ORDER AND DISORDER IN ELECTROCHEMICAL DEPOSITS OF COPPER ON GRAPHITE

Electrochemical deposition of copper on highly oriented pyrolytic graphite (HOPG) from aqueous 0.05M H2SO4 and 0.1M NaCl media has been studied using cyclic voltammogram (CV), single-pulse potentiostatic current vs. time (i vs. t) transient, and in situ scanning tunneling microscopic (STM) techniques. At potentials positive to the Nernst reversible potential, Cu(II) ions tend to adsorb on the HOPG surface. In the NaCl medium, the adsorbed Cu(II) ions form a stable, ordered layer. In this medium, the i vs. t transients indicate that the kinetics of bulk deposition is limited by planar diffusion, and the STM images reveal well-ordered bulk deposits. In contrast, in the 0.05M H2SO4 medium, the copper ions, even if adsorbed, do not form any stable or ordered layer on the HOPG surface. In this medium, the bulk deposition is a nucleated process, whose kinetics is limited by spherical diffusion, and the bulk deposit is disordered. It is inferred from these observations that the presence of an adsorbed and ordered layer of Cu(II) ions, prior to the onset of the bulk deposition, is responsible for the formation of ordered bulk deposits.