Leseprobe
DECLARATION
CERTIFICATION BY SUPERVISOR
DEDICATION
ACKNOWLEDGEMENT
INTRODUCTION AND LITERATURE REVIEW
1.2.1. Nickel Sulfide (NixSy) Nanoparticles and Thin Films
1.2.2. Cobalt Sulfide (CoxSy) nanoparticles and thin films
1.2.3. Ternary Nanomaterials with Thiospinel Structure
1.2.5.1. Energy storage applications (supercapacitors)
1.2.5.2. Hydrogen evolution reaction (energy generation)
1.2.5.3. Water Splitting Reaction
1.3. Statement of the research problem
1.5. Aim and objectives of the work
CHAPTER TWO
SYNTHESIS OF RARE PURE PHASE Ni3S4 AND Ni3S2 NANOPARTICLES IN DIFFERENT PRIMARY AMINE COORDINATING SOLVENTS
2.1 Introduction
2.2 Experimental details
2.2.1. Materials
2.2.2. Synthesis of ligands
2.2.3. Synthesis of complexes
2.2.3.1. Bis(piperidinedithiocarbamato)nickel(II) complex (1)
2.2.3.2. Bis(tetrahydroquinolinedithiocarbamato)nickel(II) complex (2)
2.2.3.3. Bis(ethylpiperazinyldithiocarbamato)nickel(II) complex (3)
2.2.4. Synthesis of NiS nanoparticles
2.2.5. Characterization techniques
2.2.5.1. Elemental CHNS/O analysis
2.2.5.2. Infra-red analysis (IR)
2.2.5.3. Thermogravimetric analyses (TGA)
2.2.5.4. 1H Nuclear Magnetic Resonance (NMR)
2.2.5.5. Single crystal X-ray crystallography
2.2.5.6. Powder X-ray diffraction (p-XRD)
2.2.5.7. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM)
2.2.5.8. Magnetic Measurement of Ni3S4 Samples.
2.2.5.9. Optical measurements
2.3. Results and discussion
2.3.1. Characterization of the ligands and complexes
2.3.2. Single-crystal X-ray structures for complexes (1) and (3)
2.3.3. Nickel sulfide nanoparticles
2.3.3.1 Powder X-ray diffraction
2.3.3.2 TEM and High Resolution TEM Analyses
2.3.3.3 Magnetic properties
2.3.3.4. Optical Properties
2.4. Conclusion
2.5 References
CHAPTER THREE
HETEROCYCLIC DITHIOCARBAMATO-NICKEL(II) COMPLEXES: SINGLE MOLECULAR PRECURSORS FOR NICKEL SULFIDE THIN FILMS
3.1. Introduction
3.2. Experimental details
3.2.1. Materials and methods
3.2.2. Synthesis of ligands and nickel complexes
3.2.3. Aerosol Assisted Chemical Vapour Deposition (AACVD) of nickel sulfide thin films
3.2.4. Characterization techniques
3.2.4.1. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX)
3.3. Results and discussion
3.3.1. Synthesis of single source precursors
3.3.2. Nickel sulfide thin films
3.3.3. SEM, EDX and Elemental Mapping analyses
3.3.4. Powder X-ray diffraction studies
3.3.5. Optical absorbance analysis
3.4. Conclusion
3.5. References
CHAPTER FOUR
FACILE SYNTHESIS OF BIFUNCTIONAL CoxSy NANOPLATES FOR EFFICIENT H2/O2 EVOLUTION AND SUPERCAPACITANCE
4.1. Introduction
4.2. Experimental details
4.2.1. Materials
4.2.2. Synthesis of ligand and cobalt complex
4.2.3. Synthesis of CoxSy nanoparticles
4.2.4. Characterization techniques
4.2.4.1. Electrochemical studies for supercapacitance application
4.2.4.2. Electrocatalytic studies for H2/O2 evolution studies
4.3. Results and discussion
4.3.1 Cobalt complex and thermogravimetric analyses
4.3.2 Single crystal X-ray crystallography structure for complex (4)
4.3.3. Phase Purity and morphology analyses
4.3.5 OER/HER electrocatalyst studies
4.3.6. Supercapacitor performances studies
4.4. Conclusion
4.5. References
CHAPTER FIVE
EFFECT OF CATIONIC DISORDER ON THE ENERGY GENERATION AND ENERGY STORAGE APPLICATIONS OF NixCO3-xS4 THIOSPINEL
5.1. Introduction
5.2. Experimental details
5.2.1. Materials
5.2.2. Synthesis of ligands and metal (Ni, Co) complexes
5.2.3. Preparation of NiCo2S4 nanosheets
5.2.4 Characterization techniques
5.2.4.1. X-ray photonelectron spectrometry (XPS)
5.2.4.2. Inductively coupled plasma-optical emission spectroscopy (ICP-OES)
5.3. Results and discussion
5.3.1. Synthesis of ligand and metal (Co, Ni) complexes and thermogravimetric analyses
5.3.2. Phase purity, christallinity and morphology analyses
5.3.3. XPS Measurements for the ternary samples (NiCoS-1 and NiCoS-2)
5.3.4. Electrocatalytic energy generation for the ternary samples (NiCoS-1 and NiCoS-2)
5.3.5. Electrochemical energy storage for the ternary samples (NiCoS-1 and NiCoS-2)
5.4. Conclusion
5.5 References
CHAPTER 6
SUMMARY OF WORK AND FUTURE OUTLOOK
6.1. Summary and Conclusion
6.2. Future Work
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 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.
List of Figures
Figure 1.1: Resonance forms of dithiocarbamic-NCS2- moiety.
Figure 1.2: A schematic diagram of AACVD apparatus assembly.
