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This work demonstrates a new strategy for controlling the evolution of twin defects in metal nanocrystals by simply following thermodynamic principles. With Ag nanocrystals supported on amorphous SiO₂as a typical example, we establish that twin defects can be rationally generated by equilibrating nanoparticles of different sizes through heating and then cooling. We validate that Ag nanocrystals with icosahedral, decahedral, and single‐crystal structures are favored at sizes below 7 nm, between 7 and 11 nm, and greater than 11 nm, respectively. This trend is then rationalized by computational studies based on density functional theory and molecular dynamics, which show that the excess free energy for the three equilibrium structures correlate strongly with particle size. This work not only highlights the importance of thermodynamic control but also adds another synthetic method to the ever‐expanding toolbox used for generating metal nanocrystals with desired properties.

Decahedral nanocrystals have received great attention owing to their unique symmetry and strain‐energy distribution. In contrast to other noble metals, it has been difficult to synthesize decahedral Rh nanocrystals. We report a robust, one‐pot method based on polyol reduction for the facile synthesis of Rh decahedral nanocrystals in high purity, with sub‐20 nm sizes. The success of the synthesis relied on our ability to manipulate reduction kinetics by systematically tuning experimental parameters. We found that the yield of Rh decahedral nanocrystals could be maximized by optimizing:i) the concentration of Rh(acac)₃(metal precursor);ii) the molecular weight and amount of poly(vinyl pyrrolidone) (colloidal stabilizer/capping agent); andiii) the chain length of the polyol (solvent/reducing agent), with tetraethylene glycol being the best. We believe the mechanisms elucidated herein can be extended to other syntheses to produce metal nanocrystals with multiply twinned structures.