Beyond the Gold Standard: Bimetallic Nanomaterials Bring New Properties and Functions

DOI: 10.1002/ppsc.201800111 interested in the synthesis, measurement, theory, and application of nanoscale systems. Innovations in synthesis are paramount to achieving bimetallic nanoparticles with the desired structural and compositional features for applications. With their manuscript, Yugang Sun and co-workers (article number 1700413) move the field beyond bulk phase diagrams by flash heating and cooling metal precursors within micelle nanoreactors to achieve alloy PtAu nanoparticles. Platinum and gold are immiscible at thermodynamic equilibrium, and a related numerical study by Riccardo Ferrando and co-workers (article number 1700425) examines the phase separation and thermodynamics of AgCu and AgNi nanoparticles. Seeded methods are promising routes to bimetallic heterostructures; however, the lattice mismatch between metals possessing similar crystal structures is typically less than 5% for conformal heteroepitaxial film deposition to be achieved. Yet, when growing atop colloidal seeds, this stringent criterion no longer appears to hold, with Hui Wang and coworkers (article number 201700361) demonstrating that both face-centered cubic and hexagonal close-packed nickel can be epitaxially grown on gold nanocrystals. Complementing this study is work from Michelle Personick and co-workers (article number 201700401), in which island-like platinum growth on gold seeds was inhibited to achieve conformal heteroepitaxial deposition by exploiting the difference in the reduction rate of gold and platinum ions. The works by Yugang Sun, Hui Wang, and Michelle Personick each report new bimetallic nanostructures, and more fundamentally, report deviations from bulk alloy and multilayer film formation which greatly enhance our understanding of colloidal nanoparticle synthesis. These synthetic innovations are rounded out by four papers on galvanic replacement from the groups of Pedro Camargo (article number 1700175), Jingyi Chen (article number 1800053), Svetlana Neretina (article number 1700420), and Emilie Ringe (article number 1700381), where nanomaterials with well-defined architectures and tunable alloy compositions are readily achieved, often through the use of new templates. Bimetallic nanoparticles are widely studied as catalysts and are even finding use in industrial processes, e.g., the production of nitric acid from ammonia requires a Pt-Rh catalyst. As highlighted in the works by the groups of Xiaohu Xia (article number 1700386) and Yan Xiang (article number 1700366), bimetallic nanomaterials can yield costeffective catalysts by either dispersing highly active but expensive materials on an underlying support or by alloying the material within an earth-abundant matrix. At the same time, these strategies can be used to impart geometric and electronic control of catalytic processes. For example, Our advances as a society and cultural heritage are intimately linked to our use of metals. Ancient ancestors hammered gold and copper into decorative items and simple blades, moving us out of the Stone Age. Moving forward, the casting of metal from naturally occurring copper-tin alloys marked the Bronze Age, when harder and more durable tools could be produced from these mixtures in comparison to tools cast from either metal alone. This pursuit for new materials with superior properties continues today as the morphologyand composition-dependent properties of metal nanomaterials are explored. In particular, bimetallic nanomaterials are of interest because the unique nanoscale properties of each metal are often integrated into one nanoparticle platform to achieve multifunctionality. In addition, completely new properties can emerge from the mixing of two metals at the nanoscale. Interestingly, synthetic advances are now enabling different types of bimetallic nanoparticles, including alloys and intermetallics, heterostructures, and even core@shell architectures, where the differences in bimetallic distribution often impart different properties and functionality (Figure 1). This special issue of Particle & Particle Systems Characterization highlights recent advances in the synthesis, characterization, and properties of bimetallic nanoparticles as they are applied to diverse applications. Although this collection of manuscripts represents only a fraction of the current research in this still-growing field, advances in both fundamental and applied science are abundant and being enabled by the growing synergy among those