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Sunlight – valued as a nearly infinite source of energy – can also provide an unusual glimpse into the nanoworld. In a new study, led by researchers from the Weizmann Institute and Ben-Gurion University of the Negev, highly concentrated solar radiation helped reveal the shapes assumed by certain inorganic nanoparticles. This research, conducted in collaboration with German scientists, sheds new light, as it were, on the behavior of particles on the nanoscale and could lead to advanced uses for various nanomaterials.
Nearly two decades ago, the Weizmann Institute’s Prof. Reshef Tenne and his colleagues in the Chemistry Faculty were the first to discover that inorganic materials could form hollow, cage-like nanostructures. Until then, only carbon molecules were known to form hollow spheres, which came to be called fullerenes due to their likeness to the geodesic domes built by the architect Buckminster Fuller. Tenne’s discovery opened up a new field of research into the inorganic fullerene-like particles, creating exciting applications, including the manufacture of superior solid lubricants, and posing a host of new questions, particularly those concerning the connection between the particles’ superior properties and their structure and shape.
The “true” inorganic fullerene – the smallest, most stable cage-like particle of inorganic material – is shaped like an octahedron resembling an eight-sided die. Such tiny octahedra have already been produced at the Weizmann Institute and elsewhere. But larger inorganic fullerene-like structures have also been produced, these being multi-layer spheres. At what point, exactly, does the octahedron become spherical in shape?
This is a crucial question – in part because the two shapes appeared to endow the nanoparticles with different properties – and it has been answered by the new study, published recently in the international edition of Angewandte Chemie. The international team worked with molybdenum disulfide (MoS2) nanoparticles just a few millionths of a meter across.
Their first goal was to create an atomic vapor of molybdenum disulfide. Laser light, used in previous nanoscale studies with this material, was found to produce only small octahedra made up of 20,000 atoms. To generate larger octahedra, Ben-Gurion University researchers headed by Prof. Jeffrey Gordon built an innovative table-top solar concentrator consisting of an elaborate system of mirrors that created an ultra-intense solar beam focused to a magnitude of about 15,000 suns. Inside a quartz capsule, molybdenum disulfide heated to 2,500oC was vaporized into a hot cloud of individual atoms. Since the concentrated solar beam is appreciably wider than the beams of typical pulsed lasers, the evaporated atoms could, upon cooling, form much larger clusters than those obtained with lasers.
Lead author of the study Dr. Ana Albu-Yaron of the Weizmann Institute and her colleagues used a series of electron microscopes – including the most advanced one in Dr. Lothar Houben’s laboratory at the Jülich Research Center in Germany – to view the architecture of these nanoparticles. The picture that emerged provided the first support for certain theoretical predictions in this area made by Prof. Gotthard Seifert’s group at the Technical University of Dresden, which turned out to be impressively accurate.
In addition to the distinct eight-sided and spherical shells, the team observed hybrid nanoparticles of molybdenum disulfide assuming an intermediate “transition” shape, and the study revealed precisely at which size each shape occurs. The smallest molecules, made up of no more than 100,000 atoms, were hollow octahedra. Larger particles, comprising about 500,000 atoms, had an intermediate structure: octahedral layers at the core, surrounded by multiple onion-like spherical shells.
Beyond addressing fundamental questions in materials science, these results can be of practical significance. Molybdenum disulfide is used as a catalyst for removing sulfur from fossil fuels to prevent acid rain. In the form of nanoparticles, the catalyst could be much more effective: Thanks to their voluminous, three-dimensional structure, such particles are likely to be more accessible to interaction with the sulfur, speeding up the removal process. The researchers plan to explore the potential of their solar-generated nanoparticles as catalysts once they manage to produce them in sufficient amounts. Such catalysts would be doubly beneficial to the environment: First, they are produced by a clean solar method; second, they promise to be more effective than existing ones at reducing the damage caused by fossil fuels.
Additional applications for the solar-synthesized fullerene-like particles might stem from the fact that the eight-sided molybdenum disulfide molecules are metallic in character whereas the spherical ones are semiconductors. The intermediate particles are hybrids: A metal-like component is embedded within a semiconductor, a structure that could find new uses in the semiconductor industry – for example, in the manufacture of advanced sensors.
The study was conducted by Dr. Ana Albu-Yaron and Prof. Moshe Levy in the lab of Prof. Reshef Tenne of the Weizmann Institute’s Materials and Interfaces Department, together with Dr. Ronit Popovitz-Biro of the Institute’s Chemical Research Support, and by Prof. Daniel Feuermann and Dr. Eugene A. Katz in the lab of Prof. Jeffrey Gordon at Ben-Gurion University, in collaboration with researchers in Germany: Marc Weidenbach, Drs. Maya Bar-Sadan and Lothar Houben of the Forschungszentrum Jülich, and Dr. Andrey N. Enyashin and Prof. Gotthard Seifert of the Technische Universität Dresden.
Prof. Reshef Tenne is Head of the Helen and Martin Kimmel Center for Nanoscale Science; and his research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement. Prof. Tenne is the incumbent of the Drake Family Professorial Chair in Nanotechnology.
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