Synthesis and characterization of fluorescent metal oxide nanostructures

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This work forms the written component of the research undertaken at Institut für Anorganische Chemie, Martin-Luther-Universitat Halle-Wittenberg, Halle (Saale), Germany and Departamentos de Química, CICECO, Universidade de Aveiro, Aveiro, Portugal, to fulfil the requirements of the Aveiro University for the PhD degree in chemistry. The work presented in this thesis has also been published in the following publications:

1- « Lanthanide-Based Lamellar Nanohybrids: Synthesis, Structural Characterization and Optical Properties »

Mohamed Karmaoui, Rute. A. Sá Ferreira, Ankush T. Mane, Luis D. Carlos, Nicola Pinna. Chem. Mater. 2006, 18, 4493.

2- « Optical Properties of Lanthanide-Doped Lamellar Nanohybrids »

Rute A. Sá Ferreira, Mohamed Karmaoui, Sónia S.Nobre, Luis D. Carlos, Nicola Pinna. Chem. Phys. Chem. 2006, 7, 2215.

3- « Lanthanide-Based Lamellar Nanohybrids: The Case of Erbium » Mohamed Karmaoui, Rute A. Sá Ferreira, Luís D. Carlos, N. Pinna. Mater. Sci. Eng. C 2007, 27, 1368.

4- « Photoluminescent Rare-Earth Based Biphenolate Lamellar Nanostructures » M. Karmaoui, Luis Mafra, Rute A. Sa´ Ferreira, João Rocha, Luis D. Carlos, Nicola Pinna.

J. Phys. Chem. C. 2007, 111, 2539.

5-«A general nonaqueous route to aluminum oxides and aluminate nanostructures» M. Karmaoui, M-G. Willinger, L. Mafra, T. Herntrich, N. Pinna.

Nanoscale (2009). DOI 10.1039/b9nr00164f

6-«The “benzyl alcohol route”: An elegant approach towards doped and multimetal oxide nanocrystals- Review and ZnAl2O4 nanostructures by oriented attachment»

Nicola Pinna, Mohamed Karmaoui, Marc-Georg Willinger, 2009 submitted

Book Chapter:

N. Pinna, M. Karmaoui, Photoluminescent rare-earth based lamellar organic-inorganic hybrid nanoparticles, In the book of « ADVANCED WET-CHEMICAL SYNTHETIC APPROACHES TO INORGANIC NANOSTRUCTURES » Edited by P. D. Cozzoli, chapter 13, pp 391-406, 2008.

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General introduction

I.1. Introduction:...- 3 -

I.2. Objectives (Motivation): ...- 4 -

I.3. Thesis organization: ...- 6 -

I.4. Hybrids materials: ...- 9 -

I.5. Sol-Gel Process: ...- 11 -

I.5.1. Introduction:...- 12 -

I.5.2. Sol-Gel chemistry and reactivity of metal alkoxides: ...- 12 -

I.5.3. Advantages and limitation of aqueous sol-gel synthesis:...- 15 -

I.6. Nonaqueous synthesis of crystalline metal oxide: ...- 15 -

I.6.1. Water-generating reactions: ...- 16 -

I.6.2. Nonhydrolytic hydroxylation reactions not involving hydrolysis: ...- 16 -

References: ... 20

-Techniques and methods

I. X-ray diffraction (XRD): ...- 34 -

II. Introduction to Electron Microscopy:...- 36 -

II.1. Transmission Electron Microscopy (TEM): ...- 37 -

III. Photoluminescence:...- 39 -

IV. The solvothermal synthesis: ...- 42 -

Refereces: ... 44

-Lanthanides doped nanomaterials

I. Lanthanide doped nanoparticles:...- 50 -

II. Lantahnide doped aluminate nanostructures:...- 52 -

II. Emission properties of lanthanides:...- 58 -

II. 1. Introduction:...- 58 -

II. 2. Optical properties of trivalent lanthanides ions: ...- 59 -

References: ... 61

-Rare-earth-based lamellar nanohybrids

IV. 1. Synthesis of rare-earth based benzoate/biphenolate hybrid material: ...- 76 -

IV. 2. Structural and Morphological Characterization: ...- 76 -

IV.2.1. X-ray Diffraction (XRD):...- 76 -

IV.2.2. Transmission Electron Microscopy (TEM): ...- 79 -

IV.2.3. Elemental analysis:...- 81 -

IV.2.4.Thermogravimetric analysis (TGA) & differential thermal analysis (DTA): - 82 - IV.2.5. Infrared spectroscopy (Fourier transform Infrared (FT-IR)):...- 84 -

IV.2.6. Formation mechanism:...- 91 -

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IV.3.I. Photoluminescent rare-earth based benzoate hybrid material: ...- 94 -

IV.3.I.1. Excitation and emission spectroscopy for (EuIII, TbIII and NdIII ) doped (YIII and GdIII)-based benzoate hybrid material : ...- 94 -

