WCA January 2020

From the Americas

Study sheds light on future chemicals and fuel production A team from the University of Oregon has used an atomic force microscope to identify how catalysts made from nickel nanoparticles can collect charges excited by light in semiconductors. The findings, described in a paper in Nature Materials and reported by The Engineer , could lead to systems that use light to make chemicals and fuels – such as by splitting water to make hydrogen or by combining carbon dioxide and water to make carbon-based fuels or chemicals. “We found a design principle that points to making catalytic particles really small because of the physics at the interface, which allows one to increase efficiency,” said Shannon Boettcher, a professor in Oregon’s Department of Chemistry and Biochemistry and member of the university’s Materials Science Institute. “Our technique allowed us to watch the flow of excited charges with nanometre-scale resolution, which is relevant for devices that use catalytic and semiconductor components to make hydrogen that we can store for use when the sun is not shining.” Mr Boettcher’s team used a model system consisting of a single-crystal silicon wafer coated with metallic nickel nanoparticles of different sizes. The silicon absorbs sunlight and creates excited positive and negative charges. The nickel nanoparticles selectively collect the positive charges and speed up the reaction of those positive charges with electrons in water molecules, pulling them apart. He explained that previous researchers have been restricted to measuring the average current moving across such a surface and the average voltage generated by the light hitting the semiconductor. By collaborating with Bruker Nano Surfaces, the manufacturer of the University of Oregon’s atomic force microscope that images the topography of surfaces by tapping a sharp tip over it, Mr Boettcher’s team was able to develop the techniques needed to measure voltage at the nanoscale. As the electrode tip touched each of the nickel nanoparticles, the researchers were able to record the build-up of holes by measuring the voltage. When the size of the catalytic particles shrinks below 100nm the collection of excited positive charges (known as “holes”) becomes much more efficient than the collection of excited negative charges. This phenomenon prevents the excited positive and negative charges from recombining and is said to increase the system efficiency. Small particles were able to better select for the collection of excited positive charges over negative charges, reducing the rate of charge recombination and generating higher voltages that better split the water molecules apart. In a statement, Mr Boettcher said that oxidation at the nickel nanoparticle surface leads to a barrier that prevents the negatively charged electrons from flowing to the catalyst and annihilating the positively charged holes. This effect has been termed “pinch-off”, and though long hypothesised to occur in solid state devices had never been observed in fuel-forming photoelectrochemical systems. “This new technique is a general means to investigate the state of nanoscale features in electrochemical environments,” said the study’s lead author, Forrest Laskowski. “While our results are useful for understanding

New material and research

Nanoparticles to the fore Are strength and conductivity no longer mutually exclusive? A new form of a familiar metal could hold the key to a broad range of advanced technologies. Studies into the nanoscale behaviour of silver have led to the creation of a new variety of the metal that is 42 per cent stronger, but retains the electrical conductivity of softer silvers. Materials scientist Frederic Sansoz, of the University of Vermont, believes the new silver will find applications from aerospace to energy. The discovery was made by a team from Vermont University and the Lawrence Livermore National Laboratory, examining natural defects in metal that bring unwanted properties, such as brittleness or softening. Unfortunately, when an alloy is created to overcome an unwanted property, the result is usually a drop in conductivity; Mr Sansoz and Lawrence Livermore scientist Morris Wang chose to address the issue. In a paper published in Nature Materials , Mr Sansoz and Mr Wang described how they doped silver with a small percentage of copper to control the behaviour of defects in the silver lattice. Using less than one per cent by weight of copper produced a marked improvement in conductivity. The scientists found that the copper impurity turned two types of inherent nanoscale defect in the silver into a much stronger internal structure. The research exploits the Hall-Petch relation, whereby the smaller the micro crystals that compose a metal, the stronger it is. However, the Hall-Petch relation breaks down when the crystals are smaller than tens of nanometres wide, when the boundaries between micro crystals become unstable. When copper is introduced into the silver it is actually attracted to the defects in the lattice. Copper atoms, being slightly smaller than silver atoms, stabilise the lattice but at such a low concentration they do not interrupt the conductivity. “The copper atom impurities go along each interface, and not in between,” Mr Sansoz explained, “so they don’t disrupt the electrons that are propagating through.” The team refers to the alloy as a “nanocrystalline-nano- twinned metal” and are claiming “unprecedented mechanical and physical properties” for the development. Not only does the new alloy overcome the softening previously seen when micro crystals and twin boundaries get too small, it also overcomes the theoretical Hall-Petch limit where crystal size reduction ceases to strengthen the material. Mr Sansoz believes that this new class of metal will have many uses, and that the principle could be applied to other metals. “This is a new class of materials and we’re just beginning to understand how they work,” he said. “When you can make material stronger you can use less of it, and it lasts longer, and being electrically conductive is crucial to many applications.”

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Wire & Cable ASIA – January/February 2020