THE SOL-GEL SYNTHESIS AND CHARACTERIZATION OF STRONTIUM
TITANATE DOPED WITH Pr AND Al IONS
MUTIA SUHAIBAH BINTI ABDULLAH
A thesis submitted in fulfillment of the
requirement for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
NOVEMBER 2010
iii
iv
ACKNOWLEDGEMENTS
I would like to present this thesis expressing the deepest and sincere gratitude
before Him for the inspiration and guidance.
My utmost gratitude goes to my supervisor, Prof. Dr Rosli Hussin for
allowing me to join his team, for his expertise, kindness, patience and most of all, for
introducing me to phosphor. I believe that one of the main gains of this 2-years
program was working with Dr. Rosli and gaining his trust and knowledge.
My thanks and appreciation goes to my group members Musdalilah Ahmad
Salim, Siti Aishah Fuzi, Suhailah Abdullah, Nur Shahira Alias, Dayang Nurfazliana,
Muhamad Syawal and Safwan Ahmad Pauzi for their continuous support, insightful
comments, friendship and pleasant discussions and also for all the fun we have had in
the last two years.
Sincere gratitude is expressed to all the lab assistants especially En Jaafar for
technical and experimental assistance especially the maintenance of the equipment in
our lab, En Zainal from Faculty of Mechanical Engineering UTM and Cik Siti
Khadijah Mohd Bakhori from N.O.R Laboratory, School of Physic, Universiti Sains
Malaysia and for their cheerful help in the running of XRD and RAMAN and PL. I
am also extremely grateful to Universiti Teknologi Malaysia generally and Physic
department, Faculty of Science especially for providing experiment facilities.
I am greatly indebted to my family; My parents, Hj Abdullah bin Daud and
Hjh Fatilah Hasni bt Hj Abd Latif for their never-ending love and support in all my
efforts, and for giving me the foundation to be who I am, my siblings for their
encouragement and prayers, which enabled me to complete this endeavor.
Thanks go to the Ministry of Science, Technology and Innovation (MOSTI)
for the financial support through National Science Fellowship (NSF) scholarship.
I also would like to thank all whose direct and indirect support helped me in
completing the thesis in time.
v
ABSTRACT
Strontium Titanate (SrTiO3) doped with Praseodymium (Pr) as an oxide
compound phosphor show a potential application for a field emission display (FED).
Addition of Aluminum ions (Al3+) has been attracting interest as a sensitizer to
improve the luminescent efficiency of phosphors. In this study, the influence of Al3+
as a dopant on the crystallization, surface morphology and luminescent properties of
SrTiO3:Pr,Al nanophosphors were investigated. Nanophosphor with the nominal
composition of SrTiO3 undoped and doped with Pr3+ and Al3+ were synthesized at
relatively low temperature by the sol-gel method. The crystal structures and average
grain sizes were examined using x-ray diffraction (XRD) and scanning electron
microscopy (SEM). XRD patterns indicated that crystalline SrTiO3 which has been
synthesized at calcining temperature of 800⁰C for 2 h present a cubic structure.
Moreover, the improvement of single crystalline phase is confirmed by increasing
the temperature. SEM micrographs indicate that the addition of Pr3+ and Al3+
influence the texture and morphology of the samples. Nanoparticles samples with
various sizes of 25-55 nm were obtained as a function of Pr and Al concentration
using Scherrer‟s equation. Fourier Transform Infrared (FTIR) spectra from SrTiO3
doped and undoped are also reported. The effect of doping on the infrared spectral of
SrTiO3 structure is clearly shown in the low-frequency regions but overlapped with
O-Ti-O bending mode. Raman spectroscopy was employed to investigate the
evolution of the cubic phase in the nanocrystals during annealing and doping
concentration. By addition of dopants, Raman spectra show formation of secondorder Raman Scattering of SrTiO3. The luminescent properties of SrTiO3:Pr,Al
phosphor were investigated by Photoluminescence (PL) spectroscopy. Under 325 nm
excitation, SrTiO3:Pr,Al phosphor exhibited a strong red emission, peaking at about
615 nm. The intensity of emission spectra was enhanced by the addition of Al3+ ions.
vi
ABSTRAK
Strontium Titanate (SrTiO3) didop dengan Praseodymium (Pr) sebagai
sebatian fosfor oksida mempunyai potensi tinggi dalam penggunaan paparan
pancaran medan (FED). Penambahan ion Aluminium (Al) sebagai pemeka bertujuan
untuk meningkatkan kecekapan pendarcahaya bahan fosfor. Dalam kajian ini,
pengaruh Al3+ sebagai bahan dop ke atas penghabluran, morfologi permukaan dan
sifat pendarcahaya nanofosfor SrTiO3: Pr telah dikaji. Nanofosfor dengan komposisi
nominal SrTiO3 tidak berdop dan dop dengan Pr3+ and Al3+ telah disintesis pada suhu
rendah menggunakan kaedah sol-gel. Struktur hablur dan purata saiz zarah diperiksa
menggunakan pembelauan sinar-X (XRD) dan mikroskopi imbasan elektron (SEM).
Corak XRD menunjukkan bahawa hablur SrTiO3 yang telah disintesis pada suhu
pengalsinan 800⁰C selama 2 jam menghasilkan struktur kubus. Tambahan lagi,
penambahbaikan fasa satu hablur disahkan dengan peningkatan suhu. Mikrograf
SEM menunjukkan bahawa penambahan Pr3+ dan Al3+ mempengaruhi tekstur dan
morfologi sampel. Sampel berzarah nano dengan pelbagai saiz antara 25-55 nm
diperolehi dengan kebergantungan pada kepekatan Pr3+ dan Al3+ menggunakan
persamaan Scherrer. Spektrum Transformasi Fourier Infra merah (FTIR) untuk
SrTiO3 yang berdop dan tidak berdop juga turut dilaporkan. Kesan pendopan pada
struktur SrTiO3 dalam spektrum inframerah jelas ditunjukkan dalam kawasan
frekuensi rendah tetapi bertindan dengan mod pembengkokan O-Ti-O. Spektroskopi
Raman digunakan untuk mengkaji evolusi fasa kiub dalam hablur nano semasa
pengalsinan dengan kepekatan bahan dop berubah. Dengan penambahan bahan dop,
spektrum Raman menunjukkan pembentukan Sebaran Raman Urutan Kedua. Sifat
pendarcahaya
fosfor
SrTiO3:Pr,Al
dikaji
menggunakan
Spektroskopi
Fotopendarcahaya. Di bawah pengujaan 325 nm, fosfor SrTiO3:Pr,Al menunjukkan
pancaran merah yang kuat pada puncak 615 nm. Keamatan spektrum meningkat
dengan penambahan ion Al3+.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xii
LIST OF SYMBOLS AND ABBREVIATIONS
xv
INTRODUCTION
1.1
General Introduction
1
1.2
Statement of Problem
4
1.3
Objectives of Study
4
1.4
Significance of the Study
5
THEORETICAL BACKGROUND
2.1
Introduction
6
2.2
Luminescence
7
2.2.1
8
2.3
Process that create luminescence
Phosphor Material
11
viii
3
2.4
Phosphor based
14
2.5
Titanium Dioxide (TiO2)
14
2.6
Alkaline earth titanate
19
2.7
Phosphor doped rare-earth
21
2.7.1
23
Praseodymium
2.8
Phosphor Synthesis
25
2.9
Sol-gel method
26
2.9.1
Preparation of precursor solutions
27
2.9.2
Hydrolysis
27
2.9.3
Gelation
28
2.9.4
Aging and drying
30
2.9.5
Annealing and porosity control.
31
2.10
X-ray Diffraction (XRD)
32
2.11
Scanning Electron Microscope (SEM)
35
2.12
Raman Studies
36
2.13
Fourier Transform Infrared (FTIR) Spectroscopy
39
2.14
Photoluminescence Studies
42
EXPERIMENTAL AND CHARACTERIZATIONS
3.1
Introduction
44
3.2
Sample Preparation
45
3.3
Experimental Characterization
47
3.3.1
X-ray Diffraction (XRD) studies
48
3.3.2
Scanning Electron Microscope (SEM)
49
3.3.3
Raman Spectroscopy
50
3.3.4
Fourier Transform Infrared (FTIR)
51
Spectroscopy
3.3.5
Photoluminescence Spectroscopy
52
ix
4
RESULTS AND DISCUSSIONS
4.1
Introduction
53
4.2
Structural Studies
54
4.2.1
54
Phase formation
4.2.1.1 The influence of calcined temperature on host
structure
54
4.2.1.2 The influence of dopant addition to the
host structure
56
4.2.1.3 The effect of dopant concentration
4.3
Grain size and morphology
4.4
Compositional analyses
62
4.5
Infrared (IR) Spectra
63
4.5.1
analysis
58
The influence of calcined temperature on host
structure
4.5.2
4.5.3
4.6
4.6.2
63
The influence of dopant addition to the host
structure
67
The influence of dopant concentration
69
4.5.3.1 Pr addition
69
4.5.3.2 Al addition
69
Raman Spectra
4.6.1
60
71
The influence of calcined temperature on host
structure
71
The influence of dopant addition to the host
75
structure
4.6.3
4.7
The effect of dopant concentration
76
4.6.3.1 Pr addition
76
4.6.3.2 Al addition
77
Photoluminescence Studies
80
4.7.1
The influence of dopant addition
80
4.7.2
The influence of Pr concentration
82
4.7.3
The influence of Al concentration
82
x
5
CONCLUSIONS AND RECOMMENDATIONS
5.1
5.2
REFERENCES
Conclusions
87
5.1.1 Structural studies
87
5.1.2 Luminescence studies
88
Recommendations
89
91
xi
LIST OF TABLES
NO
TITLE
PAGE
2.1
Luminescence phenomena and the methods of excitation
9
2.2
Phosphor Devices
15
2.3
Typical physical and mechanical properties of titania
16
2.4
Optical properties of titania.
17
2.5
Electronic Configurations of Trivalent Rare-Earth Ions in the
23
Ground State
2.6
List of some commonly used Metal Alkoxides and
29
recommended solvents for solids
3.1
Concentration of praseodymium and aluminum in the prepared
47
samples of strontium titanate phosphor
4.1
Position (cm-1) and Assignment of IR Bands
65
4.2
Raman modes comparison with the literature values
73
xii
LIST OF FIGURES
NO
2.1
TITLE
Marine creatures like jellyfish produce their own light through
PAGE
7
phosphorescence
2.2
Possible physical process following absorption of a photon by a
11
molecule
2.3
Properties and applications of Titanium Dioxide
18
2.4
Crystal structure of TiO2 group (a) Rutile (b) Anatase (c)
18
Brookite
2.5
SrTiO3 (perovskite) structure
20
2.6
Structure of SrTiO3 in the cubic phase
20
2.7
Structural units of the cubic SrTiO3crystal sectioned by three
21
different planes shown as shaded: a) the (100) surface contains
O2−and Sr2+ ions, b) the (110) surface contains Ti4+, O2−and Sr2+
atoms, c) the (001) surface contains Ti4+ and O2−ions.
2.8
Emission spectrum of Y2O2S:Pr3+ (0.3%) at room temperature
24
2.9
Energy level of Pr3+ ions
25
2.10
Common configurations for an XRD unit
33
2.11
Principle of X-ray Diffraction
33
2.12
A schematic diffractogram showing the presence of two phases
34
(with peaks at different angular positions) originating from a
material with small and large crystallites. The broad hump is due
to an amorphous phase.
2.13
Principle of Scanning Electron Microscope
36
2.14
Example of SEM images of (a) Materials (b) Insects (c) Cabbage
36
leaf
xiii
2.15
Light scattered from a molecules
38
2.16
Schematic representation of a Raman spectrometer
38
2.17
Schematic sketch of the essential features of a Fourier transform
40
infrared (FTIR) spectrometer.
2.18
Major types of vibrational modes a) Stretching vibrations b)
41
Bending vibrations
2.19
An example of an FTIR spectrum
41
2.20
Process of luminescence
43
3.1
Schematic steps involved in the sol-gel process used for the
46
preparation of SrTiO3: Pr, Al
3.2
X-ray Diffractometer (Siemens Diffractometer D5000) at Faculty
48
of Mechanical Engineering, Universiti Teknologi Malaysia,
Skudai.
3.3
Scanning Electron Microscopy (JEOL JSM-6701F) at Institute of
49
Ibnu Sina, Universiti Teknologi Malaysia, Skudai.
3.4
Equipment used for Raman spectroscopy at N.O.R laboratory,
50
School of Physics, Universiti Sains Malaysia.
3.5
FTIR spectroscopy at Chemistry Department, Universiti
51
Teknologi Malaysia, Skudai.
4.1
XRD pattern of SrTiO3 at various calcination temperatures
55
4.2
XRD pattern of SrTiO3 at 800⁰C
56
4.3
XRD pattern for (a) SrTiO3 (b) SrTiO3:1mol%Pr (c)
57
SrTiO3:1mol%Pr,1mol%Al
4.4
XRD pattern for various concentration of (a) Pr3+ (b) Al3+ when
59
3+
Pr fixed at 1 mol %
4.5
Illustration of substitution process in SrTiO3 sample as addition
60
of Pr and Al
4.6
Particle size obtained using the Scherrer‟s equation as a function
61
of (a) Pr concentration (b) Al concentration when Pr3+ fixed at 1
mol %
4.7
Scanning electron micrographs of (a) SrTiO3 (b)SrTiO3:1mol%Pr
(c) SrTiO3:1mol%Pr,1mol%Al calcined at 800⁰C
62
xiv
4.8
EDAX spectra of (a) SrTiO3 (b) SrTiO3:1mol%Pr (c)
64
SrTiO3:1mol%Pr, 1mol%Al phosphor
4.9
FTIR spectra for undoped SrTiO3 with different calcinations
66
temperatures
4.10
FTIR spectra of (a) undoped SrTiO3 (b) SrTiO3: Pr (c) SrTiO3:
68
Pr, Al phosphor which calcined at 800⁰C
4.11
FTIR spectra of the SrTiO3: xPr with 0 ≤ x ≤ 1 mol % which
70
calcined at 800⁰C
4.12
FTIR spectra of the SrTiO3: 1mol%Pr, yAl with 0 ≤ y ≤ 1 mol%
72
which calcined at 800⁰C
4.13
Raman spectrum of the SrTiO3 samples calcined at 600-800⁰C
74
for 2 h.
4.14
Raman spectra of SrTiO3 sample by addition of dopant which
76
calcined at 800⁰C
4.15
Raman spectra of SrTiO3:xPr with various Pr concentrations
78
(0≤ x ≤1 mol %) which calcined at 800⁰ C
4.16
Raman spectra of SrTiO3:1 mol% Pr, yAl with 0≤ y≤1 mol %
79
which calcined at 800⁰ C
4.17
Photoluminescence (PL) spectra of SrTiO3, SrTiO3:Pr3+ and
81
SrTiO3:Pr, Al
4.18
Energy level of Pr3+
81
4.19
Photoluminescence spectra of the SrTiO3:xPr (0≤x≤1 mol %)
83
phosphors under 325 nm excitation
4.20
PL spectra of SrTiO3:Pr, yAl (Pr3+:1 mol%) with various molar
84
ratios of Al (0≤y≤1 mol %) under the 325nm laser excitation at
room temperature
4.21
Proposed scheme of energy transfer from the defects created
around Al3+ levels to the (3P0, 1, 2 , 1I6, 1D2) energy levels of Pr3+.