Figure 1.4: Model of normal spinel structure with the crystal model inset for NiCo2S4.
Figure 1.6: Schematic representation of supercapacitors.
Figure 1.7: Schematic tabulation of different types of supercapacitors.
Figure 1.8: A comparison chart for performance of some energy storage devices.
Figure 2.1: Thermogravimetric analysis (TGA) plots of complexes (1), (2) and (3)
Figure 2.6: One-dimensional chain of [Ni(Etpz-dtc)2] complex (3)
Figure 2.8: p-XRD patterns for DDA-capped nickel sulfide nanoparticles from complex (2).
Figure 2.9: p-XRD patterns for DDA-capped nickel sulfide nanoparticles from complex (3).
Figure 2.10: p-XRD patterns for HDA-capped nickel sulfide nanoparticles from complex (1)
Figure 2.11: p-XRD patterns for HDA-capped nickel sulfide nanoparticles from complex (2)
Figure 2.12: p-XRD patterns for HDA-capped nickel sulfide nanoparticles from complex (3).
Figure 2.15: p-XRD patterns of OLA-capped nickel sulfide nanoparticles using complex (3).
Figure 3.1: The AACVD setup during thin film deposition.
Figure 3.2: A graph showing film thickness growth versus temperature of deposition
Figure 4.1: Thermogravimetric curve for [Co(Mordtc)3] complex (4).
Figure 4.5: TEM images of OLA-capped CoxSy platelets synthesized at (a-b) 230 °C (c-d) 260 °C.
Table 4.4: Comparison of OER performance of some recently reported Co-based electrocatalysts.
Figure 4.10: The electrochemical double layer capacitance (Cdl).
Figure 5.2: p-XRD pattern for OLA-capped NiCo2S4 synthesized at (a) 220 °C and (b) 250 °C.
Figure 5.8: Chronoamperometry experiment for NiCoS-2 sample.
List of Tables
Table 2.1: Selected bond parameters of crystal structure for complex (1).
Table 2.2: Crystal data and structure refinement details for complex (1).
Table 2.3: Selected bond parameters describing the coordination sphere of [Ni(Etpz-dtc)2].
Table 2.4: Crystal data and structure refinement details for complex (3) [Ni(Etpz-dtc)2].
Table 3.1: Film thickness of nickel sulfide thin films deposited on glass by AACVD.
Table 4.3: Geometric parameters of the intermolecular interactions.
Table 4.4: Comparison of OER performance of some recently reported Co-based electrocatalysts.
Table 5.4: Comparison of OER performance of some recently reported electrocatalysts.
List of abbreviations
TMS Transition Metal Sulfide
SSP Single Source Precursor
LPCVD Low Pressure Chemistry Vapour Deposition
HCP Hexagonal Closed-packed
FCC Face-centred Cubic
HER Hydrogen Evolution Reaction
OER Oxygen Evolution Reaction
OLA Oleylamine
HDA Hexadecylamine
DDA Dodecylamine
TOP Tri-n-octylphosphine
VSM Vibrating Sample Mangetometer
CSD Cambridge Structure Database
PIPDTC Piperidine Dithiocarbamate
THQDTC Tetrahydroisoquinoline Dithiocarbamate
DSSC Dye-sensitized Solar Cells
TOPO Tri-n-octylphosphine oxide
CHAPTER ONE
INTRODUCTION AND LITERATURE REVIEW
1.1. General introduction
Nanotechnology in recent years has brought a revolution in the mindset of the materials sciences community, with different materials exhibiting interesting properties at the nano-scale size regime. Generally, the field focuses on the understanding and control of matter at nano-scale dimensions between 1 nm to 100 nm. The resulting nanomaterials (consisting of nanoparticles, nanorods, nanowires, nanoflowers), depending on the morphology of the materials, would portray unique optoelectronic, magnetic, and physicochemical properties when compared to their bulk counterparts [1, 2]. Thus, these properties lead to exploitation in diverse novel applications. As a result, control and manipulation of particle size and shape at the nanometer scale have been identified as the key driving force in the field of nanotechnology [3].
The synthesis protocols of nanomaterials have been dominated by two approaches, namely the bottom-up approach and top-down approach. The former is a typical self-assembly process through a nucleation mechanism at the atomic level, whereas the latter is concerned with the breaking down (mostly a physical process) of bulk materials to afford nanostructures. Regardless of the type of approach, the breakthrough in nanotechnology over recent years has been advocated predominantly by the synthesis of colloidal quantum dots [4]. As a result, it is mostly common that synthesizing monodispersed nanostructures with well-defined morphology and properties, scaling up for green industrial production, and more understanding on the tuning of physical-chemical related properties, are likely to be the main current trends in nanotechnology research projects [5].
Transition metal sulfide (TMS) nanomaterials, a class of metal chalcogenides, have been recognized for their rich structural diversities in applications. They receive considerable attention due to their interesting electronic and magnetic properties. Furthermore, they are relatively cheap and abundant; most occur naturally in different mineral forms [6]. Such nanomaterials, including thin films, have been widely researched in fields focusing on photovoltaic technologies, semiconductors, telecommunication, bioimaging and solar cells, amongst others [7, 8].
