COMSATS Institute of Information Technology...

Rare Earth Nano Compounds: Preparation and

Thermophysical Characterization

By

Ali Abdullah

CIIT/FA07-PPH-002/ISB

PhD Thesis

In

Physics

COMSATS Institute of Information Technology

Islamabad-Pakistan

Spring, 2013

ii

COMSATS Institute of Information Technology



A Thesis Presented to

COMSATS Institute of Information Technology, Islamabad

In partial fulfillment

of the requirement for the degree of

PhD Physics

By

Ali Abdullah


Spring, 2013

iii



A Post Graduate Thesis submitted to the Department of Physics as partial

fulfillment of the requirement for the award of Degree of PhD (Physics).

Name Registration No.

Ali Abdullah


Supervisor

Dr. Muhammad Anis-ur-Rehman

Associate Professor, Department of Physics,

Islamabad Campus.

COMSATS Institute of Information Technology (CIIT),

Islamabad.

June 2013

iv

Final Approval

This thesis titled

Rare Earth Nano Compounds: Preparation and Thermophysical Characterization

By

Ali Abdullah


has been approved

For the COMSATS Institute of Information Technology, Islamabad

External Examiner: ___________________________________________

Prof. Dr. Asghari Maqsood Professor, Department of Physics, CESET, # 61, Sector I10/3, Islamabad

External Examiner: ___________________________________________

Dr. Misbah-ul-Islam,

Associate Professor, Department of Physics, Bahauddin Zakariya University, Multan

Supervisor: ____________________________________________

Dr. M. Anis-ur-Rehman

Associate Professor, Department of Physics/Islamabad

Head of the Department: __________________________________________

Prof. Dr. Arshad Saleem Bhatti

Department of Physics/Islamabad

Chairman of the Department: ________________________________________

Prof. Dr. Sajid Qamar

Department of Physics/Islamabad

Dean, Faculty of Sciences: ____________________________________________


v

Declaration

I Mr. Ali Abdullah, Reg. # CIIT/FA07-PPH-002/ISB, hereby declare that I have

produced the work presented in this thesis, during the scheduled period of study. I also

declare that I have not taken any material from any source except referred to wherever

due that amount of plagiarism is within acceptable range. If a violation of HEC rules on

research has occurred in this thesis, I shall be liable to punishable action under the

plagiarism rules of the HEC.

Date: _________________

___________________________

Ali Abdullah


vi

Certificate

It is certified that Mr. Ali Abdullah, CIIT/FA07-PPH-002/ISB has carried out all the

work related to this thesis under my supervision at the Department of Physics,

COMSATS Institute of Information Technology, Islamabad and the work fulfills the

requirement for award of PhD degree.

Date: _________________

Supervisor:

_________________________

Dr. Muhammad Anis-ur-Rehman,

Associate Professor

Head of the Department:

_____________________________


Department of Physics

vii

To

rational, logical and

questing minds of all

times

…………a people who think deeply. Al-Quran, Chapter Al-Jasia 45, part of Verse 13

...……………men of understanding. Al-Quran, Chapter A’l-e-Imran 3, part of Verse 190

.………….a people who understand. Al-Quran, Chapter Al-Nehl 16, part of Verse 12

viii

Acknowledgements

In the name of Allah, the most Gracious, the ever Merciful.

All the praises be to Allah Almighty, ‗He who taught the man which he did not know’ and

Countless prays for his Holy Prophet Muhammad (P.B.U.H).

It is a pleasure to express my truthful gratitude to my supervisor, Dr. Muhammad Anis-

ur-Rehman, who provided me the opportunity to complete my PhD in Applied Thermal Physics

Laboratory. I appreciate his scientific guidance, skilled suggestions, patience and

encouragement to me during these years.

I pay my gratefulness to members of my supervisory committee; Dr. Ishaq Ahmad, Dr.

Ahmar Naweed and former members Dr. Zuhair S. Khan, Dr. M. Kamran, for their useful

suggestions. Dr. Timothy Tan, Dr. Zhang Yan (SCBE, Singapore) and Dr. Aqif Anwar, Dr.

Abdus-Samad (IRCBM, CIIT, Lahore) are thanked for their scientific support.

I am grateful to Dean, FoS and Head of the Department Prof. Dr. Arshad S. Bhatti,

Chairman Prof. Dr. Sajid Qamar and former Head of the Department Prof. Dr. Mahnaz Q.

Haseeb for allowing me to work in my desired field and using CIIT facilities.

The Higher Education Commission, Pakistan is highly acknowledged for its financial

support through 5000 Indigenous Fellowship Program for funding my PhD, International

Research Support Initiative Program to visit Nanyang Technological University, Singapore and

Research grant project NRPU # 893. The School of Chemical and Biomedical Engineering,

NTU, Singapore is acknowledged for facilities, there.

I am thankful to my colleagues, Dr. Anwar ul Haq, Dr. G. Asghar, Dr. M. Yasin, Dr.

Nasir Khisro, Mr. G. Hasnain Tariq, Mr. M. Akram, M. Ali and others at ATPL for their

helpful discussions and maintaining a scientific environment. Special thanks to my fellows Mr.

Awais Siddique Saleemi, Mr. M. Mubeen, Mr. M. Saqib and Mr. S. Muzammil H. Shah for

their assistance, support and friendship.

I will cherish memory of my stay at ―Scholars‘ Island‖, Islamabad, with, Awais,

Hafeez, Saleemi, Arslan, Shahid, Farooq, Mohsin, Aftab, Shakoor, Irfan, Ramzan, Dr. Azeem

and at ―Pak House‖, Singapore, with Arshad, Basit, Jamil, Asif, Aftab, Vinod, Shahzad,

Zeeshan, Husnain and Adnan sb.

The Acknowledgement remains incomplete without mentioning my family (abbu,

ammi, begum, bhai, baji, bhatijian (Hamnah Fatimah, Rohah Ayshah and Umaimah Zaineb)

and son (M‘aaz Abdullah)), without whose support, affection and love, I was not able to reach

at such a point.

I pray for all a happier future and express my best wishes.

Ali Abdullah

ix

Abstract

Rare Earth Nano Compounds: Preparation and Thermophysical

Characterization

Rare earth compounds are a big group of functional materials which have varied

applications in many fields ranging from Solid Oxide Fuel Cells (SOFCs) to biological

labeling/imaging. The newly developed materials and techniques are nontoxic,

ultrasensitive, and chemically and physically stable. The main focus of this research

work was to attempt to enhance the ionic conductivity of ceria based compounds.

Factors like decrease in grain size, doping of trivalent cations and multiple doping are

mainly focused to increase the conductivity. Also, Rare earth doped inorganic matrix is

synthesized and fluorescence is observed in stabilized fluorophore as bimodal probe for

bioimaging.

A comparative study for synthesis and characterization of nanocrystalline ceria

was done with a range of wet chemical methods including composite mediated

hydrothermal method (CMH), co-precipitation method and sol-gel method. The

calcination and sintering temperatures were 500 0C and 750

0C respectively for all the

samples. X-ray diffraction (XRD) confirmed the cubic fluorite structure. Raman

spectroscopy seconded the XRD results and characteristic feature of ceria was observed

ca. 465 cm-1

. The dc conductivities of the samples were determined in temperature

range 200-700 0C. The highest value obtained was for the sample prepared with CMH

method having value 0.345 S-cm-1

at 7000C. So, CMH was selected as the synthesis

method for the later samples.

Further, the synthesis conditions of CMH method were optimized for

nanocrystalline samples. The practical parameters were heat treatment time and

temperature. The heat treatment temperature during synthesis was held at 180 0C and

220 0C whereas treatment time was 45, 70 and 90 minutes. Better values of

conductivities were observed for sample with heat treatment time of 45 minutes and

heat treatment temperature of 180 0C. The maximum electrical dc conductivity of the

sample was 0.3386 S-cm-1

at 700 0C in this case.

To further enhance the conductivity, the doping of Gd was done in ceria and

composition made was Ce1-xGdxOδ; x = 0.1, 0.15, 0.2, 0.25. The fluorite F2g band

around 465 cm-1

reconfirmed the Gd doped ceria. No peak of Gd2O3 (480 cm-1

) was

observed. DC conductivity was measured in temperature range 300-700 0C and ac

x

conductivity was determined in frequency range 1 kHz to 3MHz at temperatures 300,

400, 500, 600 and 700 0C. The larger values of conductivities were obtained for

Ce0.75Gd0.25Oδ. The jump relaxation model can be used to explain the dc conductivity

behavior. By jump of ions to available sites, a hopping motion started thus contributing

to dc conductivity. The ‗step‘ ac conductivity in dispersion curves is confirmation of

the grain interior and grain boundary conductivities as ionic conduction is dependent on

the defect formation due to thermal energies which create vacancies to aid in hopping

motion of ions. The maximum conductivity, achieved for Ce0.75Gd0.25Oδ, was 7.4x10-3

S-cm-1

at 700 0C. The thermal conductivity values obtained using Advantageous

Transient Plane Source (ATPS) method was in low thermal conductivity region. The

thermal conduction is dependent on the scattering and mean free path, so the less mean

free path and more scattering gave rise to low conductivity values.

The effect of multiple doping on conductivity was also studied. La and Nd were

co-doped in Gd doped ceria for two samples which showed maximum conductivities in

the earlier studies i.e. Ce0.9Gd0.1Oδ and Ce0.75Gd0.25Oδ. Samples with nominal

compositions Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.1, 0.25) were prepared. The

Ce-O fluorite breathing mode was observed in Raman spectroscopy to confirm the ceria

and doping in ceria. The strong ceria band appeared at ca. 465 cm-1

and weak oxygen

vacancy bands appeared ca. 570 and 600 cm-1

. The formation of oxygen vacancies and

defects was confirmed through Raman spectroscopy. The jump relaxation model is

applicable for dc conductivity and Jonscher power law described the ac conductivity

behavior. The maximum dc conductivity achieved was 1.78 S-cm-1

for Ce0.5Gd0.25

Nd0.25Oδ. The relaxation reorientation peaks can be realized in dielectric constant and

dielectric loss plots which shifted toward higher frequencies with increase in

temperature.

Rare earth hydroxides (R(OH)3) were synthesized by hydrothermal method and

stoichiometric change in composition and morphology was observed. Ce(OH)3,

La(OH)3 and Nd(OH)3 samples were synthesized. XRD confirmed the hexagonal

structures of the prepared samples. The crystallite size corresponding to the most

intense peaks were 18, 33 and 41 nm for Nd-, La- and Ce- hydroxides. SEM revealed

very interesting and fascinating morphologies. Ce(OH)3 has belts like structures,

Nd(OH)3 has needles like structures and La(OH)3 has wires like structures. The growth

of structures can be ascribed to chemical potential, maintained through precipitating

xi

agent, the pressure inside the vessel, the temperature provided for the hydrothermal

treatment and time for hydrothermal treatment. The shape evolution can be explained

by Gibbs-Curie-Wulff model which relate the shape evolution with the face energies.

When the equilibrium energy is obtained for respective faces the Ostwald ripening is

stopped. On heat treatment, the La(OH)3 first converted into LaOOH at ca. 400 0C and

finally into La2O3 at ca. 600 0C as observed in DSC plot. The increase of conductivity

with temperature is evident from the plots. Nd(OH)3 achieved maximum conductivity

and Ce(OH)3 acquired minimum among the three possibly due to smaller crystallite

sizes in the former case. The smaller grains increase the grain boundaries and charges

can pile up on boundaries which increase the conductivity. The corresponding dc

conductivity values of Ce(OH)3, La(OH)3 and Nd(OH)3 were 0.372, 6.648 and 20.369

S-cm-1

, respectively.

The fluorescence characteristics of rare earths with intense emissions and

stabilized structures were observed with Yb, Er, and Tm doping in F based inorganic

matrix NaMnF3. Yb has served as sensitizer and Tm and Er were utilized as activators.

The synthesis of NaMnF3 co-doped with Yb;Er/Tm was successfully achieved through

solvothermal method. The ethylene glycol (EG) was used as stabilizing agent. Another

important feature of this synthesis method was surface functionalization of particles

with the synthesis process in a single step. Also, the choice of precursors of Na & F

and choice of stabilizing agent (EG) rendered the nanostructures to be rods like. The

PEI polymer was used for surface modification. An intense green emission is observed

for NaMnF3: Yb, Er, with increase in Yb concentration and for fixed Er at 2 mol%.

The observed emission was around 550 nm between levels 4S3/2 and

4I15/2. Yb20 Mn78

Er2 revealed red emission at 660 nm between levels 4F9/2 and

4I15/2 which became

intense with increase of Er concentration. With Tm as dopant, NEAR IR emission was

observed at 800 nm between levels 3H4 and

3H6 although blue emission was also

observed at 480 nm between energy levels 1G4 and

3H6.

The highest value of conductivity achieved for Ce0.75Gd0.25Oδ made this material

a potential candidate as an electrolyte for SOFCs. The low thermal conductivities of

R(OH)3 can be utilized in thermal barrier coatings. The pure red emission from Yb20

Mn78 Er2 and presence of Mn made this material prospective applicant in bimodal

bioprobe.

xii

Table of Contents

Chapter 1 Introduction.................................................................................................. 1

1.1 The Nano ................................................................................................................ 2

1.2 Rare Earths ............................................................................................................. 3

1.2.1 Importance and Applications ........................................................................... 4

1.3 Aims and Objectives .............................................................................................. 5

1.4 Thesis Synopsis ...................................................................................................... 7

1.5 Literature Review/Background .............................................................................. 9

Chapter 2 Synthesis Methods ..................................................................................... 15

2.1 Physical and Chemical Methods .......................................................................... 16

2.2 Wet-Chemical Methods........................................................................................ 16

2.2.1 Composite Mediated Hydrothermal Method ................................................. 16

2.2.2 Co-precipitation Method ................................................................................ 18

2.2.3 Sol-gel Method .............................................................................................. 19

2.2.4 Solvothermal Method/Hydrothermal Method ............................................... 20

Chapter 3 Characterization Techniques .................................................................... 21

3.1 Structural and Morphological Analysis................................................................ 22

3.1.1 X-Ray Diffraction .......................................................................................... 22

3.1.2 Scanning Electron Microscopy and Transmission Electron Microscopy ...... 23

3.1.3 Differntial Scanning Calorimetry .................................................................. 23

3.1.4 Raman Spectroscopy ..................................................................................... 24

3.1.5 Thermal Conduction Measurements .............................................................. 24

3.2 Conductivity Measurements ................................................................................. 25

3.2.1 AC Conductivity Measurements ................................................................... 25

3.2.2 DC Conductivity Measurements ................................................................... 28

3.3 Fluorescence Measurements ................................................................................ 29

xiii

Chapter 4 Synthesis of Ceria and Choice of Synthesis Method............................... 30

4.1 Structural Analysis ............................................................................................... 32


4.1.2 Differential Scanning Calorimetry (DSC) ..................................................... 33


4.2 Electrical Measurements ...................................................................................... 34

4.2.1 DC Conductivity ............................................................................................ 34

4.2.2 AC Conductivity ............................................................................................ 36

4.2.3 Dielectric Constant ........................................................................................ 38

4.3 Conclusions .......................................................................................................... 40

Chapter 5 Effect of Synthesis Parameters on Ceria Synthesized by Composite

Mediated Hydrothermal Method ............................................................................... 41

5.1 Structural Analysis ............................................................................................... 42


5.2 Electrical Properties ............................................................................................. 44

5.2.1 DC Conductivity ............................................................................................ 44

5.2.2 AC Conductivity ............................................................................................ 45


5.2.4 Dielectric Loss ............................................................................................... 49

5.3 Raman Spectroscopy ............................................................................................ 51

5.4 Conclusions .......................................................................................................... 52

Chapter 6 Effect of Gd Doping on Conductivity of Ceria ........................................ 53

6.1 Structural and morphological studies ................................................................... 54


6.1.2 Scanning Electron Microscopy ...................................................................... 55



xiv

6.2.1 DC Conductivity ............................................................................................ 57

6.2.2 AC Conductivity ............................................................................................ 58


6.2.4 Dielectric Loss (tanδ) .................................................................................... 64

6.3 Thermal Conduction ............................................................................................. 66

6.4 Conclusions .......................................................................................................... 67

Chapter 7 Conductivity Enhancement in Co-Doped Rare-Earth Oxides .............. 68





7.2.1 DC Conductivity ............................................................................................ 71

7.2.2 AC Conductivity ............................................................................................ 72


7.2.4 Dielectric Loss ............................................................................................... 77

7.3 Conclusions .......................................................................................................... 80

7.4 Comparison Table ................................................................................................ 80

Chapter 8 Synthesis and Thermophysical Characterization of Rare-Earth

Hydroxides .................................................................................................................... 82


8.1.1 Structural Analysis ........................................................................................ 83

8.1.2 Surface Morphology ..................................................................................... 84

8.1.3 Differential Scanning Calorimetry ................................................................ 85

8.2 Electrical measurements ....................................................................................... 85

8.2.1 DC Conductivity ............................................................................................ 85

8.2.2 AC Conductivity ............................................................................................ 87

8.3 Thermal Conduction ............................................................................................. 89

xv

8.4 Conclusions .......................................................................................................... 90

Chapter 9 Synthesis and Fluorescence in NaMnF3: Yb;Er/Tm .............................. 91

9.1 Structural and Morphological Analysis................................................................ 92


9.1.2 Transmission Electron Microscope Analysis ................................................ 92

9.2 Fluorescence Measurements ................................................................................ 93

9.2.1 NaMnF3:Yb;Er............................................................................................... 93

9.2.2 NaMnF3:Yb;Tm ............................................................................................. 95

9.3 Conclusions .......................................................................................................... 97

Chapter 10 Summary and Conclusions ..................................................................... 98

