We have been developing a platform based on ultramicroelectrodes (UMEs) for spatially resolved, in situ microscale measurements to probe further the diffusion layer chemistry during electrodeposition of thin metal films, using of Ni on Au as an example . Advantages of UMEs include minimized solution resistance and well-controlled mass transport. Cyclic voltammetry of Au UMEs in 5.0 mmol/L NiCl2-NaCl, pH 3.0, electrolytes showed that at modest potentials between –0.6 V and –1.4 V vs. Ag/AgCl, the current was distributed between steady-state [Ni(H2O)6]2+ and H3O+ reduction. However, an unsual sharp current spike appeared at the onset of H2O reduction, demonstrating a unique and reproducible electrochemical signal corresponding to a metal self-termination process. The sharpness of this feature is reminiscent of a phase change or the initiation of a catalytic process. The generation/collection mode of scanning electrochemical microscopy (SECM), where a second UME was used as the collector electrode, indicated that H2 is generated during the process giving rise to this cathodic spike. One explanation is that a highly-active species for the hydrogen evolution reation (HER) is generated upon H2O reduction in the presence of Ni2+ , such as the heterogeneous nucleation of Ni(OH)2 in the presence of increasing pH in the diffusion layer. Using micrometer-sized pH probes in the SECM configuration, we determined that the pH of the diffusion layer increased to least 10, thus exceeding the solubility product constant, Ksp, of Ni(OH)2. The dimensions of UMEs and their diffusion layers are also amenable to optical microscopy. Thus, we took advantage of the contrast between Ni and Au to image the electrodepositon of Ni as a function of potential. In a phenomenon separate from the cathodic spike, as the rate of H2O reduction was accelerated at potentials below –1.5 V vs. Ag/AgCl, homogeneous precipitation of a [Ni(OH)a(Cl)b]2–a–b·xH2O gel occurred within the nearly hemispherical diffusion layer of the UME. In some cases, hydrogen bubbles formed, destroying the Ni films.
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