Initial stages of hydroxide formation and its reduction on Co(0001) studied by in situ STM and XPS in 0.1 M NaOH

in situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E < -0.55 V (SHE) the formation of a Co(OH)(2)(0 0 0 1) superstructure is observed. It shows a hexagonal symmetry with an average periodicity of P = 1.25 +/- 0.20 nm. The coincidence cell of the observed structure consists of 16 unit cells Co(OH)(2)(0 0 0 1) showing an average lattice parameter of a = 0.33 +/- 0.05 nm and thus the Co(OH), monolayer forms a 5 x 5 superstructure with respect to the underlying metallic Co(0 0 0 1) substrate. XPS results clearly prove the presence of hydroxide and exclude the formation of CoO in the subpotential range. At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH--Co2+-OH- trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)(2) layer is found to be not completed which is confirmed by XPS results. (C) 2004 Elsevier B.V. All rights reserved.