86
xv
LIST OF SYMBOLS AND ABREVIATIONS
ZnS
Zinc Sulphide
TiO2
Titanium Dioxide
UV
Ultraviolet
SrTiO3
Strontium Titanate
Pr3+
Praseodymium (III) ions
Al
3+
Aluminum (III) ions
SEM
Scanning Electron Microscopy
XRD
X-ray Diffraction
HCl
Hydrochloric Acid
TMOS
Tetramethoxysilane
TEOS
Tetraethoxysilane
EDAX
Energy Dispersive X-ray Analysis
TTIP
Titanium tetraisopropoxide
Pr(NO3)3.6H2O
Praseodymium Nitrate Hexahydrate
Al(NO3)3.9H2O
Aluminium Nitrate Nonahydrate
Sr(NO3)2
Strontium Nitrate

Wavelength
d
spacing between the planes
D
averaged dimension of crystallites
FTIR
Fourier Transform Infrared
ICDD
International Centre for Diffraction Data
SiO2
Silicon Dioxide
TeO2
Tellurite Dioxide
P2O5
Phosphorous Dioxide
CHAPTER 1
INTRODUCTION
1.1
General Introduction
Luminescence is a science related to spectroscopy. First observed in an
extract of Ligrium nephiticiem by Monardes in 1565, it took until 1852 to be fully
described by Stokes who reported the theoretical basis for the mechanism of
absorption (excitation) and emission. Today luminescence, in its varied forms, is one
of the fastest growing and most useful analytical techniques in science. Applications
can be found in areas as diverse as materials science, environmental science,
microelectronics, physics, chemistry, biology, biochemistry, medicine, toxicology,
pharmaceuticals, and clinical chemistry. This rapid growth occurred only in the past
couple of decades and is principally driven by the unique needs of the life sciences.
Luminescence is defined as the generation of light without heat which usually
occurs at low temperatures, and is thus a form of cold body radiation. It can be
caused by the movement of electrons within a substance from more energetic states
to less energetic states. Examples of luminescence in nature that had been observed
from ancient times are fireflies, insects, fishes, mushrooms and luminescent bacteria.
A luminescent material is often called phosphor, which means „light bearer‟ (Joseph,
2
2001). Phosphor is a substance that exhibits the phenomenon of phosphorescence
(sustained glowing after exposure to energized particles such as electrons). Of course
we are familiar with phosphor; we meet them everyday. If this should come as a
surprise, switch on the fluorescent lamp, relax in front of the television set or take a
look at the screen of our computer. All of that are the examples of phosphor
application in our daily life. Research on phosphors and their applications requires
the use of a number of fields in science and technology. For advanced application,
phosphors are widely used in plasma display panels (PDPs), field emission displays
(FEDs), luminous paint and safety indicator.
The long afterglow phosphors are a special kind of luminescent materials
with long persistent phosphorescence lasting for several hours at room temperature.
As novel functional materials, these long afterglow phosphors are drawing more and
more attention in recent years because of a constantly growing market for their
applications. In the early time, Co and Cu doped zinc sulphide (ZnS: Cu, Co) was
considered a main kind of phosphorescent materials. However, the material itself is
not stable enough during its application. It can only maintain phosphorescence no
more than a few hours. Sulfide absorb the moisture from the surrounding
environment to form sulfate that causes the destruction of sulfide lattice, and thus the
material no longer shows long afterglow (Chang and Mao, 2004). Therefore, it is
substituted by the strontium aluminates. In the past decade, new kinds of long
persistent phosphors, which overcome the shortcoming of the above mentioned
sulfides, were invented. In the mean time, titanate-based phosphors also were
increasingly investigated due to the potential interest of these materials.
Phosphors based on oxide matrices are attractive host materials for the
development of advanced phosphors due to their ease of synthesis and stability.
Titanium dioxide (TiO2) could be a possible candidate among oxide compounds
which have been known to exhibit an absorption band in the soft ultra violet (UV)
range, because TiO2 has served as a photonic catalyst in this energy range. Recently,
perovskite titanate MTiO3 (M=Mg, Ca, Sr, Ba) have attracted great interest both in
scientific and technological field. The oxide perovskite strontium titanate (SrTiO3) is
3
expected to be chemically stable and good candidates for optical host materials.
SrTiO3 is a well-known material because of its good properties, such as high
dielectric constant, high charge storage capacity, good insulating property, excellent
chemical and physical stability and excellent optical transparency in the visible
range. In addition, SrTiO3 is suitable for host matrix as phosphors (Guo et al., 2006;
Yamamoto et al., 2002).
Up to now, rare earth ion doped luminescent materials, due to their
characteristic emission bands ranging from ultraviolet to visible, to infrared
wavelength region, have attracted much attention and become an interesting topic in
the field of luminescent material. The luminescence properties of titanate perovskite
doped with trivalent praseodymium were increasingly investigated since the mid1990s due to the potential interest of these red emitting phosphors for display
applications. Several studies have already reported on the luminescence of Pr3+
doped SrTiO3 powder samples. The addition of Al3+ or Ga3+ into SrTiO3: Pr reported
to greatly enhance the emission intensity.
Traditionally, phosphor are synthesized in powder form by procedures
involving crushing, grinding, ball milling and high-temperature solid state reactions.
However, it is difficult to obtain reliable emission intensities with such methods,
probably because of inhomogeneous distributions of metal ions, phase separations or
the accumulation of impurities. Moreover as luminescent materials, the
phosphorescent properties are greatly affected by the grain size, when the grain size
reaches nano scale, many new properties can be obtained. For this reason, recent
investigations have addressed the development of alternative synthetic procedures
and more homogenous materials with improved emission efficiency achieved by
using sol-gel process.
The sol-gel method, possessing advantages of well controlling the
stoichiometry, particle size and morphology, is a potential method for preparing
inorganic materials. The sol-gel method of phosphor preparation is regarded as a wet
4
method. A kind of metal organic compounds known as alkoxides of metals is used as
precursors. These metal-organic alkoxides could either be in liquid form or are
soluble in certain organic solvents. Through the use of the appropriate reagents, the
processes of hydrolysis and gelation can be induced to produce homogeneous gels
from the mixture of alkoxides. To obtain powder or ceramic samples, gels can be
baked, sintered and powderized as in other traditional methods. The sol-gel method
is advantageous in as much as thin films or coatings of the phosphor can be formed
on substrates directly and/or the sol-gel can be molded into designated forms. In this
study, the sol-gel method was utilized for preparing SrTiO3 phosphors.
1.2
Statement of problem
Many works on luminescence properties of other materials like aluminates,
silicates and phosphate via sol gel method were done. Even though there are several
reports on luminescence properties of SrTiO3:Pr, Al, there is no clear-cut
understanding on the relation between luminescence properties and structure. Thus,
in this study we present the luminescence spectra of this system which is prepared
via sol-gel method and indicate the close relationship between such emission and the
molecular arrangements in the respective structures.
1.3
Objectives of study
The objectives of this study are as follows:

To synthesize nano-particle phosphor strontium titanate doped with
rare-earth or transition metal.

To determine the crystalline phase and structure of host material.
5

To determine the influence of the dopant concentration on
luminescent characteristic of phosphor system.

1.4
To determine the mechanism of luminescence of SrTiO3; Pr, Al.
Significance of the study
In the fast growing field of luminescent material, there are lots of these
material applications in many fields such as painting, safety sign and so on. In this
research, we want to develop phosphor based titanates because of its promising
luminescence performance.
By the end of this research, we expected to have a system that has excellent
properties as phosphor materials which has low calcination temperature, nanoscale
size and high intensity.
This study also looks forward to find out the knowledge base of phosphor
properties and applications in scientific and technological field.
CHAPTER 2
THEORETICAL BACKGROUND
2.1
Introduction
As stated in Chapter 1, luminescent materials have wide applications in areas
like laser technology, display device biomedical engineering, pollution monitoring
etc. The tremendous interdisciplinary appeal for luminescence has resulted in a
growing number of researchers desiring to quickly employ new and emerging
luminescence techniques without the time consuming effort of becoming an expert in
physical spectroscopy. Chapter 2 gave an overview of literature relevant to this
study. This chapter commence with general explanations about luminescence, its
history and process involved in luminescence. This is followed by discussions about
phosphor materials associated with its history and applications of these materials. A
review of the works already done, the various properties and applications of
phosphor were discussed below. The use of alkaline earth and rare-earth in phosphor
system were also discussed along with their properties. The most important factor in
this phosphor material system is the host and dopant which will be discussed in
section 2.5 onwards. A discussion of the experimental characterization will end up
this chapter.
7
2.2
Luminescence
Luminescence phenomena have attracted the attention of human since time
immemorial. Puzzling questions were related to the observation of emission of
visible light by a number of living organisms including fireflies, insects, fishes,
mushrooms and luminescent bacteria. How can an animal or a plant produce light?
Figure 2.1 shows an example of a living organism which is luminous.
Figure 2.1 Marine creatures like jellyfish produce their own light through
phosphorescence
This interesting and mysterious phenomenon attracted the attention of many
scientists during the last four centuries. However, it was not subjected to systematic
study until the middle of the 19th century. The phenomenon of certain kinds of
substances emitting light on absorbing various energies without heat generation is
called luminescence. Luminescence is a science closely related to spectroscopy,
which is the study of the general laws of absorption and emission of radiation by
matter (Vij, 1998). In other words, it is "cold light", light from other sources of
energy, which can take place at normal and lower temperatures. In luminescence,
some energy source kicks an electron of an atom out of its ground state which is the
lowest-energy state into an excited state which is also called the higher-energy state;
then the electron gives back the energy in the form of light so it can fall back to its
ground state.
8
A systematic scientific study of the subject of luminescence is of recent
origin, from the middle of nineteenth century. In 1852 English Physicist Stokes
identified this phenomenon and formulated his law of luminescence now known as
Stoke‟s Law, which state that the wavelength of the emitted light is greater than that
of the excited radiation. The phenomenon of certain kinds of substance emitting light
on absorbing various energies without heat generation is called luminescence.
Luminescence is obtained under variety of excitation sources (Fouassier, 1984). The
wavelength of emitted light is characteristic of the luminescent substance and not of
the incident radiation. The various luminescence phenomena are given names based
on the type of radiation used to excite the emission (Table 2.1).
2.2.1
Process that create luminescence
The changes that take place in atoms during luminescence are the same as
those that occur during incandescence. Electrons in atoms absorb energy from some
source and jump to a higher energy level. After a fraction of a second, the electrons
fall back to their original level, giving off the energy they had previously absorbed.
Various procedures can be used to get this process started, that is, to get
electrons to jump higher energy levels. In incandescence, the process is heating. In
luminescence, it can be any one of a number of other processes. Luminescence
emission occurs as a result of photo effect inside the complicated discrete
luminescence centers. The process generally involves two steps:
(a) Excitation
It is a process where a primary particle or a photon of energy E gives up some
or all of its energy to raise electrons from occupied low energy levels to
unoccupied higher energy levels. On the basis of band theory of solids with
respect to electronic levels, the forbidden gap can be imagined to contain
9
Table 2.1: Luminescence phenomena and the methods of excitation
Luminescence phenomena
Methods of excitation
Photoluminescence
Light or photons
Bioluminescence
Bio-chemical energy
(results from a biologically catalyzed reaction)
Sono-luminescence
Sound waves
Electro-luminescence
Electric field
(light created from a gas in the path of an
electrical discharge)
Chemi-luminescence
Chemical energy
(results from a chemical reaction)
Tribo-luminescence
Mechanical energy
(results from the crushing of crystals)
Cathodo-luminescence
Electrons or cathode rays
Radio luminescence
Nuclear radiation or Ionizing radiation
Fluorescence
Ionizing radiation, UV and visible light
Phosphorescence
Ionizing radiation, UV and visible light
Thermoluminescence
Ionizing radiation, UV and visible light
some acceptor/donor levels. Interaction of ionizing radiation with the solids
can transfer sufficient energy to electrons in the valence band for transferring
them to the conduction band. A good number of these liberated electrons
return immediately to the ground state accompanied or unaccompanied by
light emission (causing fluorescence or phosphorescence or internal heating).
A fraction of these can be captured at the donor levels call traps with the
corresponding holes at the acceptor levels or traps. The donor/acceptor levels
are metastable states associated with crystal defects including impurities.
10
(b) Emission
Emission occurs when internal energy from one system is transformed into
energy that is carried away from that system by electromagnetic radiation. An
emission spectrum for any given system shows the range of electromagnetic
radiation it emits. When an atom has energy transferred to it, either by
collisions or as a result of exposure to radiation, it is said to be experiencing
excitation, or to be "excited."
Fluorescence
Refer to Figure 2.2, absorption of UV radiation by a molecule excites it from
a vibrational level in the electronic ground state to one of the many
vibrational levels in the electronic excited state. This excited state is usually
the first excited singlet state. A molecule in a high vibrational level of the
excited state will quickly fall to the lowest vibrational level of this state by
losing energy to other molecules through collision. The molecule will also
partition the excess energy to other possible modes of vibration and rotation.
Fluorescence occurs when the molecule returns to the electronic ground state,
from the excited singlet state, by emission of a photon. If a molecule which
absorbs UV radiation does not fluoresce it means that it must have lost its
energy some other way.
Phosphorescence
A molecule in the excited triplet state may not always use intersystem
crossing to return to the ground state. It could lose energy by emission of a
photon. A triplet/singlet transition is much less probable than a singlet/singlet
transition. The lifetime of the excited triplet state can be up to 10 seconds, in
comparison with 10-5 s to 10-8 s average lifetime of an excited singlet state.
Emission from triplet/singlet transitions can continue after initial irradiation.
Internal conversion and other radiationless transfers of energy compete so
successfully with phosphorescence that it is usually seen only at low
temperatures or in highly viscous media (Figure 2.2).
11
Figure 2.2
Possible physical process following absorption of a photon by a
molecule
2.3
Phosphor Material
Phosphors are transition metal compounds or rare earth compounds of
various types. It is mainly inorganic materials, which are prepared by proper heat
treatment. Almost all good inorganic phosphors consists of crystalline “host
material” in which small amounts of certain impurities, the “activators” are dissolved
(Ronda et al., 1998; William et al., 2006). The activators are primarily responsible
for the luminescence. Other impurities, the “co-activators”, are necessary in some
(not in all) cases to dissolves the activator impurities into the host crystal. Coactivators do not, or only to a very minor degree, participate in the luminescence
process. Both activators and (if necessary) co-activators are diffused into the host
crystal at elevated temperatures, the “firing”. The firing temperature often is little
below the melting temperature of the host material.
