AEROGELS are an impressive feat of science and technology. These highly porous, low-density, lighter-than-air structures have become exceedingly useful at the Laboratory in laser target fabrication, energetic composites, sensors, ceramics, and coatings. Over the last decade, researchers have tailored the properties of aerogels in an effort to optimize them for different applications. Using a process referred to as doping, they incorporate additional chemical elements into the material. However, this doping process can be challenging because of aerogel’s complex pore structure. Recently, a team of Livermore researchers helped develop an efficient method to dope carbon aerogel—an electrically conductive material—with the noble metal platinum. This method could lead to the development of new designer materials for fuel cells, hydrogen storage, and pollution-control technologies.
Platinum is an effective catalyst because it can accelerate chemical reactions without being consumed or altered. As an example, catalytic converters on gas-powered vehicles use platinum to convert poisonous carbon monoxide into carbon dioxide through oxidation. In hydrogen fuel cells, platinum is used to catalyze the electrochemical reactions that produce electrical energy. For technologies such as these, more platinum means higher conversion rates. Unfortunately, platinum is expensive, making technologies that require large amounts of the material too costly for everyday applications. However, by incorporating platinum into carbon aerogels, scientists from the Laboratory, Stanford University, and the University of Bremen in Germany have shown that when it comes to platinum, less can do more.
Using atomic layer deposition (ALD)—a gas deposition process that provides atomic-level control of thin films—the team added minute amounts of platinum to uniform discs of carbon aerogel. These platinum-loaded carbon aerogels were then tested for how well they converted carbon monoxide to carbon dioxide. “The catalytic oxidation of carbon monoxide using platinum is fairly well understood and is an easy way to benchmark the material,” says Ted Baumann, a chemist in Livermore’s Advanced Materials Synthesis Group. The results showed that platinum-loaded carbon aerogels achieved nearly 100 percent conversion efficiency with minimal amounts of platinum, as little as 0.05 milligrams of platinum per square centimeter.
A Complex Kind of Jell-O
Carbon aerogels, despite their high porosity, can be very strong and are capable of supporting hundreds of times their own weight. Because they are so porous, they also have very high surface areas. For example, the aerogels used in the platinum catalysis experiments have surface areas of approximately 480 square meters per gram. This high surface area allows more of the catalyst to be exposed when reacting with external chemicals. As a result, less catalyst is needed to produce efficient results.
A Successful Sequence
In ALD, chemical precursors, or species, in their vapor (gas) form are sequentially pulsed into a reaction chamber containing a sample surface—in the case of the catalysis experiments, a carbon aerogel substrate. Inside the chamber, each species chemisorbs to the sample surface one atom at a time, eventually creating a thin monolayer film. Once the first chemical occupies all available adsorption sites, the surface is passivated, and the film growth stops until the surface is reactivated by exposure to the second gas precursor. Thus, in ALD, film growth is self-limiting, that is, restricted by the number of adsorption sites on the sample surface and the number of reaction cycles.
Baumann says ALD is different from other film-producing techniques, such as chemical vapor deposition, where one or more elements are added to a substrate all at once. Chemical vapor deposition is used in the semiconductor industry to create thin films for transistors. “If we used chemical vapor deposition to coat the inner surfaces of an aerogel, the material being deposited would quickly plug the aerogel’s pores, and the coating would not be uniform,” says Baumann.
The platinum ALD experiments were conducted at Stanford University in the laboratory of professor Stacey Bent. Uniform disks of carbon aerogel measuring 500 micrometers thick and approximately 1 centimeter wide were placed in a warm-wall chemical reactor heated to 120°C. The team used methylcyclopentadienyl-trimethylplatinum (MeCpPtMe3) and oxygen as the two precursor gases in the experiment. First, MeCpPtMe3 was pulsed into the reactor for 20 minutes, providing ample time for the gas to infuse the aerogel. Then, the reactor was purged with nitrogen for 10 minutes to remove excess precursor molecules remaining in the chamber. Next, oxygen was added for another 10 minutes. The oxygen reacted with the MeCpPtMe3 already on the substrate, transforming the platinum into its solid, metallic state. This step was followed by another 10-minute pulse of nitrogen. Because the team wanted to test how little platinum was needed to produce an efficient catalyst, the carbon aerogels were exposed to 2, 5, and 10 deposition cycles for comparison.
Team members Sergei Kucheyev and Morris Wang characterized the 10-cycle platinum-loaded aerogels at Livermore using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy. The characterization revealed that the deposited platinum does not form a continuous thin film on the surface of the carbon aerogel. Rather, the platinum–aerogel interaction produces hemispherical platinum nanoparticles less than 5 nanometers in diameter. Forming such small platinum nanoparticles is key to a high catalytic efficiency because more of the catalyst’s atoms are exposed at the surface, thus increasing the catalyst’s ability to induce chemical reactions.
The catalytic properties of the platinum-loaded aerogels were then tested at the University of Bremen. Aerogel substrates exposed to the various deposition cycles were individually placed inside a glass continuous-flow reactor. A combination of nitrogen and oxygen was mixed with carbon monoxide and injected into the reactor. The team compared the conversion rates of each of the aerogels for oxidizing the carbon monoxide into carbon dioxide. Surprisingly, they found the aerogel that had undergone only two ALD cycles had the same conversion efficiency—nearly 100 percent conversion in the 150 to 250°C range—as those that had been through 5 and 10 cycles. In other words, less platinum could be used to obtain the same catalytic effect.
Another “Noble” Cause
According to these scientists, the potential use for nanoporous materials is virtually untapped. As Baumann, Biener, and other researchers continue their work on nanoporous structures, the scientific and technological innovations they bring about could have a significant effect on the way electric, catalytic, and waste management processes are performed today. In the future, nanoporous materials such as platinum-loaded aerogels could play a key role in applications for water treatment, hydrogen storage technology, and more efficient fuel cells that use less but do more.
Key Words: atomic layer deposition (ALD), carbon aerogels, catalysis, catalyst, fuel cell, hydrogen storage, nanoporous materials, oxidation, platinum.
Lawrence Livermore National Laboratory
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