Presence of Room Temperature Ferromagnetism in Co 2+ Doped TiO 2 Nanoparticles Synthesized through Shape Transformation J. Kuljanin-Jakovljevic ´, † M. Radoic ˇic ´, † T. Radetic ´, ‡ Z. Konstantinovic ´, § Z. V. S ˇ aponjic ´,* ,† and J. Nedeljkovic ´ † Vinc ˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Institut de Cie `ncia de Materials de Barcelona, CSIC, Campus UAB, 08193 Bellaterra, Spain ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 24, 2009 New approach for synthesis of Co 2+ doped TiO 2 nanoparticles showing room temperature ferromagnetic behavior, through shape transformation of hydrothermally treated scrolled titania nanotubes in the presence of Co 2+ ions is described. The XRD and ICP measurements demonstrated successful incorporation of 0.46 at. % Co 2+ ions in preserved anatase crystal structure of TiO 2 nanoparticles, without presence of Co-oxide clustering, metallic Co, or various Co-Ti oxide species. HRTEM measurements revealed that majority of the nanoparticles have polygonal shapes with average dimension of ∼ 6-10 nm. Obtained value of 3.06 eV for band gap energy of Co 2+ doped TiO 2 nanoparticles explained altered optical properties of TiO 2 matrix and indicates narrowing of the electronic properties in respect to the undoped anatase TiO 2 nanomaterials. The Co 2+ doped TiO 2 nanocrystals enabled synthesis of optically transparent film that shows room temperature ferromagnetic ordering with a saturation magnetic moment of 0.25 μB per Co atom. The proposed explanation for room temperature ferromagnetic behavior is based on the presence of critical amount of oxygen vacancies that mediate interaction between Co 2+ spins trapped in the lattice structure of titania nanoparticles with undercoordinated surface defect sites. 1. Introduction The development of spin-based electronic (spintronics) devices has recently attracted a great deal of attention. The possibility of simultaneous control of charge currents and spin polarized currents in those systems might provide new qualities to information processing technologies. Nanoscale diluted magnetic semiconductors (DMS) are the main components of proposed spintronic devices. DMS are the mostly II-VI or III-V compounds, in which the host cations are replaced with magnetic impurities up to a few atomic percent. Some of those materials exhibit interesting magnetic and magnetooptical behavior characterized by a high-Curie-temperature ferromagnetism. Theoretical predictions have identified wide band metal oxide semiconductors (ZnO and TiO 2 ) as good candidates for the host materials, which, after doping with transition metal ions, could support room temperature ferromagnetism. Consequently, those doped semiconductors can be used for the development of the spin-based electronics devices. 1 Recently, DMSs such as Co doped TiO 2 , Fe doped TiO 2 , and Mn and Co doped ZnO have been reported to show the room temperature ferromagnetism. 2 TiO 2 is a well-known wide band gap (3.2 eV) semiconductor and as such has the potential for a wide range of applications. As inexpensive, nontoxic and biocompatible material, TiO 2 is one of the most important photocatalysts. 3 In addition, Co doped TiO 2 is promising for the spintronics applications due to the excellent optical transmission in the visible and near-infrared regions and high n-type carrier mobility. 4 Since Matsumoto et al.’s 5 discovery of high T C ferromag- netism in Co 2+ doped TiO 2 anatase films, there was an expansion of experimental and theoretical work focused on transition metal doped TiO 2 as a ferromagnetic material. However, most of the research was confined on the thin films, 5-10 whereas there was significantlylessfocusonthetransitionmetaldopednanoparticles. 11-13 Paucity of the published research on a synthesis of the free- standing high-quality nanoparticles doped with the transition metal ions is related to a number of challenges such as high surface to volume ratio, requests for uniform size distribution of the nanocrystals, as well as successful doping, i.e., control of exact position of the dopant ions within a crystal. The last condition is the most important one and hardest to satisfy. The main obstacle to successful doping is, according to the classical nucleation model, due to the kinetic noncompetitiveness between the nucleation process of doped crystals and the nucleation process of pure crystals. 14,15 According to this model, an increase of the ratio between the dopant and host radius and charge incompatibility reduces the probability for the critical nucleus formation containing impurity ions. The nucleation process in a mixed solution of precursor ions is ruled most frequently by a favorable reaction pathway that leads toward formation of the critical nucleus made up of pure host material. For example, during the synthesis of Co 2+ doped ZnO as reported by Gamelin et al., 16 the addition of 2% Co 2+ ions to the starting solution eliminates 35% of the nucleation events in comparison to pure ZnO; the result was explained by an increase in the activation energy and critical radius of the nuclei upon introduction of the impurity ions. 16 In this paper we report on the preparation procedure of Co 2+ doped TiO 2 nanoparticles through shape transformation by solution chemical route. Using titania nanotubes in the presence of Co 2+ ions as precursors for the synthesis of Co 2+ doped TiO 2 * To whom correspondence should be addressed. Tel: +381-11-8066428. Fax: +381-11-2453986. E-mail: saponjic@vinca.rs. † Vinc ˇa Institute of Nuclear Sciences. ‡ Lawrence Berkeley National Laboratory. § Institut de Cie `ncia de Materials de Barcelona. J. Phys. Chem. C 2009, 113, 21029–21033 21029 10.1021/jp905042k  2009 American Chemical Society Published on Web 11/18/2009