Influence of pH on the formulation of TiO 2 nano-crystalline powders with high photocatalytic activity Andreia Molea a , Violeta Popescu a,b, ⁎, Neil A. Rowson c , Adrian M. Dinescu b a Technical University of Cluj-Napoca, Faculty of Material and Environmental Engineering, Physics and Chemistry Department, No.103-105 Muncii avenue, 400641 Cluj-Napoca, Romania b National Institute for Research and Development in Microtechnologies, IMT, 126A Erou Iancu Nicolae Street, 077190, Bucharest, Romania c University of Birmingham, School of Chemical Engineering, Edgbaston, Birmingham B15 2TT, United Kingdom abstract article info Article history: Received 15 May 2013 Received in revised form 16 September 2013 Accepted 21 October 2013 Available online 29 October 2013 Keywords: Titanium dioxide Hydrolysis pH effect Catalysis Photodegradation process Methylene Blue dye This paper describes the effect of synthesis conditions on the formation of anatase and rutile crystalline phases and photocatalytic activity of synthesised TiO 2 phase. The synthesised powders were characterised by X-ray diffraction, Raman microscopy, Scanning Electron Microscopy and UV–Vis spectroscopy. Using these character- isation techniques, the structural, morphological and optical properties as a function of formulation pH were determined. Since photocatalysis is a surface process, the mass surface charge of the powders was also measured using a Faraday Cage connected to an electrometer. The structural, morphological, optical and surface properties were correlated with the photocatalytic activity of the formulated TiO 2 powders. The influence of synthesis condition on the photocatalytic activity of TiO 2 powders was determined by the degradation of Methylene Blue dye under both UV-A and visible light. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO 2 ) has potential applications in environmental fields such as wastewater treatment and more specifically in cases where the water has been contaminated with azo dyes from textile and oil spillages [1–6]. Titanium dioxide occurs in nature as anatase, ru- tile and brookite mineral phases. Anatase is stabilised by heat treatment at 400–600 °C, whilst, rutile, the most thermodynamically stable crys- talline phase of titanium dioxide, is stabilised at 900 °C. Between 600 and 900 °C, both anatase and rutile phases co-exist [7,8]. Photocatalysis is a surface process; therefore, structural, morphologi- cal and optical properties are critical parameters for controlling the pho- tocatalytic activity of the synthesised materials [9–12]. According to Cassaignon et al. [13], using titanium tri-chloride as a precursor, in an acid medium, at pH between 3 and 4, a rutile phase is stabilised after 24 h at 60 °C. In an alkaline medium, at pH N 6.5 and the same synthesis conditions, anatase is the main phase (65%) accom- panied by brookite. Anatase and brookite are transformed by heat treatment into rutile, but the synthesis conditions influence this transition temperature. As a result, if the material is prepared under acidic conditions and after heat treatment at moderate temperature, i.e. at 400 °C, a rutile phase is stabilised [14]. However, in alkaline environments, (pH N 7) an anatase phase is stabilised, even at 800 °C [15]., However, only a few studies have characterised the formulation of TiO 2 powders at different pHs with respect to photocatalytic activity [16]. According to some scientific literature, an anatase phase has the highest photocatalytic activity [5,15], whereas other researches have in- dicated that a mixture of crystalline phases such as anatase and rutile [1,17,18] or anatase, brookite and rutile [19] exhibits a higher photocat- alytic activity than pure anatase. Xie et al. [18] studied the photocatalyt- ic activity of TiO 2 catalysts containing various rutile mass fractions using the degradation of benzene under 24 mW/cm 2 UV radiation. The sam- ple with a higher concentration of rutile exhibits the highest photocat- alytic activity, due to the lower potential of the conduction band of rutile compared to anatase, giving more holes on the anatase surface for oxidation reactions. Lopez et al. [19] studied the photocatalytic activ- ity of TiO 2 synthesised at various pHs. Also the authors [19] demonstrat- ed that the photodegradation process depends on the energy band gap (E g ). Bulk anatase has E g = 3.2 eV, while rutile has E g = 3 eV. The highest activity corresponds to catalysts that contained anatase–brook- ite–rutile and anatase–rutile, respectively. This correlates to the lowest energy band gap of the rutile crystalline phase compared with the ener- gy band gap of pure anatase phase. As such, the material could absorb a Powder Technology 253 (2014) 22–28 ⁎ Corresponding author at: Technical University of Cluj-Napoca, Faculty of Material and Environmental Engineering, Physics and Chemistry Department, No.103-105 Muncii ave- nue, 400641 Cluj-Napoca, Romania. E-mail addresses: andreia.molea@chem.utcluj.ro (A. Molea), violeta.popescu@chem.utcluj.ro (V. Popescu), n.a.rowson@bham.ac.uk (N.A. Rowson), andrian.dinescu@imt.ro (A.M. Dinescu). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.10.040 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec