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Ceria Nanoparticle Hybrid Materials:

Interfacial Design and Structure Control

Eric Johansson Salazar-Sandoval

Doctoral Thesis

Kungliga Tekniska Högskolan, Stockholm 2015

AKADEMISK AVHANDLING

som med tillstånd av Kungliga Tekniska Högskolan i Stockholm

framläggs till offentlig granskning för avläggande av teknisk

doktorsexam fredagen den 18 september 2015, kl. 10:00 i sal

Kollegiesalen, Brinellvägen 8, KTH, Stockholm. Avhandlingen försvaras

på engelska. Fakultetsopponent: Prof. Dr. Wolfgang Bremser, Universität

Paderborn, Tyskland.

ii

Principal supervisor: Prof. Mats Johansson (KTH)

Supervisor: Dr. Anwar Ahniyaz (SP)

Copyright © 2015 Eric Johansson Salazar-Sandoval

All rights reserved

Paper I © 2014 Royal Society of Chemistry

Paper III © 2014 Royal Society of Chemistry

Paper III © 2014 Royal Society of Chemistry

Paper IV © 2015 N/A

Paper V © 2015 N/A

TRITA-CHE 2015:46

ISSN 1654-1081

ISBN 978-91-7595-674-9

iii

Abstract

This doctoral thesis addresses the challenge of bringing two very different

materials into intimate chemical contact: inorganic metal oxide nanoparticles

and acrylic polymers. In order to achieve this ambitious goal, the work has

been divided into a series of more accessible tasks. Pedagogically designed,

these tasks build upon one another to finally develop the knowledge and

skills necessary to successfully formulate novel nanocomposites.

A fundamental study on the bulk and surface bonding of ceria was carried

out to show that, due to the ceria content in small and highly charged ions,

which are difficult to polarize, the preferred chemical interactions are ionic.

Among the different capping agents, the carboxylate ligands —through the

rich and localized electron density of their oxygen atoms— formed an ionic

bond with cerium oxides. This provided stability to the ceria nanoparticles

and opened up a vast robust and versatile library of carboxylates to us. This

is exemplified by the development of synthetic routes for understanding and

modifying ceria nanoparticles with carboxylic acids carrying reactive

moieties, which were used to extend the stability of the nanoparticle

dispersions. This allowed us to perform in situ polymerization, which

resulted in homogeneous ceria–polymer hybrid nanocomposites. This

interfacial design offers not only structure control but also strong bonding

between the covalent polymer network and the ionic nanocrystals.

The focus of the present work, however, is not on characterization of the

polymeric materials used but rather on how the embedded nanoparticles

interact with the polymeric matrix with respect to chemical interfacial

aspects. The following cases were studied: i) unreactive nanoceria dispersed

in a polymer matrix; ii) dispersed nanoceria endowed with the ability to

initiate polymerizations; and iii) dispersed nanoceria capable of

copolymerizing with the propagating chains of the polymer.

These processes led to the development of novel hybrid nanocomposites that

preserved the optical properties of ceria (e.g., UV absorption) while

enhancing mechanical properties such as stiffness and glass transition

temperature.

iv

Sammanfattning

Denna doktorsavhandling angriper utmaningen att skapa en nära kemisk

kontakt mellan två mycket olika material: oorganiska

metalloxidnanopartiklar och akrylatpolymerer. För att uppnå detta mål

delades arbetet upp i en serie mer hanterbara deluppgifter, där kunskap byggs

upp efter hand för att till slut nå den nivå som krävs för att lyckas formulera

nya nanokompositer.

En fundamental studie av bindningsförmågan hos cerium i bulk och på en yta

genomfördes som visade att på grund av ceriuminnehållet i små och laddade

joner, som är svåra att polarisera, är de föredragna interaktionerna joniska.

Bland de olika ”inkapslingsföreningarna” bildade karboxylatliganderna,

genom den rika och lokaliserade elektrondensiteten hos sina syreatomer, en

jonisk bindning med ceriumoxider. Detta gav stabilitet till

ceriumnanopartiklarna och öppnade upp möjligheter att utnyttja robustheten

och variationer av det stora antalet karboxylater som finns tillgängliga. Dessa

är exemplifierade i påföljande studier av syntetiska vägar mot förståelse och

modifiering av ceriumnanopariklar med reaktiva karboxylsyror, vilka

användes för att utöka stabiliteten av nanopartikeldispersionerna. De reaktiva

karboxylaterna kunde användas för en in situ polymerisation som resulterade

i homogena cerium-polymer hybrid nanokompositer. Denna

gränsskiktsdesign erbjuder inte bara strukturkontroll men även stark

bindning mellan kovalenta polymernätverk och de joniska nanokristallerna.

Fokus i detta arbete är dock inte karakterisering av de polymera material som

använts utan snarare hur de inbäddade nanopartiklarna interagerar med den

polymera matrisen, med avseende på ”ytkemiska” aspekter. Följande fall

studerades: i) oreagerad nanoceria dispergerad i en polymermatris; ii)

dispergerad nanoceria med förmågan att initiera polymerisationer och iii)

dispergerad nanoceria med förmågan att sampolymerisera med de

propagerande kedjorna av polymeren.

Dessa processer ledde till nya hybridnanokompositer som bevarade de

optiska egenskaperna hos cerium, t.ex. UV absorption, och visade förbättring

av de mekaniska egenskaperna, så som styvhet och glastransitionstemperatur.

v

List of appended papers

Paper I

Aminopolycarboxylic acids as a versatile tool to stabilize ceria

nanoparticles – a fundamental model experimentally demonstrated. Eric

Johansson Salazar-Sandoval, Mats K. G. Johansson, Anwar Ahniyaz.

RSC Advances, 2014, 4, 9048-9055 – DOI: 10.1039/C4RA09044F

Paper II

Radical Initiator Modified-Cerium Oxide Nanoparticles for Polymer

Encapsulation via Grafting From the Surface. Eric Johansson Salazar-

Sandoval, Miren Aguirre, María Paulis, José Ramón Leiza, Mats

Johansson, Anwar Ahniyaz. RSC Advances, 2014, 4, 61863-61868 –

DOI: 10.1039/C4RA09044F

Paper III

Hybrid acrylic/CeO2 nanocomposites using hydrophilic spherical and

high aspect ratio CeO2 nanoparticles. Miren Aguirre, Eric Johansson

Salazar-Sandoval, Mats Johansson, Anwar Ahniyaz, María Paulis, José

Ramón Leiza. J. Mater. Chem. A, 2014, 2, 20280-20287 – DOI:

10.1039/C4TA03620D

Paper IV

A versatile synthesis route to prepare Ceria nanoparticles with

polymerizable capping ligands. Eric Johansson Salazar-Sandoval,

Niklas Ihrner, Cheuk-Wai Tai, Kenneth Möller, Anwar Ahniyaz, Mats

Johansson. Manuscript

Paper V

Thermodynamically driven ligand exchange on ceria nanoparticle

surfaces. An efficient route to tailor ceria nanostructure properties. Eric

Johansson Salazar-Sandoval, Mats Johansson, Anwar Ahniyaz.

Manuscript

vi

My contribution to the appended papers

Paper I

All the experimental work, characterization and interpretation of results.

The preparation of the manuscript.

Paper II

All the experimental work, characterization and most of the interpretation

of results. Most of the preparation of the manuscript

Paper III

The synthesis and surface modification of the ceria nanoparticles and its

associated analyses and their interpretation. Part of the preparation of the

manuscript.

Paper IV

The synthesis and surface modification of the ceria nanoparticles. The

analysis of it and its interpretation. Part of the preparation of the

manuscript.

Paper V

All the synthetic work, characterization and interpretation. The

preparation of the manuscript.

vii

Scientific contributions not included in this thesis

Aizat Turdalieva, Volodymyr Chmyrov, Eric Johansson Salazar-Sandoval,

Hao Xu, Anwar Ahniyaz, Jerker Widengren, Hjalmar Brismar, Ying Fu,

Multivariable study on the fluorescence of colloidal ZnO nanoparticle

synthesized by sol-gel method at room temperature, To be submitted, 2015

Shanghua Li, Mattias Karlsson, Rongsheng, Liu, Anwar Ahniyaz, Andrea

Fornara, Eric Johansson Salazar-Sandoval, The Effect of Ceria

Nanoparticles on the Breakdown Strength of Transformer Oil, IEEE 11th

International Conference on the Properties and Applications of Dielectric

Materials, 19-22 July, Sydney, Australia, 2015

Bardage, S., Henriksson, M., Olsson, S., Collins, P., Meng, D., Ahniyaz, A.,

Salazar-Sandoval, E.J., Rahier, A., Gasparini, M., Lamproye, N.

Nanoparticles for UV protection of clear wood coatings - field and laboratory

trials. Surface Coatings International, 96, 2; 94-99, 2013

Sandin G, Pilgård A, Peters GM, Svanström M, Ahniyaz A, Fornara A,

Johansson Salazar-Sandoval E, Xu Y, Location dependency of the

sustainability of textile fibres. Avancell Conference (Poster contribution),

Gothenburg, Sweden, 2012

G. A. Sandin, G. Peters, M. Svanström, A. Pilgård, A. Ahniyaz, A. Fornara,

E. Johansson Salazar-Sandoval, Y. Xu, Environmental evaluation of a

clear coating for wood: toxicological testing and life cycle assessment.

PRA's 8th International Wood Coatings Congress (conference proceeding

paper), 30-31 Oct., Amsterdam, the Netherlands 2012

ix

Abbreviations

1D mono-dimensional

2D 2-dimensional

3D 3-dimensional

ACVA 4,4’-azobis(4-cyanovaleric acid)

ACVA-Ce cerous 4,4’-azobis(4-cyanopentanoate)

ATR Attenuated total reflection

ca. circa (Latin); “approximately”, “around”, “about”

CCD charge-coupled device

CMOS complementary metal–oxide semiconductor

CMP chemical mechanical polishing

CWO catalytic wet oxidation

DLS dynamic light scattering

DNA deoxyribonucleic acid

DTGS deuterated tryglycine sulfate

e.g. exempli gratia (Latin); “for example”

EDTA ethylenediaminetetraacetic acid

EELS electron energy loss spectroscopy

eq. equation

E.S.I Electronic Supporting Information

et al. et alia (Latin); “and others”

etc. et cetera (Latin); “and so on”

FCC fuel catalytic cracking

FEG field emission gun

Fig. figure

FT-IR Fourier transform infra-red

FTIR see FT-IR above

H3NTA same as NTA, with emphasis that it is fully protonated form

HEMA 2-hydroxyethyl methacrylate

i.e. id est (Latin); “that is more precisely”, “that is”

ITO indium-tin oxide

IR infra-red

x

Ka acidity constant

Keq equilibrium constant

KRS-5 thallium bromoiodide

KTH Kungliga Tekniska Högskolan

MIR Mid infra-red

Mn number average molecular weight

Na3NTA sodium nitrilotriacetate

NTA nitrolotriacetic acid

NTA-Ce cerous nitrolotriacetate

OSC oxygen storage capacity

PEG-DA polyethylene glycol diacrylate

PEGDA see PEG-DA above

PhD Philosophiae Doctor (Latin); “Doctor of Philosophy”

PXRD powder X-ray diffraction or diffractometer

R.T. room temperature

ROS reactive oxygen species

SEM scanning electron microscope or microscopy

SOFC solid oxide fuel cell

SP SP Sveriges Tekniska Forskningsinstitut

STEM scanning transmission electron microscope or microscopy

TEM transmission electron microscope or microscopy

TGA thermogravimetric analysis

TWC three-way catalyst

UV ultraviolet

UV-Vis ultraviolet-visible spectroscopy or spectrophotometry

UK United Kingdom

vide (Latin); “see”, vide followed by a number refers to a page

VOC volatile organic compound

vs. versus (Latin); “turned against”

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction or diffractometer

xi

Table of content

Abstract ......................................................................................................... iii

Sammanfattning .............................................................................................iv

List of appended papers................................................................................... v

My contribution to the appended papers ........................................................vi

Scientific contributions not included in this thesis ....................................... vii

Abbreviations .................................................................................................ix

Table of content ..............................................................................................xi

Preface ........................................................................................................ xvii

Purpose of the study ........................................................................................ 1

1. Introduction ............................................................................................. 3

1.1 Organic vs. Inorganic ......................................................................... 6

1.1.1 Size of organic and inorganic compounds ................................. 7

1.1.2 The organic/inorganic interface ............................................... 10

1.2 Hybrid nanocomposites .................................................................... 12

1.3 Selecting materials to work with ...................................................... 13

1.3.1 Ceria. Why ceria? .................................................................... 14

1.3.1.1 Occurrence/availability ........................................................ 14

1.3.1.2 Atomic structure of cerium oxides ...................................... 14

1.3.1.3 Surface of cerium oxides ..................................................... 15

1.3.1.4 Versatility of ceria ............................................................... 16

1.3.1.4.1 Synthetic strategies ..................................................... 16

1.3.1.4.2 Morphologies .............................................................. 18

xii

1.3.1.4.3 Applications ............................................................... 20

1.3.2 Acrylic polymers for coating applications .............................. 25

1.3.2.1 Synthesis of acrylic polymers for coating applications ...... 27

1.3.2.1.1 Emulsion and miniemulsion polymerization ............. 27

1.3.2.1.2 100 % solids polymerization...................................... 28

1.4 Need to optimize the interphase towards different surroundings .... 29

1.5 Need to tailor the interface............................................................... 29

1.6 Previous work in ceria nanocomposites ........................................... 30

2. Theoretical background ........................................................................ 33

2.1 Coordination compounds ................................................................. 33

2.1.1 Ligands as capping agents ...................................................... 33

2.1.2 Inner and outer sphere models ............................................... 34

2.1.3 Hydrolysis of cations .............................................................. 35

2.2 Understanding ceria ......................................................................... 37

2.2.1 Oxygen .................................................................................... 37

2.2.2 Cerium .................................................................................... 39

2.2.3 Cerium oxides ......................................................................... 40

2.2.4 Doping of ceria and mixed oxides .......................................... 40

2.2.5 Coordination chemistry of cerium cations .............................. 41

2.2.6 Surface of ceria ....................................................................... 43

2.2.7 Coordination chemistry on ceria surface ................................ 45

2.3 Bond valence method ...................................................................... 47

3. Experimental ......................................................................................... 49

3.1 Materials .......................................................................................... 49

3.2 Characterization techniques ............................................................. 50

3.2.1 FT-IR ...................................................................................... 50

xiii

3.2.2 DLS ......................................................................................... 50

3.2.3 TEM......................................................................................... 51

3.2.4 UV-Vis .................................................................................... 52

3.2.5 TGA ......................................................................................... 52

3.2.6 XRD......................................................................................... 52

3.2.7 PXRD ...................................................................................... 53

3.2.8 XPS .......................................................................................... 53

3.3 Synthetic procedures ........................................................................ 54

Route 1 – Ligand screening ................................................................... 55

3.3.1.1 Route 1A .............................................................................. 55

3.3.1.2 Route 1B .............................................................................. 56

3.3.2 Route 2 – pH screening ........................................................... 57

3.3.2.1 Route 2A .............................................................................. 58

3.3.2.2 Route 2B .............................................................................. 59

3.3.3 Route 3 – Temperature sensitive capping agents .................... 60

3.3.3.1 Route 3A .............................................................................. 61

3.3.3.2 Route 3B .............................................................................. 62

3.3.3.3 Route 3C .............................................................................. 63

3.3.3.4 Route 3D .............................................................................. 64

3.3.4 Route 4 – functional ligands .................................................... 65

3.3.4.1 Route 4A .............................................................................. 65

3.3.4.2 Route 4B .............................................................................. 65

3.3.4.3 Route 4C .............................................................................. 66

3.3.4.4 Route 4D .............................................................................. 66

3.3.4.4.1 Route 4D1 ................................................................... 66

3.3.4.4.2 Route 4D2 ................................................................... 66

xiv

3.3.4.5 Route 4E ............................................................................. 67

3.3.5 Route 5 – polymerization strategies - enabling ....................... 68

3.3.5.1 Route 5A – emulsion polymerization ........................ 69

3.3.5.2 Route 5B – Miniemulsion polymerization ................. 70

3.3.5.3 Route 5C – precipitation polymerization ................... 70

3.3.5.4 Route 5D – Photopolymeriztion ................................ 71

3.3.5.5 Route 5E – Thermal curing ........................................ 71

4. Results and discussion .......................................................................... 73

4.1 Basic principles ................................................................................ 74

4.2 Predictions of the Bond Valence Model .......................................... 84

4.3 Robustness of the carboxylic acid based systems ............................ 88

4.4 Encapsulation of nanoparticles - nanocomposites ........................... 89

5. Conclusions ........................................................................................ 103

6. Final remarks ...................................................................................... 105

7. Future work ........................................................................................ 105

8. Acknowledgements ............................................................................ 107

9. References .......................................................................................... 113

xv

Dedicated to my mother and the

loving memory of my late father

xvii

Preface

While this thesis is generally useful for chemists, it is written for organic

polymer chemists that want to understand how encapsulated inorganic

particles affect polymer matrices, and for inorganic chemists or materials

engineers that want to disperse metal oxide nanoparticles into solvents,

including water, monomers, and polymers.