10.1 Summary and Conclusions ................................................................................ 99

10.2 Future Recommendations ................................................................................. 103

Chapter 11 References ............................................................................................... 104

xvi

List of Figures

Figure 1.1 Abundance of elements [9] ............................................................................ 3

Figure 1.2 Representative applications of rare earths (a) in SOFCs (as electrolyte

material) (b) in automobile industry [13]......................................................................... 5

Figure 1.3 Plan of work performed in different parts ...................................................... 6

Figure 2.1 Eutectic point of NaOH-KOH composite ..................................................... 17

Figure 2.2 Schematic of composite mediated hydrothermal synthesis .......................... 18

Figure 2.3 Schematics for synthesis process of co-precipitation method ...................... 19

Figure 3.1(a) Unit cell of CeO2, light atoms are O2-

and dark atoms are Ce4+

(b) Crystal

structure of ceria ............................................................................................................ 23

Figure 3.2 Block diagram of Advantageous Transient Plane Source (ATPS) method..25

Figure 3.3 Polarization mechanisms for dielectric mediums [102] ............................... 26

Figure 3.4 Types of defects helpful in ionic transport a) oxide vacancy in perovskite

structure, b) edge dislocation c) defective grain boundaries where space charges pile up

[104] ............................................................................................................................... 27

Figure 3.5 Conductivity as a function of frequency ...................................................... 27

Figure 3.6 Basis of the jump relaxation model, (a) ions (O) on a sublattice, (b) the

effective single particle potential, (c) development of potential after a hop [107]. ....... 29

Figure 4.1 XRD pattern of ceria prepared by different wet-chemical methods ............. 32

Figure 4.2 DSC plot of ceria synthesized with CMH method ....................................... 33

Figure 4.3 Raman spectrum of ceria synthesized by different wet-chemical methods . 34

Figure 4.4(a) DC conductivity of CeO2 prepared by CMH and sol-gel method ........... 35

Figure 4.4(b) DCconductivity of CeO2 prepared by co-precipitation method …….….35

Figure 4.5(a) AC conductivity of ceria synthesized by CMH method .......................... 37

Figure 4.5(b) AC conductivity of ceria synthesized by co-precipitation method……..37

Figure 4.5(c) AC conductivity of ceria synthesized by sol-gel method……………….38

Figure 4.6(a) Dielectric constant of ceria synthesized with CMH method.................... 39

Figure 4.6(b) Dielectric constant of ceria synthesized by co-precipitation method …39

Figure 4.6(c) Dielectric constant of ceria synthesized by sol-gel method…………….40

Figure 5.1 XRD patterns of CeO2 samples synthesized by different synthesis conditions

........................................................................................................................................ 43

xvii

Figure 5.2 Temperature dependent dc conductivity of the prepared samples ............... 44

Figure 5.3 AC conductivity (ζac S-cm-1

) as a function of frequency at different

temperatures for all samples (H14, H17, H19, H24, H27 and H29). ............................. 46

Figure 5.4 Comparison of ac conductivity (ζac S-cm-1

) at 3 MHz for all samples at

500 0C, 600

0C and 700

0C. ............................................................................................ 47

Figure 5.5 Dielectric constant (ε΄) as a function of frequency at different temperatures

for all the samples (H14, H17, H19, H24, H27 and H29). ............................................ 48

Figure 5.6 Dielectric constant at 3 MHz for all the samples at different temperatures

(500 0C, 600

0C and 700

0C). ......................................................................................... 49

Figure 5.7 Dielectric loss (tanδ) as a function of frequency at different temperatures for

all samples. ..................................................................................................................... 50

Figure 5.8 Raman spectra of the prepared ceria samples at 514 nm excitation laser line

........................................................................................................................................ 51

Figure 6.1 X-ray diffraction patterns of Ce1-xGdxOδ (x= 0.10- 0.25) ............................ 55

Figure 6.2 Scanning electron micrographs of Ce1-xGdxOδ (x= 0.10 - 0.25) .................. 56

Figure 6.3 Raman spectroscopy of Ce1-xGdxOδ (x= 0.10 - 0.25) ................................... 57

Figure 6.4 DC conductivity of Ce1-xGdxOδ (x= 0.10-0.25) as a function of temperature.

........................................................................................................................................ 58

Figure 6.5(a) AC conductivity of CG10 at different temperatures ................................ 59

Figure 6.5(b) AC conductivity of CG15 at different temperatures……………………60

Figure 6.5(c) AC conductivity of CG20 at different temperatures……………………60

Figure 6.5(d) AC conductivity of CG25 at different temperatures……………………61

Figure 6.6(a) Dielectric constant of CG10 at different temperatures ............................ 62

Figure 6.6(b) Dielectric constant of CG15 at different temperatures …………………62

Figure 6.6(c) Dielectric constant of CG20 at different temperatures …………………63

Figure 6.6(d) Dielectric constant of CG25 at different temperatures ………………...63

Figure 6.7(a) Dielectric loss (tanδ) of CG10 at different temperatures……………….64

Figure 6.7(b) Dielectric loss (tanδ) of CG15 at different temperatures……………….65

Figure 6.7(c) Dielectric loss (tanδ) of CG20 at different temperatures……………….65

Figure 6.7(d) Dielectric loss (tanδ) of CG25 at different temperatures……………….66

Figure 7.1 X-ray diffraction patterns of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,

0.25), the starred peaks are of Nd2O3 (*) and La2O3 (#). ............................................... 69

xviii

Figure 7.2 Raman spectroscopy of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x=0.10,0.25)

........................................................................................................................................ 71

Figure 7.3 DC conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) as

function of temperature. ................................................................................................. 71

Figure 7.4(a) AC conductivity of Ce0.8Gd0.1 La0.1Oδ(CGL10) at different temperatures

…………………………………………………………………………………………73

Figure 7.4(b) AC conductivity of Ce0.5Gd0.25La0.25Oδ (CGL25) at different temperatures

……….………………………………………………………………………………...73

Figure 7.4(c) AC conductivity of Ce0.8Gd0.1Nd0.1Oδ (CGN10) at different temperatures

……….………………………………………………………………………………...73

Figure 7.4(d) AC conductivity of Ce0.5Gd0.25Nd0.25Oδ(CGN25) at different temperatures

……….………………………………………………………………………………...73

Figure 7.5(a) Dielectric constant of Ce0.8Gd0.1La0.1Oδ(CGL10) at different temperatures

........................................................................................................................................ 75

Figure 7.5(b) Dielectric constant of Ce0.5Gd0.25La0.25Oδ(CGL25) at different

temperatures ...…………………………………………………………………………75

Figure 7.5(c) Dielectric constant of Ce0.8Gd0.1Nd0.1Oδ(CGN10) at different

temperatures……………………………………………………………………………75

Figure 7.5(d) Dielectric constant of Ce0.5Gd0.25Nd0.25Oδ (CGN25) at different

temperatures……………………………………………………………………………75

Figure 7.6(a) Dielectric loss of Ce0.8Gd0.1La0.1Oδ (CGL10) at different temperatures..78

Figure 7.6(b) Dielectric loss of Ce0.5Gd0.25La0.25Oδ(CGL25) at different temperatures.78

Figure 7.6(c) Dielectric loss of Ce0.8Gd0.1Nd0.1Oδ(CGN10) at different temperatures..78

Figure 7.6(d) Dielectric loss of Ce0.5Gd0.25Nd0.25Oδ(CGN25) at different temperatures

…………………………………………………………………………………………79

Figure 8.1 XRD pattern of Ce(OH)3, Nd(OH)3 and La(OH)3 samples .......................... 83

Figure 8.2 SEM micrographs of Ce(OH)3, Nd(OH)3 and La(OH)3 samples ................. 84

Figure 8.3 DSC plot of La(OH)3 .................................................................................... 85

Figure 8.4(a) DC conductivity as a function of temperature for Ce(OH)3 sample ........ 86

Figure 8.4(b) DC conductivity as a function of temperature for Nd(OH)3 sample…....86

Figure 8.4(c) DC conductivity as a function of temperature for La(OH)3 sample……87

Figure 8.5(a) AC conductivity as a function of frequency of Ce(OH)3 sample ............ 88

Figure 8.5(b) AC conductivity as a function of frequency of Nd(OH)3 sample……....88

Figure 8.5(c) AC conductivity as a function of frequency of La(OH)3 sample……….89

Figure 9.1 XRD pattern of PEI-capped NaMnF3:Yb,Er ............................................... 92

xix

Figure 9.2 TEM images (a, b) for PEI-NaMnF3:Yb, Er ; Yb:Er 20/2 Mn 78 mol %

sample and (c, d) for PEI-NaMnF3:Yb, Er ; Yb:Er 60/2 Mn 38 mol % sample ........... 93

Figure 9.3 Upconversion spectra of NaMnY3:Yb,Er capped with PEI for different

molar percentage. ........................................................................................................... 94


molar percentage. ........................................................................................................... 95

Figure 9.5 Upconversion of Tm doped PEI capped NaMnF3: Yb,Tm. ......................... 96

Figure 9.6 Dispersivity of NaMnF3:Yb;Er/Tm in water ................................................ 96

xx

List of Tables

Table 4.1 Crystallite size and lattice constant crossponding to the most intense peak .. 33

Table 4.2 Comparison of ac conductivity, dc conductivity and dielectric constant for the

ceria samples synthesized by CMH, co-precipitation and sol-gel method. ................... 36

Table 5.1The nomenclature designed for the samples at different optimized parameters.

........................................................................................................................................ 42

Table 5.2 The average crystallite size and that of the most intense peaks (D (1 1 1)) along

with lattice constants (a) for the samples. ...................................................................... 43

Table 5.3 DC conductivity (ζdc S-cm-1

) at different temperatures of the samples ........ 45

Table 5.4 Frequency dependent ac conductivity (ζac S-cm-1

) at different temperatures 47

Table 5.5 Frequency dependent dielectric constant (ε΄) and dielectric loss tangent (tanδ)

values at different temperatures ..................................................................................... 51

Table 6.1 The crystallite sizes (Ds (111) = Crystallite size corresponding to the most

intense peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding

to the most intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average

crystallite size estimated by Scherrer formula, Dw = Average crystallite size estimated

by Stokes and Wilson‘s formula), lattice constant and porosity of Ce1-xGdxOδ (x= 0.10-

0.25) ............................................................................................................................... 55

Table 6.2 Activation energies, ‗s‘ and conductivities of Ce1-xGdxOδ (x= 0.10 - 0.25) at

different temperatures .................................................................................................... 58

Table 6.3 AC conductivity at 3 MHz frequency for different temperatures of

Ce1-xGdxOδ (x= 0.10 - 0.25) samples ............................................................................. 59

Table 6.4 Values of dielectric constant at 1 kHz and 3 MHz for different temperatures

of Ce1-xGdxOδ (x= 0.10 - 0.25) samples. ........................................................................ 61

Table 6.5 Variation in tanδ as a function of frequency at different temperatures for

Ce1-xGdxOδ (x= 0.10 - 0.25) samples ............................................................................. 64

Table 6.6 Values of thermal conductivity and thermal diffusivity for Ce1-xGdxOδ

(x= 0.10 - 0.25) at room temperature ............................................................................. 66

Table 7.1 The crystallite sizes (Ds (111) = Crystallite size corresponding most intense

peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding most

intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average crystallite size

estimated by Scherrer formula, Dw = Average crystallite size estimated by Stokes and

xxi

Wilson‘s formula), lattice constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,

0.25). .............................................................................................................................. 70

Table 7.2 DC conductivities of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) at

different temperatures .................................................................................................... 72

Table 7.3 Values of dielectric constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ

(x = 0.10, 0.25) at 500, 600 and 700 0C for 1 kHz and 3 MHz ...................................... 75

Table 7.4 Comparison of conductivity values with literature. ....................................... 81

Table 8.1 Crystallite size corresponding to the most intense peak and lattice constants

for Ce(OH)3, Nd(OH)3 and La(OH)3. ............................................................................ 84

Table 8.2 DC conductivity as a function of temperature for hydroxide samples .......... 87

Table 8.3 AC conductivity as a function of temperature for hydroxide sample ............ 89

Table 8.4 Thermal conductivity and thermal diffusivity of R (OH)3 ............................ 89

xxii

List of Abbreviations

ATPS Advantageous Transient Plane Source

β Full Width at Half Maximum

CMH Composite Mediated Hydrothermal Method

CG10 Ce0.9Gd0.1Oδ

CG15 Ce0.85Gd0.15Oδ

CG20 Ce0.8Gd0.2Oδ

CG25 Ce0.75Gd0.25Oδ

CGL10 Ce0.8Gd0.1La0.1Oδ

CGN10 Ce0.8Gd0.1Nd0.1Oδ

CGL25 Ce0.5Gd0.25La0.25Oδ

CGN25 Ce0.5Gd0.25Nd0.25Oδ

Cop. Co-precipitation

Ds Crystallite size calculated by Scherrer formula

Dw Crystallite size calculated by Stokes and Wilson formula

D s (1 1 1) Crystallite size calculated by Scherrer formula corresponding to

most intense peak

D w (1 1 1) Crystallite size calculated by Stokes and Wilson formula

corresponding to most intense peak

DSC Differntial Scanning Calorimetry

Strains

εo Permittivity of free space

ε’ Dielectric constant

EG Ethylene Glycol

Thermal Diffusivity

Thermal Conductivity

λ Wavelength

NIR Near Infrared

PEI Polyethylenemine

xxiii

SEM Scanning Electron Microscopy

SOFCs Solid Oxide Fuel Cells

Sol. Sol-gel

TEM Transmission Electron Microscopy

tanδ Dielectric loss

UC Upconversion

UV Ultraviolet

XRD X-ray Diffraction

xxiv

List of publications

Papers part of the thesis

1. Comparative study of nano crystalline ceria synthesized by different wet chemical

methods

A. Abdullah, A. S. Saleemi and M. Anis-ur-Rehman, J. Supercond. Nov. Magn.

(2013) DOI 10.1007/s10948-013-2261-x

2. Conductivity dependence on synthesis parameters in hydrothermally synthesized

ceria nanoparticles

M. Anis-ur-Rehman, A. S. Saleemi and A. Abdullah, J. Alloy. Comp. 579 (2013)

450-456.

3. Synthesis and conductivity of nanocrystalline Ceria

A. S. Saleemi, A. Abdullah and M. Anis-ur-Rehman, J. Supercond. Nov. Magn. 26

(2013) 1065-1069

4. Synthesis and structural properties of Ce(OH)3 for useful application

M. Anis-ur-Rehman and A. Abdullah, J. Supercond. Nov. Magn. 24 (2011) 1095-

1098

Other publications

1. Electrospun proficient polymer based nanofibers with ceramic particles,

A. S. Saleemi, A. Abdullah and M. Anis-ur-Rehman, J. Supercond. Nov. Magn. 26

(2013) 1027-1030

2. Effects of Sintering Temperature on Structural and Electrical Transport Properties

of Zinc Ferrites Prepared by Sol-Gel Route,

M. Anis-ur-Rehman, M.A. Malik, I. Ahmad, S. Nasir, M. Mubeen, A. Abdullah,

Key Engineering Materials 510–511 (2012) 585-590

3. Preparation and thermoelectric studies of spinel ferrites in ―Thermoelectric Power‖

M. Anis-ur-Rehman and A. Abdullah, Ed. W. P. Dempsey, Nova Science

Publishers, NY, USA, (2011) 363-380

1

Chapter 1 Introduction

2

Introduction

1.1 The Nano

Nanoscience and nanotechnology mainly deals with synthesis, characterization,

probe, and utilization of nanostructured materials. Significant changes happen in the

physical and chemical properties of nanomaterials in comparison with those of the bulk

materials. The influence of nanoscale can be observed on the structural properties,

kinetics, reaction, governing forces and chemical makeup of nanostructures. Novel

devices and technologies can emerge by apt switching of the properties and response of

nanostructures. The significant foci in nanoscience are related with effects of size,

evolution of shape and quantum confinement [1].

The synthesis leitmotifs relating to nanoscience and nanotechnology are twin:

one is the bottom-up approach which is the reduction of the mechanisms as voiced by

Feynman, in 1959 lecture that ―there is plenty of room at the bottom‖ and the other is

the methodology of the self-assembly of molecular components which is affiliated with

Jean- Marie Lehn [2].

Taniguchi used the word Nanotechnology, first time, in 1974 in a paper

entitled ―On the basic concept of Nanotechnology‖ although the nanomaterials are

being used specifically in field of chemistry as old as chemistry itself e.g. the medieval

stain glass which utilized the precious metal colloids and use of cement by Romans [2-

3]. The Nanoscience can be defined like ―Nanoscience is the study of phenomena and

manipulation of materials at atomic, molecular and macromolecular scales, where

properties differ significantly from those at a larger scale‖ whereas ―Nanotechnologies

are the design, characterization, production and application of structures, devices and

systems by controlling shape and size at nano meter scale‖ [4].

From point of view of the length scale, the NASA defined it as ―The creation of

functional materials, devices and systems through control of matter on the nanometer

scale (1-100 nm) and exploitation of novel phenomena and properties (physical,

chemical, and biological) at that length scale‖ [5]. Normally the 1-100 nm length scale

is considered to be the nanoscale although it may vary depending on the property of

interest under observation [6]. Rogers et al have defined the Nanotechnology as;

―Nanotechnology means putting to use the unique physical properties of atoms,

molecules and other things measuring roughly 0.1 to 1000 nanometers‖ [2]. Size effects

3

are a vital feature of nanomaterials. The effects dependent on size trace the evolution of

structure, thermodynamics, electronics, spectroscopic and chemical characteristics of

these restricted systems.

