High-index faceted noble metal nanocrystals

The formation of novel and complex structures with specific morphologies from nanocrystals via a direct assembly of atoms or ions remains challenging. In recent years, researchers have focused their attention on nanocrystals of noble metals and their controlled synthesis, characterization, and potential applications. Although the synthesis of various noble metal nanocrystals with different morphologies has been reported, most studies are limited to low-index facet-terminated nanocrystals. High-index facets, denoted by a set of Miller indices {hkl} with at least one index greater than unity, possess a high density of low-coordinated atoms, steps, edges, and kinks within these structures and serve as more active catalytic sites. With the potential for enhanced catalytic performance, researchers have used the insights from shape-controlled nanocrystal synthesis to construct noble metal nanocrystals bounded with high-index facets. Since the report of Pt tetrahexahedral nanocrystals, researchers have achieved significant progress and have prepared nanocrystals with various high-index facets. Because of the general order of surface energy for noble metals, high-index facets typically vanish faster in a crystal growth stage and are difficult to preserve on the surface of the final nanocrystals. Therefore researchers have had limited opportunities to examine high-indexed noble metal nanocrystals with a controlled morphology and investigate their resultant behaviors in depth. In this Account, we thoroughly discuss the basic concepts and state-of-the-art morphology control of some noble metal nanocrystals enclosed with high-index facets. We briefly introduce high-index facets from both crystallographic and geometrical points of view, both of which serve as methods to classify these high-index facets. Then, we summarize various typical noble metal nanocrystals terminated by different types of high-index facets, including {hk0} (h > k > 0), {hhl} (h > l > 0), {hkk} (h > k > 0), and {hkl} (h > k > l > 0). In each type, we describe several distinct morphologies including convex, concave, and other irregular shapes in detail. Based on these remarks, we discuss key factors that may induce the variations of Miller indices in each class, such as organic capping ligands and metallic cationic species. In a look at applications, we review several typical high-indexed noble metal nanocrystals showing enhanced electrocatalytic or chemical catalytic activities.