Browsing by Author "Shimizu, George K.H."

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    Synthesis of Multifunctional One-dimensional and Two-dimensional Highly Conductive Nanomaterials for Charge Storage Applications
    (2020-01) Shayesteh-Zeraati, Ali; Sundararaj, Uttandaraman; Roberts, Edward P.L.; Kim, Seonghwan; Shimizu, George K.H.; Cakmak, Mukerrem
    In this thesis, highly conductive 1D and 2D nanomaterials including silver nanowire (AgNW), graphene, and MXene were synthesized and used for charge storage. AgNW with high purity and aspect ratio was incorporated in poly(methyl methacrylate) (PMMA) to fabricate a high performance dielectric material. To improve the dielectric performance of the nanocomposites, a hybrid structure, containing AgNW and MnO2NW, was utilized to not only enhance the dielectric constant but also minimize the dielectric loss. The fabricated AgNW:MnO2NW (2.0:1.0 vol %) hybrid nanocomposite showed a high dielectric constant (64 at 8.2 GHz) and low dielectric loss (0.31 at 8.2 GHz), which were among the best reported values in the literature in the X-band frequency range (8.2–12.4 GHz). This superior dielectric performance of the hybrid nanocomposites was attributed to (i) dimensionality match between the nanofillers, (ii) better dispersion state of AgNW in the presence of MnO2NW, (iii) positioning of ferroelectric MnO2NW in between AgNWs, (iv) barrier role of MnO2NW, and (v) the aligned structure of AgNWs, as nanoelectrodes. In the second phase of this thesis, graphene was synthesized via electrochemical exfoliation of graphite in different inorganic salt electrolytes. Electrochemically exfoliated graphene (EEG) sheets synthesized in this thesis had a good dispersibility in water as well as a high electrical conductivity. This eliminated extra steps to increase conductivity such as reduction of graphene oxide. By utilizing these advantages, polyvinyl alcohol (PVA)/EEG nanocomposites were prepared for dielectric application. The prepared nanocomposites had an aligned structure of co-doped EEG (doped with nitrogen and sulfur) and demonstrated excellent dielectric performance. The aligned 4.0 wt% co-doped EEG in the PVA matrix led to a high dielectric constant (201) and low dielectric loss (0.2) in X-band frequency. Different chemical and structural characteristics of the nanocomposite account for the enhanced dielectric properties: (i) good dispersion of co-doped EEG in the polymer matrix, (ii) enhanced polarization centers in the graphene due to nitrogen/sulfur doping, (iii) aligned co-doped EEG sheets, (iv) functional groups at the surface of EEG as barriers between graphene sheets. In the third phase of this thesis, a modified minimally intensive layer delamination (MILD) synthesis approach was introduced to synthesize a highly conductive Ti3C2Tx MXene. By utilizing the modified approach, the electrical conductivity was significantly improved to 2.4 ×〖10〗^4 S/cm, five times more than that obtained from traditional MILD approach which is about 5.8 ×〖10〗^3 S/cm. This significant improvement in electrical conductivity was attributed to better quality of the synthesized MXene with the modified approach as well as higher flake sizes. Furthermore, the modified approach enhanced the synthesis aspects such as synthesis yield (up to ⁓ 60%) and MXene colloidal concentration (up to 31 mg/ml). The prepared MXene demonstrated a promising application as a supercapacitor, with a high capacitance of ⁓ 490 F/g at 1 A/g. Our synthesis approach has great potential to be used as the next MXene synthesis (etching) approach since it demonstrates improved synthesis yield, high colloidal concentration, and enhanced electrical conductivity.