Tailoring band gap and light absorption in M-TiNT (M = Cu2+, Ni2+, Co2+, and Fe3+) for water remediation

Growing environmental concerns and the persistence of organic pollutants underscore the urgent need for effective wastewater treatment. Among the various strategies developed to address this challenge, photocatalysis has emerged as a promising approach due to its potential for sustainable and efficient degradation of pollutants. In this study, we investigate the simultaneous incorporation of transition metal cations (Cu ² ⁺, Ni ² ⁺, Co ² ⁺, and Fe ³ ⁺) into the crystalline structure of titanate nanotubes (H₂Ti₃O₇, TiNT) via a straightforward ion-exchange method. This modification promotes the formation of a p-n heterostructure between TiNT and the corresponding metal oxides (CuO, NiO, CoO, and Fe₂O₃). Remarkably, metal cation incorporation leads to a substantial reduction in the band gap, from 3.3 eV to 1.5 eV, and induces a new absorption feature associated with the formation of p-n heterojunctions. These modifications effectively extend the light absorption of the materials into the visible region. Furthermore, the formation of the p-n heterojunction increased charge carrier density compared to that obtained in pristine TiNT. The photocatalytic activity of the resulting metal-doped TiNT (M-TiNT) semiconductors was evaluated for the degradation of ibuprofen and indigo carmine under both UV and visible light irradiation. The enhanced photocatalytic performance is attributed to improved light harvesting and increased availability of charge carriers, facilitating the generation of reactive redox species. The importance of the hydroxyl radical as a reactive species was confirmed using a hydroxyl radical scavenger, which led to a significant reduction in photocatalytic activity compared to the control experiment without the scavenger. Notably, Cu–TiNT remained stable after the reuse cycles, retaining 90% of its initial photoactivity. These findings provide valuable insights for the rational design of nanostructured photocatalysts and underscore the potential of metal-doped TiNTs for efficient environmental remediation.