12
Phosphor materials characteristics:

High luminescence efficiency

Excellent mechanical strength

High flow ability

High packing density of phosphor
The scientific research on phosphors has a long history going back more than
100 years. A prototype of the ZnS-type phosphors, an important class of phosphors
for television tubes, was first prepared by Théodore Sidot, a young French chemist,
in 1866 rather accidentally (Bol et al. 2002). It seems that this marked the beginning
of scientific research and synthesis of phosphors.
From the late 19th century to the early 20th century, Philip E.A. Lenard and
co-workers in Germany performed active and extensive research on phosphors, and
achieved impressive results (William and Marvin, 2004). They prepared various
kinds of phosphors based on alkaline earth chalcogenides (sulfides and selenides)
and zinc sulfide, and investigated the luminescence properties.
They established the principle that phosphors of these compounds are
synthesized by introducing metallic impurities into the materials by firing. The
metallic impurities, called luminescence activators, form luminescence centers in the
host. Lenard and coworkers tested not only heavy metal ions but various rare-earth
ions as potential activators.
Pohl (1920) and co-workers in Germany investigated Tl+-activated alkali
halide phosphors in detail in the late 1920s and 1930s (Bose, 1992). They grew
single-crystal phosphors and performed extensive spectroscopic studies. They
introduced the configurational coordinate model of luminescence centers in
cooperation with F. Seitz in the United State and established the basis of present-day
luminescence physics.
13
Humbolt Leverenz and co-workers at Radio Corporation of America (U.S.)
also investigated many practical phosphors with the purpose of obtaining materials
with desirable characteristics to be used in television tubes. Detailed studies were
performed on ZnS type phosphors.
Since the end of World War II, research on phosphors and solid-state
luminescence has evolved dramatically. Chang-Tai Xia et al reported on
BaLiF3(Eu2+): A promising X-ray storage phosphor (Xia and Shi, 1997), Yong Gao
et al. (1996) works on luminescence properties of SrB4O7: Eu, Tb phosphors while
Qin Fei et al. (2005) studied on luminescent properties of Sr2MgSi2O7 and
Ca2MgSi2O7 long lasting phosphors activated by Eu2+, Dy3+.
Turning to the applications of phosphors, one notes the more recent
appearance of various new kinds of electronic displays using phosphors, such as
electroluminescent displays, vacuum fluorescent displays, plasma displays, and field
emission displays; this is, of course, in addition to the classical applications such as
fluorescent lamps, television tubes, X-ray screens, etc.
The major and important applications of phosphors are in light sources,
display devices, and detector systems. Table 2.2 lists various kinds of phosphor
devices according to the method used to excite the phosphor. It gives a summary of
phosphor devices by the manner in which the phosphors are applied. No further
explanation of the table is necessary. Research and developments of these
applications belong to the fields of illuminating engineering, electronics, and image
engineering. Therefore, research and technology in phosphors require a unique
combination of interdisciplinary methods and techniques, and form a fusion of the
above mentioned fields.
14
2.4
Phosphor based
Host materials play an important role in order to make the best phosphor. The
host materials are typically oxides, sulfides, nitrate, halides or carbonates. Among
these host materials, oxide phosphors have been found to be a candidate in field
emission display (FED) and plasma display panel (PDP) devices as they are
sufficiently conductive to release electric charges stored on the phosphor particle
surfaces. The luminescent efficiency and thermal stability can be maintained under
prolonged Coulomb loading because of oxides are resistant to high-density electron
irradiation. Various long lasting oxide phosphors have been investigated such as
SiO2, TiO2, TeO2 and P2O5.
Among them, TiO2 could be a good host to excite rare earth ions more
efficiently to result in with intense luminescence phenomenon from them (Wang et
al., 2007). Luminescence of the titanate-type compounds has the potential for their
use in optoelectronic applications. TiO2 possesses a good mechanical resistance and
stability even in some corrosive environments and therefore it is widely used in the
development of stable host matrices such as BaTiO3, SrTiO3 and CaTiO3. TiO2 has
been investigated widely by many researchers. Byong Kee Moon et al. (2006)
studied on spectroscopy of nanocrystalline TiO2:Eu3+. Lucas Alonso Rocha et al
(2005) also studied on europium incorporated into titanium oxide by the sol-gel
method. Zhimin Liu et al (2005) investigated on solvothermal synthesis of
mesoporous Eu2O3–TiO2 composites.
2.5
Titanium Dioxide (TiO2)
Titanium dioxide, also known as titania is the naturally occurring oxide of
titanium, chemical formula TiO2. Pure titanium dioxide does not occur in nature but
15
is derived from ilmenite or leuxocene ores. It is also readily mined in one of the
purest forms, rutile beach sand.
Table 2.2: Phosphor Devices
16
These ores are the principal raw materials used in the manufacture of titanium
dioxide pigment. The first step is to purify the ore, and is basically a refinement step.
Either the sulphate process, which uses sulphuric acid as an extraction agent or the
chloride process, which uses chlorine, may achieve this. After purification the
powders may be treated (coated) to enhance their performance as pigments.
Physical and mechanical properties of sintered titania are summarized in
Table 2.3, while the optical properties of titania are provided in Table 2.4.
Table 2.3: Typical physical and mechanical properties of titania.
Property
Density
4 gcm-3
Porosity
0%
Modulus of Rupture
140MPa
Compressive Strength
680MPa
Poisson‟s Ratio
0.27
Fracture Toughness
3.2 Mpa.m-1/2
Shear Modulus
90GPa
Modulus of Elasticity
230GPa
Microhardness (HV0.5)
880
Resistivity (25°C)
1012 ohm.cm
Resistivity (700°C)
2.5x104 ohm.cm
Dielectric Constant (1MHz)
85
Dissipation factor (1MHz)
5x10-4
Dielectric strength
4 kVmm-1
Thermal expansion (RT-
9 x 10-6
1000°C)
Thermal Conductivity (25°C)
11.7 WmK-1
17
Table 2.4: Optical properties of titania.
Phase
Refractive
Density
Crystal
Index
(g.cm-3)
Structure
Anatase
2.49
3.84
Tetragonal
Rutile
2.903
4.26
Tetragonal
Applications for sintered titania are limited by its relatively poor mechanical
properties. It does however find a number of electrical uses in sensors and
electrocatalysis. By far its most widely used application is as a pigment, where it is
used in powder form, exploiting its optical properties.
The most important function of titanium dioxide however is in powder form
as a pigment for providing whiteness and opacity to such products such as paints and
coatings (including glazes and enamels), plastics, paper, inks, fibres and food and
cosmetics. Titanium dioxide is by far the most widely used white pigment. Titanium
dioxide is very white and has a very high refractive index – surpassed only by
diamond. The refractive index determines the opacity that the material confers to the
matrix in which the pigment is housed. Hence, with its high refractive index,
relatively low levels of titania pigment are required to achieve a white opaque
coating.
The high refractive index and bright white colour of titanium dioxide make it
an effective opacifier for pigments. The material is used as an opacifier in glass and
porcelain enamels, cosmetics, sunscreens, paper, and paints. One of the major
advantages of the material for exposed applications is its resistance to discoloration
under UV light.
Figure 2.3 below shows major properties and applications served by titanium
dioxide.
18
Properties
Applications
 High dielectric constant
 Photocatalyst
 Good insulating properties
 PVC (rigid and flexible)
 High refractive index
 Solar cells
Figure 2.3 Properties and applications of Titanium Dioxide
Titanium dioxide occurs in nature as the well-known naturally occurring
minerals rutile, anatase and brookite. The crystal structures of them are shown in
Figure 2.4. The most common form is rutile, which is also the most stable form.
Anatase and brookite both convert to rutile upon heating.
(a)
(b)
(c)
Figure 2.4 Crystal structure of TiO2 group (a) Rutile (b) Anatase (c) Brookite
2.6
Alkaline earth titanate
19
The Alkali Earth Metals comprise the second column from the left of periodic
table. They are reactive metals that tend to oxidize in air. This makes it irritating to
try to keep samples of them. Recently several alkali earth oxides such as MgO, SrO,
CaO, and BaO were mixed up with TiO2 in an attempt to enhance the luminescence
efficiency.
Alkaline earth metal titanate having the formula MTiO3, wherein M is an
alkaline earth metal (Ca, Sr, Ba), Alkaline-earth metal titanate belong to the
important functional ceramics materials such as piezoelectricity, electro optic
devices, semiconductor etc. MTiO3 has attracted interests as one kind of the
promising oxide-based phosphors.
Among of this alkaline earth titanate, recently strontium titanate (SrTiO3)
becomes a choice for researchers to study. SrTiO3 is a well-known material because
of its good properties, such as its high dielectric constant, high charge storage
capacity, good insulating property, its chemical and physical stability and its
excellent optical transparency in the visible range. In addition, it‟s vibrational
frequency is quite low, which makes it suitable as host matrix for upconversion
phosphors (Guo et al., 2006). SrTiO3 single crystal provides a good lattice match to
most materials with perovskite structure which is show in Figure 2.5. At room
temperature it exists in the cubic form, but transforms into the tetragonal structure at
temperatures less than 105 K.
The perovskite-type oxides, ABO3, are materials of which the physical
properties, such as electric, magnetic, dielectric properties, and so on, have been
widely investigated in relation to the crystal structure because the materials have
relatively simple but various crystal structures that can be changed by substitutions
of both A and B site ions with large fraction. The perovskite-type titanates show high
levels of ionic and electronic conductivities and significant thermal, chemical and
mechanical stability in a wide range of oxygen activities. Due to these properties, the
20
titanates have a lot of potential applications as electrodes for high temperature fuel
cells and other electrochemical devices, catalysts, oxygen permeable membranes for
oxygen separation and hydrogen production.
Figure 2.5 SrTiO3 (perovskite) structure
The SrTiO3 has cubic structure. The SrTiO3 primitive unit cell contains five
atoms which is also the case for other ABO3 perovskites. Its unit cell possesses a
simple cubic symmetry Pm3m and a lattice constant a= 3.89 Å. The structure of
SrTiO3 in the cubic phase is a simple one (Figure 2.6), with Sr2+ ions at the cube
corners, Ti4+ ions at the body centers, and O2− ions at the face centers. The structure
can be considered as a rigid grouping of oxygen octahedra linked at their corners by
shared oxygen ions. The Ti4+ ions thus lie at the center of each octahedron, while the
Sr2+ ions lie outside the octahedral.
Figure 2.6 Structure of SrTiO3 in the cubic phase
The unit cells of cubic SrTiO3 lattice sectioned by three different planes,
(100), (110) and (111), are shown in Figs. 2.7a-c.
21
Figure 2.7 Structural units of the cubic SrTiO3crystal sectioned by three different
planes shown as shaded: a) the (100) surface contains O2−and Sr2+ ions, b) the (110)
surface contains Ti4+, O2−and Sr2+ atoms, c) the (001) surface contains Ti4+ and
O2−ions.
2.7
Phosphor doped rare-earth
Luminescence of rare-earth ions is an active field of research. Much of this
interest stems mainly from the unique physical properties of lanthanide oxides,
which makes them promising candidates for a wide variety of applications, such as
laser materials, optical amplifiers, phosphors and photocatalyst.
Rare-earth elements are dopants donating interesting and often useful
properties to host crystal. Rare-earth-doped materials have potential applications for
phosphors, display monitor, X-ray imaging, scintillators, lasers and amplifiers for
fiber-optic communications (Blasse and Grabmaier, 1994). Rare earth-based
phosphors are one of the most important trichromatic luminescent materials, with
which full-color display is produced by emissions in the blue, green and red regions,
respectively, at 450,550 and 610 nm. The phosphor materials contain one or more
22
impurity ions or activators (A), typically present in 0.01-100 mol % concentrations.
The actual emission is generated on these activator ions. Typical activators are rare
earth- or transition-metal ions. The rare earths are used as activator due to their
sensitivity to changes in the surroundings. The activators are primarily responsible
for the luminescence. Doped ions and elements lead a band gap narrowing.
The rare-earth elements usually comprise 17 elements consisting of the 15
lanthanides from Lanthanum (La: atomic number 57) to Lutetium (Lu: atomic
number 71), of Scandium (Sc:atomic number 21), and of Yttrium (Y: atomic number
39). The electronic configurations of trivalent rare-earth ions in the ground states are
shown in Table 2.5. As shown in the table, Sc3+ is equivalent to Argon, Y3+ to
Krypton, and La3+ to Xenon in electronic configuration. The lanthanides from Ce3+
(Cerium ions) to Lu3+ have one to fourteen 4f electrons added to their inner shell
configuration, which is equivalent to Xe. Ions with no 4f electrons, i.e., Sc3+, Y3+,
La3+, and Lu3+, have no electronic energy levels that can induce excitation and
luminescence processes in or near the visible region. In contrast, the ions from Ce3+
to Yb3+ (Ytterbium ions), which have partially filled 4f orbitals, have energy levels
characteristic of each ion and show a variety of luminescence properties around the
visible region. Many of these ions can be used as luminescent ions in phosphors,
mostly by replacing Y3+, Gd3+ (Gadolinium ion), La3+, and Lu3+ in various compound
crystals. Up to now, praseodymium has been developed for luminescence
applications.
23
Table 2.5: Electronic Configurations of Trivalent Rare-Earth Ions in the Ground
State
2.7.1
Praseodymium
Trivalent praseodymium is well-known to emit efficiently between the blue
and the red regions, depending on the host material, the concentration and the
pumping conditions (Diallo et al., 1997). Pr3+-doped phosphor materials with full
color luminescence have received substantial interest because the Pr3+ ions show a
number of different emissions depending on the host lattice in which they are
incorporated, for example red (from the 1D2 level), green (from the 3P0 level), blue
(from the 1S0 level) and ultraviolet (from the 4f–5d state).
Luminescence of Pr3+ consists of many multiplets, as follows: ~515 nm (3P0
→ 3H4), ~670 nm (3P0 → 3F2), ~770 nm (3P0 → 3F4), ~630 nm (1D2 → 3H6), ~410
nm (1S0 → 1I6), and ultraviolet (5d → 4f) transitions. The relative intensities of the
24
peaks depend on the host crystals. As an example, the emission spectrum of
Y2O2S:Pr3+ is shown in Figure 2.8. The radiative decay time of the 3P0 → 3HJ or 3FJ
emission is ~10–5 s, which is the shortest lifetime observed in 4f → 4f transitions. For
example, in Y2O2S host, decay times until 1/10 initial intensity are 6.7 μs for Pr3+,
2.7 ms for Tb3+, and 0.86 ms for Eu3+. The short decay time of Pr3+ is ascribed to the
spin-allowed character of the transition. Since the short decay time is fit for fast
information processing, Gd2O2S(F):Pr3+, Ce3+ ceramic has been developed for an
Xray detector in X-ray computed tomography.
The emission of the Pr3+ ion can be interpreted according to the energy level
diagram of Pr3+ shown in Figure 2.9
Figure 2.8 Emission spectrum of Y2O2S:Pr3+ (0.3%) at room temperature.
25
Figure 2.9 Energy level of Pr3+ ions
2.8
Phosphor Synthesis
Numerous methods have been reported for the synthesis of phosphor
depending on the material of interest, size regime and application requirement.
