The effect of phosphonate-based growth modifiers on the morphology of hematite nanoparticles formed via acid hydrolysis of ferric chloride solutions

Abstract

The effect of organic phosphonate-based additives on α-Fe₂O₃ (hematite) crystallization via the forced hydrolysis of ferric chloride solutions has been studied using a range of additives containing 2, 3 or 4 pendant phosphonate groups. The hydrolysis reactions were carried out at pH 1.1 with an iron concentration of 0.01 mol L⁻¹, and with Fe ∶ additive ratios ranging from >8000 ∶ 1 down to 200 ∶ 1. In the absence of additive, the hematite particles are very uniform rhombic single crystals, with an average length of ≈100 nm. In the presence of phosphonates the particles become hexagonal, with the additives acting by inhibiting growth at (2) faces. At Fe ∶ additive ratios lower than about 200 ∶ 1, hematite formation is completely inhibited. Our results are consistent with a mechanism in which the akaganéite and hematite phases may both precipitate from the ferric chloride solution, with akaganéite apparently forming more rapidly than hematite. Inhibition of hematite formation by addition of the phosphonate additive can lead to β-FeOOH (akaganéite) being the dominant product phase, provided the concentration of Fe³⁺ in solution is above the equilibrium solubility of the akaganéite phase.