910 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 4 Sang Wook Kim et al. DOI 10.5012/bkcs.2010.31.04.910 γ-ray Radiation Induced Synthesis and Characterization of α-Cobalt Hydroxide Nanoparticles Sang Wook Kim, † Bob Jin Kwon, Jeong Hoon Park, † Min Goo Hur, † Seung Dae Yang, † and Hyun Jung * Advanced Functional Nanohybrid Material Laboratory, Department of Chemistry, Dongguk University-Seoul Campus, Seoul 100-715, Korea. * E-mail: chemphile@dongguk.edu † Radiation Research Division for Biotechnology, Korea Atomic Energy Research Institute (KAERI), Jeongeup 580-185, Korea Received December 17, 2009, Accepted February 8, 2010 A novel synthetic route has been developed to prepare α-cobalt hydroxide with intercalated nitrate anions. It was success- fully synthesized by γ-ray irradiation under simple conditions, i.e., air atmosphere, without base. Under γ-ray irradiation, it leads to the formation of layered cobalt hydroxynitrate compounds which have small crystalline size and have the role of a generator of hydroxyl anion. Structural and morphological characterizations were performed by using power X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high resolution transmission electron microscopy (HR-TEM). The component and thermal stability of the sample were respectively measured by Fourier trans- form infrared (FT-IR) spectroscopy, elemental analysis, and thermal analyses, including thermogravimetry (TG) and differential thermal analysis (DTA). Key Words: α-Cobalt hydroxide, Layered compound, Radiolysis Introduction Research on layered transition-metal hydroxide materials such as nickel hydroxides and cobalt hydroxides has received considerable attention in recent years due to these materials po- tential applications as catalysts, supports, anion adsorbents, mag- netic materials and ion exchangers as well as high-performance electrode materials of alkaline secondary batteries and superca- pacitors. 1-7 Layered divalent metal hydroxides crystallize in the structure of mineral brucite, Mg(OH) 2 . Brucite structure is com- posed of two hydroxide layers sandwiching a Mg 2+ layer. These two-dimensional sheets are stacked to form a three dimensional structure that is stabilized by hydrogen bonding between the layers. This results in the stacking of charge-neutral layers of composition [M(OH)2; M = Mg, Co, or Ni] with an interlayer distance of around 4.6 Å. When some of the hydroxyl groups are missing from their positions in the layer sheets, the structure incorporates anions in the interlayer region to restore charge neutrality yielding hydrotalcite-like structures with the general formula [M(OH)2-x]·(A n‒ )x/n (A n‒ = Cl ‒ , NO3 ‒ , or CO3 2‒ ). The in- terlayer spacing of these structures varies from 6.9 to 9.2 Å de- pending upon the size and orientation of anion. 8 Among a variety of layered transition-metal hydroxide ma- terials, cobalt hydroxides have attracted interest due to their lay- ered structure with large interlayer spacing, their high electro- chemical redox activity, and the possibility of enhanced per- formance depending upon preparation conditions. 9 Cobalt hy- droxides are well-known to crystallize in two polymorphic forms, α and β. The first one is an isostructural with hydrotalcite- like compounds while the second is a brucite-like structure with a stoichiometric phase of the composition Co(OH)2. 10 The α- phase cobalt hydroxide is theoretically expected to exhibit high- er electrochemical activity as compared to the β-one because of its turbostratically disordered structure. 11 There are several methods to prepare the α-cobalt hydroxides by electrochemical and chemical syntheses including urea and ammonia precipita- tion as well as a hydrolysis agent. 12-14 In those cases, stabilizer or basic condition are generally needed. Recently, radiation techni- ques have been utilized as a catalyst to solve those problems in material synthesis. 15 In the present study, we have developed a novel synthetic route to generate the α-cobalt hydroxide from cobalt nitrate hexahydrate isopropanol solution using a 60 Co gamma ray with a dose rate of 25 kGy/h without stabilizer or chemical agent. The physicochemical characterizations of obtained cobalt hy- droxide have been carried out, along with the comparison of physicochemical properties by conventional chemical method. Experimental Sample preparation. Cobalt(II) nitrate hexahydrate and 2-pro- panol were obtained from Aldrich in analytical grade and were used without further purification. Decarbonated water was pre- pared by boiling deionized water from a Milli-Q water purifica- tion system. 3 g (4.6 mmol) of cobalt(II) nitrate hexahydrate was dissolved in 30 mL of 2-propanol and then the sample solution was radiated with gamma rays using 60 Co source as a function of time (4, 6, 8 and 10 hours). Gamma ray radiation was carried out in a 60 Co Gamma cell irradiator with a dose rate of 25 kGy/h pro- vided by the Korea Atomic Energy Research Institute. After ra- diation, the obtained product was centrifuged, washed several times with decarbonated water and dried in a vacuum. For comparison of physicochemical properties with obtained cobalt hydroxide obtained by radiolysis, α-cobalt hydroxide was also prepared by urea hydrolysis method. 16 In typical synthesis, 18 g (27.6 mmol) of cobalt (II) nitrate hexahydrate was mixed with 2 g (33.3 mmol) of urea and 2 mL of water in a beaker and the mixture was placed inside a pre-heated oven at 140 o C for