Magnetic, Optical, and Electrochemical Characterization of Tetracyanonickelate-Based Metal-Organic Frameworks
Halim, Md Abdul
The need for downscaling the device size in electronics, photovoltaics, and electrochemical storage systems requires functional nanomaterials to have atomic or molecular level thickness. Two-dimensional (2D) materials, such as transition metal carbides, carbonitrides and nitrides, and intercalants commonly known as metal-organic frameworks (MOFs), offer unprecedented possibilities in such applications due to their reported manufacturability at single- or few-atomic layer dimensions with higher specific surface area, and tunable chemical and physical properties. But the cost-effective large-scale production, and successful assembly of these nanosheets into macrostructural devices remains a challenge. In this work, a series of tetracyanonickelate (TCN)-based MOFs with the typical chemical formula M(H2O)2Ni(CN)4.4H2O [where M = Mn, Fe, Co, Ni, Cu, and Zn] was fabricated using wet chemical precipitation method and alloying with the transition metal ions located in the alternating pseudo-octahedral lattice positions. The energy band gap, (Egap) and magnetic Curie temperature (Tc) of the frameworks with different transition metal ions were calculated using Ultraviolet–visible spectrophotometry and Vibrating Sample Magnetometry (VSM) respectively. Firstly, we experimented on the spontaneous magnetism in TCN bulk crystals and compared their magnetic properties for different transition metals in the framework. The Mn-TCN MOF, Mn(H2O)2Ni(CN)4.4H2O exhibits a paramagnetic behavior above 50K and shows increased magnetic susceptibility on cooling with a paramagnetic to weak ferromagnetic (wFM) transition below 50K. The highest saturation magnetization of 125 emu/g was measured for this variant at 2K temperature with 90000Oe applied magnetic field. A field-dependent variation in the magnetic ordering temperature from 34 to 44K was also observed for this Mn-TCN complex. Secondly, the Ni-variant of TCN, Ni(H2O)2Ni(CN)4.4H2O was exfoliated into single- or few-layer nanosheets using 1-dodecyl-2 pyrrolidinone (DDP) and integrated into thin film by depositing the nanosheets onto flexible polyimide (Kapton) and rigid Indium Tin Oxide (ITO) coated glass substrates by inkjet printing technique in a layer-by-layer approach. Multiple absorption peaks for the printed film were observed in the visible and near Infrared range of spectrum. The bandgap shows a transition from 2.82 eV for bulk → 3.34 eV for the exfoliated structure. The inkjet printing of nanosheets allows reducing the number of complicated steps in solid-state processing of semiconductor materials and eventually bring great promise for this set of 2D nanomaterials to be used as stacked layers in multi-junction solar cells, and other optoelectronic devices. Finally, we reported on the improved electrochemical charge storage behavior of exfoliated manganese tetracyanonickelate (Mn-TCN) MOF produced by the liquid phase exfoliation (LPE) technique using DDP as the exfoliating agent. The specific capacitance of the exfoliated nanosheets was measured as the working electrode using a two-electrode configuration in 1M KOH aqueous electrolyte solution. Upon exfoliation, the material shows a six-fold increment (from 13.0 Fg-1 in bulk to 72.5 Fg-1 in exfoliated at 5 mVs-1) in specific capacitance than its bulk counterpart. According to our research, liquid phase exfoliation techniques can significantly increase the active surface area and storage performance of twodimensional TCN MOFs, making them suitable materials for a variety of electrochemical applications.
metal-organic framework (MOF), tetracyanonickelate
Halim, M. A. (2023). Magnetic, optical, and electrochemical characterization of tetracyanonickelate-based metal-organic frameworks (Unpublished dissertation). Texas State University, San Marcos, Texas.