Carbon nanotubes/titanium dioxide (CNTs/TiO 2 ) nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting enhanced photocatalytic activity Bin Gao a , George Z. Chen b , Gianluca Li Puma a, * a Photocatalysis & Photoreaction Engineering, Department of Chemical and Environmental Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom b Electrochemical Technologies, Department of Chemical and Environmental Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom 1. Introduction Heterogeneous photocatalysis is a process in which the surface of an oxide semiconductor promotes reactions upon irradiation with photons with band-gap energy or greater. Under these conditions, valence band electrons are excited and move to the conduction band leaving behind holes. The electron and holes must then migrate to the surface of the semiconductor to promote reduction and oxidation reactions of adsorbed species or of species that are in close proximity to the surface of the semiconductor. However, electron–hole recombination, at the surface or in the bulk, competes with the above process and limits the quantum yield of photocatalytic reactions [1]. Titanium dioxide (TiO 2 )(E bg = 3.2 eV for anatase) has been one of the most common semiconductors studied for the oxidation or reduction of inorganic and organic species. Reactive radicals (primarily OH  and O 2  ) or direct hole oxidation are believed to initiate the oxidation of most organic species in water and air. For an organic molecule, the process may lead to partial oxidation or can proceed toward total mineralization and formation of CO 2 and mineral acids. The overall efficiency of photon utilisation by TiO 2 is however limited by electron–hole recombination, photon scatter- ing and the intrinsic physical properties of TiO 2 that limit the absorption of photons to those with UV-A or greater energy [2]. The coupling of TiO 2 with carbon nanotubes (CNTs) has been shown to provide a synergistic effect which can enhance the overall efficiency of a photocatalytic process. CNTs/TiO 2 nano- composites have attracted attention in the literature in relation to the treatment of contaminated water and air by heterogeneous photocatalysis [3,4], hydrogen evolution [5], CO 2 photo-reduction [6], dye sensitised solar cells [7] and sensors devices [8]. The conductive structure of the CNTs scaffolds is believed to favour the separation of the photo-generated electron–hole pairs by formation of heterojunctions at the CNTs/TiO 2 interface [9]. TiO 2 is Applied Catalysis B: Environmental 89 (2009) 503–509 ARTICLE INFO Article history: Received 18 November 2008 Received in revised form 15 January 2009 Accepted 18 January 2009 Available online 24 January 2009 Keywords: Carbon nanotubes Titanium dioxide Photocatalysis Nanocomposite Surfactant Wrapping Sol–gel ABSTRACT In this work, a conventional sol–gel method was used to prepare CNTs/TiO 2 nanocomposites with different carbon loading in the range up to 20% CNTs/TiO 2 by weight. The bare CNTs (multi-walled), and the composites were characterized by a range of analytical techniques including TEM, XRD, BET and TGA–DSC. The results show the successful covering of the CNTs with discrete clusters of TiO 2 and bare CNTs surfaces which after annealing at 500 8C led to mesoporous crystalline TiO 2 (anatase) clusters. The photocatalytic activities of the nanocomposites were monitored from the results of the photodegrada- tion of methylene blue (MB). The optimum CNTs/TiO 2 ratio in the composites prepared by conventional sol–gel method was found to be in the range from 1.5% to 5% by weight under the experimental conditions investigated. The maximum increase in activity was found to be 12.8% compared to the pure TiO 2 sample. In contrast, the synthesis of CNTs/TiO 2 nanocomposites by a novel surfactant wrapping sol–gel method [B. Gao, C. Peng, G.Z. Chen, G. Li Puma, Appl. Catal. B: Environ. 85 (2008) 17.] led to a uniform and well-defined nanometer-scale titania layer on individual CNTs. The nanocomposites were found to enhance the initial oxidation rate of methylene blue by onefold compared to the pure TiO 2 sample. This larger degree of rate enhancement is attributed to the supporting role of the CNTs and surface properties prepared by this novel modified sol–gel method. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +44 115 951 4170; fax: +44 115 951 4115. E-mail address: gianluca.li.puma@nottingham.ac.uk (G. Li Puma). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.01.009