SYNTHESIS AND CHARACTERIZATION OF TIN IV OXIDE (SnO2) NANOPARTICLES FOR ENERGY APPLICATIONS BY RANIA AFIA NUAMAH (ID NO.: 10336720) UNIVERSITY OF GHANA, LEGON COLLEGE OF BASIC AND APPLIED SCIENCES DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL MATERIALS SCIENCE AND ENGINEERING DEGREE. 2016 University of Ghana http://ugspace.ug.edu.gh ii SYNTHESIS AND CHARACTERIZATION OF TIN IV OXIDE (SnO2) NANOPARTICLES FOR ENERGY APPLICATIONS BY RANIA AFIA NUAMAH (ID NO: 10336720) UNIVERSITY OF GHANA, LEGON COLLEGE OF BASIC AND APPLIED SCIENCES DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING, THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL MATERIALS SCIENCE AND ENGINEERING DEGREE. JULY 2016 Copyright © 2016 University of Ghana All rights reserved Email: afianuamah@yahoo.com University of Ghana http://ugspace.ug.edu.gh iii DECLARATIONS Candidate’s Declaration I hereby declare that this MPhil thesis which is the result of my own original research under strict supervision was prepared in accordance with the University of Ghana’s academic regulations and that no part of it has been presented for another degree in this University or elsewhere. Candidate’s name: ........ Miss Rania Afia Nuamah................................................................ Signature: ...........................................Date: ............11th July, 2016......................................... Supervisors’ declaration We hereby declare that the preparation and presentation of this MPhil thesis were supervised in accordance with the guidelines on supervision of MPhil thesis laid down by the University of Ghana. Supervisor: ……….Dr. David Dodoo-Arhin………………………………………………….. Signature.......................................................... Date: …... 11th July, 2016…………………….. Co-supervisor: Prof. Pushpendra Kumar Jain………………………………………… Signature……………………………Date: …….11th July, 2016……………………………… Head of Department’s declaration I hereby declare that this MPhil thesis has been prepared, supervised and accepted in accordance with the guidelines on the MPhil thesis laid down by the University of Ghana. Head of Department’s name: ………Dr. David Dodoo-Arhin…………………....................... Signature...........................................Date: 18th July, 2016……………………………………. University of Ghana http://ugspace.ug.edu.gh iv ABSTRACT Electrochemical capacitors (ECs) or Supercapacitors (SCs) are energy saving devices which have excellent properties that include high power density, long cycle life, low temperature sensitivity and low maintenance cost. However these devices have lower energy densities than conventional batteries. To improve on its energy density, SnO2 which is a metal oxide was considered as an electrode material because of its chemical inertness and thermal stability. SnO2 nanoparticles were synthesized using the hydrothermal and water-in-oil microemulsion techniques. Well crystalline particles with different morphologies and crystallite size in the range of 2nm to 10nm were obtained by using Urea and Soduim Borohydride as reducing agents, Dioctyl Sulfosuccinate sodium salt (AOT) and Cetyl Trimethyl ammonium bromide (CTAB) as the surfactants in the hydrothermal techniques. X-ray diffractometry, Scanning Electron microscopy, Energy Dispersive X-ray spectroscopy and Fourier Transform Infrared spectroscopy revealed the presence of tin oxide (SnO2). The electrochemical properties were investigated using cyclic voltammetry, electrochemical impedance spectroscopy and potentiostatic charge-discharge in aqueous KOH electrolyte. The SnO2 electrode material showed properties as a pseudocapacitor with a maximum capacitance value of 1.6 Fg-1 at a scan rate of 5 mVs-1, an efficiency of 52 % at a current of 1mA and a maximum capacitance retention of about 40% after 10 cycles at a current of 1 mA. From the results obtained it can be concluded that Sodium borohydride is a better reducing agent in the synthesis of SnO2 nanoparticles. Also, although the crystallite size of the SnO2 particles were small they exhibited very low pseudocapacitance as a result of the low conductivity of the nanoparticles and the lack of electrochemical active sites. To improve on University of Ghana http://ugspace.ug.edu.gh v the capacitance of the SnO2 nanoparticles, conductive carbon can be added and composite SnO2 composite materials with carbonaceous materials can also be an alternative. University of Ghana http://ugspace.ug.edu.gh vi ACKNOWLEDGEMENTS My sincere gratitude goes to the almighty God for giving me life and strength to be able to complete this MPhil research thesis work. My profound gratitude goes to the African Materials Science and Engineering Network (AMSEN), Science Initiative Group of the Regional Initiative on Science Education (SIG-RISE) with support from the Carnegie Corporation of New York for funding my MPhil program. I also thank my supervisors Dr. David Dodoo-Arhin (supervisor- University of Ghana) and Prof. P.K Jain (co-supervisor- University of Botswana) for their immense contribution towards my MPhil research thesis work. I thank all the lecturers and students of the Department of Materials Science and Engineering and the entire School as a whole for making my years of studies at the University of Ghana a successful one. University of Ghana http://ugspace.ug.edu.gh vii DEDICATION I dedicate this thesis to my parents, Mr Yaw Opuni and Madam Alice Asare and my siblings for their advice, support and care throughout my life. I also dedicate this thesis to everyone who has played an important role in my life. University of Ghana http://ugspace.ug.edu.gh viii TABLE OF CONTENTS DECLARATIONS .................................................................................................................... iii ABSTRACT .............................................................................................................................. iv ACKNOWLEDGEMENTS ...................................................................................................... vi DEDICATION ......................................................................................................................... vii LIST OF TABLES .................................................................................................................... xi LIST OF FIGURES .................................................................................................................. xii LIST OF ACRONYMS ........................................................................................................... xiv CHAPTER ONE ........................................................................................................................ 1 1.0 Introduction ................................................................................................................. 1 CHAPTER TWO ........................................................................................................................ 4 2.0 Literature Review ........................................................................................................ 4 2.1 Nanoscience and Nanotechnology ............................................................................... 4 2.1.1 Properties of Nanomaterials ................................................................................. 4 2.2 Structure and main bulk properties of Tin Dioxide ..................................................... 5 2.2.1 Comparison between stannic (SnO2) and stannous oxide (SnO) ......................... 7 2.3 Applications of tin dioxide (SnO2) .............................................................................. 8 2.3.1 Transparent conductors ........................................................................................ 8 2.3.2 Heterogeneous catalysis ....................................................................................... 9 2.3.3 Solid state gas sensors ........................................................................................ 10 2.4 Electrochemical Capacitors (Supercapacitors) .......................................................... 10 2.4.1 Electric Double Layer Capacitors (EDLC’s) ..................................................... 12 2.4.2 Pseudocapacitors/ Faradaic Supercapacitors (FS) .............................................. 14 2.4.3 Factors affecting the Capacitance of Supercapacitors ........................................ 15 2.4.4 Electrolyte .......................................................................................................... 15 2.4.5 Comparison of EDLC and Pseudocapacitor [23] ............................................... 18 University of Ghana http://ugspace.ug.edu.gh ix 2.4.6 Fabrication and Manufacturing of Electrochemical Supercapacitor (ES) ......... 18 2.5 Cyclic voltammetry ................................................................................................... 19 2.6 Constant Current Charge/Discharge (CCCD) ........................................................... 20 2.7 Electrochemical Impedance Spectroscopy (EIS) ...................................................... 20 2.8 Equivalent Series Resistance ..................................................................................... 21 2.9 Synthesis of Metal Oxides ......................................................................................... 22 2.9.1 Chemical Reduction ........................................................................................... 22 2.10 Synthesis of tin dioxide nanostructures ..................................................................... 24 2.10.1 Hydrothermal Synthesis ..................................................................................... 24 2.10.2 Instrumentation in hydrothermal processing of nanomaterials .......................... 26 2.10.3 Influence of Surfactants on particles .................................................................. 27 2.10.4 Microemulsion Synthesis method ...................................................................... 28 2.11 Characterization Techniques ..................................................................................... 30 2.11.1 Scanning Electron Microscopy (SEM) .............................................................. 30 2.11.2 X-Ray Diffraction (XRD) .................................................................................. 31 CHAPTER THREE .................................................................................................................. 35 3.0 Experimental Design ................................................................................................. 35 3.1 Materials .................................................................................................................... 35 3.2 Equipment .................................................................................................................. 35 3.3 Synthesis of SnO2 Nanoparticles ............................................................................... 36 3.3.1 Water- In – Oil (reverse micelle) Microemulsion Synthesis of SnO2 particles . 36 3.3.2 Hydrothermal Synthesis ..................................................................................... 37 3.4 Chemical characterization of SnO2 nanoparticles. .................................................... 42 3.4.1 X-Ray Diffraction .............................................................................................. 42 3.4.2 Scanning Electron Microscopy (SEM) .............................................................. 43 3.4.3 Fourier Transform Infrared (FTIR) Spectroscopy ............................................. 44 University of Ghana http://ugspace.ug.edu.gh x 3.5 Preparation and Electrochemical Characterization of SnO2 Electrodes .................... 44 3.5.1 Pretreatment of the Ni foam substrate ................................................................ 44 3.5.2 Preparation of the SnO2 Working Electrodes. .................................................... 45 CHAPTER FOUR .................................................................................................................... 47 4.0 Results and Discussions............................................................................................ 47 4.1 pH of Precursor Solutions.......................................................................................... 47 4.2 X-Ray Diffraction Analysis ....................................................................................... 48 4.3 Fourier Transform Infra-Red Spectroscopy (FTIR) .................................................. 52 4.3 SEM micrographs of the as-synthesized SnO2 nanoparticles .................................... 59 4.4 Energy Dispersive X-ray Spectrum (EDX) Analysis of the as-synthesized SnO2 nanoparticles. ........................................................................................................................ 62 4.5 Electrochemical Characterization of the obtained SnO2 nanoparticles ..................... 65 CHAPTER FIVE ...................................................................................................................... 76 5.0 Conclusions and Recommendations .......................................................................... 76 5.1 Conclusions ............................................................................................................... 76 5.2 Recommendation ...................................................................................................... 76 REFERENCES ......................................................................................................................... 78 University of Ghana http://ugspace.ug.edu.gh xi LIST OF TABLES Table 2.1: Comparison of EDLC and Pseudocapacitor Supercapacitors [21] ......................... 