Preparation of length-modulated TiO_2 nanotube arrays and their application in DSSC

A two-electrode cell has been utilized to anodize Ti foils for obtaining highly oriented TiO2 nanotube(NT) arrays.The morphology and composition of the TiO2 NT arrays were characterized by scanning electron microscope(SEM) and energy dispersive spectra(EDS).Depending upon the anodization time(8-20h),the lengths and diameters of TiO2 NT arrays was arranged from 6.7 to 19.5μm,and 90 to 110nm,respectively.As confirmed by X-ray diffraction(XRD) and selected area electron diffraction(SAED),the as-anodized TiO2 NTs were amorphous but transformed into anatase phase after thermal annealing at 450℃ for 3h.Reflectance spectrum of TiO2 NT arrays showed that the layer with longer NTs can lower the reflectance in the visible spectrum,thus enhancing light harvesting for dye-sensitized solar cells(DSSC).The photocurrent density-voltage(J-V) characteristics of TiO2 NT-based DSSCs showed that higher energy conversion efficiencies can be achieved with longer TiO2 NT lengths.