Luminescence characteristics of cobalt doped TiO 2 nanoparticles Biswajit Choudhury, Amarjyoti Choudhury n Department of Physics, Tezpur University, Napaam 784028, Assam, India article info Article history: Received 5 April 2011 Received in revised form 7 August 2011 Accepted 12 August 2011 Available online 22 August 2011 Keywords: Doped nanoparticles Defects Luminescence Defect centers Crystal field Decay time abstract TiO 2 nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co 2 þ in TiO 2 lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co 2 þ in TiO 2 lattice. Compared to undoped TiO 2 nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co 2 þ . However, this ratio decreases profoundly at 0.04 Co 2 þ , due to concentra- tion quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co 2 þ . Time resolved photo- luminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites. & 2011 Elsevier B.V. All rights reserved. 1. Introduction TiO 2 exists in three phases, anatase, rutile and brookite, of which anatase and rutile are known to be potentially active materials for many applications; such as in photocatalysis, water splitting, solar cells, sensors, paints etc. [1–3]. But the main drawback of these systems, for many of these applications, is their absorption in the UV region, which corresponds to only 3–5% of solar radiation [4,5]. However, incorporation of a small concentration of impurity ions shifts its absorption onset to the visible region, making it an efficient candidate for photocatalysis applications [6–8]. Both anion and cation doping is performed in TiO 2 to tune the electronic, structural properties and likewise to enhance the photocatalytic activity of this material [9,10]. Out of the transition metal ions, Co 2 þ is an important dopant because of its optically active nature and also because of the involvement of this metal ion in imparting interesting magnetic properties to TiO 2 semiconductor nanostructures, useful for spintronic applica- tions [11,12,17]. Cobalt doped TiO 2 materials are also reported to show enhanced photocatalytic response [13,14]. Iwasaki et al. [14] reported that TiO 2 doped with 0.03% of Co (II) has high photocatalytic activity under UV–vis light. Bulk TiO 2 does not exhibit any photoluminescence at room temperature, but nanoparticles of TiO 2 show many luminescence peaks in visible region, which are associated with the presence of self trapped excitons, surface trap states, interstitial Ti 3 þ , oxygen defects etc. [9,15,21,22]. TiO 2 incorporated with cobalt are reported to show weak PL intensity than that of its undoped one, due to inhibition of electron–hole recombination after doping [3]. The dopants introduce new defect levels, which act as a charge carrier trapping site and suppresses the electron–hole recombination, thus increasing their average lifetimes [3,5]. Steady state and time resolved luminescence spectroscopy is a way to get collective information on defect related luminescence in doped TiO 2 and also to determine the lifetime of the photogenerated electrons and holes in presence of these defects [16]. The main objective of this article is to analyze the lumines- cence characteristic of TiO 2 nanoparticles both before and after cobalt incorporation. The attempt is to correlate the dopant concentration effect with the intensity of UV and visible emission peaks, and also with luminescence decay behavior related to different trap states. 2. Experimental details 2.1. Preparation method The preparation of cobalt doped TiO 2 nanoparticles were carried out with two different cobalt concentrations, 0.02 and Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.020 n Corresponding author. Tel.: þ913712267120; fax: þ913712267005. E-mail address: ajc@tezu.ernet.in (A. Choudhury). Journal of Luminescence 132 (2012) 178–184