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Al-Azhar University-Gaza
Deanship of Postgraduate Studies
Faculty of Science
Depart. of Chemistry
Silica and Mesoporous Silica Coated Metal Oxides
Nanoparticles: Synthesis , Characterization and
Application .
By
Sara Taha Shaker Albhaisi
Supervisors
Prof. Issa M. El-Nahhal Prof. Jamil K. Salem
Professor of Inorganic Chemistry Professor of Physical Chemistry
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of
Master in Chemistry
2015
i
Dedication
To my father and my mother and Dear
brothers and sisters
ii
Acknowledgements
First of all, I am grateful to Allah, who granted me the power and courage
to finish this study.
I would like to thank my supervisors, Prof. Issa M. El-Nahhal and Prof.
Jamil K. Salem, for their great support, enthusiasm, and guidance over the
last year. I had the privilege of being involved in many interesting scientific
discussions, which tremendously influenced and improved my scientific
work.
My deepest appreciation and sincere gratitude to my parents, who helped
me at various stages during my work.
I also extend my profound thanks to the department of chemistry in Al-Azhar
University of Gaza and to the staff of the department of chemistry in Al-
Azhar University for making the facilities available for me to use.
SaraT. Sh. Albhaisi
iii
Abstract
Pure metal oxides nanoparticles such as magnesium oxide and zinc oxide (MgO and
ZnO) were successively synthesized using a co-precipitation method (oxalate route).
Silica and meso silica coating of metal oxides microsphers of general formula
MO@SiO2, MO@mSiO2 or MO@SiO2@mSiO2 were prepared using simple sol gel
method (Stöber method). In this method the metal oxide nanparticles were firstly
dispersed into the solution prior to silica encapsulation by sol-gel method. Amine or
thiol functionalized of pure metal oxide, meso silica or silica-mesosilica coated metal
oxide are also prepared by grafting method, using the appropriate silane coupling agents.
Meso silica or silica-meso silica free of metal oxide were obtained by treatment of silica
coated metal oxides with dilution HCl. It is found that these metal oxide nanoparticles
were fully dispersed into silica microsphers and their outer meso silica shells were
functionalized by amine or thiol functional ligand groups. It is found that, the coating
with silica or mesosilica shells does not altered the particle size of the encapsulated metal
oxide nanoparticles. It is found that these nanoparticles are well dispersed in one or both
silica shells. The silica and meso silica coating of magnesium oxide and its amine
functionalized were used for the adsorption and removal of BTB and heavy toxic metals
from water, UV-VIS studies indicate that some of these materials showed high potential
for the removal of toxic dyes and heavy metal ions. The synthesized MgO and ZnO and
their silica meso silica coating samples were characterized by thermal gravimetric
analysis (TGA), Powder X-ray diffraction (XRD), Transmission electron microscopy
(TEM), Energy dispersive X-ray spectroscopy (EDX), UV–VIS and Photoluminescence
spectroscopy. FTIR and TGA and 13
C NMR analysis have proved that the
organofunctional groups were introduced into the meso silica outer shell. XRD results
confirmed that there was no significant change in particle size after silica coating
process. TEM analysis showed that the metal oxide particles are well dispersed into
silica microspheres and into the meso outer shell. Three shells around metal oxide core
are well observed, silica , meso silica and functionalized silica.
iv
المغلفة بالسيليكا والميزو سيليكااسيذ المعادن والتطبيقات لجسيمات أك تحضير ودراسة الخواص
الملخص العربي
oxalateذى ذحضز أكاسذ انعاد انقح يثم أكسذ اناغسىو وأكسذ انشك تطزقح ذزسة الأوكسالاخ )
route ,)الأذح غحتانظ )أكسذ اناغسىو وأكسذ انشك( داانسهكا وانشو سهكا كغلاف لأكاسذ انع وكذنك
(MO@SiO2,MO@mSiO2, MO@SiO2@mSiO2) )تاسرخذاو طزقح انسىل جم انثسطح )أسهىب سرىتز .
يجىعاخ وظفح يثم الأي وانثىل عهى سطح أكسذ انعذ أو عهى سطح انسهكا وانشو سهكا ذزكةذى و
ذى انحظىل عهى نقذ .(silane coupling agent) اوتطزقح انرطعى تاسرخذانغهفح لأكسذ انعذ وإضافرها
انغهفح لأكسذ انعذ تحض انسهكا أو انشو سهكا خانح ي أكسذ انعذ ع طزق يعانجح انسهكا
غلاف داخم ج تشكم كايم فرشزاناىح ي أكاسذ انعاد ي انهذروكهىرك انخفف. وذث أ هذ انجساخ
هذعهى سطح غلاف انشو سهكا نح يثم الأي وانثىل إضافح يجىعاخ وظفذى كا. انسهكا وانشو سه
عهح ذغهف أكاسذ انعاد حجى انجساخ تق ثاتد حرى تعذ أ أثثرد TEM .و XRDذحانم انجساخ . و
يع حض انهزوكهىرك تانسهكا وانشوسهكا. وهذ انجساخ اناىح ك إسانرها تسهىنح ع طزق يعانجرها
( وذحهم (TGAانىس انحزاري هموذح (FTIR)انخفف. وأثثرد ذحانم الأشعح ذحد انحزاء13
C NMR أ
انجىعح انىظفح انر أضفد عهى طثقح انغلاف انخارج ي انسهكا قذ ارذثطد عهى طثقح انسهكا. وذى
(BTBاسرخذاو تعض ي عاخ أكسذ اناغسىو انغهفح تانسهكا وانشو سهكا وانطعح تالأي لإسانح طثغح
هذ انعاخ واسرخذيدوجىد الأي كجىعح وظفح. أ سثح الإسانح ذشداد يع UV-visوأظهزخ ذحانم (,
انخىاص انضىئح وانرزكثح لأكاسذ وقذ درسدنهرخهض ي انعاطز انثقهح انسايح يثم انحاص وانكىتهد .
,TGA, FTIR)انعاد وعاذهى انغهفح تانسهكا وانشو سهكا تىاسطح ) 13
C NMR, XRD, TEM, EDX,
UV-VIS, PL.
v
Contents
Page
No.
Chapter One
Introduction
1.1 Metal oxide nano particle 2
1.2 Methods of preparations of Metal Oxide Nano Particle 2
1.2.1 Liquid-solid transformations methods 2
1.2.1.1 Co-precipitation methods 2
1.2.1.2 Sol-gel processing 3
1.2.1.3 Microemulsion technique 3
1.2.1.4 Solvothermal methods 3
1.2.1.5 Template/Surface derivatized methods 3
1.2.1.6 Thermal Decomposition 3
1.2.2 Gas-solid transformation methods 4
1.2.2.1 Chemical Vapor Deposition (CVD) 4
1.2.2.2 Pulsed Laser Deposition (PLD) 4
1.3 Metal Oxide nanoparticles (NPs) 5
1.3.1 Zinc oxide nanoparticles 5
1.3.1.1 Zinc oxide nanoparticlesApplications 5
1.3.2 Magnesium oxide nanoparticles 6
1.3.2.1 Magnesiumoxide nanoparticles Applications 7
1.4 Stöber method 7
1.5 Coating of Metal Oxides Nanoparticles 8
1.5.1 Coating with organic polymer 8
1.5.2 Coating with inorganic polymer 9
1.6 Advantages silica and meso silica coating material 10
1.7 Applications of functionalized of mesoporous coating metal oxide
nanoparticle 11
1.7.1 Adsorption of toxic heavy metal ions 11
1.7.2 Adsorption of dyes 12
1.8 Literature review 13
1.8.1 Literature review of mesoporous silica coated metal oxide 13
1.8.2 Literature review about functionalization of mesoporous silica coated metal
oxidenanoparticles 14
1.9 Aim of present work 16
vi
Chapter Two
Experimental 2.1 Chemicals and Reagents 19
2.2 Synthesis of Materials 19
2.2.1 Preparation of Metal Oxide nanoparticles 19
2.2.2 Synthesis of silica coated metal oxides nanoparticles MO@SiO2 microsphere 19
2.2.3 Synthesis of meso silica coated metal oxides
microspheresMO@mesoSiO2microsphere 20
2.2.4 Synthesis of silica-meso silica coated metal oxides nano particles
MO@SiO2@meso SiO2 nanoparticles 20
2.2.5 Synthesis of amine-functionalization of MO-NH2 , MO@mSiO2-NH2,
MO@SiO2@mSiO2-NH2 20
2.2.6 Synthesis of thiol-functionalization MO@mSiO2-SH and
MO@SiO2@mSiO2-SH 21
2.2.7 Free metal oxide silica and meso silica microspheres 21
2.2.8 Preparation of Colloidal solution 21
2.3 Adsorption study dyes and metal ions 22
2.3.1 Adsorption of dyes 22
2.3.2 Adsorption kinetics for different heavy metal ions 22
2.4 Methodology 22
2.4.1 Fourier Transform Infra Red (FTIR) 22
2.4.2 Ultraviolet-Visible (UV-Vis) spectroscopy 23
2.4.3 Photoluminescence spectroscopy (PL) 24
2.4.4 Thermal gravimetric Analysis (TGA) 25
2.4.5 X-Ray Diffraction (XRD) 26
2.4.6 Transmission electron microscopy (TEM) 27
2.4.7 Cross polarization/ Magic-angle spinning , Nuclear magnetic resonance
(13C CP/MAS NMR) 28
Chapter Three
Synthesis and Characterization of silica and functionalized silica coated
zinc oxide nanomaterials ( ZnO).
3.1 Introduction 31
3.2 Synthesis 32
3.2.1 Synthesis of ZnO Nanoparticles 32
3.2.2 Synthesis of ZnO@SiO2 microspheres 32
3.2.3 Synthesis of ZnO @meso SiO2 nanoparticles 33
3.2.4 Synthesis of ZnO@SiO2@meso SiO2 nanoparticles 33
3.2.5 Amine functionalization of silica coated zinc oxide nanoparticles 33
3.2.6 Thiol functionalization of silica coated zinc oxide nanoparticles 35
3.2.7 Free ZnO spheres coated silica 35
33 Infrared spectra 38
3.4 Thermal gravimetric analysis (TGA)spectra 39
vii
3.5 X-Ray Diffraction (XRD) 40
3.6 Ultraviolet-Visible (UV-vis) spectroscopy 42
3.7 Energy gap spectra 43
3.8 Photoluminescence spectroscopy (PL) spectra 45
3.9 13
C CP/MAS NMR results 45
3.10 Transmission electron microscopy (TEM) and EDAX 46
3.11 Conclusion 49
ChapterFour
Synthesis and Characterization of silica and functionalized silica coated
magnesium oxide nanomaterials ( MgO).
4.1 Introduction 52
4.2 Synthesis 53
4.2.1 Synthesis of MgO Nanoparticles 53
4.2.2 Synthesis of MgO@SiO2 microspheres 53
4.2.3 Synthesis of MgO @meso SiO2 nanoparticles 54
4.2.4 Synthesis of mesoporous silica coated metal oxides Nano particles
MgO@SiO2@meso SiO2 nanoparticles 54
4.2.5 Amine functionalization of silica coated magnesium oxide nanoparticles 54
4.2.6 Thiol functionalization of silica coated magnesium oxide nanoparticles 54
4.2.7 Free MgO spheres coated silica 54
4.3 Infrared spectra 56
4.4 Thermal gravimetric analysis (TGA)spectra 59
4.5 X-Ray Diffraction (XRD) 61
4.6 Ultraviolet-Visible (UV-VIS) spectroscopy 63
4.7 Energy gap spectra 64
4.8 Photoluminescence spectroscopy (PL) spectra 65
4.9 Transmission electron microscopy (TEM) and EDAX 66
4.10 Application of magnesium oxide and its silica material and its functionalized 70
4.10.1 Adsorption of (BTB )dyes 70
4.10.1.1 Introduction of Bromothymol blue ( BTB ) pH – Indicator 70
4.10.1.2 Adsorption-release of BTB 71
4.10.2 Adsorption kinetics for different heavy metal ions 74
4.11 Conclusion 75
Conclusion 77
References 78
viii
List of Tables
TableNo.
Page
No.
3.1 Experimental data for ZnO 37
3.2 Particles size for ZnO 42
3.3 Energy gap for ZnO 44
4.1 Experimental data for MgO 55
4.2 Particles size for MgO 63
4.3 Energy gap for MgO 65
ix
List of Schemes
Scheme
No.
Page
No.
1.1 Metal Oxide nanoparticles preparation equation 16
2.1 Structure of Praepagen HY 21
3.2.1 Zinc oxide nanoparticles preparation equation 32
3.2.2 ZnO@SiO2 32
3.2.3 ZnO@mSiO2 33
3.2.4 ZnO@SiO2@mSiO2 33
3.2.5 ZnO@SiO2@mSiO2-NH2 34
3.2.6 ZnO@mSiO2-NH2 34
3.2.7 ZnO -NH2 34
3.2.8 ZnO@SiO2@mSiO2-SH 35
3.2.9 ZnO@m-SiO2-SH 35
3.2.10 ZnO free@SiO2 36
3.2.11 ZnO free@m-SiO2 36
3.2.12 ZnO free@SiO2@m-SiO2 36
4.10.1 BTB indicator structure 71
4.10.2 (a) Adsorbtion BTB and (b) released BTB 73
x
List of figures
Fig.No.
Page
No.
3.3.1 FTIR spectra of (a)ZnO pure , (b) ZnO@m-SiO2,(c) ZnO@SiO2@m-SiO2,
(d) ZnO@SiO2@m-SiO2-NH2 39
3.4.1 TG/DTA analysis of ZnO@SiO2@mSiO2 40
3.4.2 TG/DTA analysis of ZnO@SiO2@mSiO2-NH2 40
3.5.1
XRD patterns of:( a) ZnO free@SiO2@mSiO2,( b) ZnO pure, (c)ZnO@SiO2,
(d) ZnO@mSiO2 , (e)ZnO@SiO2@mSiO2, ( f)ZnO@SiO2@mSiO2-NH2 ,(g)
ZnO@mSiO2-SH, (h) ZnO@mSiO2-NH2 ,(i) ZnO free@mSiO2
41
3.6.1 UV-vis absorption spectra of (a)ZnO pure, (b)ZnO@SiO2@m-SiO2, (c) ZnO
@SiO2 43
3.6.2 UV-vis absorption spectra of (a) ZnO@SiO2, (b)ZnO free@SiO2 43
3.7.1 Energy gap spectra of (a)ZnO pure , (b) ZnO@SiO2, (c) ZnO@SiO2@m-
SiO2 44
3.8.1 PL spectra of (a)ZnO pure , (b) ZnO@SiO2 45
3.9.1 CP/MAS
13C NMR spectra of (a) ZnO@SiO2@mSiO2-(CH2)3-NH2 , (b)
ZnO@SiO2@mSiO2-(CH2)3-SH 46
3.10.1 Structural characterization of ZnO nanoparticles : (a) TEM image and (b)
EDAX spectra 48
3.10.2 Structural characterization TEM image of ZnO@mSiO2 :(c) low resolution
TEM (d) high resolution 48
3.10.3 Structural characterization TEM image of: (e) ZnO @SiO2 and (f) ZnO
@SiO2 @m-SiO2 48
3.10.4 Structural characterization of ZnO@mSiO2-(CH2)3-SH nanoparticles : (g)
TEM image and (h) EDAX spectra 49
3.10.5 Structural characterization of ZnO@mSiO2-(CH2)3-NH2nanoparticles : (i)
TEM image and (j) EDAX spectra 49
4.3.1 FTIR spectra of (a)MgO pure , (b) MgO@SiO2, (c) MgO@mSiO2, (d)
MgO@SiO2@m-SiO2, (e) MgO free @SiO2@m-SiO2 56
4.3.2 FTIR spectra of (a) MgO@SiO2@m-SiO2,(a) MgO@SiO2@m-SiO2-NH2,
(b) MgO free @SiO2@m-SiO2-NH2 57
4.3.3 FTIR spectra of (a)MgO@SiO2@mSiO2 with CTAB , (b)
MgO@SiO2@mSiO2 CTAB removed 58
4.3.4 FTIR spectra of (a)MgO@SiO2@mSiO2, (b) MgO free@SiO2@mSiO2 58
4.4.1 TG/DTA analysis of MgO@SiO2@mSiO2 60
xi
4.4.2 TG/DTA analysis of MgO free@SiO2@mSiO2-NH2 60
4.4.3 TG/DTA analysis of MgO free@SiO2@mSiO2 61
4.5.1
XRD patterns of (a) standard MgO pure, (b) MgO pure, (c) MgO@SiO2,
(d) MgO@ m-SiO2, (e) MgOfree@ m-SiO2 , (f) MgO@SiO2 @m-SiO2, (g)
MgO free @SiO2@m-SiO2, (h) MgO@SiO2@m-SiO2-NH2, (i) MgO-NH2
62
4.6.1 UV-Visabsorption spectra of (a)MgO pure,(b) MgO@SiO2, (c)
MgO@SiO2@m-SiO2, (d) MgO free @SiO2@m-SiO2,(e) MgO @m-SiO2 64
4.7.1 Energy gap spectra of (a)MgO pure , (b) MgO@SiO2, (c) MgO@m-SiO2 ,
(d) MgO –NH2 65
4.8.1 PL spectra of (a)MgO pure , (b) MgO@SiO2 66
4.9.1 Structural characterization of MgO nanoparticles : (a) TEM image and (b)
EDAX spectra 67
4.9.2 Structural characterization of MgO @SiO2: (a) TEM image and (b) EDAX
spectra 67
4.9.3 Structural characterization of MgO@mSiO2: (a) TEM image and (b) EDAX
spectra 68
4.9.4 Structural characterization of MgO@SiO2@mSiO2: (a) TEM image and (b)
EDAX spectra 68
4.9.5 Structural characterization of MgO@SiO2@mSiO2-NH2: (a) TEM image and
(b) EDAX spectra 69
4.9.6 Structural characterization of MgO free@SiO2@m-SiO2-NH2: (a) TEM
image and (b) EDAX spectra 69
4.9.7 Structural characterization of MgO free @SiO2@m-SiO2: (a) TEM image
and (b) EDAX spectra 70
4.10.1 UV-Vis absorption spectra of (a) MgO free@SiO2@mSiO2-NH2 +BTB , (b)
BTB dyes 72
4.10.2 UV-Vis absorption spectra of Adsorption-release study of BTB dye in the
MgOfree@SiO2@meso SiO2-NH2 73
4.10.3 Metal uptake capacity of metal ions (Cu2+
) +MgO@SiO2@meso SiO2-NH2 75
4.10.4 Metal uptake capacity of metal ions (Co2+
) +MgO@SiO2@meso SiO2-NH2 75
1
CHAPTER ONE
Introduction
2
1.1 Metal oxide nanoparticle
Metal oxides play a very important role in many areas of chemistry, physics and
materials science. Metal oxide nanoparticles can exhibit unique physical and chemical
properties due to their limited size and a high density of corner or edge surface sites.
Particle size is expected to influence three important groups of basic properties in any
materialare: 1) The structural characteristics, namely the lattice symmetry and cell
parameters [1]. Bulk oxides are usually robust and stable systems with well-defined
crystalographic structures. However, the growing importance of surface free energy and
stress with decreasing particle size must be considered. 2) Changes in thermodynamic
stability associate with size can induce modification of cell parameters and/or structural
transformations [2,3,4] and in extreme cases the nanoparticle can disappear due to. 3)
Interactions with its surrounding environment and a high surface free energy [5]. In
order to display mechanical or structural stability, a nanoparticle must have a low
surface free energy. As a consequence of this requirement, phases that have a low
stability in bulk materials can become very stable in nanostructures.
1.2 Methods of preparations of metal oxide nanoparticle
The first requirement of any novel study of nanoparticle oxides is the synthesis of the
material. The development of systematic studies for the synthesis of oxide nanoparticles
is a current challenge and, essentially, the corresponding preparation methods may be
grouped in two main streams based upon the liquid-solid [6] and gas-solid [7] nature of
the transformations.
1.2.1 Liquid-solid transformations methods
Liquid-solid transformations are possibly the most broadly used in order to control
morphological characteristics with certain “chemical” versatility and usually follow a
“bottom-up” approach. A number of specific methods have been developed, among
which those broadly in use are:
1.2.1.1 Co-precipitation methods
This involves dissolving a salt precursor (chloride, nitrate, etc.) in water (or other
solvent) to precipitate the oxo-hydroxide form with the help of a base. Very often,
control of size and chemical homogeneity in the case of mixed-metal oxides are difficult
3
to achieve. However, the use of surfactants, sonochemical methods, and high-gravity
reactive precipitation appear as novel and viable alternatives to optimize the resulting
solid morphological characteristics [6,8,9].
1.2.1.2 Sol-gel processing
The method prepares metal oxides via hydrolysis of precursors, usually alkoxides in
alcoholic solution, resulting in the corresponding oxo-hydroxide. Condensation of
molecules by giving off water leads to the formation of a network of the metal
hydroxide. Hydroxyl-species undergo polymerization by condensation and form a dense
porous gel. Appropriate drying and calcinations lead to ultrafine porous oxides [10].
1.2.1.3 Microemulsion technique
Micro emulsion or direct/inverse micelles represent an approach based on the formation
of micro/nano-reaction vessels under a ternary mixture containing water, a surfactant
and oil. Metal precursors or in water will proceed precipitation as oxo-hydroxides
within the aqueous droplets, typically leading to mono dispersed materials with size
limited by the surfactant-hydroxide contact [11].
1.2.1.4 Solvothermal methods
In this case, metal complexes are decomposed thermally either by boiling in an inert
atmosphere or using an autoclave with the help of pressure. A suitable surfactant agent
is usually added to the reaction media to control particle size growth and limit
agglomeration.
1.2.1.5 Template/Surface derivatized methods
Template techniques are common to some of the previous mentioned methods and use
two types of tools; soft-templates (surfactants) and hard-templates (porous solids as
carbon or silica). Template- and surface-mediated nanoparticles precursors have been
used to synthesize self-assembly systems [6].
1.2.1.6 Thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat.
The decomposition temperature of a substance is the temperature at which the substance
4
chemically decomposes. The reaction is usually endothermic as heat is required to break
chemical bonds in the compound undergoing decomposition. If decomposition is
sufficiently exothermic, a positive feedback loop is created producing thermal runaway
and possibly an explosion [12]. The Thermal Decomposition Change (TDC) is the
temperature at which a substance begins to undergo a detectable chemical or physical
change of state under a specified set of other environmental conditions (e.g. atmospheric
pressure, chemical environment, mechanical strain, and rate of temperature change)
[13]. The synthesis of nanoparticles (NPs) by thermal decomposition of suitable metal
precursors in liquid phase has received attention as a reliable synthetic route to prepare
metal NPs of controlled size and shape [14-17]. This method potentially offers control
of morphological parameters that are not easily achieved by other methods [18,19]. The
NPs are synthesized by solution-phase reduction of the metal precursor in the presence
of capping ligands and reducing agents.
1.2.2 Gas-solid transformation methods
Gas-solid transformation methods with broad use in the context of ultrafine oxide
powder synthesis are restricted to chemical vapor deposition (CVD) and pulsed laser
deposition (PLD).
1.2.2.1 Chemical Vapor Deposition (CVD)
There are a number of CVD processes used for the formation of nanoparticles among
which we can highlight the classical (thermally activated / pydrolytic), metal organic,
plasma-assisted, and photo CVD methodologies[20]. The advantages of this
methodology consist of producing uniform, and reproduce nanoparticles and films
although requires a careful initial setting pure up of the experimental parameters.
1.2.2.2 Pulsed Laser Deposition (PLD)
Multiple-pulsed laser deposition heats a target sample (4000 K) and leads to
instantaneous evaporation, ionization, and decomposition, with subsequent mixing of
desired atoms. The gaseous entities formed absorb radiation energy from subsequent
pulses and acquire kinetic energy perpendicularly to the target to be deposited in a
substrate generally heated to allow crystalline growth [21].
5
1.3 Metal oxide nanoparticles (NPs)
1.3.1 Zinc oxide nanoparticles
ZnO is a n-type and wide band gap semiconductor with band gap of 3.37 eV and an
exciton (e-h pair) binding energy of 60 meV. These properties make ZnO unique in
many applications as a piezoelectric materials, UV light-emitting diodes, lasers,
photovoltaic solar cells, UV-photodetectors, gas-sensors, and varistors [22-25]. For
many applications of ZnO require high surface area, the synthesis of mesoporous ZnO
is also very important [26,27]. To the best of our knowledge, only one example of
mesoporous ZnO prepared by EISA method was demonstrated by Schüth and
coworkers [28,29]. They have used a special organometallic single-source precursor for
the controllable condensation of ZnO precursor. The mesoporous ZnO has also been
prepared through nanocasting method [30]. Generally, ZnO nanoparticles were
embedded into mesoporous silica to obtain high surface area ZnO without particle
aggregation through solid state grinding method [31], templating chelating ligand [32]
and functionalization of silica walls [33]. However, all these methods do not ensure
high loading of zinc oxide into mesoporous silica.
1.3.1.1 Zinc oxide nanoparticles applications
1- Among the various metal oxides studied for their antibacterial activity, zinc
oxide nanoparticles have been found to be highly toxic. Moreover, their stability
under harsh processing conditions and relatively low toxicity combined with the
potent antimicrobial properties favours their application as antimicrobials [34].
2- Zinc oxide, with its unique physical and chemical properties, such as high
chemical stability, high electrochemical coupling coefficient, broad range of
radiation absorption and high photostability, is a multifunctional material
[35,36].
3- ZnO has a wide range of applications in optoelectronic devices [37] such as
light-emitting diodes, photodetectors, and p-n homojunctions.
4- ZnO has properties which accelerate wound healing, and so it is used in
dermatological substances against inflammation and itching. In higher
concentrations it has a peeling effect.
6
5- ZnO is used in suppositories. In addition it is used in dentistry, chiefly as a
component of dental pastes, and also for temporary fillings.
6- ZnO is used in various types of nutritional products and diet supplements, where
it serves to provide essential dietary zinc [38].
1.3.2 Magnesium oxide nanoparticles
Magnesium oxide continues to receive attention due to its interesting properties in bulk
as well as in nano scale and wide ranging applications in microelectronics,
heterogeneous catalysis, plasma display panels, etc. [39-42]. Its optical properties with
oxygen vacancies retaining one or two electron(s), known as F+ and F centres,
respectively, in bulk and on the surface [43-45]. An F-center, Farbe center or color
center (from the original German Farbzentrum; Farbe means color, and zentrum center)
is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by
one or more electrons. Electrons in such a vacancy tend to absorb light in the visible
spectrum such that a material that is usually transparent becomes colored. This is used
to identify many compounds, especially zinc oxide (yellow). Color centers can occur
naturally in compounds (particularly metallic oxides) because when heated to high
temperature the ions become excited and are displaced from their normal
crystallographic positions, leaving behind some electrons in the vacated spaces. F-
centers are often paramagnetic and can then be studied by electron paramagnetic
resonance techniques. The greater the number of F-centers, the more intense is the color
of the compound. A way of producing F-centers is to heat a crystal in the presence of an
atmosphere of the metal that constitutes the material, e.g., NaCl heated in a metallic Na
atmosphere [46].
Accordingly, strong photo-absorption peaks observed at 4.96 and 5.03 eV have been
assigned to the gap states generated by bulk F and F+ centres, respectively; the
absorption by surface F and F+ centres however occurs at relatively lower energies
[47,48]. The nature of various F centres in MgO depends on the synthesis process,
treatment conditions, etc. For example, neutron or ion irradiation generates F+ centres,
thermo-chemical reduction leads to neutral F centres, and electron-irradiation gives rise
to both the F and F+ centres [49-55]. If the concentration of F (or F
+) centres becomes
high, aggregates/dimmers, i.e., FF, FF+, and F
+F
+ type centres (termed as F2, F2
+ and
F22+
, centres, respectively) may possibly be formed as well. Their presence causes
7
reduction in the energy of absorption drastically ,e.g. , F2 dimers exhibit absorptions at
3.5 and 1.5 eV whereas F22+
centres show absorption at 3.8 eV [57,50,56-59]. The
optical properties of MgO have also been investigated by emission resulting due to
excitation with UV–vis radiation. These reveal huge Stokes shift and energy levels
corresponding to F and F+ centres lie below the conduction band by 2.55–3.03 and
3.43– 3.46 eV, respectively [43,44].
1.3.2.1 Magnesium oxide nanoparticles applications
1- MgO nanoparticles are found to possess many properties that are desirable for a
potent disinfectant [61]. Because of their high surface area and enhanced
surface reactivity, the nanocrystals adsorb and carry a high load of active
halogens. Their extremely small size allows many particles to cover the bacteria
cells to a high extent and bring halogen in an active form in high concentration
in proximity to the cell [60]. Standard bacteriological tests have shown excellent
activity against E.coli and Bacillus megaterium and a good activity against
spores of Bacillus subtilis [61].
2- Fire retardant used for chemical fiber and plastics trades.
3- High-temperature dehydrating agent used for the production of silicon steel
sheet, high-grade ceramic material, electronic industry material, adhesive and
additive in the chemical raw material.
4- High-frequency magnetic-rod antenna, magnetic device filler, insulating
material filler and various carriers used in radio industry.
5- Refractory fiber and refractory material, magnesite-chrome brick, filler for
refractory coating, refractory and insulating instrument, electricity, cable, optical
material, material for steel-smelting furnace and other high- temperature
furnaces, heating material and ceramic base plate.
6- Electric insulating material for making crucible, smelter, insulated conduit
(tubular component), electrode bar, electrode sheet.
7- Fuel additive, cleaner, antistatic agent and corrosion inhibitor [62].
1.4 Stöber method
The Stöber process is a physical chemistry process for the generation of mono disperse
particles of silica. The process was discovered in 1968 by Werner Stöber et al. building
8
on earlier work by G. Kolbe in published in 1956. The topic has since been widely
researched. Tetraethylorthosilicate is added to an excess of water containing a low
molar- mass alcohol such as ethanol and containing ammonia. The resulting solution is
then stirred. The resulting silica particles have diameters between 50 and 2000
nanometers depending on type of silicate ester used, type of alcohol used and volume
ratios. A particle size up to 1000 micrometres has been reported in a modified emulsion
technique. The reactions taking place are hydrolysis of the silyl ether to a silanol
followed by condensation reactions. The particles have been analysed by light
scattering. The process is believed to take place via a LaMer model (monomer addition)
in which nucleation is a fast process, followed by a particle growth process without
further nucleation. In an alternative model called controlled aggregation, the particles
grow by aggregation of smaller particles. This model is supported by microgravity
experiments and by SAXS analysis. Kinetics have been investigated with variation in
pH [63].
