Ultrathin black TiO2 nanosheet-assembled microspheres with high stability for efficient solar-driven photocatalytic hydrogen evolution

Abstract

Ultrathin black TiO₂ nanosheet-assembled microspheres with the advantages of both high specific surface area and broad optical absorption are promising photocatalysts, which, however, also suffer from the problem of low thermal stability. Herein, ultrathin black TiO₂ nanosheet-assembled microspheres are prepared with high-thermal stability using small amine molecules as protective agents to efficiently reduce the aggregation of TiO₂ nanosheets during the high-temperature hydrogenation. The resultant black TiO₂ nanosheet-assembled microspheres retain the anatase crystal phase and possess a relatively large specific surface area of 140.5 m² g⁻¹. After the hydrogenation treatment, surface Ti³⁺ and bulk oxygen vacancies (O_v) are constructed, inducing a narrow bandgap of 2.89 eV to expand the photoresponse from the ultraviolet to the visible-light region. The black TiO₂ nanosheet-assembled microspheres exhibited an excellent solar-driven photocatalytic hydrogen evolution rate (6.54 mmol h⁻¹ g⁻¹), which is almost three times that of pristine TiO₂ nanosheet-assembled microspheres (1.94 mmol h⁻¹ g⁻¹) under AM 1.5 irradiation. The improved hydrogen evolution performance is ascribed to the high specific surface area and efficient hydrogenation, which increase sunlight utilization, provide abundant surface active sites and thus significantly improve the separation efficiency of photogenerated charges. The stable ultrathin black TiO₂ nanosheet structure has wide applications in visible photocatalysis and is also an excellent host for the construction of heterojunction composites.