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Seed Projects
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Current Seed Projects
New Routes to Biorenewable Plastics: Converting Natural Resources to Environmentally Friendly Materials
Natural, renewable resources are abundant and can be converted into commodity
chemicals with commercial value. New, sustainable processes beginning with these renewable resources that yield both today's established products and novel replacements are of paramount importance given dwindling global oil reserves. We propose to chemically convert naturally occurring feedstocks to new monomers susceptible to catalytic polymerization processes that, in turn, yield new polyesters with tunable properties. Simple transformations that convert widely available carbohydrate, vegetable oil, and terpene derivatives into useful polymeric materials (e.g., plastics, rubbers, thermosets) with a diverse set of properties will be emphasized. To this end, we will combine organic, inorganic, and polymer chemistry expertise in creative ways to develop new and useful classes of sustainable, environmentally-friendly materials.
Over the past several years we have prepared several new cyclic esters that can be readily converted to their polymeric (i.e., ring-opened) forms using novel catalysts. For example, we prepared a polyester comprised of perfectly alternating lactic acid and ethylene oxide repeat units as a potential macromolecular plasticizing agent for the biorenewable plastic polylactide. We also prepared poly(3-hydroxypropionic acid) from large macrocyclic esters derived directly from 3-hydroxypropionic acid (a product of glucose fermentation); this new route is of practical significance. Finally,
using a highly active zinc catalyst developed at Minnesota, we converted (-)-menthide (derived from the natural terpene menthol) to a new high molecular weight amorphous polyester with a low glass transition temperature that could be used for toughening other bio-based polyesters. These successful efforts provide a foundation for our proposal to significantly expand the synthetic methods to new monomers from a broader pallet of renewable resources. There are three interrelated, integrated, and important aspects to our proposed work: (a) conversion of renewable resource starting materials into cyclic esters and derived polymeric materials, (b) development of new, highly active ring-opening
polymerization catalysts, and (c) evaluation of the new polyester properties.
Monodispersed Zinc Oxide Nanoparticle-Dye Dyads and Triads: Characterization of the Early events in Dye Sensitized Solar Cells
The overall energy conversion efficiency of solar cells depends on the combined efficiencies of light absorption, charge separation, charge and ion transport. Although zinc oxide is emerging as a potential semiconducting component in dye sensitized solar cells, there is little known about the factors controlling charge separation across the dye/ZnO interface. The lack of site homogeneity in binding dye molecules to TiO2 films and nanoparticles (NPs) has been widely acknowledged as an impediment to extracting details about the electron transfer across the interface. This project studies methods to surmount these problems and help the R&D community realize zinc oxide's full potential in solar cell applications. The overarching goal of this research is to study the relationship between structure, energetics, and dynamics in a set of synthetically controlled donor (dye) - acceptor (ZnO nanoparticle) dyads and triads. These studies will provide unprecedented understanding of the light absorption and charge transfer steps that lie at the heart of dye sensitized solar cells, thus enabling significant future advances in cell efficiencies. Our approach begins with the construction of extremely well-defined dye-NP dyads that will be sufficiently dispersible to allow the use state of the art pulsed laser spectroscopic and kinetic methods to understand the charge transfer events at a fundamental level.
Modeling of Dye Sensitized Solar Cells
There is no doubt that harnessing solar energy with inexpensive materials and
manufacturing methods is one of the most important challenges facing humanity. Solarto-
electrical energy conversion methods that make use of photosensitized nanostructures
are emerging as an inexpensive alternative to the p-n junction solar cell. A very recent
example of this approach is the nanowire-based dye sensitized solar cell where a dense
network of wide band gap semiconductor nanowires is sensitized with an organometallic
dye. The dye harvests the incident light and injects an electron into the semiconductor,
where it is transported through the nanowire to reach an electrode connected to the load.
The dye is regenerated by hole injection into an electrolyte. For example, when I-/I3
-
redox couple is used in an electrolyte, I- is oxidized to I3
- at the dye-electrolyte interface
and I3
- is reduced back to I- at the counter electrode to complete the solar electrochemical
cell. The first dye sensitized solar cells were made from TiO2 nanocrystals abutted
against each other to form a mesostructured film which served the same purpose as the
nanowires in Fig. 1. However, there is evidence that the improved electron transport in
the nanowire geometry may be useful. Furthermore, nanowire geometry is much better
defined than the random, mesostructured TiO2 photoelectrodes and, because of its more
regular geometry, will be easier to construct realistic models to describe system behavior
and to aid the design experiments which elucidate the fundamental physical mechanisms
controlling the DSSC operation. Much of the current research focuses on improving the range of spectral
absorbance by modifying the dye, on improving hole transport and cell stability by
replacing the liquid electrolyte with ionic solids or conducting polymers and on
improving electron transport by using alternative wide band gap semiconductor materials
or core-shell structures. The experimental approaches are largely empirical and time
consuming; progress is slow as the fundamental principles of how the dye sensitized solar
Figure 1. Schematic of a nanowire based dye sensitized solar cell (left). Scaning electron micrograph
of ZnO nanowires grown from methenamine and zinc nitrate solution at 60 oC (right). Nanowires are
~8 £gm long and 100 nm in diameter.
cell operates is still being debated. It is certainly fair to say that there is no
comprehensive theoretical and modeling framework that could both guide experiments
and provide a better understanding of experimental observations. Towards this end, we
are proposing to start a modeling and simulation research project aimed at providing such
a framework. Two key areas are targeted; (i) growth mechanism and growth kinetics of
nanowires and (ii) charge transport and kinetics in dye sensitized solar cells.