The synthesis of TMS nanomaterials can either follow multiple-source or single-source precursor routes, depending on compatibility with the synthetic protocols. The single-source precursor (SSP) route is mostly preferred due to pre-existing metal-sulfur bonds in the compounds, thus, requiring only careful decomposition of the compounds to afford nanomaterials of desired stoichiometry. Furthermore, the route eliminates multiple reaction steps which would adversely result to challenges in controlling the properties and/or features of nanomaterials, as well as preventing contamination issues. Dithiocarbamate metal complexes have, for some time, demonstrated their outstanding performance as SSPs desirable for the fabrication of TMS nanomaterials. Dithiocarbamate ligands are widely observed for their ability to bind effortlessly to transition metals centers of different oxidation states [9, 11, and 12]. Dithiocarbamate ligands can readily produce chelating compounds, with an ability to coordinate to a wide range of transition metals via its two donor atoms of sulfur. They can also stabilize low and high oxidation state metal ions, thus, rendering them as important compounds in the synthesis protocol of single source precursors. The complexes inherently become desirable for their diversity, easy preparation and long shelf life in ambient conditions. These complexes have also been reported in literature to have widespread applications such as in the material science, biomedical and agricultural fields [10].
Though dithiocarbamate complexes as single source precursors have been studied widely in the area of nanotechnology, the heterocyclic dithiocarbamate class have recently been investigated in detail [13]. From literature reports, distinguished research groups such as Revaprasadu and co-workers have demonstrated the attractive qualities of heterocyclic dithiocarbamates in the fabrication of TMS nanomaterials, e.g. piperidine and tetrahydroisoquinoline dithiocarbamato cadmium(II) complexes to afford CdS nanomaterials [14]. The same group has also reported other TMS nanomaterials such as PbS and ZnS fabricated from the corresponding metal heterocyclic dithiocarbamates [14, 15].
Dithiocarbamate ligands, like other related dithiolates, are formed through the nucleophilic attack on the carbon disulphide molecule, as shown in Scheme 1.1. Dithiocarbamates bonding mode with metals is shown in Figure 1.1 where the coordination capabilities of the ligands are shown to proceed through the monodentate (I & II) or bidentate manner (III). Such mode of coordination has been reported to contribute to the overall electronic structure of the resulting complexes.
Scheme 1.1: Nucleophilic attack of carbon disulfide.
Figure 1.1: Resonance forms of dithiocarbamic-NCS2- moiety.
Dithiocarbamates as common compounds that bind strongly and selectively to a range of metal ions, have in recent times attracted and controlled self-assembly of many beneficial supramolecular methods for preparation of nanomaterials [16]. While dithiocarbamates, in general, share the characteristic disulfur motif that binds to the metal as a bidentate ligand (Fig. 1.1), the nitrogen may be functionalized in various ways to modify the physico-chemical properties of the ensuing metal complex, in particular its solubility and lipophilicity. This gives them a very useful characteristic when it comes to their biological applications [17].
Since different dithiocarbamates derivatives may end up having different physico-chemical properties for the ensuing metal complex, it is important to select a group of dithiocarbametes that can beneficially suit the synthesis protocol for a particular application. For instance, iron dithiocarbamate complexes may easily bind free NO radicals to form stable mononitrosyl iron complex. For that matter, iron dithiocarbamate complexes can be applied industrially to scavenge free NO [18]. In a biological study by Pieper et al. [18] iron(II) diethyldithiocarbamate and iron(II) N-methyl-d-glucaminedithiocarbamate complexes were observed to displayed hydrophobic and hydrophilic behavious. Hence, were we found to be suitable for different biological applications. Another example is the organotin(IV) dithiocabamate complexes which are found to have the ability to stabilize specific stereochemistry in their complexes and subsequently find diverse applications in agriculture, biology and catalysis research fields [19]. Their usefulness as single source precursors for tin sulfide nanoparticles has made them relevant in modern times [20], as a result, devising easier access to SnS nanoparticles which are desirable for their stereo-electronic properties most relevant in the field of medicinal chemistry [21-23]. Organotin(IV) dithiocarbamates, therefore, owe their functionalities and usefulness to the individual attributes of the organotin and the dithiocarbamate moieties present within the molecule.
Similarly, different functionalities of metal dithiocarbamate complexes have different decomposition pattern, thus, leading to different nanomaterials. Hollingsworth et al. [24] examined complexes of nickel dithiocarbamates and concluded that the properties of the inorganic products dependend on the amide-exchange intermediates formed in the course of their decomposition. The phase of nanoparticles produced is dictated by the extent of the amide-exchange mechanism. In their study, Hollingsworth et al. [24] established that nickel dithiocarbamate complexes which decompose via a single amide-exchange mechanism form NiS, while those that decompose through a double amide-exchange machanism favoured the formation of Ni3S4 nanoparticles. Gervas et al. [25] have indicated that a choice of a capping agent also plays key role in directing the decomposition pattern and hence influencing the type of amide exchange mechanism which inadvertently controls the phase of the Ni-S system for exploitation in a specific application. It is therefore important to search for reaction parameters (particularly the choice of dithiocarbamate ligand and capping agents) which will fine-tune the properties of the nanoparticles to achieve high efficiencies in various applications.
1.2. Literature Review
As it has already been established, TMS nanomaterials play a major role in the current state of technological advances due to their continuously improved and/or novel properties. Access to these properties is easily achieved through tweaking reaction parameters during synthesis and/or processing of the as-synthesized nanomaterials. However, careful consideration of the type of TMS plays a major role since they possess different chemical behaviours. The current theme in the nanoscience field is the search of alternative non-toxic materials which exhibit similar or improved properties to the existing toxic counterparts. Thus, TMS materials such as NixSy and CoxSy have received a lot of attention due to this respect. NixSy and CoxSy nanomaterials, like other transition metal sulfides, have a wide range of potential applications. NixSy sulfide nanomaterials have potential use in rechargeable lithium batteries [26], catalytic degradation of organic dyes and magnetic devices [8]. CoxSy nanomaterials can directly be used in solar selective coatings, photo detectors, magnetic devices, lithium ion batteries and biological labelling [27, 28].
