Modulation of the Work Function of TiO2 Nanotubes by Nitrogen Doping: Implications for the Photocatalytic Degradation of Dyes

The work function engineering in metal-oxide nanostructures by judicious doping of impurities is not straightforward as it introduces multiple defects in the system. In this regard, understanding the nitrogen (N) doping induced modulation of Fermi levels in TiO2 nanotubes (TNTs) is challenging for visible-range photocatalytic applications. Here, 50 keV N ions are implanted in TNTs with a fluence range of 1014-1016 ions/cm2. X-ray diffraction and micro Raman analyses demonstrate the formation of anatase-TiO2 in pristine TNTs, while the crystalline quality is significantly affected by increasing ion fluence. The evolution of Ti3+ is also established by X-ray photoelectron spectroscopy, whereas ultraviolet photoelectron spectroscopy reveals the reduction in work function due to the formation of oxygen vacancies, in good agreement with X-ray absorption spectroscopy and photoluminescence results. The electron paramagnetic resonance study further identifies the evolution of Ti3+/N-substitutional defect centers. Finally, an enhancement in visible-light-assisted methylene blue and Rhodamine B dye degradation is recorded up to a fluence of 1 x 1015 ions/cm2, and it is correlated with the N-ion implantation-induced formation of electrochemically active states near the conduction band minimum and the valence band maximum. The decrease in degradation efficiency beyond a critical fluence of 1015 ions/cm2 is discussed on the ground of ion-beam-mediated amorphization and the subsequent increase in electron-hole recombination in the defect states.

Automated summary

The engineering of work function in metal-oxide nanostructures through doping of impurities is complicated due to the introduction of multiple defects. In this study, the modulation of Fermi levels in nitrogen-doped TiO2 nanotubes (TNTs) is investigated for visible-range photocatalytic applications. Nitrogen ions are implanted in TNTs with varying fluence levels, and the effects on the crystalline quality and work function are analyzed using various techniques. The results show that nitrogen ion implantation leads to the formation of electrochemically active states near the conduction band minimum and the valence band maximum, resulting in enhanced visible-light-assisted dye degradation.