Oxidation of surfactant stabilized magnetic cobalt nanoparticles [Elektronische Ressource] / Britta Vogel. Fakultät für Physik

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DissertationOxidation ofsurfactant stabilizedmagnetic cobalt nanoparticlesvorgelegt vonBritta VogelJuly 18, 2011Universität BielefeldFakultät für PhysikFor AleksErklärungHiermit erkläre ich, dass ich die vorliegende Arbeit selbständig verfasst und keine anderenals die angegebenen Hilfsmittel verwendet habe.Bielefeld, 18. Juli 2011Gutachter:Prof. Dr. Andreas HüttenProf. Dr. Dario AnselmettiDatum des Einreichens der Arbeit: 22. Juli 2011Contents0 Introduction 11 Theoretical Background 31.1 Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.1 Forms of magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1.2 Magnetism on nanoscales . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Formation of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.2.1 Wet chemical approach . . . . . . . . . . . . . . . . . . . . . . . . . . 91.2.2 Kinetical description of particle formation . . . . . . . . . . . . . . . 101.2.3 Thermodynamical description of particle formation . . . . . . . . . . 112 Synthesis of cobalt nanoparticles and oxidation 132.1 Synthesis of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.1.1 Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2 Surfactants and surfactant exchange . . . . . . . . . . . . . . . . . . . . . . 142.2.1 Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.
Publié le : samedi 1 janvier 2011
Lecture(s) : 45
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Source : D-NB.INFO/1015762840/34
Nombre de pages : 173
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Dissertation
Oxidation of
surfactant stabilized
magnetic cobalt nanoparticles
vorgelegt von
Britta Vogel
July 18, 2011
Universität Bielefeld
Fakultät für PhysikFor AleksErklärung
Hiermit erkläre ich, dass ich die vorliegende Arbeit selbständig verfasst und keine anderen
als die angegebenen Hilfsmittel verwendet habe.
Bielefeld, 18. Juli 2011
Gutachter:
Prof. Dr. Andreas Hütten
Prof. Dr. Dario Anselmetti
Datum des Einreichens der Arbeit: 22. Juli 2011Contents
0 Introduction 1
1 Theoretical Background 3
1.1 Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Forms of magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Magnetism on nanoscales . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Formation of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.1 Wet chemical approach . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.2 Kinetical description of particle formation . . . . . . . . . . . . . . . 10
1.2.3 Thermodynamical description of particle formation . . . . . . . . . . 11
2 Synthesis of cobalt nanoparticles and oxidation 13
2.1 Synthesis of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Surfactants and surfactant exchange . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.2 Surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1 Oxidation of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.2 Mathematical description of the oxidation process. . . . . . . . . . . 20
3 Devices 24
3.1 TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3 STEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.1 Image modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.2 Lens aberrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.4 EDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 FIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5.1 LMIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5.2 Interactions of electrons and ions with the sample . . . . . . . . . . . 33
3.5.3 Ion etching and sputtering . . . . . . . . . . . . . . . . . . . . . . . . 34
3.5.4 Gas assisted ion beam etching . . . . . . . . . . . . . . . . . . . . . . 34
3.5.5 Gas assisted ion beam deposition . . . . . . . . . . . . . . . . . . . . 34
3.5.6 Sample manipulation with the micromanipulator . . . . . . . . . . . 35
3.6 XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 AGM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.1 Function of an AGM . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.2 Low temperature setup . . . . . . . . . . . . . . . . . . . . . . . . . 38
viContents
3.8 IR-spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4 Nanoparticles - characteristics 41
4.1 Cobalt nanoparticles - overview . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3 Crystallinity of Co particles . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Oxidation of particles prepared with different surfactants . . . . . . . . . . . 45
5 Surfactants and surfactant exchange 47
5.1 Basic particles prepared with trioctylphosphin oxide (TOPO) . . . . . . . . 47
5.2 Surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2.1 Used surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.2.2 Amine group as head molecule . . . . . . . . . . . . . . . . . . . . . 51
5.2.3 Carboxyl group as head molecule . . . . . . . . . . . . . . . . . . . . 56
5.2.4 Size reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2.5 Inter particle distance . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.6 Convolution of the surfactant molecules . . . . . . . . . . . . . . . . 64
