n-p Heterojunction of TiO₂-NiO core-shell structure for efficient hydrogen generation and lignin photoreforming

Hydrogen evolution from biomass photoreforming has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution, as it could generate sustainable H₂ and value-added bioproducts simultaneously. Combining p-type semiconductors with n-type semiconductors to form n-p heterojunction is an effective strategy to improve the photocatalytic quantum efficiency by enhancing the separation of photogenerated electrons and holes, which could greatly facilitate the realization of such biomass photorefinery concept. However, the incompact contact between the n-type and p-type semiconductors often induces the aggregation of photogenerated electrons and holes. In this work, we design and synthesize an ultrafine n-p heterojunction TiO₂-NiO core-shell structure to overcome the incompact contact in the n-p interface. When the n-p heterojunction photocatalysts are evaluated for photocatalytic water splitting and biomass lignin photoreforming respectively, the as-fabricated TiO₂-NiO nanocomposite with 3.25% NiO demonstrates the highest hydrogen generation of 23.5 mmol h^-1 g^-1 from water splitting and H₂ (0.45 mmol h^-1 g^-1) and CH₄ (0.03 mmol h^-1 g^-1) cogeneration with reasonable amount of fatty acids (palmitic acid and stearic acid) production from lignin photoreforming. The excellent photocatalytic activity is ascribed to the synergistic effects of high crystallinity of TiO₂ ultrafine nanoparticles, core-shell structure and n-p heterojunction with NiO nanoclusters. This present work demonstrates a simple and efficient method to fabricate ultrafine n-p heterojunction core-shell structure for noble-metal free catalyst for both water splitting and biomass photoreforming.