18 Table 3.1: Sample groups and designation for the hydrothermal synthesis ............................. 39 Table 4.1: pH of precursor solutions for as-synthesized SnO2 nanoparticles by different treatments. ................................................................................................................................ 47 Table 4.2: EDX atomic % of the synthesized SnO2 nanoparticles ........................................... 64 Table 4.3: Capacitance values estimated form the Cyclic Voltamogram ................................ 66 Table 4.4: Internal resistance values of the SnO2 nanoparticles. ............................................. 71 Table 4.5: Efficiency values of the SnO2 nanoparticle electrodes ........................................... 72 Table 4.6: Capacitance retention values of the SnO2 electrode ............................................... 72 University of Ghana http://ugspace.ug.edu.gh xii LIST OF FIGURES Figure 2.1: Crystal structure of tin dioxide (SnO2) [15] ............................................................ 6 Figure 2.2: Sn-O phase diagram [15] ......................................................................................... 8 Figure 2.3: Ragone plot showing the specific power against specific energy for different Electrochemical Devices [18]. ................................................................................................. 11 Figure 2.4: Schematic Diagram of an EDLC [21] ................................................................... 14 Figure 2.5: SEM Machine ........................................................................................................ 30 Figure 2.6: Illustration of the Bragg’s law ............................................................................... 31 Figure 2.7: XRD Machine ........................................................................................................ 34 Figure 3.1: The process flow chart used for synthesis of nanosized SnO2 powders by the reverse micelle microemulsion synthesis technique. ............................................................... 37 Figure 3.2: The process flow chart used for synthesis of nanosized SnO2 powders by hydrothermal synthesis. ............................................................................................................ 40 Figure 3.3: Synthesis process of SnO2 particles ....................................................................... 41 Figure 3.4: Panalytical Empyrean X-ray Diffractometer ......................................................... 42 Figure 3.5: Zeiss ultraplus FESEM .......................................................................................... 43 Figure 3.6: Vertex 70v (Bruker) FTIR spectrometer ............................................................... 44 Figure 3.7: SnO2 electrode making and electrochemical testing process ................................ 46 Figure 4.1: XRD patterns of SnO2 nanoparticles ..................................................................... 48 Figure 4.2: SnO2-CT modelled using WPPM .......................................................................... 50 Figure 4.3: Lognormal Size Distribution Analysis of the SnO2 particles ................................ 51 Figure 4.4: FTIR for the water-in-oil microemulsion synthesis (E) of SnO2 nanoparticles .... 52 University of Ghana http://ugspace.ug.edu.gh xiii Figure 4.5: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent (A). .................................................................................................................................. 53 Figure 4.6: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent and AOT as the surfactant (AA) ..................................................................................... 54 Figure 4.7: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent and CTAB as the surfactant (AT). .................................................................................. 55 Figure 4.8: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent (C) ............. 56 Figure 4.9: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent and AOT as the surfactant (CA) ................................................................................................................... 57 Figure 4.10: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent and AOT as the surfactant (CA) ................................................................................................................... 58 Figure 4.11: SEM Micrographs of SnO2 nanoparticles of the various treatments ................... 60 Figure 4.12: EDX spectrum of SnO2 nanoparticles of the various treatments ......................... 63 Figure 4.13: Cyclic Voltagrams of the as-obtained SnO2 nanoparticles at different scan rates. .................................................................................................................................................. 66 Figure 4.14: Specific capacitance of the synthesized SnO2 particles ....................................... 67 Figure 4.15: CCCD of the SnO2 nanoparticles at different current densities .......................... 70 Figure 4.16: Internal resistance estimated from ohmic drop of the CCCD curves. ................. 71 Figure 4.17: Capacitance retention of the SnO2 electrodes after 10cycles .............................. 73 Figure 4.18: The Nyquist plot for the as-synthesized SnO2 nanoparticles ............................... 74 University of Ghana http://ugspace.ug.edu.gh xiv LIST OF ACRONYMS AOT Dioctyl Sulfosuccinate sodium salt BET Brunner Emmet Teller CCCD Constant Current Charge/Discharge CNT’s Carbon nanotubes CTAB Cetyl Trimethyl ammonium bromide CV Cyclic voltammetry ECs Electrochemical Capacitors EDLC Electrical Double Layer Capacitance ESR Equivalent Series Resistance SnO2 Tin (iv) oxide TCOs Transparent Conducting Oxides TEMABF4 Triethylmethylammonium tetrafluoroborate W/O Water-in-oil XRD X-Ray Diffraction Zdia Real axis University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE 1.0 Introduction For some time now, considerable efforts have been made to develop new energy storage devices which have high energy and high power density to meet the worlds demand for clean energy. Supercapacitors (SCs) which are also known as Electrochemical Capacitors (ECs) are a kind of promising energy storage devices. They can produce a large amount of energy in a short period of time and they are usually preferred for energy storage systems due to their excellent cyclability and very good power performance as compared to conventional capacitors and batteries [1]. Supercapacitors currently, are used in a wide range of consumer electronics, memory back-up systems, and industrial power and energy management. A new development as far as supercapacitors are concerned is their use in emergency doors on Airbus A380. Another promising application is their use in low-emission hybrid electric vehicles and fuel cell vehicles to serve as a temporary energy storage device with a high- power capability to store energies when braking. Supercapacitors can be grouped into two types, (Electrical Double Layer Capacitor and Pseudocapacitor), according to their charge/discharge mechanisms [2]. The Electrical Double Layer Capacitor stores energy by electrostatic adsorption/desorption between polarized solid electrode vertical line liquid electrolyte interfaces, and Pseudocapacitor stores energy by surface faradaic redox reactions on the interface of the electroactive material and the electrolyte. The large capacitance of the Pseudocapacitor is possible by electrode material that can be reversibly oxidized and reduced over a wide potential range [2-4]. The major electrode materials that are used in supercapacitors are those made of graphene and other carbonaceous materials (carbon nanotubes, activated carbon, etc.). The desire to use carbon materials come from the fact that they store the charges electrostatically using University of Ghana http://ugspace.ug.edu.gh 2 reversible adsorption of ions of the electrolyte onto active materials that are electrochemically stable and have a high accessible surface area. However, their low specific charge has made scientists look into finding new materials which are environmentally friendly and have high energy storage capacity [5]. Transition metal oxides have also been studied widely for use as electrode materials for supercapacitors. Although RuO2 has good capacitive properties as a supercapacitor electrode material (specific capacitance: 1300 Fg-1) [6], its toxic nature, rarity and high production cost will exclude it from wide and commercial applications. Manganese oxide is another transition metal oxide studied as an electrode material for supercapacitor application. Manganese can exist in three different valence states and its oxides are highly complex. The capacitance of manganese oxides theoretically, reaches to 1100 Cg-1 (from Mn (IV) to Mn (III)) but the electrochemical reversibility of redox transition of manganese dioxide is usually too low to be applicable, and the pure manganese dioxide possess poor capacitive response due to high resistance of bulk manganese oxide [7]. Tin dioxide (SnO2), a transparent conducting oxide and a wide band gap n-type semiconductor, has been used in many applications such as gas sensors, electrodes in solid- state ionic devices, solar cells, etc., due to its unique properties such as chemically inert, mechanically hard, and thermally heat stability. Only one stable phase (there is no metastable one) is known, which has a tetragonal arrangement of the atoms exhibiting either a rutile or cassiterite structure. Tin dioxide can be synthesized using a variety of techniques such as sol- gel, hydrothermal method, precipitation, carbothermal reduction and polymeric precursor. Techniques such as precipitation provide particle sizes of less surface area and smaller pore size as a result of agglomeration, which makes them unsuitable for applications such as gas sensors, electrode materials for supercapacitors and batteries as compared to hydrothermal and microemulsions techniques [8-10]. University of Ghana http://ugspace.ug.edu.gh 3 The objectives of this research are to synthesize tin dioxide nanoparticles via the hydrothermal technique under mild conditions (120°C) and water-in-oil microemulsions technique, characterize the as-synthesized particles and use the as-synthesize particles in a supercapacitor application. In order to be able to realize the objectives mentioned afore, different base additives, urea and sodium borohydride, will be used to determine their effects on the tin oxide nanoparticles. Physical and electrochemical characterizations such as X-ray Diffraction, Brunner Emmet Teller (BET) technique, transmission electron microscopy, and cyclic voltammetry measurements shall be used to determine the phase, surface area and pore size, particle size, charge and discharge properties of the as-synthesized tin oxide particles. This thesis has been organized into five chapters: First chapter gives an overview of the background of study, the key objectives, and how the results of this work could be used. Chapter two examines the relevant theories from both an appreciative inquiry and critical analysis point and gives some perspectives on some empirical works in the area. Chapter three gives the experimental design of the study and the key analytical methods. The fourth chapter presents the results and discussions of the findings and Chapter five draws conclusions and makes recommendations. University of Ghana http://ugspace.ug.edu.gh 4 CHAPTER TWO 2.0 Literature Review In this section nanomaterials and supercapacitors will be discussed. The various materials used for supercapacitors along with their pros and cons will also be elaborated. Current methods of preparation of tin dioxide nanoparticles will be discussed with much focus on the hydrothermal and microemulsions route of synthesizing tin dioxide nanoparticles. 2.1 Nanoscience and Nanotechnology Nanotechnology was first introduced in the famous lecture of Nobel laureate Richard P. Feynman, “There’s Plenty of Room at the Bottom”, given in 1959 at California Institute of Technology [11]. Since then, there have been developments in physics, chemistry and biology over the years which have proven Feynman’s ideas of manipulating and controlling matter at extremely small scale, even to the level of molecules and atoms, i.e. nanoscale. Nanotechnology refers to any technology performed on a nanoscale that has applications in the real world. It deals with the production and application of physical, chemical and biological systems at scales ranging from a few nanometers to submicron dimensions, as well as integration of the resulting nanostructures into larger systems. Changes occur significantly in materials when their particle or grain size is decreased from the macrosize to nanosize and properties such as chemical reactivity, mechanical properties and others are also affected [11]. 