E. Budevski

Electrocrystallization: Nucleation and growth phenomena

Abstract A review of the present status of the problem of metal deposition and electrochemical phase formation and growth is made. The historical background of the problem is given with an overview of the major contributions of different electrochemical schools. Phase formation is treated in classical and atomistic terms. Some of the basic consequences of the theory concerning the equilibrium form, the influence of the substrate, the size-overpotential dependence of low-dimensional systems, and the nucleation kinetics are discussed. The basic modes of crystal growth under electrochemical conditions are described. The influence of substrate surface modification in the UPD region concerning the formation of low dimensional phases and their stability ranges are discussed. The development of recent in situ techniques for nanostructuring and nanomodification of solid surfaces and their importance for electrochemical nanotechnologies are also shortly presented.

Form and step distance of polygonized growth spirals

Abstract The forms and the step distances of polygomized spirals of growth arising from emergence points of screw dislocations are calculated taking into account that the propagation rate of steps depends on step length. A rather unexpected result is obtained showing that the ratio between the step distance d and the radius ρ c of the inscribed circle in the geometrical form of the nucleus is independent of the geometrical form (i.e. number of corners) of the spiral and equal to approximately 19, very near to the value of this ratio for a circular spiral calculated by Cabrera and Levine.

Underpotential—overpotential transition phenomena in metal deposition processes

The transition phenomena in metal deposition processes have been studied in the system Ag(hkl)/Pb2+ by potentiostatic pulse experiments from the underpotential to the overpotential range. The analysis of the current-time transients indicates a model of progressive nucleation and hemispherical diffusion to the growing 3-D crystallites. An important influence of the initial state in the underpotential range on the nucleation rate has been found. Furthermore, the nucleation rate strongly depends on the overvoltage and on the substrate nature. All experimental results are explained by a stepwise bulk phase formation process including rearrangement of adatoms in the 2-D adsorbed layer, formation of critical clusters and further rearrangement and growth forming epitaxially orientated 3-D crystallites. The nucleation process must be treated by an atomistic model due to the derived low number of atoms in the critical cluster.

STM studies of real and quasi-perfect silver single crystal surfaces used in electrochemical experiments

Abstract Real and quasi-perfect silver single crystal surfaces, which have been used intensively in different electrochemical investigations, are studied by STM to determine the surface profile on an atomic level. The measurements were carried out on chemically polished macroelectrodes and electrolytically grown microelectrodes in contact with air as well as under potentially controlled electrochemical conditions. The real macroelectrodes show a much higher surface corrugation than the quasi-perfect microelectrodes which have large atomically flat terraces separated by monoatomic steps. The possibility to observe the dynamics of monoatomic steps under anodic and cathodic polarization conditions on quasi-perfect single crystal surfaces is demonstrated. Nucleation and crystal growth processes are observed by means of in situ STM investigations of lead electrodeposition on Ag(111) macroelectrodes. The results demonstrate the possibility to study fundamentally the initial steps of electrocrystallization by in situ STM under electrochemical well-defined conditions.

Slow transformation phenomena of lead adsorbates on electrolytically grown Ag(111) and Ag(100) electrode surfaces

Abstract Previous study of underpotential deposits of Pb and Tl on chemically polished Ag(111) substrates has shown that under certain conditions slow structural transformations in the submonolayer coverage take place. Voltammetric measurements taken by us on electrolytically grown Ag(111) and Ag(100) electrodes reveal new features of the Pb-adsorbates. The rate of structural transformations on Ag(111) increases with the step density. This effect is not observed on Ag(100) but kinetics limitations attend the formation of the adsorbate. An attempt is made to explain these phenomena.

Metallionenadsorption und kathodische metallabscheidung—I. Thallium- und bleiabscheidung auf (111) und (100) silbereinkristallflächen

The morphology and kinetics of the cathodic thallium and lead deposition on Ag (111)- and Ag (100) single crystal faces was studied by potentiostatic pulse experiments. The formed crystallites are oriented with their close packed planes (0001) and (111) parallel to the substrat surfaces. There exists a strong correlation between the close packed crystallographic directions of the substrates [110] and those of the crystallites [1120] and [110] respectively. The experimental current—time-transients are discussed on the basis of various nucleation-growth-models. From the morphological results and from the dependencies of the current—time-transients on the substrate orientation as well as on the inital state at different underpotentials it can be concluded that the nucleation process is connected with reorganization phenomena of the metal ion adsorbate.

STM studies in underpotential—overpotential metal deposition

Abstract The deposition of lead on “real” silver single crystal faces in the so-called underpotential ( upd ) and overpotential ( opd ) ranges was studied by in situ STM with atomic resolution. The results are compared with previous electrochemical, morphological, and in situ EXAFS and GIXS investigations. The STM images on bare substrates show regular hexagonal and quadratic lattices for the (111) and (100) planes, respectively, without any reconstructions. At high coverages in the upd range, well-ordered lead adlayers are observed on both substrate orientations. The lead monolayer on Ag(111) was found to be incommensurate, close packed, and rotated by about 4–5° with respect to the substrate. This agrees with previous GIXS measurements and indicates internal compressive strain within the adlayer. In contrast, lead on Ag(100) forms a superlattice structure which has a much lower adatom density than a hexagonal close-packed layer. The symmetry and interatomic distances are close to a c (2 × 2)Pb overlayer. In the opd range, 3-D lead crystallites are formed. On both substrate orientations, the contact plane of the lead crystallites is the close-packed (111) plane. At low overpotentials, the lead crystallites on Ag(111) grow epitaxially, rotated by about 5° with respect to the substrate. On Ag(100), two different orientations of lead crystallites in parallel to both [110] crystallographic directions of the substrate are observed. The nucleation rate of lead depends not only on the overpotential, but also on the initial polarization state in the upd range using an upd  opd transient technique. Nucleation and growth may proceed either inside the imperfect and strained upd -adlayer or on top of it according to the Stranski-Krastanov mechanism.

Non-equilibrium phenomena at the early stage of formation of lead underpotential adsorbates on electrolytically grown Ag(111) electrode surface

Abstract The phenomena of slow structural transformation in Pb underpotential adsorbate on Ag(111) surface at low coverages are quantitatively investigated.It is shown that these phenomena occur as a result of incorporation of lead atoms mainly in the crystal lattice of the terraces on the adsorbent surface and not in the growth steps as it is the case at high coverages.

Substrate-induced strain of UPD monolayers and two-dimensional—three-dimensional transition in metal electrodeposition

Abstract Various reasons for the appearance of substrate-induced strain in underpotential deposition (UPD) monolayers on strongly attractive substrates are discussed. An expression for the dependence of monolayer compression on underpotential is obtained considering the two-dimensional film as isotropic and elastic. A theoretical model is proposed for the nucleation of the three-dimensional metal bulk phase taking into account the compressive strain of the UPD monolayer. It is shown that previous results from electrochemical measurements for lead deposition on Ag(111) as well as recent experimental data obtained by in-situ X-ray scattering and scanning tunnelling microscopy are in good agreement with the theoretical considerations.

Deposition and Dissolution of Metals and Alloys. Part A: Electrocrystallization

There are two major factors which have to be considered in the process of the electrolytic metal deposition: (i) the thermodynamic and growth properties of the crystalline phase which can be treated as largely independent of the presence and character of the ambient phase and (ii) the properties of the ionic solution affecting primarily the structure of the interface boundary and the kinetics of the mass and charge transfer across it. In the first part of this chapter the problems connected with the formation and growth of the crystals of the metal deposit will be discussed more closely, while the problems arising from the ionic solution side will be treated as simply as possible (see also Vol. 1 of this series).