Photoelectrochemical properties of N-doped self-organized titania nanotube layers with different thicknesses
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The present work reports nitrogen doping of self-organized TiO2 nanotubular layers. Different thicknesses of the nanotubular layer architecture were formed by electrochemical anodization of Ti in different fluoride-containing electrolytes; tube lengths were 500 nm, 2.5 m, and 6.1 m. As-formed nanotube layers were annealed to an anatase structure and treated in ammonia environment at 550 °C to achieve nitrogen doping. The crystal structure, morphology, composition and photoresponse of the N-doped were characterized by scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and photoelectrochemical measurements. Results clearly show that successful N-doping of the TiO2 nanotubular layers can be achieved upon ammonia treatment. The magnitude of the photoresponse in ultraviolet and visible light is strongly dependent on the thicknesses of the layers. This effect is ascribed to recombination effects along the tube length.
I. INTRODUCTION
For several decades, titanium dioxide (TiO2) has been known as an excellent photocatalyst.1,2 TiO2 is an n-type semiconductor with band gap energy Eg ∼ 3.2 eV for anatase and 3.0 eV for the rutile phase. Due to these relatively high band gap energies, a significant photoresponse can only be excited by ultraviolet (UV) light ( < 380 nm). To make TiO2 more responsive in the visible light range (vis) and thus to the natural solar spectrum, several modifications have been investigated. There have been essentially two extensively investigated approaches, on the one hand, dye-sensitization3,4 and on the other hand, doping with suitable impurities. For the latter approach, transition metals5,6 or other elements, including C,7 P,8 and N9 were more or less successfully used. At present, in appears N-doping of TiO2 is the most promising path toward narrowing the band gap energy.9–11 Under optimal conditions, oxygen atoms in the TiO2 lattice are substituted with nitrogen ions, and thus the corresponding N (2p) states are located above the valence band edge. In other words, mixing of N (2p) states with O (2p) states can result, and the narrowing of the band gap occurs. This leads to higher photocurrents achieved under vis irradiation. To form N-doped TiO2 photocatalyst the common approaches include sputtering of TiO2 in a gas mixture of N2 with Ar,9 annealing in pure
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0344 2824
J. Mater. Res., Vol. 21, No. 11, Nov 2006
ammonia gas,10 or direct current (dc) magnetron sputtering of TiO2 electrodes in an Ar/O2/N2 mixture.11 In view of an efficient solar cell, except for the light absorption in the visible light region, the surface area of TiO2 (e.g., provided by the TiO2 nanoparticle size) and crystallinity are also very important factors. Therefore the recently developed technique for fabricating selforganized TiO2 nanotube layers12–14 opens new perspectives for “high surface area” applications. Recently, we have shown that the tubes can be grown to a length of sev
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