Given the tremendous technological importance of NixSy and CoxSy nanomaterials, different synthetic routes have been reported. As a rule of thumb in nanotechnology, the control over particle size can tune the band-gap for nanomaterials to suit specific applications. In order to attain this goal, a number of synthetic routes have been applied. One of the efficient routes involves the wet chemical approach where nanoparticles form through growth processes in solutions of a molecular precursor. Murray and co-workers pioneered this technique by injecting volatile metal alkyl compound and an organic chalcogen source in hot tri-n-octylphosphine oxide preheated at temperatures of between 120 °C and 300 °C [4]. Although the method affords high quality nanomaterials, it suffered from severe challenges and limitations. One of the limitations was the use of the metal alkyl such as [(CH3)2Cd], which is extremely toxic, expensive, pyrophoric, and explosive at higher temperatures. Since then, the method has undergone drastic improvements, such as the employment of alternative, safer and non-toxic precursors.
The SSPs route is one of the exemplary efforts which overcame issues such as that encountered in Murray and co-workers route. It substitutes the undesirable multiple precursors; reactions parameters remain similar, e.g. the use of the boiling coordinating solvent at elevated temperatures. The choice of these solvents depends on their ability to promote the decomposition of the SSP while efficiently passivating the surface of the nanomaterials formed [29]. Trindade and O’Brien demonstrated the use of cadmium (II) dithiocarbamate complexes as SSPs to synthesize CdS nanomaterials. The works of Revaprasadu and co-workers have reported the continued use of various metal dithiocarbamate precursors to synthesize corresponding metal sulfide nanomaterials [13, 14, 30]. The works further demonstrated particle size, shape and morphology control by varying reaction parameters such as precursor concentration, type of capping agent, reaction temperature and time. Good precursors for the deposition of thin films should have high purity to exclude substances which at times can act as dopants on thin films (external impurity) and it should be volatile to facilitate its transport from the reactor to the substrate in the furnace. Clean substrate surface is also crucial, to avoid intrinsic impurities [7]. Deposition of various 3d TMS thin films from the corresponding dithiocarbamates complexes by the aerosol-assisted chemical vapour deposition AACVD route, have been reported [31, 32].
The AACVD protocols follow 8 basic steps, namely: 1) precursor generation of reacting gaseous species, 2) transport media to deliver the precursor in the reaction chamber/furnace, 3) adsorption of the precursor into the hot substrate surface, 4) precursor atomic decomposition so as to give the required film, 5) movement of the atoms to a strong binding site, 6) nucleation which can lead to the growth of thin films, 7) desorption of the unwanted by-products, and 8) the removal of unwanted products. A schematic representation of the AACVD assembly is shown in Figure 1.2.
Figure 1.2: A schematic diagram of AACVD apparatus assembly.
1.2.1. Nickel Sulfide (NixSy) Nanoparticles and Thin Films
NixSy nanoparticles and thin films are in the class of important TMSs exhibiting potential properties for a number of applications including cathode materials for rechargeable lithium battery, catalysts, magnetic devices, solar coating, photoelectrochemical storage devices and a range of other important applications [33, 34]. It exists in species exhibiting different stoichiometries, such as Ni6S5, Ni7S6, Ni3S4, Ni3S2, and NiS. NixSy nanoparticles and thin films have been synthesized by following reaction routes such as the soft solution method, solvothermal process, UV irradiation, laser ablation, sol-gel method, chemical precipitation method, and dual or single-source-incorporated CVD [8, 34]. O’Brien and co-workers have reported the deposition of NixSy thin films by both low pressure (LP) CVD and AACVD methods utilising nickel dithiocarbamate SSPs [35,36]. Phases of NiS1.03 or a mixture of NiS1.03 and NiS were deposited on glass substrate with variation of temperature. Alam and co-workers observed that the thin films deposited from the nickel(II) xanthates SSPs by AACVD [24] at temperatures not exceeding 300 °C were either amorphous or sparsely deposited on the substrates. However, an increase in temperature afforded orthorhombic Ni7S6 and hexagonal NiS1.03 thin films [36].
The Ni-S phase diagram is known for its complexity [37, 38]. The wide range of different phases has prompted some of the phases to be widely researched, while other phases though equally important, are not widely studied. The different phases of NixSy results in different applications; for instance, NiS2, a p-type semiconductor with a band gap of about 0.5 eV, is among the phases widely researched due to its suitability for use in photoelectrochemical solar cells, IR detectors, catalysis and sensors [39]. On the other hand, Ni3S2 manifests metallic properties [40]; it changes into Ni3+xS2 phase at temperatures above 256 °C [41, 42]. Similarly, other NixSy phases are able to be transformed into other phases by varying temperature.
Some nickel-based SSPs have been effectively used to produce phase-pure NixSy nanoparticles and thin films; phase and size control can be achieved if the role of ligands and capping agents are well understood in the preparation method [34]. There are exemplary literature reports such as the synthesis of Ni3S4 affording different morphologies (wires, rods, spheres, and triangles) via a solvent thermolysis of nickel(II) thiobiuret SSPs [43]. The variation of temperature and capping agent concentration gave different morphologies for Ni3S4, while NiS only exhibited the nanowire morphology when the SSPs were dispersed in oleylamine then injected in a hot solvent, 1-octadecene.
1.2.2. Cobalt Sulfide (CoxSy) nanoparticles and thin films
CoxSy is commonly known to be a ferromagnetic material, thus, mostly used as an alloying compound in permanent magnets. It exists in basically two crystallographic systems, namely: hexagonal close-packed (HCP) and face-centered cubic (FCC). Normally HCP is stable at room temperature while FCC is stable at higher temperatures above 450 °C [44].