IV.3.I.2. Luminescence lifetime:...- 100 -

IV.3.I.4. Ligand emission:...- 101 -

IV.3.I.5. Energy level diagrams of the EuIII , TbIII and NdIII doped (YIII...- 103 -

and GdIII) -based benzoate hybrid material: ...- 103 -

IV.3.I.6. CIE chromaticity diagram of the EuIII and TbIII doped (YIII and GdIII) -based benzoate hybrid material and chromaticity coordinate: ...- 104 -

IV.3.I.7. Near InfraRed (NIR) Emission: ...- 106 -

IV.3.I.8. Photoluminescence spectroscopy for (SmIII_{and Nd}III_{)-based benzoate} hybrid material: ...- 108 -

IV.3.I.9. Excitation and emission spectroscopy for ErIII-based benzoate hybrid material:...- 110 -

IV.3.I.10. Luminescence lifetimes of NIR-emitting ions:...- 112 -

IV.3.II. Photoluminescent rare-earth based biphenolate hybrid material:...- 112 -

IV.3.II.1. Excitation and emission spectroscopy for EuIII doped (YIII and GdIII)-based biphenolate hybrid material:...- 112 -

IV.3.II.2. Excitation and emission spectra of NdIII doped (YIII and GdIII)-based biphenolate hybrid material:...- 115 -

IV.3.II.3. CIE chromaticity diagram of the (YIII_{and Gd}III_{) doped with Eu}III_-based lamellae and chromaticity coordinate:...- 116 -

Conclusions: ...- 118 -

References: ... 120

-Aluminate nanostructures

V. 1. Synthesis of aluminates and aluminium oxide hybrid lamellar nanostructures: Experimental protocol: ...- 126 -

V.2. Structural and Morphological Characterization: ...- 127 -

V.2.1. X-ray Diffraction (XRD):...- 128 -

V.2.2. Electron Microscopy (Transmission and scattering (SEM&TEM):...- 129 -

V.2.3. Elemental analysis: ...- 137 -

V.2.4. Differential thermogravimetric analysis (DTG) & differential scattering calorimetric (DSC): ...- 138 -

V. 2.5. Infrared spectroscopy (Fourier Transform- Infrared (FT-IR)): ...- 140 -

V. 3.6. Nuclear Magnetic Resonance Spectroscopy (NMR) :...- 142 -

13_{C Solid State NMR: ...- 142 -}

V.3. Optical Properties: ...- 145 -

V.3.I. Emission and excitation spectroscopy for non-doped strontium and barium aluminate nanostructures:...- 145 -

V.3.II.1. Excitation and emission spectroscopy for (PrIII, EuII) doped and (EuII DyIII and EuII NdIII) co-doped strontium aluminate nanostructures:...- 146 -

V.3.II.1.1. UV/VIS spectral region: ...- 146 -

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V.3.II.2. Excitation and emission spectroscopy for (EuII_{) doped and (Eu}II_DyIII_and

EuII NdIII) co-doped barium aluminate nanostructures : V.3.II.2.1. UV/VIS spectral

region:...- 150 -

V.3.II.2.2.Near Infrared (NIR) spectral region: Excitation and emission: ...- 153 -

V.3.II.3. Excitation and emission spectroscopy for SmIII doped zinc aluminate nanoparticles:...- 153 -

Conclusion:...- 154 -

References: ... 156

-Conclusions

... 167

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General introduction

I.1. Introduction: ...- 3 -

I.2. Objectives (Motivation):...- 4 -

I.3. Thesis organization:...- 6 -

I.4. Hybrids materials: ...- 9 -

I.5. Sol-Gel Process:...- 11 -

I.5.1. Introduction: ...- 12 -

I.5.2. Sol-Gel chemistry and reactivity of metal alkoxides: ...- 12 -

I.5.3. Advantages and limitation of aqueous sol-gel synthesis:...- 15 -

I.6. Nonaqueous synthesis of crystalline metal oxide: ...- 15 -

I.6.1. Water-generating reactions: ...- 16 -

I.6.2. Nonhydrolytic hydroxylation reactions not involving hydrolysis: ...- 16 -

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I.1. Introduction:

Nanoscience and nanotechnology are the two research topics that bring into play the study and the produce of matter on the scale of 1 to 100 nanometers, a scale at which novel chemical, biological, physical and mechanic properties enable novel applications. Nanotechnology is an emerging interdisciplinary field of research that has been booming in many areas during the recent decade, including materials science, mechanics, electronics, optics, medicine, energy and aerospace. Today the advances in materials science have brought us to the point where materials can be tailored on the atomic scale to suit specific application. This development has been possible through increased knowledge and understanding of the materials chemistry and physics, and advances in processing methods. To develop these new nanotechnologies applications it is important to investigate how the catalytic, optical, optoelectronic, electric, magnetic and mechanical properties of nanomaterials differ from that of bulk materials and molecules or atoms. As the particles’ size approaches nanometric dimensions, materials often show different properties that are only found in this size region.

Synthesis of different novel nanoparticles is a fundamental focal point of chemical research, and this interest is mandated by development in all areas of industry and technology. Materials science and engineering have qualified tremendous growth in the field of nanoparticles development, with enhanced properties of the final product. Metal oxides are very common commodities, which are finding wide variety of uses including catalysts, supports for heterogeneous catalysts, sensors, etc., and the interest in them became an additional thrust by the fast developing nanotechnology. The new strategies of synthesis that make it possible to obtain materials with well-defined size and the morphology have been undergoing a very extensive development within the scientific community since 1990 Chemical processes, including sol-gel synthesis, are of great interest in the regard that they offer relatively inexpensive industry-scale processing of a wide range of materials. They benefit from high control over stoichiometry and morphology, and they are in many aspects more flexible than others synthesis. Obviously, the sol-gel synthesis in the last decades has been so widely developed that many metal oxides nanoparticles and organic-inorganic hybrid materials are available now. Hence thousands of papers related to metal oxides nanoparticles appeared over the last decade and their number is still rising.

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Aqueous routes have much prospective for adoption by industry, but in many cases a comprehensive understanding of the structure and chemistry is lacking, partly due to the complicated aqueous chemistry of many metal oxides nanomaterials.

Organic solvents of varying viscosities and coordinating ability have been found to influence synthetic control on the final structure of the product.

Recently, the development of crystalline material has led down several interesting pathways, one of which is synthesis in non-aqueous media. Nanoparticles producted in non-aqueous media have been widely investigated experimentally. There has been a desire to detemine their behavior in absence of water in order to improve the crystalline final product.

New materials having different properties than the classical materials are opening new fields in the technology. Nanomaterials are materials with sizes less than one micrometer, usually less than 100 nm; as a particle decreases in size, a greater proportion of atoms are found at the surface compared to those inside, thus nanomaterials have a much greater surface area per unit mass compared with larger particles. They have attracted tremendous research interest in both academic institutions and industry. The new strategies of synthesis that make it possible to obtain materials with well-defined size and the morphology have been undergoing a very extensive development within the scientific community since 1990. They can be utilized in numerous applications because of some of their novel application catalysis, physical, chemical, biological and mechanic characteristics. To develop the application of these nanomaterials, it is important to investigate how those properties differ from that of bulk materials.

The field of nanoparticle research covers a wide range of interests; in fact, the large number of publications on nanoparticles can be explained by the fact that nanoscience and nanotechnology encompass a wide range of fields, including chemistry, physics, materials engineering, biology, medicine, and electronics. Nanoparticles are generally classified based on their dimensionality, morphology, composition, uniformity, and agglomeration behavior.

I.2. Objectives (Motivation):

The aim of this thesis was to investigate non-aqueous sol-gel synthetic strategies for the preparation of rare-earth-based lamellar nanohybrids and aluminates nanostructures.

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This thesis also reports explorative research concerning the photoluminescence properties of rare-earth-based lamellar nanohybrids and aluminate nanostructures. The functionality of these nanomaterials relies on their enormous potential for technological applications, such as in fluorescent lamps, display monitors, X-ray imaging, optoelectronics and medicine. Finally, the understanding of the relationship between luminescence properties and structure /composition is also an important objective of this work.

The synthesis of new compounds is the basis of solid state chemistry. Our major stimuli drive us to the synthesis of new architecture of materials. The original idea is our curiosity to create solids with novel design, new compositions, new or unusual structures as well as novel optical properties. In such particular, hybrid materials, aluminium oxide hybrid and aluminate nanostructures were produced using the non-aqueous media. The development of advanced materials forms the basis of technological progress, and requires reliable and flexible synthesis methods. Alkoxides based-nonaqueous sol-gel offers aversatile route to nanomaterials. In this thesis, we have developed an alkoxide nonaqueous sol-gel route to hybrid materials and aluminate nanostructures.

The experimental work is divided into two main parts, rare-earth-based lamellar nanohybrids and aluminum oxide hybrid material, Boehmite, aluminate nanostructures. The former deals with ordered lamellar organic-inorganic hybrid nanoparticles, the latter present the synthesis and the structural characterisation of novel aluminates nanostructures. One group of fascinating compounds with many potential applications in luminescence is the hybrid materials. Elaborating nanostructured “organic-inorganic hybrid material” with new physical properties practice nowadays a colossal growth and appeared as one of the most active research fields. This doctorate work consists on the synthesis and the study of a new family of rare-earth-oxide-based lamellar nanohybrids.

The hybrid materials are constructed form rare-earth metal ions and bridging organic ligands derived from alcohol such as benzyl alcohol or biphenyl-methanol. The versatility of the synthesis of this class of luminophores allowed tuning optical properties of this nanohybrids materialby appropriate selection of the ligands.

