ARTICLE DOI: 10.1002/zaac.201300649 Synthesis and Properties of Cobalt Sulfide Phases: CoS 2 and Co 9 S 8 Nitesh Kumar, [a] Natarajan Raman, [b] and Athinarayanan Sundaresan* [a] Dedicated to Professor C. N. R. Rao on the Occasion of His 80th Birthday Keywords: Cobalt sulfide; Ferromagnetism; Resistivity; Nanoparticles; Thermal decomposition Abstract. Single phase cobalt disulfide (CoS 2 ) nanoparticles were pre- pared by thermal decomposition of cobalt-thiourea complex at a low temperature (400 °C). CoS 2 nanoparticles exhibit ferromagnetic order- ing at 122 K below which the temperature dependent resistivity of cold pressed nanoparticles deviates from metallic behavior and shows a broad maximum. Just below T C , it also exhibits a large magnetoresis- Introduction Transition metal sulfides are very important class of materi- als known for their rich structural diversities along with inter- esting and technologically relevant electronic and magnetic properties. [1] One additional advantage is that they are cheap and abundant, many of which are found in nature in the form of minerals such as heazlewoodite (Ni 3 S 2 ), chalcocite (Cu 2 S), pyrite (FeS 2 ) etc. Among all, sulfides which crystallize in py- rite structure (FeS 2 , CoS 2 , NiS 2 , CuS 2 , and ZnS 2 ) show great variation in electronic properties by changing transition metal ion and also by substituting Se or Te in the anion site. [2-4] According to the band theory FeS 2 and ZnS 2 are semiconduct- ing while CoS 2 and CuS 2 are metallic. [2] NiS 2 is a Mott insu- lator. [5] In CoS 2 , Co 2+ remains in the low spin state with one electron in the e g sub-band. Because of the intermediate strength of the e g electron correlation, ferromagnetic transition temperature (Curie temperature, T C ) is found to be less (120 K) with fractional magnetic moment (0.85 B.M.). [6–8] The elec- trons, which are responsible for ferromagnetic interaction, also give rise to electronic conduction in CoS 2 . The phase diagram of cobalt sulfide is fairly complex con- taining Co 4 S 3 , Co 9 S 8 , CoS, Co 3 S 4 , Co 2 S 3 , and CoS 2 phases. [9] Bulk polycrystalline materials of these phases are synthesized by heating elemental sulfur and cobalt together in silica am- poules. Corresponding single crystals are prepared by adding a flux such as CoBr 2 to the elemental reactants. Recently, chemical synthesis of nanoparticles and submicron sized par- * Dr. A. Sundaresan Fax: +91-80-2208 2766 E-Mail: sundaresan@jncasr.ac.in [a] Chemistry and Physics of Materials Unit Jawaharlal Nehru Centre for Advanced Scientific Research Bangalore-560064, India [b] Department of Chemistry VHNSN College Virudhunagar-626 001, India Z. Anorg. Allg. Chem. 2014, 640, (6), 1069–1074 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1069 tance effect (6.5 %). A mixture of CoS and Co 9 S 8 phases were obtained between the temperature interval 400 °C  T  1000 °C. At 1000 °C, a pure bulk Co 9 S 8 phase was obtained. It exhibits magnetic hysteresis typical of a ferromagnet at room temperature and a peak in magnetiza- tion at low temperature with a strong relaxation indicating possible spin glass state. ticles of phases like CoS 2 , CoS, and Co 9 S 8 are carried out hydrothermally at relatively lower temperatures (120–230 °C). Bao et al. demonstrated bio-assisted CoS nanowires synthesis by hydrothermal method using cysteine as the sulfur source. [10] The same sulfur source is used to synthesize CoS 2 nanopar- ticles on graphene surface. [11] CoS nanocrystals are grown on graphene hydrothermally starting with thioacetamide as the sulfur source for supercapacitor applications. [12] Other sulfur sources used for the synthesis of cobalt sulfide nanostructures are thiourea, [13] H 2 S, [14] Na 2 S, [15,16] Na 2 S 2 O 3 , [17] etc. High electrial conductivity in many cobalt sulfide phases has made these materials ideal for many energy devices like dye sensi- tized solar cells (DSC), Li-ion batteries, and supercapacitors. Phases like CoS 2 , CoS, and Co 9 S 8 have been used as counter electrode in DSC, where it collects electrons from the external circuit and speeds up the reduction of I 3– for dye degrada- tion. [18–22] CoS 2 and Co 9 S 8 are also used as anode material in Li-ion battery. [23,24] Wang et al. have shown discharge capacity as large as 1210 mAhg –1 and good cyclability for CoS 2 hollow spheres. [23] Cobalt sulfides can also store charge as pseudocapacitors in redox processes during charging and dis- charging. Different morphologies and composite of cobalt sulf- ide with graphene have been tried for supercapacitor applica- tions. [12,25–29] Nanocrystalline CoS 2 phase has also been tried to be used as a material for electrocatalysis of oxygen re- duction reaction in fuel cell. [30] Pyrite type CoS 2 is particularly interesting in the cobalt- sulfur phase diagram for its interesting magnetic and electronic properties. Properties of bulk polycrystals and single crystals have been extensively studied but studies of corresponding properties of nanocrystalline CoS 2 are scarce in the literature. Herein, we have demonstrated the synthesis of CoS 2 nanopar- ticles from a single source precursor, namely, cobalt-thiourea complex, by decomposing it at 400 °C. The electrical and mag-