Free-standing TiO2 nanotube array films sensitized with CdS as highly active solar light-driven photocatalysts

Self-organized, large-scale, free-standing TiO2 nanotube (TiNT)-array films were fabricated via anodization of titanium (Ti) foil in fluorine-containing ethylene glycol, followed by a facile detachment method to flake the TiNT arrays off the Ti substrate. After annealing, the resulting film consisted of well-ordered, vertically oriented, nanocrystalline TiNT arrays of similar to 130 nm pore diameter, similar to 25 nm wall thickness, and similar to 46 mu m length, corresponding to a high aspect ratio (the length/diameter) of similar to 250. The surface modification of the TiNT-array film using a simple solution-based method was carried out to fabricate a US nanoparticle-sensitized TiNT heterostructure-array (CdS/TiNT) film. Except for the significant shift in spectral photoresponse to a lower energy, the induced electron-hole pairs were extracted efficiently by their type-II band alignments in the CdS/TiNT arrays. This was confirmed by the incident photon-to-electron efficiency measurements and the kinetics of photocurrent decay in response to on-off irradiation. As a result of the strong absorption within the solar spectrum and the effective suppression of electron-hole pair recombination, a significant increase of similar to 10 times in the apparent first-order rate constant (k(obs)) of methylene blue (MB) photo-degradation under simulated sunlight (AM 1.5G) illumination was obtained as compared with the commercial Degussa P25 film. This free-standing film provides the advantage of the use of back-side illumination where light directly strikes the CdS/TiNT arrays, largely reducing incident light loss through the solution. In this case, the k(obs) value was further increased by similar to 30% compared with that in front-side illumination. The ease of freestanding TiO2 nanotube array film fabrication and US sensitization in this process will facilitate the development of environmental contaminants' solar light-driven decomposition. (C) 2009 Elsevier B.V. All rights reserved.