This Research award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports work by Professor Thomas Mallouk at The Pennsylvania State University to carry out fundamental studies on the synthesis, reaction chemistry, and physical properties of new layered metal oxides. This class of inorganic materials lends itself to rational design because of the availability of intercalation, ion-exchange, and exfoliation/re-stacking reactions. Oriented attachment reactions of oxide nanosheets will be developed to realize three-dimensionally bonded solids with tailored magnetic and electronic properties. New topochemical reactions will be used to introduce dopants and defects, which will modulate the electronic and protonic conductivity of layered solids and thin film superlattices derived from them. Oriented particle membranes, which are of interest as solid electrolytes for intermediate temperature fuel cells, will be made by combining the magnetic orientation of nano- and microcrystals with electrophoretic assembly. This research will be coupled to an outreach effort that provides high school students from under-represented groups with hands-on experience in nanomaterials chemistry through an Upward Bound Math and Science summer program. The project will continue to develop guided-inquiry capstone projects for the materials- and environmentally-focused sections of the general chemistry laboratory at Penn State, and will contribute to K-12 science outreach efforts in local schools.
Layered metal oxides are a class of inorganic solids with unique ferroelectric, magnetic, superconducting, and catalytic properties. The elaboration of their chemistry is important to realizing new functional materials that combine, for example, ferroelectricity and magnetism, and to exploiting their reactivity for applications in catalysis and energy conversion.
Research in this project investigated new chemical reactions, properties, and applications of layered inorganic solids. Layered solids - for example mica and graphite - contain strong chemical bonds in two directions and weak bonds in the third. This gives their crystals a platy texture and makes it possible to peel the layers apart, either chemically or mechanically. One of the important reactions of layered solids is intercalation, in which guest molecules or ions slip between the layers. Intercalation reactions were the primary focus of this research project. Almost all layered oxides contain negatively charged sheets. We discovered in this project that the intercalation of positively charged polymers could invert the layer charge, so that any layered oxide could be converted to an anion exchanger. This discovery has proved important for making brightly colored pigments in which negatively charged dye molecules or metal nanoparticles are intercalated between clay sheets. These layered anion exchangers are also selective sponges for certain contaminants - such as perchlorate, chromate, and pertechnetate - that are difficult to remove from ground water by other means. Intercalation reactions are important in energy storage and energy conversion technologies, such as batteries, supercapacitors, and fuel cells. For example, the movement of lithium ions between the sheets of layered oxides is the reaction that charges and discharges lithium ion batteries. In this project, new layered materials were designed that could exploit intercalation reactions in supercapacitors and fuel cells. We synthesized composite solids in which conductive carbon sheets were interleaved with nanosheets or nanowires of transition metal oxides that store and release charge. This "lasagna" structure makes all the metal ions in the oxide component accessible to the liquid electrolyte, which permeates the sheets and also ensures electrical contact to all the metal ions. Because of this unique structure, the supercapacitor materials have unusually high energy storage capacity and unprecedented stability over thousands of charge/discharge cycles. We also synthesized and studied electrically insulating oxides in which the interlayer cations are protons (H+). When these materials are hydrated, they are good proton conductors, and thus they are potentially useful in fuel cells as the membrane that separates the anode and cathode. Inorganic oxides are much more tolerant of high temperatures than the polymeric proton conductors that are currently used in fuel cells. In principle, inorganic proton conductors could enable the development of less expensive, more efficient high temperature fuel cells for cars and portable electronics. However, protons intercalated in layered oxides conduct primarily along the surface of the sheets and do not conduct in the perpendicular direction. We devised a method, exploiting the diamagnetism of the layered oxides, to align them so that membranes could be formed with the highly conducting directions of the microcrystals oriented perpendicular to the membrane plane. We also developed and tested a theory to explain why crystals of layered compounds orient the way they do in magnetic fields. In this project, new scientific discoveries were integrated with educational and outreach projects that involved all the students who were engaged in the research. In the summer months, teams of visiting students from disadvantaged high schools worked on guided inquiry nanoscience projects as part of the Upward Bound Math and Science program at Penn State. Students in this program - most of whom will be the first person in their family to attend college - typically visit for 2-3 summers and work on different projects in participating laboratories. High school teachers from local schools were also involved in these projects and developed nanoscience-based lesson plans for use in their classrooms. This research project also had an international connection with visiting postdocs and sabbatical visitors from Japan, Taiwan, and India with whom we continue to collaborate. We have are also working with two companies on the development of environmental remediation technology based on materials synthesized and studied in this project.
