On the origin of visible light activity in carbon-modified titania

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 SENSex 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 SENSex do not induce significant visible light mineralization of 4-chlorophenol. However, after heating them together in suspension, followed by calcination at 200 degrees C the resulting powder VLPreas 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.