Crystal-phase and surface-structure engineering of ruthenium nanocrystals

Metal nanocrystals with controlled shapes or surface structures have received increasing attention, owing to their desirable properties for applications ranging from catalysis to photonics, energy and biomedicine. Most studies, however, have been limited to nanocrystals with the same crystal phase as the bulk material. Engineering the phase of metal nanocrystals while simultaneously attaining shape-controlled synthesis has recently emerged as a new frontier of research. Here, we use Ru as an example to evaluate recent progress in the synthesis of metal nanocrystals featuring different crystal phases and well-controlled shapes. We first discuss synthetic strategies for controlling the crystal phase and shape of Ru nanocrystals, with a focus on new mechanistic insights. We then highlight the major factors that affect the packing of Ru atoms and, thus, the crystal phase, followed by an examination of the thermal stability of Ru nanocrystals in terms of both crystal phase and shape. Next, we showcase the successful implementation of these Ru nanocrystals in various catalytic applications. Finally, we end with a discussion of the challenges and opportunities in the field, including leveraging the lessons learned from Ru to engineer the crystal phase and surface structure of other metals. Engineering the phase of metal nanocrystals while simultaneously achieving shape-controlled synthesis can enable new and desirable properties. This Review highlights the synthetic strategies for generating Ru nanocrystals with different crystal phases and surface structures, and outlines their implementation in catalytic applications.