Page 64 - Nanotechnology an introduction

6.1. Nanoparticles
Currently one can use either a top–down (comminution and dispersion, see Section 6.1.1) or a bottom–up (nucleation and growth, see Section
6.1.2) approach. The decision which to adopt depends, of course, on which can deliver the specified properties, and on cost. A third approach is
bottom-to-bottom mechanosynthesis; that is, “true” nanofacture, according to which every atom of the material is placed in a predefined position. At
present this has no commercial importance because only minute quantities can be prepared. Nevertheless, it is the only approach that would
forever solve the imperfections associated with top–down and bottom–up processes (the presence of defects, polydispersity, etc.).
Despite being the largest current commercial segment of nanotechnology, nanoparticles have as yet relatively few direct large-scale commercial
uses (excluding photographic emulsions and carbon black, both of which long precede the nano-era); mostly their applications are in composites
(i.e., a mixture of component A added to a matrix of component B, the latter usually being the majority component)—a nanocomposite differs from a
conventional composite only insofar as the additive is nanosized and better dispersed in the matrix. Applications such as reagents for the
remediation of contaminated soils (e.g., iron nanoparticles for decomposing chlorinated hydrocarbons) are being investigated, although the
present very imperfect knowledge about the effects of such nanoparticles on soil ecosystems is preventing rapid exploitation in this area.

6.1.1. Comminution and Dispersion

This top–down approach involves taking bulk material and fragmenting it. Crushing and grinding have typically been treated as low-technology
operations; theoretical scientists seeking to formalize the process beginning with the formulation of mechanistic phenomenological rules (e.g., the
random sequential fragmentation formalism) have hitherto had little industrial impact. The main advantages are universality (i.e., applicability to
virtually any material) and low cost. Even soft organic matter (e.g., leaves of grass) can be ground by first freezing it in liquid nitrogen to make it
brittle.

The main disadvantages are polydispersity of the final particles, the introduction of many defects (including contamination by the material used to
make the grinding machinery—the smaller the particles the worse the contamination because it is introduced at the surface of the particles) and the
impossibility of achieving nanoscale comminution, depending on the material: as a compressed brittle body is made smaller, its fracture strength
increases until at a certain critical size crack propagation becomes impossible [90], cf. Section 2.7. This explains the well-known size limit to the
crushing of materials—typically above the nanoscale (e.g., around 0.8 μm for calcium carbonate).

Crushing and grinding are venerable industrial processes in the form of hammering, rolling or ball-milling, but the advent of nanotechnology has
given rise to novel, very well-controlled methods of achieving monodisperse nanoparticle generation by comminution and dispersion. One such
process is electroerosion dispersion (EED) [120], in which granulated metal is ground into a fine powder by electrical discharges—typically a few
hundred volts are discharged in a microsecond. The plasma temperature in the discharge filament is 10,000 to 15,000 K, sufficient to melt any
metal.

6.1.2. Nucleation and Growth

This process involves a first order phase transition from an atomically dispersed phase to a solid condensed phase. During the first stage of the
transition fluctuations in the homogeneous, metastable parent phase result in the appearance of small quantities of the new phase. The unfavorable
process of creating an interface opposes the gain in energy through the reduction in supersaturation of the parent phase, leading to a critical size of
nucleus, n*, above which the nucleic develop rapidly and irreversibly into the new “bulk” (albeit nanoscale) phase. Many syntheses were reported in
the 19th century (e.g., [50] and [63]).
When atoms cluster together to form the new phase, they begin to create an interface between themselves and their surrounding medium, which
costs energy. Denoting the interfacial tension by γ, and using subscripts 1 and 2 to denote the new phase and surrounding medium, respectively
(see Section 3.2), the energy cost is A , where A is the area of the cluster's surface, equal to (4π) {1/3} (3nv) {2/3} , where n is the number of atoms in
γ 12
the cluster, and v the volume of one atom. At the same time each atom contributes to the cohesive energy of the new phase. Summing these two
contributions, at first the energy will increase with increasing n, but ultimately the (negative) cohesive energy of the bulk will win (Figure 6.3).

Figure 6.3 Sketch of the variation of free energy of a cluster containing n atoms (cf. Figure 2.2). The maximum corresponds to the critical nucleus size. Clusters that have managed through fluctuations to climb up the free energy slope
to reach the critical nucleus size have an equal probability to shrink back and vanish, or to grow up to microscopic size.
In order to synthesize nanoparticles via nucleation and growth, firstly the atoms are dispersed (dissolved) in a medium under conditions such that
the dispersion is stable. Then, one or more of the external parameters is changed such that the bulk phase of the material now dispersed is stable.
This could be accomplished, for example, by cooling the vapor of the material. The formation of the new bulk phase is a first order phase transition
involving nucleation. Chance fluctuations will generate critical nuclei (see Figure 6.3).
Compound particles can be synthesized by chemical reaction [135]. Suppose the formula of the desired substance is MX, where M represents a
metal such as silver or cadmium, and X a metalloid such as sulfur or selenium. One then prepares two solutions of soluble compounds of M and X
(for example, silver nitrate and sodium sulfide), which are then mixed together. Figure 6.4 and Figure 6.5 show some examples.