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
13
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).