CoxSy, like NixSy, exhibits a number of phases such as Co4S3, Co9S8, CoS, Co3S4, Co2S3 and CoS2 [6]. Thus, there is a challenge of optimizing synthetic routes to obtain a specific, desired phase. However, hexagonal Co1-xS nanoparticles have been easily prepared by a solvent thermolysis of [Co{N(SCNMe2)2}3]. The particle size was found to be influenced by reaction time and nature of the capping agent used. When the [Co{N(SOCNiPr2)2}2] complex is decomposed under the same reaction conditions, a mixture of hexagonal and cubic cobalt sulfide exhibiting irregular shapes was obtained [45-47]. Thus, this demonstrates a need for continued search on fabrication routes and reaction conditions to achieve high quality nanoparticles of specific phase, particle size, shape and morphology.
The challenges in the fabrication of CoxSy nanomaterials and thin films can be attributed to the chemistry of Co. For example, challenges in obtaining monodispersed cobalt nanoparticles are caused by agglomeration as a result of nanoparticles having difficulties in trying to reduce their high surface energy, typical for metal nanoparticles. These are caused by attractive forces exerted on the Brownian motion and van der Waals forces in nanoparticles. For Co nanoparticles, such forces are even greater, due to the additional influence of their magnetic properties [48]. To overcome such challenges, strategies have to be designed so as to give high yield of the nanoparticles in the desired shape, size and morphology.
General strategies for obtaining monodispersed nanoparticles include separation of the nucleation and growth stages during synthesis, controlling the mode of growth and prevention of random growth of nanoparticles. Factors affecting the shapes of inorganic nanocrystals have been reported to involve competition between thermodynamic and kinetic factors [49]. According to this model, reactions at higher temperatures are controlled by thermodynamic parameters while at lower temperatures they are mostly under kinetic control. The work of O’Brien and co-workers have reported the shape control of cobalt sulfide nanoparticles, under different temperature conditions; a mixture of spherical and irregular shapes was obtained at thermolysis temperature of 230 °C, while only the spherical shape is obtained when the temperature is increased to 250 °C [35]. This phenomenon is due to the influence of thermodynamic parameters, although at higher temperatures, some other factors play a role, e.g. Ostwald ripening process [50]. Furthermore, the study equally indicated that the concentration of the reactants has an effect on the nanoparticles. This observation is explained by the delicate balance between kinetic and thermodynamic control, which is why anisotropic growth is observed at high flux of the monomer [51]. Deposition of CoxSy thin films have also been reported in literature. Solution (chemical bath deposition) technique and AACVD are the two mostly used approaches to prepare CoxSy thin films [52, 53].
1.2.3. Ternary Nanomaterials with Thiospinel Structure
Thiospinel structures of transition metal sulfides in the form of AB2S4 have emerged to be important materials for various applications in areas such as catalysis, energy; (generation, storage and conversion) and also in electronics [54]. The advantage of such structure lies in its unit cell having metal atoms A and B occupying the tetrahedral and octahedral sites, thus, enabling their position to shift within the cell, and hence giving rise to a normal and inverse spinel structure [55]. A normal spinel structure has A+2 in tetrahedral position and B+3 in an octagonal structure, where the general formula is AB2X4 (e.g. MgAl2O4). An inverse spinel structure has a general formula of B(AB)X4 with B+3 tetragonal position (B+2 B+3) in octahedral position. An example of an inverse spinel is CoFe2O4. A typical inverse spinel crystal structure is indicated in Figure 1.3, showing both octahedral and tetrahedral positions.
Figure 1.3: The (a) crystal structure of inverse spinel AB2O4 and (b) corresponding octahedral and (c) tetrahedral sites [56].
On the other hand, the normal thiospinel model of crystal structure is represented in Figure 1.4 below:
Figure 1.4: Model of normal spinel structure with the crystal model inset for NiCo2S4.
Figure 1.4 shows a normal spinel, crystal structure (Ni)A[Co2]BS4 where A still represent a divalent metal and B a trivalent metal. In such material both nickel and cobalt occupy tetrahedral sites (A) and octahedral site (B). If you consider a unit cell of such material, it would be only one eighth of A sites which will be occupied by Ni+2 and half of the B sites are occupied by Co+3 [57].
Such kind of structures in Figures 1.3 and 1.4, which have multivalence metals A and B, give thiospinel materials special properties in redox reaction, good electrical conductivities and an abundance of many sulphur vacancies at the surface of the material [58]; the bimetallic sulfide of cobalt and nickel form materials with the same structure. These properties can be harnessed to make devices which can generate or store energy [59]. Supercapacitors are considered best candidate as energy storage devices.
1.2.4. Magnetic properties
Structural, electronic and magnetic properties of a material are highly dependent on particle size as it approaches the nanoscale level [60]. Magnetic properties are further affected by other parameters such as particle shape, chemical composition, electronic bond-to-bond interactions and crystal structure of the material [61]. The evolution of nanomaterials has opened a window of opportunities to study magnetic properties with respect to those of the bulk counterparts [62].
Magnetism or magnetic properties of a material is a result of electronic interaction in an orbital when subjected to external magnetic field. This interaction gives rise to magnetic moments whose origin is mainly from electron spinning and the motion of the orbital itself. Hence, net magnetic moment is the sum of these interactions. Since all materials have electronic orbit and electronic interactions, all materials are thus considered to be magnetic in nature. The only difference is the magnitude of magnetism in particular material [63]. In some materials, there is no collective interaction from the atomic magnetic moments, whereas other materials have very strong interaction between atomic moments [64]. Magnetic properties of the material can be classified as being diamagnetic, paramagnetic, antiferromagnetic and ferromagnetic. Diamagnetic means having permanent magnetic moments because all electrons in their orbitals are fully paired, thus making materials (e.g. water, protein, fats, etc) tend to repel flux lines of applied electric field weakly. Paramagnetic materials have net magnetic moment due to unpaired electrons in the electronic structure to such material applying magnetic field would tend to orient the dipole moments. Antiferromagnetic property means the individual dipoles have magnetic moments due to antiparallel magnetic moment such that the net result for magnetism is zero); in such materials (mostly metal oxides), exchange of interactions favours antiparallel orientation of atomic magnetic dipole. Ferromagnetic materials have individual magnetic dipoles but since the dipoles have different magnitude then the net results is not zero. Thus, in ferromagnetic materials, quantum mechanical exchange interactions would favour parallel alignment of moments [65, 66].
