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 × 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.