CHAPTER II: THEORETICAL BACKGROUND OF ELECTRODEPOSITION, ELECTROCHEMICAL SUPERCAPACITOR AND THIN FILM CHARACTERIZATION TECHNIQUES CHAPTER II

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1 Sr. No. CHAPTER II THEORETICAL BACKGROUND OF ELECTRODEPOSITION, ELECTROCHEMICAL SUPERCAPACITOR AND THIN FILM CHARACTERIZATION TECHNIQUES Name Page 2.1 Thin Film: Introduction 35 SECTION- A THEORETICAL BACKGROUND OF ELECTRODEPOSITION 2.A.1 Introduction 39 2.A.1.1 Basics of Electrodeposition 40 2.A.1.2 Steps Involved in Electrodeposition process 43 2.A.1.3 Pathways for the growth of an electrodeposits 44 2.A.2 Different modes of electrodeposition method 45 2.A.3 Effect of preparative parameters 48 SECTION- B THEORETICAL BACKGROUND OF ELECTROCHEMICAL SUPERCAPACITOR 2.B.1 Supercapacitors 52 2.B.1.1 Supercapacitors Based on Double Layer 53 2.B.1.2 Supercapacitors Based on Pseudocapacitor 55 2.B.1.3 Supercapacitors Based on Hybrid Capacitors 57 SECTION C THIN FILM CHARACTERIZATION TECHNIQUES 59 2.C.1 Thickness Measurement 59 2.C.2 X-ray diffraction (XRD) technique 60 2.C.3 Fourier Transform Infrared (FTIR) Spectroscopy 2.C.4 Surface Morphological Studies 64 2.C.4.1 Scanning Electron Microscopy and Energy dispersive analysis by X-rays (EDAX) No

2 2.C.4.2 Transmission Electron Microscopy 68 2.C.5 Contact Angle Measurement 69 References 71 34

3 2.1 Thin Film: Introduction Because of two-dimensional solid's potential, technical values and scientific curiosity in the properties, the field of thin films has become evergreen in recent years. Until, sufficient technology progress has not been made to give reasonable scientific confidence to thin film research. The usefulness of the thin films and scientific curiosity about twodimensional solids have been responsible for the immerse interest in the study of the science and technology of thin films. Thin films have been used in the study of the relationship between the structure of solids and their physical properties. Practical applications include electrical circuits, optical instruments and magnetic informationstorage devices. Thickness of the thin film is usually discussed in terms of angstrom units (Å). Thin films are formed by depositing material onto a clean supporting substrate to build up film thickness, rather than by thinning down bulk material. A surface bounded between two parallel plane extending infinitely in two directions restricted in the dimension along third direction. OR Any solid or liquid system possesses at most two-dimensional order or periodicity is called as thin film. The thin solid films were probably first obtained by electrolysis in The conventional bulk material is characterized by threedimensional order in which the constitute atoms or molecules find themselves. This order or periodicity is responsible for the structure/nature of the material, which in turn is at the heart of distinct physico-chemical properties of the materials. In case of thin films, the system possesses at most two-dimensional order or periodicity. This accounts for the vast difference in physico-chemical properties between bulk material and its thin film counterpart. Applications of thin film technology have revolutionized the field of optics, electronics, sensors, 35

4 energy storage device (supercapacitor) and magnetism. Many of applications like sensors, electrodes in fuel/electrochemical cells, photosensitive coatings etc. requires material is to be in the nanocrystalline form for enhancement in their properties. Nanocrystalline materials offers them large surface area, enhanced diffusivity, reduced density, higher electrical resistivity, increased strength/hardness, high thermal expansion coefficient, lower thermal conductivity and superior soft properties in comparison with conventional coarse-grained materials. There are two general ways available to produce nanomaterials. The first way is to start with a bulk material and then break it into smaller pieces using mechanical, chemical or other form of energy (top-down). An opposite approach is to synthesize the material from atomic or molecular species via chemical reactions, allowing for the precursor particles to grow in size (bottom-up). Most of researcher and manufacturers are interested in the ability to control: a) particle size, b) particle shape, c) size distribution, d) particle composition and e) degree of particle agglomeration. And many novel as well as traditional chemical routes of synthesis (bulk/thin films) allow materials to be in the nanocrystalline form. The properties of thin films depend on the method of deposition. The required properties and versatility can be obtained by choosing proper method of thin films deposition. Thin film deposition methods can be broadly classified as either physical or chemical. Under physical methods, we have vacuum evaporation and sputtering, where the deposition takes place after the material to be deposited has been transferred to a gaseous state either by evaporation or an impact process. Under chemical methods, we have the gas phase chemical processes such as conventional chemical vapor deposition (CVD), laser CVD, photo CVD, metal organo-chemical vapor deposition (MOCVD) and plasma enhanced CVD. Liquid phase chemical methods include electrodeposition, chemical 36

5 bath deposition (CBD), modified chemical bath deposition (M-CBD), successive ionic layer adsorption and reaction (SILAR), anodization, spray pyrolysis, liquid phase epitaxy etc. The broad classification of thin film deposition techniques is outlined in Chart 2.1. Chart 2.1: Broad classification of thin film deposition techniques The physical methods have many drawbacks such as small area of deposition, requirement of sophisticated instruments, high working cost of 37

