Aqueous growth mechanism of ferrite (intermediate between Fe
3O
4 and γ-Fe
2O
3) nanoparticles
via the precipitation and oxidation of green rust was studied by monitoring
pH and ORP of the reaction solution. While Fe
3O
4 was growing, both
pH and ORP reached plateaus of
pH=6 ∼ 7 and ORP=−400 ∼ −300 mV. The
pH was kept constant because the formation of Fe
3O
4 from green rust is expressed by the formula [Fe
II2Fe
III1(OH)
6]
+ · [Cl
1 ·
mH
2O]
−+NO
3−→Fe
3O
4+(3+
m)H
2O+Cl
−+NO
2↑ where H
+ is not released. The observed value of ORP was ascribed to the interfacial potential between Fe
3O
4 and FeOH
+, a dissolved species from the green rust. After ending the growth by exhausting the green rust, the surface of Fe
3O
4 was oxidized to γ-Fe
2O
3, which gave the final values of
pH=4 ∼ 5 and ORP ≈ 0 mV. Adding sucrose, a disaccharide, to the reaction solution decreased the growth rate of the ferrite particles. This supported our proposal that a disaccharide suppresses the growth of each crystal plane and thus enables the particles to grow spherical in shape.
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