1.5 Coating of Metal Oxides Nanoparticles
1.5.1 Coating with organic polymer
Since the metal-organic core-shell nanoparticles have surfaces with amount of
functional groups, they are easily modified for bio conjugation purposes [64-66]. As an
example, the distinctive optical properties including localized surface plasmon
resonance of the nanoscale noble metals like Au and Ag lead to a considerable attention
on them. For metal or metalloid oxide-organic core-shell nanoparticles , the formation is
easy by a chemical reaction. But it is difficult to attain the particles with smaller size in
a dispersing media, because they highly tend to grow and easily in agglomeration.
Coating the particles with a simple shell of polymer materials is an excellent method to
solve that problem because the shell can be removed easily [67].
Metal oxides, especially transition metal oxides coating on the organic materials can
help them have excellent applications in a broad range of fields such as material
additives [68-70], controlled release [71], catalysis [72], optics [73-75], and so on. It
is a liquid-like assembly owing to that there is no interaction between the chains. There
are some common polymers always used as the organic shell materials for silica cores:
polystyrene (PS) [76], poly(methylmethacrylate) (PMMA) [77-79], poly(vinyl chloride)
(PVC) [80], and poly(3-aminophenylboronic acid) [81].
9
Although most studies have focused on the development of small organic molecules and
surfactants coating up to now, owing to the advantages of polymers coating will
increase repulsive forces to balance the magnetic and the van der waals attractive forces
acting on the NPs. In addition, polymers coating on NPs offer a high potential in the
application of several fields. Moreover, polymer functionalized NPs have been
extensively investigated due to interest in their unique physical or chemical properties.
To make use of these materials for fundamental or applied research, access to well
defined NPs samples whose properties can be „„tuned‟‟ through chemical modification
is necessary. In a number of cases it has now been shown that, through careful choice of
the passivating and activating polymers and/or reaction conditions, can produce NPs
with tailored and desired properties. Polymer coating materials can be classified into
synthetic and natural. The saturation magnetization value of NPs will decrease after
polymers functionalization. Currently, there are two major developing directions to
form polymers functionalized NPs [82].
1.5.2 Coating with inorganic polymer
Silica-polymer core-shell nanoparticles improve colloidal stability. They are important with
a wide range of applications in electrical devices, material additives, controlled release,
catalysis, sensors, and optical devices [83]. Silica based materials are the most ancient
and widely used inorganic materials known to man. Likewise, silica (SiO2) is a very
common metal oxide material in daily life. The chemical inertness and rigid structure of
this material has promoted its extensive usage over a very long period of time and has
led to the development of various applications, which include corrosion protection and
chromatographic applications [84-86]. The porosity of the silica based materials also
allows wide applications such as immobilization and catalysis [87-89] . In the last
decades sol–gel : chemistry has played a major role in preparation of silica based hybrid
materials by bridging organic and inorganic in coupling [90]. The attractive
characteristics of these materials with electrochemical science is now in progress
[90,91]. Preparation of sol-gel matrices doped with some chemically and biologically
active molecules is a promise route to chemical solid-state sensors [90-92]. Sol-gel
matrices appear as a very important technique for immobilization, entrapment,
encapsulation for large variety of materials such as organic, inorganic, and bimolecules
[93,94].
11
1.6 Advantages silica and meso silica coating material
1- Its reaction can be controlled easily by chemical methods.
2- It allows to introduce a permanent organic groups to form inorganic – organic
hybrid materials [95].
3- The process takes place at low temperature [96].
4- High porous material prepared.
5- The matrix is chemically inert and low poisoning [97].
6- Mesoporous silicates synthesized by using a surfactant template method are
really stable, and they have narrow pore sizes. They can be used in different
fields, such separation techniques, adsorption, catalysis, drug delivery, sensors,
or photonics for instance [98].
7- Silica becomes a promising candidate for the coating due to excellent charge and
heat insulation properties [99].
8- These surface coatings allow manipulation of the interaction potential and make
it possible to disperse colloids in a wide range of solvents from very polar to
apolar.
9- Silica is chemically inert and optically transparent (so that chemical reactions
can be monitored spectroscopically) and does not affect redox reactions at the
core surface, except by physical blocking of the surface [100,101].
10- Silica coated nanoparticles are easy to centrifuge during preparation,
functionalization, and other treatment processes in solution because of higher
density of silica [102].
11- Silica is relatively chemically inert meaning that it will not modify or interfere
with surface adsorbents [103].
12- Silica is an inert and non-toxic material with high potential for functionalization
with different ligands assigned to a wide range of biomedical applications [104].
These properties make silica a convenient material for Metal oxide shielding and
functionalization.
13- Silica coating material can also act as protection of the reactive metal is an inert
material and oxide material.
11
1.7 Applications of functionalized of mesoporous coating metal oxide
nanoparticles
1.7.1 Adsorption of toxic heavy metal ions
Heavy metals can be defined as any metals or metalloids have density more than 4
g/cm3, or 5 times or more greater than water [105,106], heavy metals could enter the
body system through food, air, water and bio- accumulate over a period of time [105].
They are introduced and spread into the environment through a number of industrial
processes [107]. Some of heavy metals cause toxicity even if the concentrations are very
low and this is the main reason behind the price in the interest all over the world [108],
whereas others are biologically essential and become toxic at relatively high
concentrations, they combine with body's biomolecules, to form stable biotoxic
compounds, thus distortion their structures and hindering them from the bioreactions of
their functions [109]. Heavy metals combine with proteins and formed toxic complexes
and they can inactivate important enzyme systems [106].
Heavy metal pollution in water has attracted much attention due to its harmful influence
on human life [110,111]. Therefore, the removal of heavy metal ions in wastewater is
becoming increasingly important. Until now, various kinds of physical and chemical
methods such as ion exchange [112], adsorption [113,114], chemical precipitation
[115], reverse osmosis [116] and membrane process [117], etc. have been employed for
separation of heavy metal ions from waste water. Among the available methods,
adsorption technology is the most promising and frequently used technique due to its
simplicity, high efficiency, and low cost. Since the discovery of M41S silica in 1992,
mesoporous materials, due to their high surface areas, well-defined pore size, and
tunable pore sizes, have been widely used in the field of adsorption of heavy metal ions
[118,119]. In addition, magnetic particles can be easily removed from the reaction
system by an external magnetic field. If the mesoporous structure and magnetic
properties can be combined together, prepared nano-complexes with high specific
surface area and the ability of magnetic recovery will be a major leap forward for
practical application. So far, a number of articles have reported the synthesis of
magnetic mesoporous silica microspheres [120-122]. For the adsorption of heavy metal
ions like Hg2+
, Pb2+
, Cu2+
, Cd2+
, and so on, unmodified mesoporous silica materials
have alittle adsorption capacity [118-119]. At present, there are several kinds of
12
modified methods. One is thiol functionalized, which exhibited a high complexation
affinity for Hg2+
[121,123,124], however, for Cu2+
, Ni2+
, Zn2+
, and Cd2+
.
The selectivity of the immobilized surface towards metal ions depends on various
factors such as size of the modifier, activity of the loaded group and characteristic of the
hard-soft acid-base. Due to the presence of many reactive sites on silica gel large
number of organic molecule could be immobilized on its surface to improve its sorption
behavior. The synthetic pathway of modified silica gel was similar to the reaction of
organosilane with silanol groups on silica gel, while the types of chelating molecules
which selectively chelate to metal ions in different way [125].
1.7.2 Adsorption of dyes
Dyes are widely used as one of the key ingredients in many industries like textile, paint
and varnishes, ink, plastics, pulp and paper, cosmetics, tannery etc., and also to the
industries that produces dyes. These industries discharged the dye in the environment
with their wastewater. The main environmental concern with dyes is their absorption
and reflection of sunlight entering the water and thus causing reduction in
photosynthesis and dissolved oxygen level in river In addition, some dyes degrade into
compounds that have toxic, mutagenic and carcinogenic effect on living organism. Azo
dyes can be particularly toxic upon degradation and this class of dyes is widely used in
many industries Among various azo dyes, methyl orange (MO) serves as a model
compound for common water- soluble azo dyes, which are widely used in chemical,
textile and paper industries and it is harmful to the environment. Among various
treatment methods available for dye removal, the adsorption has been proved to be the
most suitable and promising technologies or has become the most popular technique
because of its effectiveness, operational simplicity, low cost and low energy
requirement. Recently, microporous inorganic adsorbents (e.g., zeolites) and
mesoporous silica (e.g., MCM-41 and SBA-15) with unique surface and pore properties
as well as high surface areas have been extensively investigated as alternatives to
carbon adsorbents for the liquid adsorption of dissolved pollutants in water [126]. these
classes of meso structured materials possess some short comings and need to be
extended to wider classes of meso structured silica materials. In response to this, meso
structured silica nanoparticles (MSN) have become increasingly important because of
their high surface area (>1000 m2 g
-1), thermal and mechanical stability, highly uniform
pore distribution, tunable pore size, and unique hosting properties [127].
13
1.8 Literature review
1.8.1 Literature review of mesoporous silica coated metal oxide
Normally, to protect magnetic core and retain magnetic properties , the core is coated
with some non-magnetic relatively inert shell such as silica. The silica shell is very easy
to be functionalized and good for binding of various catalytic species including
transition metal complexes [128,129]. Esmaeilpour et al. coated magnetite nanoparticles
with a layer of silica and loaded a copper salen complex inside the shell, which
exhibited catalysis of substituted 1- and 5- tetrazoles in high yield [130]. In addition the
same researchers [131] utilized silica coated iron oxide . Yeung et al. produced
mesoporous silica coated magnetic nanoparticles by a flow-through synthesis. The
researchers showed that with the addition of propyl amine and propyl di ethylene amine
these nanoparticles may be used as a catalyst in knoevenagel condensation reactions
[132]. Abdolalian et al. reported the development of a magnetic iron oxide nanoparticle
with a mesoporous silica shell to which a molybdenum (IV) catalyst had been bound.
This magnetic mesoporous catalytic system catalyses the epoxidation of olefins in the
presence of hydrogen peroxide [133]. Aleksandr Marinin produced super para magnetic
iron oxide nanoparticles (SPIONs) were synthesized by two chemical methods – co-
precipitation and thermal decomposition of organic iron precursor. The next step was
coating SPIONs with silica shell. For this purpose inverse microemulsion method was
chosen. TEOS was used as a silica precursor. Mean size, size distribution, magnetic
properties, structure of silica shell were studied [134]. Mingwei Zhang et al. prepared
well-defined iron oxide/mesoporous silica core−shell nanostructure. Iron oxide
nanocubes with a narrow size distribution were synthesized through a novel
ethanol/acetic acid system using Fe(NO3)·9H2O as an iron source and
polyvinylpyrrolidone as a capping agent under mild solvothermal conditions (200 °C).
These mono disperse nanoparticles were used directly as the core for the deposition of
amesoporous silica shell via a sol−gel process, resulting in uniform core−shell
Fe2O3@SiO2 composites with tailored silica shell thickness and controllable core
morphology. In addition, composition as well as magnetization of the reduced
core−shell composites could easily be controlled by a reduction process. Furthermore,
the iron oxide core in Fe2O3@SiO2 could be completely etched to produce hollow SiO2
nano spheres [135]. Rafique Ullah et al. have synthesized silica coated iron-oxide
composites of different ratios (iron oxide / SiO2 = 3:7, 1:1 and 7:3). Iron oxyhydroxide,
14
as the precursor of iron oxide, has been prepared by electrochemical method. The iron
oxide and tetraethylorthosilicate (TEOS), as precursor of silica, have been used at
different ratios to synthesize silica coated iron-oxide composite of different ratio. The
results reveal that well dispersed silica coated iron-oxide composites of different ratios
were formed. The prepared silica coated iron-oxide composite material can be used for
treatment of wastewater to remove heavy metal [136]. Teeraporn Suteewong et al. have
synthesized iron oxide nanoparticles and transferred to an aqueous phase using the
cationic surfactant, hexa decyl trimethyl ammonium bromide (CTAB). nanoparticles are
fabricated via sol–gel synthesis. Aliquots are taken from the solution during synthesis to
capture the particle formation process [137]. P.B. Lihitkar et al. have synthesized
mesoporous silica (MS) and zinc loaded MS composites have been and characterized
using high resolution transmission electron microscopy, X-ray diffraction, UV–visible
spectroscopy, photoluminescence spectroscopy, N2 adsorption–desorption isotherms, X-
ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Thermal
treatment of the zinc loaded MS composite lead to the formation of ZnO–MS
composite. The well ordered uniform pore structure of MS (pore size∼3.4 nm) is found
to remain stable even after 30% Zn loading albeit decrease in the pore size 1.2 nm
indicates the formation of ZnO inside the pores [138]. Edwin Escalera Mejia
synthesized and characterization of functionalized ordered mesoporous silica were
performed . Mesoporous silica with a large surface area on which organic functional
groups are grafted. The synthesis of mesoporous silica involves three steps. The first
step is the formation of the mesoporous structure using surfactants and silica precursors.
The second step is the hydrothermal treatment at moderate temperatures. The final step
is removal of surfactants from the mesoporous silica in which various techniques can be
applied [139].
1.8.2 Literature review about functionalization of mesoporous silica coated metal
oxide nanoparticles
Qing Yuan et al. have synthesized multifunctional microsphere with a large pore size
mesoporous silica shell (ca. 10.3 nm) and a magnetic core (Fe3O4) has been successfully
synthesized via a facile two-step sol– gel method. In the synthesis process, CTAB was
first dissolved in water to form spherical micelles, then added to the mixture in which
Fe3O4@SiO2 was dispersed in ethanol, next dropping TEOS to form
Fe3O4@SiO2@CTAB/SiO2 composites. This approach helps to form mesoporous silica
15
shell with a large pore size, which is propitious to the modification of much more amino
groups in order to enhance the adsorption capacity of heavy metal ions. The metal-
loaded multifunctional microspheres can be easily removed from aqueous solution by
magnetic separation and regenerated easily by acid treatment [140]. Srisuda Sae-ung
and Virote Boonamnuayvitaya have synthesized amine-functionalized mesoporous
adsorbents [141]. Amine (-NH2) groups from different amine precursors were chosen
for introducing onto pore surface because they can be used as a linker between the silica
surface and any organic species via a nucleophilic substitution reaction [142]. Othman
Hakami et al. used thiol-functionalised silica-coated magnetite nano-particles (TF-
SCMNPs) prepared by co-condensation and characterised using a variety of phyico-
chemical techniques. The composite particles were then used in an adsorption process
for Hg(II) removal, and the langmuir and freundlich isotherm models used to process
the adsorption isotherm data [143].
16
1.9 Aim of present work
Since most known silica coated metal oxides nanoparticles are those of iron oxide, and
only few studies about silica or mesosilica coated MgO or ZnO have been reported.
Our main aim is to prepare silica, meso silica or both silica-meso silica coated metal
oxides (MgO and ZnO) and their amine and thiol functionalized ligand systems . The
coating with silica shell was aim to protect the high surface activity of metal oxides
nanoparticles and to be establish an inert silica or mesosilica shells, which can be
immobilized with functionalized ligands. Our results are very encouraging and visible
and were outline below. These silica coated mesoporous metal oxides and their
functionalized ligand systems were used for removal heavy toxic metals as well as
removal of dyes ions from drinking and waste water. Therefore removal of dyes and
toxic heavy metal ions from water is very important issues because water quality is
greatly affected by these two pollutants on both national and international levels. Sol-
Gel chemistry offers a possible solution for these problems by providing very promising
synthetic routes to prepare such mesoporous functionalized materials.
