Metal deposition

An important effect in the initial stages of electrochemical metal deposition is the formation of a metal monolayer at potentials positive relative to the Nernst potential, known as underpotential deposition (UPD). These metal adlayers often form ordered structures, which can strongly depend on the anion species in the electrolyte. The most prominent example of the latter is the UPD adlayer of Cu on Au(l 11) in sulfuric acid, where a (a/3 x V3)R30° superstructure is observed [10, 15-19]. According to recent X-ray scattering results [20], a 1/3 monolayer of sulfate is embedded in a honeycomb of a 2/3 monolayer of Cu in this structure. In the presence of strongly co-adsorbing anions, such as chloride, other superstructures are observed, signaling a structure-decisive influence of the respective anions on the Cu UPD behavior [17, 21]. Similar effects were found also on the other low-index Au surfaces [22, 23]. The formation of the ordered phases in the UPD range is usually too fast to be observed directly in STM experiments. Only by using very low metal concentrations in the electrolyte can the metal monolayer be grown under diffusion control on a time scale compatible with the STM measurements. As an example, the growth of the (a/3 x"v/3)R30° superstructure on

Au(lll) is shown in two successively recorded STM images obtained in sulfuric acid containing 10"6 M CuSCU (Fig. 3) [10]. In the lower half of the images formation of a highly defective (^3 x V3)R30° island is observed. Such islands coexist with areas where no atomic structures are discernible. Most likely, the Au(l 11) substrate in these featureless areas is covered by a dilute adlayer of mobile Cu atoms. The observation of an island growth mechanism is in good agreement with electrochemical measurements of the kinetics of adlayer phase formation [24] and indicates clearly that the gross lateral interactions between the adsorbed anions and the Cu atoms in the (V3 x V3)R30° structure are attractive.

Fig. 3. Formation of the (V3 x V3)R30° superstructure on Au(lll) in 0.01 M H2S04 + 10"6 M CuS04 at 0.13 Vsce (100 x 90 A2) [10].

Comparative studies in an electrochemical environment and under UHV conditions gain detailed information on the mechanistic role on anions and solvent molecules. This is demonstrated for nucleation and growth of thin epitaxial Ni films on Au(l 11). In contrast to Cu, no UPD or anion effects were found for Ni on Au [25]. In addition, due to the negative deposition potential of Ni the Au(lll) electrode can be kept in a potential regime in which the surface exhibits the "herringbone" reconstruction, well known from UHV studies [26], In Fig. 4(a), recorded in a modified Watts electrolyte [25], the zigzag pattern of the "herringbone" reconstruction is clearly visible on the electrode surface.

Fig. Deposition of Ni on Au(l 11) in modified Watts electrolyte (with 10"3 M NiS04) at (a) -0.64 VSCE (1100 x 1100 A2), (b) -0.64 VSCE (1500 x 1500 A2), and (c) -0.68 VSCE (1500 x 1500 A2) [25],

At low overpotentials nucleation of the Ni deposit starts at the "elbows" of the reconstruction (Fig. 4(b)), followed by anisotropic growth of monolayer islands perpendicular to the double rows of the reconstruction (Fig. 4(c)). For multilayer coverages a layer-by-layer growth of the Ni thin film was observed up to thicknesses of six layers. Under UHV conditions, this system exhibits a similar nucleation behavior but the subsequent growth proceeds isotropically and in a more three-dimensional fashion [27]. Both in UHV and in the electrochemical environment the nucleation of islands is preceded by the formation of depressions at the elbows. This indicates that Au surface atoms at these sites are replaced by Ni atoms, which subsequently act as centers for adlayer island nucleation [28]. This demonstrates far-reaching mechanistic similarities for deposition at the metal-vacuum and the metal-electrolyte interface, even in complex cases.

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