Nanoparticles: A simpler route to hollow carbon spheres
Hollow carbon nanoparticles are strong, conduct electricity well and have a remarkably large surface area. They show promise in applications such as water filtration, hydrogen storage and battery electrodes—but commercial use would demand reliable, low-cost ways for their production.
Xu Li of Singapore's A*STAR Institute of Materials Research and Engineering and co-workers have developed a simple manufacturing technique that offers precise control over the size and shape of hollow carbon nanospheres.
A current method for preparing these particles involves coating a hard template, such as silica nanoparticles, with a carbon-based material that can be fused into a shell using extreme heat. This is a laborious process, and etching away the template requires harsh chemicals. Heating hollow polystyrene nanospheres achieves similar results but offers poor control over the size and shape of the resulting carbon nanoparticles.
Li and co-workers combined a block copolymer called F127, consisting of poly(ethylene oxide) and poly(propylene oxide), with donut-shaped ?-cyclodextrin molecules in water. After heating the mixture to 200 °C, the molecules self-assembled into hollow nanoparticles with a 97.5% yield.
The water-repelling poly(propylene oxide) parts of the polymer stuck together to form hollow spheres, leaving poly(ethylene oxide) molecules dangling from the outside. The ?-cyclodextrin rings then threaded onto these strands, packing around the outside of the sphere to form a stable shell. Using a higher proportion of F127 in the mix produced larger nanospheres, ranging from 200 to 400 nanometers in diameter. Heating these particles to 900 °C in inert gases burned off the polymer to make hollow carbon nanoparticles.
The smallest nanospheres were 122 nanometers across and had 14 nanometer-thick walls dotted with tiny pores roughly 1 nanometer wide. Each gram of this material had a surface area of 317.5 square meters, which is greater than a tennis court.
The researchers used a slurry of particles to coat a copper foil and tested it as the anode in a lithium-ion battery. They found that the particles had a reversible charging capacity of 462 milliampere hours per gram—higher than graphite, a typical anode material—and could be recharged at least 75 times without significant loss of performance. The pores apparently allow lithium ions to migrate to the inside surfaces of the spheres. "Changing the porosity could improve the transport process for higher performance," suggests Li. The team now plans to incorporate metal and metal oxide materials into the hollow carbon nanospheres to further enhance their properties.