News Article
CBE researcher’s work in film growth featured in Nature publication
Contacts:
Kurt Hebert, Chemical and Biological Engineering, 515 294-6763, krhebert
iastate.edu
Mary Jo Glanville, Engineering Communications and Marketing, 515 294-8787, mglanviliastate.edu
Kurt Hebert, a professor of chemical and biological engineering at Iowa State University, is working to understand the growth of self-ordered porous aluminum oxide films. The intriguing problem has implications for the further refinement of solar cells and sensors.
Over the last 15 years, researchers have developed methods to prepare films on aluminum and other metals in which the pores are arranged in a nearly perfect hexagonal pattern. But they don’t understand how the pores initiate or what accounts for their regularity.
These films arise spontaneously on metal surfaces when an electric voltage is applied to the metal in contact with an acidic water bath. The length of the pores, which is measured in micrometers, is much bigger than the width and spacing, which are measured in nanometers.
This regular arrangement of pores and very large surface area of the oxides make these films a desirable trait. “Many applications benefit from a large surface area,” Hebert explains. “In solar cells, for example, the more energy gathered on the surface means more chemical conversion and more power generated. Sensors are another example. The sensing molecules are positioned on a surface. A larger surface area means more sensing molecules and an improved ability to detect the target substance.”
While scientists know that the pore geometry can be adjusted to meet the requirements of different applications, they are seeking answers about the film formation process itself. “If you can understand the growth mechanism, then you can learn how to manipulate the geometry and tailor the properties to be exactly what you want,” Hebert says. “In addition, understanding the mechanism is key to streamlining the formation process. Currently, the experimental procedures that lead to their formation are quite long and present a barrier to commercialization.”
In the April 12 advance online publication of Nature Materials, Hebert and recent PhD graduate Jerrod Houser report on their work examining what happens during the growth of the films. Previous research provided them some valuable information.
“In 2006, researchers at the University of Manchester in England embedded tungsten metal tracers in the substrate metal and used electron microscopy to follow the motion of the tracers after they entered the oxide,” Hebert says. “Their results showed clearly that ions in the film move not only because of electrical forces, as had previously been thought, but due to mechanical stress as well.”
In a brand new approach, Hebert and Houser developed a model integrating transport of ions due to both mechanical stress and electric fields. In the model, stress causes the solid oxide material to flow according to the same physical laws that govern the flow of liquid water, but with a much larger viscosity. The results were in detailed agreement with the experimental findings from England.
Hebert notes that the understanding of porous oxide formation gained in this work may be the initial step toward an ability to rationally design these porous materials with a specific geometry, tailored for a particular application.
Hebert and Houser’s article, “The role of viscous flow of oxide in the growth of self-ordered porous anodic alumina films,” can be found on the Nature Materials Web site.