5.2.7 Change of magnetic properties . . . . . . . . . . . . . . . . . . . . . 65
5.3 Comparison - surfactant exchange on TOPO particles . . . . . . . . . . . . 68
5.4 Basic particles prepared with oleylamine . . . . . . . . . . . . . . . . . . . . 70
5.4.1 Used surfactants and size reduction . . . . . . . . . . . . . . . . . . . 70
5.4.2 Conclusion for self-assembly changes by surfactant exchange . . . . . 71
5.4.3 Change of magnetic properties . . . . . . . . . . . . . . . . . . . . . 72
5.5 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6 Oxidation at room temperature 74
6.1 Calculation of the effective magnetic particle volume and radius . . . . . . . 77
6.2 Second surfactant exchange on TOPO . . . . . . . . . . . . . . . . . . . . . 80
6.3 Surfactant exchange based on oleylamine . . . . . . . . . . . . . . . . . . . . 81
6.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7 Temperature dependence 84
◦7.1 Particles stored at a temperature of −18 C . . . . . . . . . . . . . . . . . . 84
◦7.2 Particles stored at a temperature of 48 C . . . . . . . . . . . . . . . . . . . 88
◦7.3 Particles stored at a temperature of 80 C . . . . . . . . . . . . . . . . . . . 88
◦7.4 Particles stored at a temperature of 121 C . . . . . . . . . . . . . . . . . . . 91
◦7.5 Particles stored at a temperature of 180 C . . . . . . . . . . . . . . . . . . . 94
◦7.6 Particles stored at a temperature of 300 C . . . . . . . . . . . . . . . . . . . 94
7.7 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8 Temperature effects on crystallinity and shape 100
9 Oxidation measurements during the first 30 minutes after fabrication 106
9.1 AGM oxidation setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
9.2 Used surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
9.3 Effects of the surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . 108
9.3.1 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
9.4 Oxidation during the first thirty minutes after fabrication . . . . . . . . . . 110
viiContents
9.4.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.5 Crystallinity of particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
9.6 Oxide shells - conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
10 Oxidation behaviour in dependence on shape 124
10.1 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.2 Finite elements simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.3 Volume consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
10.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
10.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
11 Oxidation of nanoparticle clusters 131
11.1 Older Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
11.2 Newer Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
11.3 Effective oxygen diffusion in clusters . . . . . . . . . . . . . . . . . . . . . . 141
11.3.1 Oxygen diffusion process in the case of one nanoparticle . . . . . . . 141
11.3.2 Oxidation process in the case of nanoparticle clusters . . . . . . . . . 144
11.3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
12 Summary 152
.0.4 Conference Contributions . . . . . . . . . . . . . . . . . . . . . . . . VIII
.0.5 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII
viii0 Introduction
“There is plenty of room at the bottom” was the name of a talk given by Richard Feynman
at the annual meeting of the American Physical Society in 1959.
Today the main idea is still more topical than ever. Research in the field of nanoscale
structures is booming in modern physics because nanomaterials exhibit outstanding prop-
erties. The surface to bulk ratio is extremely large and, therefore, surface effects gain in
importance, leading to unique magnetic, electronic and optical effects [1,2,3]. The boom
has been made possible by the fast development of technology during the last few decades,
allowing the visualization of structures in the nanometer range. Improved methods to
fabricate nanoparticles have contributed to this development as well. Nanoparticles and
physics of nanostructures already play an important role in economy. Products built of
or containing nanoparticles are widely spread although the risks and advantages have not
been evaluated completely yet [4].
History shows, that nanoparticles accidentally appeared much earlier. By the end of the
middle ages colloidal gold was used as heat stable pigment to fabricate red glass [5].
1At the present, one of the most impressive discoveries are no doubt ferrofluids .
Who would not be fascinated if a previously even, black, oily liquid suddenly exhibits
ripples and drops upwards from a petri dish, defying gravity, to a conical tip of an electro-
magnetlocatedinsomedistanceabovethepetridish. Itflows ontheconicalsurface, forms
afilmandthengrowsspikeswithsizesintheseveralmillimeter range,whichthensuddenly
start to shrink. They collapse and flow and drop back as liquid into the petri dish, while
a magnetic field is varied over the electromagnet during the whole process. Several videos
canbefoundontheinternetplatformyoutube thatcanbedeclaredas(somekindof)art[7].
