Toward Rationally Designing Surface Structures of Micro- and Nanocrystallites: Role of Supersaturation.

Tailoring the surface structures of nanocrystals is an exciting research area on account of appealing surface-dependent properties in various applications. Although significant progress has been made in recent years, current synthetic approaches are mainly dependent upon trial and error because of the ambiguous roles of various influencing factors in complicated environments. Therefore, a general theory for predicting and guiding the rationally controlled synthesis of micro- and nanocrystallites with specific surface structures is highly desired. Of note, previous research attention was mainly focused on the crystal growth in near equilibrium conditions. However, in supersaturated growth environments (nonequilibrium conditions), the corresponding crystal growth theories are still limited. Recently, the supersaturation-controlled surface structure strategy, which is derived from thermodynamics and the Thomson-Gibbs equation, has opened up a new avenue for the control the surface structures of crystals. This strategy involves manipulating the supersaturation of growth units to control the surface structure of micro- and nanocrystallites, as the surface energy of exposed facets is correlated to the supersaturation of growth blocks. Based on the proposed theory, micro- and nanocrystallites with various surface structures, especially high-energy facets, have been successfully synthesized by our group and other researchers in past years. In order to draw lessons from previous studies, it is imperative to give a timely research account related to the supersaturation strategy and corresponding applications in controlling surface structures of different crystallites. In this Account, we explore the supersaturation-controlled surface structure strategy to construct functional nanomaterials with desired architectures. First, we highlight the role of supersaturation of growth units from theoretical analysis after a short introduction of fundamental principles for crystal growth. Then, some detailed cases concerning evolution of surface structures are presented to highlight the key experimental factors involved in manipulating the supersaturation of growth units during synthetic processes. These factors include solvents, reaction rates, and additives in wet chemical routes as well as overpotential in electrochemical routes. In addition, we briefly discuss the role of supersaturation in growth kinetics with focus on explaining the formation of spherical micro- and nanocrystallites at extremely high supersaturation. Finally, a general summary of the supersaturation-dependent surface structure control and future prospects in this field are provided. It is expected that this Account will deepen the current understanding on fundamental principles behind the control of surface structures of micro- and nanocrystallites, which can help us to construct desirable nanomaterials and promote their practical applications.