The subject of this work is the chemical interaction at the interface

between two very different materials. Consequently, this work is aimed at

the interdisciplinary, condensing enough inorganic chemistry and

polymer science to give a comprehensive understanding to: i) the

inorganic chemist (or material scientist), to decide how to synthesize and

modify the surface of a nanoparticle to be dispersed in a solvent or

encapsulated into a polymer matrix; and ii) the organic chemist (or

polymer scientist), to design new surface modifiers or to gain an

understanding of the consequences (positive and negative) of

incorporating nanoparticles into a polymeric material.

A doctor of philosophy (PhD) in chemistry or chemical engineering does

need to develop new and advanced understanding of chemical

interactions or chemical processes. Furthermore, being the doctorate

awarded from this prestigious technical university and the study financed

by SP Technical Research Institute of Sweden, the applicability and

industrial relevance of the results were always encouraged.

Stockholm, August 2015

E. J. S-S.

Purpose of the study | 1

Purpose of the study

In order to improve or design new composite materials, it is important to

understand the interaction governing the compatibility of the

components; more specifically, the dispersibility of the filler in the

continuous phase or matrix. Thus, the aim of this doctoral thesis is the

development of a general strategy to tailor the inorganic–organic hybrid

interface in metal oxide-polymer nanocomposites.

The main underlying question is how the bond between the inorganic

surface and the surrounding polymer phase should be designed to obtain

colloidal stability (i.e., a good dispersion with minimum agglomeration)

when formulating nanocomposite systems. This can be divided into

specific sub-questions such as: What chemical structures should the

surface ligands have? Should they be of an inorganic nature like the

nanoparticle or should they be organic like the surrounding matrix? Does

the size of the ligand matter, that is, does it need to be a macromolecule

or would a simple and small molecule suffice? What chemical or physical

interaction should exist between the nanoparticle and the surface ligand,

and what sort of interaction should exist between the ligand and the

polymer matrix? Would the size or morphology of the nanoparticles

affect the design of the interface and interphase?

The intentions of this work were: i) to develop the skills to synthetize

metal oxide nanoparticles, with full control over the synthetic variables;

and ii) to gain enough understanding to describe the surface of metal

oxides in order to discern which capping agents should result in stable

surface modifications. Furthermore, the intention was to unveil, in as

2 | Purpose of the study

much detail as possible, the chemical or physical interactions between the

two. With this information, chemists will be able to select or even design

suitable capping agents that result in a good dispersion of the

nanoparticles, resulting in worthy encapsulation of the metal oxide

nanoparticles in the polymer matrix. The term “worthy encapsulation”

means that the incorporation of nanoparticles does not compromise the

positive properties of the matrix, and the load of nanoparticles can be

increased to result in a significant change. This thesis would accomplish

its goal if it leads to the production of nanocomposites of non-aggregated

and homogeneously distributed nanoparticles.

Introduction | 3

1. Introduction

Humanity has always progressed stepwise by extending our control over

new materials and their transformation. Starting from generating the

spark that could light a fire or selecting stones that could be cut in desired

orientations, to forging metal blends; all contributed, not only to the

advance of civilization and improvement of living standards, but to the

rise of chemistry; the science that studies the properties, composition,

structure and transformation of matter.

There is interest in optimizing the consumption of both energy and

material resources. For instance, reducing the weight of structural

materials for the automotive industry, without compromising the

mechanical performance, has been pursued to reduce fuel consumption.

For example, the reduction in size, i.e. amount of material needed, for a

structural application usually requires to enhance the mechanical

properties (strength–wise, for instance). In addition, sustainability

comprises not only the optimization in manufacturing processes of

materials in terms of energy and yield, but also ensures biodegradability

or recyclability while extending the service life in reasonable terms.

All this collective needs are translated into scientific and engineering

challenges. And many interdisciplinary fields are rising to contest those

challenges with the creation of novel composite materials that not only

bring together the properties of their individual components but are also

capable of showing new, sometimes unexpected properties. A composite

material is thus, a multicomponent system where at least one material

(usually referred as phase) is dispersed in a different continuous phase. In

4 | Introduction

most cases, the origin of the new properties resides in the intimate contact

between the components in the composite material, because it forces new

chemistries to occur and in consequence, new features or responses

appear. In order to enhance the number of these interactions, the

dispersed phase is reduced to the nanoscale (1 000 000 to 10 000 times

smaller than a millimeter). When a material is reduced to the nanoscale

the relative surface area increases tremendously per unit weight, thus

explaining the increase in number of chemical interactions at the

interface. However, the increase in surface area is not the only effect of

such size reduction. Due to the confinement of the electronic interatomic

interactions in the small particle volume, new physical properties can

arise. Additionally, these very reactive surfaces, rearrange the surface

atoms to minimize the energy of the system, commonly referred as

surface relaxation. This new atomic arrangement can be unique in

nanostructures and because chemical structures are the ultimate origin for

physical properties, new properties can be manifested.

Many are the reasons for the design of new, better performing and/or

more sustainable materials. It could very well be overcoming the

challenges that sharing the limited resources of a planet with 7 billion

(109) people represent; or be the survival of a business or a local economy

in a competitive world where outperforming others is more than essential.

For many years, the chemist’s approach to new materials was to create

new molecules, mixed oxides or alloys. This was accompanied by the

development of new synthetic routes and finer characterization

techniques. As promising as it seemed, there has been a major drawback:

synthetic chemistry has been traditionally divided into organic and

Introduction | 5

inorganic chemistry. While some concepts were shared between the two

branches, notably differences kept most chemists on either branch. For

organic chemists, despite concentrating in a smaller number of elements

from the Periodic Table, the possible carbon–carbon bonding patterns is

only limited to the imagination. This was so, that organic chemistry split

in numerous sub-fields that concentrated in specific compounds such as

polymers, macromolecules, biomolecules, etc. During that time,

inorganic chemists were also entertained combining different elements in

different ratios. Similarly, inorganic chemistry progressed and split into

numerous sub-groups such as nuclear chemistry, solid–state or

electrochemistry. Fortunately, some sub-groups of organic chemistry

moved towards inorganic chemistry and vice-versa and fruitful

interdisciplinary fields like bioinorganic or organometallic chemistry

have been intensively researched.

Despite the division between inorganic and organic chemistry, it is not

uncommon that new technological applications demand materials with

properties that cannot be attained by neither organic nor inorganic

materials alone but only in combination, with the so-called hybrid

materials.

Before continuing, it feels necessary to review the difference between

organic and inorganic compounds. The distinction between the two could

be intuitive, but formally, it is far from a straightforward answer. For

pedagogical reasons and better understanding of the following sections in

this thesis, the difference between the two shall be further elaborated.

6 | Introduction

1.1 Organic vs. Inorganic

In the 19th century, Berzelius, in agreement with most of his coetaneous,

divided all compounds in two main categories. Those extracted from

living organisms, such animals and plants, were called organic; and those

isolated from ores or minerals were classified as inorganic (as opposite to

organic and to emphasize they were of inanimate origin). This division,

even inaccurate, is still widespread. To remark the inconsistency, note

that living organisms such humans synthesize the mineral hydroxyapatite

in the tooth enamel and the frustule in diatom algae is made of silica.

However, both silica and hydroxyapatite are generally accepted as

inorganic compounds. Also opposing to this widespread division was

Wöhler’s synthesis of urea from the inorganic reactants ammonium

cyanate and ammonia reported as early as in 1828. Moreover, Chemistry

has evolved to a point where we can synthesize new compounds and

materials from their elements and the resulting compounds are no longer

of animate or inanimate origin. These man-made compounds are

generally termed synthetic. Whether a synthetic compound is organic or

inorganic should be decided depending on to which group it resembles

the most, when considering its physical properties and reactivity.

A number of reaction mechanisms are typical of organic compounds;

likewise, among inorganic compounds there are trends or similarities in

their reactivity. Another differentiating aspect is the predominance of one

bonding character: covalent, metallic or ionic. It is widely accepted that

organic compounds are based on hydrocarbon skeletons, with our without

heteroatoms such as oxygen, nitrogen, sulfur and phosphorous. Regarding

its main bond type, organic molecules generally consist of well-oriented

Introduction | 7

covalent bonds. The concatenation of atoms through covalent bonds

results in compounds that can be very complex. Inorganic compounds, on

the contrary, are usually tridimensional arrays of only few elements

combined together, where the bond can be covalent (e.g. diamond, silicon

dioxide) but it is far more frequent to be either metallic or ionic. For

combinations of elements of different electronegativity the structure is

held together by means of ionic interactions. Due to the ubiquity of

oxygen, the most frequent case is one or few different metal cations

combined with the oxygen anion.

1.1.1 Size of organic and inorganic compounds

Organic compounds, due to their oriented covalent bond form discrete

entities whose dimension is generally well defined and it extends for as

long as the covalent sequence of atoms does. Such an entity is known as

molecule. The physical and chemical properties of the molecule depend

on its atomic structure and thus breaking an organic molecule into two

will necessarily alter either its chemical or physical properties, or

sometimes both simultaneously.

The intermolecular interactions define the melting and boiling points. For

small molecules, the number of intermolecular interactions in solid state

is limited and raising the temperature usually breaks enough of those

interactions to freeing the molecules to their gaseous state. An

intermediate state between solid and gas is the liquid state, where the

intermolecular interactions are only weaken but not vanished.

8 | Introduction

Inorganic compounds can be either molecules (e.g. mineral carbonates,

nitrate salts) or tridimensional arrays (e.g. rock salt). This study focuses

on those inorganic compounds where atoms are bonded through metallic

or ionic bonding. Carbon based materials, combine covalently in three

different geometries, linear, planar or tridimensional depending on the

hybridization of the orbitals (sp, sp2 or sp

3). All these geometries can be

found in a single molecule. Differently to the covalent bond, which is

based on well-defined and oriented molecular orbitals; the ionic or

metallic bonds are not based on oriented orbitals and bonding involves all

spatial directions. In consequence, compounds based on ionic, metallic

(or sp3 geometries of covalent compounds) do not exist as molecules but

as three-dimensional arrays or networks that achieve much larger sizes

than molecules. The inorganic compounds relevant to the present study

belong to this category.

This size difference between compounds can be relevant when the

species react. If a small molecule loses an atom, the chemical and

physical properties would be affected. However, for large inorganic

tridimensional arrays, losing an atom, or even splitting into two fractions

would not compromise its chemical properties. The only exception occurs

when the size is reduced to the nanoscale and reducing the volume would

increase the surface to bulk ratio to a point where the large number of

surface interactions outweighs the physical and chemical properties

arising from the bulk.

Because materials are more reactive on the surface (and Nature tends to

minimize reactivity) inorganic compounds tend to grow in order to

minimize the surface area. This leads to phenomena like crystal growth,

Introduction | 9

particle aggregation and adsorption of molecules on the surface. This

tendency to grow means that, in general, inorganic ionic compounds

reach very large sizes —compared to those of molecules— and therefore

form solids. Because in the bulk, all interactions are strong intramolecular

and lack the weak intermolecular, their melting temperatures are high.

Remarkably, for some of the very complex organic molecules such as

DNA, it is possible to describe the complete sequence of atoms with high

accuracy. However, for inorganic compounds that do not exist as

molecules but as tridimensional arrays (either amorphous or crystalline),

describing all atoms sequentially results impractical, if not impossible,

due to difficulties in determining all defects or identifying the entire

assortment of species adsorbed on the surface. However, for amorphous

materials the ratio between the different atoms, what chemists know as

the molecular formula can be easily depicted and for ordered materials

such as crystals, the unit cell is (when neglecting the defects) sufficient to

describe the repeating pattern of atoms in the bulk.

Organic compounds can be very complex in terms of elemental

composition and structure. Organic molecules can form linear or

branched chains and even networks. The complexity does not necessarily

compromise stability. For inorganic materials, however, the

tridimensional arrays are relatively simple, and symmetries are limited in

number (up to 219) and generally involve from one (pure metals) to only

a handful number elements. Ideally, the tridimensional arrangement of

atoms packs to minimize the energy by optimizing the number and

distance of neighboring atoms according to their size and charge, if any.

When the network consists of positively and negatively charged atoms

10 | Introduction

(known as cations and anions, respectively) the most favorable

arrangement results from alternating the positive and negative charges. If

the inorganic structure consists of many different atoms in terms of size

or charge, a positive-negative charge sequence is difficult to maintain and

the compound is generally unstable, hence more reactive. This has been

exploited in the design of new materials where the introduction of only a

fraction of new atoms of a different element in a known structure has led

to a completely new behavior. For instance, introducing impurities, such

as boron or arsenic in semiconductors like silicon has led to a fine tuning

of the semiconducting behavior and to a revolution that has transform our

society: microelectronics

1.1.2 The organic/inorganic interface

The backbone of organic molecules is generally a repetition of carbon-

carbon single or multiple bonds. The remaining valence of each carbon

atom is saturated by bonding single-valence hydrogen atoms. This

hydrogenated “surface” prevents the molecule from reacting with the

surrounding environment. In case two neighboring carbons form a

multiple (double or triple) bond, the molecule exposes reactive electron

density in the form of π–bonds. To highlight the existence of this reactive

center, a multiple bond is also known as an unsaturation (more precisely,

hydrogen unsaturation). When a heteroatom (O, N, S or P) is intercalated

in the carbon–carbon sequence, the heteroatom exposes its electron pairs

and reactivity increases at those points in the sequence. Additionally,

these dangling electron pairs are likely to participate in intermolecular

electrostatic interactions such as hydrogen bonds that associate molecules

Introduction | 11

together (raising the melting and boiling points, for instance) or

participate in intramolecular hydrogen bonds that result in particular

conformations (structures) of crucial–to–life molecules such as proteins

or DNA.

For inorganic solids, the repeating pattern is necessarily disrupted at the

surface and the outermost layer of atoms has a considerable amount of

unsaturated valence. All these dangling bonds explain the reactivity of

their surface, and offer a good explanation for the commonly found

adsorption of organic molecules that hinder the surface reactivity. When

talking about nanoparticles, these adsorbed molecules are usually referred

to as capping agents. Capping agents can be selected to not only stop the

particle’s surface from reacting, but they can also introduce heteroatoms

or unsaturations that through intermolecular electrostatic interactions help

dispersing the modified nanoparticles into a solvent or matrix of choice.

Further, the capping agent can be “loaded” with reactive groups

(e.g. insaturations) that open the door to achieve stronger interactions

(i.e. chemical bonds) with the surrounding matrix.

The interactions between inorganic model surfaces and simple organic

molecules have been, and still are, subject of study for the surface

chemists and surface physicists. However, the scope of this thesis is not

to approach the problem from a surface chemistry perspective, but to first

describe the surface of metal oxides. Secondly, recognize suitable

capping agents that interact with the surface of metal oxides and allow

dispersing metal oxide nanoparticles in a given environment (solvent).

Thirdly, find capping agents that can form both strong bonds with the

metal oxide and with a hosting polymeric matrix. Finally, describe the

12 | Introduction

alterations in the mechanical properties of the resulting nanocomposites

compared to the unloaded polymer.

1.2 Hybrid nanocomposites

How inorganic materials can be classified has been already defined in the

Introduction. For the purpose of this work, inorganic materials are the

metal oxide particles, e.g. ceria nanoparticles. Organic phases are the

solvents other than water, the monomers and polymer materials.

Composites are multiphase materials where one phase, called filler, is

dispersed in a second continuous phase, called matrix. At macroscale

level, they are prepared by physical mixing procedures where different

techniques yield different degrees of homogeneity. In composite

materials, one expects to combine the individual properties of the filler

and matrix.

Nanocomposites are a subclass of composites where the dispersed phase

is a nanomaterial. In nanocomposites, not only the individual properties

of filler and matrix shall remain, but since the interface of the dispersed

and continuous phase increases when the particle size of the dispersed

constituent decreases, in nanocomposites (provided that the nanoparticles

are well dispersed, i.e. not aggregated), the interface is maximized. Thus

new properties can arise as a result of the chemical interactions at the

boundaries of the two phases.

Inorganic–organic hybrid materials, herein referred simply as hybrid

materials are those which comprise an organic and inorganic phase.

Introduction | 13

Therefore a hybrid nanocomposite would consist of an inorganic

nanoparticle in an organic continuous phase (e.g. nanoclays in acrylic

polymers).1 The interface in a hybrid nanocomposite is the surface

created where the inorganic phase meets the organic phase.

1.3 Selecting materials to work with

Regarding the experimental investigations, the present study focuses on

cerium oxides, in particular ceria nanoparticles, as the inorganic phase

and to acrylic polymers as the organic phase. In principle there is no

restriction on what capping agents to work with. Since the organic and

inorganic phases are chemically different, one could expect the capping

agents to resemble surfactants. Surfactants are organic molecules with

affinity to two very different systems. They usually comprise a head polar

group (primarily carboxylates, sulfates or phosphates) with a hydrophobic

organic tail (hydrocarbons, esters, amides, etc.). Both the head and tail

can be chosen to solubilize in the targeted environments.

Despite the fact the experimental part focuses on only a reduced set of

materials of industrial relevance, special attention is given to emphasize

the aspects that are common to other metal oxides and polymers. In this

way the achievements presented here can find a broader field of

application.

14 | Introduction

1.3.1 Ceria. Why ceria?

1.3.1.1 Occurrence/availability

The selection of cerium oxide nanoparticles as the inorganic phase for

this study is easily justified by the substantial availability of cerium

(0.0046% of the Earth's crust by weight)2 and the significant industrial

relevance of this material. Cerium is present in the minerals allanite (also

known as orthite), monazite, bastnasite, cerite, and samarskite. The

occurrence is spread around the Earth’s crust. Known deposits of

monazite are located in southern tip of India and in river sands of Brazil.

Allanite can be found in the western United States and deposits of

bastnasite are present in southern California.3

1.3.1.2 Atomic structure of cerium oxides

The arrangement of atoms in a crystalline structure is based on the

number of neighboring atoms, which depends on the nature of the

electron shell (the oxidation number) of the atoms forming the

compound; and their relative sizes, which depend on the atomic number

of the elements involved. Further details on the atomic structure are given

in the Theoretical background section below, but for the purpose of

understanding the origin of the applications that follow, it is enough to

understand that cerium oxides consists of divalent oxygen anions

combined with trivalent, tetravalent or a combination of tri- and

tetravalent cerium cations to form, respectively: Ce2O3, CeO2 or CeO2-x

(0 < x < 0.5).4 The proximity of the energies of the orbitals O(2p) and

Ce(4f) results in a charge transfer band, of ca. 3.2 eV, that confers

Introduction | 15

semiconducting properties and light absorption in the ultraviolet range,

remarkably the absorption at 400 nm is the strongest for any oxide.5 The

refractive index (2.1 – 2.2) and the fluorescence properties are thus

consequence of the particular arrangement of atoms in cerium oxide

structures.6

1.3.1.3 Surface of cerium oxides

Cerium is a rare earth element of the lanthanide sub-group. All rare earth

elements are similar in their chemistry, however a particularity of cerium

is that the energy of the inner 4f level is sufficiently similar to the 5d

valence orbital and thus cerium is able promote a fourth electron to the

valence shell.7 When combined to very electronegative elements like

fluorine or oxygen, the tetravalent oxidation state can be stabilized.8 For

cerium oxide structures, the change in valence from Ce4+

to Ce3+

is

associated with the formation of an oxygen vacancy, accompanied by a

change in coordination number and a slight lattice expansion.9-12

This is a

reversible process that is associated with an intake or release of oxygen,

particularly on the surface, depending on the partial pressure of oxygen to

which the cerium oxide is exposed.13, 14

This singularity can be exploited

in many different fields, hence the vast number of applications.

16 | Introduction

1.3.1.4 Versatility of ceria

Not only cerium is adaptable in terms of adopting a valence to best fit the

surrounding environment, cerium oxide is versatile in many aspects. The

size of ceria particles can easily be reduced to the nanoscale, i.e. ceria

nanoparticles (also known as nanoceria). Employing different synthetic

techniques one can also produce various morphologies, such as spherical

nanoparticles; nanorods of assorted widths and lengths; octahedral

nanoparticles; or even films with thicknesses in the nanoscale. All this

versatility when making ceria nanostructures results in particles of

different sizes, shapes and degree of crystallinity (the content of atomic

defects) that are exploited in many different applications.

1.3.1.4.1 Synthetic strategies

There are many ways to prepare ceria nanoparticles. A variety of

precursors such as cerium hydroxide, chloride, nitrate, carbonate or

acetate can be converted into ceria in a wide range of temperatures,

pressures and in a selection of solvents or atmospheres, to name only a

few of the possible variables. To highlight the versatility of nanoceria

preparation, a compilation of the most popular ones is found in Table I.

Introduction | 17

TABLE I

Method for ceria synthesis REFs

Direct precipitation at room temperature 15-20

Homogeneous precipitation 21, 22

Precipitation + calcination 18, 23-30

Reverse precipitation + calcination 31, 32

Precipitation + hydrothermal 120°C 33

Precipitation + hydrothermal 180°C 34

Thermal decomposition 20, 35

Urea decomposition 36

Flash combustion 37

Spray pyrolysis 38, 39

Mist pyrolysis 40

Electrochemical synthesis 41, 42

Hydrothermal synthesis 43-49

Hydrothermal synthesis in near critical-H2O and supercritical-H2O 19, 50-55

Hydrothermal synthesis in near critical- and supercritical-alcohols 56

Sol-gel deposition 57, 58

Non-hydrolytic condensation 59-61

Reverse micelle sol-gel technique 62, 63

Reverse precipitation (gel-to-sol) 64

Microwave – ionic liquids 65

MOCVD* 66, 67

EBE* 68, 69

EBE-IBAD* 6, 68

Laser ablation deposition* 70

Laser-pulsed ablation deposition* 71-73

Low energy dual ion beam* 74

*for production of thin films with thickness in the nanoscale

18 | Introduction

1.3.1.4.2 Morphologies

The morphology can be critical depending on the intended use of the

nanoceria. Since different lattice planes expose the different type of

atoms in variable levels, certain applications (e.g. OSC) are highly

dependent on the morphology of the nanoparticles.49, 75

Fortunately, ceria

nanoparticles can be obtained in different shapes, ranging from spheres,76

cubes,46, 77

octahedrons,45, 77

rods78-80

and platelets36, 81

to name a few.

Figure 1 highlights different ceria morphologies; some found in the

literature, others obtained during the course of this study. There are two

different synthetic strategies for controlling the shape of the ceria

nanoparticles. One alternative consists on preparing a precursor of a

given shape and maintain that particular morphology during the

conversion to cerium oxide.81

Alternatively, one could hinder the growth

of certain lattice plains, which would lead to anisotropic growth.78

Many

are the parameters influencing the morphology of either the precursors or

the actual ceria nanoparticles: temperature, pressure, solvent, counter-

ions, pH value, ionic strength, solvent and so on.

Figure 1. From left to right: Ceria octahedrons; ceria octahedrons and nanorods and ceria

nanorods, from Ref. 45; ceria platelets, from Ref. 81.

Introduction | 19

An alternative approach to creative morphologies is the aggregation of

small nanoparticles in clusters. This could lead, for instance, to hollow

spheres82

or raspberry–like aggregates.44

Templating using silicates or

polymer particles has also resulted in ceria structures and patterns based

on the morphology of the material used as template. Perhaps the most

interesting cases are those where the templating material can be

subsequently removed leaving only cerium oxide behind. Figure 2 shows

examples of these.

Figure 2. Top-left: Raspberry-like ceria clusters. From Ref. 44. Top-right: Ceria pattern after

removal of PMMA template, from Ref. 83. Bottom-left: Hollow ceria spheres, from REF 82.

Bottom-right: Ceria nanotubes, from Ref. 84

20 | Introduction

1.3.1.4.3 Applications

Cerium oxide finds application in many everyday products such as the

active component in the Three-Way catalyst (TWC),85

which is one of the

alternatives (mainly for gasoline–driven engines) for the mandatory

emission control device mounted in the exhaust of automobiles to reduce

the release of pollutants. For diesel engines, one alternative is to add ceria

nanoparticles as fuel additive.86

Its catalytic activity is also exploited in

self-cleaning ovens to fully oxidize the cooking residues (hydrocarbons)

that stick to the walls and glasses in the oven.87

Due to the UV absorbing

features of cerium oxides, combined with the invisibility attained at the

nanoparticle level, nanoceria is currently being introduced as UV–

screening additive in sunscreens and cosmetic.88

A more comprehensive

list of the current and prospective applications of cerium oxides are

briefly mentioned below.

1.3.1.4.3.1 Biomedical

To neutralize reactive oxygen species (ROS), and thus minimize the

effects of associated diseases, the radical scavenger properties of cerium

oxides have attracted attention for biomedical applications.89, 90

1.3.1.4.3.2 Use as polishing agents

The outstanding hardness of CeO2 has found application, through the

method known as chemical mechanical planarization (CMP), in polishing

glasses or removing silicon dioxide films.91, 92

Introduction | 21

1.3.1.4.3.3 Uses as catalysts

The oxygen storage capacity (OSC) of ceria14

translates into a catalytic

activity that is exploited in the oxidation of toxic pollutants such as CO

and hydrocarbons to carbon dioxide.93, 94

This is widely found in the

TWC process92,95

. Industrially, denitrification (de-NOx) and

desulfurification (de-SOx), which are the conversion of NOx and SOx

gases to NO2 and SO3, respectively, are important in the removal of such

contaminants from the flue gases in power plants.96, 97

Cerium oxides also

assist in the de-SOx process in the fuel catalytic cracking (FCC) in oil

refineries.85

FCC is one of the most important processes to convert the

rich in hydrocarbon fraction of crude oils into valuable gasoline and

olefinic gases. Catalytic cracking yields more gasoline and of higher

octanes compared to the now into abeyance thermal cracking.98

The catalytic activity of ceria is continuously investigated and it is

anticipated that cerium-based oxides such as ceria–zirconia contribute in

facilitating the reduction of the emissions of the very polluting volatile

organic compounds (VOCs) that several industries produce as

by-products and have to be properly processed before being released to

the atmosphere.98, 99

Ceria is not only found as catalyst for gases, the catalytic wet oxidation

(CWO) of manganese–cerium oxide (CexMn1-xO2-y) has been investigated

for the treatment of wastewater carrying toxic pollutants.85, 100, 101

As recoverable oxidizing agent, cerium oxides have proven the direct

conversion of methane into synthesis gas with an H2/CO2 ratio of 2.102

22 | Introduction

Cerium oxide is also employed as catalyst in the production of chemicals.

Thus, ceria can be used as catalyst in the synthesis of ethyl carbonate

from ethanol, carbon dioxide and butylene.103

CeO2–based catalysts have

shown selectivity for the different oxidation, dehydrogenation,

hydrogenolysis and transesterification routes of glycerol.104

Ceria is

commonly found in the catalyst for the dehydrogenation of ethylbenzene

to yield styrene.85

However, from a research perspective, the catalytic

application that nowadays attracts the most attention is the utilization of

cerium–based oxides in the electrodes of solid-oxide fuel cells (SOFC).

As early as 1988, due to the high ionic conductivity of (CeO2)0.9(Y2O3)0.1

ceria–based oxides were proposed as effective electrolyte in SOFC.105

The high ionic conductivity allowed lower temperatures of operation.

However, the limitation was the increased in electric conductivity when

the surface of the oxide was poisoned with hydrogen. Despite this

drawback, ceria has still be found useful within this field of application

and the current trend is to employ cerium–based oxides as the anode in

SOFC.97, 106

In 1999 ceria was proposed as anode for the electrochemical

oxidation of methane in a SOFC.107

In 2000, the direct conversion of

simple hydrocarbons was postulated as an alternative to hydrogen–based

fuel cells.108

Later, in 2002, the anode was improved by adding ceria into

a copper cermet.109

To reduce the amount of ceria needed in SOFC

applications, thin films of ceria were proposed as anodes for SOFC in

2005.110

These catalytic features of ceria are consequence of its OSC, which

resides on the ability of ceria to reversibly create oxygen vacancies on the

metal oxide surface that, in turn, are possible because of cerium ability

Introduction | 23

(in cerium oxides) to shift from trivalent to tetravalent oxidation states

depending on the partial pressure of oxygen to which cerium oxide is

exposed to.

1.3.1.4.3.4 Uses in gas sensors

This reversible valence change is also exploited in the design of fast and

long-term reliable gas sensors. Using mist pyrolysis, thin films of only

100 nm consisting of even smaller nanoparticles have been grown to

develop oxygen and carbon monoxide sensors. It has been demonstrated

that the time response of the sensor in proportional to the nanoparticle

size and that it could be controlled by adjusting the firing temperature

during synthesis.40, 111, 112

In addition, ceria can be combined with other

metals to result, for instance, in a Pt/Ce0.8Gd0.2O1.9/Au sensor that

responds linearly to carbon monoxide concentration.113

1.3.1.4.3.5 Corrosion protection

The reversible Ce4+ + e− ⇌ Ce3+ redox reaction that cerium can

undergo in cerium oxides can be exploited in corrosion protection

applications. The cathodic electrolytic deposition of ceria nanoparticles as

conversion coatings, as thin as 100 nm, are also successfully employed in

the corrosion protection of steel and aluminum alloys,114-116

Cerium

oxides nanoparticles, as small as 5 nm, deposited on steel alloys have

been proven successful in preventing oxidation even at high

temperatures.58

24 | Introduction

1.3.1.4.3.6 Use in microelectronics

Growing CeO2 films epitaxially on Si substrate allows tailoring the

refractive index (n) and dielectric constant (k) as the thickness is

increased from 100 to 500 nm. The resulting ceria film has been proposed

as ultra-thin gate oxide in CMOS technology.68

In addition, growing

high–k ceria films epitaxially on Si(111) substrate could suppress gate

leakage current. This facilitates miniaturization of microelectronic

components for next generation electronics.117

It has been demonstrated

that ceria can be grown epitaxially not only on Si(111),70, 73, 74

but also on

Si(100),66

Ge(001)72

and on InP(001);71

fact that broadens the potential

applications in the field of microelectronics.

1.3.1.4.3.7 Use in solar cells

CeO2 grown epitaxially on Si substrate acts as buffer layer if

subsequently, tin–doped indium oxide (ITO) is grown on ceria. The

thickness of the ceria buffer layer can be as thin as 3 nm. The resulting

system finds applicability in solar cells.118

1.3.1.4.3.8 Applications as UV-absorbing additives

One particular application that motivates most of the present study is the

incorporation of cerium oxide nanoparticles as UV absorbing additives in

clear coatings. Ceria is a well stablished and understood UV absorbing

material.5, 23, 65, 73, 119, 120

Introduction | 25

Incorporating UV–screening agents such as cerium oxide prevents

radiation damage of the substrate in outdoor applications. Two might

seem the most promising alternatives, either used in cosmetics and

sunscreens81, 121, 122

or used as additives in varnishes to produce clear

coatings with enhanced UV protection of the substrate.123-126

Noteworthy,

when UV light is irradiated, ceria is less photoactive than ZnO or TiO2

towards DNA damage.127

1.3.2 Acrylic polymers for coating applications

Polymers are macromolecules that exhibit properties depending on the

specific monomer constituents and how they are connected within the

structure. Similarly to LEGO building blocks, a given number of

monomers concatenate in endless combinations, resulting in polymers of

various structures, such as linear, branched or networks. Polymers in

general have found numerous uses due to good availability, cost, good

material performance and easy processing. Most remarkable is their

ability to be synthesized or later processed into intricate shapes. One

specific group of polymers that have found widespread use is acrylics

derived from different acrylate/methacrylate monomers. Acrylic

polymers are used in numerous applications ranging from engineering

thermoplastics to acrylate resins for organic coatings. In organic coatings

acrylics are either used as a thermoplastic binder e.g. latex, or as a

thermosetting binder e.g. hydroxyfunctional polyacrylates. A third group

is polyfunctional acrylate monomers/oligomers that are polymerized

directly into a thermoset e.g. UV–coatings.128

26 | Introduction

Acrylic coatings for protective and decorative applications are commonly

employed for both industrial and consumer solutions. Among the

industrial-oriented applications, acrylic coatings are frequently found in

architectural coatings and product finishes, as well as for special-purpose

coatings. Consumer finishes based on acrylic polymers range from nail

lacquers to normal exterior house paints.

Acrylic polymers are in many cases preferred for outdoor applications

due to their performance towards hydrolysis and photoinduced

degradation.128

The overall performance of the coating is directly coupled

to the type of resin used and its interaction with other components in the

film e.g. fillers, pigments, and additives.

An understanding of the chemical interactions at the surface of ceria is

thus of great value in the encapsulation of dispersed nanoparticles in a

film-forming polymeric matrix.79, 80, 126, 129, 130

When the polymer is

transparent, this allows for a discreet coating that can embed the

UV absorbing ceria nanoparticles, —which due to their small size are

also non-visible— thus protecting sensitive substrates such as wood or

other UV–sensitive organic based materials. Since most of the UV

damage comes from sunlight and not from artificial illumination,

UV protection is associated with outdoor exposure. Many different

families of polymeric materials show film-forming abilities. However,

consumer varnishes or paints for outdoor exposure are in many cases

based on acrylic latexes because they are easy to apply, offer enough

hydrophobicity to prevent growth of fouling and the glass transition

temperatures can be tuned to allow film forming and withstanding the

typical outdoor environment to which they are exposed.

Introduction | 27

1.3.2.1 Synthesis of acrylic polymers for coating applications

Acrylic binder resins are produced in several different ways depending on

the end use i.e. if it is a waterborne, solvent borne or 100% solids coating.

Two of the more common routes are emulsion, and miniemulsion

polymerization to form waterborne dispersion for exterior applications.

1.3.2.1.1 Emulsion and miniemulsion polymerization

As waterborne paints, they are mainly produced through emulsion (and to

a lesser extent miniemulsion) polymerization processes.131

The monomer

and water phases are stabilized as oil–in–water emulsion with the aid of

anionic surfactants. The addition of water-soluble radical initiators that

decompose upon heating starts polymerizing the fraction of monomers

dissolved in the aqueous phase as consequence of the distribution

equilibrium that ensures constant concentration of the monomer droplets

in the aqueous phase. As soon as the oligomer grows its hydrophobicity is

accentuated and it precipitates. However, the anionic surfactant shall

stabilize it to form stable dispersed polymer particles.

In miniemulsion polymerization132

small monomer droplets are formed

after applying additional energy to a previously formed emulsion. In this

case, the radical initiator can be dissolved in the either of the phases. Its

decomposition will trigger the polymerization that eventually transforms

the monomer droplets into polymer particles.

An advantage of emulsion and miniemulsion polymerization processes is

that the resulting polymer particles have very little gel content and

28 | Introduction

therefore, the particles can coalesce upon drying to yield a homogeneous,

defect free, thin film that can be used in coating applications.

1.3.2.1.2 100 % solids polymerization

For coating applications where an acrylic thermoset is desired, the use of

waterborne formulations is not generally viable. In order to avoid the use

of toxic and pollutant solvents that evaporate during film forming, and

thus generate undesired volatile organic compounds (VOC), the curing of

solvent-free systems is the alternative preferred by the industry. One

common curing method is photopolymerization where the formulations

comprise exclusively the monomer, oligomers or unsaturated polymers;

optionally additives and a photoinitiator. The curing is started by

irradiating with UV wavelengths that cleave the initiator, forming

radicals responsible of initiating the polymerization and propagate chain

growth. This process tends to yield high molecular weight crosslinked

films.133, 134

Despite numerous advantages, this process requires a sufficient degree of

transparency for the UV light to penetrate and cause homogeneous

curing. For this reason, highly pigmented coatings or those with UV

absorbing additives might not always suit this process.

Oxygen inhibition is the most relevant side reaction when curing in open

environments. It causes tackiness or stickiness on the surface. In

photocuring, however, oxygen inhibition can be reduced to acceptable

limits by optimizing the formulation, the irradiating wavelength and

intensity or exposure time.135

Introduction | 29

1.4 Need to optimize the interphase towards

different surroundings

The ultimate goal of this study is to optimize the interphase between the

metal oxide nanoparticle and the matrix where it is dispersed (solvent or

polymeric). This is possible if one first addresses the chemistry at the

interface of the nanoparticle, modifying it in order to transform the

effective interface towards the surrounding. The modification will create

an intermediate layer between the nanoparticle and the dispersing media,

known as interphase.

1.5 Need to tailor the interface

The reduction in size of the inorganic metal oxide to the nano-level,

prospectively introduces a large number of applications and allows for

optimizing existing ones. Bare ceria and nanoceria, without any molecule

adsorbed on the surface, already have numerous applications (e.g. as

catalysts or polishing agents). However, there are cases where the surface

modification of ceria nanoparticles is of paramount importance. The

surface reactivity of bare ceria cannot be contained, and unmodified ceria

nanoparticles will irremediably aggregate as soon as they become in

touch. This reduces the exposed surface area and thus minimizes the

catalytic effect on the surface. As a powder, the mobility of the

nanoparticles is limited and weakly agglomerated particles can be

obtained. However, in solution, nanoparticles move freely and the

number of collisions, which is function of mechanical and thermal

agitation, inevitably leads to the aggregation of unmodified nanoparticles.

30 | Introduction

Therefore, when nanoparticles are to be kept dispersed in a solvent, the

stabilization of the nanoparticles through surface modification is

indispensable. Furthermore, the nanoparticle dispersion might not be the

definitive product, but a necessary step in a more complex preparation.

Even for applications where ceria is desired to be surface-clean, it is not

uncommon to synthesize nanoparticles with a particular surface treatment

that is later conveniently removed by facile processes such as calcination.

1.6 Previous work in ceria nanocomposites

The catalytic and the chemical mechanical polishing properties of ceria

are among the ones that had attracted more interest in the past few

decades, however, there is a current trend to formulate ceria–polymer

nanocomposites to exploit the features of ceria in a broader scope of

applications. Indeed, the number of articles and patents devoted to ceria

nanocomposite preparations has increased during recent years. Many of

the most notable achievements in this field have occurred simultaneously

to this study.

In 2007, Shang et al. incorporated ceria nanoparticles (of 50 to 100 nm)

in concentrations up to 2 wt% into a polyimide (PI) matrix.136

This

resulted in enhancing significantly the mechanical properties, i.e. storage

modulus and Tg, respect to the PI reference. However, the incorporation

of nanoceria, which was physically mixed, diminished the elongation at

break.

Introduction | 31

Parlak et al. reported in 2011, ceria-poly(methyl methacrylate) core shell

particles that were encapsulated in polystyrene at ceria concentrations of

5 wt%. This core-shell approach increased the transparency in the visible

range compared to the encapsulation of nanoceria in polystyrene without

the poly(methyl methacrylate) shell.137

In order to exploit the UV-screening properties of ceria in clear coating

applications, the incorporation of ceria nanoparticles in film-forming

polymer matrices has been sought. In 2012, Saadat-Mofared et al.

introduced ceria nanoparticles into a water-based polyurethane resin.138

With ceria concentrations up to 1.44 wt%, the thin films prepared of this

nanocomposite formulation had enhanced UV-absorbance, especially in

the UV-B region and this ability lasted longer after accelerating

weathering tests. In 2013, Bardage et al. incorporated 2.0 wt%

nano-CeO2 and nano-Fe–CeO2 in water-born acrylic clear coating

formulation and found that resulting clear coating is effective in UV

protection of wood.123

In 2013, Itoh and co-workers incorporated ceria

nanoparticles coated with a poly(vinylpyrrolidone) shell into a

pentaerythriol triacrylate matrix at ceria concentration of 17 wt%.139

Aguirre et al. reported the encapsulation of nanoceria in latex particles

for water-borne clear coating formulations.124

Films with up to 1 wt%

ceria content could be cast without defects and showed enhanced UV-

absorbance.

In 2014, Zhang et al. reported the incorporation of ceria nanoparticles in

concentrations from 10 to 40 vol.% in a polystyrene matrix.140

The

nanocomposites were aimed at dielectric material applications and

showed low dielectric loss and weak frequency dependence.

32 | Introduction

The combination of ceria nanoparticles with polyaniline was shown in

2014 by Ecco et al. to have a positive effect in the anticorrosion

properties of alkyd resins.141

In 2015 Li et al. reported that the

encapsulation of 1.0 wt% nanoceria into water-borne acrylic coating

significantly improved anticorrosion performance for carbon steels.142

This expanded the potential application of nanoceria in water-borne

acrylic coatings.

Theoretical background | 33

2. Theoretical background

2.1 Coordination compounds

A coordination complex is the array of a central metal atom or ion,

known as the coordination center, surrounded by molecules or ions of

non-metal elements (charged or neutral) that are called ligands.

Whenever a ligand forms two or more separate bonds (not be confused

with multiple bonds) with the coordination center, the ligand is said to be

multi-dentate and is called chelating agent. When a chelating agent is

involved in the coordination reaction, the complex is referred as chelate.

It is well documented that chelation is accompanied with an increase in

stability, compared to an equivalent number of coordination bonds from

multiple monodentate ligands;143

and that this gain in stability is an

entropic effect.144-146

In fact, to emphasize that mono-dentate ligands lack

the stability effect of chelation, they are sometimes referred as non-

chelating ligands.

2.1.1 Ligands as capping agents

To synthesize dispersed ceria nanoparticles, one has to still favor

nucleation over growth, and at the same time prevent aggregation. This is

usually achieved using capping agents during their synthesis. Capping

agents are thus molecules, generally organic, that complex the surface of

ceria and prevent other particles from colliding and sinter. The limitation

is that organic molecules cannot withstand high temperatures, where in

the presence of oxygen they burn to CO2. It is for this reason that the

34 | Theoretical background

synthesis of dispersed particles and nanoparticles at moderate

temperatures (those save for the capping agents) has been extensively

studied.15-20

2.1.2 Inner and outer sphere models

When an atom or ion is solvated, the molecules of the solvent surround

the atom in a more or less ordered fashion. In case of aqueous solutions

the water molecules form what are called layers of hydration. If a ligand

complexes a metal (or metal ion) replacing the water molecules in the

first layer of hydration, the coordination compound is defined as inner–

sphere. That is, there is no other molecule between the metal and the

ligand.147

On the contrary, if the metal prefers to be surrounded by water

molecules and the ligand is, loosely associated, in the second or

subsequent layers of hydration, then the complex is called outer–sphere.

Figure 3 shows a graphical representation of these two behaviors.

Figure 3. Representation of an inner–sphere complex (left); and an outer–sphere complex

(right). From Paper I.


2.1.3 Hydrolysis of cations

To understand what it is coming, we need to review the hydrolysis of the

cations. First, H2O is a dipole; this is illustrated in Figure 4. If a metal

cation is exposed to water and water solvates (surrounds) the cation with

layers of water molecules, the positive charge on the metal ion interacts

with the dipole of water molecules resulting, as it is represented on Figure

5, in the orientation of the water molecules in such a way that at least one

lone pair of the oxygen atom of the water molecule points towards the

positive metal ion. The water molecule is now seized by the metal ion

and as the metal–oxygen bond becomes stronger, one of the two oxygen–

hydrogen bonds weakens, and if it finally breaks, a proton is released.

Releasing a proton (H+) changes the pH of the aqueous solution and that

is why metals are said to behave like Brønsted acids. The equilibrium

constant (Keq) for the reaction can be measured by the monitoring the pH

drop. Note the similarity between equation for the hydrolysis of a

hydrated cation (eq. 2) and the equation of ionization of a weak acid in

water (eq. 4). This similarity makes it common to represent the

equilibrium constant for cations with the same notation as for the acids,

that is Ka. The tabulation of different Ka helps us comparing the different

acidity of different metal cations. Ce3+

is a weak acid cation (pKa = 8.3),

but Ce4+

is an extremely strong acid (pKa = -0.8) that is able to hydrolyze

the surrounding water.148


[𝑀(𝐻2𝑂)𝑛]𝑧+ + 𝐻2𝑂 → [𝑀(𝐻2𝑂)𝑛−1(𝑂𝐻)](𝑧−1)+ + 𝐻3𝑂+ eq. 1

𝐾𝑒𝑞 =[𝐻3𝑂+][𝑀(𝐻2𝑂)𝑛−1(𝑂𝐻)](𝑧−1)+

[𝑀(𝐻2𝑂)𝑛]𝑧+ eq. 2

𝐻𝑥𝐴 + 𝐻2𝑂 → (𝐻𝑥−1𝐴)− + 𝐻3𝑂+ eq. 3

𝐾𝑎 =[𝐻3𝑂+][(𝐻𝑥−1𝐴)−]

[𝐻𝑥𝐴] eq. 4

Solvated anions can also hydrolyze and contrary to cations, they behave

like Lewis bases. Following the same reasoning, the negative charge of

the anion will orient surrounding water molecules to “offer” the hydrogen

atoms. If the anion manages to snatch the hydrogen from the water, an

OH- is released and in consequence, the pH of the aqueous solution

increases.

Figure 4. Dipole moment of the H2O molecule.


Figure 5. Hydrolysis of the cation. Adapted from REF 148

2.2 Understanding ceria

Ceria consists of cerium and oxygen only, and as this work aims to bring

into light, understanding the electronic configuration of both elements,

their needs to satisfy the Octet rule and the energy of the resulting bonds

is crucial to understand the requirements of the dangling bonds on the

surface of nanoceria and thus select the optimum capping agents.

2.2.1 Oxygen

Oxygen is the element with atomic number of 8. Oxygen is by mass the

third-most abundant in the universe.

The electronic configuration of oxygen on its ground state is [He] 2s2 2p

4.

To achieve a closed-shell configuration [He] 2s2 2p

6 in which the low-

lying energy levels are full and the higher energy levels are empty, each

oxygen atom needs to incorporate into its valence shell two additional


electrons. As a sole atom, or in an ionic environment, it needs to form the

oxygen anion (O2-

) while as a molecule it can form a double covalent

bond and thus fulfill the octet rule, Figure 6.

Atomic

orbitals

Molecular

orbitals

Atomic

orbitals

O O

Figure 6. Molecular orbital diagram for O2. Adapted from Ref. 149

Because of the electronegativity of oxygen (i.e. its willingness to fill up

its valence shell) and the ability to form ionic, as well as covalent bonds,

oxygen readily combines with most elements on the periodic table.

Oxygen reacts with most metals oxidizing them and thus forming the

metal oxide (note oxidation comes originally from combining with

oxygen). As already reduced (negatively charged) element, it combines

with already oxidized (positive) cations and prevents them from reducing

(keeps them oxidized). Note that in Chemistry, the term oxidation has


been extended and the loss of electrons it is also known as oxidation of

the element even when the electrons are transferred to an element other

than oxygen. The bond between oxygen and metals is generally ionic or

at least ionic in a large extent. However, when oxygen combines with

non-metals, it tends to form covalent bonds.

2.2.2 Cerium

The element with atomic number 58 is known as cerium. It was

discovered in 1803 by Swedish chemists Berzelius and Hisinger; and

simultaneously in Germany by Kaproth.

The electronic configuration of cerium on its ground state is [Xe] 4f1 5d

1

6s2. This electronic configuration makes it possible for cerium to cede up

to three electrons to elements of moderate electronegativity such as

chlorine or bromine. However, the f-electron is highly shielded and only

the two most electronegative elements, oxygen and fluorine, are capable

to form tetravalent compounds with cerium.8 Cerium is most frequently

found in combination with the most electronegative elements: F, O,

Cl, Br. And for these elements, the thermodynamic stability is

Ce–F > Ce–O > Ce–Cl > Ce–Br.8 Because fluorine is not as abundant as

oxygen, cerium oxides are the most common.


2.2.3 Cerium oxides

Cerium combines with oxygen in both of its common oxidation states: +3

and +4, resulting in Ce2O3 and CeO2 respectively. In many cases, a mixed

valence is formed to yield CeO2-x where 0 < x < 0.5.4

From the chemical perspective, cerium as it is common among the

lanthanides has a rather stable trivalent oxidation state. However, in

combination with most electronegative elements oxygen and fluorine, the

tetravalent oxidation can be attained. In addition, the energy of the inner

4f level is sufficiently similar to the 5d valence orbitals and thus cerium

can adjust its oxidation state to changes in the surrounding environment.

This singularity can be exploited in many different fields; hence the vast

number of applications.

2.2.4 Doping of ceria and mixed oxides

The fluorite structure of cerium (IV) oxide can be relaxed to

accommodate trivalent cerium and oxygen vacancies. This explains the

ability of ceria to take and release oxygen depending on the partial

pressure of O2 in the environment. This uncommon ability can be

exploited to dope cerium oxides with other elements. For instance, ceria

has been successfully doped with Al, Fe, Gd, Zr, Mg, Sr, among many

others.150-155

When the concentration of the third element is significant, it is more

accurately described as mixed oxide. Elements like zirconium can be

incorporated in the ceria lattice.156

The properties of the resulting mixed

oxide would differ from those of ceria as the concentration of the third


element is increased. The OSC and the catalytic performance of ceria

have been enhanced as a ceria-zirconia mixed oxide.157-160

2.2.5 Coordination chemistry of cerium cations

In 1963, Pearson published a list of cation and anion combinations that

are usually found forming stable compounds. Pearson argued that ions

could be classified as hard or soft depending on their polarizability.161

Cations would behave like hard spheres if they are difficult to polarize,

that is, if their electron cloud remains symmetrical and cannot be

deformed. That happens to small and highly charged cations, similarly

small anions of low charge would be “harder” than large anions where

the electron cloud is malleable or soft and can easily result in dipoles.

Following Pearson’s reasoning, soft cations are those of large size and

low charge that can be polarized. The usefulness of his classification is as

predictive tool: He showed that hard cations usually combine with hard

anions and their bond is predominantly ionic, while soft cations and

anions form compounds where the covalent character prevails. Pearson’s

method found successful application in, for instance, the selection of

effective complexing ligands for the different lanthanides.162

The

complexation of metals is crucial in chemical speciation where different

metals are separated from others by phase transfer, precipitation or

chromatographic separation.

In the particular case of cerium ions (both Ce3+

and Ce4+

), Pearson’s

model predicts that its small size and high charge makes it a hard cation

that preferentially combines with hard anions, such as fluorine or oxygen,


rather than with softer anions like chlorine or bromine, exactly as the

thermodynamics showed later.8 The experimental observations that

showed that the most effective organic ligands to aquatic cerium ions

were carboxylic acids were in agreement with Pearson’s model. The

carboxylic group can deprotonate to produce a carboxylate, and this

results in an oxygen–rich anionic environment, that can coordinate to

cerium cations. Indeed, Choppin et al. studied various cerium

carboxylates, especially those of aminopolycarboxylic acids and

demonstrated that the complexation of carboxylates and aquatic cerium

ions follows the inner-sphere model.163

That is, the carboxylate replaces

enough water molecules of the first layer of hydration to position the

oxygen atoms of the carboxylic group in direct contact to the cerium

ion.164

Dehydrating the cerium cation is enthalpy–costly and the

coordination of the ligands do not compensate for it. Thus, this is an

unfavorable process in terms of enthalpy. However, dehydration of the

metal by inner-sphere ligands liberates enough water molecules to

increase the entropy of the system to the point that the whole process

becomes energetically favorable and feasible.165, 166

This delicate balance

of enthalpy and entropy gains is known as the compensation effect.147, 167

When both ΔH and ΔS are negative (e.g. nitrates), the cations retain most

of its primary hydration sphere and form outer-sphere complexes, where

the ligand occupies a position in the second or third layers of hydration,

see Figure 3.168


2.2.6 Surface of ceria

The arrangement of atoms in the bulk can explain its physical properties

(light absorption, refractive index, etc). However, since they are not

accessible to react, from a chemical perspective the interactions at the

surface are far more important when explaining the reactivity of ceria (as

catalyst, for instance). Considering cerium and oxygen exclusively, the

oxide —at the surface— has to terminate in either a cerium or oxygen

ion. However, in the presence of water, the strong acid Ce4+

ions sitting

on the surface will hydrolyze water and thus coordinate the resulting OH–

to the surface.169

Interestingly, if the oxygen is the outermost ion it can

also hydrolyze water, capturing a proton, which results as well in an –OH

on the surface.170

In addition to water hydrolysis, CeO2 has an additional intrinsic and

efficient mechanism to reduce the number of dangling bonds on the

surface. Ceria, by expanding the crystal lattice (increasing the distance

between atoms) can allow oxygen vacancies. These oxygen vacancies go

accompanied by a reduction of the oxidation state of cerium from

tetravalent to trivalent state. The overall result is that the surface energy is

effectively reduced.

