Experimental and DFT Study of Transition Metal Doping in a Zn-BDC MOF to Improve Electrical and Visible Light Absorption Properties

A combined experimental and density functional theory (DFT) study of metal–organic framework-5 (MOF-5) and its doped variants (M@MOF-5, M = Co, Fe, Ni, and Mn) has been performed to elucidate changes in electronic properties. The in situ doping of transition metals in MOF-5 has remained an unexplored area. All MOFs are synthesized using the hydrothermal method and characterized by X-ray, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, thermogravimetric analysis (TGA), and diffuse reflectance spectroscopy. TGA indicates that the structure of MOF-5 and doped MOF-5 is stable up to 450 °C; however, the Ni-doped MOF is less stable. The band gap is calculated by the Kubelka–Munk method. Doping remarkably decreases the band gap from 3.8 eV of pristine MOF-5 to 3.7, 3.6, 3.3, and 2.6 eV of Ni-, Mn-, Co-, and Fe-doped MOF-5s, respectively. DFT/time-dependent DFT computations reveal changes in electronic properties for 25–75% dopant substitutions in the secondary building unit for each M@MOF-5. Unoccupied d orbitals on dopants cause a reduction in the band gap and ligand to metal charge transfer (LMCT) in M@MOF-5. Because of the LMCT, the UV–vis spectra appear in the visible region, thus lowering the band gap. The presence of more empty orbitals and facile LMCT give rise to the smallest band gap in Fe@MOF-5, whereas the filled d10 and half-filled d5 configurations of Zn2+ and Mn2+, respectively, in MOF-5 and Mn@MOF-5 are responsible for the largest band gaps. Due to their lower band gaps and semiconducting characteristics, the doped MOF-5s will have a wide range of applications, from optoelectronic devices to sensors. Fe@MOF-5 can act as a visible-light-driven photocatalyst.