Our outline of present work
Synthesis :
1- Synthesis of metal oxide nanoparticles (magnesium oxide and zinc oxide) using
decomposition oxalate route (Scheme 1.1).
MSO4 + H2C2O4 MC2O4 calcinations 500oC Metal oxide
Scheme( 1.1) Metal oxide nanoparticles preparation equation M=(Zn , Mg)
2- Silica or mesosilica coating of metal oxide (MO@SiO2, MO@mSiO2,
MO@SiO2@mSiO2) by sol-gel modified Stöber method (M=Zn , Mg).
3- Functionalization of meso silica coated metal oxide nanoparticles using 3-
(trimethoxysilyl)-propylamine and (3-mercaptopropyl)tri-methoxysilane.
(MO@SiO2@mSiO2-NH2, MO-NH2, MO@SiO2@mSiO2-SH, MO@mSiO2-SH)
(M=Zn , Mg).
4- Formation of free metal oxide silica spheres (MO free@m-SiO2, MO free@SiO2, MO
free @SiO2@mSiO2-NH2, MO free@SiO2@mSiO2) (M=Zn , Mg).
Structure characterization :
17
1- Several methods were used for structural characterization of materials using FTIR
,13
C NMR ,TGA.
2- Particle size and optical properties and structure morphology were examined by
XRD, TEM , PL and UV-VIS spectra.
Applications :
1- Adsorption of BTB dye .
2- Extraction and removal of heavy metal ions ( Cu+2
and Co+2
) .
18
CHAPTER TWO
Experimental
19
2.1 Chemicals and reagents
All the chemicals used were analytical grade and directly used as received without
further purification. Zinc sulfate hepta hydrate, magnesium sulfate pentahydrate, oxalic
acid, absolute ethanol and 3-(trimethoxysilyl)-propylamine were purchased from Merck
Company and used as received. Tetraethylorthosilicate (TEOS), cetyl trimethyl
ammonium bromide (CTAB), toluene and bromothymol blue dye (BTB) were
purchased from Aldrich Company and used without further purification. (3-
Mercaptopropyl)tri-methoxy silane, these reagent were purchased from Alfa Aesar
Company and used without further purification. Ammonium hydroxide from Frutarom
Company. Copper chloride and cobalt chloride were purchased from from Oxford
Company. All the glasswares used in this experimental work were washed with acid and
dried at 100oC.
2.2 Synthesis of materials
2.2.1 Preparation of metal oxide nanoparticles
Metal oxides nanoparticles were prepared as previously reported [144], when (20 mmol) of
metal sulfate of zinc sulfate hepta hydrateand magnesium sulfate penta hydrate was dissolved
into 25 mL of deionized water. (20 mmol) of oxalic acid was dissolved in an equal volume of
deionized water and dropwise to metal sulfate solution under magnetic stirring for 60 min.
The mixture was then stirred for further 30minute, the mixture was settled down and
decantation 3-times. The precipitate of metal oxalate was isolated, washed with water several
times and dried at 100oC for 24 hours. The dried material was grounded using mortar and
pestle to produce fine powder precursor. Subsequently, the precursor, metal oxalate was
annealed in muffle furnace under air at 500oC for 2 h to produce ZnO and MgO
nanomaterials.
2.2.2 Synthesis of silica coated metal oxides nanoparticles MO@SiO2 microsphere
The coated silica metal oxide microsphere labeled as ZnO@SiO2 or MgO@SiO2 were
prepared in similar reported Stöber method [129] through a simple sol–gel process.
Briefly, 0.10 g as-prepared metal oxide nanoparticles ZnO or MgO were dispersed in a
mixture of ethanol (40 mL), deionized water (10 mL), and 1.2 mL concentrated
ammonia solution (28 wt %) by ultrasonication for 1 h. Tetraethylorthosilicate (TEOS)
21
(0.4 mL) was added dropwise to stirred mixture, after 6 h the product was isolated and
dried in vacuum at 80°C for 12 h over phophorus (V) oxide at 60 oCfor 8 h.
2.2.3 Synthesis of meso silica coated metal oxides microspheres MO@mesoSiO2
microsphere
Meso silica coated metal oxides microsphere labeled as ZnO@mSiO2, MgO@mSiO2.
When metal oxide (0.1g) was added to stirred ethanolic solution containing 0.3 gram
CTAB, followed by adding 1.2 M concentrated ammonia solution (28 wt%). The
mixture was sonicated for half hour at 40 o
C before 0.43ml of TEOS was added
dropwise to stirred mixture, the product was isolated after six hours by centerfegation
washed by ethanol and dried in vacuum at 80 oC for 12 h over phophorus (V) oxide. The
product was calcinated at 500oC for 3h.
2.2.4 Synthesis of silica-meso silica coated metal oxides nano particles
MO@SiO2@meso SiO2 nanoparticles
The silica-meso silica coated metal oxides microspheres labeled as ZnO@SiO2@meso-
SiO2 or MgO@SiO2@meso-SiO2 microspheres were prepared in a similar method
described above when 0.1g of ZnO@SiO2 or MgO@SiO2, was added to an ethanolic
solution containing 0.3 gram CTAB, followed by adding 1.2 M concentrated ammonia
solution (28 wt%). The mixture was sonicated for half hour before adding 0.43ml
TEOS to the mixture. The product was isolated, and dried in vacuum at 80 o
C for 12 h
over phophorus (V) oxide. The product was calcinated at 500 oC for 3h.
2.2.5 Synthesis of amine-functionalization of MO-NH2 , MO@mSiO2-NH2,
MO@SiO2@mSiO2-NH2
Amine-functionalized of pure metal oxide, meso silica or silica-meso silica coated metal
oxides were prepared as previously described [145] by disperse 0.10 g of pure MO
(ZnO, MgO) or silica coated MO (ZnO@mSiO2 , MgO@mSiO2) materials or
(ZnO@SiO2@mSiO2 , MgO@SiO2@mSiO2) in 30 ml dry toluene, followed by adding
0.37g ,0.002 mol of 3-aminepropyltrimethoxy silane coupling agent. The mixture was
refluxed for 24 h at 110oC. The material was filtered off washed with ethanol and dried
in vacuum at 80oC.
21
2.2.6 Synthesis of thiol-functionalization MO@mSiO2-SH and MO@SiO2@mSiO2-
SH
Thiol-functionalized of meso silica or silica- meso silica coated metal oxides were
prepared as previously described [145] by disperse 0.10 g of silica coated MO
(ZnO@mSiO2 , MgO@mSiO2) materials or (ZnO@SiO2@mSiO2,MgO@SiO2@mSiO2)
in 30 ml dry toluene, followed by adding 0.37g, 0.0019 mol of 3-thiolpropyltrimethoxy
silane coupling agent. The mixture was refluxed for 24 h at 110oC. The material was
filtered off washed with ethanol and dried in vacuum at 80oC.
2.2.7 Free metal oxide silica and meso silica microspheres
Free metal oxide silica, meso silica microspheres were obtained by treating 0.5 gram
silica coated metal oxides materials (ZnO@SiO2, ZnO@mSiO2, ZnO@SiO2@mSiO2
,MgO@SiO2@mSiO2 , MgO@SiO2@mSiO2-NH2, MgO@mSiO2,) microsphere with 20
ml 6M concentrated hydrochloric acid (HCl) with continuous stirring. The free metal
oxide were separated and dried in vacuum at 60oC for 8 hours. Washed with distilled
water and dried in vacuum at 80oC.
2.2.8 Preparation of colloidal solution
For optical absorption and photoluminescence measurements, 0.0015 g of sample was
dissolved in 3 mL of concentration 20% HY solution (20 mL HY/100 mL H2O).
Optical absorption measurements was taken after one day. The surfactant selected in
this study Praepagen HY classified as a typical cationic surfactant whose
hydrophilic characteristics are improved by the presence of a hydroxyl group in its
structure as seen in (scheme2.1) The HY can affect local aggregation which in turn
can substantially enhance the stablity, the optical and photoluminescence properties.
Scheme(2.1) Structure of Praepagen HY
22
2.3 Adsorption study dyes and metal ions
2.3.1 Adsorption of dyes
Batch method was used by shaken 0.10g of the sampl (MgO free@SiO2 @meso SiO2-
NH2) with 3 ml of BTB solution (5 × 10-5
M). The removal of BTB was examined by
UV-Vis spectra .
2.3.2 Adsorption kinetics for different heavy metal ions
Batch method was used by shaken 0.10g of solid sample (MgO@ SiO2 @ meso SiO2-
NH2) with 30 ml metal ion in phosphate buffer solution (Cu+2
, Co+2
) of concentration
for 24 hour. The uptake metal ions was determined by atomic absorption in triplicate .
2.4 Methodology
The following technique were used for structure characterization of free metal oxides
(pure), silica coated metal oxides and amine or thiol functionalized silica, meso silica
materials .
2.4.1 Fourier Transform Infra Red (FTIR)
Fourier Transform Infrared (FTIR) spectroscopy is a powerful tool for identifying types of
chemical bonds in a molecule by producing an infrared absorption spectrum that is like a
molecular fingerprint. Infrared (IR) spectroscopy is one of the most common spectroscopic
techniques used by organic and inorganic chemists. Simply, it is the absorption measurement
of different IR frequencies by a sample positioned in the path of an IR beam. The main goal
of IR spectroscopic analysis is to determine the chemical functional groups in the sample.
Different functional groups absorb characteristic frequencies of IR radiation. Using various
sampling accessories, IR spectrometers can accept a wide range of sample types such as
gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for
structural elucidation and compound identification [146].
FTIR spectra were recorded using a fourier transform infrared spectrophotometer (Frontier
Perkin Elmer); The samples are measured on a zinc selenide crystal, it is working as a
multiple reflection ATR system (Attenuated Total Reflection).
23
2.4.2 Ultraviolet-Visible (UV-vis) spectroscopy
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV-vis or UV/vis)
refers to absorption spectroscopy in the ultraviolet-visible spectral region. This means it uses
light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption in
the visible range directly affects the perceived color of the chemicals involved. In this region
of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is
complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from
the excited state to the ground state, while absorption measures transitions from the ground
state to the excited state, UV-vis spectroscopy is routinely used in the quantitative
determination of solutions of transition metal ions highly conjugated organic compounds,
and biological macromolecules [147].
Ultraviolet–visible absorption spectra were recorded on a UV-vis spectrophotometer
Shimadzu, UV-2400 in the wavelength range from 200 to 800 nm.
For a direct band gap material the following equation was used to determine the bandgap at
zero absorbance.
(αhv)2
= hv-Eg
α= absorbance coefficient (μm ) .
24
(αhv)2 = absorbance coefficient (μm eV).
E g = E = Band gap energy (eV) .
hv = photon energy = 1.24 μm-eV.
2.4.3 Photoluminescence spectroscopy (PL)
Photoluminescence (PL) is the spontaneous emission of light from a material under optical
excitation. The excitation energy and intensity are chosen to probe different regions and
excitation concentrations in the sample. PL investigations can be used to characterize a
variety of material parameters. PL spectroscopy provides electrical (as opposed to
mechanical) characterization, and it is a selective and extremely sensitive probe of discrete
electronic states. Features of the emission spectrum can be used to identify surface, interface,
and impurity levels and to gauge alloy disorder and interface roughness. The intensity of the
PL signal provides information on the quality of surfaces and interfaces. Under pulsed
excitation [148].
PL spectra were recorded on a spectrofluorometer JASCO, FP- 6500 , the extinction
wavelength was selected to be 310 nm. The scan rate was set at 600 nm/min with the
entrance and exit slit width of 5 nm.
25
2.4.4 Thermal gravimetric Analysis (TGA)
Thermogravimetric analysis or thermal gravimetric analysis (TGA) is a method of
thermal analysis in which changes in physical and chemical properties of materials are
measured as a function of increasing temperature (with constant heating rate), or as a
function of time (with constant temperature and/or constant mass loss). TGA can
provide information about physical phenomena, such as second-order phase transitions,
including vaporization, sublimation, absorption, adsorption, and desorption. Likewise,
TGA can provide information about chemical phenomena including chemisorptions,
desolvation (especially dehydration), decomposition, and solid-gas reactions (e.g.,
oxidation or reduction). TGA is commonly used to determine selected characteristics of
materials that exhibit either mass loss or gain due to decomposition, oxidation, or loss
of volatiles (such as moisture). Common applications of TGA are (1) materials
characterization through analysis of characteristic decomposition patterns, (2) studies of
degradation mechanisms and reaction kinetics, (3) determination of organic content in a
sample, and (4) determination of inorganic (e.g. ash) content in a sample, which may be
useful for corroborating predicted material structures or simply used as a chemical
analysis. It is an especially useful technique for the study of polymeric materials,
including thermoplastics, thermosets, elastomers, composites, plastic films, fibers,
coatings and paints [149].
26
Thermal gravimetric Analysis (TGA) was carried out using Melter Toledo SW 7.01 analyzer
of 25-600 oC under nitrogen with rate 10
oC/1 minute.
2.4.5 X-Ray Diffraction (XRD)
The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic
level. X-ray diffraction has been in use in two main areas, for the fingerprint characterization
of crystalline materials and the determination of their structure. Each crystalline solid has its
unique characteristic X-ray powder pattern which may be used as a "fingerprint" for its
identification. Once the material has been identified, X-ray crystallography may be used to
determine its structure, i.e. how the atoms pack together in the crystalline state and what the
inter atomic distance and angle are etc. X-ray diffraction is one of the most important
characterization tools used in solid state chemistry and materials science. We can determine
the size and the shape of the unit cell for any compound most easily using X-ray diffraction
[150].
The X-ray diffraction (XRD) patterns of the dried as-prepared and classified samples were
obtained using an X-ray diffractometer PANalytical X 0 pert (PAN alytical) with Cu Ka
radiation (0.154 nm wavelength) under 40 kV and 200 mA. The Scherrer Equation was used
27
to determine bulk crystallite diameter of a sample. The following is a general form of the
Scherrer Equation ( d = Kλ / Bcos Ө ):
d = crystallite diameter (nm).
K = crystallite shape constant (using 0.9 for spherical shape)
λ = x-ray wavelength (0.154 nm)
B = Full-width at half max at Bragg angle of interest
Ө = Bragg Angle (angle of interest)
2.4.6 Transmission electron microscopy (TEM)
TEM is a powerful and unique technique for structure characterization. The most important
application of TEM is the atomic-resolution real-space imaging of nanoparticles. TEM is
unique in identifying and quantifying the chemical and electronic structure of individual
nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and
mapping the valence states are even more attractive. In situ TEM is demonstrated for
characterizing and measuring the thermodynamic, electric, and mechanical properties of
individual nanostructures, from which the structure-property relationship can be registered
with a specific nanoparticle/structure [151].
The TEM analysis was done with JEM2010 (JEOL) transmission electron microscope with
energy dispersive X-Ray Spectrometer INCA (Oxford Instruments).
28
2.4.7 Cross polarization/ Magic-angle spinning , Nuclear magnetic resonance ( 13
C
CP/MAS NMR)
Solid-state NMR spectroscopy serves as an analysis tool in organic and inorganic chemistry.