One example that illustrates well the ability of ceria to force the

dissociation of oxygen-hydrogen bonds in order to seize electron density

is the dissociation of methanol into methoxy (CH3CO–). Process that

happens even at room temperature after methanol is adsorbed onto ceria

surface.171


Surface Cerium terminated Oxygen terminated

[010]

facing

upwards

[010]

seen

from

above

[111]

facing

upwards

[111]

seen

from

above

[221]

facing

upwards

[221]

seen

from

above

Table II. Some of the different lattice planes of ceria.


Table II shows the fluorite structure of ceria where the atomic

arrangements seen are result from the projection of the basic cubic

geometry of the crystal. The different surfaces are not flat at atomic scale

and therefore the adsorption of molecules has to take the 3D distances

(between cations, for instance) into account and not only apparent

distances of the vertical projection. Table II highlights that the only two

distances between oxygen atoms are those along the edge of the cell and

that of the diagonal: 2.706 and 3.826 Å, respectively.

2.2.7 Coordination chemistry on ceria surface

Capping agent is the term used to refer to the first monolayer of the

adsorbed species, usually organic compounds, on the surface of particles.

They adsorb to reduce the surface energy of the bare particle. Capping

agents can be very different in nature, from polymers to simple

molecules. In many cases, the capping agent is simply adsorbed by means

of weak but numerous electrostatic interactions. However, it is not

uncommon to find molecules that form fewer but stronger bonds. These

are commonly referred as coordination bonds and the molecules that

coordinate are called ligands.

Ligands reported in the vast literature as effective in dispersing ceria

nanoparticles are usually carboxylic acids. To name a few, Masui

employed citric acid,172

Sehgal used polyacrylic acid173

and Ahniyaz


utilized oleic acid.78

However, very little has been done in asserting the

chemical reasons for selecting carboxylic acids over other organic

functions such as alcohols, amines or sulfates. Indeed, a successful

nanoparticle dispersion seems to be the consequence of an iterative trial–

and–error process rather than based on fundamental chemistry. To make

things worse, some authors have defined the interactions between

carboxylic acids and ceria surface as covalent bonds, that without

corroborating such affirmations or citing any previous work that defined

these surface modifications as covalent grafting or lacking any sort of

model of the chemical interactions at the surface.174

The present work

will argue that since cerium oxides are more ionic than covalent, on their

surface, the successful ligands are those that can interact ionically.

As mentioned before, few different successful surface modifications of

ceria have been accomplished and most of them have in common to

consist of carboxylic acid ligands. However, only few of the papers

published in the open literature have looked into the reasons for the

carboxylic acids to outperform other organic groups and failed to give a

satisfactory explanation of the chemical interactions at the very interface

between the ceria crystalline structure and what are the geometries or

bonds formed between the two entities. Recent master and doctoral thesis

have started to investigate this promising field175

but presumably without

taking the care, dedication and detail that is be given on this work.


2.3 Bond valence method

The bond valence method is a popular method in coordination chemistry

that combines three concepts: oxidation state, coordination number and

bond length with Pauling's rules. Its popularity is due to its simplicity and

robustness. The bond valence method provides a more intuitive and

graphical representation of chemical structures, based on simple electron

counting (valence) rules. It is mostly used to correlate bond distances

with the valence of the atoms or vice versa, the valence of the atom with

the strength of its bonds.176

The Bond Valence method consists in splitting the total valence of the

atom into individual bond valences. The division takes into account the

observed bond lengths: the shorter the distance, the higher the bond

valence. This method is very convenient when the total bond valence is

known and for systems where the delocalization of electrons is negligible.

The total bond valence (bond valence sum) of each atom in the structure

can in most cases be equated to the oxidation number. Figure 7, gives

examples on different bond valence values for the different bond in the

carboxylate and carboxylic molecules.

Figure 7. Bond valence for the bonds in carboxylate (left) and carboxylic (right) molecules.

Adapted from REF 176.

Experimental | 49

3. Experimental

3.1 Materials

Below follows a brief description of the most relevant materials

employed in this study. All chemicals were purchased from commercial

suppliers: Sigma–Aldrich, Alfa–Aesar, Merk, Ciba Specialty Chemicals

and used as received.

The cerium salts: cerium (III) nitrate hexahydrate, cerium (III) acetate

hydrate and cerium (III) bromide were used as precursors for the cerium

oxides and/or cerium complexes. The acids and bases employed to adjust

pH of aqueous solutions were: Nitric acid, ammonium hydroxide, sodium

hydroxide, potassium hydroxide. Hydrogen peroxide has been used to

oxidize the aquatic cerium cations and cerium hydroxides. To form

coordination compounds and surface complexes the following organic

ligands were utilized: Oleic acid, benzoic acid, nitrilotriacetic acid,

methyl methacrylate, methacrylic acid, acrylic acid, n-butyl acrylate,

styrene, glycidyl acrylate, 2-hydroxyethyl methacrylate, PEG-diacrylate.

The radical initiators used were: 4,4’-azobis(4-cyanovaleric acid),

potassium persulphate and 2,2-dimethoxy-2-phenylacetophenone. Among

a variety of solvents, the most used were: double distilled water, n-

hexane, n-heptane, THF, chloroform, ethanol, acetone, isopropanol, 1,4-

butylene glycol, ethyleneglycol.

50 | Experimental

3.2 Characterization techniques

3.2.1 FT-IR

A PerkinElmer Fourier transform infrared (FTIR) spectrometer, Spectrum

One with Attenuated Total Reflection (ATR) sampling accessory was

used with a MIR (mid-infrared) beam source. The instrument is equipped

with KRS-5 and diamond ATR crystals on the top plate and with a MIR-

DTGS (mid-infrared deuterated tryglycine sulfate) detector. When

needed, samples were dried overnight in a vacuum oven at room

temperature. A few milligrams of sample were placed directly on the

ATR crystal. Spectra were recorded with 16 scans and a resolution of 2

cm−1

. The baseline was corrected as suggested by the accompanying

sorftware, Spectrum, from Perkin Elmer.

3.2.2 DLS

Particle size distribution was analyzed by Dynamic Light Scattering

(DLS) using a Zetasizer (Nano ZS, 2003, Malvern Instruments, UK). The

measurement angle was 173° (backscatter). Both, the standard general

purpose algorithm and the so-called high resolution mode were used for

the analyses. Disposable plastic cuvettes were used once per

measurement.

An aliquot of the dispersion was diluted with double-distilled water and

the diluted sample was then analyzed by DLS.

Experimental | 51

3.2.3 TEM

Transmission Electron Microscopy (TEM) and high-resolution TEM

images were obtained using two different microscopes.

A JEOL JEM-2100 transmission electron microscope (Cs = 1.4 mm,

point resolution = 0.25 nm) equipped with a LaB6 crystal as the electron

source was operated at 200 kV. A drop of a diluted aliquot of the sample

was deposited on ultrathin carbon film–coated copper grids (TED Pella).

Images were recorded by CCD camera (Gatan Orius SC1000).

A JEOL-2100F transmission electron microscope (Cs = 0.5 mm, point

resolution = 0.19 nm) equipped with a Schottky–type field emission gun

(FEG) as electron source was used to record HRTEM images by a Gatan

Ultrascan CCD camera (2k x 2k pixels), and electron energy loss spectra

and elemental maps by a Gatan Image Filter (GIF Tridiem 863 with 2048

channels). Before acquiring the element maps by energy–filtered TEM

(EF-TEM), the thickness map and core–loss spectrum of the region of

interest were obtained. The thickness of a region was estimated by the

mean free path of the inelastic electrons (λinelectric), which was calculated

by the ratio between elastic (ZLP) and inelastic (energy lost) electrons.

The thickness of the regions for elemental study was less than λinelectric.

The core-loss spectrum was obtained to confirm the presence of the

particular elements and their particular edge energy. In element mapping

obtained by EFTEM, the width of the slit in GIF and the selected energy

ranges used depended on the elements and were suggested by the

software, Gatan DigitalMicrograph, which was responsible for

acquisition as well as used for the analysis of the results.

52 | Experimental

3.2.4 UV-Vis

UV-Vis spectra of the samples were collected with a Perkin Elmer

Lambda-650 spectrometer. Two light sources were used: deuterium lamp

between 250 and 218 nm and tungsten lamp between 218 and 800 nm.

Plastibrand disposable UV–cuvettes with 1 cm of optical path length

were used for each measurement and its corresponding background.

Single scan spectra were recorded with a resolution of 1 nm.

3.2.5 TGA

To quantify the cerium oxide content of the dispersions,

thermogravimetry analyses (TGA) were carried out in a Perkin-Elmer

TGA analyzer. The temperature was set to increase from 25 °C to 800 °C

at the rate of 20 °C ·min−1

with an air flow of 40 mL·min-1

.

3.2.6 XRD

X-ray diffraction (XRD) analysis was carried out to verify the crystal

structure of the stabilized cerium oxide nanoparticles.

XRD analysis was performed using an Agilent Xcalibur III single–crystal

diffractometer with a 4-circle kappa geometry, equipped with Sapphire-3

CCD detector and Molybdenum radiation source. On the diffractometer a

capillary containing the stabilized ceria dispersion was mounted, and the

2D-data were converted and merged to a conventional powder diffraction

pattern.

Experimental | 53

3.2.7 PXRD

Powder X-ray diffraction (PXRD) studies were conducted in a

PANanalytical X'Pert Pro diffractometer using Cu radiation. An aliquot

of the polymerized sample was first dried at room temperature for 72

hours. This yields a film which was rinsed with double distilled water to

remove any residues of surfactants or even eventual dust deposited during

drying. The dried film was placed onto silicon wafers on which a drop of

chloroform was previously added to help attaching the film onto the

wafer. The solvent evaporated at room temperature prior analysis.

3.2.8 XPS

X-ray photoelectron spectroscopy (XPS) studies were conducted to verify

the valence of surface cerium ion in CeO2 nanoparticles.

The spectra were recorded using a Kratos AXIS UltraDLD X-ray

photoelectron spectrometer (Kratos Analytical, Manchester, UK). The

sample was analyzed using a monochromatic Al X-ray source. The

analysis area was below 1 mm2, with most of the signal from an area of

700 × 300 μm.

54 | Experimental

3.3 Synthetic procedures

Many different synthetic routes have been explored during the extent of

this study. The ones of most academic and industrial relevance have been

published and are described in detail in the appended papers. This section

is devoted to describe the different procedures conducted with emphasis

in the parameters that were found to alter the size, morphology or

dispersibility of the resulting nanoparticles.

Chart I. Process chart summarizing the evolution of the different synthetic routes

deleteA

Route 1A

Route 2A

Route 3A

Route 4A

deleteB

Route 1B

Route 2B

Route 3B

Route 4B

Route 5B

deleteC

Route 3C

Route 4C

Route 5A

deleteD

Route 3D

Route 4D

Route 5D

deleteE

Route 4E

Route 5E

Thermosensitive

Capping Agent

Functional

Capping Agent

Carboxylate

Capping Agent

Polymerization Process

Overview of synthetic routes

Experimental | 55

Route 1 – Ligand screening

This route is based in the precipitation of metal hydroxides for their

subsequent oxidation and stabilization with capping agents.

Two alternatives were explored: Route 1A and Route1B.

3.3.1.1 Route 1A

The different steps were:

1) Dissolution of cerium nitrate in water of different temperatures

(either at R.T. or in the interval of 60 to 90 °C)

2) Addition of acetic acid.

3) Start of high-speed homogenizer (Ultra–Turrax).

4) Addition of ammonium hydroxide. At this point, the system shall

be stirred for times between 5 to 15 minutes.

5) Increase of temperature to values between 70 and 90 °C.

6) Addition of hydrogen peroxide.

7) Addition of the capping agent in its carboxylic form.

8) The system shall be left under heavy stirring for a period between

30 minutes and 2 hours.

9) Optionally, the system can be left under gentle stirring (with

magnetic bar) for times spanning up to 48 hours.

*Steps 6 and 7 can be interchanged.

56 | Experimental

3.3.1.2 Route 1B








5) Centrifugation of the system at 4,000 rpm for 20 minutes.

6) Removal of supernatant

7) Re-dispersion (through heavy stirring) of the precipitate in

double distilled water at 70 – 90 °C.


9) Addition of the capping agent in its carboxylic form.





The various amounts of the different ingredients are shown on Table III.

Experimental | 57

TABLE III

Component Typical values Variation range units

H2O 2 1 – 5 Liters

Cerium nitrate hexahydrate 30 30 – 120 grams

Acetic acid 20 0 – 30 grams

Ammonium hydroxide 22 15 – 30 grams

Hydrogen peroxide 8 5 – 20 grams

Capping agent 6 5 – 18 grams

The different capping agents tested were: itaconic, oleic, palmitic,

benzoic, isovaleric, isobutyric, 2-(2-Methoxyethoxy)acetic,

2-[2-(2-Methoxyethoxy)ethoxy]acetic, nitrilotriacetic, mandelic and

ethylenediaminotetracetic acid.

3.3.2 Route 2 – pH screening

Both the addition of H2O2 and the use of a carboxylic acid as capping

agent result in a drop of the pH of the system. The pH values can be

maintained in the neutral range (7 ±1) if the carboxylic capping agent is

replaced by its corresponding carboxylate (either by neutralizing with

NH4OH or by using the sodium salt of the acid).

58 | Experimental

3.3.2.1 Route 2A








5) Increase of temperature to values between 70 and 90 °C.


7) Addition of the capping agent in its carboxylate form.





*Steps 6 and 7 can be interchanged.

Experimental | 59

3.3.2.2 Route 2B




2) Optionally, addition of acetic acid.






7) Redispersion (through heavy stirring) of the precipitate in double

distilled water at 70 – 90 °C.


9) Addition of the capping agent in its carboxylate form.





Carboxylates employed as capping agents were Na3NTA, sodium

benzoate, sodium oleate and sodium methoxyethoxi acetate.

The various amounts of the different ingredients are shown on Table IV

60 | Experimental

TABLE IV


H2O (first step) 2 400 - 600 mL

Cerium nitrate hexahydrate 30 30 – 120 grams

Acetic acid 20 0 – 30 grams

Ammonium hydroxide 22 15 – 30 grams

H2O (7th

step) 2 1.5 – 3 Liters

Hydrogen peroxide 8 5 – 20 grams

Capping agent 6 5 – 18 grams

A more detailed example of the application of Route 2B is found in the

synthesis of NTA-stabilized ceria nanoparticles reported in Paper I.

3.3.3 Route 3 – Temperature sensitive capping agents

This Route can be divided into 3 subcategories (3A, 3B and 3C). Routes

3A and 3B are modifications of Routes 1A and 1B. Route 3C was

specifically designed to tackle the issue of employing two different

capping agents, where one is thermosensitive.

Two thermosensitive capping agents were studied: 4,4’-azobis(4-

cyanovaleric acid) and acrylic acid

Whenever the capping agent decomposes at the moderate temperatures of

the reaction (70 and 90 °C) the syntheses described above as Route 1A or

Route 1B can be modified if after the addition of the hydrogen peroxide

Experimental | 61

the heating is stopped and the beaker, where the reaction is conducted, is

removed from any source of heat in order to accomplish a quick but

controlled cooling. Once the system reaches a temperature that the

capping agent can withstand, the capping agent is added.

The different steps can be described as:

3.3.3.1 Route 3A

1–5) Steps 1 to 5 are equivalent of those of Route 2A


7) Heating is stopped.

8) The system shall be left under heavy stirring until it reaches a

temperature that does not significantly decompose the capping

agent.

9) At this point, the capping agent shall be added.

10) The intense stirring shall continue for at least 30 more minutes.


magnetic bar) for times spanning up to 48 hours. Since the

capping agent is thermosensitive, it is recommended that this last

step take place in a refrigerated environment.

62 | Experimental

3.3.3.2 Route 3B

1–7) Steps 1 to 7 are those of Route 2B





agent.

11) At this point, the capping agent shall be added.





step takes place in a refrigerated environment.

Experimental | 63

3.3.3.3 Route 3C









6) Re-dispersion (through heavy stirring) of the precipitate in

double distilled water at 70 – 90 °C.

7) Addition of the first (non-thermosensitive) capping agent as

carboxylic, carboxylate or their combination.





agent.

11) At this point, the second capping agent shall be added.





step takes place in a refrigerated environment.

64 | Experimental

For routes 3A and 3B, the amounts of the different reactants coincide

with the values shown in Tables III and IV, respectively.

