Role of Anions During the Cu Underpotential Deposition on Au(111): A Microcalorimetric Investigation

Abstract

We measured the molar Peltier heat during the course of Cu underpotential deposition (UPD) in sulfate-containing solutions by electrochemical microcalorimetry. The molar Peltier heat during Cu UPD deviates considerably from that of Cu bulk deposition in the corresponding solution. Since the molar Peltier heat directly reflects the reaction entropy of the involved electrochemical reaction, including also charge-neutral side processes, this finding signals significant contributions of non-Faradaic side reactions. For the first stage of the Cu UPD this process is known to be coadsorption of Cu2+ and sulfate species. We retrieved the potential dependent surface coverages of Cu and sulfate during the first Cu UPD stage with the single additional assumption that the Cu coverage in the first stage of the Cu UPD reaches 2/3 ML. In addition, we found that both HSO4(ad) and SO4(ad) are adsorbed on the surface with a noticeable surplus of bisulfate. For the second stage of Cu UPD, i.e., the completion of the first (1 × 1) Cu monolayer, our data indicates a charge-neutral side process, which cannot be inferred from the current potential relationship as measured, e.g., by cyclic voltammetry. This side process has considerably high positive reaction entropy and is present also for Cu UPD in perchlorate solutions. We interpret this side process as substitution of adsorbed sulfate or perchlorate anions by oxygen species upon completion of the first Cu monolayer, whereby the potential of zero charge shifts negatively.

Underpotential deposition of Cu on Au(111) from neutral chloride containing electrolyte

“…12 Numerous studies have established the crucial role of the anion in the formation and structure of the UPD layer 10,12,32,34,38,[48][49][50][51][52][53][54][55][56][57][58][59][60] which is highlighted by the stabilising effects of the sulfate anion on the (√3×√3)R30° honeycomb structure 57 and the commensurate (1×1) structure of Cu on Au(111), 10 or the transition of a Cu-UPD layer on Au(111) from a (√3 x √3)R30° to a (5×5) structure upon the exchange of the sulfate by chloride. 31,37,38,50,[61][62][63][64] Notably, Cu-UPD has overwhelmingly been studied at pH ≤ 1 whereas studies employing less acidic conditions 43,48,65,66 or even neutral pH 67,68 are rather scarce. Here we present the structural characterisation of a Cu-UPD layer on Au(111) prepared from a neutral chloride containing solution.…”

Standard Underpotential Deposition Shift. A New Parameter to Characterize the UPD Phenomenon.

“…Besides, we have used solutions with acid concentrations differing by one order of magnitude and also obtained a result with a single crystal surface. Despite the fact that these huge differences are known to severely affect the UPD behaviour of the system Cu +2 /Au [36][37][38] , we have surprisingly obtained constant values for ΔE UPD 0 and E ¿ ¿ . More experiments are necessary to understand these results from a microscopic point of view.…”

“…Thus, valuable information was obtained through experiments involving single crystal electrodes in acid media using X-rays absorption and diffraction techniques in synchrotron facilities and Scanning Tunnelling Microscopy in situ. Even if it is not the aim of our work to discuss the Cu structures on the Au p electrode, it is worth noting that the literature show that the deposition strongly depends on the Au crystalline plane 29 and also of the composition of the electrolyte [36][37][38] . These facts explain the difference in the CV profiles shown in Figures 1 and S1.…”

Standard Underpotential Deposition Shift. A New Parameter to Characterize the UPD Phenomenon.

“…Thus, valuable information was obtained through experiments involving single crystal electrodes in acid media using X-rays absorption and diffraction techniques in synchrotron facilities and Scanning Tunnelling Microscopy in situ. Even if it is not the aim of our work to discuss the Cu structures on the Aup electrode, it is worth noting that the literature show that the deposition strongly depends on the Au crystalline plane 29 and also of the composition of the electrolyte [36][37][38] . These facts explain the difference in the CV profiles shown in Figures 1 and S1.…”

“…Besides, we have used solutions with acid concentrations differing by one order of magnitude and also obtained a result with a single crystal surface. Despite the fact that these huge differences are known to severely affect the UPD behaviour of the system Cu +2 /Au [36][37][38] . More experiments are necessary to understand these results from a microscopic point of view.…”