SSNMR is also a valuable tool to study local dynamics, kinetics, and thermodynamics of a
variety of systems. Objects of SSNMR studies in materials science are inorganic/organic
aggregates in crystalline and amorphous states, composite materials, heterogeneous systems
including liquid or gas components, suspensions, and molecular aggregates with dimensions
on the nanoscale, where different nuclei can be used as NMR probes. In many cases, NMR is
the uniquely applicable method for measurement of porosity, particularly for porous systems
containing partially filled pores or for dual-phase systems [152].
29
31
CHAPTER THREE
Results and Discussion
Synthesis and Characterization of silica and
functionalized silica coated zinc oxide
nanomaterials ( ZnO).
31
3.1 Introduction
Zinc oxide (ZnO) is an inorganic multifunctional material with very useful properties, it is
used in various applications in catalysis, in cosmetics as UV absorber, in biomedical
applications, in chemical sensors, electroluminescent and photochemical properties [153-
164]. Zinc oxide nano materials can be prepared by several methods: hydrothermal methods
[165], electrochemical depositions [166], sol–gel method [167], chemical vapor deposition
[168], thermal decomposition [169], and combustion method [170,171]. Recently, ZnO
nanoparticleswere prepared by ultrasound [172] and co-precipitation [144]. Silica coated
structured ZnO nanomaterials have recently been used to improve their stability and their
dispersability in suspensions [173] and therefore they can be used for wide range of
applications [173-176]. Silica-based coatings are of particular interest because it has good
environmental stability with different materials [177], ease of surface modification [178] and
reduce potential for photocatalysis and formation of free radicals [179]. Silica coating of
metal oxide nanoparticles has been widely studied such as Fe2O3, TiO2 and others [180, 181,
145]. However, the introduction of silica coating on ZnO nanomaterial is substantially rather
difficult, due to its high surface energy and its large surface area and therefore zinc oxide
nanoparticles could be easily aggregated. There were some reported articles in which
dispersant agents were used as coupling agents for the fabrication of silica coated ZnO
nanoparticles [182,183]. Two main strategies were recently applied for the fabrication of
silica coating of zinc oxide nanoparticles using modified Stöber method. The first strategy is
to incorporated silane precursors during the growth process of zinc oxide as coating agent
[184-190], the second strategy is to use sol-gel process for coating of previously prepared
zinc oxide nanoparticles [190]. In our research article, we used the second strategy, where
ZnO nanoparticles were firstly ultrasonicated in an ethanolic solution containing base in
order to decrease aggregation and activate zinc oxide nanoparticles, followed by sol-gel
coating process. CTAB was used as coupling agent to incorporate with zinc oxide
nanoparticles and also formation of meso silica microspheres. Twelve different silica or
meso silica and their functionalized silica coated zinc oxide microspheres are prepared.
Several methods and techniques were used for structural characterization of these new
materials. These methods include: X-ray diffraction (XRD) , transition electron microscopy
with energy dispersive X-Ray Spectrometer (TEM-EDX) , CP/MAS 13
C nuclear magnetic
resonance (NMR) spectra, Fourier transform spectroscopy (FTIR), Ultra violet-visible
spectra(UV/VIS) and thermal analysis (TGA).
32
3.2 Synthesis
3.2.1 Synthesis of ZnO nanoparticles
Zinc oxide nanoparticles (ZnO NPs) were preparated by the co-precipitation method
[144] by the reaction of zinc sulfate pentahydrate with oxalic acid, then the metal oxides
were obtained by calcinations at 500 oC (Scheme 3.2.1).
ZnSO4.5H2O. + C2O4H2→ ZnC2O4 calcinations → ZnO
Scheme (3.2.1) Zinc oxide nanoparticles preparation equation
3.2.2 Synthesis of ZnO@SiO2 microspheres
The coated silica zinc oxide microspheres labeled as ZnO@SiO2 were prepared using
modified Stöber method [129] through a simple sol–gel process. Briefly, by
ultrasonicated of ZnO nanoparticles in presence of ammonium hydroxide and
tetraethylorthosilicate (TEOS). The ultrasonication of nanoparticles prior the coating
process in ethanol was used to obtain well homogeneous dispersion of nanoparticles and
prevent agglomerization of particles [190], no dispersant agents are used. In this sol–gel
method, TEOS acts as a silica coating precursor and NH4OH act as catalyst. In the basic
medium, the surface of the ZnO NPs was activated [190]. When TEOS was added to the
solvated ZnO nanoparticle, the silane precursor began to undergo hydrolysis and
condensation process to form ZnO–Si–OH linkages onto the nanoparticle surface
[145,190]. A polymerization reaction occurred with the excess silane material and a
three-dimensional siloxane bonds (Si–O–Si) are formed.The use of CTAB as cationic
surfactant has two functions, it acts as coupling agent to incorporate with zinc oxide
nanoparticles to obtain well homogeneous dispersion and to prevent aggregations of
zinc oxide nanoparticles (Scheme 3.2.2).
Scheme (3.2.2) ZnO@SiO2
33
3.2.3 Synthesis of ZnO @meso SiO2 nanoparticles
In this sol–gel method, TEOS acts as a silica coating precursor and NH4OH act as the
catalyst. In the basic medium, the surface of the ZnO NPs was probably activated [190].
When TEOS was added, the silane precursor undergo hydrolysis and polycondensation
process to establish ZnO–Si–O- linkages onto the nanoparticle surface [145,190]. This
leads to formation of silica or meso silica coating zinc oxide (ZnO@SiO2,
ZnO@mSiO2) microspheres of diameter 260 nm and 300 nm, respectively. That was
confirmed by TEM discussed later. (Scheme 3.2.3).
Scheme (3.2.3) ZnO@mSiO2
3.2.4 Synthesis of mesoporous silica coated metal oxides nano particles
ZnO@SiO2@meso SiO2 nanoparticles
Synthesis of ZnO@SiO2@mSiO2 microspheres of two-shells of silica microspheres of
diameter over 300 nm were obtained by treating ZnO@SiO2microspheres with
concentrated ammonia solution (28 wt%) and CTAB and TEOS as confirmed from
TEM results. The nanoparticles of ZnO were only embedded into the silica shell
without appearance of zinc oxide nanoparticles in the meso silica shell as confirmed by
TEM discussed later. (Scheme 3.2.4).
Scheme (3.2.4) ZnO@SiO2@mSiO2
3.2.5 Amine functionalization of silica coated zinc oxide nanoparticles
Functionalization of ZnO or ZnO@SiO2@m-SiO2 or ZnO@m-SiO2 nanomaterials were
prepared by treating ZnO nanoparicles or their silica coated material with 3-
34
aminopropyltrimethoxysilane (APTMS) in dry toluene at 110 oC [145] (Schemes 3.2.5,
3.2.6 and 3.2.7).
Scheme (3.2.5) ZnO@SiO2@mSiO2-NH2
Scheme (3.2.6) ZnO@mSiO2-NH2
Scheme (3.2.7) ZnO@mSiO2-NH2
35
3.2.6 Thiol functionalization of silica coated zinc oxide nanoparticles
Functionalization of ZnO@SiO2@m-SiO2 or ZnO@m-SiO2 nanomaterials were
prepared by treating ZnO nanoparicles or their silica coated material with 3-
thiolpropyltrimethoxysilane (TPTMS) in dry toluene at 110 oC [145]. (Scheme 3.2.8
and 3.2.9).
Scheme (3.2.8) ZnO@SiO2@m-SiO2-SH
Scheme (3.2.9) ZnO@m-SiO2-SH
3.2.7 Free ZnO spheres coated silica
Free spheres of coated silica and free spheres of functionalized silica coated materials
are obtained easily by treating silica coated zinc oxide or functionalized silica coated
zinc oxide with hydrochloric acid to remove the zinc oxide. (Scheme 4.2.10, Scheme
3.2.11, Scheme 3.2.12). The experimental data are summarized in Table 3.1.
36
Scheme (3.2.10) ZnO free@SiO2
Scheme (3.2.11) ZnO free@mSiO2
Scheme (3.2.12) ZnO free@SiO2@mSiO2
37
Table 3.1 Experimental data.
Notes Synthesis Description Material
Agglomeration hexagonal ZnSO4 + H2C2O4 Zn C2O4
calcinations ZnO pure
ZnOpure
Microspheres are formed,260 nm ZnO +TEOS+NH4OH + sonication
1 h, annealed at 500 °C for 4h.
ZnO@SiO2
Microspheres, wormlike meso
silica, 300 nm diameter, ZnO
NPs are dispersed into
mesopores
ZnO+TEOS+NH4OH+CTAB+
sonication1h, annealed at 500 °C
for 4 h.
ZnO@mSiO2
Microspheres of two layers, >
300 nm diameter, ZnO NPs
dispersed into silica shell.
ZnO@SiO2+TEOS+NH4OH+CTAB
+sonicationfor 1h→
ZnO@SiO2@mSiO2
Thiol-functionalized and meso
silica two layers of 13 and 5 nm
thickness.
ZnO@mSiO2 + thiol silane , reflux
in dry toluene at 110 °C →
ZnO@mSiO2-SH
ZnO@SiO2@mSiO2 + amino silane
,reflux in dry toluene at 110 °C →
ZnO@SiO2@mSiO2-NH2
Meso silica and functional
silicalayersof 13 and 5 nm
thickness,
ZnO@SiO2@m-SiO2+ thiol silane ,
reflux in drytoluene at 110 °C →
ZnO@SiO2@mSiO2-SH
Low density MgO free silica
(silica and meso layers)
ZnO@SiO2@ +6MHCl → ZnO free @SiO2
Low density ZnO free meso
silica layer, no ZnO is present.
MgO@m-SiO2+6MHCl → ZnO free@mSiO2
Low density ZnO free silica and
meso layers, no Zn content is
observed
ZnO@SiO2@m-SiO2-NH2+ 6MHCl
→
ZnO free @SiO2@mSiO2
ZnO nanoparticles + amine silane →
reflux indrytoluene at 110 °C →
ZnO-NH2
38
3.3 Infrared spectra
Fig. 3.3.1(a-d) presents, the FT-IR spectra of ZnO, silica coated zinc oxide
microspheres (ZnO@SiO2, ZnO@SiO2@mSiO2) and amine functionalized silica and
meso silica coated zinc oxide spheres (ZnO@SiO2@mSiO2-NH2), respectively. The
peaks at 1550–1600 cm-1
and 3100–3600 cm-1
can be attributed to the hydroxyl groups
(O-H) and amine (N-H) bending and stretching vibrations, respectively. The Si–O–Si
asymmetric stretching vibration band of 980–1190 cm-1
was found only in the
ZnO@SiO2 NPs spectrum compared to the ZnO NPs [185,190,194]. The Si-ZnO bond
formation was confirmed by the reduction in peak intensity at 450-500 cm-1
of Si–O–Si.
This manifests the existence of a silica layer and successfully grafted over the ZnO NPs.
The absoption bands at 2980 and 1560 cm-1due to the ν(C-H) of aliphatic hydrocarbons
and δ(N-H) Fig.3.3.1(d), respectively as well as the disappearance of the shoulder at
960 cm-1
due to free silanol hydroxide groups (Si-OH) provide evidence for the
introduction of the organofunctional ligand groups onto the coated silica layers. The
removal of CTAB was confirmed from the FTIR spectra were the peaks assigned due to
CTAB at 2984 and 1475cm-1
due to ν(C-H) vibrations are removed.
Amine-functionalized and meso
silica two layers are formed
around ZnO.
ZnO@mSiO2 + amine silane , reflux
in toluene at 110 C →
ZnO@mSiO2-NH2
39
4500 4000 3500 3000 2500 2000 1500 1000 500
(d)
(c)
(b)
(a)
Tra
nsm
itta
nce (
%T
)
wavenumber (cm-1
)
Fig.)3.3.1) FTIR spectra of (a)ZnO pure , (b) ZnO@m-SiO2,(c) ZnO@SiO2@m-SiO2, (d)
ZnO@SiO2@m-SiO2-NH2
3.4 Thermal gravimetric analysis (TGA) spectra
Thermogravimetric analysis (TGA) and defferential thermogravimetric analysis (DTA)
for ZnO@SiO2@mSiO2 and its amine functionalized ZnO@SiO2@mSiO2-NH2
nanoparticles were examined under nitrogen atmosphere at 20–600 oC at rate 10
oC/minute. The thermogram of the ZnO@SiO2@mSiO2 nanomaterialFig. 3.4.1 shows
two peaks the main preak occurs at 75oC due to loss of 9.9 % of its initial weight. This
attributed to loss of physisorbed water and alcohol from the system pores [193]. The
second peak is at 390 o
C due to loss of 3.6 %, which is probably due to dehydroxylation
and loss of water or alcohol from silica [193]. The total loss of weight was 13.3 %.
Fig.3.4.2 shows the thermogram of the ZnO@SiO2@mSiO2-NH2 nanomaterial, three
peaks were observed, the first preak occurs at 75oC due to loss of 7.2 % of its initial
weight. This attributed to loss of physisorbed water and alcohol from the system pores
[193]. The second peak and third peaks at 350 o
C and at 450, are due to lost of the
amine functional groups from the system loss 17.6 % and dehydroxylation and loss of
water or alcohol from silica [193]. The total loss of weight was 23.8 %. The difference
between the total loss of the two materials (10.5%) is probably due to tne amine
organofunctional group.
41
0 100 200 300 400 500 600
7.0
7.5
8.0
8.5
9.0
9.5
10.0
We
igh
t (m
g)
Temperature (oC)
ZnO@SiO2@mSiO2
Derivative Y1
-0.018
-0.016
-0.014
-0.012
-0.010
-0.008
-0.006
-0.004
-0.002
0.000
0.002
De
riva
tive
Y1
Fig.(3.4.1) TG/DTA analysisof ZnO@SiO2@mSiO2
0 100 200 300 400 500 600
8.2
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
We
igh
t (m
g)
Temperature (oC)
ZnO@SiO2@mSiO2-NH2
Derivative Y1
-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
De
riva
tive
Y1
Fig.(3.4.2) TG/DTA analysisof ZnO@SiO2@mSiO2-NH2
3.5 X-Ray Diffraction (XRD)
The XRD patterns for pure ZnO powder prepared by the co-precipitation method [144],
silica, meso silica coated zinc oxide and their amine or thiol functionalized materials
are shown in Figure 3.5.1( a-i). All the diffraction peaks are well indexed to the
41
hexagonal ZnO wurtzite structure (JCPDS no. 36–1451) [195,196]. Diffraction peaks
corresponding to the impurity were not found in the XRD patterns, confirming high
purity of the synthesized products. The mean crystallite size of ZnO particles was
determined by Sherrer‟s equation ( D = 0.89λ/β cosθ ) where D is the crystallite size
(nm) λ is the wavelength of incident X-ray (nm), (β is the full width at half maximum
and θ is the diffraction angle). The obtained particle size was 13 nm for pure ZnO.
There was a small shift to lower angle with and change in the line width revealing that
silica coating was successfully conducted Figs 3.5.1( b-h) . This leads in slight change
of the particle size of the coated zinc oxide. The zinc oxide free silica Figs. 3.5.1 (a& i)
showed no XRD peaks, this may confirmed complete removal of zinc oxide
nanoparticles form silica coated zinc oxide micorspheres, when treated with
hydrochloric acid.
30 40 50 60 70 80
(112)(103)(110)
(102)
(101
(002)(100)
Inte
ns
ity
(a
.u.)
2 Degrees)
(i)
(h)
(g)
(b)
(c)
(d)
(e)
(f)
(a)
Figs.(3.5.1) XRD patterns of:( a) ZnO free@SiO2@mSiO2,( b) ZnO pure, (c)ZnO@SiO2, (d)
ZnO@mSiO2 , (e)ZnO@SiO2@mSiO2, (f)ZnO@SiO2@mSiO2-NH2 ,(g) ZnO@mSiO2-SH, (h)
ZnO@mSiO2-NH2 ,(i) ZnO free@mSiO2.
The Particles size of materials are summarized in Table 3.2.
42
Table 3.2 Particles size of materials.
3.6 Ultraviolet-Visible (UV-vis) spectroscopy
The UV/vis spectra for the solution contain ZnO coated silica ZnO@SiO2 and
ZnO@SiO2@mSiO2 NPs in HY surfactant Figs. 3.6.1(a-c). HY surfactant was used to
obtain a homogenous solution of un-coated and coated silica zinc oxide particles. Two
peaks at 359, 290 nm are shown in Figs. 3.6.1 (a-c), that characteristic for the presence
of ZnO nanoparticles [173,190]. It is found that there is a decreasing of the absorption
band intensities as the number of shells of coated silica is build up on zinc oxide
(ZnO@SiO2 to ZnO@SiO2@mSiO2). There was a shift of the absorption band at 307
nm for the free uncoated ZnO to shorter wavelength at 290 nm for ZnO@SiO2 and at
283 nm for ZnO@SiO2@mSiO2. The blue shift of the absorption band 307 nm to shorter
wavelength is probably due to decreasing of the particle size. The was no peaks are
observed for the ZnO free coated silica-mesosilica indicating the absence of shell core of
zinc oxide Fig.3.6.2b when treated with HCl acid in comparison with that of
ZnO@SiO2Fig. 3.6.2b.
Particles size(nm) Material
13 ZnO pure
16.2 ZnO@SiO2
18.4 ZnO@SiO2@m-SiO2
18.1 ZnO@SiO2@m-SiO2-NH2
19.5 ZnO@m-SiO2
16.6 ZnO@m-SiO2-SH
18 ZnO@m-SiO2-NH2
43
300 6000
1
Ab
s.
Wavelength(nm)
(a)
(b)
(c)
296nm
283nm
307nm
Fig.(3.6.1 ) UV-visabsorption spectra of (a)ZnO pure, (b)ZnO@SiO2@m-SiO2, (c) ZnO @SiO2
300 600
0.0
0.7
Ab
s.
Wavelength(nm)
(a)
(b)
Fig. (3.6.2) UV-visabsorption spectra of (a) ZnO@SiO2, (b)ZnO free@SiO2
3.7 Energy gap spectra
The optical band gap of the nanoparticles has been evaluated from the Tauc relation
[197].
(ɛ hv ) = C ( hv - Eg ) m
(1)
44
where C is a constant, ɛ is molar extinction coefficient, Eg is optical band gap of the
material and m depends on the type of transition. The value of molar extinction
coefficient for the synthesized nanoparticles is more than 900; thus, we can assume that
the transitions in the nanocrystals are allowed direct transitions [198]. For m= 0.5, Eg in
Eq.1 is directly allowed band gap. The optical band gap was estimated from the linear
portion of the (ɛhv )2 versus hv.There is no change in the energy gap be in the range of
2.9 to 3.4. The energy gap spectra for the ZnO coated silica ZnO@SiO2 and
ZnO@SiO2@mSiO2 NPs Figs. 3.7.1(a-c). The energy gap of materials are summarized
in Table 3.3.
2 3 4 5
2 3 4 5
(h
v)2
(a)
3.4
3.0
Eg(hv)
(b)
2.92
(c)
Fig.(3.7.1) Energy gap spectra of (a)ZnO pure , (b) ZnO@SiO2, (c) ZnO@SiO2@m-SiO2
Table 3.3 Energy gap of materials.
Eg Material
3.4 eV ZnO pure
3.0 eV ZnO@SiO2
2.9 eV ZnO@SiO2@m-SiO2
45
3.8 Photoluminescence spectroscopy (PL) spectra
The luminescence of ZnO nanoparticles is one of particular interest from view points of
both physical and applied aspects. Figure 3.8.1 shows the room temperature
photoluminescence spectrum of the ZnO nanoparticles excited at 362 nm. Green
emission was observed from the hydrothermally synthesized ZnO nanoparticles. It can
be attributed to the transition between singly charged oxygen vacancy and photo excited
hole or Zn interstitial related defects [199,200]. The inset in the Figure 3.8.1 shows the
photoluminescent excitation spectra of the ZnO nanoparticles (λem=545 nm) which
indicates that the excitation is at 362 nm. The excitation peak corresponds to the band
transition which also confirms the blue shift in the band gap of ZnO nanoparticles. After
coating ZnO the photoluminescence spectrum of the ZnO@SiO2 excited at 362 nm.
400 500 600
0
1000
2000
3000
4000
5000
6000
Inte
ns
ity
Wavelength (nm)
(b)
(a)
Fig. (3.8.1) PL spectra of (a)ZnO pure , (b) ZnO@SiO2
3.9 13
C CP/MAS NMR results
The CP/MAS 13
C NMR spectra for amine-functionalized coated silica-meso silica zinc
oxide and thiol-functionalized coated silica-meso silica zinc oxide are depicted in Fig.
3.9.1(a&b) respectively. The spectrum of amine-functionalized composite shows three
methylene carbons at 11, 23 and 43 ppm correspond to the Si-CH2 , C-CH2 and CH2-
N. the signal at 165 ppm is probably due to absorbed CO2 [184]. The spectrum of thiol-
46
functionalized composite shows three methylene carbons at 11 and 28 ppm correspond
to the Si-CH2 and the two carbons C-CH2 and CH2-S, respectively. The signal at 129
ppm is probably due to presence of some impurities. These assignments are based on
spectral data reported for similar systems [191,192].
Fig.(3.9.1) CP/MAS 13
C NMR spectra of (a) ZnO@SiO2@mSiO2-(CH2)3-NH2 , (b)
ZnO@SiO2@mSiO2-(CH2)3-SH .
3.10 Transmission electron microscopy (TEM), EDX
TEM images along with EDX of pure ZnO, coated silica zinc oxide (ZnO@SiO2),
coated meso silica zinc oxide (ZnO@mSiO2) and coated silica-mesosilica
(ZnO@SiO2@mSiO2) are given in Figs. 3.9.1 (a-g). TEM image of pure zinc oxide
shows different bar sizes of hegxagonal shape of 20-70 nm length and 15-30 nm width
47
Fig. 3.10.1 a. The TEM-EDX of ZnO nanomaterial shows only zinc and oxygen peaks
with no silicon is present Fig. 3.10.1 b . Figs. 3.10.2(c & d) illustrate low and high
resolution TEM images of meso silica coated zinc oxide (ZnO@mSiO2) microspheres
of 300 nm diameter. The meso silica microsphere was evident from the grey wormlike
structure with zinc oxide appear as dark nanoparticles (dark color) which were
capsulated into silica mesopores as shown in Fig. 3.10.1d. The small particles of zinc
oxides are probably formed through the ultra sonication process of colloidal solution in
presence of CTAB, so these small particles are fitted into the mesopores of the meso
silica [174]. For silica coated zinc oxide ZnO@SiO2 it can be seen from TEM-image
Fig. 3.10.3e, that roughly microspheres of size 260 nm of silica, in which ZnO
nanoparticles are dispersed. EDX confirm the presence of peaks due to components of
silicon and oxygen and zinc.In the case of ZnO@SiO2@mSiO2 microsphere, it can be
seen from TEM image Fig. 3.10.3f that zinc oxide nanoparticles of different sizes are
dispersed only into the silica microspheres and none are present in the meso silica shell.
This is in consistence with experimental work, where zinc oxide nanpaticles were firstly
accommodated into the silica microsphere, and later coated with a meso silica shell. The
TEM-EDX of ZnOfree@mSiO2 and ZnOfree@SiO2@mSiO2 microspheres showed
only the presence of silicon and oxygen and absence of zinc content which is evident for
complete removal of ZnO core upon treatment with HCl. This also was confirmed by
UV/vis spectra discussed below. The TEM and TEM-EDX images Figs. 3.10.4(g &h)
and Figs 3.10.5(i&j) of ZnO@mSiO2-SH and ZnO@mSiO2-NH2 NPs, respectively
showed ZnO core bare covered with distinguishable meso silica layer,13 nm in
thickness covered by thin layer of functionalized thiol silane precursor layer, 5 nm in
thickness resulting from the consensation of thiol-silane over the meso silica layer
[190,191,194] as marked in Fig. 3.10.4g. This was also evident from TEM-EDX
Figs.3.10.4(i&j) in which the silicon content has increased from 1.3% for ZnO@mSiO2
to 9.13% after coating with thiol-silane and the zinc content has decreased from 96% to
68% after.
48
Fig.(3.10.1) Structural characterization of ZnO nanoparticles : (a) TEM image and (b) EDAX
spectra
Fig. (3.10.2) Structural characterization TEM image of ZnO@mSiO2 :(c) low resolution TEM (d)
high resolution
Fig. (3.10.3) Structural characterization TEM image of: (e) ZnO @SiO2 and (f) ZnO @SiO2 @m-SiO2
49
Fig.(3.10.4) Structural characterization of ZnO@mSiO2-(CH2)3-SH nanoparticles : (g) TEM image
and (h) EDAX spectra
Fig.(3.10.5) Structural characterization of ZnO@mSiO2-(CH2)3-NH2nanoparticles : (i) TEM image
and (j) EDAX spectra
3.11 Conclusion
Silica, meso silica or both silica coated ZnO microspheres were synthesized by
modified Stöber method. ZnO particles are dispersed into silica microspheres or into
mesopores of the meso silica shell as confirmed from TEM analysis. Functionalization
of the meso silica shell with amine or thiol silane coupling agent, where two shells,
meso and functional silica layers of 13 nm and 5 nm thickness are formed, respectively.
XRD results of pure ZnO NPs and the silica coated zinc oxide materials showed
hexagonal ZnO wurtzite structure with mean average crystallite size of 13 nm are found.
51
Zinc oxide free silica materials have low density silica spheres showed no XRD peaks
due to complete etching of core ZnO particles from silica microspheres. TGA and FTIR
spectra revealed that the amine and thiol organofunctional groups are covalently
attached to the meso silica layer. TGA revealed that 10.5 % of the weight of the
functionalized coated silica zinc oxide is due to organofunctional groups. These
materials have been testing for removal of toxic of heavy metals and dyes as proposed
coming further research.
51
CHAPTER FOUR
Results and Discussion
Synthesis and Characterization of silica and
functionalized silica coated magnesium oxide
nanomaterials ( MgO).
52
4.1 Introduction
Metal oxides with high surface area and porosity have attracted considerable interest for
scientific research due to their potential application such as functional components for
nanoelectronics, optoelectronics and sensing devices [201]. In particular, magnesium
oxide (MgO) as a versatile oxide materials with assorted properties finds extensive
applications in catalysis, ceramics, toxic waste remediation, or as an additive in
refractory, paint and superconductor products [202,203]. Also, owing to its very large
band gap, excellent thermodynamical stability, low dielectric constant and refractive
index, it has been used as a transition layer for growing various thin film materials
[204,205]. Over the past years, the specific surface area, dispersion stability and
morphological characteristic of the magnesia particles have been identified as important
parameters that influence high-performance for such applications [206,207]. Therefore,
many approaches to control these properties of magnesia particles have been
extensively investigated. The most conventional method for synthesizing magnesia
particles is the decomposition of various magnesium salts obtained from liquid phase
processes such as sol–gel [208], precipitation [209], surface-initiated in-situ
polymerization [210] and hydrothermal synthesis [211]. Silica-based coatings are of
particular interest because it has good environmental stability with different materials
[177] and their low refractive index, water-compatibility, lower toxicity and ease of
surface functionalization [178]. Another reason for coating nanoparticles is to reduce
the potential for photocatalysis and formation of free radicals [179]. Silica coating of
nanoparticles has been widely studied on some materials such as Fe2O3, TiO2 and others
[180,181,145]. There were some reported articles in which dispersant agents were used
as coupling agents for the fabrication of silica coated MgO nanoparticles [182,183].
More recently silica coated magnesium oxide nanomaterials were recently reported
using two main strategies for the fabrication of silica coating of magnesium oxide
nanoparticles, the first strategy is to incorporated silane precursors during the growth of
MgO oxide as coating agent [184-190], the second strategy is to use sol-gel process for
coating of previously prepared magnesium oxide nanoparticles [190]. In our research ,
we used the second strategy, where MgO nanoparticles prepared by the co-precipitation
method of magnesium sulfate. We have used ultrasonication of the ethanolic solution of
MgO particles in presence of base, prior to the sol-gel coating process in which
multilayer with silica, mesosilica and functionalized silica coated magnesium oxide
53
nanoparticles are synthesized, eleven material silica , meso silica , and functionalized
silica coated magnesium oxide and their MgO free silica materials are also prepared.
Free silica coated materials are obtained by etching the MgO nanoparticles using HCl.
Several methods and techniques were used for structural characterization of these new
materials. These methods include: X-ray diffraction (XRD) , transition electron
microscopy with energy dispersive X-Ray Spectrometer (TEM-EDX) , Fourier
transform spectroscopy (FTIR), Ultra violet-visible spectra(UV/VIS),
Photoluminescence spectroscopy (PL) and thermal analysis (TGA).
4.2 Synthesis
4.2.1 Synthesis of MgO nanoparticles
In typical synthesis of MgO nanopowders [144], magnesium oxide nanoparticles were
preparation by the co-precipitation method by the reaction of magnesium sulfate penta
hydrate MgSO4.5H2O with oxalic acid. The metal oxides were obtained by calcination
of magnesium oxalate at 500 oC in similar to (Scheme 3.2.1).
4.2.2 Synthesis of MgO@SiO2 microspheres
The coated silica magnesium oxide microspheres labeled as MgO@SiO2 were prepared
using modified Stöber method [129] through a simple sol–gel process. Briefly, by
ultrasonicated of MgO nanoparticles in presence of ammonium hydroxide and
tetraethylorthosilicate (TEOS). The ultrasonication of nanoparticles prior the coating
process in ethanol was used to obtain well homogeneous dispersion nanoparticles and
prevent agglomerization of particles [190], no dispersant agents are used. In this sol–gel
method, TEOS acts as a silica coating precursor and NH4OH act as the catalyst. In basic
medium, the surface of the MgO NPs was activated [190]. When TEOS was added to
the solvated–nanoparticle mixture, the silane material began to undergo hydrolysis and
condensation process to establish MgO–Si–OH linkages on the nanoparticle surface
[145,190] in a similar to (Scheme 3.2.2) . A polymerization reaction occurred with the
excess silane material and a three-dimensional siloxane bonds (Si–O–Si) are formed.
The experimental data are summarized in table 4.1.
54
4.2.3 Synthesis of MgO @meso SiO2 nanoparticles
A meso silica shell has introduced to encapsulated MgO nanoparticle by using
tetraethylorthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB) as cationic
surfactant in ethanol with presence of ammonium hydroxide similar to (Scheme 3.2.3).
CTAB was then removal by calcinations at 500°C. The nanoparticles of MgO were
probably dispersed into the meso silica region. This was confirmed by TEM results
discusses later.
