Characterizing cavity containing materials using electron microscopy
Characterizing cavity containing
materials using electron
microscopy
A study of metal oxides, mesoporous crystals and porous material
containing nanosized metal-particles
Miia Klingstedt
Cover: A twig composed of transmission electron microscopy images
c Miia Klingstedt, Stockholm 2011
ISBN 978-91-7447-247-9
Printed in Sweden by US-AB, Stockholm 2011
Distributor: Department of Materials and Environmental Chemistry, Stockholm University
"Nature composes some of her loveliest poems for the microscope and
the telescope." Theodore Roszak
Abstract
This thesis concerns the characterization of novel materials by utilizing electron microscopy techniques. The examined materials contain cavities with
certain attributes that enables desired properties for applications such as gas
separation, catalysis and fuel cells. The specimens concerned herein belong
to the following groups of materials: Metal oxides in the Sb-W-Mo-O system; ordered mesoporous silicas and carbons; hollow spheres containing Aunanoparticles; zeolite LTA incorporated with mesopores; metal organic frameworks doped with nickel.
With scanning electron microscopy (SEM) and transmission electron microscopy (TEM) you get vast possibilities within the field of characterization.
This thesis utilizes conventional electron microscopy techniques such as imaging, energy-dispersive spectroscopy and electron diffraction as well as reconstruction techniques, such as exit-wave reconstruction, electron tomography
and electron crystallography. Furthermore, the sample preparation technique
cross-section polishing has been used in conjunction with low voltage SEM
studies.
The scientific approach is to gain knowledge of nano-sized cavities in materials, in particular their shape, size and content. The cavities often have irregularities that originates from the synthesis procedure. In order to refine
the synthesis and to understand the properties of the material it is required
to carefully examine the local variations. Therefore average characterization
techniques such as crystallography needs to be combined with local examination techniques such as tomography. However, some of the materials are
troublesome to investigate since they to some extent bring limitations to or
gets easily damaged by the applied characterization technique. For the development of novel materials it is essential to find means of overcoming also
these obstacles.
7
List of Papers
This thesis is based on the following papers in order of publication.
I
"Ordered Mesoporous Pd/Silica-Carbon as a Highly Active Heterogeneous Catalyst for Coupling Reaction of Chlorobenzene in
Aqueous Media"
Y. Wan, H. Wang, Q. Zhao, M. Klingstedt, O. Terasaki and D.
Zhao, J. Am. Chem. Soc., 2009, 131, pp 4541-4550
II
"An Appraisal of High Resolution Scanning Electron Microscopy
Applied To Porous Materials"
S. M. Stevens, K. Jansson, C. Xiao, S. Asahina, M. Klingstedt,
D. Grüner, Y. Sakamoto, K. Miyasaka, P. Cubillas, L. Han,
S. Che, R. Ryoo, D. Zhao, M. Anderson, F. Schüth, and O.
Terasaki, JEOL news, 2009, 44, pp 17-22
III
"Mesopore generation by organosilane surfactant during LTA
zeolite crystallization, investigated by high-resolution SEM and
Monte Carlo simulation"
K. Cho, R. Ryoo, S. Asahina, C. Xiao, M. Klingstedt, A.
Umemura, M. W. Anderson and O. Terasaki, Solid State
Sciences, 2011, 13, pp 750-756
IV
"A new HRSEM approach to observe fine structures of novel
nanostructured materials"
S. Asahina, S. Uno, M. Suga, S. M. Stevens, M. Klingstedt, Y.
Okano, M. Kudo, F. Schüth, M. W. Anderson, T. Adschiri and
O. Terasaki, Microporous and Mesoporous Materials, 2011, 146,
pp 11-17
V
"Advanced electron microscopy characterization for pore structure of mesoporous materials; a study of FDU-16 and FDU-18"
M. Klingstedt, K. Miyasaka, K. Kimura, D. Gu, Y. Wan, D. Zhao
and O. Terasaki, Journal of Materials Chemistry, 2011, 21, 13664
9
VI
"Exit wave reconstruction from focal series of HRTEM images,
single crystal XRD and total energy studies on Sbx WO3+y (x∼
0.11)"
M. Klingstedt, M. Sundberg, L. Eriksson, S. Haigh, A.
Kirkland, D. Grüner, A. De Backer, S. Van Aert and O. Terasaki,
In manuscript
Reprints were made with permission from the publishers.
10
Abbreviations
2D
two dimensional
3D
three dimensional
BF
bright-field
BSE
backscattered electrons
Cs
spherical aberration
CBED
convergent beam electron diffraction
CMK
carbon mesostructured by KAIST
CP
cross-section polisher
CTEM
conventional transmission electron microscopy
CTF
contrast transfer function
DF
dark-field
DMF
dimethylformamide
EC
electron crystallography
EDS
energy dispersive x-ray spectroscopy
EFTEM
energy-filtered TEM
EISA
evaporation induced self-assembly
EWR
exit wave reconstruction
FD
Fourier diffractogram
FDU
Fudan university
FEG
field emission gun
FOLZ
first order laue zone
GB
gentle beam
11
HAADF
high angle annular dark field
HRTEM
high-resolution TEM
HTB
hexagonal tungsten bronze
ITB
intergrowth tungsten bronze
IUPAC
International union of pure and applied chemistry
KAIST
Korea advanced institute of science and technology
LTA
Linde type A (zeolyte A)
MCM
Mobile composition of matter
MOF
metal organic framework
NMP
N-methylpyrrolidone
OSS
organo silane surfactant
PTB
perovskite tungsten bronze
SAED
selected area electron diffraction
SDA
structure directing agent
SE
secondary electrons
SEM
scanning electron microscopy
SG
space group
STEM
scanning transmission electron microscopy
TEM
transmission electron microscopy
TEOS
tetraethyl orthosilicate
TTB
tetragonal tungsten bronze
WBP
weighted back projection
WD
working distance
WDS
wavelength dispersive x-ray spectroscope
WPOA
weak phase object approximation
XRD
x-ray diffraction
ZOLZ
zero order laue zone
12
Contents
1
2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Background to electron microscopy . . . . . . . . . . . . . . . . . . . . . . . .
