Capping Groups Induced Size and Shape Evolution of Magnetite Particles Under Hydrothermal Condition and their Magnetic Properties Song Li, z,y Gaowu W. Qin, T. w,z Wenli Pei, z Yuping Ren, z Yudong Zhang, y Claude Esling, y and Liang Zuo z z Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China y LETAM, CNRS-UMR 7078, Universite´ de Metz, Metz 57045, France Three simple capping molecules (urea, 3-aminopropanol, and polyethylene glycol) with different functional groups (ÀNH 2 for urea, ÀNH 2 and ÀOH for 3-aminopropanol, and ÀOÀ for polyethylene glycol) have been designed to prepare magnetite (Fe 3 O 4 ) particles with various shapes. The crystal structure, morphologies, and magnetic properties of the products have been characterized by X-ray diffraction, electron microscopy, and vibrating sample magnetometer. The results indicate that the capping functional groups play a major role in determining the size, shape, and thus magnetic properties of the magnetite nanocrystals. The morphology evolution of the magnetite under the hydrothermal condition is discussed in detail, particularly for the interaction between the various capping groups and surface structure and chemistry of the nanocrystals. I. Introduction I T is generally accepted that physical and chemical properties of nanoparticles are sensitive to their particle morphology (size and shape). 1,2 Various chemical synthesis methods and posttreatment approaches, such as polyol method, 3,4 reverse mi- celle technique, 5,6 sol–gel process, 7,8 seed-mediated process, 9,10 and hydrothermal treatment, 11,12 have been applied for overall control of size and shape of nanocrystals in order to manipulate their properties, especially for noble metals or chalcogenide semiconductors. In most cases, the shape tailoring has been achieved by the inhibited anisotropic growth of nanocrystals, which can be controlled through the selective adsorption of surfactants or ligands on specific crystallographic facets. For example, the selective binding of thiol molecules onto the {111} facets of tetradecahedron PbS seeds leads to higher growth rate in {100} facets, producing cross- and star-shaped nanoparti- cles. 13 Considering that the binding of capping molecules on the surface of colloid particle could be attributed to the electronic structure of end functional group of the molecule, to investigate the effects of various end functional groups of stabilizer on nanocrystals shape evolution will enable us to better understand the dynamic crystal growth process on the organic–nanocrystal interface. However, although many reports have appeared on surfactant-assisted shape control of nanocrystals, little work has been performed on the detailed effects of different capping groups on the size and shape evolution of nanocrystals during the wet chemical process. Magnetite nanoparticles exhibit many interesting magnetic, electronic, and catalytic properties depending on their size, shape, and crystalline structure. Based on these unique proper- ties, magnetite nanoparticles offer a high potential for applica- tions such as drug delivery carrier, 14 magnetic resonance imaging contrast agent, 15 magnetoresistive devices, 16 solenoid RF inductor cores, 17 and noise suppressor. 18 Magnetite nano- structures with various morphologies have been prepared by using different techniques. 11,12,14,15 In this work, magnetite, as a model nanomaterial, was synthesized by hydrothermal method using three simple molecules, urea, 3-aminopropanol, and polyethylene glycol (PEG) as capping molecules to clarify how different capping groups affect final shape of nanoparticles. The influence of stabilizers with different functional groups on size, shape, and magnetic properties of final products were examined and a possible shape evolution mechanism was proposed. II. Experimental Procedure (1) Preparation of Fe 3 O 4 Nanoparticles All chemical reagents were purchased from Shanghai Chemical Co. Ltd. (Shanghai, China) with analytical quality and were used without any further purification. In a typical synthesis ex- periment, solutions of ferric chloride and ferrous chloride in 100 mL deionized water with Fe 31 /Fe 21 ratio of 1.8:1 were prepared as precursor solutions. The iron cation concentration was 0.1 mol/L. A fixed amount of stabilizer was added into the solution. Urea, 3-aminopropanol, and PEG (molecule weight 5 20 000) were used as stabilizers. Subsequently, 4.0 g NaOH was added to the above solution with stirring under N 2 flow. Then the re- actant mixture was placed in a 120 mL Teflon-sealed autoclave and heated up to 2001C for 10 h. The particles synthesized were washed with ethanol and deionized water several times and then dried at 501C. (2) Characterization of Fe 3 O 4 Nanoparticles The structure of synthesized magnetite was characterized on a Philips X’Pert Pro X-ray powder diffractometer (XRD, the Netherlands), using CoKa radiation (l 5 0.17903 nm). The mor- phology of products was observed on a JEOL 6500F scanning electron microscope (SEM, Tokyo, Japan) and a Philips CM200 transmission electron microscope (TEM). A small amount of nanoparticles were milled with KBr and then pressed into a disc for Fourier transform infrared (FTIR) analysis (Spectrum 100, Perkin Elmer, MA). The magnetic properties of the products were measured on a Lake Shore 7407 vibrating sample magne- tometer (VSM) at room temperature. III. Results and Discussion In order to investigate the effect of end group of capping mol- ecules on the shape evolution process of Fe 3 O 4 , three simple molecules with different functional groups, including 3-amino- propanol, urea, and PEG, have been used as stabilizer, respec- V. Hackley—contributing editor This work was supported by the Ministry of Education of China, under Projects No. 108039, No. B07015, and No. IRT0713. w Author to whom correspondence should be addressed. e-mail: qingw@smm.neu.edu.cn Manuscript No. 25158. Received August 26, 2008; approved December 5, 2008. J ournal J. Am. Ceram. Soc., 92 [3] 631–635 (2009) DOI: 10.1111/j.1551-2916.2009.02928.x r 2009 The American Ceramic Society 631