Cu₂O/TiO₂ p–n heterojunction photoelectrodes were prepared by depositing different amounts of p-type Cu₂O nanoparticles on n-type TiO₂ nanotube arrays (i.e., forming Cu₂O/TiO₂ composite nanotubes) via an ultrasonication-assisted sequential chemical bath deposition. The success of deposition of Cu₂O nanoparticles was corroborated by structural and composition characterizations. The enhanced absorption in the visible light region was observed in Cu₂O/TiO₂ composite nanotubes. The largely improved separation of photogenerated electrons and holes was revealed by photocurrent measurements. Consequently, Cu₂O/TiO₂ heterojunction photoelectrodes exhibited a more effective photoconversion capability than TiO₂ nanotubes alone in photoelectrochemical measurements. Furthermore, Cu₂O/TiO₂ composite photoelectrodes also possessed superior photoelectrocatalytic activity and stability in the degradation of Rhodamine B. Intriguingly, by selecting an appropriate bias potential, a synergistic effect between electricity and visible light irradiation can be achieved.

This Review highlights the recent developments pertaining to pure and modified TiO₂ nanotube arrays for photocatalysis.

ZnFe₂O₄ coupled TiO₂ nanotube arrays (ZnFe₂O₄–TiO₂ NTAs) were synthesized by modifying TiO₂ nanotube arrays with ZnFe₂O₄ nanoparticles. Self-organized TiO₂ NTAs with a tube diameter of 100 nm and length of 400 nm were prepared by an electrochemical anodization method. ZnFe₂O₄ nanoparticles were deposited on the surface of TiO₂ nanotubes through a facile hydrothermal route. The modification of ZnFe₂O₄ nanoparticles extended the photoresponse of TiO₂ nanotube arrays into the visible light region. Under visible light irradiation, a 12-fold enhancement in photocatalytic activity was achieved using ZnFe₂O₄–TiO₂ NTAs compared with TiO₂ NTAs. The enhanced photogenerated electron–hole separation and improved transfer of photogenerated charge carriers were responsible for the increased photocatalytic activity of ZnFe₂O₄–TiO₂ NTAs. This approach provides a simple method for synthesizing a variety of nanoparticle-modified one-dimensional nanomaterials.

An ultrasound-assisted deposition technique was adopted to construct NiO nanoparticles on TiO₂ nanotube arrays (NiO–TiO₂ NTAs) which were prepared by three-step electrochemical anodization in an ethylene glycol system. The NiO nanoparticles were found to deposit on the surface of the highly oriented TiO₂ nanotubes. NiO–TiO₂ NTAs exhibited improved photochemical capability under UV irradiation. Compared with TiO₂ NTAs, a more than 2.56-fold enhancement in photocurrent response was achieved using the NiO–TiO₂ NTAs. The maximum incident photon to charge carrier generation efficiency (IPCE) values of NiO–TiO₂ NTAs and TiO₂ NTAs were 83.5% and 22.2%, respectively. The photocatalytic activities of NiO–TiO₂ NTAs were evaluated by the photodegradation of methylene blue (MB) aqueous solution. The kinetic constant of photocatalytic degradation of MB using NiO–TiO₂ NTAs prepared by ultrasonic deposition for 1.0 h was 1.9 times higher than that using TiO₂ NTAs. The enhanced photocurrent response and photocatalytic activity with NiO–TiO₂ NTAs were attributed to the formation of the NiO–TiO₂ p–n junctions.