Whatever be the method, the attraction of the method depends on its capability for
the reproduction of luminescence emission and the grain size. The various methods
used for the preparation of phosphors are listed below.
1. Co-precipitation methods
4. Solid-state reaction method
2. Combustion methods
5. Sol-gel method
3. Spray pyrolysis
The solid-state reaction process is used intensively for phosphor synthesis,
but this process often results in poor homogeneity and requires high calcinating
temperature. Moreover, the grain size of phosphor powders prepared through solid-
26
state reaction method is in several tens of micrometers. Phosphors of small particles
are obtained by grinding the larger phosphor particles. These processes easily
introduce additional defects and greatly reduce luminescence efficiency. With the
development of scientific technologies on materials, several chemical synthesis
techniques, such as coprecipitation, sol–gel, microwave, Pechini and combustion
synthesis methods have been applied to prepare phosphors (Zhao and Chen, 2007).
Sol–gel process is an efficient technique for the preparation of phosphors due
to the good mixing of starting materials and relatively low reaction temperature
resulting in more homogeneous products than those obtained by direct solid state
reactions (Jiang et al., 2004).
2.9
Sol-gel method
The sol-gel method has been widely used to prepare a number of phosphors
for displays and other materials that are of technical importance. The sol-gel method
is a chemical technique that uses metal alkoxides for the synthesis and production of
glasses or ceramics through a series of chemical processes, including hydrolysis,
gelation, drying and thermal treatment. The sol-gel technique was developed as early
as 1864; T. Graham prepared gels of silica from aqueous salts, while M. Ebelmen
obtained silica gels from metal alkoxides. The potential of the sol-gel process was
not appreciated until 1980, when it was “rediscovered” and found to be very useful in
synthesizing various materials of practical importance, such as optical glasses and
solid-state laser materials. Since then this method has received considerable attention
and was investigated extensively.
In general, a sol is defined as a colloid of solid particles suspended in a
liquid; the particles consist of dense oxide or polymetric clusters formed by the
27
precursors and reagents. A gel, on the other hand, is a composite substance
consisting of a continuous solid skeletal structure which results from the gelation of
the sol; the gel forms cells which encapsulate colloidal liquids.
2.9.1
Preparation of precursor solutions
The initial raw materials for sol–gel preparations consist of metal alkoxides
either in the solid or in the liquid form (Table 2.6). An alkoxide is a metalorganic
compound in which a hydrogen atom belonging to the hydroxyl (OH) group of an
alcohol is replaced by a metal atom. As the sol–gel method is a wet chemical
method, a proper solvent is needed to convert solid alkoxides, if used, into liquid
form. Some alkoxide solutions are commercially available (see Table 2.6). Doping or
activator ions are introduced either through another alkoxide solution or by using an
aqueous solution of the doping ions. This liquid mixture of the metal alkoxides is
stirred for an extended period. To stimulate hydrolysis, a mixture of water, alcohol,
hydrochloric acid is added.
2.9.2
Hydrolysis
A mixture of water, alcohol, and HCl is prepared, with HCl acting as a
catalyst in this process. This acidic solution is added slowly (dropwise) into the
precursor alkoxide mixture. The reaction of alkoxides with water is called hydrolysis.
In hydrolysis, a hydroxyl (OH) group attaches itself to the metal atom by replacing
the alkoxide (OR) group.
Since the chemical and physical processes involved are similar for all metal
alkoxides, silicon alkoxides are use as an illustrative example for the preparation of
sol–gel materials. For these matrices, TMOS (tetramethoxysilane, Si(OCH3)4, liquid)
or TEOS (tetraethoxysilane, Si(OC2H5)4, liquid) is commonly used. They react
28
readily but are not soluble in water. A solvent such as methanol (MeOH) or ethanol
(EtOH) is normally used to produce the precursor solution. A typical hydrolysis
reaction for these materials goes as
OR3  Si  OR  H 2O  OR3  Si  OH  ROH
where R is the alkyl (alkylic) radical, CnH2n+1. R = CH3 for TMOS and R = C2H5 for
TEOS. Hydrolysis can occur with any one of the OR groups of the molecule. If the
sol–gels are to be doped, an aqueous solution containing the doping ion/ions is also
blended in during the hydrolysis step.
2.9.3
Gelation
With proper thermodynamic conditions, gelation occurs. Gelation is a continuous
process in which two partially hydrolyzed molecules begin to connect and intertwine
with each other with the release (condensation) of water when in an aqueous solution
(OR) 3  Si  OH  HO  Si  (OR) 3  (OR) 3  Si  O  Si  OR3  H 2O
Alcohol, ROH, is released when an alcohol solution is employed:
(OR) 3  Si  OR  HO  Si  OR3  OR3  Si  O  Si  (OR) 3  ROH
where ROH = C2H5OH for TEOS and CH3OH for TMOS.
29
Table 2.6: List of some commonly used Metal Alkoxides and recommended solvets
for solid
30
With continuing gelation, larger structures are produced by polymerization,
wherein chains of polymers can cross-link to form three-dimensional clusters. Small
clusters suspended in the liquid constitute the sol, and through the gelation process
these clusters begin to grow by combining with monomers or other clusters while
releasing or condensing water or alcohol. Different metal alkoxides can also coalesce
to form “compound” clusters.
Several factors affect the rate of sol and gel formation including the temperature,
the relative concentration of the alkoxide precursors, water, and solvent, and the pH
of the total admixture. In most cases, sol–gel synthesis is carried out at room
temperature, though both the sol and gel formation rates are known to increase with
increasing temperature. Because water and alkoxysilanes are immiscible, a common
solvent such as alcohol is also normally used as a homogenizing agent.
2.9.4
Aging and drying
Aging leads to changes in the structure and other properties of the gel through
further condensation, dissolution, and reprecipitation of monomers or oligomers.
Syneresis or spontaneous shrinkage of the network of the gel takes place as bond or
attraction between clusters induces a contraction of the network and expulsion of
liquid from the pores.
Drying by evaporation under normal conditions gives rise to pressure within
the pores, which causes shrinkage of the gel network. Pressure gradients develop
through the volume of the gels, so that the networks are compressed more at the
surfaces than in the bulk. This may cause, if the gradients are too large, cracking of
the sample.
31
After shrinkage stops, further evaporation drives the meniscus of the liquids
into the bulk and the rate of evaporation decreases. The resulting dried gel is called a
xerogel. Xerogels are useful in the preparation of dense ceramics and are also
interesting because of their high porosity and large surface area; these materials are
useful as catalytic substrates, filters, and vapor sensors.
2.9.5
Annealing and porosity control.
Additional heat treatment of the sol–gel is required to produce pore-free
ceramic materials; sintering at high temperatures results in densification driven by
interfacial energy considerations. By heating, the gel constituents move by viscous
flow or diffusion so as to reduce the solid–vapor interfacial areas, and hence reduce
porosity. Removal of organics takes place by endothermic carbonization near 200°C,
followed by exothermic oxidation at temperatures between 300 and 400°C. For the
silicate system of our example, the exothermic process is suppressed if the gels are
heated under inert conditions, where oxidation is prevented. The temperature interval
between 400 and 525°C represents a region where considerable skeletal densification
occurs with little associated weight loss. Structural relaxation, a process by which
free excess volume is removed by diffusive motion of the network, is the
predominant shrinkage mechanism in this temperature interval. The condensation
(water or alcohol) and pyrolysis reactions that occur during heating liberate a large
volume of gas that can generate high pressures, and because of low permeability of
the small pores in the network, this may cause cracking when the samples are heated
between room temperature and 400°C. At 800°C, there is partial densification of the
sol–gel; by 900°C, the gel is completely densified leaving only a trace of silanols (SiOH).
The sol–gel technique has the following advantages:
a) High homogeneity of the chemical composition of the materials produced.
b) High uniformity of doping ion distribution
c) Processing temperature can be very low.
32
d) The microstructure (porosity and size of pores) of the materials can be
controlled.
e) Thin films and multilayer coatings of sol–gel materials can be readily
prepared by spinning or dipping methods during the gelation period.
f) The sol–gel procedures produce little unintentional contamination. No
milling and grinding are needed, as these processes are known to
contaminate samples. Fluxes such as B2O3, H3BO3 and NH4Cl which are
commonly used in ceramic technology and contaminate the end products
are no longer needed. When phosphor powders are prepared by the sol–gel
method, powdering may be used and trace of foreign particles can be
blended in. This “contamination” does not enter into the lattice of powders
and will not affect the optical properties of the phosphor.
2.10
X-ray Diffraction (XRD)
X-Ray Diffraction (XRD) is a high-tech, non-destructive technique for
analyzing a wide range of materials, including fluids, metals, minerals, polymers,
catalysts, plastics,
pharmaceuticals, thin-film coatings, ceramics, solar cells
and semiconductors.
The XRD pattern is often spoken as the “FINGERPRINT” of a mineral or a
crystalline substance, because it differs from pattern of every other mineral or
crystalline substance. Common configuration of XRD units is shown in Figure 2.10
33
Figure 2.10 Common configurations for an XRD unit
When a monochromatic x-ray beam with wavelength  is incident on the
lattice planes in a crystal planes in a crystal at an angle , diffraction occurs only
when the distance traveled by the rays reflected from successive planes differs by a
complete number n of wavelengths. This is described by Bragg‟s equation:
n  2d sin 
where d is the spacing between the planes (see Figure 2.11).
Figure 2.11 Principle of X-ray Diffraction
(2.1)
34
By varying the angle , Bragg‟s Law conditions are satisfied by different dspacing in polycrystalline materials. A diffractometer collects intensities over an
angular range by measuring 2θ, the angle between the incident and the diffracted
beam. The resulting plot of the intensity as a function of the 2θ value is known as a
diffractogram (see Figure 2.12) containing peaks (reflections) that are characteristic
of the particular atomic arrangement(s) of the crystallites in the sample – the socalled „phases‟. When a mixture of different phases is present, the diffractogram
exhibits the sum of the individual patterns.
From such a diffractogram, information can be obtained about the identity of
the phases (qualitative information), their atomic arrangement (structural
information), their relative concentrations (quantitative information), as well as the
possible presence of a non-crystalline amorphous phase.
Figure 2.12 A schematic diffractogram showing the presence of two phases (with
peaks at different angular positions) originating from a material with small and large
crystallites. The broad hump is due to an amorphous phase.
One possible method of calculating crystallite size from XRD line broadening
is to use the method developed by Scherrer in 1918, which uses the Scherrer equation
( Cullity, 1978).
D  0.9 /  cos
(2.2)
35
where D is the averaged dimension of crystallites; λ is the wavelength of X-ray; θ is
Bragg angle; and β is the full width at half maximum of the peak.
2.11
Scanning Electron Microscope (SEM)
Scanning Electron Microscopy (SEM) is a high resolution imaging technique,
with a great depth of field. It shows topographical, structural and elemental
information at magnifications of 10X to 200,000X.
SEM utilizes electrons in place of light beams to achieve both higher
magnification and greater clarity and depth of field over traditional microscopes.
Preparing a sample for examination by an SEM is a complicated process. First, the
sample must be dehydrated before placing in a vacuum chamber. If it is a non-metal,
it must then be covered with a thin conductive covering to prevent "charging" by the
electrons, usually gold foil. This process is called "sputter coating."
Working principle of SEM is shown in Figure 2.13. A beam of electrons is
produced from the "electron gun" located at the top of the SEM. These electrons are
usually produced by the heating of a metallic filament like tungsten. The electron
beam from the "gun" follows a path down through the microscope past
electromagnetic lenses which focus the beam towards the sample. When the beam
hits the sample, other electrons are dispersed or ejected from the sample. These
electrons are called back scattered or secondary electrons.
Specialized detectors placed in the SEM collect the ejected electrons and
convert them to a signal sent to a viewing screen. The screen assembles the signal
36
into an image. SEM images can be controlled to a magnitude of x250 000 with a
resolution around 10nm. Figure 2.14 show the example of SEM images.
Figure 2.13 Principle of Scanning Electron Microscope
(a)
(b)
(c)
Figure 2.14 Example of SEM images of (a) Materials (b) Insects (c) Cabbage leaf
2.12
Raman Studies
Raman is an invaluable technique for characterisation of materials. In the
field of semiconductor characterisation, the use of Raman microscopes is now
37
widespread because it offers a non-destructive and quantitative microanalysis of
structures and electronic properties. Because of its higher resolution with respect to
FTIR spectroscopy and its versatility and simplicity in terms of sample handling and
the possibility of acquisition of the whole spectra (4000 to 10 cm-1) with the same
instrument.
Raman has actually an increasing use versus other similar spectroscopic
techniques. In particular, in addition to the well known application of identification
of compounds, a large number of reported works rule for example on the
characterization of concentration and mobility of free carriers, characterization of
strain and crystal quality, determination of local crystal orientation, etc. It is worth to
comment that while IR and Raman spectra analyze basically the same vibrational
behavior of the molecule they are not exact duplicates since the selection rules and
relative band intensities differ in many cases.
When monochromatic radiation is incident upon a sample the light interact
with the sample in the some fashion. It may be reflected, absorbed or scattered in
some manner. It is the scattering of the radiation that occurs which can tell the
Raman spectroscopist something of the samples molecular structure.
If the frequency (wavelength) of the scattered radiation is analyzed, not only
is the incident radiation wavelength seen (Rayleigh scattering) but also, a small
amount of radiation that is scattered at some different wavelength (Stokes and AntiStokes Raman scattering). (approx. only 1 x 10-7 of the scattered light is Raman). It is
the change in wavelength of the scattered photon which provides the chemical and
structural information.
Light scattered from a molecule has several components - the Rayleigh
scatter and the Stokes and Anti-Stokes Raman scatter (Figure 2.15). In molecular
systems, these frequencies are principally in the ranges associated with rotational,
38
vibrational and electronic level transitions. The scattered radiation occurs over all
directions and may also have observable changes in its polarization along with its
wavelength.

The scattering process without a change of frequency is called Rayleigh
scattering, and is the same process described by Lord Rayleigh and which
accounts for the blue color of the sky.

A change in the frequency (wavelength) of the light is called Raman
scattering. Raman shifted photons of light can be either of higher or lower
energy, depending upon the vibrational state of the molecule.
Figure 2.15 Light scattered from a molecule
The schematic diagram in Figure 2.16 represent the Raman spectrometer.
Figure 2.16 Schematic representation of a Raman spectrometer
39
2.13
Fourier Transform Infrared (FTIR) Spectroscopy
Fourier Transform Infrared Spectroscopy, or simply FTIR Analysis, is a
failure analysis technique that provides information about the chemical bonding or
molecular structure of materials, whether organic or inorganic. It is used in failure
analysis to identify unknown materials present in a specimen.
The technique works on the fact that bonds and groups of bonds vibrate at
characteristic frequencies. A molecule that is exposed to infrared rays absorbs
infrared energy at frequencies which are characteristic to that molecule. During FTIR
analysis, a spot on the specimen is subjected to a modulated IR beam. The
specimen's transmittance and reflectance of the infrared rays at different frequencies
is translated into an IR absorption plot consisting of reverse peaks. The resulting
FTIR spectral pattern is then analyzed and matched with known signatures of
identified materials in the FTIR library.
