Solid State Sciences 7 (2005) 367–374 www.elsevier.com/locate/ssscie Characteristic of hydrous iron (III) oxides prepared by homogeneous precipitation of iron (III) sulphate with urea S. Bakardjieva, V. Štengl ∗ , J. Šubrt, E. Veˇ cerníková Institute of Inorganic Chemistry AS CR, 250 68 ˇ Rež, Czech Republic Received 9 August 2003; received in revised form 3 December 2004; accepted 29 January 2005 Available online 11 March 2005 Abstract Hydrous iron (III) oxides agglomerated into uniform porous spherical clusters with submicrometre size and high specific surface area were obtained. Homogeneous precipitation of iron (III) sulphate was carried out in boiling aqueous solution that contained excess of urea. The prepared precursor precipitates were heated for 2 h at various temperature (from 200 to 800 ◦ C) to yield fine iron oxides powders. The struc- ture evolution during heating of powders was investigated by X-ray diffraction (XRD). The morphology and microstructure characteristics were also obtained by means of scanning electron microscopy (SEM), (BET) and (BJH). In order to see the phase transition temperature thermogravimetric analysis/differential thermal analysis studies were performed. It was found that formation of different hydrous iron (III) oxides was affected by experimental conditions: aging time, mixing procedure, pH and temperature.  2005 Elsevier SAS. All rights reserved. Keywords: Homogeneous precipitation; Uniform spherical particles; Ferrihydrite; Haematite 1. Introduction Submicrometre size reactive metal (Fe, Ti, Zn, Al) ox- ide powders of desired characteristics (purity, particle size, distribution and morphology, high specific surface area and porosity) are required for advanced applications—catalyst, ceramics, ferrites, sorption materials. One of the most widely applied processes to obtain such powders is a con- trolled solution method—homogeneous precipitation [1]. Inorganic precursors of this metal (like sulphate, chloride, nitrate etc.) in aqueous solution are extensively hydrol- ysed into small polynuclear cationic species. With the in- crease of pH or continued refluxing, they grow further by polymerisation–condensation reaction into bigger polynu- clear species, which finally yields in small discrete solid units called primary particles [2]. An increase of pH is achieved by adding ammonium hydroxide solution (hetero- geneous precipitation) or generating it slowly in situ (homo- * Corresponding author. Tel.: +420 2 6617 3534; Fax: +420 2 2094 0157. E-mail address: stengl@iic.cas.cz (V. Štengl). geneous precipitation). During heterogeneous precipitation, when the reactants exist in two phases before reaction ow- ing to the fast changes in solution concentration, no control of the size and shape can be exercised. It is well known, that in the homogeneous precipitation the reactant have been dissolved in the reaction phase before the start of the reac- tion, or one of the reactants is evolved slowly and uniformly throughout the solution [3]. M. Ocana et al. [4] reported a procedure for preparation of uniform ellipsoidal α-Fe 2 O 3 particles by precipitation from aqueous iron salts solutions [Fe(NO 3 ) 3 , Fe(ClO 4 ) 3 and FeCl 3 ] in the presence of urea at 100 ◦ C. S. Music et al. [5] described synthesis of β -FeOOH using hydrolysis of 0.1 M FeCl 3 solution and preparation of mixture contains α-Fe 2 O 3 and FeOOH using hydrolysis of 0.1 M Fe(NO 3 ) 3 solution at 90 ◦ C. Homogeneous precipitation from aqueous solution of Fe 2 (SO 4 ) 3 with urea in the temperature range 60–100 ◦ C has previously been used to produce poorly crystalline iron (III) hydrous oxides and basic salts (ferrihydrite, schwertmannite and jarosite) [6]. 1293-2558/$ – see front matter  2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.01.008