Construction and Visible-Light-Driven Photocatalytic Properties of LaCoO3-TiO2 Nanotube Arrays

TiO2 nanotube arrays (NTAs) have high photocatalytic activity; however, their weak visible light absorption limits their solar energy utilization and environmental application. Perovskite (ABO(3))-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 ABO(3) 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 ABO(3)-TiO2 NTA heterostructured composites has been extremely challenging owing to the different crystallization temperatures of anatase TiO2 NTAs and ABO(3). 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.