Ferromagnetic materials have a tendency of forming what is known as domains, i.e. magnetization is in a uniform direction. Domain is a basic concept in magnetism. Although all domains are spontaneous, the direction of magnetization will vary from one domain to the other. The sum total magnetization becomes near to zero. Domain structure minimizes energy due to stray fields. When an electric field is applied on the domains, their structure changes; domains whose magnetization is in the direction of the field tend to grow while domains in other directions shrink. This characteristic is very important because if strong field is applied then magnetization can be saturated, thus, creating a single domain and the removal of the field does not necessarily change the domain structure to its original size, hence resulting into ‘magnetic hysteresis’ [67].
A general practice to identify the magnetism of a material is by studying and extracting information from its hysteric loop. Figure 1.5 shows an example of a hysteric loop and information which can be drawn from the loop. Figure 1.5 refers to a material (a) magnetized to saturation by the alignment of domains, (b) the magnetic material retains a considerable level of magnetization (useful for memory devices), (c) coercivity stage where magnetic field must be reversed and increased to a large value to drive the magnetization, (d) saturation points but towards the opposite direction, (e) material has been magnetized and the magnetic field can be dropped to zero because magnetic material will retain its history, and (f) the applied magnetic field intensity.
Figure 1.5: Hysteresis loop indicating information to be learned on magnetic properties of materials.
Magnetic nanoparticles (MNPs) are currently providing substantial knowledge on magnetic interaction and magnetic phenomenon at the nanoscale level. Most of critical magnetic lengths (whether single or multi-domain structure, ferrimagnetic or superparamagnetic) are found at the nanometer-size regime (1-100 nm). This makes size tuning in nanomaterials an important factor to equally tune the desired magnetic properties of material.
Single-domain nanoparticles are very important in the establishment of well-understood prototypes which are useful in probing more complex magnetic phenomena [68]. MNPs have important applications in pharmaceuticals and biomedical [69], magnetic fluid [70], data storage [71], magnetic resonance imaging [72], and environmental remediation [73], among others. All these applications require precise control of MNPs in the synthesis protocol such that the magnetic properties to suit specific applications are attained.
Non-precious metal chalcogenides with magnetic properties become desirable over other materials. They are of low cost, high abundance, less toxic and mostly possess unique magnetic, electric and optical properties [74- 77].
1.2.5. Energy Applications
Consumption of electricity is estimated to be growing annually at a rate of about 2.5%. Fossil fuel combustion account to almost 70% of the total power produced globally, while 11.7% comes from nuclear power plants. Hydroelectric power generates about 16% and wind turbines contribute to global power consumption by 2.5%. Photovoltaic produces 0.6% and other sources like geothermal, solar power give 1.4% to the total of the global power production. To reduce global warming, fossil fuel consumption should be replaced by renewable clean energy. The switch to clean energy is inevitable since energy is among the topmost problems facing human society today [78].
The International Renewable Energy Agency (IRENA), International Energy Agency (IEA) and the Renewable Energy Policy Network for the 21st century (REN21) have reported that almost 29% of total energy produced globally is consumed by the transportation sector [79]. The transport vehicles traditionally use internal combustion engines whose efficiency is approximately 25%. If the internal combustion engines would switch to grid electricity for wheels, its efficiency would increase up to 80 %. If then hydrogen gas can be used as fuel with the provision of grid electricity to wheels, it can play an important role in the transportation sector. This is because a car running on fuel cell-generated hydrogen coupled in connection to grid electricity to wheels has overall efficiency of ~28%. Thus, energy storage is an important element in this regard. Electricity storage is more important in renewable energy systems due to the nature of its source and the variability of electricity load required by the user.
1.2.5.1. Energy storage applications (supercapacitors)
Energy storage and energy generating materials have recently become a key area of interest among material scientists and engineers, in pursuit of meeting the technological demands of modern world industrialization and population increase [80]. Important energy storage devices include fuel cells, batteries and supercapacitors, amongst others. Some important applications of supercapacitors is their incorporation in electric vehicles, industrial equipment and memory backup equipment. Figure 1.6 represents basic components and operation of a typical supercapacitor. There are three classifications of supercapacitors, namely: double layer capacitors, pseudo-capacitors and hybrid capacitors, which have different applications as depicted in Figure 1.7.
Figure 1.6: Schematic representation of supercapacitors.
Figure 1.7: Schematic tabulation of different types of supercapacitors.
But what are supercapacitors (ultracapacitors) or electrochemical capacitors? These are energy storage systems which have recently gained popularity due to their potential application in energy storage devices. They can be thought of as a hybrid device which possesses qualities of both an ordinary capacitor and a battery, but technologically different from them. Ultracapacitor cells have a positive and negative electrode separated by an electrolyte, similarly to a battery. However, they store energy electrostatically (like a capacitor) rather than chemically (typical of a battery).
Supercapacitors have recently gained popularity because of safety issues related to Li-ion batteries which have been reported to catch fire or explode when used in devices like mobile smartphones, laptops and related technologies [81]. Supercapacitors have also proved to perform better than Li-ion batteries when it comes to charging time. Researchers have now focused their attention on supercapacitors so that the challenges faced by battery technology can be addressed and hence developing more reliable and safer source of energy [82].
