Materials Chemistry and Physics 124 (2010) 140–144 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Synthesis of cobalt nanoparticles in supercritical methanol Nae Chul Shin a , Young-Ho Lee a , Young Ho Shin a , Jaehoon Kim b , Youn-Woo Lee a,∗ a School of Chemical and Biological Engineering, Seoul National University, 599 Gwanangno, Gwanak-gu, Seoul 151-744, Republic of Korea b Energy & Environment Research Division, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea article info Article history: Received 12 June 2009 Received in revised form 8 October 2009 Accepted 5 June 2010 Keywords: Cobalt Nanoparticles Supercritical Methanol abstract Cobalt nanoparticles with a face-centered cubic (fcc) structure were synthesized by the reduction of Co(NO 3 ) 2 in supercritical methanol. The reduction of Co(NO 3 ) 2 to fcc Co was completed within 15 min at 400 ◦ C and 300 bar. The reduction mechanism was investigated by varying the reaction tempera- ture (200–400 ◦ C) and the reaction time (0.5–15 min). The results suggested that cobalt methoxynitrate formed at an initial stage was converted to CoO, and then reduced to Co. In addition, an introduction of oleic acid as a stabilizer achieved the formation of Co nanoparticles (∼10 nm). © 2010 Elsevier B.V. All rights reserved. 1. Introduction Nanoscale magnetic materials have received considerable atten- tion due to their extensive applications in data storage, spintronics, and biomedical fields [1–4]. Among the magnetic materials, Co nanoparticle has been investigated intensively because of their high saturation magnetization and high coercivity [5]. In addition, cobalt nanoparticles show its structure-dependent magnetic and electronic properties. There have been a number of approaches to synthesize Co nanocrystals with controlled structures such as face-centered cubic (fcc), hexagonal-close packed (hcp) and  phases. Co nanocrystals with an fcc and hcp structures have been synthesized the chemical reduction, polyol process, ther- mal decomposition of organometallic precursors, thermolysis and hydrothermal/solvothermal methods [6–11]. In general methods, the synthesized product is mainly hcp Co or a mixture of hcp Co and fcc Co, while the formation of the fcc Co at a low temperature is also possible [12]. -Co nanoparticles were synthesized by the decomposition of Co 2 (CO) 8 in the presence of surfactant mixture in o-dichlorobenzene or the solution phase reduction of CoCl 2 in dioctylether [13,14]. In recent years, the syntheses of inorganic nanoparticles in supercritical fluids have been extensively reported [15,16]. Super- critical fluids at high temperature and pressure have a low dielectric constant, which induces the fast nucleation, giving small parti- cles. Supercritical water and alcohol are promising reaction media for producing the inorganic nanoparticles with controlling size, ∗ Corresponding author. Tel.: +82 2 880 1883. E-mail address: ywlee@snu.ac.kr (Y.-W. Lee). morphology and crystalline structure of the particles. Moreover, it is possible to obtain inorganic materials having high crystallinity without a calcination, and thus the agglomeration of the nanopar- ticles can be avoided. It is well known that alcohol acts not only as the reac- tion medium but also as the reducing agent. In this study, Co nanoparticles were synthesized by the solvothermal method in supercritical methanol without any reducing agent. The formation of Co nanoparticles occurred under supercritical conditions, which induced fast decomposition of precursor and nucleation of metal particle within a reaction time of 15 min. In addition, the reduc- tion mechanism of Co(NO 3 ) 2 to Co 0 was investigated by varying the reaction temperature and reaction time. 2. Experimental 2.1. Procedure A certain amount of Co(NO3)2·6H2O methanolic solution (0.1 M) was introduced into a SUS 316 reactor with the inner volume of 23 ml. After being tightly sealed, the reactor was immersed into the molten salt bath (KNO3, NaNO3, and Ca(NO3)2 in the weight ratio of 46:24:30) which was heated to a desired temperature, and constantly shaken for a certain time. After the reaction, the reactor was rapidly cooled to room temperature by immersing in cold water. The resulting powders were isolated by a centrifugation, and washed with water and methanol several times to remove the residues. Then, the products were dried in a vacuum oven overnight at 40 ◦ C. The reaction temperature ranged from 200 to 400 ◦ C at 300 bar, and the reaction time was varied from 30 s to 15 min. The effect of the initial Co(NO3)2 concentration (0.02–0.1 M) on the morphology of the product was also studied. 2.2. Characterization The morphologies of the products were observed by field emission scan- ning electron microscopy (FE-SEM, JEOL model JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL model JEM-3010). The crystalline 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.06.005