Photochemical & Photobiological Sciences PAPER Cite this: Photochem. Photobiol. Sci., 2015, 14, 591 Received 14th July 2014, Accepted 24th October 2014 DOI: 10.1039/c4pp00271g www.rsc.org/pps Transparent thin films of Cu–TiO 2 with visible light photocatalytic activity Marcin Janczarek,* Anna Zielińska-Jurek, Irmina Markowska and Jan Hupka Thin films of Cu–TiO 2 with a high level of transparency were prepared bya dip-coating procedure on the glass surface. CuCl 2 was used as a copper precursor added during sol–gel synthesis of TiO 2 . The exten- sion of optical absorption into the visible region of as-prepared thin films was indicated by UV/Vis spectroscopy. Only the anatase phase was detected by X-ray diffraction analysis (XRD). The presence of copper in the structure of thin films was confirmed by energy dispersive X-ray spectrometry (EDS). The significant rate of phenol and 4-chlorophenol mineralization was observed during visible light irradiation. The photocatalytic activity of the prepared thin films is correlated with the optimum copper content in the structure. Copper in metallic form and cupric oxides were not detected by XRD and scanning electron microscopy analysis. It is suggested that copper may exist as dispersed ions in the TiO 2 lattice. Introduction Intensive industrial growth, improvement of living standards and increasing human population have resulted in the in- tensified presence of pollutants in the environment. Thus the development of effective methods aimed at removal of con- taminants from air and water has become highly important. Heterogeneous photocatalysis using titanium dioxide can be an efficient method for complete destruction of pollutants in the liquid and gaseous phases. Most organic and inorganic compounds can successfully be degraded in this way. 1,2 Problems with scale-up photocatalytic processes pertain to the design of the most applicable form of the photocatalyst, especially for liquid phase reactors. The common approach in the case of most photocatalytic experiments is the use of a sus- pended system. The slurry system has the advantages of the absence of the mass transfer limitation and the large specific surface area of the nanometer-scale particles but, owing to the small size of the photocatalyst and its ability to adsorb onto various elements of the reactor, a serious problem with the separation of the photocatalyst after the process appears. In this case it is necessary to apply an additional step of treat- ment to separate the photocatalyst from the liquid phase. Another disadvantage of the slurry system is connected with the problem of the photocatalyst reuse in the next process cycle. Also the penetration depth of the light into the slurry is limited. In order to solve the above limitations of the slurry system and to make the photocatalysis a more applicable technology for wastewater treatment it is necessary to establish an easily separable and stable form of the photocatalyst. 3–5 One of the possible solutions to overcome these limitations is the application of titania thin films as the main part of photocatalytic systems to remove contaminants from air and water. Weak points of such a system concern the fact that the mass transfer limitation can easily occur and the available specific surface area of the photocatalyst is limited for com- pounds being photodegraded. To extend the application of photocatalysis based on TiO 2 it is substantial to incorporate visible light photocatalytic activity into this material. It can be realized by the sensitization of the titania surface: by dyes, 6 by colorless compounds via sub- strate–catalyst surface complexes, 7 and by coupling titania with other semiconductors. 8 The other approach is the modifi- cation of TiO 2 with non-metal 9–11 and metal elements. 12–14 A large number of studies on photocatalysis have been con- ducted using copper to modify TiO 2 . 15–35 The presence of copper species can enhance the photocatalytic degradation due to electron trapping by the copper ions leading to the pre- vention of electron–hole recombination. The majority of papers about Cu–TiO 2 concerns this topic. 15,16,19–21,26,30,32 It has also been found out that copper may extend the light absorption of titania to the visible light region 18 and incorpor- ate photocatalytic properties into this material in this wave- length range. 23–25,27,28,35 In both cases the role of copper species with multivalent states in the photocatalytic reaction still remains unclear. The discussion about the type of copper species responsible for visible light photocatalytic activity pro- ceeded in this paper. In comparison with the titania powders the modification of thin films to obtain an efficient visible light active material Department of Chemical Technology, Gdansk University of Technology, Narutowicza Str. 11/12, 80-233 Gdansk, Poland. E-mail: mjancz@pg.gda.pl This journal is © The Royal Society of Chemistry and Owner Societies 2015 Photochem. Photobiol. Sci. , 2015, 14, 591–596 | 591