Beyond nitrogen: an oxygen-chemical-potential framework for defect-engineered TiO₂ nanotube architectures in energy conversion and environmental catalysis

Nitrogen-doped titanium dioxide (TiO₂) nanotube arrays (N-TNTs) have become a reference platform for solar energy conversion and electrochemical storage, yet the field remains organised around a material-centric narrative in which nitrogen is treated as the primary functional agent and each application is optimised in isolation. Here we argue for a different reading. Drawing together evidence from lithium-ion storage, supercapacitive charge storage, photoelectrochemical water splitting and visible-light photocatalysis, we show that the performance-determining quantity in these systems is not the nitrogen content itself but the coupled population of shallow oxygen vacancies (V_o) and Ti³⁺ centres-a single defect state whose equilibrium is set, during synthesis and post-treatment, by the oxygen chemical potential (µ _o). In this view nitrogen acts chiefly as a thermodynamic stabiliser that pins this defect state and shifts its formation window, rather than as an independent visible-light 'switch'. Re-examining the literature through this lens reveals that the four applications are not separate optimisation problems but different projections of the same underlying defect equilibrium, and that their apparently disparate optimal conditions collapse onto a common design window in the µ _o-defect-density plane. We formalise this as a defect-state design map, propose a compact and transferable defect-state descriptor together with a minimum defect-reporting protocol to make results comparable across laboratories, and outline how operando defect metrology and closed-loop, machine-learning-guided synthesis can convert empirical doping into predictive defect-equilibrium engineering. This reframing offers a unifying and testable basis for designing N-TNT architectures-and, more broadly, defective oxide nanomaterials-for robust, scalable energy and environmental devices.