Elucidating the Lability and Diversity of Self-Assembled Structures of N-Heterocyclic Carbenes at the Liquid/Metal Interface

N-heterocyclic carbenes (NHCs) represent one of the most promising ligands for the surface functionalization of metals and for the design of catalytic materials, but controlling the growth of an ordered self-assembled monolayer of NHCs has rarely been achieved at the liquid/metal interface. By means of scanning tunneling microscopy, here we show that the presence of a liquid phase makes the self-assembled structures of NHCs on the metal surface highly variable. A few new findings were revealed in the surface functionalization with NHCs, including the appearance of up to five polymorphs in a narrow range of temperatures, bilayer stacking of grafted NHCs, and temperature-induced surface reconstruction. With theoretical modelings, we reason that the transition in surface structures is mainly driven by the switching of binding and packing modes of NHCs at surfaces. The properties of distinct surface structures of NHCs are manifested as the structure-dependent activities of NHC-functionalized gold substrates in electrochemical CO2 reduction reactions. While strong NHC coordination has been frequently recognized as a strategy for the preparation of gold nanoparticles with superior stability, our results highlight the high mobility of the NHC-Au complex at the liquid/solid interface, which is beneficial to the controlled functionalization of the metal surface for tailored properties.