Copper Adlayer Research Articles

Influence of the additives (TU) and (PEG) on the underpotential deposition of copper on platinum electrodes: a coulometric, microgravimetric and topographic study

We have studied the influence of additives such as thiourea (TU) and polyethyleneglycol (PEG) on the underpotential deposition of Cu on Pt using coulometry, in situ microgravimetry and STM. STM images taken with and without additives are consistent with evenly dispersed copper adlayers, while in the bulk region microcrystallites are formed. The sputtered Pt samples may obtain a preferential orientation after extended potential cycling in electrolyte solutions due to electrochemical faceting. The electrosorption valency τ of Cu on Pt for these samples is 1.2 ± 0.3. τ remains unaffected if TU and PEG are present in the electrolyte.

Metal deposition and dissolution monitored by In situ scanning tunneling microscopy

The possibilities of scanning tunnelling microscopy (STM) for the study of metal deposition and dissolution reactions under in situ electrochemical conditions will be described and illustrated by examples. The metal reaction being studied is copper deposition and dissolution on foreign substrates such as Au(111), or polycrystalline copper. Data on atomically-resolved copper adlayers and on bulk copper deposition on gold will be presented. In addition, time resolved in situ STM for the study of copper deposition and dissolution on polycrystalline copper will be shown as well. From the latter local reaction rates can be estimated. The necessity to consider a possible influence of the tip on the results will be emphasized and the reasons for such an interference of the tip will be briefly discussed.

In situ x-ray absorption spectroscopy study of underpotential deposition of copper on platinum (100)

X-Ray absorption spectroscopy (XAS) spectra have been recorded for the electrochemical adsorption of Cu on Pt(100) in the fluorescence detection mode at grazing incidence, with polarization parallel to the surface. During data collection, the Pt(100) surface was under potentiostatic control in a 10 −4 M Cu 2+ electrolytic solution, in order to maintain a monolayer coverage. The near-edge structure of the copper K-edge resembles that of Cu +. The Fourier transform of the EXAFS spectra shows a first peak at 0.15 nm, and a broad unresolved peak with maximum at 0.30 and a shoulder at 0.37 nm. The first single peak is associated with oxygen neighbours at 0.197 nm. The unresolved peak is built up from contributions of copper first and second neighbours. The distance of first copper neighbours is 0.309 nm, and that of second copper neighbours is 0.393 nm, equal to the substrate lattice parameter (0.3929 nm). This suggests that a first copper adlayer of half monolayer density is in registry with the platinum unreconstructed (100) surface net. A second copper adlayer of the same density lies over the first adlayer, at a distance of 0.135 nm. The copper atoms in this second layer are in the fourfold geometry with respect to the underlying copper half-monolayer. These layers form a c(2x2) periodicity on the Pt (100) substrate. The copper—oxygen distance is about half the second copper—copper distance. The sharing of oxygen between the two copper adlayers is unclear.

Copper adlayer formation on Au(111) from sulfuric acid electrolyte

We have used in situ scanning tunneling microscopy to examine submonolayer Cu deposition from a sulfuric acid electrolyte on Au(111), sampling over large areas of the surface. We describe the growth of three distinct superlattice phases of Cu on the Au(111) surface before the onset of bulk deposition of Cu. In the earliest stages of deposition, during an electrochemical current peak, we observe the rapid growth of a low height phase, with an apparent height of 0.46±0.10 Å. During this time, a second, monolayer-height phase forms as small islands. We identify these phases as the (√3×√3)R30° and (1×1) Cu superlattices, respectively. A third phase appears at about 175 mV (versus Cu/Cu+2), and grows from nucleation points at the top edges of atomic steps and around the small (1×1) Cu islands. This phase, which we believe is a (5×5) Cu adlayer, slowly supplants the (√3×√3)R30° layer.

