Nanosystems

Discussion in the previous section clearly leads to the conclusion, that in the precipitation process nanoparticles represent either a transition stage or they are building blocks in the formation of colloids. In view of this observation, a general comment is in order. Due to the preponderant present interest in nanomaterials, it is noted that "nano" is often unjustifiably used or emphasized. Larger particles, built of small subunits, should not be considered as nanosystems, because the properties of the latter are lost. Instead, the designation must be reserved for fully dispersed nanometer particles. Nanocomposites should represent systems in which unaggregated nanoparticles are incorporated in a medium, such as in a polymer film, so that the properties based on their smallness can be utilized.

The understanding of the aggregation mechanism can help in the design of methods for the preparation of nanoparticles, which may proceed in two directions. The first is to prevent the aggregation stage in the precipitation process, commonly accomplished by the addition of stabilizers, such as surfactants or macromolecules. The second, a more rarely described case, is to peptize the colloidal particles built from a nano precursor. Matijevi'c's group has followed both routes. Figure 5 illustrates recently obtained silver particles, prepared in collaboration with Dr. Sondi, using highly concentrated silver nitrate solutions in the presence of a reducing agent and a polymeric sulfonate stabilizer. In contrast, nanosized indium hydroxide was produced by peptization of internally aggregated colloidal particles of the same composition.

Films of fully dispersed nanoparticles are of interest in many applications, such as in the production of color filters, of conducting layers, or in photolithography. However, the preparation of such thin films is not a trivial project, especially since hydrophilic particles need frequently to be incorporated in nonpolar media, without causing coagulation of the former. The task can be accomplished by modifying the nanoparticle surfaces by thin coatings, making them compatible with the carriers, as was done with 8-11 nm silica for the use in the encapsulated inorganic resist technology.

One special feature of such films is their transparency, which was documented with highly conductive polymer layers containing nanosized indium-tin oxide particles. In another example, fully dispersed nano-pigments in polymers have resulted in transparent films of very high color purity. Figure 6 shows the structure of such a color filter containing 60% of the pigment, yet still being 100% transparent (Figure 7).

The described systems represent just a few examples of the many projects that are being investigated by Professor Matijevic' and his collaborators. They may also help one to understand why a lifetime can be spent in the field of fine particles. The latter certainly offer exciting challenges, but also cause a sense of satisfaction because of their usefulness in numerous applications that are beneficial to our well-being.

Figure 5. TEM of nanosized silver particles.

Figure 6. SEM of a fully dispersed red pigment in a polymer matrix. The pigment was prepared by interacting D&C red dye with nanosized titania particles.

Figure 7. Transmittance spectra of the film prepared with the pigment shown in Figure 6.

For more information about Professor Matijevic' and his research, you may call him at 315-268-2392 or send e-mail to metcalf@clarkson.edu.

Some relevant literature

Fine Particles: Synthesis, Characterization and Mechanisms of Growth, T. Sugimoto, Ed., Marcel Dekker, New York, (2000).

E. Matijevic': "Uniform Colloid Dispersions - Achievements and Challenges". Langmuir, 10, 8-16 (1994).

E. Matijevic': "Preparation and Properties of Uniform Size Colloids". Chem. Mater., 5, 412-426 (1993).