Band gap modulation of functionalized metal–organic frameworks

Abstract

Metal–organic frameworks (MOFs) have been envisioned as alternatives to planar metallic catalysts for solar-to-fuel conversion. This is a direct result of their porous structure and the ability to tailor their optical absorption properties. This study investigates the band gap modulation of Zr-UiO-66 MOFs from both the computational and experimental points of view for three linker designs that include benzenedicarboxylate (BDC), BDC–NO₂, and BDC–NH₂. Emphasis in this study was aimed at understanding the influence of the bonding between the aromatic ring and the functional group. A ground state density functional theory (DFT) calculation was carried out to investigate the projected density of states and the origins of the modulation. A time-dependent density functional theory (TDDFT) calculation of the hydrogen terminated linkers confirmed the modulation and accounted for the electron charge transfer providing comparable optical band gap predictions to experimental results. Computational results confirmed the hybridization of the carbon–nitrogen bond in conjunction with the donor state resulting from the NH₂ functionalization. The NO₂ functionalization resulted in an acceptor configuration with marginal modification to the valence band maximum. The largest modulation was BDC–NH₂ with a band gap of 2.75 eV, followed by BDC–NO₂ with a band gap of 2.93 eV and BDC with a band gap of 3.76 eV. The electron effective mass was predicted from the band structure to be 8.9 m_e for all MOF designs.