SYNTHESIS AND CHARACTERIZATION OF COBALT DOPED NICKEL OXIDE NANOPARTICLES PREPARED BY THE MODIFIED OXALATE COPRECIPITATION ROUTE.

ABSTRACT

NiO materials are suitable for super capacitor applications and the relevant properties can be enhanced by defect engineering. This can be achieved by doping which changes either the morphology, and /or the microstructure. Cobalt doped nickel oxide (Ni_1-xCo_xO_1-δ, where x = 0.1, 0.3, 0.5, 1, 3, 5 40 and 50) nanostructured materials were synthesized from chloride salts by a facile green co-precipitation route via a single molecular precursor and using carambola fruit juice as the solvent. The precursors were characterized by Thermogravimetric Analysis (TGA) and Fourier Transform Infra-Red Spectroscopy (FT-IR). Base on the TGA, the precursors were decomposed at 450 ºC for four hours to constant weight and the residue characterized by Powder X-ray Diffraction (PXRD) and Scanning Electron Microscopy (SEM). PXRD pattern showed prominent peaks at 2𝜃 values of 37.5, 43.27, 62.82, and 75.42, corresponding to the database 2𝜃 values of JCPDS card # 47-1049 suggesting phase pure nickel (II) oxide.  The particle size determinations from XRD results using Sherrer’s equation with size ranging from 7.3 to 7.8 nm revealed that the particle were all in the nano size range. FT-IR results showed peaks at ~450 cm⁻¹ which should be attributed to the metal oxide. TGA/DTG revealed that decomposition occurred in three different stages. The SEM results revealed that the morphology of the particle varied with the percentage mole fraction doping of cobalt ions from rough and continuous to smooth and discontinuous nano fibers. The EDS indicated the presence of excess oxygen atoms in the micro structure of the synthesized samples. Variability in particle morphology and presence of the excess Oxygen atoms in the micro structure of the cobalt doped Nickel oxide nano particles synthesized in this study is indicative of the fact that the carambola fruit juice method turns out to be a choice of interest because of its simplicity and ease of use, solubility advantage, eco-friendliness, economy, scaling capacity and control over the particle morphology even at high percentage doping fractions of cobalt.

CHAPTER ONE

 1.1 INTRODUCTION

The rapid growth in the population especially in Africa has not been matched with the energy supply. Like other parts of the world, Africa has experienced a technological boom particularly in the continent’s burgeoning cities which have profound implications for the energy sector both regionally and globally (Report, 2019).

The demography in Cameroon and Africa indicates that, the greater part of the population is found in enclave zones where the usual energy grid fails to reach them due to cost.  In large cities like Yaounde, Baffousam, Bamenda and Douala with difficult topographies, transportation of electricity to the hill tops has proven to be very expensive. The stage is thus set for a new wave of dynamism among African policy makers and business communities, with fallen costs of key renewable technologies opening up avenues for innovation and growth. Chief among the challenge is providing universal access to reliable, modern, affordable and sustainable energy.

This problem can easily be solved by using energy sources which can be managed by small communities and even single households.  One of such energy source is solar energy. Solar energy is sustainable, clean, cheap, and can be generated anywhere provided there is sunlight. The energy however needs to be harvested from sunlight and stored, or used directly. In both cases, the harvesting and storage are done by specific materials (transition metal oxides) such as NiO. This oxide however has its limitations and thus can be modified by incorporating impurity elements at proportions dosage which leads to no changes in the physical structure of the material. In this case the material is called the dopant. On the other hand, if the impurity elements is at a dosage such that the physical property and structure changes, it is called a system.

Therefore, engineering of new material for energy is a function of the ratio of the constituent elements to the impurity element.

• BACKGROUND

Growth in global economy has considerably increased the demand for energy consumption and has led to the development of alternative uninterrupted energy sources and storage devices for supply reduction (Kate R. S., Deokate R. J, 2020). Combustion of fossil fuels which are very rich form of energy containing around 30– 50 MJ of energy per kilogram results in the emission of CO₂ , CH₄, N₂O etc. in the atmosphere which trap the solar radiation in the atmosphere (Roy A., 2018) resulting to environmental problems such as global warming and climate change. The rapid depletion level of fuel reservoirs along with the increasing effect of environmental pollution as a result of the gases released are the two most important concerns of twenty-first century.

It is estimated that it will take around 40 more years to run all the known oil deposits dry leaving the whole world into complete darkness considering the rate at which fossil fuels is depleted. (Apurba Ray, 2018).

Focusing on the alternative green energy sources which are sustainable will further reduce the world’s hunger for fossil fuels while maintaining the same living standards.

