Multi-Scale Study of Branched Electrodeposits

This study concerns the growth of metallic ramified branches formed by galvanostatic electrolysis of a stagnant metal salt aqueous solution inside a horizontal Hele-Shaw (thin gap) cell. Indeed, without supporting electrolyte, the electroneutrality constraint forces the deposit to grow rapidly in the form of long ramified branches made up of crystals whose size might be as lower than 100 nm [1-2] . The presentation focuses on the study of these branches growth, the understanding and the elucidation of the formation mechanisms of its nanostructure (crystals assembly) in order to exploit it as an alternative process for nanoparticles synthesis [2]. Besides, we propose here a new experimental protocol allowing to recover the whole electrodeposit assembly without damages, in order to process to its multiscale characterization and also the determination of the local microscopic morphology in each location of one branch. This presentation focuses specifically on the growth of the branches of copper, silver and iron at i) the macroscale for the branch pattern (10 µm to 1 mm, direct optical visualizations) and ii) also the scale of the nanocrystals (from 50 up to 500 nm, Scanning Electron Microscopy SEM). For each experiment, the branch pattern and structure are collected, and the measurements enabling correlating the impact of large scale transport phenomena on the resulting micro/nano-structure. The data are analysed with the support of theoretical models at both macro and micro scales. The effects of the current density J (16-266 mA/cm 2 ), the electrolyte concentration C (0.1-0.5M), the type of metal (copper, silver, iron) and the cell thickness (25-100 µm) are investigated. At the macroscale, the experimental results are compared to the prediction of a specifically developed growth model based on a “2D dielectric breakdown” [4] scheme assuming a diffusion-limited process for the metallic cation (M λ + ) and taking into account a finite growth velocity v g for the electrodeposit. For low J/C, the experimental and theoretical patterns (see the figure) are fractal up to a cut-off length, corresponding to the diffusion length L d = D/ v g (D: diffusion coefficient). However, there are some differences between the theoretical and experimental apparent density of the deposit as well as in Sand time (time for the full depletion of the cations at the electrode marking the onset of the branch growth). These differences are related to non-diffusive mass flux generated