Multivariate optimisation of TiO 2 /carbon nanocomposites for photocatalytic degradation of a reactive textile dye Elias da Costa, Aldo J.G. Zarbin, Patricio Peralta-Zamora * Departamento de Quı´mica, Universidade Federal do Parana ´. CP 19081, 81531-990, Curitiba-PR, Brazil 1. Introduction Because of its high degradation efficiency against many substrates of environmental relevance, heterogeneous photocata- lysis occupies a prominent place within the group of advanced technologies for the treatment of liquid wastes. In general, advanced oxidation processes based on the use of semiconductors are efficient in the degradation of resistant pollutants and often result in their complete mineralisation. Within this context, the use of titanium dioxide (TiO 2 ) is particularly relevant, especially in its anatase crystalline form, which has allowed for efficient photocatalyst degradation of pollutants using either artificial or solar radiation [1–3]. The principles of photocatalysis [4], along with the practical problems that preclude their application in large-scale treatment systems, are well established. Attention has also been given to the difficulties in separating the photocatalysts, which are usually of nanometric size. The need for artificial radiation sources has also drawn attention because the band gaps of most of the photo- catalysts are in the ultraviolet region. To overcome these draw- backs, new approaches that involve the use of immobilised [5] and modified [6] photocatalysts have been devised. Admittedly, the use of nanocomposites based on TiO 2 and carbonaceous materials can improve the efficiency of the photo- catalytic process through a synergistic effect, which can be explained by the previous adsorption of the substrate onto the carbonaceous materials followed by mass transfer to the photo- active TiO 2 [6]. Moreover, the high electrical conductivity of carbon allows for an alternative path for the electron in the conduction band, thus preventing the electron–hole recombination [6]. Therefore, the number of published works in this area has grown substantially since the mid-1990s [6]; most of these reports concern sol–gel processes, which are followed by the polymerisa- tion of a carbon precursor and a thermal treatment. For obvious reasons, nanoparticles (TiO 2 ) and carbon nano- composites (TiO 2 /C) exhibit characteristics that strongly depend on the synthesis conditions. Thus, the effects of relevant operational parameters (e.g., synthesis temperature of TiO 2 nanoparticles, TiO 2 /polymeric precursor ratio, pyrolysis tempera- ture, etc.) on the main characteristics of the nanocomposites (e.g., crystalline phase, particle size, etc.) are commonly investigated, along with the photocatalytic activity of the nanocomposite against model substrates. With this objective, most researchers in this subject area use univariate optimisation strategies to assess the effect of one variable at a time; these strategies thus neglect the existence of the effects of interactions between the variables [7]. In the current literature, a few papers have considered multivariable systems (factorial design of experiments) to evaluate the effect of relevant experimental variables on the properties of the synthesised materials; thus, highlighting the production of films and other TiO 2 hybrid materials (TiO 2 /quitosane [8], TiO 2 / SiO 2 [9] and TiO 2 /paper [10]) by sol–gel processes. In general, these studies expose the existence of strong interaction effects that cannot be observed by conventional optimisation systems. For this study, factorial design systems were used to evaluate the effects of experimental variables on the main characteristics of Materials Research Bulletin 48 (2013) 581–586 A R T I C L E I N F O Article history: Received 20 June 2012 Received in revised form 12 September 2012 Accepted 6 November 2012 Available online 15 November 2012 Keywords: A. Semiconductors B. Sol–gel chemistry D. Catalytic properties A B S T R A C T In this study, the effect of synthesis variables on the photocatalytic activity of TiO 2 nanoparticles and TiO 2 /C nanocomposites was evaluated by factorial design using a reactive dye as a model substrate. The most significant result demonstrated a significant effect of the pyrolysis temperature on the photocatalytic performance of both materials. For TiO 2 nanoparticles, the lower temperatures of pyrolysis enhanced the photocatalytic activity because of the lower rutile content. Conversely, for nanocomposites, the optimum condition was represented by higher temperatures of pyrolysis, which resulted in greater concentrations of the rutile phase. In the latter case, the higher activity observed for the rutile phase was clear evidence of the favourable effect of the presence of carbon. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +55 41 33613297; fax: +55 41 33613186. E-mail address: zamora@ufpr.br (P. Peralta-Zamora). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.11.045