Green Synthesis of Ca-Doped ZnO Nanosheets with Tunable Band Structure via Cactus-Juice-Mediated Coprecipitation for Enhanced Photocatalytic H₂ Evolution

The development of efficient, stable, and sustainably-synthesized photocatalysts for solar-driven hydrogen production remains a critical challenge. Here, we report a novel, green coprecipitation route for the synthesis of calcium-doped zinc oxide (Ca-ZnO) nanosheets, utilizing cactus juice as a natural, multifunctional precipitation medium. This method enables the in-situ incorporation of 3-7% Ca2+ ions into the wurtzite ZnO lattice without the need for harsh chemical agents. Comprehensive analysis confirms that Ca2+ substitutionally replaces Zn2+, preserving the crystal structure while inducing a uniform nanosheet morphology. This doping strategy effectively modulates the electronic band structure, progressively narrowing the bandgap from 3.19 eV to 2.90 eV and significantly enhancing visible-light absorption. Crucially, the incorporation of Ca2+ also generates oxygen vacancies, which act as efficient electron traps to suppress charge recombination. The optimized 5%Ca-ZnO photocatalyst demonstrates an exceptional hydrogen evolution rate of 889 μmol·g-1·h-1 under visible light, with outstanding stability, retaining 94.8% of its activity after four cycles. This work not only presents a high-performance material but also establishes a paradigm for the sustainable design of advanced semiconductor photocatalysts.