Bidirectional light harvesting in a hierarchical H-TiO₂/NiO photocatalyst enables orientation-independent CO₂...

Solar-driven photocatalytic CO₂ conversion represents a promising route to generate renewable fuels while mitigating anthropogenic carbon emissions. Yet conventional photocatalysts face fundamental limitations: poor visible-light harvesting, rapid charge recombination, and monofacially activated catalysts that wastes up to half of the available surface area and restricts operation to fixed illumination geometries. Here, we overcome these constraints with a hierarchical, transparent bifacial photocatalyst that functions efficiently under illumination from either side. Our design integrates electrochemically hydrogenated TiO₂ nanotube arrays with sculptured NiO nanostructures on a fluorine-doped tin oxide substrate. Hydrogenation introduces Ti³⁺/oxygen vacancies that narrow the bandgap and improve charge transport, while oblique-angle-deposited NiO forms a p-n heterojunction with tailored band alignment and preserved optical transparency. The resulting architecture establishes a Z-scheme charge-transfer pathway, directionally tunable, that suppresses recombination and maximizes redox potential. Under simulated solar irradiation, the optimized H-TiO₂/NiO heterostructure achieves CO and CH₄ production rates of 1.5 and 0.2 μmol·cm^-2·h^-1, respectively, under front illumination, with back-side activity retaining 67-72.5% of this performance. Spectroscopic and electrochemical analyses further identify that annealing at 200 °C optimally balances Ni³⁺ preservation with defect engineering. By synergizing hierarchical junction design with bidirectional light harvesting, this work establishes a direction-independent photocatalyst platform that outperforms conventional architectures in both efficiency and operational flexibility. The approach offers a generalizable strategy for scalable, orientation-independent solar fuel generation.