Properties of Carbon-doped TiO 2 (Anatase) Photo-Electrodes
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Properties of Carbon-doped TiO2 (Anatase) Photo-Electrodes Cristina S. Enache, Joop Schoonman, and Roel van de Krol Delft Institute for Sustainable Energy, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands. ABSTRACT To enhance the visible light absorption of anatase TiO2 photo-electrodes, the material was doped with carbon by two different methods: i) by spray pyrolysis under a CO2 atmosphere, and ii) by a post-deposition thermal treatment in a hexane-containing environment. For the hexanetreated samples, most of the carbon is located at the surface, from which it can be removed by reoxidation at elevated temperatures. In addition, both methods seem to result in the presence of small amounts of carbon in the bulk of the material, as deduced from a small red-shift of the absorption edge of TiO2.
INTRODUCTION Since the early 1970s, after Fujishima and Honda’s discovery that TiO2 can split water into H2 and O2 upon illumination with ultraviolet (UV) light [1], TiO2 has continued to attract many researchers’ attention as a water-splitting photo-catalyst. The main advantages of TiO2 over other metal oxides are its excellent photochemical stability, its suitable band edge positions, and its natural abundance and low cost. Its main drawback, however, is that it only absorbs in the UV region of the spectrum due to its large band gap (3.2 eV for the anatase form of TiO2). Despite much research on TiO2 photoelectrodes doped with cations such as Fe [2,3] and Cr [4] to enhance the visible-light absorption, little progress has been made. Cation dopants generally introduce additional energy levels deep in the band gap which act as efficient recombination centers [5]. Moreover, these energy levels have little overlap with the valence band states, which precludes effective transport of photogenerated holes to the surface of the photoelectrode. Recently, various anion dopants, such as nitrogen [6-8], carbon [9-12], and sulfur [13] have been reported to significantly enhance the visible light absorption of TiO2. Theoretical calculations by Asahi et al.[14] suggest that the p-orbitals of these anions show a significantly overlap with the valence band O-2p orbitals, which would facilitate the transport of holes to the surface. Until now, virtually all studies on carbon-doped titanium oxide have been carried out on suspensions of micro- or nano-sized particles [10,11,15,16]. In contrast, the number of publications on carbon-doped TiO2 photo-electrodes is limited [9,17]. In this paper, a comparison between the optical properties of C-doped anatase TiO2 photo-electrodes prepared by two different methods is presented.
EXPERIMENTAL Thin dense films of undoped and C-doped TiO2 were prepared using the spray pyrolysis technique. The precursor consisted of a 6 vol% titanium-tetraisopropoxide (>99%, Acros Organics) solution in ethanol (>99.9% J.T. Baker). To prevent cracking, 2,4-pentadione was
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added. A substrate temperature of 350 0C was used with a time sequence of 3s on, 60s off to
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