Synthesis and Characterization of TiO2-GeO2 ...

Synthesis and Characterization of TiO2-GeO2-Azobenzene nanocomposite as Photoanode for Dye Sensitized Solar Cells and other Optoelectronic devices Applications A dissertation submitted to the Department of Physics, Pondicherry University in partial fulfilment of the requirements for the degree of Master of Science In Physics

Submitted by Aashutosh Kumar Reg.No.11380083

Prof. N. Satyanarayana (Supervisor)

Department of Physics Pondicherry University Puducherry-605014

MAY-2016

CERTIFICATE

This is to certify that the project entitled “Synthesis and Characterization of TiO2-GeO2Azobenzene nanocomposite as Photoanode for Dye Sensitized Solar Cells and other Optoelectronic devices Applications” submitted by Mr Aashutosh Kumar (Reg. No.11380083) to the Pondicherry University in partial fulfilment of the requirements for the degree of Master of Science Integrated in Physics is a bona fide record of project work carried out by the candidate.

Place: Pondicherry Date:

Prof. N. Satyanarayana (Supervisor)

DECLARATION

I hereby, declare that this project report entitled “Synthesis and Characterization of TiO2-GeO2-Azobenzene nanocomposite as Photoanode for Dye Sensitized Solar Cells and other Optoelectronic devices Applications” is the result of investigation carried out by me under the supervision of Prof. N. Satyanarayana, Department of Physics, Pondicherry University, Pondicherry.

Place: Pondicherry: Date:

Aashutosh Kumar

ACKNOWLEDGEMENT

First and foremost, I would like to express my sincere gratitude to project supervisor Prof. N. Satyanarayana, Department of Physics, Pondicherry University for his constant support and encouragement. In the midst of all his official assignment, he was attentive to spare his valuable for me. Without his guidance, this work never have success. I regard it as a pleasant duty to express my deep thanks to Prof. R. Murugan, Head of the Department of Physics, Pondicherry University, Pondicherry, for providing all the facilities and encouragement. I take this opportunity to thank Prof. G. Govindraj, Dr. Alok Sharan, Dr. D. Bharthi Mohan Department of Physics, Pondicherry University, for their valuable suggestions and constant support in all respects during the course of Master program in Physics. I express my gratitude to Mr. S. Vivekanandan and Mrs. P.C Karthika, Department of Nanotechnology, SRM University Kattankulathur, for their advice and characterization support. I am thankful to Mr. A. Ratnakar, Center for Nanoscience and Technology, for his guidance and suggestions due which I am able to complete my project. I would like to express my heartfelt thanks to CIF Pondicherry University and Materials and Characterization laboratory, Department of Physics and Nanotechnology, Nanotechnology Research Centre, SRM University who helped during my experimental works in CIF. I am also thankful to Ph.D. scholars Anil Kumar Pal, Giridhar Chakrobarthy, K Hari Prasad, Paramanand Jena, D. Narasimulu, S. Vinoth, and N. Naresh who helped me in my difficulties during the project work. I’am thankful to my friends Vasundhara Shaw, Mukund P.M, Ashwani N, Pragnya Satpati Vinod Kumar, Tanmoy Chakroborty, and Manoshij Benergy for their help during project work and valuable advices. I wish to express my sincere gratitude to my parents and my family members for their unconditional support and encouragement throughout my life. I explore my strength, my parents, who showered up on me their unconditional love and fathomless care all through my life.

Aashutosh Kumar

CONTENTS

Abstract List of figure Chapters 1. Introduction 1.1

Introduction

1

1.2

Need for alternate energy source

3

1.3

Solar energy and technology

3

1.4

Objective of the study

5

2. Basic of Semiconductor 2.1

P-n junction devices

6

2.2

Photovoltaic effect

8

2.3

Solar cells

9

2.4

Operation of solar cells

10

2.5

Equivalent circuits and Important parameters

10

2.5.1

Current-voltage (I-V) characteristics

11

2.5.2

Efficiency and fill factor

12

2.5.3

Quantum efficiency

13

2.5.4

Spectral Response

14

2.6

Dye sensitized solar cells

16

2.7

Structure and components of DSSC

16

2.7.1

Transparent Conductive Oxides

17

2.7.2

The Mesoporous semiconductor film (photoanode)

18

2.7.3

Dye (photosensitizer)

18

2.7.4

Electrolyte

19

2.7.5

Counter electrode

20

2.8

Operation Principle of Dye-sensitized Solar Cells

20

2.9

TiO2 nanoparticles as Photoanode

22

2.9.1

TiO2 nanoparticles

22

2.9.2

TiO2 doped with GeO2

23

2.9.3

Effect of azobenzene functionalization

23

3. Experimental and characterization technique 3.1

Sol-gel process

25

3.2

Characterization Techniques

24

3.1.1

X-ray diffraction technique

27

3.1.2

Fourier transform infra-red spectroscopy

28

3.1.3

Raman spectroscopy

29

3.1.4

Scanning electron microscope

30

3.1.5

UV-Vis spectroscopy

31

4. Synthesis and characterization result 4.1

Reagents

34

4.2

Synthesis of titania nanoparticles

34

4.3

Preparation of GeO2-TiO2-azobenzene nanocomposite

35

4.4

Results and Discussions

37

4.4.1

XRD

37

4.4.2

FTIR

38

4.4.3

RAMAN

39

4.4.4

SEM

41

4.4.5

UV-Visible

42

5. Conclusion and future work

45

6. References

47

7. Picture references

54

ABSTRACT

Titanium dioxide (TiO2) nanoparticles are most preferred metal oxide nanoparticles for the optoelectronics applications since it provides most favorable condition for rapid transfer of electrons. TiO2 has been used in Dye sensitized solar cells application as the photoanode material for quite a long time. Various morphology such as nanofibers, nanorods, nanocones, quantum dots etc. are also preferred to fabricate photoanode which gives high efficiency for Dye sensitized solar cells. But no effort has done so far to study of the effect of functionalization of TiO2 or any other metal oxides that could be used for photoanode Dye Sensitized solar cells. In this study we proposed a new type of methodology of functionalizing TiO2 nanoparticle with an optically responsive dye, azobenzene (a diazene) which helps in maximum absorption of light. With this we propose a composite TiO2 (anatase + rutile)-GeO2 functionalized by azobenzene and its improved optical responsiveness through UV-Visible studies for optoelectronic device. Their morphology and structures have been studied by SEM, XRD, FTIR and Raman.

LIST OF FIGURES

Figure 1.1: Renewable energy and nonrenewable energy consumption, a pictorial representation Figure 1.2: Classification of energy resources Figure 2.1: p-n junction in thermal equilibrium with zero bias voltage Figure 2.2:.a: working of p-n junction in forward and reverse bias. Figure 2.2.b: V-I characteristics Figure 2.3: Photovoltaic effect of light-electricity conversion in semiconductor Figure 2.4: A schematic structure of photovoltaic device Figure. 2.5: Equivalent Circuit for a solar cell Figure 2.6: V-I characteristics for maximum power Figure 2.7: Quantum efficiency of silicon based solar cells. Figure 2.8: Spectral response of solar cells Figure 2.9: A schematic representation structure and component of DSSC Figure 2.10: The operating principles of the DSSC Figure 2.11: Structure of different phases of TiO2 Figure 2.12: Azobenzene isomerization and transitions Figure 3.1: Schematic representation of the different stages sol-gel process Figure 3.2: XRD principle and working Figure 3.3: FTIR assembly, principle and working Figure 3.4: Raman spectroscopy principle and operation Figure 3.5: A schematic representation of SEM

Figure 3.6: A schematic representation of UV-Vis spectrometer Figure 4.1: Flow chart of the experimental procedure adopted for synthesis and characterization of azobenzene functionalized TiO2-GeO2 composite.

Figure 4.2.a: XRD pattern of the TiO2anatase and rutile phase Figure 4.2.b: XRD pattern of the nanocomposite material Figure 4.3: FTIR of the nanocomposite Figure 4.4.a: FT Raman spectroscopy of the TiO2 and GeO2 composite Figure 4.4.b: FT Raman spectroscopy of the nanocomposite material Figure 4.5: Morphology of the prepared nanocomposite material Figure 4.6: UV-Vis (transmittance) of the prepared samples; a. TiO2anatase, b. TiO2 anatase and rutile mixed phase, c. TiO2 and GeO2 composite d. Azobenzene, e. TiO2 -GeO2 – Azobenzene nanocomposite

CHAPTER 1 INTRODUCTION

1.1 Introduction Every physical, chemical and biological activity requires an input – a scalar quantity; energy, to initiate, terminate or complete a process. Energy is an imperative input required to organize, run and maintain industries/sectors such as automobiles, telecommunications, agriculture, life science and research, which in turn supports and satisfies the needs of human society. This imperative input has/is been/being harvested by processing raw materials that once belonged to Carboniferous period of Paleozoic era (360-280 million years ago). These raw materials are otherwise commonly known as fossil fuels and they are extremely rich in carbon content. These fossil fuels rich in carbon content are burned to achieve an oxidation reaction whose products are carbon monoxide, water and several other by products along with ‘energy’. The energy derived is utilized to run mechanisms, heavy machineries etc., to obtain heat, electricity, light. The utilization of fossil fuels for the last couple of centuries has resulted in a huge loss of resources, fossil fuels. The lack of energy conservation has forced us to find an equivalent substitute for the rapidly depleting fossil fuels through focusing on harvesting the other natural resources such as solar energy, wind energy, hydrothermal, geothermal, biomass etc., and we have been successful to an extent in achieving sustainability. The gradual progress in shifting the acquisition of energy by various sectors and firms from fossil fuels to alternate fuels have also resulted in design and development of electrochemical cell based device such as batteries, fuel cells, fuel cells etc. The development in conservation of energy and utilization of energy could be understood from the following pictographs; Fig 1.1 & Fig 1.2.

1

Figure 1.1: Renewable energy and non renewable energy consumption, a pictorial representation

Figure 1.2: Classification of energy resources

2

1.2 Need for alternate energy resources At present scenario, the requirement of energy mostly dependent other than fossils fuels, on biofuels, hydroelectric energy, nuclear energy and other non-renewable sources, along with the threat of global warming which may be a reason for constantly increasing greenhouse gas emissions in energy production. Nuclear energy can also pose health hazards from radioactive waste released from nuclear power plants. These effects are responsible for the future challenges for major environmental pollution and increasing health issues, have made research, development and implementation of renewable energy one of the most crucial challenges of today’s mankind. A step towards the sustainable development and growth of renewable energy is a safest way to overcome from these problems but also a big challenge for mankind and the scientific community in particular.

1.3

Solar energy and technology Solar energy technology is one of the most promising technology which make more

extensive use of renewable sources of energy derived from sun, which motivates the improvement of eco-friendly and green energy technologies. Solar energy is a largely non-exploited energy source and available abundantly which can be used both directly and indirectly. The earth surface receives 1021 Joules on energy per day from solar radiation which is many times greater than the energy presently used by the entire world within a year. Research on uses of solar energy technologies have expanded rapidly. However, extensive applications of solar energy technology depends on considering factors such as efficiency, reliability, availability, competitiveness and cost-effectiveness. This is why it requires developing a new, more advanced, cheaper and efficient solar energy devices which is called for to bring this form of renewable energy available to even larger number of customers. Solar cells are the p-n junction device which traps the light energy from sunlight and converting it in electrical energy based on the photovoltaic effect and also referred as photovoltaic cells. The operations of solar cells are based on the generation of a voltage or potential across the p-n junction when sunlight (photons) strikes on the device. During the operation of solar cells, it does not produce any harmful gases or emission of any radiation or noise. Because of this solar cells provide us with the most important technology to meet our future requirement of eco-friendly 3

energy [1]. To increase the photovoltaic power generation capacity economically, it is important to choose the semiconductor material based on their performance. First generation of solar cells were based on crystalline silicon, which favors the technology to produce worldwide material for solar cells. Crystalline silicon wafers solar cells have thickness of about 200-300 microns and are the most efficient (efficiency is about 15-20%). Currently 80-90% of solar cells are commercially available in market at high cost based on these technologies. The second generation solar cells are based on amorphous silicon and cadmium telluride (CdTe), where the typical performance is 10 - 15%. Since the second generation solar cells avoid use of silicon wafers and have a lower material consumption it has been possible to reduce production costs of these types of solar cells compared to the first generation, thin film technology introduced to silicon wafer solar cells. These solar cells have the thickness about 1-2 microns but lesser efficiency than first generation, because of limitation in thin film solar cells is low absorption of light (long wavelength photons) beyond 0.5 microns. Third generation solar cells based nanotechnology and organic materials such that variety of new materials besides silicon, including nanotubes, silicon wires, solar inks using conventional printing press technologies, organic dyes, and conductive plastics and polymers [2-4]. Dye-Sensitized Solar Cells (DSSC) is one of most promising third generation hybrid solar cells with organic and inorganic materials as active components of solar cells, which have been subjected in the present thesis. DSSC developed by Professor Grätzel in 1991[5]. DSSC based on a TiO2 nanoparticle photoelectrode sensitized with a light-harvesting metallo-organic dye, which attached onto the TiO2 act as light absorbers to discharges free electrons. This thesis intends to support the development of material for the DSSC by assisting the optimization of nanoparticles and the supporting techniques. The efficiency of a DSSC has been reported to be around13% [6].

1.4 Objective of the study The main objective of this thesis is to develop and optimize innovative materials for photoanode application for Dye-sensitized Solar Cells and other optoelectronic devices, with the aim of overcoming the drawbacks in the conventional anode materials. It has been aimed for improvement in anode material which will contribute to achieve efficient solar cells. Hence, for this purpose a material required which absorbs maximum photons from light energy and also to supports dye 4

harvest the maximum light energy to discharge the maximum electron after the absorption of photon of light energy. Titanium dioxide (TiO2) nanoparticles are most preferable semiconductor material for this purpose, since it has favorable and easily tunable band gap by inducing changes to the morphology, structures and dimensions. Through this project it has been attempted to tune the bandgap of TiO2 to achieve efficient DSSC. To tune the bandgap of TiO2, a new concept of combining the two phase anatase and rutile in 3:1 ratio and then doped with GeO2 (a well-known semiconductor material for optoelectronic applications) and Azobenzene (a photoresponsive material).

5

CHAPTER 2 BASIC OF SEMICONDUCTORS

Semiconductors are materials, that have an electrical conductivity intermediate between the electrical conductivity of conductors and good insulators. A semiconductor has electrical conductivity can be controlled over a wide range, either permanently or dynamically. There are many materials that exhibit semiconducting behavior but only a very few of them are of much interest for electronics. At the room temperature semiconductors behave like insulators because very few electrons gain enough thermal energy to leap the band gap, which is necessary for conduction. Due to this reason, pure semiconductors and insulators have a similar electrical properties in the absence of applied fields. Silicon and Germanium are the most important semiconductor and is the active material in almost all electronic devices. A few other semiconductors for example, such as gallium arsenide and cadmium sulfide are essential because they can be used to make optoelectronic devices. Semiconductors are of two types, intrinsic semiconductors (the pure semiconductor) and extrinsic semiconductors (intrinsic semiconductor doped with the impurities such doping with Group 3rd & 5th elements). Intrinsic semiconductors have electrical properties that are very often permanently modified by introducing impurities, in the process known as doping. Usually it is reasonable to approximate that each impurity atom adds one electron or one hole that may move freely. Doping a semiconductor such as silicon with a small amount of impurity atoms, such as phosphorus, arsenic or boron etc., gives an excess amount of either free electrons or holes within the semiconductor. 2.1 p-n junction devices: When the doped semiconductor contains majority of holes it is known as a "p-type", and when it contains majority free electrons it is known as an "n-type". The semiconductor materials, which are used for devices and are doped under highly controlled conditions in a fabrication facility to control precisely the location and concentration of p- and n-type dopants.

6

The junctions which form by the combination of n-type and p-type semiconductors are called p–n junctions and the device based on it, known as p-n junction devices. The addition of a sufficiently large proportion of dopants, semiconductors conduct electricity nearly as well as metals [7]. The junctions between regions of semiconductors that are doped with different impurities contain built-in electric fields. Operation of p-n junctions can be understood by the figure (2.1).

Figure 2.1: p-n junction in thermal equilibrium with zero bias voltage

Basic operation of p-n junction device is based on diffusion of electrons from n-type to ptype and holes from p-type to n-type across the junction by due density gradient and this process is continued till equilibrium established at both sides. An electric field built between the positive ions cores in n-type and negative ions cores in p-type which exposed due to movement of electrons in n-type and holes in p-type respectively. The region at junction is also known as “space charge region or depleted region” because the region gets depleted by sweeping out mobile carriers due to electric field. When a forward bias is applied to p-n junction, initially junction barrier gets lowered and then charge carrier move from one to another by crossing the junction and hence, results a current flow through the circuit. When a revere bias is applied to a p-n junction, causes a separation in space charge region due to attraction of charge carriers towards applied potential polarity (holes attracted towards the negative terminal and electrons

7

towards the positive terminals). After a saturation limits, breakdown occurs in the p-n junction due to an increment in potential across space charge region.

Figure 2.2.a: working of p-n junction in forward and reverse bias.

Figure 2.2.b: V-I

characteristics

2.2 Photovoltaic effect: Photovoltaic effect is a mechanism in which light energy is directly converted to electrical energy. When sunlight incident on a material, its get reflected back and a part of light partially absorbed by the material. This absorbed light energy is enough to create electron-hole pair in the material, in which electrons are usually in bounded state or not free to move from atom to atom within the crystal, provides enough energy to excite few electrons at their bounded states and move to conduction band [8].

8

Figure 2.3: Photovoltaic effect of light-electricity conversion in semiconductor

2.3 Solar cells: Solar cells are the p-n junction device which converts light energy into electrical energy by the photovoltaic mechanism. A solar cell consist of two terminals or electrodes (same as cathode and anode) of two different ohmic metal semiconductor materials. Top surface of solar cells is photon absorbing layer referred as anti-reflection coating transparent glass, which prevent the reflection of light. Absorbed light energy excite the p-n junction to create electron-hole pair [8-9].

Figure 2.4: a schematic structure of photovoltaic device

9

2.4 Operation of solar cells: Basic Operation of solar cells explained into following steps: (i) absorption of light to generate electron-hole pair, (ii) the separation of elections and holes to prevent recombination and (iii) collection of photo-generated charged carriers at electrode. When light exposed on the surface of solar cells device, the material absorbs photons from light energy partially which energize electrons and make them free from valence state. Free electrons and holes are accumulated in n-type and p-type materials respectively. These movement of electron and hole builds up a potential or voltage across its two terminals which is attaches n-type and p-type material. The free electrons and holes are responsible to generate an electric current through external circuit of solar cells. This process will continues as long as light irradiates the surface of semiconductor material.

2.5 Equivalent circuit and important parameter of a Solar Cells: In order to understand the operation of solar cells we consider an equivalent circuit give in figure (2.4). An ideal solar cell consider as diode which connected parallel to a current source, a shunt resistance and a series resistance are added to the equivalent circuit. The corresponding current and voltage (I-V) characteristics can be given for this equivalent circuit by the Shockley solar cell equation

=

- 1)

2.1

where, Iph is the current due to absorbed photons, Io is current at equilibrium, q is the electric charge of electrons, V is the potential difference between the terminals, KB is the Boltzmann’s constant (values 1.38064852 × 10-23 m2 kg s-2 K-1) and T is the thermal temperature [8-12].

