A NEW LYOTROPIC LIQUID CRYSTALLINE SYSTEM:

OLIGO(ETHYLENE OXIDE) SURFACTANTS WITH TRANSITION METAL

COMPLEXES (M(H2O)nXm) AND THE SYNTHESIS OF

MESOPOROUS METAL SULFIDES

A THESIS

SUBMITTED TO THE DEPARTMENT OF CHEMISTRY

AND THE INSTITUTE OF ENGINEERING AND SCIENCES

OF BILKENT UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

By

ÖZGÜR ÇELİK

July 2001

ii

I certify that I have read this thesis and in my opinion it is fully adequate, in scope and

in quality, as a thesis of the degree of Master of Science

Asst. Prof. Dr. Ömer DAĞ

I certify that I have read this thesis and in my opinion it is fully adequate, in scope and

in quality, as a thesis of the degree of Master of Science

Prof. Dr. Atilla AYDINLI

I certify that I have read this thesis and in my opinion it is fully adequate, in scope and

in quality, as a thesis of the degree of Master of Science

Asst. Prof. Dr. Margarita KANTCHEVA

Approved for the Institute of Engineering and Sciences

Prof. Dr. Mehmet Baray

Director of Institute of Engineering and Sciences

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ABSTRACT

A NEW LYOTROPIC LIQUID CRYSTALLINE SYSTEM:

OLIGO(ETHYLENE OXIDE) SURFACTANTS WITH TRANSITION METAL

COMPLEXES (M(H2O)nXm) AND THE SYNTHESIS OF

MESOPOROUS METAL SULFIDES

ÖZGÜR ÇELİK

M.S. in Chemistry

Supervisor: Asst. Prof. Dr. Ömer Dağ

July 2001

In this study a new templating method, which can be used to synthesise

mesporous materials, has been developed. The main objective of this work is to form

organic mesophase in the presence of inorganic salts. This is an organic-inorganic

hybrid mesophase, which can be used to template the growth of inorganic materials.

Here for the first time, a new lyotropic liquid crystalline (LLC) system has been

presented from oligo (ethylene oxide) type surfactant and transition metal aqua

complexes.

The temperature and the metal aqua complex concentration range of the

complex/surfactant mixtures have been determined, where the mixtures have a liquid

crystalline (LC) phase. Here, the complex refers to Ni(NO3)2·6H2O, Co(NO3)2·6H2O,

Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, and CoCl2·6H2O and the surfactant is

C12H25(CH2CH2O)10OH, (C12EO10). The addition of the metal aqua complexes

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directly to the surfactant produces a LC phase. The LC phase obtained from the

mixture of these two is more stable than the LC phase obtained from a mixture of free

water and surfactant. The FT-IR and UV-Vis absorption, Polarised Optical

Microscopy (POM) and Powder X-ray Diffraction measurements show that the

coordinated water molecules mediate the formation of the LC phase. Our observations

also show that the coordinated water molecules make a stronger interaction with

ethylene oxide (EO) chains than free water molecules.

The LC templating approach, which is demonstrated as a new system has been

used for synthesis of meso-structured metal oxides, metal sulphides and even metal

mesh. From all these studies, it is well known that in order to maintain LC phase the

metal ion concentration should correspond to metal ion to surfactant mole ratio below

0.8. However, this work shows that the amount of metal aqua complex concentration

can be increased up to a 6.5 complex to surfactant mole ratio by maintaining the

integrity of the hexagonal and/or cubic structure of the LC phase. This may open a

new area for the realisation of new mesostructured materials with better qualities and

much higher yields.

In the first part of the thesis, the thermal and structural properties of the new

LLC phase has been established by using polarized optical microscopy (POM) with

an attached hot plate, PXRD, FT-IR and UV-Vis absorption methods. In the second

part, the new phase has been used as a template to synthesise mesoporus metal

sulfides. The second part of the thesis deals mainly with the structure and synthesis of

mesostructured CdS and ZnS. It has been demonstrated that the LC phase of

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Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O in oligo(ethylene oxide) surfactant survive

partially during the reaction with H2S to produce the corresponding metal sulfides.

Keywords: Lyotropic liquid crystal, mesophases, transition metal aqua complexes,non-ionic surfactants, mesoporous materials, metal sulfides.

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ÖZET

YENİ BİR LİYOTROPİK SIVI KRİSTAL SİSTEMİ:

OLİGO(ETİLEN OKSİD) YÜZEY-AKTİFLER İLE GEÇİŞ METAL

KOMPLEKSLERİ (M(H2O)nXm) VE MEZOGÖZENEKLİ METAL SÜLFÜRLERİN

SENTEZLENMESİ

ÖZGÜR ÇELİK

Kimya Bölümü Yüksek Lisans Tezi

Tez Yöneticisi: Asst. Prof. Dr. Ömer Dağ

Temmuz 2001

Bu çalışmada mezogözenekli malzemelerin sentezlenmesinde kullanılabilecek,

yeni bir kalıplama metodu geliştirildi. Bu çalışmanın ana amacı organik mezo fazı

inorganik tuzlarla hazırlamaktır. Bu, yeni inorganik malzemelerin oluşumunu

kalıplamada kullanılabilir, bir organik-inorganik melez mezo fazdır. Oligo (etilen

oksid) yüzey-aktifi ve geçiş metallerinin sulu komplekslerinden oluşan yeni bir

liyotropik sıvı kristal (LSK) sistemi, ilk kez bu çalışmada sunulmaktadır.

Metal kompleks/yüzeyaktif karışımlarının, sıvı kristal fazı içeren örneklerinde

sıcaklık ve metal sulu komplekslerin derişim aralıkları belirlenmiştir. Burada

kompleksler; Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, ve

CoCl2·6H2O tuzları ve yüzey-aktif; C12H25(CH2CH2O)10OH, (C12EO10) molekülüdür.

Metal sulu komplekslerinin yüzey-aktife direkt eklenmesi, LSK fazını

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oluşturmaktadır. Bu ikisinin karışımından elde edilen sıvı kristal (SK) faz, serbest su

ve yüzey-aktif karışımında elde edilen SK fazından daha kararlıdır. Polarize optik

mikroskobu (POM), PXRD, ve FT-IR ve UV-Vis soğurma spektroskopisi ölçümleri

SK fazının koordine olan su molekülleri tarafından yönlendirildiğini döstermektedir.

Ayrıca gözlemlerimiz, koprdine olan su moleküllerinin serbest su moleküllerine

nazaran, etilen oksit zinciri ile daha kuvvetli etkileşim içinde olduğunu göstermiştir.

Tamamen yeni bir sistem olan, SK kalıplama metodu mezo-yapılı metal oksit,

metal sülfür ve hatta metal ağların sentezlenmesinde bir süredir kullanılmaktadır. Bu

çalışmaların hepsinde, bilinen şu ki, metal iyon derişimi, SK fazı koruyarak,

metal/yüzey-aktif mol oranında 0.8 den daha yukarıya çıkılamamıştır. Fakat bu

çalışmada metal/yüzeyaktif mol oranının, altıgensel ve/veya kübik SK fazını

koruyarak, 6.5’e kadar yükseltilebileceği gösterilmiştir.

Bu çalışmanın birinci kısmında, POM, PXRD ve FT-IR ve UV-Vis soğurma

metodları kullanılarak, yeni SK sisteminin ısıya bağlı ve yapısal özellikleri

saptanmıştır. Bu çalışmanın ikinci kısmında, yeni sistem metal sülfürlerin

sentezlenmesinde kalıp olarak kullanıldı. Bu tezin ikinci kısmı CdS ve ZnS

sentezlenmesi ve yapısıyla ilgilidir. Ayrıca Oligo (etilen oksit) yüzey-aktif

içerisindeki Zn(NO3)2·6H2O, ve Cd(NO3)2·4H2O komplekslerin metal sülfürleri

oluşturmak için H2S ile reaksiyonları sırasında SK fazının tam olarak korunmadığı

açığa çıkarıldı.

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Anahtar kelimeler: Liyotropik sıvı kristal, mezofaz, geçiş metalleri ve sulukompleksleri, nötral yüzey-aktifler, mezogözenekli malzemeler,metal sülfürler.

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ACKNOWLADGEMENT

I would like to express my deep gratitude to Asst. Prof. Dr. Ömer DAĞ for his

encouragement and supervision throughout my studies.

I would like to thank to Mr. Murat GÜRE (Bilkent University Department Of

Physics) and Mr. Erdem YAŞAR (Kırıkkale University Department Of Physics) for

their help and support for recording SEM and TEM images, respectively.

I am very thankful to Ol’ga SAMARSKAYA, Özlem DEMİR, Sinan BALCI

A.Çağrı ATEŞİN, and all present and former members of Bilkent University

Chemistry Department for their kind helps and supports during all my study.

I whis to thank to Ahmet GÜNAY, for his help in preparation of some of the

samples.

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TABLE OF CONTENTS

1.INTRODUCTION……………………………………………………………………..1

1.1. FROM BULK TO MOLECULAR MATERIALS……………………………….1

1.2. LIQUID CRYSTALS…………………………………………………………….6

1.3. MESOPOROUS INORGANIC MATERIALS…………………………………12

1.4. LIQUID CRYSTALLINE PHASE; TEMPLATE FOR INORGANIC

MATERIALS……………………………………………………………………20

2.EXPERIMENTAL……………………………………………………………………23

2.1.MATERIALS…………………………………………………………………….23

2.2.SYNTHESIS……………………………………………………………………..23

2.2.1.Synthesis of Liquid Crystal Phase of Inorganic Salts……………………...23

2.2.2. Synthesis of CdS and ZnS………………………………………………...24

2.3.INSTRUMENTATION………………………………………………………….25

2.3.1. Polarized Optical Microscopy……………………………………………..25

2.3.2. X-Ray Diffraction…………………………………………………………26

2.3.3. FT-IR Spectroscopy……………………………………………………….27

2.3.4. UV-VIS Spectroscopy…………………………………………………….27

2.3.5. Scanning and Transmission Electron Microscopies (SEM and TEM)……28

3.RESULTS AND DISCUSSIONS…………………………………………………….29

CHAPTER-1…………………………………………………………………………29

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3.1.1. Lyotropic Liquid Crystalline (LC) Phase Behavior of Poly(oxyethylene)

Type Nonionic Surfactants with Transition Metal Aqua Complexes as a

Second Component……………………………………………………….29

3.1.2. Polarised Optical Microscopy (POM) and Thermal Properties…………...34

3.1.3.PXRD Analysis…………………………………………………………….43

3.1.4.FT-IR Spectral Studies……………………………………………………..54

3.1.5.Vis-Near-IR Spectral Studies………………………………………………74

CHAPTER-2…………………………………………………………………………77

3.2.1.Synthesis of Mesoporous Metal Sulfides…………………………………..77

3.2.2. FT-IR Spectral Analysis…………………………………………………..78

3.2.3. X-Ray Analysis……………………………………………………………80

3.2.4. SEM and TEM Analysis…………………………………………………..84

3.2.5.UV-Vis Spectral Analysis………………………………………………….86

4.CONCLUSION……………………………………………………………………….92

5.REFERENCES………………………………………………………………………..95

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LIST OF TABLES

1. PORE-SIZE REGIMES AND REPRESENTATIVE POROUS INORGANIC

MATERIALS……………………………………………………………………12

2. THERMAL PROPERTIES AND COMPOSITION OF MX2/NO

MIXTURES……………………………………………………………………...36

3. THERMAL PROPERTIES OF FOR MX2/NO MIXTURES WITH FREE

WATER. THE SURFACTANT/WATER IS 50 WT %. M; Ni(H2O)62+,

Co(H2O)62+, AND X; NO3

-, Cl–…………………………………………………42

4. FIRST AND SECOND DIFFRACTION LINES OF HIGH MX2/NO MOLE

RATIOS OF Cd AND Zn SAMPLES…………………………………………...52

5. DIFFRACTION LINES AND THEIR ASSIGNMENTS FOR VARIOUS

MX2/NO MOLE RATIOS. M= Cd(H2O)42+, Co(H2O)6

2+, Zn(H2O)62+, Ni(H2O)6

2+ ,

X= NO3-, Cl-……………………………………………………………………..53

6. VIBRATIONAL FREQUENCIES (cm-1) AND ASSIGNMENTS OF

NITRATES………………………………………………………………………61

7. IR PEAKS AND THEIR ASSIGNMENT OF POE IN SOLID AND IN

MOLTEN STATE, SURFACTANT (C12H25(CH2CH2O)10OH), AND

CoCl2.6H2O/SURFACTANT MIXTURE………...…………………………......67

8. OPTICAL BAND GAPS OF MESOPOROUS CdS AND ZnS SYNTHESIZED

WITH DIFFERENT MX2/NO MOLE RATIOS AND THE BULK VALUES….89

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LIST OF FIGURES

1. Representative phase transition, from solid to liquid crystal and then to liquid

phase…………………………………………………………………………………..7

2. Various types of surfactants…………………………………………………………...8

3. General lyotropic liquid crystalline phases, formed when a solvent and amphiphilic

macromolecules are mixed…………………………………………………………..10

4. Schematic phase diagram for C16TMABr in water………………………………….11

5. Formation of microporous and mesoporous molecular sieves by using short and long

alkyl chains…………………………………………………………………………..14

6. Two possible pathways for LCT mechanism………………………………………...15

7. Hydrolysis and polymerization-condensation reactions of silica alkoxide…………..18

8. Lyotropic liquid crystalline phases used as structure directing media (Template)…..21

9. Glass cells, used in the synthesis of metal sulfides. The cell A used for the synthesis

of thin film samples on a quartz substrate and the cell B (shlenk) used for large

quantities……………………………………………………………………………..25

10. Representation of structure of LC phase formed directly with metal aqua complexes

by the help of hydrogen bonding…………………………………………………….32

11. POM images of (a) Ni(NO3)2.6H2O/No hexagonal, (b) Co(NO3)2.6H2O/No hexagonal,

(c) Cd(NO3)2.4H2O/No hexagonal, (d) cubic phase of Cd(NO3)2.4H2O/No with a mole

ratio of 3.6…………………………………………………………………………....35

12. Temperature profiles for Co and Ni complexes with C12EO10….…………………...38

13. Temperature profiles for Cd and Zn complexes with C12EO10……………………….39

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14. Temperature profiles for MX2/No mixtures with free water. The surfactant/water is 50

wt %. M; Ni(H2O)62+ , Co(H2O)6

2+ and X; NO3-, Cl-………………………………..41

15. Schematic representation of X-ray diffraction from atomic planes………………….43

16. First three diffraction lines of hexagonal structure……………………….………….44

17. X-ray diffractogram of Cd(NO3)2.4H2O/Surfactant, mole ratio is; 2………………..45

18. PXRD patterns of Cd(NO3)2.4H2O/Surfactant, with increasing mole ratios………...46

19. X-ray diffractogram of Ni(NO3)2.6H2O/Surfactant, mole ratio is; 2.6………………47

20. X-ray diffractogram of Ni(NO3)2.6H2O/Surfactant, with increasing mole ratios……48

21. X-ray diffractogram of (A) Co(NO3)2.6H2O/Surfactant, mole ratio is; 2.6, (B)

Co(NO3)2.6H2O/Surfactant, with increasing mole ratios…………………………….49

22. X-ray diffractogram of (A) CoCl2.6H2O/Surfactant, mole ratio is; 2.2, (B)

CoCl2.6H2O/Surfactant, with increasing mole ratios………………………………...50

23. X-ray diffractogram of (A) Zn(NO3)2.6H2O/Surfactant, mole ratio is; 2.6, (B)

Zn(NO3)2.6H2O/Surfactant, with increasing mole ratios…………………………….51

24. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt %), (c)

Cd(NO3)2.4H2O/surfactant, mole ratio of 2………………………………………….55

25. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt %), (c)

Cd(NO3)2.4H2O/surfactant, mole ratio of 2…………………………………………56

26. FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant, mole ratio

is 2, (c) Co(NO3)2. 6H2O/ surfactant, mole ratio is 2, (d) Co(NO3)2. 6H2O crystal…57

27. FT-IR spectra of (a) pure (molten) surfactant (b) Zn(NO3)2.6H2O/ surfactant, mole

ratio is; 0.5, (c) Ni(NO3)2.6H2O/ surfactant, mole ratio is; 0.5, (d) Co(NO3)2.6H2O/