There is unlimited vigor in the area of nanoscience & nanotechnology and vast

prospects as the nanoscience is an interdisciplinary field covering physics, chemistry,

biology, materials and engineering. New materials, new scientific observations and

innovative technological potentials are emerging from the collaboration among

scientists with different qualifications. Along with the academia, industry is an equally

important beneficiary of nanoscience and nanotechnology. The global market of

nanotechnology is expected to be $70-160 billion by 2015 [7]. The society is also

getting benefits as the nanotechnology has applications in fields like health care,

medicine, environment and energy [1,8].

1.2 Rare Earths

Rare earths are 4f-block elements which are also called Lanthanides and/or

Lanthanones. They get their name ―Rare earth‖ due to their extraction from oxides

which were known as earths and these were considered to be rare although now many

of these are plentiful than other elements in the periodic table.

Figure 1.1 Abundance of elements [9]

4

Owing to their close resemblance of chemical and physical properties all the

fifteen elements from La to Lu (51-71) are taken as the members of lanthanide series.

Rare earths form one of the major industries in the world and market of $1.4 billion in

2010 is expected to reach $4.1 billion by 2017 [10].

These elements form the tripositive lanthanide cations. These show oxidation

states of +2, +3 and +4. The +3 is considered as more stable than di- and tetra-positive

cations. Although the change in oxidation state can change the properties very

remarkably but in general, these elements show metals like behavior. Rare earths have

electrode potential values comparable to those of alkaline earths and the rare earth

oxides resemble those of alkaline earth oxides. These oxides are insoluble in water but

absorb CO2 and H2O. The rare earth hydroxides are not amphoteric but basic like

oxides and have hexagonal structure and are definitely not hydrous oxides. These also

absorb CO2 and on heating decompose to form oxides.

A characteristic feature of rare earths is what is known as lanthanides

contraction and is the decrease in atomic and ionic radii. This decrease in size is more

evident in ions but not so regular in case of atoms. An interesting effect in rare earths is

that, the additional electron enters 4f- subshell but not in the valence shell. The mutual

shielding effect of 4f electrons is very diminutive due to the shape of f sub shell which

is very much diffused although d electrons are larger than f electrons. However, with

the increase in each step the atomic number increases, whereas for the 4f electrons,

there is no similar increase in the mutual shielding effect. That‘s why; the atomic and

ionic radii go on decreasing from La to Lu due to the electrons in the outer most shell

practice increasing nuclear attraction. The chemical properties of an element or an ion

depend on the size of the atom or ion. The greater the atomic or ionic radius, greater is

the ease with which the ions or atoms will lose electrons [11].

1.2.1 Importance and Applications

Rare Earth compounds are a large group of functional materials with varied

applications in electric, magnetic, optical and catalytic fields mainly due to their unique

4f electrons. Owing to the amazing reducing property of rare earths, they are used in

metallothermic reactions. Different brands of steel are made through the alloys of these

materials. The Rare earth oxides are widely used for decolorizing glass. Also, these can

absorb ultra violet rays. Owing to their higher melting points the rare earth oxides are

5

used in refractories. Some other applications include; abrasives, paints, textile and

leather industries, lamps, oxidizing agents and ferromagnetic garnets among others.

Rare earth nanocrystals have major interest of scientists and engineers with their special

properties such as consistent optical characteristics and boosted catalytic acts allowing

them to aid in preparation of functional assemblies. The fruitful uses of rare earth bulk

materials as phosphors, magnets, catalysis, superconductors, electrolytes and electrode

materials in SOFCs, hard alloys, etc. have stimulated great research interest in their

nanoscale complements [12].

(a)

(b)

Figure 1.2 Representative applications of rare earths (a) in SOFCs (as electrolyte

material) (b) in automobile industry [13]

1.3 Aims and Objectives

This system (Rare earth compounds) is chosen for its numerous applications in

various technological, industrial and research purposes. The anticipated findings of this

6

proposed work are many fold; new synthesis routes, study of the varying behavior of

various properties at nano scale and functionalization of nano structures. The rare earth

compounds have various applications which are very attention-grabbing and beneficial

from research point of view; the R-oxides have gas sensing and electrolyte material for

SOFCs applications and R-hydroxides are potential candidates for biolabeling

applications.

Rare earth compounds including R-oxides and R-Hydroxides are synthesized.

Structural, electrical and thermal properties of synthesized compounds are investigated.

The transport phenomenon is examined in these compounds. An attempt is made to

explain the obtained results with some appropriate theories. The plan of work is

detailed in figure 1.3.

Figure 1.3 Plan of work performed in different parts

The major focus in these compounds was to seek their application to be

functional in areas of energy and nanobiotechnology, namely, electrolyte material for

SOFCs and bimodal bio probe for small animals imaging. A safe long term energy

resource is one of the major challenges in the 21st century. Energy is vital to universal

human development including the ecology, economic growth, occupation, success and

parity. The disappearing fossil based reserves and other related issues (environment,

social) demand for dedicated and vigorous efforts to change the present energy system

to sustainable one. The SOFCs, which convert chemical energy into electrical are such

CeO2

I. Rare earth Oxides

Ce1-x

GdxO

X=0.10, 0.15, 0.20, 0.25

Ce1-2x

GdxLa

xO

X=0.10, 0.25

Ce1-2x

GdxNd

xO

X=0.10, 0.25

II. Rare Earth doped & co-doped Ceria

III. Rare Earth Hydroxides

Ce(OH)3

Nd(OH)3

La(OH)3

NaMnF3:Yb,Er/Tm

IV. Rare Earth doped F based inorganic Matrix

7

an area of interest of researchers as alternate to fossil fuels. Rare earth doped ceria

based compounds are widely studied as electrolyte material for SOFCs. The researchers

are trying to enhance the ionic conductivity and lower the operational temperature of

these cells. Various strategies are utilized to increase the conductivity like material,

doping, multiple doping, composites, decrease in grain size and synthesis methods. In

this research work synthesis methods are compared for the achievement of enhanced

conductivity in ceria. The doping and multiple doping of rare earths in ceria are also

studied for the same.

The next generation personalized and targeted drugs are blessings of the

marriage of nano and bio technologies. The advent of functionalized, nontoxic and bio

compatible materials with sophisticated and precise, smart, par excellence tools and

devices have made researchers believe in a disease free world. The nanomaterials are

being used for imaging, diagnosis and treatment of various diseases these days. Rare

earth doped therapeutics and imaging probes are one of the widely synthesized and

fascinated materials. The physical properties significant to materials often made these

materials utilized for more than one modalities. Another focus in this research work

was to synthesize materials having potential for bimodalities. The tuning of emission

bands in rare earth doped inorganic matrix is also studied in this work.

1.4 Thesis Synopsis

Thesis is constructed on introduction, synthesis & characterizing techniques and

on different parts of research work. Introduction to chapter contents is presented here.

Chapter 1: The introduction to nano, rare earths, importance of rare earths and

objectives of the research work are discussed in this chapter. The vital application of

rare earths in the fields of energy and biotechnology grew the interest to work on

these materials.

Chapter 2: The requirements for the synthesis of functional nanomaterials are phase

purity, ease to prepare complex compositions and desired structures, The synthesis

techniques are categorized as physical methods and chemical methods or can be

termed as bottom-up and top-down approaches. The wet chemical methods have

provided easy, simple, often low temperature and precise control of synthesis of

nanomaterials. These different synthesis methods are discussed in this chapter and

8

brief description of wet chemical methods is given. Representative synthesis

schemes utilized in this research work are also presented.

Chapter 3: The progress in nanotechnology was highly based on new characterization

tools as the study of physical properties at nanoscale was not possible using

conventional tools. Physical, chemical, optical, thermal and electrical properties are

being studied with the aid of new state of the art characterization tools with

approaching- to- ideal precision in measurements. The importance of tools is

manifested by giving brief introduction to those employed in this research work.

Corresponding theories / models and formulae are also discussed.

Chapter 4: Ceria is one of the most studied and industrially produced rare earth

compounds. The significant applications in various fields make this compound one

of the most interesting materials for researchers. The synthesis of ceria is done using

range of wet chemical methods. The main focus of interest (conductivity) is

compared in prepared samples as a function of synthesis method. Conclusion is

drawn for the choice of better method for synthesis of ceria to achieve enhancement

in conductivity.

Chapter 5: The optimization of some synthesis process is very much important as the

properties of materials also depend on the synthesis process. A facile wet chemical

route, composite mediated hydrothermal method, was utilized to synthesize ceria,

focused for its application as electrolyte material. Different synthesis parameters

were tuned and results of the conductivity were compared to have idea of better set

of parameters.

Chapter 6: The ceria based compounds are widely used as electrolyte material in

SOFCs in quest by researchers to seek alternate energy sources instead of dying

conventional sources. The doping of cations in ceria influences the conductivity and

normally aid in enhancement in conductivity. The doping of Gd in ceria was done

and its effect on different properties is discussed in this chapter.

Chapter 7: The samples which showed maximum conductivity were further co-doped

with other rare earths (La, Nd) and different properties were observed. Multiple

doping is another noted process to help increase the conductivity. The results are

discussed in this chapter.

9

Chapter 8: The stoichiometry of oxygen in rare earth compounds is one of the very

important features as the properties like reduction, catalysis and conductivity are

highly dependent on it. The stoichiometry change with synthesis method also

influences the crystal structure, for example, R-oxide has cubic structure and R-

hydroxide has hexagonal structure at nano regime whereas the starting material has

cubic structure in both the cases. The morphology becomes attention grabbing as

many 1-D structures can be observed like rods, wires and belts. 1-D structures are

swiftly growing attention due to their fascinating properties and exclusive

applications. Nanowires are developing as important building blocks helping as

interconnects and active components in nanoscale electronic, magnetic and photonic

devices. The synthesized 1-D structures of rare earth hydroxides (La, Ce, Nd) and

their properties are discussed in this chapter.

Chapter 9: The blissful merger of nano and bio technologies promises the diagnosis

and treatment of various diseases. The area of multimodal imaging utilizes physical

materials at nanoscale to seek information from living organisms. Rare earth doped

inorganic matrices are being prepared in this regard. One such material with

potential of optical and magnetic imaging NaMnF3: Yb,Er/Tm, was synthesized and

its different properties are discussed in this chapter.

Chapter 10: Conclusions are drawn in this chapter and the future recommendations are

discussed.

Chapter 11: References and citations are provided in this chapter.

1.5 Literature Review/Background

Rare earth compounds are a large group of materials having multi applications

in fields like electric, magnetic, optical and catalysis. They have got immense interest

with special properties like optical and catalytic, reduction. Their usage in different

fields is increasing day by day due to synthesis techniques and functionalization. The

solution based or wet chemical synthesis routes have made easier, their synthesis and

functionalization and in many cases both in single step although high temperature solid

state reaction methods (bulk synthesis) are also being used. With advent of

nanotechnology the rare earth applications like phosphors, magnets, catalysis,

superconductors, electrolytes and electrode materials in solid oxide fuel cells, hard

10

alloys etc are being made with nanoscale rare earth compounds. Although the intra-4f

transitions are independent of size or shape of the materials but with reduction of size

nanocrystals may display electron-photon interaction. Also, the increase in the surface

areas improve the catalytic activity. The size and shape at nanoscale also influence the

application oriented studies, e.g. for small animals imaging , the probes need to be of

nanometer scale and for a smart, compact design of fuel cell nanometer scale is desired

[14-19].

Ceria based rare earth compounds have far-reaching applications as UV

absorbers, 3-way catalysis, and electrolyte in solid oxide fuel cells. Ceria is one of the

most studied rare earth and also one of the major industrially produced compounds with

Carbon, Titanium oxide, Silicon and Zirconia [2]. A large number of workers have

synthesized ceria utilizing different methods and by varying crystal growth techniques

[20-26].

One dimensional nanostructures include rods, wires and belts. The interesting

features mainly conduction and optical make these structures property of interest for

researchers. The template, supersaturation and capping are deciding parameters for the

directional growth. The rare earth hydroxides, orthophosphate and orthovanadates are

1-D rare earth compounds among others. Rare earth hydroxides have hexagonal

structure with P63/m which also becomes evident in morphology of the structures.

Hydrothermal method is widely utilized to get 1-D structures. Rare earth hydroxides

were prepared with hydrothermal method reported by Wang and Li [27]. KOH was

used to make precipitating gels at room temperature and then hydrothermal treatment

was given at 180 0C. As a representative reaction, 5 mol KOH was used to precipitate

the solution. When pH was between 6 and 7 nanosheets were observed for Sm(OH)3

and nanorods for pH 9-10. The crystal structure (hexagonal) of R(OH)3 manifest itself

through anisotropic crystal growth. The 1-d structures formed due to the chain of the

OH- and R 3+ cations connected to each other.

The films and quantum wells like structures fall in category of 2-d structures.

Mainly, these are made using molecular beam epitaxy, physical vapor deposition or

surface ligands. Hydrothermal synthesis provides facilities 2-d nonstructural growth

without surfactants. The control of pH with other parameters decides the visible faces

[28-30].

11

Re2O3 or RO2 have three phases; the hexagonal phase, monoclinic phase and

cubic phase. These rare earth oxides were mainly prepared using solid state reaction

methods as the hydrothermal methods mostly went through hydration. The synthesis of

Gd2O3 nanoplates in organic solvents is starting from gadolinium acetate to the thermal

decomposition of rare earth benzoylacetonate, acetylacetonate (acac) and acetate

precursors [25, 28-30]. Rare earth fluoride nanocrystals can be obtained by the

thermolysis. Different sizes and shapes can be obtained through reaction temperature,

and reaction time. The phase, cationic radii and synthesis parameters can be varied to

have shape control of RF3. Light rare earths fluorides are formed in trigonal, truncated

trigonal and hexagonal shapes. Large cations like La favored trigonal shapes whereas

small cations form hexagonal phases. In LaF3 short reaction times favored trigonal

phases whereas longer times favor hexagonal shapes. The shape evolution can be

explained through Ostwald ripening [25, 31-32].

R(OH)3 nanotubes can be observed through corrosion in RNi5 with help of

KOH. Eu2O3 assemblies can also be obtained with carbon nanotubes as templates in a

temperature range of 80 0C -180

0C. In this way rare earth oxides, oxysulfidde and

oxyfluoride nanotubes can be synthesized [33-36]. Yada at al has reported synthesis of

rare earth oxides using sodium dodecylsufate as surfactant [37]. The nitrates and

chlorides of rare earths can be mixed with urea, surfactant and water at 40 0C, and then

heating at 80 0C, the hydrolysis of urea raises pH values and precipitate rare earth

hydroxide. Rare earth nanotubes are thus obtained with smaller inner diameters of 3nm

after calcination.

Surfactant free synthesis of nanotubes of rare earth hydroxides is also reported.

Xu et al have reported hydrothermal synthesis of dysprosium and terbium hydroxides

from colloidal hydroxide precipitation at 120-160 0C. With similar procedures CeO2

and Y2O3: Er has also been reported. The nanotubes formation can be achieved with

combination of low temperature and high basicity and high temperature and low

basicity [38].

Colloidal up conversion nanoparticles of rare earth phosphates, oxides and

oxysulfides have been reported. NaRF4 are widely studied material. It has two

polymorphs, cubic α-NaRF4 and hexagonal β-NaRF4 and β phase is found to be

excellent up-conversion hosts. OA/OM/ODE solvents are used to get NaRF4 with

12

controlled size composition and surface state. The phase transitions are dependent on

Na/R ratios, solvent compositions, reaction temperatures and time. For β phase high

temperatures, long time and large Na/R ratios are required [39-45].

Adding aliovalent cations produces oxygen vacancies which increase the

conductivity. In case of ceria, Gd or Sm doping is widely studied. In addition to single

doping multiple doping is more effective in increase of conductivity [46-47]. Lee et al

have studied the CeO2 and Sc2O3 co-doped with ZrO2 for conductivity improvement

and phase stability. They have reported that the system showed much higher

conductivities than YSZ at same temperature range [48].

Ralph and coworkers reported the effect of oxide addition for Gd doped ceria on

different concentrations. With addition of Ca, Fe and Pr. The conductivity increased

than the simple Gd doped ceria [49-50]. Maricle et al have studied the Pr and Sm

doping in Ce0.8Gd0.19M0.01O2 and reported that by the two orders of magnitude, oxides

addition improved the lower oxygen partial pressure limit [51].

Mori and coworkers introduced the concept of effective index in ceria based

compounds. The effective index is multiple of ratios of average ionic radius to oxygen

ion radius and dopant radius to host radius. They observed increase in conductivity in

reducing atmospheres [52].

The decrease in defect formation energy improved the conductivity in

nanocrystalline ceria as described by Chiang et al [53].

Thermal decomposition of ceria complexes is utilized by Veranitisagul et al to

prepare Gd doped ceria, sintered at 1500 0C. The maximum conductivity achieved, for

Ce0.85Gd0.15O1.925, was 0.025 S/cm at 600 0C [54].

The effect of microwave heating and conventional heating on conductivity is

reported by Acharya [55]. Nanosized Dy doped ceria was prepared by combustion

method and sintering was done with microwave heating and resistive heating. The MH

heating improved the conductivity of the material.

Dielectric and electrical properties are observed in Gd doped ceria synthesized

by sol-gel method reported by Liu. The composition studied was Ce0.8Gd0.2O2-δ [56].

Anderson et al have theoretically optimized the ionic conductivity in ceria with

different dopants. They have predicted the relation between defect association and ionic

13

conductivity. The balance between elastic and electronic defect interactions is required

for low association and high conductivity [57].

Balaguer et al have synthesized different rare earth doped ceria with co-

precipitation method. Gd, La, Tb, Pr, Eu, Er, Yb and Nd were used as dopants. The

addition of Co is also studied. The samples with Pr and Eu doping showed higher

values of conductivity. The same compounds with Co addition showed higher values of

conductivity [58].

