Theoretical Perspectives on the Influence of Solution-Phase Additives in Shape-Controlled Nanocrystal Synthesis

Shape-selective solution-phase nanocrystal growth is facilitated by capping agents, or structure-directing agents (SDAs), which guide shape evolution. It is often stated that these chemical additives impart shape selectivity by promoting nanocrystals with a majority of facets to which they bind most strongly. However, little is known on the mechanisms through which they impart shape selectivity. In this Feature Article, we highlight our recent studies aimed at understanding the thermodynamic and kinetic influences of SDAs using theory and computational tools. We review our studies of the poly(vinylpyrrolidone) (PVP)-mediated synthesis of {100}-faceted Ag nanocubes in ethylene glycol solution, which has been studied experimentally. Our studies of the interfacial free energy of Ag-PVP solution interfaces show that while solution-phase PVP does bind more strongly to Ag(100) than to Ag(111), this selectivity is not sufficient to thermodynamically change the Wulff shape of a PVP-covered Ag nanocrystal in solution from that of the bare metal in vacuum. These studies indicate that a strong facet binding selectivity is needed for a SDA to thermodynamically alter the solution-phase crystal shape from that of the bare metal. Interestingly, the binding selectivity of PVP for Ag(100) is sufficient to regulate the atom deposition fluxes to Ag(100) and Ag(111), so that cubic Ag(100) nanocrystals form kinetically. Altogether, our studies indicate that kinetic control of metal nanocrystal shapes is likely more prevalent than thermodynamic control. We outline some current challenges in understanding shape-selective solution-phase nanocrystal syntheses.