Nickel and cobalt sulfide nanomaterials for magnetic and energy applications
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Date
2018
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University of Zululand
Abstract
This thesis reports the symthesis of five metal complexes, namely bis(piperidinylldithiocarbamato)nickel(II) (1), bis(tetrahydroquinolinyldithiocarbamato)nickel(II) (2), bis(N’-ethyl-N-piperazinyldithiocarbamato)nickel(II) (3), tris(morpholinodithiocarbamato)cobalt(III) (4) and tris(N’-ethyl-N-piperazinyldithiocarbamato)cobalt(III) (5). These heterocyclic dithiocarbamate complexes have been characterised using common techniques such as Fourier Transform Infrared spectroscopy, elemental analysis and nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance spectroscopy measurements were not conducted for complexes, due to their paramagnetic behaviour which adversely interferes with the technique. Single-crystal X-ray diffraction was used instead, which aided in the accurate elucidation of novel chemical structures of the complexes. Three complexes were characterised using the technique; the chemical structures of the rest are already known in literature. Generally, dithiocarbamate complexes have been identified as compounds of technological importance, particularly as single-source molecular precursors for the fabrication of nanomaterials for widespread applications. However, interest has mainly been on alkyl derivatives. Thus, this thesis focuses on the use of heterocyclic dithiocarbamates complexes as single-source molecular precursors for the fabrication of the corresponding metal sulfide thin films and nanoparticles through thermal decomposition routes.
Thermal decomposition of the complexes (1)-(5) produced Ni-S, Co-S and Ni-Co-S nanoparticles and thin films which exhibited interesting morphological and optoelectronic properties. The above-mentioned systems were particularly chosen for their increased interest in magnetism, as well as energy generation and storage applications. In this thesis, the nature of the complexes and other reaction parameters were demonstrated to have an influence on the particle size, morphology, and phase purity of the nanoparticles and thin films produced. These properties have a profound impact on the efficiency of the nanoparticles and thin films, towards specific applications.
The work presented in chapter two aimed at establishing reactions protocols suitable to to produce good quality nanoparticles using the solvent thermolysis approach. The complexes (1), (2) and (3) afforded NixSy nanoparticles displaying various morphologies (spheres, rods, irregular tetrahedral, nanospheres and irregular shapes) which were identified mainly by the x
transmission electron microscopy imaging technique. Furthermore, it was discovered that docecylamine and hexadecylamine capping agents produced phase pure Ni3S4 and Ni3S2, respectively; phase identification was conducted using the powder X-ray diffraction technique. On the contrary, oleylamine capping agent produced mixed phase NixSy nanoparticles from complexes (1) and (2) while complex (3) afforded phase-pure Ni3S2. Magnetic measurements identified Ni3S4 and Ni3S2 nanoparticles to possess ferromagnetic and paramagnetic behaviours, respectively. Chapter 3 reports the deposition of thin films using the same three complexes by the aerosol-assisted chemical vapour deposition method; NiS phase thin films were predominantly formed. The scanning electron microscopy imaging technique showed the films to display various morphologies.
Chapter 4 focuses on the catalytic evaluation of Co3S4 nanoparticles (with minor CoS2 impurities) in the oxygen evolution reaction (OER), hydrogen evolution reaction reaction (HER) and supercapacitance applications. The nanoparticles were prepared from complex (4) by a facile olelylamine-mediated hot injection method. The OER catalytic performance of nanoparticles prepared at 230 ºC and 260 ºC showed the overpotential of 307 mV and 276 mV, respectively. The specific capacitance and specific stability for the nanoparticles prepared at 230 ºC are 298 F/g and 73%, respectively. Nanoparticles prepared at 260 ºC achieved 440 F/g and 97%. The efficiency was measured after 5000 cycles. These results indicated that the prepared materials are good candidates for efficient energy generation and energy storage devices.
Chapter 5 reports the use of complexes (3) and (5) as dual precursors for the Ni-Co-S ternary material. Though thiospinels structure show interesting catalytic and energy storage applications, the cationic disorder can have major influence on the energy generation and/or energy storage applications. In this work, the effect of stoichiometric variation of metals in a thiospinel i.e. NixCo3-xS4 was examined on energy generation and storage properties. Nickel or cobalt-rich NixCo3-xS4 nanosheets were prepared by the oleylamine-mediated hot injection method. It was observed that nickel-rich and cobalt-rich nanosheet have different performances when tested for OER and HER, as well as supercapacitance performance. It was observed that the nickel-rich NixCo3-xS4 nanosheets have superior energy generation and storage properties.
Description
A dissertation submitted in fulfilment of the requirements for the Degree Doctor of Philosophy in the Department of Chemistry, Faculty of Science and Agriculture at the University of Zululand, 2018.
Keywords
Sulfide nanomaterials, Magnetic and energy applications