Dr. Jimmy Mills' Homepage
Interesting changes in the physical and chemical properties of metals and semiconductors take place by reducing the dimensions of these substances into the nanometer size range. These materials, called nanomaterials, exhibit unusual properties and are expected to find a variety of applications in electronic and optical devices. Research in our lab is centered on the development of new simple ways to generate nanometer-sized particles of metals and semiconductors, and on their uses in adaptive systems, fuel cells, diodes, as well as in thermal and photochemical catalysis. For example, quantum-sized CdS crystallites with a diameter of 4 nm have been prepared inside the micropores of hollow polymer fibers. Since the insulating polymers can be converted into semiconducting materials with simple chemical steps, the goal of this research is the generation of hybrid inorganic-organic semiconductors.
One of the methods for the preparation of small metal crystallites that we have developed is based on the spontaneous reduction of metal ions by solvent molecules in basic solutions of alcohols, or by polymers such as poly(ethylene glycol)s in water. We have used these methods to prepare stable colloidal metals and alloy particles having superlattice structures, or to deposit metal films on several supports. A very interesting procedure is the light initiated formation of metal crystallites inside gels of poly(DADMAC) made by swelling polymers of diallyldimethylammonium chloride with methanol. Small Au or Pd particles are generated by photoreactions of metal ions present in the gels, but the particles decay in the dark to reform the starting metal ions. The gels are a new class of photoadaptive, or "smart", systems that change their properties reversibly as a response to external stimulus. A qualitative energy diagram of the reversible behavior of gold crystallites is presented in the scheme. Small metal crystallites are a metastable form of the bulk metal, they agglomerate and precipitate in solutions containing high chloride ion concentrations. In the gels particle growth is inhibited but the crystallites are unstable toward oxidation promoted by chloride ions. Presently we are investigating the properties of gels swollen with methanol/water mixtures, which exhibit a novel thermoadaptive behavior. Au particles are stable in these gels at 25°C; they decay at high temperatures and reform when the gels are cooled back to room temperature.
Another area of interest is the transformation of pollutants into desirable chemicals initiated by illumination of semiconductor particles. In general, these photoreactions proceed with low efficiencies, but we have shown that efficient free radical chain reactions, such as the reduction of CFCs to form HCFCs, can be initiated under conditions favoring phase transfer photocatalysis, and proceed with high photonic efficiencies. The degradation of toxic chemicals present in air is also being investigated using semiconductors grafted to textiles. These materials are a new class of protective clothing that degrade toxins adsorbed onto cotton surfaces. Semiconducting materials that attack toxic chemicals via thermal as well as photochemical processes have also been developed, and their reactivity is currently under investigation.