Hu et al have studied the effect of dopants on energy of oxygen vacancy

formation theoretically. They observed that the low valence dopants lower the energy

of oxygen vacancy formation. This effect occurred due to the creation of hole at the top

of valence band [59].

Li and coworkers studied the defect formation using Raman in doped ceria. The

dopants were Gd, Zr, La, Sm, Y, Lu and Pr. The samples were synthesized by citrate

sol-gel method. They observed two defect sites, the difference of ionic valence state

and ionic radii gave rise to creation of defect sites [60].

Synthesis of ceria with co-precipitation method is described by Shih et al. The

influence of calcination temperature is observed on crystallite size. With the increase in

calcination temperature the crystallite size is found to be increased [61].

The effect of grain size on conductivity is observed by Lenka and coworkers.

They studied that at low temperatures smaller grain size show high conductivities

whereas at higher temperatures larger grain size show higher conductivities [62].

Ren at al have reported the synthesis of hexagonal NaYF4:Ce, Tb, Gd

nanocrystals. The solvothermal process is used for the synthesis with PEI as

functionalization agent. The nanocrystals showed efficient fluorescence and T1 contrast

MRI [63].

The synthesis and biological studies of amine-functionalized Er doped La(OH)3

nanoparticles is reported by Sun et al [64]. The in vitro experiments with HeLa cells

and cytotoxicity have been studied. Mu et al [65] described the synthesis and

photoluminescence of La(OH)3. The electrical and photoluminescence behaviors of

La(OH)3 nanobelts are discussed by Zuo et al with Ce and Er used as dopants [66].

The cytotoxicity and luminescence of Gd(OH)3 nanostructures are discussed by

Hemmer et al [67]. The in vitro toxicity analysis was done with human lung A549 and

14

Caco2 cells. The photoluminescence of Gd2O3 nanocrystals doped with Eu and Tb is

reported by Seo et al [68]. The review by Mader et al included La2O3 as upconversion

material with Yb-Eras dopant ions [69].

Amongst others, an important and well-studied rare earth system is Y2O3. Das

and Tan have reported the rare earth doped and codoped Y2O3 as potential bioimaging

probes. The photoluminescence, upconversion and cytotoxicity of amine functionalized

nanoparticles are reported. Tb, Eu and Er are used as dopants. The cell viability data of

nanoparticles attached with Hep-G2 cells has been given [70]. The photoluminescence

in Y2O2S nanotubes is reported by Wang and Li [35]. The upconversion and down

conversion is obtained by doping with Yb-Er. Tb- Y2O3 nanocrystals synthesis and

luminescence properties are reported by Zhang et.al. The influence of different alkyl

amines is discussed [71]. The cell viability data and luminescence and magnetization

are also discussed [72]. Yang et al [73] used Ce-Tb and Yb-Er ion pairs for down- and

upconversion in LaF3 and NaLaF4. The other systems mentioned by Mader et al are

NaYF4, NaYbF4, CaF2, GdOF, BaTiO3, Lu2O3 and Y2O3 whereas Yb, Er, Tm and Ho

were used as dopants [69].

The review by Yuan et al included Eu, Sm, Tb and Dy complexes with β-

diketone and aromatic-derivative ligands as luminescence probes. The nanoparticles of

Ho- Y2O3, Er/Yb-ZrO2, Er/Yb-La2(MoO4)3, Tm/Yb-LuPO4 and Er/Yb-NaYF4 are also

mentioned [74]. The review by Chao et al [75] included NaYF4, CeF3, PrF3 NdF3 and

LaF3 systems and Er,Yb as dopants are used. The synthesis and emission spectrum of

YPO4-Eu nanobundles is reported by Wang et al [76]. The NaYF4 system doped with

Yb,Er is reported by Xiong et al [77]. The luminescence, toxicology studies and in vivo

& ex vivo examination with HeLa and MCF-7 cells is discussed. The in vivo toxicity of

Eu(OH)3 in mice is detailed by Patra et al [78]

Tian and coworkers have observed the effect of Mn doping in NaYF4: Yb/Er.

Solvothermal technique was utilized to obtain nanocrystals. The hexagonal to cubic

phase change and green to red UC emissions are related with the Mn contents [79].

15

Chapter 2 Synthesis Methods

16

Synthesis Methods

The control of size, shape and structure of nanomaterials are addressed in

synthesis process. Also, the knowledge of application plays its role in synthesis as the

nanomaterials are made functional and operational. Nano scale materials are generally

synthesized by following two approaches

Physical Methods

Wet-chemical Methods

The physical and wet-chemical methods are utilized for the controlled synthesis of

functional nanomaterials having pure phase, anticipated composition, even morphology

and tunable properties.

2.1 Physical and Chemical Methods

The physical methods include 1) physical vapor deposition, 2) pulsed laser

depositions and 3) sputtering methods. The chemical synthesis techniques can be

categorized as; 1) vapor-phase, 2) solution precipitation and 3) solid-state processes, 4)

water oil microemulsions 5) polyol method [1].

2.2 Wet-Chemical Methods

The wet-chemical methods consist of chemical reaction. The precipitating

colloids of nanomaterials from a solution of chemical compound have been a striking

scheme because of the simplicity. A key advantage of solution synthesis is the capacity

to form functional nanoparticles improving their stability and broader applications.

There are many wet chemical methods; some of them which are utilized in this research

work are given below

Composite Mediated Hydrothermal Method

Sol-gel Method

Co-precipitation Method

Solvothermal Method/Hydrothermal Method

2.2.1 Composite Mediated Hydrothermal Method

The composite mediated hydrothermal (CMH) method has simple procedure

and is a subsidiary of hydrothermal method. It gives less impurities (practically zero) in

final product and is relatively environmental affable. The method made use of the

eutectic point of NaOH-KOH composite. The melting temperatures of KOH and NaOH

17

are 406 0C and 323

0C respectively but at the mole ratio 0.515/0.485 of NaOH-KOH

system the eutectic point is 170 0C i.e. these melt at this temperature for this ratio. [80-

83].

Figure 2.1 Eutectic point of NaOH-KOH composite

By this technique the NaOH-KOH not only served as reactants but also as

precipitating agent. Very complex chemical compositions can be obtained with this

method. The synthesis parameters like heat treatment time and temperature can be

varied to obtain different sizes of nanomaterials.

Synthesis of Rare-earth oxides

For a typical reaction, Ce(NO3)3.6H2O, 4 gm, NaOH, 10.3gm and KOH, 9.7gm

were used. All chemicals in powder form were mixed in Teflon chamber, sealed and

were heat treated at 180 °C for 45 minutes in a pre-heated resistive heating oven. Once

the heat treatment was done and room temperature was achieved, the sample was

shifted to oven for drying after washing several times with de-ionized water. The

calcination was done at 500 0C for 2hrs. The pellets, made with 1gm powder of 13mm

diameter, were then sintered at 750 0C for 5hrs. The proposed chemical reaction is.

Ce(NO3)2+ KOH + NaOH CeO2+ by products

Synthesis of rare earth oxide NP‘s consists of following experimental steps given in

flow chart.

18

Figure 2.2 Schematic of composite mediated hydrothermal synthesis

2.2.2 Co-precipitation Method

Nucleation, growth and agglomeration processes sequentially happen in co-

precipitation. This method is considered to be appropriate and cost-effective for

synthesis of nanomaterials with narrow particle size. The chemical reactivity is

achieved through the pH maintained by some precipitating agent. The particles‘

formation, nucleation and growth are dependent on the rate with which the precipitating

agent is mixed. The molarities of precursor solution and precipitating agent solution

along with the pH are decisive parameters in co-precipitation method [84-88].

Synthesis of Ceria

Ce(NO3)3. 6H2O (cerium nitrate) was used to get cerium oxide nanoparticles by

co-precipitation method. The precipitating agent used was sodium hydroxide (NaOH)

provided the chemical reactivity through maintaining the pH. The molarity of cerium

nitrate solution was 0.2M and the molarity of sodium hydroxide solution was kept at

1M. De-ionized water was used as solvent for both the solutions.

19

Figure 2.3 Schematics for synthesis process of co-precipitation method

To achieve the required value of pH, the precipitating agent (sodium hydroxide

solution) was added to the cerium nitrate solution. After the continuous stirring for 30

minutes, washing of the sample was done several times with de-ionized water. Then the

sample was shifted to the conventional oven for overnight at 105 0C for drying. The

calcination temperature and time for the prepared powder was same as used in

composite mediated hydrothermal method (at 500 0C for 2-hours). The calcined powder

was then used to made the pellets which were further sintered for 5-hours at 750 0C.

2.2.3 Sol-gel Method

In sol-gel process, hydrolization of metal precursors is done with water which

yields suspension of colloid (the sol) following the condensation to produce a gel. In

condensation water or alcohol molecules are released. The rate of hydrolization and

condensation play an important role for the formation of final product [1].

Synthesis of Ceria

Ce(NO3)3. 6H2O was used to obtain cerium oxide nanoparticles by sol-gel

method. 0.4M solution of cerium nitrate was prepared using water. Ethylene glycol

20

(EG) was used as chelating agent. Initially at low temperatures the solution was

gradually changed into gel. With increase of temperature the gel was dried. After

rigorous stirring it was burned and nanoparticles of cerium oxide nanoparticles in

powder form were formed. The calcination of the prepared powder was done and

pellets made were sintered as previously described [89-90].

2.2.4 Solvothermal Method/Hydrothermal Method

In solvothermal process soluble metal species undergo precipitation followed by

a pressurized heat treatment in an autoclave. The temperatures more than 100 0C and

pressures more than the atmospheric pressure are utilized to form nanostructures. The

products thus achieved are phase pure and show homogeneity. If some solvent is used

other than water, the method is known as solvothermal [91].

Synthesis of rare earth hydroxides with hydrothermal method

For the synthesis of rare earth hydroxides hydrothermal method was adopted. A

nominal amount of cerium oxide was dissolved in nitric acid. Sodium hydroxide was

utilized for precipitation. These precipitates were then heat treated in a Teflon chamber

at 180 0C for overnight period of time. After the product was allowed to cool down to

room temperature the nanostructures were achieved after the washing and filtering with

de-ionized water [92].

Synthesis of NaMnF3 Yb; Er/Tm with solvothermal method

The solvothermal approach [93-94] was utilized to synthesize elongated

structures of rare earth doped NaMnF3. NH4F, NaCl and MnCl2 (Aldrich made,

analytical grade) were used to get F, Na and Mn ions respectively. Ethylene Glycol was

used to stabilize the ions and PEI was used as polymer to render nanorods hydrophilic.

Reaction:

Ethylene Glycol + NaCl (sonication)

+

MnCl2 + Yb(NO3)3 + ErCl3+PEI + NH4F + Ethylene Glycol (sonication)

Hydrothermal treatment @200 0C for 2hr

Collection of particles, washing, drying @50 0C

21

Chapter 3 Characterization

Techniques

22

Characterization Techniques

3.1 Structural and Morphological Analysis

3.1.1 X-Ray Diffraction

X-ray diffraction (XRD) is a multipurpose, nondestructive and investigative

method to study the crystallographic features. The phase purity, unwanted phases,

amorphous or crystalline structure, crystallite size, crystal structure and stresses in the

specimens can be determined using XRD data. The machine works on Bragg‘s law

[95]. As the lattice planes in materials are comparable with the x-rays wavelength so a

diffraction pattern is formed when x-rays interact with the crystalline substance which

is characteristic signature of the crystal structure. PANalytical X‘Pert pro XRD

diffractometer with Cu-Kα x-ray source was used to obtain diffraction patterns.

1.5406Å wavelength (λ) x-rays were utilized and 40kV, 30mA were the operational

parameters.

The Scherrer formula was used to estimate the crystallite sizes,

(3.1)

where, D is the crystallite size, λ; the wave length of the incident x-rays, θ ; the

diffraction angle and β; full width at half maximum (FWHM) expressed in radians.

The Stokes and Wilson‘s formula which incorporated the effect of strains in crystals is

[96]

. (3.2)

The strains were calculated using the relation

. (3.3)

The lattice constant ‗a‘ of the cubic system was calculated using the relation

√ . (3.4)

The formula to calculate the porosity of the samples was,

, (3.5)

where,

, (3.6)

and m = mass, r = radius, h = thickness

23

, (3.7)

where, 4 = formula units for fluorite cubic structure, M is Molecular weight, N is

Avogadro‘s number and V is volume of the unit cell.

Crystal structure of ceria

Cerium oxide (CeO2) is an oxide of cerium also known as ceric oxide, ceria or

cerium dioxide. The cerium oxide (CeO2) has cubic fluorite-type with FCC

arrangement. In such arrangement each cerium site is surrounded by eight oxygen

atoms [97-98].

Figure 3.1 (a) Unit cell of CeO2, light atoms are O2-

and dark atoms are Ce4+

(b)

Crystal structure of ceria

3.1.2 Scanning Electron Microscopy and Transmission Electron Microscopy

For its nondestructive nature SEM is utilized to have high resolution images of

samples to study the surface morphology. Electrons beam and sample interaction

expose information about surface morphology. The samples were studied using SEM

(HITACHI SU-1500). To study the morphology and particle size measurements of the

samples, TEM is one of the most powerful microscopy techniques. It can achieve

atomic resolution of crystals under study. Both spectroscpies rely on the interaction of

electron beam with material and the resulting elastic and inelastic scattering phenomena

provide with the information of the sample under study. The TEM analyze the

transmitted or forward scattered beam. TEM utilized was JEOL JEM 2100F operating

at 200 kV. The samples were dispersed in water and were supported on a 200 mesh,

3mm copper grid.

3.1.3 Differntial Scanning Calorimetry

Different phase transition can be observed using DSC as these transitions occur

due to change in temperatures. The machine works on the observation of differential

24

heat flow to sample and an inert reference material. The rate of increase in temperature

for both the sample and reference is kept constant. The thermal analysis was done on

DSC (TA Q200) in Ar environment at a rate of 10 0C/min. from room temperature to

1000 0C.

3.1.4 Raman Spectroscopy

Raman spectroscopy is an authoritative device to study the phase and structural

properties of materials as it is dependent on the local phonon characteristics. The

interaction of applied frequency with the material excites certain phonon modes which

are signatures of the materials. The Raman studies are done on DongWoo Optron

Raman & PL device with excitation laser of 514 nm [99].

3.1.5 Thermal Conduction Measurements

The thermal conductivity and diffusivity values were obtained by Advantageous

Transient Plane Source (ATPS) by employing modified bridge circuit as shown in

figure 3.2. Three dimensional heat flows inside the sample is studied. Resistive element

is used for this technique (TPS, a hot disk made out of a bifilar spiral) both as the

temperature sensor and the heat source. The TPS sensor was sandwiched between two

similar pellets. The change in the electrical resistance (t) increases with the increase

in time dependent temperature to be expressed by

( ) ( ( )) (3.8)

The thermal diffusivity, thermal conductivity and specific heat are related

through

(3.9)

where = thermal diffusivity, = thermal conductivity and the product in denominator

( ) is the volumetric heat capacity. The power of heat pulse and change in

temperature are related by

( )̅̅ ̅̅ ̅̅ ̅̅

⁄ ( ) (3.10)

Where = power, = thermal conductivity, r and ( ) are design parameters of the

sensor. This equation is an exact solution of hot disk using ring-source solution [100-

101].

25

Figure 3.2 Block diagram of Advantageous Transient Plane Source (ATPS) method

3.2 Conductivity Measurements

3.2.1 AC Conductivity Measurements

Wayne Kerr LCR meter 6440B was used to measure the capacitance and

dissipation factor at different temperatures (300 0C to 700

0C ±5

0C) for the frequency

ranging from I kHz to 3 MHz. The formula used to calculate the dielectric constant (ε′)

was,

. (3.11)

Where, ‗C‘ represent the capacitance, ‗d‘ symbolize the thickness of pellet sample, ‗A‘

denote the area, and ‗εo‘ ; the permittivity of free space (= 8.854 × 10-12

F/m).

The ac conductivity was calculated using the formula

, (3.12)

The error in measurement in capacitance was 0.2% and 0.002 % in dissipation

factor. The ε′, dielectric constant, is the ability of charge storing capacity of the

dielectric material. The dielectric constant at lower frequencies is due to polarization

processes. At higher frequencies the charges in polarization mechanisms become

unable to follow the applied frequency. The high values of dielectric constant at low

frequencies are due to electrode-electrolyte interface. With the increase in temperature

the relaxation peaks shifted toward the higher frequencies. These relaxation and

reorientation peaks are due to the response of ions to applied frequency The dielectric

loss has higher values at lower frequencies due to loss of energy in relaxation and

reorientation of ions whereas at higher frequencies dielectric loss has lower values

because of the higher frequencies, the ions were not able to respond to frequencies. The

mechanisms are shown in figure 3.3.

26

Figure 3.3 Polarization mechanisms for dielectric mediums [102]

Ceria exist in Ce3+

and Ce4+

and redox reaction is the reason of its use as

electrolyte and in catalysis.

( ) (3.13)

The Kroger-Vink notations describe the defects formation equations. In fluorite

structures, anti Frenkel type defects exist. The defect formation is governed by

(3.14)

which mean that an O atom existing on its original site with neutral charge can

be shifted to an interstitial site with two negative charges and two positive charges with

one vacancy will be generated at original site of oxygen. The reaction for ceria is

(3.15)

Similarly, when doping is done defects and vacancies are generated as shown in

figure 3.4. The Kroger-Vink relation for Gd doping in ceria is

(3.16)

The vacancy, Vo.. , can occupy any of the eight equivalent sites (cerium is surrounded

by eight oxygen atoms) around A3+

trivalent cation [62,103].