3.3.3.4 Route 3D

In contrast to the procedures followed in Routes 3A, 3B or 3C; Route 3D

is exclusively conducted at room temperature and therefore. Route 3D

was performed either using cerium nitrate or cerium acetate as precursors,

in combination with either carboxylates or carboxylic acids as capping

agents.

The steps followed in Route 3D were:

1) Dissolution of cerium precursor in double distilled water at R. T.

2) Addition of capping agent

3) Addition of hydrogen peroxide under heavy stirring

Amounts used in Route 3D are shown in Table V.

TABLE V


H2O 150 50 - 250 mL

Cerium precursor 10 5 - 40 mmol

Capping agent Same mole as

cerium precursor

From 0.2 to 3 times

the moles of cerium mmol

H2O2 (30 %) 1.2 0.5 – 1.8 grams

Experimental | 65

3.3.4 Route 4 – functional ligands

The ligands used as capping agents can be selected to have additional

reactive moieties within their molecular structure. In the present work,

two moieties were exploited: the azo bond, which decomposes with

temperature to form radicals able to start the polymerization of a variety

of monomers; and ligands with polymerizable double bonds. The

functional capping agents were selected to be carboxylic acids (or any of

its salt carboxylates) so they could be used in any of the Routes described

above.

3.3.4.1 Route 4A

This route follows the Route 1A, where the carboxylic acid used as

capping agent had reactive double bonds such as itaconic acid.

3.3.4.2 Route 4B

This route is based on Routes 2B or 3B, where the carboxylate salt had a

reactive (polymerizable) moiety. A detailed example is found on Paper

II, where surface-modified nanoparticles are prepared following the steps

described on Route 2B and selecting 4,4’-azobis(4-cyanovaleric acid)

(ACVA) as capping agent.

66 | Experimental

3.3.4.3 Route 4C

Route 4C is based on the developments achieved by Route 3B, where the

thermosensitive capping agent added on step 12 is the radical initiator

ACVA. This route is exemplified in the preparation of NTA/ACVA-

modified nanoparticles described in Paper III. (Please note that ACVA is

noted as V501 in the cited Paper).

3.3.4.4 Route 4D

Route 4D is based on Route 3D. The introduction of new steps into the

process of split Route 4D into two alternatives Route 4D1 and 4D2.

3.3.4.4.1 Route 4D1

Route 4D1 follows precisely the steps of Route 3D. The precursors

employed were cerium acetate and cerium nitrate and the capping agents

explored were acrylic and methacrylic acid.

3.3.4.4.2 Route 4D2

The steps followed in Route 4D2 were:

1) Dissolution of cerium precursor in double distilled water at R. T.

2) Addition of capping agent

3) Addition of monomer

4) Addition of hydrogen peroxide under heavy stirring

5) Stop stirring to allow the phase separation of the water and

monomer phases

6) Collection of monomer phase

Experimental | 67

Routes 4D1 and 4D2 were employed in the synthesis of methacrylate

modified nanoceria reported in Paper IV. The route there referred in the

Paper as Ac1 corresponds to 4D1 and the route referred in the Paper as

Ac2 corresponds to 4D2 here.

3.3.4.5 Route 4E

This route does not involve the synthesis of ceria nanoparticles but rather

the modification of commercially available dispersions of nanoceria with

acrylic acid. The surface modification reported on Paper V was based on

this Route 4E.

The steps followed in Route 4E were:

1) Addition of NYACOL® CeO2(AC)

2) Optionally, addition of double distilled water for dilution

3) Optionally, addition of solvent

4) Addition of acrylic acid

5) Stirring for 6 hours

Optionally, air or nitrogen gas can be flushed to remove acetic acid;

followed by a second stirring run of 3 hours

6) Addition of monomer

7) Addition of AIBN (10% in acetone)

8) Stir or agitate

9) Flush N2 gas to remove oxygen from headspace

10) Place in water bath or oven at 70 °C

68 | Experimental

In case the monomer is water soluble, such as 2-hydroxyethyl

methacrylate (HEMA), it can be added as step 3 instead of step 6.

Solvents used in step 3 were: acetone, ethanol, isopropanol or mixtures of

them.

Typical amounts for the reactants of Route 4E are shown on Table VI.

TABLE VI

Reagent Typical values Variation range Respect to

NYACOL CeO2(AC) 15 wt% 1 – 20 wt% monomer

Acrylic acid 10 wt% * 5 – 20 wt% * NYACOL

Solvent 50 wt% * 0 – 100 wt% monomer

Route 4E was exploited in the preparation of acrylate-modified ceria

nanoparticles for the subsequent formation of nanocomposites reported

on Paper V.

3.3.5 Route 5 – polymerization strategies - enabling

Four polymerization routes were pursued to encapsulate the formed

nanoceria into a polymer matrix: emulsion and miniemulsion

polymerization, precipitation polymerization and photocuring.

Experimental | 69

3.3.5.1 Route 5A – emulsion polymerization

An emulsion polymerization process was used to encapsulate solvent-

borne nanoceria prepared following Routes 1 and 2 and also to form

hybrids with ceria nanoparticles modified with initiators and

polymerizable ligands, as prepared in Route 4B and 4C. Route 5A was

also the one selected on Paper III to conduct the encapsulation of the

nanoceria. A monomer-in-water emulsion was stabilized by anionic

surfactants. The aqueous phase was added first, then the monomer and

finally the surfactant. Energy was applied in form of stirring or agitation

to form monomer droplets. The air in the headspace was replaced by N2

gas. The temperature was raised to 70 – 75 °C and then the initiator was

added in those cases where the nanoceria was not already surface

modified with radical initiators.

Monomer concentration was usually between 5 and 30 wt% respect to

water content. The surfactant (Dowfax 2A1) was added in the minimum

necessary amount to yield a stable (for at least for 24 hours) emulsion

(typically 2 wt% respect to the monomer). When the water phase is of

high ionic strength or when nanoparticles are dispersed in it, the

surfactant concentration could reach 10 wt% of the monomer. And the

initiator is typically added as 0.5 wt% respect to the monomer. An

emulsion polymerization is usually kept at 70 °C for 4 hours.

It shall be noted that when preparing the blanks (ceria–free), it was

usually not possible to reproduce a blank with the concentrations of

surfactants needed to emulsify nanoceria–containing solutions.

70 | Experimental

3.3.5.2 Route 5B – Miniemulsion polymerization

Miniemulsion polymerization was chosen to try to encapsulate the

modified nanoceria prepared following Route 4B, as in Paper II. It

proceeded similarly to Route 5A, however when the emulsion was

obtained and corroborated stable, the amount of surfactant was doubled

and higher energy was applied into the system by means of an ultrasonic

probe. To avoid undesired heating up during this step, the emulsion was

placed in ice bath.

It shall be noted that when preparing the blanks (ceria–free), it was

usually not possible to reproduce a blank with the concentrations of

surfactants needed to prepare a miniemulsion of nanoceria-containing

solutions.

3.3.5.3 Route 5C – precipitation polymerization

Precipitation polymerization was carried out for samples prepared in

Route 4D. The monomer-modified ceria nanoparticles were placed in a

suitable vial or bottle under stirring. A second monomer, normally

methyl(methacrylate) was added to the solution. Then 0.5 wt% of KPS

initiator, respect to the monomer, was added. The oxygen of the

headspace was removed by flushing N2 gas for a time no shorter than 2

minutes. The vial was then placed in a water bath at 70 °C and kept under

stirring for 4 hours. An example is provided in Paper IV.

Experimental | 71

3.3.5.4 Route 5D – Photopolymeriztion

In order to photopolymerize the ceria in monomer dispersions obtained

by Route 4D, to a given amount of ceria in monomer dispersion 1 wt%

(respect to the monomer) of photoinitiator Irgacure 651 was added and

stirred until completely dissolved. Films were made either on glass or in

Teflon mold and then the sample was irradiated for 5 minutes with the

UV source at ambient temperature. The light source used for curing was a

Black-Ray B-100AP (100 W, λ=365 nm) Hg UV lamp, which after the

aforementioned irradiation time subjects the sample to a total dose of

4.8 J·cm-2

, as determined using a Hamamatsu light power meter

(C6080-3) calibrated for the emission line at 365 nm. Route 5D was used

in the preparation of thermosets described in Paper IV.

3.3.5.5 Route 5E – Thermal curing

The acrylate modified NYACOL®CeO2(AC) nanoparticles can be

polymerized in situ if monomer and a radical initiator are added to the

system. One example that exemplifies the thermal curing conducted on

Paper V is:

To the vial containing the acrylate-modified nanoceria, desired volume of

monomer was added. The necessary amount AIBN dissolved in the

minimum amount of acetone was then added to match a final AIBN

concentration of 2 wt% respect to the monomer. The vial was then stirred

or agitated, N2 gas was flushed to remove oxygen from the heaspace. And

finally the mixture was placed in water bath or oven at 70 °C.

72 | Experimental

Results and discussion | 73

4. Results and discussion

Summary of the focus of each of the appended Papers:

Paper I established the scientific grounds regarding the coordination of

carboxylic acids, particularly aminopolycarboxylates, to the surface of

ceria. Defined the interaction as ionic, and characterized the interaction

by shifts in the vibration bands of the carboxylate group as shown by

FTIR.

Paper II utilized the main findings from Paper I to produce nanoparticles

with azo moieties on the surface that could trigger polymerization

through radical formation and thus resulted in nanoparticle encapsulation

in latex polymer particles.

Paper III made use the developments accomplished so far to formulate

not only encapsulated particles, but also a stable hybrid through an

emulsion polymerization process, compatible with those use by the

Industry to produce water-borne acrylic paints.

In Paper IV a new surface modification was carried out using

methacrylate moieties on nanoceria surface that were later polymerized in

100% systems that led to novel nanocomposites of enhanced mechanical

properties.

Paper V described the modification of commercial ceria nanoparticles by

ligand exchange. In this case, the resulting nanoparticles contained

acrylate moieties on their surface that could be polymerized leading to

novel nanocomposites with ceria content exceeding 15 wt%.

74 | Results and discussion

4.1 Basic principles

The architecture of cerium oxides consists on only two elements in their

ionic form: cerium cations and oxygen anions alternated in a

tridimensional network that tends to form an ordered structure

(amorphous cerium oxides are also possible). This is true for cerium

oxides of all sizes, and that includes cerium oxide nanoparticles.

However, at the surface the ordered architecture is disrupted and —in the

absence of any other elements— the outermost atoms can only be cerium

or oxygen. In CeO2, each bulk cerium tetravalent cation coordinates

(form bonds) to eight neighboring oxygen atoms; in Ce2O3 the

coordination of the trivalent cerium cations is reduced to six. In both

cases the coordination number of the oxygen anion is four. A Ce4+

sitting

on the surface would only coordinate half of the bonds, the ones pointing

to the core of the particle, and would have four more uncoordinated

bonds —dangling bonds— pointing outwards from the surface of the

particle and avid to react. For Ce3+

the number of dangling bonds is

reduced to three and this is energetically more favorable and thus it is

expected that whenever a surface atom can reduce its valence it would do

so. In ceria, the reduction in valence of surface cerium ions is

accompanied by the generation of an oxygen vacancy, which explains the

ability of cerium oxides to act as oxygen buffers, an ability which is

known as oxygen storage capacity (OSC).98, 117

As reviewed above, OSC

results in a number of applications and for that reason, the mechanism

behind the formation of oxygen vacancies has been extensively studied.

Cerium oxide nanoparticles have been described as consisting of a

surface layer of Ce2O3 and a core of CeO2.177

Interestingly, while the

enrichment in trivalent surface cerium has been corroborated, and


explained in terms of the facile redox reaction between Ce3+

and Ce4+

, the

driving force for this singularity has not been thoroughly discussed.

According to thermodynamics, in aqueous solution Ce3+

is more stable

than Ce4+

. However, in the bulk, the dioxide (CeO2) is energetically more

stable than the sesquioxide (Ce2O3).178

This is the reasoning behind the

hypothesis for the thermodynamic origin of the surface valence reduction

proposed in Paper I. However, this thermodynamic explanation holds for

aqueous environments. If the same phenomenon happens in the absolute

absence of water there must be other forces driving it, perhaps the energy

gained by reducing the number of dangling bonds is sufficient. While the

aforementioned hypothesis on the thermodynamic origin of the valence

reduction for cerium oxides exposed to water could not corroborated, the

coexistence of both trivalent and tetravalent cerium ions was indeed

asserted by the XPS analysis reported in Paper I, see Figure 8. This

supported the hypothesis for the surface valence reduction in aqueous

ceria nanoparticle dispersions.

In case oxygen occupies the outermost surface positions, it will still have

very reactive dangling bonds. However, oxygen is more versatile to

satisfy its uncoordinated bonds and two reasons seem to prevail when

giving a plausible explanation. First, oxygen is highly electronegative and

thus combines chemically with virtually any other element that might be

present (with the notable exception of the noble gases and metals).

Additionally, this reactive oxygen anion can hydrolyze water and bond a

proton (vide 35) and thus terminate the metal oxide surface in hydroxyl

groups.170


Figure 8. XPS spectra of NTA–modified ceria nanoparticles. From E.S.I. of Paper I

Since ceria is an ionic crystal,179

the surface cerium atoms shall be

regarded as ions, with the implications this has in its chemistry and the

bonds they form. The model described in Paper I predicted that for ceria

particles in aqueous solution, the chemistry of those surface cerium ions

would be equivalent to that of aqueous cerium ions. In consequence, ceria

is subjected to surface chemical reactions such as the hydrolysis of the

surface cations. This contributes to explaining the origin of hydroxyl

groups on ceria surface. There are now two simultaneous but not

competing processes that result in the coverage of this metal oxide

surface by –OH. One is the hydrolysis of the anion, by which the surface

oxygen takes a proton from the surrounding water. The other one is the

hydrolysis of the cerium cation, which would seize an –OH, Figure 6.

Independently of what hydrolyzes, the cation or the anion, the result is

equivalent: a hydroxylated surface, Figure 9. Most likely, both hydrolysis

happen simultaneously. However, if one is more significant than the

other, the change in pH would discern between the two.180


Figure 9. Hydroxylated ceria surface viewed from different angles.

The model this work is elaborating is now refined by assuming that not

only the hydrolysis between the solvated Ce3+

and the solvated ceria

surface shall be equivalent but the coordination chemistry should also

resemble. This realization unveiled very interesting consequences. There

has been extensive research in coordination chemistry and particularly for

cerium, the stability constants of many different organic ligands can be

found in the literature.181, 182

As a result, the stability of the surface

complex can be predicted, which brings an immediate benefit by reducing

the tedious trial–and–error that experimentalists face when testing their

assumptions. In addition, if the complex is known to be stable in solvents

other than water, the ceria nanoparticles will be dispersible in such

phases.


The most stable coordination compounds for aquatic cerium ions are the

carboxylic acids, Table VII. Their stability is pH dependent but in general

the ranges of pH that led to stable complexes are wide enough and

fortunately, cover the region of moderate acidity (6 to 4) where metals do

not suffer extensive side reactions such as precipitation due to hydrolysis.

The logarithmic values of the stability constant for the metal–carboxylate

complexation (log β) is proportional to the sum of the pKa of the acid

(ΣpKa), which indicates that the complex is actually formed between the

carboxylate group and the metal.183

Furthermore, it has been shown that

the oxygen atoms in the carboxylate are at a bonding distance to the

metal, thus taking the responsibility for the chemical interaction between

the two species.184

It should not come as a surprise since the basic oxygen

atoms of the carboxylate are electron–rich and able to form π–bonds (in

addition to σ–interactions) with coordinating metals. However, the

implications of the proportionality between log β and ΣpKa are far

deeper. This has been interpreted as clear evidence for ionic interaction

between the carboxylates and the lanthanide cations (e.g. cerium).183, 185

For the oxygen atoms of the carboxylate to be able to form ionic bonds

with a metal, the carboxylate has to first or simultaneously deprotonate,

meaning that the complexation is actually a cation exchange process.186

The ionic interaction also exhibits less directional restrictions when

bonding. Compared to a covalent bonding situation that is highly

restricted by well define orbital orientations; the ionic bond is only

restricted by steric considerations.187

In addition, most carboxylic acids

coordinate in the inner–sphere;163-165

hence the stability of the resulting

coordination compounds. In a display of brilliant chemistry, Choppin

related the thermodynamics of the dissociation of different carboxylic


acids, with the structure of the resulting complex and concluded that the

weaker the carboxylic acid, the more stable the complex.162

This is, of

course, treasured information for the selection of capping agents for ceria

nanoparticles. It also allows to efficient exchange of capping agents when

there is need for it. Thermodynamic considerations in ligand exchange

reactions are discussed in more detail on Paper V. Recent papers that are,

obviously, not aware of this valuable information, report the exchange of

surface ligand as the simple addition of excess of the desired capping

agent,188

which is only a demonstration of brute force versus chemical

refinement.

