Decoration with nanoparticles creates intricate surface patterns full of nooks and crannies, twists and turns that greatly improve surface area. Image courtesy of the Stanford Nanocharacterization Laboratory. |
Like
a lead actress on the red carpet, nanowires—those superstars of
nanotechnology—can be enhanced by a little jewelry, too. Not the
diamonds and pearls variety, but the sort formed of sinuous chains of
metal oxide or noble metal nanoparticles.
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 novel and more effective
method of “decorating” nanowires that is simpler and faster than
previous techniques. The results of their study were published recently
in the journal Nano Letters.
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.
Tree-like structures
“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,”
explained 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,” said 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.
A spark of an idea
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. These
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.
Prof. Xiaolin Zheng has discovered a new way to “decorate” nanowires with coatings of metal nanoparticles that greatly improve surface area. The decorated nanowires look like tiny pipe cleaners. Image courtesy of the Stanford Nanocharacterization Lab. |
Zheng
dipped the nanowires in a solvent-based gel of metal and salt, then
air-dried them before applying the flame. In her process the solvent
burns away 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,” said 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,” said Zheng. The bejeweled nanowires look like pipe
cleaners. The resulting structure increases the surface many fold over
what went before, she said.
Dramatic performance, unprecedented control
“The
performance improvements have so far been dramatic,” said In Sun Cho, a
post-doctoral fellow in Zheng’s lab and co-author of the paper.
Zheng
and 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 importantly, 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 Stanford 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,” said Zheng.
Pratap M. Rao and Lili Cai also contributed to this research. The study was supported by the ONR/PECASE program.
Source: Stanford University
