Nanoparticle synthesis: A trick of the light

Nanocrystals are transformed into nanotubes after illumination with ultra-violet light.

Researchers at the University of Science and Technology of China (USTC) in Hefei, China have demonstrated the transformation of nanocrystals into one-dimensional hollow nanotubes by irradiation with ultraviolet (UV) light.¹

Fig. 1: CdCl₂ nanotube obtained by illuminating CdSe nanocrystals with UV-light.

Hollow nanoparticles are of interest because of their potential applications, such as drug delivery and chemical sensing. Magnetic or electric fields can be used to control the structural properties of these types of nanoparticles and here, Xiaoping Wang and colleagues tested the effects of light on the synthesis of nanoparticles by dispersing CdSe nanocrystals–approximately 2nm in diameter– in o-dichlorobenzene and then irradiated them with UV light for two hours.

“It is significant if we can control the morphology of nanostrucures in synthesis processes by use of external forces such as magnetic, optical and electric fields,” says Wang. “And some work has shown that light may induce electric dipoles on the nanostructure to promote their assembly.”

The researchers found that the nanocrystals had transformed into CdCl₂ nanotubes. A careful study of the different stages of the transformation by transmission electron microscopy revealed that during the first stage the nanocrystals become hollow CdCl₂nanospheres, which then coalesce and form nanotubes in the second stage.

The researchers believe that the UV radiation produces free Cl radicals from the o-dichlorobenzene. At this point the first stage of the transformation occurs through the Kirkendall effect. Namely, the difference in concentration between Cd, Se and Cl pushes both Cd and Cl towards the surface of the crystals, while Se atoms are initially trapped in the core and then expelled through the CdCl₂ shell.

In the second stage, the electric dipole attraction between hollow spheres produces long chains of spheres that eventually merge into a single tube to minimise their surface energy.

“Our findings shows that one can control the dimensions and configuration of nanostructures simultaneously during synthesis,"says Wang. “In addition, this light induced method not only have potential applications for fabrication of inorganic fullerene-like nanostructures in mild conditions, but this method is also an effective approach for head-to-end self-assembly. We are on the way.”