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.

中文翻译：

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阐明液/金属界面 N-杂环卡宾自组装结构的不稳定性和多样性

N-杂环卡宾（NHC）是金属表面功能化和催化材料设计中最有前途的配体之一，但在液体/金属中很少实现控制 NHC 有序自组装单层的生长界面。通过扫描隧道显微镜，我们发现液相的存在使得金属表面 NHC 的自组装结构高度可变。 NHC 表面功能化方面揭示了一些新发现，包括在狭窄的温度范围内出现多达五种多晶型物、接枝 NHC 的双层堆叠以及温度诱导的表面重建。通过理论模型，我们推断表面结构的转变主要是由 NHC 在表面的结合和堆积模式的切换驱动的。 NHCs独特表面结构的特性表现为NHC功能化金底物在电化学CO ₂还原反应中的结构依赖性活性。虽然强 NHC 配位经常被认为是制备具有优异稳定性的金纳米粒子的策略，但我们的结果强调了 NHC-Au 配合物在液/固界面处的高迁移率，这有利于金属的受控功能化定制特性的表面。