Electrochemical interfaces between Au(100) and deep eutectic solvents: An in situ scanning tunneling microscopy study

Deep eutectic solvents (DESs) are promising electrolytes for electrochemical fields due to their excellent properties. Electrochemical behaviors of Au(100) electrode in two choline chloride-based DESs ethaline and reline with different hydrogen bond donor molecules have been investigated by cyclic voltammetry and in situ scanning tunneling microscopy (STM). Cyclic voltammograms present three pairs of current peaks for Au(100) electrodes in the two DESs. On the basis of the results of in situ STM, the three pairs of current peaks are assigned to the formation and lifting of Au(100) surface reconstruction, adsorption and desorption of chloride ions, and phase transition of chloride ion adlayer. Compared with Au(111), one extra current peak and the corresponding (√5 × √5)R26.6° ordered adlayer of chloride ions were observed for Au(100) electrode. An order-order phase transition of chloride ion adlayer was observed in ethaline, while an order-disorder phase transition was observed in reline. Furthermore, no Au islands was observed on Au(100) surface in ethaline after the lifting of reconstruction when the potential is moved positively and gradually. However, gold nanowires could form and stably exist on Au(100) surface, which are different from that formed on Au(111) surface in reline. Our results provide detailed insights into the interfacial structures between Au(100) electrode and DESs on the atomic scale that hydrogen bond donor molecules and the surface crystallography of the electrode interface could significantly affect the arrangement of gold atoms of the electrode surface and chloride ions in the innermost Stern plane.