The fabrication, formation mechanisms and applications of novel nanoporous TiO2 membranes prepared by anodization

Abstract

In this paper, the TiO 2 nanoporous membranes were prepared by a two-step anodization method on Ti foils. The novel overlapped porous nanostructure, core-shell nanotube arrays and normal nanotubes arrays were successively observed during the anodization process. The formation mechanisms of such novel porous nanostructures were proposed based on the results and analysis of the surface morphology, crystallographic structure, X-ray photoelectron spectroscopy analysis and time-dependent current density curves. The overlapped porous nanostructure may originate from the exponential decay of current density during the initial stage. The nanopore diameter significantly depends on the current density; the large current density causes large nanopores on the top layer and the successive deceased current density results in the small nanopores in the under layer. The core-shell nanotubes arrays structure was suggested to relate with the composition and solubility of the anodized nanotubes. From the inner to the outer, each tube consists of several kinds of titanium oxide layers with different composition and different solubility. With further anodization, the inner layer titanium oxide, with high solubility in the electrolyte, could be gradually dissolved by the solution, which results in the formation of the normal nanotubes arrays. The finally obtained stable TiO 2 nanotube arrays were used as anodes to assemble dye-sensitized solar cells (DSSCs), and the photovoltaic performance of the cells were studied. The cell based on TiO 2 nanotube arrays without any surface modification gives a power conversion efficiency ( η ) up to 5.88%. After treating the TiO 2 nanotube arrays by TiCl 4 sol solution, the efficiency of the assembled cell increases to 8.47%, exhibiting a well prospect in energy conversion.