Alternating morphology transitions in electrochemical deposition

Formation of magnetic nanowire arrays by cooperative lateral growth

Nanowires typically grow along their longitudinal axis, and the long-range order among wires sustains only when a template exists. Here, we report an unprecedented electrochemical growth of ordered metallic nanowire arrays from an ultrathin electrolyte layer, which is achieved by solidifying the electrolyte solution below the freezing temperature. The thickness of the electrodeposit is instantaneously tunable by the applied electric pulses, leading to parallel ridges on webbed film without using any template. An array of metallic nanowires with desired separation and width determined by the applied pulses is formed on the substrate with arbitrary surface patterns by etching away the webbed film thereafter. This work demonstrates a previously unrecognized fabrication strategy that bridges the gap of top-down lithography and bottom-up self-organization in making ordered metallic nanowire arrays over a large area with low cost.

Ag fractal structures in electroless metal deposition systems with and without magnetic field

Metal electrodeposition systems display tree-like structures with extensive ramification and a fractal character. Electrolysis is not a necessary route for the growth of such dendritic metal deposits. We can grow beautiful ramification patterns via a simple redox reaction. We present here a study of silver (Ag) deposits from the reduction of Ag⁺ in (AgNO₃) solution by metallic copper. The experiments are carried out in discotic geometry, in a Petri dish hosting a thin AgNO₃ solution film. A variety of deposited structures and patterns is obtained at different Ag⁺ concentrations, yet with essentially the same fractal dimension averaged at 1.64, typical of diffusion-limited aggregation (DLA). A linear magnetic field of low induction (0.50-1.0 T) applied across the medium causes a notable transformation in the morphology of the deposits. In both the field off and the field on cases, the effect of vertical (hence 3D) heaving seems to be dominant, perhaps explaining the nearly constant fractal dimension.

Fractal growth of platinum electrodeposits revealed by in situ electron microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth - either dense branching or ramified islands - are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Nature of inclined growth in thin-layer electrodeposition under uniform magnetic fields

Electrochemical deposition (ECD) in thin cells in a vertical position relative to gravity, subject to an external uniform magnetic field, yields a growth pattern formation with dense branched morphology with branches tilted in the direction of the magnetic force. We study the nature of the inclined growth through experiments and theory. Experiments in ECD, in the absence of magnetic forces, reveal that a branch grows by allowing fluid to penetrate its tip and to be ejected from the sides through a pair of symmetric vortices attached to the tip. The upper vortices zone defines an arch separating an inner zone ion depleted and an outer zone in a funnel-like form with a concentrated solution through which metal ions are carried into the tip. When a magnetic field is turned on, vortex symmetry is broken, one vortex becoming weaker than the other, inducing an inclination of the funnel. Consequently, particles entering the funnel give rise to branch growth tilted in the same direction. Theory predicts, in the absence of a magnetic force, funnel symmetry induced through symmetric vortices driven by electric and gravitational forces; when the magnetic force is on, it is composed with the pair of clockwise and counterclockwise vortices, reducing or amplifying one or the other. In turn, funnel tilting modifies particle trajectories, thus, growth orientation.

Fractal structures in two-metal electrodeposition systems II: Cu and Zn

In this second part of our study on fractal co-electrochemical deposition, we investigate the Cu-Zn system. Macroscopic and microscopic inspection shows a sensitive dependence of the morphology of the final pattern on initial concentrations. The pattern is seen to undergo a transition from classical dendrites to randomly ramified deposits, with each slight increase in [Cu(2+)](0), while [Zn(2+)](0) is maintained constant. The variational trends in chemical composition, growth velocity, and fractal dimension with increasing [Cu(2+)](0) are analyzed. The latter is seen to generally increase with copper (II) ion concentration. In contrast, the growth rate of the deposits is seen to decrease with increasing concentration of Cu(2+) ions. A new probe of dense ramified morphology, the pattern density, is introduced and seen to increase with [Cu(2+)](0). XRD measurements reveal that the observed properties correlate with the birth of copper-rich nuclei, which disrupt the crystalline anisotropy of the two-metal alloy.

Fractal structures in two-metal electrodeposition systems I: Pb and Zn

Pattern formation in two-metal electrochemical deposition has been scarcely explored in the chemical literature. In this paper, we report new experiments on zinc-lead fractal co-deposition. Electrodeposits are grown in special cells at a fixed large value of the zinc ion concentration, while that of the lead ion is increased gradually. A very wide diversity of morphologies are obtained and classified. Most of the deposited domains are almost exclusively Pb or Zn. But certain regions originating at the base cathode, ranging from a short grass alley to dense, grown-up bushes or shrubs, manifest a combined Pb-Zn composition. Composition is determined using scanning electron microscopy/energy dispersive x ray measurements as well atomic absorption spectroscopy. Pb domains are characterized by shiny leaf-like and dense deposits as well as flowers with round, balloon-like corollas. The Zn zones display a greater variety of morphologies such as thick trunks and thin and fine branching, in addition to minute "cigar flower" structures. The various morphologies are analyzed and classified from the viewpoint of fractal nature, characterized by the box-count fractal dimension. Finally, macroscopic spatial alternation between two different characteristic morphologies is observed under certain conditions.

