Abstract
Gas diffusion electrocrystallization (GDEx) was explored for the synthesis of iron oxide nanoparticles (IONPs). A gas-diffusion cathode was employed to reduce oxygen, producing hydroxyl ions (OH−) and oxidants (H2O2 and HO2−), which acted as reactive intermediates for the formation of stable IONPs. The IONPs were mainly composed of pure magnetite. However, their composition strongly depended on the presence of a weak acid, i.e., ammonium chloride (NH4Cl), and on the applied electrode potential. Pure magnetite was obtained due to the simultaneous action of H2O2 and the buffer capacity of the added NH4Cl. Magnetite and goethite were identified as products under different operating conditions. The presence of NH4Cl facilitated an acid–base reaction and, in some cases, led to cathodic deprotonation, forming a surplus of hydrogen peroxide, while adding the weak acid promoted gradual changes in the pH by slightly enhancing H2O2 production when increasing the applied potential. This also resulted in smaller average crystallite sizes as follows: 20.3 ± 0.6 at −0.350 V, 14.7 ± 2.1 at −0.550 and 12.0 ± 2.0 at −0.750 V. GDEx is also demonstrated to be a green, effective, and efficient cathodic process to recover soluble iron to IONPs, being capable of removing >99% of the iron initially present in the solution.
Highlights
A gas diffusion cathode is used for the electroreduction of oxygen (O2) contained in a gas phase which, in turn, drives the precipitation of crystalline iron oxide nanoparticles (IONPs) at the electrochemical interface
For comparison purposes between experiments, a one-hour period was chosen to measure the concentration of H2O2, which was normalized to the amount of charge consumed for each experiment from which the corresponding electrolyte samples were extracted
The buffering ability of NH4Cl in aqueous solutions of 0.1 M NaCl enhanced the generation of H2O2 via the cathodic deprotonation process
Summary
Nanoparticle production methods that are economical, clean, safe, easy-to-implement, and upscalable are an active research subject.[1,2,3] A key disadvantage of most alternatives[4,5,6,7] is the use of non-aqueous molecular solvents and reducing and capping agents, which are o en environmentally hazardous and added in excess.[8,9] most methods involve several processing steps, o en including different upstream and downstream units, which can appear to be suitable at the lab scale but are challenging to implement industrially.[5,10,11,12]The development of environmentally friendly synthesis methods has gained particular attention,[13,14,15,16,17] intensifying the interest in electrochemistry as a synthetic platform under mild conditions.[18,19,20,21,22,23,24,25] In most instances, known electrochemicalA compilation of pathways and methods for the electrochemical formation of magnetite nanoparticles is available in the scienti 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 (Fe2+). The development of environmentally friendly synthesis methods has gained particular attention,[13,14,15,16,17] intensifying the interest in electrochemistry as a synthetic platform under mild conditions.[18,19,20,21,22,23,24,25] In most instances, known electrochemical. A compilation of pathways and methods for the electrochemical formation of magnetite nanoparticles is available in the scienti c literature.[23,26,29,30,31,32]. 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 (Fe2+). A gas diffusion cathode is used for the electroreduction of oxygen (O2) contained in a gas phase (air) which, in turn, drives the precipitation of crystalline IONPs at the electrochemical interface
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