Platinum nanostructures on alumina become highly faceted at temperatures greater than 700 °C, making them potentially useful for high-temperature fuel-cell catalyst applications. The new, simple self-assembly technique to make such metal-oxide systems is simpler than existing chemical methods, says Deeder Aurongzeb of Texas Tech University in the US.
Self-assembly is an attractive technique for producing nanoscale structures because it is relatively simple compared with conventional patterning techniques, such as focused ion beam, lithography or template nanoimprinting. Although planar platinum is already widely used in fuel-cell applications, faceted platinum nanoclusters should be even more efficient catalysts. However, little is known about how stable such nanostructures are and how they form. Aurongzeb has now grown faceted platinum nanodots in alumina by rapidly heating platinum thin-films to 700 °C. He found that at a critical film thickness of 2 nm, the clusters form facets with multifaceted sidewalls. These structures greatly increase the surface area of the material, so increasing efficiency.
According to Aurongzeb, the shape of the nanoclusters can be controlled with temperature and time. "It is interesting to see a cubic close-packed metal turn into various geometric shapes, such as hexagonal, pentagonal and triangular, at high temperatures," he told nanotechweb.org. "These structures are very stable, thus useful for high-temperature applications." The rapid annealing technique could be extended to other metal-oxide systems. Applications include gas sensing and catalytic decomposition of environmentally unfriendly material, says Aurongzeb.
The experiment itself is "really simple", he adds. "Metal diffuses inside an oxide if you heat the combination to a very high temperature, so the trick is to find the right film thickness and temperature where the atomic self-diffusion is faster than the diffusion of atoms in the host. This is achieved by rapid thermal annealing of films at various film thicknesses and temperatures to find the exact point at which this happens."
High-temperature, long-life fuel cells use oxide surfaces like alumina to support platinum. However, the problem is that there are few reaction sites available on the surface of the platinum. "Crystal facets are sources for reaction sites and they are tightly bonded" explained Aurongzeb "and we showed that up to 18 facets/steps can be formed on the platinum nanodots."
He goes on to say that it is very difficult to fabricate platinum with such a large number of facets using conventional chemical methods that have much less control.
Aurongzeb now plans to nanoalloy and layer the nanodots with other metals like osmium, iridium and conducting oxides and oxinitrides to form even higher numbers of facets and reaction sites. "I believe more-complex combinations with other supports like carbon or carbides will lead to very interesting shapes and sizes that will be stable at high temperatures," he said. "Not to mention the interesting physics that will unfold with all of these studies."
Self-assembly is an attractive technique for producing nanoscale structures because it is relatively simple compared with conventional patterning techniques, such as focused ion beam, lithography or template nanoimprinting. Although planar platinum is already widely used in fuel-cell applications, faceted platinum nanoclusters should be even more efficient catalysts. However, little is known about how stable such nanostructures are and how they form. Aurongzeb has now grown faceted platinum nanodots in alumina by rapidly heating platinum thin-films to 700 °C. He found that at a critical film thickness of 2 nm, the clusters form facets with multifaceted sidewalls. These structures greatly increase the surface area of the material, so increasing efficiency.
According to Aurongzeb, the shape of the nanoclusters can be controlled with temperature and time. "It is interesting to see a cubic close-packed metal turn into various geometric shapes, such as hexagonal, pentagonal and triangular, at high temperatures," he told nanotechweb.org. "These structures are very stable, thus useful for high-temperature applications." The rapid annealing technique could be extended to other metal-oxide systems. Applications include gas sensing and catalytic decomposition of environmentally unfriendly material, says Aurongzeb.
The experiment itself is "really simple", he adds. "Metal diffuses inside an oxide if you heat the combination to a very high temperature, so the trick is to find the right film thickness and temperature where the atomic self-diffusion is faster than the diffusion of atoms in the host. This is achieved by rapid thermal annealing of films at various film thicknesses and temperatures to find the exact point at which this happens."
High-temperature, long-life fuel cells use oxide surfaces like alumina to support platinum. However, the problem is that there are few reaction sites available on the surface of the platinum. "Crystal facets are sources for reaction sites and they are tightly bonded" explained Aurongzeb "and we showed that up to 18 facets/steps can be formed on the platinum nanodots."
He goes on to say that it is very difficult to fabricate platinum with such a large number of facets using conventional chemical methods that have much less control.
Aurongzeb now plans to nanoalloy and layer the nanodots with other metals like osmium, iridium and conducting oxides and oxinitrides to form even higher numbers of facets and reaction sites. "I believe more-complex combinations with other supports like carbon or carbides will lead to very interesting shapes and sizes that will be stable at high temperatures," he said. "Not to mention the interesting physics that will unfold with all of these studies."
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