6 system, wastage of depositing material, cleaning after each deposition etc. The stoichiometric problem encounters in producing ternary compounds (oxide) thin films because of creation of point defects and secondary phases. Keeping drawbacks of physical methods in mind, recently much emphasis has been put on the soft solution chemical processes for the preparation of advanced inorganic materials such as pervoskite-type oxides [1], spinel-type oxides [2], nanowires [3] and nanodots with quantum size effect. Chemical methods are simple, economic and convenient for the deposition of metal chalcogenide thin films. The preparative parameters such as concentration, ph, nature of the complexing agent, temperature etc are easily controllable. The low temperature deposition avoids oxidation and corrosion of metallic substrates. Chemical method results into pinhole free and uniform deposition. Therefore, these soft solution chemical processes are important for the preparation of the nanocrystalline particles. Such processes include sol-gel, atomic layer deposition, electrochemical deposition, spray pyrolysis, dip coating, solution hydrolysis, chemical bath deposition etc. Among all these thin film deposition techniques, electrochemical deposition offers a wide range of advantages over more expensive other methods of thin film deposition. Along with being a simple and economic method, it has its own advantage of no wastage of material, does not require very pure starting material etc. The art and science of electrodepositing metal and metallic alloys and anodization have been developed from more than a century [4]. This chapter is divided into three sections; Section-A deals with theoretical background of electrodeposition, Section-B deals with theoretical background of supercapacitors and section C deals with experimental characterization techniques of thin films. 38

7 2. A. 1 Introduction: SECTION- A: THEORETICAL BACKGROUND OF ELECTRODEPOSITION To begin, electrodeposition is a fascinating phenomenon that one can put a shiny coating of one metal on another simply by donating electrons to ions in a solution is remarkable, and studies of the process at an atomic level continue to yield surprises. Electrodeposition is exceptionally versatile, and valuable applications keep being invented. This method is well known due to inexpensive and simple process of fabrication of metallic coatings. Also the electrodeposition can made a wide range of nanostructured materials. While electrodeposition continues to be widely used for protective or decorative coatings, challenging new applications have been found in the electronics industry, particularly exciting developments include the development of thin film magnetic recording heads for hard disks, and the recent replacement of aluminum and its alloys by electrodeposited copper for interconnects in ultra large scale integrated circuits [5]. Electrodeposition has become attractive due to its advantages over the other physical and chemical deposition techniques. The semiconductor properties like n- or p- type conductivity; band gap variation, control over stoichiometry, doping etc. can be controlled with a resemble accuracy. In recent years, materials (metals, semiconductors, ceramics, superconductors, conducting polymers, etc) in thin film form have been prepared by electrodeposition technique. Preparation of thin films using electrodeposition technique has several attractive features. 1. Structurally and compositionally modulated alloys and compounds can be deposited which are not possible with other deposition techniques. 2. In most of the cases the deposition can be carried out at room temperature enabling to form the semiconductor junctions without interdiffusion. 39

8 3. Deposition on complex shapes is possible. 4. Toxic gaseous precursors need not to be used (unlike gas phase methods). 5. The deposition rate is higher than other physical and chemical methods. 6. The equipments needed are not expensive and does not require sophisticated instrumentation and vacuum. 7. Reactions involved in the deposition process occur closer to the equilibrium than many gas phase methods and the deposition process can be controlled more accurately and easily. Even with above advantageous points, electrodeposition has interesting feature that, direct cathodic/anodic electrodeposition from aqueous and non-aqueous baths is possible and it can be employed as one of the steps in the preparation of semiconductors or oxides. There are three modes of electrodeposition 1) Potentiostatic (At constant potential) 2) Galvanostatic (At constant current) 3) Potentiodynamic (potential and current variables with time) 2.A.1.1 Basics of Electrodeposition Electrodeposition is process of depositing metal atoms on a conducting substrate by passing direct current through solution containing the metal(s) ions to be deposited. The schematic diagram explaining the electrodeposition is shown in Fig. 2.A.1. The typical electrodeposition set up consists of following components, 1. Electrolyte 2. Cathode and anode 3. Source of electricity. 40

9 Fig. 2.A.1 The schematic diagram of electrodeposition When the direct current is passed through cathode and anode immersed in electrolyte containing the metal(s) ions, the metal ions get attracted towards the cathode, neutralized electrically by receiving electrons and get deposited on cathode. The deposition is controlled by controlling the amount and the rate of charge passing through the electrolyte. Thus the electrical energy is used to cause chemical change. The net result is that metal (cation) is deposited on the cathode from the solution of metal ions according to following process. M n+ + ne - M (2.1) On the other hand, if the electrolyte contains more than one ionic species that can be simultaneously deposited, then the electrodeposition process for, say two types of ionic species can be written as M + + e - M (2.2) N + + e - N (2.3) or M + + N + + 2e - MN (2.4) Accordingly, one can deposit a compound or an alloy of a multicomponent system. When an electrolysis (chemical changes due to electricity) is carried out in the electrolyte, metal is deposited on cathode at the same time anode (metal) is dissolved in the solution. The amount of 41

