8556 Chem. Commun., 2012, 48, 8556–8558 This journal is c The Royal Society of Chemistry 2012 Cite this: Chem. Commun., 2012, 48, 8556–8558 Enhanced performance of surface-modified TiO 2 photocatalysts prepared via a visible-light photosynthetic routew Ayyappan Ramakrishnan, Susann Neubert, Bastian Mei, Jennifer Strunk, Lidong Wang, Michal Bledowski, Martin Muhler and Radim Beranek* Received 13th June 2012, Accepted 10th July 2012 DOI: 10.1039/c2cc34243j Benzene can be activated by visible light (k > 455 nm) in the presence of TiO 2 , which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation. Heterogeneous semiconductor photocatalysis is widely recog- nized as one of the cheapest and most efficient methods for solar decontamination of water and air from toxic organic pollutants. 1,2 The most commonly used semiconductor is titanium dioxide which excels by the unique combination of low cost, non-toxicity, excellent stability against photo- corrosion, and possibility for further functionalization. 2–4 Moreover, apart from photocatalytic degradation reactions, in recent years we have become also increasingly aware of the great potential of TiO 2 for photocatalytic synthetic reactions, including selective photooxidation of organic compounds like alkanes or alcohols. 5–11 In this communication we commu- nicate an interesting example of successful unification of these two concepts: a novel type of surface-modified TiO 2 with improved photocatalytic degradation performance is prepared by a photosynthetic route involving visible light-induced activation of benzene. The fact that irradiation of TiO 2 in benzene or in the presence of benzene vapours with UV light (l o 400 nm) can lead to brownish coloration of TiO 2 is well-known from the literature. 12–15 However, rather surprisingly, recently we observed that brownish coloration of suspensions of TiO 2 (anatase, Hombikat UV 100) in benzene occurs even under prolonged (15 hours) irradiation with visible (l > 455 nm) light (Fig. 1a). The resulting powder (TiO 2 -B-VIS) contained ca. 0.3 wt% of carbon as revealed by elemental analysis. Similarly to the case of UV light-induced brown coloration of TiO 2 , 12,13 we assume that the brownish color is due to carbonaceous polymeric deposits on the titania surface formed through polymerization of benzene cation radicals which are formed by one-hole oxidation of benzene molecules (Fig. 1b). This is corroborated by IR and XPS (see ESIw, Fig. S1 and S2), the latter showing that hydrocarbons with a C1s binding energy of 285 eV are chiefly responsible for the increased surface carbon content. As compared to pristine TiO 2 , the optical absorption spectrum of TiO 2 -B-VIS revealed a very weak and rather broad absorption shoulder in the visible region down to more than 800 nm (Fig. 2a), in accordance with the brown color and the presumably randomly branched structure of the polymeric deposit. The polymeric deposit is clearly very stable, in line with a literature report, 15 and the brown coloration of TiO 2 -B-VIS could not be completely removed even upon boiling in NaOH (0.1 M). Since the bandgap of anatase is 3.2 eV (B390 nm) the fact that benzene photoactivation occurs also under visible light irradia- tion (l > 455 nm) is intriguing. We assume that intra-bandgap electronic states which are associated with surface structural features different from the bulk (so-called surface states) 16 Fig. 1 (a) Color change of a TiO 2 (Hombikat UV 100) suspension in benzene after irradiation with visible light (l > 455 nm); (b) suggested mechanism of formation of brown polymeric deposits. Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universita ¨tstr. 150, D-44780 Bochum, Germany. E-mail: radim.beranek@rub.de; Fax: +49-234-3214174; Tel: +49-234-3229431 w Electronic supplementary information (ESI) available: Experimental details and Fig. S1–S6. See DOI: 10.1039/c2cc34243j ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Published on 10 July 2012. Downloaded by Ruhr Universitat Bochum on 01/08/2013 09:07:04. View Article Online / Journal Homepage / Table of Contents for this issue