Growth mechanism and growth kinetics-The quality and microstructure of the
ZnO nanowire array determines the solar cellÕs performance, but we do not understand
quantitatively the effects of key experimental variables on the microstructure. Nanowires,
shown in Fig. 1, are grown from methenamine and zinc nitrate solution at temperatures
less than 100 oC. Anisotropic growth results from different growth rates of the various
crystallographic faces; however, the kinetics of growth are neither understood nor
quantified. Nanowire grow is an example of a special type of solution crystal growth; the
general area of solution crystal growth has received much attention in DerbyÕs prior
research. Models describing the growth of these nanowires will be constructed by
combining simple theories of face growth with analyses of mass transfer through the
liquid phase. These models will rely on experimental data to tune phenomenological
model parameters and will, in turn, provide a mathematical basis to explain how process
changes will affect growth characteristics. We expect the synergy between model and
experiment to enable advances not possible by experimentation alone.
Charge transport and kinetics in dye sensitized solar cells-During the operation of
the cell, electrons are continually generated, injected into and transported in the
nanowires while the hole undergoes a redox reaction at the semiconductor-dye-electrolyte
interface. The ions in the electrolyte penetrate the region between the nanowires and
mediate the positive charge transport between the dye and the counter electrode. The
spatial variation of the electric field is determined by the charge distribution both in the
nanowire and in the electrolyte. The solar cell operation spans 15 and 6 orders of
magnitude in time and length scales, respectively. For example, the electrons are injected
from the dye into the nanowire within several femtoseconds but may take milliseconds to
reach the anode. The nanowire and the electrolyte determine the solar cell thickness
which may be 10-50 £gm; in contrast, the electrical double layer that forms at the
nanowire-dye-electrolyte interface maybe on the order of nanometers. This disparity in
scales presents one of the modeling challenges. We propose to initiate a hierarchical
modeling effort starting from the simplest models of self-consistent electron transport and
kinetics in the nanowires and ion transport and kinetics in the electrolyte. Drift-diffusion
equations for electrons in the solid and for the ions in the electrolyte will be solved
together with the Poisson equation to provide a framework within which experimental
data such as the solar cell current-voltage characteristics can be examined. Such
systematic examinations can be used to infer information about the rate limiting
processes in the solar cell and lead to improved designs. Thus a hierarchy of models,
beginning with analytical, lumped parameter approaches, ranging to 2D, and possibly 3D,
finite element solutions of the governing electrochemical and electromagnetic equations
will be developed and applied to this problem. Derby has had past experience with
modeling electrochemical reactions and the solution of Maxwell equations (see CV), and
we anticipate significant progress can be made in a relatively short time frame.
Zeolite Growth in Confined Spaces
At the present time, the fundamental growth mechanisms of zeolites are poorly understood. Zeolites are ordered framework materials with regular cages and channels of sub-nanometer size. Their tailored structure, stability, and activity have led to a broad variety of applications in industry as catalysts, adsorbents, and ion exchangers. Achieving control over crystallite shape and the microstructure of thin films produced from these materials is a major goal in Materials Science and Chemistry. A fundamental understanding of zeolite growth mechanisms will lead to improved control over defect concentrations, size, and morphology of zeolite crystals.
This project began through a collaboration between Profs. Penn and Tsapatsis, and results were recently published in Natural Materials (vol. 5, 400-408, 2006). Previously, the role of noncrystalline nanoparticles that spontaneously form upon mixing of reagents (i.e., as spectators versus direct participants) in zeolite nucleation and growth was not understood. Results from this project show that such noncrystalline silica nanoparticles, formed rapidly in the presence of tetrapropylammonium (TPA) cations, are metastable and evolve slowly to zeolite (MFI, in this case) by a mechanism that appears to involve nanoparticle structure evolution and aggregation. The slow transformation allowed us to observe, for the first time, zeolite crystals at initial stages of formation (i.e., zeolite yield less than 5%). A new mechanism for aggregative crystal growth and a mathematical model formulated according to this mechanism were shown to be in agreement with the experimental findings. According to this mechanism, the metastable precursor nanoparticles, although they do not have the MFI structure, are not mere spectators but rather contribute to crystal growth by direct addition to the growing crystal after a certain stage of evolution. The indispensable element of the proposed mechanism is that the evolving population of silicate nanoparticles, although homogeneous in size and shape, exhibits functional diversity towards aggregative crystal growth due to structural differentiation among the nanoparticles.