As shown in Figure 2.17, in the interferometer the light passes through a
beam splitter, which sends the light in two directions at right angles. One beam goes
to a stationary mirror then back to the beam splitter. The other goes to a moving
mirror. The motion of the mirror makes the total path length variable versus that
taken by the stationary-mirror beam. When the two meet up again at the beam
splitter, they recombine, but the difference in path lengths creates constructive and
destructive interference pattern called an interferogram. The recombined beam passes
through the sample. The sample absorbs all the different wavelengths characteristic
of its spectrum, and this subtracts specific wavelengths from the interferogram. The
detector now reports variation in energy versus time for all wavelengths
simultaneously. A laser beam is superimposed to provide a reference for the
instrument operation.
40
Figure 2.17 Schematic sketch of the essential features of a Fourier transform infrared
(FTIR) spectrometer.
The infrared spectrum can be divided into two regions, one called the
functional group region and the other the fingerprint region. The functional group
region is generally considered to range from 4000 to 1500 cm-1 and all frequencies
below 1500 cm-1 are considered characteristic of the fingerprint region. The
fingerprint region involves molecular vibrations, usually bending and vibration
motions that are characteristic of the entire molecule or large fragments of the
molecule. Thus these are used for identification. The functional group region tends to
include motions, generally stretching vibrations, which are more localised and
characteristic of the typical functional groups, found in organic molecules. While
these bands are not very useful in confirming identity, they do provide some very
useful information about the nature of the components that make up the molecule.
The major types of molecular vibrations are stretching and bending. The
various types of vibrations are illustrated in Figure 2.18 and the example of IR
spectra is shown in Figure 2.19.
41
Figure 2.18 Major types of vibrational modes a) Stretching vibrations b)
Bending vibrations
Figure 2.19 An example of an FTIR spectrum
According to the earlier study by Um and Kumazawa (2000), in their study a
broad absorption band in the range of 2700 to 3600 cm-1 suggests the presence of
considerable amounts of H2O and OH- in the particle. According to Yan et al. (2009),
from the FT-IR spectrum of TiO2, the peaks of 3420 and 1630cm−1 correspond to the
stretching vibrations of O-H and bending vibrations of adsorbed water molecules
42
(δH2O), respectively, indicating the existence of O-H on the surface of TiO2. All
studies on carbonate surface species including a recent work on iron oxide studied by
IR spectroscopy showed that peak for CO32 appears at 1490–1450 cm−1 (Chang et al.
2008). It was confirmed by Kumar and Buddhudu (2009) that bands at 1426 and 866
cm-1 are due to the vibration of CO32 anions. In particular, the sharp absorption band
observed at 568 cm-1 characteristic for an octahedrally coordinated Ti-O stretching
mode, which was assigned by Park et al. (2001). Last in his study found that cubic
SrTiO3 shows two strong infrared bands around 395 and 610 cm-1. He considered the
vibrations of this material in terms of the TiO3 octahedra and suggested that these
two bands could be assigned to the O-Ti-O bending and Ti-O stretching modes,
respectively (Last, 1957).
According to Monica Popa et al. (2001) on his studies about praseodymium
oxide, band at 380, 420, 440, 450, 590, 660 cm-1 are due to praseodymium oxide, in
accordance with Hussein study (1994). In his paper, Monica Popa also stated that
absorption bands due to bending and stretching vibrations of the bond in CO32
appear at 855, 878 and 1455 cm-1.
2.14
Photoluminescence Studies
Photoluminescence spectroscopy is a contactless, nondestructive method of
probing the electronic structure of materials.
Light is directed onto a sample, where it is absorbed and imparts excess
energy into the material in a process called photo-excitation. One way this excess
energy can be dissipated by the sample is through the emission of light, or
luminescence. In the case of photo-excitation, this luminescence is called
43
photoluminescence. The photoluminescence spectrum provides the transition
energies, which can be used to determine electronic energy levels (Gfroerer, 2000).
Photo-excitation causes electrons within the material to move into
permissible excited states. When these electrons return to their equilibrium states, the
excess energy is released and may include the emission of light (a radiative process)
or may not (a nonradiative process). The energy of the emitted light
(photoluminescence) relates to the difference in energy levels between the two
electron states involved in the transition between the excited state and the
equilibrium state. The quantity of the emitted light is related to the relative
contribution of the radiative process. This process is shown in Figure 2.20
Figure 2.20 Process of luminescence
CHAPTER 3
EXPERIMENTAL AND CHARACTERIZATION
3.1
Introduction
A proper understanding of the experimental techniques and the instruments
used is imperative before they are subjected to fruitful research. This chapter deals
with a brief account of the tools and techniques employed to carry out this work.
The synthesis of nano-crystalline phosphors imposes a challenge on the
traditional solid-state synthesis methods, which fail to offer a low temperature and
desired homogeneity at the nanometer level. Preparation of definite size is very
difficult task, but is essential since in the nano world the properties are size
dependent. Nowadays, numerous methods are employed for the preparation of
phosphors of desired quality and size. In this study, the simplest but most precise
methods of preparation and characterization of samples are employed. Low
temperature calcinations can be achieved using sol-gel method. Visual observations
on morphology and grain size were done with scanning electron microscopes and
EDAX. The samples were identified and characterized by X-ray diffractometry,
Infrared and Raman Spectroscopy. Photoluminescence Spectroscopy was used to
study the luminescence properties.
45
3.2
Sample preparation
The phosphors were prepared by sol-gel method. The process for preparing
SrTiO3 phosphor is schematically indicated in Figure 3.1. Titanium tetraisopropoxide
(TTIP) was used as starting material and strontium nitrate, Sr(NO3)2 as modifier.
Ethanol was selected as solvents for TTIP and hydrochloric acid (HCl) as catalyst.
Praseodymium nitrate, Pr(NO3)3.6H2O and aluminium nitrate, Al(NO3)3.9H2O were
used as dopant. The range of the system is
SrTiO3; xPr, yAl
where
0 mol % x 1mol %
0 mol % y 1mol %
25 ml of ethanol dropwise in 5 ml TTIP and stir during the process. The
white colour sol obtained was labeled as SOL 1. Then, 0.5 ml distilled water and 0.5
ml of HCl were added to 25 ml of ethanol which then assume as SOL 2. SOL 2 was
dropwised into SOL 1. The obtained solutions were vigorously stirred at 80⁰C for
about 15 minutes. A stoichiometric amounts of Sr(NO3)2 was added to the obtained
sol. The praseodymium and aluminum salt were dissolved in a small amount of water
and added slowly to the obtained sol with different molar ratios of Pr3+ and Al3+. The
sols obtained were heated at 120⁰C for 24 hours to form gels. The samples of
crushed gels were heated at 450⁰C, 600⁰C, 700⁰C and 800⁰C for 2 hours to form
nanocrystalline SrTiO3 powders doped with Pr3+ and Al3+. The phosphors thus
obtained were finely powdered and used for the studies. A number of SrTiO3
phosphor doped with Pr3+ and Al3+ ions were prepared as shown in Table 3.1.
46
Ethanol
Distilled water
TTIP
HCl
Ethanol
Transparent
solution
White color sol
SOL 1
SOL 2
SOL 1
Dropwise
SOL 2
White color sol
Stirr at 80⁰C
(15 min)
Dropwise
White color gel
Sr(NO3)2
Pr(NO3)3.6H2O
Al(NO3)3.9H2
O
Evaporation and
drying at 1200C
Powder
Calcinations
Figure 3.1 Schematic steps involved in the sol-gel process used for the preparation
of SrTiO3: Pr, Al
47
Table 3.1: Concentration of praseodymium and aluminum in the prepared samples of
strontium titanate phosphor
Sample Code
Concentration (mol %)
Praseodymium(x) Aluminium(y)
SrTiO3
3.3
0
0
SrTiO3:x1Pr
0.1
0
SrTiO3:x2Pr
0.2
0
SrTiO3:x3Pr
0.5
0
SrTiO3:x4Pr
1
0
SrTiO3: x4Pr ,y1Al
1
0.1
SrTiO3: x4Pr ,y2Al
1
0.2
SrTiO3: x4Pr ,y3Al
1
0.5
SrTiO3: x4Pr ,y4Al
1
1
Experimental Characterizations
There are few measurement were used to characterize the samples. The
morphologies of the phosphors were observed using scanning electron microscopy
(SEM). Phase identification of the obtained powders was performed by X-ray
diffraction (XRD) analysis. Average particle sizes were evaluated from the XRD
patterns using the Scherrer‟s equation (Deren, 2008) and from scanning electron
micrographs. The photoluminescent properties were investigated by measuring the
emission spectra employing photoluminescent spectrum. For the structural
characterization of the system Raman and Infrared spectroscopy were used.
48
3.3.1
X-ray Diffraction (XRD) studies
X-Ray Diffraction (XRD) is a high-tech, non-destructive technique for
analyzing a wide range of materials, including fluids, metals, minerals, polymers,
catalysts, plastics,
pharmaceuticals, thin-film coatings, ceramics, solar cells
and semiconductors. Throughout industry and research institutions, XRD has become
an indispensable method for materials investigation, characterization and quality
control. Example areas of application include qualitative and quantitative phase
analysis, crystallography, structure and relaxation determination, texture and residual
stress investigations, controlled sample environment, micro-diffraction, nanomaterials, lab- and process automation, and high-throughput polymorph screening.
In this research, the phase purity and phase structure of powder samples were
characterized by the X-ray powder diffraction (patterns) with Cu-K radiation
operating at 40 kV, 30 mA at room temperature using Siemens Diffractometer
D5000, equipped with diffraction software analysis. Identification of the diffraction
peaks of the XRD patterns was carried out by using the International Centre for
Diffraction Data (ICDD) database. Scherrer equation was used to determine the
particle size. Figure 3.2 shows a XRD that has been used.
Figure 3.2 X-ray Diffractometer (Siemens Diffractometer D5000) at Faculty
of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai.
49
3.3.2
Scanning Electron Microscope (SEM)
Scanning Electron Microscopy (SEM) is a high resolution imaging technique,
with a great depth of field. It shows topographical, structural and elemental
information at magnifications of 10X to 200,000X.
In this study, morphology of the phosphor powder was examined on a JEOL
JSM-6701F Field Emission Scanning Electron Microscopy as in Figure 3.3. An
electron beam with 2.0 kV acceleration voltages was used for imaging. Samples were
coated with platinum to avoid charging effects. The compositions of the powders
were investigated using EDX JED 2300F.
Figure 3.3 Scanning Electron Microscopy (JEOL JSM-6701F) at Institute of Ibnu
Sina, Universiti Teknologi Malaysia, Skudai.
50
3.3.3
Raman Spectroscopy
Raman is an invaluable technique for characterisation of materials. In the
field of semiconductor characterisation, the use of Raman microscopes is now
widespread because it offers a non-destructive and quantitative microanalysis of
structures and electronic properties. Because of its higher resolution with respect to
FTIR spectroscopy and its versatility and simplicity in terms of sample handling and
the possibility of acquisition of the whole spectra (4000 to 10 cm-1) with the same
instrument.
Measurement of Raman spectra were obtained on a Jobin Yvon HR 800 UV
as shown in Figure 3.4. Raman spectra were excited with argon ion laser emission at
514.5 nm and recorded by monocromators. The spectra were monitored in the range
of 0-4000 cm-1 with the laser power on the sample of 20 mW.
Figure 3.4 Equipment used for Raman spectroscopy at N.O.R laboratory, School of
Physics, Universiti Sains Malaysia.
51
3.3.4
Fourier Transform Infrared (FTIR) Spectroscopy
Fourier Transform Infrared Spectroscopy, or simply FTIR Analysis, is a
failure analysis technique that provides information about the chemical bonding or
molecular structure of materials, whether organic or inorganic. It is used in failure
analysis to identify unknown materials present in a specimen.
FTIR spectra were recorded with a Perkin-Elmer (Spectrum One FT-IR)
spectrometer in the frequency range 400-4000 cm-1 at room temperature as shown in
Figure 3.5. Pellets were prepared for FTIR measurements by mixing and grinding a
small quantity of powder sample with spectroscopic grade dry KBr powder and then
compressing the mixtures to form pellets for measurements. All measurements were
at 4 cm-1 resolution.
Figure 3.5 FTIR spectroscopy at Chemistry Department, Universiti Teknologi
Malaysia, Skudai.
52
3.3.5 Photoluminescence Spectroscopy
Photoluminescence spectroscopy is a contactless, nondestructive method of
probing the electronic structure of materials.
Photoluminescence spectra were also recorded with a Jobin Yvon HR
800 UV by HeCd laser excitation at wavelength of 325 nm which is attached
together with Raman Spectroscopy as shown in Figure 3.4.
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1
Introduction
A structural study is an essential part in this research in order to study the
characteristic of phosphor material. Knowledge of the structure of the compounds
involved is essential to understand the processes taking place during a chemical
reaction. Chemical and physical properties of a substance can only be understood
when its structure is known. The structure determination is mainly performed by Xray diffraction (XRD). Besides, spectroscopy is also used as a necessary tool for
structure determination. In this research, Infrared (IR) Spectroscopy and Raman
Spectroscopy are used. Their bands were analyzed to identify the functional groups.
This chapter reports the analysis of phosphor system in terms of structural studies.
In term of photoluminescence studies, SrTiO3 doped with Pr,Al was
successfully synthesis using sol-gel technique with the addition of Al together with
Pr
to
SrTiO3
as
a
luminescent
center.
The
dopant
may
act
as
trapping/recombination/luminescent centers in the host materials. In this chapter, the
doping effects of Pr3+ and Al3+ on the luminescence properties of SrTiO3 will be
discussed.
54
4.2
Structural Studies
4.2.1
Phase Formation
4.2.1.1 The effect of calcined temperature on host structure
XRD was taken to examine the crystal structure and phase purity of the
products, and the typical XRD patterns of the SrTiO3 host structure prepared using
sol-gel technique at various temperatures ranging from 450℃ to 800℃ for 2 h were
shown in Figure 4.1. When considering 450⁰C, it is obvious that only the Sr(NO3)3
phase is present, no diffraction pattern of other phases were identified. When the
calcination temperature was increased to 600⁰C, peaks of new phases, SrTiO3 and
TiO2 (anatase) are identified from the XRD pattern, which can be indexed to the
cubic phase structure of SrTiO3 (ICDD card no. 84-0444) and ICDD card no 711169 for TiO2 (anatase). As a result, the creation of the perovskite SrTiO3 phase
occurs at 600⁰C. From this point of view, this method can form SrTiO3 at low
temperature in comparison with the conventional solid state method. The same
pattern was observed for phosphor at calcinations 700⁰C which are SrTiO3 and TiO2
(anatase). However, with the calcinations temperature increasing to 800⁰C, almost all
the diffraction peaks can be indexed to the cubic SrTiO3 phase. It is evident from the
sharpness of the peaks that the crystallinity and uniformity is continuously increasing
with increasing calcination temperature.