27

Figure 3.4 Types of defects helpful in ionic transport a) oxide vacancy in perovskite

structure, b) edge dislocation c) defective grain boundaries where space charges pile up

[104]

The ionic conduction is dependent on the defect formation due to thermal

excitations which create vacancies and these help in ionic motion to proceed. Also the

most common explanation of the increase in conductivity as a function of frequency is

the presence of the inhomogeneities which may be micro and/or macro in nature. The

ac conductivity follows a power law at higher frequencies and on lower frequencies

there is steady transition to a frequency independent region (figure 3.5). A dielectric

loss peak is also observed. The numeric value of power is less than 1 (0.5< s <0.8)

[105]. The power law is known as Jonscher power law or universal dielectric response,

( ) ( ) (3.17)

Figure 3.5 Conductivity as a function of frequency

28

The Wheatstone bridge circuit is utilized in Wayne Kerr LCR meter 6440B as

working principal.

3.2.2 DC Conductivity Measurements

The resistivity can be measured with two methods (two and four probe)

depending on the range of resistivity. If the resistivity of the sample is much higher

than the contact resistance, two probe method is engaged and if the resistivity of the

sample is comparable to the resistivity contacts then four probe method is used. The use

of common and independent set of probes for current and voltage is the difference

between these two methods. Wayne Kerr LCR meter 6440B was used to measure

resistance in temperature range 300 0C to 700

0C (±5

0C) with 1 volt applied. The

conductivity was calculated by using

(3.18)

where,

The error in measurement of resistance is 0.1 %. The physical and chemical

processes which depend on the temperature can follow what is known as Arrhenius

relation which describes the rate of change, is proportional to ( ⁄ ) where Ea

is a distinctive energy parameter specific to the process. The ionic hopping is

considered to be such a property. As the ceria based compounds are insulators at room

temperature and become conductive at higher temperatures, ( ⁄ ). The

hopping phenomenon is considered to be happening in rare earth compounds. The jump

relaxation model can be used to explain the ionic conductivity at higher temperature

more easily than only with Arrhenius relation [106]. The model can be understood with

figure 3.6. The following are the suppositions [107];

1) The hopping ions are of same kind.

2) The mobile ions are less than the sites available and sites are also of the same kind.

3) The mobile ions have repulsive interaction which results in a cage effect and ions

remain at a distance with each other.

29

Figure 3.6 Basis of the jump relaxation model, (a) ions (O) on a sublattice, (b) the

effective single particle potential, (c) development of potential after a hop [107].

An ion can hop to neighboring site, at t = 0, due to available thermal agitation.

The hops create a mismatch in terms of cage effect potential. Once this mismatch is

created, this can be reversed in two competing ways; to relax the ion may jump back to

original site or neighboring ions may arrange a hopping motion which produces another

cage effect potential for the ion at new site. In this later way, hop is successful and

contributes to dc conductivity.

3.3 Fluorescence Measurements

Fluorescence is the emission of light within nanoseconds after the striking of

light on the material which is of shorter wavelength than the emitted. The process is

explained with the upconversion phenomenon. The emissions are characteristic of the

transitions occurring in the material under observation.

UC is the process to generate visible light from NIR. It is a nonlinear process.

The process needs absorption of two or more photons so that sufficient energy for the

process is available. The processes which lead to UC include; excited state absorption,

energy transfer and photo avalanche. The excited state absorption process is considered

to be happening in materials with low dopant concentrations. The energy transfer

process comprises of absorption of photons that transfer energy from excited ion

(sensitizer) to another ion (activator) [108].

Fluoromax-4, Horiba Jobin Yvon Spectrofluometer with photon counting

detector was used in this work. The samples, dispersed in water were excited with 980

nm laser at 1W power working at 2.1 A.

30

Chapter 4 Synthesis of Ceria and

Choice of Synthesis Method

31

Synthesis of Ceria and Choice of Synthesis Method

There are many synthesis techniques employed for the enhancement of the

conductivity. Synthesis methods are optimized depending upon their synthesis

conditions to reduce the grain size. Grain size is an important factor for which the

increase of conductivity is observed as the grain size is reduced.

Following are some factors which influence the conductivity [109]

Processing conditions can change the conductivity as the sample density; level

of impurities, size of grains etc can influence the motion of ions which

contributes to conductivity.

The microdomains influence the conductivity of the materials.

Doping of aliovalent cation significantly increase the conductivity.

Multiple doping is another way to enhance the conductivity.

Space charge polarization and segregation of space charges also enhance the

conductivity which can be achieved by reducing the grain size.

In general, the conductivity of material is dependent on the composition,

microstructure and processing of the material. The phase pure composition and

microstructure, themselves are dependent on the synthesis process. Moreover, the

reduction in grain size makes charges to pile up on grain boundaries which in result

increase the conductivity [110]. The grain size and microstructure are dependent on the

synthesis routes therefore the choice of synthesis method plays an important role for the

application of enhancement of conductivity.

It has been reported that the reduction in grain size has increased the conductivity. It

might be due to the enhanced electronic conductivity and decrease in defect formation

energy at grain boundaries. The ceria samples were prepared with range of wet

chemical methods (Composite mediated hydrothermal method, Co-precipitation

method and Sol-gel method). One can hardly find direct comparison in the already

reported literature. In the present study, it is clear from the results that the composite

mediated hydrothermal method is better among the three for the enhancement in

conductivity.

The samples are named as; CMH for Composite mediated hydrothermal

method, Cop. for Co-precipitation method and Sol. for sol-gel method.

32

4.1 Structural Analysis


The XRD patterns of CeO2 prepared by three wet chemical routes are shown in

figure 4.1. All the prepared samples are of single phase with cubic crystal structure

(card # 00-043-1002). The lattice constants were calculated by equation 3.4 which

varied between 5.38 Å to 5.41 Å. The crystallite size of samples were calculated using

Scherer formula, as given in equation 3.1.

20 30 40 50 60 70 80

2Theta (degrees)

Inte

ns

ity

(a.u

.)

CMH

Cop.

(331)(400)(222)

(311)(220)

(200)

(111)

SOl.

Figure 4.1 XRD pattern of ceria prepared by different wet-chemical methods

The pattern shown in figure 4.1 confirmed the cubic fluorite structure of ceria.

Table 4.1 is showing the values of crystallite size and lattice constants corresponding to

the most intense peak.

33

Table 4.1 Crystallite size and lattice constant crossponding to the most intense peak

4.1.2 Differential Scanning Calorimetry (DSC)

The DSC plot given in figure 4.2 showed a hump ca. 750 0C and no appreciable

change afterward. The sintering temperature can be 750 0C and above. The sintering at

higher temperature increases the density and the grain size. For uniformity in all

samples, the sintering temperature was chosen to be 750 0C and pellets were made

under similar conditions of pressure and time. The effect of density was same on all

samples.

0 200 400 600 800 1000 1200

-140

-120

-100

-80

-60

-40

-20

0 CeO2

He

at

Flo

w (

mW

)

Temperature (0C)

Crystallization

Figure 4.2 DSC plot of ceria synthesized with CMH method

CeO2 D s (1 1 1)

(nm)

Lattice Constant

Å

Sol. (Sol-gel) 10 5.38(1)

Cop. (co-precipitation) 47 5.41(1)

CMH (Composite mediated

hydrothermal method) 60 5.40(2)

34


The oxygen-cerium F2g mode vibration for cubic fluorite showed the single

intense peak at ca. 465 cm-1

. The bands around ca. 570 and 600 cm-1

correspond to

vacancy sites and intrinsic oxygen [58].

400 500 600 700 800

520 540 560 580 600 620

2835

2870

2905

2940

Raman shift (cm-1

)

Raman shift (cm-1

)

Ram

an

in

ten

sit

y (

a.u

)

465

CMH

Cop.

Sol.

Hydro

Sol.

Cop.

600

570C546

Figure 4.3 Raman spectrum of ceria synthesized by different wet-chemical methods

4.2 Electrical Measurements

4.2.1 DC Conductivity

The dc conductivities of ceria samples synthesized by different methods

are shown in figure 4.4 (a-c). All samples show Arrhenius type dependence of

conductivity on temperature. A significant feature of CeO2 to show conductivity at high

temperature is observed whereas it is insulator at room temperature. The values of dc

conductivities at different temperatures are mentioned in table 4.2. It is clear from the

values that the CMH method has maximum value of dc conductivity.

35

200 300 400 500 600 700

0.02

0.04

0.06

0.08

0.10

0.12

0.14

d

c (

Scm

-1)

Temperature (oC)

d

c (

Scm

-1)

Sol

0.00

0.07

0.14

0.21

0.28

0.35

CMH

Figure 4.4 (a) DC conductivity of CeO2 prepared by CMH and sol-gel method

400 450 500 550 600 650 7002.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0 Cop.

dcx10

8(S

-cm

-1)

Temperature (oC)

Fig. 4.4 (b) DC conductivity of CeO2 prepared by co-precipitation method

36

Table 4.2 Comparison of ac conductivity, dc conductivity and dielectric constant for

the ceria samples synthesized by CMH, co-precipitation and sol-gel method.

CMH 400oC 500

oC 700

oC

DC Conductivity σdc

(S-cm-1

)

0.004 0.085 0.338

AC Conductivity σac

(S-cm-1

) at 3MHz

0.063 0.226 2.661

Co-precipitation 400oC 500

oC 700

oC


(S-cm-1

)

2.351x10-8

2.672x10-8

3.963x10-8


(S-cm-1

) at 3MHz

0.247 0.472 2.344

Sol-gel 400oC 500

oC 700

oC


(S-cm-1

)

0.010 0.011 0.095


(S- cm-1

) at 3MHz

0.019 0.316 2.511

4.2.2 AC Conductivity

The ac conductivities of ceria samples are shown in figure 4.5 (a-c). The values

of ac conductivities at 3MHz are mentioned in table 4.2 also.

The jump relaxation model can be applied to explain the behavior. At lower

frequencies the ac conductivity is due to the hopping of ion from one available site to

other. At higher frequencies dispersion occur due to hopping and relaxation of ions.

Dispersion region shifted towards the higher frequency region with the increase in

temperature. The step in dispersion is the confirmation of grain boundaries and grains

interior conduction. The confirmation of ionic hopping in addition to Arrhenius relation

is given by the Jonscher‘s law [107, 111].

37

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

700oC

600oC

500oC

400oC

300oC

log

ac (

Scm

-1)

log f (Hz)

Figure 4.5 (a) AC conductivity of ceria synthesized by CMH method

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-2.0

-1.5

-1.0

-0.5

0.0

0.5

log f (Hz)

log

ac (

S-c

m-1

)

700 0C

600 0C

500 0C

400 0C

300 0C

Figure 4.5 (b) AC conductivity of ceria synthesized by co-precipitation method

38

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

700oC

600oC

500oC

400oC

300oC

log f (Hz)

log

ac (

Scm

-1)

Figure 4.5 (c) AC conductivity of ceria synthesized by sol-gel method

4.2.3 Dielectric Constant

The dielectric constant values of all three samples are given in figure

4.6. The sample prepared by composite mediated hydrothermal method showed higher

values of the dielectric constant. At lower frequencies the dielectric constant values are

much higher but those decreased with increase in frequency. Because, at higher

frequencies, the dipoles were not able to follow the applied field. The ‗universal‘

dielectric response is evident from the plots. The Jonscher‘s power law is applicable to

such materials which do not show loss peaks [111]. The high values of dielectric

constant at lower frequencies are the manifestation of the electrode-electrolyte interface

[112]. The shift of peaks toward higher frequencies is evident from plots.

39

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

log

Die

lec

tric

co

ns

tan

t ('

) 700

0C

600 0C

500 0C

400 0C

300 0C

log f (Hz)

Figure 4.6 (a) Dielectric constant of ceria synthesized with CMH method

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

1

2

3

4

5

700 0C

600 0C

500 0C

400 0C

300 0C

log

Die

lectr

ic c

on

sta

nt ('

)

log f (Hz)

Figure 4.6 (b) Dielectric constant of ceria synthesized with co-precipitation method

40

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

1.2

1.4

1.6

1.8

2.0

2.2

2.4

log

Die

lec

tric

co

ns

tan

t ('

) 700

oC

600oC

500oC

400oC

300oC

log f (Hz)

Figure 4.6 (c) Dielectric constant of ceria synthesized with sol-gel method

4.3 Conclusions

Successful synthesis of phase pure ceria was done by composite mediated

hydrothermal method, co-precipitation method and sol-gel method. The dc

conductivities of the prepared samples were found to be increasing with the increase in

temperature and the values as high as 0.345 S-cm-1

and 0.095 S-cm-1

(at 700 0C) were

achieved for CMH and sol-gel samples, respectively. The samples prepared by CMH

method showed better values of ac and dc conductivities. Also, the higher values of

dielectric loss tangent and dielectric constant were observed for same samples. These

properties made this method suitable to prepare materials which can be utilized as the

electrolyte material in SOFCs. The presence of oxygen vacancies and cerium oxide was

also confirmed through Raman spectrum. The intensity in Raman spectrum for CMH

sample was observed very high as compared with samples prepared by co-precipitation

and sol-gel methods. Hence, to obtain higher values of conductivity in ceria, CMH

method is better synthesis approach.

41

Chapter 5 Effect of Synthesis

Parameters on Ceria

Synthesized by Composite

Mediated Hydrothermal

Method

42

Effect of Synthesis Parameters on Ceria Synthesized by

Composite Mediated Hydrothermal Method

Composite mediated hydrothermal (CMH) method was used to synthesize

cerium oxide nanoparticles. The enhancement in conduction was the focus in

optimization of synthesis conditions in addition to smaller crystallites. Hydrothermal

treatment temperature (180 oC and 220

oC) and hydrothermal treatment time (45 min,

70 min and 90 min) were optimized. The calcination and sintering temperature were

the same (500 0C for calcination and 750

0C for sintering). Structural properties of ceria

were observed through X-Ray diffraction (XRD) data. DC conductivity was observed

to be increasing with the increase in temperature and was done in temperature range

200 0C to 700

0C. Frequency dependent dielectric properties and ac conductivity were

observed at different temperatures in frequency range from 20Hz to 3MHz. 514 nm

excitation laser was used to observe the Raman spectrum of samples. The designed

nomenclature for the samples is given in table 5.1.

Table 5.1The nomenclature designed for the samples at different optimized parameters.

5.1 Structural Analysis


Figure 5.1 showed the indexed XRD patterns of cerium oxide samples. All the

prepared samples were single phase with cubic structure. The value of lattice constant

varies between 5.40 Å to 5.43 Å. The Scherrer formula given in equation 3.1 was

utilized to calculate crystallite sizes.

The lattice constants, average crystallite sizes and crystallite size corresponding

to the most intense peak are mentioned in table 5.2.

43

Figure 5.1 XRD patterns of CeO2 samples synthesized by different synthesis conditions

Table 5.2 The average crystallite size and that of the most intense peaks (D (1 1 1)) along

with lattice constants (a) for the samples.

44

5.2 Electrical Properties


Temperature dependent resistance was measured from 200 0C to 700

0C for the

prepared samples. Equation 3.18 was used to calculate the resistivity. Figure 5.2 is

showing the dc conductivity (ζdc S-cm-1

) behavior for the samples as a function of

temperature from 200 0C to 700

0C. The conductivity increased with the increase in

temperature but the increase is much sharp at higher temperatures (5000C – 700

0C) than

lower temperatures.

Figure 5.2 Temperature dependent dc conductivity of the prepared samples

The comparison of values of dc conductivity (at 300 0C, 500

0C and 700

0C)

given in table 5.3. Maximum dc conductivity is achieved for the sample synthesized at

180 0C (H14) and 220

0C (H24) for 45 minutes using Arrhenius relation [106]. Jump

relaxation model can be used to explain the behavior [107, 111].

45

Table 5.3 DC conductivity (ζdc S-cm-1

) at different temperatures of the samples


By using relation 3.12, the frequency dependent ac conductivity at different

temperatures (from 200 0C to 700

0C) was measured and the plots are given in figure

5.3.

Hopping and relaxation of ions led to dispersion for higher frequencies. The ion

can relax in two ways after jumping from one site to other. The neighboring ions may

rearrange in the sequence of their hopping motion or it may jump back to its original

site. Frequency dispersion region in the conductivity spectra can be observed due to the

relaxation of ions.

The dispersion region is found to shift to the higher frequency region with

increase in temperature. The presence of both the grain boundary and the grain interior

conductivities is confirmed with the ‗step‘ in dispersion region [21]. Table 5.4 shows

the values of ac conductivities at frequency of 3 MHz for different temperatures.

46

Figure 5.3 AC conductivity (ζac S-cm-1

) as a function of frequency at different

temperatures for all samples (H14, H17, H19, H24, H27 and H29).

47

Table 5.4 Frequency dependent ac conductivity (ζac S-cm-1

) at different temperatures

Figure 5.4 is showing the comparison of values of ac conductivities at different

temperatures on 3 MHz frequency. The behavior is dependent on temperature as well as

on the synthesis method. The plot shown here is actual data beyond error bars. The

possible sources of error in ac measurements are mentioned in section 3.2.1. The value

of ac conductivity increased with increase in temperature. The samples H14 and H24

have maximum conductivity. AC conductivity is found to be higher for the samples

with narrow range of crystallite size at all the temperatures [113].

.

Figure 5.4 Comparison of ac conductivity (ζac S-cm-1

) at 3 MHz for all samples at

500 0C, 600

0C and 700

0C.

48


Figure 5.5 is showing the trend of dielectric constant at different temperatures

as a function of frequency from 20 Hz to 3 MHz.

Figure 5.5 Dielectric constant (ε΄) as a function of frequency at different temperatures

for all the samples (H14, H17, H19, H24, H27 and H29).

49

The plots are showing that the values of dielectric constant increased with the

increase in temperature. Dielectric constant decreased sharply with the increase in

frequency which is due to the frequency increase and the dipoles did not follow the

applied field. Tables 5.5 show the comparison of values for the dielectric constant at

different temperatures at frequency of 3 MHz.

