New Insights into the Electrochemical Formation of Magnetite Nanoparticles

Abstract

The electrochemical mechanism of the formation of magnetite nanoparticles is studied. The proposed mechanism suggests the formation of iron hydroxide Fe(OH) 2 in the presence of oxygen which produces lepidocrocite (γ-FeOOH) followed by its chemical dehydration. This is in contrast to other reported mechanisms that suggest the reduction of Fe(OH) 3 at the cathode. Video frames captured during the electrosynthesis of magnetite, in a typical two-electrode cell, indicate that the nanoparticles form in the region close to the anode. The pH value near the anode and cathode changes with time, indicating the formation of nanoparticles. Additional experiments in a two-compartment cell fitted with a cationic membrane, to avoid direct intermixing of Fe 2+ and OH − and possible oxide or oxyhydroxide reduction at the cathode, support this mechanism. The amount of dissolved oxygen in the electrolyte was found to be a key factor to produce magnetite by promoting the transformation of Fe(OH) 2 into (γ-FeOOH). Hydrogen bubbling during electrosynthesis does not contribute to the reduction of the oxyhydroxides, according to X-ray diffraction results. The paper presents a proposed mechanism for the formation of magnetite, based on previous and new evidence.

Progress in the Preparation of Magnetite Nanoparticles through the Electrochemical Method

“…The last mechanism which involves dissolved oxygen as an oxidizing agent for Fe(OH) 2 in solution to produce magnetite seems to be the most likely appropriate to describe the electrochemical formation of magnetite nanoparticles. This mechanism was first proposed by Fajaroh F. et al (2012), and the validity was confirmed by Lozano I. et al (2017) …”

“…As noted by Lozano I. et al (2017), however, it is unlikely that solid species such as Fe(OH) 3 can reach the cathode to undergo reduction. Additionally, due to its high propensity to escape from the reacting medium, it is almost impossible for hydrogen gas to react with iron hydroxide (Eqn.…”

“…The proposed mechanisms have been compiled and discussed thoroughly by Lozano I. et al (2017). In general, the proposed mechanisms can be divided into four groups although the detail steps for each group may be different.…”

“…et al, 2002), (ii) hydrogen gas as a reducing agent of iron hydroxide (Franger S. et al, 2004;Manrique-Julio J. et al, 2016;Melnig V. and Ursu L., 2011), (iii) direct reaction between Fe 2+ and Fe 3+ (Karami H. and Chidar E., 2012;Ying T.Y. et al, 2002), and (iv) oxygen as oxidant agent and subsequent in-solution precipitation Gopi D. et al, 2016;Lozano I. et al, 2017;Pascal C. et al, 1999;Starowicz M. et al, 2011).…”

pH Transitions and electrochemical behavior during the synthesis of iron oxide nanoparticles with gas-diffusion electrodes

“…[18][19][20][21][22][23][24][25] In most instances, known electrochemical approaches for the synthesis of nanoparticles [26][27][28] involve the growth of lms onto inert substrates, mostly by one-step electrodeposition, at working temperatures between 70 and 90 C. A compilation of pathways and methods for the electrochemical formation of magnetite nanoparticles is available in the scien-tic literature. 23,26,[29][30][31][32] In the present study, a new electrochemical method, called gas-diffusion electrocrystallization (GDEx), 14 is employed for the formation of iron oxide nanoparticles (IONPs) 33 from a soluble iron precursor (Fe 2+ ). A gas diffusion cathode is used for the electroreduction of oxygen (O 2 ) contained in a gas phase (air) which, in turn, drives the precipitation of crystalline IONPs at the electrochemical interface.…”