Fabrication And Photocatalytic Activity Of TiO₂-Based Nanotube Composites

Environmental pollution and energy shortage have emerged as two major obstacles for the sustainable development of human society. Semiconductor photocatalysis shows great potential in solving the above problems. Titanium dioxide (TiO2), as a typical semiconductor photocatalytic material, has been widely applied in energy conversion, such as solar cells, water splitting and photocatalytic systems. In particular, highly ordered TiO2 nanotube arrays (TNTs) fabricated on Ti surfaces by anodization have gained increasing attention because of their large specific surface area, unique charge transport and strong adhesion to the substrate. However, its narrow light-response range and its low quantum efficiency limites its practical applications. Consequently, we try to seek an efficiently approach of modifying TNTs to improve the visible-light active photocatalysis of TiO2. In this paper, we successfully fabricated a series of TNT-based nanotube composite photocatalysts, such as NiO/N-TNTs, CSA-PANI/ TNTs, La2O3/TNTs and CuxO (x= 1,2)/TNTs. The photocatalytic activities of these composites were evaluated by the degradation of Rhodamine B (RhB) aqueous solutions under visible-light irradiation (λ> 420nm). The main content in this work is as follow:Firstly, N-doped TiO2 nanotube arrays modified with NiO (denoted NiO/N-TNTs) were prepared by electrochemical process of anodization, followed by a facile impregnation-calcinations-cycle technique. The coverage of NiO in TNTs can be tuned by changing the concentration of Ni2+. NiO/N-TNTs photocatalyst exhibited a higher photocatalytic activity than pure TNTs, NiO/TNTs and N-doped TNTs photocatalysts. In the co-modified catalysts, N doping led to the visible-light absorption through band gap narrowing, and the loaded NiO caused the high separation efficiency of photo-induced electron-hole pairs. Interestingly, after many recycling experiments, the degradation ratios of RhB are well remained. This confirms that the NiO/N-TNTs photocatalyst possesses good stability and reusability.Secondly, camphorsulfonic acid doped polyaniline (CSA-PANI) was prepared by a doping-dedoping-redoping process, and subsequently used for modifying the TiO2 nanotube arrays (TNTs) on titanium surfaces via a facile impregnation method. The as-prepared organic-inorganic composites (CSA-PANI/TNTs) exhibited a higher visible-light photocatalytic activity for the degradation of Rhodamine B than pure TNTs. The photocatalytic activity of the hybrids was related to CSA-PANI concentration used in doping treatment, and an optimal CSA-PANI concentration was 100mg/L. The activity of CSA-PANI (100 mg/L) hybridized TNTs photocatalyst displayed 2.4 times as high as that of pure TNTs. The superior performance of CSA-PANI/TNTs is mainly determined by the following three properties such as the adsorption ability of the photocatalyst to target pollutants, light harvest efficiency, and the charge transportation and separation. After modifying TNTs by CSA-PANI, the positively charged amine (-NH2δ+) may undergo electrostatic attraction with the carboxyl groups of RhB molecule, and adsorbance of RhB on the CSA-PANI/TNTs photocatalyst was higher than that on TNTs. Moreover, CSA-PANI can be used as a stable photosensitizer to arouse visible-light absorption, which extended the spectral response of TiO2 to the visible light range. Certainly, CSA-PANI as an excellent electron donor and hole transporter, could facilitate the separation of photogenerated charges and then lead to high photocatalytic performance.Thirdly, Lanthanum-doped TiO2 nanotube arrays (La2O3/TNTs) were prepared by a facile impregnation-calcinations technique. Lanthanum nitrate solution was used as lanthanum source. The as-prepared La2O3/TNTs samples exhibited a higher visible-light photocatalytic activity for the degradation of Rhodamine B than pure TNTs. The photocatalytic activity of the hybrids was related to impregnation time of lanthanum nitrate solution, and an optimal time was 2h. The activity of 2-La2O3/TNTs displayed 3.8 times as high as that of pure TNTs. In the La2O3/TNTs catalysts, La2O3 nanoparticles were dispersed on the surface of TNTs. At the interface Ti atoms might substitute La atoms in the lattice of La2O3 resulting in formation of Ti-O-La bonds. Ti3+species could lead to the visible-light absorption through band gap narrowing, and the loaded La2CO3 nanoparticles caused the high separation efficiency of photo-induced electron-hole pairs. Consequencely, La2O/TNTs hybrids exhibit enhanced photocatalytic activity.Finally, TiO2 nanotube arrays modified with CuxO (denoted CuxO/TNTs) were prepared by an impregnation-calcinations technique. Copper nitrate solution was used as copper source. The coverage ratio of CuxO in TNTs can be tuned by changing impregnation time. TNTs modified with CuxO can response the visible light. Moreover, the heterojunction built between CuxO and TNTs could stimulate the separation of photogenerated charges. When dipping time was 2h, the activity of 2-CuxO/TNTs displayed 4.9 times as high as that of pure TNTs in photocatalytic degradation of Rhodamine B.