News — UPTON, NY鈥擥old bars may signify great wealth, but the precious metal packs a much more practical punch when shrunk down to just billionths of a meter. Unfortunately, unlocking gold鈥檚 potential often requires complex synthesis techniques that produce delicate structures with extreme sensitivity to heat.
Now, scientists at the U.S. Department of Energy鈥檚 Brookhaven National Laboratory have discovered a process of creating uniquely structured gold-indium nanoparticles that combine high stability, great catalytic potential, and a simple synthesis process. The new nanostructures鈥攄etailed online June 10 in the 鈥攎ight enhance many different commercial and industrial processes, including acting as an efficient material for catalytic converters in cars.
鈥淲e discovered a room-temperature process that transforms a simple alloy into a nanostructure with remarkable properties,鈥� said physicist Eli Sutter, lead author on the study. 鈥淏y exposing the gold-indium alloy nanoparticles to air, ambient oxygen was able to drive an oxidation reaction that converted them into an active core-shell structure.鈥�
Larger chunks of gold show the lowest chemical reactivity of all metals, but divided into discrete nanoparticles, gold can become a highly active chemical catalyst. But keeping gold in this active state is an ongoing challenge. Under even moderate heat, the tiny gold particles tend to sinter鈥攆using together into much larger pieces鈥攁nd lose that crucial reactivity. Linking gold with other elements, however, can both increase durability and retain catalyst qualities鈥攂ut only if the structure is perfect.
鈥淲hen we saw these new nanoparticles with gold throughout their amorphous shells, we immediately recognized the exceptional potential of the material,鈥� said physicist and study coauthor Peter Sutter. 鈥淭he presence of gold embedded in the outermost atomic layers of the oxide shell not only excites us as scientists, but it is key to the catalytic activity and enhanced stability of the new material.鈥�
The Brookhaven Lab researchers were studying oxidation processes through which metals and alloys combine with oxygen when they made the discovery. For this study, they examined alloys of a noble metal and a non-noble metal through a remarkably simple reaction technique: giving gold-indium nanoparticles a little room to breathe. Once nanoparticles of the metal alloy were exposed to oxygen, highly reactive shells of gold-indium oxide formed across their surfaces.
鈥淐onventional wisdom would say that oxidation should push the gold atoms into the center while pulling the less noble indium to the surface, creating a noble metal core that is surrounded by a shell of non-reactive indium-oxide,鈥� Peter Sutter said. 鈥淚nstead, the oxygen actually penetrated the alloy. After oxidation, the alloy core of the nanoparticles was encapsulated by a newly formed thin shell of mixed gold-indium oxide.鈥�
Trapping gold in the amorphous oxide shell retains its catalytic properties and prevents the gold from sintering and becoming inert. The new nanostructures proved capable of converting oxygen and carbon monoxide into carbon dioxide, demonstrating their activity as a catalyst.
鈥淭he indium and gold in the shell are not mobile, but are frozen in the amorphous, oxide,鈥� Eli Sutter said. 鈥淚mportantly, the structural integrity holds without sintering at temperatures of up to 300 degrees Celsius, making these remarkably resilient compared to other gold nanocatalysts.鈥�
The research was conducted at Brookhaven Lab鈥檚 Center for Functional Nanomaterials (CFN), whose unique facilities for nanoscale synthesis and characterization proved central to the discovery of this new process.
鈥淭he CFN brings a wide range of state-of-the-art instruments and expertise together under one roof, accelerating research and facilitating collaboration,鈥� Eli Sutter said. 鈥淲e used transmission electron microscopy to characterize the structures and their composition, x-ray photoelectron spectroscopy to determine the chemical bonding at the surface, and ion-scattering spectroscopy to identify the outermost atoms of the nanoparticle shell.鈥�
Further investigations will help determine the properties of the gold-indium oxide particles in different catalytic reactions, and the same oxidation process will be applied to other metal alloys to create an entire family of new functional materials.
The Center for Functional Nanomaterials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the U.S. Department of Energy, Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please .
DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
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