Two dimensional metal–oxianion surface complexes formation during the upd process on a Au(1 1 1) electrode studied by in situ surface X-ray diffraction and infrared reflection absorption spectroscopy

Unconventional Electrochemical Behaviors of Cu Underpotential Deposition in a Chloride-Based Deep Eutectic Solvent: High Underpotential Shift and Low Coverage

The (5 × 5) Moiré pattern resulting from coadsorption of Cu atoms and chloride ions on the Au(111) electrode is one of the most classical structures for underpotential deposition (UPD) in electrochemical surface science. Although two models have been proposed to describe the pattern, the details of the structure remain ambiguous and controversial, leading to a question that remains to be answered. In this work, we investigate the UPD behaviors of Cu on the Au(111) electrode in a chloride-based deep eutectic solvent ethaline by in situ scanning tunneling microscopy (STM). Benefiting from the properties of the ultraconcentrated electrolyte, we directly image not only Cu but also Cl adlayers by finely tuning tunneling conditions. The structure is unambiguously determined for both Cu and Cl adlayers, where an incommensurate Cu layer is adsorbed on the Au(111) surface with a Cu coverage of 0.64, while the Cl coverage is 0.32 (only half of the expected value); i.e., the atomic arrangement of the observed (5 × 5) Moiré pattern in ethaline matches neither of the models proposed in the literature. Meanwhile, STM results confirm the origin of the cathodic peak in the cyclic voltammogram, which indicates that the underpotential shift of Cu UPD in ethaline indeed increases by ca. 0.40 V compared to its counterpart in a sulfuric acid solution, resulting in a significant deviation from the linear relation between the underpotential shift and the difference in work functions proposed in the literature. The unconventional electrochemical behaviors of Cu UPD reveal the specialty of both the bulk and the interface in the chloride-based deep eutectic solvent.

Real-time observation of interfacial ions during electrocrystallization

Understanding the electrocrystallization mechanisms of metal cations is of importance for many industrial and scientific fields. We have determined the transitional structures during underpotential deposition (upd) of various metal cations on Au(111) electrode using time–resolved surface X–ray diffraction and step–scan IR spectroscopy. At the initial stage of upd, a characteristic intensity transient appears in the time–resolved crystal truncation rod depending on metal cations. Metal cations with relatively high coordination energies of hydration water are deposited in two steps: first, the hydrated metal cations approached the surface and are metastably located at the outer Helmholtz plane, then they are deposited via the destruction of the hydration shell. However, Tl⁺ and Ag⁺, which have low hydration energy, are rapidly adsorbed on Au(111) electrode without any metastable states of dehydration. Therefore, the deposition rate is strongly related to the coordination energy of the hydration water. Metal cations strongly interacting with the counter coadsorbed anions such as Cu²⁺ in sulfuric acid causes the deposition rate to be slower because of the formation of complexes.

Catalytically active structure of Bi deposited on a Au(111) electrode for the hydrogen peroxide reduction reaction

The surface structure of underpotentially deposited Bi has been determined on Au(111) in perchloric acid solution using surface X-ray diffraction (SXD), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. SXD analysis and STM images reveal that the catalytically active structure for the hydrogen peroxide reduction reaction (HPRR) is the (2 x 2)-Bi honeycomb structure with theta(Bi) = 0.50. The stability is supported by DFT calculations. The hydrated perchlorate anion is located in the center of the honeycomb structure without hydrogen peroxide. DFT calculations predict that the Bi honeycomb structure promotes the dissociation of the O-O bond of hydrogen peroxide. Hydrogen peroxide expels the hydrated perchlorate anion, and then HPRR takes place at the honeycomb center.