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surfactant, mole ratio is; 0.5, (e) Cd(NO3)2.4H2O/ surfactant, mole ratio is; 2. All

sample contain 50 wt % water with respect to surfactant……………………………59

28. FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant, mole ratio

is; 2, (c) Co(NO3)2.6H2O/ surfactant, mole ratio is; 2, (d) Co(NO3)2.6H2O crystal…60

29. Trans and gauche conformers of 1,2-dimethoxyethane and GTTG conformer of -

OCH2CH2OCH2CH2O- group………………………………………………………..62

30. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt %), (c)

CoCl2.4H2O/ surfactant, mole ratio is; 2……………………………………………..63

31. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt %), (c)

Cd(NO3)2.4H2O/surfactant, mole ratio is 2…………………………………………..64

32. FT-IR spectra of (a) pure (molten) surfactant (b) Cd(NO3)2. 4H2O/ surfactant, mole

ratio is; 2, (c) CoCl2.6H2O/ surfactant, mole ratio is; 2………………...……………66

33. Representation of helical structure of POE, GTTG conformation. Blue ; C, red ; O,

white ; H……………………………………………………………………………..68

34. IR spectrum of CoCl2.6H2O/Surfactant with the mole ratio of (a) 1.4, (b) 1.8, (c) 2.2,

(d) 2.6, (e) 3.0………………………………………………………………………..69

35. IR spectrum of Co(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2, (b) 1.6, (c)

2.0 (d) 2.4, (e) 2.8, (f) 3.2……………………………………………………………70

36. IR spectrum of Zn(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2, (b) 1.6, (c)

2.0 (d) 2.4, (e) 2.8, (f) 3.2……………………………………………………………71

37. IR spectrum of Ni(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2, (b) 1.6, (c)

2.0, (d) 2.6, (e) 3.0…………………………………………………………………...71

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38. IR spectrum of Cd(NO3)2.4H2O/Surfactant with the mole ratio of (a) 1.2, (b) 1.6, (c)

2.0, (d) 2.6, (e)3.0……………………………………………………………………72

39. IR spectrum of MX2/No with the mole ratio of 2, M: (a) Ni, (b) Zn, (c) Co (d) Cd…73

40. Vis-Near-IR spectrum of various Ni(NO3)2.6H2O/surfactant mixtures (a) 0.1 M

Ni(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6……………………………………74

41. Vis-Near-IR spectrum of various Co(NO3)2.6H2O/surfactant mixtures (a) 0.1 M

Co(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6…………………………………...75

42. A; IR spectum of Cd(NO3)2.4H2O/No (a) After H2S exposure, (b) before H2S

exposure. B; after washing…………………………………………………………...79

43. A; IR spectum of Zn(NO3)2.6H2O/No (a) After H2S exposure, (b) before H2S

exposure. B; after washing…………………………………………………………...80

44. X-ray diffractograms of (a) before H2S gas exposure, (b) after washing (pure CdS),

(c) after H2S gas exposure (CdS in organic matrix). Cd(NO3)2.4H2O/surfactant mole

ratio is 6.5……………………………………………………………………………81

45. X-ray diffraction patterns of (a) before H2S gas exposure, (b) after H2S gas exposure

(ZnS in organic matrix), (c) after washing (pure ZnS), Zn(NO3)2.6H2O/surfactantmole

ratio is; 4……………………………………………………………………………..82

46. Wide angle diffractogram of CdS after washing. Synthesised from

Cd(NO3)2.4H2O/surfactant mole ratio is 1.5. ………………………………………..83

47. SEM image of CdS after washing the samples, showing the morphology of the

mesostructured CdS………………………………………………………………….84

48. TEM image of washed CdS sample, showing the mesoporosity in the sample……..85

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49. Absorption spectrum of CdS. Synthesised from Cd(NO3)2.4H2O/surfactant mole ratio

is 1.8………………………………………………………………………………….88

50. Absorption spectrum of ZnS. Synthesised from Zn(NO3)2.6H2O/surfactant mole ratio

is 2.4………………………………………………………………………………….88

51. Absorption spectrum of CdS (A) and ZnS (B) synthesized from different MX2/No

mole ratios……………………………………………………………………………90

1

1. INTRODUCTION

1.1.From Bulk to Molecular Materials

The synthesis of well-defined, monodispersed nanoparticles, in the 1-100 nm

range, has attracted great attention in the materials community, because the nanoparticles

have properties in between atoms and bulk materials. Therefore the research on nano-

structured materials has developed very rapidly over last decade. The materials

community is looking for new materials for specific purposes. The major aim of the

current research in materials chemistry is to be able to control the synthesis in a

molecular level to design materials with novel property [1].

The nano-structures are assemblies of bonded atoms that have dimensions in the

range of 1 to 100 nanometers (nm). In bulk materials, the property is determined by the

bulk atoms. However, if we reduce the particle size of any material to nanometers, the

surface atoms become effective in determining the electronic [2], optic [3], catalytic [4]

and magnetic [5] properties of the materials, because the thermodynamic properties of the

atoms at interior is different than the atoms at the surface. The nanoparticles also show

reversible metal-insulator transitions due to inter-particle interactions, which may be

controlled by adjusting size of the particles and the inter-particle distance between the

particles by capping with ligands. While the bulk materials are considered to be spherical,

in nanoparticles shape is very important and may influence the optical properties of, for

instance, the metal nano-particles [6]. The factors are [7]:

2

● number of atoms on the surface to be close to the number of atom in the

interior.

● the ratio of the surface energy to the total energy to be close to unity.

● conduction or valance electrons to be confined to a small length of volume

(Particle in 3-D box).

As we reduce the size of the particles, the surface area of the particles and the

interface between particles and surroundings increase. Therefore, the surface free energy

of the particles increases [8]. This makes the colloidal particles very unstable [8]. The

small particle stability depends only on the secondary minimum [8] in their potential

energy diagram. At the secondary minimum point, the nanoparticles are meta-stable.

Therefore, the bare nanoparticles are not stable by themselves. However a stabilizer,

which is a ligand, reduces this excess free energy and organizes the nano-particles with

each other to form super-lattices. When the bare metal particles make contact with each

other, the immediate reaction is an aggregation [8]. The coordinated ligands or adsorbed

surfactant onto the surface of the clusters prevent the aggregation. The ordered

superlattices, which are composed of micrometer-sized particles, have been known in

colloid chemistry for quite a long time [8]. This is also not a new strategy for biological

systems, because from proteins to viruses, they all have nanometer dimensions.

There are many synthetic methods reported in the literature that have been applied

to synthesize ordered, monodisperse metal superlattices in the literature. Self-assembly

[9], for example, which is a spontaneous assembly process of molecules into a stable

3

structure and non-covalently joined aggregates, is one of the mostly applied methods for

size and shape tailoring. The ordered precipitation of a ligand stabilized Au has been

extensively studied [9-13]. For instance, the AuCl4- solution in water-toluene yields

colloidal micro emulsions upon reducing with sodium borohydride [9]. These colloidal

particles were stabilized with dodecanethiol, C12H25SH [10], and dithiol to synthesize

gold monolayer over a substrate [11]. These thiol derivatives coordinate to the gold

nanoparticles and protect them from coalescence. Gas-phase synthesis of bare clusters

followed by solution phase encapsulation with aryl ditihols or aryl diisonitriles has also

been applied to Au nanoparticles [12]. The basic steps in colloidal self-assembly [12-13]

of nanoparticles are:

i) synthesis of ultra-fine metal clusters with uniform diameter.

ii) adsorption of self assembled monolayer of organic surfactant on the surface of

these particles to prepare macro molecular entities.

iii) displacement of organic surfactant with a molecular interconnect that bonds

adjacent particles without destroying the order.

The colloidal pathway is widely used to prepare nanoparticles.

Inorganic/surfactant composites, dispersible in a suitable dispersion, give colloidal metal

particles stabilized in water-in-oil micro emulsions [14]. These micro emulsions have

been used as micro reactors to synthesize desired materials [15]. Polymeric processes

have also been applied. For instance, the block co-polymer micelles have been used as a

reaction vessel to prepare monodispersed particles. In these methods, the particle size is

determined by host-guest effects [16-17]. The use of 3-D crystalline superlattices [18],

4

such as zeolites, which control the geometry and shape of the nanoparticles, have been

widely demonstrated in the literature [18]. Porous glasses, [19] STM tips, [20] epitaxial

methods [21] have also been applied to get materials with desired dimensions and

geometry.

The reaction conditions are very important to obtain monodispersed crystalline

nanoparticles. For example, in the synthesis of CdS, CdSe, CdTe [22], which are capped

with tri-n-octylphosphine and tri-n-octylphosphine oxide, the modulation of reaction

temperature gives a homogenous size distribution. Different reaction conditions [23] may

lead to a distinct morphology, such as faceted crystals to ordered thin films. It is also

possible to obtain cubic [24], hexagonal [25] and triangular [26] nanocrystals depending

on the structure of the micro reactor, which is basically an organic template.

The small metal islands, separated by a barrier (stabilizer) with enhanced stability

may show very different behaviors compared with their bulk counterparts.

Semiconductors and metals, which are smaller than the diameter of bulk Bohr exciton,

show drastic changes in their electronic properties, due to zero dimensionality [27]. The

nanoparticles are better photoemitters [28] than their bulk counterparts, due to three-

dimensional confinement of electrons and holes within a particle (size quantization). The

band gap depends strongly on the particle size. For example, CdSe [28] nanoparticles

with different sizes emit from 450 nm to 650 nm, blue to red portion of the

electromagnetic spectrum.

5

The catalytic properties of colloidal particles and metal clusters have also been

extensively studied for a long time [4]. Most of the small metal clusters and colloids have

better catalytic properties compared to the bulk counterparts. The metal sols show

increasing catalytic activity with decreasing particle size [29], due to increased number of

active sites on the surface. However in some cases, like in H2-D2 exchange reaction by Pt

catalyst [30] the particle size has opposite effects. This is due to increase in the binding

energy of the molecular species on the surface of platinum nanoparticles by the decreased

particle size. As a consequence the reaction activation energy increases.

The semiconductor quantum dots have been used in photo-catalytic reactions to

increase the quantum yield, because the band gap can be tuned by changing the size of

the particles [31]. Therefore, the electron and hole redox potential can be tuned to achieve

increased redox power [31]. For example, NO3- reduction [31] has been achieved by the

CdS nano-crystals at natural pH, which is impossible, by bulk CdS.

Naturally, magnetic particles smaller than 10 nm are usually super paramagnetic

and behave as single magnetic domain. For example, Co nanoparticles synthesized, from

organometallic precursor, show larger magnetic moment per atom compared to the bulk

cobalt [32]. It has also been reported [33] that, the saturation magnetization is not only

depended on the size of the particles but also on the surface and structural characteristics

of the particles. The 2-D arranged nanoparticles have different magnetic behaviors than

the ones dispersed in solution [34].

6

As a result of their unique properties, molecule-based materials are very attractive

and represent rapidly developing area of materials chemistry. New methodologies for

synthesis and characterization of novel materials are growing very rapidly.

1.2. Liquid Crystals

Generally, three phases of the matter are considered to be solids, liquids and

gasses. Due to strong forces between the molecules, all the molecules have rigid

arrangement in solid state, because each molecule occupies a certain place (positional

order). Also the molecules are oriented in a specific direction (orientational order).

Because of the highly ordered arrangement, the attractive forces are additive. In liquid

state, molecules are neither occupy a specific average position nor remain oriented in a

particular direction. Therefore, the attractive forces are not additive. The molecules in gas

phase have less order than the molecules in the liquid phase. Therefore, the forces

between the molecules are not strong enough to hold the molecules together.

The liquid crystalline (LC) state is the fourth state of the matter and it has

properties of both liquid and solid states. When the solids melt they lose their positional

and orientational order. However in LC phase positional order may be lost but molecules

or aggregates in LC phase still have some orientational order [35], as shown in Figure

1[35].

7

Figure 1. Representative phase transition, from solid to liquid crystal and then to liquidphase [35].

There is only a slight tendency for the molecules to point in one direction than the

others or to spend more time in various directions than other directions. This gives an

anisotropic nature to the liquid crystals.

The LC phases can be divided into two main groups, the thermotropic and

lyotropic. The thermotropic LCs are made up of rod like (calamitic LC), disc like

(discodic LC), or polymeric (polymeric LC) molecules, in which the LC is formed phase

by heating. Briefly, if the temperature increases, the thermotropic mesogens change

phases from crystal to smectic then to nematic and finally to the liquid phase [35-36].

This sequence also shows the degree of order for these phases.

The lyotropic LCs are formed depending on the concentration of the mesogen in

an appropriate solvent [35-38]. The LC phase can also be formed not only by changing

the temperature of the medium but also by changing the concentration of one substance in

8

another. In this study, the lyotropic LCs are studied, therefore only lyotropic LC phase

will be explained in detail.

The lyotropic liquid crystals (LLC) can be obtained by mixing surfactant and

water (as solvent). The surfactants have dual character and so called as amphiphilic

molecules, which have both hydrophilic (affinity for water) and hydrophobic (affinity for

oil) parts. The surfactants are surface-active molecules, which reduce the surface tension

of the solvent [39]. As a result, the surfactants adhere to the solvent surface [39]. Water is

the main solvent used. The surfactants have dual character (Figure 2) because the non-

ionic surfactants have long alkyl chains (tails) which is hydrophobic and a hydrophilic

polyoxyethylene (-CH2CH2O-, POE) head group.

Figure 2. Various types of surfactants

R

O

O

O

O

O

O

O

OH

O

S

R O

OO

-Na

+

H

H

N+

R

H Cl- Cationic

Anionic

Non ionic

9

There are various kinds of surfactants as shown in Figure 2. They can be

classified as anionic, cationic, amphoteric and non-ionic based on the type of their head

group. The anionic and cationic surfactants are electrolytes and they give the charge to

the adsorbed surface and align alkyl group away from the surface.

If a surfactant is added to water, the amphiphilic surfactant molecules orient

themselves at the air/water interface. In order to reduce water-hydrocarbon interaction,

the hydrocarbon tail orients itself toward air [35-39]. As a result, addition of surfactant

reduces the surface tension of water until a point where it stays constant. Adding more

surfactant will cause phase separation like benzene-water mixture. However neither

complete phase separation nor molecularly dispersed solution occurs. The surfactant

molecules self-assemble to create a micro phase in which the hydrocarbon chains

sequester themselves inside the aggregate and the polar head groups orient themselves

toward the aqueous phase. This is the point where micellisation is discerned and is called

critical micelle concentration (CMC). As shown in Figure 3, the addition of surfactant to

the solvent first produces dissolved surfactant molecules and upon increasing the

surfactant concentration leads to formation of micelles. Further increase in surfactant

concentration leads to the ordered mesostructures.

10

Figure 3. General lyotropic liquid crystalline phases, formed when a solvent andamphiphilic macromolecules are mixed.

The surfactants form reverse micelles (head group inside the micelle) in organic

solvents, and micelles (head group outside the micelle) in polar solvents. Such phases are

characterised by having some physical properties of both crystalline and fluid structures.

These phases will have at least one dimension, which is highly ordered and, as a result,

will exhibit relatively sharp x-ray diffraction patterns and optical birefringence.

Lamellar

Hexagonal

Cubic

Increasing Surfactant Concentration

Hydrophilic Head

Hydrophobic Tail

11

Figure 4. Schematic phase diagram for C16TMABr in water [40].

The amphiphilic molecules are associated physically not chemically [35-39], so

that they can change the size or shape of their microstructure in response to small

changes in concentration, salt content, temperature, pH, and pressure. Therefore, it is

possible to get different kind of mesophases and mesostructures by changing the

surfactant concentration and/or temperature, see Figure 4 [40].

Surfactant Concentration wt %

12

1.3. Mesoporous Inorganic Materials

Porous materials have very important features due to their micro structures, which

enable to obtain high surface area. As a result, these materials may get enhanced catalytic

activity and adsorption capacity. A lot of research has done to tailor the pore size and

shape. Many mesoporous materials have been synthesized [41] in different pore-size and

shape regimes, (see Table 1).

Table 1. Pore-size regimes and representative porous inorganic materials [41].

The zeolites with three-dimensional microporous framework structures are known

for a long time. The frameworks forming the channels and the cavities are constructed

from linked tetrahedra with many elements, which form MO4 in their structure (M = Al,

Si, P, Be, Ga, Ge, Zn). The different ways in which these tetrahedra are connected

produce different type of three-dimensional crystalline frameworks and so form different

zeolites. Due to the periodic structure of these crystalline solids, they exhibit an

extremely narrow pore size distribution and are very important in size-selective catalysis

13

[42], ion exchange (water softener in detergents) [43], desiccation, sorption [43] etc. For

example, zeolite films are used as stable gas-separation inorganic membranes and sensors

[44]. Also the use of zeolite frameworks has been extended to application as nanoreactors

[18] in the host-guest chemistry. These zeolites modified with various metals and

semiconductors (nano-scale inclusion chemistry) have potentials in electronic and optical

applications [44]. Typical areas of applications are: [45] 1) selective adsorption of large

organic molecules, 2) catalysis such as processing of tar sand, and 3) the high distillates

of crude oils to valuable low-boiling products.

There has been growing interest in the extension of microporous size dimensions

and the applications of zeolites to mesoporous size-regimes. Therefore, there have been

studies to carry the crystallinity and well–defined pore size distribution of zeolites to the

mesoporus regime by using quaternary ammonium salts [46-47]. However, these attempts

did not give desired results. Pillaring clay minerals [48], has been used as a method to

synthesize mesoporous materials. The clays are naturally occurring three-layer sheet

silicates. The structure of these clays has a net negative charge resulting from substitution

either the tetrahedral (Al3+, Fe3+ for Si4+) or octahedral (Fe2+, Mg2+ for Al3+) sites. The

charges are compensated by alkaline or alkaline earth metal ions. These metal ions can be

ion-exchanged for larger oligomeric cations [48]. The calcination of the exchanged

materials leads to connection of layers by these pillars. The pillaring increases the surface

area of the layered clay minerals [42], however majority of the pore size is in micro-

porous range due to small size of oligomeric cations.

14

Recently, Kresge et al. and Beck et al. [49-50] synthesised mesoporous molecular

sieves which is named as MCM-41 (MCM stands for Mobil’s Composition of Matter).

They have applied a liquid crystal templating approach to synthesise these materials in

the hexagonal, MCM-41, cubic, MCM-48 and lamella, MCM-50 forms. Here the

template is not small alkyl chain quaternary directing agents, which have been used in the

synthesis of zeolites, but self-assembled supramolecular quaternary ammonium ions as

shown in Figure 5 [63].

Figure 5. Formation of microporous and mesoporous molecular sieves by using short andlong alkyl chains [63].

These long chain quaternary ammonium salts are used as surfactants, which can

form various kind of supramolecular assemblies in different water/surfactant ratios. One

of the best features of the MCM family is the ability to tailor the pore size and shape.

15

Using the surfactant with different alkyl chain length leads to mesoporous materials with

different pore size and shape [49-50].

There are many mechanisms [41], which have been proposed by different

research groups to explain the formation of mesoporous silica framework. The Mobil

researchers have proposed the liquid crystalline templating mechanism for first time [49-

50]. Two kinds of pathways are assumed (see Figure 6 [41]);

1) the aluminosilicate precursor species occupy the space between a preexisting

hexagonal lyotropic liquid crystals (LC) phase and deposited on the micellar

rods of the LC phase, (see the LC section).

2) the inorganics mediate the ordering of the surfactants into the hexagonal

arrangement, (see next section).

Figure 6. Two possible pathways for LCT mechanism [41].

16

A “charge density model” was later proposed by Stucky et al. [51] based on their

X-ray analysis. In this model, the mesophase formation results from strong Coulombic

interaction between the surfactant and silicate species at the micelle interface.

A generalized liquid crystal templating mechanism [52] later has been proposed

based on electrostatic interaction between a given inorganic precursor (I) and the

surfactant head group (S). In this model, the synthesis of MCM type mesoporous

materials was based on the condensation of cationic surfactant (S+) and anionic inorganic

species (I-). However various charge matching pathways which are (S-I+), (S+X-I+) X= Cl-

, Br- or (S-M+I-) M= Na+, K+ are employed to get mesoporous materials [52]. In highly

acidic media (pH<2) the silicate species are cationic (I+) but cationic surfactant (S+) can

be used to synthesize mesoporous materials because anionic counter ion X- can behave as

buffer between the S+ and I+. Therefore X- mediates formation of mesoporous silica. The

resulting materials are known as “acid prepared mesostructures” or SBM materials

[41,52]. The SBM-1, for cubic, SBM-3, for 2D hexagonal and SBM-2, for 3D hexagonal

structures have different pore wall thickness and framework charge due to different

precipitation conditions compared with base-derived mesoporous materials. The SBM-2

is the regularly caged mesoporus silica, however the other mesoporus silica structures

posses uniform pore channels but not cages [53].

In contrast to the charge matching methods, the mesoporous materials have also

been synthesised using neutral templates [54]. This method is based on hydrogen bonding

and self-assembly between natural primary amine micelle (So) and neutral inorganic

17

precursor (Io). In contrast to the previous methods SoIo pathway allows facile recovery of

the template by a simple solvent extraction. Natural alkyl amines are costly and toxic and

consequently they are not suited to large-scale synthesis of mesoporous structures. The

non-ionic poly ethyleneoxide, CnH2n+1(CH2CH2O)mOH, (PEO) surfactants are used to

mediate the mesoporus structures [55]. The PEO type surfactants are biodegradable and

also offer synthetic versatility. In this method, it is possible to prepare mesoporous metal

oxides that are not accessible by electrostatic templating method. The metal alkoxides

that readily hydrolyze to the corresponding metal oxide can be templated by PEO type

nonionic surfactants [55].

The lyotropic liquid crystalline phase as a templating media was first used by

Attard et al. [56] for the synthesis of mesoporous silicates. Here, the sol-gel synthesis of

the inorganic nanostructure takes place in the ordered environment of a bulk surfactant

mesophase. The polycondensation of water-soluble silicates confined within the aqueous

domains of a microphase-separated medium yields monolithic mesoporous silicates. This

method has an advantage of having a high degree of control over the structure. These

monolithic silica materials are large crack-free objects. The continuous silica films with

large cage and pore structures have been synthesised [57-58] by poly(ethylene oxide)-

block-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers. It is possible to

extend the pore size up to 300 Å [59] and get various type of mesoporous structure [60]

by changing the type of copolymer. Note also that the PEO type surfactants and

amphiphilic block copolymers enabled to synthesise [61-62] mesoporous TiO2, ZrO2,

Al2O3, Nb2O5, Ta2O5, WO3, HfO3, SnO3, Pt, CaHPO4, CdS, and ZnS inorganic materials.

18

The EO units in the surfactant complex with inorganic ions through weak coordination,

and forms a mesoscopic structure [61-62].

Both sol-gel derived oxides and oxide molecular sieves are usually prepared from

oxide gels. In a certain sense, both can be regarded as sol-gel derived oxides, differing

only in their degree of structural ordering [63-64]. In sol-gel derived oxides, neither the

oxide framework nor the pore structure is ordered. The mesoporous oxide molecular

sieves have ordered and uniform pores. As shown in Figure 7, the hydrolysis and then

polymerisation-condensation reaction produce amorphous oxide framework. However

usage of supramolecular structure directing agents produce well ordered structures with

adjustable pores.

Figure 7. Hydrolysis and polymerization-condensation reactions of silica alkoxide.

The hybrid mesoporous materials have also been synthesised to change the

properties of bare mesoporous materials [65-66]. These hybrid organic-inorganic groups

can be inside the inorganic framework [65] or over the pore wall [66].

OR Si OR

OR

OR

+ H2O OR Si OH

OR

OR

+ ROH Hydrolysis

OR Si OH

OR

OR

+ ORSiHO

OR

OR

O Si O

O

O

O Si

O

O

+ H2O Condensation

19

Polymerization-condensation reactions can be acid or base catalyzed and both

pathways gives different structures and pore sizes [65-66]. Adjusting reaction conditions

can give desired materials. Also functionalizing the pore walls of the mesoporous

structures gives many materials with unique properties [67]. The functionalization may

be done by co-condensation of original silica source, tetraethoxysilane, with derivatives

of silanes [67] or through ion exchange [68]. There are numerous reports that concern

the modification of the internal surfaces of MCM type materials with a variety of metals

and non-metals. These composite materials can open new areas for application. For

example MCM-41 modified with nano-structured Si [69], CdSe [70], or GaN [71], may

have application in optoelectronic devices. The mesoporous materials modified with

LiCF3SO3 [72], shows high ionic conductivities at room temperature. The Ag

nanoclusters in mesoporus [73] materials show drastic changes in optical properties.

Now, for the synthesis of better molecular devices and nanoelectronics, chemist

faced with extension of mesoporous materials beyond the oxides. The synthesis of metal

sulfides is still not developed very well. The problem is mainly in the method of

synthesis, because the reaction of metal precursors with H2S is fast and does not allow

controlling the morphology and the particle size. The first LC templated CdS, which has

been synthesized in the presence of non-ionic surfactant, [74-77] gave hexagonally

ordered mesostructure. The syntheses of microporous metal sulfides have also been

reported [78-79]. However, these used LC templating methods have some drawbacks,

like low metal ion concentration. Therefore, new methods are essential in order to

improve the materials quality and property.

20

1.4. Liquid Crystalline Phase; Template for Inorganic Materials

Designing new molecular materials with novel properties is a newly growing

scientific and technological area [1]. The templating step is crucial in order to be able to

make materials with desired shape and size. In template based synthesis, inorganic

materials copy the shape and the size of the organic template [80]. The organic templates

enable to manipulate the microstructure at molecular level, such as biomaterials [81]

semiconductor materials synthesised in phosphatylcholine vesicles, nano reactors, [82]

and lamellar liquid crystalline phase [83]. In a newly growing area, the 3D photonic

crystals, [84] silica colloidal crystals have been used as a template to replicate interstitial

void space, in the form of high refractive index materials. As stated in the previous

sections, the liquid crystalline phase is used as a template for sculpting and shaping

mesoporous materials [41,49-50,52-53,55,58,74-77].

The organic template is a building block and forms a network. It directs the

structure of the inorganic material during the synthesis. This ordered network has

periodic voids. If it is possible to carry the polymerisation and condensation reactions of

the inorganic precursors in these voids, it allows to mimic the shape and size of organic

building block. Removing organic template from inorganic matrix leaves porous

inorganic material in desired shape and pore size. There are several factors that are

important [40] to copy the template: (1) the nature of the interaction between the template

and embedding matrix, (2) the ability of the matrix to conform to the template, (3) the

relative sizes of the template and the primary units used to construct the matrix.

21

Figure 8. Lyotropic liquid crystalline phases used as structure directing media (Template)[109a].

The supramolecular surfactant molecules form aggregates when mixed with

solvent. These aggregates (micelles) produce the liquid crystalline (LC) phase. In the LC

phase templating approach, see Figure 8 [109], the inorganic precursor polymerizes or the

reaction takes place in the polar region of the template. Therefore, inorganic matrix

mimics the shape and the size of LC phase, depending on which type of LC phase is

used. The interactions between the surfactant and the inorganic precursors, depending on

the surfactant type, can be electrostatic (if anionic or cationic surfactant is used) or

hydrogen-bonding type (if non-ionic surfactant is used) [85]. Also changing the length of

alkyl group of the surfactant molecule changes the sizes of the miceller rods, therefore it

is possible to change the pore size of the inorganic material. However, this templating

method is mostly applicable to the synthesis of mesoporous silica [41,49-50,52-54] and

also other metal oxides [61-63]. In order to be able to synthesise other inorganic

materials, such as metal sulfides in desired size and shape, new templating methodologies

should be developed.

22

In this thesis, for the first time, we demonstrate that the LC phase of non-ionic

surfactants can be formed using aqua complexes of transition metals. In this content, the

second component, water, which is the solvent in lyotropic LCs, is replaced by

coordinated water molecules. Unlike of the free water molecules, the coordinated water

molecules organize the surfactant molecules into more stable structures (hexagonal and

cubic). The LC behaviour of the non-ionic surfactant, C12H25(CH2CH2O)10OH, in the

presence of Ni(II), Co(II), Cd(II), and Zn(II) and the formation of mesoporous CdS and

ZnS have been demonstrated. These structures (cubic and hexagonal) template the

synthesis of mesoporous metal sulfides.

23

2.EXPERIMENTAL

2.1.Materials

All chemicals and solvents were reagent grade and used as received without any

further treatment.

The surfactant used throughout this work, homogeneous polyoxyethylene 10

lauryl ether (C12E10), is commercially available from Sigma, Germany. Cobalt (II) nitrate

hexahydrate (Co(NO3)2·6H2O, 98 % pure) , cobalt(II) chloride hexahydrate (CoCl2·6H2O,

98 % pure) and hydrogen sulfide (H2S, 99.5 %pure) were obtained from Aldrich,

Germany. Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 97 % pure), zinc (II) nitrate

hexahydrate (Zn(NO3)2·6H2O, 99 % pure) and cadmium (II) nitrate tetrahydrate

(Cd(NO3)2·4H2O, 99 % pure ) were obtained from Merck, Germany.

2.2. Synthesis

2.2.1. Synthesis of liquid crystal phase of inorganic salts

All samples were prepared by direct mixing the surfactant,

C12H25(CH2CH2O)10OH (represented No), and metal complex salts (Co(NO3)2·6H2O,

CoCl2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O, (which are denoted

as MX2) in solid phase. One gram surfactant (1.595x 10-3 mole) is mixed with metal

24

salts, in mole ratios of (MX2/No); 0.1-7.0. Then, the mixture was either heated up to

isotropisation temperature of the sample or dissolved in acetone which can be pumped

out under vacuum to obtain homogenous mixtures. However, most of the samples used

through out this work were prepared by heating over the melting point and shaking

constantly then cooling to room temperature (RT). This heating and cooling cycles were

repeated several times to achieve homogeneity. Finally, the samples were kept below

their isotropisation temperature (IT) for several hours. However over heating, especially

in the case of (Cd(NO3)2·4H2O) samples, may destroy the desired liquid crystal (LC)

phase.

The surfactant-water-metal salt samples were prepared by mixing 50:50 wt % of

water and surfactant (1gr water, 1gr surfactant) and then CoCl2·6H2O),

(Co(NO3)2·6H2O)) and (Ni(NO3)2·6H2O) salts are added to this mixture with a MX2/No

mole ratio 0.0-5.0. The same procedure, which was applied to the water free samples,

was used to homogenize the samples.

2.2.2. Synthesis of CdS and ZnS

First, the liquid crystal phase, containing Cd(NO3)2·4H2O and Zn(NO3)2·6H2O

complexes have been prepared as described above. The thin films of the LC phases were

prepared on quartz substrates and then these samples were exposed to H2S gas in a

specially designed, evacuated glass cell. (Figure 9). This gives CdS and ZnS film

samples over quartz substrates. Another pathway is that the samples prepared in a shlenks

25

(Figure 9) can be purged with H2S gas under vacuum. Then, the samples were washed

with ethanol-diethylether solution several times to remove unreacted complexes and the

surfactant molecules. To collect the products, the ethanol-diethylether solutions were

centrifuged and the products were dried at RT.

Figure 9. Glass cells, used in the synthesis of metal sulfides. The cell A used for thesynthesis of thin film samples on a quartz substrate and the cell B (shlenk) used for largequantities.

2.3. Instrumentation

2.3.1. Polarized Optical Microscopy

Polarized optical microscopy (POM) has been applied to characterize the

mesophases formed from MX2/No mixtures. The LC phases were most of the time

identified by a birefringent texture observed under POM. The samples for the POM

A B

26

images were prepared by sandwiching the LLC samples in between two glass slides, and

heating it above its IT, and then cooling to RT.

The POM images were recorded in transmittance mode on a Meije Techno

ML9400 series Polarising Microscope with reflected and transmitted light illumination,

using covergent white light between parallel and crossed polarisers. The thermal

properties of the mixtures were studied using a Leica Microscope Heating Stage 350

attached to the above microscope. The hot-stage was operated with 3 oC/min heating rate.

The attached hot-stage was calibrated against the melting point of naphthalene, which is

80 oC.

Stereo microscope Stemi 2000 from Carl Ziess Jena GmbH with halogen lamp

6V/10W equipped for bright field and phase contrast was used to record the images.

Power of the objective was 10x/0.25.

2.3.2. X-Ray Diffraction

The powder x-ray diffraction, PXRD, patterns were collected on a Rigaku

Miniflex diffractometer using a high power Cu-Kα source operating at 30kV/15mA. The

samples, which are in the LC phase, were prepared on a 0.5 mm glass sample holder. The

PXRD patterns were recorded twice for each sample. The first measurements were

carried on a less ordered sample (un-oriented) and the second one was carried using a

sample heated up to IT and cooled back to RT to obtain well ordered LC phase

27

(oriented). This was found to be essential in order to see all diffraction lines. The PXRD

patterns of ZnS and CdS powder samples were recorded with a 0.2 mm glass sample

holder. All the measurements were recorded using 0.20 theta/min scan rate and 0.01 data

interval the 2θ range between 1.0 and 10.0. Between 1.0 and 10.0 2θ, the scan rate was

1.00 theta/min and at higher theta values, the scan rate was 4 theta/min.

2.3.3. FT-IR Spectroscopy

The transmission FT-IR spectra were recorded with a Bomem Hartman MB-102

model FTIR spectrometer. A standard DTGS detector was used with a resolution of 4

cm-1 and 128 scan for all samples. The MX2/No samples were prepared as a thin film over

a Si(100) wafer surface. The samples, MX2:No:H2O, prepared using 50 wt %

water/surfactant were sandwiched between two Si(100) wafers. The IR spectra of the

powder CdS and ZnS samples were recorded as KBr pellets. The IR spectra of the

powder CdS and ZnS samples were also recorded by preparing colloidal dispersion of the

sample with ethanol and by evaporating several drops of this suspension over a Si(100)

wafer. The FT-IR spectra of all of the samples were recorded in 200-4000 cm-1 range.

2.3.4. UV-VIS Spectroscopy

UV-Vis absorption spectroscopy was used for characterization and also to obtain

information about the electronic properties of the mesostructured CdS and ZnS. The UV-

Vis spectra were recorded using a Varian Cary 5 double beam spectrophotometer with

28

150 nm/min speed with a resolution of 2 nm over the wavelength range from 1400 to 200

nm. The UV-Vis absorption measurements were recorded using thin films of

mesotructured CdS and ZnS samples over quartz slides and the MX2 /No samples were

sandwiched between two glass slides.

2.3.5. Scanning and Transmission Electron Microscopies (SEM and TEM)

The TEM images were recorded at 300 kV using a JEOL 3010. TEM specimen of

metal sulfide is prepared under ambient conditions by depositing a droplet of ethanol-

metal sulfide suspension on to carbon films supported on copper grid.

The SEM images were recorded at 16 and 25 kV using a JEOL 6400. The SEM

specimen of the metal sulfide is prepared under ambient conditions by depositing a

droplet of ethanol-metal sulfide suspension onto a gold coated silica wafer.

29

3.RESULTS AND DISCUSSIONS

CHAPTER 1

3.1.1. Lyotropic Liquid Crystalline (LC) Phase Behavior of Poly(oxyethylene) Type

Nonionic Surfactants with Transition Metal Aqua Complexes as a Second

Component.

We have studied the liquid crystal (LC) phase behavior of different transition

metal aqua complexes, MYx.nH2O (M= Co2+, Ni2+, Cd2+, Zn2+, Fe2+,) (Y= NO3-, Cl-,

SO42-, CH3COO-), with polyoxyethylene type nonionic surfactants (No). The mixing of

MYx.6H2O with No, depending on the concentration range and type of counter ion, has

produced a new phase. It is well known [35-36] that amphiphilic molecules form

lyotropic LC phase depending on amphiphile concentration in water solution. Here, water

molecules are the second component for such LC phases. The lyotropic LC phase occurs,

because the oil-like tail group (in this work, C12H25-) of the surfactant tends to minimize

the interaction with water and forms micelle in diluted water solutions. However, the

polar EO groups (-(CH2CH2O)10OH) tend to stay outside the micelle. Usually, the metal

complexes were added to the media as a third or forth component of the mixture [74-77,

83,86-87]. In such systems, the LC phase is obtained by using water, where the polar-

30

apolar interactions organise the structure and determine the structure type. In these

studies, it is widely accepted that metal salts dissolve in the water (polar) region.

This approach has also produced numerous solid materials with mesoprous

structures. It is well established that, for example, the polymerization of silica species

takes place in polar region of the LC phase and the final product is the cast of the LC

structure [56]. This method has also been used to synthesise various inorganic solid

materials [88-89]. Here the LC phase is a structure-directing agent (template). However,

in all these procedures, there is a well-known problem. The addition of electrolytes to the

pre-constructed LC phase would affect the shape of micelles and mostly reduce the

stability of the LC phase [90-95]. Therefore dilute concentrations of metal salts have been

used in the synthesis of mesoporous CaPO4, CdS, ZnS, [74-77,83] by preserving the LC

phase [96-97]. Attard et al. has shown a stabilisation of the LC phase in the presence of

hexachloroplatinicacid (HCPA) as the third component [98]. Again in this study, the LC

phase was constructed from free water, and salt concentration was very low [98].

The inorganic electrolytes can be classified into two groups [99-100] according to

their effect on mutual solubility of water and No. The first, lyotropic, reduces the mutual

solubility between surfactant and water (salting out effect) such as Cl- and SO42- ions; and

the second hydrotropic, increases the mutual solubility between the surfactant and water

(salting in effect) such as NO3- and ClO4

- ions [99-100]. This effect had been first studied

by Hofmeister more than a century ago [101]. According to his observations the mutual

solubility is decreased by the electrolyte (inorganic solids) in the order of; SO42->HPO4

2-

31

>CrO4->CO3

2->Cl->Br->NO3->I-> ClO4

->SCN-. These additives reduce or increase the

hydrophilicity of the surfactant. The studies on discontinuous cubic [102] and hexagonal

[103] phases revealed that the addition of NaCl reduces the melting and cloud point

(reduction of stability) of the LC phase. This is believed to be due to dehydration of the

polyethylene oxide (EO) chain. Also note that lyotropic salts cause the effective cross-

sectional area per one surfactant molecule to shrink in hexagonal phase due to

dehydration of EO chain. However, if NaSCN is added, the melting point shows almost

no change at low concentrations but decreases at higher concentrations [102-103]. In

addition it is well known that [104] some electrolytes make water a better solvent

(structure breakers) and some make water a poorer solvent (structure makers) for EO

chain. The structure breakers disrupt the association of water molecules and the structure

makers enhance it as shown below:

nH2O (H2O)n

The structure breaking anions [91] I- and SCN-, have low electronegativity, high

polarizability and weak electrostatic fields, therefore they disrupt the structure of water

and “salt in” the surfactant. However the SO42- and PO4

3- anions have high

electronegativity and low polarizability therefore they “salt out” the surfactant [91].

In this work, we demonstrate the construction of a LC phase directly from metal

aqua complexes. The coordinated water molecules mediate the formation of the LC

phase. It is well known [105] that crown ethers can form different kind of hydrogen

32

bonding with the metal aqua complexes, as shown in Figure 10. Our surfactant molecule

has an EO chain, that acts like crown ethers, forms hydrogen bonds with metal aqua

complexes. This interaction organises the surfactant molecules into hexagonally ordered

rods that build the LC hexagonal structures, illustrated in Figure 10.

Figure 10. Representation of structure of LC phase formed directly with metal aquacomplexes by the help of hydrogen bonding.

We observed the same trend as it is given in Hofmeister series. The Cl-, SO42- and,

H3CCOO-salts of metal complexes did not produce the LC phase with the surfactant.

These salts are lyotropic and reduce the hydrophilicity of the EO chain of the surfactant.

Therefore, the solubility of these metal complexes in surfactant is very low and the

mixtures undergo complete phase separation (as pure surfactant and complex crystals).

However, the LC phase can be constructed with water in the presence of a very low

concentration of the transition metal salts of lyotropic anions. Because of high

33

concentration of free water, these salts dissolve in the water region and the LC phase can

tolerate small amount of these lyotropic anions. However, there is no free water used, in

our systems therefore such lyotropic counter anion effects on hydrophilicity of EO chain

can be directly observed. No mutual solubility of the Cl-, SO42- and, H3CCOO- salts of

these transition metal hexahydrate complexes is observed. However, the metal nitrates

are hydrotropic salts, and they increase the solubility. Therefore, relatively high solubility

between No and MYx.nH2O has been observed. Due to the hydrogen bonding between the

coordinated water and No, the LC phase is constructed. In contrast to the previous works,

we have increased the salt concentration. In addition, we observed a higher LC phase

stability with an increasing salt concentration up to a saturation point.

In the Fe(NO3)3.nH2O/No systems, the LC phase is not stable in that the oxidative

reduction of EO chain [106] with Fe(III) ion takes place. Due to low hydrophilicity of EO

chain by Cl-, SO42- and, H3CCOO- counter ions; the salts of metal aqua complexes were

excluded from this study. However, CoCl2.6H2O is an exception, because it undergoes

dehydration followed by dimerization reaction and forms CoCl42- ion. In this complex,

the counter ion is no longer Cl- but is CoCl42-. The CoCl4

2- ion also behaves similarly to

the NO3- ion. Upon mixing CoCl2.6H2O with No, one observes a sharp color change from

reddish pink to blue, which is a good indication of CoCl42- ion [107].

The LC phase behaviours of Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O,

CoCl2·6H2O and, Cd(NO3)2·4H2O salts with polyoxy ethylene type nonionic surfactant

have been extensively studied.

34

3.1.2. Polarised Optical Microscopy (POM) and Thermal Properties

The polarised optical microscopy (POM) is a very powerful tool in the

determination of the structure of liquid crystals. An optical texture generated by liquid

crystals and other mesophases enables us to identify the structure type. Anisotropic

mixtures or materials have different indices of refraction in different directions. This

birefringence allows us to see different textures for different anisotropic LC samples. For

example, a hexagonally ordered LC phase produces a focal conic fan texture between

cross polars, and cubic phase does not produce any kind of a texture. The POM images

obtained between cross polars of this phase are completely black (no light passes through

the analyzer of the microscope) Figure 11.

The POM images of most of the samples studied in this work show very similar

textures. In the Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and CoCl2.6H2O samples, we observed

focal conic fan texture, Figure 11, between 1.2 and 3.2 MX2/No mole ratios. This is an

indication of a hexagonally ordered LC structure. Above this concentration range, the

mixtures undergo crystallization. Depending on the metal complex, the crystallization

starts at different mole ratios. For example, the crystallization starts at 3.4, 3.6, 3.2, mole

ratios for Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and CoCl2.6H2O, respectively. At higher

concentrations of these complexes, the LC phase is still present, but it is mixed with the

salt crystals. However, the fresh samples of higher concentrations may show a

homogeneous LC phase but are stable only for several hours. All measurements were

recorded 24 hours after preparation.

35

Figure 11. POM images of (a) Ni(NO3)2.6H2O/No hexagonal, (b) Co(NO3)2.6H2O/No

hexagonal, (c) Cd(NO3)2.4H2O/No hexagonal, (d) cubic phase of Cd(NO3)2.4H2O/No witha mole ratio of 3.6. The scale bar is 200 µm.

The Zn(NO3)2.6H2O (ZnX2) and Cd(NO3)2.4H2O (CdX2) salts show different

behaviours when compared with the other three metal complexes. They also show a focal

conic fan texture, Figure 11, under the POM. The CdX2/No and ZnX2/No mixtures are

optically anisotropic between the ranges of 1.4-3.2 and 1.2-3.4, respectively. The

mixtures do not show crystallization above these mole ratios but they are optically

isotropic at RT. The ZnX2/No mixture shows LC phase till 5.0 mole ratio. However it is

not very stable, in that it crystallizes within one day.

A B

C D

36

The CdX2/No mixtures do not show any crystallization up to a 7.0 mole ratio.

However, around the 7.0 CdX2/No mole ratio, the mixture seems to decompose from gel

phase (mesophase) to a liquid phase. The CdX2/No and ZnX2/No mixtures do not show

focal conic fan texture above a 3.0 and 3.2 mole ratio, respectively at RT. They have

completely dark appearance, between the cross polars Figure 11, at RT. This may be an

indication of a cubic phase. The high viscosity and no fluidity also support this proposal

(also see, the section on PXRD).

Table 2. Thermal properties and composition of MX2/No mixtures. Start is starting andend is end point of hexagonal phase.

Transition Metal Aqua Complexes Isotropisation Temperature (oC)MX2/N0

(mole) Ni(NO3)2 Co(NO3)2 CoCl2 Zn(NO3)2 Cd(NO3)21,1 27-30,41,2 31-31,9 38-391,3 32-34,31,4 33,3-34 35-35,8 29-31 49,8-51 48,3-49,81,5 45-48,4 56-591,6 50,5-51,3 47-48,2 41,2-42,8 66-67 62-63,31,7 53,5-55,11,8 64,5-67 59-60,8 49,5-50,7 70,2-72 71-73,11,9 68,5-69,52 72,7-74,8 67,2-68,9 60,8-62,3 75,5-77,5 81,2-83

2,1 74,5-76,12,2 79-81,2 76,7-79,3 64,5-66,7 79-80 89-912,3 82-842,4 82,3-83,5 79,5-80,5 78,5-80,3 84,5-86,5 94-96,32,5 87-88,72,6 88,4-90 88,3-90,3 79,9-81,8 86,5-88,3 97-100,52,8 89,7-90,2 89,3-91,3 88-91 87,5-90 100,5-105,53 91-91,7 91-93,5 93,5-96 86-90 102-106

3,2 84,5-85 92-95,5 Crystalization 82,5-90,5 42,5-50 (start)110-114,3(end)

3.4 Crystalization 92-95,5 35-45 (start)84,5-90 (end)

3.6 Crystalization 50-55 (start)80,5-83 (end)

51-60 (start)110-113 (end)

3.8 71,5-85 (start)106,5-115 (end)

4.0 50,5-65 (start)80-83 (end)

37

The thermal properties of these lyotropic LC mixtures were determined using a

hot stage attached to the microscope. The phase transition, from a LC phase to a liquid

phase or from a cubic phase to a hexagonal phase then to a liquid phase, temperatures

were determined for a broad range of MX2/No mixtures. Note that the anisotropic

mixtures show optical birefringence between cross polars while the melts are isotropic,

which show no texture between the cross polars. The isotropisation temperatures (IT)

were determined by watching the disappearance of the characteristic texture, using POM.

The ITs for metal complexes studied in this work are given in Table 2. In the table, the

first temperature is where anisotropy starts to disappear during heating and the second

temperature is where anisotropy starts to appear again during the cooling. The cubic

phase starts to appear above 3.0 and 3.2 mole ratio for the CdX2/No and ZnX2/No

mixtures, respectively. This phase is optically isotropic and shows a phase transition to an

anisotropic phase. Therefore it is possible to determine the anisotropisation temperature

(AT). Above 4 mole ratio for the CdX2/No and ZnX2/No mixtures, only the cubic phase

starts to appear (no hexagonal phase). The cubic phase is optically isotropic, therefore it

is not possible for us to determine the phase transition temperatures of these mixtures by

using POM.

The ITs show a constantly increasing trend with complex concentrations. This

contradicts with previous observations [76,83,91]. The disappearance of the LC phase at

high salt concentrations was observed for hydrotropic salts. Note also that, as stated in

previous works, [76,91] the addition of hydrotropic salts does not affect the IT a lot, also

our observations on the samples, which are prepared with free water show the same

38

result. However, our new approach, yields stable LC phase until the saturation point. As

shown in Figure 12 for Co and Ni complexes the IT increases with increasing MX2/No

mole ratio. For the Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and CoCl2.6H2O salts, the 3.0

MX2/No mole ratio appears to be the saturation point. Note also that there is a small

decrease above this point, and then the crystallization starts.

Figure 12. Temperature profiles for Co and Ni complexes.

The CdX2/No and ZnX2/No mixtures do not show hexagonal fan texture after 3.0

and 3.2 at RT, respectively. Above these mole ratios, the samples are optically isotropic.

