This study presents a multi-step strategy to achieve a high figure-of-merit (zT) value of approximately 0.486 at 580 K by interfacing lowly-oxidized graphene quantum dots (GQDs) with 3D nanostructured zinc oxide (ZnO). The fabricated 3D GQD@ZnO heterostructure exhibits a significantly low thermal conductivity of 0.785 W m⁻¹ K⁻¹ and a remarkably high Seebeck coefficient of -556 μV K⁻¹ at 580 K. The high zT value is attributed to the interfacial energy barrier formed by the energy offset between the lowest unoccupied molecular orbital (LUMO) of GQDs and the conduction band minimum (CBM) of ZnO, which enables effective low-energy electron filtering and phonon scattering. The 3D ZnO nanostructure, with a shell thickness of 70 nm, was fabricated using a 3D lithographic technique, and grain boundary engineering was employed to further enhance phonon scattering and low-energy electron filtering. The interfacial energy barrier of 0.63 eV contributes to the high zT value by modifying the electron and phonon transport behavior. The study demonstrates that the combination of structural and interface engineering can significantly improve the thermoelectric performance of ZnO-based materials, achieving a record high zT value of 0.486 at 580 K. The results highlight the potential of ZnO-based thermoelectric materials for energy conversion applications, particularly in waste heat recovery. The study also emphasizes the importance of controlling the interfacial energy barrier and optimizing the nanostructure to achieve high thermoelectric performance. The findings suggest that the proposed approach can be extended to wearable and flexible thermoelectric devices, leveraging the biocompatibility of GQDs and the unique mechanical properties of 3D ZnO systems.This study presents a multi-step strategy to achieve a high figure-of-merit (zT) value of approximately 0.486 at 580 K by interfacing lowly-oxidized graphene quantum dots (GQDs) with 3D nanostructured zinc oxide (ZnO). The fabricated 3D GQD@ZnO heterostructure exhibits a significantly low thermal conductivity of 0.785 W m⁻¹ K⁻¹ and a remarkably high Seebeck coefficient of -556 μV K⁻¹ at 580 K. The high zT value is attributed to the interfacial energy barrier formed by the energy offset between the lowest unoccupied molecular orbital (LUMO) of GQDs and the conduction band minimum (CBM) of ZnO, which enables effective low-energy electron filtering and phonon scattering. The 3D ZnO nanostructure, with a shell thickness of 70 nm, was fabricated using a 3D lithographic technique, and grain boundary engineering was employed to further enhance phonon scattering and low-energy electron filtering. The interfacial energy barrier of 0.63 eV contributes to the high zT value by modifying the electron and phonon transport behavior. The study demonstrates that the combination of structural and interface engineering can significantly improve the thermoelectric performance of ZnO-based materials, achieving a record high zT value of 0.486 at 580 K. The results highlight the potential of ZnO-based thermoelectric materials for energy conversion applications, particularly in waste heat recovery. The study also emphasizes the importance of controlling the interfacial energy barrier and optimizing the nanostructure to achieve high thermoelectric performance. The findings suggest that the proposed approach can be extended to wearable and flexible thermoelectric devices, leveraging the biocompatibility of GQDs and the unique mechanical properties of 3D ZnO systems.