In a second experimental part of this thesis aluminum oxide hybrid material, Boehmite, alkaline earth aluminate nanostructures (BaAl2O4, CaAl4O7, and SrAl4O7) and zinc

aluminate nanocrystal (Gahnite, ZnAl2O4) of various structures, sizes and morphologies

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We will demonstrate that the one-step reaction provided a simple, successful route to various metal oxide and several complex metal oxide nanostructures. The preparation of a large variety of nanomaterials by nonaqueous strategy is already discussed and detailed in the chapter 1. Non-aqueous route to achieve nanomaterials with benzyl alcohol are introduced in chapter 5. The optical properties of the novel synthesized metal aluminates phosphors were investigated in detail. The investigation on Eu-ion doped and Dy, Nd co-activated nanostructures compounds prepared via a “Benzyl Alcohol route” indicates that europium ions were partially or totally oxidize form Eu2+ ions to Eu3+ in all matrixes. In this work, a combination of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), differential thermal analysis / differential scanning calorimetry (DTA/DSC), ICP (inductively-coupled plasma spectrometer), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), Solid-State NMR (SS-NMR) and elemental analysis were employed to characterize the structure, morphology, composition, thermal behavior and photoluminescence spectroscopy of the final products.

I.3. Thesis organization:

This thesis is divided into five chapters:

This chapter 1 is divided into three sections. It is intended to give some description of the developments of the hybrid materials and sol-gel (aqueous and non-aqueous) synthesis of nanostructured materials. A large number of references are provided throughout the chapter for readers interested in further or specific details of the sol-gel route (aqueous and non-aqueous). In the section I.4 will provide some background on historical and recent reports in the very broad field of organic-inorganic hybrid materials.

The overall goal was to gain a better insight into physicochemical phenomena that predominantly to define the properties of organic-inorganic hybrid materials, in general, and to convey scientifically valuable knowledge regarding properties of such materials on the basis of the particular organic–mineral systems chosen. In the section I. 5, the study and understanding of the sol-gel chemistry of metal alkoxides and the relationship between synthesis conditions and properties of the obtained oxidic metal oxide nanoparticles will be described in detail.

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-Introduction -Objectives (motivation) -Thesis organization -Hybrids materials - Sol-Gel process - Nonaqueous synthesis of

crystalline metal oxide

Chapter I

- X-ray diffraction

- Introduction to electron microscopy - Photoluminescence

- The solvothermal synthesis

- X-ray diffraction

- Lanthanide doped nanoparticles - Emission properties of lanthanides- Lanthanide doped nanoparticles_{- Emission properties of lanthanides}

- Synthesis of rare-earth based benzoate/ biphenolate hybrid material

- Structural and morphological characterization - Optical properties:

Photolumenescence rare-earth based-benzoate /biphenolate hybrid material

- Introduction

- Synthesis of aluminates nanostructures and aluminium oxide hybrid lamellar

- Structural and morphological characterization - Optical properties

- Introduction

- Structural and morphological characterization - Optical properties Chapter IV Chapter IV Chapter III Chapter III Chapter II Chapter II Chapter V Chapter V -Introduction -Objectives (motivation) -Thesis organization -Hybrids materials - Sol-Gel process - Nonaqueous synthesis of

Chapter I

- X-ray diffraction

- Lanthanide doped nanoparticles - Emission properties of lanthanides- Lanthanide doped nanoparticles_{- Emission properties of lanthanides}

- Introduction

- Structural and morphological characterization - Optical properties

- Introduction

- Structural and morphological characterization - Optical properties Chapter IV Chapter IV Chapter III Chapter III Chapter II Chapter II Chapter V Chapter V

Figure I. 3-

Schematic of the five chapter addressed in this thesis.

In addition, the section also describes the reactivity of metal alkoxides used for preparation of different metal oxide employed in this present work, underlying the unique properties of these precursors’ metal complexes. In the section I. 6 of this chapter is to give a brief general introduction of non-aqueous sol-gel routes to metal oxide nanoparticles. This section also discusses the nonaqueous route-to-oxide nanoparticles evolution, the process was studied in detail and is described under “Water-generating reactions”, “Nonhydrolytic hydroxylation reactions not involving hydrolysis” and “Aprotic condensation reactions” including their mechanism reactions.

Chapter 2 gives a brief preamble to only some experimental techniques used in this work such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and photoluminescence spectroscopy. It is not the intention to give a complete and stringent presentation, as this can be found in numerous publications elsewhere. Rather, it is the intention to give a more phenomenological explanation of the techniques.

Chapter 3 presents background information relevant to the thesis work including lanthanide ions and their optical properties. The goal of this chapter is to improve the luminescence properties of lanthanide ions by doping them in the core of inorganic

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Generally, lanthanide ions have good optical properties in inorganic matrices and long luminescence lifetimes and high quantum yields are observed in these materials. An overview of the luminescence of lanthanide ions will be given also in this chapter as well the doping behaviour of lanthanide ions in nanoparticles. The synthesis and properties of different nanoparticles will be discussed in detail.

The hybrid organic-inorganic materials that are the subjects of the chapter 4 are rare-earth-based lamellar nanohybrids were investigated. Nanoparticles with ordered lamellar organic-inorganic are a new class of materials and are studied in detail in this chapter. They may present the advantage of exhibiting the stability and optical properties of inorganic materials on one side, and the flexibility and processability of organic molecules metal complexes on the other side. The motivation of this chapter was the investigation of novel synthetic routes to organic–inorganic hybrid materials. The reaction of RE alkoxides in different alcohols (such as benzyl alcohol or biphenyl-methanol) leads to hybrid materials possessing different sizes, morphologies and temperature stability. Such systems have attracted interest, because of their optical properties which are intrinsically connected with their unique structure.