Figure 5.6 is showing the comparison of the values for the dielectric constant

for the samples synthesized with different synthesis conditions, which show that the

value of dielectric constant increased as temperature was achieved.

Figure 5.6 Dielectric constant at 3 MHz for all the samples at different temperatures

(500 0C, 600

0C and 700

0C).

5.2.4 Dielectric Loss

Dielectric loss plots are shown in figure 5.7 on different temperatures. When a

material acts like a dielectric medium between two conductor plates these losses occur

due to the polarization effects.

In lower frequency region, space charge polarization effects occur and in higher

frequency region ionic polarization effects appear [102]. The polarization peak shifted

to higher frequency with the increase in temperature which is very clear for higher

temperatures.

50

Figure 5.7 Dielectric loss (tanδ) as a function of frequency at different temperatures for

all samples.

At the higher temperatures the losses due to polarization mechanisms are very

much sharp. Table 5.5 is showing the comparison values of dielectric loss values at

different temperatures (500 0C, 600

0C and 700

0C) for the frequency 1 kHz and

dielectric constant at 3 MHz frequency.

51

Table 5.5 Frequency dependent dielectric constant (ε΄) and dielectric loss tangent (tanδ)

values at different temperatures

5.3 Raman Spectroscopy

Raman spectrum is observed at 514nm excitation laser and is given in figure

5.8. High intensity 465 cm-1

band of ceria is observed for all the samples. The

magnified view is shown in inset plot. 546 cm-1

band could be assigned to the oxygen

vacancies introduced into the ceria. The intrinsic oxygen vacancies are related to

570cm-1

and 600 cm

-1 bands [58].

400 500 600 700 800 900 1000

540 560 580 600 620

1500

1600

1700

1800

Ra

ma

n in

ten

sit

y (

a.u

)

Raman shift (cm-1

)

H29

H27

H24

H19

H17

H14

465

570

600

546

Figure 5.8 Raman spectra of prepared ceria samples at 514 nm excitation laser line

52

5.4 Conclusions

Optimization of different synthesis parameters of composite mediated

hydrothermal method was done to obtain phase pure nanocrystalline ceria. The crystal

structure, crystallite size, phase purity, and lattice constants were determined using

XRD data. 0.3386 S-cm-1

was the maximum value of electrical dc conductivity of the

samples at 700 0C. For sample H14 the maximum ac conductivity was found with a

value of 2.661 S-cm-1

at 700 0C. Strong ceria band was observed at 465 cm

-1 through

Raman spectrum which confirmed the presence of ceria. At 546 cm-1

, 570 cm-1

and 600

cm-1

week bands of oxygen vacancies are present. The material can be a strong

candidate for intermediate temperature range SOFCs.

53

Chapter 6 Effect of Gd Doping

on Conductivity of Ceria

54

Effect of Gd Doping on Conductivity of Ceria

The effect of Gd doping in ceria prepared by composite mediated hydrothermal

method is studied here. In general, the conductivity of material is dependent on the

composition, microstructure and processing of the material. The phase pure

composition and microstructure, themselves are dependent on the synthesis process. As

discussed in chapter 4, the composite mediated hydrothermal method is found to be

better in achieving the enhancement of the conductivity; hence this method is adopted

to prepare rare earth oxides (Gd doped ceria). In addition to synthesis process the

addition of doping produces oxygen vacancies which also help to enhance the

conductivity. Four nominal compositions were made with varying contents of Gd in

CeO2 i.e. Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25) and corresponding samples were

named as CG10 (x= 0.10, Ce0.9Gd0.1Oδ), CG15 (x= 0.15, Ce0.85Gd0.15Oδ), CG20 (x=

0.20, Ce0.8Gd0.2Oδ) and CG25 (x= 0.25, Ce0.75Gd0.25Oδ).

6.1 Structural and morphological studies


The XRD patterns shown in figure 6.1 of prepared samples, showed the

formation of cubic structure. All peaks were indexed with cubic fluorite structure

(space group 225 and card # 01-075-0161). All the compositions are phase pure. The

crystallite sizes were in range of 14 nm to 88 nm by using Scherrer formula given in

equation 3.1 and in range of 19 to 159 nm using Stokes and Wilson‘s formula given in

equation 3.2. The crystallite size obtained using both formulas; lattice constant and

porosity are given in table 6.1.

55

20 30 40 50 60 70 80

CG25

CG20

(420)(331)(400)(222)

(311)(220)

(200)

(111)

Inte

ns

ity

(a

.u.)

2degrees

CG10

CG15

Figure 6.1 X-ray diffraction patterns of Ce1-xGdxOδ (x= 0.10- 0.25)

Table 6.1 The crystallite sizes (Ds (111) = Crystallite size corresponding to the most

intense peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding

to the most intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average

crystallite size estimated by Scherrer formula, Dw = Average crystallite size estimated

by Stokes and Wilson‘s formula), lattice constant and porosity of Ce1-xGdxOδ (x= 0.10-

0.25)

6.1.2 Scanning Electron Microscopy

The SEM micrographs are shown in figure 6.2. The sample CG10 has

needles-like structures and the other three samples have agglomerated particles. No

conclusive information can be obtained for sizes of particles, due to magnification

limitation.

Sample D s (1 1 1)

(nm)

D w (1 1 1)

(nm)

D s

Average

(nm)

D w

Average

(nm)

Lattice

Constant

Å

Porosity

CG10 66 88 34 76 5.40(2) 0.62

CG15 30 40 24 44 5.38(3) 0.64

CG20 83 111 32 55 5.40(2) 0.62

CG25 83 111 41 76 5.40(3) 0.62

56

Figure 6.2 Scanning electron micrographs of Ce1-xGdxOδ (x= 0.10 - 0.25)


The Raman spectra of Ce1-xGdxOδ are given in figure 6.3. The spectra show the

single intense peak corresponding to the oxygen-cerium F2g mode vibration for cubic

fluorite structure at ca. 465 cm-1

. The doping of Gd in ceria is confirmed from the

absence of Raman bands at ca. 480 cm-1

which is vibrational mode of cubic Gd2O3.

Also, the increase in dopant concentration of the Gd increased the peak bandwidth

which is due to the intermixing of bands. The increase in conductivity can be attributed

to more vacancy sites generation with increase in Gd concentration in ceria [114-115].

CG10 CG15

CG20 CG25

57

400 500 600 700

CG25CG20CG10

Sample Peak Width

(cm-1

)

CG10 16.27

CG15 14.35

CG20 18.26

CG25 19.68

CG10

CG15

CG20

CG25

Raman Shift (cm-1)

Ra

ma

n I

nte

ns

ity

(a

.u.)

465

CG15

Figure 6.3 Raman spectroscopy of Ce1-xGdxOδ (x= 0.10 - 0.25)



The dc conductivity of prepared samples is shown in figure 6.4. The

activation energies (section 3.2.2) calculated are given in table 6.2. The Ceria and Gd

doped Ceria are very high resistive at room temperature. At higher temperature these

become conductive as the oxygen ion conductivity is dependent on temperature. The

values of conductivities of prepared samples at different temperatures are mentioned in

table 6.2. The sample CG25 has the highest value of conductivity. The increase in

conductivity might be credited to the increase in vacancy sites due to the increase in Gd

contents and the microstructure distribution due to the synthesis process [104, 107, 114,

116].

58

300 400 500 600 700

0.000

0.002

0.004

0.006

0.008

d

c (S

-cm

-1)

CG10

CG15

CG20

CG25

Temperature (oC)

Figure 6.4 DC conductivity of Ce1-xGdxOδ (x= 0.10-0.25) as a function of temperature.

Table 6.2 Activation energies ‗s‘ and conductivities of Ce1-xGdxOδ (x= 0.10 - 0.25) at

different temperatures

Sample

Activation

Energy

(eV)

‘s’ DC Conductivity (S-cm-1

) at

300-700 °C 300

oC 500

°C 550

°C 600

°C 650

°C 700

°C

CG10 0.89(1) 0.60 1.14×10-4

4.02×10-4

6.74×10-4

13.3×10-4

24 ×10-4

CG15 0.87(1) 0.49

4.10×10-5

1.42×10-4

2.10×10-4

2.76×10-4

4.5×10-4

CG20 1.19(2) 0.60

2.89×10-5

1.20×10-4

1.57×10-4

4.25×10-4

6.52×10

-

4

CG25 1.13(2) 0.59 1.95×10-4

4.78×10-4

9.30×10-4

25.8 ×10-4

74 ×10-4


The frequency spectra of Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25) at

different temperatures are given in figure 6.5 (a-d). The jump relaxation model

explained the behavior of conduction mechanism [107]. At lower frequencies dc

conductivity occurs due to the jumping of ion from one vacant site to the other. At

higher frequencies dispersion occur due to hopping and relaxation of ions. The

59

dispersion region shifted toward the high frequency region with the increase in

temperature. The step in dispersion is the confirmation of grains boundaries and interior

conduction. The confirmation of ionic hopping in addition to Arrhenius relation is

given by the Jonscher‘s law [105-106, 111]. The value of exponent ‗s‘ in equation 3.17

is determined for ceria and expressed in table 6.2. The ac conductivities as a function of

frequency at different temperatures are summarized in table 6.3.

Table 6.3 AC conductivity at 3 MHz frequency for different temperatures of

Ce1-xGdxOδ (x= 0.10 - 0.25) samples

Sample name

AC Conductivity (S-cm-1

)

at 3 MHz

500 °C 600

°C 700

°C

CG10 0.0025 0.0023 0.0081

CG15 0.0014 0.0017 0.0023

CG20 0.0012 0.0023 0.0037

CG25 0.0025 0.015 0.189

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

700 0C

600 0C

500 0C

400 0C

300 0C

log

ac (

S-c

m-1

)

log f (Hz)

Figure 6.5 (a) AC conductivity of CG10 at different temperatures

60

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-14

-13

-12

-11

-10

-9

-8

-7

-6

700 0C

600 0C

500 0C

400 0C

300 0C

log

ac (

S-c

m-1

)

log f (Hz)

Figure 6.5 (b) AC conductivity of CG15 at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-14

-13

-12

-11

-10

-9

-8

-7

-6

700 0C

600 0C

500 0C

400 0C

300 0C

log

ac (

S-c

m-1

)

log f (Hz)

Figure 6.5 (c) AC conductivity of CG20 at different temperatures

61

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-12

-10

-8

-6

-4

-2

700 0C

600 0C

500 0C

400 0C

300 0C

log

ac (

S-c

m-1

)

log f (Hz)

Figure 6.5 (d) AC conductivity of CG25 at different temperatures


The dielectric constants as a function of frequency are shown in figure

6.6 (a-d). The ‗universal‘ dielectric response is evident from the plots. The Jonscher‘s

power law is applicable to such materials which do not show loss peaks. The high

values of dielectric constant at lower frequencies are the manifestation of the electrode

-electrolyte interface [117-118]. The shift of peaks toward higher frequencies is

evident from plots. Values of dielectric constant at different temperatures and lowest

and highest frequency applied are given in table 6.4.

Table 6.4 Values of dielectric constant at 1 kHz and 3 MHz for different temperatures

of Ce1-xGdxOδ (x= 0.10 - 0.25) samples.

Sample

name

Dielectric constant (′)

at 3 MHz at 1 kHz

500 °C 600

°C 700

°C 500

°C 600

°C 700

°C

CG10 25.83 25.53 34.45 315.21 857.62 4077.75

CG15 20.22 24.05 26.06 202.21 375.91 794.14

CG20 26.62 32.45 38.85 200.46 398.12 1229.72

CG25 27.33 39.98 24.70 758.53 7033.89 108676.66

62

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52

3

4

5

6

7

8

9

700 0C

600 0C

500 0C

400 0C

300 0C

log

Die

lec

tric

co

ns

tan

t ('

)

log f (Hz)

Figure 6.6 (a) Dielectric constant of CG10 at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5 700 0C

600 0C

500 0C

400 0C

300 0C

lo

g D

iele

ctr

ic c

on

sta

nt ('

)

log f (Hz)

Figure 6.6 (b) Dielectric constant of CG15 at different temperatures

63

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52

3

4

5

6

7

8 700

0C

600 0C

500 0C

400 0C

300 0C

lo

g D

iele

ctr

ic c

on

sta

nt ('

)

log f (Hz)

Figure 6.6 (c) Dielectric constant of CG20 at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

3

4

5

6

7 700 0C

600 0C

500 0C

400 0C

300 0C

log

Die

lec

tric

co

ns

tan

t (')

log f (Hz)

Figure 6.6 (d) Dielectric constant of CG25 at different temperatures

64

6.2.4 Dielectric Loss (tanδ)

The frequency dependent tanδ plots are given in figure 6.7 (a-d) for

Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25). At lower frequencies tanδ has larger values and

at higher frequencies the value decrease and became independent of frequency due to

the fact that the dipoles were not able to respond and reorient themselves with applied

frequency. Also, the relaxation and reorientation peaks are shifting towards higher

frequencies with increase in temperature. Values of tanδ for different temperatures and

applied frequency are shown in table 6.5.

Table 6.5 Variation in tanδ as a function of frequency at different temperatures for

Ce1-xGdxOδ (x= 0.10 - 0.25) samples

Sample

name

Dielectric loss tangent (tanδ)

at 3 MHz at 1 kHz

500 °C 600

°C 700

°C 500

°C 600

°C 700

°C

CG10 0.57 0.54 1.41 16.94 30.59 22.29

CG15 0.40 0.42 0.52 9.65 12.41 15.82

CG20 0.26 0.41 0.57 11.68 13.73 20.63

CG25 0.54 2.26 3.45 22.14 28.72 38.72

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

4

8

12

16

20

24

28

32 700 0C

600 0C

500 0C

400 0C

300 0C

tan

log f (Hz)

Figure 6.7 (a) Dielectric loss (tanδ) of CG10 at different temperatures

65

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

2

4

6

8

10

12

14

16

18

700 0C

600 0C

500 0C

400 0C

300 0C

ta

n

log f (Hz)

Figure 6.7(b) Dielectric loss (tanδ) of CG15 at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

4

8

12

16

20 700

0C

600 0C

500 0C

400 0C

300 0C

ta

n

log f (Hz)

Figure 6.7(c) Dielectric loss (tanδ) of CG20 at different temperatures

66

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

5

10

15

20

25

30

35

40

45

700 0C

600 0C

500 0C

400 0C

300 0C

ta

n

log f (Hz)

Figure 6.7(d) Dielectric loss (tanδ) of CG25 at different temperatures

6.3 Thermal Conduction

The room temperature thermal conductivity and thermal diffusivity values are

measured using ATPS method. This method provides the simultaneous measurements

of the conductivity and diffusivity [100, 101]. The values obtained are given in table

6.6. The samples showed the values in lower conductivity range. These low thermal

conductivity samples can be utilized for the thermal barrier coatings. The conduction of

heat inside solids is aided through the scattering of phonons. The decrease in

conductivity with increase of Gd contents might be due to the increase of vacancies

which slowed down the phonons and the mean free path reduced hence the conductivity

is reduced [119-120].

Table 6.6 Values of thermal conductivity and thermal diffusivity for Ce1-xGdxOδ

(x= 0.10 - 0.25) at room temperature

Sample Thermal Conductivity

(Wm-1

K-1

)

Thermal Diffusivity

(mm2s

-1)

CG10 0.42 0.64

CG15 0.15 0.91

CG20 0.39 0.71

CG25 0.40 0.79

67

6.4 Conclusions

Nanocrystalline Gadolinium doped Ceria, Ce1-xGdxOδ (x = 0.10, 0.15, 0.20,

0.25), were successfully synthesized with facile composite mediated hydrothermal

method. The X-ray diffraction confirmed the phase and composition of the synthesized

material. The crystallite sizes were estimated using Scherrer and Stokes and Wilson‘s

formulae. The range of crystallite size was 30-83 nm corresponding to the most intense

peak using Scherrer‘s formula. The x-ray diffraction data was used to calculate lattice

constant and porosity. The ranges obtained for lattice constant and porosity were

5.38Å-5.40Å and 0.62-0.64 respectively. The scanning electron microscopy was used

to get morphology of the materials. Ce0.9Gd0.1Oδ has needles like structures whereas

other three samples have agglomerated particles. DC conductivity was measured in

temperature range 300-700 °C and ac conductivity was determined in frequency range 1

kHz to 3MHz at temperatures 300, 400, 500, 600 and 700 °C. The larger values of

conductivities were obtained for Ce0.9Gd0.1Oδ and Ce0.75Gd0.25Oδ as well as for increase

in temperature. Arrhenius plots were used to calculate activation energies, obtained in

the range 0.87-1.19 eV for 300-700 °C. The ‗universal‘ dielectric response with

Jonscher power law and jump relaxation model explained the conduction phenomena in

the synthesized material. The maximum conductivity achieved for Ce0.75Gd0.25Oδ to be

9.30x10-4

S-cm-1

at 600 °C. The thermal conductivity values for these samples lie in low

thermal conductivity region and can be utilized for thermal barrier coatings. The

Raman spectroscopy seconded the structural, electrical and thermal results.

68

Chapter 7 Conductivity Enhancement

in co-Doped Rare-Earth Oxides

69

Conductivity enhancement in co-doped rare-earth oxides

The maximum conductivity achieved in samples with Gd doping, were further

co-doped with La and Nd. With the doping of La, Gd and Gd, Nd in ceria the nominal

compositions were Ce1-2xGdxLaxOδ (x= 0.10, 0.25) and Ce1-2xGdx NdxOδ (x= 0.10,

0.25). The samples were named as CGL10 (Ce0.8Gd0.1La0.1Oδ), CGN10

(Ce0.8Gd0.1Nd0.1Oδ) and CGL25 (Ce0.5Gd0.25La0.25Oδ), CGN25 (Ce0.5Gd0.25Nd0.25Oδ).