However, when they are heated, they display a phase transition from the isotropic to

anisotropic, which shows a focal conic fan texture, see Table 2. Like other metal

complexes, the ITs increase with the salt concentration. Between a 3.0 and 3.6 mole ratio

1,0 1,5 2,0 2,5 3,0 3,520

30

40

50

60

70

80

90

100 Isotropic Liquid

Anisotropic Hexagonal

Ni(NO3)2.6H2O Co(NO3)2.6H2O CoCl2.6H2O

Tem

pera

ture

(o C)

Mole Ratio (Metal Salt/Surfactant)

39

the LC phase has a melting point over 100 oC, which is very high compared with the LC

phase prepared by adding the free water and salt as a third component. The ZnX2/No and

CdX2/No mixtures are optically isotropic at all temperature above mole ratios of 4.0 and

3.8, respectively.

Figure 13. Temperature profiles for Cd and Zn complexes withC12EO10.

1,0 1,5 2,0 2,5 3,0 3,5 4,040

50

60

70

80

90

100

110

120

Isotropic Cubic

Anisotropic Hexagonal

Isotropic Liquid

Cd(NO3)2.6H2O

Tem

pera

ture

(o C)

Mole Ratio (Metal Salt/Surfactant)

1,0 1,5 2,0 2,5 3,0 3,5 4,0

30

40

50

60

70

80

90

100

Isotropic Cubic

Anisotropic Hexagonal

Isotropic Liquid

Zn(NO3)2.6H2O

Tem

pera

ture

(o C)

Mole Ratio (Metal Salt/ Surfactant)

40

They do not show any phase change. The saturation point is reached at a mole

ratio of 5 for the ZnX2/No mixture. However, it is not stable more than 24 hours and

crystallizes within a day Figure 13.

The CdX2/No mixtures do not show crystallization up to a mole ratio of 7, but it

appears that this concentration does not have a LC phase. At these concentrations, the

samples are isotropic at RT. Therefore the POM is useless for determining whether there

is a mesophase or not. We used powder x-ray diffraction, PXRD, to establish the

mesophase and LC behaviour of the high concentrations. It was determined that the

CdX2/No shows LC phase behavior up to a 6.5 mole ratio (see text for PXRD). This is a

very high salt concentration compared with the metal ion concentration applied in

conventional method [74-77, 83] (the maximum concentration achieved so far is around

0.8 MX2/No mole ratio).

To make our point clear, the LC phase was also constructed in free water (50/50

wt %), then Ni(NO3)2.6H2O or Co(NO3)2.6H2O, or CoCl2.6H2O salt is added to this

mixture. The ITs were recorded in the same way as before. As it is shown in Table 3 and

Figure 14, the Ni(NO3)2.6H2O and Co(NO3)2.6H2O salts have very little effect on the

isotropisation temperature. The LC phase surfactant/water (50 wt %) has an IT around

60oC. There is only 4-5 oC increase in ITs in a broad range of the salt concentrations.

Because there are many free water molecules in the mixtures, the metal complex are

solvated, therefore the surfactant molecules have very little interaction with the metal

aqua complexes and all the salt is in free ions form, see text for FT-IR.

41

Figure 14. Temperature profiles for MX2/No mixtures with free water. Thesurfactant/water is 50 wt %. M; Ni(H2O)6

2+ , Co(H2O)62+ and X; NO3

-, Cl-.

Notice also that in the case of Cl- salt, the ITs decrease with metal complex

concentration, (Figure 14). Since Cl- is known as a lyotropic anion, which has a “salting–

out” effect, it is expected to dehydrate the EO chain. The sample viscosity also decreases

with salt concentration, especially around 1.0 mole ratio and the samples become more

like a liquid. However, the LC phase, which is directly prepared from metal aqua

complexes, becomes thicker with an increasing salt concentration. There is no harm in

stating that, if one classifies the LC phase prepared by using free water, as liquid-like, the

LC phase prepared directly from metal complex, is crystal-like.

0,0 0,2 0,4 0,6 0,8 1,045

50

55

60

65

70

Anisotropic Hexagonal

Isotropic Liquid

Ni(NO3)2

Co(NO3)2

CoCl2

Tem

pera

ture

(o C)

Mole Ratio (Metal Salt/Surfactant)

42

Table 3. Thermal Properties of for MX2/No mixtures with free water. TheSurfactant/Water is 50 wt %. M; Ni(H2O)6

2+, Co(H2O)62+, and X; NO3

-, Cl–.

Transition Metal Aqua Complexes ITsTemperature (oC)MX2/N0-H2O

(mole) Ni(NO3)2 Co(NO3)2 CoCl20,0 60,50,1 61,35 57,150,2 60,90,3 62,5 53,750,4 62,40,5 60,55 53,20,6 63,50,7 63,25 50,950,8 63,750,9 62,7 50,41,0 64,35 62,9 50,05

The lyotropic LC phase occurs, because the oil-like tail group of the surfactant

tends to reduce the interaction with the water molecules to a minimum and forms micelle

in dilute water solutions. On the other hand, the polar EO groups (-(CH2CH2O)10OH)

tend to stay outside the micelle. If one compares the ITs, it is clear that the hydrogen

bonding interaction between the coordinated water and surfactant is much stronger than

the hydrogen bonding between the free water molecules and the surfactant.

43

3.1.3.PXRD Analysis

The powder X-ray diffraction (PXRD) is a powerful technique in establishing

the meso-phase and the structure type. It provides information about the crystallinity

and degree of order in the samples. In the crystalline samples, X-rays are reflected

from the parallel lattice planes spaced d apart (d-spacing). Constructive interferences

of the reflected beams give us a diffraction line according to Bragg’s law, given

below

nλ = 2d Sin θ

Figure15. Schematic representation of X-ray diffraction from atomic planes [109b].

In general, the crystalline samples have order in atomic scales, therefore any

reflection will be due to the spacing of parallel atomic planes (Figure 15). In

mesoscopic materials, micellar aggregates (rods) form ordered meso-structures

(hexagonal, cubic, lamella), and has dimensions in the nano-meter range. Therefore,

X-rays will be diffracted from these ordered molecular aggregates, which diffracts at

44

small angles, mostly within the 1-10 2θ range (Figure 16). The diffraction lines

obtained from these ordered molecular rods give the type of structure of the

mesophase.

Figure 16. First three diffraction lines of hexagonal structure [109a].

PXRD pattern of the mixtures prepared in this study display the first three

lines of a hexagonal structure around 48, 28, and 24 Å d-spacings. However, some of

the samples display up to 5 diffraction lines corresponding to (100), (110), (200),

(210) and (300) associated with P6m 2D-hexagonal symmetry [108]. For 2D-

hexagonally (P6m) ordered structures, the assigned [108-109a] reciprocal spacing, 1/

dhk, ratios of XRD lines, are 1, 31/2, 2, 71/2, 3, 121/2, 131/2, 4 . The diffraction lines

observed in our samples generally coincide well with this assignment. However in

some samples, only (100) and (200) lines were observed. This has been observed in

many meso-structured materials and explained that the (110) is missing due to the

45

orientation of the micellar rods in one direction [110]. Note that, this has been

observed in mesoporous silica materials, [111-112] for instance, rubbing the substrate

in a parallel direction yields silica channels highly oriented in one direction. These

samples, prepared on Si(100) surface, also give a diffraction pattern without (110) and

(210) lines. However, it has been observed that crushing these highly oriented

samples or destroying the orientation results in a strong (100) with relatively weak

(110), (200) and (210) lines [111-112].

Figure 17. X-ray diffractogram of Cd(NO3)2.4H2O/Surfactant, mole ratio is; 2

As shown in Figure 17, only (100) and (200) lines are observed from the

Cd(NO3)2.4H2O/No mixtures. However, from some of the samples, a weak (300) line

is also observed. This is most likely due to the high orientation of meso-channels

parallel to the sample holder. Heating and cooling the samples between IT and RT

yield highly oriented samples, which display a very intense (100) line. However, any

0 2 4 6 8 100

1000

2000

3000

4000

(*10)

200

100

Inte

nsity

2Θ

46

of the CdX2/No samples, oriented or un-oriented, do not display the (110) and (210)

lines. With the help of the POM images, it is possible to determine that these samples

have 2D-hexagonal structures. At higher concentrations of cadmium salt, two signals

corresponding to (100) and (200) diffraction lines are again observed. In order to

assign the structure type, at least three lines are required. With the help of the POM

images observed from these samples, the correct structure type can be assigned. The

samples with high concentrations of cadmium salt (MX2/No> 3.2) display completely

dark image between the cross polars, which means they are isotropic. The

bicontinious cubic phase is also isotropic, however the lamellar and hexagonal phases

are anisotropic. Therefore higher concentrations of Cd salt can be easily assigned to a

cubic phase.

Figure 18. PXRD patterns of Cd(NO3)2.4H2O/Surfactant, with increasing mole ratios.

0 2 4 6 8 10

6.5

6.0

5.04.5

2.82.42.01.6

Nor

mal

ized

Inte

nsity

2Θ

47

Figure 18 shows the normalized diffraction lines of Cd(NO3)2.4H2O/surfactant

with increasing salt concentration. There is a clear shift at the first diffraction line to

lower 2θ values. The d-spacing obtained from (100) line, increases with an increasing

salt concentration.

Figure 19. X-ray diffractogram of Ni(NO3)2.6H2O/Surfactant, mole ratio is; 2.6

The Ni(NO3)2.6H2O/Surfactant mixtures generally show five diffraction lines

around 48, 28, 24, 18, and 16 Å which are assigned to (100), (110), (200), (210) and

(300) respectively, see figure 19. These coincide well with the d-spacing ratios for the

2D-hexagonal structure. Also note that the birefringence that is observed in Ni

samples can be assigned to a hexagonal phase, and it will be used as a reference to

assign structures for the other samples, which show just one or two diffraction lines.

0 2 4 6 8 10-10000

0

10000

20000

30000

40000

50000

60000

70000

80000

300210(*40)

200110

100

Intensity

2Θ

48

In all samples, excluding those with high concentrations of zinc and cadmium

salts, the same optical texture is observed under the POM between the cross polars.

There is no correlation between the d-spacings observed and the metal concentration

in the nickel samples, see Figure 20.

Figure 20. X-ray diffractogram of Ni(NO3)2.6H2O/Surfactant, with increasing moleratios.

0 2 4 6 8 10

3.0

2.6

2.2

1.8

1.4

Rel

ativ

e In

tens

ity

2Θ

49

Figure 21. X-ray diffractogram of (A) Co(NO3)2.6H2O/Surfactant, mole ratio is; 2.6,(B) Co(NO3)2.6H2O/Surfactant, with increasing mole ratios.

The Co(NO3)2.6H2O samples, which generally display three diffraction lines,

and are assigned to the (100), (200), and (300) of the 2D-hexagonal structure, are

shown in Figure 21. The samples are highly oriented parallel to the sample holder in

that the (110) and (210) lines are not observed. However, the POM images indicate

that the samples have a hexagonal fan-textures between cross polars. Note also that

the d-spacing does not have any correlation with an increasing salt concentration.

The CoCl2.6H2O/surfactant samples also do not display more than two

diffraction lines. These two lines have been assigned to the (100) and (200) diffraction

line of the 2D-hexagonal phase, according to their d-spacing ratios, Figure 22. At

higher mole ratios, there is only one line in the PXRD patterns. However, by referring

to their POM images, the structure type for high concentration can be assigned to a

hexagonal phase.

2 4

3.0

2.6

2.2

1.8

1.4

Relat

ive In

tensit

y

2Θ2 4 6 8 10

0

5000

10000

15000

20000

25000

30000

35000

*20300200

100Int

ensit

y

2Θ

A B

50

Figure 22. X-ray diffractogram of (A) CoCl2.6H2O/Surfactant, mole ratio is; 2.2, (B)CoCl2.6H2O/Surfactant, with increasing mole ratios.

The CoCl26H2O/surfactant mixtures display diffraction lines similar to those

of Co(NO3)26H2O/surfactant and Ni(NO3)26H2O/surfactant samples. They also do not

show any correlation between the d-spacing and the complex concentration.

As shown in Figure 23, the Zn(NO3)2.6H2O/surfactant samples generally show

five diffraction lines. The POM images and the d-spacing ratios reveal that zinc

samples have a 2D-hexagonal structure up to 3.4 mole ratio of MX2/No. These lines

are assigned to the (100), (110), (200),(210), and (300) of the 2D-hexagonal structure,

Figure 23. As observed in the oriented thin mesoporous silica film samples by Ozin

et.al [110] and later by others [111-112], crushing the oriented film samples bring the

(110) and (210) lines, upon loosing the orientation. Upon breaking the orientation in

our samples, we have observed the (110) and (210) lines of the hexagonal structure.

2 4 6 8 10

2.6

2 .2

1 .8

1 .6

Rel

ativ

e In

tens

ity

2Θ

2 4 6 8 10

0

5000

10000

15000

20000

*10200

100

Inte

nsity

2Θ

B

A

51

The diffraction patterns in Figure 23 A were recorded using a thin film in the oriented

form (prepared by heating to IT and cooling to RT) and using thin film in the

disordered form (prepared by packing the sample from the vial as in the disordered

form). The disordered form can be easily obtained for these samples by mixing the

oriented sample at room temperature. These samples are very soft in that mixing or

stirring with a spatula or pressing with a glass slide can easily break the orientation. In

these un-ordered samples, the diffraction lines originating from (100) and (200) are

not as intense as in the oriented samples, sometimes the (200) lines completely

disappear in the diffraction patterns, see Figure 23. However, this procedure creates a

neat orientation of the samples with respect to X-ray beam and detector in that the

(110) and (210) signals appear in the diffraction pattern. This is a very well

established procedure to determine the structure type of the oriented thin film samples

[110-112].

Figure 23. X-ray diffractogram of (A) Zn(NO3)2.6H2O/Surfactant, mole ratio is; 2.6,(B) Zn(NO3)2.6H2O/Surfactant, with increasing mole ratios.

2 4 6 8 10

0

2000

4000

6000

8000

10000

*10*5

300

200110

100

Inten

sity

2Θ2 4

5.0

4.0

2.62.2

1.4

Relat

ive In

tensit

y

2Θ

A B

52

The Zn samples also show a correlation between the d-spacing and the

concentration of the complex. As shown in Figure 23, the first diffraction line shifts to

lower angles, therefore the d-spacing increases by increasing the zinc salt

concentration, but there is no linear correlation between the concentration and the d-

spacing.

Table 4 shows the first and second diffraction lines of Cd and Zn samples,

which have high metal/surfactant mole ratio (MX2/No > 4.0). Their structures are

assigned as cubic, but with two lines it is not possible to determine the space group of

the cubic phase, therefore the diffraction lines are not assigned to any plane.

Table 4. First and second diffraction lines of high MX2/No mole ratios of Cd and Znsamples

Metal Salts Diffraction Lines

Mole ratios 2theta d-value (Å) 2theta d-value(Å)

Cd/Surf4.5

1.810 49.00 3.570 24.70

Cd/Surf5.0

1.800 49.04 3.630 24.32

Cd/Surf6.0

1.710 51.62 3.430 25.74

Cd/Surf6.5

1.690 52.23 3.350 26.35

Zn/Surf4.0

1.670 52.86 3.320 26.59

Zn/Surf5.0

1.630 54.15 3.280 26.91

The diffraction lines, their 2-theta and d-spacing values, of all the samples

prepared throughout this work are listed in Table 5. The assignments were done

53

according to the d-spacing ratios. Generally, the samples have d-spacing ratios, given

at the beginning of this section, for a 2D-hexagonal structure with a space group of

P6m.

Table 5: Diffraction lines and their assignments for various MX2/No mole ratios. M=Cd(H2O)4

2+, Co(H2O)62+, Zn(H2O)6

2+, Ni(H2O)62+ , X= NO3

-, Cl-.

