This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 379–385 379 Cite this: Catal. Sci. Technol., 2012, 2, 379–385 Macro–meso-porous TiO 2 , ZnO and ZnO–TiO 2 -composite thick films. Properties and application to photocatalysisw Jayasankar Mani, Hazem Sakeek,z Salah Habouti, Matthias Dietze and Mohammed Es-Souni* Received 4th August 2011, Accepted 28th October 2011 DOI: 10.1039/c1cy00302j We report on a new and versatile method for preparing homogenous, crack-free and macro–meso-porous thick films of TiO 2 , ZnO and ZnO–TiO 2 composites using a single step coating procedure and fairly low annealing temperatures of a maximum of 400 1C. The method relies on an oxide nanopowder filler in an optimized precursor solution. The films are well adherent to the substrate, show homogeneously distributed open porosity, and are hydrophilic. The photocatalytic activity of these films was characterized using methyl orange as a model dye. We show that the TiO 2 films have better photocatalytic activity than ZnO and ZnO–TiO 2 composite films. The advantages of these composite films over particulate photocatalysts lie in their robustness and ease of application as no filtration is needed. Furthermore it is possible to apply them on suitable large area membranes. 1. Introduction TiO 2 and ZnO are the most versatile semi-conductor oxides with applications spanning a wide range from cosmetics to air purification. 1,2 Most of the applications rely on the generation of electron–hole pairs upon excitation with electromagnetic waves that have energies higher than their band gaps. TiO 2 and ZnO are also reported as good photocatalysts for the degradation of environmental contaminants. 3,4 When illuminated with appropriate light sources, the photocatalyst generates electron–hole-pairs, which initiate a series of chemical reactions and produce the hydroxyl radical (OH  ) and superoxide anions (O 2  ), when the semiconductor is in contact with water and oxygen. 5 Most of the applications require thick films with tailored porosity, and until now the method of choice for their processing is screen-printing that requires a number of steps, including paste formulation and processing, transfer to the substrate, burning and sintering at high temperatures. Sol–gel may constitute an alternative to screen-printing, though it is more appropriate for thin film processing. Thick films may be fabricated using layer-by-layer coating, but this is tedious, cost-intensive, can generate cracks, and the thickness achieved rarely exceeds 1 mm. We have shown in a number of studies that thick film fabrication using sol–gel can be made possible by loading sols with nanopowders at high loadings (hybrid sol–gel–powder method). 6 We demonstrated that this method worked well for lead–zirconate–titanate (PZT) thick films with which high functional properties, e.g. ferroelectric polarization, piezoelectricity and pyroelectricity, could be obtained. Advan- tages of the method encompass the possibility of combining different materials, and thus making nanocomposite films, and tailoring porosity. In the present paper we extend the hybrid sol–gel–powder method to the processing of thick, macro–meso-porous TiO 2 , ZnO and ZnO–TiO 2 composite films, and show that these films can be advantageously applied for the photocatalytic degradation of dye molecules, as one of possible applications. Degradation of toxic organic compounds by using UV irradiated TiO 2 and ZnO in the form of powders (e.g. Evonik P25 30 nm particles of TiO 2 ), thin film coatings and membranes has been studied and practiced both at the laboratory and industrial scales. 7–16 The application of powder is generally accompanied by complications arising from the need for filtra- tion and separation of the powder from the treated solution, which is considered a challenge to large scale applications. 17,18 From a practical point of view, it is therefore advantageous to use semiconductor coatings on suitable substrates to immobilize the catalyst. 19,20 In this respect sol–gel thick films with micro- and nanoporous structure were proposed as potential candidates, 21,22 and some attempts have been undertaken to process thick films containing TiO 2 nanoparticles fillers, 21,22 though also in these reports numerous dip-coating sequences were necessary to fabricate thick films that resulted in microstructures with rather small active surface and fairly clumped nanoparticles (see Fig. 3 in ref. 22). Institute for Materials & Surface Technology, University of Applied Sciences Kiel, Grenzstrasse 3, 24149 Kiel, Germany. E-mail: me@fh-kiel.de w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cy00302j z Al-Azhar University-Gaza, Department of Physics, Gaza, Palestine. Catalysis Science & Technology Dynamic Article Links www.rsc.org/catalysis PAPER