Investigation of TiO2 nanostructures fordye-sensitized solar cells applications

著者 Jayaram Archanayear 2013-06出版者 Shizuoka UniversityURL http://doi.org/10.14945/00007932

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DOCTORAL THESIS

Investigation of TiO2 nanostructures for

Dye-sensitized solar cells applications

J. Archana

Graduate School of

Science and Technology, Educational Division

Department of Optoelectronics and Nanostructure Science

Shizuoka University

June 2013

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ACKNOWLEDGEMENT

First of all, I express my sincere and esteemed gratitude to my Research guide and

Supervisor, Prof. Yasuhiro Hayakawa, Professor, Research Institute of Electronics,

Shizuoka University, for his constant encouragement, committed guidance at every stage of

my research work.

My profound gratitude to Prof. A. Konno, Prof. A. Ishidha and Prof. H. Tatsuoka,

Shizuoka University for the evaluation of the thesis and valuable comments to improve the

research in future.

I like to convey my sincere thanks to Prof. K. Murakami, Research Institute of

Electronics in Shizuoka University, for the instrumentation support during the material

analysis.

I wish to thank Mr. T. Koyama and Mr. W. Tomoda for their assistance in

instrumentation handling and characterizations of the samples.

I am also very grateful to my laboratory members for all their supports on me during the

research period.

I would like to thank MEXT – Japan for the financial assistance to complete the research

work.

Finally, very special thanks to my husband Dr. M. Navaneethan, my parents and family

members for their patience, assistance and constant source of support throughout my research

work.

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Contents

Page

Abstract i

Chapter 1 Introduction

1.1 Background 1

1.1.1 Dye – sensitized solar cells 2

1.1.2 Basic working principle 5

1.1.3 Factors affecting the efficiency of DSSC 8

1.1.4 Organic sensitizers 8

1.2 Review of literature 10

1.3 Problem statement 14

1.4 Purpose of the research 15

References 15

Chapter 2 Hydrothermal growth of high surface area mesoporous anatase TiO2

nanospheres and investigation of dye-sensitized solar cell properties

2.1 Background 20

2.1.1 Experimental Section 21

2.1.2 Hydrothermal growth of mesoporous TiO2 spheres 21

2.1.3 Dye-sensitized solar cell fabrication details 21

2.1.4 Characterization techniques 22

2.2 Results and Discussion 27

2.3 Conclusions 45

References 46

Chapter 3 Synthesis of template assisted mesoporous anatase TiO2

nanospheres by hydrothermal method and dye-sensitized solar cell

properties

3.1 Background 47

3.2 Experimental Section 48

3.2.1 Hydrothermal growth of mesoporous TiO2 spheres 48

3.2.2 Dye-sensitized solar cell fabrication 49

3.3 Result and discussion 49

3.4 Conclusions 62

References 63

Chapter 4 Functional properties of citric acid capped TiO2 nanoparticles by

hydrothermal growth and dye-sensitized solar cell performance

4.1 Background 64

4.2 Experimental procedure 66

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4.2.1 Synthesis of TiO2 nanoparticles 66

4.2.2 Dye sensitized solar cell fabrication 66

4.3 Results and Discussion 67

4.4 Conclusions 82

References 82

Chapter 5 Hydrothermal growth of monodispersed rutile TiO2 nanorods

and functional properties

5.1 Background 85

5.2 Experimental procedure 86

5.2.1 Synthesis of TiO2 nanorods 86

5.2.2 Dye sensitized solar cell fabrication 87

5.3 Results and Discussion 87

5.4 Conclusions 97

References 97

Chapter 6 Summary and future work

6.1 Summary 100

6.2 Future works 102

List of publications and conferences 103

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ABSTRACT

Even before the industrial revolutions, human life quality is greatly affected by the

availability of energy. The escalated and savage consumption of conventional sources of

energy has been leading to forecasted energy and environmental crises. Renewable energy

sources such as solar energy are considered as a feasible alternative because more energy

from sunlight strikes Earth in 1 hour than all of the energy consumed by humans in an entire

year. Facilitating means to harvest a fraction of the solar energy reaching the Earth may solve

many problems associated with both the energy and global environment. Therefore, intensive

research activities have focused on different classes of organic and inorganic based solar cells.

Dye-sensitized solar cells (DSSCs) have attracted significant attention as low-cost

alternatives to conventional semiconductor photovoltaic devices. These cells are composed of

a wide band gap TiO2 semiconductor deposited on a transparent conducting substrate, an

anchored molecular sensitizer, and a redox electrolyte. Ruthenium sensitizers have shown

very impressive solar-to-electric power conversion efficiencies, reaching 11% at standard AM

1.5 sun light.

TiO2 semiconducting material has the wide band gap of 3.2 eV. Nanocrystalline

semiconductor TiO2 particles are of interest due to their unique properties and several

potential technological applications such as photo catalysis, solar energy harvesting cell,

memory devices, antibacterial coating and photonic crystals. TiO2 is regarded as the most

efficient material to be used as the electron transporting materials for DSSC. In order to

synthesize the TiO2 nanoparticles, several methods have been adapted such as sol – gel, laser

ablation, hydrothermal, microwave, wet chemical, solvothermal etc. In comparison with other

methods, hydrothermal method is a simple and inexpensive method to prepare the well

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crystalline materials.

The objectives of the thesis are to investigate

(1) To synthesize various TiO2 nanostructures (mesoporous spheres, nanoparticles,

nanorods) by simple hydrothermal method

(2) Preparation of photoanode using the synthesized TiO2 nanostructures by spray

deposition method and study the DSSC characteristics.

Mesoporous anatase TiO2 nanospheres were successfully synthesized by a simple

hydrothermal method without the aid of templates. Experimental conditions were optimized

to achieve the high surface area and well-defined TiO2 mesoporous spheres. The effects of

systematic growth periods on the morphological, structural and optical properties of the

mesoporous TiO2 spheres were investigated. Crystal structure, morphology and phase

formation were characterized by X-ray diffraction (XRD), ultraviolet visible spectroscopy

(UV), Raman spectroscopy, fourier transform infrared spectroscopy (FTIR), field emission

scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray

photoelectron spectroscopy (XPS). It was found that the samples prepared for 25 h growth

period yielded good interparticle connection with a well-defined sphere-like morphology

when compared with the samples for 15 and 20 h growth periods. High surface area of 188

m2g

-1 was obtained from the BET analysis for the 25 h grown TiO2 mesoporous spheres.

Mesoporous TiO2 spheres with different growth periods were used to prepare a photoanode

layer by spray pyrolysis deposition for DSSC fabrication. The ruthenium dye (N719) and

indoline dye (D205) were used as sensitizers in the devices. The effect of the photoanode

active layer thickness on the DSSC conversion efficiency was investigated. It was found that

the maximum efficiency () of 7.02 (N719) and 6.97 % (D205) were achieved for a layer

thickness of 16 m. TiO2 mesoporous spheres was used as a scattering layer for standard P25

titania coated DSSC and the enhanced efficiency of 5.92 (N719) and 5.12 % (D205) were

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obtained.

Well-crystallized mesoporous TiO2 spheres were prepared by the hydrothermal

method using ethylene glycol as a template. The amount of titanium tetra isopropoxide

(TTIP) was varied as 0.5, 1.0, 1.5 and 2.0 mL. The role of the precursor amount on the

formation of mesoporous and the functional properties were investigated. The morphological

studies results that the spheres with defined boundaries were obtained for 0.5 ml when

compared to the other amounts (1.0, 1.5 and 2.0 mL) of the samples. The as synthesized

mesoporous TiO2 with various amounts of precursors were used as the photo electrode

material of the DSSCs. The ruthenium based dye (N719) and indoline dye (D205) were used

as sensitizers in the devices. The overall maximum efficiency () of 8.96 (N719) and

9.02 % (D205) were achieved for the precursor amount of 0.5 mL.

Anatase TiO2 nanoparticles were successfully synthesized by a simple hydrothermal

method with citric acid as a capping agent. The effects of systematic growth periods on the

morphological, structural and optical properties of TiO2 nanoparticles were investigated.

XRD results confirmed the formation of anatase phase when citric acid was used. TEM

measurement revealed that the particle size increased by increasing the growth period. TiO2

particles synthesized under different growth periods such as 5, 15, 25 and 45 h were used to

prepare a photoanode layer by spray deposition technique for DSSC fabrication using N719

ruthenium as a sensitizer. It was found that the maximum efficiency () of 7.66 % was

achieved for 15 h growth period due to enhanced light harvesting caused by absorption of

greater numbers of dye molecules.

Monodispersed rutile TiO2 nanorods were synthesized by hydrothermal method.

Citric acid is used as a capping agent to prevent agglomeration. XRD pattern revealed the

formation of rutile phased TiO2. The prominent UV absorption was detected and the band gap

was found to be 3.22 eV. Spectroscopic studies evidenced the presence of inorganic and

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organic compounds. FESEM and TEM images illustrated the formation of monodispersed

TiO2 nanorods with 1 – 1.5 m in length and 20 – 30 nm in thickness. The photoanode layer

based on the synthesized TiO2 nanorods was fabricated by spray deposition technique for

DSSC fabrication using N719 ruthenium as a sensitizer. It is indicated the maximum

efficiency () of 4.0 %.

The above results clearly confirm the morphology of the TiO2 nanostructures was a

crucial factor in the device performance. Among the synthesized nanostructures, the best

efficiency of efficiency () of 8.96 (N719) and 9.02 % (D205) were obtained for the ethylene

glycolate assisted TiO2 mesoporous spheres.

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Chapter 1

Introduction

1.1 Background

The Greek word ‘nano’ is referred to the length scale of one billionth of a meter.

Thus nanoscience deals with the science of materials and technologies in the scale range of ~

1-100 nm. This means that the nanoscience deals with a few hundred to a few thousand atoms

or atomic clusters, whereas the microscopic world is made out of trillion of atoms or

molecules. Nanoparticles are larger than individual atom and molecules, but are smaller than

bulk solid; hence they obey neither absolute quantum chemistry nor laws of classical physics.

Nanotechnology has been steadily receiving significant attention during the past decades both

in scientific and engineering communities. The nanoscience and technology represents the

most active discipline in the all around the world and is considered as the fastest growing

technology revolution which the human history has ever seen. This intense interest in the

science of the materials confined within the atomic scales the fact that these nanomaterials

exhibit fundamentally unique properties with great potential of next generation technologies

in electronics, computing, optics, biotechnology, medical imaging, medicine, drug delivery,

structural materials, aerospace, energy, etc.

Nanostructured materials are materials with the characteristic length scale of the

order of (typically 1 to 100) nanometers. The structure refers to the chemical composition, the

arrangement of the atoms and the size of a solid in one, two or three dimensions. The factors

controlling the properties of nanostructure materials are size where critical length scales of

physical phenomenon become comparable with the characteristic size of the building blocks

of the micro structure. The synthesis, characterization and processing of nanostructure

materials are part of an emerging and rapidly growing field. Research and development in

this field emphasizes scientific discoveries in the generation of materials with controlled

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micro structural characteristics.

In nanoparticles, the properties (physical, chemical, biological etc.,) can be

selectively controlled by engineering the size, morphology and composition of the particles.

Nanomaterials are known to exhibit markedly different properties compared to micron sized

ones. These new substances will have enhanced or entirely different properties from their

bulk counterparts [3, 4]. It has been shown that the various material properties such as

electrical, mechanical, optical, magnetic etc., are highly influenced by the fine - grained

structure. Using a variety of synthesis methods, it is possible to produce nanostructured

materials in the various forms such as thin films, powder, quantum wires, quantum wells,

quantum dots, etc.

In the past decades, chemical routes for nanomaterials fabrication have matured and

there is a very good control over the size [6,7], shape [8, 9] and most importantly, yield when

considering a per - batch basis [10]. Applications of nanomaterials cover a wide range of

fields including bio - medicine [11], electronics [12], optoelectronics [13] and water

purification [14], amongst many others. Traditionally nanomaterials investigated for

optoelectronic applications were fabricated by vapor deposition techniques [15], However

recent advances in the chemical routes allow synthesis of nanomaterials with good control

over its shape, and size [16]. This work focuses on the synthesis of TiO2 nanomaterials to the

dye sensitized solar cells.

1.1.1 Dye – sensitized solar cells

In today's society, it is becoming important to find alternative sources of energy that

are both cheap and efficient. Solar cells have become one of the most widely-researched

methods of obtaining energy in "greener" ways than burning fossil fuels, etc. One of the new

variants on the solar cell that is currently being researched is the dye-sensitized solar cell

(DSSC), which was invented by Michael Gratzel and Brian O'Regan in 1991. Recently,

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commercial applications of the DSSC have been under intensive investigation. The cost of

commercially fabricating DSSCs is expected to be relatively low because the cells are made

of low-cost materials and assembly is simple and easy.

Photo electrochemical solar cells (PSCs), which consist of a photo electrode, a redox

electrolyte and a counter electrode, have been studied extensively. Several semiconductor

materials, including single-crystal and polycrystalline forms of n- and p-Si, n- and p-GaAs, n-

and p-InP, and n-CdS, have been used as photo electrodes. These materials, when used with a

suitable redox electrolyte, can produce solar light-to-current conversion efficiency of

approximately 10 %. However, under irradiation, photo corrosion of the electrode in the

electrolyte solution frequently occurs, resulting in poor stability of the cell, so efforts have

been made worldwide to develop more stable PSCs.

Oxide semiconductor materials such as TiO2, ZnO and SnO2 have good stability

under irradiation in solution. However, stable oxide semiconductors cannot absorb visible

light because they have relatively wide band gaps. Therefore, photo sensitizers such as

organic dyes are required to absorb visible light. TiO2 is an important semiconductor material

for use in a wide range of applications, including photo catalysis, environmental pollution

control and solar energy conversion [17-24]. The TiO2 materials have good chemical stability

under visible irradiation in solution, nontoxic and inexpensive. It is well known that TiO2

exists in three crystalline polymorphs namely rutile (tetragonal), anatase (tetragonal) and

brookite (orthorhombic). Rutile is the most stable phase whereas the anatase and brookite are

metastable phases and transform to rutile upon heating. However, the anatase phase has been

widely used in the photo catalyst due to its high photo activity.

The basic structure of the DSSC is shown in the Fig.1.1. The original Grätzel

designed cell has three primary parts. First one is glass sheet with transparent conducting

oxide coating (ITO or FTO) as anode on top of it. Second is the semiconductor oxide film

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deposited on the conductive side of the glass sheet. Third is a mixture of a photosensitive

ruthenium- polypyridine dye (also called molecular sensitizers) and a solvent. After soaking

the film in the dye solution, a thin layer of the dye is covalently bonded to the surface of the

TiO2. A thin layer of the iodide electrolyte is spread over a conductive sheet, typically

platinum metal. The front and back parts are then joined and sealed together to prevent the

electrolyte from leaking.

