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Structural Defect‐Induced Bandgap Narrowing in Dopant‐Free Anodic TiO 2 Nanotubes
Author(s) -
Nasirpouri Farzad,
Peighambardoust PhD student Ms NaeimehSadat,
Samardak Alexander,
Ognev Alexey,
Korochentsev Vladimir,
Osmushko Ivan,
Binions Russell
Publication year - 2017
Publication title -
chemelectrochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.182
H-Index - 59
ISSN - 2196-0216
DOI - 10.1002/celc.201700038
Subject(s) - band gap , materials science , anatase , amorphous solid , nanotube , x ray photoelectron spectroscopy , anodizing , fermi level , optoelectronics , nanotechnology , chemical engineering , photocatalysis , crystallography , carbon nanotube , chemistry , composite material , electron , aluminium , biochemistry , physics , quantum mechanics , engineering , catalysis
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.