Synthesis of submicrometric dendritic iron particles in an Electrochemical and Vibrating Hele-Shaw cell: study of the growth of ramified branches

The purpose of this study is to explore a new synthesis way for the production of iron nanoparticles exploiting the nanometric structure of long ramified iron branches formed by electrodeposition in a Hele-Shaw cell. After the growth, these branches are fragmented by the action of a vibrating element (piezoelectric disk) integrated into the cell. The emphasis is put on the growth of the ramified iron branches which is performed by galvanostatic electrodeposition in a stagnant electrolyte (FeCl2) inside the Hele-Shaw cell (50 μm deep). The competition between the co-formation of H2 bubbles (H+ reduction) and the growth of ramified iron branches (FeII reduction) is analyzed by varying both the applied current density j and the FeCl2 concentration. Two regimes, depending mainly on j, are highlighted: below a threshold current density of 8 mA/cm2 only H2 bubbles are formed, while above this threshold, iron branches grow accompanied by the formation of H2 bubbles which nucleate and grow at the top of the branches during their formation. The H2 bubbles influence the branches growth especially at low j (<24 mA/cm2) when the growth velocity of the branches is low compared to the growth rate of the bubbles. At higher j (>24 mA/cm2), the branches follow a columnar growth with a constant front velocity, well predicted by the theory. Scanning Electron Microscopy (SEM) of the iron branches shows a dendritic structure constituted of nanometric crystallites, whose size depends on the local growth velocity: increasing the growth velocity from 3.6 μm/s to 40 μm/s leads to a decrease in the crystallites size, from ∼1 μm to ∼10 nm. Using the acoustic vibrations (4 kHz) of the piezoelectric disk, these fragile branches are successfully fragmented into submicrometric fragments of dendrites exhibiting high specific surfaces S/V (equivalent to the S/V of nanoparticles of 30 nm diameter). Advantages/Drawbacks compared to other synthesis ways as well as the optimization of the proposed synthesis are discussed.