Fig.1.1 Schematic diagram of the DSSC.

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1.1.2. Basic working principle

Fig.1.2 Schematic illustration of the working principle of DSSC.

Figs.1.2 and 1.3 show the working principle and overview of kinetics of electron

transfer process in DSSC, respectively. Monolayer of dye is attached to the surface of a

mesoscopic film of TiO2 (wide-bandgap oxide). The mechanism of the DSSC is as follows.

(1) The dye serves to harvest the solar light and generate the electrons.

(2) Electrons are injected into the conduction band of TiO2

(3) Electrons travels through the nanoparticle network and collected by the anode by

diffusion process.

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(4) Then the electron subsequently passes through the external circuit, performs the

electrical work and moves to the cathode

(5) Meanwhile the dye injects holes to the hole conductors and transport to the

counter electrode with the outside circuit which finishes the loop.

Fig.1.3 Over view of kinetics of electron transfer process in DSSC.

Under illumination photocurrent generation is taken place as described above and

photo voltage is defined by the difference between the Fermi level of the electron in the TiO2

and the redox potential of the electrolyte. Generation of electrical power is achieved by the

capability of the photovoltaic device to produce voltage over an external load and passing a

current through the load at the same time. This is characterized by the current-voltage (I-V)

curve of the cell at certain illumination and temperature as shown in Fig. 1.4. The power

output is given by the product of current and voltage through the load (Fig. 1.4). In the

present time, the software draws these two graphs at the same time when solar cell is

measured under computer controlled I-V set up in simulated sunlight.

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Fig.1.4 Current – Voltage characteristic of a solar cell.

When the cell is short circuited under illumination the short circuit photocurrent (Isc)

is the maximum current that generated. Under open circuit conditions no current can flow and

the voltage, which is at its maximum, is called the open circuit voltage (Voc). The point at

which the product of current and voltage is maximum in the I-V curve is called the maximum

power point (MPP). These points are shown in the Fig. 1.4 in the I-V curve. Another

important characteristic of the solar cell performance is the fill factor (FF), defined as

FF = VMPP . IMPP / Voc . Isc

Where VMPP and IMPP are the voltage and current at the maximum power point in the I-V

curve of the cell respectively.

The maximum power output of the solar cell then can be written as

Pmax = Voc . Isc . FF

Although the operation principle of different types of photovoltaic cells are not identical the

shape of I-V curve of well performing cells are similar and compared with each other in

terms of FF, Voc and Isc. Finally, the energy conversion efficiency of the solar cell is defined

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as the maximum power produced by the cell (Pmax) divided by the power of the incident light

on the representative area of the cell (Plight)

= Pmax / Plight

(or)

= Voc . Isc . FF / Plight

1.1.3. Factors affecting the efficiency of DSSC

There are several parameters influencing or depending the efficiency of the DSSC such as,

1. Working function and resistivity of indium tin oxide electrode (ITO or FTO).

2. Morphology and carrier transport properties of semiconductor (oxide materials).

3. Absorption wavelength and interface alignment of dye molecule and semiconductor

material.

4. Electron transfer from redox electrolyte to excited dye molecule.

5. Catalytic reaction of platinum counter electrode with the iodide electrolyte.

1.1.4. Organic sensitizers

Ruthenium sensitizers have indicated very impressive solar-to-electric power

conversion efficiencies and reached 12.3 % at standard AM 1.5 sun light [25, 26]. Several

groups have developed metal-free organic sensitizers and obtained efficiencies in the range of

4-8 % [27-30]. The critical factors that influence sensitization are (1) the excited-state redox

potential, which should match the energy of the conduction band edge of the oxide. (2) Light

excitation associated with electron flow from the light-harvesting moiety of the dye toward

the surface of the semiconductor surface. (3) Conjugation across the donor and anchoring

groups and electronic coupling between the lowest unoccupied molecular orbital (LUMO) of

the dye and the TiO2 conduction band. The major factors for low conversion efficiency of

many organic dyes in the DSSC are due to the dye aggregation on the semiconductor surface

and recombination of conduction-band electrons with triiodide [31]. Therefore, to obtain

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optimal performance, aggregation of organic dyes and recombination of electron, have to be

avoided by appropriate structural modification [32]. On the basis of the strategy, the best

photovoltaic performance having high conversion yield and long – term stability has been

achieved with polypyridyl complexes of ruthenium and osmium. Sensitizers having the

general structure ML2(X)2 where L stands for 2,2’-bipyridyl-4,4’-dicarboxylic acid, M is Ru

or Os and X presents a halide, cyanide, thiocyanate, acetyl acetonate, thiacarbamate or water

substituent, are particularly promising. The ruthenium complex cix-RuL2(NCS)2, [N3 dye] is

shown in the Fig.1.5 and Fig.1.6, respectively.

Fig.1.5 The ruthenium complex cis –bis (isothiocyanato)bis (2,2’- bipyridyl- 4,4’-

dicarboxylato) - ruthenium(II) – [N3 dye].

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Fig.1.6 The ruthenium complex cis- doosptjopcuamatp –bis (2,2-bipyridyl-4,4-

dicaboxylato) ruthenium (II) bis(tetrabutylammonium) –[N719 dye].

1.2 Review of literature

Sun et al., prepared the DSSC with P25 electrode and demonstrated the short circuit

photocurrent desnity (ISC) as 5.04 (mA/cm2) and the overall conversion efficiency of 2.70 %

[33]. Yun et al., reported that the P25 electrode for DSSC yielded the efficiency of 5.62 %

with the Isc of 9.50 mA/cm2 [34]. Hamadanian et al., prepared the P25 electrode at various

thickness of 1.5, 4.2, 7.1, 12.2, 17.0, 21.4, 24.0, 26.3 µm and the electrode with the thickness

of 24 µm showed the highest efficiency of 6.56 % with the Isc of 16.4 mA/cm2 [35]. The P25

electrode synthesized by Alam Khan et al., resulted in the efficiency of 6.59 % where the Isc

was 22.30 mA/cm2 [36]. De Zhao et al., investigated the effect of annealing temperature on

the photo electrochemical properties of DSSCs. The P25 electrode is annealed at 500 º C

indicated the efficiency of 6.33 % with the Isc of 5.68 mA/cm2 [37]. The dependency of

efficiency on the thickness of the film was studied by the Marco et al. They varied the

thickness of P25 electrode as 4, 6, 8, 10 and 14 µm. The maximum efficiency was 5.5 % for

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the thickness of 14 µm with the Isc of 11.30 mA/cm2 [38]. Niu et al., prepared the P25

electrode based DSSC. It resulted in the efficiency of 5.8 % with the Isc 12.55 mA/cm2 [39].

The conventional P25 electrode with various thickness of 6, 10 and 13 µm was studied by

Hao et al. The highest efficiency of 6.59 % was obtained for 13 µm with the Isc of

12.84 mA/cm2 [40]. Agarwala et al., studied the performance of P25 electrode for DSSC. It

resulted the efficiency of 4.0 % with Isc of 7.6 mA/cm2 [41]. Xu et al., fabricated the DSSC

on TiO2 nanoparticles based electrode. The obtained efficiency was 4.25 % with the Isc of

8.9 mA/cm2 [42]. Fan et al., synthesized the anatase TiO2 fusiform nanorods for DSSC with

the diameter of 20 -80 nm and lengths of 200 – 400 nm. It resulted the efficiency of 2.45 %

with the Isc of 4.56 mA/cm2 [43]. Pan et al., prepared the TiO2 nanorods based photoanode

for the DSSC. The efficiency was 0.93 % with the Isc of 4.08 mA/cm2 [44]. The TiO2

nanorod prepared by Guo et al., for the DSSC had the efficiency of about 0.76 % where the

Isc was 2.57 mA/cm2 [45]. Koo et al., studied the I – V characteristics for the DSSC

fabricated with the TiO2 nanorods. It was observed that the obtained efficiency was 3.54 %

with the Isc as 9.07 mA/cm2 [46]. Yang’s group had fabricated the DSSCs using 2.5 µm long

nanorods. The overall conversion efficiency was 1.31 % where the Isc is 2.55 mA/cm2 [47].

Guang et al., prepared the mesoporous based photoanode at various thickness of 5.3, 8.2, 12.5,

15.4, and 22.3 µm. The highest efficiency of 8.20 % was obtained for the thickness of

15.4 µm with the Isc as 16.67 mA/cm2 [48].The porous TiO2 spheres were synthesized and

I –V characteristics were studied by Wang. The efficiency of the porous material resulted as

5.0 % with the Isc as 15.6 mA/cm2 [49]. Kim et al., prepared the ordered mesoporous

structures for the application of DSSC. The observed efficiency was 5.88 % with the Isc as

16.07 mA/cm2 [50]. Hou et al., fabricated the DSSC with the highly crystallized mesoporous

TiO2 films. They had varied the thickness of the films as 1.0, 2.5 and 4.0 µm. The highest

overall efficiency of 5.31 % was obtained for the thickness of 2.5 µm with the Isc as

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13.30 mA/cm2 [51].The mesoporous structure with high surface area were synthesized by

Jung et al., for the applications of DSSC. The thickness of the photo anode material was

varied as 4.7 and 8.7 µm. The maximum conversion efficiency of 7.54 % resulted for the

thickness of 8.7 µm with the Isc as 14.4 mA/cm2 [52].

Table.1 TiO2 nanostructures and obtained efficiencies from literature reports.

Group Material / Method

Isc

(mA/cm2)

Voc

(V)

FF Eff (η)

(%)

Sun et al., P25 nanoparticles

chemical synthesis

5.04 2.70

Yun et al., P25 nanoparticles

chemical synthesis

9.50 0.79 0.74 5.62

Hamadanian et al., P25 nanoparticles

chemical synthesis

16.40 0.72 0.55 6.56

Alam Khan et al., P25 nanoparticles

chemical synthesis

22.30 0.67 0.43 6.59

De Zhao et al., P25 nanoparticles

chemical synthesis

5.86 0.64 0.70 6.33

Marco et al., P25 nanoparticles

chemical synthesis

11.30 0.70 0.70 5.50

Niu et al., P25 nanoparticles

chemical synthesis

12.55 0.75 0.6 5.80

Hao et al., P25 nanoparticles

chemical synthesis

12.84 0.77 0.66 6.59

Agarwala et al., P25 nanoparticles

chemical synthesis

7.60 0.70 0.67 4.0

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Xu et al., P25 nanoparticles

chemical synthesis

8.90 0.72 0.66 4.25

Fan et al., Nanorods

hydrothermal synthesis

4.56 0.75 0.71 2.45

Pan et al., Nanorods

hydrothermal synthesis

4.08 0.67 0.34 0.93

Guo et al., Nanorods

hydrothermal synthesis

2.57 0.63 0.47 0.76

Koo et al., Nanorods

sol gel synthesis

4.08 0.67 0.34 0.93

Yang et al., Nanorods

Microwave synthesis

2.65 0.85 0.60 1.31

Guang et al., Mesoporous

hydrothermal synthesis

16.67 0.74 - 8.20

Wang et al., Mesoporous

microwave synthesis

15.60 0.60 0.53 5.0

Kim et al., Mesoporous

sol gel synthesis

16.03 0.72 0.50 5.88

Hou et al., Mesoporous

chemical synthesis

13.03 0.71 0.56 5.31

Jung et al., Mesoporous

hydrothermal synthesis

13.2 0.73 0.72 6.99

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1.3 Problem statement

Recently, the good efficiency of 12.3 % has been achieved in DSSCs using TiO2 as a

photoanode material. However, further increase of the energy conversion efficiency of

DSSCs remains a great challenge. Much effort have been made for improving the

performance of DSSC by means of increasing light harvesting, increasing the electron

transport in the conduction band of semiconductor oxide and reducing the interfacial

recombination of the charge carriers at the semiconductor oxide and electrolyte interfaces.

The above issues can be implemented through optimizing the photo sensitizers [53, 54],

photo anodes [55 - 60], redox electrolytes [61] and counter electrodes [62 - 64]. Among these,

the structure and morphology of the photoanodes play a very important roles in determining

the light harvest and charge transport properties, which significantly influence the final cell

performance.

The most efficient semiconductors as photoanode are typically composed of

randomly clustered TiO2 nanoparticles 20 – 40 nm in size [65]. The nanoparticles possess

high internal surface for dye adsorption, thereby giving rise to high energy conversion

efficiency. However, the electron transport in such photoanode film is slow due to the random

walk up of electrons and trapping / de-trapping events along the electrons path due to defects,

surface states, grain boundaries and self trapping [66]. In this regard, the tailoring of TiO2

nanostructures is a crucial aspect of increasing the current photovoltaic efficiency of DSSCs.

The oriented one dimensional structure such as TiO2 nano tubes, nano rods or nano wires

have been synthesized and fabricated as a device in order to overcome these problems. So far,

it is reported the array nanostructures prove the direct pathways for the electron transport and

reduce the degree of charge recombination. But, such one dimensional arrays have

insufficient internal surface area to adsorb dye molecules which greatly limit the optical

absorption efficiency and thus the low conversion efficiency [67]. The mesoporous titania

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thick films have attracted increasing interest as photoanodes in DSSCs. It is also considered

to be a promising candidate as a nano porous electrode of DSSC, because of its high surface

area, few grain boundaries and the uniform interconnected titania skeleton with regular nano

crystal junctions. Thus the uniform pore structures with excellent connectivity of mesoporous

are expected to achieve the efficient transfer of electrons and diffusion of electrolytes [68].

Therefore, further size reduction of nanoparticles, aligned one dimensional nanostructures

and mesoporous sphere like nanostructures are required to overcome the above issues.

1.4 Purpose of the research

The aim of the research is as follows:

1. Synthesize of the monodispersed TiO2 nanostructures using hydrothermal method.

2. Investigation of the functional characteristics of TiO2 nanostructures by various

characterization techniques.

3. Fabrication of the DSSC using TiO2 nanostructure and study the device performance.

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20

Chapter 2

Hydrothermal growth of high surface area mesoporous anatase TiO2

nanospheres and investigation of dye-sensitized solar cell properties

2.1 Background

The development of the DSSCs requires a multidisciplinary research approach

combining several fields of physics and chemistry. The main factors that affect the efficiency

are the electron transport in the TiO2 conduction band and the interfacial recombination of the

charge carriers at the electrolyte interfaces [1]. The morphology of the photoanode plays an

important role in determining the electron transport properties. The use of mesoporous TiO2

spheres as photoanode materials with uniform pore sizes has attracted considerable attention

because of their special functionality, where the interconnected junctions with open pores in

the mesoporous structure will speed up the electron transport [2-5]. Because the mesoporous

structure has an interconnected titania skeleton with regular nanocrystal junctions and an

internal surface area, it allows greater adsorption of dye molecules between the pores and

promotes efficient light harvesting compared with the TiO2 nanoparticles alone [6]. The

mesoporous TiO2 spheres have a higher surface area over 10 times than nanotubes and

nanowires and the uniform nano channels can be accessed easily by the I3- transport

electrolyte [7]. In addition to that, the mesoporous TiO2 spheres were utilized as scattering

layer on P25 titania coated DSSC to collect the more amount of incident light which was not

interacted on dye molecules adsorbed on P25 nanoparticles. However, the effect of the

growth period on the formation of mesoporous anatase TiO2 without a template has not been

investigated. Moreover, the electron transport was limited in the thicker films, leading to a

21

decrease in short-circuit current Isc in the DSSCs[8]. This suggested that an optimum

thickness is required to improve the conversion efficiency.

In this research, well-defined high surface area mesoporous TiO2 spheres were

successfully synthesized using a template-free hydrothermal method. The growth period

dependence of the morphological, structural and optical properties of the mesoporous TiO2

spheres was investigated. Mesoporous TiO2 spheres with different growth periods were used

to prepare photoanodes by spray pyrolysis deposition for DSSC fabrication using N719

ruthenium and D205 indoline dyes as sensitizers. The effect of the photoanode active layer

thickness on the conversion efficiency was also investigated. Mesoporous TiO2 spheres were

used as a scattering layer on standard P25 titania active layer and the device performance was

studied.

2.1.1. Experimental procedure

2.1.2 Hydrothermal growth of mesoporous TiO2 spheres

Titanium (IV) isopropoxide (TTIP) and 1-butanol (CH3(CH2)2CH2OH) were

purchased from Wako Chemicals, Japan and were used as received without further

purification. TTIP (0.5 M) was added to 200 ml of butanol. The solution was maintained at

room temperature while being stirred vigorously for 30 min; 60 ml of deionized water was

slowly added to the above solution and was then stirred for 1 h. The white-colored solution

was then transferred to a 50 ml Teflon-lined stainless steel autoclave and hydrothermal

growth was carried out at 150 °C for periods of 15, 20 and 25 h, respectively. Finally, the

resulting products were collected and annealed at 350 °C.

2.1.3. Dye-sensitized solar cell fabrication details

The prepared mesoporous TiO2 powders were dissolved in ethanol and ground using

an ultrasonic processor for 30 min, and 5 drops of triton-X were added to the solution as a

binder. The solution was sprayed on a transparent conducting glass (F-doped SnO2 (FTO),

22

Nippon Sheet Glass, 8.7 Ω/square, transparency of 80 % in the visible range) at 150°C by

spray pyrolysis. The prepared TiO2 films were annealed at 450 °C for 2 h. The resulting

photoanodes were then soaked in an ethanol solution containing 0.03 M of

di-tetrabutylammonium cis-bis (isothiocyanato) bis (2,2”-bipyridyl-4,4’ dicarboxylato)

ruthenium (II) (N719) for 12 h. The DCCS photoanode was clamped firmly with a Pt coated

counter electrode (FTO) to form a sandwich type cell. A redox electrolyte solution was filled

in between the electrodes to form the cell by capillary action. The electrolyte was composed

of 0.6 M dimethylpropylimidazolium iodide, 0.1 M lithium iodide, 0.01 M iodide and 0.5 M

tetrabutylpyridine in acetonitrile (FUNCHEM, Tomiyama electrolyte company, Japan).

2.1.4 Characterization techniques

XRD spectra were recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200)

with CuKα radiation at 0.02 °/sec step interval as shown in Fig.2.1.

Fig.2.1 Rigaku X-ray diffractometer at Center for Nano device Fabrication and

Analysis, Shizuoka University

23

UV-visible absorption analyses were performed using a Shimadzu (Japan) 3100 PC

spectrophotometer with ethanol as dispersing medium as shown in Fig.2.2.

Fig.2.2 UV 3100PC – UV visible absorptions spectrophotometer at Center for Nano

device Fabrication and Analysis, Shizuoka University

24

Raman spectra were obtained using a JASCO NR 1800 Raman spectrophotometer equipped

with Nd:YAG laser as shown in Fig.2.3.

Fig.2.3 JASCO NR 1800 Raman spectrophotometer at Center for Instrumental Nano

device Fabrication and Analysis, Shizuoka University

25

FESEM images were recorded using a JEOL JSM 7001F and 6320F field emission

scanning electron microscopes as shown in Fig.2.4.

Fig.2.4 JEOL – JSM 7001F field emission scanning electron microscope at Center for

Instrumental Nano device Fabrication and Analysis, Shizuoka University

26

TEM images were recorded using a JEOL JEM 2100F transmission electron microscope at

an accelerating voltage of 200 kV as shown in Fig.2.5.

Fig.2.5 JEOL JEM 2100F transmission electron microscope at Center for Nano device

Fabrication and Analysis, Shizuoka University

27

I-V characteristics (1.5 AM 1000 W m-2

simulated sunlight) were recorded with a

calibrated solar-cell evaluation system (JASCO, CEP-25BX) as shown in Fig.2.6.

Fig.2.6 I-V curve measurements system at Prof. Kenji Murakami laboratory,

Shizuoka University.

2.2 Results and Discussion

Fig.2.7 (a) depicts the XRD pattern of the mesoporous TiO2 spheres prepared at

350 °C for the different time intervals of 15, 20, and 25 h. All of the diffraction peaks were

well matched with the pure anatase phase (JCPDS, no. 21-1272). The broadening of the

diffraction peaks indicated a nanocrystalline structure. No other peaks related to other TiO2

phases were observed. Fig.2.7 (b) represents the Raman spectra of the mesoporous TiO2

28

nanoparticles. Ohsaka et al. [9] performed a Raman analysis of the TiO2 material and reported

that the anatase TiO2 nanocrystalline material has six fundamental vibrational modes denoted

by [A1g + 2 B1g + 3 Eg]. The six allowed bands were ascertained as being 144 (Eg), 197 (Eg),

399 (B1g), 513 (A1g), 519 (B1g2), and 639 cm-1

(Eg3) [10]. From Fig.2.7 (b), we confirmed that

all of the peaks represented the anatase phase. It was noted that these peaks showed a slight

shift towards higher wave numbers at 149 (Eg), 402 (B1g), 520.5 (B1g2), and 641.5 cm-1

(Eg3).

Choi et al. [11] studied the size effects in the Raman spectra of TiO2 nanoparticles and

proposed that the Raman band shift towards the higher wave numbers was caused by the

reduction in the particle size of the nanoparticles. When the particle size was reduced, the

vibrational properties of the material changed. Volume contraction may occur within the

nanoparticles because of size induced radial pressure, which in turn increases the force

constants as a result of the reduction in the inter-atomic distances.

29

Fig. 2.7 (a) XRD patterns, and (b) Raman spectra of mesoporous TiO2 spheres

with growth periods of 15, 20 and 25 h.

Fig. 2.8 Optical absorption spectra of mesoporous TiO2 spheres with growth

periods of 15, 20 and 25 h.

30

Fig. 2.9 FTIR spectra of mesoporous TiO2 spheres with growth periods of 15, 20

and 25 h.

UV-visible absorption spectrum of anatase mesoporous TiO2 spheres was recorded for

the three different growth periods as shown in Fig.2.8. All three samples indicated significant

absorption at shorter wavelengths below 400 nm. This may be attributed to the intrinsic band

gap absorption of TiO2. It was found that the incident light was greatly absorbed by the

mesoporous spheres and this enriched the light harvesting. The intensity of the absorbance

was more pronounced in the 25 h samples than in the 20 h and 15 h samples. FTIR spectra of

the mesoporous TiO2 spheres are illustrated in Fig.2.9. The peak at 1040 cm-1

was assigned to

the asymmetric stretching vibration of Ti-O. The peaks at 1440 and 1640 cm-1

were attributed

to the titanium acetate complex and O-H bending, respectively [12]. The broad transmission

around 3400 cm−1

was assigned to stretching vibrations of the Ti-OH groups. The peak at

3400 cm−1

was sharper in the 25 h sample than in the 15 h and 20 h samples. This indicated

that the interactions between the Ti and the hydroxyl ions were more intense in the 25 h

sample.

31

To investigate the elements present in the mesoporous nanoparticles, XPS

measurements were carried out as shown in Fig.2.10. The binding energies of the specimens

were corrected by reference to the Mg peak at 463.8 eV. In Fig.2.10 (a), a strong peak at 459

eV corresponded to Ti 2p3/2 [13]. The broad peak in Fig.2.10 (b) was related to O1s, which

represented the presence of oxygen in the synthesized material. The main peak located at

530.6 eV was produced by the signature of the lattice oxygen in the Ti-O-Ti bonds [14]. The

small shoulder peak originating around 532.3 eV was attributed to physically absorbed

oxygen [15]. This peak was more dominant in the 25 h samples than in the 15 h and 20 h

samples.

32

Fig. 2.10 XPS spectra of mesoporous TiO2 spheres with growth periods of 15, 20

and 25 h. (a) Ti 2p3/2, (b) O1s.

33

Fig. 2.11 (a-1) FESEM, (a-2), (a-3)TEM and (a-4) HRTEM images of growth

period of 15 h.

To interpret the morphological properties of the synthesized mesoporous TiO2 spheres,

the properties of the nanoparticles for various growth periods were analyzed using FESEM

and TEM measurements. Fig.2.11 (a-1), (b-1) and (c-1) shows the FESEM images of

mesoporous TiO2 from the 15, 20 and 25 h samples, respectively, which showed spherical

morphology with average sizes of approximately 100–200 nm. TEM images are shown in

Fig.2.11 (a-2, a-3), (b-2, b- 3) and (c-2, c-3) which indicated the morphological changes for

the different growth periods from the 15, 20 and 25 h samples, respectively. The

34

morphologies of the samples at 15 and 20 h were spherical however, the walls of the spheres

were not well defined. The morphology of the sample at 25 h showed a defined spherical

structure of about 200 nm with interconnected channels, as shown in TEM images of

mesoporous-TiO2. These images confirmed the porosity and the interconnectivity of the

material.

Fig. 2.11 (b-1) FESEM,(b-2), (b-3)TEM and (b-4) HRTEM images of

growth period of 20 h.

Fig.2.11 (a-4), (b-4) and (c-4) shows high-resolution TEM images corresponding to

the samples synthesized at 15, 20 and 25 h, respectively. The nanoparticle size of about 5 nm

was consistent with the XRD results. It was noted that the sample prepared with the longer

35

growth time of 25 h provided a good morphology with interconnected junctions. It would

alleviate the flow of electrons through the TiO2 nanoparticles and reduce the recombination

processes.

Fig. 2.11 (c-1) FESEM, (c-2), (c-3) TEM and (c-4) HRTEM images of growth period of

25 h.

36

Fig. 2.12 Nitrogen adsorption-desorption isotherm (a-c) and Barret-Jyner-Halenda

(BJH) pore size distribution plot (d-f) of the mesoporous TiO2 spheres at different

growth periods 15h, 20h and 25h

The mesoporous network formation of the TiO2 spheres is confirmed by

Brunauer-Emmett-Teller (BET) analysis. Fig.2.12 (a-c) show the N2 adsorption-desorption

isotherms for the mesoporous TiO2 spheres for the growth period at 15, 20 and 25 h,

respectively. All the three samples displayed a typical type-IV isotherm curve with H4

hysteresis loop in the range of 0.6-0.85 P/P0, which is clearly evidenced the mesoporous

network of the samples. The surface area and pore size of the samples were obtained from the

37

Barrett-Joyner-Halenda analysis. The surface areas of the samples were 168.46 m2g

-1 (15

h),178.44 m2g

-1 (20 h) and 188.40 m

2g

-1 (25 h). The surface area was the highest for the

mesoporous TiO2 spheres grown at 25 h when compared to that of the samples grown at 15

and 20 h. The surface area analysis showed higher value than the standard P25 titania

nanoparticles (30 – 50 m2g

-1). Fig.2.12 (d-e) show the pore size distribution curve of

mesoporous TiO2 spheres for the growth period at 15, 20 and 25 h, respectively. All the three

samples exhibited the narrow distribution of the pore size less than 5 nm and the maximum

number of pores with diameter of 4 nm.

The as-prepared 25 h mesoporous TiO2 spheres were used for the photoanodes to fabricate

DSSCs. The effects of various photoanode thicknesses on the conversion efficiency were

investigated. Fig.2.13 (a) depicts the I-V characteristics for mesoporous TiO2 sphere with

various thicknesses of 3, 7, 12, 16 and 23 m. The short-circuit current densities (Isc) for

these samples were 3.08, 6.55, 9.23, 13.11 and 8.70 mA cm-2

, respectively. The associated

energy conversion efficiencies () were 0.56, 2.80, 4.50, 6.4 and 4.07 %, respectively. The

measurements indicated that the DSSC characteristics depended on the photoanode thickness.

Table. 2.1 confirmed that Isc increased as the thickness increased up to 16 m, and began to

decrease at 23 m. This may be because of the charge recombination process in the active

layer. The top-view SEM image of the 16 m mesoporous TiO2 spheres is shown in Fig.2.13

(b). A spherical structure was observed with an average diameter of 200 nm. Also, the

mesoporous TiO2 spheres exhibited a well-defined spherical morphology, even annealed at

450°C. This clearly indicated that the mesoporous TiO2 spheres were highly stable at high

temperatures. A cross sectional view of the mesoporous TiO2 spheres is presented in Fig.2.13

(c). A closely packed layer of uniformly arranged mesoporous TiO2 spheres with a thickness

of about 16 m was formed on the FTO substrate. The mesoporous TiO2 spheres were

interconnected through the edges of the particles.

38

39

Fig. 2.13 (a) Illustration of J-V characteristics and photocurrent spectra of

mesoporous TiO2 spheres at growth times of 15, 20, and 25 h, and (b) top view and (c)

cross sectional views of the photoanode (thickness of 16 m).

Table. 2.1 Photovoltaic performance of DSSC devices made with various meso-TiO2

thicknesses at AM 1.5 and irradiance of 100 mW/cm2.

Thickness

(m)

3 7 12 16 23

FF 0.39 0.63 0.66 0.70 0.67

Voc (V) 0.46 0.67 0.73 0.68 0.68

ISC(mA/cm2) 3.08 6.55 9.23 14.45 8.70

EFF (%) 0.56 2.80 4.50 7.02 4.07

40

Fig. 2.14. Illustration of J-V characteristics and photocurrent spectra of

mesoporous TiO2 spheres (thickness of 16 m) with growth periods of 15, 20, and 25 h.

For better understanding of the effects of the growth period on the solar cell

performance, the thickness was fixed at 16 m and the growth periods were changed from 15

to 20 h. The short-circuit current densities (Jsc) were 8.96 and 11.44 mA cm-2

and the energy

conversion efficiencies () were 3.93% and 4.83%, respectively, as shown in Fig.2.14 and

Table. 2.2. The efficiency increased as the growth period increased because of the improved

morphologies of the mesoporous nanoparticles. Moreover, the well-defined spherical

morphology of mesoporous TiO2 network enhanced the dye adsorption compared to the low

growth period samples.

41

Table. 2.2 Photovoltaic performance of DSSC devices made using meso-TiO2 with

various growth periods at AM 1.5 and irradiance of 100mW/cm2

Growth period 15 h 20 h 25 h

Thickness (m) 16 16 16

FF 0.60 0.63 0.70

Voc (V) 0.72 0.66 0.68

ISC(mA/cm2) 8.96 11.44 14.45

EFF (%) 3.93 4.83 7.02

In addition to that, the indoline dye D205 was used as sensitizer to replace the

ruthenium dye N719 and the device performances were studied. Fig 2.15 shows the I-V

characteristic curves of the N719 and D205 sensitized devices. The obtained device

parameters were Isc of 16.03 mA cm-2

, Voc of 0.68 V and FF of 0.63. The metal free

sensitizer D205 showed higher Isc value of 16.03 mA cm-2

than the N719 sensitized device.

This is due to the collection of more number of incident photons by the D205 dye molecules.

However, the low fill factor value of 0.63 resulted the decline in efficiency of 6.97 %

compared to N719 cell. There was no significant change observed in Voc of both cells. This is

the highest efficiency (6.97 %) so far obtained using mesoporous TiO2 nanospheres by

sensitized with D205 dye.

42

Fig. 2.15 I-V characteristics and photocurrent spectra of mesoporous TiO2 spheres

sensitized with N719 and D205 dyes.

Fig. 2.16 I-V characteristics curves of P25 coated device and mesoporous TiO2 spheres

as a scattering layer coated device sensitized with N719 dye.

43

Fig. 2.17 I-V characteristics curves of P25 coated device and mesoporous TiO2 spheres

as a scattering layer coated device sensitized with D205 dye.

Table 2.3 Photovoltaic performance of DSSC devices made using P25 and scattering

layer of meso-TiO2 nanospheres (25 h sample) sensitized with N719 and D205 dyes at

AM 1.5 with irradiance of 100 mW/cm2

Sample P25@N719

P25-Scattering

layer of

mesoporous

@N719

P25@D205

P25-Scattering

layer of

mesoporous

@D205

FF 0.68 0.64 0.58 0.57

Voc (V) 0.69 0.71 0.68 0.63

Isc(mA/cm2) 11.11 12.82 11.12 13.96

EFF (%) 5.23 5.91 4.44 5.12

44

Fig. 2.18 Schematic diagram of dye-sensitized solar cell (a) mesoporous TiO2

nanospheres coated device, (b) standard P25 titania coated device and (c) mesoporous

TiO2 nanospheres as scattering layer on P25 titania coated cell.

The mesoporous TiO2 nanospheres were used as a light scattering layer (4 m) on top

of the P25 active layer (16 m) and the device performances were studied using N719 and

D205 dyes. Fig 2.16 shows the I-V characteristic curves of P25 coated device and

mesoporous TiO2 nanospheres coated on P25 layer sensitized by N719 dye. Fig. 2.17 shows

the I-V characteristic curves of P25 coated device and mesoporous TiO2 nanospheres coated

on P25 layer sensitized by D205 dye. The obtained device parameters were summarized in

Table 2. The P25 titania coated DSSC sensitized with N719 shows an efficiency of 5.23 %.

Whereas, the light scattering layer of mesoporous TiO2 nanospheres coated device shows an

increased efficiency of 5.91 %. The enhancement of the efficiency is due to the collection of

more number of photons from the internal reflections by the scattering effect of mesoporous

TiO2 nanospheres as shown schematically in Fig.2.18. The collection of internally reflected

45

photons incident on the dye molecule thus results the enhancement of Isc value of 12.82 mA

cm-2

. This was higher than that of P25 titania coated device. P25 titania coated device

sensitized with D205 shows an efficiency of 4.44%. Whereas, the scattering layer of

mesoporous TiO2 nanospheres coated device shows an increased efficiency of 5.12 %. The

similar behavior of Isc was observed as compared to that of the device sensitized with N719

dye. It clearly evidenced the significant role of mesoporous TiO2 nanospheres as a scattering

layer to collect the more photons by internal reflections. However, the decrease of Voc and

fill factor resulted the low efficiency in D205 device when compared to that of N719 device.

This may be due to the electrolyte diffusion in the mesoporous network. This can be

significantly reduced by introducing the blocking layer of TiO2 through surface treatment and

this will be solved in the further investigations.

2.3 Conclusions

A simple hydrothermal method has been adapted to synthesize mesoporous anatase TiO2

spheres. The effects of the systematic growth periods on the morphological, structural and

optical properties of the mesoporous TiO2 spheres were investigated. The functional

properties of the TiO2 spheres were investigated by XRD, Raman spectroscopy, UV-visible

spectrophotometery, FTIR spectroscopy, XPS analysis, FESEM and TEM. It was shown that

the sample prepared for 25 h yielded excellent interparticle connection with a well-defined

sphere-like morphology when compared with the 15 and 20 h growth samples. The effect of

the photoanode active layer thickness on the DSSC conversion efficiency was also

investigated. It was found that the maximum efficiency () of 7.42 % was achieved for a

thickness of 16 m.

46

References

[1] Kim Y. J, Lee Y. H, Lee M. H, Kim H. J, Pan J. H, Lim G. I, Choi Y. S, Kim K, Park N,

Lee C, Lee W, Langmuir. 24 (2008) 13225.

[2] Chen D, Huang F, Cheng Y. B, Caruso R. A, Adv. Mater. 21 (2009) 2206.

[3] Shao W, Gu F, Li C, Lu M, Inorg.Chem. 49 (2010) 5453.

[4] Yang W.G, Wan F.R, Chen Q.W, Li J.J, Xu D.S, J. Mater. Chem. 20 (2010) 2870.

[5] Wei M, Konishi Y, Zhou H, Yanagida M, Sugihara H, Arakawa H, J. Mater. Chem. 16

(2006) 1287.

[6] Satyanaran Reddy G, Krishnamoorthy A, Cristopher Y, Gratzel M, Palani B, Energy

Environ. Sci, 3 (2010) 838.

[7] Mingdeng W, Yoshinari K, Haoshen Z, Masatoshi Y, Hideki S, Hironori A, J. Mater.

Chem, 16 (2006) 1287.

[8] Wei Guang Y, Fa – Rong W, Qing Wei C, Jing Jian L, Dong Sheng X, J. Mater. Chem.

20 (2010) 2870.

[9] Ohsaka T, Izumi F, Fujiki Y, J. Raman Spectrosc. 7 (1978) 321.

[10] Ohsaka T, J. Phys. Soc. Jpn. 48 (1980) 1661.

[11] Hyun Chul C, Young Mee J, Seung Bin K, Vibrational Spectroscopy 37 (2005) 33.

[12] Venkatachalam N, Palanichamy M, Murugesan V, Mater. Chem. Phys. 104 (2007)

454.

[13] Shamaila S, Sajjad A. K. L, Feng C, Jinlong Z, Chem. Eur. J. 16 (2010)13795.

[14] Li J, Wang D, Liu H, He Z, Zhu Z, Appl. Surf. Sci, 257 (2011) 5879.

[15] Ming L, Kui C, Wenjian W, Chenlu S, Piyi D, Ge S, Gang X, Gaorong H, Mater. Lett,

62 (2008) 1965.

47

Chapter - 3

Synthesis of template assisted mesoporous anatase TiO2 spheres by

hydrothermal method and dye-sensitized solar cell properties

3.1. Background

Dye sensitized solar cells (DSSCs) have attracted significant attention on account of

their potential for converting solar energy to electrical energy at low cost compared to the

commercial solar cells. However energy conversion efficiency is still low [1-6]. Therefore

many efforts have been taken in order to improve the energy conversion efficiency. Though

there were several factors to limit the cell performance factor, the light harvesting efficiency

is considered to be very important factor. Since the oxide semiconducting material with the

mesoporous structure results high internal surface area when compared with the

nanocrystalline materials, it is expected to enhance energy conversion efficiency [7 - 11].

Considering the above factor, researchers have been taking the steps to modify the

structure of the photoanode material. Sung Hoon et al., had prepared the mesoporous TiO2

films using a template of graft co-polymers for DSSCs. The maximum efficiency of 4.6 %

was achieved [12]. Satyanarayana Reddy et al., designed a soft template method for preparing

the mesoporous TiO2 by using various cationic surfactants as structure directing and pore

forming agent. They achieved the efficiency of 7.5 % [13] Hun-Gi jung et al., synthesized the

mesoporous TiO2 spheres by simple urea assisted hydrothermal process. They found that the

mesoporous TiO2 electrode resulted in better efficiency of 7.54 % when compared with the

commercial P25 TiO2 electrode (5.69 %) [14].

Thus it is considered that the photoanode material made of mesoporous framework is

considered as a better choice for yielding good efficiency. Since it has the large surface area,

it facilitates the dye loading process and improves the light scattering effect. I prepared the

48

mesoporous anatase TiO2 spheres by simple hydrothermal method without any templating

agent as presented in the previous chapter. In this chapter, ethylene glycol was used as a

template to prepare mesoporous TiO2. The effect of the amount of the precursor material

(titanium tetraisopropoxide) on the morphology, optical properties and DSSC performance

was investigated. The obtained mesospheres were taken as the photoanode material in the

preparation of DSSC. The performance of DSSC made of commercial available P25 deguassa

powder was investigated for reference.

3.2 Experimental procedure

3.2.1 Hydrothermal growth of mesoporous TiO2 spheres

All the chemicals were commercially purchased from Wako chemicals and used without

further purification. In a typical experiment, two steps were involved for preparing the TiO2

mesospheres.

Formation of Titanium glycolate spheres

0.5 ml of Titanium tetra isopropoxide (TTIP) was added to the ethylene glycol (50 ml).

The amount of TTIP was varied as 1.0, 1.5 and 2.0 ml. The mixture solution was allowed to

stir for 5 h at room temperature. Then this solution was added to the acetone of 150 ml with

the trace of water. The stirring continued for 2 h to form a white suspension. The solution was

centrifuged and the resultant precipitate was washed with distilled water and ethanol several

times to remove the impurities. The as-prepared product was dried at 80° C for 10 h. It

resulted the formation of glycolate spheres.

Formation of mesoporous TiO2 spheres

The prepared ethylene glycolate spheres were dispersed in equal amount of water and

ethanol of 30 ml. It was stirred for 2 h. Then the white-colored solution was transferred to a

100 ml Teflon-lined stainless steel autoclave and hydrothermal growth was carried out at

150 °C for periods of 12 h. Finally, the resulting products were collected and annealed at

49

300 °C for 2 h.

3.2.2 Dye-sensitized solar cell fabrication

The prepared mesoporous TiO2 powders were dissolved in ethanol and ground using an

ultrasonic processor for 30 min, and 5 drops of triton-X were added to the solution as a binder.

The solution was sprayed on a transparent conducting glass (F-doped SnO2 (FTO), Nippon

Sheet Glass, 8.7 Ω/square, transparency of 80 % in the visible range) at 150° C by spray

pyrolysis. The prepared TiO2 films were annealed at 450° C for 2 h. The resulting

photoanodes were soaked in an ethanol solution containing 0.03 M of di-tetrabutylammonium

cis-bis (isothiocyanato) bis (2,2”-bipyridyl-4,4’ dicarboxylato) ruthenium (II) (N719) and

D205 for 12 h. The DCCS photoanode was clamped firmly with a Pt coated counter electrode

(FTO) to form a sandwich type cell. A redox electrolyte solution was filled in between the

electrodes to form the cell by capillary action. The electrolyte was composed of 0.6 M

dimethylpropylimidazolium iodide, 0.1 M lithium iodide, 0.01 M iodide and 0.5 M

tetrabutylpyridine in acetonitrile (FUNCHEM, Tomiyama electrolyte company, Japan).

3.3 Result and discussion

Fig.3.1 shows the XRD pattern of the synthesized material. All the diffraction peaks (101),

(004), (220), (105), (204), (220), (215) were assigned to the anatase phase crystal structure of

TiO2. It was well matched with the (JCPDS, no. 21-1272). The peaks related to other crystal

polymorphs such as rutile, brookite were not observed. Fig.3.2 represents the Raman spectra

of the mesoporous TiO2 nanoparticles. It is reported that the anatase TiO2 nanocrystalline

material has six fundamental vibrational modes denoted by [A1g + 2 B1g + 3 Eg]. The Raman

spectra confirmed that all of the peaks represented the anatase phase. The bands were

ascertained as 148 (Eg), 400 (B1g), 518 (A1g), and 640 cm-1

(Eg3) .

50

Fig. 3.1 XRD patterns of mesoporous TiO2 spheres for the amount of

0.5, 1.0, 1.5, 2.0 ml of TTIP

Fig.3.2 Raman spectra of mesoporous TiO2 spheres for the amount of

0.5, 1.0, 1.5, 2.0 ml of TTIP

51

Fig.3.3 exhibits the UV-visible absorption spectra of the TiO2 mesospheres. All the

samples showed a significant absorption onset at the shorter wavelength below 380 nm. It is

attributed to the band gap absorption of the anatase TiO2. FTIR spectra were measured to

characterize the structural analysis of TiO2 as shown in the Fig.3.4. A broad band was

observed at 3380 cm-1

which was ascribed to the stretching vibration of the H-O-H and it

indicated the presence of the hydroxyl groups on the TiO2 surfaces. The stretching band at

1640 cm-1

corresponded to the –OH bending vibrations. Moreover, there were no peaks

observed for the functional groups of organic material. It indicated that the alkyl group of

ethylene glycol was completely removed from the synthesized TiO2 mesospheres

Fig.3.3 Optical absorption spectra of mesoporous TiO2 spheres for the amount of

0.5, 1.0, 1.5, 2.0 ml of TTIP

52

Fig.3.4 FTIR spectra of mesoporous TiO2 spheres for the amount of

0.5, 1.0, 1.5, 2.0 ml of TTIP

To analyze the electronic state of the mesoporous TiO2 spheres, X-ray photoelectron

spectroscopy measurement was carried out. The XPS spectra of mesoporous TiO2 were

shown in Fig.3.5. It represented the core level spectra of Ti2p and O1s. The two strong peaks

at 459.5 and 464.0 eV were observed in Fig.3.5 (a). It is attributed to the binding energies of

Ti2p3/2 and Ti2p1/2 which represented to Ti3+ and Ti4+ ions. The single peak at 529.3 eV in

Fig.3.5 (b) corresponded to the O1s.

The morphologies of the synthesized product were characterized by FESEM and TEM

techniques. Fig.3.6 (a-1, a-2, a-3, a-4) represents the typical FESEM images of ethylene

glycolate spheres with the average size of 100 – 200 nm. Close inspection of the surface of

the ethylene glycolate spheres revealed that the surface was very smooth. After the

hydrothermal treatment, the porous TiO2 spheres were formed as shown in Fig.3.6 (b-1, b-2,

53

b-3, b-4). The TEM images of the porous TiO2 spheres were shown in Fig.3.6 (c-1, c-2, c-3,

c-4). From these images it is clear that the formation of spherical structure highly depended

on the amount of the precursor. When the amount of the TTIP was 0.5 ml, the formation of

the TiO2 spheres had good interactivity and defined boundaries (Fig.3.6 (c-1)). As the amount

of the TTIP was increased to (1.0, 1.5, 2.0 ml), the morphology of the products became more

and more irregular. No more defined boundaries were observed in Fig.3.6 (c-2, c-3, c-4). The

high magnified TEM images for various amount of TTIP (0.5, 1.0, 1.5, 2.0 ml) are shown in

Fig.3.6 (d-1, d-2, d-3, d-4) where the HRTEM images are given as the inset. It indicated that

the synthesized material had the good crystalline nature and the average size of the particles

were about 5 – 8 nm. These results indicated that the morphology of the TiO2 mesospheres

could be effectively adjusted by the amount of the precursor material. From the above

discussion, the defined interconnected structures were obtained for 0.5 ml amount of TTIP.

54

Fig. 3.5. XPS spectra of mesoporous TiO2 spheres for the amount of

0.5, 1.0, 1.5, 2.0 ml of TTIP (a) Ti 2p3/2, (b) O1s.

55

Fig. 3.6 (a-1) FESEM, (b-1), (c-1)TEM and (d-1) HRTEM images for the amount of

0.5 ml of TTIP

56

Fig. 3.6 (a-2) FESEM, (b-2), (c-2)TEM and (d-2) HRTEM images for the amount of

1.0 ml of TTIP

57

Fig. 3.6 (a-3) FESEM, (b-3), (c-3)TEM and (d-3) HRTEM images for the amount of

1.5 ml of TTIP

58

Fig. 3.6 (a-4) FESEM, (b-4), (c-4)TEM and (d-4) HRTEM images for the amount of

2.0 ml of TTIP

Fig. 3.7 shows the formation mechanism of TiO2 mesoporous spheres. In general the

reactivity of titanium precursor is very high when compared with other metals due to the

variable oxidation capability. The presence of vacant d- orbital in a transition metal enables to

increase its co ordination number. Thus the titanium tetra isopropoxide (TTIP) reacts with the

alkyl chain of ethylene glycol and it undergoes the hydrolysis and condensation reaction to

form titanium glycolates. The following chemical equation illustrates the formation of

titanium glycolates.

Ti (OCH (CH3)2)4 + 2HO (CH2)2OH → Ti((OCH2)2)2 + 4(CH3)2CHOH.

59

The resultant titanium glycolate were very stable. These titanium glycolates were

further treated with acetone and water, then they underwent the hydrothermal treatment.

During the process, titanium glycolate reacted with acetone. Since the amorphous titanium

glycolates have a loose bond with alkoxy groups, they have the tendency to lose the alkoxy

group and initiate the formation of TiO2 nuclei. The removal of alkoxy group from the surface

of the spheres leads to the formation of vacant pores which in turn results the formation of

mesoporous TiO2 spheres

Fig.3.7 Formation mechanism of mesoporous TiO2 spheres.

Fig.3.8 (a) shows the I-V curves of N719 sensitized DSSCs fabricated with four different

photo electrodes, as a function of the amount of the precursor. Their photovoltaic parameters

were summarized in Table.3.1. The DSSC fabricated with 0.5 ml amount of TTIP showed

energy conversion efficiency of 8.96 % due to the higher Isc value of 19.09 (mA/cm2). On the

other hand, the photo electrodes prepared with the higher amount of TTIP as 1.0, 1.5, 2.0 ml

60

indicated that the Isc values gradually decreased as 17.72, 14.77 and 12.32 mA/cm2 which led

to the decrease in the efficiency as 8.43, 7.22 and 6.05 %. In addition, the metal free D205

was used as sensitizer to replace the ruthenium dye N719 and the device performances were

studied. Fig.3.8 (b) shows the I-V characteristic curves of D205 sensitized cells. Similar to

the previous results of N719 sensitized DSSC, the higher energy conversion efficiency was

obtained for the 0.5 ml amount of TTIP as 9.02 % with the Isc 19.74 mA/cm2. The Isc values

decreased as 17.77, 17.40, 14.74 mA/cm2 with the corresponding decrease in the efficiency as

7.92, 7.43, 6.44 % for the amount of 1.0, 1.5, 2.0 ml of TTIP, respectively. It is worthy to note

the highest efficiency of 8.96 % (N719) and 9.02 % (D205) was obtained for the 0.5 ml

amount of TTIP, however the efficiency decreased for the higher amount. The sample with

0.5 ml amount of TTIP had the good interconnectivity and defined boundaries. It enhanced

the dye adsorption and facilitated the electron transport when comparing with other amounts

of TTIP.

Table 3.1: Photovoltaic performance of DSSC devices made using meso-TiO2 sensitized

by N719 dye at AM 1.5 and irradiance of 100 mW/cm2

Amount of

TTIP 0.5 ml 1.0 ml 1.5 ml 2.0 ml

Thickness (m) 16 16 16 16

FF 0.66 0.70 0.70 0.71

Voc (V) 0.70 0.67 0.69 0.68

Isc (mA/cm2) 19.09 17.72 14.77 12.32

EFF (%) 8.96 8.43 7.22 6.05

61

Table 3.2: Photovoltaic performance of DSSC devices made using meso-TiO2 sensitized

by D205 dye at AM 1.5 and irradiance of 100 mW/cm2

Amount of TTIP 0.5 ml 1.0 ml 1.5 ml 2.0 ml

Thickness (m) 16 16 16 16

FF 0.67 0.66 0.61 0.65

Voc (V) 0.67 0.68 0.67 0.68

Isc (mA/cm2) 19.74 17.40 17.77 14.74

EFF (%) 9.02 7.92 7.43 6.44

62

Fig. 3.8 I-V characteristics and photocurrent spectra of mesoporous TiO2 spheres for

the amount of 0.5, 1.0, 1.5, 2.0 ml of TTIP using (a) N719 and (b) D205 as sensitizers.

3.4 Conclusion

To synthesize high surface area mesoporous anatase TiO2 nanospheres, a simple

hydrothermal method was adapted. The effects of the amount of the precursor (TTIP) on the

morphological, structural and optical properties of the mesoporous TiO2 nanospheres were

investigated. The functional properties of the TiO2 nanospheres were investigated by XRD,

Raman spectroscopy, UV-vis spectrophotometry, FTIR spectroscopy, XPS analysis, FESEM

and TEM. It was observed that the sample 0.5 ml yielded excellent interparticle connection

with well-defined boundaries when compared with the other amounts (1.0, 1.5, 2.0 ml). The

effect of the photoanode active layer on the DSSC conversion efficiency was also

investigated with two sensitizers (N719) and (D205). It was found that the maximum

efficiency () of 8.96 % was achieved using N719 and 9.02 % was obtained for D205 for a

thickness of 16 m.

63

References

[1] O’Regan B Gratzel M, Nature. 353 (1991) 737.

[2] Hagfeldt A, Gratzel M, Acc.Chem.Res. 33 (2000) 269.

[3] Gratzel M, Nature. 414 (2001) 338.

[4] Snaith H.J, Adv.Funct.Mater 20 (2010) 13.

[5] Meyer G.J, ACS Nano, 4 (2010) 4337.

[6] Ho W, Yu J.C, Lee S, Chem Commun (2006)115.

[7] Wang Y, Tang X, Yin L, Huang W, Hacohen Y.R, Gedanken A, Adv.Mater, 12

(2000)1183.

[8] Kluson P, Kacer P, Cajthaml T, Kalaji M, J.Mater.Chem, 11 (2001) 644.

[9] Gratzel M, Curr.opin.Colloid Interface.Sci 4 (1999) 314.

[10] Lu X, Li G, Yu J.C, Langmuir 26 (2009) 3031.

[11] Liu S, Yu J, Jaronie C, J.Am.Chem.Soc 132 (2010) 11914.

[12] Sung Hoon Ahn, Joo Hwan Koh, Jin Ah Seo, Jong Hak kim, Chem.Commun 46

(2010) 1935.

[13] Satyanarayana Reddy Gajjela, Krishnamoorthy Ananthanarayanan, Chrisotpher Yap,

Michael Gratzel, Palani Balaya, Energy.Environ.Sci, 3 (2010) 838.

[14] Hun Gi Jung, Yong Soo Kang, Yang kook sun, Electrochimica Acta, 55 (2010)

4637.

64

Chapter 4

Functional properties of citric acid capped TiO2 nanoparticles by

hydrothermal growth and dye-sensitized solar cell performance

4.1 Background

Dye-sensitized solar cells (DSSCs) have been considered as alternative to

semiconductor solar cells due to their good potential and cost effectiveness [1-5]. It is known

that the DSSCs consist of FTO substrate, photoanode material for dye absorption, platinum

counter electrode and the electrolyte (iodide/tri-iodide). Among these, the photoanode is

considered to be an important factor for the light harvesting and charge transfer properties. In

DSSCs, the semiconductor oxide materials with the wide band gap are used as the

photoanode material [6]. Titanium-di-oxide (TiO2) has gained good attention due to their

unique properties such as well matched band alignment with dyes [7]. It is regarded as a

promising material preferred as a heterogeneous photo catalyst in solar cells [8,9].

TiO2 exists in three crystalline polymorphs such as rutile, anatase and brookite.

Among those, rutile is the most stable phase, whereas anatase and brookite are in metastable

phases [10]. However, the anatase phase has been highly employed in wide applications such

as DSSCs, photo catalysts, sensors etc [11, 12]. In order to synthesis TiO2 nanoparticles,

several methods such as solvothermal, sol gel laser ablation, hydrothermal were adopted

[13-15]. Jong Ho Park et al., synthesized the TiO2 nanoparticles by solvothermal method and

investigated the fractal dimension of the material [16]. N.Okubo et al., fabricated the anatase

TiO2 by pulsed laser ablation method. They suggested that the particle size increased with the

increase of gas pressure irrespective with the increase of flow rate [17]. Huaming yang et al.,

successfully prepared the TiO2 nanoparticles with the crystal size of about 16 nm by sol gel

method and performed the photo catalytic studies [18]. In comparison with the other methods,

65

hydrothermal method is a simple and inexpensive method to prepare well crystalline

materials. However, the fast hydrolysis process leads to the formation of irregular phase and

morphology. In order to synthesize the nanoparticles without agglomeration, it is necessary to

use the capping agent. It is reported that the carboxylic acids have strong affinity with TiO2

material [19]. The carboxylic acid with a long hydrocarbon chain is considered as an

important surfactant for the synthesis of titania nanoparticles. Wang et al., synthesized the

TiO2 nanoparticles using decyl amine as the capping agent [20]. Weller et al., reported the

oleic acid capped TiO2 nanoparticles [21]. It is very important to identify the capping ligand

which offers unique size reduction and better morphology. Graham et al., [22] investigated

the nanoparticle-nanotube interactions in the solution and studied the effect of pH and the

ionic strength using citric acid as the capping agent. The ionogenic carboxylic acid groups on

the surface of citrate capped gold nanoparticles and multi walled carbon nanotubes

determined the surface charge of the nanostructures in the solution. Dmitri et al., synthesized

the silver nanoparticles and studied the initio preferential surface coordination with the citric

acid. They investigated the chemical reduction and demonstrated that the blocking of

different surfaces of crystals can be used to prevent chemical activity at some surfaces. In

particular, citric acid is considered as an effective capping agent capable of blocking surfaces

from chemical reactivity [23].

In this chapter, I described the synthesize of anatase TiO2 nanoparticles by facile

hydrothermal method, using citric acid as a capping agent. The systematic investigations

were carried out to investigate the effect of the growth period on the functional properties.

The photoanodes were fabricated using spray technique and DSSC performances were

studied.

66

4.2. Experimental procedure

4.2.1. Synthesis of TiO2 nanoparticles

All the chemicals were used as received without further purification from WAKO

chemicals, Japan. A 25 ml of Titanium tri chloride was added to the 250 ml of water under

vigorous magnetic stirring. 15 g of citric acid was added to the above solution as the capping

agent. The stirring was continued to obtain the transparent color. Then the solution was

transferred to a Teflon-lined stainless steel autoclave and hydrothermal growth was carried

out at 200 °C. The growth period was varied as 5, 15, 25 and 45 h, respectively. Finally, the

resultant powder was annealed at 350 °C for 1 h.

4.2.2. Dye sensitized solar cell fabrication

The photoanodes were prepared by TiO2 powder synthesized at different growth period. The

TiO2 powders were dispersed in ethanol and ground using mortar for 15 min. The solution

was ultrasonicated for 30 min and 5 drops of triton-X were added to the solution as a binder.

The solution was sprayed on a transparent conducting glass (F-doped SnO2 (FTO), Nippon

Sheet Glass, 8.7 Ω/square, transparency of 80 % in the visible range) at 150 °C by spray

deposition method. The prepared TiO2 films were annealed at 530 °C for 2 h. The resulting

photoanodes were soaked in an ethanol solution containing 0.03 M of di-tetrabutylammonium

cis-bis (isothiocyanato) bis (2,2”-bipyridyl-4,4’ dicarboxylato) ruthenium (II) (N719) for 15 h.

The DCCS photoanode was clamped firmly with a Pt coated counter electrode (FTO) to form

a sandwich type cell. A redox electrolyte solution was filled in between the electrodes to form

the cell by capillary action. The electrolyte was composed of 0.6 M

dimethylpropylimidazolium iodide, 0.1 M lithium iodide, 0.01 M iodide and 0.5 M

tetrabutylpyridine in acetonitrile (FUNCHEM, Tomiyama electrolyte company, Japan).

67

4.3 Results and Discussion

Fig.4.1. (a-1) and (a-2)TEM and HRTEM images of uncapped -TiO2 nanostructures.

Fig. 4.1. (b-1), ( b-2), (b-3)TEM, HRTEM images and histogram of size distribution of

TiO2 nanoparticles for growth of 5 h period.

68

Fig. 4.1 (a-1), (a-2) shows the TEM and HRTEM images of the uncapped TiO2

nanoparticles respectively. The overview TEM image (Fig. 4.1 (a-1)) of the uncapped -

nanoparticles indicated the irregular morphology due to the presence of both the spherical

shaped nanoparticles with the size of 20 nm and the nanorods with the size of 100 - 200 nm.

Fig. 4.1(b-1), (b-2) represented the TEM and HRTEM images of the citric

acid-capped TiO2 nanoparticles synthesized for 5 h, respectively. The TEM image clearly

showed the formation of spherical particles without any agglomerations. It demonstrated that

the citric acid effectively passivated the surface during the nucleation which avoided the

agglomerations and restricted the polydispersity in morphology. The lattice fringes were

clearly seen in the HRTEM image which was the evidence of the crystalline nature of the

citric acid-capped TiO2 nanoparticles. Fig. 4.1 (b-3) shows the size distribution. From the

histogram it was observed the particle size distributed in the range of 6 – 14 nm with the

maximum at 9 nm. Fig. 4.1 (c-1), (c-2) represents the TEM and HRTEM images of the TiO2

nanoparticles synthesized for 15 h, respectively. It represented the same morphology of

spherical shape but the size of the particles increased when compared to the particles grown

for 5 h. Fig. 4.1(c-3) shows that the size distribution was in the range of 10 - 20 nm with the

maximum distribution at 14 nm. Fig. 4.1(d-1), (d-2) represents the TEM and HRTEM images

of the TiO2 nanoparticles grown for 25 h, respectively. The morphology of the particles were

irregular in shape. From the HRTEM it was found that some particles showed elongated

shape. Fig. 4.1(d-3) shows that the size distribution was in the range of 18 - 35 nm with the

maximum distribution at 27 nm. Fig. 4.1(e-1), (e-2) represent the TEM and HRTEM images

of the TiO2 nanoparticles grown for 45 h, respectively. The particles had the rod-like

morphology. From the HRTEM it was evidenced that the particle size increased both in

length and diameter. Fig.4.1(e-3) shows that size distribution was in the range of 23 - 52 nm

with the maximum distribution at 34 nm. It clearly showed that the size of the nanoparticles

69

gradually increased from 9 to 34 nm by increasing the growth period from 5 to 45 h. From

the TEM analysis, it was clear that the prolonged growth period up to 45 h resulted the

polydispersity in size and irregular morphology. This is due to smaller nanoparticles tending

to attach with the bigger particles when the growth period is over 25 h under hydrothermal

growth conditions.

Fig. 4.1. (c - 1), (c - 2) and (c - 3) TEM, HRTEM images and histogram of size

distribution of TiO2 nanoparticles for growth of 15 h period.

70

Fig. 4.1. (d - 1), (d - 2) and (d - 3) TEM, HRTEM images and histogram of size

distribution of TiO2 nanoparticles for growth of 25 h period.

Fig. 4.2 (a) and (b) shows the TEM and HRTEM images of commercial P25 Degussa

TiO2 nanoparticles. The images show that P25 TiO2 had irregular morphologies with

spherical nano particles, elongated nano cubes. Sizes of the nanoparticles were in the range of

20 – 80 nm. Moreover, the HRTEM image represented the amorphous and crystalline nature

of the P25 TiO2 nanoparticles.

71

Fig. 4.1. (e - 1) (e - 2) and (e - 3) TEM, HRTEM images and histogram of size

distribution of TiO2 nanoparticles for growth of 45 h period.

Fig. 4.2 (a) TEM and (b) HRTEM images of commercial P25 Degussa TiO2

nanoparticles

72

Fig. 4.3(a) depicts the XRD pattern of the uncapped and citric acid capped TiO2 at

different growth periods of 5, 15, 25 and 45 h. The phase compositions of all the samples

were identified from the XRD pattern. All the diffraction peaks were indexed to (101), (004),

(200), (105), (211), (204), (116), (220) and (215) planes of the crystal structure of anatase

TiO2 phase and it matched with card (JCPDS: 21-1272). Whereas the uncapped- TiO2 shows

rutile phase such as (110), (101), (111), (211) and (220) which indicated the rutile and anatase

phases. It was demonstrated that citric acid was acted as a phase directing ligand to achieve

only the anatase phase.

Fig. 4.3(b) illustrates the Raman spectra of the prepared samples. Generally, the anatase

phase has six fundamental vibrational modes such as [A1g + 2 B1g + 3 Eg] and the rutile phase

has four fundamental vibrational modes such as [A1g + B1g + B2g + Eg]. The citric-acid

capped nanoparticles grown at different growth period had the four Raman peaks at 145, 395,

519 and 642 cm-1

which can be assigned to Eg, B1g, A1g and Eg modes of the anatase phase

[24]. The uncapped TiO2 had the Raman peaks at 237, 395, 450, 518 and 640 cm-1

. Where the

peaks 237 and 450 cm-1

can be assigned to Eg modes of the rutile phase and the remaining

lines belong to the anatase phase as mentioned above [25]. Thus it confirms that the

uncapped-TiO2 material had the mixture phase of anatase and rutile. It has good agreement

with the XRD data.

73

Fig. 4.3 (a) XRD patterns and (b) Raman spectra of TiO2 nanoparticles with growth

periods of 5, 15, 25 and 45 h.

74

The hydrolytic stability is considered to be an important issue. Hobbel et al., [26]

studied the effect of the multi ligands on the hydrolysis process of various metal complexes

such as Al, Zr and Ti. In addition they had explained that the hydrolytic stability was strongly

dependent on the structure of the ligand. Livage et al., [27] reported that the suppression of

the hydrolysis was possible by complexing the metal ions with the ligands such as EDTA.

They explain that the condensation reactions will be forced by this complexation due to the

charge generated from these complexes. The hydrolysis rate will be directly affected by the

molecular fragment which departs with the pair of electrons in the bond cleavage (leaving

group). The leaving group will donate the electron and weaken the bond of the other ligand.

Thus it separates the nucleofugal group from the metal center. In the present case, the citric

acid played a determinative role as a ligand to obtain the agglomerated free TiO2

nanoparticles. The carboxylic functional group favors the conjugate system which reduces the

Lewis basicity of the bonding oxygen and it limits the charge donation to the metal center

[28]. It is worthy to note that the existence of mixed phases of anatase and rutile were

observed when there was no ligand. When the citric acid was added, only anatase phase was

formed. The main reason is that the citrate ions substitute the chlorine ions of Titanium tri

chloride during the hydrolysis process. Thus the citrate ion forms a strong coordination with

the Ti4+

ions and highly stabilizes the molecule. By face shared linking it favors the formation

of anatase TiO2 molecule. The possible formation mechanism is illustrated in the Fig.4.4. It is

evidenced that the citric acid-capped TiO2 nanoparticles show the anatase phase with the

average size of 6 - 14 nm for 5 h growth period. Then the average size of the particle

increased as 10 - 20 nm and 18 – 35 nm with the spherical morphology for the higher growth

period of 15 and 20 h, respectively. The rod-like morphology occurs with the size of 23 – 52

nm for 45 h growth. The driving force of the crystal growth is the reduction of the surface

energy [29]. The two primary nanocrystals attach together and result the rod - like

75

morphology. In hydrothermal reaction the two possible growth mechanisms are reported as

oriented attachment and repeated nucleation. In our work the favorable growth mechanism is

considered as the oriented attachment of the primary crystals.

Fig. 4.4. Formation mechanism for the citric acid capped TiO2 nanoparticles

The optical absorption spectra of the TiO2 nanoparticles are shown in Fig.4.5 (a).

From the spectra it is clear that the uncapped-particles did not show any significant onset in

the region of 300 – 400 nm. There was no significant absorption onset in the uncapped

particles. This may be due to the presence of both the rutile and anatase phase. The

capped-nanoparticles showed the clear absorption onset in the region of 350 – 380 nm. This

may be attributed to the intrinsic band gap absorption of TiO2. It was found that the incident

light was greatly absorbed by the citric acid capped nanoparticles and enriched the light

harvesting. Fig.4.5 (b) shows the typical FTIR absorption spectra of the uncapped and citric

acid-capped TiO2 nanoparticles at various growth periods. The uncapped-TiO2 does not show

any significant vibration peaks in the region of 1000 – 3500 cm-1

. It indicates that the

uncapped-TiO2 did not have any organic molecules. The IR band at 3400 cm-1

indicated the

presence of the Ti-OH stretching vibrations. The peak at 2400 cm-1

corresponded to the

76

atmospheric CO2. The citric acid-capped TiO2 shows significant vibrational peaks in the

region of 1000 - 3500 cm-1

. In particular it had several vibrational peaks in the region of 1100

- 1800 cm-1

which was considered to be the finger print region of citric acid. The peak at

1195 cm-1

was corresponded to the C-O stretching of citric acid. The peak at 1400 cm-1

was

corresponded to COO- and the peak at 1720 cm

-1 attributed to the C=O stretching of the

carboxyl group of citric acid as can be seen from the spectra of 5 and 15 h grown samples [30,

31]. It clearly demonstrates that the citric acid was effectively passivated the surface of the

TiO2 nanoparticles. On the other hand it is observed that the 25 and 45 h grown samples did

not have the strong peaks related to carboxylic group. It confirmed that the citric acid can be

liberated from the surface of the TiO2 nanoparticles at higher growth temperature of 200 ◦C

for longer growth period at hydrothermal condition. These results can be directly correlated

with the increase of the size of the TiO2 nanoparticles at longer growth period as evidenced

by TEM analysis.

77

Fig.4.5 (a) Optical absorption spectra and (b) FTIR spectra of f TiO2 nanoparticles with

growth periods of 5, 15, 25 and 45 h

78

Fig.4.6 XPS spectra of TiO2 nanoparticles with growth periods of 5, 15, 25 and 45h.

(a) Ti 2p3/2, (b) O1s.

Further confirmation for the electronic levels of the samples was analysed by X-ray

photoelectron spectroscopy (XPS). The binding energies obtained in the XPS analysis were

corrected by reference to C1s at 284.60 eV. Figure 6 represents the XPS spectra obtained

from Ti and O regions of TiO2 nanoparticles. Fig.4.6 (a) shows two strong peaks at 459.5 and

464.9 eV which correspond to the binding energies of Ti 2p3/2 and Ti 2p1/2. In Fig.4.6 (b),

there was a strong peak at 530.8 eV, which was attributed to signature of the lattice oxygen

O1s in the Ti-O-Ti bonds [32, 33]. All the samples exhibited the similar peak values in the Ti

and O core level spectra. No obvious peaks for other elements of impurities were observed.

Photovoltaic performance

The DSSCs were fabricated using the nanoparticles synthesized at various growth periods.

Fig.4.7 shows the current density versus voltage (I-V) characteristics measured for 5, 15, 25

and 45 h. Fig.4.8 shows the dependency of device parameters at various growth periods. The

79

values of Voc, Jsc, FF and conversion efficiencies () of the DSSCs are listed in the Table.4.1.

From the table it is clear that the Voc and FF show somewhat constant whereas the Jsc shows

an increasing behavior from 12.02 to 16.59 mA cm-2

when the average particle size

increased from 9 nm (5h) to 14 nm (15 h). When the particle size increased as 27 nm (20 h)

and 34 nm (45 h), the Jsc started to drop from 16.59 to 13.44 and 12.16 mA cm-2

. The overall

conversion efficiency () shows the similar behavior of Jsc thus the efficiency increased from

5.44 to 7.66 % ((5 h) to (15 h)) and it decreased as 6.45 and 5.61 % ((25 h) to (45 h)). It is

clear that the DSSC performance was highly dependent on the Jsc factor. When compared

with the above data, the growth period 15 h was optimized to yield the maximum efficiency

7.66 % with the average particle size of 14 nm. The decrease in the efficiency as the particle

size increased may be due to the minimal of surface area for the greater absorbance of dye

molecules. For the comparison, J-V characteristics were measured for uncapped and P25

TiO2 nanoparticles coated devices as shown in Fig.4.9. The uncapped-TiO2 nanoparticles

coated device exhibited the efficiency of 3.86 %, Jsc of 10.38 mA cm-2

, Voc of 0.59 V and

FF of 0.62. The low efficiency can be attributed to the mixed crystal structure, irregular

morphology and polydispersity in size. Whereas, P25 TiO2 nanoparticles coated device shows

the efficiency of 5.23 % with the following device parameters such as Jsc of 11.11 mA cm-2

,

Voc of 0.69 V and FF of 0.68. However, the obtained efficiency from P25 TiO2 nanoparticles

coated device and uncapped-TiO2 nanoparticles coated device were less as compared to that

of citric acid capped-TiO2 nanoparticles coated device. Therefore, size confinement and

monodispersity in morphology significantly improves the efficiency of DSSC.

80

Fig.4.7. I - V characteristics curves of citric acid capped TiO2 nanoparticles at growth

periods of 5, 15, 25 and 45 h.

Fig. 4.8. Relationship between DSSC device parameters at various growth periods

81

Fig.4.9. I - V characteristics curves of uncapped and P25 Degussa TiO2 nanoparticles

coated devices.

Table. 4.1 Device parameters of DSSC

Growth period (h) 5 15 25 45

FF 0.64 0.67 0.66 0.65

Eff (%) 5.54 7.66 6.45 5.61

Isc (mA/cm2) 12.02 16.59 13.44 12.16

Voc (V) 0.64 0.69 0.72 0.69

82

4.4 Conclusions

TiO2 nanoparticles were successfully synthesized using facile hydrothermal method.

The effect of citric acid on TiO2 nanoparticles was studied. The functional properties of the

TiO2 nanoparticles were investigated. TEM analysis revealed that the size of the TiO2

nanoparticles increased by increasing growth period and narrow size distribution was

obtained for 15 h growth. XRD and Raman results confirmed the formation of pure anatase

phase of citric acid capped TiO2 nanoparticles. It was found that citric acid promoted the

nucleation for anatase phase formation through the coordination of carboxylic groups with

the titanium complexes. The effect of the photoanode (with various growth periods) on the

DSSC conversion efficiency was investigated. It was found that the maximum efficiency ()

of 7.66% was obtained for 15h growth period and the obtained efficiency was higher than the

commercial P25 coated DSSC of 5.23 %.