10 dissolution and deposition is determined by the quantity of electricity passed. Faraday s Laws of Electrolysis Michael Faraday (1834) established the relationship between the electricity passed through the electrolyte and the chemical change produced in terms of solid material liberated/deposited at the electrode. Faraday s first law The amount of substance liberated or deposited on the electrode is proportional to the quantity of electricity passed. Mathematically, W Q where, W is the amount of substance liberated in grams, and Q is the quantity of electricity passed through electrolyte, in coulombs. If current strength I is passed for t seconds, then the quantity of electricity. Quantity of electricity = Current strength x time Q = I t (2.5) W I t or W = z I t (2.6) Here z is the proportionality constant, known as Electrochemical equivalent. It can be defined as, The amount of substance liberated (in gm) on the electrode on passing a current 1 A for 1 Sec. or passing 1 coulomb of electricity. Faraday s Second Law If same quantity of electricity is passed through different electrolytes, then the amount of substance liberated on the respective electrodes will be in the ratio of their equivalent weights. An important implication of the Faraday s second law is that the ratio of the mass of electrodeposit to its gm-equivalent weight is a constant equivalent to 1 Faraday or 96,500 Coulombs (c) or 26.3 ampere-hour (Ah). 42

11 2.A.1.2 Steps involved in electrodeposition process Electrodeposition of ionic species from the electrolyte occur in following successive steps (Fig. 2.A.2) 1. Ionic transport 2. Discharge 3. Breaking of ion-ligand bond (if the bath is complexed) 4. Incorporation of adatoms on to the substrate followed by nucleation and growth. All above steps occur within A o from the substrate; however each has its own region of operation. These various processes can be classified with respect to distance from the electrolyte as: a. In the electrolyte b. Near the electrode c. At the electrode Fig. 2.A.2: Approximate regions in which various stages of ion transport occur leading to electrodeposition. a. Process in the electrolyte The ions in the electrolyte can move towards the electrode under the influence of 1. Potential gradient leading to ion drift, dφ/dx 43

12 2. Concentration gradient leading to diffusion of ions, dc/dx 3. A density convective current, dρ/dx due to consumption of ions at the electrode. The general mathematical equation including all these processes can be written as Nernst-Planck equation, DC dϕ dc j = zf + D + cv RT dx dx (2.7) Where, F is the Faraday s constant, ν is the viscosity of the electrolyte, R is the gas constant and D is the diffusion coefficient. The three terms in the parenthesis respectively describe the contributions of migration, diffusion and convection processes to the mass transport towards the electrode. b. Processes near the electrode but within electrolyte Ionic species are normally surrounded by a hydration sheath or by other complex forming ion or legand present in the electrolyte. They move together as a single entity and arrive near the electrode surface where the ion-legand system either accepts electrons from the cathode (or donates electrons to the anode). This ionic discharge occurs between 10 to 1000 Å from the electrode. c. Processes that occur on the electrode surface The discharged ions arrive near the electrode, where step by step they lead to the formation of a new solid phase or the growth of an electrodeposit. The atoms deposited have a tendency to form either an ordered crystalline phase or a disordered amorphous phase. The electrodeposit formation steps of transport, discharge, nucleation, and growth are interlinked. 2.A.1.3 Pathways for the growth of an electrodeposit The entire pathway for the growth can be divided into following steps (Fig. 2.A.3) [2]. 44

13 1. Transport of ions in the electrolyte bulk towards the interface. 2. Discharge of ions reaching the electrode surface, giving rise to generation of adatoms. 3. Nucleation and growth, where again alternative routes are possible i. Growth assisted by surface diffusion. ii. Growth assisted by formation of clusters and critical nuclei. iii. Formation of monolayer and final growth of electrodeposit. Surface defects such as steps, kinks and dislocations generally control the growth kinetics. The kinks sites and screw dislocations together sustain the growth of the electrodeposit. Transport (Step-I) Ion in the electrolyte bulk Ion at the interface (Step II) Diffusion to a growth site Step III (Route A) Adatom formation and surface diffusion Step III (Route B) Cluster Formation of growth centre Step IV (Monolayer formation) Step IV (Monolayer formation) Fig. 2.A.3 Schematic representation of steps involved in electrodeposition 2.A.2 Different modes of electrodeposition 1) Potentiostatic- In potentiostatic (i.e. constant dc potential) electrodeposition, different charge transfer reactions proceed under a steady-state condition at rates appropriate to the steady-state interfacial 45

14 overpotential and exchange current density. The choice of overpotential depends on the composition of the bath, the substrate, and the reversible potential of the species to be deposited. The standard electrode potentials serve as an approximate guide in finding the potential at which electrodeposition of a particular species will be possible, but in practice the actual deposition potential depends on a number of factors such as the substrate-depositant interaction, hydrogen overvoltage, interaction between the components during compound electrodeposition, and the polarization characteristics of the bath. Potentiostatic deposition is carried out under either pure activation, diffusion, or mixed control depending on the choice of the deposition potential. Important features of potentiostatic deposition are, i) It controls the potential of working electrode with respect to reference electrode. ii) The potential at counter electrode is driven to the required potential to establish the desired working electrode potential. iii) The output waveform gives the variation of cell current with time. 2) Galvanostatic - With this mode, the deposition of elemental or binary (compound) semiconductor films can be obtained by keeping constant dc current between working and counter electrodes during deposition. For having uniform deposition it is necessary to have constant current density during deposition. Usually cathodic deposition (working electrode at negative potential) is preferred in this mode. The rate of deposition is proportional to the corresponding limiting current densities. The galvanostatic electrodeposition of compound semiconductor is complex as the codeposition depends upon optimum value of current density to be chosen correctly so that steady state overpotential satisfies thermodynamic conditions for codeposition. Important features of galvanostatic deposition are, 46

