Chemistry Reference
In-Depth Information
3. Their synthesis may be achieved by either solution or solid-state methods. Many
nanomaterials are made directly as dry powders, and it is a common myth that
these powders will stay in the same state when stored. In fact, they sometimes
rapidly aggregate through a solid bridging mechanism in as little as a few
seconds. The dangling bonds at the surface of the nanoparticle make them
reactive. If the nanoparticles need to be kept separate, then they must be
prepared and stored in a liquid medium designed to facilitate sufficient inter-
particle repulsion forces to prevent aggregation.
4. If the nanoclusters are made in solution, then their properties depend on the
ability of the ligands to co-ordinate to the surface of the particle. Strong
chelating ligands lead to more clearly defined materials.
5. Some nanoparticles may be re-dissolved to give clear solutions in organic
solvents and water.
6. They have reproducible electronic and catalytic properties, which depend
closely on the sizes of the particles.
Nanoparticles may be synthesised either by attrition methods which involve the
fragmentation of bulk solids (top-down) or by pyrolysis, inert gas condensation,
solvothermal reactions and sol-gel fabrication (bottom-up) techniques. The attri-
tion methods involve grinding bulk solids by ball mill, planetary ball mill or
related size reducing techniques. These result in a broad distribution of particle
sizes - 10-1,000 nm - and a variety of shapes. Applications include nano-
composites and nano-grained bulk materials. Bottom-up methods may be executed
in the gas and liquid phases. For metals this involves evaporation followed by
gas-phase reactions. Nucleation/condensation at the substrate interface results in
agglomeration and grain growth. Liquid-phase fabrication involves the initial
reaction in the liquid phase followed once again by agglomeration and grain
growth. The liquid-phase methods include hydrothermal and solvothermal
methods, sol-gel methods and micro-emulsion syntheses in structured materials.
Pyrolysis involves the evaporation of a volatile precursor and it is then forced
through a hole or opening at high pressure and burned. Instead of evaporation a
thermal plasma, which achieves temperatures of 10,000 K, can cause the evapo-
ration of small (micrometre) particles, which on cooling form nanoparticles. The
plasmas may be dc plasma jet, dc arc plasma and radio-frequency induction
plasmas.
Nanoparticles may be classified according to their dimensionality, morphology,
composition, uniformity and degree of aggregation. The definition of the dimen-
sionality of colloids has caused some confusion and it is generally more common to
define the dimensionality (nD) in terms of the number of pseudo-infinite axes.
Therefore a nanosurface is 2D and a nanoparticle is 0D. 1D nanomaterials are one
dimensional in the nanometre range. Their applications in electronics include
circuitry in computer chips and anti-reflection coatings on lenses. 2D nanomaterials
frequently have their nanostructures attached to a substrate or nanopore filters used
for molecular separation. Asbestos fibres represent examples of 2D nanoparticles.
The difference between nanorods and nanowires is subtle, since although the
former are typically thicker and shorter than the latter there are regions where the
characteristics overlap.