10

Figure 2.5: Equivalent Circuit for a solar cell

Based on the equivalent circuit we can define few important parameters of solar cell. 2.5.1 Voltage-current (V-I) Characteristics: V-I characteristics is important quantities to characterize a solar cell with following parameters: 

Open circuit voltage (Voc): The voltage between two terminals or electrodes and counterelectrodes when no current is drawn which means that an infinite load (resistance) in the circuit. i.e. I=0,



Short circuit current (Isc): The current when two terminals or electrodes and counterelectrodes are connected to each other. The potential becomes zero and hence load (resistance) tends to zero. i.e. V=0. Therefore, Isc= I= Isc

The short circuit current depends on intensity of light. Its increases, intensity of light increase which means that maximum number photons of absorbed by device turn in more number of electrons. Also the current depends on area on which sunlight irradiates. Since we know that current density is flow of current per unit area, that’s applied for short circuit current density, i.e. Jsc = Isc/A,

2.2

which often used to decide the current generate by the solar cells. A voltage or potential develops across the terminals, when a load or resistance is connected to the solar cell. It decreases the flow of current through the circuit. Load is responsible for develop a dark current which flows in the opposite direction [13]. The dark current density is given by 11

2.3 where, Jo is current density at the equilibrium and V is the voltage across the terminals. Now, both currents get superposed, giving resultant current 2.4 Now to obtain an expression of open circuit voltage (Voc) by setting the current density as J = 0, which means that there is no current flow as the currents cancels each other. The expression for Voc is

)

2.5

2.5.2 Efficiency and Fill factor: The efficiency of a solar cell is defined as the ratio of power output by the power output. If the incoming light has a power density Ps and Po is output power density, the efficiency will be 2.6

The output power delivered by a device is product of current and voltage i.e. Po= I.V . in terms of power density,

Pd = J.V

2.7

The maximum power achieved by the device somewhere between both short circuit and open circuit condition, i.e. V = 0 for short circuit and V = Voc for open circuit at a voltage Vm. The corresponding current density is called Jm, and thus the maximum power density is

Pmax = Jm Vm

2.8

12

Now we can define another quantity fill factor (FF) which is used to characterize a solar cell. It is defined as maximum power to the power output at both the open circuit voltage and short circuit current together and given by 2.9

And

2.10

We refer to the V-I characteristics graph figure (2.5). The open circuit voltage and short circuit current is "utilized" at maximum power. Using FF we can obtain an expression for the efficiency

of the solar cells 2.11

Figure 2.6: V-I characteristics for maximum power

2.5.3 Quantum efficiency: The terms “quantum efficiency (QE)” is the ratio of the number of charge carriers collect by solar cell to the number of photons (of energy E = hυ) strike the solar cell. The quantum efficiency of a solar cell which shows the number of photons of a particular wavelength converted in current when sunlight irradiated on the device. The quantum efficiency of solar cell may consider as either as a function of energy or as wavelength. If all absorbed photons of certain wavelength results in free electrons collected, then the quantum efficiency at that particular wavelength will unity. The quantum efficiency for photons with energy less than bandgap of semiconductor material will be zero. 13

Figure 2.7: Quantum efficiency of silicon based solar cells.

The energy-production value will be highest possible value over whole spectrum for the cell gives overall energy conversion efficiency value [13-14]. The expression for quantum efficiency of solar cell can be given by

2.12

where, Jsc is short circuit current, λ is the particular wavelength of photons, q is the charge of minority carrier or free electrons, φo is spectral photon flux and R(λ) is reflectance of solar cell surface. 2.5.4 Spectral response: Spectral response is important since it is the spectral response that is measured from a solar cell, and from this the quantum efficiency is calculated. The quantum efficiency gives output, the number of electrons compared to the number photons incident on the solar cell device whereas spectral response gives the ratio of current produced by solar to the power incident on the solar cell. The spectral response may use to determine the quantum efficiency by replacing

14

the power of the light with the photon flux for a particular wavelength [14]. The spectral curve has shown above in figure 2.7.

Figure 2.8: Spectral response of solar cells The ideal spectral response is limited at long wavelength by inability of device to absorb the incident photons of energies lesser than the bandgap of semiconductor material. This limit is the same as that light represents a significant power loss in the solar cell consisting of p-n junction. The inability to fully utilize the incident energy at high energies and the inability to absorb low energies of spectral response. The spectral response can be given by the expression

2.13

15

2.6

Dye Sensitize Solar Cells Dye sensitize solar cells (DSSC) are the hybrid, oregano-metallic nanostructured third

generation of photovoltaic based on process similar with photosynthesis occurs in plants. In recent years, right from the invention of Grätzel cell by Professor Michael Grätzel and Dr. Brian O’Regan in 1991, TiO2 have been regarded as the most versatile conducting metal oxide nanocrystalline photoanode for constructing dye sensitized solar cells [5]. With Mesoporous nanostructured electrodes (TiO2) and efficient charge injection dyes, professor Grätzel and his co-workers invented a solar cell with efficiency exceeding 7% in 1991, (O'Regan & Grätzel) and 10% in 1993 (Nazeeruddin et al.) [15- 16]. This solar cell is called also called as the Grätzel cell after its inventor. DSSC is regarded the next generation solar cell for their ability to generate reasonable quantity of energy by harvesting meager percentage of visible light from sun [14-17]. Dye sensitizer absorbs the light incident on DSSC and exploits the light energy to induce electron transfer process [24]. A few of the reported laboratory models and mathematical assisted by theoretical models with overall efficiencies 13% and more offers a lot of room for improvement in their fundamental design and model [6, 17]. Most of the study involves engineering their bandgap by adding dopants; either to obtain a material with wider bandgap or to obtain a highly photocatalytic material.

2.7

Structure and Component of Dye-sensitized Solar Cells: The structure of the Dye-sensitized solar cell in the original Grätzel design DSSC: glass

sheet with transparent conducting oxide coating as anode on top layer and semiconductor oxide (mesoscopic TiO2) film deposits on the conductive side of the glass sheet supported by a mixture of a photosensitive ruthenium-polypyridine dye. On the other side a thin layer of the iodide electrolyte spread over a platinum metal. These two parts are then joined and sealed together to prevent the electrolyte from leaking [14].

16

Figure 2.9: A schematic representation structure and component of DSSC

The modern DSSC consist of major five components[15-22].: (1) Transparent Conductive Oxides or a coated glass substrate; (2) the Mesoporous semiconductor film (photoanode), usually TiO2; (3) dye (photosensitizer) or a sensitizer adsorbed onto the surface of the semiconductor; (4) an electrolyte containing a redox mediator; (5) a counter electrode capable of regenerating the redox mediator such as platinum. Fig. 2.9, shows a schematic representation of the dye-sensitized solar cell. 2.7.1 Transparent conducting oxide or coated glass substrates: The transparent conducting oxide (TCO) or glass substrate is upper most part of DDSC which transmit the sunlight to the electrode and sensitizer [15-18]. The electrodes are prepared onto transparent conducting oxide, between which the cell is assembled. The conducting coating of the substrate works as a current collector and the substrate material itself both as a support structure to the cell. TiO2 nanoparticles are deposited TCO by sputtering or by low pressure chemical vapor deposition [17]. A variety of TCOs were investigated for DSSC’s application, among this Fluorine-doped Tin Oxide (FTO) and Indium Tin Oxide (ITO) are most commonly used since it compromise in terms of fabrication process, optical and electrical properties. Despite of the lower optical 17

transparency and electrical conductivity as compared to ITO, FTO is the preferred solution in DSCC application, because of the high cost and the limited supply of indium, and the weak flexibility of ITO layers.

2.7.2

Mesoporous semiconductor (photoanode): The electrode or photoanode consists of TiO2 mesoporous nanoparticles, in the size of 15-

30 nm and form a transparent photoelectrode, with a typical thickness of 1-15 μm. The mesoporous TiO2 nanocrystalline layer is deposited on a glass substrate called transparent conducting oxide (TCO) or plastic substrate [15-18]. As compared to the bulk material, the nanocrystalline TiO2 possesses properties which are unique and tunable. The mesoporous structure of the TiO2 film provides high internal surface area to accommodate sufficient amount of dye for efficient light absorption. It also ensures that each dye molecule is in direct contact with both the TiO2 and the electrolyte that fills the pores of the film [17-18]. In a semiconductor, further charge separation takes place due to a charge flow in the existing electrical field formed by the space charge region. Such phenomenon of developing an electrical field does not exist in the nanoparticle. However, the appropriate kinetics of the redox reactions develops charge separation can still be attained. In a dye-sensitized TiO2 nanoparticle, the charge separation takes place across the interface due dye-excitation and consequently electron injection, and hence, an electron transferred to the TiO2, whereas the hole stays in the dye molecule (here oxidation occurs in dye [19-20]. In the absence of a space charge layer in nanostructured TiO2 and the screening by the quite concentrated redox electrolyte, diffusion provides the movement of electrons which are suggested to move through the porous network [21-22]. TiO2 nanoparticles are fused together or doped with other compound to enhance the porosity of working electrode which has been discuss in thesis.

2.7.3 Dye (photosensitizer): This sensitizer dye is one which harvest solar energy by absorption of maximum photon from sunlight. The absorption spectrum of the dye molecules should cover the whole visible region and few part of the near-infrared band of a threshold wavelength of 920 nm [23-26]. The dye molecules have suitable anchoring groups, which attached with the molecule to the 18

mesoporous TiO2 nanoparticle which works as anode of the device [24-25]. It injects electrons into the TiO2 nanoparticles by exciting the dye molecules when it absorbs the light energy. In the process of injection electrons in DSSC’s, the energy difference between the conduction band of the TiO2 and the excited state of the sensitizer is a determining by the HUMO and LUMO factor[27].and for the dye LUMO level must be higher in energy than the conduction band of the TiO2 semiconductor[28]. In a molecule with a metal-to-ligand charge transfer (MLCT) transition, the HOMO is located near the metal, meanwhile the LUMO is located near the molecule ligands. After injection, the oxidized dye is reduced by electrolyte which works as a hole transport medium (HTM) for the DSSC’s. Ruthenium-complex dyes are the most used as sensitizers for DSCs application. These dyes exhibit a strong absorption in the visible region of spectrum and a stay for a mean life 20ns60 ns in excited state which occurs due to the metal-to-ligand charge transfer (MLCT) property [20-30]. In a Ru-complex dye, upon irradiation of light, electrons are stimulated from the ruthenium metal center to the carboxylated bipyridil ligands. These carboxylate groups are directly coordinated to the Ti-ions producing intimate electronic contact between the sensitizer and the TiO2 anode [29].Therefore, the Ru-complex dye possesses directionality in the excited state and due to this the fast electron transfer process occurs at the Ru-dye/TiO2 interface.

2.7.4 Electrolyte: Electrolyte which plays different roles in the functioning of the device, are composed of a solvent and a redox couple. Electrolyte is responsible for the hole transport medium in the DSSC by redox process between the dye and electrolyte. The electrolyte must have long-term stability which can prevent the degradation of the dye at TiO2 interface [30]. The electrolyte transport the carries between the working electrode and the counter electrode. After the injection of the electrons into electrode, the oxidized dye needed to be regenerated and back to its ground state. The electrolyte ensures the fast diffusion of the charge carriers into the device and produce contact with electrode and the counter electrode. -

The traditional DSSCs using ruthenium dye and volatile liquid electrolyte containing I /I3

-

redox couple, has achieved high efficiency [31]. The highly used electrolytes as redox couple are 19

-

-

iodide/triiodide (I /I3 ) and CsSnI3, mainly because of the slow recombination process. Electrolytes based on redox couple are commonly prepared by dissolving iodide salts with metals cations such as Li+, Na+, Mg+, in the liquid solvent. However the corrosive nature of iodine also affecting the DSSC’s efficiency and have led to the investigation of alternative redox couples [20].

2.7.5 Counter electrode: Although most of the efficient DSSCs counter electrodes are based on Pt counter electrode. The counter electrode of a DSSC is usually fabricated by sputtering a platinum layer (~200 nm) -

-

on FTO substrate by using I /I3 electrolyte as redox couple. There are two basic advantages of make Pt based counter electrode, one is the electrocatalytic activity of the platinum which improves the reduction of electrolyte by facilitating electron exchange [32] and the second, it is the working as a reflector which reflects back light within the device due to the mirror-effect and increases light absorption. The stability of DSSCs, which is caused by the leakage and -

-

evaporation of liquid electrolyte, the corrosion of Pt counter-electrode by I /I3 , and the degradation of dye molecules [33].

2.8 Operation Principle of Dye-sensitized Solar Cells The dye-sensitized solar cell, which consists of a transparent conducting glass electrode coated with porous nanocrystalline TiO2 and dye molecules attached to the surface of the nanocrystalline TiO2. Sunlight irradiates on the device the dyes will absorb photons and become photoexcited. The adsorbed dye molecules will discharge an electrons after excitation and it injected into the TiO2 photoelectrode and thus it become oxidized by producing holes after discharge of electron [15-24]. Charge separation is required to stop recombination process across the semiconductor interface where an electron in the TiO2 and a hole is in the oxidized dye molecule. The electrons will then transfer through the porous TiO2 and reach to contact of electrode where charge gets collected at FTO and extraction of charge occurs. The extracted charge generate electricity in the external circuit and eventually return to the counter electrode by which it transfer to electrolyte where reduction of the redox mediator takes place [17]. The 20

electrolyte will complete the circuit by reducing the oxidized dye and transfer charge back to dye and this process continue till to absorption of photon from energy source and converting in electricity.

Figure 2.10: The operating principles of the DSSC.