15
17
2.1 Scanning electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
19
22
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24
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2.1.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Transmission electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Resolution in TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Electron diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Electron crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5 Electron tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Scanning transmission electron microscopy . . . . . . . . . . . . . . . . . . . . . . .
2.4 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 Intergrowth bronzes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Ordered mesoporous carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Ordered mesoporous silica-carbon with Pd-nanoparticles . . . . . . . . . . . . . .
3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Mesoporous LTA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Hollow spheres containing Au-nanoparticles . . . . . . . . . . . . . . . . . . . . . . .
3.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6 MOF-5 with Ni-metal particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 Mesoporous MOF-1 with Ni-metal particles . . . . . . . . . . . . . . . . . . . . . . . .
3.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
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1. Introduction
Materials today are getting more complex due to a demand for more specific
properties which we know are interrelated to the structure of the material.
Solving the structure enables us to better understand and tweak the material
structure so that we therefore get better performance for the desired application. The structural features relevant to characterize depends upon the material
and the application. A careful characterization contributes with information
that can lead to improvements in the synthesis of the material. Material development is in other words both dependent on advanced information gathering
and production techniques on a nanoscopic level. The trend of increasing complexity of the materials also puts higher requirements on the characterization
methods.
In practice, several methods often have to be combined, refined or developed specifically for the material in focus. For example crystallography is a
viable method for materials with a highly ordered structure, but it does not
show aberrations in the structure itself. To study the morphological properties
using imaging methods such as low voltage SEM is a powerful tool.
This thesis’s subject is characterization by electron microscopy of inorganic
materials and starts with a brief background to electron microscopy.
15
2. Background to electron microscopy
Electron microscopy is a technique that utilizes a ray of electrons for imaging
objects and obtaining information from the signals generated in the interaction between the electron beam and specimen. One of the benefits of electron
microscopy is the strong interaction of the beam with matter which leads to
a series of signals that can be utilized. The strong interaction can also lead
to one of the drawbacks of electron microscopy, radiation damage [1], which
Figure 2.1 shows an example of. For creating the electron beam there are two
main types of electron guns, (i) the field emission guns and (ii) thermionic
emission guns, such a LaB6 and a tungsten filament. Electromagnetic lenses
are used for focusing and magnifying the beam. There are two main types of
microscopes; the scanning electron microscope (SEM) which scans the surface with an electron beam; and transmission electron microscope (TEM)
where the beam is transmitted through the specimen. A technique where a
beam scans over the specimen and the transmitted beam is collected called
Scanning Transmission Electron Microscopy (STEM) is available in both dedicated SEMs and TEMs. The acceleration voltages are commonly used in
TEM is 100-300 kV whereas for the SEM it stretches from a few kV to 30
kV.
The resolution of a SEM is typically from a few nanometers to hundreds of
nanometers whereas the resolution in the TEM is better then a few Ångström.
TEM was invented by Ruska and Knoll already in 1931, for which Ruska
received the Nobel prize in 1986. The first commercially available TEM came
already in 1939 whereas the first commercial SEM appeared 20 years later in
the 1960’s.
2.1
Scanning electron microscopy
In SEM an electron beam scans the specimen surface where mostly the electrons and signals reflected back are utilized for within imaging to compositional information. The beam only penetrates and interacts within a specific
volume of the specimen, called the interaction volume, and information is
gained from this volume only. The size of this volume is approximately proportional to the accelerating voltage and the probe current of the beam. A
small interaction volume means that the area where the signals originates from
is more localized, and this leads to higher resolution. However, a lower accelerating voltage leads to lower resolution due to chromatic aberration. A lower
17
Figure 2.1: Electron beam radiation damage observed for SiO2 nano-particles during
TEM studies. The particles are melting due to heating caused by the electrons that
has suffered energy loss. Observation done by JEM-2100 LaB6 at 200 kV and current
density of 12 pA/cm2 s for a) 0 min, b) 5 min, c) 10 min and d) 15 min.
probe current will make the images more noisy which in turn yields to a decreased quality. So there is an apparent need to balance these two points for
obtaining the optimum resolution. Resolution limit in the SEM today is a few
nanometers but which varies depending on the properties of the material.
There are commonly two main types of scattered electrons generated by
the electron beam used for image formation: the secondary electrons (SE) in
the low energy region with energies in the range from 0 to 50 eV and the
backscattered electrons (BSE) in the high energy region with energies larger
than 50 eV. The difference in energies makes it possible to filter the SE and
BSE signals. This makes it possible to generate SE and BSE-images but they
can also be mixed to form an image.
There are four types of SE signals generated (as shown in Figure 2.2). The
SE1 signal is generated by the primary electrons and is the only type of SE
signals that contribute positively to high resolution. SE2 are produced by the
BSE at the surface layer. The SE3 signal originates from BSE interacting with
the specimen chamber and the SE4 signal originates from the incident beam
interacting with the column. To obtain topographical information the SE1s
are preferred since these originate from the outermost surface layer. The BSE
electrons are scattered at wide angles, elastically scattered from deeper within
the specimen where the intensity is strongly dependent on the atomic number
Z. This means that compositional information can be obtained from BSEimages.
In SEM the conductivity of the sample and the way it is mounted is of
great importance. The excess of electrons needs to be transported away from
18
Figure 2.2: Schematic representation of the signals generated in an SEM.
the sample to avoid charge build up which leads to detector problems. The
sample must therefore be mounted in such a way that it can be discharged to
electrical ground. It is possible to place the material directly on a conducting
brass or carbon stub with or without using a medium in between.
If the material itself is non-conductive and causes “charging” problems a
way to overcome it is by lowering the accelerating voltage [2, 3]. However,
there can still be a significant charge build up and a bias current can be applied
to the sample neutralizing the charge build up by balancing the number of the
incident electrons to that of the emitted electrons, this mode is called gentle
beam (GB) mode. A downside with this technique is that it takes more time
to adjust since the magnetic fields also get affected. The time factor itself
can be critical if the material is beam sensitive. Another remedy is to sputter
the surface with a conductive layer such as carbon or gold with the obvious
downside that the layer itself hides some topographical information. The issue
with charge build up can also be mitigated by reducing the size of the sample
and/or having a smooth surface.
2.1.1
Sample preparation
The sample preparation in electron microscopy is of great importance, specially in high-resolution imaging where the best performance of the microscope is required. First and foremost the sample needs to be clean, this means
that solvents or gases need to be carefully removed before inserting the specimen into the chamber. This can be obtained by heating the sample or keeping
it in vacuum. By crushing the sample or using a sample preparation technique
that grinds and polishes the sample, it is possible to investigate the internal
structure of the specimen.
19
Cross-section polishing
Cross-section polisher (CP) provides a systematic way to characterize the internal structure of a material. The technique uses a beam of argon ions to
polish the sample which creates a flat surface. This is done by removing a thin
layer typically around 75 µ m, with the help of a shield plate placed in front of
the sample.
A drawback with the CP is the possible damage and deformations induced
on the sample. One type of damage is melting of the surface which leads to
that the fine structure is lost. Another type is deformations of the shape where
elongations along the Ar-beam direction are common. A way to overcome
deformations is to reduce the accelerating voltage of the argon beam or to
cool the specimen during the polishing. Another aspect to keep in mind is
the possible redeposition caused by sputtering of the polished matter causes
malformations. In order to investigate the presence of a redeposition layer the
samples can be coated by a gold layer before polishing, as shown in Figure 2.4.
In order to minimize the redeposition as much as possible, the specimen are
commonly glued on the backside of a Si-wafer [4] as shown in Figure 2.3.
Figure 2.3: SEM sample preparation techniques for powders a) placed on a stub commonly made of brass or carbon mounted with a paste or tape. In b) a specialized way to
mount powders for polishing in the CP is shown, where the powder is on the backside
of a silica-wafer to reduce damage and redeposition.
20
Figure 2.4: The redeposition in CP was investigated by first covering the specimen,
mesoporous LTA, in a gold layer before polishing the particle. The polished surface
was examined by both SE- and BE- modes. In c) a SE-image from a) (taken at accelerating voltage 1.0 kV and working distance 1.9 mm) is overlaid in by the areas of
brighter contrast in the b) BE-image (taken at accelerating voltage 1.0 kV and working
distance 1.9 mm) colored in green. This visualizes the thickness of the redeposition of
the LTA material since all the matter on top of the gold cover is redeposited material
sputtered from the polishing.
21
2.2
Transmission electron microscopy
Figure 2.5: Ray diagrams for the electron beam in TEM after the beam has passed
the specimen for diffraction and imaging mode. The electrons transmitted through the
specimen continue through the objective lens. The objective lens creates a diffraction
pattern in the back focal plane and the first intermediate image at the image plane.
After this the strength of the first intermediate lens is set either in such a way that i)
the second intermediate image is in the object plane for the intermediate lens or ii)
for diffraction the back focal plane of the objective lens is in the object plane for the
intermediate lens. The projector lens then enlarges the diffraction pattern or the second
intermediate image in the object plane out to the viewing screen.
In TEM the electrons are transmitted through the specimen and the image is
a 2D-projection of the 3D object. The TEM-images give real space information and electron diffraction (ED) patterns give reciprocal space information,
the ray diagrams for these two modes are shown in Figure 2.5. 3D information is much more preferable and for that reconstruction techniques such as
electron crystallography and electron tomography can be used.
The electron wave that leaves the object, also referred to as the object wave,
has the amplitude a(x, y) and phase φ (x, y) where x and y are coordinates
in the image plane. After the electron wave passes through the microscope,
the electron wave imaged, the image wave, is no longer in agreement with
the object wave. The amplitudes and phases are now affected by the transfer
22
functions sin[X(R)] and cos[X(R)] where X(R) is the wave aberration and R
is the spatial frequency vector. The electron wave now have an amplitude and
phase which are denoted A(x, y) and Φ(x, y) respectively. In perfect imaging
conditions there are no aberrations for the wave and thus X(R) = 0. [5]
2.2.1
Imaging
Several contrast forming mechanisms contributes to the intensity of the images in TEM. For absorption contrast, or mass-thickness contrast, the degree
of attenuation of the incident beam comes from variations in density and follows the Beer-Lambert law, Equation 2.1, where x is the path length for the
incident beam of intensity I0 ; and µ is the linear absorption coefficient:
I = I0 e−µx
(2.1)
Images containing absorption contrast can be obtained by inserting an objective aperture. This will make areas that are thicker or have a higher density
to appear darker since they scatter electrons at an angle wider than α , Figure 2.6.
Figure 2.6: Ray diagram showing how the objective aperture screens scattering from
higher angle than α. Specimen will scatter more to wider angles depending on its
thickness and density of the matter. (Image drawn after Goodhew et al. [6])
Diffraction contrast arises from the beams that are diffracted by the crystallographic planes in the material. This means that a variety of effects are
visible due to changes the planes, such as dislocations, stacking faults and
buckled crystalline particles. Diffraction contrast is also responsible for the
thickness fringes that can be seen on the particles. By inserting an aperture
(objective aperture) in the back focal plane of the objective lens, it is possible
23
to narrow electrons for the image formation. This means that the contrast is
dependent on which spots are inside the aperture. If the direct beam, the beam
that has not changed direction in the microscope, is transmitted it is called a
bright-field (BF) image. If the direct beam is excluded from the image it is
called a dark-field (DF) image. So by changing the diffraction condition, e.g.
by tilting and thus changing the θ angle, we can observe a contrast change in
the image.
Phase contrast arises from interference between electrons of different phase
inside the objective aperture. The phases of the electron beam are changed by
the atom in the specimen and gives rise to the contrast in high-resolution TEM
(HRTEM) images that can be used for crystallographic studies.
However, it should be noted that the image is also affected by electron optical distortions such as spherical aberration or astigmatism of the lenses as
well as chromatic aberration. Spherical aberration scatters the electrons from
the optical axis which then blurs the image. Chromatic aberration arises from
an energy spread of the incident beam and inelastic scattered electrons. These
two result in the formation of a disk from a point object. Astigmatism originates from an inhomogeneous, non-cylindrical, magnetic field. Also dynamical scattering, multiple scattering processes, occurs and causes the images to
be difficult to interpret, especially in HRTEM images. Thin specimen samples
should be used, since for these the weak phase object approximation (WPOA)
is valid and the dynamical scattering processes are minimized.
The electron-matter interactions and image formation processes are quite
well understood today except for the Stobbs factor [7] and dynamical scattering. It is possible to generate computer simulated images and these are often
used for interpreting the HRTEM images.
2.2.2
Resolution in TEM
The resolution of the TEM is dependent on both the spherical aberration and
the wavelength of the electron beam in accordance with the following equation:
dS = 0.71CS 1/4 λ 3/4
(2.2)
When imaging at the atomic scale (Ångström) using a conventional TEM
the best point resolution, where the contrast can be interpreted easiest, is
gained at the Scherzer defocus ∆ fsch . This can be calculated for a microscope
from the spherical aberration Cs and wavelength λ [8]:
1
∆ fsch = −1.2(CS λ ) 2
(2.3)
Why the Scherzer defocus is preferable to use in HRTEM imaging can be
explained by the the contrast transfer function (CTF), Equation 2.4, given
without the envelope function which dampens the amplitudes. The pink curve
24
in Figure 2.7 shows that the Scherzer defocus gives the largest window since
the curve is crossing the x-axis at this point. This function explains how the
resolution is changing with the distance in reciprocal space q. The equation
shows that q and is dependent on the defocus ∆ f , λ and Cs of the microscope:
1
2
3 4
CT F(q) = 2 sin π∆ f λ q + πCs λ q
(2.4)
2
Figure 2.7: CTF curves at Scherzer defocus (pink) and at a defocus of -200 nm (blue)
calculated for a 300 kV TEM.
In conventional HRTEM-images the lighter atoms are much harder to visualize and usually very hard to resolve. For studying atomic scale events,
such as displacements from a regular lattice, a sub-Ångström resolution is desired. Many efforts have been made to reach a resolution at the information
limit of the microscope. The possible resolution is now beyond the Scherzer
point resolution and under 1 Å [9]. The methods for obtaining better resolution are divided into direct and indirect methods. The direct methods, such as
STEM, spherical aberration corrected TEM and high voltage TEM, refer to
approaches where all the work is done while on the microscope and require
specialized microscopes.
Spherical aberration can be corrected with electromagnetic hexapoles and
additional lenses [10]. But as Equation 2.2 shows, the accelerating voltage and
wavelength λ will have much greater effects than a decrease of the spherical
aberration Cs for the resolution of the microscope ds . In high-voltage microscopes accelerating voltages typically in the range 1-1.5 MV are applied and
the technique enables to image thicker specimens due to better transmission of
the electron beam [11]. However, the microscopes used for this technique are
usually large and expensive. Most importantly, specimen damage is relatively
common at these voltages.
The indirect methods, such as holography [5] and exit wave reconstruction [12, 13], require processing of the data after the imaging which are referred to as post-imaging methods. Paper VI in this thesis utilizes exit wave
reconstruction (EWR), which is a method that reconstructs the exit wave and
mathematically resets the Cs , astigmatism, coma and higher-order terms to
nearly zero and thus diminishes some of the electron optical distortions by
calculation. A benefit of EWR is that it can be applied for the commonly used
25
microscopes with an accelerating voltage in the mid-voltage range. The reconstruction of the exit wave is made by using a series of conventional HRTEMimages of different phase shift. The phase shifts can be obtained by changing
the astigmatism, beam tilt or the defocus. The reconstruction calculation is
using a method described by Meyer et al [14, 15]. The observed 0.89 Å C-C
spacing in diamond for the crystallographic direction [110] and the 0.78 Å
Si-Si spacing for [112]-direction are considered to be milestones in the obtained resolution for the focal series restorations. It is still hard to correct for
the dynamical scattering and the specimen should be thin to avoid multiple
scattering.
2.2.3
Electron diffraction
There are two main types of electron diffraction (ED), Selected Area ED
(SAED) and Convergent Beam ED (CBED), as shown in Figure 2.8 a) and
b), respectively. The diffraction patterns are related to the wavelength of the
radiation, the object and the settings of the magnetic lenses. In SAED a parallel beam is utilized which gives a pattern of spots. In CBED the beam is converging so that the ED pattern consists of disks. A single CBED pattern gives
3D structural information which can be used for determination of the spacegroup symmetry [16]. Kikuchi lines are a third form of diffraction shown in
Figure 2.8 c), these lines always come in pairs, one dark and one bright parallel
to each other. They originate from inelastically scattered electrons that forms
an point electron source which elastically scatters electrons. These lines can
be used for obtaining orientational information of the sample and for aligning
the crystal into proper orientation since the stronger lines leads to the zone
axes.
The spacings between diffraction spots and the origin observed in diffraction patterns are are inversely proportional to spacings in the crystal (d -values)
following Bragg’s law. The irradiation is diffracted at the scattering angle θ
when the incident beam satisfies the Bragg condition which is dependent on d
and the wavelength of the radiation λ , Equation 2.5. The intensities of these
diffraction spots are related to the arrangement of the atoms.
nλ = 2dsinΘ
2.2.4
(2.5)
Electron crystallography
When a material consists of periodically arranged matter it is denoted as a
crystal. It has a unit, referred to as the unit cell, which is repeating itself with
translational symmetry. The unit cell has an atom, a void or a group of atoms
called a basis which is placed on points that defines the lattice type (summarized in Figure 2.9) of the unit cell. The unit cell belongs to a crystal system,
Figure 2.9, which is determined by the symmetry and dimensions. The dif26
Figure 2.8: ED-patterns: a) Shows a typical SAED pattern of Lapis Lazuli and in b) a
CBED pattern of Si (Silicon) along the [112]-direction. c) Kickuchi lines from GaAs,
taken with a camera length of 40 cm on a JEOL JEM-2000FX operated at 200 kV.
Figure 2.9: The 14 Bravais lattices and the 7 crystal systems which together with the
32 point groups and translational symmetry will combine to the 230 space groups.
ferent combinations of these lattices and crystal systems will form the space
groups. Most crystals will have an arrangement in accordance with a specific
space group. However, there are examples of ordered structures, referred to as
quasi-crystals, which are not arranged in any translational order [17].
Crystallography is the study of the periodicity. Historically the crystal shape
or morphology was used to determine the point-group symmetry but today the
diffraction intensity from interaction with electromagnetic wave, or matter
wave is used. Where electromagnetic waves includes X-ray, gamma-ray and
visible light and matter wave includes electrons, positrons, ions and neutrons.
The three sources typically used for solving the crystal structure are X-rays,
electrons and neutrons. The radiation interacts differently with matter, X-rays
are scattered by the electrons in the atoms whereas electrons are scattered by
the electrostatic charge from both the electrons and the nuclei. Neutrons are
scattered by the nuclei and electrons with magnetic moment. The most widely
used technique for solving crystal structures is single crystal X-ray diffraction.
27
In electron crystallography [18] only small particles are needed as specimen
as electrons interacts strongly with matter. The smallest size needed is a crystal that has 10 times the unit cell which is a clear benefit over single-crystal
XRD that requires a specimen larger than 5 µ m. Due to the strong interaction
it is possible to observe weak diffraction phenomena such as super-lattice reflections and diffuse scattering, that can be used for understanding fluctuations
in the material. However there are also drawbacks. Only a small area can be
investigated at a time, yet it has to be made sure that this area is representative
for the material.
To solve a structure the amplitudes and phases of the structure factors are
needed. The structure factors are the Fourier components of the Fourier series
that describes the periodic functions of the crystal. The structure factor is defined as follows where f j (u) is the atomic scattering factor for atom j, u is the
reciprocal space vector, and r the position of atom j:
N
F(u) =
∑ f j (u)e2πi(ur )
j
(2.6)
j=1
Diffraction intensity I(u) is related to the structure factor F(u) and the
Fourier transform of the total electrostatic potential Φ(u).
I(u) ∝ |F(u)|2 ∝ |Φ(u)|2
(2.7)
Unfortunately the experiments by XRD, ED and ND yields only the amplitudes and the phases Φ(u) needs to be retrieved by other methods, such as
direct methods or Patterson method. Importantly, the Fourier transformation
of HRTEM images contain both the amplitudes and the phases of the crystal
structure factors [19, 20]. The possibility of extracting the phases is one of the
largest advantages of using HRTEM images in crystallography.
For an accurate crystallographic determination several structural directions
should be collected all of an area with a large number of unit cells. The data
needs to be corrected for the CTF in order to extract the correct amplitudes
and phases. The data is used to obtain the symmetry which is imposed to the
amplitudes and the phases. Symmetry related reflections will have the same
amplitudes and also phases if there is no translational symmetry elements such
as glide planes or screw axes present. After the structure factors have been
obtained these can, by an inverse Fourier transform, be used to create a 3D
electrostatic potential map. From this it is possible to deduce a model of the
structure. However, a structural solution might not come that easy, since the
experimental data is seldom complete and without any noise. For further reading Electron Crystallography by Zou et al. is recommended [20].
28
2.2.5
Electron tomography
In electron tomography a sequence of angular projections are collected for
reconstructing the object in 3D. Tomography itself is important in many sciences, including medicine where it is used on a daily basis. In conventional
medical X-ray tomography the sectional images are sampled by moving the
focal plane inside the body. This technique has been developed for biological
science and has been used for more than 30 years. Recently an increased interest for applications within material science has arisen due to the nanoscaled
features within various materials [21].
Figure 2.10: In tomography the projected images of different tilt angles of the object
are used for reconstructing the 3D-object.
The four major steps in tomographic studies: acquisition, alignment, reconstruction and visualization. There are automated data collection systems that
will do both image tracking and focus tracking during the acquisition. The
image tracking is in order to compensate for the image shift while tilting the
sample since the object imaged is commonly not in the centre of the rotation
axis. In the alignment-step the exact axis of rotation is identified by leastsquare fitting. Focus changes can also happen while tilting the object due to
changes in height which in turn affects the defocus. In practice however, these
automated systems do not always work sufficiently so the image position and
focus has to be adjusted manually.
The mathematical basis for the reconstruction calculations is the Radon
transform, Equation 2.8 shows it for 2D. Where p(r, Θ) is the 2D projection
at position r on the projection angle Θ of the original object f (x, y).
Z∞ Z∞
p(r, Θ) =
f (x, y)δ (x cos Θ + y sin Θ − r)dxdy
(2.8)
−∞ −∞
With the results of the Radon transform each 2D image is back-projected
along the direction of the projection with a method such as weighted back
29
projection (WBP) or algebraic reconstruction techniques [22, 23]. WBP is
a linear method and the outcome is thus predictable from the experimental
data. The micrographs used have to follow the projection criterion, stating
that the signal varies strictly monotonically with the thickness. In the BFTEM images of strongly scattering crystalline material, the contrast will be
dominated by a contribution from diffraction contrast and the image will not
follow the projection criteria. One way to overcome this problem is to use
STEM-HAADF and energy-filtered TEM imaging where the contrast from
diffraction is avoided [24].
If there is an infinite number of projections of the object taken at an infinite
amount of directions the inverse of the Radon transform can perfectly reconstruct the object. It is of course, in practice, impossible to collect the full 180
degrees for a single axis tilt series. However larger tilt range gives a better
reconstruction result. But there are mechanical limitations of the microscope
and objects shielding the view of the specimen. It is common to be able to
sample projected images at the range of ± 60 degrees. This means that a parts
of the information is missing referred to as the missing wedge and leads to
artifacts in the reconstruction [25]. The missing wedge can be minimized by
using techniques such as dual axis or conical tilting [22, 26].
30
2.3
Scanning transmission electron microscopy
In STEM a highly focused electron beam scans over an area and the transmitted electrons collected. STEM uses a converging beam. The size of this beam
is a key aspect for obtaining higher resolution. An advantage over TEM is the
absence of contrast reversal. Furthermore it is a way to localize the beam so
that it is possible to control which area is providing the information, which is
especially important in analytical EM. A drawback with the technique is that
drift in the microscope can create distortions since STEM detector records the
pixels sequentially.
It is common to combine STEM with a High-Angle Annular DF (HAADF)
detector. This detects the beam which is scattered at angles wider than 50
mrad, as visualized in Figure 2.11. Since heavier atoms scatter more due to
their pronounced inelastic interactions, the detector is more sensitive to compositional differences in the sample [27]. This leaves out the central beam
itself and also diffracted beam and elastic scattering events.
Figure 2.11: Principal drawing of a STEM HAADF detector showing the inner and
outer semiangles.
The scattering at high angles consist mainly of thermal diffused scattering. This is highly dependent on the atomic number Z and the intensity I in
HAADF images is proportional Zn . The exponent n will be slightly less than
2 due to screening of the atomic electron cloud [28].
2.4
Spectroscopy
Excitation of core electrons by the incident electrons leads to emission of characteristic X-rays that are induced by the core electron excitations followed by
downward transition. These processes give the analytical opportunities (see
Figure 2.12). The resulting X-ray emission spectrum gives information on the
atomic composition of the material since the wavelength (energy) of the Xrays are related to the electron energy levels of the atoms. Both in the SEM and
31
TEM it is possible to analyze the emitted X-ray signal with either wave-length
dispersive X-ray spectrometer (WDS) or energy dispersive X-ray spectrometer (EDS). The two techniques work in different ways. In WDS the X-rays are
filtered by a crystal and a narrow wavelength range is detected at a time. The
detector in EDS will convert the X-ray signal to a current with a magnitude
proportional to the wavelength. EDS is the faster of the two providing a possibility to measure the whole energy spectrum at the same time. WDS on the
other hand, has a better energy resolution than EDS and it is therefore more
powerful for light elements.
Incident electrons with high energy will lose energies by exciting inner shell
electrons of matter to higher energy states and create a vacancy. These inelastic scattering processes are useful for obtaining elemental and electronic state
information from the specimen. In EELS the transmitted electrons are spectroscopically analyzed which yields an energy spectrum where the electrons that
suffer energy loss will be distinguishable due to their relatively lower energy
content. This technique provides information regarding the electronic states
of the atoms in the sample.
The signal from EELS, WDS and EDS can furthermore be used for elemental mapping and energy filtered imaging [29]. Images are produced by
selecting the signal of an energy gap corresponding to the energy levels of
either the X-rays in WDS and EDS or the energy of the electron.
Figure 2.12: The inelastic interaction of the electron beam leads to emission of characteristic X-rays which are detected in EDS analysis where the energy of the X-rays
corresponds to a specific energy gap in the atom. The process gives loss of energy
for electrons. These are used in EELS to obtain information about, among others, the
elemental composition, electronic state, and specimen thickness.
32
3. Studies
In this chapter the results from characterization of materials are presented with
emphasis put on the role of electron microscopy. The length scale of interest
varies from a few Ångström to hundreds of nanometers. For the intergrowth
bronzes the interest is on an atomic scale. This is to be compared with hollow
spheres with Au-nanoparticles which are in the order of 100 nm in size.
33
3.1
Intergrowth bronzes
The project presented in this section has partially been published in paper VI.
3.1.1
Introduction
A mixed metal oxide is composed of metal cations and oxygen of oxidation
state -II. New compounds are synthesized pretty much every day and thus
there exists a large number of different metal oxide compounds. The properties are highly related to the structure which makes the structural characterization a very important step in the development process of metal oxides. Tungsten bronzes are a family of metal oxides with complex structures due to their
non-stoichiometric composition. They were discovered early as 1823 by Wöhler. The name bronze is due to the metallic shimmer and intense color that are
characteristic for the family of tungsten bronzes. There are four types of tungsten bronzes: hexagonal tungsten bronzes (HTB) [30], intergrowth tungsten
bronzes (ITB) [31], perovskite tungsten bronze (PTB) and tetragonal tungsten bronzes (TTB). A corner-sharing WO6 -octahedra constructs the main
framework and they follow the general formula Ax WO3 . Where A typically
is an electropositive metal located in one of the cavities of the framework,
with the amount varying in the range 0 < x < 1 [32]. Of the four structure
types ITB and PTB is created for low x-values and HTB and TTB for higher
amounts of x [33]. The tungsten bronzes are of interest since they have been
reported for applications within humidity sensors, electrochromic devices and
fuel cells [34–36].
Here the ITB studies of type materials is in focus which consists of HTB
type and WO3 domains intergrown epitaxially in the direction of the b-axis
(see Figure 3.1). The structure contains 6-membered rings forming hexagonal
tunnels, where the positive A-ions are located, in the HTB-slabs. These slabs
are joint together by the areas of ReO3 -type. A whole family of phases is
created from the variations of the thickness of the domains. The materials in
this study are from the Sb-W-O, Sb-Mo-O and Sb-W-Mo-O systems. For these
systems there has previously been reported a (2)-ITB phase both Sb-W-O and
Sb-Mo-O systems [37–40].
Although the ITB phases seem very simple at a first glance they are structurally complex. With variations and fluctuations in both positions and occupancies of the atoms. Variations in the rotations of the octahedra have been
found in the phases [41]. The occupancy of the A ion, in the hexagonal tunnel, is commonly reported to be less than what structurally is the maximum
[42, 43]. This variation of the occupancy in the tunnel content is also believed
to influence the variations in the octahedral network around. The part of the
project described in this thesis and paper VI was undertaken to get insight of
the fluctuations in the positions and occupancies in the hexagonal tunnels.
34
Figure 3.1: Tungsten atoms is drawn in gray, oxygen in red and the tunnel atom A
in green, the WO6 octahedra in dark grey. The two phases which by lamellar intergrowth forms the ITB phases a) the HTB and b) WO3 structure. The thickness of the
HTB and WO3 slabs will determine which phase it is. Image in c) shows a theoretical
structure model of the (1,2)-ITB as an example of an intergrowth phase where the tunnels consists of an A-atom displaced from the middle and an additional oxygen. The
WO6 -octahedra is drawn to the left, this phase has HTB slabs that are two tunnel rows
wide. To distinguish these different ITB-type of phases from each other the number of
octahedra in between the hexagonal tunnel rows is given in brackets (m,n,o,...)-ITB.
Thus a single row of hexagonal tunnels will be denoted (m)-ITB whereas if the HTB
slab is two tunnel rows wide (1,n)-ITB and so on.
3.1.2
Experimental
As starting materials Sb2 O3 , WO3 , W-metal, MoO3 and MoO2 powders, were
used with MoO2 and W as reducing agents. The purity of the starting material
was checked with powder X-ray diffraction. The reactants were grounded in
an agate mortar with compositions of Sbx Moy W1−y O3+z 0.1 ≤ x ≤ 0.5, 0 ≤
y ≤ 1 and 0 ≤ z ≤ 0.4. Reaction took place in evacuated and sealed quartz
tubes at 500 - 900 ◦ C for 4 to 10 days.
Powder XRD films were recorded in a Guiner-Hägg camera using Si as an
internal standard (a = 5.4301 Å). The films were evaluated by the processing
system Scanpi version 9. Pirum, version 921204, was used to index and refine
the powder patterns.
SEM studies were made in a JEOL JSM-7000F equipped with an Oxford
EDS system. The specimen was prepared by mounting crystals on a Cam35
Table 3.1: ITB phases
Phase
Composition a
xmax b
c
Unit-cell dimensions
a (Å)
b (Å)
c (Å)
(2)-ITB
Sb0.18 WO3+z
0.200
10.1983(4)
7.4231(3)
3.8037(2)
(2)-ITB
Sb0.21 MoO3+z
0.200
10.0164(5)
7.2003(3)
4.0448(4)
(2)-ITB
Sb0.18 Mo0.45 W0.55 O3+z
0.200
20.086(1)
7.2861(5)
3.9670(3)
(3)-ITB
Sb0.14 WO3+z
0.143
27.790(4)
7.3671(5)
3.8687(5)
(4)-ITB
Sb0.10 Mo0.55 W0.45 O3+z
0.111
17.474(1)
7.2860(4)
3.9483(3)
(5)-ITB
Sb0.11 Mo0.22 W0.78 O3+z
0.091
42.46(1)
7.3247(5)
7.8427(6)
(1,2)-ITB Sb0.27 MoO3+z
0.250
32.23(1)
7.263(1) 4.0274(7)
composition calculated from the Sb:Mo:W ratios measured with EDS in
both SEM and TEM from in average 10 particles. b Structurally the maximum amount
of cation, A, in the hexagonal tunnels. c Unit-cell dimensions from powder X-ray
diffraction studies.
a Average
bridge aluminium stub with a conducting carbon tape. The EDS analyses were
made on several different particles.
TEM. A JEOL JEM-2000FXII (200 kV) with Link EDS-system was used
for SAED studies and JEOL JEM-3010 microscope (300 kV), with an Oxford EDS system for HRTEM and SAED studies. For the STEM and STEM
- HAADF imaging a JEOL JEM 2100F microscope (200 kV), equipped with
a field emission gun, with a probe size of 0.2 nm, and HAADF detector with
the inner and outer semi-angles of 76 and 203 mrad respectively. For the TEM
studies a small amount of the sample was crushed in an agate mortar and then
slurred with n-butanol. The slurry was treated with ultrasound and a droplet
was put on a holey carbon film supported copper grid. EDS analyses were
combined with both imaging and SAED studies.
3.1.3
Results and discussion
The series of samples made in the three systems Sb-W-O, Sb-Mo-O and SbW-Mo-O was investigated with powder x-ray diffraction and SEM as a first approach. This revealed ITB-type phases, as listed in Table 3.1. Of these species
only the (2)-ITB was previously reported in the Sb-W-O and Sb-Mo-O systems but not in the Sb-Mo-W-O system and the rest of the ITB type of phases
were not previously reported in these systems.
Diffraction studies in the TEM were used to extract more information from
the samples. The ED patterns for the ITB type of phases, Figure 3.2, shows
characteristic brighter spots which corresponds to the size of an octahedra.