15 i) It controls the current between counter electrode and working electrode at selected current range. ii) The counter electrode is driven to the potential required to establish the desired cell current. iii) Reference electrode is not used for controlling current but it is used to measure potential at some point in electrochemical cell. iv) The output waveform gives the variation of cell potential with time. 3) Potentiodynamic (Cyclic voltammetry) - This technique is used as an electrochemical analytical as well as varying potential electrodeposition technique. Potentiodynamic or cyclic voltammetry is a method in which electrolysis currents are measured as a function of imposed potential. This technique is a tool for studying the various electrochemical processes taking place during electrolysis such as charge transfer, electrode kinetics etc. It gives qualitative information of electrochemical reaction. By studying cyclic voltammogram which is an electrochemical hysteresis redox potential of an electroactive species in the solution can be determined. In this technique, electrochemical process taking place entirely in the solution phase, a new solid phase is formed on the substrate surface due to the redox state. The solid phase must be formed by oxidation process. Metal oxides can be deposited in single step by using the potentiodynamic method. Film formation takes place by the reduction of metal ion (at reduction potential) on substrate followed by oxidation (at oxidation potential) of metal ions. Several repeated cycles can furnish a desired thickness of the film by following layer by layer deposition. These processes may in general written as, (Z) M-X + ne - (1-Z) M + nx - (2.8) In this solution X - refers to the anion in the solution, Where Z signifies total amount of product form after oxidation, (1-Z) signify the small amount of formed product reduced after deposition. After first cycle 47

16 monolayer of the required material deposited on the substrate. Important features of potentiodynamic (cyclic voltammetry) deposition are, i) It measures the current during triangular potential sweep and observes both the anodic and cathodic responses. ii) The output waveform has both forward and reverse peaks that give information about the electroactivity of electrode or the solution. 2.A.3 Effect of preparative parameters The preparative parameters directly affect the structural, morphological and optical properties of the electrodeposits. The various preparative parameters like substrate, applied field and current density, bath temperature, complexing agents and ph of the bath etc. should be controlled to obtain uniform, smooth and stoichiometric electrodeposits [6, 7]. Some preparative parameters are discussed below. 1. Substrate Substrates play an important role in electrodeposition. Besides providing mechanical support to the electrodeposit, influences on the morphological characteristics of the growing layer and on the electronic and optical properties of the electrodeposit. For the choice of suitable substrate following criteria should be applied for their selection [8]. i. It should have good conductivity because it is one of the electrodes in electrodeposition. A good conductivity is beneficial in improving carrier collection efficiency. ii. The thermal expansion coefficient should match with that of electrodeposit. A mismatch may cause cracking or peeling of the film. iii. It should have good mechanical strength. iv. It should be stable in electrolyte bath. 48

17 v. It should be smooth. Uneven, porous, voids and other irregularities influence the local current distribution. Metals have been widely used as substrates because of their good conductivity, easy availability, lower cost and relative ease of handling. 2. Bath temperature The rise in bath temperature enhances the rate of diffusion and increases ionic mobility, hence the increase in conductivity of the bath. The increase in temperature increases the rate of crystalline growth favoring the coarse deposits. This increase in crystal size corresponds to decrease in polarization. At higher temperature, current densities increase, which increases the rate of nucleation, hence fine-grained, smooth deposits can be obtained. The rise in bath temperature decreases the hydrogen overvoltage so facilitates the evolution of gas, as well as precipitation of basic salts. The opposing effects make it difficult to predict the choice of bath temperature; however, it can be optimized by performing actual experiments. 3. Current density At lower current densities (or overpotentials) the discharge of ions occurs slowly, so the growth rate decreases but increases the crystallinity forming closely packed structures. As the current density is raised, the nuclei formation rate will increase and the deposit becomes fine grained. At higher current densities, the rate of discharge of ions becomes greater compared to rate of supply of ions and there is duplicity of ions near the cathode, which favors the growth perpendicular to the substrate surface. Usually spongy, dendritic growth can be observed under this condition. Secondly, at very higher current densities, hydrogen evolution occurs at faster rate, which interferes the crystal growth and spongy, porous 49

18 deposits may be obtained. This can also favor the precipitation of hydrous oxides or basic salts due to increase in local ph. 4. Metal ion concentration The plating bath is always an aqueous solution containing compound of metal to be deposited. It is always advantageous to use higher concentration of metal components in the bath solution. A high current density can be employed in high metal bearing bath. An increase in metal concentration, under given condition decreases the cathode polarization and increases the crystallite size. 5. Hydrogen ion concentration (ph) In order to operate a bath with optimum efficiency and maintain the desired physical properties of the deposit, control of ph of plating bath is necessary. Besides too low ph may lead to accumulation of hydroxide ions in the vicinity of the cathode and consequent precipitation of basic salts, which may get included in electrodeposition, thereby altering deposit properties. All aqueous solutions contain H + ions; infact in every deposit from an aqueous bath, there is a possibility of the hydrogen gas evolution at the cathode due to H + ions. It takes place, the efficiency of metal deposition is lowered. As this efficiency and hydrogen discharge potential partly depends upon hydrogen ion concentration, at low ph, the bath permits the use of higher current density to produce a sound deposit with relatively high efficiency. 6. Addition agents In almost all cases of electrodeposition of metals, it is observed that addition of small quantities of certain substances often results in the production of smooth, fine grained and nanocrystalline deposits. Such substances are known as addition agents. Addition agent such as 50

