Addition of carbon to anatase TiO2 by n-hexane treatment-surface or bulk doping?

Anatase TiO2 can be sensitized to visible light by adding carbon as a dopant. Towards this end, TiO2 photoelectrodes were subjected to a thermal treatment in a hexane-rich environment. By comparing the optical and photoelectrochemical characteristics of both thin film and nanocrystalline nanoporous photoelectrodes, carbon is found to be located mainly at the surface of the TiO2. The amount of carbon that diffuses into the bulk of the material is too small to significantly enhance the visible light response and only a small shift of the absorption edge towards higher wavelengths is observed. The presence of carbon in TiO2 shifts the anatase-to-rutile transformation temperature beyond 800 deg. C, and X-ray diffraction shows that spray deposition of TiO2 under a CO2 atmosphere results in a higher bulk carbon concentration than a post-deposition thermal treatment in a hexane-rich environment