Formation of iron oxides in a highly alkaline medium in the presence of palladium ions Stjepko Krehula * , Svetozar Music ´ Division of Materials Chemistry, Ruder Boškovic ´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia article info Article history: Received 26 August 2008 Accepted 6 October 2008 Available online 12 October 2008 Keywords: Palladium TMAH Magnetite Hematite Mössbauer FE-SEM abstract The effect of the presence of palladium ions in a highly alkaline precipitation system on the formation of iron oxides was investigated using X-ray powder diffraction (XRPD), Mössbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular a-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylam- monium hydroxide (TMAH) were used as reference material. The initial addition of palladium ions to that precipitation system had a significant effect on the formation of iron oxide phases and their properties. In the presence of palladium ions, the initially formed a-FeOOH has been transformed to a-Fe 2 O 3 crystals in the form of hexagonal bipyramids via a dissolution–reprecipitation mechanism with a simultaneous for- mation of metallic palladium nanoparticles. These palladium nanoparticles acted as a catalyst for the reductive dissolution of a-Fe 2 O 3 particles and the formation of Fe 3 O 4 crystals in the form of octahedrons. Increase in the initial concentration of palladium ions in the precipitation system accelerated the trans- formation process a-FeOOH ? a-Fe 2 O 3 ? Fe 3 O 4 and influenced changes in the shape of a-Fe 2 O 3 and Fe 3 O 4 particles. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Iron oxides and oxyhydroxides (usually referred to as iron oxides) are common compounds which are widespread in nature and pos- sess properties suitable for a broad range of applications [1–3]. They are used as pigments (paints, coatings, rubber fillers, construction sealants, cosmetics, ceramic glaze, etc.), catalysts, materials for mag- netic recording devices, abrasives, gas sensors, etc. For most of these applications it is very important that a particular iron oxide material has specific characteristics which depend on its microstructural properties (particle size and shape, crystallinity, porosity, etc.). For example, the rod-like c-Fe 2 O 3 (maghemite) and metallic iron parti- cles, produced via the rod-like a-FeOOH (goethite) particles, are used as materials for magnetic recording devices [4–6] and it is owing to the acicular shape of the particles that these materials can be used for magnetic recording. In the synthesis of iron oxides by precipitation from ferric salts solutions the final product of the precipitation system (phase com- position, particle size and shape, crystallinity) depends on the con- centration of ferric ions, pH, temperature, time of aging and the nature of the anions and other ligands present [1,7,8]. The presence of various metal cations also affects the formation of iron oxides but they usually require higher concentrations than anions. The major- ity of investigated metal cations (Cd 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) sup- press the formation of a-FeOOH by stabilizing ferrihydrite against dissolution [9,10]. The presence of Cu 2+ or Cd 2+ ions in a precipita- tion system [11–13] influences the formation of an a-Fe 2 O 3 phase in the form of rhombohedral crystals instead of the plates typically obtained from ferrihydrite in highly alkaline media [14]. The present investigation is concerned with the effect of Pd 2+ ions addition on the kinetics of the formation of iron oxides in a highly alkaline medium and the microstructural properties of ob- tained phases. Former investigations of Pd/Fe oxide systems were mainly focused on solid-state synthesis and there is a lack of data on the effects of the presence of palladium ions on the transforma- tions of iron oxides in a solution. Wang et al. [15] synthesized a series of Pd-doped a-Fe 2 O 3 powders by calcining precipitated Pd-ferrihydrite. These powders possess improved gas (H 2 S) sensing properties compared with the undoped a-Fe 2 O 3 . Urasaki et al. [16] prepared Pd-impregnated hematite by calcining hematite powder with surface adsorbed palladium ions. They investigated the effects of adding small amounts of palladium on the reaction of iron oxide with hydrogen and metallic iron with steam. The aim of the present work is to investigate the influence of Pd 2+ ions on the formation of iron oxide phases in a highly alkaline medium with focus on the microstructural properties of the prod- ucts and the mechanism of their formation. Acicular and monodis- perse a-FeOOH particles prepared in our previous work [17] were used as reference material. 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.10.008 * Corresponding author. Tel.: +385 1 4561 094; fax: +385 1 4680 098. E-mail address: krehul@irb.hr (S. Krehula). Journal of Molecular Structure 924–926 (2009) 201–207 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc