The effect of anatase crystal orientation on the photoelectrochemical performance of anodic TiO₂ nanotubes

TiO₂ nanotube films were prepared by anodizing Ti plates in ethylene glycol based electrolytes containing variable concentrations of ammonium fluoride and water. The morphology, optical and semiconducting properties, as well as the composition of TiO₂ films were shown to be dependent on the anodizing bath composition. Among different film properties, only the preferential orientation of anatase crystals, quantified with the texture coefficient of the (004) plane, TC₍₀₀₄₎, showed the same dependence of photoelectrochemical performance on the electrolyte composition. The increased value of TC₍₀₀₄₎ was related to anatase crystals piling up in the [001] direction (normal to the plane of the Ti substrate), forming a fiber like texture structure along the tube that facilitates the transport of photogenerated electrons toward the conducting substrate.