PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences On the origin of visible light activity in carbon-modified titania Przemyslaw Z ˛ abek, Joachim Eberl and Horst Kisch* Received 25th July 2008, Accepted 8th December 2008 First published as an Advance Article on the web 14th January 2009 DOI: 10.1039/b812798k Characterization of a commercial carbon-modified titania visible light photocatalyst (VLP) reveals a quasi-Fermi level of -0.50 V at pH 7 and characteristic C1s binding energies of 284.8 eV and 286.3 eV as measured by XPS. Treatment with sodium hydroxide affords a soluble brown extract SENS ex exhibiting in the IR spectrum intense peaks at 1420 cm -1 and 1580 cm -1 , tentatively assigned to an arylcarboxylate group. Both the residue and the solution SENS ex do not induce significant visible light mineralization of 4-chlorophenol. However, after heating them together in suspension, followed by calcination at 200 ◦ C the resulting powder VLP reas exhibits the same quasi-Fermi level and C1s binding energies as the original VLP. Furthermore, within experimental error its visible light activity is identical with that of VLP. These results clearly indicate that, at least for VLP but probably also for other “carbon-doped” titania materials, an aromatic carbon compound and not substitutional or interstitial carbon is the origin of visible light activity. Introduction Titanium dioxide has received great attention both in fundamental and applied photocatalysis due to its low cost, non-toxicity, and stability against photocorrosion. 1–6 Unfortunately it can utilize only a very small UV part (about 3%) of solar light arriving at the earth surface. However, also the visible part (l > 400 nm) may induce photocatalysis if titania is modified by transition or main group elements. Out of the latter, especially nitrogen and carbon were reported to give the most active photocatalysts. Recently, we have reported on a carbon-modified titania (TiO 2 –C) prepared from TiCl 4 and tetrabutylammonium hydroxide. 7–9 Thereafter a technical process was developed rendering this visible light photocatalyst commercially available (VLP). It consists of calcining titania in the presence of an organic compound as carbon source. 9 Several reports concerned with this type of modification employing solid, liquid or gaseous carbon sources have been published recently. 10–15 Alternatively, also the organic substituent in a titanium alcoholate may serve as carbon precursor. 14,16–18 In all cases calcination temperatures were in the range of 250–500 ◦ C. Carbon modified titania was obtained also by annealing titanium carbide at about 600 ◦ C. 19 All these so called “C-doped” titania materials exhibit a weak absorption shoulder between 400 and 800 nm, the intensity of which increases with increasing carbon content. 7,16 However, maximum photocatalytic activity is observed at intermediate carbon concentrations. Thus, in the case of TiO 2 –C the absorbance steadily increases with carbon contents of 0.03%, 0.42%, and 2.98%, whereas the reaction rate of 4-chlorophenol oxidation exhibits a maximum at 0.42%. 7 Most of these carbon-modified anatase materials are active in visible light photo-oxidations of various organic pollutants. In addition to 4-chlorophenol, 11,20 also isopropanol, 19 gaseous Department of Chemistry and Pharmacy, Institute of Inorganic Chemistry, Friedrich-Alexander-Universit¨ at Erlangen-N¨ urnberg, Egerlandstraße 1, 91058, Erlangen, Germany. E-mail: Horst.Kisch@chemie.uni-erlangen.de; Fax: +49 (0)9131 85–27363; Tel: +49 (0)9131 85–27363 benzene, 15 and nitrogen oxides 10,14,18 were photo-oxidized by visible light irradiation of “C-doped” titania. It was generally proposed that the presence of some carbon species in titania is responsible for the visible light activity. However, the chemical nature of the “carbon dopant” is still a matter of discussion. In general the C1s binding energy, as easily obtained by XPS, was taken as diagnostic tool for the type of carbon present. From corresponding values of 284.8– 285.7 eV, 7,8,12,19–23 the presence of elemental carbon and graphitic or coke-like carbon was proposed. 16,23 It is noted that the binding energies of carbidic carbon of 281.8–284.3 eV 10,19,21,22,24,25 and aromatic ring carbon atoms of 284.3–284.7 26–28 fall in the same range. Also surface carbonates were proposed as relevant species 286.5–289.4 eV, 7,8,15,23,29,30 but it was shown that their presence is not responsible for visible light activity. 31 Binding energies of 288.6 and 288.9 eV were thought to arise from structural fragments like Ti–O–C 12 and Ti–OCO. 32 Density functional theory calculations suggest that substitutional (of lattice oxide) and interstitial carbon atoms are present. 33 Contrary to the common opinion it was proposed that it is not a carbon species itself but oxygen vacancies, generated only in the presence of a carbon source, that are respon- sible for visible light activity. 34,35 It is noted that the relevant species may be different, depending on the nature of the carbon source. In anatase powders prepared from alcoholates as carbon source a symmetric paramagnetic signal was observed at g = 2.005 by EPR spectroscopy, assigned to an aromatic coke like species. 16 Similar results were obtained for the commercial VLP product revealing that signal intensity increases with carbon content. 8 Although the intensity increased upon Vis irradiation, it could not be concluded that the corresponding radical is involved in the photocatalysis process since the concentration of radicals was about five to six orders of magnitude lower than the total carbon content. 8 Contrary to this, it was proposed that in carbon-modified titania prepared from gaseous cyclohexane this paramagnetic signal arises from an electron trapped at an oxygen vacancy. 15 To get further information on the basic question whether a carbon species or just oxygen vacancies are responsible for the 264 | Photochem. Photobiol. Sci., 2009, 8, 264–269 This journal is © The Royal Society of Chemistry and Owner Societies 2009