Controlling the phase of iron oxide nanoparticles fabricated from iron(III) nitrate by liquid flame spray

Iron oxide nanoparticles were synthesized in a liquid flame spray process from
iron(III) nitrate. The choice of chemicals and all other process parameters affects the crystallographic phase composition and the quality of the material. Adjustment of the solvent composition and the gas flow rates was used to control the phase composition of the produced particles. All samples consisted of pure maghemite (γ‐Fe2O3) or a mixture of maghemite and hematite (α‐Fe2O3). When using pure alcohols as solvents, the maghemite/hematite phase ratio could be adjusted by changing the equivalence ratio that describes the oxidation conditions in the flame zone. A large residual particle mode formed in the size range of ~20‐700 nm along with a dominant very fine particle mode (2‐8 nm). Both phases seemed to contain large particles. A partial substitution of methanol with carboxylic acids turned the hematite phase into maghemite completely, even though some of particles were possibly not fully crystallized. Residual particles were still present, but their size and number could be decreased by raising the heat of combustion of the precursor solution. 30 vol‐% substitution of methanol with 2‐ethylhexanoic acid was adequate to mostly erase the large particles.