Environmental Engineering Reference
In-Depth Information
Fig. 1.1 Intuitive representation of micro and nanoparticle fields, with examples
to self-organise based on their high reactivity. These properties characterise fractal
systems known as clusters, aggregates or filaments [
13
,
14
].
Nanoparticles are of great scientific interest, as they are a bridge between bulk
substances and their molecular or atomic structure. A bulk substance has constant
physical and chemical properties irrespective of its size; nevertheless, at the
nanoscale, these properties depend on more or less discreet molecular or atomic
phenomena. Regardless of the nature of nanoparticles, their most important phys-
ical properties are the following:
1. Surface area. It has been found that properties vary with particle size. In
sub-micrometre particles, forces that govern the atomic or molecular universe
dominate to the detriment of statistical aspects, which are revealed at the
macroscale.
2. Optical properties. Nanoparticles often have particular optical properties, as they
are small enough to limit the thickness of the common electron layer of metals;
this phenomenon generates quantum effects. Although it is common knowledge
that gold is yellow and silicon is grey, gold and silicon nanoparticles are bright
red to black. Moreover, gold nanoparticles melt at a much lower temperature
(~300
C, 2.5 nm size) than gold slabs that melt at 1,064
C. Solar energy
absorption in photovoltaic cells made of silicon-based nanomaterials is much
