Crystal phase regulation in noble metal nanocrystals

Noble metal nanocrystals (NCs) are often densely packed in their most stable forms that are determined by a combination of effects arising from the electronic, magnetic, geometric, and phononic properties of the NCs. These packing modes usually include the densest packed polytypes of Barlow packings and more open or distorted packings with slightly lower atomic packing factors. The structural modulation strategies of NCs towards the better performances for diverse applications are usually limited to the crystal size, shape, and surface control, which have been robustly studied and documented. An exciting emerging field related to structural engineering of noble metal NCs turns out to be the crystal phase control, which allows the chemical synthesis of energetically high-lying phases of NCs and leads to intriguing performances in catalysis and energy conversion. This article provides a comprehensive review of crystal phase regulation that endows both noble metal and noble-metal-based alloy NCs with unique electronic structures and enhanced performances. The basic principles, general design rationale, synthetic approaches, and structural characterizations for a variety of successful case studies related to crystal phase engineering are reviewed and discussed. In the end, the perspectives and challenges associated with the development of a more controllable chemical synthetic strategy towards the high-energy phases of noble metal NCs are put forward. (C) 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.