Elucidation of the Mechanism of Electrochemical Formation of Magnetite Nanoparticles by In Situ Raman Spectroscopy

Abstract

The formation of magnetite nanoparticles (Fe 3 O 4 -NPs) by electro-oxidation process was studied by in situ and ex situ techniques in chloride electrolytes with and without ethanol. The electrochemical synthesis is characterized by the application of a current disturbance that promotes the oxidation of a low carbon steel electrode in solution to an oxidized state (Fe 2+ ) which is subsequently transformed into magnetite by reactions in solution. The electrochemical synthesis results in a final product of pure and crystalline magnetite nanoparticles (20-40 nm). The presence of ethanol in the electrolyte does not modify the mechanism of magnetite formation but it extends the lifetime of some precursors during electrosynthesis and promotes the formation of low size magnetite nanoparticles. In situ Raman spectroscopy measurements were used in order to identify the precursor species formed prior to the formation of magnetite nanoparticles during the electro-oxidation of the low carbon steel in electrolyte containing chloride and ethanol. It was corroborated that the electrochemical synthesis of magnetite follows the sequence: Fe(OH) 2 → GR(Cl − ) → γ-FeOOH → Fe 3 O 4 , with a redox interaction between Fe(OH) 2 and γ-FeOOH precursors.

Reaction Mechanisms of the Electrosynthesis of Magnetite Nanoparticles Studied by Electrochemical Impedance Spectroscopy

“…This species then reacts in the bulk solution with dissolved dihydrogen formed in the cathode to obtain the magnetite (reaction ). However, the anodic reaction involving three electrons and three hydroxide molecules is unlikely to occur in a single step.At the cathode, water reduction (reaction ) occurs, and in the bulk solution Conversely, Lozano et al showed that hydrogen is not a reducing agent of lepidocrocite (γ-FeOOH) (reaction ), which is also in agreement with the work of Montoya et al…”

“…Indeed, Lozano et al 12 and Montoya et al 13 devised mechanisms in which only Fe 2+ is generated at the anode, while the other reactions of the mechanism take place in the homogeneous phase.…”

“…Additionally, they reported a number of reactions related to the formation of green rust in the homogeneous phase, which consisted of unstable compounds containing a mixture of ferrous and ferric iron. 13 …”

“…The probability of having enough H 2 in the bulk solution is minimal because the coalescence and buoyancy phenomena favor its release from the solution, so that this synthesis pathway cannot be the main one. Lozano et al 12 and Montoya et al 13 have assumed that the reaction occurring on the anodic surface is simply iron oxidation followed by subsequent homogeneous reactions. However, anodic iron dissolution is a process that has been extensively investigated.…”

“…The probability of having enough H 2 in the bulk solution is minimal because the coalescence and buoyancy phenomena favor its release from the solution, so that this synthesis pathway cannot be the main one. Lozano et al and Montoya et al have assumed that the reaction occurring on the anodic surface is simply iron oxidation followed by subsequent homogeneous reactions. However, anodic iron dissolution is a process that has been extensively investigated. − Indeed, studies by electrochemical impedance spectroscopy (EIS) have concluded that iron oxidation involves several steps, which are observed from the Nyquist diagrams as more than one time constant in the spectra, some involving adsorbed intermediate species. , All of these results suggest that additional studies are still required to accurately describe the NP synthesis mechanism.…”