Anisotropic Defect-Induced Orbital Reordering Enables Coherent Charge Redistribution and Selective Photocatalytic CO2 Reduction in Metal–Organic Frameworks

Abstract

Photocatalytic CO2 reduction in metal–organic frameworks (MOFs) is often limited by inefficient charge separation and poorly defined active sites, while the role of crystallographic defect heterogeneity remains unclear. Taking MIL-125-NH2 as a prototypical system, we demonstrate that inequivalent oxygen vacancies dictate photocatalytic behavior through defect-site-dependent orbital reconstruction. By combining time-dependent density functional theory with nonadiabatic molecular dynamics, we show that axial and equatorial vacancies of TiO6 cage induce distinct Ti 3d electronic configurations, leading to markedly different carrier dynamics. Axial vacancies trigger cooperative t2g–eg orbital reordering, suppress coherent electron–hole overlap, accelerate decoherence, and prolong carrier lifetime, whereas equatorial vacancies exhibit the opposite behavior. This electronic disparity directly translates into catalytic outcomes: axial vacancies enable efficient CO2 activation via synergistic σ-type and π-type interactions, stabilizing bent intermediates and selectively driving HCOOH formation with overpotentials of less than 0.05 eV, while equatorial vacancies remain largely inactive. These findings identify defect-site anisotropy as a decisive descriptor and establish orbital-level defect engineering as a general strategy for designing high-performance photocatalysts.