the atoms in these structures can be the result of either hexagonal or cubic crystal structures. As we see in Figure 4.10, there is a sharp decline in the percentage of surface atoms with increasing particle size.

For a given material, the crystal structure of a particle is not necessarily the same as the structure found in the bulk. Ruthenium particles 2–3 nm in diameter can have BCC and FCC structures not found in bulk ruthenium. Platinum and indium can also take on different crystal structures in particle form.

van der Waals forces dominate the interaction of particles, which behave almost like atoms.

The high surface-to-volume ratio of particles also makes them inherently more reactive as catalysts in chemical reactions, a property exploited by engineers to quicken commer-cial chemical production. Atoms at the corners and edges of the particles are even more reactive than those along surface planes. As we can see in Figure 4.9, smaller particles have more of their atoms at edges and corners than larger particles do. Gold is a useful catalyst in numerous chemical reactions, including oxidation processes crucial to production of

FIGURE 4.9 Particles. The fewer the atoms in the particle, the higher the percentage of atoms on the surface, endowing such particles with heightened chemical reactivity and size-dependent melt-ing properties.

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agrochemicals, pharmaceuticals, and other products. As part of one such process, gold particles 2–15 nm in diameter are small enough to squeeze into specific locations along certain long-chain hydrocarbon molecules and insert a single oxygen atom—making the process more efficient and eco-friendly because the harsh chemicals traditionally used, such as chlorine and organic peroxides, are no longer necessary.