The project has now expanded to include the expertises of Prof. Stein (Chemistry) and McCormick (Chemical Engineering and Materials Science). Through a combination of advanced materials characterization methods (e.g., Cryo-transmission electron microscopy, small angle X-ray and neutron scattering, and high-resolution transmission electron microscopy), our group will address the following fundamental questions:
o Is aggregation of the noncrsytalline nanoparticles a necessary event that must occur before zeolite nucleation and crystal growth or can individual nanoparticles evolve into zeolite crystallites?
o What is the structure of the interfaces in the aggregate-like crystals?
o Can zeolite nanocrystals with size similar to that of the initial, noncrystalline nanoparticles (ca. 5 nm) be made?
Our hypothesis is that the amorphous silica nanoparticles evolve to zeolite nanocrystals prior to contributing to crystal growth by oriented aggregation. We propose to test this hypothesis by an innovative approach: We propose to examine nanoparticle evolution in confined spaces so as to minimize or eliminate particle aggregation. If the hypothesis is correct, we expect a high yield of zeolite nanocrystals with sizes on the order of few unit cells. If, on the other hand, the hypothesis is incorrect, no zeolite nucleation will be observed.
High Speed Characterization Methods for Polymer Substrates
The goal of this project is to develop material characterization tools able to predict the electronic properties of polymers - insulating and/or conductive. As electronic circuits become more prolific in everyday use, a need for low cost, small size and light weight technologies to accommodate mobile applications has become imperative. Insulating polymer materials, available for over a decade, have found many important uses in a broad range of electronic applications. For example, at low frequencies it has been instrumental for the creation of flexible interconnects that allow large circuit designs to be compressed into smaller more compact spaces. In aerospace high frequency (10's of GHz) communication system applications, it has been used to develop multilayer integrated circuit designs that combine polymers and semiconductor material. The main advantages are that it offers increased circuit miniaturization and higher circuit performance at a much lower cost to all-semiconductor versions of the same design. To understand the potential implications of conducting polymer materials in this rapidly growing electronic world, key information about the electronic material properties such as dielectric constant and material conductance are needed. Hence, we will investigate the use of high frequency planar circuit design methods on conducting materials in collaboration with Prof. Dan Frisbee's lab in order to obtain data on material dielectric constant and conductance properties. This data will be used to extend our current material extraction techniques, developed for insulating materials, to include conductive material properties. This extension will enhance the material characterization capability at the University of Minnesota by adding the ability to evaluate material use for high frequency electronic applications.
Molecular Engineering of Photo-Responsive Bioactive Surfaces
Photo-responsive bioactive surfaces are capable of presenting or withdrawing bioactive molecules in response to light. These surfaces provide valuable tools in biomedical engineering research. For example, they allow dynamic regulation of artificial cellular microenvironments to control cell behavior and engineer functional tissues. The microenvironments cells sense in vivo are highly dynamic. However, it has remained challenging to mimic dynamic presentation of signaling molecules immobilized on extracellular matrix or neighboring cells. Photo-responsive bioactive surfaces will allow us to manipulate immobilized signaling molecules in a temporally controlled manner by programming photo stimuli. We are developing such surfaces through molecular engineering of polymers and proteins. It has been reported that ligand accessibility to cell surface receptors could be controlled by the relative height difference between bioactive ligands and inert polymer chains that are co-immobilized on a substrate. We are engineering surfaces on which the height of the bioactive ligands and their accessibility can be controlled by photo stimuli through engineering a photo-responsive phase transition behavior of the polymer spacer that links the bioactive molecules to the surfaces. Once a photo-responsive bioactive surface is developed, a Notch ligand will be engineered for dynamic presentation and its impact on stem cell fates will be examined. During animal development, highly dynamic Notch signaling with sequential positive and negative activation plays a crucial role in assuring correct stem cell fates and tissue and organ formation. Notch ligands must be immobilized on a surface to activate the signaling pathway. The stimuli-responsive bioactive surfaces proposed here will provide elegant systems to study and manipulate stem cell fates by regulating dynamic Notch signaling in vitro.
Past Seed Projects
Structural Properties of Silicon Nanoparticles -- Theory and Simulation
Oriented Aggregation of Zeolite Nanocrystals: Controlling Twinning and Thin Film Microstructure
Novel Magnetic Materials and Nanodevices for Spin Transfer
Novel Biopolymers for Targeted Drug Delivery: Peptide-Functionalized Polymersomes
Computational and Experimental Design of Nucleation Templates for Controlling Crystal Growth and Polymorphis
Designed Heterogeneous Nucleation of Polymeric Foams
Measurement and Synthetic Optimization of Nanomechanical Properties of Three-Dimensionally Ordered Macroporous (3DOM) Solids
Microfabrication of Cell/Material Hybrid Constructs Via Laser-Guided Direct Writing: Application to Artificial Tissue Vascularization
Modeling and Design of Crystals Grown from Solution
Nonlinear Spectroscopic Studies of Nanoscopic Materials Targeted at Phototonic Switching Applications
Organic Semiconductors
Programmable Reconfigurable Integration of Nanowires on Silicon Substrates
Using Biological Structures as Templates for Magnetic Nanostructure Fabrication
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Last
modified on April 3, 2008