Concrete may have found it's killer app in graphene super-sensitivity
The use of metal nanoparticles has found broad applications ranging from high-sensitivity sensors and bio-imaging to catalysis, lubrication and energy storage. Scientists have tried to use graphene to make similar particles for an even wider range of applications, including sensing, photonic and energy devices. But most studies of these devices have been limited to either 2D graphene sheets or, if one went up in dimensionality, graphene nano-ribbons.
No one has tried applying 3D graphene. Until now.
The research team, led by Zhong Yao, senior scientist in the Department of Materials Science and Engineering at the University of California, Berkeley, made metal particles of different sizes by growing them in hollow carbon nanotubes. They then added one, two or three of these metal particles to the graphene material and grew it in three dimensions. Graphene-metal particles larger than 10 nanometers formed sheets, while particles ranging from 8 to 6 nanometers grew the largest individual flakes. Graphene with between four and eight metal nanoparticles formed a sheet, but these were small and likely to have a rough surface, the researchers explain in a paper that will be published online this week in the journal ACS Nano.
“If you look at the size of these metals, you don’t see it in the X-ray diffraction data,” Yao said, adding that the method used to see the particles in this study could enable other researchers to track down the exact location of other nanoparticles.
“It’s not that we can’t see them, it’s that there’s not enough signal to detect them in a very well-ordered system like this.”
Graphene is a semi-conductor that’s very much like Silicon in how it conducts electricity, but it’s 200 times thinner and stronger than a sheet of glass. Graphene’s combination of electronic properties and physical strength, combined with its high biocompatibility, make it a very attractive material for use in biosensors and energy devices. It could also be used as a platform to make more efficient energy storage devices, Yao said.
Yao and her team made the graphene-metal samples by synthesizing hollow carbon nanotubes filled with metal and then removing the metallic nanoparticles that had formed on the tube walls, leaving a metallic shell outside the tube and a hollow interior. They grew sheets and flakes of the nanocomposite by inserting the material into a furnace set to 500°C.
“The metals like to bond with the [carbon] nanotubes because they have to do work,” Yao explained. “That work draws on a lot of energy in a process called reduction, and that reduces the nanotubes and the metal particles.”
But the reduction of the carbon nanotube is very localized, which is why it’s important that the material used is an isolated particle instead of a large, disordered aggregate. “If you have a big aggregate of metal nanoparticles, you don’t have this reduction.”
Yao said that in a very organized system, reduction can go on in three dimensions, making more metal particles, “but those are small-area reduction.” The large-scale reduction that creates the layers of graphene is driven by the carbon nanotubes inside the system. That process could make the graphene grow in height, instead of across the horizontal plane.
At 400°C, the process became chaotic, she added. “But above 500°C, the metal particles become oxidized, which makes them very soft and not easily reducible.” The researchers found that their nanocomposite can be reduced to graphene down to the atomic level by using liquid nitrogen at 300°C. The graphene they made is also similar to that made using other techniques, such as chemical vapor deposition.
Yao says that the addition of different metals is an effective method to tailor graphene’s electronic properties, giving it even better potential to be a more efficient device than graphene, and other related materials, have shown so far. The research could help other people find new ways to make graphene and graphene-based materials, and enable them to make new graphene-based devices.
Yao explained that it’s difficult to make these types of devices using the two-dimensional sheet of graphene.
“Many materials, especially at the nanoscale, have to go through reduction. If you make a sheet of graphene, you’ve never really had the reduction in the horizontal direction to make a very clean product.” She’s also been talking with physicists about using the technique she developed to grow multi-layer graphene and study how the materials’ properties change when they are placed together. But she thinks graphene sheets are going to have the broadest potential, because they can be manufactured easily.
“They are also better than the nanoribbons because they are so strong and have low defects,” she said. “We’re now looking into how we can use the horizontal direction of the layered graphene to make better devices.”
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The use of metal nanoparticles has found broad applications ranging from high-sensitivity sensors and bio-imaging to catalysis, lubrication and energy storage. Scientists have tried to use graphene to make similar particles for an even wider range of applications, including sensing, photonic and energy devices. But most studies of these devices have been limited to either 2D graphene sheets or, if one went up in dimensionality, graphene nano-ribbons.
No one has tried applying 3D graphene. Until now.
The research team, led by Zhong Yao, senior scientist in the Department of Materials Science and Engineering at the University of California, Berkeley, made metal particles of different sizes by growing them in hollow carbon nanotubes. They then added one, two or three of these metal particles to the graphene material and grew it in three dimensions. Graphene-metal particles larger than 10 nanometers formed sheets, while particles ranging from 8 to 6 nanometers grew the largest individual flakes. Graphene with between four and eight metal nanoparticles formed a sheet, but these were small and likely to have a rough surface, the researchers explain in a paper that will be published online this week in the journal ACS Nano.
“If you look at the size of these metals, you don’t see it in the X-ray diffraction data,” Yao said, adding that the method used to see the particles in this study could enable other researchers to track down the exact location of other nanoparticles.
“It’s not that we can’t see them, it’s that there’s not enough signal to detect them in a very well-ordered system like this.”
Graphene is a semi-conductor that’s very much like Silicon in how it conducts electricity, but it’s 200 times thinner and stronger than a sheet of glass. Graphene’s combination of electronic properties and physical strength, combined with its high biocompatibility, make it a very attractive material for use in biosensors and energy devices. It could also be used as a platform to make more efficient energy storage devices, Yao said.
Yao and her team made the graphene-metal samples by synthesizing hollow carbon nanotubes filled with metal and then removing the metallic nanoparticles that had formed on the tube walls, leaving a metallic shell outside the tube and a hollow interior. They grew sheets and flakes of the nanocomposite by inserting the material into a furnace set to 500°C.
“The metals like to bond with the [carbon] nanotubes because they have to do work,” Yao explained. “That work draws on a lot of energy in a process called reduction, and that reduces the nanotubes and the metal particles.”
But the reduction of the carbon nanotube is very localized, which is why it’s important that the material used is an isolated particle instead of a large, disordered aggregate. “If you have a big aggregate of metal nanoparticles, you don’t have this reduction.”
Yao said that in a very organized system, reduction can go on in three dimensions, making more metal particles, “but those are small-area reduction.” The large-scale reduction that creates the layers of graphene is driven by the carbon nanotubes inside the system. That process could make the graphene grow in height, instead of across the horizontal plane.
At 400°C, the process became chaotic, she added. “But above 500°C, the metal particles become oxidized, which makes them very soft and not easily reducible.” The researchers found that their nanocomposite can be reduced to graphene down to the atomic level by using liquid nitrogen at 300°C. The graphene they made is also similar to that made using other techniques, such as chemical vapor deposition.
Yao said that the addition of different metals is an effective method to tailor graphene’s electronic properties, giving it even better potential to be a more efficient device than graphene, and other related materials, have shown so far. The research could help other people find new ways to make graphene and graphene-based materials, and enable them to make new
