Defect engineering using surface linkage modification is an efficient method to tailor solar-to-chemical energy conversion performance of a metal-organic framework (MOF), albeit the nature and impact of the defects remain unexplored. The present study explored that the alteration of electronic and morphological properties due to linkage modification augments the intrinsic charge transfer in MOF but is not reflected in the overall hydrogen production activity when integrated with a cocatalyst. This is illustrated with the simply prepared judicious bulk heterostructure between defect-regulated NH2-MIL-125 and Co3O4. The study further demonstrates that the subtle use of the photosensitizer can multi-fold improve the activity while anchored onto a semiconductor surface. Several analytical methods including X-ray absorption spectroscopy revealed the unique anchoring of Co3O4 on the MOF surface that pertains to its catalytic activity. The composite Co3O4@PANI@NH2-MIL-125, without defects, showed significant spatial separation of the excited-state charge carriers thereby improving the rate of H2 evolution reaction (∼1208 μmol h-1 g-1), with apparent quantum yield of ∼3% under simulated visible-light irradiation. The separation of photogenerated charge carriers at the MOF/cocatalyst interface was unequivocally confirmed by the time-dependent emission spectra and steady-state electrochemical measurements. The photocatalytic activity is correlated with the compatible charge transfer kinetics and density functional theory calculation on the Co3O4@NH2-MIL-125 heterostructure. Further, femtosecond transient absorption spectroscopy studies revealed the initial photoexcited charge transfer from polyaniline (PANI) to hybrid PANI@NH2-MIL-125, which favorably occurs in picoseconds time scale to boost the photocatalytic activity of the system. This investigation will bestow a beneficial blueprint for structural design on MOF to precisely manipulate cocatalyst morphology and structural positions for developing an efficient photocatalyst. © 2022 American Chemical Society.