Junction-Engineered TiO2 Nanotube Photocatalysts for Efficient Solar Hydrogen Generation: Doping Strategies, Structure–Activity Relationships, and Future Perspectives

The escalating global energy demand and the imperative to decarbonise the energy sector have established green hydrogen—produced via solar-driven photocatalytic water splitting—as a critical technology for a sustainable future. Titanium dioxide (TiO₂) nanotubes (TNTs) represent a particularly promising photocatalytic platform, combining the inherent advantages of TiO₂ (chemical stability, non-toxicity, low cost) with the structural benefits of one-dimensional nanotube architecture (directed electron transport, high surface area, and tunable morphology). However, pristine TNTs are fundamentally limited by a wide bandgap (~ 3.2 eV), confining photoactivity to the UV region (~ 4–5% of the solar spectrum), and by rapid electron–hole recombination. This review presents a critical, framework-guided analysis of the strategies developed to overcome these limitations, organised around a unified three-tier design hierarchy: (Tier I) electronic structure tuning via elemental doping and defect engineering; (Tier II) charge carrier management via heterojunction formation and carbonaceous coupling; and (Tier III) surface kinetic optimisation via co-catalyst integration. For each tier, we not only summarise reported performance gains but critically evaluate the mechanistic evidence, identify active controversies and contradictions in the literature, and propose specific operando characterisation protocols to resolve them. Particular attention is given to: the metal-specific concentration windows governing dopant activity versus recombination; the definitive characterisation of Type-II versus Z-scheme heterojunction mechanisms using operando EPR, hydroxyl radical fluorescence probing, and surface photovoltage spectroscopy; the fundamental kinetic timescale mismatch between ex situ transient absorption spectroscopy and operando catalytic turnover; and an explicit techno-economic analysis of immobilised versus slurry reactor configurations at pilot scale. A standardised performance normalisation protocol is proposed to enable reliable cross-study benchmarking. The review concludes with a quantitative forward research roadmap identifying specific targets (AQY > 30% under visible light, STH > 2% for unassisted splitting) and research directions for the 2025–2030 horizon, including high-entropy doping, single-atom co-catalysis, photothermal-photocatalytic synergy, and AI-guided materials discovery. A unified three-tier mechanistic design framework (light absorption → charge separation → surface kinetics) is introduced as the organisational principle of this review. Metal-specific optimal doping concentration windows (Fe: 0.1–0.5 at%; Cr: 0.05–0.15 at%) are quantified and linked to d-orbital configuration and redox midpoint potential. A three-technique operando protocol (EPR + hydroxyl radical tracking + SPS) is proposed for unambiguous Type-II versus Z-scheme mechanistic assignment. Explicit structure–activity relationships linking nanotube diameter, wall thickness, and length to photocatalytic performance are derived from comparative literature data. A standardised AQY/STH normalisation protocol is presented to enable reliable benchmarking across studies with heterogeneous testing conditions. A pilot-scale techno-economic analysis reveals that immobilised TNT arrays offer a cost advantage over slurry systems when noble metal co-catalysts are used. A quantitative forward roadmap with specific targets (AQY > 30%, STH > 2%) is proposed for the 2025–2030 research horizon.