Formation of a crystalline nanotube-nanoparticle hybrid by post water-treatment of a thin amorphous TiO₂ layer on a TiO₂ nanotube array as an efficient photoanode in dye-sensitized solar cells

An atomic layer deposited, conformal thin layer (∼15 nm) of amorphous TiO₂ on a primary TiO₂ NT array is effectively converted into partly crystalline nanoparticles on the inside walls of TiO₂ nanotubes by water mediated dissolution-precipitation phenomena without disturbing the original state of the TNTAs. This method helps to achieve the exclusive conversion of the secondary shell of amorphous TiO₂ layer coated by ALD into nanoparticles and the preservation of original crystalline nanotube (core) properties such as mechanical stability and the charge transport properties. As a consequence, the resultant core-shell type nanotube-nanoparticle composite showed higher photoanode performance (25% increment in efficiency) than conventional untreated TiO₂ NT array photoanode.