The following processes occur in the DSSC during light to electricity conversion; Symbols: S (fundamental state of the dye), S* (excited state of the dye), S+ (oxidized state of the dye), SC (semiconductor or photoanode), CB (conduction band), TCO (transparent conducting oxide). Photoexcitation:

S + hυ

Electron injection

S*

e-(CB) (SC) + S+

Relaxation

S*

S + hυ

Electron transport

e- (CB) (SC)

Recombination with the dye

S+ + e-(CB) (SC) 21

S*

e-(TCO) S

Recombination

2e-(CB) (SC) + I3-

Dye regeneration

2S+ + 3I-

Reaction at the counter electrode

I3- + 2e-(Pt)

3I-

2S + I33I-

2.9 TiO2 nanoparticles as Photoanode 2.9.1 TiO2 nanoparticles Titanium dioxide (TiO2) also called as titania nanoparticles is the most widely used nanoparticles for photoanode for DSSC and other optoelectronic application since it favors most favorable condition for rapid electron transfer and tunable bandgap[35].TiO2 nanoparticles has an important property such as high brightness, a high refractive index and a good photocatalysts with a bandgap of 3.0-3.2 eV. It is a more effective compound as a Ultra-violet (UV) radiation absorber [35]. Titanium dioxide is used in as a photocatalyst; in solar cells for the production of hydrogen and electric energy; as varistors in electric devices such as; in MOSFETs as a gate insulator and in magnetic spin-valve systems as a spacer material and in the storage devices [3643].

Titania nanoparticles (TiO2), the oxide of the metal titanium, occurs naturally in bulk TiO2 in different kinds of rock and mineral sands. Titanium is the ninth most common element in the earth’s crust and typically thought of as being chemically inert. Titania nanoparticles have three different phases; rutile, anatase and brookite [44]. Rutile is most stable phase of TiO2 at all temperature among the anatase and brookite. Rutile and anatase has the tetragonal structure with 2 and 4 groups of TiO2 per unit cell of volume respectively and rutile possesses the lowest density among the all three [45]. Whereas anatase is kinetically stable and it transform to rutile above 500oC temperature with the specific gravity increases to 4.5. Brookite has 8 TiO2 groups per unit cell and hence, a larger cell volume than either anatase or rutile.

22

Figure 2.11: Structure of different phases of TiO2 Anatase is also the better candidate for photocatalytic activity than rutile because of their Fermi level position. Furthermore, the lower dielectric constant and low density makes anatase to be better than rutile [46-48]. Another most important feature of anatase is, it’s easily tunable bandgap, which can be tuned down from 3.2eV to around 2.6-2.7eV by doping [49]. After the extreme condition for rutile, it is anatase that is most used for applications such as solar cells, field effect transistors and other semiconductor devices due to their ability to conduct and facilitate rapid transfer of electrons. The performance of a photoanode depends on various factors such as surface morphology, particle size, surface area, porosity, crystalline phase, and dispersion of TiO2 nanoparticles [19-20, 41-50]. Several metals and their oxides have been doped to achieve a stable and desired band gap [22-23]. TiO2 based photoanodes are most successful and achieved the highest efficiency [6].

2.9.2 TiO2 doped with GeO2 The term nanocomposite could be addressed as a misnomer in the field of Nanoscience and technology because the actual definition of a composite in material science goes by the following, “Composite is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components”. The ingredients preferred to develop a nanocoposite here are GeO2 and TiO2 which are well known for their semiconducting properties. These two when combined together could result in a composite structure that could result in generation of excitons or surface plasmon resonance when irradiated by sunlight. This phenomena could be exploited to develop solar cells that could help in reducing the overall loss in efficiency [51-53]. 23

2.9.3 Effect of azobenzene functionalization Azobenzene has the formula C12H10N2 and is one of the known versatile and strong diazene. They are preferred as dye for various applications and one among them is, for functionalizing metal oxides to induce high light absorption property. This functionalization could be exploited for achieving a coupling or adsorptive mechanism on any substrate with the dye or with another molecule to which the dye has got adhered to. The isomerization of azobenzene is unique and isomerization result in the cis to trans transition resulting in a change in the overall dimension of the molecule from a lateral 9 A0 dimension to a flipped 5 A0 dimension. These transitions occurring at molecular level when exposed to 365nm-395nm and 396nm-435nm respectively can also be noticed in bulk. Azobebnzene molecules have a unique ability to grip on to a surface through their structure resulting in a surface that can be easily programmed by a mere exposure to UV and the same can be reversed by using visible light. Thus, when azobenzene is used to functionalize GeO2-TiO2 nanocomposite, the resultant could be a material which will have light absorption property better than individual GeO2 and TiO2. Azobenzene could bind on to the reflective surfaces and enable higher light absorption by responding to visible and UV range [54-55].

24

Figure 2.12: Azobenzene isomerization and transitions When it comes to applications such as photovoltaic or solar cells and other optoelectronic devices, the dopants have to induce an enhancement in the absorption of the light which followed by an immediate response towards the same. To facilitate such behavior, azobenzene is preferred which is a prominent organic material preferred to design liquid crystal based light responsive shape memory polymers. They are basically dyes which get adsorbed to other materials and when subjected to shape fixing they tend to oscillate between their cis and trans isomerization. The molecular level change can be noticed in bulk materials too, provided necessary engineering had been done to facilitate the same. GeO2 has absorption spectrum in IR wavelength and azobenzene, which can undergo isomerization when exposed to UV range can be of huge importance while designing electronic switches of nanodimension. Azobenzene is used to functionalize mixture of anatase and rutile TiO2 and GeO2 to develop a nanocomposite with enhanced optical absorption at electrode. Thus, when azobenzene is used to functionalize GeO2TiO2 nanocomposite, the resultant could be a material which will have light absorption property better than individual GeO2 and TiO2 and also a possibility of absorption of a wide range of wavelength of light on the electromagnetic spectrum

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CHAPTER 3 EXPERIMENTAL AND CHARACTERIZATION TECHNIQUES 3.1

Sol-gel process Sol-gel process is technique preparation of a sol with its subsequent transition into a gel. A

colloidal system consisting of a liquid medium contained in a spatial grid formed by connected particles of the dispersed phase. In the sol-gel process, the first stage the hydrolysis and polycondensation reactions lead to the formation of a colloidal solution. Increasing bulk concentration of the dispersed phase changes in external conditions leads to the intense formation of contacts between particles and the formation of a monolithic gel, in which the solvent molecules are enclosed in a flexible, but fairly stable, three-dimensional grid formed by particles of hydroxides. Concentration of sols followed by gelation is carried out using various process such as dialysis, ultrafiltration, electrodialysis, evaporation at relatively low temperatures, or extraction [56-59]. A crucial role in the sol-gel process is played by the processes of solvent removal from the gel (drying). Depending on the method the synthesis can result in various products (xerogels, ambigels, cryogels and aerogels). The common features of these products include the preservation of the nanosizes of the structural elements and sufficiently high values of specific surface area (hundreds of m2/g), although the bulk density can vary by hundreds of times. Most products of solgel synthesis are used as precursors in obtaining oxide nanopowders, thin films or ceramics. The sol-gel method is also effective for obtaining xero-gels with a pronounced quasi-one-dimensional structure. For example, the V2O5·nH2O xero-gel is the basis for the synthesis of vanadium oxide nanotubes. Synthesis of TiO2 nanoparticles through sol-gel technique is still considered to be the most versatile technique because of their simplicity and ease in engineering the particle size from simple spherical ones to the complex core-shell-corona types. Several reports exists with respect to the synthesis of TiO2 through sol-gel technique. The conceptualization of sol gel synthesis of TiO2 involves transition of the sol-gel (from a mixture to a gel followed by aging and drying) formed out of the precursors [60-64].

