Explaining the Enhanced Photoelectrochemical Behavior of Highly Ordered TiO2 Nanotube Arrays: Anatase/Rutile Phase Junction

The effect of calcination temperature on the photoelectrochemical properties of TiO2 nanotube arrays (TNTAs) has been investigated in many studies. Most work focused on improving the photoelectrochemical properties through optimization of the microstructure. In this paper, however, an anatase/rutile phase junction formed in TiO2 nanotubes has been demonstrated to account for the enhancement of the photoelectrochemical performance. Observations by the UV-visible diffuse reflectance spectra, glancing incidence angle X-ray diffraction (GIA-XRD), and electrochemical impedance spectroscopy indicate that the rutile fraction is at the bottom of the nanotubes while the anatase fraction is at the body of the nanotubes. The TNTAs with a coexistence of about 60% anatase and 40% rutile exhibit optimal performance and show a 1.4-times improved photocurrent density compared with the pure anatase TNTAs. Detailed synchrotron radiation photoemission spectroscopy further confirms the existence and effect of the phase junction. The results suggest photogenerated electrons transfer from the rutile phase to the anatase phase in the nanotubes due to the band edge alignment, which facilitates the photogenerated carriers separation and transport along the nanotubes and leads to apparent enhancement of the photoelectrochemical behavior.