Surface Modification of TiO ₂ Nanorods for Dye Removal: Photodegradation vs Adsorption Activity

Colloidal titanium dioxide nanoparticles (NPs) represent a versatile class of semiconductors due to their tunable morphologies and organic shell formulations via the tools offered by synthetic nanochemistry. While largely exploited to remove industrial pigments from wastewater, scarce attention has been thus far devoted to modulating and rationalizing the different capacities of TiO₂-based NPs in photodegradation and adsorption processes, posing the different dye structures in relation to the surface chemistry and morphology of the relevant semiconductor. This study aims at effectively understanding the potentialities and limits of surface modification in TiO₂ nanorods (NRs) for dye removal from wastewater by adsorbing or photodegradation pathways. The NRs are initially synthesized with oleic acid (OA) as capping ligands (NR-OA) and subsequently functionalized with catechol (Cat) to generate the surface-modified analogue (NR-Cat) preserving the original aspect ratio of the inorganic core. Notwithstanding the partial replacement of the pristine capping ligands (supported by TGA and XPS analyses), an evident reduction of the band gap in NR-Cat (1.72 eV) in comparison to NR-OA (2.75 eV) with positive implications in its light-harvesting properties is observed. However, NR-Cat demonstrates to be highly efficient in water remediation solely by adsorption processes and only for cationic colorants, such as methylene blue and rhodamine B. This behavior is attributed to the increasingly negative surface charge of NR-Cat promoted by the surface modification, as proven by ζ-potential measurements. At the same time, the photoactivity of NR-Cat in dye removal is heavily affected by the sole activation of hydroxyl radicals for the degradation process, as suggested by the position of the material conduction band (hampering the formation of reactive oxygen species) and confirmed by scavenging experiments. Conversely, the photoinduced processes are revealed to be the preferential pathway for dye removal in the case of NR-OA, acting only on xanthene-based (eosin B, rhodamine B, Rose Bengal) and methylene blue colorants. Therefore, this study demonstrates that applying a systematic NP/dye approach could be a promising strategy for unlocking deeper insights into the fundamental behavior of semiconductors during adsorption and photocatalytic pathways, thus paving the way to innovative nanomaterials designed for mitigating the environmental problems raised by wastewaters.