Synergistic Effect of Ultrasonication-Induced Surface Activation and A-Site Sr/Ba Distribution Unlocks Cocatalyst-Free Hydrogen Evolution in Sillén–Aurivillius Photocatalysts

The application of Sillén–Aurivillius (SA) photocatalysts for hydrogen evolution has remained limited, with prior studies relying heavily on noble-metal cocatalysts and offering only modest activity. We identify and address a major gap in the field by demonstrating that SA oxychlorides can be activated for efficient cocatalyst-free hydrogen generation. A dual design strategy is implemented that intrinsically activates SA oxychlorides: A-site cation engineering to optimize the band structure, coupled with controlled surface disorder engineering to generate oxygen vacancies and reduced metal centers, promote carrier separation, water adsorption, and visible-light utilization. The study reveals that A-site Sr content governs the intrinsic reduction potential of the designed Sr x Ba 2– x Bi 4 Ti 2 NbO 14 Cl ( x = 0.8, 0.7, and 0.5) photocatalyst, enabling more efficient proton reduction with increasing Sr content. Moreover, mild ultrasonication of the compounds creates an optimally disordered surface with oxygen vacancies and reduced metal centers, which collectively enhance carrier separation, strengthen water adsorption, and introduce band tailing that extends visible-light absorption. This combined effect boosts the hydrogen-evolution rate of the SA catalyst by 8.7 times, reaching 1.22 mmol g –1 h –1 even without a cocatalyst, showing remarkable improvement. These results demonstrate a scalable approach to activate standalone SA photocatalysts and offer critical insights into the role of cation distribution and defect chemistry in layered perovskite systems for efficient solar-to-fuel conversion.