Practical applications of supercapacitors are limited by the lack of high-performance electrode materials, which could be easily prepared at reasonably low cost [83]. Exemplary materials include graphene and carbon nanotubes [84], transition metal derivatives like (metal oxides, metal hydroxides, metal sulphates and metal sulfides), and conducting polymers [18]. Unfortunately, these materials are limited by poor conductivity, low capacitance, mechanical degradation and high cost. Relatively cheaper and high performance materials thus become the main objectives and goals in the field of sustainable energy. Ni, Co and Mn have been recognized as alternative candidates, due to their natural abundance [85]. Furthermore, researchers have highlighted the Ni-Co-S ternary system to exhibit better electrochemical performance than other alternative such as the Ni-Co-O system [18]. Hence, the former ternary system has been recently crowned a hot topic in the field of nanomaterials and related applications [86-91].
Researchers have prepared Ni-Co-S nanomaterials exhibiting various supercapacitance efficiencies, by using various synthetic protocols. Liu et al. [103] used the hydrothermal method to prepared flowerlike NiCo2S4 nanomaterials whose specific capacitance was 1516 Fg-1 at 2 Ag-1. Similarly, Li et al. [104] prepared NiCo2S4 materials with hollow sphere morphology using the hydrothermal method. The material exhibited a specific capacitance of 1753.2 F g-1 at 1A g-1 compared to 1036 F g-1 at 1 A g-1 reported by Shen et al. [105]. Furthermore, the nanomaterials achieved a rate capability of up to 77.8% from 1 to 10 A g-1. Microwave approach to prepare NiCo2S4 nanomaterial has been reported by Yan et al. [106], where they obtained a tremella-like morphology and a specific capacitance of 1410.7 F g-1 at 1 A g-1. The nanomaterial also showed a very high rate capability of 92.7% at 20 Ag-1. A specific capacitance of 10.82 F cm2 was obtained at 10 mA cm-2 [107] from NiCo2S4 nanomaterial electrodeposited on nickel foam.
Figure 1.8 depicts the performance chart for commonly available technology for energy storage devices. Supercapacitors are, by far, the attractive devices in the field. For supercapacitors to be highly efficient, they need materials that will have high power density, fast charge-discharge kinetics and long-life cycle as compared to other battery counterparts [108].
Figure 1.8: A comparison chart for performance of some energy storage devices.
Traditionally, performance and durability of both batteries and supercapacitors largely depend on electrodes which are conductive and can manage to survive the hurdles of charging cycles. Very often, electrodes have polymeric binders which stabilize the electrode materials and subsequently the conductive charges during operation or charging cycles. However, binders fail to perform well because they are not mechanically strong enough to withstand the stress experienced when the device is operating, this can lead to cracking of electrode which automatically will affect conductivity within the electrode. Search for reliable material with good electrical conductivity and remarkable mechanical properties are envisaged to enhance performance of energy storage devices. Another solution to manage the physical deterioration of storage devices is to increase surface area of electrodes thereby improving the chemical reaction taking place within the device. Nanomaterials improve drastically with regards to surface area, hence ideal for electrode materials [109].
The charge storage mechanism for any supercapacitor is analyzed based on its energy and power densities defined by equation (1) below:
Where C is the capacitance in Farad, V is the operating potential in volt; R is the resistance in Ohm. Power density is improved with an increase in the potential range across the supercapacitor and reducing the resistance, but capacitance depends on the type of electrode material used and the storage mechanism [110].
Evaluation of supercapacitors employs similar criteria for energy storage systems, i.e. cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). These are employed to evaluate parameters like specific capacitance, energy density, power density, series resistance, cycling life and rate capability. [111-113]. It is these electrochemical properties which are used to characterize supercapacitors (electrode materials) [114-116]. Collectively, these techniques complement each other, giving a broad understanding of the energy storage mechanism and the surface phenomena between the electrode and the electrolyte.
1.2.5.2. Hydrogen evolution reaction (energy generation)
Hydrogen is a good candidate to substitute the use of fossil fuels due to its high energy density and it is pollution-free [117]. Hydrogen, as abundant, clean and renewable energy source, is a suitable candidate to replace the depleting fossil fuels in the near future [118, 119].
There are two important technologies for industrial production of hydrogen, namely: electrochemical and photoelectrochemical electrolysis of water. While these approaches are environmental-friendly, the efficiency of sunlight-driven water splitting reaches only 12.3% for generating H2. This calls for the use of electrolysis of water as the most viable route towards scaling up in the production of H2.
The modern large-scale production of H2 depends on the method of steaming methane gas, as shown in the following equations:
Equation (2) involves high energy input which goes up to ∆H298 = -206 kJ mol-1 while equation (3) needs an energy input of ∆H298 = -41 kJ mol-1. These reactions need high energy input (heat) and produce huge amount of carbon dioxide into the atmosphere, hence, contributing to global warming. This cannot be considered to be a favourable green route for the production of H2 [120]. The greener route would be the electrolysis of water which produces H2 with no greenhouse gas emissions.
1.2.5.3. Water Splitting Reaction
Hydrogen has been regarded as the cleanest chemical fuel and hence it is among the major sources of sustainable energy [121]. It is thus considered a sustainable energy carrier that can replace depleting fossil fuels and also tackle environmental problems that comes with fossil fuels [122, 123].
Hydrogen is the most abundant element in the universe, accounting for 75% by mass though it is not commonly found in its pure form due to its high reactivity. Hydrogen is also abundantly present in water, thus, any endeavor to split water into H2 and O2 would be a promising means to renewable and sustainable energy. Progress in water splitting reactions has been achieved by use of Pt as cathodic material. However, poor abundance and high cost for Pt has limited its use for cost-effective hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [124, 125].