The formation of SrTiO3 phases after calcinations at 600⁰C is almost similar
to the results reported by Park et al. (2001) in SrTiO3: Al, Pr phosphor from a
complex precursor polymer. In their study, after calcinations at 600⁰C, the crystalline
SrTiO3 and minor amounts of SrCO3 have formed.
55
800oC
SrTiO3
TiO2 - Anatase
Intensity (a.u)
700oC
600oC
450oC
10
Sr(NO3)3
20
30
40
50
60
70
2
Figure 4.1 XRD pattern of SrTiO3 at various calcination temperatures
80
56
For further analysis, calcinations temperature at 800⁰C was chosen as the
optimum synthesis temperatures of the sample because of the phases are well
matched and it‟s highly crystalline.
Figure 4.2 shows the XRD pattern of SrTiO3 at 800⁰C. The patterns show the
main peaks at 2θ~32.4⁰, 39.9⁰, 37.4⁰, 57.8⁰, 67.8⁰ and 77.2⁰ which correspond to the
(110), (111), (200), (211), (220) and (320) respectively. Our data are in good
agreement with the reported work by Potdar et al. (1992).
Intensity (a.u)
(110)
(111)
10
20
30
40
(200)
(211)
50
60
(220)
70
(310)
80
2
Figure 4.2 XRD pattern of SrTiO3 at 800⁰C.
4.2.1.2 The influence of dopant to the host structure
The XRD patterns for doped and undoped sample are shown in Figure 4.3.
The patterns show well-defined peaks, which indicate the crystalline and phase
formation of the synthesized compounds. The diffraction patterns of SrTiO3: Pr, Al
and SrTiO3:Pr phase are found to be in agreement with that of the host material
SrTiO3 from the ICDD card no 84-0444 and also those are in coincidence with the
57
reported data (Kakihana et al., 1998). Comprising of these three patterns, it is found
that they are in cubic structure with a space group Pm3m.
Pr3+ ions could easily be substituted (in the place Sr2+) in the host matrix
without any change in the host matrix structure. As for that, a change is not observed
in the XRD pattern by the addition of Pr and Al ions. This correspond to the research
done by Kim et al. (2004) which reported that the addition of group-IIIb ions,
resulted in no changes being observed.
Comparing the XRD patterns of the samples, there are also no identifiable
differences of diffraction peaks even by addition of Al3+. Due to the closed ionic
radius between Al3+ (0.535 Å) and Ti4+ (0.605 Å), it can be considered that Al3+ has
substituted for Ti4+ site in the SrTiO3: Pr, Al sample. Tang et al. (2006) also reported
that no changes were observed when Al was added to Al–CaTiO3:Pr sample which is
consistent with our observation.
SrTiO3
Intensity (a.u)
(c) SrTiO3:1mol%Pr,
1mol%Al
(b) SrTiO3:1mol%Pr
(a) SrTiO3
10
20
30
40
50
60
2
Figure 4.3 XRD pattern for (a) SrTiO3 (b) SrTiO3:1mol%Pr (c)
SrTiO3:1mol%Pr,1mol%Al
70
80
58
4.2.1.3 The influence of dopant concentration
Figure 4.4 (a) showed the XRD patterns for various concentration up to 1 mol
% of Pr3+ which was calcined at 800⁰C while Figure 4.4(b) showed patterns for
various concentration up to 1 mol % of Al3+ when Pr3+ was fixed at 1 mol%. The
diffraction peak positions and the relative intensities of the prepared sample were
well matched with the standard powder diffraction file (ICDD card no. 84-0444). It is
evident that the phosphor powders show all the peaks attributed to the SrTiO3 phase.
In the XRD studies, it can be conclude that the results proved that all
phosphor samples prepared in this work are almost single SrTiO3 phase. It also
indicates that the little amount of co-doped Pr3+ ions and Al3+ have almost no effect
on the SrTiO3 phase composition. There are no identifiable differences of diffraction
peaks even addition of Pr and Al. Due to the closed ionic radius between Pr3+ and
Sr2+ and Al3+ with Ti4+, it can be considered that Pr3+ substituted for Sr2+ and Al3+
substituted for Ti4+ in the SrTiO3: Pr, Al sample. This process is illustrated in Figure
4.5. This is corresponding to the research that has been done by Tang et al. (2006)
which was mentioned in previous section. A small quantity of doped rare earth active
ions Al3+and Pr3+ has negligible effect on the basic crystal structure.
59
(a)
(b)
SrTiO3
doped
with Pr3+
SrTiO3
doped
with Al3+
1mol%
1 mol %
0.5mol%
0.5 mol %
Intensity (a.u)
0.2mol%
Intensity (a.u)
0.2 mol %
0.1mol%
0.1 mol %
0mol%
0 mol %
10
20
30
40
50
60
70
80
10
20
30
2
40
50
60
2
Figure 4.4 XRD pattern of SrTiO3 doped with various concentration of (a) Pr3+ and
(b)doped with Al3+ when Pr3+ was fixed at 1 mol %
70
80
60
Sr2+ addition
Substitution of Sr2+
by Pr3+
Substitution of Ti4+ by
Al3+
Figure 4.5
Illustration of substitution process in SrTiO3 sample as addition of Pr
and Al
4.3
Grain Size and Morphology Analysis
On the basis of XRD patterns, full-width at half-maximum (FWHM) data
could be analyzed by Scherrer formula (Equation 2.2) to determine the average
crystallite sizes
Figure 4.6 shows the grain size obtained by Scherrer‟s equation for (110)
cubic direction as a function of the dopant concentration. We observe that the grain
are nanosize, ranging approximately from 25 to 55 nm and is dependent on the
dopant concentration, which has also been reported by Buscaglia et al. (2000). In this
research, increasing of Pr concentration increased the grain size which is show in
61
Figure 4.6 (a) while increasing Al concentrations with Pr fixed causes a decrease in
the grain size which is shown in Figure 4.6 (b).
SEM study was carried out to investigate the surface morphology and the
crystallite sizes of the synthesized phosphor powder. The powder samples calcined at
temperature 800⁰C for 2 hours were used for these experiments. Figure 4.7 (a-c)
shows the representative SEM micrographs taken for SrTiO3, SrTiO3:1mol%Pr and
SrTiO3:1mol%Pr, 1mol%Al, respectively. It can be seen that the crystallite
morphology of phosphors varied with addition of dopant. This indicated that the
addition of dopant and co-activator influence the texture and morphology of SrTiO3.
(a)
(b)
Figure 4.6 Grain size obtained using the Scherrer‟s equation as a function of (a) Pr
concentration (b) Al concentration when Pr3+ fixed at 1 mol %
62
(a)
(b)
(c)
Figure 4.7 Scanning electron micrographs of (a) SrTiO3 (b) SrTiO3:1mol%Pr (c)
SrTiO3:1mol%Pr, 1mol%Al calcined at 800⁰C
4.4
Compositional analyses
Figure 4.8 represent the elemental analysis of the synthesized products was
performed using the energy dispersive X-ray analysis (EDAX) technique. Figure
4.8(a)–(c) present the EDAX spectra of SrTiO3, SrTiO3:1mol%Pr, SrTiO3:1mol%Pr,
1mol%Al phosphor. Respectively, it shows that the compositions of phosphor
samples mainly consist of Sr, Ti and O with a trace of Al and Pr elements. EDAX
analysis was carried out mainly to confirm the presence of rare-earth ions in the
phosphor prepared and the results confirm their presence.
63
Figure 4.8(a) shows the EDAX result for undoped SrTiO3. In this figure it
clearly shows that Sr become a major element in this sample. Cl elements which
origin from the raw material (Hydrochloric acid, HCl) was also observed in this
spectrum. As Pr was added to the host structure, Figure 4.8(b) shows their presence
although the concentration of Pr is small. The same phenomena was observed when
small amounts of Al concentration was added and Figure 4.8(c) confirmed the Al
presence in the sample.
Pr and Al elements trace by EDAX confirm that this instrument can be used
to detect doping although in small amount which is difficult to detect using XRD.
4.5
Infrared (IR) spectra
4.5.1
The influence of calcined temperature on host structure
FTIR spectra of SrTiO3 host matrix were recorded and studied in the wave
number range 400-4000 cm-1. FTIR spectra of SrTiO3 samples at different
calcinations temperatures which are 600⁰C, 700⁰C and 800⁰C are shown in Figure
4.9. Phase sample calcined at 600⁰C for 2 hours shows prominent absorption peaks
obtained at 3405,1625, 1462, 861, 560 and 456 cm-1. In the case of SrTiO3 with
calcined at temperature 700⁰C absorption peaks were obtained at 3414, 1633, 1468,
861, 576 and 439 cm-1. At a higher temperature of 800⁰C, five peaks also develop at
3407, 1627, 1469, 861, 565 and 433 cm-1.
64
Counts
(a)
4400
4000
3600
3200
2800
2400
2000
1600
1200
800
400
0
Sr
0.00
Counts
(b)
Ti
TiO
Ti
C
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Sr
0.80
ClCl
1.60
2.40
Ti
3.20
4.00
Energy (keV)
4.80
5.60
6.40
7.20
S
r
C
T
i
O
T
i
T
i
0.00
Pr
Pr
T
Pr Pr
i
S
r
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
7.00
8.00
9.00
10.00
Energy (keV)
Counts
(c)
4400
4000
3600
3200
2800
2400
2000
1600
1200
800
400
0
S
r
T
Ti
iO
C
0.00
C
Cl
l
S
A
Pr r
l
1.00
2.00
3.00
T
i
Pr
T Pr Pr
i
4.00
5.00
Energy (keV)
6.00
Figure 4.8 EDAX spectra of (a) SrTiO3 (b) SrTiO3:1mol%Pr (c) SrTiO3:1mol%Pr,
1mol%Al phosphor
65
In the case of this SrTiO3, the vibrational peaks at 3405, 3414 and 3407 cm-1
corresponding to calcinations at 600⁰C, 700⁰C and 800⁰C are due to hydrogen
bonded water. The area and intensity of the bands which accounts the presence of
water diminishes with rise in calcinations temperature. The peak at 3405 cm-1
corresponding to calcinations temperature 600⁰C has maximum area and intensity.
The peak at 3407 cm-1 developed during calcined at 800⁰C has the least area and
intensity. The bands at 1625, 1633 and 1627 cm-1 are due to the deformation
vibrations of water molecules (δH2O). The appearance of this mode is probably due
to the adsorption of water during the compaction of the powder specimens with KBr.
The peaks at 861, 1462, 1468 and 1469 cm-1 are due to cation coordination by
carboxylic groups, indicating the absorption of CO2 molecules on the surface of the
samples, tend to disappears with increasing the calcinations temperature. Absorption
peaks at 560, 576 and 565 cm-1 characteristic for an octahedral coordinated Ti-O
stretching mode is observed which correspond to Last (1957) and Park et al. (2001)
studies. Band around 400 cm-1 can be assigned as O-Ti-O bending mode. Our data
are in good agreement with the reported work on poly crystalline SrTiO3 by Last
(1957). The most important bands and their assignments are listed in Table 4.1.
Table 4.1: Position (cm-1) and Assignment of IR Bands
IR bands (cm-1)
Assignments
Our work
Literature
3444
3420
υ(O-H)
1627
1630
δ(H2O)
1468
1490–1450
υ(CO3)2-
563
568
υ(Ti-O)
395
δ(O-Ti-O)
380, 420, 440, 450, 590, 660
Pr2O3
445, 425,540
δ(O–Al–O)
399-450
Intensity (a.u)
v[Ti-O]
565
1469
1627
3407
861
800C
576
456
1468
1633
861
439
700C
3414
433
v[CO3]2v[CO3]2-
δ[HOH]
v[OH]
δ[O-Ti-O]
66
4000
3600
3200
2800
2400
2000
1800
1600
560
1462
1625
3405
861
600C
1400
1200
1000
800
Wavenumber (cm-1)
Figure 4.9 FTIR spectra for undoped SrTiO3 at different calcinations temperatures
600
400
67
4.5.2
The influence of dopant addition to the host structure
Figure 4.10 shows the FTIR spectra for undoped SrTiO3, SrTiO3: Pr and
SrTiO3: Pr, Al which were calcined at 800⁰C. Undoped SrTiO3 show prominent
peaks at 3407, 1627, 1469, 861, 565 and 433 cm-1. When Pr was added, absorption
peaks were obtained at 3425, 1633, 1465, 861, 559 and 438 cm-1. Four peaks existed
when Al was added which is at 3434, 1632, 556 and 436 cm-1.
Referring to the discussion on infrared studies in the previous chapter, the
bands at 3407, 3425 and 3434 cm-1 show the presence of considerable amounts of
H2O (Um and Kumazawa, 2000). The absorption peaks around 1600 cm-1 correspond
to the bending vibrations of adsorbed water molecules (δH2O) (Yan et al., 2009).
According to Kumar and Buddhudu (2009), bands at 1400 and 861 cm-1 are due to
the vibrations of CO32 anions. The width and intensity of 1400 cm-1 band decrease
with addition of Pr and vanishes when Al was added while band at 861 cm-1 vanishes
with the addition of the Pr and Al, showing removal of organics.
With SrTiO3: Pr and SrTiO3: Pr, Al patterns, there appears to be no difference
in the finger print region (400-600 cm-1) with respect to SrTiO3 host matrix which
does not allow us to determine the influence of dopant addition to the host matrix.
438
433
559
4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800
565
861
1469
1627
436
v[Ti-O]
556
861
1465
1633
(a) SrTiO3
3407
v[CO3]2-
v[CO3]2-
(b) SrTiO3: Pr
3425
Intensity a.u
δ[HOH]
1632
(c) SrTiO3: Pr, Al
3434
v[OH]
68
600
400
Wavenumber cm-1
Figure 4.10 FTIR spectra of (a) undoped SrTiO3 (b) SrTiO3: Pr (c) SrTiO3: Pr, Al
which calcined at 800⁰C
69
4.5.3
The influence of dopant concentration
4.5.3.1 Pr addition
FTIR spectra of powder sample SrTiO3: xPr (0 ≤ x ≤ 1 mol %) in the
frequency range 399–1500 cm−1 is shown in Figure 4.11. As can be seen from the
figure, main bands appearing along the spectrum are in the region of 600-400 cm-1.
The effect of Pr doping on the infrared spectral feature of local structure of
SrTiO3 is clearly shown in the low-frequency regions. In this study, it can be seen
that band around 400-450 cm-1 clearly appear with increasing Pr content. These
bands are due to praseodymium oxide but by Last (1957) band around 400 cm-1 also
can be assigned as O-Ti-O bending mode. It can be assume that there are overlapping
band in these region. The bending vibration band is still considered as a scattering
effect, but not a real absorption band.
4.5.3.2 Al addition
The FTIR pattern for SrTiO3: 1mol%Pr, yAl (0 ≤ y ≤ 1 mol %) in the
frequency range 399–1500 cm−1 is illustrated in Figure 4.12. With increasing Al3+
doping concentration (y=0.1-1 mol %), there are only have slight changes in the
finger print region especially in the low region.
