Preparation of TiO2 Nanotube Arrays with Different Cerium Mixing Morphology and Their Photoelectrochemical Response in Visible Light

Well-ordered TiO2 nanotube arrays were prepared by electrochemical anodization onto a pure Ti sheet in an organic solution. Reduced cerium and oxidized cerium were deposited onto the TiO2 nanotube arrays by electrochemical cathodic reduction and then anodic oxidation. The morphology and crystalline phase were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), which indicated that the reduced cerium was in the form of elemental cerium nanofibers and they were dispersed over the surface of the TiO2 nanotube. The oxidized cerium was present as elemental cerium and cerium dioxide after oxidation. The photocurrent was measured and the bandgap energies were calculated. The results showed that the photocurrent response of the TiO2 nanotube arrays modified by the reduced cerium was enhanced in the visible spectra rather than in the UV spectra. The optimum amount of deposition was found for the sample prepared using the 10 mmol . L-1 cerium nitrate solution and with a bandgap energy of 2.88 eV. Also, the carrier density increased and the flat band potentials moved to the negative direction as the cerium content increased. After anodic oxidation, the photocurrent response of the samples increased in the visible spectra and in the UV spectra. However, the photocurrent response modified by oxidized cerium in visible spectra is weaker than that of the TiO2 nanotube arrays modified by reduced cerium.