The optical properties of such nanohybrids doped with various lanthanides emitting ions (Eu3+, Tb3+ and Nd3+) which are good emitters in the red, green and near infrared were prepared giving rise to different luminescence spectra. It was found that an efficient charge transfer from the organic moieties to the Ln (III) emitting ion takes place. We have demonstrated that the organic ligand can act as light collectors transferring absorbed energy to lanthanide emitting ions, with a high yield. In fact, an active energy transfer between the phenyl rings of the benzoate/biphenolate complexes to the inorganic part is found.

Finally due to the presence of organic molecules which naturally emit in the blue and green region of the visible, the CIEchromaticity can be tuned by the excitation energy without losing the high radiance values. In summary, this study successfully and systematically synthesized several kinds of organic-inorganic nanoparticles. The resulting materials were characterized by various advanced techniques such as X-ray powder diffraction (XRD), small angle X-ray scattering (SAXS), high resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), differential thermal analysis / differential scanning calorimetry (DTA/DSC), ICP (inductively-coupled plasma

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spectrometer), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), Solid-State NMR (SS-NMR) and elemental analysis were employed to characterize the resulting materials.

Chapter 5 describes a comprehensive study about the synthesis, characterization and photoluminescence properties of oxide organic-inorganic hybrid material, Boehmite and metal aluminate phosphor.

A nonaqueous route has been explored to prepare these large varieties of nanomaterials. The presence of different structures from aluminum oxide hybrid, Boehmite to aluminates nanostructureMAlxOy (M = Sr, Ca, Ba and Zn; (2 ≤ x ≤ 4; 4 ≤ y ≤ 7)) was studied by XRD,

SAXS, SEM, TEM, HRTEM, ED, CHN, TGA/DSC, FT-IR, NMR, Solid-State NMR (SS-NMR) and photoluminescence spectroscopy.

We also have developed several new metal aluminate phosphors including the red phosphor SrAl4O7:Pr3+; the bright orange-reddish phosphor ZnAl2O4:Sm3+; the blue-bluish

phosphor SrAl4O7:Eu2+; SrAl4O7:Eu2+Nd3+; BaAl2O4:Eu2+Nd3+ and persistent green-blue

phosphor SrAl4O7:Eu2+Dy3+; BaAl2O4:Eu2+Dy3+.

For this purpose, divalent cations (Ca2+, Ba2+ and Zn2+) were used to substitutionally replace the strontium. In addition, we have also investigated the near Infrared luminescence of SrAl4O7:Eu*+Nd3+; BaAl2O4:Eu*+Nd3+. To the best of our knowledge, the

NIR luminescence properties of SrAl4O7:Eu2+Nd3+ and BaAl2O4:Eu2+Nd3+ have not been

reported until now.

Photoluminescence emission spectra have shown that, in this host Eu*+Nd3+ exhibits excellent emission in UV/VIS and (NIR) and this emission is significantly enhanced in presence of Nd3+. The luminescence properties of some other phosphor such as zinc aluminate (ZnAl2O4) were also studied and described in detail in this chapter of thesis.

I.4. Hybrids materials:

Definition, classification and advantages

A composite material is defined by Kelly [1] as a multiphase solid material mixed at the nanometric scale. Hybrid materials are homogeneous systems derived from monomers and miscible organic and inorganic components with size range of 1-100 nm. They show peculiar physical-chemical and mechanical properties when compared to the conventional composites [2-7]

.

The distinction between composite and hybrid material is more often subtle and the appellation organic and inorganic particles will be used in the following

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Depending on the nature of the constituting phases, Sanchez[2] divided organic–inorganic materials into two classes (figure I.1):

A/ Class I hybrid organic-inorganic compounds:

corresponds to hybrid

systems in which organic and inorganic entities interact only weakly; in this class, both components interact via weak bonds such as van der Waals, hydrogen, or electrostatic interactions.

B/ Class II hybrid organic-inorganic compounds:

where organic and

inorganic phases are bonded via stronger covalent or iono-covalent bonds [2]

.

A) Hybrid class I B) Hybrid class II

Figure I. 1-

Schematic representation of the two kind classes of hybrids materials.

In general, the properties of organic-inorganic hybrid materials depend on their structure as well on their organic-inorganic junctions. Thus, the precise design of such hybrids is crucial for the development of desired functions[2, 6-8]

.

Many investigations concerning the development of incorporation of organic molecules into inorganic matrices have been published [9-17]

.

They principally describe the preparation of organic-inorganic hybrid materials, including different classes of inorganic building blocks such as transition metal (IV) phosphates and phosphonates, transition metal oxyhalides, alkali-transition metal oxides, and smectites. The specific interest of hybrid materials can be mainly attributed to organic-inorganic interface; in fact the host-guest interaction usually changes significantly the chemical, catalytic, electronic, optical, and mechanical properties[7, 8, 13, 16-35]

.

