Chemistry of Nanoscale Inorganic Materials
The Mallouk group uses nanoscale assembly techniques to make complex materials with unusual properties or specific functions. Often these properties arise because the system is mesoscopic, meaning that the physical size of the object corresponds to some characteristic physical length, such as the wavelength of light, the coherence length of Cooper pairs in a superconductor, or the width of the depletion layer in a semiconductor liquid junction.
Overview of current research projects in the group
Solar Photochemistry and Photoelectrochemistry
An important goal of this aspect of our research is to develop new kinds of nanomaterials that will lead to more efficient and less expensive solar energy conversion devices. Dye-sensitized solar cells, developed over two decades ago by Michael Gratzel and coworkers, can use the solar spectrum more efficiently when they are coupled to nanostructures that trap visible light or selectively re-direct infrared light to a silicon solar cell. In a collaborative project with the Lakhtakia
groups, we are now exploring solar cell designs that combine plasmonic (metal) nanostructures and periodic dielectrics (photonic crystals) to control the flow of light. By incorporating nanoparticles that catalyze water oxidation into dye sensitized solar cells, it is possible to split water to hydrogen and oxygen using visible light. We use biomimetic principles to control electron and proton transfer reactions in these cells and transient spectroscopic techniques to measure their kinetics. These studies have recently led to a better understanding of system-level problems in the photoelectrolysis of water, and to the design of more efficient electrolyzers for converting CO2
to fuels and chemicals.
Template Synthesis of Nanowires and Metamaterials
Several projects in the group use porous membranes and colloidal crystals as templates for making nanomaterials. Multi-segment nanowires have interesting electronic properties, such as transistor and diode behavior and in some cases unusual low temperature transport properties. In collaboration with the Sen
, and Crespi
groups, we are studying the movement of multi-segment nanorods powered by spontaneous catalytic reactions. These nanorods were the first examples, outside of biological systems, of autonomously powered nano- and micromotors. In many ways, they resemble living microbial motors and exhibit similar kinds of collective behavior. The principles of catalytically driven movement have now been used to design microscale pumps and rotors, and to study the powered motion of individual enzyme molecules. In collaboration with Mauricio Hoyos
and Angelica Castro at ESPCI in Paris, we have recently discovered that micron-size metal "rockets" undergo a range of autonomous and cooperative motion when propelled by acoustic waves. These new motors are biocompatible and exhibit fast directional motion at ultrasonic power densities that are typically used in medical imaging. In collaboration with the Badding
group in the Penn State MRSEC, we are using colloidal crystal templates to synthesize and study metalattices of semiconductors, ferromagnets, and other materials.
Functional Inorganic Layered Materials
We are developing a set of soft chemical reactions that topochemically interconvert different structural families of layered and three-dimensionally bonded oxides. Layer perovskites, metal phosphates, clays, and other lamellar solids can be grown layer-by-layer and converted to other interesting nanoscale morphologies (such as nano-scrolls and tubes) by means of intercalation, exfoliation, and restacking reactions. In collaboration with the Crespi
groups, we are devising new ways to intercalate and exfoliate metal dichalcogenides, boron nitride, and graphite, without using redox cycles that damage the sheets. These unilamellar compounds are of particular interest as novel low-dimensional electronic conductors, as catalyst supports, as electrode materials for batteries, and as ionic conductors.