The research focus of my group is the characterization of the structure and dynamics of macromolecules and their multi-component systems. The goal is to understand molecular level structure/properties of these systems and correlate them with the materials macroscopic properties. While the primary tool for these investigations is solid-state NMR, the complimentary techniques of FTIR, fluorescence, powder X-ray diffraction and scanning electron microscopy are also used in the characterization of materials. With solid-state NMR (SSNMR), materials are studied in their functional forms as solids, powders, films and fibers without molecular weight and solubility restrictions.
Polymer - Nanoparticle Composites for Photovoltaics
Hybrid photovoltaic (PV) devices consist of three components: semiconducting nanoparticles or films, a capping agent and a conjugated polymer. While there are many benefits to this device design, currently, these PV systems suffer from low power conversion efficiencies, and these systems are poorly characterized. Our recent research efforts have focused on composites consisting of CdS nanoparticles, capping agents that make the nanoparticles water-soluble and conjugated and functionalized water-soluble polymers. The structures of the nanoparticles, polymer and the nanoparticle - polymer interface of these materials are being investigated by using a variety of state-of-the-art 1H, 113Cd, 77Se, 15N and 13C solid-state NMR techniques. The structure determined by SSNMR is being correlated with the materials optical properties, determined from absorption and fluoroescence methods. These results are being used to aid in the design and selection of PV materials with enhanced power conversion efficiencies.
Figure 1. High-resolution TEM (HRTEM) micrograph of CdS nanoparticles capped with thioglyceral.
Figure 2. 13C SSNMR spectra from CdS-thioglyceral/polystyrene and CdS-thioglyceral/polyvinylpyrrolidine composites.
Beyond providing water solubility to the nanoparticle and preventing surface reactions from occurring, the capping agents are also involved in the electron transfer between the conjugated polymer and nanoparticles. Capping agents that include conjugated and aromatic groups are being investigated as a means to enhance the electron transfer rates. This includes capping agents with specific functional groups that are being paired with polymers containing complimentary functionalities that promote specific intermolecular interaction between these two compounds. Our characterizations have revealed that in these systems the polymer interacts more strongly with the nanoparticles and produce better PV materials.
Figure 3. Conjugated polymer/CdS-capping agent composite with complimentary functional groups.
Figure 4. Model of the structure of the interface for CdS-thioglyceral/polystyrene sulphonate composite based on solid-state NMR characterization.
Nanometer diameter fibers of alumina (Al2O3) and other metal-oxides are being produced by using electrospinning techniques, for use in the adsorption and decomposition of chemical warfare agents (CWA) and as the electrolyte in Li ion batteries. Simulants of CWA's are reacted with nanofibers calcined under different conditions to determine the effect of processing conditions on the efficacy of CWA simulant detoxification. Characterization of the bulk and surface portions of the nanofibers, as well as the adsorption and decomposition products of the simulants are being conducted using 27Al, 31P and 13C SSNMR. The results are being used to optimize the nanofiber fabrication conditions.
Figure 5. Scanning electron micrograph (SEM) of alumina nanofibers. The nanofiber diameters are 200-500 nm.
Electrolytes in Li-ion batteries need to conduct the Li ions effectively, and the Li-ion conductivity is enhanced by combining lithium salts with alumina. We have been adding Li salts to our alumina nanofibers, either by coating them onto the surface of nanofibers or incorporating the salts within the fibers during the electrospinning process. The extent of Li ion solubility into the nanofibers, the size of the Li-salt/nanofiber interface and the Li ion conductivity are being investigated in an effort to improve the electrolytes conductivity.
B.S., 1984, Illinois State University
M.S., 1986, Illinois State University
Ph.D., 1993, Michigan State University
Postdoctoral fellow, 1993-1996, Washington University (St. Louis)
Office: KNCL 221
Lab: KNCL 219