Some possible configurations of hybrid materials are given in figure I.2. The structures can be grouped into three categories depending on their arrangement at the nanoscale

:

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System I:

hybrid organic-inorganic materials, where the organic is surrounded (or connected) by the inorganic core and thus can form hybrid materials [8, 33, 36-38] or core-shell hybrid material, where the inorganic is spherical core encircled by a organic shell which is a few nanometers thick (both species with mutual chemical bonds are formed) [39-45]

.

System II:

where alternate layers of the two materials are stacked in a

regular manner (the guest organic molecules are confined in inorganic host layers) [13, 16, 18-25, 46]

.

System III:

In this system, the organic component acts as continuous

matrix where the nanoparticles are dispersed inside this matrix [6, 8, 27, 47-57] or in the contrary; the inorganic matrix, where organic guest are embedded in an inorganic host [8, 58, 59].

System I

System II System III

Organic core A coated Intercalation of layers of A Organic species A incorpored with inorganic B between the inorganic layers B in matrix B

(Core-Shell)

Figure I. 2-

Different types of hybrid materials.

Incorporation of organic molecules between the inorganic layers corresponds to the second system. Intercalation materials are multilayer structures, in which guest molecules are inserted within the space existing between the inorganic layers (In this thesis chapter four, I will present the systemII).

I.5. Sol-Gel Process:

The scope of this section is to give a brief introduction on sol-gel chemistry and its advantages for nanomaterials and organic-inorganic hybrid materials synthesis.

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I.5.1. Introduction:

Sol-gel chemistry provides a convenient approach for the preparation of different materials in a wide variety of forms such as: ultra-fine powders, thin films, fibers, porous materials, monolithic ceramics and glasses. It appears attractive because it offers several advantages that will be discussed later. The process is well discussed and more detailed in the literature[2, 30, 33, 48, 60-62]

.

The sol-gel reaction occurs via hydrolysis and condensation of molecular precursors. The sol-gel process is a wet-chemical procedure in which a solution of a metal compound or a suspension of very fine particles in a liquid (referred to as a sol) is converted into a semi rigid mass (a gel).

The term, ‘sol’, describes a colloidal suspension in a liquid (1-1000 nm), and a gel is defined as a two-phase system including a molecular backbone and an interstitial liquid. ‘Sol-gel’ refers to the process of forming metal oxides from molecular precursors; both a ‘sol’ and a ‘gel’ are formed in the intermediate stages of this process. These sol and gel formations can be carried out at low temperature (<100°C) and an oxide can be finally obtained after burning off the organic compounds by thermal treatment [2, 30, 33, 48, 60, 61]

.

Hydrolytic sol-gel methods have been employed in the preparation of many metal oxide ceramics and glasses [63], silica nanotube membrane [64], metal oxides nanoparticles [65]

,

f

erroelectric nanomaterials [66], Silica films [67] and many other interesting nanomaterials that are characterized by high homogeneity of the product.

I.5.2. Sol-Gel chemistry and reactivity of metal alkoxides:

The sol-gel chemistry is generally performed in a solution of metal complexes in an alcohol or other organic solvent and water.

The nature and reactivity of precursor materials are one of the most important parameters for the gel structures. The selected precursor materials used in the preparation of the sols are usually inorganic metal salts such as metal (-chlorides, -acetates, -nitrate, -sulfates), metal organic compounds (metal alkoxides) or organometallic compounds (Metal carbonyls). Amongst these,the metal alkoxides are almost widely used in sol-gel research

[62]_{. The chemical reactivity of the metal complexes depends} _{mainly on the coordination}

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M

etal alkoxides are represented by the following chemical formula M(OR)n, wherein M

design metallic atom of valence n that can bond with organic compounds through oxygen donor ligands and R is an alkyl group. The chemical reactivity of metal alkoxide is also linked to the stability of the M-O bond and depends on the nature of metal atom M and alkoxy group R. The reactivity of an alkoxide also increases with the capacity of the metal centers to increase its coordination by reacting with nucleophilic ligands or by solvatation

[2]

_.

Most metal alkoxides are very reactive towards hydrolysis and condensation, and controlling this reactivity with inorganic acid inhibitors such as β-diketones, carboxylic acids or other complexing ligands can inhibit hydrolysis and condensation reactions and therefore prevent precipitation.

In the sol-gel process, many parameters, such as solution pH, temperature, reagent concentration, stirring rate, addition rate, and aging time, must be controlled precisely to achieve uniform nucleation throughout the solution.

I.5.2.1. Hydrolysis:

The hydrolysis of metal alkoxide, takes place in the presence of water, and the overall reaction can be written as follows

(shema I.1.):

Scheme I. 1

The first step is a nucleophilic substitution of a metal-alkoxide groups (OR), ivolving the addition of a water molecule and the breaking of both M–O and O–H bonds. Its kinetics will be driven mainly by the lability of the metal–oxygen bond.