2.1.1 Properties of Nanomaterials Nanostructure materials are single phase or multiphase polycrystalline solids with a typical average particle size of a few nanometers (1nm = 10-9m). Nanomaterials are usually in the range of 1 to 100 nm, [12] according to the National Nanotechnology Initiative in the US. The University of Ghana http://ugspace.ug.edu.gh 5 size of hydrogen atom is considered as the lower limit of nano whereas upper limit is arbitrary. The grain sizes are so small; a significant volume fraction of the atoms resides in grain boundaries. Material is characterized by a large number of interfaces in which the atomic arrangements are different from those of crystal lattice. The basic classification of nonmaterial is done based on the confinement. Bulk structures show no confinement whereas nano-wells and nanowires can be obtained by 2-D and 1-D confinement respectively. The quantum realm comes to the picture when there is a 3-D confinement and leads to zero- dimension quantum structures that is quantum dot. Recent trends in Electrochemical Supercapacitors (ES) also involve the development of nanostructured materials, such as nanoaerogels, nanotubes/rods, nanoplates, nanospheres, and so on. Nanostructured materials possess high specific surface area. They can provide short transport/diffusion path lengths for ions and electrons, leading to faster kinetics, more efficient contact of electrolyte ions, and more electroactive sites for faradaic energy storage, resulting in high charge/discharge capacities even at high current densities. Material morphology is closely related to the specific surface area and the diffusion of ions in the electrode, and one-dimensional nanostructure materials seem to be very promising for ES application due to their reduced diffusion paths and larger specific surface areas [13]. 2.2 Structure and main bulk properties of Tin Dioxide Tin dioxide is a semiconductor material, transparent, of high chemical and mechanical stability. Only one stable phase is known, which has a tetragonal arrangement of the atoms receiving the names of either rutile or cassiterite. The heat of formation is, H = 1.9 *103 J mol- 1, the heat capacity of the material is, Cp = 52.59 J mol-1 K-1, the density at 300 K is 6.95 g cm-3 and the melting point is 16300C. This crystalline structure contains metal atoms in octahedral coordination and oxygen in planar three-coordination. Lattice parameters are a = University of Ghana http://ugspace.ug.edu.gh 6 4.737 Å (a=b in the tetragonal structure) and c = 3.186 Å. The atomic position at 300 K in the unit cell is (0, 0, 0) and (½, ½, ½) for the metal and ± (u, u, 0) and (½+u, ½-u, ½), being u=0.387, for the oxygen. The rest of the atoms of the unit cell can be obtained applying the rutile symmetry D144h (C2v, P42/mnm point group). Chemical bonding is mainly governed by the linear combination of oxygen 2s and 2p orbitals with tin 5s and 5p orbitals. With respect to its chemical properties, tin dioxide has a high chemical stability, as it is only attacked by hot concentrated alkalis, this being one of the reasons of the wide research done with this material [12]. It is well known that the size and the morphology of the nanomaterials greatly affect their properties as well as their applications due to their high surface-to volume ratio, enhanced characteristics of quantum size effects, and high fraction of chemically similar surface sites [14]. Figure 2.1: Crystal structure of tin dioxide (SnO2) [15] Sn O University of Ghana http://ugspace.ug.edu.gh 7 2.2.1 Comparison between stannic (SnO2) and stannous oxide (SnO) Oxides of tin exist in two main forms: stannic oxide (SnO2) and stannous oxide (SnO). These two oxides show the dual valency nature of tin, with oxidation states of 2+ and 4+. SnO2 is better characterized than stannous oxide (SnO). For instance, SnO’s electronic band gap is not accurately known but lies somewhere in the range of 2.5–3 eV. Thus SnO exhibits a smaller band gap than SnO2, which is commonly quoted to be 3.6 eV [15]. A detailed studies of stannous oxide cannot be facilitated due to the non-availability of single crystals of stannous oxide. Stannic oxide has the rutile structure and stannous oxide has the less common litharge structure. Stannic oxide the more abundant form of tin oxide and is of technological importance in gas sensing applications and oxidation catalysts. There also exists a slightly denser orthorhombic high pressure phase in addition to the common rutile (tetragonal) structured SnO2 phase. Figure 2.2 shows the Sn–O phase diagram for atmospheric pressure. The diagram indicates the existence of an intermediate tin-oxide phase between SnO and SnO2 at elevated temperature. In these intermediate oxides Sn is present as a mixture of Sn (II) and Sn (IV). Stannic oxide is the thermodynamically most stable form of tin oxide[16]. University of Ghana http://ugspace.ug.edu.gh 8 Figure 2.2: Sn-O phase diagram [15] 2.3 Applications of tin dioxide (SnO2) Tin dioxide is in the class of materials that have high electrical conductivity with optical transparency and as such constitutes an important component for optoelectronic applications [16]. 2.3.1 Transparent conductors SnO2 belongs to the oxide materials family that combine low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. These properties are useful in a number of applications; notably as electrode materials in solar cells, light emitting University of Ghana http://ugspace.ug.edu.gh 9 diodes, flat panel displays, and other optoelectronic devices where an electric contact needs to be made without obstructing photons from either entering or escaping the optical active area, and in transparent electronics such as transparent field effect transistors. Another property of SnO2 and other Transparent Conducting Oxides (TCOs) is that although they are transparent in the visible spectral range they are highly reflective for infrared radiation. This property is reason for the dominant use of SnO2 as an energy conserving material. Architectural windows coated with SnO2, for example, allow transmitting light but keeping the heat out or in the building depending on the climate region. More sophisticated architectural windows, rely on TCOs to electrically contact electrochromic films that change their coloring and transparency by applying a voltage across the films. There is a large number of TCOs, the most commonly known ones are the binary systems, i.e. SnO2, ZnO, and In2O3. Often the highest possible conductivity is sought. Many of the binary TCOs already possess a high conductivity due to intrinsic defects, i.e. oxygen deficiencies. This is also the case for SnO2, which, as a wide band-gap semiconductor, is in its stoichiometric form a good insulator. Non-stoichiometry, in particular oxygen deficiency, makes it a conductor [16]. 2.3.2 Heterogeneous catalysis Many oxides act as a support material for dispersed metal catalysts; however, tin dioxide, is an oxidation catalyst in its own right. Tin-oxide based catalysts show good activity towards CO/O2 and CO/NO reactions. In most oxide catalysts the oxidation reactions are supposed to follow the Mars–van Krevelen mechanism. In the Mars-van Krevelen mechanism, the molecules are oxidized by consuming lattice oxygen of the oxide catalyst which in turn is re- oxidized by gas-phase oxygen. This is possible because transition and post transition oxides have multivalent oxidation states that allow the material to easily give up lattice oxygen to react with adsorbed molecules and can be subsequently re-oxidize by gas-phase oxygen. It is shown that surfaces with Sn4+ or Sn2+ are stable for different oxygen chemical potentials. University of Ghana http://ugspace.ug.edu.gh 10 Which is an indication that, an easy re-oxidation and reduction of SnO2 surfaces can be expected in catalytic oxidation reactions [16]. 2.3.3 Solid state gas sensors Gas sensing materials can be made from materials that change their properties depending on the ambient conditions. Normally, changes in the electrical conductance in response to environmental gases are monitored. For instance, all the following oxides show a gas response in their conductivity: Mn2O3, NiO, Co3O4, CuO, CdO, MgO, SrO, BaO, In2O3, WO3, La2O3, TiO2, V2O3, Fe2O3, GeO2, Cr2O3, Nb2O5, MoO3, Ta2O5, CeO2, Nd2O3. ZnO and SnO2 however, are the most commonly used gas sensing materials. Bulk- and surface-sensitive materials are the main divisions of gas sensitivity of oxides. For example, TiO2 increases its conductivity due to the formation of bulk oxygen vacancies under reducing conditions and is thus categorized as a bulk sensitive gas sensing material. On the other hand, SnO2, although bulk defects affect its conductivity, belongs to the category of surface sensitive materials [15]. 2.4 Electrochemical Capacitors (Supercapacitors) It’s a device that stores electrical energy in the electrical double layer that forms at the interface between an electrolytic solution and an electronic conductor [17]. According to the Rangone plot in Figure 2.3, supercapacitors have very good properties in terms of the specific energy as well the specific power in comparison to batteries [18, 19]. High-capacitance supercapacitors and the overall capacitance of the electrode materials can be realized when the pseudo capacitance due to the presence of foreign electro-active species is coupled with the Electrical Double Layer Capacitance (EDLC) [18]. The ultimate performance of the supercapacitor can be achieved when the most critical aspects which is to optimize the energy University of Ghana http://ugspace.ug.edu.gh 11 density without reducing their power capability is paid attention to. The use of high- capacitance materials (high surface area or pseudo-active species) ensures high energy density. For EDLC’s, carbonaceous materials such as carbon aerogel is an ideal electrode material as it allows an interconnected three- dimensional network structure, a high specific surface-area, good electrical conductivity and a suitable pore size for common electrolytes [2]. Transition metal oxides such as RuO2, NiO, MnO2 and SnO2 and conducting polymers with various oxidation states are considered as promising materials for Pseudocapacitors as they generate their capacitance from the reversible redox transitions of the electroactive materials [2]. Figure 2.3: Ragone plot showing the specific power against specific energy for different Electrochemical Devices [18]. University of Ghana http://ugspace.ug.edu.gh 12 2.4.1 Electric Double Layer Capacitors (EDLC’s) Electric double layer capacitor (EDLC) is an electrical storage device, which stores much more energy than conventional capacitors and offer much higher power density than batteries. EDLC works on the principle of double-layer capacitance at the electrode/electrolyte interface where electric charges are accumulated on the electrode surfaces and ions of opposite charge are arranged on the electrolyte side. The electrode material used for the construction of the cell EDLC is mainly carbon material (activated carbon, carbon nanotubes and graphene) [1]. Although activated carbon has a high specific surface area, the low electrical conductivity of activated carbon limits its applications in high power density supercapacitors for example, a commercial activated carbon for supercapacitors can only achieve a specific capacitance of 26 Fg-1). Carbon nanotubes (CNT’s) which have been studied to replace activated carbon for supercapacitors because of their excellent electrical conductivity and high surface area unfortunately showed a low energy density as reported by Cheng et.al. [1]. The electrolytes can be either aqueous or non-aqueous depending on the mode of construction of EDLC cell. Since 1978 EDLCs based on carbon electrodes have been used for memory back-up device for many electrical appliances like camera and VCRs. The EDLCs in 1980s were used for the energy source to drive wrist watches with solar cells. In 1990s, they were used as actuator back-up sources for toys, electric appliances, home equipment and others. Recently, EDLCs with higher capacitances are under development for higher electric power sources in electric vehicle systems and electric power storage systems. Specific capacitance theoretically, of an activated carbon is directly proportional to the specific surface area. In reality however, it does not happen. A major disadvantage of carbon based EDLC is the lower specific stored energy. To rectify these problems, researchers now incorporate transition metal oxides along with carbon in the electrode materials. When the electrode materials consist of transition metal oxides, then the electrosorption or redox processes enhance the University of Ghana http://ugspace.ug.edu.gh 13 value of specific capacitance for 10-100 times depending on the nature of oxides. In such a situation the EDLC is called a hybrid supercapacitor [7]. Thus, the development of electroactive materials with high specific surface areas, such as nanostructured metal oxides becomes a trend in order to increase the performance of supercapacitors. These electrode materials generally show the combined characteristics of high double-layer capacitance and Faradaic pseudocapacitance, resulting in the significant increase in both energy density and power density [19]. As reported by Wang and Hu [18] the addition of SnO2 into RuO2 matrix considerably increased the electrochemical stability and the electrochemically active sites of RuO2 coatings. During the process of charging, the electrons travel from the negative electrode to the positive electrode through an external load. Within the electrolyte, cations move towards the negative electrode while anions move towards the positive electrode. During discharge, the reverse processes take place. In this type of ES, no charge transfers across the electrode/ electrolyte interface, and no net ion exchanges occur between the electrode and the electrolyte. This implies that the electrolyte concentration remains constant during the charging and discharging processes. In this way, energy is stored in the double-layer interface [20]. Carbon composites with transition metal oxides such as SnO2, RuO2 and MnO2 as electrode materials for SCs have shown very good specific capacitance. Qian Chen et al [1] reported on SCs using MnO2-coated graphene electrode which exhibited a high specific capacitance of 328 Fg-1 as compared to the graphene electrode supercapacitor which had a specific capacitance of 245 F g-1. EDLCs have low energy density and University of Ghana http://ugspace.ug.edu.gh 14 Figure 2.4: Schematic Diagram of an EDLC [21] 2.4.2 Pseudocapacitors/ Faradaic Supercapacitors (FS) The term “Pseudocapacitance” is usually used to describe the behavior of electrode materials (MnO2, RuO2) that have the electrochemical signature of a capacitive electrode, that is, a linear dependence of the charge stored with changing potential within the window of interest, but in which charge storage originates from electron-transfer mechanisms, rather than simply relying on the accumulation of ions in the electrochemical double layer (as with activated carbons). When a potential is applied to a FS, fast and reversible faradaic reactions (redox reactions) take place on the electrode materials and involve the passage of charge across the double layer, similar to the charging and discharging processes that occur in batteries, resulting in faradaic current passing through the supercapacitor cell [20]. Materials University of Ghana http://ugspace.ug.edu.gh 15 undergoing such redox reactions include conducting polymers and several metal oxides, including RuO2, MnO2, and Co3O. Reversible adsorption (for example, adsorption of hydrogen on the surface of platinum or gold), redox reactions of transition metal oxides (e.g. RuO2), and reversible electrochemical doping–dedoping in conductive polymer based electrodes are the three types of faradaic processes occur at FS electrodes. It has been proven that these faradaic electrochemical processes not only increase the working voltage but also increase the specific capacitance of the supercapacitors. An FS exhibits far larger capacitance values and energy density than an EDLS since the electrochemical processes occur both on the surface and in the bulk near the surface of the solid electrode, [20]. 