19 brightening agents, surfactants, complexants etc. are often added to the bath to obtain smoother, brighter deposits, controllable reaction rates, better adhesion and better texture. The adsorbed additives influence the rate of deposition by i) changing Helmholtz layer potential, ii) acting as a bridge for mediating electron transfer reactions between electrode and discharging species iii) forming complexes between the adsorbed additive and the ionic species to be plated. 7. Complexing agent The unstable metal ions are capable of combining chemically with neutral molecules and with ions of opposite sign to form stable complex ion. The combination is through the covalent bond, when neutral molecules interact with positively charged metal ions to yield negatively charged complex ions. Complex compound in a plating bath serves two purposes. Firstly they make possible to maintain a high metal concentration but low metal ion concentration. The complex ions of the complex compound serve as a reserver and continuously supply of the simple ions necessary for the discharge at the cathode occurs. A low metal ion concentration enables the production of deposits with small grains and improves the throwing power. Secondly, complex formation enables us to enhance appreciably the solubility of slightly soluble salts. 51

20 SECTION- B THEORETICAL BACKGROUND OF ELECTROCHEMICAL 2.B.1 Supercapacitors SUPERCAPACITOR Electrical energy is able to be stored either electrostatically in conventional capacitors, or electrochemically in cells or batteries (electrochemical power sources). For capacitors the electrical energy stored is given by: 1 CV 2 E = 2 (2.9) where V is the electrical potential drop across the capacitor when electrical charge Q resides on its plates; C is the capacitance of the condenser defined as: Q C = (2.10) V The amount of charge as well as the capacitance C is directly proportional to the surface area of the plates. For ideal capacitors, C is a constant and Q and V are linearly related. During charge and discharge electrical charges do not cross the capacitor but move in and out through the external circuit. Batteries store and supply electrical energy through a quite different mechanism. Electrical energy is stored as the chemical energy of reactants which are then transformed at the interface of the two electrodes of an electrochemical cell. A supercapacitor is a compact, electrochemical capacitor that can store an extremely high amount of energy, and discharge that energy at rates demanded specifically by the application. Total energy in supercapacitors is measured in farads, whereas microfarads, nano-farads and pico-farads are used for regular capacitor. The first supercapacitor was developed by General Electric in 1957 using a porous carbon electrode, based on the assumption that carbon increased the surface area of an electrode, producing a higher degree of capacitance. 52

21 Today, supercapacitor can pack up to 100 times the energy of conventional capacitors and deliver ten times the power of ordinary batteries. The modern supercapacitor users on of the three different types of electrodes, including thin fibers of carbon nanotubes which can increase the surface area of an electrode, also called Double Layer Capacitor (DLC) are the most common. Supercapacitors manufactured with metal oxide and conducting polymers, the other two electrode types, are less common and considerably more costly to manufacture. 2.B.1.1 Supercapacitors Based on Double Layer Capacitor: The Electrochemical Double Layer Capacitor store charges in the electrode/ electrolyte interface. High surface area electrodes are used in electrochemical capacitors resulting in large double layer capacitance, and much of the storage capacity which is due to the charging/discharging of the double layer. At some surface oxidation/reduction also occurs, but in contrast to reactions occurring in batteries, this is limited to a monolayer or two on the electrode surfaces. Consequently, the device behaves more like a capacitor than a battery. EDLC is also called "supercapacitor" and "ultracapacitor" depending upon the materials and electrodes used. EDLC typically have much larger power density but much smaller energy density than batteries. Charge is stored electrostatically in polarized liquid layers between an ionically conducting electrolyte and an electrochemicallyconducting electrode. This energy storage mechanism is either based on capacitive (non-faradic) or pseudocapacitive (faradic)[9]. Non-Faradaic The charge accumulation is achieved electrostatically by positive and negative charges residing on two interfaces separated by a vacuum or a molecular dielectric (The double layer or, e.g., a film of mica, a space of air or an oxide film, as in electrolytic capacitors) [10]. 53

22 Faradaic The charge storage is achieved by an electron transfer that produces chemical or oxidation state changes in the electroactive materials according to Faraday s laws (hence the term) related to electrode potential. This is known as pseudocapacitance in supercapacitors. The energy storage is indirect and is analogous to that in a battery [10]. Electrochemical double layer capacitor stores energy using the double layer concept. This double layer is realized when two electrodes immersed in an electrolyte, are polarized. The polarized charges at both the positive and negative electrodes resemble two capacitors connected in series (Fig. 2.B.1). Analyzing the exploded view of the individual electrode, Helmholtz has identified the existence of a 'double layer' in the electrochemical cell in And it is named after him as Helmholtz layer. Helmholtz double layer is a structure of charge accumulation and charge separation that always occurs at the interface when an electrode is immersed into an electrolyte solution. This layer is formed as the solvated ions are blocked and accumulated at the electrode/electrolyte interface. This would create a charge separation between the solvated ions and the electrode material that resembles the charge storage mechanism similar to conventional capacitor, where d is distance between conducting plate and insulating material. In general, positive and negative charges are arrayed at counter position with an extremely short distance, such as atomic distance (~ 10 Å), between both at the contact interface of two different phases where, in this case, the two phases would be the solid carbon electrode and the electrolyte. This interfacial charge distribution layer is explained as the Helmholtz layer in Fig. 2.B.2. The excess charge on the electrode surface is compensated by an accumulation of excess ions of the opposite charge in the solution. This structure behaves essentially as a capacitor as it 54

