Applied Catalysis B: Environmental 176 (2015) 288–297 Contents lists available at ScienceDirect Applied Catalysis B: Environmental j ourna l h omepa ge: www.elsevier.com/locate/apcatb Effect of surfactant concentration on active species generation and photocatalytic properties of TiO 2 Ekemena Oghenovoh Oseghe, Suresh Maddila, Patrick Gathura Ndungu, Sreekanth Babu Jonnalagadda ∗ School of Chemistry and Physics, University of Kwa-Zulu Natal, Westville Campus, Private Bag X 54001, Durban 4000, South Africa a r t i c l e i n f o Article history: Received 8 December 2014 Received in revised form 30 March 2015 Accepted 6 April 2015 Available online 8 April 2015 Keywords: Surfactant concentration TiO2 nanoparticle Photocatalysis Hydroxyl radical Oxygen vacancy a b s t r a c t The importance of varying surfactant concentration in TiO 2 synthesis and its effect on the formation of OH radical, oxygen vacancy, and photocatalytic properties is the focus of this work. The sol–gel method was adopted in the synthesis of TiO 2 nanoparticles and calcined in air/argon at 400 ◦ C. In a typical synthesis, the concentration of non-ionic surfactant, pluronic F127, was varied by mass relative to the concentration of TiO 2 precursor. The as-prepared materials were characterized by X-ray powder diffraction (XRD), scan- ning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), N 2 sorption, Raman spectroscopy, fluorescence spectrometry, and electron spin resonance spectroscopy (ESR). Pho- tocatalytic properties of the materials were evaluated by monitoring the decoloration of 10 mg/L model dye, methylene blue at pH 3, 7, and 11 in the presence of different light sources. Pore size distribution calculated from the Barrett–Joyner–Halenda (BJH) shows that the materials are both meso- and macro- porous with surface area of 48–110 cm 2 /g. The as-prepared TiO 2 nanoparticles existed in the anatase phase with a decrease in crystal size as the concentration of the surfactant used in synthesis increased. An increase in OH radical and surface oxygen vacancy formation was also observed as the concentra- tion of surfactant used in synthesis increased. The as-prepared TiO 2 , calcined in air and argon, T 1 :S 3 AA, showed the best photocatalytic activity resulting from its relatively higher hydroxyl radical generated and surface oxygen vacancy. © 2015 Elsevier B.V. All rights reserved. 1. Introduction A number of semi-conductors of high quality have been exten- sively studied in order to develop a more efficient photocatalytic system. TiO 2 has proven to be one of the best semi-conductors for this process. This is because it is stable; most efficient; cheap and environmentally benign. The photocatalytic mechanism for the decoloration of model dyes by TiO 2 has been classified into direct and indirect pathway [1]. OH radical and photogenerated holes are key species needed by the two mechanisms for photodecoloration of dyes. The direct mechanism is based on the photo-induced gen- eration of electrons and holes by the catalyst. The photo-generated hole is then trapped by either the adsorbed dye on the surface of the catalyst or surface defects to form reactive radicals or sur- face active centers, respectively. The adduct species formed when dyes reacts with either the holes or surface active centers, would ∗ Corresponding author. Tel.: +27 31 2607325; fax: +27 31 2603091. E-mail address: jonnalagaddas@ukzn.ac.za (S.B. Jonnalagadda). either decompose or recombine with electrons to form intermedi- ates and products [2,3]. In the indirect mechanism, OH radical is formed when the photogenerated electron and holes reacts with adsorbed water and O 2 . The generated OH radical is responsible for the decoloration of the dye forming intermediates and products [4]. Some other factors considered influential in improving the photo- catalytic properties of TiO 2 are particle size, specific surface area, crystal structure and size [5,6]. Several efforts have been made to synthesize TiO 2 with good photocatalytic properties by changing variables during prepara- tions. Some of these variables include solvent variation [5], pH of the reaction media [7], calcination temperatures [8,9], dopant con- centration [10–12], surfactant [13] and surfactant concentration [14]. Xiao et al. [15] observed an enhanced photocatalytic degra- dation of methylene blue under UV irradiation resulting from high specific surface area and increased formation rate of hydroxyl rad- ical by Sm 3+ doped TiO 2 . Tryba et al. [8] also showed a relationship between crystallinity stemming from heat treatment temperature and the formation rate of OH radical. http://dx.doi.org/10.1016/j.apcatb.2015.04.010 0926-3373/© 2015 Elsevier B.V. All rights reserved.