Radiochemical Assay of Adsorption at Single Crystal/Solution Interfaces

By using a radioactive labeling method and electrochemistry, results were obtained that show that interactions of bisulfate anion with well‐ordered single‐crystal electrodes are different from those with the polycrystalline electrodes. In particular, the anion surface concentration on the polycrystalline rhodium electrode increases monotonically with the electrode potential and then decreases when the surface becomes electrooxidized. With Rh(111), the surface stability of bisulfate is observed in a broad electrode potential range. Likewise, the hydrogen adsorption process apparently overcomes a higher energy barrier to nucleate into surface water‐bisulfate network on Rh(111) than it does on the polycrystalline surface. These findings, and the corresponding results obtained with platinum electrodes, demonstrate some unique electrochemical properties of electrode materials that have a regular atomic periodicity and a long‐range crystallographic order. Extension of this work to surfaces covered by underpotential‐deposited‐metal (UPD) adlayers illustrates the principles of “enhanced” adsorption.4 That is, the anionic adsorption from solutions containing cationic UPD precursors is observed in the potential range where no anion adsorption occurs on the clean substrate surfaces. It is shown here for the first time that there are some inactive and active UPD metal adlayers toward adsorption. In the case of adsorption of bisulfate on platinum covered by UPD copper, it is concluded that in the inactive electrode potential range perchlorate successfully competes with bisulfate for the surface sites. This observation is easy to embrace if the UPD copper adlayer is composed of Cu1+ cations41 rather than neutral copper adatoms. In general, it is demonstrated that a simple electrostatic approach to the anion bonding with metal electrodes is not appropriate. There is a chemical component involved in adsorption that merits further investigation.

Adsorption of anions on ultra-thin metal deposits on single-crystal electrodes: II: Voltammetric and radiochemical study of bisulfate adsorption on Pt(111) and Pt(poly) electrodes containing copper adatoms

The formation of ultra-thin metal deposits of copper on Pt(111) and polycrystalline platinum electrodes, as well as the adsorption of bisulfate on the copper-covered platinum surfaces, were studied by cyclic voltammetry and radioactive labeling. The highest charge obtained by voltammetry in the underpotential stripping range nearly corresponds to a close-packed monolayer of copper. The radioactive labeling data indicate that there are inactive and active copper adlayers toward bisulfate adsorption. The transition from inactive to active behavior is interpreted in terms of an increase in surface—bisulfate interactions at the expense of surface—perchlorate interactions. Based on recent X-ray absorption near-edge spectroscopy (XANES) analysis of copper deposition onto platinum, the site for bisulfate adsorption is most probably a Cu + surface species. Combining this spectroscopic information with coulometry shows that an additional electron is confined to surface platinum atom(s) covered by the copper species. The copper film attains bulk copper properties when approximately 2.5 monolayers of copper are deposited.

Atomic structure of ordered copper adlayers on single-crystalline gold electrodes

We have investigated submonolayer electrodeposits of copper on the three low-index planes of gold in situ by STM. The atomic structure of the clean electrode surfaces and of different ordered Cu adsorbate layers were resolved. For submonolayer Cu coverages on Au(111) and Au(100), deposited under potential control from sulfuric acid solutions, various ordered structures were observed, where the Cu adatoms are arranged in hexagonal or quasihexagonal lattices. The larger Cu-Cu distances in these structures, as compared to the pseudomorphic (1×1) Cu islands formed in vacuum deposits even at low coverages, demonstrate the structure determining role of the coadsorbed anions. For Cu deposition on Au(110) only a (1×1) Cu adlayer was observed. These results are in good agreement with recent ex situ LEED data.

Combined optical second harmonic generation/quartz crystal microbalance study of underpotential deposition processes: copper electrodeposition on polycrystalline gold

Optical second harmonic generation and quartz crystal microbalance techniques are used as in situ probes of copper underpotential deposition on polycrystalline gold surfaces in sulfuric acid electrolyte. The second harmonic signal from a polished bulk gold substrate is observed to decrease by >60% as a result of copper underpotential deposition on gold. Also, the mass of an underpotentially deposited copper adlayer is monitored in situ by an oscillating quartz crystal microbalance, yielding an estimated coverage of ~8.0 x 10(-10) mol cm(-2) and an electrosorption valency of 1.5 for a copper adlayer on the surface of vapor-deposited polycrystalline gold.