Titanium dioxide (TiO₂) has long served as both a model photocatalyst and an interfacial scaffold for photocatalytic H₂ production because of its stable oxide lattice, well-defined band structure, and tunable nanoscale morphology. In this review, sacrificial H₂ evolution denotes reduction-side H₂ generation assisted by a hole scavenger; photoreforming couples organic-substrate oxidation with H₂ generation and may derive hydrogen from water, the substrate, or both; and overall water splitting (OWS) uses water as the sole redox substrate and requires simultaneous H₂ and O₂ evolution near a 2:1 molar ratio. TiO₂-based systems now emphasize coupled control of absorption states, directional charge migration, atomic- or nanoscale reaction sites, and immobilized mass-transfer architectures. Their principal losses arise from insufficient light absorption, carrier recombination, HER/OER kinetic mismatch, H₂/O₂ back reaction, bubble retention, and gas management. This review evaluates doping and defect engineering, sensitized absorption, crystal-phase and facet regulation, heterointerfaces, cocatalysts, and nanofiber or immobilized structures through linked criteria: effective absorption, directional migration, HER/OER closure, gas desorption and recovery, long-term stability, and reaction-boundary consistency. The resulting framework uses evidence strength to interpret performance across different reaction regimes.