Direct Spectroscopic Evidence of an Atom-Transfer Mechanism during Copper Electrodeposition on Gold

Understanding how metals deposit at electrified solid−liquid interfaces is essential to electrochemistry, yet the molecular mechanism of electrodeposition has remained largely unresolved due to the intrinsic opacity of buried interfaces. Here, we combine operando, interface-sensitive soft X-ray absorption spectroscopy with first-principles electronic-structure simulations to directly track copper electrodeposition on gold with elemental and chemical specificity. A newly developed lock-in total electron yield detection scheme enables selective probing of the topmost nanometers of an operating electrochemical interface, overcoming long-standing limitations imposed by Faradaic currents. Using this approach, we find unambiguous evidence that copper deposition does not proceed through classical outer- or inner-sphere electron-transfer pathways, but instead follows an atom-transfer mechanism mediated by a transient cuprous oxide (Cu2O) layer. This Cu(I) oxide forms at moderate cathodic potentials at pH ∼4.5 and subsequently reduces to Cu0, establishing a stepwise sequence governed by interfacial chemistry rather than direct electron transfer. Our results redefine the mechanistic landscape of metal electrodeposition and establish a generalizable platform for resolving complex electrochemical transformations at buried interfaces.