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85

Chapter 5

Hydrothermal growth of monodispersed rutile TiO2 nanorods and

functional properties

5.1. Background

For the past decades, dye sensitized solar cells (DSSCs) have attracted a great interest due

to the conversion of light to electrical energy [1-5]. Titanium (TiO2) has been considered as a

promising semiconductor material for sensors, photo catalytic and photo voltaic applications

due to the wide band gap [6-8]. Hierarchical one dimensional nanostructures of TiO2 receive

much attention due to the enhanced properties compared to that of the bulk TiO2. The one

dimensional structures have high surface to volume ratio and the unidirectional channels

possess a direct pathways which increases the electron mobility and enhances the

performance of the DSSCs. The TiO2 has three crystalline structures such as anatase, rutile,

and brookite. It is reported that the rutile phase has a good physical properties and is used in

various applications such as lithium ion batteries, DSSCs etc [9, 10]. The rutile nanorods are

more stable at high temperatures when compared to brookite and anatase phases. However,

synthesis of the well aligned rutile TiO2 nanorods is difficult due to the high hydrolysis rate.

There are several methods to prepare the well aligned TiO2 nanorods such as sol gel, chemical

vapor deposition, hydrothermal, electro spinning methods and so on [11, 12]. Jian shi et al.,

had synthesized rutile TiO2 nanorods by pulsed chemical vapor deposition and studied the

effects of purging time coating of Au and the temperature on the product. The obtained

morphologies were nanorods, nanowires, nano flakes and nanoparticles [13]. Wenxi Guo et al.

had prepared the rectangular branched rutile TiO2 nanorods arrays by a new technique called

dissolve, grow and etch grow method [14]. They studied the photovoltaic measurement for

86

the prepared nanorods and obtained the efficiency of 1.68 %. M.Ge et al had synthesized the

rutile phased 3D TiO2 hierarchical structures by one step template free hydrothermal method

and obtained excellent photo catalytic performance [15]. M.N. Tahir et al. had reported the

hydrothermal growth of rutile TiO2 nanorods using 3-hydrosytytramine as the

functionalization agent [16] hydrothermal method is simple and inexpensive to extend for

large scale production when compared to sol – gel, chemical vapor deposition and electro

spinning technique. However, the monodispersed synthesis is a challenging task due to the

ripening and agglomeration processes. The organic capping molecules such as triethylamine,

ethylenediaminetetraacetic acid, N-methylaniline were effectively used to synthesis the

monodispersed semiconducting nanostructures [17–19]. In this chapter, I describe the

synthesis of rutile TiO2 nanorods by simple hydrothermal method using citric acid as a

capping agent. The role of capping agent in the formation of TiO2 nanorods and the detailed

functional properties were investigated.

5.2. Experimental procedure

5.2.1. Synthesis of TiO2 nanorods

All the chemicals were purchased from WAKO chemicals, Japan and used without

further purification. Synthesis of the TiO2 nanorods is as follows: 1 ml of titanium trichloride

was dissolved in the mixture of 10 ml of de-ionized water and 10 ml of hydrochloric acid

under vigorous magnetic stirring of 450 rpm at room temperature. 1 mg of citric acid was

added to the above solution. The reaction was continued for 8 h, and then the solution was

transferred to the Teflon-lined stainless steel autoclave and hydrothermal growth was carried

out at 180 °C for 24 h. After the growth, the precipitates were collected and annealed at

200 °C for 3 h.

87

5.2.2. Dye sensitized solar cell fabrication

The prepared mesoporous TiO2 powders were dissolved in ethanol and ground using

an ultrasonic processor for 30 min, and 5 drops of triton-X were added to the solution as a

binder. The solution was sprayed on a transparent conducting glass (F-doped SnO2 (FTO),

Nippon Sheet Glass, 8.7 Ω/square, transparency of 80 % in the visible range) at 150 °C by

spray pyrolysis. The prepared TiO2 films were annealed at 450 °C for 2 h. The resulting

photoanodes were then soaked in an ethanol solution containing 0.03 M of

di-tetrabutylammonium cis-bis (isothiocyanato) bis (2,2”-bipyridyl-4,4’ dicarboxylato)

ruthenium (II) (N719) for 12 h. The DCCS photoanode was clamped firmly with a Pt coated

counter electrode (FTO) to form a sandwich type cell. A redox electrolyte solution was filled

in between the electrodes to form the cell by capillary action. The electrolyte was composed

of 0.6 M dimethylpropylimidazolium iodide, 0.1 M lithium iodide, 0.01 M iodide and 0.5 M

tetrabutylpyridine in acetonitrile (FUNCHEM, Tomiyama electrolyte company, Japan).

5.3. Results and discussion

Fig.5.1(a) depicts the XRD pattern of TiO2 nanorods. All the diffraction peaks indicated

the formation of rutile phase of the crystal structure. It was good agreement with the standard

JCPDS card no: 89-0554. No other diffraction peaks of other phases such as anatase and

brookite were observed [20]. The optical absorption spectrum of the rutile TiO2 nanorods is

shown in Fig. 5.2. From the spectrum it was found that the maximum absorption onset was

observed at 385 nm. The bandgap was calculated as 3.22 eV using the band gap plot (hυ Vs

(αhυ)2

as shown in the inset of (Fig. 5.2). Fig. 5.3 shows the Raman spectrum of TiO2

nanorods. The two major peaks located at 447 and 612 cm-1

represented the Eg and A1g modes

of the rutile phase, respectively. The peak observed at 237 cm-1

(Eg) was weak and broaden

which was due to the phonon confinement effect [21].

88

Fig. 5.1 (a) XRD pattern of rutile TiO2 nanorods.

Fig.5.2 UV visible absorption spectrum (inset: band gap plot) of rutile TiO2 nanorods.

89

Fig.5.3. Raman spectrum of rutile TiO2 nanorods.

Fig. 5.4 represents the FTIR measurement of the TiO2 nanorods. The peaks at 620 and

1200 cm-1

indicated the symmetric stretching vibration of Ti-O-Ti and the asymmetric

stretching vibrations of Ti-O, respectively. The peaks at 1440 and 1600 cm-1

corresponded to

the –OH bending and C=O stretching vibrational modes for the presence of the carboxylic

group. It indicated the presence of the citric acid [22, 23]. The morphological studies are

described in Fig.5.5. FESEM images of rutile TiO2 nanorods at lower magnifications are

provided in Fig. 5.5 (a). It revealed the formation of monodispersed nanorods with the length

of 1- 1.5 m. It is seen that the branched structure was composed of rod - like array geometry.

Fig. 5.5 (b) revealed that the nanorods were uniformly aligned with the smooth surface at the

side walls of the entire length. The inset figure represents that they had the square facets at

90

the top surface which was the growth habit for the tetragonal crystal structure. The

corresponding TEM image shown in the Fig. 5.5 (c) confirmed that the tip of the nanorods

was having the square facets (indicated by the white dotted lines) with the thickness of 20 -

30 nm. The HRTEM image as shown in Fig. 5.5 (d) indicated that the nanorods were well

crystalline. These measurements confirmed that the nanorods were a 1 – 1.5 m in length and

about 20 – 30 nm in diameter.

Fig.5.4 FTIR spectrum of rutile TiO2 nanorods.

91

92

Fig. 5.5 (a,b) FESEM images at various magnifications, (c) TEM image, and (d)

HRTEM image of rutile TiO2 nanorods.

93

Fig. 5.6 (a) and (b) presents the core level spectra of Ti 2P and O 1s of the rutile

nanorods. Since the binding energies of Ti 2p3/2 and Ti 2p1/2 are 459.5 and 464.9 eV, the peaks

at 459.5 and 464.9 eV attributed to the Ti3+

and Ti4+

ions, respectively [24]. The O 1s peak

showed an asymmetric shape and was deconvoluted into two peaks using the Gaussian fitting

curve. The main peak located at 530.6 eV was produced by the signature of the lattice oxygen

O1s in the Ti-O-Ti bonds. Whereas, the peak at 531.9 eV corresponded to the defect level

oxygen in the TiO2 nanorods [25, 26].

94

Fig. 5.6 XPS spectra (a) Ti 2p3/2 state and (b) O 1s state of rutile TiO2 nanorods.

Fig. 5.7 Formation mechanism of rutile TiO2 nanorods

Fig.5.7 describes the growth mechanism of TiO2 nanorods. The hydrolysis of TiCl3

resulted in the formation of TiO2. Usually a rapid hydrolysis occurs during the hydrothermal

growth of TiO2. When the citric acid is added, it increases the ionic strength of the solution

95

and slows down the hydrolysis rate of the solution which promotes the establishment of

smaller crystals by electrostatic screening [27]. Moreover, the interaction between the Ti4+

and carboxylated group of citric acid controls the chemical kinetics of the hydrothermal

method.

The as-prepared rutile TiO2 nanorods were used for the photoanodes to fabricate

DSSCs. Several groups had studied the DSSC performance by using rutile TiO2 nanorods as

photo anode material. Wenxi et al., had synthesized the rutile TiO2 nanorod arrays grown on

carbon fibers by dissolve and grow method. DSSC were fabricated using TiO2 nanorod coated

carbon fibers as photoanode. The maximum conversion efficiency of 1.28 % with the short

circuit current density as 4.58 mA/cm2

was obtained. Weiguang group prepared rutile TiO2

nanorods with the length of 40 – 130 nm and the diameters of 8 – 15 nm by surfactant

assisted hydrothermal method. It exhibited the conversion efficiency of 6.03 % with Isc of

13.5 mA/cm2. Young Hee et al., studied the effects of TiO2 nanorods on the photoelectrodes

of DSSCs. They observed that an excessive quantity of rutile TiO2 nanorods created an

obstacle for the electron movement in the TiO2 thin film. They had optimized the quantity of

TiO2 nanorods as 7 wt% and achieved the good efficiency of 6.16 % with the Isc (12.29

mA/cm2).

In the present work, the effects of various dyes N719 and D205 as sensitizers were

investigated. Fig.5.8 depicts the I-V characteristics for TiO2 nanorods. From the Table.5.1,

The short-circuit current densities (Isc) for these samples were 8.36 and 6.19 mA/cm2 and the

conversion efficiency were 4.08 % and 2.46 % for the N719 and D205, respectively. It is

observed that the maximum efficiency of 4.08 % was obtained for the sample using N719 as

sensitizer where as the efficiency of the sample using D205 gave the efficiency of 2.46 %

since there is a decrease in the Isc value. This may be due to the recombination effect.

96

Table. 5.1: Photovoltaic performance of DSSC cells of TiO2 nanorods with various

sensitizers at AM 1.5 with the irradiation of 100 mW/cm2.

Sensitizer N719 D205

FF 0.69 0.59

Voc (V) 0.70 0.67

ISC(mA/cm2) 8.36 6.19

EFF (%) 4.08 2.46

Fig. 5.8. I - V characteristics and photocurrent spectra of TiO2 nanorods.

97

5.3 Conclusion

The rutile TiO2 nanorods with the diameter of about 20-30 nm and length of 1 - 1.5 µm

were successfully synthesized by facile one-step hydrothermal method. The addition of citric

acid to the solution retarded the hydrolysis and favored the formation of one dimensional

structure with monodispersed size distribution. The obtained TiO2 nanorods had the sharp

edges, thus it would be the affirmative material for the dye adsorption in the dye-sensitized

solar cell. The effect of the DSSC conversion efficiency was also investigated. It was found

that the maximum efficiency () of 4.08 % was achieved for the sample using N719 as

sensitizer.

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100

Chapter – 6

Summary and Future work

6.1. Summary

Hydrothermal method was adapted for the preparation of various TiO2 nanostructures

such as mesoporous nanospheres, nanoparticles and nanorods. The growth condition, amount

of the precursors, growth period were optimized to obtain the desired morphology of TiO2

structures. Functional properties of the synthesized TiO2 structures were measured by X-ray

diffraction pattern, UV-visible absorption analysis, Raman spectroscopy, X-ray photoelectron

spectroscopy, field emission scanning electron microscopy and Transmission electron

microscopy. The synthesized TiO2 nanostructures were used as the photoanode material in the

DSSC fabrication. In addition, two different sensitizers (N719 and D205) were used to study

the device performance.

A simple hydrothermal method was adapted to synthesize mesoporous anatase TiO2

spheres without any organic additivie. The effects of the systematic growth periods on the

morphological, structural and optical properties of the mesoporous TiO2 spheres were

investigated. The sample prepared for 25 h yielded excellent interparticle connection with a

well-defined sphere-like morphology when compared with the 15 and 20 h growth samples.

The effect of the photoanode active layer thickness on the DSSC conversion efficiency was

investigated. It was found that the maximum efficiency () of 7.02 % was achieved for a

thickness of 16 m. The use of the mesoporous TiO2 layer as the photoanode in the DSSC

was beneficial for the photo conversion efficiency.

Glycol assisted hydrothermal growth was adapted to synthesize the mesoporous

anatase TiO2 nanospheres. The effects of the amount of the precursor (TTIP) on the

morphological, structural and optical properties of the mesoporous TiO2 nanospheres were

investigated. The sample 0.5 ml yielded excellent interparticle connection with well-defined

101

boundaries when compared with the other amounts (1.0, 1.5, 2.0 ml). The effect of the

photoanode active layer on the DSSC conversion efficiency was investigated using two

sensitizers (N719) and (D205). It was found that the maximum efficiency () of 8.96 % was

achieved using N719 and 9.02 % was obtained for D205 for a thickness of 16 m.

TiO2 nanoparticles were successfully synthesized using facile hydrothermal method

using citric acid as an organic ligand. The effect of citric acid on TiO2 nanoparticles was

studied. Citric acid promoted the nucleation for anatase phase formation through the

coordination of carboxylic groups with the titanium complexes. The effects of various growth

periods (5, 15, 25 and 45 h) were investigated. The effect of the photoanode (with various

growth periods) on the DSSC conversion efficiency was investigated. It was found that the

maximum efficiency () of 7.66% was obtained for 15h growth period.

Table 1. Summary of the results

Morphology/

crystal phase

Template

Capping

agent

Parameter

optimized

Dye

Isc

(mA/cm2)

Voc

(V)

FF

Eff

(%)

Mesoporous

spheres

(Anatase)

Free

Free

Growth

period

N719

14.45

0.68

0.70

7.02

Mesoporous

spheres

(Anatase)

Ethylene

glycol

Free

Amount

of precursor

D205

19.74

0.67

0.67

9.02

Nanoparticles

(Anatase)

Free

Citric

acid

Growth

period

N719

16.49

0.69

0.67

7.66

Nanorods

(rutile)

Free

Citric

acid

Growth

period

N719

8.36

0.70

0.69

4.08

102

The rutile TiO2 nanorods with the diameter of about 20-30 nm and length of 1 - 1.5

µm were successfully synthesized by facile one-step hydrothermal method. The addition of

citric acid to the solution retarded the hydrolysis and favored the formation of one

dimensional structure with monodispersed size distribution. The obtained TiO2 nanorods had

sharp edges, thus it would be the affirmative material for the dye adsorption in the

dye-sensitized solar cell. The obtained TiO2 nanostructures and the I – V characteristics

with their maximum efficiency were summarized in the Table.6.