23 possesses the double layer. The amount of charge is a function of the electrode potential [10, 11]. Fig. 2.B.1 Behavior of electrolyte in ion (positive charge) in the pore when charged and discharge Fig. 2.B.2 Helmholtz double layer 2.B.1.2 Supercapacitors Based on Pseudocapacitor There are two basic reactions, which lead to electrochemical cell. Both occur at the interface between a conductor and an electrolyte and both benefit from very high specific surface areas at the electrode. Surface areas around 2,000 m 2 /g are commonly available for carbons while 140 m 2 /g commonly available for metal oxide. The first mechanism commonly 55

24 referred to as charge separation, which is well documented as a non- Faradic mechanism and is the basis for EDLC. The charges are basically blocked at the electrode/electrolyte interface, preventing the charges to diffuse. The second reaction commonly referred to as an oxidation reduction reaction (redox) due to faradic mechanism, which is the basis for pseudocapacitance. In here, the charges are partially blocked where some charges diffuse into the electrode material and intercalate. The term pseudocapacitor is commonly used to explain the pseudocapacitance behavior of such double layer capacitors. Carbon is an example of a charge separation of non-faradic electrode material and ruthenium, manganese and nickel oxide are an examples of faradic electrode material [10]. Although, pseudocapacitor has electrochemical reaction similar to battery (the faradic mechanism), the distinction between a battery and an electrochemical double layer capacitor is not explicit. A battery relies on electrochemical reactions that involve active materials in the electrode, where the charges are not blocked, diffuse into the material and intercalate. In this case, active means that the materials participate in the reaction and a transfer of electrons between the active material and ionic species in solution occurs across the solid/liquid interface. On the other hand, in an "ideal" electrochemical capacitor that utilizes carbon or metal oxides, the electrodes play passive role. That is, the electrode surface only participates by serving as sites for charged species to accumulate and no electron transfer occurs across the solid/liquid interface. In a "practical" electrochemical capacitor there may occur some surface oxidation/reduction on the electrodes, that is, it operates partly as a capacitor and partly as a battery. Of course, this phenomenon is depends on the material properties. So, in order to characterize a double layer capacitor, one has to investigate the above behavior. The metal oxide technology of the pseudocapacitor utilizes an electrochemical reaction similar to battery technology for energy storage, thus improving potential 56

25 energy density. Since the pseudocapacitor uses a dense metal oxide as the electrode material, the load of the oxide is three times that of the EDLC for the same-coated area. With this advantage, pseudocapacitor cell needs to be only 60% in volume as compared to an EDLC of the same capacitance. Conversely, it also means the pseudocapacitor holds 80% more energy than the equivalent-size EDLC. Finally, the pseudocapacitor uses the same manufacturing processes and facilities as EDLC production. 2.B.1.2 Supercapacitors Based on Hybrid Capacitors A hybrid capacitor is a double layer capacitor fabricated with one electrode purely double layer (carbon based as the negative electrode) and another electrode with pseudocapacitance (e.g., metal oxide based as the positive electrode). The energy density of these devices is found to be significantly higher than that of pure EDLCs. Like the activated carbon cathode in EDLC, an activated carbon in the cathode of the hybrid capacitor absorbs anions at the electric double-layer and linearly polarizes to positive side in the charging process. On the other hand, as shown in Fig. 2.B.3, a Li doped graphite in the anode of the hybrid capacitor that intercalates Li + ions into its interlayer in the charging process and deintercalates Li + ions in the discharging process. When the anode is charged, potential of the anode becomes very negative and approaches the deposition potential of Li-metal anode as illustrated in Fig. 2.B.3. The voltage is largely developed at anode but negligibly at cathode. Voltage developed in the electrolyte is due to a solution resistance the socalled ohmic drop (IR drop). The voltages developed at each electrode are made from resulting charge stored and electrode capacity. Because the cathode capacity is much larger than that of anode, voltage development occurs mostly at anode when a cell is charged. 57

26 Fig. 2.B.3 A Li-doped graphite in the anode of the hybrid capacitor that intercalates Li + ions into its interlayer in the charging process and deintercalates Li + ions in the discharging process. 58

27 2. C Introduction SECTION- C THIN FILM CHARACTERIZATION TECHNIQUES In the past years the advancement in science has taken place mainly with the discovery of new novel materials. Characterization is an important step in the development of exotic materials. The complete characterization of any material consists of phase analysis, compositional analysis, structural elucidation, micro-structural analysis and surface characterization, which have strong bearing on the properties of materials. This has led to the emergence of variety of advanced techniques in the field of materials science. In this section different analytical instrumental techniques used for thin films characterization are described with relevant principles of their operation and working. 2.C.1 Thickness Measurement The thickness of film is the most significant parameter that affects the properties of the thin films. It may be measured either by in-situ monitoring of the rate of the deposition or after the film is taken out from deposition chamber. Technique of the first type often referred to as monitor methods generally allow both monitoring and controlling of deposition rate of film thickness. Any known physical quantity related to film thickness can be used to measure the thickness. The method chosen should be convenient, reliable and simple. One of the most convenient and reliable method for determining film thickness is gravimetric method. In this method, area and weight of the film are measured. The thickness is obtained by using the formula [12] t = M/Aρ (2.11) M = m1- m2 (2.12) where, t is film thickness, M is mass of the film material, A is area of the film, m1 is mass of the substrate with film, m2 is mass of the substrate 59

