Properties of nanoparticles

Nanoparticles are of great scientific interest as they are, in effect, a bridge between bulk materials and atomic or molecular structures. Materials of dimensions close to the atom (≤ 100 nm) show very different properties and great potentiality. These new properties are derived from the very small size of the nanoparticles, very close to the size of the atoms (1-100 nm) and causing substantial changes in the material.

A bulk material should have constant physical properties regardless of its size, but at the nano-scale size-dependent properties are often observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of the surface in relation to the percentage of the volume of a material becomes significant.

Because almost all the atoms in a nanoparticle are on the surface, these particles have chemical and physical properties that differ from those of individual molecules or larger aggregates of atoms or molecules.

The causes of these behavioral differences in their properties are mainly two:

1. The large increase in the surface area of the nanoparticle.

2. The quantum confinement of the electrons inside the nanoparticle (new quantum effect).

Nanomaterials have a much greater surface area to volume ratio than their conventional forms, which can lead to greater chemical reactivity and affect their strength. Also at the nanoscale, quantum effects can become much more important in determining the materials properties and characteristics, leading to novel optical, electrical and magnetic behaviors.

The extraordinary surface area of the nanoparticles causes a redistribution of the atoms, increasing the fraction of atoms that are in its surface. The number of atoms in the surface grows in a parabolic form, as the nanoparticle is smaller. When the particle size is 3 nm there is about 50% of atoms on the surface and when a nanoparticle size is 1nm there is more than 90% of its atoms located on its surface. This is the main reason for the highest catalytic activity of nanoparticles in chemical and biochemical reactions.

The most popular term in the nano world is quantum confinement effect which is essentially due to changes in the atomic structure as a result of direct influence of ultra-small length scale on the energy band structure. The length scale corresponds to the regime of quantum confinement ranges from 1 to 25 nm. In which the spatial extent of the electronic wave function is comparable with the particle size. As a result of these “geometrical” constraints, electrons “feel” the presence of the particle boundaries and respond to changes in particle size by adjusting their energy. This phenomenon is known as the quantum-size effect.

Nanoparticles have unique properties compared with their bulk counterparts. Many of these properties, including physical, chemical, optical, electrical, and magnetic, can be controlled by relatively simple tuning of their sizes, shapes, compositions, protecting organic ligands, and interparticle distance.