Search for a replacement of Pt as cathodic material for HER would need an understanding of the mechanism that is undertaken to break water into H2 and O2. The cathodic reaction is represented in equation (4) below:
The burning of hydrogen would result into equation (5). The energy produced by burning one molecule of hydrogen is given by ∆H = -286 kJ/mol which can be equated to about 142 MJ/Kg energy density at 700 bar pressure. This is far greater than the energy density for methane which is just 55.5 MJ/Kg when in a compressed state. Such huge amount of energy can be used in so many other technologies such as cars running on H2 engines.
The OER is another half electrochemical cell for water splitting process. Equation (6) represents this process which is driven by four electrons and hence becoming much more complicated if compared to HER reaction.
The best catalysts for this reaction are iridium and ruthenium oxides, particularly in acidic media. However, their natural abundance is very limited. They are also costly and they cause electrochemical instability during reaction, hence, rendering them ineffective material for OER [126].
Extensive work has been carried out to look for a catalyst that can replace Pt for the HER/OER reaction. Layered transition metal dichalcogenides like WS2, MoS2, MoSe2 and WSe2 have been shown to have higher catalytic activity to HER [127-129]. Other potential candidates are metal phosphides and metal alloys such as Ni3S2, CoS2 and NiCo2S4 which seem to have surpassing character for the catalytic performance in water splitting reactions [130, 131].
1.3. Statement of the research problem
Dithiocarbamates are versatile ligands, i.e. they provide endless opportunities regarding their derivatization. In organic synthesis, they are important synthetic intermediates; they also can bind to a range of transition metals of different oxidation states to form corresponding metal complexes [132]. As a result, using dithiocarbamates as SSPs for nanomaterials synthesis and thin films broadens the scope that needs to be explored [14,36,45]. Nanomaterials synthesized by using selected dithiocarbamate metal complexes give specific pure phases which can be tuned to a particular technological application. Energy crisis around the globe has prompted researchers to search into targeted dithiocarbamate ligands, which will ultimately produce nanomaterial applicable in magnetism and energy application.
According to various energy specialists, the fossil-fuel reserve is rapidly depleting and it can support a limited number of years for petroleum, 60 years for natural gas and ~156 years for coal [133, 134]. The predicted scarcity of fossil-fuel reserve coupled with the increasing energy consumption threatens energy and economic security worldwide. Thus, to search into the use of nanostructured materials which has proved to be promising candidate for synthesis of supercapacitors with higher efficiency and making electrode nanomaterial that can catalyse electrolysis of water for HER and OER reaction is highly needed as a means to solve the energy crises worldwide and hence alleviate the severe environmental threats associated with the use of fossil fuels.
The main motivation of this work is to attain a great energy security by searching into nanomaterials that can reduce the dependence on depleting nonrenewable sources of energy. This can be achieved by synthesizing nanomaterials which would effortlessly generate and/or store energy, hence, be able to sustain our energy economy and environment. A secondary objective is to prepare nanoparticles and subsequently study their magnetic properties. To achieve this goal, some methods need to be devised so as to obtain different, pure phases of nickel and cobalt sulfide nanoparticles and thin films produced through variation of reaction parameters. This would thus allow their evaluation in different applications with aims of expanding contributions to the body of knowledge in the field of nanosciences. Selected dithiocarbamate ligands and their Ni(II) and Co(III) complexes are synthesized, characterized fully and evaluated as single source precursors to fabricate binary and ternary nanoparticles and thin films. This provides an opportunity to evaluate the as-prepared nanoparticles and thin films in magnetic, H2/O2 evolution reactions and supercapacitance applications.
1.4. Scope of the work
The technological application of TMS materials in their thin film and monodispersed nanoparticulate forms have motivated continued research on novel and/or improved SSPs. In this research project, binary and ternary nanomaterials (including thin films) of cobalt and nickel sulfides were fabricated from the corresponding heterocyclic dithiocarbamate SSPs. The SSPs were derived from four different dithiocarbamate ligands namely: piperidine, tetrahydroisoquinoline, 4-morpholine and 1-ethylpiperazine. Reaction parameters which include temperature, time, nature of capping agent, among others, were investigated to gain particle size, shape and morphology control of the nanomaterials and thin films. Well-suited capping agents capable of stabilizing and passivating the nanoparticles during solvent thermolysis reactions are key research areas in nanotechnology; oleylamine (OLA), hexadecylamine (HDA), and dodecylamine (DDA) were used to passivate the fabricated nanoparticles. Thin films were deposited from the solutions of complexes in chloroform by the AACVD technique. The resulting nanomaterials were thoroughly investigated, where applicable, for their efficacy in applications which include magnetism, H2/O2 evolution and capacitance.
1.5. Aim and objectives of the work
The aim of this project was to explore the use of different heterocyclic dithiocarbamate complexes as SSPs for the fabrication of Ni and Co sulfide nanoparticle and thin films for magnetic and energz applications.
The objectives included:
1. Synthesis and characterization of Co and Ni piperidine, tetrahydroisoquinoline, 4-morphiline and 1-ethylpiperazine dithiocarbamates complexes.
2. Synthesis and characterization of DDA, HDA and OLA-capped NixSy and CoxSy nazoparticles by the solvent thermolysis of the complexes.
3. Synthesis of multiple-phase NiS thin films by AACVD at different decomposition temperatures
4. Synthesis of Ni and Co-rich ternary NixCo3-xS4 nanoparticles through the solvent thermolysis method using a dual molecular precursor route.
5. Establishment and effectiveness of NixCo3-xS4 ternary material in the electrocatalytic and electrochemical reactions, i.e. energy generation and energy storage application.
1.6. Thesis layout
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