6.2. Future work

In the present work, various TiO2 nanostructures were synthesized and their

functional properties were studied. The DSSC were fabricated and the device studies were

performed by using two different sensitizers N719 and D205.

In future, it is aimed to synthesis various hierarchical TiO2 nanostructures. During

the synthesis the parameters such as growth period, growth temperature, amount of the

precursor will be optimized. Phase, structure and morphology of the synthesized TiO2

nanostructures will be characterized by XRD, Raman, FESEM and TEM. The optical

properties and elemental analysis will be studied by UV and XPS analysis.

The synthesized TiO2 nanostructures will be used as the photoanode in the DSSC

fabrication. In addition, various dyes such as C518, black dye will be used as sensitizers. Dye

loading onto the TiO2 nanostructures in the photoanode will be optimized to achieve the high

efficiency. The fabricated DSSCs will be evaluated by I – V measurements to study the

device parameters. The impedance spectroscopy analysis will be measured to know the

resistance of the device and charge transfer mechanism between the interfaces of dye

molecules and TiO2 nanostructures and electron transfer from the iodine electrolytes to dye

molecules through redox couple effect.

103

List of publications and conferences

(A) Journal publications

1) J. Archana, M. Navaneethan, Y. Hayakawa, (2013) “Solvothermal growth of high surface

area mesoporous anatase TiO2 nanospheres and investigation of dye- sensitized solar cell

properties" J. Power Sources. 242:803-810

2) J. Archana, M. Navaneethan, Y. Hayakawa, (2013) “Hydrothermal growth of

monodispersed rutile TiO2 nanorods and functional properties” Mater. Lett. 98:38-41.

(B) Other publications (Journal)

1) T. Prakash, M. Navaneethan, J. Archana, S. Ponnusamy, C. Muthamizhchelvan, Y.

Hayakawa, (2013) “Preparation of N-methylaniline capped mesoporous TiO2 spheres by

simple wet chemical method”, Mater. Res. Bulletin. 48:1541–1544.

2) G. Arthi, J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan,

(2013) “Hydrothermal growth of ligand-passivated high-surface-area TiO2 nanoparticles

and dye-sensitized solar cell characteristics”, Scripta Materialia, 68:396–399.

3) M. Navaneethan, J. Archana, M. Arivanandhan, Y. Hayakawa, (2012) “Functional

properties of amine-passivated ZnO nanostructures and dye-sensitized solar cell

characteristics”, Chemical Engineering Journal. 213:70-77.

4) T. Prakash, M. Navaneethan, J. Archana, S. Ponnusamy C. Muthamizhchelvan,

Y.Hayakawa, (2012) “Synthesis of TiO2 nanoparticles with mesoporous spherical

morphology by a wet chemical method” Mater. Lett. 82:208-210.

5) M. Navaneethan, J. Archana, M. Arivanandhan, Y. Hayakawa, (2012) “Chemical synthesis

of ZnO hexagonal thin nanodisks and dye-sensitized solar cell performance”, Phys. Status

Solidi - Rapid Research Letters. 6:120 -122.

(C) Other publications (Journal)

1) J. Archana, M. Navaneethan, T. Prakash, S. Ponnusamy, C. Muthamizhchelvan, Y.

Hayakawa, (2013) “Chemical Synthesis and functional properties of magnesium doped

ZnSe nanoparticles”, Mater. Lett. 100:54-57. .

2) J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy,

C. Muthamizhchelvan, (2012) “Effects of multiple organic ligands on size uniformity and

optical properties of ZnSe quantum dots” Mater. Res. Bulletin. 47:1892-1897.

3) J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan (2012)

“Chemical synthesis of monodispersed ZnSe nanowires and its functional properties”

Mater. Lett. 81:59-61.

104

4) M. Navaneethan, J. Archana, K. D. Nisha, S. Ponnusamy, M. Arivanandhan, Y. Hayakawa,

C. Muthamizhchelvan, (2012) “Organic ligand assisted low temperature synthesis of lead

sulfide nanocubes and its optical properties”, Mater. Lett. 71:44-47.

5) M. Navaneethan, J. Archana, K. D. Nisha, Y. Hayakawa, S. Ponnusamy, C.

Muthamizhchelvan, (2012) “Synthesis of highly size confined ZnS quantum dots and its

functional characteristics”, Mater. Lett. 68:78 - 81.

6) M. Navaneethan, J. Archana, K. D. Nisha, S. Ponnusamy, M. Arivanandhan, Y. Hayakawa,

C. Muthamizhchelvan, (2012) “Synthesis of wurtzite ZnS nanorods by microwave

assisted chemical route”, Mater. Lett. 66:276 - 279.

7) J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan, (2011)

“Organic Molecules passivated Mn doped Zinc Selenide quantum dots and its properties”

Appl. Surf. Sci. 257:7699-7703.

8) M. Navaneethan, J. Archana, K. D. Nisha, Y. Hayakawa, S. Ponnusamy, C.

Muthamizhchelvan, (2010) “Temperature dependence of morphology, structural and

optical properties of ZnS nanostructures synthesized by wet chemical route”, J. Alloys

Compd. 506: 249 – 252.

9) J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan, (2010)

“Synthesis of organic ligand passivated Zinc selenide nanorods via wet chemical route”,

Mater. Lett. 64:2094–2097.

10) J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan, (2009)

“Optical, structural and surface morphological studies of bean-like triethylamine capped

zinc selenide nanostructures”, Mater. Lett.63:1931–1934.

(D) Conferences

1) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama, Y. Hayakawa,

“Monodispersed growth of ZnO nanostructures for efficient charge collection in dye

sensitized solar cells” (P35) (ICONN 2013, SRM University, Chennai, India, Mar 18 - 20,

2013).

2) J. Archana, M. Navaneethan, Y. Hayakawa, “Growth and investigations of mesoporous

TiO2 nanospheres and dye-sensitized solar cells performance” (P39) (ICONN 2013, SRM

University, Chennai, India, Mar 18 - 20, 2013).

3) R. Karthikeyan, M. Navaneethan, J. Arhcana, M. Arivanandhan, Y. Hayakawa, “Facile

synthesis of activated carbon from organic waste for DSSC” (P86) (ICONN 2013, SRM

University, Chennai, India, Mar 18 - 20, 2013).

4) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa, “Surface

passivation and functional property studies of ZnO nanostructures for the development of

dye sensitized solar cells”, The 14th

Takayanagi Kenjiro Memorial Symposium, S-3-5

(Hamamatsu, Japan, Nov 27 – 28, 2012).

105

5) J. Archana, M. Navaneethan, T. Koyama and Y. Hayakawa, “Synthesis of mesoporous

TiO2 microspheres for dye sensitized solar cells”, The 14th

Takayanagi Kenjiro Memorial

Symposium, S-3-6 (Hamamatsu, Japan, Nov 27 – 28, 2012).

6) J. Archana, M. Navaneethan, T.Koyama and Y. Hayakawa, “Functional properties and

dye-sensitized solar cell performance of citric acid capped TiO2 nanoparticles”, 42nd

National Conference on Crystal Growth (NCCG-42), 11aD04 (Kyushu, Japan, Nov 9 – 11,

2012)

7) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Monodispersed chemical synthesis of ZnO quantum dots: Influence of annealing and

functional properties”, 42nd

National Conference on Crystal Growth (NCCG-42) 11aD05

(Kyushu, Japan, Nov 9 – 11, 2012)

8) J. Archana, M. Navaneethan and Y. Hayakawa, “Formation of mesoporous anatase TiO2

spheres by hydrothermal method and dye-sensitized solar cells properties”, International

conference on Solid State Devices and Materials (SSDM 2012), M-6-6 (Kyoto, Japan, Sep

25-27, 2012)

9) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Hydrothermal growth of 3 dimensional porous ZnO nanoflowers and functional

properties”, International conference on Solid State Devices and Materials (SSDM 2012),

PS-8-5 (Kyoto, Japan, Sep 25-27, 2012)

10) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Hydrothermal growth of ZnO nanostructures: Nanorods to nanoflowers and functional

properties”, The 73rd

Japanese society of applied physics- Autumn meeting 2012,

14a-PB1-1 (Ehime, Japan, Sep 11 – 14, 2012).

11) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Monodispersed growth of ZnO nanostructures: Efficient charge collection photoanode

materials for dye sensitized solar cells”, 2012 MRS Spring Meeting W3.2 (Moscone West

Convention Center, San Francisco, California, USA) (April 9th

– 13th

, 2012).

12) J. Archana, M. Navaneethan, T.Koyama and Y.Hayakawa, “Growth and investigations of

mesoporous TiO2 nanospheres and the applications to the dye sensitized solar cells”, The

59th

Spring Meeting of the Japan Society of Applied Physics and Related Societies,

16p-GP7-6 (Waseda University, (Tokyo, Japan, March 2012).

13) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Hydrothermal growth of ZnO nanostructures and their dye sensitized solar cell

characteristics”, The 59th

Spring Meeting of the Japan Society of Applied Physics and

Related Societies, 17p-DP7-1 (Waseda University, (Tokyo, Japan, March 2012).

14) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama and

Y. Hayakawa, “Growth of highly monodispersed zinc oxide nanodisks and dye sensitive

106

solar characteristics”, International Conference on Advanced Materials (ICAM 2011)

K010 (December 12th

- 16th

, Coimbatore, India).

15) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama Y. Hayakawa, “Controllable

growth of highly monodispersed zinc oxide nanodisks and dye sensitized solar

characteristics”, PVSEC-21 (21st International Photovoltaic Science and Engineering

Conference) 2D-5P-28 (November 28- December 2, 2011, Fukuoka Sea Hawk, Japan).

16) M. Navaneethan, J.Archana, M. Arivanandhan, T. Koyama and Y. Hayakawa,

“Monodispersed synthesis of ZnO nanostructures for the development of dye sensitized

solar cells”, 13th

Takayanagi memorial symposium (Hamamatsu, Japan, November 17-18,

2011).

17) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama, Y. Hayakawa, “Chemical

synthesis of ZnO nanosheets using organic ligand for the application of dye sensitized

solar cells”, 41st National Conference on Crystal Growth, NCCG-41, 05aD01 (Tsukuba

International Conference Center) (Tsukuba, Ibaragi, Japan) (November 3rd- 5th, 2011).

18) J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy, C. Muthamizhchelvan,

“Thioglycerol capped ZnSe quantum dots in polymer matrix”, 41st National Conference on

Crystal Growth, NCCG-41, 05aD02 (Tsukuba International Conference Center) (Tsukuba,

Ibaragi, Japan) (November 3rd- 5th, 2011).

19) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama, Y. Hayakawa, “Synthesis of

monodispersed ZnO nanostructures and their dye sensitized solar cell characteristics”. The

72nd

Japanese society of applied physics- Fall meeting 2011, 1P-ZA-4 (Yamagata, Japan,

Sep 1. 2011).

20) J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy, C. Muthamizhchelvan,

“Synthesis of ZnSe quantum dots by passivating organic ligands”, The 72nd

Japanese

society of applied physics- Fall meeting 2011, 1P-ZA-5 (Yamagata, Japan, Sep 1. 2011).

21) M. Navaneethan, J. Archana, M. Arivanandhan, T. Koyama, Y. Hayakawa, “Controllable

growth of one dimensional ZnO nanorod and its photovoltaic property”, The 58th

Japanese

society of applied physics- spring meeting 2011, 27a-BQ-10 (Kanagawa, Japan, March 27.

2011).

22) J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy and C.Muthamizhchelvan,

“Synthesis and characterization of ethylenediamine capped ZnSe nanosheets”, The 58th

Japanese society of applied physics- spring meeting 2011, 27a-BQ-10 (Kanagawa, Japan,

March 27. 2011).

23) M. Navaneethan, J. Archana, M. Arivanandhan and Y. Hayakawa, “Amine

functionalized ZnO nanoparticles and its dye sensitized solar cell characteristics”, Asian

Conference on Nanoscience and Nanotechnology 2010, PB004 (November 1-3, Miraikan,

Tokyo, Japan).

107

24) M. Navaneethan, J. Archana, M. Arivanandhan, S. Ponnusamy, C. Muthamizhchelvan, Y.

Hayakawa “Synthesis of ZnS quantum dots using organic ligands by wet chemical route”,

17th

Japan Society of Applied physics – Tokai Region seminar (Hamamatsu, Japan, May

13-14, 2010).

25) M. Navaneethan, J. Archana, K. D. Nisha, M. Arivanandhan, S. Ponnusamy, C.

Muthamizhchelvan, Y. Hayakawa, “Synthesis of monodispersed organic capped lead

sulfide nanocubes by chemical route”, The 16th

International conference on crystal growth

in conjunction with The 14th

international conference on vapor growth and epitaxy

(ICCG-16/ICVGE-14), P.83 (Beijing, China. August 8-31, 2010). (Invited)

26) M. Navaneethan, J. Archana, M. Arivanandhan, S. Ponnusamy, C. Muthamizhchelvan, Y.

Hayakawa, “Synthesis of wurtzite phase ZnS quantum dots and nanospheres by wet

chemical route”, The 37th International Symposium on Compound Semiconductors

(ISCS- 2010), MoP5 (Takamatsu, Kagawa, Japan, May 31- June 3, 2010).

27) J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy and C. Muthamizhchelvan,

“Synthesis of ZnSe quantum dots by passivating organic ligands”, The 37th International

Symposium on Compound Semiconductors (ISCS- 2010), MoP21 (Takamatsu, Kagawa,

Japan, May 31- June 3, 2010).

28) M. Navaneethan, J. Archana, K. D. Nisha, M. Arivanandhan, S. Ponnusamy, C.

Muthamizhchelvan, Y. Hayakawa, “Effect of temperature on the formation of ZnS

nanostructures and properties”, IEICE Technical Report, ED 2010-17-ED2010-32,

Electron Devices, P.39 (Hamamatsu, Japan, May 13-14, 2010).

29) M. Navaneethan, J. Archana, M. Arivanandhan, S. Ponnusamy, Y. Hayakawa, “Synthesis

of PVP capped ZnS nanorods under microwave irradiation”, The 57th Japanese society of

applied physics- spring meeting 2010, 18a-TM-9 (Tokyo, Japan, March 17-20. 2010).

30) J. Archana, M. Navaneethan, S. Ponnusamy, Y. Hayakawa, C. Muthamizhchelvan,

“Optical and structural properties of N-Methylaniline passivated ZnSe:Mn2+

Quantum

dots”, International conference on Nanoscience and Technology (ICONN-2010), P77

(Chennai, India, February 24-26, 2010).