We are principally interested in exploring the interface between traditional inorganic chemistry and materials science. To this end, techniques for studying the reactivity, structure, and bonding of inorganic complexes, which have their roots in traditional inorganic chemistry, are applied to important classes of inorganic materials. From the understanding gleaned from such studies it is hoped that new and potentially useful materials can ultimately be engineered.
A particular class of materials that are both technologically important and amenable to study by our approach are glasses derived from the sol-gel process. This process utilizes the condensation reactions of silicon or metal alkoxides to afford a low temperature, synthetically flexible route to inorganic glasses and ceramics. For the synthesis of a silicate glass, for example, tetramethylorthosilicate is simply treated with water which causes it to gel.
The gelled materials are then aged and dried producing a material known as a "xerogel". Xerogels have a number of unique properties. They are hard and optically transparent much like conventional silicate glass, however, they are also highly porous, allowing small molecules to permeate them.
By exploiting the synthetic flexibility of the sol-gel process we have succeeded in incorporating vanadium oxide transition metal centers into the silica matrix.
The resultant material has remarkable properties which are a hybrid of the reactive oxovanadium center and the inert silica matrix. For example, it is hard and optically transparent, yet since the pore structure of the xerogel allows small molecules to penetrate, chemical reactions can take place inside the matrix. Our studies of these hybrid materials have uncovered a broad range of interesting and diverse reactivity; much of what may prove useful in a number of applications. Ongoing areas of study in our laboratory include:
Coordination Processes/Chemical Sensing. The vanadium center forms coordination complexes with small molecules. The process of coordination is frequently reversible and is often associated with strong color changes in the bulk glass. For example, water turns the material orange, hydrogen sulfide turns it amber, and formaldehyde turns it green. Experiments are being carried out to fully understand this process so that these materials may ultimately find application as chemical sensors for pollution detection and process monitoring.
Heterogeneous Catalysis. The oxovanadium center catalyses a variety of thermal and photochemical transformations including the oxidation of both aliphatic and aromatic hydrocarbons. The inherently superior optical properties of the hybrid xerogel material makes it a unique medium with which to study these reactions. We have been able to spectroscopically characterize the oxovanadium active site establishing for the first time its detailed electronic structure. Using various spectroscopic techniques we are conducting investigations into the mechanism of specific catalytic transformation.
Polymer/Silica Nanostructured Materials. The oxovanadium center has also been found to photoinitiate the polymerization of vinyl and acetylenic functional groups. When photopolymerization is carried out on gaseous monomers infused into the xerogel glass, the resulting material is a nanocomposite in which an organic polymer fills the pores of the silica matrix. In this way we fabricated the first polymer/silica nanocomposite containing the conducting polymer polyacetylene. Research in our lab is currently being directed at developing new composites containing different polymers and at exploring their chemical and optical properties.
"The Low Energy, Charge-Transfer Excited States of 4-Amino-4'-nitrodiphenylsulfide," O'Conner, D.B.; Scott, G.W.; Tran, K.; Coulter, D.R.; Miskowski, V.M.; Stiegman, A.E.; Wnek, G.E., J. Chem. Phys. 97, 4018 (1992).
"Vanadia/Silica Xerogels and Nano-Composites," Stiegman, A.E.; Eckert, H.; Plett, G.; Kim, S.-S.; Anderson, M.; Yavourian, A., Chemistry of Materials 5, 1590 (1993).
"Ultraviolet and Vacuum-Ultraviolet Radiation Effects on spacecraft Thermal Control Materials," Stiegman, A.E.; Liang, R.H. in The Behavior of Systems in the Space Environment, DeWitt, R.N.; Duston, D.; Hyder, A.K. editors; NATO ASI Series, Vol. 245, Kluwer Academic, Dordretch, 1993.
"Electronic Structure of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix: Implications for Catalysis and Photocatalysis," Tran, K.; Hanning-Lee, A.; Biswas, A.; Stiegman, A.E.; Scott, G.W., J. Am. Chem. Soc. 117, 2618 (1995).