Multiple Modifications Of TiO₂ Nanotube Arrays And Their Photoelectrochemical Performance

TiO₂ nanotube arrays have caused great interest due to their high surface area, strong absorption capacity, and high photo-generated electron conduction velocity, showing great potential applications in photocatalytic degradation of pollutants, water splitting, solar cells,sensitive devices and other aspects. However, owing to the inherent large band gap（3.0-3.2eV）, the activity of TiO₂ NTs are largely restricted to the ultraviolet region, and the photo-generated electron-holes are easy to recombine, therefore, they need to be modified to improve the photoelectrochemical properties. At present, the methods for their modification include transition metal ion doping, nonmetal ion doping, semiconductor compounding and loading of nobel metal nanoparticles. But each method has its own advantages, and some disadvantages, the TNTAs performance are still relatively limited. Therefore, we need to use a variety of means comprehensively to achieve multiple modifications, optimizing their photoelectrochemical properties.We first prepared nickel ions, cobalt ions, and iron-nickel ions codoped TNTAs respectively in NH4F+H₃PO₄ system in situ using anodic oxidation, and selected the optimal performance of TNTAs according to photoelectric test results mainly, then CdS or Ag nanopaticles were depositied on them. The as-prepared TiO₂ nanotube arrays were characterized by scanning electron microscopy（SEM）, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, UV-visible absorption（Uv-vis） to obtain a different doping concentration, oxidation time, the heat treatment temperature and other factors which have an influence on the morphology, crystal type, and photoelectric property of TiO₂ nanotube arrays. And we also get the optimum preparation conditions of different modification methods. The mechanism of multipe modifications are explored preliminarily.The results are shown as follows:We found that the transition metal ions, such as Ni, Co, Fe-Ni ions, can improve photoelectrochemical properties of TiO₂ nanotube arrays on different levels in 0.5wt%NH4F+0.3MH₃PO₄ system. TiO₂ nanotube arrays doped with 0.06 M nickel nitrate and oxidation at 20 V for 1h had the best photoelectrochemical properties. The photocurrent density and photoconversion efficiency of as-prepared Ni/TNTAs were 0.45mA/cm2 and0.29%, respectively. While the photocurrent density and photoconversion efficiency of undoped TNTAs were 0.26mA/cm2 and 0.16% under the same conditions. Cobalt nitrate doped TiO₂ nanotube arrays, which had the best photoelectrochemical properties when they were prepared with 0.015 M of cobalt nitrate, and oxidation at 20 V for 1h. The as-preparedCo/TNTAs were with photocurrent density of 0.68mA/cm2 and photoconversion efficiency of0.44%, which is 2.75 times of undoped TNTAs. TiO₂ nanotube arrays co-doped with ferric nitrate and nickel nitrate had the best photoelectrochemical properties when they were prepared with 0.005 M of ferric nitrate and nickel nitrate, and oxidation at 20 V for 1h. The as-prepared Fe-Ni/TNTAs were with photocurrent density of 0.92mA/cm2 and photoconversion efficiency of 0.66%, which is 4.12 times of undoped TNTAs. By impregnation method, CdS were compound to Ni/TiO₂, Co/TiO₂, Fe-Ni/TiO₂ with the best photoelectrochemical properties. CdS can absorb visible light, the electrons in the conduction band transfer directly to the TiO₂ conduction band, and further improve the photoelectrochemical properties. The photoconversion efficiency of Cd/Fe-Ni/TiO₂ was1.12%, which was 1.67 times of the Fe-Ni/TiO₂ and 7 times of undoped TNTAs. By UV reduction method, silver nanoparticles were deposited on the surface of Ni/TiO₂, Co/TiO₂,Fe-Ni/TiO₂. The amount of silver deposition has a great impact on the optical and electrical properties. 1mM Ag/Co/TNTAs with the maximum photoconversion efficiency of 1.15% is2.61 times of the Co/TiO₂ and 7.18 times of undoped TNTAs.Multiple modifications take advantage of each dopant and further improve the photoelectrochemical properties of TNTAs, which makes it possilble to prepare the devices with high performances.