It’s really important to point out that many parameters can influence this hydrolysis, among the parameters mentioned above; the pH value not only plays a major role in the mechanism but also in the final desired materials.

The hydrolysiscan be promoted by acid or base catalysts and commonly catalysts used are HCl, NaOH orNH4OH, but fluorides can be also used as catalysts leading to short reaction

times. Applying acid-catalysis is favorable for the hydrolysis reaction leading to condensation of small clusters. Contrarily, the base-catalysis promotes condensation process leading to highly cross-linked sol particles [69, 70].

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II.5.2.2. Condensation:

Referring to the general mechanism of this step, the reaction may occur following three special pathways:alcoxolation, oxolation and olation.

i. Alcoxolation:

Alcoxolation is condensation reaction between hydroxyl-group and an adjacent metal-alkoxy group to produce a bridging oxo group under the elimination of on alcohol as shown in the reaction (scheme I.2.):

Scheme I. 2

ii. Oxolation:

In this process, the condensation occurs when two hydroxyl groups react as shown in scheme I.3. It

produces water in addition to forming oxo bridges.

Scheme I. 3

Condensation reactions proceed through several additional stages until a pure M-O-M network is formed.

iii. Olation:

In this chemical reaction, the formation of a hydroxo bridge takes place between two cationic centers (M) with the elimination of a solvent molecule. In the reaction, a hydroxo-bridge occurs via reaction (scheme I.4.)

:

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Condensation can be performed via the nucleophilic addition of negatively charged OH groups onto positively charged metal cation. This leads to the departure of coordinated water molecules and formation of “ol” bridges (M–OH–M bridge).

The idea behind sol-gel chemistry is: to form a metal-oxide network, starting from precursors in solution, in a controlled manner. According to these four reactions, the exact structure and morphology of the resultant gels depends on the relative contribution of each reaction. The reaction parameters, cited above, have an important influence during the formation of solid phases

.

I.5.3. Advantages and limitation of aqueous sol-gel synthesis:

The complexity of sol-gel systems allows for synthesis of many different structures and paves the way for the development of a variety of materials for many applications. Sol-gel processes also yield extremely pure materials.

The advantages of hydrolytic sol-gel methods become apparent:

- The chemical reactions occur at much lower temperatures than other methods. - High purity and homogeneious of resulting material.

- Control of the chemical and physical composition. - Preparation of highly porous materials.

- Possibility of various forming process in a range of shapes, such as thin film, fibers, monoliths or powder.

The main disadvantage of the hydrolytic sol-gel route is due to the difficulty in controlling the rapid hydrolysis of transition metal oxide precursors. This can result in direct precipitation rather than in the formation of gels. Therefore, thesize and morphology of the precipitate is difficult to control. Furthermore, in most cases amorphous precipitates are obtained and a calcination step is needed to induce crystallization.

I.6. Nonaqueous synthesis of crystalline metal oxide:

Non aqueous sol-gel chemistry was introduced to overcome the main drawback of aqueous sol-gel chemistry cited above [71-73]

.

Based on initial results, we briefly describe the basic chemical reactions involved on the preparation of the metal oxide under non-hydrolytic sol-gel conditions.

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Secondary and with regard to the applicability of the process, we present some examples showing that non-hydrolytic sol-gel is a promising route to produce metal oxides nanoparticles, illustrating the benefits that can be expected compared to conventional aqueous sol-gel process.

Non-hydrolytic gel chemistry may be performed in a manner similar to hydrolytic sol-gel (described above), except that it is performed essentially in the absence of water, in other term the traditional hydrolysis and condensation reactions are replaced by direct condensation or non-hydrolytic hydroxylation reactions

.

I.6.1. Water-generating reactions:

Vioux classified two routes in the non-hydrolytic sol-gel reactions: non-hydrolytic hydroxylation reactions and aprotic condensation reactions, according to whether or not hydroxyl groups are produced [71]. Nanosized alumina powders have been prepared via non-hydrolytic sol-gel process. It is shown that high-quality powder can be produced by this route when dispersion of alcoholic solution of aluminum sec-butoxide, without adding water, which is provided by the dehydration of sec-butanol used as a solvent, at 250 ºC [74]

.

T

itanium and zirconium oxides were successfully synthesized by using an esterification reaction between acetic acid and an alcohol[75-80]

.

However, the action of the carboxylic acid (generally acetic acid) is complicated by complexation reactions, where the reactivity of the metal alkoxide completely changes [75-81]

.

I.6.2. Nonhydrolytic hydroxylation reactions not involving

hydrolysis:

The most well-known non-hydrolytic hydroxylation reaction is the thermal decomposition of metal alkoxides. In this process, hydroxyl groups are produced on metal cations along with alkenes side products during thermal decomposition [72, 82]

,

as shown in schemeI.5.

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χ

-alumina powder with nanometric size (4-20 nm) was synthesized by Massashi using the thermal decomposition of aluminium isopropoxide in inert organic solvent heated at 300 ºC [83].

Scheme I. 6-

Hydroxylation reaction “non hydrolytic”.

