Nanoparticle Synthesis via Lyotropic Liquid Crystals
Tim Dellinger
Former Graduate Student in Materials Science and Engineering
Lyotropic liquid crystals are made by mixing water with amphiphiles, which are molecules with hydrophobic blocks and hydrophilic blocks. When mixed with water, the hydrophobic blocks cluster together, as they are insoluble in water, and the hydrophilic blocks dissolve in the water. Because the hydrophobic and hydrophilic blocks are covalently linked, microphase separation does not occur, but rather structures with 1-10 nm periodicity can result. The packing geometry of the two portions of the amphiphile, along with the water to amphiphile ratio, determines the phase of the resulting liquid crystal. Liquid crystalline phases possible include hexagonal, lamellar, and inverse hexagonal.
The hydrophilic and hydrophobic domains of lyotropic liquid crystals have dimensions on the order of nanometers, so chemistries designed to operate exclusively in one domain of the liquid crystal are effectively constrained inside nanoreactors.
We primarily use aqueous chemistries in order to produce our nanoparticles. Bismuth, for instance, is produced via the reduction of bismuth (III) chloride to bismuth (0) metal by chrome (II) chloride.
Bi Nanoparticles and Nanocomposites through Novel Synthetic Techniques
Nanosized bismuth has attracted interest recently as a potential room temperature thermoelectric material. Bismuth is unusual in that its electrons have a very low effective mass, making their wavefunctions very large, and so bismuth exhibits quantum confinement effects in nanoparticles as large 50 nm, as the size of the particle becomes smaller than the size of what the electron's wave functions are in bulk bismuth. Further reducing nanoparticles size causes a semimetal to semiconductor transition.
Thermolectic theorists have long held that bismuth's low thermal conductivity and high carrier mobilities would make it an ideal thermoelectric material - if only the electrons would stop combining with the holes. Recent theoretical work has proposed that the semimetal to semiconductor transition could achieve this, and thus the quest for nanosized bismuth began.
Alas, bismuth nanoparticles, nanowires, and nanocomposites have proved difficult to synthesize. There is no easy surface passivation chemistry available, such as thiols passivating gold surfaces. In addition, bismuth has a very low melting temperature: 271 C, so room temperature is already 55% of the melting temperature.
Novel Synthetic Techniques
We are exploring two main synthetic routes for making nanosized bismuth. We seek not only nanosized bismuth, but perhaps just as important is developing the science of nanoparticle and nanocomposite fabrication techniques. In a certain sense, bismuth is just a Very Good Excuse to explore new nano-synthesis routes.
Professor Paul Braun • Phone: +1.217.244.7293 • Fax: +1.217.333.2736 • Email: pbraun@illinois.edu
Department of Materials Science and Engineering • University of Illinois at Urbana-Champaign