Metal Salts hkl planesMole ratio 100 110 200 210 300

2theta d-value(Å)

2theta

d-value(Å) 2theta d-value

(Å) 2theta d-value(Å) 2theta d-value

(Å)Cd/surf 1.4 2.010 43.92 3.940 22.41Cd/surf 1.6 1.960 45.04 3.810 23.17Cd/Surf 2.0 1.910 46.21 3.940 22.41Cd/Surf 2.4 2.080 42.44 4.030 21.91Cd/Surf 2.8 1.950 45.27 3.940 22.41 5.860 15.07

Ni/Surf 1.4 1.790 49.31 3.570 24.73Ni/Surf 1.8 1.740 50.73 3.170 27.85 3.550 24.87 4.400 20.07 5.370 16.44Ni/Surf 2.2 1.820 48.50 3.030 29.13 3.560 24.80 4.550 19.40 5.300 16.66Ni/Surf 2.6 1.830 48.23 3.050 28.94 3.610 24.45 4.550 19.40 5.310 16.63Ni/Surf 3.0 1.760 50.15 2.950 29.92 3.420 25.81 4.290 20.58 5.090 17.35

CoNO3/surf 1.4 1.770 49.87 3.550 24.87CoNO3/surf 1.8 1.840 47.97 3.560 24.80CoNO3/surf 2.2 1.780 49.59 3.520 25.08 5.340 16.53CoNO3/surf 2.6 1.740 50.73 3.490 25.29 5.230 16.88CoNO3/surf 3.0 1.850 47.71 3.640 24.25 5.390 16.38

CoCl/surf 1.4 1.840 47.97 3.590 24.59CoCl/surf 1.8 1.760 50.15 3.580 24.66CoCl/Surf 2.2 1.710 51.62 3.450 25.59CoCl/Surf 2.6 1.920 45.97CoCl/Surf 3.0 1.740 50.73

Zn/Surf 14 1.950 45.27 3.790 23.29Zn/Surf 18 1.900 46.46 3.800 23.23 4.720 18.71 5.700 15.49Zn/Surf 2.2 1.810 48.77 3.090 28.57 3.620 24.39 4.630 19.07 5.620 15.71Zn/Surf 2.6 1.810 48.77 3.090 28.57 3.610 24.45 4.700 18.79 5.650 15.63Zn/Surf 3.0 1.830 48.23 3.000 29.42 3.530 25.01 5.330 16.57

54

3.1.4.FT-IR Spectral Studies

FT-IR spectroscopy is extensively used to obtain information about the

structure of non-ionic surfactants in the presence of metal complexes [113]. A free

poly oxyethylene, POE type surfactant, such as the one used throughout this work,

C12H25(CH2CH2O)10OH, shows drastic changes in conformation upon mixing with

water [114]. This is due to the interaction between polar head group of the surfactant

and water molecules. The Raman [115] or FT-IR [113-114,116-117,121-122]

spectroscopic techniques are very useful to elucidate these local structural and

conformational changes. These structural changes may even take place by heating,

therefore all the measurements were carried out at RT throughout this work. Thus any

conformational changes on the POE backbone are due to the hydrophilic interactions

between the surfactant head group and water molecules, which are coordinated to a

transition metal center.

It is the general trend that the FT-IR spectra of transition metal

hydrates/surfactant (represented as MX2/No) mixtures display drastic changes in most

regions of the spectrum. Therefore, it is better to divide the spectrum into several

segments and examine in detail. Some of the regions and their assignments [114] are

listed as: (1) the broad band at around 3200-3700 cm-1 due to ν-(OH) stretching

(symmetric and anti symmetric), (2) the symmetric and antisymmetric stretching of C-

H at around 3000-2850 cm-1, (3) the CH2 scissoring vibrations at 1500-1450 cm-1, (4)

the CH2 wagging vibrations, symmetric with respect to C2 axis of OCH2- CH2O, at

1420-1389 cm-1, (5) the CH2 wagging vibrations, antisymmetric with respect to C2

55

axis of OCH2- CH2O, at 1380-1320 cm-1, (6) the CH2 twisting vibrations, symmetric

with respect to C2 axis of OCH2- CH2O, at 1310-1270 cm-1, (7) the CH2 twisting

vibrations, antisymmetric with respect to C2 axis of OCH2-CH2O, at 1280-12300 cm-1,

(8) the hybridized vibrations of skeletal stretching (C-O and C-C stretching) and the

CH2 rocking at 1160-810 cm1, (9) the skeletal deformation vibrations (CCO and COC

bending and C-O and C-C torsion) below 600 cm-1.

Figure 24. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio of 2.

The most informative regions for the conformational changes are the region of

the hybridized vibrations of the –CH2CH2O- (EO) skeleton, the CH2 scissoring,

wagging, and twisting regions. It is also clear to see the effect of the hydrogen

bonding, (Figure 24) in the OH stretching region. The ν-(OH) stretching vibrations for

molten surfactant, surfactant/water (50 wt %) and CdX2/No mixtures are observed at

3480 cm-1, 3425 cm-1 and 3370 cm-1 respectively, (Figure 24). The terminal ν-(OH)

group of the surfactant has stretching vibrations at around 3480 cm-1 [116]. The

addition of water shifts the water bands (free and crystallohydrate) to a lower energy,

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due to the hydrogen bonding between the water molecules and the surfactant head

group (poly ethylene oxide group)[113,116]. The LC phase formed by metal aqua

complexes has an even lower ν-(OH) stretching frequency, observed at 3370cm-1, due

to the hydrogen bonding between surfactant (No) and metal aqua complex (MX2).

Figure 25. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio of 2.

It is also possible to observe the structural and conformational changes on the

ethoxy methylenes in the ν-(CH2) stretching region, (Figure 25). The ethoxy

methylenes absorb at distinctly higher frequencies than the alkyl methylenes [117]. As

shown in Figure-25, the ν-(CH2) stretching band of EO units has been observed as a

shoulder at 2869 cm-1. This band shifts to 2873 cm-1 and 2888 cm-1, by adding water

and metal aqua complex, respectively. This is also a very good indication of the

strength of the interaction between the free-water or coordinated-water and the

surfactant molecules. It is worth saying that the aqua complexes make stronger

hydrogen bonds with the surfactant molecules. These interactions organize and form

2790 2880 2970

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more ordered and stable LC phases. The temperature profiles, as it was shown in the

previous section, also prove this, see Figure 12 and 14. Figure 25 shows that, while

going from the molten surfactant to the MX2/No mixture, the signals become better

resolved and sharper. It also shows that, the intensity of the antisymmetric stretching

of the ν-(CH2) group increases. The broadening of methylene stretching bands can be

explained by the conformational disorder, (will be discussed later).

Figure 26 . FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant,mole ratio is 2, (c) Co(NO3)2. 6H2O/ surfactant, mole ratio is 2, (d) Co(NO3)2. 6H2Ocrystal.

The NO3- ion has a very broad signal at around 1400 cm-1 and it covers almost

all of the CH2 scissoring, wagging, and twisting regions. Therefore it prevents us from

obtaining information about conformational changes in this region of the spectrum.

However the NO3- ion signals also provide information about the local interactions

between the metal complex and the nature of the NO3- ions. In order to understand

fully the EO signal, it is crucial to examine the NO3- ion signals first. Figure 26

clearly shows that the NO3- ion in the crystalline metal nitrates has a broad single

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band at around 1385 cm-1 and the free nitrate ion in water solution is observed at

around 1350 cm-1, which is due to the doubly degenerate antisymmetric NO stretching

[118]. When the metal aqua complex is mixed with the surfactant, this broad signal

splits into two sharp signals, which are centered at around 1303 cm-1 and 1468 cm-1.

Such kind of splitting has been observed for bounded NO3- ions [118-119]. This

shows that the nitrate anions are not free but that they interact with metal aqua

complex cation. Most likely, there are two kinds of nitrate ions in LC phase; one is the

dissociated NO3- (free nitrate) and the other is the associated NO3

- (ion-pair). When

NO3- counter anion makes ion-pair it lowers its symmetry from D3h to C2v, therefore

the doubly degenerate single peak splits into two peaks [118] Another possibility of

splitting of NO3- signals can enter to the coordination sphere and coordinate to metal

the center like [M(H2O)4(NO3)2] or [M(H2O)5NO3]+. However, there is no evidence

for the coordination of NO3- ions to the metal center, observed from the Vis-Near-IR

absorbance measurements, (see Vis-Near-IR section).

The spectra of molten surfactant and the CoCl2.6H2O/surfactant mixtures are

also shown in Figure 26. The two spectra, obtained from molten surfactant and

CoCl2.6H2O/surfactant, also confirm that the two intense peaks at 1303 and 1468 cm-1

originate from NO3- ions. Note also that, the FT-IR spectra after addition of the

CoCl2.6H2O complex to the surfactant does not display the big changes in the CH2

scissoring wagging, and twisting vibrational region. As shown in Figure 27, in the

case of the LC phase prepared with free water and aqua complexes, added as a third

component, the two strong NO3- peaks at around 1300-1460 cm-1 are not observed.

There is a single broad peak around 1350 cm-1, which is due to the free NO3- ion. This

shows that the metal aqua complex dissolves in the water region. However there is no

59

free water molecules in the system established in this thesis work, therefore, the NO3-

ions are in closer contact with the metal complex cations.

The C-O stretching around 1100 cm-1 in the ternary system has a very little

shift to lower energy with respect to MX2/No system. This demonstrates that the metal

complex has weaker interaction with the EO group in the ternary system. See also the

thermal properties of the ternary systems in previous section. The metal complex in

ternary system is solvated in the water region, but if the LC phase is prepared directly

from metal complex (binary system) the metal aqua complexes mediates the LC phase

formation.

Figure 27. FT-IR spectra of (a) pure (molten) surfactant (b) Zn(NO3)2.6H2O/surfactant, mole ratio is; 0.5, (c) Ni(NO3)2.6H2O/ surfactant, mole ratio is; 0.5, (d)Co(NO3)2.6H2O/ surfactant, mole ratio is; 0.5, (e) Cd(NO3)2.4H2O/ surfactant, moleratio is; 2. All sample contain 50 wt % water with respect to surfactant.

The other NO3- signals, which are marked, are also shown in Figure 28. The

peak, located around 1044 cm-1 as a shoulder, is due to the symmetric NO stretching

[118]. The band is shifted to a lower frequency in comparison to the Co(NO3)2 aqua

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complex. Note also that this band overlaps with some other signals originated from

the surfactant molecules, (Figure 28). The sharp signal, which is located around 810

cm-1, is due to the out of plane bending mode of the nitrate ion [118], which is also

shifted to a lower frequency.

Figure 28 . FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant,mole ratio is; 2, (c) Co(NO3)2.6H2O/ surfactant, mole ratio is; 2, (d) Co(NO3)2.6H2Ocrystal.

Around 750 cm-1, the nitrate ion has another signal, due to an ONO bending

mode [118]. It is difficult to assess whether this band shifts to a higher or a lower

energy due to the broad feature of Co(NO3)2.6H2O aqua complex at around 800 cm-1.

The shifts observed in these spectra, are also good indications for the interactions of

the nitrate anions with the metal aqua complex cations and also likely an indicator of

the interactions with the surfactant molecules. A list of vibrational frequencies and

assignments of metal aqua complex salts with the NO3- counter anion are given in

Table 6.

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Table 6. Vibrational Frequencies (cm-1) and Assignments of nitrates.

Antisym. NOstretch. (cm-1)

Sym. NOstretch. (cm-1)

Out-of-planebend. (cm-1)

ONO bend.(cm-1)

KNO3[118]

(melt) 1388 1045 829 720

Co(NO3)2(solid) 1385 1045 824 730

Co(NO3)2/No

Mole ratio:2 1468 1303 1044 810 750

Cd(NO3)2/No

Mole ratio:2 1477 1291 1025 816 744

Ni(NO3)2/No

Mole ratio:2 1450 1310 1044825816810

758

Zn(NO3)2/No

Mole ratio:2 1488 1298 1042 814 751

In an aqueous solution of surfactant or molten surfactant, the vibrational bands

are broad as compared to the crystalline POE, because the POE chain in crystalline

state has a well ordered helical structure [114]. The energy difference between gauche

(G) and trans (T) forms of X–C-C-Y is not so high [120]. The X–C-C-Y prefers

gauche conformation when electronegative groups are attached to X and/or Y position

upon hydrogen-bonding [120]. Some of the conformers of 1,2-dimethoxyethane are

given in Figure 29 as an illustration. In the crystalline state, the POE internal rotations

along series of bonds -CH2-CH2-O-CH2-CH2- are GTTG, which gives a helical

structure to the POE chain, (Figure 29).

62

Trans Gauche

CH2 CH2

O CH2

CH2

O

O

G

G

TTCH3

CH3

O

CH2 CH2

O

CH3CH3 OO

CH2 CH2

GTTG

Figure 29. Trans and gauche conformers of 1,2-dimethoxyethane and GTTGconformer of -OCH2CH2OCH2CH2O- group.

The addition of a metal aqua complex to the POE type non-ionic surfactant

induces conformational changes in the EO backbone. Figure 30 shows the CH2

scissoring, wagging, and twisting regions. However, this region does not respond a lot

to the conformational changes. The ν-(CH2) scissoring band, which is around 1465

cm-1, does not show any change with the addition of water or metal aqua complex.

The band at around 1350 cm-1 has been assigned [114] to the antisymmetric, CH2

wagging vibration of -CH2-CH2-. The CH2 wagging vibration appears at around 1380-

1350 cm-1, if the C-C bond with gauche (G) conformer and at around 1355-1320 cm-1,

if it has a trans (T) conformer.

63

Figure 30 . FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) CoCl2.6H2O/ surfactant, mole ratio is; 2

The band located at around 1350 cm-1 is most likely due to the gauche

conformer but this band may also contain a small extent of trans conformer as well,

(Figure 30). The band at 1325 cm-1 was assigned [114] to the trans conformation of

the C-C bond. This band loses its intensity and shifts to a higher energy in the

presence of water. However, upon addition of a metal aqua complex, this band almost

disappears, Figure 30. This is a clear indication of a trans to gauche conformational

change of a C-C bond. The symmetric twisting vibrations of CH2, associated with the

gauche and trans conformer around a C-O bond, are expected at 1310-1290 cm-1 and

1295-1270 cm-1, respectively. Therefore, the band is located at around 1301 cm-1, see

Figure 30, is originated from the gauche conformer. Note also that, the antisymmetric

twisting vibrations of the O-CH2-CH2-O segment at around 1265-1250 cm-1 and

1245-1230 cm-1 are assigned [114] to the TGG and TGT conformers, respectively.

The band, which is observed at around 1251 cm –1, most likely originates from the

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TGG conformer, (Figure 30). These two, symmetric and antisymmetric twisting

vibration modes of the CH2-CH2 units show that, in the MX2/No mixtures, not all of

the EO chain has GTTG conformer for the -CH2-CH2-O-CH2-CH2- units like in

crystalline POE [114]. However some of the C-O bonds have G conformation as well.

Figure 31. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio is 2.

The skeletal stretching modes (C-O and C-C stretching) and the CH2 rocking

modes give IR bands in at around, 1160-810 cm-1. The CO stretching vibrations

between 1160-1050 cm-1 do not show any conformational dependence [114].

However, they show dependence on hydrogen-bonding [113]. As shown in Figure 31,

the ν-(CO) stretching band for the molten surfactant is at 1115 cm-1 and shifts to 1100

cm-1 and 1087 cm-1, in the presence of water and metal aqua complex, respectively.

When hydrogen-bonding occurs, the electron density of the C-O bond decreases,

therefore a shift to a lower energy is observed [113]. It is also known that the size of

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the shift shows the strength of the hydrogen-bonding [113]. The LC phase prepared

by metal complexes has a lower C-O stretching vibration frequency than the LC phase

prepared with free water. This indicates that hydrogen bonding in MX2/No mixtures is

stronger, therefore the electron density on a C-O bond is lower. The shift of the band

around 1100 cm-1 due to C-O stretching to a lower frequency proves this. Here, also,

coordinated water molecules can make hydrogen-bonding just with EO chain but free

water molecules makes hydrogen-bonding also with each other and form small

aggregates. Because of the interaction between the free water molecules, they make

less hydrogen-bonding compared to coordinated water molecules. In the previous

section, see Figure 12 and 14, it was indicated that the LC phase prepared with metal

aqua complexes has higher isotropisation temperature. The spectroscopic evidence

observed here also supports our temperature profile results obtained using POM.