4.2.4 Synthesis of MgO@SiO2@meso SiO2 nanoparticles
MgO@SiO2@meso-SiO2 two layers microspheres was prepared in a similar method
described [129], by introducing a meso silica shell onto the MgO@SiO2 microspheres
using tetraethylorthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB) as
cationic surfactant in presence of ammonium hydroxide similar to (Scheme 3.2.4).
CTAB was then removal by calcinations at 500°C.
4.2.5 Amine functionalization of silica coated magnesium oxide nanoparticles
Amine functionalization of MgO@SiO2@m-SiO2 or MgO nanomaterials were prepared
by treating MgO nanomaterials or its silica coated material with 3-
aminopropyltrimethoxysilane (APTMS) in dry toluene at 110 o
C [145], as shown in
similar (Schemes 3.2.5, 3.2.6 and 3.2.7).
4.2.6 Thiol functionalization of silica coated magnesium oxide nanoparticles
Thiol functionalization of MgO@SiO2@m-SiO2 or MgO@m-SiO2 nanomaterials were
prepared by treating MgO nanomaterials or its silica coated material with 3-
thiolpropyltrimethoxysilane (TPTMS) in dry toluene [145], as shown in similar
(Schemes 3.2.8 and 3.2.9).
4.2.7 Free MgO spheres coated silica
Free spheres of coated silica and their functionalized silica coated materials are obtained
easily by treating silica coated magnesium oxide or functionalized silica coated
magnesium oxide with hydrochloric acid to remove the magnesium oxide, as shown in
similar (Schemes 3.2.10 , 3.2.11 and 3.2.12).
55
Table 4.1 Experimental data.
Notes Synthesis Description Material
Agglomeration nano rode. MgSO4 + H2C2O4
MgC2O4 calcinations
MgO pure
MgOpure
MgO nanoparicles
dispersed into silica .
MgO +TEOS+NH4OH + sonication
1 h, annealed at 500 °C for 4h.
MgO@SiO2
Microspheres,wormlike meso
silica, MgO NPs dispersed into
mesopores
MgO +TEOS+NH4OH+CTAB +
sonication1h, annealed at 500 °C
for 4 h.
MgO@mSiO2
Microspheres,50nm diameter,
MgO@SiO2 dispersed into
silica.
MgO@SiO2+TEOS+NH4OH+CTA
B + sonicationfor 1h→
MgO@SiO2@mSiO2
Meso silica and functional
silicalayers.
MgO@m-SiO2+ thiol silane , reflux
in drytoluene at 110 °C →
MgO@mSiO2-SH
MgO@SiO2@m-SiO2+amino silane
, reflux in dry toluene at 110 °C →
MgO@SiO2@mSiO2-NH2
Meso silica and functional
silicalayers.
MgO@SiO2@m-SiO2+ thiol silane ,
reflux in drytoluene at 110 °C →
MgO@SiO2@mSiO2-SH
Low density MgO free silica
(silica and meso layers)
MgO@SiO2@ +6MHCl → MgO free @SiO2
Low density meso layer MgO@m-SiO2+6MHCl → MgO free@mSiO2
MgO@SiO2@m-SiO2-NH2+ 6MHCl
→
MgO free @SiO2@mSiO2-
NH2
MgO nanoparticles + amine silane
→ reflux indrytoluene at 110 °C →
MgO-NH2
56
4.3 Infrared spectra
The FT-IR spectra of pure MgO, silica coated magnesium oxide , mesosilica coated
magnesium oxide, silica-meso silica coated magnesium oxide and magnesium oxide free
coated silica meso silica microspheres are shown in Figs .4.3.1 (a-e). Three regions of
absorption at 3300–3650 cm-1
,1400–1700 cm-1
and 500–1100 cm-1
are observed due to ν(O-
H), δ(O-H) and ν(Si–O–Si) vibrations, respectively. The two peaks at 3660 cm−1
(sharp) and
3500 cm−1
(broad) Figs.4.3.1 (a&b) are associated with free ν(O-H) and hydrogen bonding
ν(O-H) of MgO crystallizing water molecules, respectively. The absorption peak at 1435
cm−1
is probably associated with δ(O-H) vibration [212]. The disappeared of the δ(O-H) at
1435 cm−1
upon coating with silica is provide evidence for the incorporated of silica onto
MgO particles an formation of Si–O–Mg linkages. The Si–O–Si asymmetric and symmetric
stretching vibration bands are observed at 960–1190 cm-1
and 480 cm-1
, respectively for the
silica coated materials, which provide strong evidence for the presence of silica coated shells
[213,214]. The decrease of broad band at 500 cm-1
for pure MgO to a shoulder at 530 cm-1
for MgO coated silica and small peak at 461 cm-1
due probably to δ (Si–O–Si) vibration.
4000 3500 3000 2500 2000 1500 1000 500
Tra
nsm
itta
nce (
%T
)
Wavenumber (cm-1
)
(a)
(b)
(c)
(d)
(e)
Si-O-Si
Mg-O
(O-H)
Fig. (4.3.1) FTIR spectra of (a ) MgO pure , (b) MgO@SiO2, (c) MgO@mSiO2, (d) MgO@SiO2@m-
SiO2, (e) MgO free @SiO2@m-SiO2
57
FT-IR spectra of amine functionalized silica-mesosilica coated magnesium oxide and its
amine functionalized magnesium oxide free and are given in Figs. 4.3.2 (a-c),
respectively. Fig.4.3.2a the absorption band at 961cm-1
is due to the Si-O-H which has
disappeared after functionalization with silane coupling agent and formation of siloxane
(Si-O-Si) network. Fig.4.3.2b the absorption bands at 3500cm-1
broad, 2930cm-1
and
1560 cm-1
are due to the ν(N-H) , ν(C-H) of aliphatic hydrocarbons and δ(N-H) due to
the amine group, respectively [215]. This provides evidence for the introduction of the
amine functional ligand groups onto the coated meso silica layer. There is a decreasing
of the broad absorption at 3500 cm-1
is due to the absence of crystallized water- MgO,
where two peaks at 3427 cm-1
and 3040 cm-1
are probably due to ν(N-H) asymmetric
stretching and symmetric ν(N-H) stretching bands, respectively. The Si–O–Si
asymmetric stretching vibration band at 1100 cm-1
and ymmetric stretching vibration
band at 462 cm-1
are observed only for the silica coated materials. Fig.4.3.2c these
assignments of FTIR peaks are based on spectral data reported for similar systems [216].
4000 3500 3000 2500 2000 1500 1000 500
100
200
sy N-H
O-H(b)
(a)
(c)
Tra
nsm
itta
nce (
%T
)
Wavenumber (cm-1
)
O-H
C-H
asy N-H
Si-O-Si
Mg-O
Si-O-Si
Si-O-H
Fig.(4.3.2) FTIR spectra of (a) MgO@SiO2@m-SiO2 , (b) MgO@SiO2@m-SiO2-NH2, (b) MgO free
@SiO2@m-SiO2-NH2
FT-IR spectra of MgO@SiO2@mSiO2 before and after removal CTAB are given in
Figs. 4.3.3 (a&b), the removal of CTAB was evident from the disappearance of
58
absorption bands at 2932 cm-1
ν(C-H) and 1485cm-1
δ(C-H) vibrations [217]. After
removal CTAB , a strong absorption band, around 3500 cm-1
is observed due to ν(O-H)
band.
Fig.(4.3.3) FTIR spectra of (a) MgO@SiO2@mSiO2 , with CTAB , (b) MgO@SiO2@mSiO2 , CTAB
removed
FT-IR spectra for silica meso silica coated magnesium oxide and its magnesium oxide
free coated silica are given in Figs. 4.3.4 (a&b), respectively. The two spectra are very
similar. The only difference in is for the absorption bands at 500 cm-1
in which it has
low intensity with no shoulder in case of MgO free coated silica Fig.4.3.3b. This is
probably due to the absence of Mg-O-Si bands.
Fig.(4.3.4) FTIR spectra of (a)MgO@SiO2@mSiO2, (b) MgO free@SiO2@mSiO2
59
4.4 Thermal gravimetric analysis (TGA) spectra
Thermogravimetric analysis (TGA) and defferential thermogravimetric analysis (DTA)
for MgO@SiO2@mSiO2, amine functionalized MgO@SiO2@mSiO2-NH2 and its
magnesium oxide free coated silica (MgOfree@SiO2@mSiO2) nanoparticles were
examined under nitrogen atmosphere at 20–600 oC of rate 10
oC/minute. The
thermogram of the MgO@SiO2@mSiO2 nanomaterial Fig 4.4.1 shows two peaks the
main preak occurs at 75oC due to loss of 9.9 % of its initial weight. This attributed to
loss of physisorbed water and alcohol from the system pores [193]. The second peak is
very broud centered at 390 o
C due to loss of 3.6 %, which is probably due to
dehydroxylation and loss of water or alcohol from silica [193]. The total loss of weight
was 13.5%. Fig4.4.2 shows the thermogram of the MgO@SiO2@mSiO2-NH2
nanomaterial. Three peaks were observed the first preak occurs at 75oC due to loss of
7.2 % of its initial weight. This attributed to loss of physisorbed water and alcohol from
the system pores [193]. The second peak and third peaks at 350 o
C and at 450, are due
to loss of the amine organofunctional groups from the system and dehydroxylation and
loss of water or alcohol from silica of 17.6 % [193]. The total loss of weight was 24.8
%. The difference between the total loss of the two materials (11.3%) is probably due to
the amine organofunctional group. The shoulder at 120oC Fig. 4.4.2 is probably due to
adsorbed CO2 molecules which was confirmed with similar mono amine functionalized
systems [218]. The thermogram for MgO free coated silica Fig4.4.3 shows asimilar
pattern as that of MgO@SiO2@mSiO2 Fig4.4.1.
61
100 200 300 400 500 600
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
390o
C ( 3.6 %)
We
igh
t (m
g)
Temperature (oC)
MgO@SiO2@mSiO2 Derivative Y1
75o
C( 9.9 %)
-0.018
-0.016
-0.014
-0.012
-0.010
-0.008
-0.006
-0.004
-0.002
0.000
De
riva
tive
Y1
Fig.(4.4.1) TG/DTA analysisof MgO@SiO2@mSiO2
100 200 300 400 500 600
6.5
7.0
7.5
8.0
8.5
9.0
390o
C( 3.6) % 350
oC (14 %)
75o
C (7.2 %)
We
igh
t (m
g)
Temperature (oC)
MgO@SiO2@mSiO2-NH2 Derivative Y1
adsorbed CO2
-0.008
-0.007
-0.006
-0.005
-0.004
-0.003
-0.002
-0.001
De
riva
tive
Y1
Fig.(4.4.2) TG/DTA analysisof MgO free@SiO2@mSiO2-NH2
61
100 200 300 400 500 600
6.0
6.5
7.0
7.5
8.0
390o
C ( 7.7 %)
75o
C ( 6 %)
We
igh
t (m
g)
Temperature (oC)
MgO free@SiO2@mSiO2
Derivative Y1
-0.025
-0.020
-0.015
-0.010
-0.005
0.000
De
riva
tive
Y1
Fig.(4.4.3) TG/DTA analysisof MgO free@SiO2@mSiO2
4.5 X-Ray Diffraction (XRD)
The XRD pattern of pure MgO are shown in Fig.4.5.1a. The diffraction pattern matched
with the face centered cubic structure of particles MgO (JCPDS No. 87-0653). The
major peaks at 2θ values of 36.8 o, 42.9
o, 62.2
o, 74.6
o, and 78.6
o can be indexed to the
lattice planes of (111), (200), (220), (311) and (222), respectively [219].
The XRD diffraction peaks showed the presence of both forms, pure cubic structure(*) of
MgO and hexagonal structure($) of Mg(OH)2 [220]. Figs. 4.5.1(b&c) but after coating
only the cubic MgO form is present and no peaks of correspond to Mg(OH)2 is detected
Figs.4.5.1(d , f , h and i).
The mean crystallite size is determined using Scherrer‟s formula d = 0.9 λ / β cos θ
where d is the crystallite size λ is wavelength of x-ray radiation β is full width at half
maximum and θ is the diffraction angle. The obtained particle size for the different
materials are summarized in Table 4.2. They were found in the range (6- 8.7nm), which
means that the particle size was almost remain unchanged after silica coated, but it's
intensity does not change with silica when silica shell or silica-mesosilica two shells when
is added. There was no obvious change in the line width which confirms that the coating
process does not alter the crystallite size of the particles Figs.4.5.1 (b-i ). The magnesium
oxide free coated silica (Fig.4.5.1e& g) showed no XRD peaks , this may confirmed
62
complete removal of magnesium oxide nanoparticles form silica coated magnesium oxide
micorspheres. These assignments were based on reported XRD data of similar systems.
The XRD pattern for MgO@SiO2 @m-SiO2 and those of amine functionalized ligand
systems Figs.4.5.1 f & h , showed same diffraction pattern and number of peaks
correspond to cubic MgO structure. A conversion from hexagonal Mg(OH)2 to cubic MgO
occurred by silica coating in which a condensation of hydroxyl groups, and formation of
Si–O–Mg linkages are formed.
The XRD pattern for coated functionalized MgO-NH2 are shown in Fig.4.5.1 i both
pattern contain high intensity peak for cubic form (*) and contain low intensity peak for
form hexagonal form($) . This is probably due to the conversion from Mg(OH)2 to cubic
MgO was not completed after coating with amine silane coupling agent where all peaks
corresponding to Mg(OH)2 spaces have decreased significantly.
30 40 50 60 70 80
30 40 50 60 70 80
(a)200 311
222220111
(b)*
** $ $$$
(c) *$ $ $*
$
$
(d) ** *
*
(e)
(f)*
* *
(g)
(h) ** *
Inte
nsi
ty (
a.u
.)
2 (Degrees)
(i) ** *
Fig. (4.5.1) XRD patterns of (a) standard MgO pure, (b) MgO pure, (c) MgO@SiO2, (d) MgO@
m-SiO2, (e) MgOfree@ m-SiO2, (f) MgO@SiO2 @m-SiO2, (g) MgO free @SiO2@m-SiO2, (h)
MgO@SiO2@m-SiO2-NH2, (i) MgO-NH2
63
Table 4.2 Particles size of materials.
4.6 Ultraviolet-Visible (UV-vis) spectroscopy
The UV/vis spectra for pure MgO and silica coated magnesium oxide nanomaterials are
shown in Figs. 4.6.1(a-e). The spectrum of MgO nanoparticles exhibit two well-defined
bands in HY at 230 and 300 nm Figs. 4.6.1(a). HY was used as coupling agent to
incorporate and stabilized MgO nanoparticles. In the literature, bulk MgO is reported to
exhibit absorptions in the UV region in between 160–200 nm [221]. It is found that
there is a decreasing of the absorption band intensities as the number of layers of coated
silica increased MgO@SiO2 to MgO@SiO2@mSiO2. Figs. 4.6.1 (c&d), there are no
absorption bands observed for MgO free @SiO2@mSiO2 due to the absence of MgO
nanoparticles, in case of MgO@SiO2@mSiO2 , weak absorption bands were observed
due to the low of concentration MgO nanoparticles for the number of silica- mesosilica
layers Figs. 4.6.1(c). The high intensity of peaks for MgO@mSiO2 in comparison with
other silica coated samples is probably due to the presence of meso silica shell and high
availability of MgO to be detected.
