This TEM and XRD investigation of the HS of mixed FeCl3 / FeCl2 precursor solution, at pH ~ 12, has
provided evidence for the development of Fe3O4 NPs that are generally isotropic and < 50 nm in size.
A HS reaction temperature of 200°C was observed sufficient to promote the formation of the
thermodynamically stable Fe3O4 iron oxide phase as the sole reaction product, whilst it is considered
that the Fe3O4 NP growth mechanism may have involved the growth and dissolution of intermediate
iron (oxy)hydroxide phases. Kine matically accessible amorphous Fe(OH)3, poorly crystalline
‘hydroxychloride green rust 1’: Fe3.6Fe0.9(O, OH, Cl)9 (JCPDS 13-88) sheets and α-FeOOH nanorods
are known to precipitate during HS at relatively low temperatures in iron-chloride salt solutions at pH
~ 12 [4, 10], and their dissolution with increasing temperature is considered to supply elemental
growth constituents for formation of the more thermodynamically stable Fe3O4.