δ[O-Ti-O] / Pr2O3
v[Ti-O]
v[CO3]2-
70
455
435
555
1468
v[CO3]2-
455
435
x = 1 mol %
576
1457
858
x = 0.2 mol %
435
559
1465
Intensity (a.u)
858
x = 0.5 mol %
427
861
x = 0.1 mol %
861
1400
565
1469
1500
433
568
x = 0 mol %
1300
1200
1100
1000
900
800
700
600
500
399.2
Wavenumber (cm-1)
Figure 4.11 FTIR spectra of the SrTiO3: xPr with 0 ≤ x ≤ 1 mol % which calcined at
800⁰C
71
Study that have been done by Mikhailov (2001) et al stated that the band due
to symmetric bending vibrations of O–Al–O or Al-O-Al bonds or deformation of
tetrahedral and octahedral units, appears at 445, 425 and 540 cm−1.
From the literature, it can be assumed that Al vibration also occurred at the
same frequencies either with Ti or Pr in the region 400- 600 cm-1. Al, Pr and Ti
vibration mode at low wavenumber cannot be measured, because the spectrum was
recorded starting at 400 cm-1. Further study is really necessary to index the Al, Pr and
Ti peak.
In IR studies the vibration of Sr2+ cannot be detected because when Sr enter
the TiO6, it formed ionic bonding. IR spectroscopy only can detect covalent bonding.
Raman spectroscopy has in general better frequency resolution for ceramic
material than IR spectroscopy and is worthy to stress here is that unlike IR
spectroscopy it is sensitive to the presence of TiO2.
4.6
Raman Spectra
4.6.1
The influence of calcined temperature on Host Structure
According to factor group analysis, anatase TiO2 has six Raman active modes
(A1g+ 2B1g + 3Eg). The Raman spectrum of an anatase single crystal has been
investigated by Ohsaka (1980), who concluded that the six allowed modes appear at
144 cm-1 (Eg), 197 cm-1 (Eg), 399 cm-1 (B1g), 513 cm-1 (A1g), 519 cm-1 (B1g), and 639
72
cm-1 (Eg). In this study, we assigned and interpreted the Raman bands of the SrTiO3
563
y = 1 mol %
440
v[Ti-O]
δ[O-Ti-O] / Pr2O3 /Al203
using earlier results obtained for the bulk phase.
566
y = 0.2 mol %
444
561
Intensity (a.u)
433
y = 0.5 mol %
550
441
y = 0.1 mol %
555
435
y = 0 mol %
1500
1400
1300
1200
1100
1000
900
800
700
600
500
Wavenumber (cm-1)
Figure 4.12 FTIR spectra of the SrTiO3: 1mol%Pr, yAl with 0 ≤ y ≤ 1 mol% which
calcined at 800⁰C
399.2
73
Figure 4.13 shows the Raman spectrum of the SrTiO3 samples calcined at
600-800⁰C for 2 hrs. The Raman bands are assigned based on the literature data on
the wave number ranges related to the corresponding vibrations of the structural units
in various glassy and crystalline titanate. The position and the relative intensity of
each component of Raman spectra related to the vibrational mode of titanate units
were determined.
All of the Raman spectrums in Figure 4.13 have five peaks which are located
at 146, 251, 406, 526 and 641 cm−1 identical with the feature of anatase type TiO2,
and a weak peak observed at 1072 cm-1. Peaks 146, 251and 406 cm-1 is attributed to
the O-Ti- bending mode while 526 and 641 cm−1 are attributed to Ti-O vibration
mode, υ(Ti-O). Weak peak at 1072 cm-1 is assigned to assigned to the υ(CO3)2symmetric stretching mode (Frost Ray, 2007). This results correspond with Hyun
Chun Choi et al. (2005) studied on size effects in the raman spectra of TiO2
nanoparticles while according to Federico et al. (2008), SrCO3 displays a strong peak
at 148 cm-1. In our experimental evidence suggests that the peak at 146 cm-1
observed in SrTiO3 raman spectra overlapped between SrCO3 and O-Ti-O bending
mode, δ(O-Ti-O). From the literature and review that have been done, the Raman
vibration for our sample can be concluded as in Table 4.2.
Table 4.2 Raman modes comparison with the literature values
Raman Shift (cm-1)
Assignments
Our work
Literature
146
144
Eg / δ(O-Ti-O)
146
148
SrCO3
251
197
Eg / δ(O-Ti-O)
406
399
B1g/ δ(O-Ti-O)
526
513,519
A1g, B1g / υ(Ti-O)
641
639
Eg / υ(Ti-O)
1072
1072
υ(CO3)2-
74
δ(O-Ti-O)
υ(Ti-O)
Intensity (a.u)
υ(CO3)2800C
251
1072
641
526
600C
406
146
700C
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman Shift (cm-1)
Figure 4.13 Raman spectrum of the SrTiO3 samples calcined at 600-800⁰C for 2 h.
75
4.6.2
The influence of dopant addition to the host structure
The doping dependence of Raman spectra of SrTiO3 is shown in Figure 4.13.
The low-frequency modes below 900 cm−1 show marked doping dependence while
the change of higher frequency modes is not remarkable as dopants were added.
As dopants were added to the SrTiO3 systems, according to Nilsen et al, we
find that the Raman spectrum to be entirely attributed to second-order Raman
scattering which is show by Figure 4.14(b) and (c). Both figures consist of two
second-order broad bands centered in the 200-400 and 600-800 cm-1 region. The
assignments proposed by previous authors which are Nilsen et al. (1968) and Perry et
al. (1967) for the two broad bands are in good agreement with our data. When Al is
added to the SrTiO3:1mol%Pr system, there are new bands appearing at lowfrequency which could be assume the band of SrCO3 overlapped with O-Ti-O
bending mode. There are also slight changes in 200-400 and 600-800 cm-1 region as
Al added to the system. The intensities of band at 303 cm-1 decrease with addition of
Al causing a reduction of the band contour around 300-400 cm-1. Same phenomenon
also happened for the 600-800 cm-1 region, which decrease by Al addition.
Unfortunately, there are few reports on the vibrational study of second-order Raman
scattering. The exact mechanism for this observation is not clearly understood as of
yet.
76
υ(CO3)2-
Intensity (a.u)
(c) SrTiO3:1%Pr, 1%Al
303
(b) SrTiO3:1%Pr
(a)SrTiO3
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman Shift (cm-1)
Figure 4.14 Raman spectra of SrTiO3 sample by addition of dopant which calcined
at 800⁰C
4.6.3
The Influence of dopiant concentration
4.6.3.1 Pr addition
Figure 4.15 presents the Raman spectra of SrTiO3:xPr with different doping
concentrations (0≤ x ≤1 mol%).When the concentration of Pr is increase from 0.1
mol% to 1 mol%, it exhibits second-order Raman spectrum of SrTiO3 and all the
Raman spectra show similar spectra.
77
4.6.3.2 Al addition
Figure 4.16 represent the Raman spectra for the phosphor sample as
concentration of Al change when concentration of Pr fixed at 1 mol%. As
concentration of Al increase from 0.1 mol% to 1 mol%, a change in the second-order
Raman spectrum was observed. When Al concentrations are 0.5 and 1 mol %, new
band is observed at 120 cm-1.A weak peak at 171 cm-1 show up at y=0.2 mol % and
clearly seen at y=0.5 and 1 mol %. The intensities of band 238 and 400 cm -1 decrease
with the increasing of Al3+ concentration causing a reduction of the band contour
around 200-610 cm-1.Also, shoulder at 780 cm-1 becoming more intense as
concentration of Al3+ increase. A weak peak at 1072 cm-1 is attributed to the υ(CO3)2.
.
The vibrational studies using IR confirmed the vibration of Ti but cannot
detect the vibration of dopant because it overlapped with the host vibration. Raman
analysis shows by addition of dopant, the patterns become second-order Raman
scattering of SrTiO3. It can be observed that by addition of dopant, there are changes
on the patterns but cannot be explained clearly because of lack of references on
second-order Raman scattering of SrTiO3. This phenomenon is interesting to study
but there are a few reports in this area and need much more time to make it clear.
78
Intensity (a.u)
υ(CO3)2-
x = 1 mol %
x = 0.5 mol %
x = 0.2 mol %
x = 0.1 mol %
x = 0 mol %
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman Shift (cm-1)
Figure 4.15 Raman spectra of SrTiO3:xPr with various Pr concentrations
(0≤ x ≤1 mol %) which calcined at 800⁰C
120 cm-1
171 cm-1
79
υ(CO3)2-
y = 0.5 mol %
780 cm-1
606 cm-1
y = 0.2 mol %
400 cm-1
238 cm-1
Intensity (a.u)
y = 1 mol %
y = 0.1 mol %
y = 0 mol %
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman Shift (cm-1)
Figure 4.16 Raman spectra of SrTiO3:1 mol% Pr, yAl with 0≤ y≤1 mol % which
calcined at 800⁰C
80
4.7
Photoluminescence Spectra
Figure 4.17, 4.19 and 4.20 represent emission spectra for the representative
samples. The samples were excited with 325 nm wavelength and the emission
spectra were recorded.
4.7.1 The influence of dopant addition
Effect of dopant on luminescence properties of SrTiO3 are show in Figure
4.17. Undoped SrTiO3 showed that bands in the region of 330 to 430 nm were
attributed to the host emission which corresponds with Pr and Al doping spectrum.
As the Pr3+ is added, the region of higher wavelength (430-730nm) exhibit the
emission spectrum consisted of four emission peaks, originating from the 3P0
→3Hj (j=6, 5, 4) and 1D2→3H4 transitions of Pr3+,respectively.This was assigned
based on energy level of Pr3+ (Figure 4.18). From the literature, had the emission
originated from the 3P0 level, it would be green as in Gd2O2S:Pr; however, if the
emission originated from 1D2 level, it would be red (Blasse, 1994).
In the presence of Al3+ addition, the intensity of 1D2→3H4 tends to be stronger
with an unchanged position. This agrees with Yamamoto and Okamoto (2000) study
on efficiency enhancement by aluminum addition to some oxide phosphors for field
emission displays.
SrTiO3:Pr,Al
3
P0 →3H6
P0 →3H5
SrTiO3:Pr
3
3
SrTiO3
P0 →3H4
1
D2→3H4
81
Figure 4.17
Photoluminescence (PL) spectra of SrTiO3, SrTiO3:Pr3+ and
SrTiO3:Pr, Al
Figure 4.18 Energy level of Pr3+
82
4.7.2
The influence of Pr concentration
Figure 4.19 presents the photoluminescence emission spectra of SrTiO3 with
different Pr3+ concentrations (0-1 mol% of Pr) under 325 nm excitation at room
temperature. Figure shows that by addition of Pr, the 3P0 →3H4 band dominates the
spectrum. Apart from this, three other emission bands corresponding to the radiative
4f → 4f transitions from some excited states of the Pr3+ also observed. The bands in
the blue-green region 480-500 nm and 517-536 nm are assigned to the transition 3P0
→3H4 and 3P0 →3H5 of Pr3+, respectively, while the band in the red region 600-630
nm is associated with the 1D2→3H4 transition of Pr3+.
Refer to the dominant peak of the spectrum which is at 490 nm, the maximum
emission intensity is at 0.5 mol% Pr3+. Beyond this doping level, emission intensity
decreased with increasing Pr3+ contents, presumably due to concentrations
quenching.
4.7.3
The influence of Al concentration
In this section we investigated Al addition effect on photoluminescence (PL)
properties of Pr-doped SrTiO3. Figure 4.20 shows the PL spectra for SrTiO3:Pr, yAl
in the range 0≤y≤1 mol % and Pr3+ was kept constant at 1 mol% under the 325 nm
laser excitation at room temperature. The PL spectrum have peak at 490, 530, 615
and 650 nm, which is attributed to transition from excited states
3
3
P0 →3H4 ,
P0→3H5, 1D2→3H4 and 3P0 →3H6 of Pr3+, respectively.
As the addition of Al, band at 615 nm which emit red emission dominates the
spectrum which is attributed to the intra-4f transition from the excited state 1D2 to
P0 →3H6
D2→3H4
1
3
P0 →3H5
3
3
P0 →3H4
83
Figure 4.19 Photoluminescence spectra of the SrTiO3:xPr (0≤x≤1 mol %) phosphors
under 325 nm excitation
the ground state 3H4 of Pr3+(1D2→3H4 emission), instead of by Pr addition, the
domain band is at 3P0 →3H4 transition. It can be noticed from Figure 4.20 that the
emission intensity increases with doping of the second ion Al3+. In fact the intensity
of emission increases with the increase in Al3+ concentration up to 0.2 mol%, beyond
which the emission intensity decreases.Leverenz has proposed an explanation of this
concentration quenching phenomenon. According to his results, optimum
concentration of Mn in ZnSiO4 is approximately 0.3 wt.%. At above 0.5 Mn wt.% in
84
the host material, it is generally found that increases in chemical complexity and
structural heterogeneity of phosphors decreased their relative emission intensity
(Leverenz, 1968). These phenomena may be attributed to concentration quenching,
3
P0 →3H6
P0 →3H5
3
3
P0 →3H4
1
D2→3H4
though the mechanism of it is not yet clearly understood.
Figure 4.20 PL spectra of SrTiO3:Pr, yAl (Pr3+:1 mol%) with various molar ratios of
Al (0≤y≤1 mol %) under the 325 nm laser excitation at room temperature
The results described above indicates that SrTiO3:Pr exhibits the typical
emission profile with a prominent luminescence in the greenish-blue region arising
from
3
P0 level, while SrTiO3:Pr,Al shows its characteristic
1
D2→3H4 red
luminescence. We are focus on the red emission of the sample which are assign by
85
1
D2→3H4 transition. The red emission of SrTiO3: Pr3+ with addition of Al3+ was
greatly intensified compared to the undoped Al samples. Pr3+ substitutes for the Sr2+
site in the SrTiO3 lattices and a positive charge defect is formed. Such charge defects
hamper the process of energy transfer from the host to Pr3+, but it can be
compensated by the negative charge defect when Ti4+ ion is substituted by Al3+ ions
and producing a vacancy in the process. This vacancy ultimately plays an important
role in enhancement of the luminescence efficiency
Pr3+ ions, which act as an activator and emit the red luminescence, cannot
directly absorb the radiation used for excitation. The excitation radiation is absorbed
by the lattice defects associated with Al3+ ions, which act as sensitizers, and the Al3+
ions subsequently transfer it to the Pr3+ ions. After this, the Pr3+ ions emit the
radiation.
