Thermochemistry of Nanomaterials

The term “nanoparticle” or “nanomaterial” is somewhat difficult to define rigorously. A nanoparticle has dimensions somewhere in the nanometer regime, that is, a diameter of 1 to 100 nm. Thus nanoparticles span the range from clusters of atoms in solution at the small end to colloidal particles at the large end. Nanoparticles may be amorphous or consist of only a few unit cells of crystalline material. A very large fraction of their atoms are near the surface, see Figure 1⇓. Nanoparticles may be surrounded by vacuum, a gaseous atmosphere, water, or other fluid. In the natural environment, nanoparticles are generally heavily hydrated. Figure 1. Fraction of atoms within 0.5 nm of the surface of a nanoparticle as a function of its diameter. [Used by permission of the editor of Materials Research Society Symp Proc , from Navrotsky (1997), Fig. 3, p. 10.] A nanomaterial can be loosely defined to be any material containing heterogeneity at the nanoscale in one or more dimensions. In the broadest sense, then, the following are nanomaterials: phase-separated glasses or crystals with domains in the nanoregime, zeolites and mesoporous materials with pores of nanometer dimensions, clays with nanometer sized alternations of aluminosilicate layers and interlayer hydrated cations, and nanoscale leach layers at the mineral-water interface. A broad definition in the sense above emphasizes the commonality of phenomena at the nanoscale. In essence “if it quacks like a nanomaterial, it is one.” A nanomaterial is any state of condensed matter whose properties diverge significantly from those of the bulk or of molecules by the emergence of new phenomena not seen at smaller or larger scales. Such properties are related to nanoscale heterogeneity created by pervasive surfaces, interfaces, chemical variability, or pores. The exact size at which this happens depends both on the system and the property …