26

Figure 3.1: Schematic representation of the different stages sol-gel process

3.2

Characterization techniques

3.2.1

X-ray diffraction technique X-ray diffraction a common technique for the study of crystal structures and atomic

spacing. X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to dspacings allows identification of the mineral because each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings with standard reference patterns. With the

27

data obtained and standards such as JCPDS, ICDD the graphs obtained for a specific sample’s crystalanity can be analyzed and studied [64-67].

Figure 3.2: XRD principle and working

X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology.

3.2.2 Fourier transform infra-red spectroscopy FTIR offers quantitative and qualitative analysis for organic and inorganic samples. Fourier Transform Infrared Spectroscopy (FTIR) identifies chemical bonds in a molecule by producing an infrared absorption spectrum. The spectra produce a profile of the sample, a distinctive molecular fingerprint that can be used to screen and scan samples for many different components. FTIR is an effective analytical instrument for detecting functional groups and characterizing covalent bonding information

28

FTIR can be used with other molecular spectroscopy techniques available in Intertek laboratories, including Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Infrared-spectroscopy coupled to Thermo-gravimetric Analysis (FTIR/TGA), Nuclear Magnetic Resonance Spectroscopy (NMR), Gas Chromatography - Mass Spectrometry (GC/MS), Liquid Chromatography - Mass Spectrometry (LC/MS), UV/Vis spectroscopy, Near Infra-red (NIR) and Raman scattering. FTIR combined with these techniques provides complementary data regarding a molecule's molecular structure [64, 68-70].

Figure 3.3: FTIR assembly, principle and working

3.2.3 Raman spectroscopy Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near

29

ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information [64, 71-74].

Figure 3.4: Raman spectroscopy principle and operation

5.2.4 Scanning electron microscope A scanning electron microscope (SEM) is an instrument that produces images of a sample by scanning it with a focused beam of electrons. When the electrons interact with atoms in the sample, it produces different signals that can be detected and gives information about the sample's surface morphology and composition. The electron beam is generally scanned in raster scan pattern process and the superposition of position of beam with the detected signal to produce an images. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the 30

incident electrons are decelerated in the solid sample. These decelerated electron are termed as secondary electrons or backscattered electrons, commonly used for imaging samples. Secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbitals of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength. Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam [64, 75-77].

Figure 3.5: A schematic representation of SEM

5.2.5 UV-Vis spectroscopy Ultraviolet–visible spectroscopy (UV-Vis) also known as absorption spectroscopy or reflectance spectroscopy since it absorbs light in the ultraviolet-visible spectral region. It uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption or reflectance

31

in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. The incident light excites the valence band electrons which lead to the collective oscillation of electrons against the restoring force of positively charged nuclei. The collective oscillation occurs with different frequencies depending upon the shape and size of the absorbing molecules. When surface plasmon resonance occurs, the molecules absorb incident light. Beer–Lambert law is most often used in a quantitative way to determine concentrations of an absorbing species in solution. The Beer-Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length. A=log(I/Io)=.c.L where A is the measured absorbance, in Absorbance Units (AU), Iois the intensity of the incident light at a given wavelength, is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure The basic parts of a spectrophotometer are a light source, a holder for the sample, a diffraction grating in a monochromator or a prism to separate the different wavelengths of light, and a detector. The radiation source is often a Tungsten filament (300-2500 nm), a deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm), Xenon arc lamp, which is

32

continuous from 160-2,000 nm; or more recently, light emitting diodes (LED) for the visible wavelengths. The detector is typically a photomultiplier tube, a photodiode, a photodiode array or a charge-coupled device (CCD) [64, 78-80].

Figure 3.6: A schematic representation of UV-Vis spectrometer

33

CHAPTER 4 SYNTHESIS AND CHARACTERIZATION 4.1

Reagents All the reagents used were of analytical grade and no further purification was done before

use. TiO2 nano-powders were prepared via sol–gel method using Titanium (IV) isopropoxide (TTIP, sigma Aldrich), distilled water (H2O), and ethyl alcohol (EtOH, Merck) as the starting materials. Nitric acid (HNO3, Qualigens) was used to adjust the pH and for restrain the hydrolysis process of the solution

4.2

Synthesis of titania nanoparticles During the synthesis the molar ratio of Titanium (IV) isopropoxidein [Ti(OC3H7)4] and

distilled water H2O preferred was 1:4. The chief precursor Titanium (IV) isopropoxidein powder form was (TIPO) dissolved in ethanol. To this mixture double distilled water was added. In the presence of water the former mixture of TIPO and ethanol reacts to give hydrolyzed Titanium (IV) isopropoxide and i-propanol as products. This initial step is the hydrolysis of the alkoxide and the same could be understood from the equation mentioned below.

The next process that follows the hydrolysis is condensation of the hydrolyzed species or the propagation step, with the bridging of oxygen. Each new alcoxolation step is accompanied by the formation of a i-propanol molecule which could be understood from the following equation.

During the propagation step the nucleation of the TiO2 beings to appear. Nucleation of the TiO2 plays a key role in determining the nature of the end product. If the nucleation is left unattended chances for the resultant to possess aggregates and unreacted isopropanol titanate salts are high. Hence, the nucleation process is attended by altering the pH of the sol-gel through

34

nitric acid (HNO3). 3.5ml of Nitric acid was added to the sol-gel to fix the pH to 2.7-3.1 during the synthesis process. The obtained solutions were kept under slow-speed constant stirring on a magnetic stirrer for 40 min at room temperature. In order to obtain desired nanoparticles, the gels were dried under 50oC for 1.5 hr to evaporate water and organic material to the maximum extent prior to calcination and annealing. The final composition of the material obtained through drying of the gel has the following structure, where every titanium atom is forming part of the network; where y can be 1 or 2.

The obtained dried gel was annealed for 6 hours at 4000C to obtain anataseTiO2 nanoparticles and the same is represented through the following equation.

while a few more grams were annealed separately at 7000C for 6 hours to obtain TiO2R(Rutile). The annealed titania particles, both TiO2-A and TiO2-R in 3:1 ratio were suspended in 10 ml of DD water. 4.3

Preparation of GeO2-TiO2-azobenzene nanocomposite To obtain a GeO2-TiO2 nanocomposite, 4-9: electronic grade GeO2 was added to the gel

obtained during the propagation process of the sol-gel synthesis. To obtain a functionalized GeO2-TiO2, to the annealed nanocomposite, 25ml of ethanol with 0.02g of azobenzene (Sigma Aldrich) is added and dried at room temperature. The final product is dried and grinded using a mortar and pestle and subjected to characterization to study their properties.

