Synthesis of acicular a-FeOOH particles at a very high pH S. Krehula a , S. Popovic ´ b , S. Music ´ a, * a Division of Materials Chemistry, Rud c er Bos ˇkovic ´ Institute, P.O. Box 180, Bijenicka Cesta 54, 10002 Zagreb, Croatia b Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, 10002 Zagreb, Croatia Received 25 July 2001; accepted 28 July 2001 Abstract The acicular a-FeOOH particles were synthesized from FeCl 3 solutions at a very high pH using tetramethylammonium hydroxide as a precipitating agent. Samples were characterized by X-ray powder diffraction, FT-IR spectroscopy and transmission electron microscopy. a-FeOOH precipitated as a single phase from room temperature to 160 jC in a broad aging time interval. The ratio of the length to the width of a-FeOOH particles depended on the temperature and concentration of the starting FeCl 3 solution. Multidomainic character of a-FeOOH particles gradually decreased with an increase in the temperature and time of aging. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Goethite; (a-FeOOH) tetramethylammonium hydroxide; Acicular particles; XRD; FT-IR; TEM; FeOOH goethite 1. Introduction Goethite (a-FeOOH) is a major constituent of many soils, sediments and iron ores. It also appears as the weathering product of various rocks containing iron. Goethite ores are used for the production of raw iron. Natural goethite was used as a pigment in paintings for many centuries. Today, the synthetic a-FeOOH pig- ment is widely used in various compositions of house paints or fine colors for painting. The color of goethite can vary from dark brown to lemon yellow, as observed with the naked eye. Synthetic a-FeOOH particles are utilized as a starting material in the production of acicular maghemite (g-Fe 2 O 3 ) particles for use as the magnetic recording medium. Generally, a-FeOOH is also very important from the academic standpoint. In colloid and surface chemistry, the synthetic a-FeOOH particles are often used as a model adsorbent in fundamental studies of the adsorption/desorption phe- nomena. The simplest way to produce a-FeOOH particles is to subject the diluted Fe(NO 3 ) 3 aqueous solutions to hydrolysis for a longer time at room temperature (slow hydrolysis). The hydrolysis of Fe 3+ ions is much accelerated with the increase in temperature (forced hydrolysis). With prolonged time of hydrolysis a- FeOOH undergoes transformation to a-Fe 2 O 3 (hema- tite) via dissolution/reprecipitation mechanism [1,2]. a-FeOOH particles also precipitate by forced hydrol- ysis of diluted aqueous solutions of Fe 2 (SO 4 ) 3 [3] or NH 4 Fe(SO 4 ) 2 salts [4,5]. At higher concentrations of these salts, the jarosites H 3 OFe 3 (OH) 6 (SO 4 ) 2 or NH 4 Fe 3 (OH) 6 (SO 4 ) 2 are precipitated. The yield of a- FeOOH is small in all these hydrolytic reactions, and a- FeOOH may also undergo a phase transformation 0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-577X(01)00546-8 * Corresponding author. E-mail address: music@rudjer.irb.hr (S. Music ´). www.elsevier.com/locate/matlet May 2002 Materials Letters 54 (2002) 108 – 113