28 without film and ρ is density of the deposited material. In order to get more accurate results, one should measure thickness using the films with maximum area so that, weight difference is accurately measurable. The value presenting to the bulk material is usually taken for ρ even when the actual density of the material in thin film form is lower. However, this method is destructive one and actual density of material is different than the film material. In case of porous thin films, the density of deposited material will not be well defined so thickness of thin film can be calculated in terms of deposited mass per unit area of the material only. So the equation (2.11) becomes t = M/A (2.13) 2.C.2 X-ray diffraction (XRD) technique The X-ray diffraction is one of the most important techniques for characterizing the structural confirmation of crystalline materials. Structure identification, determination of lattice parameters and grain size are based on the X-ray diffraction pattern. Improved detection methods for X-ray, the availability of commercial monochromators and intense microfocus X-ray sources have made X-ray diffraction method applicable to films as thin as 100 A 0. The basic principles of X-ray diffraction are found in textbooks e.g. by Buerger [13], Cullity [14], Tayler [15] and Guinier [16]. Figure 2.C.1 shows the schematics of X-ray diffractometer. Diffraction in general occurs only when the wavelength of the wave motion is of the same order of magnitude as the repeat distance between scattering centers. This condition of diffraction is nothing but Bragg s law and is given as, 2d sinθ = nλ (2.14) 60

29 where, d = interplaner spacing θ = diffraction angle λ = wavelength of x-ray n = order of diffraction X-ray Source Diverging Slit Receiving Slit Detector Sam ple θ 2θ Figure 2.C.1 Schematics of X-ray diffractometer. The d values are calculated using above relation for known values of θ, λ and n. The X- ray diffraction data thus obtained is compared with American Standard for Testing of Materials (ASTM) or Joint Committee Powder Diffraction Standards (JCPDS) powder diffraction data to identify the unknown material. The sample used may be in the powder, single crystal or thin film form. The crystallite size of the deposits is estimated from the full width at half maximum (FWHM) of the most intense diffraction line by Scherrer's formula as follows [17]. K. λ D = (2.15) β.cosθ where, D is grain size, β is full width at half maxima of the peak (FWHM) in radians, θ is Bragg's angle and K is constant. Value of K varies 61

30 from 0.89 to 1.39, but for most cases it is closer to 1. Though this technique is applicable for determination of crystal structure, lattice parameters, particle size etc, it is not useful for identification of individuals of multilayer s or percentage of doping material. Moreover this technique does not give any idea about surface morphology. 2. C.3 Fourier Transform Infrared Spectroscopy Vibrational spectroscopy deals with the changes in vibrational motion of atoms in a molecule which are greatly influenced by the masses of atoms, their geometrical arrangement and strength of their chemical bonds. The IR spectra involve IR radiation and results from transition between quantized vibrational states and provide a complementary image of molecular vibrations. The interaction of IR radiation with vibrating molecule is only possible if the molecular dipole moment is modulated by the vibration (IR active) while a molecular vibration is only in Raman spectrum if there is modulation of molecular polarizability by the vibration (Raman active). Fig. 2.C.2 shows the schematic ray diagram of FTIR spectrometer. The sample is irradiated with IR radiations from a heated ceramic source. The radiations travels form IR source to beam splitter where 50% of radiations are reflected to a fixed mirror and 50% are transmitted to a movable mirror. The reflected radiations from the mirrors are combined at beam splitter before passing through the sample is separated into individual frequencies. Depending on the composition of the sample, at certain frequencies absorption will be observed because each distinct molecular structure in the sample vibrates at distinct frequency. When that frequency is matched by the frequency of the infrared radiation, the molecules absorbs this energy and hence a very little radiation reach detector. By noting both frequencies and the relative amount of absorbed 62

31 radiation, the nature of the molecular structure and its constituent can be deduced [18]. It is possible to investigate how the infrared vibrational frequencies and thus the inter-atomic forces are affected by the onset of the semiconductor states. If the electron with two energy levels E1 and E2 is placed in an electromagnetic field and the difference in the energy between the two states is equal to a constant 'h' multiplied by the frequency of the incident radiation ν, a transfer of energy between the molecules can occur, giving therefore ΔE = hν (2.16) where, the symbols have their usual meanings. When the ΔE is positive, the molecule absorbs energy; when ΔE is negative, radiation is emitted during the energy transfer and emission spectra are obtained. When the energies are such that the equation (2.16) is satisfied, a spectrum unique to the molecule under investigation is obtained. The spectrum is usually represented as a plot of the intensity Vs the frequencies and peaks occur when the condition (equation 2.16) is satisfied. Frequency ranges that can be encountered in this spectrum vary from those of γ rays, which have wavelength of about cm to radio waves which have wavelength of cm. In the atom, the absorption represents transition between the different allowed levels for the orbital electrons. In case of molecules, however, the atoms within the molecules vibrate and the molecule as a whole rotates and the total energy contributions are represented by the equation [19, 20] Etot = Eelect + Evib + Erot + Etrans (2.17) where, Eelect is the electronic energy, Evib is the vibrational energy, Erot is the rotational energy and Etrans is the translation energy. The separate energy levels are quantized and only certain transitions of electronic, vibrational and rotational energy are possible. Translational energy is usually sufficiently small to be ignored. The vibrational spectrum 63

