Highly Conductive Ni-Co Based Bimetallic Metal-Organic Framework and Coordinated Water Induced Selective Adsorption and Separation of Trace Amount of Methyl Blue from Aqueous System

Abstract

A bimetallic MOF (NiCo-F) was solvothermally crystallized in monoclinic unit cell with P2₁/c space group containing in situ generated ligand, Ni²⁺ and Co³⁺ ions. The theoretical band gap was found to be 2.5 eV indicating its semiconducting nature. However, conductivity was higher with respect to the theoretical band gap. Therefore, besides through bond electrical conduction, coordinated water facilitated proton conductivity to significantly increase the overall conductivity. The MOF exhibits capacitance of 497 F/g at scan rate of 10 mV/s which impressive considering the specific capacitance of MOF derived supercapacitors. The coordinated water molecule induced selective adsorption of methyl blue (MeB) from the mixture of dyes in 100% aqueous system. High sensitivity enabled adsorption of trace amount of the target dye (upto 0.5 ppm) and efficiency reaches upto 90% within 40 min. The selective adsorption of MeB was attributed to the synthon type interactions between coordinated water molecule of NiCo-F and ─SO₃⁻ group of MeB. The adsorption kinetics follows pseudo-second-order with Langmuir type adsorption indicating monolayer chemisorption at uniform sites. Moreover, NiCo-F exhibits impressive dye desorption behaviors which establishes the recyclability of the dye, stability of the MOF, and also the potentiality of reuse of the adsorbent for multiple times.