Angling for a better (nano) surface
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A promising method for creating and studying chemically tailored nanocrystalline surface materials was recently developed by researchers at the U.S. Department of Energy's Pacific Northwest National Laboratory. Results are reported in the April 11, 2002, issue of the Journal of Physical Chemistry B.
June 10, 2002—Mankind has been tinkering with surfaces ever since we tried chipping different rocks to see which gave us the best edge for a hunting spear. The tinkering goes on, but researchers are now delving far beyond what is humanly perceptible, and peering into the nanoenvironment. Nanoenvironments—where structures are created and studied atom by atom or molecule by molecule—are new to research, the products of highly advanced technologies.
Researchers at the Pacific Northwest National Laboratory (PNNL) are creating and then studying thin nanostructure films to determine how their chemical properties are related to their nanostructure. The results answer disparate questions that are ultimately related. An astrophysics question—"How does the surface ice on comets store and release large quantities of gas?"—provides the basis for responding to more earth-bound needs, such as "What properties make the best chemically reactive surfaces?"
A promising method for creating and studying chemically tailored nanocrystalline surface materials was recently developed by PNNL researchers. The method lays down a specific surface structure quickly, reliably, and in amounts useful for large-scale research and, possibly, commercial production. The researchers used reactive ballistic deposition (RBD) to form a nanocrystalline surface that is stable at high temperatures (1200K, or 927° C) and have a significantly higher surface area and greater porosity than films produced by other deposition methods.
These properties are critical to creating surfaces that are chemically active and that have a large and uniformly distributed number of high-energy binding sites—in short, in creating surfaces that are effective for controlled chemical reactions. Stability at high temperatures means that the material will work effectively in "real world" conditions. The greater the surface area, the greater the number of sites upon which reactions can occur. Finally, the presence of a large number of high-energy binding sites indicates that a material has good potential for supporting high catalytic activity.
Cometary Ice Suggests an Ideal Surface
The PNNL researchers began their work by studying amorphous solid water at 20K (the temperature at which amorphous ice forms in comets). They were driven by curiosity about why cometary ices are so efficient at storing and releasing large amounts of gas. They found that the amorphous solid water, which forms very slowly and at very low temperatures, is nanoporous and has a very high surface area. These properties enable cometary ice to store much greater quantities of gas than ice formed at higher temperatures. The chemical aspects of the research were so interesting, and the potential for applications so great, that the researchers embarked on a study of how to grow highly structured, nanoporous thin films of other materials.
"The general phenomenon of nanocrystal orientation has been targeted for various applications because they have interesting optical and magnetic properties," says PNNL researcher Zdenek Dohnálek. "However, no one, to our knowledge, has looked at the chemical properties of these materials before."
There are many ways of creating a thin film. The most familiar are the stuff of household science: a base material is painted with or dipped into a liquid; or is placed into a vapor which condenses on it as it cools; or is held in front of a sprayer that is moved across the surface until the coating is evenly applied—as with a paint-spray gun. This last is most analogous to the "ballistic deposition" method, in which a directed beam of material in gas phase is "shot" onto a substrate.
Thinking in Angles
The PNNL researchers refined ballistic deposition into the RBD method when they found that by changing the angle of the deposition from directly overhead to 85° and applying a beam of gaseous magnesium to a substrate in an oxygen-rich environment at a low-temperature, the resultant film was made up of a thin layer of MgO filaments that were uniformly angled at 30° relative to the substrate.
Using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HTREM), and X-ray diffraction, the researchers scrutinized the MgO filaments-to their knowledge, this is the first time that the nanostructure of individual filaments has been reported on. They found that the filaments are crystalline, with feather-like structures—the result of being grown at low temperature. These structures make the filament surface area approximately 50 times greater (~1000m2/g) than it would be were the filament smooth sided.
By comparison, the surface area of traditionally prepared MgO powders is 2-100m2/g, and that of nanocrystals is 400-500m2/g. Porosity of the RBD-grown surface is also greater than that of surfaces grown by more traditional methods, because the angle of deposition creates a "shadow," or interfilament space, behind each crystal. The overall porosity of the RBD film is 90% and is the result of the large interfilament spaces. These spaces allow for facile transport of reagents to the high-energy, intrafilament binding sites. Materials with these qualities are being sought after for use in catalysis, hydrogen storage, chemical sensors, and geochemistry.
The PNNL researchers also tested the properties of the MgO surface by examining how it interacted with nitrogen (N2). The degree to which the surface adsorbs the gas and then desorbs (releases) it when it is heated, and the range of temperatures over which the desorption occurs, indicates the total number and distribution of binding sites of the surface, and thus the potential of the surface for supporting catalysis. The N2 desorbed from the nanoporous surface over a temperature range that is much broader (25-140K) than the desoption from the surface formed by dense MgO(100) film. These tests confirmed the estimates of surface area and related physiosorption properties, and indicate that the surface is thermally stable at 1200K.
Now that the initial proof of RBD and low-temperature deposition has been demonstrated, the next step is to see if the results can be replicated with metals.
"We have the ability to create chemically tailored materials with various nano-structural features that can lead to enhanced selectivity and reactivity," said Bruce Kay, a senior chief scientist on the project. "It takes specialized equipment to grow these kinds of materials and even more sophisticated instrumentation—like what we have available here—to study them."
Key PNNL contributors to this project are Zdenek Dohnálek, Greg A. Kimmel, David E. McCready, James S. Young, Alice Dohnálková, R. Scott Smith, and Bruce D. Kay. The findings were published in "Structural and Chemical Characterization of Aligned Crystalline Nanoporous MgO Films Grown via Reactive Ballistic Deposition", 2002, J. Phys. Chem. B. 106(14); 3526-3529.
Media Contacts: PNNL Media Relations, pnl.media.relations@pnl.gov , (509) 375-3776
Technical Contacts: Bruce D. Kay, Pacific Northwest National Laboratory, bruce.kay@pnl.gov
Related Web Links
Nanostructure Science and Technology—A Worldwide Study
Virtual Journal of Nanoscale Science and Technology
Nanotechnology Research Project Links
Societal Implications of Nanoscience and Nanotechnology
Funding: Funding for this work was provided by the Chemical Sciences Division of the U.S. Department of Energy's Office of Basic Energy Sciences, and the Pacific Northwest National Laboratory's laboratory-directed research and development program.
Pacific Northwest National Laboratory is a DOE research facility and delivers breakthrough science and technology in the areas of environment, energy, health, fundamental sciences and national security. Battelle, based in Columbus, Ohio, has operated the laboratory for DOE since 1965.
The William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), the Department of Energy's newest national scientific user facility, is located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. The EMSL is operated by PNNL for the DOE Office of Biological and Environmental Research.
Author: Michaela Mann is a science writer and electronic communications specialist at Pacific Northwest National Laboratory in Richland, Washington. She was formerly the managing editor and original website developer of Energy Science News, an award-winning online newsletter for DOE's Office of Science.