Type-II engineered p-Co3O4/n-ZnO heterojunctions: Mechanistic insights into high-efficiency solar-driven hydrogen evolution and dye degradation

Cobalt oxide–zinc oxide (p-Co 3 O 4 /n-ZnO) nanocomposites with varied molar ratios were successfully synthesized via a facile hydrothermal method and systematically investigated for photocatalytic applications. Comprehensive structural, optical, and surface characterizations using XRD, UV–Vis, Raman, FTIR, FESEM, EDAX, XPS, UPS, BET, and HR-TEM confirmed the formation of well-defined heterojunctions comprising cubic Co 3 O 4 and hexagonal ZnO phases. Incorporation of Co 3 O 4 induced a pronounced red shift in absorption and band-gap narrowing, rendering the composites highly responsive to visible light. Ultraviolet photoelectron spectroscopy revealed a high work function of 5.90 eV, indicating strong surface electron binding and promoting effective charge separation. Raman spectroscopy validated the interfacial coupling, while HR-TEM provided direct evidence of coherent lattice fringes between ZnO and Co 3 O 4 , highlighting the robust construction of the heterojunction. FESEM images displayed uniform nanoscale assemblies (<30 nm), and the increased surface area further enhanced photocatalytic activity. Photoluminescence spectroscopy confirmed suppressed recombination of photogenerated charge carriers. Mechanistic studies revealed that the heterojunction operates via a Type-II scheme, effectively preserving highly reducing electrons in the ZnO conduction band and strongly oxidizing holes in the Co 3 O 4 valence band, which underpins the enhanced photocatalytic hydrogen evolution and dye degradation. As a result, the optimized composite achieved remarkable photocatalytic efficiency, degrading 91% of orange-red dye within 10 min and exhibiting a 3.2- and 2.4-fold enhancement compared to pristine Co 3 O 4 and ZnO, respectively. Moreover, the nanocomposite demonstrated a high hydrogen generation rate of about 2643 μmol h −1 g −1 under direct sunlight, governed by pseudo-first-order kinetics. These findings highlight the synergistic role of band-gap tuning, high work function, and interfacial heterojunction engineering, positioning p-Co 3 O 4 /n-ZnO nanocomposites as promising candidates for sustainable energy and environmental remediation technologies.