Subnanometer silver clusters exhibiting unexpected electrochemical metastability on graphite

The anodic dissolution of silver particles on highly oriented pyrolytic graphite (HOPG) electrode surfaces is investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and atomic force microscopy (AFM). As a discontinuous and granular silver electrodeposit is anodically stripped from HOPG surfaces at a potential of +500 mV vs Ag-0/Ag+, the number and diameter of silver particles larger than 20 nm in diameter visible by SEM decreases from similar to 10(9) cm(-2) to zero in 10 min. On these same surfaces, AFM and TEM images acquired at longer times show that; silver nanoparticles continue to diminish in size until a high density (similar to 10(9) cm(-2)) of clusters having a diameter of 0.3-0.6 nm are obtained within 30 min. The coverage of the anodically poised HOPG surface by these subnanometer silver clusters gradually decreases to zero over the course of an hours. SAED analysis confirms the composition of the clusters seen by AFM is FCC silver. A decrease in the coverage of the surface by silver nanoparticles is correlated with an increase in the nucleation overpotential for silver as measured by cyclic voltammetry. Possible origins for the metastability of silver clusters at positive potentials are discussed.