Chemistry of Nanoscale Inorganic Materials
The Mallouk group takes a building block or "Lego" approach to the synthesis of interesting inorganic materials from the bottom up. Some of this research is very fundamental in nature and is designed to learn the rules of assembly of objects on the nanometer and micron length scales. The available building blocks that the group uses to make materials are nanoscale spheres, sheets, rods, and tubes, glued together by organic molecules and polymers, nucleic acids, and proteins. The inorganic building blocks are made by using a variety of techniques, including high temperature synthesis, sol-gel chemistry, aqueous ion-exchange and acid-base chemistry, and replication of porous templates.
Another aspect of this work is to use 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
Dye Sensitized Solar Cells
The spectral response of dye sensitized solar cells can be significantly enhanced by manipulating light using nanoparticle assembly to produce photonic crystals. In a collaborative project with the Redwing group, semiconductor nanowire arrays are being studied as photoelectrodes. Crystalline Si and CdSe nanowires can be made by vapor phase and electrochemical deposition techniques, respectively. The vertical nanowire morphology separates the length scales of light absorption and minority carrier diffusion, and in principle offers a low-cost route to very efficient multi-junction cells.
Catalytic Motors and Pumps
Several projects in the group use porous membranes as templates for growing nanowires and nanorods. Multi-segment nanowires have interesting electronic properties, such as transistor and diode behavior and in some cases unusual quasi-1D superconductivity. In collaboration with the Sen Group, we are now studying the movement of multi-segment nanorods powered by spontaneous catalytic reactions. These nanorods are the first examples, outside of biological systems, of autonomously powered nano- and micromotors. The same electrochemical principles have recently been used to design catalytic micropumps.
Functional Inorganic Layered Materials
We are developing a set of soft chemical reactions that topochemically interconvert different structural families of layered and three-dimensional perovskites. Layered 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. These materials are of particular interest to us as catalyst supports and ionic conductors in intermediate temperature fuel cells.
Nanowire Sensor Arrays
Using electrodeposition in porous membrane templates, metal-tipped nanowires of conducting polymers can be prepared. These are aligned in a controlled manner, using a technique developed by our collaborators in the Mayer group, into cross-reactive sensor arrays. The nanowire morphology and very high redunancy of sensor elements provide high sensitivity and low limits of detection for gaseous analytes.
In-Situ Remediation of Contaminants in Soil and Groundwater Using Nanoscale Reagents
Layer-by-layer assembly on nanoparticle surfaces is being studied as a means of controlling core-shell structure and particle aggregation for optimized sub-surface transport, targeting of insoluble contaminants, and concentration of soluble contaiminants at the reactive nanoparticle surface.