Magnetic nanoparticles can be applied directly or as ferrofluid. In the case of the fer-
rofluidthemanipulationoftheliquidbytheparticlesisthemaintask. Furtherapplications
of ferrofluids are printing ink for magnetic bar codes or circuit paths [6]. They are also
used as sealant for rotatable parts in vacuum devices and space flight vessels, where the
particle suspension is kept in place by magnets. They are also used in grease to improve
thebreakingeffectof accelerators orasdamping materialin loudspeakers andmotors [6,8].
A possibility for a direct application of magnetic nanoparticles is their use as magneto-
resistive random access memory (MRAM) which would increase the storage density. [9]
Further use of nanoparticles lies in the biological and medical sector. This involves the
necessity of a biocompatible coating of the particles. Possible applications are phagoki-
netic studies, separation and purification of biological molecules and cells, bio detection of
pathogens, tissue engineering, probing of DNA structure, detection of proteins and gene
delivery. Drug delivery is another application which could improve further cancer therapy
1Ferrofluids are a colloidal suspension of magnetic (nano)particles, which are stabilized e.g. with surfac-
tants against agglomeration in the liquid and to improve their solubility [6].)
10 Introduction
by transporting the drug directly into the tumor. This would enable the use of more po-
tent drugs while the rest of the body would remain less affected [10]. Today particles are
already applied successfully in studies as a contrast agent in MRI and for hyperthermia,
where the particles are injected directly into a tumor and heated by alternating magnetic
fieldsresultinginanoverheatingofthesurroundingtissueandthereforedyingoftheheated
cells [10,11,12]. Another use lies in the field of biotechnology, where the particles could be
attached to molecules as magnetic markers to gain control over the molecules on the one
hand and to monitor them with magnetoresistive biochips on the other hand [13].
IronoxideFe O particles embedded in amatrix aremostly used intodays applications.x y
The utilization of other magnetic materials offers a decreased particle size with an equal
or increased saturation magnetization. Such materials are among others Fe, FeCo and Co.
One problem bears the oxidation propensity of the particles. Metal oxides exhibit lower
or none of the desired magnetic properties.
In a ferrofluid the particles have to be stabilized against agglomeration; as well as oxida-
tion. The case of agglomeration can be prevented by covering the particle surface with
surfactant molecules. It bears an interesting ansatz to find out, to what extent the surfac-
tants prevent the particles from oxidation as well.
To gain information concerning the protection effects of the surfactants and to improve
understandingoftheoxidationprocessonnanoscales,thechangeofthemagneticproperties
during the oxidation process as well as the investigation of the microstructural properties
of the particles can be used, which are part of this thesis.
Firstashort introduction tothe theory of magnetism, synthesis and formation of nanopar-
ticles isgiven. The possibilitiesforstabilization, especiallythe covering of thenanoparticle
surfacebyamphiphilemolecules,calledsurfactants,andtheoxidationprocessofnanoparti-
clesisdescribed,followed byanoverviewoverthedevicesusedforanalysisandadesription
of the corresponding sample preparation.
The preparation of cobalt nanoparticles is described and the results of the stabilization
with different surfactants are displayed including the influence on size, shape and crystal
structure.
Afterwards the reasons for a surfactant exchange are explained.
First the used surfactants and the process of the surfactant exchange are described. Two
batches of particles were fabricated with TOPO, one was fabricated with oleylamine. The
surfactants deployed during the exchange posses either a amine or a carboxyl headgroup
and different chainlengths and number. Investigated were the influence on the size, shape,
crystallinity, inter particle distance and surfactant conformation as well as magnetic prop-
erties. The oxidation of the samples at room temperature and under ambient conditions,
◦ ◦ ◦ ◦ ◦ ◦and temperatures of −18 C, 48 C, 80 C, 121 C, 180 C and300 C was investigated, start-
ing a few hours after an initial exposure to air during sample preparation. A change in the
◦crystallinity of the particles stored at 180 C was detected. To gain more insight in the ox-
idation influence of the surfactants the sample preparation and measurement method were
modified and samples were investigated directly after fabrication and surfactant exchange,
beginning with the first measurements about one minute after initial exposure to air. The
influence of the surfanctant on the crystal structure of the particles is described. Finally a
closer look is taken at the oxidation behaviour of large multilayer particle clusters.
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