The most promising sustainable energy sources are: solar energy, wind energy, ocean energy which is done by smart materials with properties like perovskite solar cells. Although these sources are capable of meeting the world’s energy requirement, their intermittent nature is an unavoidable problem which significantly stimulated the motivation of research on the energy storage systems. (Apurba Ray, 2018)       

Today a variety of energy storage and conversion devices are available such as batteries, conventional capacitors, fuel cell, and super capacitors. Among such energy storage systems electrochemical capacitors or super capacitors have drawn attention as one of the most promising energy storage systems because of their high power density, less charging time and long life span though having moderate energy density 10–15 mWh/g which is still very less compared to batteries. Several research associations in the world are trying to enhance the energy density and overall life span of the device by suitably choosing different electrode materials. (Wang J., 2017)

In recent years, nano materials like metal oxides, sulfides, hydrides and conducting polymers received great attention in energy storage applications. NiO with wide range of applications is one of the most commonly used transition metal oxides. Furthermore it is considered to be a model semiconductor with p-type conductivity films due to its wide band gap (Sato H., 1993) . NiO is a promising alternative transition metal electrode material for energy storage applications due to its low cost, easily available, good electrochemical stability, abundant in nature and high super capacitance in a relatively large voltage window.

Uniform sized with well dispersed NiO nanoparticles as a kind of functional material has attracted extensive interests due to its novel optical electronic, magnetic, thermal and mechanical properties and potential applications in catalysis, battery electrodes, gas sensors, electro chromic films, photo electronic devices, magnetic materials and so on (Alcantara R., 1998) (Yoshio M., 1998).

In order to enhance the capacitive behavior of nano materials such as maximum current density, good reversibility, increased electronic conductivity, good ion storage capacity and cyclic stability and durability; doped metal oxides have been broadly used because of the confinement of electronic states and the tendency to occupy the sites in crystalline structure. (Kate R. S., Deokate R. J, 2020). Doping of cobalt in NiO without causing much lattice strain is possible because of several reasons such as; both NiO and CoO have a cubic structure with lattice parameters 4.17A° (Walls B., 2021) and 4.26A° (Villars, 2016) with low lattice mismatch between them and also Ni and Co have similar ionic radii.

It is important to note that many studies have reported on high amounts of doping fractions, where an increase in doped Cobalt fraction has resulted in a decrease in some properties.

For the synthesis of nanoparticles, various chemical and physical methods have been used which include; high-temp solution phase, thermal decomposition, hydro-thermal micro-emulsion, and reduction, etc. (Remya V., 2017). However, green synthesis of nanoparticles is considered a significant branch of nanotechnology (Patra J. K., 2016) (Phukan A., 2017). This method is cost-effective and eco-friendly as compared to the conventional synthesis methods, where chemical additives, high-temperature pressure, and energy are used. Therefore, it is necessary to develop and use safe methods that must be low cost, efficient, and none hazardous, and environment friendly. Due to this reason, many researchers adopted the green synthesis method to fabricate nanoparticles.

Materials that are derived from plants are used to prepare nanoparticles and are also the best alternative to the chemical and physical method. (Yameen A., 2018).

 In this research we will be using modified oxalate route, with Averrhoa carambola fruit juice as solvent and doping cobalt at very lower concentrations than reported in the literature for the synthesis of Nickel oxide.

Research Problem

In spite of all the progress made, the synthesis of Co-doped NiO nano materials of controlled size and shape remains a challenge.

Tailoring the size and shape can be done by an appropriate choice of the synthesis methods and conditions. Fine-tuning of the morphology is of key importance, since the surface energy, the electronic structure, bonding, and the chemical reactivity of nano materials are all directly related to surface morphology. Ekane et al., (2017) reported that the purity and stoichiometry (quality of the nanoparticles generated) depend on the synthesis route.

Some methods of synthesis of Cobalt doped NiO method are; hydrothermal process (Xu Hui, 2015), solvo-thermal method (Trang Vu Thi, 2015), chemical bath deposition (Mai Y. J., 2011), thermal decomposition (Liang D., 2016), by thermal decomposition of the prepared nickel hydroxide precursor (Wang X, 2006) with some requiring use of solvents that maybe environmentally unfriendly.

 A Carambola fruit juice has been used as a precipitating agent (Ekane et al., 2017) to synthesis single molecular precursors with similar or better results. Though the fruits cannot be found all the time, and sometimes the concentration of oxalate is low due to seasonal changes; it is fortunately that oxalate salts which have a solubility problem are very soluble in the fruit juice. Therefore, it is possible to prepare soluble oxalate solutions using Averrhoa fruit juice as a solvent which is renewable and environmentally friendly.

Lastly, Co-doped NiO has been synthesized before but at relative high doping fractions. It has been believed that at lower doping fractions of about 0.5%, the materials synthesized have better modified properties (Tedjieukeng K., 2018).

 Objectives

The main objective

The main objective of this project was to synthesis cobalt doped NiO by modified oxalate co-precipitation route using A. Carambola fruit juice as solvent to generate phase pure nano-composites by thermal decomposition of corresponding single molecular precursors.

Specific Objectives

In order to achieve this, our basic objectives were to:

• Harvest the fruits and prepare the oxalate juice.
• Synthesize the single molecular metal oxalate precursors and characterize the precursors by FT-IR, TGA
• Thermally decompose the molecular metal oxalate precursors to metal oxide nano composite and characterize the products (PXRD, FT-IR, SEM).

The influence of the concentration of the dopant on the composition and morphology of precursor and calcined samples were investigated.

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