Effect of Metal Substitution on the Optical Band Gap of MIL-125-NH₂

Targeted modification of optical band gap is an important step for photovoltaic, sensing, and photocatalytic applications of metal-organic frameworks (MOFs). Herein, effective optical band gap modification of MIL-125-NH2 has been realized by replacing titanium(Ti) in MIL-125-NH2 with cobalt(Co), iron(Fe), and copper(Cu), respectively. Optical band gaps of the original MIL-125-NH2, MIL-125-NH2-Co-25%, MIL-125-NH2-Cu-25%, and MIL-125-NH2-Fe-12.5% were determined by diffuse reflectance spectra (DRS) to be 2.62 eV, 2.06 eV, 2.2 eV, and 2.3 eV, respectively. The mechanism of band gap variation of MIL-125-NH2 was discussed to be related to the band gap and conduction band minimum of metal oxides of doped metals. The photoluminescence (PL) properties of the materials was investigated, and the PL performance was influenced by doping metals: MIL-125-NH2-Co-25%, MIL-125-NH2-Cu-25%, and MIL-125-NH2-Fe-12.5% exhibited red shifted compared to the original MIL-125-NH2. Moreover, the intensity of the PL was found to be decreased with doping metals. The origins of luminescence tunability was discussed accordingly.