The XRD patterns given in figure 7.1 of prepared samples showed the

formation of cubic fluorite structure (space group 225). The starred peaks are of Nd2O3

(*) and La2O3 (#).

20 30 40 50 60 70 80

2Degrees)

CGL10

Inte

ns

ity

(a

.u.)

CGL25

#

CGN10

CGN25

(111)

(2 0 0)(2 2 0)

(3 11)(3 31)

(4 2 0)

Figure 7.1 X-ray diffraction patterns of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x =

0.10, 0.25), the starred peaks are of Nd2O3 (*) and La2O3 (#).

70

The crystallite sizes were in the range of 12 to 70 nm using Scherrer formula

and in range of 22 to 92 nm using Stokes and Wilson‘s formula. The crystallite size

using both formulas and lattice constant are given in table 7.1.

Table 7.1 The crystallite sizes (Ds (111) = Crystallite size corresponding most intense

peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding most

intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average crystallite size

estimated by Scherrer formula, Dw = Average crystallite size estimated by Stokes and

Wilson‘s formula), lattice constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,

0.25).


The Raman spectra of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ are given in figure

7.2. The oxygen-cerium F2g mode vibration for cubic fluorite showed the single intense

peak at ca. 465 cm-1

. The bands ca. 570 and 600 cm-1

correspond to vacancy sites and

intrinsic oxygen in samples, respectively. Also, the increase in dopant concentration

increased the peak bandwidth which is due to the intermixing of bands. The increase in

conductivity can be attributed to more vacancy sites generation with increase in dopants

concentration in Ceria [58, 114-115, 121].

Sample D s (1 1 1)

(nm)

D w (1 1 1)

(nm)

D s

Average

(nm)

D w

Average

(nm)

Lattice

Constant

Å

CGL10 57 73 36 55 5.74(2)

CGL25 42 54 39 64 5.91(1)

CGN10 43 56 31 53 5.45(2)

CGN25 49 64 38 58 5.46(2)

71

400 500 600 700

CGN10

CGL15

CGN15

Sample Peak width

CGL10 15.83

CGL25 14.19

CGN10 26.91

CGN25 31.35

CGL10

CGL25

CGN10

CGN25

600570

Raman Shift (cm-1)

465

Ra

ma

n I

nte

ns

ity

(a

.u.)

CGL10

Figure 7.2 Raman spectroscopy of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x=0.10,0.25)



The dc conductivity of prepared samples is shown in figure 7.3. At higher

temperatures ceria and doped ceria become conductive and the oxygen ion conductivity

is dependent on temperature although these are very high resistive at room temperature.

The values of conductivities of prepared samples at different temperatures are given in

table 7.2.

Figure 7.3 DC conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) as

function of temperature.

300 400 500 600 700

0.00

0.02

0.04

0.06

0.08

0.10

0.12

d

c (S

-cm

-1)

Temperature (oC)

CGL10

CGL25

300 400 500 600 700

0

1

2

3

4

dc (S

-cm

-1)

Temperature (oC)

CGN10

CGN25

0.00

0.02

0.04

0.06

0.08

0.10

0.12

72

Table 7.2 DC conductivities of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25)

at different temperatures

Sample DC Conductivity dc (S-cm

-1) at

500 0C 550

0C 600

0C 650

0C 700

0C

CGL10 0.0082 0.0177 0.0318 0.0611 0.1120

CGL25 0.0034 0.0074 0.0159 0.0317 0.0610

CGN10 0.0066 0.0141 0.0298 0.0602 0.1051

CGN25 0.3490 0.8830 1.7830 2.9990 4.4591


The frequency spectra for ac conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx

NdxOδ (x = 0.10, 0.25) at different temperatures (3000C – 700

0C) are given in figure 7.4

(a-d). The conductivity increased with the increase in frequency at all temperatures.

This behavior is manifestation of phenomenon explained by jump relaxation model.

Hoping of ions from one vacant state to other is responsible for the successful dc

contribution towards total conductivity at lower frequencies. Jumping and relaxation of

ions led to dispersion at higher frequencies. There are two ways in which these ions can

relax, either to go back to its original site or may reorder their jumping motion

[105,107]. There appeared a frequency dispersion region in the conductivity spectra

due to the relaxation of ions associated with the conduction. The dispersion region

present in conductivity spectrum, associated with relaxation of ions shifted to the

higher frequency region on increase of temperature. The contribution of grain interior

and grain boundary conduction can be confirmed from the presence of step in the

dispersion region [111-112, 117-118, 122-123].

73

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

700oC

600oC

500oC

400oC

300oC

log

a

c (

S-c

m-1)

log f (Hz)

Figure 7.4(a) AC conductivity of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-3.6

-3.2

-2.8

-2.4

-2.0

-1.6

-1.2

-0.8

-0.4

700oC

600oC

500oC

400oC

300oC

log

a

c (

S-c

m-1)

log f (Hz)

Figure 7.4(b) AC conductivity of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different

temperatures

74

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

-3.2

-2.8

-2.4

-2.0

-1.6

-1.2

-0.8

700oC

600oC

500oC

400oC

300oC

log

ac (

S-c

m-1)

log f (Hz)

Figure 7.4(c) AC conductivity of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different temperatures

700oC

600oC

500oC

400oC

300oC

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

log

a

c (

S-c

m-1)

log f (Hz)

Figure 7.4(d) AC conductivity of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different

temperatures


The dielectric constants as a function of frequency are shown in figure 7.5 (a -

d). The behavior of plots is explained by the ‗universal‘ dielectric response. The

absence of loss peaks in such materials can be explained through Jonscher‘s power law

[105, 124]. Higher values of dielectric constant at lower frequencies are due to the

polarization of charge carriers at the electrode-electrolyte interface. With the increase in

75

temperature, the polarization increases due to the enhancement of mobility of charge

carriers [112, 117-118]. Values of dielectric constant at different temperatures and

lowest and highest frequency applied are given in table 7.3.

Table 7.3 Values of dielectric constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ

(x = 0.10, 0.25) at 500, 600 and 700 0C for 1 kHz and 3 MHz

Sample

name

Dielectric constant (′)

at 3 MHz at 1 kHz

500 °C 600

°C 700

°C 500

°C 600

°C 700

°C

CGL10 10.95 13.26 16.61 71.60 117.31 346.71

CGL25 12.99 15.39 18.34 89.46 368.29 1082.92

CGN10 15.86 17.84 21.26 88.23 190.41 398.81

CGN25 30.81 35.63 21.49 10969.08 58910.74 198212.12

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4 700oC

600oC

500oC

400oC

300oC

log

Die

lec

tric

co

ns

tan

t (')

log f (Hz)

Figure 7.5(a) Dielectric constant of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different

temperatures

76

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51

2

3

4

5

6

700oC

600oC

500oC

400oC

300oC

log

Die

lec

tric

co

ns

tan

t (')

log f (Hz)

Figure 7.5(b) Dielectric constant of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different

temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6 700

oC

600oC

500oC

400oC

300oC

log

Die

lec

tric

co

ns

tan

t (')

log f (Hz)

Figure 7.5(c) Dielectric constant of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different

temperatures

77

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51

2

3

4

5

6

700oC

600oC

500oC

400oC

300oC

log

Die

lec

tric

co

ns

tan

t (')

log f (Hz)

Figure 7.5(d) Dielectric constant of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different

temperatures

7.2.4 Dielectric Loss

The frequency dependent dielectric loss (tanδ) plots are given in figure

7.6 (a-d) for Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.1, 0.25). At lower frequencies

tanδ has larger values and at higher frequencies the value decreased and became

independent of frequency. For Ce1-2xGdx NdxOδ the peak shift is clearly showing the

polarization phenomenon even at higher frequencies. Also, Vo.. can occupy any of the

eight equivalent sites around A3+

can jump from one site to the other site giving rise to

reorientation and relaxation process. With increase in temperature, the peaks shift

towards the higher frequencies. The effect is widely observed in ceria based

compounds by various researchers [56, 112, 117-118].

78

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

20

40

60

80

100

120 700oC

600oC

500oC

400oC

300oC

tan

log f (Hz)

Figure 7.6(a) Dielectric loss of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

5

10

15

20

25

30

700oC

600oC

500oC

400oC

300oC

tan

log f (Hz)

Figure 7.6(b) Dielectric loss of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different temperatures

79

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

10

20

30

40

50

60 700

oC

600oC

500oC

400oC

300oC

tan

log f (Hz)

Figure 7.6(c) Dielectric loss of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different temperatures

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

20

40

60

80

100 700

oC

600oC

500oC

400oC

300oC

tan

log f (Hz)

Figure 7.6(d) Dielectric loss of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different temperatures

80

7.3 Conclusions

Nanocrystalline Gadolinium doped Ceria, Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ

(x = 0.10, 0.25), were successfully synthesized with facile composite mediated

hydrothermal method. The X-ray diffraction confirmed the phase and composition of

the synthesized material. The crystallite sizes were estimated using Scherrer and Stokes

and Wilson‘s formulae. The range of crystallite size was 30-83 nm corresponding to

most intense peak using Scherrer‘s formula. The x-ray diffraction data was used to

calculate lattice constants. DC conductivity was measured in temperature range 300-

700 0C and ac conductivity was calculated in frequency range 1kHz to 3MHz at

temperatures 300, 400, 500, 600 and 700 0C. The ‗universal‘ dielectric response with

Jonscher power law and jump relaxation model explained the conduction phenomena in

the synthesized material. The Raman spectra confirmed the doping and increase of

vacancy sites. The maximum conductivity achieved is 1.78 S-cm-1

for Ce0.5Gd0.25

Nd0.25Oδ.

7.4 Comparison Table

A comparison of values of conductivities with literature is given in table 7.4 .

Ce0.5Gd0.25 Nd0.25Oδ prepared with CMH method got maximum conductivity overall.

The Gd and Dy doped materials which have larger values than this work (Gd doped

ceria) are synthesized with high temperature preparation methods.

81

Table 7.4 Comparison of conductivity values with literature.

# Composition Synthesis method Temp

.

oC

Conductivity

S-cm-1

Ref.

1 Ce0.85Gd0.15O1.9

25

Thermal

decomposition

600 0.025 Veranitisagul et al,

Ceram Int 2012 [54]

2 Ce0.85Dy0.15O2-δ Combustion

MH(microwave

heating)

CH(convention

al heating)

550

550

7.42x10-2

9.79x10-3

Acharya,

J Power Sourc 2012 [55]

3 Ce0.8Gd0.2O2-δ Citrate auto

ignition

550 1.8x10-4

Baral et al, Nanoscal Res

Lett 2010 [118]

4 Ce0.8Eu0.2O2-δ Citrate auto

ignition

550 1.39x10-4


Lett 2010 [118]

5 Ce0.8Dy0.2O2-δ Citrate auto

ignition

550 1.36x10-4


Lett 2010 [118]

6 Ce0.8Ho0.2O2-δ Citrate auto

ignition

550 1.4x10-4


Lett 2010 [118]

7 Ce0.85Gd0.15O2-δ Citrate auto

ignition

550

600

1.2x10-4

2.6x10-4

Baral et al, Appl Phys A

2010 [117]

8

Ce0.9Gd0.1Oδ

CMH

(wire like

morphology)

550

600

4.02×10-4

6.74×10-4

This work

9

Ce0.75Gd0.25Oδ CMH 550

600

4.78×10-4

9.30×10-4

This work

10 Ce0.5Gd0.25

Nd0.25Oδ CMH

550

600

0.88

1.78

This work

82

Chapter 8 Synthesis and

Thermophysical

Characterization of

Rare-Earth Hydroxides

83

Synthesis and Thermophysical Characterization of Rare-

Earth Hydroxides

Introduction

The preparation and depiction of Ce(OH)3, La(OH)3 and Nd(OH)3 is given

here. The precipitated hydrothermal method was used to grow the nanostructures. The

prepared samples got very interesting morphologies which make these fascinating for

further functionalization in various applications. The synthesis method and

stoichiometric change in composition affect the crystal structure and morphology.


8.1.1 Structural Analysis

The XRD patterns of rare earth hydroxides are given in figure 8.1. The

corresponding pattern confirmed the phase and hexagonal structure of Ce(OH)3 (card #

01-074-0665), Nd(OH)3 (card # 00-006-0601) and La(OH)3 (card # 00-006-0585). The

crystallite sizes corresponding to most intense peaks and lattice constants are

mentioned in table 8.1.

20 30 40 50 60 70 80

(110)

(103) (302)(311)

2degrees

Inte

nsity

(a.u

.)

(002)(202)

(112)

Ce(OH)3

(110)

(310) (410)(311)

(220)

(300)

(120)

(201)(200)

(311)

(310)

(112)(211)

(300)

(210)

(201)

(200)

(101)

(110)

La(OH)3

(302)

Nd(OH)

3

Figure 8.1 XRD pattern of Ce(OH)3, Nd(OH)3 and La(OH)3 samples

84

Table 8.1 Crystallite size corresponding to the most intense peak and lattice constants

for Ce(OH)3, Nd(OH)3 and La(OH)3.

8.1.2 Surface Morphology

The SEM micrographs are shown in figure 8.2. The Ce(OH)3 has belts like

structures, Nd(OH)3 has needles like structure and La(OH)3 has wires like structures

with lengths in microns.

Figure 8.2 SEM micrographs of Ce(OH)3, Nd(OH)3 and La(OH)3 samples

Sample D s (1 1 0)

(nm)

Lattice Constant Å

a c

Ce(OH)3 41 6.52(2) 3.81(1)

La(OH)3 33 6.53(2) 3.86(1)

Nd(OH)3 18 6.40(2) 3.74(1)

Ce(OH)3

Nd(OH)3

La(OH)3

85

The growth of samples can be attributed to the chemical potential maintained

through the precipitating agent, the pressure inside the vessel, the temperature provided

for the hydrothermal treatment and time for heat treatment. Moreover, the crystal

structures of precursors also play role in growth formation. The resultant shape is

dependent on the equilibrium energy of the respective faces according to Curie-Wulff-

Gibbs model [27, 35, 125].

8.1.3 Differential Scanning Calorimetry

The DSC plot showed the two step conversion of La(OH)3 to LaOOH and

finally La2O3. The conversions occur at ca. 400 0C and ca. 600

0C [35].

0 200 400 600 800 1000

-20

-15

-10

-5

0

5

10

La2O

3

LaOOH

He

at

Flo

w (

mW

)

Temperature (0C)

La(OH)3

Figure 8.3 DSC plot of La(OH)3

8.2 Electrical measurements


The dc conductivities of R(OH)3 are shown in figure 8.4 (a-c). The

conductivities are measured in temperature range 300 0C to 500

0C. Nd(OH)3 achieved

maximum conductivity whereas Ce(OH)3 got minimum among the three. The

conductivity increase with temperature shows the Arrhenius type dependence. The high

conductivity of Nd(OH)3 might be due to smaller crystallite size as the smaller grains

86

increase the grain boundaries and charges pile up on boundaries which help in

enhancement of conductivity [110, 113, 127-129].

300 350 400 450 500-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

dc (S

-cm

-1)

Temperature (oC)

Figure 8.4 (a) DC conductivity as a function of temperature for Ce(OH)3 sample

300 350 400 450 500

0

5

10

15

20

Temperature (oC)

d

c (S

-cm

-1)

Figure 8.4 (b) DC conductivity as a function of temperature for Nd(OH)3

87

300 350 400 450 5000

1

2

3

4

5

6

7

d

c (S

-cm

-1)

Temperature (oC)

Figure 8.4 (c) DC conductivity as a function of temperature for La(OH)3 sample

Table 8.2 DC conductivity as a function of temperature for hydroxide samples


The AC conductivities of R(OH)3 are given in figure 8.5 (a-c). These

conductivity plots also followed jump relaxation model. At lower frequencies the dc

conductivity is due to the jumping of ion from one vacant state to other. At higher

frequencies dispersion occur due to hopping and relaxation of ions. The increase in

temperature moved the dispersion region towards the higher frequency region. The step

in dispersion is the confirmation of grain boundaries and interior conduction. The

confirmation of ionic hopping in addition to Arrhenius relation is given by the

Jonscher‘s law.

σdc (S-cm-1

) 300oC 400

oC 500

oC

Ce(OH)3 0.001 0.018 0.372

La(OH)3 0.751 1.361 6.648

Nd(OH)3 1.284 4.582 20.369

88

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

500oC

400oC

300oC

log f (Hz)

log

a

c (

S-c

m-1)

Figure 8.5(a) AC conductivity as a function of frequency of Ce(OH)3 sample

3.0 3.5 4.0 4.5 5.0 5.5 6.0

1.6

1.7

1.8

1.9

2.0

2.1

2.2

500oC

400oC

300oC

log

a

c (

S-c

m-1)

log f (Hz)

Figure 8.5(b) AC conductivity as a function of frequency of Nd(OH)3 sample

89

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0.9

1.0

1.1

1.2

1.3

1.4

1.5 500

oC

400oC

300oC

log f (Hz)

log

a

c (

S-c

m-1)

Figure 8.5(c) AC conductivity as a function of frequency of La(OH)3 sample.

Table 8.3 AC conductivity as a function of temperature for hydroxide sample

8.3 Thermal Conduction

The thermal conduction and thermal diffusivity values were measured with

ATPS method. The values of conductivities and diffusivities are given in table 8.4.

