Abstract:

TiO₂ nanotube arrays (NTAs) have high photocatalytic activity; however, their weak visible light absorption limits their solar energy utilization and environmental application. Perovskite (ABO₃)-type oxides with a narrow band gap can absorb visible light in a wide wavelength range and have excellent stability; however, their photocatalytic activity is relatively low. Coupling TiO₂ NTAs with ABO₃ to form heterojunctions is one of the most promising approaches to extend the optical absorption of TiO₂ NTAs into the visible-light range and promote the separation rate of photogenerated electron–hole pairs. However, to date, constructing ABO₃-TiO₂ NTA heterostructured composites has been extremely challenging owing to the different crystallization temperatures of anatase TiO₂ NTAs and ABO₃. In this work, LaCoO₃ nanoparticles were first synthesized using a sol-gel method. The as-prepared LaCoO₃ nanoparticles were then modified on the surface of the TiO₂ NTAs using an electrophoretic deposition technique, and a series of LaCoO₃-TiO₂ NTAs photocatalysts were thus constructed by controlling the deposition time. Results of the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) demonstrated that the nanoparticles prepared through the sol-gel method were LaCoO₃ with a uniform size and high crystallization. The average diameter of the LaCoO₃ nanoparticles was 100 nm. The binding strength between the LaCoO₃ nanoparticles and the TiO₂ NTAs was strong. The UV-visible absorption spectra (diffuse reflectance spectroscopy; DRS) demonstrated that the absorption band edge of the LaCoO₃-TiO₂ NTAs was gradually red-shifted into the visible light region with the increase in electrophoretic time. The LaCoO₃-TiO₂ NTAs prepared by the electrophoretic deposition technique for 15 min exhibited a strong light absorption in the wide wavelength range from 250 to 700 nm, which was the same as that of the LaCoO₃ nanoparticles loaded on a Ti foil. The results of the photocatalytic degradation of methyl orange (MO) under visible light irradiation demonstrated that the photocatalytic degradation rate of MO over LaCoO₃-TiO₂ NTAs was considerably higher than those of TiO₂ NTAs and LaCoO₃ nanoparticles loaded on a Ti foil. The LaCoO₃-TiO₂ NTAs prepared by the electrophoretic deposition technique for 15 min showed the highest photocatalytic degradation rate of MO, which was a four-fold enhancement compared to that of TiO₂ NTs under the same conditions. The p-n heterojunctions between the LaCoO₃ nanoparticles and the TiO₂ nanotubes were responsible for the enhanced visible light photocatalytic activity. The results of the electrochemical impedance spectroscopy (EIS) and photoluminescence spectroscopy (PL) tests demonstrated that the loading of the LaCoO₃ nanoparticles effectively promoted the separation and transport of photogenerated charges, thereby enhancing the visible light photocatalytic activity of the TiO₂ NTAs.

Key words: LaCoO₃, TiO₂ nanotube arrays, Visible light, Photocatalysis, Methyl orange