As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

A versatile method for selectively synthesizing single-crystalline rhombic dodecahedral, cubic, and octahedral palladium nanocrystals, as well as their derivatives with varying degrees of edge- and corner-truncation, was reported for the first time. This is also the first report regarding the synthesis of rhombic dodecahedral palladium nanocrystals. All the nanocrystals were readily synthesized by a seed-mediated method with cetyltrimethylammonium bromide as surfactant, KI as additive, and ascorbic acid as reductant. At the same ascorbic acid concentration, a series of palladium nanocrystals with varying shapes were obtained through manipulation of the concentration of KI and the reaction temperature. The formation of different palladium facets were correlated with their growth conditions. In the absence of KI, the 100 palladium facets are favored. In the presence of KI, the concentration of KI and the reaction temperature play an important role on the formation of different palladium facets. The 110 palladium facets are favored at relatively high temperatures and medium KI concentrations. The 111 palladium facets are favored at relatively low temperatures and medium KI concentrations. The 100 palladium facets are favored at either very low or relatively high KI concentrations. These correlations were explained in terms of surface-energy and growth kinetics. These results provide a basis for gaining mechanistic insights into the growth of well-faceted metal nanostructures.

This paper reports a versatile seed-mediated growth method for selectively synthesizing single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals. In the seed-mediated growth method, cetylpyridinium chloride (CPC) and CPC-capped single-crystalline gold nanocrystals 41.3 nm in size are used as the surfactant and seeds, respectively. The CPC-capped gold seeds can avoid twinning during the growth process, which enables us to study the correlations between the growth conditions and the shapes of the gold nanocrystals. Surface-energy and kinetic considerations are taken into account to understand the formation mechanisms of the single-crystalline gold nanocrystals with varying shapes. CPC surfactants are found to alter the surface energies of gold facets in the order {100} > {110} > {111} under the growth conditions in this study, whereas the growth kinetics leads to the formation of thermodynamically less favored shapes that are not bounded by the most stable facets. The competition between AuCl(4)(-) reduction and the CPC capping process on the {111} and {110} facets of gold nanocrystals plays an important role in the formation of the rhombic dodecahedral (RD) and octahedral gold nanocrystals. Octahedral nanocrystals are formed when the capping of CPC on {111} facets dominates, while RD nanocrystals are formed when the reduction of AuCl(4)(-) on {111} facets dominates. Cubic gold nanocrystals are formed by the introduction of bromide ions in the presence of CPC. The cooperative work of cetylpyridinium and bromide ions can stabilize the gold {100} facet under the growth condition in this study, thereby leading to the formation of cubic gold nanocrystals.