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Kaumudi Yadav

Thesis title: Synthesis of NiCo2O4 nanoneedles and their growth on carbon substrates for multifaceted applications

Phd Thesis Abstract

Nickel cobaltite (NiCo2O4), an inverse spinel with mixed valance transition metal oxide exhibits excellent redox reaction, magnetic and optoelectronic properties. Due to its versatile properties, NiCo2O4 is used as a multifunctional material in various applications. However, its structural evolution due to heat-treatment and integration with carbonaceous materials has not been fully explored. The present thesis attempts to study a few aspects in this direction. Toward this, NiCo2O4 has been synthesized and grown onto various carbon substrates and the resulting multifunctional materials have been used in various applications such as photocatalytic degradation of a model explosive, electromagnetic interference (EMI) shielding and microwave absorption, and as a supercapacitor electrode for energy storage.

The work began with the synthesis of one-dimensional (1D) nanoneedle-like NiCo2O4. The NiCo2O4 powder was subjected to annealing at different temperatures, i.e., 350, 550, and 750°C to study the effect of the annealing temperature on various properties of NiCo2O4. Subsequently, the powder samples were used for photocatalytic treatment of 2,4,6-trinitrotoluene (TNT) contaminated water under UV irradiation. The dependency of the magnetic and photocatalytic properties of NiCo2O4 on annealing temperature has been investigated and it was found that there exists a strong correlation between the annealing temperature, morphology and the above properties of NiCo2O4. The sample annealed at 550°C displayed the sharpest 1D nanoneedle-like structure with an optimum surface area of 44 m2 g−1), pore size of 11 nm, band gap energies of 1.87 and 2.52 eV, and saturation magnetization of 5.09 emu g−1 at 5T magnetic field. The photocatalytic degradation study is extended to different annealed samples by using different concentrations of TNT and NiCo2O4 in aqueous solution and pH values as well. Notably, the complete photodegradation of TNT (50 mg L−1) was found to occur within 3 h using NiCo2O4-550 of the concentration of 300 mg L−1 in water under UV−vis light. The degradation rate showed first-order kinetics with a rate constant of 1.6 h−1. Due to the magnetic behaviour of the catalyst, it can be fully recovered after use and the material can also be used repeatedly. The degradation product of TNT, as elucidated by gas chromatography−mass spectroscopy (GCMS), was found to be completely mineralized into CO2 and H2O.

Moving forward, needle-like magnetic NiCo2O4 nanoparticles were in situ grown onto a light-weight open cell reticulated vitreous carbon (RVC) foam by hydrothermal method leading to the formation of hierarchical nanostructured foam. The RVC foam was synthesized by carbonization of flexible polyurethane foam followed by acid-treatment. Here the aim of the study was to improve the EMI shielding and microwave absorption properties of RVC foam through NiCo2O4 coating. The coating structure, stability and performance were systematically investigated by various complementary techniques using XRD, XPS, Raman and FTIR spectroscopies, compression test and ultrasonication. The foam coated for 12h exhibited considerably improved shielding effectiveness (SE), i.e., ~36 dB from ~23 dB for bare RVC foam in X-band, leading to ~99.97% shielding efficiency. Though the foams mainly showed reflection-dominated properties regardless of the coating, however, the EM absorption improved with coating. The shielding mechanism is explained by means of reflection and absorption coefficients and, dielectric and magnetic losses manifested by NiCo2O4 coating and interfacial polarization.

In this work, nanoneedle-like NiCo2O4 particles were in-situ grafted onto woven carbon fibres (WCF) by hydrothermal method with reaction time varying between 6 and 18 h. The SE, measured in the X-band for 12 h coated WCF showed a striking value of ~72 dB, which is considerably higher than that of the bare WCF (~40 dB). Moreover, the coated WCF showed dominant contribution from absorption. The significant improvement of SE in NiCo2O4-coated WCF is attributed to it’s an anomalously large dielectric constant (~300–600, over a range of 8.2 to 12.4 GHz) resulting from interfacial polarization between NiCo2O4 nanoneedles and WCF. In this study, a technique useful for large scale production of low density, flexible and thin (~0.4 mm) NiCo2O4-coated WCF composite fabrics, with excellent absorption of X-band waves, is developed.

The thesis concludes with the study on use of NiCo2O4-coated RVC foam as a supercapacitor electrode. In this work, electrochemical properties of nanoneedle-like NiCo2O4-coated RVC foam samples have been studied. Among the coated samples for different time periods, the RVC foam coated for 8h showed the best electrochemical properties, i.e., the areal capacitance of 1.2 at scan rate of 5 mV.s-1 with the lowest charge transfer resistance. A symmetric charge storage device made using the NiCo2O4-coated RVC foam showed the highest areal capacitance of 432 at a current density of 1 Energy density of the symmetric device was found to be 0.29 Wh.Kg-1 at a power density of 208 W.Kg-1. The device showed capacitance retention of ~96.7% after 10,000 galvanostatic charge/discharge cycles at 5 mV.s-1. Further, the charged device was used to glow a 2.5 V LED for about 15 min, demonstrating the practical application of the coated RVC foam in supercapacitor charge storage.

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