Professor Francis P. Zamborini of the University of Louisville is supported by the Analytical and Surface Chemistry Program in the Division of Chemistry to study the electrochemical reactivity and oxidation potential of silver and gold nanostructures as a function of size, shape, and surface chemistry. A second thrust of this supported research is to electrochemically synthesize Silver nanowires for sensing and molecular electronics applications. The proposed nanostructures will be characterized by spectroscopic, electrochemical and microscopic techniques. This research will lead to a better understanding of the effect of the size and shape of chemically and electrochemically synthesized nanostructures on the oxidation potential and the mechanisms of electron transfer at these nanoscale dimensions, which will push the field forward towards technological applications in miniaturized electronics, chemical sensing, and catalysis. Graduate and undergraduate students will participate in all aspects of the proposed research, and high school students will participate through outreach activities.
One of the major goals of this work was to study the chemical stability of metals as a function of their size. We tested the metal stability against oxidation reactions, which is the same reaction that occurs when a metal corrodes, such as rust formation. We discovered that the stability of the metal (gold and silver in this case) decreases when it becomes smaller than about 50 nanometers. The stability decreases even more dramatically when the metal becomes smaller than about 4 nanometers. Spherical shapes were the focus of this study. The results show that metals are much more prone to oxidation, especially below the 4 nanometer size. This work is very important because small pieces of metal are being explored for a wide variety of applications. They can be used as small electronic components, in chemical detection systems, as pigments, antibacterial agents, in drug delivery, medical imaging, fuel cells, batteries, solar cells, and a large number of other devices. It is often the case that the very small pieces of metal are the most useful for these applications. For example, small pieces of platinum are needed to catalyze the reactions in a fuel cell that produce electricity from hydrogen and oxygen. This is one of the cleanest energy producing devices, but it relies on very small pieces of platinum. If the stability of these small pieces of platinum is not adequate, then the fuel cell will not function properly. This can be generally stated for any device or technology that relies on small pieces of metal. With the use of very small pieces of metal in the nanotechnology field, many people are also worried about the toxicity of these small metals. If the metals are more prone to chemically oxidize and change, then toxicity could become an important consideration, making this work important for environmental reasons. Larger pieces of metal are much more stable, making toxicity less of an issue for them. A second goal of this work was to use small wires of silver as electronic switches. With the small silver wires, if they are close to another metal (or electrode), but not in contact, they can be forced into contact by applying a voltage between the wire and the electrode. This leads to electrical connection. The electrical connection can be broken by changing the voltage. The connection occurs at a specific voltage by the formation of a small silver filament between the nanowire and the electrode. This filament may only be a few atoms thick in some cases. At certain voltages, the filament is stable and current flows between the wire and the electrode. At other voltages the filament is not stable and current does not flow. Controlling the current to be on and off can serve as the basis of memory devices. The wires are on a very small size scale, which potentially allows a large number of switches in a small area. This could lead to high density memory storage devices. We explored the chemistry of these filaments forming and breaking. Our work has led to new methods for forming the silver nanowires and the devices that operate as on/off switches. We also learned about the chemistry of the filaments and found conditions where they form and do not form. A better understanding of their chemistry and physics may allow us to fabricate stable and highly dense switching elements for memory device applications. We also explored these silver nanowire as part of electronic junctions for sensor applications.