2.4.3 Factors affecting the Capacitance of Supercapacitors  Particle sizes,  Concentration of electrolyte,  Scan rate,  Surface activation under the electrochemical conditions,  Surface oxygen content,  Surface oxides, and  Lattice defects resulting from the method of preparation [20]. 2.4.4 Electrolyte The general requirements for an electrolyte in ES include: high electrochemical stability, wide voltage window, high ionic concentration, low solvated ionic radius, low resistivity, low viscosity, low volatility, low toxicity, low cost as well as availability of constituent chemicals of high purity. The electrolyte used in an ES can be classified into three types: (1) aqueous electrolyte, (2) organic electrolyte, and (3) ionic liquids (ILs). University of Ghana http://ugspace.ug.edu.gh 16 (1) Aqueous electrolyte: aqueous electrolytes (such as H2SO4, Na2SO4, KOH and NH4Cl aqueous solution), compared with organic electrolytes can give a higher ionic concentration and lower resistance. ES containing aqueous electrolyte may display higher capacitance and higher power than those with organic electrolytes, probably due to higher ionic concentration and smaller ionic radius. Also, aqueous electrolytes can be utilized and prepared without strictly controlling the preparing processes and conditions, while organic ones need strict processes and conditions to obtain ultra-pure electrolytes. A large disadvantage of aqueous electrolytes unfortunately, is their small voltage window (1.2 V) which much lower than those of organic electrolytes. Aqueous electrolytes have a big limitation in terms of improving both power densities and energy as a result of their narrow voltage window. This is one of the main reasons why organic electrolytes are often recommended [20]. (2) Organic electrolyte: Organic electrolytes can provide a voltage window as high as 3.5 V compared to aqueous electrolytes. This is one big advantage of organic over aqueous electrolytes. Acetonitrile and propylene carbonate (PC) are the most commonly used solvents among organic electrolytes. Larger amounts of salt can be dissolved with Acetonitrile than other solvents, but suffers from environmental and toxic problems. PC-based electrolytes are environmentally friendly and can offer a wide electrochemical window, a wide range of operating temperature, as well as good conductivity. Besides, organic salts such as tetraethylammonium tetrafluoroborate, tetraethylphosphonium tetrafluoroborate, and triethylmethylammonium tetrafluoroborate (TEMABF4) have also been used in ES electrolytes. Salts with less symmetric structures have lower crystal-lattice energy and increased solubility. However, one issue which should be kept in mind is that the water content in organic electrolytes must be kept below 3–5 ppm. Otherwise, the ES’s voltage will be significantly reduced [20]. University of Ghana http://ugspace.ug.edu.gh 17 (3) Ionic liquids (ILs): A salt may be turned into a ‘liquid’, by providing heat to the system to counterbalance the salt lattice energy. Such a system is called molten salts or ILs. ILs can exist in liquid form at the desired temperatures. Their desirable properties make them promising candidates for ES electrolytes. These properties include low vapor pressure, high thermal and chemical stability, low flammability, wide electrochemical stability window ranging from 2 to 6 V, typically about 4.5 V, and conductivity at a level of ca. 10 mScm-1 [21]. Since ILs are solvent-free, there is no solvation shell in ILs, and thus ILs can offer a well identified ion size. The main ILs studied for ES applications are imidazolium, pyrrolidinium, as well as asymmetric, aliphatic quaternary ammonium salts with anions such as tetrafluoroborate, trifluoromethanesulfonate, bis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide or hexafluorophosphate. Room temperature ILs are usually quaternary ammonium salts such as tetralkylammonium [R4N]+, and cyclic amines such as aromatic pyridinium, imidazolium and saturated piperidinium, pyrrolidinium. Low temperature molten salts based on sulfonium [R3S]+ as well as phosphonium [R4P]+ cations are also explored in the literature [22]. The chemical–physical properties of these ILs strongly depend on the type of cation and anion. For example, aliphatic quaternary ammonium and pyrrolidinium salts display a wider potential range, sometimes exceeding 5 V, but their conductivity is generally lower than that of 1-ethyl-3- methylimidazolium-based ILs which remain liquid down to low temperature (< -50°C) and display higher conductivities as high as 10-2 S cm-1 at room temperature. A number of electrodes and devices have been prepared using ILs. For example, ethyl-methyl imidazolium-bis(trifluoromethane- sulfonyl)imide was used to investigate the relationship between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance.1 N-Butyl-N-methyl pyrrolidinium bis-(trifluoromethane sulfonyl)imide (NBNMPBTSI), which has low melting temperature, hydrophobic and highly cycleable properties within a wide voltage window, allowed hybrid ES to be operated University of Ghana http://ugspace.ug.edu.gh 18 between 3.4 and 1.5 V. 1-Ethyl 3-methyl imidizolium bis-(trifluoromethylsulfonyl)imide showed better fluidity than NBNMPBTSI [20-22]. 2.4.5 Comparison of EDLC and Pseudocapacitor [23] Table 2.1: Comparison of EDLC and Pseudocapacitor Supercapacitors [21] EDLC Pseudocapacitor Indefinitely reversible Highly reversible High-voltage operation Low-voltage operation, limited by electrochemistry and decomposition voltage of solvent Has state-of-charge self-indication Has state-of-charge self-indication Capacitance constant with voltage Capacitance not constant with voltage 2.4.6 Fabrication and Manufacturing of Electrochemical Supercapacitor (ES) Main processes in the fabrication of cells include five steps: (1) coating the electrode; (2) winding; (3) filling with an electrolyte; (4) testing; and (5) welding and sealing. The electrode, the heart of the cell, determines the ES performance in terms of self-discharge, life expectancy, capacity, resistance, and so on. Therefore, the electrode fabrication including an active material coating process is the most important step. As a result, stringently controlling the preparation process is necessary for achieving both durability high and performance. Normally, the charge collector is made from aluminium because of its high current-carrying capability, chemical stability and low cost. In the coating process of electrodes, firstly, binders, active materials and conductive additives are mixed to obtain a homogeneous slurry University of Ghana http://ugspace.ug.edu.gh 19 with the desired density, and then the slurry is spread onto an etched aluminium foil, followed by drying the electrode and roll pressing to achieve a uniform electrode coating layer. Once the electrodes are prepared, they are brought into a glove box with ultra-low moisture. A pair of such electrodes, with a separator layer inserted between them, is wound around a central mandrel into the desired shape. Then the electrolyte is filled into this separator. The electrolyte-filling process generally requires special care. The amount of electrolyte in the cell is critical, because excess electrolyte can lead to excessive gassing and leakage in operation. Generally, the process is slow, typically taking one to two days, and is relatively labor intensive. This is because of the nanopore level of internal porosity of active materials, making the wetting process slow. After the completion of electrolyte filling, the cell is subjected to cycling test [20]. 2.5 Cyclic voltammetry The capacitance of a supercapacitor material can be assessed using cyclic voltammetry (cv) which is one of the commonest tools used. During CV testing, a varying electric potential which is linear is applied between positive and negative electrodes for two-electrode systems, or between reference and working electrodes for three-electrode configurations. The rate of the potential change in mVs −1 is designated as the sweep rate or scan rate, ν, and the range of potential change is called the potential window or operating potential. The instantaneous current during the cathodic and anodic sweeps is recorded to characterize the electrochemical reactions involved. The data are plotted as current (A) vs. potential (V) or sometimes as current (A) or potential (V) vs. time (s). The three-electrode setup is regarded as the most suitable approach to examine the charge storage mechanisms of supercapacitor (SC) materials [24]. University of Ghana http://ugspace.ug.edu.gh 20 2.6 Constant Current Charge/Discharge (CCCD) Under direct current the most widely used method for the characterization of SCs is the CCCD testing. It is conducted by repetitive charging and discharging of the SC device or the working electrode at a constant current level with or without a dwelling period (a time period between charging and discharging while the peak voltage Vo remains constant), and normally the output is a plot of the potential (E ) vs. time (s). Choosing a proper level of the constant current is critical to produce consistent and comparable data from a CCCD test. CCCD test is regarded as the most versatile and accurate approach in characterizing SC devices. All three core parameters of SC devices, C T, RES, and Vo, can be tested from it and subsequently used to derive most of the other properties, such as the time constant, power and energy densities, and leakage and peak current. It can also be conveniently used to study the cycling stability of SC devices. Moreover, by using a three-electrode setup, the specific capacitance, reversibility, and potential window for SC materials can also be obtained via CCCD test [24]. 2.7 Electrochemical Impedance Spectroscopy (EIS) EIS testing, which is also known as the dielectric spectroscopic testing, measures the impedance of a power cell as a function of frequency by applying a low-amplitude alternating voltage (normally 5 mV) superimposed on a steady-state potential. The data that results from the test are usually expressed graphically in a Bode plot to demonstrate the cell response between the phase angle and frequency, and in a Nyquist plot to show the imaginary and real parts of the cell impedances on a complex plane. In addition to the frequency response and impedance, EIS has also been used to characterize the charge transfer, mass transport, and charge storage mechanisms, as well as to estimate the capacitance, energy, and power properties. To distinguish the contribution of individual structure component in a cell system to the total impedance, different equivalent circuits and models have been developed. The real University of Ghana http://ugspace.ug.edu.gh 21 parts of the complex impedance at selected frequencies are used in literature to represent RES when SC devices are tested. However, one needs to remember that the RES from an EIS test is often much smaller than that derived from the CCCD test, and is as such, limited in describing the power performance of SC devices. The impedance, specific capacitance, charge transfer, mass transport, and charge storage of SC materials can be studied using EIS testing mechanisms. [24]. 