In My Element: Silver

Abstract

The In My Element series celebrates the personal accounts from Chemistry – A European Journal Editorial Board members for the 2019 International Year of the Periodic Table. In this contribution, Younan Xia gives his story on silver. Silver (Ag) is my favorite chemical element due to its broad spectrum of unique properties and applications. Although a silver medal may not be as valuable as a gold medal, silver easily beats gold in an array of applications. Silver has the highest electrical and thermal conductivities among all metals. Since ancient times, the antimicrobial capability of silver was utilized to purify water, preserve fresh milk, and treat wounds. Before digital cameras came of age, silver-based photographic films were everyday commodities. Silver is also an effective catalyst for oxidation reactions as molecular oxygen can adsorb and dissociate into atomic oxygen on its surface under ambient conditions. In fact, silver is the only catalyst capable of converting ethylene to ethylene oxide (on a scale of 20 million tons in 2009!). Oh, did I mention that silver is probably the best material for the production of shiny mirrors? The last but not least, the abundance of silver in Earth's crust is much higher than that of gold (79 vs. 3.1 ppb) so the price of silver is always a small fraction of that of gold ($17 vs. $1299 per oz.). Silver is also my lucky metal. Since 2001, my group has worked on the development of methods for the colloidal synthesis of silver nanocrystals with controlled shapes in an effort to tailor their surface plasmon resonance properties. We began with the silver mirror reaction, in which a silver ammonia complex is mixed with an aldehyde compound for immediate reduction and thus formation of a shiny mirror on the surface of the container (see the photo, left panel). With the addition of a polymer stabilizer, we could use the same reaction to produce a stable suspension of silver nanoparticles. The suspension displayed a yellow color (see the photo, right panel) because of the surface plasmon resonance peaked at 400 nm. In an effort to better control the reduction kinetics, we later switched to polyol reduction, in which a polyol such as ethylene glycol is thermally oxidized to glycoaldehyde. Since the reductant can be generated in situ and then consumed to maintain a relatively stable concentration in the reaction solution, we were able to synthesize silver nanocrystals with controlled shapes and facets, including cubes, octahedrons, cuboctahedrons, bipyramids, bars, rods, and wires. These novel nanocrystals have opened up a world of opportunities for applications in areas ranging from catalysis to plasmonics, photonics, and surface-enhanced Raman scattering, as well as fabrication of touchscreens, solar cells, flexible electronic devices, and metamaterials. In addition, our technology on silver nanowires has been commercialized by various companies for the production of flexible, transparent, and conductive electrodes. Photographs of (left) a silver mirror and (right) a colloidal suspension of silver nanoparticles. Position Brock Family Chair Professor and GRA Eminent Scholar E-mail [email protected] Homepage http://nanocages.com Education B.S (University of Science and Technology of China, 9/1982 to 7/1987, Hefei, Anhui, P. R. China) M.S. (9/1991 to 5/1993, Alan G. MacDiarmid, University of Pennsylvania, Philadelphia, PA, USA) Ph.D. (9/1993 to 8/1996, George M. Whitesides, Micro- and Nanofabrication Based on Microcontact Printing and Replica Molding, Harvard University, Cambridge, MA, USA) Postdoctoral Fellow (8/1996 to 7/1997, George M. Whitesides, Harvard University, Cambridge, MA, USA) Awards MRS Medal (Materials Research Society, 2017); ACS National Award in the Chemistry of Materials (American Chemical Society, 2013); NIH Director's Pioneer Award (National Institutes of Health, 2006); Camille Dreyfus Teacher Scholar (2002); David and Lucile Packard Fellow in Science and Engineering (2000); Sloan Research Fellow (2000); CAREER Award (National Science Foundation, 1999); Victor K. LaMer Award (American Chemistry Society, 1999); New Faculty Award (Camille and Henry Dreyfus Foundation, 1997) Research interests Nanocrystals, electrospinning, self-assembly, colloid science, catalysis, plasmonics, nanomedicine, regenerative medicine Hobbies Reading, cooking, table tennis