Most of our research interests are toward the fundamental studies of complex systems at the nanoscale with regard to applications of materials at the macroscale. The Poler Research Group is particularly interested in how large supramolecular systems interact with and “Mechanically wrap” about nanoparticles like carbon nanotubes (SWCNTs, DWCNTs, and MWCNTs), metal nanoparticles (NPs), and quantum dots (QDots). Specifically, we are trying to elucidate energy and charge transfer mechanism between these systems while we work toward efficient manufacturing methods of nanomachines, nanosensors, nanotransducers, nanoparticle based composites, energy storage, and water purification materials.
Dispersions and Aggregation of Nanoparticles
To effectively use nanoparticles in new materials or new devices, they must be assembled into a useful structure. Liquid processing facilitates transport and positioning of these particles. The interactions of nanoparticles in solution or dispersion are complex. The stability of a dispersion is essential to enable downstream processing and viable manufacturing.
Nanomaterials are extremely sensitive to processing conditions, as we demonstrated in a recent JACS (2010) paper. Certain solvents that are commonly used to disperse pristine SWCNTs (i.e. DMF, DMA, etc.) can be rendered unusable for SWCNT dispersion due to ultrasonication under inappropriate conditions. If the temperature or oxygen concentration of the solution is wrong, ultrasonication can initiate an autoxidation reaction that leads to the formation of a contaminant, methyl hydroperoxide, that destabilizes SWCNT dispersion. Understanding these processing limitations is important for scaling up liquid processing of SWCNTs.
There is significant interest in optimizing solvents to maximize the quality and stability of SWCNT dispersions. We have shown significant gains in dispersion stability by mixing solvents to adjust solvent properties. As observed by others, solvent parameters, such as the Hildebrand/Hansen parameters, are important for understanding why some solvents are better at dispersing SWCNTs. We have shown that mixing DMF and NMP optimizes the dispersive Hansen parameter, allowing us to increase the stability of SWCNT dispersions by 60-115% when the coagulant concentration is near the “onset of aggregation”, as reported in our recent J. Phys. Chem. C (2011) paper. We also model the aggregation profile of SWCNT dispersions with a physical model that relates Maxwell-Boltzmann statistics to the DLVO model of colloidal stability.
Energy Storage Materials
We have recently discovered that mechanically wrapping our ruthenium complexes around pristine SWCNTs leads to an interesting charge transfer effect. Thin films of these composite materials lead to significantly enhance specific capacitance in our supercapacitor devices. We are pursuing new technology to enable low cost, low weight energy storage capacitors. These devices are ideally suited to integration with renewable energy sources such as solar, wind, and wave.
Hybrid Nanoparticle Synthesis
We have synthesized large rigid molecules that can “Mechanically dock” around small nanoparticles such as SWCNTs and metal NPs and semiconducting QDots. Some of our compounds non-covalently bind only to the ends of a SWCNT. These same compounds bind strongly to Au, Ag, Pt, and Fe NPs. We have directed the assembly of amphiphilic hybrid nanoparticles. We are currently investigating the supraparticle structures assembled from these for use as new tunable colloidal systems.
One focus is to develop techniques for solution based processing of SWCNTs centering our attention on industry scalable processes to form SWCNTs into thin-film materials and then transfer these films to various substrates. Ruthenium complexes synthesized in our group are combined with SWCNTs and NPs. The interaction of our ruthenium complexes are designed to modulate charge transfer between the nanoparticles.
Another focus is to apply calorimetric and spectroscopic techniques to measure the interaction between these materials in solution. When Ru and SWNTs are processed into composite films, we characterize these films using spectroelectrochemistry, SEM, TEM and AFM, and study their properties for energy generation and catalytic activity using electrochemistry techniques.
We have built a thermal CVD system that we use to grow SWCNTs and MWCNTs at atmospheric pressure. We can grow forests of CNTs randomly- or vertically-aligned. The alignment, forest height, and number of walls can be controlled by adjusting parameters like the thickness of the Fe catalyst film, growth time, and carbon feedstock flow rate. We are using home-grown CNTs and CNT arrays to develop novel nanoactuators and to study systems where the CNT length distribution is relatively monodisperse.
We are developing nanoactuators that operate by modulating the interaction of vertically-aligned CNTs grown on top of microcantilevers. Preliminary data suggests that our system will be able to demonstrate significantly greater deflections than other microcantilever-based actuators that have been presented in the literature. Further, this platform will allow us to study interactions between SWCNTs and Ru coordination complexes, after the VA-CNT forest has been functionalized with the Ru coordination complexes that our group synthesizes.
Synthesis of Supramolecular Complexes
We synthesize and characterize charged multinuclear Ruthenium (Ru) complexes whose properties make them suitable for pairing with nanomaterials in order to achieve a highly tunable final product. We control the shape and size of a π-pocket on the complex that interacts strongly and specifically with nanoscale particles. The optical absorption, photoluminescent, and electrochemical properties of the Ru complexes are of particular interest. We are interested in studying how modification of the charge states and structural isomerism of the Ru complexes affect these properties. A variety of organic and inorganic ligands can be used to tune the charge of each Ru center and serve as a bridge between two Ru centers.
An overarching goal of our research is to develop a better fundamental understanding of hybrid-nanoparticle interactions. This goal is central to the field of nanoscale science, as highlighted by a recent review in Chem. Soc. Rev. (2010). Specifically, we hypothesize that our amphiphilic hybrid-nanoparticles can be directed to form novel supraparticle assemblies with unique characteristics and potential uses. We synthesize and characterize supraparticle structures to be stable in water and enable new advances in the targeted removal of selected toxic organic compounds from contaminated aqueous environments (e.g., removal of pesticides, prescription medications, or gasoline additives like MTBE from contaminated drinking water sources). We collaborate with Dr. James Amburgey’s group in Environmental Engineering department.