Monitoring the hydrothermal precipitation of α-Fe2O3 from concentrated Fe(NO3)3 solutions partially neutralized with NaOH

The influence of amidosulfonate anion on the crystallization of α-Fe2O3 from a dense ferrihydrite suspension in an acidic pH medium was investigated using XRD, 57Fe Mössbauer and FT-IR spectroscopies and FE-SEM + EDS. The ferrihydrite suspensions contained traces or a small amount of α-FeOOH. A reference ferrihydrite suspension was obtained by partial neutralization of Fe(NO3)3 solutions with a concentrated NaOH solution and brief autoclaving at 160 °C. Upon a prolonged autoclaving time a mixture of pseudospheric α-Fe2O3 particles was produced. The addition of amidosulfonate anion into the reference precipitation system suppressed the formation of α-Fe2O3 particles and completely changed the shape of the particles. Near spindle-shaped and hollow α-Fe2O3 particles were produced consisting of elongated primary α-Fe2O3 particles which were laterally arrayed, thus forming hollow particles. The elongation of primary α-Fe2O3 particles according to the crystallographic c-axis was caused by the specific adsorption of sulfate anions perpendicular to the c-axis. Sulfate anions were formed by the decomposition of amidosulfonates in the precipitation system.