Photoelectrochemical (PEC) water reduction is one of the key reaction routes to harness solar energy for the production of green hydrogen (H₂). Copper oxides are promising semiconductor materials for PEC applications as it possesses a narrow bandgap and favourable energy band positions. Owing to its severe photocorrosion, poor charge separation and rapid recombination of photogenerated electron-hole pairs, however, the use of conventional pristine Cu₂O photocathodes is not technically feasible for PEC reduction of water. Moreover, it is widely known that the morphology and structure of copper oxide-based photocathodes are pivotal to their PEC performance as they heavily influence the intrinsic properties, such as the density of active sites and specific surface areas. Modulation of these properties is crucial in increasing the available active sites for surface redox reactions and improving the separation efficiency of photogenerated charge carriers. Therefore, it is highly desirable to construct an efficient nanostructured heterojunction of Cu₂O-based photocathode to improve its photoconversion efficiency. Herein, we report a carefully engineered multi-component cuprous-cupric oxide heterostructure with distinct 2D-nanoflake morphologies (Cu₂O-CuO-NF) to overcome the technical bottlenecks encountered in conventional Cu₂O photocathodes. This was achieved through hot alkali treatment and subsequent thermal oxidation on the electrodeposited Cu₂O/FTO, which partially oxidises Cu₂O into 2D-CuO nanoflakes. Positive characteristics brought upon through this synthesis technique resulted in an enhanced specific surface area, which could facilitate a greater photocathode-electrolyte interfacial contact and higher water reduction activity during the PEC application. Enhanced charge separation efficiency is achieved as a result of the band bending at the Cu₂O-CuO interface, which provides a strong driving force for electron-hole separation and therefore, reduces the electron-hole pairs’ recombination process. Through morphological and compositional analyses using FE-SEM, XRD, and XPS, the formation of CuO nanoflakes on the surface of the Cu₂O photocathode was proven and validated. From the optical and electrochemical measurements, the charge carrier density was enhanced while the interfacial charge transfer resistance was reduced. This confirmed that the formation of heterojunction coupled with the surface nanoflake morphology of CuO plays a pivotal role in facilitating charge separation of photogenerated charge carriers. A high photocurrent density of -2.96 mA/cm² at -0.6 V vs Ag/AgCl under AM 1.5 G solar irradiation (100 mW/cm²) was achieved, which was approximately 27 times, and 1.5 times higher than that of the pristine Cu₂O (-0.11 mA/cm²) photocathode and Cu₂O-CuO (-1.96 mA/cm²) photocathode without 2D surfaces nano-structuration, respectively. Ultimately, experimental-derived information and data were deployed to construct a theoretical band diagram that elucidates the heterojunction band alignment of Cu₂O and CuO over the PEC water reduction mechanism.