Table 8.4Thermal conductivity and thermal diffusivity of R (OH)3

σac (S-cm-1

) 300oC 400

oC 500

oC

Ce(OH)3 0.806 0.056 0.964

La(OH)3 1.006 1.317 1.522

Nd(OH)3 1.721 2.001 2.207

Samples Thermal Conductivity

(Wm-1

K-1

)

Thermal Diffusivity

(mm2/s)

Ce(OH)3 0.75(2) 0.78(2)

La(OH)3 0.59(2) 0.96(2)

Nd(OH)3 1.05(2) 0.57(2)

90

The thermal conductivity is dependent on the scattering of phonons during their

motion due to thermal agitation. The mean free path is the measure of the thermal

conduction. The larger the mean free path, lager the thermal conduction and smaller

mean free path, lower the thermal conduction [119-120, 130].

8.4 Conclusions

The synthesis of Ce(OH)3, La(OH)3 and Nd(OH)3 was done by using the

hydrothermal method. Hexagonal structures were confirmed with XRD. The crystallite

size corresponding to most intense peaks were 18, 33 and 41 nm for Nd, La and Ce

hydroxides. SEM revealed very interesting and fascinating morphologies. Ce(OH)3 has

belts like structures, Nd(OH)3 has needles like structures and La(OH)3 has wires like

structures. The growth of structures can be ascribed to chemical potential maintained

through precipitating agent, the pressure inside the vessel, the temperature provided for

the hydrothermal treatment and time for hydrothermal treatment time. Two step

transformations from hydroxide to oxide was observed in DSC plot. Nd(OH)3 achieved

maximum conductivity and Ce(OH)3 acquired minimum among the three. The larger

values of conductivities for Nd(OH)3 and smaller in other two samples might be due to

smaller crystallite size. The smaller grains increase the grain boundaries and charges

can pile up on boundaries which increase the conductivity. The thermal conductivity

values were determined using ATPS method and were found in low thermal

conductivity region. The thermal conduction is dependent on the scattering and mean

free path. The less mean free path and more scattering gave rise to low conductivity

values. Nd(OH)3 have larger value of thermal conductivity which might be due to its

smaller crystallite size.

91

Chapter 9 Synthesis and Fluorescence

in NaMnF3: Yb;Er/Tm

92

Synthesis and Fluorescence in NaMnF3: Yb;Er/Tm

9.1 Structural and Morphological Analysis


The X-ray diffraction was done on as prepared samples of NaMnF3:Yb/Er and

the obtained pattern can be indexed with reference pattern PDF-number 00-018-1224

having orthorhombic crystal system and space group number 62. The represented

pattern is shown in figure 9.1.

20 25 30 35 40 45 50 55 60 65

(3 2 1)(2 0 2)

(1 2 1)

(2 0 0)

Inte

ns

ity

(a

.u.)

2Theta (degrees)

PEI@NaMnF3:Yb,Er (Yb18Er2Mn80)

(1 0 2) (1 3 1)(2 1 2)

(1 0 3)

(1 1 3)

(2 4 0)

Figure 9.1 XRD pattern of PEI-capped NaMnF3:Yb,Er

9.1.2 Transmission Electron Microscope Analysis

TEM revealed the nanorod structures with diameter range 50-60 nm and length

of few hundreds nm. The representative images are given in figure 9.2. The uniform

distribution, crystallinity and narrow range of particles‘ size can be observed from the

figure. The elongated structures have more intense emissions than the normal

nanocrystals [41]. Although the lack of in situ studies of solvothermal process [131]

makes it difficult to exactly assign a parameter to a specific job but the elongated

structured growth can be attributed to crystal structures of the reactants with ethylene

93

glycol as solvent [132] along with temperature and pressure [35, 133]. The fluoride

source itself is a deciding element [134-135]. Moreover, the shape evolution of crystals

is dependent on the compromise of the free energies of the faces of the crystal [125].

Figure 9.2 TEM images (a, b) for PEI-NaMnF3:Yb, Er ; Yb:Er 20/2 Mn 78 mol %

sample and (c, d) for PEI-NaMnF3:Yb, Er ; Yb:Er 60/2 Mn 38 mol % sample

9.2 Fluorescence Measurements

9.2.1 NaMnF3:Yb;Er

It is difficult to obtain up conversion (UC) system capable of pure single UC

emission because there are more than one metastable excited states in lanthanide ions,

generally [79]. For Yb/Er co-doped systems, two emissions are possible; bright green

emission ca. 550 nm and weak dark red emission ca. 660 nm. With the low penetration

depth of green and low intense red emission, this becomes a limit for tissue imaging

applications. The pure red and intense emission can penetrate into tissues (in vivio

imaging) [136].

94

The figure 9.3 show the fluorescence spectra for NaMnF3:Yb, Er. The red color

(4F9/2 to

4I15/2) emission is observed for Yb/Er 20/2. The increase in Yb concentration

gave rise to dual band red and green (4S3/2 to

4I15/2) emissions with green band

dominating [137]. To further tune the red emission obtained at Yb 20, the

concentration of Er was varied. The 10 and 20 mol % increase in Er gave rise to high

intensity red emissions as shown in figure 9.4. Both the green and red emissions in

respective samples are seen with naked eyes when excited by 980 nm laser. With such

intense emissions it can be anticipated that the deep tissue penetration is achievable.


molar percentage.

The red UC emission obtained put forward that energy transfer mechanism

exchanged between Er3+

and Mn2+

ions is exceptionally proficient, which can be

mainly credited to the resonances between the Mn2+

absorption bands and available

many metastable Er3+

levels. The refinement of emission color, red, could be ascribed

to non-radiative energy transfer from the 2H9/2 and

4S3/2 levels of Er

3+ to the

4T1 level of

Mn2+

, followed by back-energy transfer to the 4F9/2 level of Er

3+ which enhance the

450 500 550 600 650 700

0

1x104

2x104

3x104

0.05.0x10

3

1.0x104

1.5x104

2.0x104

2.5x104

0.0

5.0x104

1.0x105

1.5x105

0.0

2.0x105

4.0x105

6.0x105

8.0x105

Wavelength (nm)

Yb18

Yb20

Yb60

4S

3/2

4F

9/2

4I15/2

Yb80

Inte

ns

ity

(c

ps

)

95

likelihood of the red emission [93, 138-139]. The fluorescence increase and decrease

was found to be non-linear. Also with increase of Mn, the fluorescence intensity

decreased due to quenching phenomenon which happens due to efficient energy

transfer among dopant ions [136].


molar percentage.

9.2.2 NaMnF3:Yb;Tm

For NaMnF3:Yb,Tm blue color emission is observed for fixed Tm 0.02 and

changing Yb, Mn concentration as shown in figure 9.5. The observed strong emission

is NIR 3H4 to

3H6 [140] which might be due to energy transfer from

1D2 and

1G4

followed by back-energy transfer to the 3F4 level [141]. The dispersivity of sample in

water is clear as shown in figure 9.6 which was obtained due to successful

functionalization of surface of prepared samples with PEI.

450 500 550 600 650 700

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

0.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x1060

1x105

2x105

3x105

4x105

5x105

Wavelength (nm)

Er02

Inte

ns

ity (

cp

s)

Er10

4F

9/2

4I15/2

Er20

96

Figure 9.5 Upconversion of Tm doped PEI capped NaMnF3: Yb,Tm.

Figure 9.6 Dispersivity of NaMnF3:Yb;Er/Tm in water

400 500 600 700 800 900

0.0

2.0x106

4.0x106

6.0x1060.0

2.0x106

4.0x106

6.0x106

8.0x106

1.0x107

0.0

5.0x106

1.0x107

1.5x107

0.0

5.0x106

1.0x107

1.5x107

Wavelength (nm)

Yb20Tm0.02

Yb40Tm0.02

Yb60Tm0.02

Inte

nsit

y (

cp

s)

1G

4

3H

6

3H

4

3H

6Yb80Tm0.02

97

9.3 Conclusions

The synthesis and surface modification of NaMnF3 co-doped with Yb;Er/Tm

was successfully achieved in single step through solvothermal method. The PEI

polymer was used for surface modification. An intense green emission is observed for

NaMnF3:Yb, Er, with increase in Yb concentration and with fixed Er at 2 mol%. The

observed emission was around 550 nm and that was between levels 4S3/2 and

4I15/2.

Yb20 Mn78 Er2 revealed red emission at 660 nm and that was between levels 4F9/2 and

4I15/2 which became intense with increase of Er concentration. With Tm as dopant,

NEAR IR emission was observed at 800 nm between levels 3H4 and

3H6 although blue

emission was also observed at 480 nm between energy levels 1G4 and

3H6. The X-ray

diffraction confirmed the structure to be orthorhombic and TEM showed the

morphology to be nanorods with diameters 50-60 nm.

98

Chapter 10 Summary and

Conclusions

99

10.1 Summary and Conclusions

Comparative study for preparation and properties of phase pure nanocrystalline

ceria and doped ceria was done to obtain higher values of ionic conductivity. Wet

chemical methods like composite mediated hydrothermal method, co-precipitation

method and sol-gel method were adopted to synthesize the samples. X-ray diffraction

(XRD) and Raman spectroscopy were used for structural characterization on prepared

samples. Ceria, Gd doped ceria and co-doped ceria (CeO2, Ce

1-xGd

xO, Ce

1-2xGd

xLa

xO

and Ce1-2x

GdxNd

xO) showed the cubic fluorite structure. Ceria crystallite size

corresponding to most intense peaks for CMH, sol-gel and co-precipitated samples

were 60, 10 and 47 nm respectively. For the optimized results of CMH method, the

average crystallite sizes were in range 31 to 64 nm estimated with Scherrer formula. In

Ce1-x

GdxO , the range of crystallite size was 30-83 nm corresponding to most intense

peak using Scherrer‘s formula and 40-111 nm using Stokes and Wilson‘s formula. The

X-ray diffraction confirmed the phase and crystal structure of Ce1-2x

GdxLax/Nd

xO,

cubic structure except one peak each in Ce0.5Gd0.25 Nd0.25Oδ and Ce0.5Gd0.25 La0.25Oδ

corresponding to Nd2O3 and La2O3 respectively. The range of average crystallite size

was 31-39 nm corresponding to most intense peak using Scherrer‘s formula and 53-64

nm using Stokes and Wilson‘s formulae. The x-ray diffraction data was used to

calculate lattice constants which were in the range 5.45 Å to 5.91 Å. XRD confirmed

the hexagonal structures of Ce(OH)3, La(OH)3 and Nd(OH)3 and were found phase

pure. The crystallite size corresponding to the most intense peaks were 18, 33 and 41

nm for Nd-, La- and Ce- hydroxides.

Raman spectroscopy seconded the XRD results and intense peak ca.

465 cm-1

characteristic of Ce-O F2g band was observed for all samples. In addition to

this band, weak bands for intrinsic oxygen (appeared for charge neutrality due to

conversion of Ce4+

to Ce3+

) and oxygen vacancies were also observed, former ca. 546

cm-1

and later ca. 570 cm-1

& 600 cm-1

. For Raman excitation 514 nm laser was used.

The doped ceria and co-doped ceria also showed the increase in bandwidth of the peak

with doping contents which also increased the oxygen vacancies.

The dc conductivities of the samples were determined in temperature range 200-

700 0C. AC conductivities were determined in frequency range 20Hz-3MHz at

100

temperatures 300, 400, 500, 600 and 700 0C. For doped and co-doped ceria the

temperature range was 300-700 0C and frequency range was 1 kHz to 3 MHz. The

values obtained were in decreasing order with the synthesis method and were 0.345 S-

cm-1

(CMH), 0.095 S-cm-1

(Sol-gel) and 3.96x10-8

S-cm-1

(Co-precipitation) at 700 0C.

The maximum conductivity, achieved for Ce0.75Gd0.25Oδ, was 7.4x10-3

S-cm-1

at 700 0C.

The maximum conductivity achieved was 1.78 S-cm-1

for Ce0.5Gd0.25 Nd0.25Oδ. The

jump relaxation model can be used to explain the dc conductivity behavior. With the

thermal energies, the ion overcame the potential and moved to another available site

which caused a mismatch in the lattice. The relaxation could be achieved in two ways;

either the ion can move back or neighboring ion can move. The observation have put

weight in later scenario and by jump of neighboring ion, a hopping motion started thus

contributing to dc conductivity. The ‗universal‘ dielectric response or Jonscher power

law elaborates the ac conductivity phenomenon. The log of ac conductivity as a

function of log of frequency plot has a power dependence in the dispersion curves. The

dispersion region moved toward high frequency region with increase of temperature as

mobility increased. This phenomenon occurred due to the relaxation and hopping of

ions. The ‗step‘ in dispersion curves is confirmation of the grain interior and grain

boundary conductivities as ionic conduction is dependent on the defect formation due

to thermal energies which create vacancies to aid in hopping motion of ions. The high

values at low frequencies are due to electrode-electrolyte interface. The shift of

relaxation peaks toward higher frequencies with increase in temperature is also clear

from plots. These relaxation and reorientation of peaks which are due to the response of

ions to applied frequency is also evident in dielectric loss plots. The dielectric loss has

higher values at lower frequencies due to loss of energy in relaxation and reorientation

of ions whereas at higher frequencies dielectric loss has lower values because of the

higher frequencies; the ions were not able to response to frequencies. The enhancement

in conductivity was successfully achieved and this property made these materials as

potential candidates for SOFCs as electrolyte material.

Nd(OH)3 got maximum conductivity and Ce(OH)3 got minimum among the

three. The larger values of conductivities for Nd(OH)3 and smaller in other two samples

might be due to smaller crystallite size. The smaller grains increase the grain

boundaries and charges can pile up on boundaries which increase the conductivity. The

101

corresponding dc conductivity values of Ce(OH)3, La(OH)3 and Nd(OH)3 were 0.372,

6.648 and 20.369 S-cm-1

, respectively.

Very interesting and fascinating morphologies were revealed with SEM.

Ce(OH)3 has belts like structures, Nd(OH)3 has needles like structures and La(OH)3 has

wires like structures. The growth of structures can be attributed to chemical potential

maintained through pH adjusted by the precipitating agent, the pressure inside the

vessel, the temperature provided for the hydrothermal treatment and time for

hydrothermal treatment. The crystal structures of precursors also have a decisive role in

growth specially in obtaining different morphologies with different compounds. The

shape evolution can be explained by Gibbs-Curie-Wulff model which relate the shape

evolution with the face energies. When the equilibrium energy is obtained for

respective faces the Ostwald ripening is stopped. The monomer concentration is also

important which was obtained with rapid adjustment of pH of the solution. The DSC

plot showed a two-step transformation of R(OH)3 to R2O3. The La(OH)3 first

converted into LaOOH at ca. 400 0C and finally into La2O3 at ca. 600

0C. The dc

conductivities of these three samples were found in temperature range 300-500 0C. The

increase of conductivity with temperature is evident from the plots. These materials can

be utilized as thermal barrier coatings due to their low thermal conductivies.

The thermal conductivity values determined simultaneously with thermal

diffusivity using ATPS method for samples Ce1-x

GdxO, and R(OH)3 were in low

thermal conductivity region. The thermal conduction is dependent on the scattering and

mean free path. The less mean free path and more scattering gave rise to low

conductivity values which is the case here. The Gd contents increased the scattering

sites and conductivity decreased which can also be confirmed by Raman spectrum as

the bandwidth increased with Gd contents. The mean free path is the measure of the

thermal conduction. The larger the mean free path, larger the thermal conduction and

smaller mean free path, lower the thermal conduction.

To explore the fluorescence properties of rare earths intense emissions and

stabilized structures (crystal as well as morphologies), rare earths (Yb, Er, and Tm)

were exploited as Yb has maximum absorption cross section and served as sensitizer

and ladder like metastable states of Tm and Er were utilized as activators. The F based

inorganic matrix NaMnF3 was synthesized with dopants Yb, Er, Tm as F based

102

matrices are most stabilized when used with NIR lasers for biological imaging. The Mn

is anticipated to be more bio compatible as compared with other elements used in

synthesis of matrices. Moreover, the magnetic effect of Mn can be utilized for MR

imaging. The synthesis of NaMnF3 co-doped with Yb;Er/Tm was successfully

achieved through solvothermal method. The ethylene glycol was used as stabilizing

agent. Another important feature of this synthesis method was surface functionalization

of particles with the synthesis process in single step. Also, the choice of precursors of

Na & F and choice of stabilizing agent (EG) rendered the nanostructures to be rods like

which are not obtained normally in bio labels synthesis. The PEI polymer was used for

surface modification. An intense green emission is observed for NaMnF3: Yb, Er, with

increase in Yb concentration and for fixed Er at 2 mol%. The observed emission was

around 550 nm between levels 4S3/2 and

4I15/2. Yb20 Mn78 Er2 revealed red emission at

660 nm between levels 4F9/2 and

4I15/2 which became intense with increase of Er

concentration. With Tm as dopant, NIR emission was observed at 800 nm between

levels 3H4 and

3H6 although blue emission was also observed at 480 nm between energy

levels 1G4 and

3H6. For the excitation 980 nm laser was used. The red, green and blue

emission can be seen and photographed digitally when prepared samples were excited.

The X-ray diffraction confirmed the crystal structure to be orthorhombic and TEM

showed the morphology to be nanorods. The diameters of the rods were 50-60 nm

approximately. The PEI polymer made samples hydrophilic and were properly

dispersed in water. The intense emissions specially pure red and prescence of Mn made

these materials excellent applicant as bimodal imaging bioprobe in optical and MR

imaging.

103

10.2 Future Recommendations

Following are a few experiments and theoretical analysis that are suggested for future

studies.

The conductivity of ceria based compounds could be studied in oxygen

atmosphere.

The gas sensing properties of ceria and doped ceria could be studied.

These synthesized materials might be combined with cathode and anode

materials for the study of SOFCs.

The temperature dependent thermal conduction properties of R(OH)3 samples

could be studied

The NaMnF3:Yb;Er/Tm samples might be studied for the MRI and cytotoxicity

analysis

104

Chapter 11 References

105

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