Tailoring the properties of immobilized titanium dioxide/carbon nanotube composites for photocatalytic water treatment Maria J. Sampaio a , Rita R.N. Marques a , Pedro B. Tavares b , Joaquim L. Faria a , Adria´n M.T. Silva a , Cla´ udia G. Silva a, * a LCM – Laborato´rio de Cata´lise e Materiais – Laborato´rio Associado LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b Universidade de Tra´s-os-Montes e Alto Douro, Centro de Quı´mica – Vila Real, Departamento de Quı´mica, 5001-911 Vila Real, Portugal Introduction One of the most active areas in heterogeneous photocatalysis has been the application to environmental cleanup [1–3]. In water and wastewater treatment, photocatalytic processes have been used for the removal of a great number of persistent organic compounds such as pharmaceuticals, dyes, pesticides and other problematic pollutants [4–6]. The major advantages of this technology include the possibility of using solar energy, ability of mineralize a great variety of hazardous compounds and the mild operational conditions. The possibility of using solar light as irradiation source in photocatalytic processes has received much attention in recent years [7,8]. Sun is an economical and ecological source of light, which will save the installation and energy consumption expenses of an artificial light source. Titanium dioxide, the benchmark material for photocatalytic reactions, shows poor efficiency under solar light irradiation mainly due to its large bandgap (3.2 eV). Nevertheless, the low cost, high robustness and photostability under most operation conditions makes it an excellent candidate to be used in photocatalytic degradation reactions [9,10]. Efforts have been made to extend the light absorption of TiO 2 into the visible region, including doping with metal and nonmetal ions, dye photosensitization, deposition of noble metals, coupling with other semiconductors, and addition of inert supports [11–16]. Recent works have emphasized the use of composite materials based on carbon nanotubes and TiO 2 in diverse fields such as dye sensitized solar cells [17], photocatalytic hydrogen generation [18] and photocatalytic oxidation of pollutants in aqueous and gaseous phase [19–24]. The conductive nature of the CNT is believed to favor the separation of the photo-generated electron–hole pairs by formation of heterojunctions at the TiO 2 /CNT interface [25]. Moreover CNT can provide spatial confinement of TiO 2 and large supporting surface areas, leading to faster observed rates of redox reactions [20]. However, there are also examples in the literature where CNT did not produce or even show a detrimental effect in the photoefficiency of TiO 2 /CNT composites. Several factors including the synthesis route, the presence of oxygenated groups Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx A R T I C L E I N F O Article history: Received 22 April 2013 Received in revised form 10 July 2013 Accepted 4 August 2013 Keywords: Photocatalysis Carbon nanotubes Titanium dioxide Films Methylene blue Phenols A B S T R A C T Sol–gel (SG) and hydration–dehydration (HD) methods were used to prepare titanium dioxide/multi- walled carbon nanotube (TiO 2 /CNT) composites with different carbon contents using functionalized (CNT f ) and pristine carbon nanotubes (CNT). Composite materials were mainly constituted by anatase crystallites of 8.0–9.0 nm. Surface area of the composite materials varied between 70 and 141 m 2 g 1 , with the higher values for the materials produced by SG using functionalized CNT. The composite materials were immobilized on glass slides and tested in the photodegradation of methylene blue (MB). Composite photocatalysts produced by SG method showed higher efficiency for MB degradation than those prepared by HD. The best photocatalytic efficiency was obtained for the material produced by SG method with the highest loading of functionalized CNT. A 62% MB removal was achieved for that catalyst contrasting with 46% obtained with bare TiO 2 . In addition, it was proved that the introduction of oxygen surface groups in CNT is crucial to prepare TiO 2 /CNT composites with high photoactivity, promoting the dispersion of TiO 2 particles and inducing the CNT action as a photosensitizer. The films with higher efficiency for MB abatement were tested in the photocatalytic degradation of four para-substituted phenols: 4-aminophenol (AP), 4-methoxyphenol (MP), 4-chlorophenol (CP) and 4-nitrophenol (NP). A relationship between the Hammett constant of each para-substituted phenol compound and its degradability by photocatalysis was obtained. A beneficial effect of the introduction of CNT in the matrix of TiO 2 was mainly observed for the degradation of AP and MP. ß 2013 Elsevier Ltd All rights reserved. * Corresponding author. Tel.: +351 225081998. E-mail addresses: cgsilva@fe.up.pt, cssilva@fe.up.pt (C.G. Silva). G Model JECE-128; No. of Pages 9 Please cite this article in press as: M.J. Sampaio, et al., Tailoring the properties of immobilized titanium dioxide/carbon nanotube composites for photocatalytic water treatment, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.014 Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering jou r n al h o mep ag e: w ww .elsevier .co m /loc ate/jec e 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.08.014