2.8 Equivalent Series Resistance A SC is not an ideal electrical component in the sense that it has its own internal resistance, and thus dissipates the energy that is stored. A SC cell can be treated as a system of a capacitor in series arrangement with a resistor RES. The resistance of this resistor is usually called the equivalent series resistance, RES, and is essential in reflecting the power performance and energy efficiency of SCs. In general, a small RES is preferred for better electrochemical performance. RES is evaluated by analyzing the IR drop or voltage variation at the initial stage of the discharging curve from CCCD tests and by applying ohm’s law to the IR drop, RES can be readily calculated: R ES VI   (1) Where ΔV and ΔI are the voltage and current of the IR drop, respectively. A larger charge/discharge current used in the test will usually give rise to a smaller RES. If I is the constant current applied, then in the case of 0 min dwelling time the current change is [24]. ΔI ≅ 2I (2) University of Ghana http://ugspace.ug.edu.gh 22 2.9 Synthesis of Metal Oxides There are many ways to synthesize metal nanoparticles and can be mainly divided into two categories: top down approach and bottom up approach. In the top down approach, the bulk materials are used as the starting materials and treated by physical means such as mechanical alloying, sputtering techniques and mechanical grinding to synthesize nanomaterials. The particles synthesized by this method generally have a broad size distribution. The metal nanoparticles synthesized by this method are typically larger and cannot be reproduced resulting in irreproducible catalytic activity. Single atoms or ions are allowed to grow into clusters or nanoparticles using wet chemical synthetic methods such as chemical reduction of metal salts, and the decomposition of precursors using thermal, sonochemical treatment in the bottom up process. The particles synthesized by this method have narrow size distribution of the particles. The method used for the synthesis of metal nanoparticles has control over the size and shape of the particles. Chemical synthesis methods are found to be more suitable methods to have better control over the particle size and shape, than physical synthesis. Chemical reduction of metal salts, thermal and photochemical decomposition of metal complexes and electrochemical reduction of metal salts are the three major routes used to synthesize transition metal nanoparticles. [25] 2.9.1 Chemical Reduction One of the most widespread method used for the synthesis of transition metal nanoparticles is the chemical reduction of metal salts. In this method, the metal salts dissociate to form metal ions. The metal ions are then reduced by a reducing agent to form metal nanoparticles that are stabilized by a stabilizer. Most of the commonly used reducing agents include solvents such as sodium borohydride, alcohol, or sodium citrate and gases such as carbon monoxide and hydrogen. Ethanol, methanol and isopropanol are the alcohols mainly used for the reduction University of Ghana http://ugspace.ug.edu.gh 23 of metal salts. The alcohols act both as a reducing agent and solvent. The transition metal nanoparticles are formed by the reduction of metal salts in the refluxing alcohol and the alcohols are oxidized to form corresponding carbonyl compounds (for example, methanol to formaldehyde, ethanol to acetaldehyde). The size distribution of the metal nanoparticles is dependent on the structure and quantity of the alcohol, the stabilizing agent, metallic precursor, temperature and the base used [26]. Borohydrides are the most commonly used reducing agents to synthesize colloidal transition metal nanoparticles. Sodium borohydride is dissolved in aqueous solution and used immediately to avoid decomposition of borohydride to borane and gaseous hydrogen that could escape from the solution. Sodium borohydride is also used as a reducing agent to synthesize platinum and palladium nanoparticles stabilized by PAMAM dendrimers. Sodium citrate is used as a reducing agent to synthesize Ir and Pt nanoparticles [27]. Turkevitch and co-workers [28] used sodium citrate as reducing agent to synthesize Au nanoparticles. The citrate anion acts both as a reducing agent and stabilizer. Larger size particles are produced as a result of the decrease in the amount of sodium citrate. The reduction in the amount of sodium citrate will result in less number of citrate ions available for stabilizing the particles and this causes the agglomeration of smaller particles to form bigger ones. Another commonly used reducing agent to synthesize colloidal transition metal nanoparticles is hydrogen gas. This method involves bubbling of hydrogen gas into a solution containing precursor metal salt and a slow reduction process allows the formation of colloidal metal nanoparticles. Hydrogen gas is used as a reducing agent to synthesize Ru, Pt, Ag, Pd, Ir, Rh and Au colloids in the presence of polyvinylalcohol. Polyphosphate is used as a stabilizer to synthesize Ag nanoparticles using hydrogen as reducing agent. Carbon monoxide is used as a reducing agent to synthesize Au nanoparticles in the presence of polyvinylsulfate [29] . University of Ghana http://ugspace.ug.edu.gh 24 2.10 Synthesis of tin dioxide nanostructures Many processes have been used to synthesize tin dioxide nanostructures, e.g., spray pyrolysis, conventional co-precipitation, and hydrothermal method: microwave assisted, reverse micelles and sol-gel method. 2.10.1 Hydrothermal Synthesis Sir Roderick Murchison, a British geologist was the first to use the term ‘hydrothermal’ to describe the action of water at elevated temperature and pressure, in bringing about changes in the earth’s crust leading to the formation of various rocks and minerals [30]. It is well known that the largest single crystal formed in nature (beryl crystal of >1000 g) and some of the large quantity of single crystals created by man in one experimental run (quartz crystals of several 1000s of g) are both of hydrothermal origin. Any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions can be defined as hydrothermal processing [30]. Definition for the word hydrothermal has undergone several changes from the original Greek meaning of the words ‘hydros’ meaning water and ‘thermos’ meaning heat. Recently, Byrappa and Yoshimura define hydrothermal as any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non- aqueous) above the room temperature and at pressure greater than 1 atm in a closed system. However, there is still some confusion with regard to the very usage of the term hydrothermal. For example, chemists prefer to use a term, viz. solvothermal, meaning any chemical reaction in the presence of a non-aqueous solvent or solvent in supercritical or near supercritical conditions. Similarly, there are several other terms like glycothermal, alcothermal, ammonothermal, and so on. Further, the chemists working in the supercritical region dealing with the materials synthesis, extraction, degradation, treatment, alteration, University of Ghana http://ugspace.ug.edu.gh 25 phase equilibria study, etc., prefer to use the term supercritical fluid technology. However, if we look into the history of hydrothermal research, the supercritical fluids were used to synthesize a variety of crystals and mineral species in the late 19th century and the early 20th century itself [24]. So, a majority of researchers now firmly believe that supercritical fluid technology is nothing but an extension of the hydrothermal technique. Hence, here authors use only the term hydrothermal throughout the text to describe all the heterogeneous chemical reactions taking place in a closed system in the presence of a solvent, whether it is aqueous or non-aqueous. The hydrothermal technique is among the various technologies available today in advanced materials processing, and it occupies a unique place owing to its advantages over conventional technologies. It covers processes like hydrothermal crystal growth leading to the preparation of fine to ultra- fine crystals, bulk single crystals, hydrothermal transformation, hydrothermal sintering, hydrothermal decomposition, hydrothermal stabilization of structures, hydrothermal dehydration, hydrothermal extraction, hydrothermal treatment, hydrothermal phase equilibria, hydrothermal electrochemical reactions, hydrothermal recycling, hydrothermal microwave supported reactions, hydrothermal mechanochemical, hydrothermal sonochemical, hydrothermal electrochemical processes, hydrothermal fabrication, hot pressing, hydrothermal metal reduction, hydrothermal leaching, hydrothermal corrosion, and hydrothermal synthesis [30]. In comparing with other techniques, hydrothermal synthesis generally has the advantage of:  Low cost for equipment.  A low temperature synthesis procedure.  Composition and particle size control by changing preparation parameters [6]. Chin–Chang et al synthesize SnO2 enriched Ru-Sn oxide composite via a two-step hydrothermal procedure which showed an excellent capacitive performances in H2SO4 as that for pure RuO2.xH2O [6]. Mild hydrothermal conditions (material construction occurring University of Ghana http://ugspace.ug.edu.gh 26 between 100 and 200 under autogenous pressures), have been applied to prepare various hydrous and crystalline transition metal oxides in the past few years, such as tungsten oxide, vanadium oxide, molybdenum oxide, titanium oxide, as well as lithium/sodium manganese oxides [18]. Lin Tan et al reported that Krishna and Komarneni [24] used both conventional- hydrothermal and microwave-hydrothermal methods to prepare SnO2 nanoparticles of very high surface areas. The hydrothermal processing is an alternative to calcination for the crystallization of SnO2 under mild temperature [10]. 2.10.2 Instrumentation in hydrothermal processing of nanomaterials A pressure vessel capable of containing a highly corrosive solvent at high temperature and pressure is required for material processing under hydrothermal conditions. Hydrothermal experimental investigators require facilities that must operate routinely and reliably under extreme pressure temperature conditions. Designing a suitable hydrothermal apparatus popularly known as an autoclave, or reactor, or pressure vessel, or high pressure bomb is the most difficult task to define, because each project has different objectives and tolerances. However, an ideal hydrothermal autoclave should have the following characteristics [23].  Ease of assembly  Inertness to acids, bases and oxidizing agents  Sufficient length to obtain a desired temperature gradient.  Leak-proof with unlimited capabilities to the required temperature and pressure range.  Rugged enough to bear high pressure and temperature experiments for long periods with no damage so that no machining or treatment is needed after each experimental run. University of Ghana http://ugspace.ug.edu.gh 27 The commonly used reactors in the hydrothermal processing of advanced nanomaterials are listed below [30]:  Morey type e flat plate seal.  General purpose autoclaves.  Stirred reactors.  TZM autoclaves.  Cold-cone seal Tuttle-Roy type.  Batch reactors.  Flow reactors.  Microwave hydrothermal reactors.  Mechanochemical-hydrothermal.  Piston cylinder apparatus.  Belt apparatus.  Opposed anvil. 2.10.3 Influence of Surfactants on particles The term surfactant (short for surface-active-agent) is used to describe a substance which exhibits some superficial or interfacial activity. It can be said that, when powders are prepared in presence of surfactants, CetylTrimethylammonium bromide (CTAB) as well as Dioctyl Sulfosuccinate sodium salt (AOT), the precipitated powders coated by a surfactant overlayer, the growth of powder is affected since the ions have to diffuse through the surfactant overlayer, which causes a reduction in grain growth. According to Jain et al [31] anionic surfactant AOT was more effective in reducing the grain growth as compared to the cationic surfactant CTAB [9], [31]. SnO2 nanoparticles in the range of 8-13nm with different University of Ghana http://ugspace.ug.edu.gh 28 morphologies were synthesized by Gnanam and Rajendran [10] using different surfactants. Therefore, the type of surfactant used, determines the morphology of the particles. 2.10.4 Microemulsion Synthesis method Among all chemical methods the microemulsion has been demonstrated as a reproducible and a very versatile method that allows control over the nanoparticle size, and yields nanoparticles with a narrow size distribution. In the macroscale microemulsions are homogeneous and in nanoscale are heterogeneous dispersion of two immiscible liquids consisting of nanosized domains of one or both liquids in the other, stabilized by an interfacial film of surface active molecules. Normal emulsion and microemulsion can be distinguished by their particle size and stability. Normal emulsions age by coalescence of droplets and Ostwald ripening. Microemulsions are thermodynamically stable, isotropic and usually form spontaneously. Microemulsions have ultralow interfacial tension, large interfacial area and capacity to solubilize both aqueous and oil-soluble compounds. Depending on the proportion of various components and hydrophilic–lypophilic balance (HLB) value of the used surfactant, microemulsions can be classified as water-in-oil (W/O), oil-in-water (O/W) and intermediate bicontinuous structural types that can turn reversibly from one type to the other. The dispersed phase consists of monodispersed droplets in the size range of 5 – 100 nm. The nanodroplet size can be modified by varying concerned parameters, e.g. the type of stabilizer, continuous phase, the precursor content dissolved within the nanodroplets, and the molar ratio of water to surfactant (W). In addition, the stability of the microemulsion can be influenced by addition of salt, concentration of reagents, temperature or pressure. The preparation procedure of metallic nanoparticles in W/O microemulsion commonly consists of mixing of two microemulsions containing metal salt and a reducing agent, respectively. After mixing two microemulsions, the exchange of reactants between micelles takes place during the collisions of water droplets, the attractive van der Waals forces and repulsive osmotic and elastic forces University of Ghana http://ugspace.ug.edu.gh 29 between reverse micelles. Successful collisions lead to coalescence, fusion, and efficient mixing of the reactants. Metal