32 of a molecule is considered to be a unique physical property and is a characteristic of the molecule. As such the infrared spectrum can be used as a finger print for identification, in support of X-ray diffraction technique for the purpose of characterization. Fig. 2.C.2 schematic of a typical FTIR spectrometer 2.C.4 Surface Morphological studies 2.C.4.1 Scanning Electron Microscopy and Energy dispersive analysis by X-rays (EDAX) Interaction of electrons with elements is well understood and has been extensively used for characterizing of the materials. As the electrons can be focused to micron or sub-micron size, it is well suited for analyzing sub-micron sized areas or features. When an electron strikes the atom, variety of interaction products are evolved. Fig. 2.C.3 illustrates these various products and their use to obtain the various kinds of information about the sample. Scattering of electron from the electrons of the atom results into production of backscattered electrons and secondary electrons. Electron may get transmitted through the sample if it is thin. Primary electrons with sufficient energy may knock out the electron from the inner shells of atom and the excited atom may relax with the liberation of Auger electrons or X-ray photons. All these interactions carry 64

33 information about the sample. Scanning electron microscope is an instrument that uses electron beams to observe the morphology of a sample at higher magnification, higher resolution and depth of focus. These, backscattered electrons, secondary electrons and transmitted electrons give information about the microstructure of the sample. Auger electron, ejected electrons and X-rays are energies specific to the element from which they are coming. These characteristic signals give information about the chemical identification and composition of the sample. The advantages of SEM over light microscopy include much higher magnification (>X 1, 00, 000) and greater depth of field up to 100 times than that of light microscopy [21, 22]. Principle of Scanning Electron Microscope A well-focused mono-energetic (~25KeV) beam is incident on a solid surface giving various signals as mentioned above. Backscattered electrons and secondary electrons are particularly relevant for SEM application, their intensity being dependent on the atomic number of the host atoms. Each may be collected, amplified and utilized to control the brightness of the spot on a cathode ray tube. To obtain signals from an area, the electron beam is scanned over the specimen surface by two pairs of electro-magnetic deflection coils and so the C.R.T. beam in synchronization with this. The signals are transferred from point to point and signal map of the scanned area is displayed on a long persistent phosphor C.R.T. screen. Change in brightness represents change of a particular property within the scanned area of the specimen. 65

34 Electron Beam Secondary Electrons X-rays Cathodolumenescence Back Scattered Electrons Auger Electrons Electromotive force Transmitted Beam Fig. 2.C.3 Variety of interaction products evolved due to interaction of electron beam and sample Energy Dispersive X-ray Analysis (EDAX) The interactions of electron with solid as mentioned above leads to different signals like secondary electrons, backscattered electrons and x- rays which are characteristic to atoms of host lattice. These x-rays can be used for element detection and its quantification for compositional analysis. Two techniques are used for analyzing x-ray spectrum. 1. Wavelength dispersive x-ray analysis (WDAX) or (WDS) 2. Energy dispersive x-ray analysis (EDAX) or EDS These techniques deals with diffraction of x-rays under analysis and can be termed as wavelength (or energy) dispersive spectroscopy also. (a) Wavelength dispersive x-ray Analysis (WDAX) or (WDS) The characteristics x-ray are identified by their wavelengths by diffracting them on a crystal monochromator of known d spacing and following Bragg s law nλ = 2d sinθ. In fully focusing spectrometer sample, crystal and detector all lie on the same circle called as Rowland circle such 66

35 that Bragg s law satisfied in all cases. The crystal moves on a linear path in order to maintain constant take off angle. The centre of Rowland circle moves on arc with x-ray source as centre. (b) Energy Dispersive X-ray Analysis (EDAX) or (EDS) Here, the characteristics x-rays are identified by their energy using solid state detectors. It consists of semiconductor Si(Li) counter and FET preamplifier, both cooled by liquid nitrogen and multichannel analyzer (MCA). The utility of this kind of spectrometer is based on two properties 1. The excellent energy resolution of Si(Li) counter. 2. The ability of MCA to perform rapid pulse height analysis. Semiconductor counter produces pulses proportional to the absorbed energy with better energy resolution than any other counter. The silicon and germanium is the best detector for x-rays and γ-rays respectively. The pure silicon in intrinsic semiconductor has very high electrical resistivity especially at low temperatures. Hence, only few electrons are thermally excited across the energy gap into conduction band. However, incident x-ray can cause excitation and thereby create a free electron in the conduction band and free hole in the valance band. If high voltage is maintained across opposite faces of the silicon crystal, the electrons and holes will be swept to these faces, creating small pulse in the external circuit. It is very difficult to get pure silicon. Hence a p-type Si, lightly doped with boron, were used and to make it intrinsic this is again doped with Li. Typically the resolution of Si(Li) detector is given as 147 ev for manganese Ka radiation of 5.9 KeV energy. The energy dispersive system can analyze whole x-ray spectrum simultaneously. Due to low ionization potential of Si (Li) the count rate is very high (>10,000 c/s/na) and hence very low current is required. It is used in conjunction with SEM; TEM where current used is very low (of the order of pa) 67