Thermodynamic model of nonequilibrium phase transitions

Within the scope of a thermodynamic description using the maximum entropy production principle, transitions from one nonequilibrium (kinetic) regime to another are considered. It is shown that in the case when power-law dependencies of thermodynamic flux on force are similar for two regimes, only a transition accompanied by a positive jump of thermodynamic flux is possible between them. It is found that the difference in powers of the dependencies of thermodynamic fluxes on forces results in a number of interesting nonequilibrium transitions between kinetic regimes, including the reentrant one with a negative jump of thermodynamic flux.

Transition between two dendritic growth mechanisms in electrodeposition

We report in this paper the observation of a transition between two different dendritic growth mechanisms in the electrodeposition of a metal from a binary electrolyte. Our results, in particular concerning the dendritic growth velocities, enable us to explain this behavior in terms of models previously proposed in the literature.

Alternating morphology transitions in crystallization of NH4Cl on agar plates

Two types of alternating morphology transitions have been observed in crystallization of NH4Cl on agar plates. One is the alternating morphology transitions between dense branching morphology and sparse branching morphology, and the other is the alternating morphology transitions between dense branching morphology and zigzag branching morphology. The appearance of them is found to depend on the mass proportion of agar to NH4Cl in the initial solution and the relative humidity. It is suggested that both the two alternating morphology transitions result from the oscillation of solute concentration in front of the growing interface caused by the competition of crystal growth and solute transfer at a moderate mass proportion. Which one of them occurs depends on the relative humidity, which controls the supersaturation.

Long-range ordering effect in electrodeposition of zinc and zinc oxide

In this paper, we report the long-range ordering effect observed in the electro-crystallization of Zn and ZnO from an ultrathin aqueous electrolyte layer of ZnSO4 . The deposition branches are regularly angled, covered with random-looking, scalelike crystalline platelets of ZnO. Although the orientation of each crystalline platelet of ZnO appears random, transmission electron microscopy shows that they essentially possess the same crystallographic orientation as the single-crystalline zinc electrodeposit underneath. Based on the experimental observations, we suggest that this unique long-range ordering effect results from an epitaxial nucleation effect in electrocrystallization.

Morphological evolution in the electrodeposition of the Pb-Sn binary system

Morphological evolution in the electrodeposition of Pb-Sn binary system is studied. As the second component increases, the morphology of the codeposit changes from dendrite to ramification, to dense branch, and finally to fractal structure, respectively. The evolution arises from the influence of crystallographic texture, which leads to a splitting of dendritic tips and the formation of ramified morphology. This work provides direct evidence to explore the crystallographic influence on the morphological evolution in electrodeposition.

Spontaneous formation of periodic nanostructured film by electrodeposition: Experimental observations and modeling

In this paper we report the spontaneous formation of a nanostructured film by electrodeposition from an ultrathin electrolyte layer of CuSO4. The film consists of straight periodic ditches and ridges, which corresponds to the alternating deposition of nanocrystallites of copper and copper plus cuprous oxide, respectively. The periodicity on the film may vary from 100 nm to a few hundred nanometers depending on the experimental conditions. In the formation of the periodically nanostructured film, oscillating voltage/current has been observed across the electrodes, and the frequency depends on the pH of the electrolyte and the applied current/voltage. A model based on the coupling of [Cu2+] and [H+] in the electrodeposition is proposed to describe the oscillatory phenomena in our system. The calculated results are in agreement with the experimental observations.

Electrochemical growth of iron and cobalt arborescences under a magnetic field

Pattern formation in the electrochemical deposition of the magnetic Fe and Co metals from thin layers of Fe(SO4) or Co(SO4) aqueous solutions were investigated in circular geometry and under magnetic field. Sparse arborescences with few thick branches and dense arborescences with many thin branches can be generated when no magnetic field is applied. Unlike for nonmagnetic metals, no tendency towards growth spiraling or asymmetric branching is found out in magnetic field normal to the plane of the growth. The morphology of the deposits appears instead to become more sparse. Under in-plane magnetic field, the sparse arborescences get into a needle morphology, oriented along the field, while the dense arborescences show a circular to rectangular morphology symmetry breaking, one edge of the rectangle being parallel to the field. Unexpected in most instances, these magnetic field effects cannot be understood without invoking the magnetic dipolar interaction inside the magnetized growing aggregate together with its interaction with the applied field.

Pattern selection induced by electroconvection in the electrodeposition of iron

The morphology of iron electrodeposit is shown to relate closely to the pH of the electrolyte solution. Macroscopically, depending on the strength of the interbranch convection, which is associated with the concentration of H3O+ in the electrolyte, the deposit morphology varies from treelike pattern to meshlike pattern and dense-branching morphology. Microscopically the deposit is ramified and dense-branching at lower concentration of H3O+, while it becomes relatively smooth and stringy at higher H3O+ concentration. The symmetry of the convective vortices on the two sides of the growing tip is observed to decide the growth behavior of the tip. We suggest that H3O+ influences the pattern formation and pattern selection in the electrodeposition of iron from FeSO4 solution by either initiating interbranch convection or changing the effective interfacial energy of the deposit and the electrolyte.