nuclei are formed as a result of the reaction between solubilizates. A metal salt is reduced to give zero valent metal atoms in the initial stage of the nucleation, which then collides with further metal ions, metal atoms, or clusters to form an irreversible seed of stable metal nuclei. Growth then occurs around this nucleation point where successful collision occurs between a reverse micelle carrying a nucleus and another one carrying the product monomers with the arrival of more reactants due to intermicellar exchange. The nucleation reaction and particle growth take place within the micelles, and the size and morphology of as-prepared nanoparticles depend on the size and shape of the nanodroplets and the type of the surfactant, whose molecules are attached on the surface of the particles to stabilize and protect them against further growth [32-34]. Hui-Yuan Wang et al [29] prepared SnO2 submicrospheres at ambient temperature with uniform morphology, high specific surface area and narrow size distributions, contributing to SnO2 submicrostructures with large reversible capacity (846 mAh·g-1 after 50 cycles at a current density of 100 mA·g-1) and significantly improved cycling performance [35]. Song et al [30] synthesized nanoparticles of tin oxide (SnO2) via water-in-oil microemulsions consisting of water, AOT (surfactant), and n-heptane (oil). Precursor hydroxides were precipitated in the aqueous cores of water-in-oil microemulsions and then calcined at 600°C for 2 hrs to form tin oxide powder. The formation of phase pure tin oxide was then confirmed by means of X-ray diffraction analysis. The tin oxide powder was found to be less than 40 nm in particle diameter and had a higher specific surface area, about 73 m2/g, when compared with tin oxide powder prepared through the conventional precipitation method (19 m2/g) [36]. University of Ghana http://ugspace.ug.edu.gh 30 2.11 Characterization Techniques 2.11.1 Scanning Electron Microscopy (SEM) The object is scanned at the surface with the electron beam point by point whereby secondary electrons are set free. The angle of inclination of the object’s surface determines the intensity of this secondary radiation. The signals that are derived from the electrons gives information about the sample including chemical composition, external morphology (texture) and crystalline structure and orientation of materials making up the sample. A detector that sits at an angle at the side above the object collects the secondary electrons. The signal is then enhanced electronically. The image appears on viewing screen and the magnification can be chosen depending on the model. An SEM machine is illustrated in Figure 2.5. Figure 2.5: SEM Machine University of Ghana http://ugspace.ug.edu.gh 31 2.11.2 X-Ray Diffraction (XRD) Diffraction produced by the reticular planes that form from the atoms of the crystal is one of the phenomena of interaction of x-rays with crystalline matter. An x-ray beam passing through a crystal will be diffracted at specific angles depending on the x-ray wavelength, the crystal orientation and the structure of the crystal. In the macroscopic version of x-ray diffraction, a certain wavelength of radiation will constructively interfere when partially reflected between surfaces (i.e., the atomic planes) that produce a path difference equal to an integral number of wavelengths. This condition is described by the Bragg law: 2d sin θ = nλ (3) Figure 2.6: Illustration of the Bragg’s law where n is an integer, λ is the wavelength of the radiation, d is the spacing between surfaces and θ is the angle between the incident beam of radiation and the atomic plane. This relation reveals that interference effects are observable only when radiation interacts with physical dimensions that are approximately the same order of magnitude as the wavelength of the radiation. Diffraction methods to characterize materials require radiation in the x-ray region of the electromagnetic spectrum, or beams of electrons or neutrons with similar wavelength, since the distances between atoms or ions in a substance are on the order of 1 Å. Therefore, through x-ray diffraction spectra one can identify and analyze any crystalline material. An x- University of Ghana http://ugspace.ug.edu.gh 32 ray diffractometer basically, consists of an x-ray generator, a goniometer and sample holder and an x-ray detector, such as photographic film or a movable proportional counter. X-ray tubes, which generate x-rays by bombarding a metal target with high energy (10-100 keV) electrons that knock out core electrons are the most usually employed instrument to generate x-rays. Thus, an electron in an outer shell fills the hole in the inner shell and emits an x-ray photon. Mo and Cu, which have strong K X-ray emissions at 0.71073 and 1.5418 Å, respectively are the two common targets used. Aside these spectral lines, other accompanying lines appear, which have to be eliminated in order to monochromatize the x-rays to enable the interpretation of the spectra. Crystal monochromators are used to partially suppress these lines. Powder cameras or powder diffractometers can be used to acquire powder diffraction patterns experimentally. Powder camera is the simplest instrument and many factors can contribute to the decision to select a camera technique rather than a powder diffractometer. These factors include the desire of maximum resolution, the lack of adequate sample quantity, and the lower initial cost. Of the various camera types the Debye-Scherrer camera is the most usually used, probably because it is the simplest and easiest to use. In it, the sample itself can be packed in a glass capillary or, if very small sized sample, mounted on the tip of a glass rod or other rigid material using rubber cement or other permanent adhesive. The sample is placed at the camera center and aligned to the incident beam. The diffracted beams intensities are proportional to the crystalline sample volume irradiated by the beam. Thus, times for exposure have to be increased as the sample size decreases. In the Debye Scherrer powder camera the whole diffraction pattern is recorded simultaneously. Diffractometer data are generally of good quality than Debye-Scherrer data, mainly because the shapes of the lines (or peaks) of Debye-Scherrer data are affected by both the adsorption of the specimen and the physical size of the specimen. University of Ghana http://ugspace.ug.edu.gh 33 The photographic film (or some annular detector) used in cameras in powder diffractometers are replaced with a photon detector. The majority of commercially available powder diffractometers use the Bragg-Brentano parafocusing arrangement. A given instrument may provide a horizontal or vertical θ/2θ configuration or a vertical θ/θ configuration. In it, all the rays diffracted by suitably oriented crystallites in the specimen at an angle 2θ converge to a line at the receiving slit. A radiation detector, usually a scintillation counter or a sealed gas proportional counter is used to detect the x-rays. The receiving slit assembly and the detector are coupled together and move around a circle in order to scan a range of 2 (Bragg) angles. For θ/2θ scans, the specimen is rotated about the same axis by the goniometer as the detector but at half the rotational speed. The surface of the specimen therefore remains tangential to the focusing circle. The goniometer which acts as a support for all of the various slits and other components which make up the diffractometer, also sets the angles θ and 2θ. The parallel-plate collimators serves the purpose of limiting the axial divergence of the beam and thus partially control the shape of the diffracted line profile. It follows that the center of the specimen surface must be on the axis of the goniometer and this axis must be also parallel to the axis of the line focus, divergence slit and receiving slit [12]. The designation for its identification of each crystalline solid is given by its x-ray diffraction pattern. XRD may be used to determine its structure, i.e. how the atoms pack together in the crystalline state and what the interatomic distance and angle are etc. From these points it can be concluded that x- ray diffraction has become a very important and powerful tool for the structural characterization in solid state physics and materials science. Figure 2.7 shows the setup for a typical XRD machine. University of Ghana http://ugspace.ug.edu.gh 34 Figure 2.7: XRD Machine University of Ghana http://ugspace.ug.edu.gh 35 CHAPTER THREE 3.0 Experimental Design This chapter gives a precise description of the procedures, methods, materials, tools and equipment used in obtaining the results. 3.1 Materials Chemicals used in this study, their purity and sources of procurement are listed below:  Tin Chloride pentahydrate (Sncl4.5H2O), 98% Sigma Aldrich,  Urea (CO (NH2)2), Paskem;  Soduim Borohydride (NaBH4), Lab. Tech chemicals; Sodium Hydroxide (NaOH), n- hexane (CH3 (CH2)4CH3), 95% Sure Chem. products Ltd.; Ethanol (C2H5OH), 96%, BDH laboratory supplies (Analar); distilled water; Dioctyl Sulfosuccinate sodium salt (AOT, C20H37NaO7S), Sigma Aldrich; Cetyl Trimethyl ammonium bromide (CTAB, C19H42BrN), Central Drug House Ltd. 3.2 Equipment The analytical tools and equipment used include:  Autolab PGSTAT205 (Metrhom BV),  XPERT-PRO diffractometer (PANalytical BV, Netherlands)  Zeiss Ultra plus 55 field emission scanning electron microscope  Vertex 70v (Bruker) spectrometer University of Ghana http://ugspace.ug.edu.gh 36 3.3 Synthesis of SnO2 Nanoparticles The SnO2 nanoparticles were synthesized using the water-in-oil microemulsion and hydrothermal techniques. 3.3.1 Water- In – Oil (reverse micelle) Microemulsion Synthesis of SnO2 particles 3.254 g of AOT was added to a solution consisting of 62 ml hexane and 62 ml of Isopropanol. The mixture was then stirred at a rate of 125 rpm until the AOT was completely dissolved. 8 ml of SnCl4.5H2O (0.2 M) was then added to the above mixture and stirred to obtain a homogeneous mixture after which 0.548 ml of NaOH (1 M) was added drop-wise. The colloid was then stirred at a steady rate of 125 rpm for 30 mins to allow time for the SnO2 particles to form. 10 ml of acetone was then added to the colloid in order to break the surface tension for the as- synthesized particles to settle. The as-synthesized particles were washed with water and ethanol in a ratio 1:1 via centrifugation several times to get rid of any impurity that might be present and dried at 100°C overnight. The as-obtained particles were then calcined at 600°C for 3 hours after which they were characterized using X-Ray Diffraction, Scanning Electron Microscope, Energy Dispersive X-ray, Brunner Emmet Teller, Fourier Transform Infrared and UV-visible Spectroscopy. The samples prepared by the water-in-oil microemulsion were designated as ‘E’. University of Ghana http://ugspace.ug.edu.gh 37 Figure 3.1: The process flow chart used for synthesis of nanosized SnO2 powders by the reverse micelle microemulsion synthesis technique. 3.3.2 Hydrothermal Synthesis For the hydrothermal synthesis of SnO2 nanoparticles, two reducing agents (Urea and Sodium Borohydride, a cationic (CTAB) and anionic (AOT) surfactants were used. In a typical synthesis, 2.6 g of urea was added to a solution consisting of 60 ml of water and 60 ml ethanol whiles stirring. The mixture was then rigorously stirred for 15 mins after which it was Surfactant Stirring to obtain a homogeneous mixture and particles formation Hydrocarbon solvent (oil) Acetone addition Precipitation of particles Metal precursor salt + NaOH Washing of particles via centrifugation with water and absolute ethanol Drying of as-synthesized particles at 100°C Calcination of as-obtained particles at 600°C in a furnace for 3 hrs. Characterization of Particles (XRD, BET, FTIR, EDX) etc University of Ghana http://ugspace.ug.edu.gh 38 autoclaved at 120°C for 7 hours for complete reaction to take place. The reaction mechanism is represented by the Equations 4 to 8. After autoclaving, the precipitates from the precursor solution were washed several times with water and ethanol via a centrifuge to rid the precipitates of any impurity. The washed precipitates were then dried in an oven at 100°C overnight after which the dried precipitates were then calcined in a furnace at 600°C for 3 hrs. The final product was characterized using X-Ray Diffraction, Energy Dispersive X-ray, Scanning Electron Microscope, Fourier Transform Infrared spectroscopy and Bruner Emmet Teller. The same procedure was repeated for the other samples. The flow chart for the synthesis process is presented in Figure 3.2. Reaction equation for precursor solution using urea + -2 2(s) 2 2(g) 4CO(NH ) +H O CO +2NH +2OH (4) 4+ - 4Sn +4OH Sn(OH) (5) heat4 2 2Sn(OH) SnO +2H O (6) Reaction equation for precursor solution using Sodium Borohydride 4(s) 4(s) (s) 4 4(s)SnCl +NaBH NaCl +Sn(BH ) (7) 4 4(s) 2 (l) 4(s) 4Sn(BH ) +H O Sn(OH) +BH (8)   heat4 2(s) 2 (g)sSn(OH) SnO +2H O (9) University of Ghana http://ugspace.ug.edu.gh 39 Table 3.1: Sample groups and designation for the hydrothermal synthesis Sample ID SnCl2.2H2O Reducing Agent Surfactant A 2.1g SnCl2.2H2O 2.6g Sodium Borohydride - AA 2.1g SnCl2.2H2O 2.6g Sodium Borohydride 2.0 g AOT AT 2.1 g SnCl2.2H2O 2.6 g Sodium Borohydride 2.0 g CTAB C 2.1 g SnCl2.2H2O 2.6 g Urea - CA 2.1 g SnCl2.2H2O 2.6 g Urea 2.0 g AOT CT 2.1 g SnCl2.2H2O 2.6 g Urea 2.0 g CTAB University of Ghana http://ugspace.ug.edu.gh 40 Figure 3.2: The process flow chart used for synthesis of nanosized SnO2 powders by hydrothermal synthesis. Surfactant and metal salt precursor Reducing agent Stirring to obtain a homogenous solution for 15 minutes Autoclaving of solution to form precipitate Washing of precipitate via centrifugation with water and ethanol Drying of as-obtained particles at 100°C Calcination of particles at 600°C for 3 hrs. Characterization of Particles (XRD, BET, FTIR, EDX) etc Mixture of water and ethanol University of Ghana http://ugspace.ug.edu.gh 41 (a)Dissolution process for SnO2 particle formation (b) Autoclaving (c)Washing of as-synthesized particles via a centrifuge (d) as-synthesized SnO2 (e) Drying of the as-synthesized particles (f) Calcination of as-synthesized particles Figure 3.3: Synthesis process of SnO2 particles University of Ghana http://ugspace.ug.edu.gh 42 3.4 Chemical characterization of SnO2 nanoparticles. 