Surfaces Interfaces

The surface phenomenon is one of the most common ones in nature. Everyone can see in daily life the direct consequence of surface effects: water droplets automatically tend to be in the shape of balls. Surface effects originate basically from the surface energy, which is mainly ascribed to the crystallographic asymmetry of the surface atoms bearing the noncompensated forces from other atoms. Surfaces effects become much more important when one system is of nano-capsule/nanoparticle size [79]. The smaller the size of the particles, the higher the surface energy because of the larger specific surface area. The specific surface area is defined as the surface area A per the unit volume V of matter, a as = y « d

where d is the diameter of a particle. When the diameter of the particle decreases, the specific surface area and the number of surface atoms increase evidently. For instance, when d = 10 nm, 20% of the total 3 x 104 atoms are located on the surface; when d = 4 nm, 40% of the total 4 x 103 atoms are the surface atoms. The large number of surface atoms, the high surface energy, and the lack of atomic coordination number lead to the high chemical activity of the surface atoms. Because the surface atoms possess less stabilized chemical bonds with neighboring atoms, they are very unstable and thus can combine easily with other atoms. The tendency to be stabilized results in the high chemical activity of the surfaces of the nanoparticles. It is very common for the metal nanoparticles spontaneously to burn in air. It is also a fact that the atoms at the surface of the inorganic particles could easily absorb the atoms/molecules, like gases, water, etc., in air. However, after the formation, the shells of some nanocapsules could prevent the particles from further oxidization.

The interfaces in the nanocapsules could be distinguished as: (1) the interface between different crystal grains of one phase, (2) the interface between different crystal grains of different phases, (3) the interface between the shell and the core, etc. For the small nanocapsules, it is possible that only a single phase exists in the core and the interface of the second type does not exist. In the limited case, the interface of the first type may not exist in the very small nanocapsules. In nanoparticles without the core/shell structure, the interface of the third type cannot be observed. However, in some cases, all interfaces of the three types may be observed in a nanocapsule with a large enough size.

The surfaces/interfaces play important roles in the physical properties of the nanocapsules, because the environments of the atoms at the surface/interface are totally different than those of the atoms inside the grains [80-83]. The physical behaviors of the atoms located inside the grains could be very close to those of atoms in bulk materials. The properties of the atoms at the surfaces/interfaces could differ from those of bulk materials, which are partially due to the lack of structural symmetry of these atoms, similar to what happens in thin films [80-83].

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