Effects of anions on the morphology and structure of hematite particles produced from forced hydrolysis of Fe(NO3)3–HNO3

The effects of anions (F−, Cl−, NO3− and Br−) on morphology and structure of hematite particles produced from a forced hydrolysis reaction of Fe(NO3)3–HNO3 were investigated by various means. In the case of NO3− and Br− systems, there was no significant change in particle morphology; the small spherical hematite particles with ca. 20–40 nm in diameter were produced. On the other hand, the addition of F− ions produced needle-like α-FeOOH particles up to 0.01 M, small rod-like β-FeOOH particles were formed at 0.02 M and finally the large rod-like β-FeOOH particles were precipitated above 0.03 M. A unique effect of anions was observed on Cl− ions; the spherical hematite particles are produced up to 0.1 M and the size was increased with increase in the concentration of Cl− ions. The size of large spherical hematite particles (ca. 350 nm in diameter) produced in the Fe(NO3)3–HNO3 system with 0.1 M Cl− ions was analogous to that steadily attained in the FeCl3–HCl one (called as control system). It was suggested that 3.2 of the Cl/Fe molar ratio in the aging solution is an optimum condition for producing the large spherical hematite particles. The formation of large spherical hematite particles only in the Cl− system was explained by the formation of small β-FeOOH particles as an intermediate for producing hematite ones via dissolution–reprecipitation process. The hematite particles produced from Fe(NO3)3–HNO3 solution were monocrystalline but those from the control system are polycrystalline. This fact describes that the system with NO3− ions provides much more condensed inner structure rather than the control system.