Figure 4.21 shows a tentative scheme of the Pr3+ energy levels and defect
levels with respect to the conduction and valence bands of SrTiO3.The band gap for
pure SrTiO3 is 3.5 eV (Kim et al. 2007); it is very close to the one for GaN (about
3.8 eV) (Dorenbos and van der Kolk, 2006). Since energy levels of rare earth ions
vary little from host to host, we assume that the relative positions of the Pr3+ energy
levels in both hosts are close to each other. This means that the 3P0,1,2 energy levels
are located close to the bottom of the conduction band and even more closer to the
defect‟s energy levels located in the forbidden gap below the conduction band. Such
a location of energy levels favors energy transfer from Al3+ ions to Pr3+ ions, as
shown in Figure 4.21. Approximate location of the Pr3+ energy levels with respect to
the valence band (VB) and conduction band (CB) of SrTiO3 was obtained in analogy
with Dorenbos et al. (2006)
86
CB
Defect levels
25000
3
P2
P1, 1I6
3
P0
3
20000
Energy, cm-1
1
D2
15000
1
10000
G4
3
3
5000
3.5 eV=28229 cm-1
F4
F3
3
F2
3
H6
3
H5
3
H4
VB
Figure 4.21 Proposed scheme of energy transfer from the defects created around
Al3+ levels to the (3P0, 1, 2 , 1I6, 1D2) energy levels of Pr3+.
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
5.1.1
Structural Studies
Using sol-gel method, fine powders of SrTiO3: Pr, Al phosphor was prepared.
This method yielded SrTiO3 formation lower temperatures than required for solid
state reaction method which is at 600⁰C. Te structural studies of the samples have
been investigated using X-Ray Diffraction, Infrared and Raman Spectroscopy. XRD
patterns show the phase formation of SrTiO3 phase which can be indexed to the cubic
phase structure of SrTiO3. No changes in phase formation were observed on the host
lattice as addition of Pr and Al dopant. This means that the structure retains the host
perovskite phase.
Grain size was obtained using the Scherrer‟s equation as a function of Pr
concentration and Al concentration. We observe that the grain size have nanoscale
range, ranging approximately from 25 to 55 nm.
88
IR spectra exhibit the stretching and bending vibration of sample. The band at
527 cm-1 is due to the Ti-O stretching vibrations. Furthermore, the bands in the region
400-500 cm-1 are attributed to the Ti-O-Ti bending mode which overlapped with
Pr2O3 and also Al2O3 vibration. As for that, FTIR cannot be used as local probe of
doping rare-earth in the host material SrTiO3.
Raman spectroscopy has in general better frequency resolution for ceramic
material than IR spectroscopy and worthy to stress here is that unlike IR spectroscopy
it is sensitive to the presence of TiO2. SrTiO3 host material has five peaks which is
located at 146, 251, 406, 526 and 641 cm−1 which is almost similar with anatase TiO2.
Peak at 146 cm-1 was overlapped between SrCO3 and O-Ti-O bending mode. With
addition of dopants, it is found that the Raman spectrum to be entirely attributed to
second-order Raman scattering. When the sample are doped, a slight shift was
observed on the characteristic position of Raman modes that can be related to the
degree of crystallization, interaction force between ions, structural defects and
influence of doping.
5.1.2
Luminescence studies
From the findings, addition of Pr doping make the 3P0 →3H4 transition has a
larger intensity than the 1D2→3H4 transition of Pr3+ but with addition of Al,
1
D2→3H4 transition became dominant.
As for that, SrTiO3:Pr has a blue-green
emission between 480-500 nm red emission for SrTiO3: Pr,Al.
The SrTiO3: Pr,Al phosphor has a red emission band between 580 and 640 nm
peaking at 617 nm. This is due to the radiative decay of the 1D2 states (1D2→3H4
transition). Emission intensity increased with the increasing activator concentration
up to maximum at 0.5 mol% of Pr3+ without addition of Al. As the Pr fixed at 1
89
mol%, the emission intensity up to a maximum at 0.2 mol% of Al3+. Beyond these
doping level, emission intensity decreased with increasing doping contents. This
decrease is understood to be due to concentration quenching.
In conclusion from this research, the blue-green color emission of SrTiO3:Pr
can be tuned to red by simply introducing a small amount of Al ions into the host to
disturb the microstructure. Consequently, this color tunable phosphors offer a flexible
choice for any potential application of phosphor material either the blue-green or red
emission.
5.2
Recommendations
Although it has been shown that the sol-gel method can synthesize SrTiO3,
the processes have not been well controlled. Hence, further investigations in the
controlled environment are needed to synthesize a quality samples.This experiment is
done in open air, so, for future work; it is suggested to run this experiment under a
controlled environment like in the glove box.
In this work, we just used small amount of dopant concentration. So, for
future work, we propose that the dopant concentration is varied up to 20 mol% or
more to observe the effect on structural and luminescence properties.
Moreover, it is an interesting observation when using raman spectroscopy, it
showed that the spectrum become second-order Raman scattering. More thorough
and systematic studies should be performed in order to understand the formation of
second-order Raman scattering of SrTiO3.The mechanisms of this formation also
requires further investigation in order to make it clear.
90
Since this material has potential application in field emission display (FED),
more measurement are needed in order to understand the influence of doping addition
on
luminescence
properties
such
as
by
using
Transmission
Electron
Microscopy(TEM) which can detect the defect in samples. Besides that, because of
only few researches have been done on the structural studies of this material; it is
recommended that for future work a detail study should be done focusing on the local
structure. As for that, Nuclear Magnetic Resonance (NMR) could be a great
technique to use besides other measurements.
91
REFERENCES
Blasse, G. and Grabmaier, B.C. (1994). Luminescent Materials. Berlin: SpringerVerlag.
Bol, A. A., Ferwerda, J., Bergwerff, J. A., and Meijerink, A. (2002). Luminescence
of nanocrystalline ZnS:Cu2+. Journal of Luminescence, 99(4), 325-334.
Bose, H. N. (1992). Luminescence and allied phenomena. Indian Journal of History
Science 27(4).
Buscaglia, M. T., Buscaglia, V., Viviani, M., Nanni, P., & Hanuskova, M. (2000).
Influence of foreign ions on the crystal structure of BaTiO3. Journal of the
European Ceramic Society, 20, 1997-2007.
Byong Kee Moon, I.-M. K., Hyun Kyoung Yang, Hyo Jin Seo, Jung Hyun Jeong,
Soung Soo Yi and Jung Hwan Kim. (2008). Spectroscopy of nanocrystalline
TiO2:Eu3+ phosphors. Colloids and Surfaces A: Physicochemical and
Engineering Aspects, 313-314, 5.
Chang, C. and Mao, D. (2004). Long lasting phosphorescence of Sr4Al14O25:Eu2+,
Dy3+ thin films by magnetron sputtering. Thin Solid Films, 460(1-2), 48-52.
Chang, C.A., Ray, B., Paul, D. K., Demydov, D., & Klabunde, K. J. (2008).
Photocatalytic reaction of acetaldehyde over SrTiO3 nanoparticles. Journal of
Molecular Catalysis A: Chemical, 281(1-2), 99-106.
Cullity, B. D. (1978). Elements of X-Ray diffraction (2nd ed ed.): Addison-Wesley
Publishing Company, Inc., Reading, MA.
Deren, P.J., R. P., W. Strek. Ph. Boutinaud, R. Mahiou. (2008). Synthesis and
spectroscopic properties of CaTiO3 nanocrystals doped with Pr3+ ions.
Journal of Alloys and Compounds, 451, 595-599.
Diallo, P., Boutinaud, P., Mahiou, R. and Cousseins, J.(1997). Red Luminescence in
Pr3+- Doped Calcium Titanates. phys. stat. sol. (a)(160), 255.
92
Dorenbos, P., & van der Kolk, E. (2006). Location of lanthanide impurity levels in
the III-V semiconductor GaN. Applied Physics Letters, 89(6), 061122061123.
Federico A. Rabuffetti, H.-S. K., James A. Enterkin, Yingmin Wang,Courtney H.
Lanier, Laurence D. Marks, Kenneth R. Poeppelmeier and, and Stair, P. C.
(2008). Synthesis-Dependent First-Order Raman Scattering in SrTiO3
Nanocubes at Room Temperature. Chem. Mater., 20, 8.
Fouassier, C. (1984). Luminescence-Encyclopedia of Inorganic Chemistry Academic
Press New York
Frost Ray L, H. M. C., Jagannadha Reddy B. (2007). Aurichalcite - : An SEM and
Raman spectroscopic study. Polyhedron, 26(13), 10.
Gfroerer, T. H. (2000). Photoluminescence in Analysis of Surfaces and Interfaces.,
Encyclopedia of Analytical Chemistry. Chichester: John Wiley & Sons Ltd.
Guo, H., Dong, N., Yin, M., Zhang, W., Lou, L., and Xia, S. (2006). Green and red
upconversion luminescence in Er3+-doped and Er3+/Yb3+-codoped SrTiO3
ultrafine powders. Journal of Alloys and Compounds, 415(1-2), 280-283.
Hussein, G. A. M. (1994). Formation of praseodymium oxide from the thermal
decomposition of hydrated praseodymium acetate and oxalate. Journal of
Analytical and Applied Pyrolysis, 29(1), 13.
Hyun Chul Choi, Y. M. J., Seung Bin Kim. (2005). Size effects in the Raman spectra
of TiO2 nanoparticles. Vibrational Spectroscopy, 37, 6.
Jiang, L., Chang, C., Mao, D., and Zhang, B. (2004). A new long persistent blueemitting Sr2ZnSi2O7:Eu2+, Dy3+ prepared by sol-gel method. Materials
Letters, 58(12-13), 1825-1829.
Joseph, J. (2001). Luminescence Study on Geological Samples and Doped Phosphors.
Mahatma Gandhi University, Kottayam.
Kakihana, M., Okubo, T., Arima, M., Nakamura, Y., Yashima, M., & Yoshimura, M.
(1998). Polymerized Complex Route to the Synthesis of Pure SrTiO3 at
Reduced Temperatures: Implication for Formation of Sr-Ti Heterometallic
Citric Acid Complex. Journal of Sol-Gel Science and Technology, 12(2), 95109.
Kim, J. Y., You, Y. C., Kang, J. H., Jeon, D. Y., and Weber, J. (2004). New
perspective in degradation mechanism of SrTiO3:Pr,Al,Ga phosphors:
Materials Research Society.
Kim, J.D., T. M., and Itaru Honma. (2007). SrTiO3 Thin Films with Visible-Light
Band Gap Fabricated by Nitrogen Reactive Sputtering. Japanese Journal of
Applied Physics, 46, 2.
93
Kumar, G. B., and Buddhudu, S. (2009). Synthesis and emission analysis of RE3+
(Eu3+ or Dy3+):Li2TiO3 ceramics. Ceramics International, 35(1), 521-525.
Last, J. T. (1957). Infrared-Absorption Studies on Barium Titanate and Related
Materials. Physical Review, 105(6), 1740.
Leverenz, H. W. (1968). An Introduction to Luminescence of Solids. New York:
Dover.
Lucas Alonso Rocha, L. R. A., Bruna Leonardo Caetano, Eduardo Ferreira Molina,
Herica Cristina Sacco, Katia Jorge Ciuffi, Paulo Sergio Calefi, Eduardo Jose
Nassar. (2005). Europium incorporated into titanium oxide by the sol-gel
method. Material Research, 8(3), 8.
Mikhailov, D. A. J. a. G. G. (2001). Phase diagram of CaO–Al2O3 system. Ceramics
International, 27(1), 4
Monica Popa, M. K. (2001). Praseodymium oxide formation by thermal
decomposition of a praseodymium complex. Solid State Ionics, 141-142, 8.
Nilsen, W. G., and Skinner, J. G. (1968). Raman Spectrum of Strontium Titanate. The
Journal of Chemical Physics, 48(5), 2240-2248.
Ohsaka, T. (1980). Journal of The Physical Society of Japan, 48, 1661.
Park, J. K., Hojin Ryu., Hee Dong Park and Se-Young Choi. (2001). Synthesis of
SrTiO3:Al, Pr phosphors from a complex precursor polymer and their
luminescent properties. Journal of the European Ceramic Society, 21(4), 8.
Perry, C. H., Fertel, J. H., and McNelly, T. F. (1967). Temperature Dependence of
the Raman Spectrum of SrTiO3 and KTaO3. The Journal of Chemical Physics,
47(5), 1619-1625.
Potdar, H.S., Deshpande, S.B., Godbole, P.D., Gunjikar, V.G., Date, S.K.. (1992).
Low temperature synthesis of ultrafine strontium titanate (SrTiO3) powders.
J. Mater. Res, 7(2).
Qin Fei, C. C. a. D. M. (2005). Luminescent properties of Sr2MgSi2O7 and
Ca2MgSi2O7 long lasting phosphors activated by Eu2+, Dy3+. Journal of Alloys
and Compounds, 390(1-2), 5.
Ronda, C.R., Justel, T. and Nikol, H. (1998). Rare earth phosphors: fundamentals and
applications. Journal of Alloys and Compounds, 275-277, 7.
Tang, J., Yu, X., Yang, L., Zhou, C., and Peng, X. (2006). Preparation and Al3+
enhanced photoluminescence properties of CaTiO3:Pr3+. Materials Letters,
60(3), 326-329.
94
Um, M. H., & Kumazawa, H. (2000). Hydrothermal Synthesis of Ferroelectric
Barium And Strontium Titanate Extremely Fine Particles. Journal of
Materials Science, 35(5), 1295-1300.
Vij, D. R. (1998). Luminescence of Solids. New York: Plenum Publishing
Corporation
Wang, N., Lin, H., Li, J., Yang, X., and Zhang, L. (2007). Photoluminescence of
TiO2:Eu nanotubes prepared by a two-step approach. Journal of
Luminescence, 122-123, 889-891.
William M. Yen, Marvin J. Weber. (2004). Inorganic Phophors: Compositions,
Preparation and Optical Properties: CRC Press LLC.
William M. Yen, S. S. and Yamamoto Hajime. (2006). Phosphor Handbook (2nd
edittion ed.): CRC Press.
Xia, C.-T., and Shi, C.-S. (1997). BaLiF3(Eu2+): A promising X-ray storage
phosphor. Materials Research Bulletin, 32(1), 107-112.
Yamamoto, H., & Okamoto, S. (2000). Efficiency enhancement by aluminum
addition to some oxide phosphors for field emission displays. Displays, 21(23), 93-98.
Yamamoto, H., Okamoto, S., and Kobayashi, H. (2002). Luminescence of rare-earth
ions in perovskite-type oxides: from basic research to applications. Journal of
Luminescence, 100(1-4), 325-332.
Yan, J.-H., Zhu, Y.-R., Tang, Y.-G., and Zheng, S.-Q. (2009). Nitrogen-doped
SrTiO3/TiO2 composite photocatalysts for hydrogen production under visible
light irradiation. Journal of Alloys and Compounds, 472(1-2), 429-433.
Yong Gao, C. S. a. Y. W. (1996). Luminescence properties of SrB4O7: Eu, Tb
phosphors. Materials Research Bulletin, 31(5), 6.
Zhao, C., and Chen, D. (2007). Synthesis of CaAl2O4:Eu,Nd long persistent phosphor
by combustion processes and its optical properties. Materials Letters, 61(17),
3673-3675.
Zhimin Liu, J. Z., Buxing Han, Jimin Du, Tiancheng Mu, Yong Wang and Zhenyu
Sun. (2005). Solvothermal synthesis of mesoporous Eu2O3–TiO2 composites.
Microporous and Mesoporous Materials, 81(1-3), 6.