Ultrathin two-dimensional metallic nanocrystals for renewable energy electrocatalysis

Even since the discovery of exfoliated graphene, exponentially growing efforts have been devoted to the search of novel 2 dimensional (2D) nanomaterials with atomic thickness and the extension of their applications. Among the huge 2D materials family, ultrathin 2D metallic nanocrystals gain vast attention due to the unique importance of transition metals to heterogeneous catalysis. Here, we review recent representative advances in ultrathin 2D metallic nanocrystals and their catalytic applications toward a variety of reactions, including the hydrogen generation, oxygen reduction/evolution, CO2 activation, and small molecules' oxidation/hydrogenation. Special focuses are placed in current understanding of bottom-up fabrication of ultrathin 2D metallic nanocrystals, the establishment of their structure-performance relationship as well as the implication of their real applications in devices. We also shed light on further structural modification of the ultrathin 2D metallic nanocrystals to achieve the optimized catalytic performance. At end of this review, we discuss the remaining challenges and future opportunities in this burgeoning research area.