Enhanced Visible Light Sensitization of N-Doped TiO2 Nanotubes Containing Ti-Oxynitride Species Fabricated via Electrochemical Anodization of Titanium Nitride

Cite this: J. Phys. Chem. C 2019, 123, 4, 2189–2201
Publication Date (Web):December 18, 2018
https://doi.org/10.1021/acs.jpcc.8b09762
Copyright © 2018 American Chemical Society
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Abstract

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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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09762.

  • List of as-sputtered Ti–TiN films at different sputtering conditions; SEM micrographs of as-sputtered Ti–TiN films sputtered at different titanium nitride sputtering powers; SEM micrographs of anodized TiO2–N films sputtered at different titanium nitride sputtering powers; model of TiO2 nanotube considered for SAXS calculations and SAXS modeling parameters; fitting of SAXS knee based on formulated SAXS model for series of anodized TiO2–N substrates; XRD analysis of TiO2–N substrates deposited at different Ti–TiN sputtering powers; variation of N atom % and Ti atom % for series of as-sputtered Ti–TiN films and anodized TiO2–N substrates; model for Ti 2p XPS high-resolution spectra with peak assignment and relative position and full width at half-maximum; high-resolution XPS spectra of Ti 2p for as-sputtered Ti–TiN films at different N concentrations; XPS depth profiling analysis of Ti 2p spectra for anodized TiO2–N films at different N atom %; XPS depth profile analysis of N 1s for as-sputtered Ti–TiN films at different N atom %; calculation of kinetic constants resulting from methylene blue degradation under 400–500 light filter for anodized TiO2–N substrates (PDF)

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