Structural Defect‐Induced Bandgap Narrowing in Dopant‐Free Anodic TiO₂ Nanotubes

Abstract

The application of TiO 2 in energy-harvesting materials has been restricted, owing to its relatively large bandgap. In accordance with theoretical predictions of lower bandgap and higher absorbance of visible light in TiO 2 nanotube arrays, we demonstrate experimentally that large-area TiO 2 nanotube arrays produced either by low-temperature anodization or by two-step anodization on electropolished Ti exhibit a narrowed bandgap; reducing the bandgap from 3.2 to 2.92 eV in an amorphous (as-anodized) state and to 2.84 eV in the anatase (annealed) phase. The bandgap energy of oxide nanotubes was measured by calculating Tauc plots, according to the diffuse reflectance spectra. Furthermore, the band structure of the TiO 2 nanotube arrays measured by using X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy revealed similar bandgap narrowing evidence. This bandgap narrowing behavior can be attributed to either surface edge states, owing to the incorporation of oxygen atoms on the high surface area of the nanotubes, or the formation of Ti 3 + defect states form nonbonding states within the bandgap below the Fermi-level edge. Annealing the tubes further reduced the energy bandgap, owing to the formation of anatase crystalline phase.

CO2 Photoreduction Activity Enhancement and Unexpected Observation of Carbon Monoxide Adsorbates on the Surface of TiO2 Nanotube Arrays Synthesized in Formamide Electrolytes

“…Simultaneously, the TNTA-FA samples showed a stronger visible and near-infrared absorption compared to TNTA-EG. There are two potential sources of the larger visible light absorptionfirst due to a greater number of defects (Ti 3+ states and oxygen vacancies) , originating from the structural disorder explained above and second due to a band gap narrowing owing to compositional doping. The TNTA-FA samples contain a larger amount of nitrogen and carbon as will be shown in the XPS section later.…”