Ligand-Engineered Structural and Physiochemical Properties of 1D Molybdenum-MOFs: A Seldom Explored System for Photocatalytic Applications.

ABSTRACT

As one of the seldom explored systems, molybdenum-based metal-organic frameworks (Mo-MOFs) with different ligands such as terephthalic acid (Mo-TA), 2-aminoterephthalic acid (Mo-ATA), benzenetricarboxylic acid (Mo-BTC), 2-methylimidazole (Mo-2MI), 2-bipyridine (Mo-2bpy), and 4-bipyridine (Mo-4bpy) were developed in this study. X-ray diffraction (XRD), Raman, and attenuated total reflectance-infrared (ATR-IR) analyses confirmed the ligand-dependent crystal structure of the Mo-MOFs along with the characteristic functional groups present in the respective systems. Interestingly, the morphology of all of these the developed Mo-MOFs was found to be a one-dimensional rod-like structure, which was attributed to the binding nature of the ligands onto the growing Mo-frameworks. Optical analysis indicated that all these Mo-MOFs exhibit ultraviolet (UV) light absorption properties with band gap energy in the range of 3.47-3.03 eV. Among the various Mo-MOFs developed, Mo-4bpy MOF degraded a maximum of ∼76 and 62% of malachite green and Congo red dyes, respectively, under sunlight irradiation. The observed improved photocatalytic efficiency of Mo-4bpy MOF was attributed to its appropriate band edge potential, confirmed by Mott-Schottky analysis, improved carrier lifetime (∼34.6 ns) estimated using the time-resolved photoluminescence (TRPL) spectrum, presence of elements with stable oxidation states in the system confirmed by X-ray photoelectron spectroscopy (XPS), improved charge transfer characteristics, and decreased recombination resistance, as confirmed by impedance and PL analyses, respectively. The degradation of Mo-4bpy MOFs mediated by superoxide (•O2-) and hydroxyl radicals (OH•) was further confirmed by scavenger studies. Cyclic studies performed for up to 5 cycles suggested that the degradation efficiency of the Mo-4bpy MOF was stable, attributed to its excellent structural, optical, and morphological features confirmed via postcharacterization of the recycled photocatalyst.