If carboxylic acids are the strongest bonding ligands among the possible

organic functions, the particular case of aminocarboxylates has been

reported in the literature as of outstanding stability for the cerous ion.145,

147, 182, 189 Indeed, aminocarboxylates such as iminodiacetic acid (IMDA);

nitrilotriacetic acid (NTA); Ethylenediaminetetracetic acid (EDTA);

Diethylenetriaminepentacetic acid (DTPA); Propylenediaminetetra-acetic

acid (PDTA); Ethyleneglycol-bis(2-aminoethylether)tetracetic acid

(EGTA) or 1,2-Diaminocyclohexanetetra-acetic acid (DCTA) have been

used in the isolation of different metals, especially the rare-earths.190

Accordingly, and to verify that the coordination chemistry of both the

aqueous Ce3+

and the surface of ceria are of similar chemistry, in

Paper I, ceria nanoparticles were synthesized and in situ modified with

NTA, Figure 10. Nitrilotriacetic acid was selected because it has been

extensively studied in regards to its coordination to lanthanides, including

cerium,147, 189, 191-201

and because the stability constants of the complexes

were available.182, 189

But perhaps more imporatantly, because the


addition of H2O2 to a system comprising the cerous nitrilotriacetate

complex (Ce-NTA) has been shown to result in the oxidation of the

cerium metal while it left the NTA molecule unaffected.202

In fact that

made NTA not only a potential successful stabilizer for ceria

nanoparticles (a hypothesis in Paper I) but made it fully compatible with

the synthetic routes designed and pursued in this study (Routes 1 to 3,

exemplified in Papers I, II and III). All the experimental data presented

in Paper I supports this hypothesis. Particles could be synthesized small

and were stable over long periods of time, Figures 11 and 12. Not only

the so prepared nanoparticles showed impressive stability over time, but

the FTIR analyses revealed the equivalence of the bonds formed between

the carboxylate groups of NTA with both cerium ions (the free ion in

aqueous solution and the cerium ions on ceria surface). This not only

supported our assumption but also demonstrated that carboxylates form

inner-sphere complexes on ceria surface. The implications are of

importance. Ceria, as well as many other metal oxides, in aqueous system

have been understood as covered by hydroxyl groups, probably

resembling the surface of covalent compounds such as silica. However,

the inner-sphere model indicates that the process is a mere ligand

substitution. In the ceria case, the –OH is replaced by a more stable,

preferred, ligand: the carboxylate. One should make a clear distinction

between hydroxyl groups coordinated ionically such as in the ceria

case203

and the cases where –OH is bound covalently to the surface. This

later case is that of covalent compounds such as silica. The oxygen atom

of the surface –OH is bound to Si covalently and because the electronic

configuration of silicon prevents it from forming ionic bonds, the

replacement of the hydroxyl group is unlikely to happen as a ligand


exchange reaction. In contrast, the surface Si–OH is susceptible of

condensation reactions that led to either Si–O–Si–R or Si–O–C–R, in

agreement with the covalent nature of the compound.

Figure 10. Nitrilotriacetic acid (NTA) molecule

The summary of the knowledge gained during the study reflected in

Paper I, is that carboxylic acids shall be pursued as ceria nanoparticle

stabilizers because they form stable inner–sphere surface complexes. In

general, such modifications would remain stable unless exposed to a

weaker acid144, 162, 204-207

or to a large excess of any other.188

In particular,

aminopolycarboxylates offer the advantage of the chelate effect besides

the coordination of the nitrogen atom to the nanoparticle surface.166, 191


Figure 11. DLS of NTA-modified ceria nanoparticles 3 and 6 months after synthesis, From Paper I

Figure 12. TEM micrographs of ceria nanoparticles, 9 months after synthesis. From Paper I


4.2 Predictions of the Bond Valence Model

Interestingly, the inner and outer sphere models are consistent with the

bond valence theory. In CeO2, the oxidation number of cerium is four.

The valence of cerium, disregarding any delocatization, coincides with

the oxidation number and thus amounts to four. Each cerium ion is

surrounded by eight oxygen anions, therefore the coordination number of

cerium is eight. Because all the oxygens are symmetrically equivalent,

the valence has to be equally divided between all the Ce-O bonds:

4 ÷ 8 = 0.5. When consider some degree of delocalization, the effective

valence would be somewhat lower than 4 and thus the bond valence

would, as well, be slightly lower than 0.5. For Ce2O3, the oxidation

number and valence are 3 and the coordination number is 6, this results in

6 ÷ 3 = 0.5. Again, no delocalization is considered here. For both cerium

oxides, and any mixed valence state, the oxygen always has a valence of

2 and it is surrounded by 4 equivalent cerium ions, which makes a

valence of 2 ÷ 4 = 0.5. The perfect match between the valence

contribution of the cerium and oxygen bonds, showcases the great

stability of such compounds. It is worth noticing that the surface atoms

need to satisfy a bond valence of 0.5 as well, since despite their

uncompleted coordination outwards, it is not possible for them to increase

the valence of the bonds towards the bulk because those are already

“saturated” with a valence of 0.5.

Successful doping and stable solid solutions (mixed oxides) can be

predicted using the Bond Valence model. Tetravalent cerium in ceria is

eight coordinated, while trivalent cerium is six coordinated. Both show a

bond valence towards oxygen of 0.5 (4/8 = 3/6 = 0.5). The prediction is


that other elements with bond valences towards oxygen of 0.5, such as Zn

or Zr, are potential candidates to be successfully incorporated into the

ceria structure, provided they have a similar size of that of the cerium

cation they are replacing. Indeed, ceria has been doped with zinc;

reversibly ZnO has been doped with Ce and mixed oxides of CeO2–ZnO

and CeO2–ZrO2 have been prepared and extensively studied.208-210

The Bond Valence Model can also be applied to organic ligands.176

A

comparison between the carboxylic and carboxylate situations is shown

in Figures 13a and 13b. In both cases, the carbon atoms have been

assigned a valence of 4, which in case of carboxylic acids, is divided as:

1.1 units to bond the next carbon in the carbon sequence, 1.8 units to the

carbonyl oxygen and 0.8 units to the hydroxyl oxygen. In contrast, when

the acid deprotonates to a carboxylate, the bonds between the two oxygen

atoms can hybridize and thus, 1.45 units of valence are assigned to each

of the carbon–oxygen bonds. This affects not only to bond distances, but

to the strength of the oxygen ligand bond. For carboxylic acids, the only

intermolecular bonding possible is through hydrogen bonds, as shown by

the dashed lines of Figure 13b. However, a carboxylate can either form

up to three hydrogen bonds or bond an electron acceptor with a valence

of 1.1 (0.55 x 2).


Figure 13. Expected structures and bond valences of different molecules.

Fig 13a) has been adapted from REF 176.

C C

O

O

H

H

H

C C

O

O

H

H

H

H

R C O

H

H

R C O

H

HH

O HO H

N

H

R

H

N

O

O O

1.45

1.45

0.55

0.55

1.1

0.97

0.03

0.97

0.03

1.1

1.1

1.8

0.8 0.2

0.2

1.0

1.0

1.0 1.0

0.8

0.2

0.2

1.01.0

1.0

0.8 0.20.4

0.4

0.4

1.0

0.8

0.2

0.41.66

1.66

1.66

0.33 0.17

0.17

1.2 0.20.8O H

0.8 0.2

0.6

0.6

0.1

O H0.8 0.2

0.4

0.8

H

(a) (b)

(c) (d)

(e) (f) (g)

(h) (i) (j)


The fact that the oxygen atoms in a carboxylate ligand can form bonds

with a valence contribution of ca. 0.5, explains well the fact that

carboxylates have been found to bond in a chelate fashion and three

carboxylates, as it happens in NTA, can precipitate a trivalent cation,

such as Ce (III), from aqueous solution. In addition, a bond strength of

ca. 0.5 units of valence per carboxylate oxygen, matches precisely the

valence of the dangling cerium surface bonds, explaining the affinity of

these ligands for cerium oxide surfaces.

One useful consequence of applying the bond valence method is that

strong bonding coordination compounds can be predicted. For instance, it

is possible to predict that carboxylic acids would bond on ceria surface,

but nitrates would not. In this respect, the Bond Valence model discerns

between inner and outer-sphere behaviors. The oxygen atoms in nitrates,

Figure 13i, can only form bonds of valence 0.17 or 0.33. All of them too

far from the 0.5 valence required to bond cerium in the inner-sphere.

Alcohols, as shown in Figure 13c, are only able to form weak hydrogen

bonds through both the oxygen and hydrogen of the hydroxyl group. In

the eventual case of deprotonation, the remaining valence of the oxygen

would lie between 0.8 and 1, depending on whether a hydrogen bond

(0.2) is formed.

In primary amines, Figure 13j, the total valence of the nitrogen is 3 and it

is split in 1 + 2 x 0.8 = 2.6 which leaves 0.4 for bonding through the lone

pair of the nitrogen. This 0.4 dative bond would be used to bind ceria

surfaces in the absence of carboxylates.


This demonstrates that the valence model can be used as a tool to predict

surface coordination and that this model fits with the experimental results

obtained in this study.

4.3 Robustness of the carboxylic acid based systems

As continuation of the work, and in order to test the robustness of this

methodology, a number of ceria modifications were conducted with the

carboxylic acids listed in Experimental section (benzoic, oleic, palmitic,

mandelic, etc.), results not included in the appended papers. The fact that

in all cases an interaction between nanoceria and the carboxylic acids was

observed, strengthen the assumption that is the carboxylic group (the only

thing all these molecules had in common) what was causing the strong

interactions. An example that illustrates well this point was the addition

of benzoic acid to synthesis such as those of Route 1A (vide 54). When

benzoic acid was added to a dispersion of unmodified ceria in aqueous

solution, the particles irremediably and almost instantly phase-separated

and aggregated at the water-air interphase. This is easily explained due to

the hydrophobic character of the aromatic ring of the benzoate, that

would now cover the surface of the ceria nanoparticles. The experiment

was very visual: the bright orange dispersion would turn colorless and the

precipitate would be of pale yellow color. If the particles were collected,

they could then be redispersed in suitable organic solvents. However, the

aggregation experienced during this rough surface modification did

compromise the particle size and therefore only short-term stable

dispersions were attained. However, these experiences were useful for the

subsequent developments.


Many carboxylate modifications were tested against dispersibility and

stability. For example, ceria was successfully dispersed in 1,4-butylene

glycol, Ethylene glycol, ethanol and water when modified with 2,2-

(Methoxyethoxy)acetic acid. When oleic acid was used, ceria dispersed

well in short–chain hydrocarbons such as hexane or heptane. Citric acid,

poly(acrylic acid) and itaconic acid provided stability in aqueous

solution. Since some of these acids were already reported in the literature

as ceria stabilizers, their use allowed guaranteeing that the synthetic

approaches carried out in this work produced ceria nanoparticles of

similar characteristics.

4.4 Encapsulation of nanoparticles - nanocomposites

To formulate novel ceria–polymer nanocomposites the first step had to be

ensuring a successful encapsulation of the ceria nanoparticles into

polymer matrices. A carboxylic acid with a reactive azo moiety was

suggested as polymerizable ligand to be used as ceria surface modifier by

Prof. Leiza (see Papers II and III). As expected, the radical initiator

4,4’-azobis(4-cyanovaleric acid) (ACVA) was a very thermosensitive

molecule and a particularly weak dicarboxylic acid, Figure 14. In view of

that, the synthetic protocol followed along previous synthesis (based on

Routes 1 and 2, vide 54) had to be optimized. This new route is described

in Paper III. The two most significant alterations were i) the need to wait

until the temperature of the reaction dropped to ca. 30 °C, in order to

minimize the cleavage of the azo moiety and ii) that the carboxylic acid


was deprotonated in high pH solution prior being added to the

nanoparticle dispersion in order to accelerate the surface coordination of

the ligand (by removing the deprotonation step in the cation exchange

reaction) and thus minimize the aggregation of ceria nanoparticles.

Interestingly, these changes affected to a large extent the morphology of

the resulting nanoparticles. It is worth recalling that the particles obtained

during the synthesis associated to Paper I were spherical. However, the

nanoparticles obtained during this combined ACVA–modifications

consisted in a mixture of nanorods and spherical nanoparticles, see Figure

15 (Figure 2 in Paper II). Previous studies suggest that for such

precipitation reactions, the morphology might be dependent on whether

the forward or reverse precipitation methods are followed.28

This suggests

that the resulting morphology is dependent on the pH or more precisely,

changes in pH during synthesis.

Figure 14. 4,4’-azobis(4-cyanovaleric acid) (ACVA) molecule


Figure 15. TEM of ACVA-modified ceria. Two morphologies can be appreciated:

nanorods and spherical particles. Adapted from Fig 2 in Paper II.

In order to corroborate that the findings from Paper I are applicable to

this new carboxylic acid, the FTIR spectra of the surface complex and the

aqueous coordination compounds were correspondingly compared and

equate. Observing a comparable shift between the carbonyl vibration

band of the protonated acid and the asymmetric vibration of the

carboxylate coordinating to cerium for both ACVA and NTA, allows us

to confirm that despite lacking values for the strength of the ACVA to

Ce3+

bond, it had to be of similar magnitude to those formed between


NTA and Ce3+

. Consequently the inner-sphere can be extended to this

system and with it all the associated benefits, vide 34.

Performing a synthesis based on the steps of Route 4C (see Experimental

section), ensures the survival of the reactivity of the azo moiety after the

synthesis and thus, the resulting dispersion could be polymerized on-

demand at a later stage. Indeed, following a conventional emulsion

polymerization process (i.e. an anionic surfactant is added to stabilized

the precipitating polymer particles), these NTA&ACVA–modified ceria

nanoparticles allowed to graft a polymer from the nanoceria surface

which resulted in stable hybrid particles dispersed in aqueous solution,

details in Paper III. It should be highlighted that the only radical initiator

present in the system was the ACVA adsorbed on the nanoparticles.

However, a better (less aggregated nanoparticle dispersion) could be

produced if the initiator capping agent was added at higher temperatures

(i.e. earlier in the process of making nanoceria, as described in Route

2B). This inevitably triggered the cleavage of the azo moieties and for

this reason the polymerization step had to be conducted straightaway. In

this case the process resulted in nanoceria successfully encapsulated in

latex particles, see Figure 16. Further details on Paper II.


Figure 16. TEM micrograph of the hybrid formed after polymerizing ACVA-modified

ceria with MMA. Adapted from Figure 4, Paper II.

A series of latex hybrids with different monomer compositions were

made, demonstrating the robustness of the ceria encapsulation process

towards different monomer compositions, Figure 17. The evaluation of

films formed after the coalescence of the resulting hybrid particles

showed that the ceria nanoparticles remained well dispersed after film

formation, Figure 18. It should be emphasized that the emulsion

polymerization process followed in Paper III is very robust and similar

to those used in the industry. This should suffice to ensure the readiness

of this approach for encapsulation processes at industrial scale.


Figure 17. Appearance of the cast hybrid films, Figure 9 in Paper III

Figure 18. TEM micrograph of the hybrid film b of Figure 9 above these lines.

From Paper III, Figure 10.

When using ACVA as initiating ligand, polymer chains are consequently

grafted–from the surface. This minimizes crosslinking of the polymer

chains, which is desired for applications such as coalescing films for


coating technology. An alternative route to grafting–from a surface is

grafting–to, which implies that the surface functionality must be

polymerizable e.g. have a reactive double bond that can react with a

propagating chain. Modifying ceria with polymerizable double bonds is

expected to yield completely different mechanical properties due to the

multitude of reactive sites originated on ceria surface, Figure 19.

Nonetheless, this new approach also introduces a limitation: because the

surface modifier is in itself a monomer, at the moderate temperatures of

synthetic reaction (70 – 80°C) the polymerization is prematurely initiated,

causing early undesired crosslinking of the particles in an emulsion

polymerization process. In order to maintain the polymerizability of the

surface modifiers intact during the synthesis of ceria, a room temperature

route was designed, see Route 4D or Paper IV. In short, a cerous

methacrylate complex was formed in aqueous solution of pH 6. Adding

the strong oxidizing agent, hydrogen peroxide, yielded the ceric complex,

that at the pH of the solution is able to hydrolyze water and precipitate as

methacrylate–modified nanoceria. The resulting aqueous dispersion can

be blended with short-chain alcohols and thus conduct a precipitation

polymerization or dispersion polymerization reaction. Alternatively, the

modified nanoceria can be transferred to certain organic solvents or,

remarkably, to monomer mixtures such as HEMA, PEG-DA, gycidyl

acrylate, and similar hydrophilic monomers (see Paper IV). Transfer of

the modified ceria nanoparticles to a monomer phase gives a 100% solids

formulation that after photo-polymerization produces transparent films

that manifest “nanocomposite improvements”: combination of the

properties of the individual components, such as UV absorbance, in

addition to new properties that neither of the individual components had


(mechanical performance, refractive index, etc). Indeed, the

photopolymerization of a sample prepared following Route 4D3 with

PEG-DA monomer yields a hybrid nanocomposite of homogeneous

appearance, Figure 20. This was exemplified in Paper IV as sample Ac3.

Figure 19. Thermoset resulting from grafting–to the surface of ceria

Figure 20. Thin films of nanoceria–PEGDA (left) and PEGDA (right).


