ZnO/Al2O3/CuO ternary nanocomposites: a bandgap-engineered solution for high-efficiency photocatalytic hydrogen generation

Abstract

In photocatalytic systems, the efficient generation, separation, and transport of photoinduced charge carriers are essential to achieving high-performance hydrogen production. Semiconductor composites offer a promising route to enhance these processes by leveraging differences in energy band alignments to facilitate charge separation. In this study, ZnO/Al₂O₃ and ZnO/Al₂O₃/CuO nanocomposites were synthesized via a facile one-step hydrothermal method. Comprehensive structural and surface analyses using X-ray diffraction (XRD), Raman spectroscopy, filed emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) confirmed the successful formation and heterojunction interface of the composites. UV-Vis diffuse reflectance spectroscopy revealed a significant bandgap narrowing, with values of ∼2.94 eV for ZnO/Al₂O₃ and ∼2.19 eV for ZnO/Al₂O₃/CuO—both lower than that of pristine ZnO—indicating improved visible-light absorption. Theoretical band structure calculations further supported the experimental findings and confirmed the energetic favorability for interfacial charge transfer. UPS measurements were employed to determine the work function, revealing efficient band alignment among the constituent semiconductors, thereby promoting effective charge separation and transport. Photocatalytic hydrogen evolution experiments demonstrated superior activity for the ternary composite, with hydrogen generation rates of 676.85 µmol h⁻¹ g⁻¹ (ZnO), 1448.62 µmol h⁻¹ g⁻¹ (ZnO/Al₂O₃), and 1888.66 µmol h⁻¹ g⁻¹ (ZnO/Al₂O₃/CuO), highlighting the interfacial charge transfer enhancement driven by heterojunction formation and bandgap engineering. These results indicate that the ZnO/Al₂O₃/CuO nanocomposite is a highly promising photocatalyst for efficient solar-driven hydrogen production.