Journal of Hazardous Materials 163 (2009) 1179–1184 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Preparation and application of visible-light-responsive Ni-doped and SnO 2 -coupled TiO 2 nanocomposite photocatalysts Romana Khan, Tae-Jeong Kim ∗ Department of Applied Chemistry, Kyungpook National University, Taegu 702-701, Republic of Korea article info Article history: Received 28 January 2008 Received in revised form 13 June 2008 Accepted 18 July 2008 Available online 25 July 2008 Keywords: Ni–TiO2–SnO2 Nanocomposites Photocatalytic activity Visible light abstract A new series of visible-light-driven TiO 2 photocatalysts constituting lattice-doped Ni and surface-coupled SnO 2 nanocomposites, xNi–TiO 2 –SnO 2 (x =0.1, 0.3, 0.5), were synthesized. TiO 2 and xNi–TiO 2 were pre- pared by a sol–gel method while the SnO 2 was coupled to these via a ligand exchange reaction and finally the catalysts were thermally treated. The presence of Ni ions in the lattice of the photocatalysts was indirectly confirmed by a red shift in DRS spectra. XRD showed crystalline peaks only assignable to TiO 2 anatase phase. XPS analysis confirmed that Sn is present on the surface of the catalysts as SnO 2 while Ni 2 O 3 is absent. The xNi–TiO 2 –SnO 2 nanocomposites showed a promising visible-light-responsive photo- catalytic activity and were found superior to TiO 2 , xNi–TiO 2 and TiO 2 –SnO 2 for the degradation of toluene in air. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Titanium oxide (TiO 2 ) is one of the most efficient semiconductor photocatalysts for extensive environmental applications because of its strong oxidizing power, non-toxicity, high photochemical corrosive resistance and cost effectiveness. Due to these inher- ent properties, TiO 2 is the most suitable candidate for degradation and complete mineralization of toxic organic pollutants in water, soil and air [1–4]. Yet, the widespread technological use of TiO 2 is impaired by its wide band gap (3.2 eV for crystalline anatase phase) which requires the use of UV light. Thus, the possibility of employ- ing solar light in TiO 2 photocatalysis is limited. It is noteworthy that the UV-radiation fraction of the global solar radiation is only 4–6% while its visible light component is 45% [5]. Therefore, development of an efficient process that can shift the optical response of TiO 2 from the UV to visible spectral range seems to be the most appropri- ate strategy to improve the photocatalytic efficiency of TiO 2 under solar light irradiation [6]. Among others, the substitution of Ti by transition metals through doping is the most widely studied approach for the syn- thesis of visible-light-active photocatalysts [7–10]. However, unlike other metals, there have been relatively few studies on Ni doping into TiO 2 lattice for photocatalytic degradation of organic pollu- tants [8]. Nevertheless, Ni 2+ has been found to be an efficient dopant for improving the photocatalytic activity of certain semi- ∗ Corresponding author. Tel.: +82 539505587; fax: +82 539506594. E-mail address: tjkim@knu.ac.kr (T.-J. Kim). conductor photocatalyst for hydrogen evolution from water [11]. In principle, transition metal with proper oxidation state replace some of the Ti(IV) from lattice producing a impurity state that reduces the band gap of TiO 2 [8,12]. However, metal doping has some drawbacks, the most crucial of which is that the doped metal centers act as electron traps, which ultimately results in higher rate of electron and hole pairs (e - /h + ) recombination [13]. There- fore, further studies are required in order to improve the activity of these transition-metal-doped TiO 2 by holding back the possi- ble accumulation of electrons on TiO 2 . This will in turn suppress the recombination of photogenerated e - /h + and thus will improve the efficiency of the net charge transfer in photocatalysis. Cou- pling of two semiconductor particles with different Fermi levels provides an interesting approach in this regard. The TiO 2 –SnO 2 system seems to be a pair of choice because of the structural anal- ogy between both oxides. The band gaps of SnO 2 and TiO 2 are 3.8 and 3.2eV, respectively. When these two semiconductor par- ticles are coupled, the conduction band of SnO 2 acts as a sink for photogenerated electrons. Since the photogenerated holes move in the opposite direction, they accumulate in the valence band of the TiO 2 particle, therefore, leading to efficient spatial separation of photogenerated charges and thus suppressing recombination [14,15]. Several methods ranging from thin films and particles to nanofibers have been developed to produce TiO 2 –SnO 2 materials [16–18]. Such a combination is certainly advantageous but can be active only under UV light. Substitution of Sn for Ti in a TiO 2 lat- tice matrix can only bring a blue shift in the absorption spectra of TiO 2 because SnO 2 have a large band gap than TiO 2 . Therefore, it 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.07.078