There is one peak located at 948 cm-1 for molten surfactant, see Figure 31. The

IR bands in the 950-945 cm-1 region have been assigned to the CH2 rocking vibration

and to the TGT conformation of O-CH2-CH2-O unit of POE [114,116]. Also note that,

the band width of this peak decreases with an increasing its intensity, by adding water

or a metal aqua complex. This shows that the surfactant has a helical structure

although not as perfect as in crystalline POE, that has some T conformation in OCH2-

CH2O unit. However the addition of water or metal aqua complex makes the

surfactant molecules more ordered in the LC phase because they have a sharper and

more intense peaks at around 950 cm-1. This means that the water or metal aqua

complexes decreases the T conformer percentage and increases the G conformer

percentage.

66

Figure 32 . FT-IR spectra of (a) pure (molten) surfactant (b) Cd(NO3)2. 4H2O/surfactant, mole ratio is; 2, (c) CoCl2.6H2O/ surfactant, mole ratio is; 2.

The spectral range, 900-800 cm-1, has been assigned to the CO stretching and

CH2 rocking modes [114,121]. Figure 32 shows that the surfactant has a weak and

broad signals, which cover almost the entire region from 800-900 cm-1. This band

becomes narrower and sharp after the addition of the metal aqua complexes. In this

region, the bands above 825 cm-1 are assigned to the G conformer and the bands at

around 810 cm-1 are assigned to the T conformer of the OCH2-CH2O unit [114,121-

122]. However, there is a sharp and relatively intense signal of the NO3- counter anion

at around 810 cm-1, in the case of the Cd (NO3)2 salt, therefore it is difficult to obtain

information about the conformation of the EO units in this region.

Note that, in the case of CoCl26H2O/Surfactant, there is a peak at around 846

cm-1, which indicates the surfactant molecules have a G conformer. Also note that

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there is no peak around 810 cm-1 for CoCl2.6H2O/No system. This shows that there is

no appreciable amount of T conformer of the surfactant in the LC phase of these

mixtures. The increase in peak intensity and centring above 825 cm-1 indicates that the

surfactant conformation changes from T to G [114,121-122].

All IR peaks and their assignment of POE in solid and in molten state, are

given in Table 7. Also IR peaks and their assignment of surfactant

(C12H25(CH2CH2O)10OH), used in this work, and the CoCl2.6H2O/Surfactant mixture

are given in Table 7.

Table 7. IR peaks and their assignment of POE in solid and in molten state, surfactant(C12H25(CH2CH2O)10OH), and CoCl2.6H2O/Surfactant mixture.vs; very strong, s;strong, m; medium, w; weak, vw; very veak, sh; shoulder.

IR Wavenumbers (cm-1) and Vibrational Assignmnts between 1500-800 cm-1

POE solid state[114]

POE melt[114]

No (this work) CoCl2.6H2O/No 2 (this work)

Assignments

1470 m1463 m1457 m1453 w

1460 m1484 sh1466 m1457 m

1466 m1457 m

CH2scissoring

1415 w1364 m1345 m

1352 m1326 w

1378 w1350 m1325 w

1378 w1350 m CH2 wagging

1283 m1244 m1236 w

1296 m1249 m 1298 m

1250 m1301 m1251 m CH2 twisting

1149 s1119 s1102 vs1062 m

1140 sh1107 s1038 m992 w

1144 sh1116 vs1040 sh994 w

1135 sh1098 vs

C-O stretch,C-C stretch, CH2 rock

963 s947 m 949 w 946 m CH2 rock

844 s945 m915 m855 m

883 w845 w

881 w846 m

C-O stretchCH2 rock

810 sh 809 vw CH2 rock, CH2 twist

68

Figure 33 . Representation of helical structure of POE, GTTG conformation. Blue : C,red : O, white : H.

It is possible to state that when metal aqua complexes are mixed with the

surfactant molecules, the head groups (EO units) undergo hydrogen bonding with the

coordinated water molecules of the metal complexes. The hydrogen-bonding leads to

order in the surfactant molecules which undergo a trans to gauche transformation and

to make a helical structure, Figure 33. However this helical structure is not as perfect

as in crystalline POE.

69

Figure 34. IR spectrum of CoCl2.6H2O/Surfactant with the mole ratio of (a) 1.4, (b)1.8, (c) 2.2, (d) 2.6, (e) 3.0

Figure 34 shows the effect of the concentration of CoCl2.6H2O on the

surfactant structure with increasing salt concentration. There is a small change in the

spectra with an increasing salt concentration, but this is not related with the

conformational changes (almost all signals remained unchanged). The ν-(CO)

stretching vibration around 1100 cm-1 shifts to a lower wave number. This indicates

that hydrogen-bonding increases with increasing the CoCl2.6H2O concentration. This

also coincides with the POM results and the thermal properties of the mixtures. Note

also that the ν-(CH2) stretching mode of the EO units around 2870 cm-1 shifts to a

higher wavenumber with an increasing salt concentration.

The band near 311-291 cm-1 is originated from the CoCl4- ion [123]. This

band proves that the CoCl2.6H2O complex undergoes a dehydration followed

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dimerization reaction and forms CoCl4-, which behaves very differently from the Cl-

ion. Therefore the LC phase can be obtained using CoCl2.6H2O complex.

Figure 35. IR spectrum of Co(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0 (d) 2.4, (e) 2.8, (f) 3.2

Figure 35 shows the spectra of the Co(NO3)2.6H2O/surfactant mixtures. The

broad feature of the NO3- counter anion around 1350 cm-1 predominates in the spectra,

where the peaks become broader with increasing metal aqua complex concentration.

This is likely due to increasing the number of free nitrate ion in comparison to

associated nitrate ion. The ν-(CO) stretching mode at around 1100 cm-1, shifts to a

lower energy with increasing salt concentrations. The ν-(CH2) stretching mode of EO

units, at around 2870 cm-1, shifts to a higher energy at higher metal complex

concentrations. Also note that the splited antisymmetric stretching of the NO3- band,

which is located around 1300 and 1460 cm-1, shows a small change at higher

concentrations. These two bands get closer at higher concentrations. The amount of

splitting decreases with increasing metal aqua complex concentrations. This is most

likely due to an increase of the free NO3- counter anion at higher concentrations.

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Figure 36 . IR spectrum of Zn(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0 (d) 2.4, (e) 2.8, (f) 3.2

The Zn(NO3)2.6H2O/surfactant mixtures as shown in Figure 36, have almost

the same changes as in the case of Co(NO3)2.6H2O/surfactant system with an

increasing salt concentration.

Figure 37. IR spectrum of Ni(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,

(b) 1.6, (c) 2.0, (d) 2.6, (e) 3.0

1 0 0 0 1 5 0 0 3 0 0 0 3 5 0 0

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The Ni(NO3)2.6H2O/surfactant mixtures, as shown in Figure 37 have the same

changes as those of the Co and Zn samples. However, in the case of the

Ni(NO3)26H2O/surfactant, the peaks are broader than other metal complexes. This

indicates that the Ni complexes are behaving differently than other metal complexes.

If one carefully inspects the spectra of the samples prepared using nickel complex, the

peak at around 1400 cm-1 due to a free nitrate ion is much more intense than in the

other metal systems. This means that the solubility of the nickel salt is higher and the

nitrate ions stay as in the free ion form.

However in the case of the Cd(NO3)2.4H2O/surfactant, Figure 38, in addition

to the changes that occur in the other metal complexes, the peaks become relatively

sharper, more intense and some signals are better resolved.

Figure 38. IR spectrum of Cd(NO3)2.4H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0, (d) 2.6, (e)3.0

500 1000 1500 3000 3500

0,0

0,7

1,4

edcba

Nor

mal

ized

Abs

orba

nce

W avenumber (cm-1

)

73

This means that Cd(H2O)42+ ions form well ordered structures with the surfactant

molecules. This is also consistent with the POM and thermal analysis results that the

ITs are relatively higher in the case of the cadmium samples, see Figure 12 and 13.

Figure 39. IR spectrum of MX2/No with the mole ratio of 2, M: (a) Ni, (b) Zn, (c) Co(d) Cd.

Figure 39, shows the comparison of all metal complexes in the 650-1750 cm-1

region. Note also that the doubly degenerate NO3- antisymmetric stretching, which

splits into two peaks around 1300 cm-1 and 1460 cm-1, displays visible changes from

complex to complex. This clearly shows that the major interaction of the nitrate ions

is with the metal complex, as in the form of ion-pairs. However, we also demonstrated

that there is no ligand exchange reaction, taking place between the coordinated water

molecules and nitrate ions. This has been proven using UV-Vis absorption

spectroscopy.

800 1000 1200 1400 1600

d

c

ba

Rel

ativ

e In

tens

ity

W avenumber (cm-1

)

74

3.1.5.Vis-Near-IR Spectral Studies

The electronic absorption spectrum of transition metal aqua complexes gives

valuable information regarding the changes in the coordination number, type of ligand

and the geometry of the complexes. Any of these changes affect the electronic

structure of the complexes and therefore the spectral features.

The Vis-Near-IR spectra were recorded for Ni(H2O)62+ and Co(H2O)6

2+ at

various concentrations in the LC phase and those in the pure water in the 400-1400

nm region, see Figure 40 and 41. These peaks originate from d-d transitions [124],

metal centred, and do not respond to the composition of the mixtures. The spectra for

the LC phases were recorded by sandwiching the samples between two glasses or two

quartz windows. The spectra for the water solutions were recorded in 1 cm quartz

cuvets. The spectral changes clearly indicate that there is no ligand substitution

reaction, which may occur between the surfactant molecules or nitrate ions and the

metal aqua complexes.

Figure 40. Vis-Near-IR spectrum of various Ni(NO3)2.6H2O/surfactant mixtures (a)0.1 M Ni(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6

400 600 800 1000 1200 1400

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

dcba

Abs

orba

nce

λ nm

75

Figure 41. Vis-Near-IR spectrum of various Co(NO3)2.6H2O/surfactant mixtures (a)0.1 M Co(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6

The only change observed is in the intensity, which increases with increasing

metal aqua complex concentration. Therefore it is obvious that the coordinated water

molecules stay in the coordination sphere and that the geometry of the complexes

does not change in the LC phase.

These results fully support our previous results obtained from the POM

images and the Ft-IR spectra. It was stated in the previous section that the LC phase is

formed by hydrogen-bonding between the coordinated water molecules and the

ethylene oxide (EO) units of the surfactant molecules. There is no complexation

and/or chemical interaction between the EO and the metal centre of the aqua

complexes. It is also true that the NO3- ions are still in the vicinity of the counter ion

sphere of the M(H2O)62+ complex and are interacting electrostatically with the

complex ion. The mixture forms a net, composed of surfactants organized in a

400 600 800 1000 1200 1400

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

dcba

Abs

orba

nce

λ nm

76

hexagonal structure, the metal aqua complexes in the hydrophilic interface, as in the

free ion and ion-pair form with the counter cation, see Figure 10. However it is hard

to determine, the number of interactions that occur between the metal aqua complexes

and the surfactant molecules, and the types of the ion-pairs. It is well know in many

polymer electrolyte mixtures that the cations and anions form different types of

species in the polymer electrolyte systems [72]. These systems attract much attention

from groups working on fast ion conductors. It is very important to determine

behaviour of the ions in these systems. For example, LiCF3SO3 is one of the major

salts used by groups working on the faster ion conductors, and it has been well

established that the Li+ ions are in many different forms, including free ion (Li+ and

CF3SO3-), ion-pairs (LiCF3SO3, molecular) and aggregates (Li(CF3SO3)x

(x-1)-, or

LixCF3SO3(x-1)+ etc.) [72]. In our system, preliminary results show that the nitrate ions

are not coordinated, but form ion-pairs, aggregates with the metal aqua complex

counter ions, and free ions in the LC phase. However, we have also demonstrated that

samples prepared in the presence of free water, samples prepared from the mixture of

water/surfactant (%50 wt/wt) and the metal nitrates, have only free ions, (see Figure

27).

77

CHAPTER 2

3.2.1.Synthesis of Mesoporous Metal Sulfides

The synthesis of II-VI semiconductor nanoparticles and films has been studied

for some time due to its technological and scientific importance. In the literature,

there are many methods, which have been applied to generate small particles

(nanoparticles) of these semiconductor materials [9,15,17,19,21-22,26,74-77,83]. In

the majority of these works, a capping agent was used to prevent the aggregation of

small particles [9,11-12,14-15,17-22]. However, the surfactant based templating

mechanism, which has mostly been applied to metal oxides, has also been used to

make metal sulfides [74-77,83]. In these metal sulphide (MS) works, capping agents

were not used. The LC phase templating approach has also been applied to make

mesoporous Pt mesh, CdS, ZnS nanowires and ZnS nanoparticles [74-77,83,86-88].

In all of these studies, the LC phase was formed using water as a second component

of the lyotropic LC phase and then the metal salt was added to the mixture as a third

component, (ternary system). In such case, there is always one problem, at high salt

concentrations, the LC phase is not stable and it may even collapse into a disordered

non-structured phase. Therefore, these studies were carried with the samples, which

contain very low metal ion concentrations (such as 0.1 M solutions). In the previous

works, the main objective was to use the structure present in the organic mesophase to

directly template the growth of an inorganic phase. However our objective here is to

form an organic mesophase directly with inorganic, which forms an organic-inorganic

hybrid mesophase. Here, we state that the organic-inorganic hybrid mesophase can be

78

used to obtain mesoporous CdS and ZnS. Upon exposing the samples to H2S gas, the

Cd(H2O)42+ and Zn(H2O)6

2+ ions immediately react with H2S gas to produce

mesoporous metal sulfides.

[M(H2O)n](NO3)2 + H2S MS + 2 HNO3 + nH2O (M: Zn and Cd)

These acid byproducts and the free water produced after the reaction may

destroy the liquid crystalline phase. Each of these species affects the liquid crystal

(LC) phase templating mechanism. FT-IR, UV-Vis spectroscopies, PXRD, SEM and

TEM techniques were used for the characterization of the samples.

3.2.2. FT-IR Spectral Analysis

In this part of the study, FT-IR spectroscopy is used in order to determine the

structural changes in the LC phase and, after H2S gas exposure, and also to see

whether the reaction by-products and unreacted species are properly discarded after

washing. The vibrational spectra of CdS samples are given in Figure 42. These

spectra were recorded before and after the H2S exposure and once more after washing,

which has been carried to obtain surfactant free mesoprous CdS or ZnS. The effect of

the formation of CdS on the LC phase is shown in Figure 42 A. The splitted NO3-

signals at around 1300-1460 cm-1 loses its intensity. This indicates that while the Cd

(II) salt reacts with H2S, NO3- counter ion becomes free and solvated due to the free

water produced. Also the other NO3- signals around 810 cm-1 lose their intensity,

which shows that the NO3- loses the ion pair interaction with the metal complex and

becomes a free ion. Note also that the C-O stretching mode peaking at around 1100

79

cm-1 shifts back to the higher frequency after the reaction with H2S gas. In the

previous chapter it was shown that due to strong hydrogen bonding between the metal

aqua complex and EO units, this band shifts to a lower wave number. After the

reaction of these samples with H2S, the peak originates from the CO bond shifts back

to the higher wave number. This means that hydrogen bonding between the EO units

and the metal complex disappears after the CdS formation. This also shows an

important feature of the templating mechanism. While the reaction is proceeding or

after the reaction is complete, the LC phase is mostly destroyed. In addition, a

peaking at around 270 cm-1 gains intensity, which is due to the CdS modes [125].

Figure 42. A; IR spectrum of Cd(NO3)2.4H2O/No (a) After H2S exposure, (b) beforeH2S exposure. B; after washing.

The IR spectrum of the washed CdS sample is also given in part B of Figure

42. It is interesting to note that there are two peaks observed around 614 cm-1 and

1114 cm-1. These are characteristic signals of SO4-2 ion [123]. The Cd (II) complex is

very reactive with H2S so that the temperature of the reaction medium during the H2S

exposure increases. Therefore, the free NO3- ion can easily be reduced to NO2

- and S2-

is oxidized to the SO4-2 ion. This indicated that the reaction temperature should be

kept at lower temperatures. Note also that the bands around 1300-1400 cm-1 indicate

200 400 600 800 1000 1200 1400 1600 1800 2000

270

A

b

a

Rela

tive I

nten

sity

Wavenumber (cm-1)

600 800 1000 1200 1400 1600

0,0

0,2

1385

1114

614

B

Abso

rban

ce

Wavenumber (cm-1)

80

the presence of by products (NO2–) in the samples where we could not get rid off by

washing. After washing, the IR spectra were recorded as a KBr pallet, which cuts

around 300 cm-1, so the Cd-S vibrational region can not be observed, (seen in part B

of Figure 42).