The so-prepared nanocomposite, with nanoceria content of ca. 2%,

exhibited a significant change in mechanical performance compared to a

reference sample that did not contain any ceria nanoparticles. From DMA

analyses, it was observed that the Tg, as determined by the tan peak

value, was shifted upwards from 90 ⁰C to 120 ⁰C, denoting a very strong

reinforcing effect of the nanoceria, Figure 21. Intriguingly, the modulus

in the rubbery region was lower for the nanocomposite compared to the

reference, Figure 22. If the theory that applies to polymer thermosets is

applied to hybrid nanocomposites, then it implies that the crosslinking

density was lower for the nanocomposite than for the reference. A lower

crosslinking density while at the same time, an enhancement of 30 ⁰C in

Tg is counterintuitive, however not unreasonable. Ceria is a UV absorber

whose absorption overlaps with that of the irradiation during curing; and

while it was not able to compromise a full polymerization, it could have

affected the kinetics of the reaction. In addition, the fact that the

methacrylate monomers were not homogeneously distributed in the

system, but concentrated on the surface of the nanoparticles could lead to

a change in the propagation and termination mechanism of the acrylate

polymer chains as they encounter and graft to the nanoparticles. It is well

known that acrylates and methacrylates undergo different termination

mechanisms.211

The lower crosslinking density, despite the uncertainties

on its origin, does nothing but stress that the changes in mechanical

properties, i.e. higher Tg, are consequence of the interphase modification

that resulted after encapsulating ceria nanoparticles with a methacrylate

interface.


Figure 21. tan δ of the nanocomposite and reference sample as function of

temperature. From Paper IV, Fig. 11

Figure 22. Storage modulus as function of temperature. From Paper IV, Fig. 10


It is worth mention here that ceria has been described as radical

scavenger212

and therefore when present during a polymerization, lower

molecular weights, i.e. shorter chains of the polymer, can be expected.

However, the mechanism of radical scavenging during polymerization

processes has not yet been described. One possibility is that ceria, due to

its OSC, releases O2 in the anaerobic conditions of the polymerization

and this O2 is then responsible for scavenging the radicals that terminates

chain growth prematurely.

To this point, the coordination chemistry of different carboxylate ligands

has been exploited to synthesize ceria nanoparticles capped with a variety

of carboxylate ligands. However, an alternative and facile route to obtain

nanoceria endowed with reactive moieties (or for that matter, any other

feature), that does not involve any synthesis of ceria nanoparticles, is the

surface modification of commercially available nanoceria dispersions.

This work has argued that the coordination of carboxylates during the

synthesis of ceria was possible due to the inner-sphere complexation that

the carboxylates are able to achieve. The inner–sphere model is a

graphical, intuitive representation for the chemical process that involves

the replacement of water molecules and hydroxyls, coordinated to the

metal by the inner–sphere capable ligand. If water and hydroxides are

regarded as ligands this process is nothing more than a simple ligand

exchange reaction. It should not come as surprise that most of the

commercially available nanoceria dispersions employ carboxylates,

polycarboxylates or polymeric carboxylates as capping agents. As already

reviewed in Paper I, the driving force for inner or outer sphere


complexation resides, primarily, in the ΔS of the reaction and this allows

to quantitatively describe the inner-sphere value of the different acids.207

A more practical, yet not as exact, method to estimate the inner or outer

sphere behavior is the pKa of the carboxylate. There is enough evidence

to classify acids with pKa below 2 as outer–sphere and it is safe to say

that the inner-sphere complexation dominates for carboxylic acids of pKa

above 3. For 2 < pKa < 3 an intermediate situation occurs.162

Special care

should be taken here. This might be misleading and one could think that

the higher the pKa value of the acid, the more inner-sphere behavior. This

is only valid for monocarboxylic acids that do not form other ligand-to-

metal bonds or that do not have conjugated insaturations with the

carboxylate.207

For chelates such as polycarboxylic acids when

coordinating ionically with for instance cerium or other lanthanides and

actinides; the stability of the complex is function of the ΣpKa.213

Furthermore, if the ligand is able to form other bonds apart from the

carboxylate(s), this should increase the stability and enhance the inner-

sphere behavior. With these warnings in mind, Paper V had the ambition

to prove whether this point would be applicable to ceria. Thus a ligand

exchange reaction based on thermodynamic considerations was tried on

ceria nanoparticles, where the second ligand was chosen to have an

additional reactive moiety. The objective of Paper V was two-fold. First,

produce ceria nanocomposites similar to those of Paper IV but with

larger ceria lattices. Second, succeeding in performing a ligand exchange

based on the considerations described in the literature for solvate Ce3+

would strengthen our early assumption that the coordination chemistry

reported in the literature lanthanide metals, in particular cerium, can and


should be extended to ceria surface in order to provide the scientific

community with a toolbox to fine tune the surface of this metal oxide.

A commercial nanoceria dispersion, claimed to be stabilized by acetate

was modified with acrylic acid following Route 4E. A more detailed

description of the synthetic methodology is found in Paper V.

Interestingly, prior modification the nanoceria dispersion did not have

any significant smell of acetic acid. However, after the modification,

which happens shortly after enough stirring or agitation, the odor of the

dispersion was that of acetic acid. To the so-prepared acrylate–modified

nanoceria dispersion, a monomer, a radical initiator, and in some cases a

solvent, were added and a polymerization, triggered by the decomposition

of the radical initiator AIBN at 70 ⁰C, was carried out.

The polymerization transformed the liquid nanoceria dispersion into a

monolithic, presumably highly crosslinked material, Figure 23. When

using HEMA as monomer, the water was not excluded, therefore a

hydrogel had necessarily be the structure obtained. In order to discern

whether all the ceria was bound to the polymer network a leaching

experiment was conducted. An aliquot of each sample was submerged,

repeatedly, in large volumes of water (water was the original solvent for

the nanoceria dispersion) for a total of 36 hours. The thermogravimetric

analysis of these aliquots was compared against specimens not subjected

to the leaching test. In all cases, the residual solids content was higher for

the leached samples, signifying that no ceria was leaching but some of the

unreacted monomer or some not uncrosslinked oligomers did. Further

details on Paper V. This, of course, implies i) that all the ceria

nanoparticles were modified with acrylate through a ligand exchange


reaction and ii) that the acrylate ligands on ceria surface were reacting in

the polymerization causing a strong bond between the nanoceria and the

polymer matrix that was resistant to hydrolysis.

Figure 23. Nanoceria–HEMA hydrogel nanocomposite obtained in Paper V

Conclusions | 103

5. Conclusions

Through a detailed study of different precipitation conditions to form

cerium oxide nanoparticles, from the aqueous solution of cerium salt

precursors, it was soon understood that due to the small size and highly

charged nature of cerium cations, the outermost surface cerium cations of

cerium oxide nanoparticles prefer to coordinate carboxylates through

ionic bond.

Furthermore, the carboxylates have been identified to coordinate in the

inner-sphere of ceria surface cations, substituting hydroxyl or water

molecules previously coordinated on the surface. The chelating power of

polycarboxylic acids was exploited to further enhance the stability of the

complex and thus the surface modification. Employing carboxylates as

surface modifiers not only prevented ceria nanoparticles from aggregate,

but also allowed the dispersion of nanoparticles in a variety of solvents.

Aggregation of cerium oxide nanoparticles in aqueous solution was

prevented for extraordinary long times by using water soluble

aminopolycarboxylates as capping agents. Understanding the ionic nature

of the chemical bond at the interface, together with the diversity of

carboxylic acids available, translates into a versatile tool to design and

tailor the cerium oxide–ligand interface.

In addition, through the use of polymerizable ligands, carrying different

reactive moieties it has been possible to design a range of ceria surfaces

used for the development of novel ceria–polymer nanocomposites where

the chemistry and structure can be controlled. The most remarkable

development was that cerium oxide nanoparticles were not only

104 | Conclusions

chemically bonded to polymer matrix but the nature of the bond

understood.

This opens up a toolbox for designing hybrid nanocomposites where the

encapsulation of ceria nanoparticles can be chosen as grafting–from or

grafting–to; and thus the structure controlled to yield different degrees of

crosslinking.

All this has been exemplified in the preparation of ceria nanoparticles

carrying azo, acrylate and methacrylate moieties on the surface, which

have been used, independently, to produce novel thermoplastic and

thermoset nanocomposites through different polymerization processes

such as emulsion, miniemulsion and photocuring.

The resulting nanocomposites were homogeneous and retained the

individual properties of the continuous and disperse phases, in addition to

new features that arise from the combination of such materials. The

interfacial design offered, in addition to structure control, a strong

bonding between the covalent polymer network and the ionic

nanocrystals. Mechanical properties of the resulting nanocomposite, such

as stiffness and glass transition temperature were significantly enhanced.

The optical properties of ceria, i.e. UV absorption and high refractive

index were successfully preserved and transferred to the nanocomposite.

Finally, it has been shown that a choice of ligand based on

thermodynamic considerations enables an efficient ligand exchange on

pre-made nanoceria. This resulted in stable dispersions of functionalized

ceria nanoparticles that could be polymerized as already described.

Final remarks | 105

6. Final remarks

As we have seen, inorganic nanoparticles such as metal oxides need to be

surface modified with organic molecules to prevent aggregation and to

facilitate dispersion into either solvents or solid polymeric materials. This

explains why nanotechnology shall be seen as an interdisciplinary field

where different branches of chemistry such as inorganic, organic,

polymer, surface and physical chemistries have to come to communion in

order overcome the challenges that formulating these new materials

represent.

7. Future work

Although the work detailed in this thesis provides understanding on

surface coordination to cerium oxide nanoparticles, it should be validated

whether the findings for cerium oxide are applicable to other metal oxides

and in which extent.

When it comes to encapsulation of nanoparticles, two main

functionalities have been explored, the radical initiating and reactive

double bond. It would be interesting both from an academic and

industrial perspective to investigate other functional surface modifiers

In order to model and describe the system with accuracy, the

simplification of considering only one coordinated carboxylic acid at a

time was necessary. Most likely, in commercial formulations these

concepts will be later applied in a more complex formulation where

106 | Future work

multiple carboxylic acids are coordinated, each bringing its own

advantage. The study and modelling of such systems should be

encouraged.

Similarly, only acrylic polymers have been used throughout this work.

The encapsulation of ceria in different polymeric matrices should be a

natural step. Furthermore, the mechanical implications of encapsulating

ceria nanoparticles at different concentrations remains unsolved. This,

together with exploring the encapsulation of different morphologies, in

particular nanorods versus nano–spheres could expand the applicability

of this hybrid nanocomposites.

Acknowledgements | 107

8. Acknowledgements

I will try to express here my gratitude to the people that deserve

recognition for having helped or contributed in one way or another to the

work that has been presented here. Without them I would not be here,

writing the last lines of this thesis. And by “try to express” I mean that

there is probably no way to put in words how grateful I feel.

My first words of gratitude are to Prof. Lennart Bergström, who

recommended me to my supervisor at YKI, Anwar Ahniyaz. Right after I

signed the contract, I went to his office and told him that I was going to

work very hard because I owed him that.

Anwar Ahniyaz has been a great supervisor and mentor. He gave me the

right balance of independence and freedom to think differently and at the

same time he was quietly but methodically supervising my work and

results. From Anwar I have received advice, not only in regards to this

thesis, but also many wise thoughts that will help me in many other

aspects of my professional development. Some of his advice has already

proven useful and I am sure his teachings will make of me a better and

more respected professional in the future.

Mats Johansson, as my academic supervisor, gave me confidence,

support and invaluable help in structuring the different ideas, experiments

and results. Mats has a gift for transmitting clear and concise ideas and

concepts. His guidance helped me to keep focus on the primary goals

while enriching this thesis with all the relevant findings that we were

developing alongside.

108 | Acknowledgements

During the first years, there was a special person to me, Galina Alvarez.

We shared not only lab but also project and language. She helped me

with almost everything I can think of. On top of that, we had very

interesting discussions about Swedish society and how difficult but

worthy it is to integrate. Whenever Galina could not help or after she

retired, I always got priceless help and support from the lab engineers and

instrument specialists. Anders Svensk, Annika Dahlman, Karin

Hallstensson, Lukas Boge, Johan Andersson, Eva Sjöström, Marie

Ernstsson and Mikael Sundin deserve recognition for this thesis, since

without their help it would had simply been impossible to undertake the

tasks and experiments that this work is based on. I could not forget

Rodrigo Robinson who always –early, late, on weekends…– has been

willing to help me with SEM or EDS. His willingness and predisposition

to help I am sure are part of his character. Together with the experimental

part of the work, a significant effort was given to finding references and

learning from the literature. In more occasions that I can recall, I needed

the assistance from our librarian, Eva Lundgren. I hope she knows how

much I needed and appreciated her help.

From Stockholm University, three people are greatly acknowledged,

Cheuk-Wai Tai, who helped with everything that involved the use of

TEM. Cheuk-Wai helped me to the extent of rightfully deserve

co-authoring one of the publications appended to this thesis. Without his

teaching and assistance I would not have been able to develop the skills I

needed to properly characterize any of my samples. Kjell Jansson and

Lars Eriksson have helped me in numerous occasions running SEM and

XRD respectively.


I cannot forget Niklas Ihrner, who partnered up with me in the

development of some of the nanocomposites shown in this work. His

expertise in preparing and characterizing photo-cured films led to a very

well thought and comprehensive publication together.

During my time in Polymat, I had the opportunity to work with Miren

Aguirre, a brilliant and devoted student of whom I keep a high level of

esteem. There I was supervised by professors José Ramón Leiza and

María Paulis. Despite our short time together, their excellent supervising

and teaching skills resulted in a very fruitful experience and the

knowledge learnt from them was essential in the development of this

thesis.

I have to admit that I feel privileged from belonging to the Coatings

Division at KTH. It is an extremely talented group of researchers and I

would like to thank all of its past and present members for being so

welcoming and openhearted. Especially to Kristina Olofsson and Martin

Wåhlander for making sure I did not have to wander the suburbs of San

Francisco by myself. Carmen Cobo deserves special mention for her help

characterizing some of my samples. Samuel Pendergraph is greatly

acknowledged for our fruitful discussions.

As industrial PhD student I had my office and the people I had spent the

most time with at SP. It has been a wonderful and enriching experience,

since SP is full of capable researchers with knowledge and skills in very

different areas. I cannot think of a place where one can learn so much

from so many. I had the opportunity to attend a few courses here at SP

and I feel honored to have had the opportunity to learn from some of the


most talented people I have ever met: Mark Rutland, Per Claesson or

Bengt Kronberg, Martin Andersson and Isabel Mira are only some

examples.

I would like to show my gratitude to the past and present doctoral

candidates at SP. Asaf Oko, Maria Badal, Hanna Dahlenborg, Christian

and Petra Mille, Lina Ejenstam, Lisa Skedung, Joakim Karlsson and

Marine Nuzzo have been very supportive and I have always enjoyed our

technical as well as the philosophical discussions.

Christian Mille, my office mate for the first two years shall be doubly

acknowledged. Christian made me feel welcome not only in his office but

made sure I joined the social activities like fika, the student’s lunch on

Fridays and the theme parties. He was also the person that answered the

most intriguing question I had for the first two months of my studies.

Why was everyone –and I mean every one– disappearing from their

office or labs at 2 pm sharp, especially on Thursdays?

One day Christian said:

—Hey! It’s fika!

And I thought: I’m not sure it’s fika, but it’s dark and it’s snowing…

So I didn’t move from my chair, despite he jumped off his. And then

he explained.

Thus, Christian deserves the credit for explaining and introducing me to

fika.

As I progressed in this study, more complex chemistry got involved, and

when it came to polymer chemistry, the teachings and assistance I got


from the exceptional Dr. Jens Sommertune (I am including his title

because the guy is German) has been inestimable.

Marie Sjöberg, my immediate manager during all these years deserves

not only my gratitude but all my respect and admiration. Marie could

have been extremely busy and still, she always had time to listen

whenever I went to her seeking help or advice. My new capo, Andrea

Fornara, deserves my appreciation for his unconditional support.

I would like to mention as well, Annika Bergström, Karin Hede and

Rositsa Larsson who have constantly helped with the very necessary

administration and paperwork. Kerstin Linsten, to whom I probably owe

her final decision to hire me, has always been very kind, somehow

protective. Knowing that there was someone higher up in the hierarchy

that would help when needed, gave me the necessary peace of mind to

focus on the work.

Most of this work has been conducted in parallel or within several

research projects. I have met very skilled and professional people through

our multiple meetings. I would like to thank them for sharing some of

their knowledge with me: Peter Collins, Robert A. Martuch, Marielle

Henriksson, Stig Bardage, Jackob Becker, Decheng Meng, Elizabeth

Bardsley, Erik Ronne, Dietrich Pantke are only a few names on a longer

list.

Working in projects within and aside of my main research project also

gave the opportunity to work with talented people within SP, Karin

Persson and Anders Larsson have been a reference for me for their

organizational skills and broad multidisciplinary knowledge.


Finally, the most important people involved in the successful outcome of

this thesis: my friends and family. As it is natural, I became closer to

some of the people I have met during these years. I owe Sara Olsson,

Ruben Alvarez, Maziar Sedighi, Ali Aboudzadeh, Shaghayegh

Hamzehlou, Hanoi Labrador, Antonio Alvarez and Sandra García not

only many good memories together but also my gratitude for their

support.

My family, and among them, Almudena deserves my deepest gratitude.

Almudena embarked on this venture with as much enthusiasm as myself

and she has kept it even when it got tough and tiring, her emotional

support has been indispensable to accomplish this thesis.

As it could not be otherwise, the financial support provided by SP

Technical Research Institute of Sweden is greatly acknowledged.

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