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Abstract Surface capping agents have been extensively used to control the evolution of seeds into nanocrystals with diverse but well‐controlled shapes. Here we offer a comprehensive review of these agents, with a focus on the mechanistic understanding of their roles in guiding the shape evolution of metal nanocrystals. We begin with a brief introduction to the early history of capping agents in electroplating and bulk crystal growth, followed by discussion of how they affect the thermodynamics and kinetics involved in a synthesis of metal nanocrystals. We then present representative examples to highlight the various capping agents, including their binding selectivity, molecular‐level interaction with a metal surface, and impacts on the growth of metal nanocrystals. We also showcase progress in leveraging capping agents to generate nanocrystals with complex structures and/or enhance their catalytic properties. Finally, we discuss various strategies for the exchange or removal of capping agents, together with perspectives on future directions. 

Abstract Despite the pivotal roles played by halide ions (e. g., Cl⁻and Br⁻) in directing the evolution of seeds into metal nanocrystals with diverse shapes, it is still unclear how halides affect the reduction kinetics of a salt precursor and thus the outcome of a synthesis. Here we report a quantitative analysis of the multiple roles played by halides in controlling the growth behaviors of Pd seeds with cubic and octahedral shapes, respectively. Our quantitative measurements clearly indicate the existence of a transition point around 10⁻³ mM min⁻¹for the reduction rate, which separates the reduction into two distinctive pathways (solutionversussurface) for the formation of completely different products. More significantly, we demonstrate that the speciation, reduction kinetics, and reduction pathway of a Pd(II) precursor can all be manipulated by varying the type and/or amount of halides introduced into a synthesis for the deterministic formation of a specific product. This work represents a critical step forward in achieving a quantitative understanding of the multiple roles of halides involved in the shape‐controlled synthesis of Pd nanocrystals, with the knowledge potentially extendible to other noble metals and their alloys.