Particles size(nm) Material
6.4 MgO pure
6.9 MgO@SiO2
6.5 MgO@SiO2@m-SiO2
8.7 MgO@m-SiO2
5.5 MgO@m-SiO2-SH
6.4 MgO@SiO2@m-SiO2-SH
6.0 MgO@SiO2@m-SiO2-NH2
8.4 MgO-NH2
64
300 400 500 600 700 800
0
2
230
nm
300
nm
Inte
ns
ity
(e)
(d)
(c)
(b)
Wavelength(nm)
(a)
Fig. (4.6.1 ) UV-Visabsorption spectra of (a)MgO pure, (b) MgO@SiO2, (c) MgO@SiO2@m-SiO2,
(d) MgO free @SiO2@m-SiO2,(e) MgO @m-SiO2
4.7 Energy gap spectra
The optical band gap of the nanoparticles has been evaluated from the Tauc relation
[197].
(ɛ hv ) = C ( hv - Eg ) m
(1)
where C is a constant, ɛ is molar extinction coefficient, Eg is optical band gap of the
material and m depends on the type of transition. The value of molar extinction
coefficient for the synthesized nanoparticles is more than 900; thus, we can assume that
the transitions in the nanocrystals are allowed direct transitions [198]. For m= 0.5, Eg in
Eq.1 is directly allowed band gap. The optical band gap was estimated from the linear
portion of the (ɛhv )2 versus hv. There is no change in the energy gap be in the range of
3 to 3.38. The energy gap of materials are summarized in Table 4.3.
65
2 3 4 50
500000
1000000
15000000
200000
400000
600000
8000000
1000000
2000000
30000000
1000000
2000000
3000000
2 3 4 5
(a)
3.38
Eg(hv)
-33
82
.72
(-3
38
2.7
2)
(b)
3.18
(c)
3.0(
hv
)2
(d)
3.35
Fig.(4.7.1) Energy gap spectra of (a)MgO pure , (b) MgO@SiO2, (c) MgO@m-SiO2 , (d) MgO –NH2
Table 4.3 Energy gap of materials.
4.8 Photoluminescence spectroscopy (PL) spectra
PL microscopic images were taken under the stimulation of a wavelength of 250 nm,
and a 370 nm filter was used. The spectra show that both samples exhibit emission
bands at 418-428 nm. It has been accepted that the emission bands at 418-428 nm result
from the formation of new energy levels in the band gap of MgO induced by defects in
the samples. The defects are probably formed during the calcinations process [222].
Fig.4.8.1a depicts the PL spectrum of the coral-like MgO crystals, in which an extensive
Eg Material
3.38 eV MgO pure
3.18 eV MgO@SiO2
3.0 eV MgO@m-SiO2
3.35 eV MgO-NH2
66
blue PL band around 428 nm is observed. And the shape of the band is just like that of
blue light emissions in porous silicon [223]. In addition, the spectrum shows a tail
around 365 nm, which is probably due to the scattering from the voids among the
nanoparticles formed in the calcinations process.
400 500 600
-2000
-1000
36
5 n
m
41
8-
42
8 n
m
Inte
ns
ity
Wavelength (nm)
(b)
(a)
Fig. (4.8.1) PL spectra of (a) MgO pure, (b) MgO@SiO2
4.9 Transmission electron microscopy (TEM), EDX
TEM images along EDX of pure MgO, MgO@SiO2 and MgO@mSiO2 are given in
Figures 4.9.1-3 (a&b). TEM image of pure MgO a shows agglomeration of MgO
nanoparticles in rode like shape Fig.3.9.1a. TEM-EDX of pure MgO show the presence
of only Mg and O components and with no Si is present in Fig. 4.9.1b.
67
Fig. (4.9.1) Structural characterization of MgO nanoparticles : (a) TEM image and (b) EDX spectra.
Fig. 4.9.2a shows the TEM image of MgO@SiO2. The MgO nanoparticles seems to be
coated by silica shell. TEM-EDX Fig. 4.9.2b for MgO@SiO2 shows the presence of C, Si,
Mg ,O components , which confirm the presence of silica coating layer around MgO
particles, the presence of carbon is probably due to presence of some un hydrolyzed alkoxy
groups.
Fig. (4.9.2) Structural characterization of MgO @SiO2: (a) TEM image and (b) EDX spectra.
TEM image of MgO@mSiO2 is given in Fig. 4.9.3a where MgO nanoparticles are seen in
dark and the coated silica are seen in grey colour. TEM-EDX Fig. 4.9.3b for MgO@m-SiO2
shows the peaks of Mg, O and Si components.
68
Fig. (4.9.3) Structural characterization of MgO@mSiO2: (a) TEM image and (b) EDAX spectra.
TEM image and TEM-EDX for MgO@SiO2@mSiO2 are shown in Fig. 4.9.4(a&b). MgO
@SiO2@mSiO2 microsphere with two shells silica and mesosilica with MgO particles
dispersed into the silica microsphere. TEM-EDX Fig.4.9.4b shows peaks of Si, O and small
peak Mg. This may explain that MgO nanoparticles are diluted dispersed into silica
microspheres.
Fig. (4.9.4) Structural characterization of MgO@SiO2@mSiO2: (a) TEM image and (b) EDAX spectra.
Fig. 4.9.5a represents TEM image of MgO@SiO2@mSiO2-NH2 , it shows different layers or
shells probably for the silica, meso-silica and amine functionalized silica with different
thicknes. MgO particles are presumably dispersed into the silica shell which covered by the
m-silica and amine functionalized silica layers. TEM-EDX Fig. 4.9.5b shows several peaks
of high intensity peaks for Si ,O and low intensity peaks for Mg and N of the amine group .
69
Fig.(4.9.5) Structural characterization of MgO@SiO2@mSiO2-NH2: (a) TEM image and (b) EDAX
spectra.
Fig.4.9.6a represents TEM image of MgO free@SiO2@m-SiO2-NH2, it shows no Mg
particles within the silica shell . TEM-EDX Fig. 4.9.6b shows almost two strong peaks for
Si and O components and no peaks for Mg . The presence of Cl peaks are probably of traces
of HCl that used for removed MgO which is protonated amine groups.
Fig. (4.9.6) Structural characterization of MgO free@SiO2@m-SiO2-NH2: (a) TEM image and (b) EDAX
spectra.
Fig. 4.9.7a represents TEM image of MgO free@SiO2@m-SiO2 microsphere of 670nm
diameter , it in value a silica microsphere covered with a mesosilica shell of 50 nm thickness
.TEM-EDX Fig. 4.9.7b shows zero MgO particles within the silica or mesosilica
microsphere. Only peaks for O and Si of equal intensity are present. The presence of small
peaks of Cl is probably due to traces of HCl used for removal of MgO.
71
Fig.(4.9.7) Structural characterization of MgOfree@SiO2@m-SiO2: (a) TEM image and (b) EDX spectra.
4.10 Application of magnesium oxide and its silica material and its
functionalized
4.10.1 Adsorption-Realease of BTB
4.10.1.1 Introduction of Bromothymol blue ( BTB ) pH – Indicator
BTB indicator acts as a weak acid in solution. It can, thus, be protonated or
deprotonated. BTB color appear as yellow color in acidic medium, and as blue color in
basic medium, with pka value 7.1 [224] . It has an orange – red color in neutral solution.
It is typically sold in solid form as the sodium salt of the acid form, and used in several
applications, such as:
. Laboratory, as a biological slide stain.
. To define cell walls or nuclei under the microscope.
. Measuring substances that would have relatively low
acidic or basic levels.
Bromothymol blue (pH indicator)
below pH 6.0
above pH 7.6
6.0 ⇌ 7.6
71
Scheme(4.10.1) BTB indicator structure.
4.10.1.2 Adsorption-release of BTB
Adsorption-release study of BTB dye in the MgOfree@SiO2@mesoSiO2-NH2
adsorbents. Amines function groups are probably present in both forms, free amine and
ammonium cation/hydrogen bonding forms [225]. When an acidic functionalized
ammonium mesoporous silica is treated with BTB dye solution, the white solid material
is readily changed into blue due to the entrapment of the BTB dye into the surface pores
of the functionalized MgOfree@SiO2@mesoSiO2. This is probably due to electrostatic
attraction between the positive ammonium cations(NH3+) and negative anionic (BTB
-)
species [226] Figs.4.10.1(a&b).
Acid medium :
1) MgO@SiO2@meso SiO2-NH2 HCL MgO free@SiO2@meso SiO2-NH3+
2) MgOfree@SiO2@meso SiO2-NH3+ + BTB
-
MgO free@SiO2@meso SiO2-NH3+_BTB
-
(electrostatic attraction)
72
200 300 400 500 600 700 800
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
(b)
Ab
s.
Wavelength(nm)
(a)
Fig. (4.10.1) UV-Vis absorption spectra of (a) MgO free@SiO2@mSiO2-NH2 +BTB , (b) BTB dyes.
When the blue material is treated with phosphate buffer solution (NaH2PO4) of pH 8,
most of the dye species are release and strong blue solution is formed in immediatly
Fig. 4.10.2(a&b). The reason for this behavior is due to presence of competitive anions,
the BTB dye uptake capacity was notably reduced due to presence of soft phosphate
anions [226]. When the functionalized ammonium groups are converted to free amine
groups no more electrostatic forces exists for adsorbtion of BTB- anion. The mesopores
loaded with guest species were captured by strong electrostatic attraction between the
ammonium cations and the anionic species of BTB.
Basic medium :
1) MgO free@SiO2@meso SiO2-NH3+_BTB
- phosphate buffer
2) MgOfree@SiO2@meso SiO2-NH2 + BTB-
(repulsion attraction)
73
200 300 400 500 600 700 800
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Ab
s.
Wavelength (nm)
MgOfree@SiO2@mSiO2-NH2
Absorption BTB dyes.
BTB dyes.
Releasing BTB dyes.
Fig.(4.10.2) UV-Vis absorption spectra ofAdsorption-release study of BTB dye in the
MgOfree@SiO2@meso SiO2-NH2
The anions of BTB were adsorbed to the ammonium functionalized mesoporous silica
upon decreasing the pH < 4. This system works as a swinging gate, these gates could
regulate the entrance where the cargo can diffuse in and out of the pore surface (scheme
4.10.2a). The anions of BTBwere released when the functionalized amine free upon
treatment with phosphate buffer solution at pH 5-7 (scheme 4.10.2b) [227].
74
Scheme (4.10.2) (a) Adsorbtion BTB and (b) released BTB.
4.10.2 Adsorption kinetics for different heavy metal ions
A first group of experiments has been carried out to test the ability of synthesized
materials to act as heavy metal adsorbents, MgO@SiO2@mSiO2-NH2 are included.
Aqueous solutions of metal ions of each amine-functionalized adsorbent at 25oC and the
essays have been repeated for copper chloride and cobalt chloride.
MgO@SiO2@mSiO2-NH2 experiments yield null adsorption at this detection level, so
metal loading of the amine functionalized MgO@SiO2@mSiO2 sample should
exclusively attribute to the presence of active amine groups anchored to the silica walls.
The metal ion uptake adsorption was determined by batch method by mixing each of
MgO@SiO2@mSiO2-NH2 systems with solutions of metal ions (Cu2+
and Co2+
).
Measurements were carried out at room temperature after 20 minutes uptake time to
reach equlibrium in case of copper and 60 minutes in case of cobalt ions. Fig.4.10.3 and
Fig.4.10.4 show the maximum metal uptake capacity of the different metal ions (Cu2+
and Co2+
), respectively. The metal uptake capacity of all ligand systems were found to
increase in the following order:
Co2+
< Cu2+
75
0 20 40 60 80 100 120-1
0
1
2
3
4
5
6
7
mm
ole
/1g
Time (min)
Cupper uptake
Fig.(4.10.3) Metal uptake capacity of metal ions (Cu2+
) +MgO@SiO2@meso SiO2-NH2
0 20 40 60 80 100 120-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
mm
ole
/1g
Cobalt uptake
Time (min)
Fig.(4.10.4) Metal uptake capacity of metal ions (Co2+
) +MgO@SiO2@meso SiO2-NH2
4.11 Conclusion
Silica, meso silica or both silica-mesosilica coated MgO microspheres were synthesized
by modified Stöber method. MgO particles are dispersed into silica microspheres or
into the pores of meso silica shell, as confirmed by TEM analysis. Functionalized of the
meso silica shell with amine or thiol silane coupling agent, two shells of meso and
76
functional silica of 40 nm and 7 nm thickness are formed. TEM results of pure MgO
NPs showed agglomeration of MgO nanoparticles in rode like shape with mean average
crystallite size of 6.406 nm are found. XRD and UV-Vis results of pure MgO and silica
or mesosilica coated MgO show almost no change of mean particle size of MgO. This
confirm that MgO particles does not change before or after coating with silica. XRD
patterns revealed that two structure forms of MgO hexagonal and cubic are exist in the
uncoated MgO, and after coating with silica only the cubic form is present. Magnesium
oxide free silica materials have low density silica spheres showed no XRD peaks due to
complete etching of core MgO. TGA and FTIR spectra revealed that the amine and thiol
organofunctional groups are covalently attached to the meso silica layer. TGA revealed
that 17.6 % of the weight of the functionalized coated silica magnesium oxide is due to
organofunctional groups. These materials have been testing for removal of toxic of
heavy metals and dyes as proposed coming in research.
77
Conclusion
Despite the fact that metal oxide nanoparticles were extensively investigated, but their silica
coating is not fully examine except that of Fe2O3. Metal oxide nanoparticles (MgO and ZnO)
were prepared by the co-precipitation method. These matal oxides were coated by silica ,
meso silica or both using modified Stober method. The metal oxide nanparticles are
dispersed into the resulting silica microspheres. The silica coating about the metal oxide was
evdent from TEM analysis and UV specra. No despersent agent was used and only
ultrasonication prior to sol-gel coating process was considered. These materials were fully
characterized using modern physical chemistry techniques including TGA,13C NMR , FTIR
, UV/vis , XRD and TEM are used.
In case of MgO nanoparticles, it was confirmed that magnesium oxide particles are present in
two forms cubic MgO and hexagonal Mg(OH)2 and are present in agglomerated rode like
shape as confirmed by TEM. Coating with silica, meso silica or both shells do not alter the
mean particle size of MgO particles with only minor changes in size of particles probably
due to the condensation between hydroxyle surface groups with silica silanols during the
silica coating process. . Coating with silica have also converted the hexagonal Mg(OH)2
strucure to the cubic MgO shape structure. The MgO core particles were removed from the
coated silica microsphere, by treatment with HCl. Such materials are capable to be used in
various applications e.g. extraction and removal of toxic heavy metals and toxic dyes from
water. They also could be used for drug delivery and as sensors for CO2 gas.
In the case of ZnO nanoparticls, it was confirmed from XRD patterns and TEM results that
ZnO particles have a hexagonal shape and their size do not alter by the coating with silica or
mesosilica. No dispesant agent was used and only ultrasound sonication was used followed
by thes sol-gel silica coating process. The ZnO particles were poly dispersed into the silica
and meso microsphere silica shells. The meso sphere are easily functionalized by amine or
thiol coupling agent where a very thin shell of the functionalized was observed. FTIR and
TGA and 13
C NMRmethods have confirmed that these silane coupling agent were
successfully introduced to the mesosilica shell coated metal oxide. TEM show clearly that
these ZnO nanomaterials were coated by silica, mesosilica and functionalized silane shells of
different thickness. These nanoparticles can be easily removed upon treatment them with
HCl leaving a very porous materials.
78
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