35

Figure 4.1: Flow chart of the experimental procedure adopted for synthesis and characterization of azobenzene functionalized TiO2-GeO2 composite.

36

4.4 Results and Discussions 4.4.1 XRD The XRD patterns of the synthesized as prepared sample through sol-gel and nanocomposite sample are presented respectively, in Figure 4.2.a. and 4.4.b. the observed patterns were compared with JCPDS data and references. From the comparison it was observed that the synthesized material is a combination of azobenzene, germanium oxide, anatase and rutile phases of titania particles through, [(002), (003)], [(100), (101), (110), (102), (111), (104), (301)] and [A(101), R(110), A(105), R(211), R(220), R(002)], respectively. The particle size was calculated by the Scherrer equation which can be given by

Where, τ is the particle size, K is a dimensionless shape factor (K=0.94),λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) and θ is the Bragg angle. The crystalline size and strain were calculated to be 22.71nm and 0.0088 respectively. The obtained patterns were in accordance with the other reports too [64-68].

Figure 4.2.a: XRD pattern of the TiO2anatase and rutile phase

37

Figure 4.2.b: XRD pattern of the nanocomposite material

4.4.2 FTIR The FTIR spectra obtained for the prepared material is presented in Figure 4.3. The FTIR spectra clearly indicates the presence of azobenzene, titania particles and GeO2 complementing and confirming the patterns obtained from XRD and FT-Raman through bands 3400 cm-1, 2865 cm-1. The other stretching bands indicate the presence of azobenzene and titania.

38

Figure 4.3: FTIR of the nanocomposite

4.4.3 Raman Spectroscopy The Raman spectroscopy pattern obtained confirms the presence of the required metal oxides within the nanocomposite material. Through the pattern obtained, the presence of anatase, rutile phases of TiO2 and rutile form of GeO2 can be confirmed; from bands at 400 cm-1 to 700 cm-1 and 174 cm-1 respectively. It can be noticed that no other bands associated with GeO2 could be noticed between 400 cm-1 to 700 cm-1. This should be due to the dominance of titania nanoparticles and due to the possibilities for the Ge4+ to have got substituted within titania crystal lattice. Hence only, 399, 514, and 639 cm−1 bands of TiO2 were detected by the laser. The wavelength of laser preferred was 514nm [65-69].

39

Figure 4.4.a: FT Raman spectroscopy of the TiO2 and GeO2 composite

Figure 4.4.b: FT Raman spectroscopy of the nanocomposite material

40

4.4.4 SEM From the Figure 4.5, it can be observed that the particles are spherical and are of optimum dimension of 80nm to 200nm to support applications such as anode material for dye sensitized solar cell or as semiconductor for diodes or for transistors. The spheres with lower diameter should be GeO2 particles while the greater ones should be TiO2 nanoparticles due to annealing at higher temperature. The shift from bright shade to the lighter shade could indicate the presence of azobenzene coating over the nanoparticles.

Figure 4.5: Morphology of the prepared nanocomposite material

41

4.4.5 UV-Visible Spectroscopy The UV-Vis spectrum presented in the Figure.4 shows the improved response of the synthesized material when compared to pure titania particles. The synthesized azobenzene based composite nanomaterial showed strong cut off greater than pure titania. From the spectra, using band Gap Energy; E = h*c/λ. The optical band gap of the synthesized material was found to be 2.78eV to 3.1eV and 3.06eV to 3.14eV for the compositions of TiO2 (anatase and rutile) and azobenzene functionalized TiO2-GeO2 respectively; from Fig 4.6, (a, b, c, d, e). It can also be noticed that the band gap observed for the functionalized nanocomposite matches closely with the commercially available TiO2 materials for solar cell and FET’s applications.

a

b

42

c

d

e Figure 4.6: UV-Vis (transmittance) of the prepared samples; a. TiO2anatase, b. TiO2 anatase and rutile mixed phase, c. TiO2 and GeO2 composite d. Azobenzene, e. TiO2 -GeO2 – Azobenzene nanocomposite. This makes the synthesized material to be one of the suitable composition for applications such as optoelectronic devices, solar cells etc. Thus a low band gap material was 43

synthesized, and the chances for the material to create an impact on dye sensitized version of solar cells is greater as the DSSC requires a wide band gap material. Also, the synthesized material composition could be further modified by necessary anatase content to support the applications that require wide and direct band gap feature. Furthermore, the presence of azobenzene can play a crucial role in functionalizing and modifying the surface of TiO 2 for ruthenium or any other dye conjugation for DSSC application. Their presence can also facilitate the prevention of rapid dye degradation due to photocatalytic activity of TiO2. The azobenzene functionalization too adds an advantage for the material’s application in the field of optoelectronics as the material can be programmed to respond to a specific wavelength of UV range.

44

CHAPTER 5 Conclusion and Future Work

6.1 Conclusion The material, azobenzene functionalized TiO2-GeO2 nanocomposite was synthesized through simple sol-gel technique and the same was characterized by XRD, FTIR, FT-Raman, SEM, UV-Vis to study the material’s crystal morphology, composition, confirm the phase formation, structural morphology and optical properties respectively. From the characterization results, it could be concluded that, the composition TiO2-GeO2 functionalized with azobenzene could serve as a good photoanode material for dye sensitized solar cells, optoelectronic devices such as optically controlled GATE, Transistors, etc. It could also be noticed from SEM data that the particles were spherical and were almost uniform proving the sol gel method adopted to prepare the composite to be a better and optimized technique. From the UV-Vis results, it can be concluded that, the composition of anatase and rutile phases do get an enhancement in optical properties when combined with GeO2 and azobenzene. From the above mentioned results and from the other results mentioned in the previous chapter, it can be concluded that, the composition TiO2 (anatase + rutile) –GeO2 with azobenzene functionalization possess the desirable and effective chemical and optical properties to support the vast research for an optimized photoanode material and semiconducting materials for optoelectronic devices. 6.2 Future perspectives The TiO2 (anatase + rutile)-GeO2 composites were functionalized with azobenzene to achieve a remarkable material that can respond to a UV light source. This was also the actual theme of the project initially. The azobenzene was preferred so that the material to which it had got adhered will enable an adsorptive forces (N=N & C-H, H-N) when dye such as ruthenium is used in developing photoanode for dye sensitized solar cell. The behavior of the material and their conductivity studies weren’t performed due to lack of material quantity and the same will be addressed in future through bulk synthesis and through a detailed Impedance studies (presence and absence of UV).

45

The TiO2 anatase and rutile phases prepared and functionalized with azobenzene could also be further studied using XPS to understand their enhanced properties and shelf life. The chosen composition could also be preferred to synthesize nanofiber material and the same can be characterized to compare and identify the best material morphology for photoanode and optoelectronic device applications.

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CHAPTER 6 REFERENCES

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