The concentration
and chemical state of nitrogen represent critical
factors to control the band-gap narrowing and the enhancement of visible
light harvesting in nitrogen-doped titanium dioxide. In this study,
photocatalytic TiO2–N nanoporous structures were
fabricated by the electrochemical anodization of titanium nitride
sputtered films. Doping was straightforwardly obtained by oxidizing
as-sputtered titanium nitride films containing N-metal bonds varying
from 7.3 to 18.5% in the Ti matrix. Severe morphological variations
into the as-anodized substrates were registered at different nitrogen
concentrations and studied by small-angle X-ray scattering. Titanium
nitride films with minimum N content of 6.2 atom % N led to a quasi-nanotubular
geometry, whereas an increase in N concentration up to 23.8 atom %
determined an inhomogeneous, polydispersed distribution of nanotube
apertures. The chemical state of nitrogen in the TiO2 matrix
was investigated by X-ray photoelectron spectroscopy depth profile
analysis and correlated to the photocatalytic performance. The presence
of Ti–N and β-Ti substitutional bonds, as well as Ti-oxynitride
species was revealed by the analysis of N 1s X-ray photoelectron spectroscopy
high-resolution spectra. The minimum N content of 4.1 atom % in the
TiO2–N corresponded to the lowest Ti-oxynitride
ratio of 13.5%. The relative variation of N-metal bonds was correlated
to the visible light sensitization, and the highest Ti–N/Ti
oxynitride ratio of 3.3 was attributed to the lowest band gap of 2.7
eV and associated with a 3-fold increase in the degradation of organic
dye. Further increase of N doping led to a dramatic drop of Ti–N/Ti
oxynitride ratio, from 3.3 to 0.4, which resulted in a loss of photocatalytic
activity. The impact of the chemical state of nitrogen toward efficient
doping of TiO2 nanotubes is demonstrated with a direct
correlation to N loading and a strategy to optimize these factors
based on a simple, rapid synthesis from titanium nitride.
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