Layer-by-Layer Electrodeposition of Copper in the Presence of o-Phenanthroline, Caused by a New Type of Hidden NDR Oscillation with the Effective Electrode Surface Area as the Key Variable

Multivariate statistical analysis of chemical and electrochemical oscillators for an accurate frequency selection

The effect of experimental parameters on the frequency of chemical oscillators has been systematically studied since the first observations of clock reactions. The approach is mainly based on univariate changes in one specific parameter while others are kept constant. The frequency is then monitored and the effect of each parameter is discussed separately. This type of analysis, however, does not take into account the multiple interactions among the controllable parameters and the synergic responses on the oscillation frequency. We have carried out a multivariate statistical analysis of chemical (BZ-ferroin catalyzed reaction) and electrochemical (Cu/Cu₂O cathodic deposition) oscillators and identified the contributions of the experimental parameters on frequency variations. The BZ reaction presented a strong dependence on the initial concentration of sodium bromate and temperature, resulting in a frequency increase. The concentration of malonic acid, the organic substrate, affects the system but with lower intensity compared with the combination of sodium bromate and temperature. On the other hand, the Cu/Cu₂O electrochemical oscillator was shown to be less sensitive to changes in the temperature. The applied current density and pH were the two parameters which most perturbed the system. Interestingly, the frequency behaved nonmonotonically with a quadratic dependence. The multivariate analysis of both oscillators exhibited significant differences - while the homogenous oscillator displayed a linear dependence with the factors, the heterogeneous one revealed a more complex dependence with quadratic terms. Our results may contribute, for instance, in the synthesis of self-organized materials in which an accurate frequency selection is required and, depending on its value, different physicochemical properties are obtained.

Quasiperiodic behavior in the electrodeposition of Cu/Sn multilayers: extraction of activation energies and wavelet analysis

The oscillatory electro-deposition of Cu/Sn in the presence of a surfactant shows quasiperiodic behavior, which is described by the coupling between a mainly activation and a mainly diffusion-controlled processes.

Tuning Electrochemical Bistability by Surface Area Blocking in the Cathodic Deposition of Copper

We report herein a precise control of the electrochemical bistability induced by surface area changes during the cathodic deposition of copper. Small additions of 1,10-phenanthroline (Phen) in the reaction media present an inhibiting effect on the global rate mainly due to the adsorption of protonated Phen. The increase of its concentration favors a shrinkage of the bifurcation (saddle-node) diagram and shifts it to less negative potentials. The dynamic instability is verified by impedance measurements, and a negative impedance is clearly found. We calculated the apparent molar mass of the adsorbents using in situ gravimetric monitoring in the electrochemical experiments, and the results indicate that mass changes occur mainly due to the reduction of copper from bivalent ions dissolved in the reaction media. Importantly, the adsorption of protonated Phen molecules does not show a considerable contribution in mass variations but prevents the formation of a copper course grained morphology over the surface. Imaging analysis indicates finer nodulations at the lower branch compared to the upper branch in the bistability domain. On the basis of these observations, a kinetic mechanism is proposed and a good agreement is obtained between the apparent molar mass extracted from experiments and the theoretical values. Altogether, our results contribute to a detailed physical chemical description of the nonlinear behavior, bringing new insights about this reaction and pointing out the possibility to design switchable surface electrodes by taking advantage of the bistable behavior.

Oscillatory electrodeposition of metal films at liquid/liquid interfaces induced by the large surface energy of growing deposits

Electrodeposition of zinc (Zn) at an aqueous ZnSO4/n-butylacetate (BuAc) interface (liquid/liquid (LL) interface) showed a potential oscillation in the region of the current density exceeding the diffusion-limited one, accompanied by formation of two-dimensional Zn film with a concentric pattern at the LL interface. In-situ optical microscopic inspections revealed that the oscillatory growth of the Zn film synchronized with meniscus oscillation of the LL interface. The vigorous growth of the deposits occurs only when the shape of the meniscus becomes hollow on the negative potential side of the potential oscillation. On the other hand, on the positive side, the meniscus becomes almost flat and the deposits formed in the preceding stage are thickened. A mechanism is proposed to explain the oscillatory Zn electrodeposition coupled with the meniscus oscillation, on the basis of the fact that the interfacial tension at the growing metal/aqueous solution interface is extremely large.

Mechanisms of Oscillations and Formation of Nano-Scale Layered Structures in Induced Co-Deposition of Some Iron-Group Alloys (Ni−P, Ni−W, and Co−W), Studied by an In Situ Electrochemical Quartz Crystal Microbalance Technique

We have investigated mechanisms of oscillations and formation of nano-scale layered structures in induced co-deposition of some iron-group alloys (Ni-P, Ni-W, and Co-W) that have unique properties and are widely used in industries. Detailed in situ electrochemical quartz crystal microbalance (EQCM) experiments have revealed that the electrodeposition (induced co-deposition) of the alloys has negative differential resistances (NDRs), from which the oscillations and the layer-structure formation arise. The NDRs, however, cannot necessarily be seen in current-potential curves owing to overlap of hydrogen evolution current, indicating that the oscillations are of a hidden-NDR (H-NDR) type. The EQCM experiments have also shown that electrolyte components (such as H2PO2- and WO4(2-)) or related species are adsorbed at the electrode (deposit) surface and act as a promoter for the co-deposition reaction and that the NDRs arise from desorption of the adsorbed promoter. Interestingly, the adsorbed promoter is drawn into the deposition reaction itself, thus resulting in the alloy deposits. This mechanism was supported by in situ EQCM investigations of the oscillation as well as Auger electron spectroscopic (AES) analyses of deposits formed during the oscillation. The present work has for the first time clarified a general mechanism for the induced co-deposition reactions of some industrially important iron-group alloys (Ni-P, Ni-W, and Co-W).