The action of certain alcohols on metal halides is represented in schemes I.6. (a) and (b)

[71]_{. Paths (a) and (b) in scheme I. 6 both involve the coordination of alone pair of electrons}

from oxygen to the metal center followed by cleavage of either the alkoxyl group (a), or hydroxyl group (b). Whether (a) or (b) is the directed pathway depends upon the alcohol.

Aprotic condensation reactions:

Discarding all functional hydroxyls group, many heterofunctional condensations may occur in scheme I.7. In rigorously non-hydrolytic condition oxo-bridges may be formed between two metal complexes, which involve the elimination of ether, ester or alkyl halide. The non-hydrolytic condensation between two metal alkoxides with ether elimination provides an attractive approach and has been invoked in the formation of metal oxides[69,

70, 84, 85].

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An interesting example of this route allows the synthesis of pure hafnium metal oxides, where formation of crystalline nanoparticles (HfO2) from hafnium alkoxides and benzyl

alcohol was observed at low temperature with concomitant formation of M-O-M bridge

[86]_.

Ester elimination, RCOOR’, is another type of aprotic condensation reaction that involves metal carboxylates and metal alkoxides, as starting materials resulting in the formation of oxygen bridges between metal atoms in M-O-M network [78, 87]

.

Recently, several groups have reported the synthesis of metal oxide nanoparticles via non-hydrolytic sol-gel with alkyl halide elimination. The condensation between metal halide and metal alkoxides leads to the formation of M-O-M bond. The by-product of this reaction is an alkyl halide [88, 89]. According to Vioux, further condensations in scheme I. 8 [71]demonstrating a possible two step process using ether as the oxygen donor with metal halides.

Scheme I. 8-

Aprotic condensation reaction.

In the first step, oxygen coordinates with the metal center of the halide resulting in a mixed alkoxy group and condensation with the loss of RX [90]

.

In the second step, the mixed alkoxy groupreacts with another metal halide resulting in condensation and the creation of M-O-M links.

Non-hydrolytic sol-gel routes have been developed in recent years as potential alternatives to the conventional hydrolytic route to inorganic oxides [71-73]

.

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The non-hydrolytic sol-gel process has become an attractive and intensive area of research, for the preparation of highly homogeneous metal oxide Al2O3 [88, 91-93], In2O3 [94-96], Y2O3 [32, 97]_, _Sm

2O3 [98], CeO2 [99], TiO2 [71, 90, 100-105], Ta2O5 [106], Nb2O5 [107], FeOx [108, 109],

Fe2O3-Fe3O4 [110], SiO2 [17, 111], HfO2/HfxZr1_xO2 [112, 113], AlF3 [114], V2O3 [107], V2O5 [107, 115]

, ZrO2[113, 116, 117], CuOx [118, 119], Cr2O3[120], Co3O4[120, 121], CoO[118, 122], BaTiO3 &

SrTiO3 [123, 124], WO3 [125], ZrTiO4 [126], WOx[115, 127], WO3 [110, 128], ReO3 [129], NiO[120, 130, 131]_{, MnO}[108, 132] _{, Mn}

3O4 [115, 118, 132], SiO2-ZrO2 [133], SnO2 [134-136]

,

Ga2O3 [94], ZnO [94, 134, 137]_{, SiO}

2-TiO2 [17, 138], SiO2-Al2O3 [17, 139] TiO2-ZrO2 [126, 140]

,

V2O5-Nb2O5 [141],

ZnO-GaO2-In2O3[94], ZrO2-WO3 [128], ZrTiO4[126]a polymorph of ZrW2O8 [128]and NASICON

(Na3Zr2Si2PO12) [142]

.

We have now found that sol-gel techniques which utilize a

non-hydrolytic procedure may be successfully used in the preparation of all this variety of nanoparticles. One of the major advantages of the non-hydrolytic sol-gel route provided a practical route to crystalline inorganic oxides nanoparticles in water-free systems. This process has some advantages over the conventional hydrolytic sol-gel route due to the ability to producehighly crystalline nanoparticles at moderate temperature.

In our group, non-aqueous approach was used to prepare a large variety of metal oxide nanoparticles. This general synthetic approach is called the “benzyl alcohol route”.

In the last few years it was shown that non-aqueous sol-gel reactions of benzyl alcohol with different metal oxides precursors (alkoxides, chlorides, acetylacetonates…) allow the controlled and straightforward synthesis of various crystalline metal oxide nanoparticles

[72]_{. In the “benzyl alcohol route”, the simple precursors used for the production of the}

nanomaterials allowed to carefully explore the chemistry taking place during particle formation [73]_{. As already mentioned above, the “benzyl alcohol route” seems not only}

promising for the preparation of metal oxide nanoparticles but also be useful for the synthesis of organic-inorganic hybrid materials currently widely investigated.

For further details, please refer to Ref [143] that contains a thoroughreview of about benzyl alcohol route, while the reviews [73, 143, 144] are moregeneral introductions to non-hydrolytic sol-gel processeses.

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