Figure 43. A; IR spectrum of Zn(NO3)2.6H2O/No (a) After H2S exposure, (b) beforeH2S exposure. B; after washing.

The IR spectra of ZnS samples are given in Figure 43. The mesoporous ZnS

also shows similar changes. The IR spectrum of the washed ZnS sample displays

peaks at around 2700-3000 cm-1 region corresponding to the CH2 stretching modes of

the surfactant molecules (Figure 43 A), which means there are still some surfactant

molecules that are not washable. The Zn-S vibrational mode has observed at around

314 cm-1. The peaks at 614 and 1106 cm-1 are originating from the SO4 2- ion (Figure

43 B).

3.2.3. X-Ray Analysis

The PXRD technique was very useful for the determination of the

mesostructures formed in this section. The small angle diffraction lines at in the 1-5

400 600 800 1000 1200 1400 1600 1800

314

A

b

a

Relat

ive I

nten

sity

Wavenumber (cm-1)

300 600 900 1200 1500 3000 3500 4000

0,00

0,05

0,10

1106

614B

Abso

rban

ce

Wavenumber (cm-1)

81

2Θ region show, whether the material has a mesostructure, and the wide angle

diffraction lines (5-60 2Θ) show the crystallinity of the materials. Note also that the

wide angle diffraction lines are very sensitive to the particle size. If the particle size

decreases, the diffraction lines loose their intensity and becomes broader. The PXRD

patterns of the Cd and Zn samples were recorded three times: before H2S gas

exposure (unreacted), after H2S gas exposure (CdS and ZnS in the organic matrix) and

after washing the samples to remove the surfactants (pure ZnS and CdS).

Figure 44. X-ray diffractograms of (a) before H2S gas exposure, (b) after washing(pure CdS), (c) after H2S gas exposure (CdS in organic matrix).Cd(NO3)2.4H2O/surfactant mole ratio is 6.5.

All three diffraction patterns, obtained from unreacted, washed and unwashed

Cd samples, are shown in Figure 44. The first diffraction line is well resolved in the

unreacted sample. However, after the reaction with the H2S gas, the followings are

observed: the line intensity decreases and the first diffraction line shifts to the higher

two theta value.

2 4 6 8 100

1000

2000

3000

4000

5000

6000

cba

*5

Inte

nsity

2Θ

82

These observations can be explained as the loose of the orientational order

upon fast reaction with the H2S gas and the polymerisation and the growth of the MS

walls, respectively. The presence of the diffraction line at around 1.85 two-theta

(corresponds to 44 Å d-spacing) is an indicator for the existence of the mesoporous

structure. However upon washing, the diffraction line shifts to lower angles and

becomes very broad. The diffraction line observed from the washed samples begins at

around 2.0 2θ (4.4 nm) and extends until 1.0 2θ (8.8 nm). Note also that this peak is

not completely detectable due to instrumental limitations (one can run the diffraction

pattern starting from 1.0 up 2 θ). This broad signal is an indication of the existence of

a broad pore size distribution.

Figure 45. X-ray diffraction patterns of (a) before H2S gas exposure, (b) after H2S gasexposure (ZnS in organic matrix), (c) after washing (pure ZnS),Zn(NO3)2.6H2O/surfactantmole ratio is 4.

Figure 45 shows all three samples of Zn(NO3)2.6H2O/Surfactant, unreacted,

unwashed and washed. After H2S exposure and washing the ZnS samples show the

same changes as observed in the cadmium samples. The wide angle patterns give

2 4 6 8 100

1000

2000

3000

4000

5000

6000

7000

8000

cba

Inte

nsity

2Θ

83

information about the crystallinity of the MS (CdS and ZnS in this work) walls, the

crystal structure and the size of the crystals. As shown in Figure 46 after washing, the

samples display three very broad signals at the wide angle region, corresponding to

the metal sulfides. The three broad lines, which have been assigned [126-128] to the

cubic structure (sphalerite or zinc-blend) are observed from the CdS and ZnS samples.

These lines, corresponds to the (111), (220) and, (311) planes of the cubic crystal

structure of bulk CdS and ZnS. These signals clearly show that the mesoporous walls

made up of MS (CdS and ZnS in this work) are nanocrystalline.

Figure 46. Wide angle diffractogram of CdS after washing. Synthesised fromCd(NO3)2.4H2O/surfactant mole ratio is 1.5. The sharp diffractions observed in thesesamples, (see Figure 42 and 43) are due to unreacted Cd(NO3)2.4H2O crystals and byproducts.

The overall picture obtained from the X-ray measurements is: 1) the metal

aqua complexes used in this work, such as Cd(NO3)2.4H2O and Zn(NO3)2.6H2O form

a mesostructure, which templates the formation of mesoporous CdS and ZnS,

respectively. 2) the mesoporous materials obtained in this method are not stable for

1 0 2 0 3 0 4 0 5 0 6 0 7 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

3 1 12 2 0

1 1 1

Inte

nsir

y

2Θ

84

washing, however, there are other methods (burning), which may be applied to

remove the surfactant molecules from the pores.

3.2.4. SEM and TEM Analysis

Electron microscopy techniques, such as Scanning Electron Microscopy

(SEM) and Tunnelling Electron Microscopy (TEM) have vital role, as they are the

only techniques that can be used to obtain real images of samples. Direct imaging can

easily identify the morphology, atomic structure in nanoparticles and mesostructure.

Figure 47. SEM image of CdS after washing the samples, showing the morphology ofthe mesostructured CdS.

The SEM image of CdS sample is given in Figure 47. General morphology shows that

the samples consist of sub-micron particles (100 nm-1000nm). Figure 47 also show

that the larger particles are formed due to aggregation of these small (sub-micron)

particles.

85

Figure 48. TEM image of washed CdS sample, showing the mesoporosity in thesample (scale bar is 20 nm).

Figure 48 shows a TEM image of one of the sub-micron particle. The porous

structures are visible in this image. However, the samples are not stable under high-

energy electron beam. It is observed that the samples usually burn during the

measurement of the SEM and TEM images when the machines are focused to a

specific region. The image, shown in Figure 48, was recorded while the sample was

burning therefore the pores have started to open, as shown in region A of the image.

However, region B is almost intact and we can consider this region is a real

representation of the mesostructured CdS. The pore dimension obtained from TEM

images, region B, is around 8-10 nm, which is consistent with the PXRD results of the

washed samples. It has been pointed out, in the previous sections that the reaction

A

B

86

conditions may (high temperature, by products, acidity) affect the templating

mechanism. Therefore it is not surprising to observe poorly organized mesostructure

in these samples. While the porous structure of metal sulfide is forming, the LLC

phase is collapsing. To understand further and elucidate the correct questions related

to the reaction conditions, more detailed study is required. However, our preliminary

results show that the LLC phase obtained using metal aqua complexes template the

formation of mesoporous metal sulphides.

3.2.5.UV-Vis Spectral Analysis

The UV-Vis absorption spectroscopy is also one of the most sensitive

technique to determine changes in the size of the semiconductor and metal

nanoparticles. Here the electron in the conduction band and the hole in the valance

band are confined spatially by potential barrier, the surface of porous material. The

lowest energy optical transition from the valance to conduction band increases due to

confinement effects. Within a simple effective-mass approximation [129], the

confined gap is given as:

Where mc* and mv

* are the conduction and valance-band effective masses,

respectively, and ωx, ωy, and ωz are the dimensions of confined region assumed to be

+

+++=

∗∗vczyx

gapbulkgapconfined mmEE 11111

2 222

22

ωωωπη

87

a box. As the particle size decreases, the band gap of the crystal increases therefore a

blue shift is expected in the absorption spectra. This is due to the confinement of holes

and electrons [130-131] in the valance and conduction bands, respectively. Figure 49

and Figure 50 show the absorption spectra of CdS and ZnS, respectively. Both bulk

CdS and ZnS crystals have direct band gaps in which the valance band maximum and

conduction band minimum have the same momentum, ∆k = 0.0. Therefore the

absorption band edges were fit to the expression for a direct allowed (da) interband

electronic transition, [7,79]

αda = Cda(hv-Eg)1/2

where αda is the absorption coefficient (in this study, the absorbance value is used as

αda), Cda is coefficient with little energy dependence, hν is photon energy, and Eg is

optical band gap [79]. The optical band gap was determined by plotting αda2 as a

function of energy, hν, and then measuring the intersection of a line passing through

the absorption edge (linear fit). The intersection with the energy axis gives the optical

band gap, Eg [79].

88

Figure 49. Absorption spectrum of CdS. Synthesised from Cd(NO3)2.4H2O/surfactantmole ratio is 1.8.

Figure 50. Absorption spectrum of ZnS. Synthesised from Zn(NO3)2.6H2O/surfactantmole ratio is 2.4.

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.80.0

0.2

0.4

0.6

0.8

1.0

Bulk ZnS

3.89

(αda

) 2

Energy hν (eV)

2.2 2.4 2.6 2.8 3.0 3.20.0

0.2

0.4 Bulk CdS

2.60

( αda

)2

Energy hν (eV)

89

There is a clear blue shift observed from both samples compared to the bulk

Eg values, see Figure 49 and 50. The increase in the band gap indicates that the

structure is made up of small particles. The bulk CdS and ZnS has band gap energy

around 2.42 and 3.60 eV at 300K, respectively [7]. There is a 0.3 and 0.2 eV increase

in band gap energy for the mesoporous ZnS and CdS, respectively. This shift may be

due to the confinement of electrons and holes in pore walls.

The mesoporous CdS and ZnS samples were also prepared by using different

MX2/No mole ratios. The optical band gaps and the chemical composition of the

samples are given in Table 8.

Table 8. Optical band gaps of mesoporous CdS and ZnS synthesized with differentMX2/No mole ratios and the bulk values.

Cd(NO3)2/Surfactantmole ratio

Optical BandGap(eV)

Zn(NO3)2/Surfactantmole ratio

Optical BandGap(eV)

Bulk CdS 2.42 Bulk ZnS 3.601.4 2.52 1.4 3.811.8 2.60 1.6 3.882.2 2.58 2.0 3.892.6 2.47 2.4 3.893.0 2.48 2.8 3.873.6 2.45

The absorption spectra of various mesoporous CdS and ZnS samples are

shown in Figure 51. The change in the absorption band edge provides information

about the templating mechanism. As shown in Figure 51 and Table 8, the absorption

edge of the sample prepared from the low MX2/No mole ratio is red-shifted, closer to

the bulk samples. The liquid crystalline phase of this composition is not as stable as

the other compositions. Note also that the samples prepared with a 1.4 mole ratio and

90

a 1.6 mole ratio have lower melting point or an IT at around 35 to 45oC. Therefore,

the LC phase can be destroyed easily during the reaction between the LC phase

samples and the gas phase H2S. Note also that the temperature increases during the

MS formation reactions. Therefore both samples, mesoporous CdS and ZnS, prepared

using low MX2/No mole ratios, are not well templated. The solvated crystals coagulate

during the reaction and large particles are produced compared to the higher molar

ratios.

Figure 51. Absorption spectrum of CdS (A) and ZnS (B) synthesized from differentMX2/No mole ratios.

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6

e

dc

ba

e ZnS 1.4d ZnS 1.6c ZnS 2.0b ZnS 2.4a ZnS 2.8

Rela

tive

Inte

nsity

Energy hν (eV)

2.2 2.4 2.6 2.8 3.0

de c

ba

e CdS 1.4d CdS 1.8c CdS 2.2b CdS 2.6a CdS 3.0

Rel

ativ

e In

tens

ity

Energy hν (eV)

A

B

91

However, as shown in Figure 51 and Table 8, at higher mole ratios of MX2/No

the absorption band edge red shifts with increasing the mole ratio. The LC phase

becomes very thick and dense at higher MX2/No mole ratios. Therefore, this time

there is a diffusion problem for the H2S gas to react with the metal ion [77]. The low

diffusion rate limits the reaction that occurs only on the surface. Stupp and co-workers

also stated that the low H2S diffusion rate causes coagulation of the CdS and ZnS

crystals [75-77]. However, the mesoporous ZnS is better templated than the

mesoporous CdS, because the LC phase of zinc nitrates are softer and thinner. This

also proves that there is a diffusion problem in the case of the cadmium samples.

The templating step is very important in order to obtain well ordered CdS and

ZnS samples. Temperature, concentration and even type of metal complex change the

quality and the order the LC phase. This part of the work is not yet complete to

determine the exact templating mechanism, more experiments must be done. It has

been shown for the first time in this thesis work that the organic-inorganic hybrid

mesophase can be used to synthesize the mesoporous metal sulfides.

92

4.CONCLUSION

The self-assembling liquid crystalline templating (SLCT) approach enables

material scientists to synthesize mesostructured materials. The mesoporous and

nanostructured materials are technologically and scientifically important due to their

unique size and/or shape dependent properties. Much works have been devoted to the

design of materials, having different properties. Controlling the organisation and

orientation of the molecular precursors, is the key step for designing materials with

desired properties. Supramolecular self-assembly templating has opened new

dimensions in porous and mesoporous materials of different length scales. However

new methods must be developed in order to make the synthesis of other mesoporous

inorganic materials possible.

Here, for the first time, we have investigated the lyotropic liquid crystalline

(LLC) behaviour of oligo(ethylene oxides) non-ionic surfactants (No) with transition

metal aqua complexes (MX2). This binary system has many advantages over the

ternary system in which the LC phase is constructed using surfactant, water and metal

salts, where metal salt is added as the third component. The MX2/No mixtures, which

show LC behaviour in a broad temperature range, were prepared using various metal

aqua complexes and mole ratios (M= Ni(H2O)6 2+, Zn(H2O)6 2+, Co(H2O)6 2+, and

Cd(H2O)4 2+, X= NO3–). The Cl- and SO4

2- salts of these metal aqua complexes do not

have an LC behaviour. However, CoCl2.6H2O is an exception.

The thermal properties of the MX2/No mixtures indicate that the new phase is

even more stable than the one prepared in the ternary system (surfactant, free water

and metal salt). It was discovered that the metal aqua complexes interact more

93

strongly through hydrogen bonding, using their coordinated water molecules with the

surfactant molecules (compared to the same interaction of the free water). The PXRD

and POM techniques have proven that the MX2/No mixtures, with different mole

ratios form different types of mesophases. At high mole ratios, the CdX2/No and

ZnX2/No mesophases show a phase transition from an isotropic cubic phase to an

anisotropic hexagonal phase. However, the higher concentrations of Ni and Co salts in

non-ionic surfactant yield crystallization of these salts. Another important observation

throughout this work is regarding the ITs, which increase with increasing salt

concentration up to a saturation point at around 80-100oC. This new binary phase

enabled an increase in the metal ion concentration in the LC phase. For instance, in

the Cd(NO3)2.6H2O/No mixtures, the complex to surfactant mole ratio has been

increased to up to 6.5, keeping the meso phase. This increase can be an advantage in

the sol-gel synthesis of various inorganic materials.

The IR and Vis-Near-IR results show that the coordinated water molecules

can mediate the LC phase formation. The LC phase is a result of complex-induced

aggregation or self-assembly of the surfactant molecules. The interaction between the

aqua complexes and surfactant molecules is of hydrogen-bonding type. There is no

covalent interaction between the metal complex ions and the surfactant molecules.

In the second part of this thesis we have demonstrated that the binary LC

phase can be used effectively, to synthesize mesoporous metal sulfides. By employing

the PXRD and electron microscopy techniques we have proven this. However the

second part of the thesis is continuing and not yet completed. A wide range of

MX2/No mole ratio should be tested in order to observe concentration based changes

in the final product. Intensive work is required on the reaction conditions in order to

94

understand the templating mechanism which is affected by reaction by-products,

temperature and the acidity of the reaction media. Understanding reaction conditions

may lead to better and more stable structures. However, our preliminary results show

that the new LC binary system, discovered in this work, template the MS formation

and yield new mesoporous materials.

95

5.REFERENCES

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