Scanning tunneling microscopy and scanning electron microscopy observations of the early stage of silver deposition on graphite single crystal electrodes

Abstract

The early stages of Ag overpotential deposition on highly oriented pyrolytic graphite (HOPG) from Ag+-ion-containing acid solutions have been studied by ex situ scanning tunneling microscopy and scanning electron microscopy imaging complemented with electrochemical, energy dispersive X-ray analysis, and Auger electron spectroscopy data. Nucleation and 3D Ag growth initiate at surface defects. Unstructured 3D Ag nuclei decorating HOW steps and flat geometric Ag islands are formed around the steps. The island structure is compatible with a local layer-by-layer growth. The entire morphology of the Ag deposit is consistent with a growth mechanism involving Ag atom diffusion from 3D nuclei at step edges toward Ag flat domains. Images with atomic resolution reveal large uncovered HOPG areas and Ag submonolayer domains with the nearest-neighbor distance d = 0.33 f 0.02 nm, whereas flat Ag islands exhibit d = 0.29 f 0.02 nm as expected for the nearest-neighbor distance in the Ag lattice. A model for these structures is discussed. The interest in the very early stages of metal electrodeposition both in the underpotential (upd) and overpotential (opd) regions encourages further investigations to demonstrate the validity of growth models and phase transitions operating at different stages of the process.' The rapid development in the applioation of Scanning tunneling microscopy (STM) to electrochemical systems involving well-defined substrates under different operating conditions allows the examination of growth processes from the early formation of a single monolayer to deposition equivalent to several atomic layers. Despite the fact that there are still no universally accepted criteria concerning the optimum STM operating mode in electrochemistry, a number of very interesting results and conclusions concerning the el~~tallization of metals have been reported in recent Investigations of the upd of metals with atomic resolutionH and the early stages of Cu opd on Au single-crystal electrodes have been made using STM.7 For upd of metals, ordered overlayers have been reported from STM and AFM imaging.3 These overlayers exhibit open structures with packaging densities which depend on the anion prtsent in the electrolyte solution. A partial charge transfer from anions to metal adatoms has been proposed to explain the experimental results. It should be noted that due to the difference in electronegativities between substrate and metal adatoms, the latter remains with a small positive chargeas STM observations indicate that the initial opd of metals occurs at step edges, dislocations, and defects in the substrate surface. Subsequently, the deposition of metals takes place on flat substrate terraces, the rate of growth of the deposit being far faster in the x- and y-directions than it is in the z-directi~n.~ For Cu electrodeposition on Pt, it has been concluded that a 2D layer-by-layer growth mechanism does not account for the process, although in this case the shape of Cu crystallites is not well-defined at the early stages of growth, it changes into a better defined morphology at later stages.