Coherent CuO-ZnO nanobullets maneuvered for photocatalytic hydrogen generation and degradation of a persistent water pollutant under visible-light illumination

CuO–ZnO nanocomposite photocatalysts synthesized via a facile co-crystallization process followed by high pressure annealing demonstrated excellent photocatalysis of H₂ gas generation via water splitting as well as methylene blue (MB) dye degradation. Morphological analyses revealed that CuO nanoparticles readily combined with ZnO crystals to give rise to unique hierarchical bullet-shaped composite particles referred to as nanobullets. Elemental analyses confirmed the uniform distribution of CuO nanoparticles in the ZnO nanobullets. X-ray photoelectron spectroscopic analysis showed peak shifts associated with changes in the electronic states of the Cu and Zn species, confirming the formation of a heterojunction. Successful heterojunction formation was further supported by several imaging and spectroscopic techniques. The composite samples demonstrated enhanced photocatalytic activity compared with both uncombined ZnO and uncombined CuO. Among the as-synthesized nanocomposites, the sample annealed for 30 min (abbreviated as CZ-30) displayed the highest activity, generating 4.094 millimoles of H₂ within 3 h and 98.6% photodegradation of MB within 80 min under exposure to visible light. Such an enhancement in photocatalytic performance was ascribed to the synergistic migration of charge carriers across the heterojunction of the composites. A mechanism of synergistic photocatalysis was proposed on the basis of the results.