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Though science has known for some time that such ornamentation can greatly increase the surface area and alter the surface chemistry of nanowires, engineers at Stanford University have found a more effective method of decorating them that is simpler and faster than previous techniques.

The results of their study were published recently in the journal Nano Letters.

Xiaolin Zheng, center, watches as In Sun Cho, left, and Yunzhe Feng, right, prepare nanowires with a solvent-based gel of metal and salt. (Credit: John Todd)

The development, say the researchers, might someday lead to better lithium-ion batteries, more efficient thin-film solar cells and improved catalysts that yield new synthetic fuels.

Tiny trees

“You can think of it like a tree. The nanowires are the trunk, very good at transporting electrons, like sap, but limited in surface area,” explains Xiaolin Zheng, an assistant professor of mechanical engineering and senior author of the study.

“The added nanoparticle decorations, as we call them, are like the branches and leaves, which fan out and greatly increase the surface area.”

At the nanoscale, surface area matters a great deal in engineering applications like solar cells, batteries, and especially catalysts, where the catalytic activity is dependent on the availability of active sites at the surface of the material.

“Greater surface area means greater opportunity for reactions and therefore better catalytic capabilities in, for example, water-splitting systems that produce clean-burning hydrogen fuel from sunlight,” says Yunzhe Feng, a research assistant in Zheng’s lab and first author of the study.

Other applications such as sensing small concentrations of chemicals in the air—of toxins or explosives, for example—might also benefit from the greater likelihood of detection made possible by increased surface area.

Burst of flame

The key to the Stanford team’s discovery was a flame. Engineers had long known that nanoparticles could be adhered to nanowires to increase surface area, but the methods for creating them were not very effective in forming the much-desired porous nanoparticle chain structures. Those other methods proved too slow and resulted in a too-dense, thick layer of nanoparticles coating the wires, doing little to increase the surface area.

Zheng and her team wondered whether a quick burst of flame might work better, so they tried it.

Zheng dipped the nanowires in a solvent-based gel of metal and salt, then air-dried them before applying the flame. In the process, the solvent burns in a few seconds, allowing the all-important nanoparticles to crystalize into branch-like structures fanning out from the nanowires.

“We were a little surprised by how well it worked,” says Zheng. “It performed beautifully.”

Using sophisticated microscopes and spectroscopes at the Stanford Nanocharacterization Laboratory, the engineers were able to get a good look at their creations.

“It created these intricate, hair-like tendrils filled with lots of nooks and crannies,” says Zheng. The bejeweled nanowires look like pipe cleaners. The resulting structure increases the surface many fold over what went before, she says.

“The performance improvements have so far been dramatic,” says In Sun Cho, a postdoctoral fellow in Zheng’s lab.

Zheng and her team have dubbed the technique the sol-flame method, for the combination of solvent and flame that yields the nanoparticle structures. The method appears general enough to work with many nanowire and nanoparticle materials and, perhaps more important, provides an unprecedented degree of engineering control in creating the nanoparticle decorations.

The high temperature of the flame and brief annealing time ensure that the nanoparticles are small and spread evenly across the nanowires. And, by varying the concentration of nanoparticle in the precursor solution and the number of times the wires are dip-coated, the team was able to vary the size of the nanoparticle decorations from tens to hundreds of nanometers, and the density from tens to hundreds of particles per square micrometer.

“Though more research is needed, such precision is crucial and could bolster the wider adoption of the process,” says Zheng.

Pratap M. Rao and Lili Cai also contributed to this research. The Office of Naval Research/PECASE program supported the study.

More news from Stanford: http://news.stanford.edu/