Dimensional Dependence of the Optical Absorption Band Edge of TiO₂ Nanotube Arrays beyond the Quantum Effect

Instead of investigating the quantum effect that influences the absorption band edge of TiO2 nanostructures, herein we report that geometrical parameters can also be utilized to manipulate the optical band gap of the TiO2 nanotube arrays. Hexagonal arrays of TiO2 nanotubes with an excellent crystalline quality were fabricated by techniques combining anodic aluminum oxide templates and atomic layer deposition. Through absorption spectroscopic analysis we observed that the optical absorption band edge of the TiO2 nanotube arrays exhibited a red shift as the diameter of the nanotube was tuned to be larger and the distance between two nanotubes became smaller accordingly, while the wall thickness of the nanotube was kept constant. Subsequent finite-difference time-domain simulations supported the observation from theoretical aspect and revealed a large near-field enhancement around the outer space of the nanotubes for the arrays with densely distributed nanotubes when the corresponding arrays were exposed to the illuminations. Thus, this paper provides a new perspective for the shift of the optical band gap, which is of great significance to the research in photoelectronics.