3.4.1 X-Ray Diffraction Figure 3.4: Panalytical Empyrean X-ray Diffractometer To determine the phases present and the microstructure of the SnO2 nanoparticles, X-ray powder diffraction (XRD) patterns were collected on an Empyrean diffractometer (Panalytical BV, Netherlands) in Figure 3.4 with theta/theta geometry, operating Cu Kα radiation tube (λ = 1.5418 Å) at 40 kV and 45 mA. The XRD patterns of all the randomly oriented powder specimens were recorded in the 20°- 70° 2θ range with a step size of 0.017° and a counting time of 14 s per step. The diffraction patterns were matched against the ICSD's PDF database and qualitative phase analysis conducted using the X’Pert Highscore plus search match software (Panalytical, Netherlands). The instrumental resolution function was characterized with the NIST SRM 640d (Si) standard [37] and all peak profiles were simultaneously fitted University of Ghana http://ugspace.ug.edu.gh 43 with symmetrical pseudo-Voigt functions whose width and shape were constrained according to the Caglioti et al. formulae [38]. Microstructural analysis was performed using the Whole Powder Pattern Modelling (WPPM) method [39], with the aid of the PM2K software [40]. 3.4.2 Scanning Electron Microscopy (SEM) A high resolution Zeiss Ultra plus 55 field emission scanning electron microscope (FESEM) operated at 2.0 KV was used in the surface morphological investigations of the as-produced particles. Figure 3.5: Zeiss ultraplus FESEM University of Ghana http://ugspace.ug.edu.gh 44 3.4.3 Fourier Transform Infrared (FTIR) Spectroscopy Transmission FTIR spectra were recorded on a Vertex 70v (Bruker) spectrometer in the 4000- 400 cm-1 range with 4 cm-1 resolution. Sample compartment was evacuated during acquisition and the contact between the sample and the diamond ATR crystal is 2 mm diameter. Spectra were recorded and analyzed with the Opus software. Figure 3.6: Vertex 70v (Bruker) FTIR spectrometer 3.5 Preparation and Electrochemical Characterization of SnO2 Electrodes 3.5.1 Pretreatment of the Ni foam substrate The Ni foam substrate was carefully etched with hydrochloric acid (HCl 2 M) for thirty minutes to ensure the removal of NiO layer on the surface. The etched Ni substrate was then rinsed in ethanol and distilled water severally via an ultrasonic cleaner. University of Ghana http://ugspace.ug.edu.gh 45 3.5.2 Preparation of the SnO2 Working Electrodes. The working electrode for electrochemical evaluation were prepared by mixing 98 wt% of SnO2 nanoparticles from the various treatments and 2 wt% polyvinylidene difluoride (PVdF) binder in a mortar. The mixture was then dissolved in 1-methyl-2-pyrrolidinone (NMP) to form a slurry. The slurry was coated on the treated Ni foam with dimensions 3 cm x 1 cm and pore size of 450 µm and dried at 80°C overnight to ensure complete evaporation of the NMP. Figure 3.7 illustrate the SnO2 particles electrode making process. 3.5.3 Characterization of the Electrodes The electrochemical test of the SnO2 electrodes was performed in a three electrode configuration using glassy carbon rod as counter electrode and Ag/AgCl (3 M KCl) as the reference electrode in 1 M KOH electrolyte. The electrochemical properties of the supercapacitor were studied using cyclic voltammetry (CV), Chrono potentiostat charge- discharge and electrochemical impedance spectroscopy (EIS) using Autolab PGSTAT workstation 101 driven by the NOVA software. The CV tests were carried out in the potential range of 0 to 0.5 V at different scan rates ranging from 5 mV/s to 100 mV/s. the EIS measurements were performed in the frequency range of 100 kHz-10 mHz. The EIS data were analyzed using Nyquist plots in Figure 4.18. University of Ghana http://ugspace.ug.edu.gh 46 (a) Mixing of particles with PVdF binder (b) Coating of slurry on the Ni substrate (c) Electrochemical measurement setup (d) SnO2 working electrode Figure 3.7: SnO2 electrode making and electrochemical testing process University of Ghana http://ugspace.ug.edu.gh 47 CHAPTER FOUR 4.0 Results and Discussions 4.1 pH of Precursor Solutions The pH of the precursor solutions were measured with a pH meter and recorded in Table 4.1. Table 4.1: pH of precursor solutions for as-synthesized SnO2 nanoparticles by different treatments. Sample Id Method pH A SnCl2.2H2O + Sodium Borohydride 9.03 AA SnCl2.2H2O + Sodium Borohydride + AOT 9.18 AT SnCl2.2H2O + Sodium Borohydride + CTAB 8.94 C SnCl2.2H2O + Urea 8.27 CA SnCl2.2H2O + Urea + AOT 8.42 CT SnCl2.2H2O + Urea + CTAB 8.12 From the Table 4.1 it can be seen that when sodium borohydride was used as the reducing agent (samples A, AA, AT), the pH was higher than when urea was used as the reducing agent (samples C, CA, CT). It can also be observed that, the addition of CTAB to both sodium borohydride and urea precursor solutions (samples AT, CT) decreased the pH and this could be attributed to the fact that, CTAB in aqueous solution dissociate to produce 4NH ions University of Ghana http://ugspace.ug.edu.gh 48 which consumes OH- ions in the aqueous medium. Hence, there is OH- deficiency and more H+ in solution, therefore there is decrease in pH. The addition of AOT (Samples AA, CA) tends to increase the pH and this is because, in aqueous medium AOT dissociates to produce 7SO ions which consumes the H + ions in solution hence increasing the OH- ions and pH. 4.2 X-Ray Diffraction Analysis 20 30 40 50 60 70 0 10000 20000 30000 40000 50000 60000 70000 80000 I n t e n s i t y ( c o u n t s ) 2Theta (degrees) (101) (200) (211) (220) (301) (310) (111) (002) (112) (110) A AA AT C CA CT E Figure 4.1: XRD patterns of SnO2 nanoparticles The diffraction peaks in Figure 4.1 are markedly broadened, which indicate that the crystalline sizes of samples are very small with the average size estimated from Figure 4.2 being 2.1 nm, 2.2 nm,5.0 nm, 4.0 nm, 9.5 nm and 8.6 nm for A, AA, AT, CA, C and CT respectively. It can be seen from the XRD data that samples produced with sodium borohydride (A, AA and AT) have broader and less intense peaks than samples produced with University of Ghana http://ugspace.ug.edu.gh 49 urea (C, CA and CT) which means that the samples produced with sodium borohydride have smaller crystallite sizes than particles produced with urea. This can be attributed to the fact that samples produced with sodium borohydride have higher pH than samples produced with urea and the higher the pH, the smaller the particles sizes [41]. In a higher pH value solutions, the coordinated water molecules suppress the agglomeration among the freshly formed nanocrystallites [41, 42]. Whole Powder Pattern Modelling (WPPM) approach using the PM2K software which connects a physical model for the microstructure with the diffraction pattern, allowing an extraction of the microstructure parameters without recurring arbitrary peak shapes to fit each diffraction peak was used for the microstructure analysis. A typical diffraction pattern of sample CT modelled with the WPPM is shown in Figure 4.2. The modelling was carried out by assuming the presence of a lognormal distribution (figure 4.3) of aligned spherical domains. The almost featureless nature of the residual line in Figure 4.2 indicates a good agreement between experimental data and model which also suggests that the shape assumption is right for the domains investigated. University of Ghana http://ugspace.ug.edu.gh 50 20 25 30 35 40 45 50 55 60 65 70 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 I n t e n s i t y ( c o u n t s ) 2 Theta (degrees) Experimental Calculated Difference Figure 4.2: SnO2-CT modelled using WPPM University of Ghana http://ugspace.ug.edu.gh 51 0 2 4 6 8 10 12 14 16 18 20 22 24 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 F r e q u e n c y ( a . u ) Domain Size (nm) A AA CA AT C CT Figure 4.3: Lognormal Size Distribution Analysis of the SnO2 particles University of Ghana http://ugspace.ug.edu.gh 52 4.3 Fourier Transform Infra-Red Spectroscopy (FTIR) 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0 20 40 60 80 100 120 140 160 180 200 T r a n s m i t t a n c e [ % ] Wavenumber [cm -1 ] E (3435) (1630) (629) (538) (1122) (2877) (2852) Figure 4.4: FTIR for the water-in-oil microemulsion synthesis (E) of SnO2 nanoparticles The peaks occurring at about 1122 cm-1, 629 cm-1and 538 cm-1in Figure 4.4 are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH [43]. The peak at about 1630 cm-1 belongs to H-OH bonds coming from the moisture content of the powder and the peak at 3435 cm-1 is attributed to Sn-OH bonds [44]. Peaks at 2877 and 2852 cm-1 can also be attributed to hydroxyl groups that are absorbed on the Tin Oxide surface. There are no impurity peaks from the surfactant used, which together with the XRD data confirm that phase pure SnO2 nanoparticles were synthesized using the water-in oil microemulsions technique. University of Ghana http://ugspace.ug.edu.gh 53 4500 4000 3500 3000 2500 2000 1500 1000 500 0 50 60 70 80 90 100 110 120 T r a n s m i t t a n c e [ % ] Wavenumber [cm -1 ] A (3405) (1645) (1008) (1357) (644) (538) Figure 4.5: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent (A). Figure 4.5 represents the FTIR spectra of SnO2 particles prepared using sodium borohydride as a reducing agent. The peak at 3405 cm-1 is attributed to Sn-OH bonds. Peaks at 1008 cm-1, 644 cm-1 and 538 cm-1 are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH. Peak at 1645 cm-1and 1357 cm-1 are attributed to H-OH bonds coming from the moisture content of the powder and Sn-OH bonds respectively. University of Ghana http://ugspace.ug.edu.gh 54 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0 20 40 60 80 100 120 140 160 180 200 T r a n s m i t t a n c e [ % ] Wavelength [cm -1 ] AA (3420) (1645) (644) (507) (1342) (1433) (993) (1114) (2449) (826) Figure 4.6: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent and AOT as the surfactant (AA) The hydrophobic group R-SO3 Na+ of AOT is responsible for the peak at 993 cm-1 and 1114 cm-1. From the above result (figure 4.6), it is obvious that AOT molecules are adsorbed in the pores of the powders [45]. 2449 cm-1 , 3420 cm-1 and 1342 cm-1 peaks are attributed to Sn-OH bonds [44]. Peak at 1645 cm-1 is also assigned to H-OH bonds from the moisture content of the powder. 1433 cm-1 peak corresponds to bending vibrations of –CH2. Which shows that a few organic groups are absorbed on the surfaces of SnO2 nanoparticles [31] which could be as a result of combustion of the organic groups. University of Ghana http://ugspace.ug.edu.gh 55 4500 4000 3500 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 80 90 T r a n s m i t t a n c e [ % ] wavenumber [cm -1 ] AT (3420) (2343) (993) (1645) (644) (2920) (542) Figure 4.7: FTIR spectra of SnO2 nanoparticles with sodium borohydride as the reducing agent and CTAB as the surfactant (AT). From the Figure 4.7, the peak at 2920 cm-1 can be attributed to CH stretching [10]. The peak at 3420 cm-1, 1645 cm-1, and 993 cm-1 are assigned to Sn-OH bonds, H-OH bonds from the moisture content of the powder and the hydrophobic group R-SO3 Na+ of AOT respectively [45]. 644 cm-1 542 cm-1 peaks are attributed to Sn–O stretching modes of Sn–O–Sn. University of Ghana http://ugspace.ug.edu.gh 56 4500 4000 3500 3000 2500 2000 1500 1000 500 0 20 40 60 80 100 120 140 T r a n s m i t t a n c e [ % ] wavenumber [cm -1 ] C (3405) (1630) (962) (644) (507) (2920) (2859) Figure 4.8: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent (C) The peaks at 3405 cm-1, 1630 cm-1, and 962 cm-1 in the Figure 4.8 are assigned to Sn-OH bonds, H-OH bonds from the moisture content of the powder. 2920 cm-1and 2859 cm-1 are attributed to alkyl CH stretching [46]. 644 cm-1 and 507 cm-1 are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH, respectively [43, 47]. University of Ghana http://ugspace.ug.edu.gh 57 4500 4000 3500 3000 2500 2000 1500 1000 500 0 20 40 60 80 100 120 140 160 T r a n s m i t t a n c e [ % } Wavenumber [cm -1 ] CA (3420) (1630) (1129) (962) (644) (538) (2343) (2874) (2935) Figure 4.9: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent and AOT as the surfactant (CA) From Figure 4.9, peaks at 644 cm-1 and 538 cm-1 are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH, respectively. 2935 cm-1and 2874 cm-1 are attributed to alkyl CH stretching. The peak at 3420 cm-1, 2343 cm-1, 1630 cm-1, and 962 cm-1 are assigned to Sn-OH bonds, H-OH bonds from the moisture content of the powder [40-44]. University of Ghana http://ugspace.ug.edu.gh 58 4500 4000 3500 3000 2500 2000 1500 1000 500 0 0 20 40 60 80 100 120 140 T r a n s m i t t a n c e [ % } Wavenumber [cm-1] CT (3420) (2343) (1630) (644) (538) (962) (2920) Figure 4.10: FTIR spectra of SnO2 nanoparticles with urea as the reducing agent and AOT as the surfactant (CA) From Figure 4.10, the peaks at 644 cm-1 and 538 cm-1 are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH, respectively. 