Leidenfrost drops prove to be versatile nanoreactors
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Synthesis of nanoporous gold: (a) photograph of macroscopic spongy gold structure prepared by the Leidenfrost drop synthesis method collected on a glass substrate; (b) Leidenfrost pool of suspended nanoporous brown gold levitating on a hot plate and (c) the same solution inside a glass container; (d) time evolution for synthesis of solid nanoporous gold sphere from initialization to final product as a sponge; (e) scanning electron microscope (SEM) image of the spongy brown gold (inset: higher magnification SEM image); (f) Leidenfrost pool of suspended nanoporous black gold levitating on a hot plate and (g) the same solution inside a glass container.
centration within the drop was increased by adding NaOH to the reaction solution, macroscopic, spongy three-dimensional (3D) metal nanostructures were formed. Nanoparticle synthesis, clustering, assembly, and fusion all occur within the levitating Leidenfrost drop (see Figure), and preliminary follow-up investigations show that the morphology, shape, and porosity of the resulting nanoporous metal can be controlled by varying the NaOH concentration. Alternatively, homogeneous nanostructured coatings can be applied to different substrates in situ after the synthesis of nanostructures within the Leidenfrost reactor. To accomplish a uniform coating the substrate is placed inside the drop, which is rotated during the levitation phase. The team led by Mady Elbahri, who holds a joint appointment at the University of Kiel and the Helmholtz-Zentrum Geesthacht, demonstrated that Leidenfrost drops can even be used as nanoreactors to synthesize functional metal–polymer hybrid foams. Such a 3D coating of a complex structure cannot be obtained by conventional immersion synthesis techniques; it requires the unique aspects of a Leidenfrost nanoreactor. “We understand the chemistry; extending the range of applications for Leidenfrost drop chemistry now necessitates a better understanding of the levitation process. This is the challenge we are working on,” said Elbahri. For now, however, the limiting factors to advancing this new type of charge-driven chemistry are the size of the levitated droplet and its stability. Birgit Schwenzer
into atomic-scale patterns and is linked to the emergence of high-temperature superconductivity. An international research team has now illuminated the origins of the socalled “stripe phase” in which electrons become concentrated in stripes throughout a material. “We’re trying to understand nanoscale order and how that determines materials properties such as supercon-
ductivity,” said Robert Kaindl, a physicist at Lawrence Berkeley National Laboratory (Berkeley Lab). “Using ultrafast optical techniques, we are able to observe how charge stripes start to form on a time scale of hundreds of femtoseconds.” Kaindl, W.-S. Lee (SLAC National Accelerator Laboratory), T. Sasagawa (Tokyo Institute of Technology), and their colleagues reported the results of
Leidenfrost drops prove to be versatile nanoreactors
T
he Leidenfrost effect, first investigated over 250
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