1. Introduction Synthesis and characterization of nanostructures is of signifcant importance because of their fundamental role in basic research and technological applications [1,2]. Among the various nano materials, metal oxides nanoparticles have attracted increasing technological and industrial interest. This interest has mainly to do with their properties associated with general characteristics such as mechanical hardness, thermal stability or chemical passivity. Transition metal oxides can be prepared through various methods, such as chemical vapor deposition [3], laser vaporization [4], hydrothermal techniques [5], plane pyrolysis [6], sol-gel rout [7] and liquid-control precipitation [8]. Sun et al. [9] have recently reported the preparation of some crystalline metal oxides by ultrasonic spray pyrolysis technique. Among the metal oxides, nickel oxide attracts a great attention because of its wide applications. Ultra fne NiO particles with a uniform size and well dispersion are desirable for many applications in the manufacture of ceramic, magnetic, electrochromic, heterogeneous catalytic materials, preparation of alkaline batteries and p-type transparent conducting flms, etc [10,11]. Nickel oxide has been successfully prepared by chemical liquid precipitation, electrodeposition and sol-gel technique [12-14]. In all these methods, calcinations at the minimum temperature of 250°C are needed to get the crystalline NiO, but other properties of this oxide such as particle size are hard to control. The chemical preparation of nano-particles NiO is composed of two stages: the formation of metstable nickel precursor precipitate and the subsequent transformation to nano-NiO by thermal treatment [15,16]. In addition to nickel oxide, Ni(II) can also form nickel peroxide (NiO 2 ). This peroxide is easily made by oxidation of Ni(II) salt by hypochlorite solution [17,18]. Ji et al. [19] have reported the synthesis of nano-sized NiO 2 powder by a wet chemical method followed by calcinations. This is the frst and the only method, to our knowledge, reported for the fabrication of nano-sized NiO 2 . Although NiO 2 was synthesized without using NaClO, as oxidant, but the prepared method is a lengthy procedure and the yield of obtained NiO 2 is rather low. NiO 2 has been used as an effcient oxidant for the oxidation of wide range of organic compounds [17-20]. In this study, nano-sized nickel peroxide is easily made via oxidation of Ni(II) by KClO in the presence of Triton ® X-100 GR surfactant. The as-prepared nano NiO 2 was used to prepare nano-sized NiO in a very simple and rapid process. Central European Journal of Chemistry Synthesis and characterization of nanosized NiO 2 and NiO using Triton ® X-100 * E-mail: m_kooti@scu.ac.ir Received 25 December 2007; Accepted 12 September 2008 Abstract: Nanosized NiO 2 particles with an average diameter of 15 nm are prepared by treating of Ni(NO 3 ) 2 •6H 2 O with an aqueous solution of KClO in the presence of Triton ® X-100. This black fne powder of nickel peroxide was characterized by XRD diffraction, energy disper- sive spectroscopy (EDS) and scanning electron microscopy (SEM). The as-prepared NiO 2 can be easily transformed to nanosized NiO merely by washing it with acetone. The obtained NiO has an average diameter of 40 nm and was characterized by the same means used for NiO 2 . The nanoparticles of NiO 2 and NiO were obtained in high yields and purities. © Versita Warsaw and Springer-Verlag Berlin Heidelberg. Keywords: Nanosized • NiO 2 • NiO • Triton ® X-100 Department of Chemistry, College of Science, Shahid Chamran University, 65355-141 Ahvaz, Iran Mohammad Kooti*, Mehdi Jorf Research Article Cent. Eur. J. Chem. • 7(1) • 2009 • 155-158 DOI: 10.2478/s11532-008-0077-5 155 155