2935 cm-1and 2874 cm-1 peaks are attributed to alkyl CH stretching. The peak at 3420 cm-1, 2343 cm-1, 1630 cm-1, and 962 cm-1 are assigned to Sn- OH bonds, H-OH bonds from the moisture content of the powder [40-44]. The FTIR results in Figures 4.4 to 4.10 indicate that synthesized powder is tin oxide containing some degree of moisture, residual hydroxide and some AOT content. Residual hydroxide content is an indication of either incomplete conversion of hydrated tin oxide or some reaction between SnO2 and H2O due to very high surface area of the synthesized powder and the residual AOT from inadequate washing of precipitates before drying [28]. University of Ghana http://ugspace.ug.edu.gh 59 4.3 SEM micrographs of the as-synthesized SnO2 nanoparticles A AA AT C at 5µm magnification C at 1µm magnification CT University of Ghana http://ugspace.ug.edu.gh 60 (E) at 5µm magnification (E) at 200nm magnification CA at 5µm magnification CA at 1µm magnification Figure 4.11: SEM Micrographs of SnO2 nanoparticles of the various treatments The SEM micrographs in Figure 4.11 clearly shows different morphologies for SnO2 prepared in different surfactants. The shape of the agglomerated sample C looked like florets of cauliflower. Nanorods were formed for samples A and AA. Uniform spherical-like morphologies were observed for sample E. For samples prepared with CTAB (CT and AT), agglomerated particles were formed and this is because, in an aqueous system, CTAB ionizes completely and results in cation with tetrahederal structure. The electrostatic interaction takes place between CTA+ cations and 2-6Sn(OH) anions, the CTA + cations condense into aggregates in which counter ions are interrelated in the interfaces condense into aggregates in University of Ghana http://ugspace.ug.edu.gh 61 which counter ions 2-6Sn(OH) are interrelated in the interfaces between the head group to form CTA+- 2-6Sn(OH) pair[10, 47]. AOT being an anionic surfactant, it was found that there is no direct interaction between the 2-6Sn(OH) ions and the 7SO head of the AOT since both are similarly charged. Both the 2-6Sn(OH) nanoparticles and micelles coexist individually with no significant change in the structure of the micelles [48]. The different morphology reveals the role of individual reducing agents and surfactants in controlling the nucleation and crystal orientation. University of Ghana http://ugspace.ug.edu.gh 62 4.4 Energy Dispersive X-ray Spectrum (EDX) Analysis of the as-synthesized SnO2 nanoparticles. E C CT CA A AT 0 5 1 0 1 5 2 0 E n e r g y ( k e V ) 0 5 1 0 1 5 2 0 2 5 c p s O N a Z r C l S n N i 4 6 8 1 0 E n e r g y ( k e V ) 0 5 1 0 1 5 2 0 2 5 c p s S n S n S n S n S n S n N i P t 0 2 4 6 E n e r g y ( k e V ) 0 5 1 0 1 5 2 0 2 5 c p s O Z r P t S n 0 2 4 E n e r g y ( k e V ) 0 1 2 3 4 5 c p s O N a P t S n 4 6 8 1 0 E n e r g y ( k e V ) 0 2 4 6 8 1 0 c p s S n N i 0 5 1 0 1 5 2 0 E n e r g y ( k e V ) 0 5 1 0 1 5 2 0 c p s O N a S i P t C l S n N i University of Ghana http://ugspace.ug.edu.gh 63 AA Figure 4.12: EDX spectrum of SnO2 nanoparticles of the various treatments 0 2 4 E n e r g y ( k e V ) 0 5 1 0 1 5 2 0 c p s O N a A l S i P t C l S n University of Ghana http://ugspace.ug.edu.gh 64 Table 4.2: EDX atomic % of the synthesized SnO2 nanoparticles Sample Element (Atomic %) Atomic ratio of Sn:O O Sn A 66.29 30.21 1:1.19 AA 54.95 30.33 1:1.81 AT 63.31 33.04 1:1.92 C 81.06 61.54 1:1.32 CA 58.37 36.45 1:1.60 CT 76.46 21.51 1:3.55 E 60.05 38.62 1:1.54 According to the EDX analysis, the atomic ratio of Sn to O (Table 4.0) for all the treatments agrees with tin oxide being an n-doped semiconductor with intrinsic carrier density primarily determined by oxygen vacancies. The EDX shows that Sn and O are present which is in agreement with the XRD results [9]. University of Ghana http://ugspace.ug.edu.gh 65 4.5 Electrochemical Characterization of the obtained SnO2 nanoparticles 0.0 0.1 0.2 0.3 0.4 0.5 -0.00015 -0.00010 -0.00005 0.00000 0.00005 0.00010 0.00015 0.00020 Potential( V vrs Ag/Agcl) 1mV/s 5mV/s 10mV/s 15mV/s 20mV/s . . . . . . -0.0006 -0.0004 -0.0002 0.0000 0.0002 0.0004 . 06 C u r r e n t ( A ) ( Cl) AA 1mV/s 5 /s 10 /s 15 /s 20 /s A AA 0.0 0.1 0.2 0.3 0.4 0.5 -0.0003 -0.0002 -0.0001 0. 0. 1 0.0002 0.0003 0.0004 C u r r e n t ( A ) otential (V vrs Ag/AgCl) AT 1mV/s 5mV/s 10mV/s 15mV/s 20 V/s . . . . . . -0.00010 -0.00005 . 00 0.00005 0.00010 0.00015 0.00020 C u r r e n t ( A ) P t ti l ( / l C 1mV/s 5 V/s 10 /s 15m /s 20mV/s AT C 0.0 0.1 0.2 0.3 0.4 0.5 -0. 4 -0.0002 . 0 0.0002 0.0004 0.0006 C u r r e n t ( A ) Potential (V vrs Ag/AgCl) /s CA 5mV/s 0 /s 15m / 20mV/s . . . . . . -0. 03 -0.0002 -0.0001 .0000 .0001 .0002 .0003 0.0004 C u r r e n t ( A ) t ti l ( / l) 1mV/s CT University of Ghana http://ugspace.ug.edu.gh 66 CA CT 0.0 0.1 0.2 0.3 0.4 0.5 -0.0004 -0.0003 -0.0002 -0.0001 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 C u r r e n t ( A ) Potential (V vrs Ag/AgCl) 1mV/s E 5mV/s 10mV/s 15mV/s 20mV/s E Figure 4.13: Cyclic Voltagrams of the as-obtained SnO2 nanoparticles at different scan rates. From the Figure 4.13 above, it can be observed that, the current under the curve increased with the increasing scan rate and this due to the reaction time being shorter and voltammetric current is increased if the reversibility is excellent [49]. Results also show two main peaks, a broad cathodic peak, and anodic peaks corresponding to redox peak of Sn4+/Sn3+. The electron addition process for SnO2 semiconductor electrode can be written as [50] IV - III -2 (external) (solution)Sn O +e +H Sn O(OH )  (11) Table 4.3: Capacitance values estimated from the Cyclic Voltamogram Sample Capacitance (F/g) 5 mV/s 10 mV/s 15 mV/s A 0.5427 0.3462 0.3732 AA 1.5687 1.2358 1.036 AT 0.937 0.835 0.748 University of Ghana http://ugspace.ug.edu.gh 67 C 0.493 0.3567 0.2743 CA 1.288 1.071 0.8969 CT 0.7 0.65 0.57330 E 1.03 0.5855 0.5027 4 6 8 10 12 14 16 18 20 22 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 C a p a c i t a n c e ( F / g ) A CA AT C AA CT E Scan rate (mV/s) Figure 4.14: Specific capacitance of the synthesized SnO2 particles From Figure 4.14, the specific capacitance generally decreases with increasing scan rate. The specific decrease of capacitance with increase in scan rate can be attributed to electrolytic ions diffusing and migrating into the active materials at low scan rates. At high scan rates, the diffusion effect, limiting the migration of the electrolytic ions, causes some active surface areas to become inaccessible for charge storage [51]. Specific capacitance of the nanoparticles was calculated from cyclic voltammetry (CV) curves in Figure 4.13 using the following equation: University of Ghana http://ugspace.ug.edu.gh 68 Cs = i sm (12) where Cs is the specific capacitance, i is the average cathodic current, s is the scan rate and m is the mass of the electrode. It is reported that, the factors affecting the capacitance are particle sizes and electrochemical conditions, some of which are type of electrolyte, concentration of electrolyte and scan rate. Some other factors affecting the capacitance are surface activation under the electrochemical conditions, oxygen content on the surface, surface oxides and lattice defects resulting from the method of preparation [49]. The highest capacitance recorded for the SnO2 nanoparticles was estimated to be 1.6 F/g at a scan rate of 5 mV/s (Table 4.3). The low values of capacitance recorded could be attributed to low conductivity of the SnO2 nanoparticles which is evident from the high internal resistance values recorded in Table 4.4. The observed specific capacitance values are comparable with reported values of SnO2 [33]. It can also be seen from Table 4.3 that samples with AOT (AA and CA) as the surfactant have higher capacitance than samples with CTAB (AT and CT) as the surfactant and samples without surfactant addition (A and C). This could be attributed to the fact that for samples prepared with CTAB there was agglomeration of particles, hence the porosity decreased as compared with samples prepared with AOT. This is confirmed by the SEM micrographs from Figure 4.11. Enhanced porosity allows for more electrolyte penetration and hence increases charge storage [52]. Although the average crystallite size for sample A was smaller and as such had a larger surface area compared to the other samples, it still had a lower capacitance value recorded. This could be attributed to the fact that there was less electro active surface for the transfer of electrons and diffusion of ions [53]. Since not all the specific surface area is electrochemically accessible when the material is in contact with an electrolyte, the measured capacitance of various materials does not linearly increase with increasing specific surface University of Ghana http://ugspace.ug.edu.gh 69 area. The pore size of the electrode material plays an important role in the electrochemical active surface area. In addition, the intrinsic electrical resistance of the material may increase because the bonded heteroatom (O) possess higher reactivity, resulting in barriers to electron transfer [20]. 5.82 5.84 5.86 5.88 5.90 5.92 5.94 5.96 5.98 6.00 -0.8 -0.6 -0.4 -0.2 0.0 0.2 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3mA 4mA 5mA A 5.86 5.87 5.89 5.90 1 5.92 5.93 5.94 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 P o t e n t i a l ( V ) Time (s) 1mA A 2mA 3mA 4mA 5mA A AA 5.82 5.84 5.86 5.88 5.90 5.92 5.94 -0.8 -0.7 - .6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3mA 4mA 5mA AT 4 5.85 5.86 5.87 5.88 5.89 5.90 . 1 5.92 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3m 4mA 5mA C AT C University of Ghana http://ugspace.ug.edu.gh 70 5.82 5.84 5.86 5.88 5.90 5.92 5.94 -0.8 -0.6 -0.4 -0.2 0.0 0.2 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3mA 4mA 5mA CA 5.90 5.91 92 5.93 5.94 5.95 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3mA 4mA 5mA CT CA CT 5.78 5.80 5.82 5.84 5.86 5.88 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 P o t e n t i a l ( V ) Time (s) 1mA 2mA 3mA 4mA 5mA E E Figure 4.15: CCCD of the SnO2 nanoparticles at different current densities University of Ghana http://ugspace.ug.edu.gh 71 Table 4.4: Internal resistance values of the SnO2 nanoparticles. Sample Ri (Ω) 1 mA 2 mA 3 mA 4 mA 5 mA A 78.0 39.8 26.7 20.1 16.1 CA 55.0 27.8 18.5 13.8 12.8 E 59.0 30.0 20.0 15.1 12.1 AT 62.0 30.8 20.5 15.5 12.4 C 83.0 41.0 27.7 20.7 16.5 CT 62.0 32.3 21.0 16.0 12.3 AA 45.0 23.0 15.3 11.8 9.4 1 2 3 4 5 10 20 30 40 50 60 70 80 90 R i ( o h m ) Current (mA) A AA AT C CA CT E Figure 4.16: Internal resistance estimated from ohmic drop of the CCCD curves. University of Ghana http://ugspace.ug.edu.gh 72 Table 4.5: Efficiency values of the SnO2 nanoparticle electrodes Sample Columbic efficiency (ƞt) % 1 mA 2 mA 3 mA 4 mA 5 mA A 51.3 50.8 50.5 50.5 50.5 AA 52.3 52.3 52.3 52.3 49.7 AT 51.3 50.3 50.3 49.5 49.5 C 50.8 50.8 50.5 50.3 50.3 CA 51.8 51.0 51.3 53.9 50.3 CT 51.0 51.0 51.0 50.8 50.8 Figure 4.15 compares the charge-discharge curves of the SnO2 particles at different currents. They all show nonlinear behavior, demonstrating the redox process of the SnO2 particles. This is also in agreement with the cyclic voltamogram in Figure 4.13. The efficiencies Ƞ of the SnO2 electrode material were estimated from the CCCD plots in Figure 4.15 using the equation: Ƞ = D C t t (13) where tD and tC are the discharge and charge durations (s). The low energy efficiencies are essentially related to the inability of the SnO2 electrodes to store enough charges as result of their high internal resistance. Table 4.6: Capacitance retention values of the SnO2 electrode University of Ghana http://ugspace.ug.edu.gh 73 Sample Capacitance of 1st cycle (F/g) Capacitance of 10th cycle (F/g) Capacitance retention (%) A 0.052102083 0.016265957 31.21939931 AA 0.04552381 0.018005556 39.55195258 AT 0.04866875 0.015778723 32.42064652 C 0.066546939 0.018553191 27.87985718 CA 0.066688421 0.024827083 37.22847676 CT 0.049791579 0.015761538 31.65502841 E 0.037070526 0.012644048 34.10808768 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 26 28 30 32 34 36 38 C A AA AT CA CT E C a p a c i t a n c e r e t e n t i o n ( % ) Cycle number Figure 4.17: Capacitance retention of the SnO2 electrodes after 10cycles University of Ghana http://ugspace.ug.edu.gh 74 The capacitance retention (Figure 4.17) was estimated from the CCCD curves in Figure 4.15 by using the equation capacitance of the first cycleCapacitance retention= 100%capacitance of the 10th cycle X (14) The capacitance retention also agrees with the discussions made for Figures 4.14. 50 75 100 125 150 175 200 1 - Z ' ' ( o h m ) Z' (ohm) E AA CT CA A AT C Rs=44ohms Figure 4.18: The Nyquist plot for the as-synthesized SnO2 nanoparticles Figure 4.18 shows the Nyquist plot, which represents the real and imaginary parts of the impedance in a sample. The x-axis intercept in the high frequency region corresponds to the resistance of the electrolyte solution (Rs) which is also referred to as the equivalent series resistance (ESR), which consist of the resistance of the aqueous electrolyte, the intrinsic resistance of the SnO2 electrode, and the contact resistance at the electrode. The impedance spectra usually show a single semicircle in the high frequency region and nearly vertical line in the low frequency region for a supercapacitor, which indicates that the electrode process is University of Ghana http://ugspace.ug.edu.gh 75 controlled by electrochemical reaction at high frequencies and by mass transfer at low frequencies [43]. The origin of the semi-circle at higher frequency range is due to ionic charge transfer resistance (Rct) at the electrode–electrolyte interface. The diameter of the semi-circle along the real axis (Zdia) gives the Rct [54]. It is also worth mentioning that for ideal supercapacitors, the Nyquist plot should be a line perpendicular to the real axis at low frequency. It also confirms the fact that porosity affects the electrolyte interaction with the SnO2 particles which in turn affects the capacitance. The ESR for the samples increase accordingly as CA (31.5 Ω)