Weak scattering seen in a few different forms in the ED patterns taken of
different ITB species. For the (2)-ITB there was diffuse scattering displayed
36
as arcs indicated by white arrows in Figure 3.2 a). The arcs had not previously
been seen and a doubling of the a- and b-axis reported earlier was not seen in
this study [40]. In Figure 3.2 d) the effect on the ED pattern from a variation
of the slab thickness can be seen as streaking in b-axis direction. To deduce
more structural information of the (2)-ITB phase an ED study with both SAED
and CBED studies were preformed. This gave valuable information of the
symmetry elements of the phase as the existence of two mirror planes was
determined and the symmetry of direction [001] was determined to be p2mm.
The HRTEM-images provides a possibility to confirm the type of ITB phase
and to see the local details in the structure which is hard to detect with any
other technique. The ITB type of phases along the [001]-direction is a beneficial direction to view the structure and shows the intergrowth arrangement, as
seen in Figure 3.4 and Figure 3.5 . STEM and STEM-HAADF studies further
confirm the location of the antimony and tungsten atoms, as shown for the
(2)-ITB from the Sb-W-O system in Figure 3.6.
Studies has shown that the position of Sb+3 ions are split [40] because of
steric reasons of the lone pair on Sb+3 . Also there are reports of additional
oxygen atoms present in the tunnels believed to balance the charge of antimony [37]. A more thorough structural study was made of particles of the
(3)-ITB structure to get insight of the location and occupation of the content of the hexagonal tunnels. The study combines exit-wave reconstruction
with a statistical estimation of the parameters and compares these results with
single-crystal refinement and energy calculations. The exit-wave reconstruction made it possible to localize the oxygen atoms which are otherwise to light
to be seen in conventional TEM images. The results of this study is summarized in paper VI.
From the results gained with EWR from TEM-images the occupation of
the split position of antimony showed variations in both their amount and positions. However, results for the ITB-type of material is obtained from only
the [001]-direction. The other directions for the ITB-type of samples do not
display a clear image of the structure and are therefore not useful for estimating the fluctuations. Since the additional oxygen in the tunnels are located in
the same column as the antimony in the [001]-direction it is not possible to
resolve it with TEM studies alone. Here the combination of characterization
methods showed to be powerful since the results from the single-crystal XRD
studies indicated that oxygen was present in the tunnels.
The study presented in paper VI clearly shows that if one wants to go into
detail with a structure solution the combination of several techniques gives the
best result. Where advanced TEM studies have the clear benefit over singlecrystal XRD since it shows local variations. At the same time single-crystal
XRD studies are also a very powerful method and provides with valuable
information where the TEM studies could not.
37
Figure 3.2: ED patterns taken in TEM of the [001]-direction for ITB species.
The longest axis, the a-axis is horizontal in the patterns and has the smallest distance between the spots. For the patterns with uneven number of octahedra inbetween the hexagonal tunnels (3)-ITB, (5)-ITB and (1,2)-ITB there are extinctions at h+k = 2n, due to the c-centering.The ED patterns were taken of particles that came from the samples with the average compositions a) Sb0.18 WOx b)
Sb0.14 WOx c) Sb0.10 Mo0.55 W0.45 Ox d) Sb0.11 Mo0.22 W0.78 Ox e) Sb0.11 Mo0.22 W0.78 Ox
and f)Sb0.27 MoOx
38
Figure 3.3: Diffraction studies made in the TEM of (2)-ITB with the composition
Sb0.18 WOx . A), B) and C) shows the SAED and CBED patterns for directions [001],
[014] and [104] respectively. In the CBED patterns lines are marking the mirror planes
and for the SAED pattern for [001] diffuse scattering is marked with arrows.
39
Figure 3.4: Conventional HRTEM-image taken at the Scherzer defocus along [001]
of ordered (5)-ITB with a composition of Sb0.11 Mo0.22 W0.78 Ox measured with EDSanalyzes. The dark spots in the images correspond to the heavy atoms in the structures:
Sb, Mo and W. The lighter oxygen atoms cannot be seen in the conventional HRTEM
images. The arrows are indicating the rows of hexagonal tunnels. Comparing the images to the structural model in Figure 3.1 where the WO6 octahedra are drawn in
black.
Figure 3.5: HRTEM-image taken at the Scherzer defocus along [001] where the arrows are indicating the position of the hexagonal tunnel rows and the numbers the
number of octahedra in-between. Here a (3)-ITB has a WO3 -slab 4 octahedra thick in
an otherwise ordered part with 3 octahedra wide WO3 -slabs in-between the hexagonal
tunnel rows.
40
Figure 3.6: STEM-Images of the (2)-ITB with composition Sb0.18 WOx taken in A)
STEM-BF and B) STEM-HAADF mode. The hexagonal tunnels are indicated with
arrows. The STEM-HAADF image only shows the heavier tungsten atoms and not
the lighter antimony atoms. In the STEM-BF image also the antimony can be detected
but the images reveals that not all of the hexagonal tunnels are filled with antimony.
Distortions in the image is due to instability in the scanning.
41
3.2
Ordered mesoporous carbon
The project presented in this section has partially been published in paper V.
3.2.1
Introduction
Porosity are voids within the material that can be present both periodically or
non-periodically. For example, zeolites are types of porous materials with ordered pores and atomic arrangement, whereas carbon black has none. International Union of Pure and Applied Chemistry (IUPAC) have classified porous
materials into three groups based on their pore size p: p < 2nm microporous;
2nm < p < 50nm mesoporous; p > 50nm macroporous.
Mesoporous crystals does not have order on an atomic scale, as for most
crystals, but the pores are arranged with long-range order. The walls are commonly amorphous, with a few exceptions [44, 45]. These materials have been
synthesized with various inorganic and organic compositions. They were first
discovered in the beginning of the 1990s; synthesized with the mineral kanetine as template by Yanagisawa et al. [46] and later by the liquid-crystal templating mechanism by Kresge et al. [47]. The synthesis routes used today are
commonly based on the latter soft-template method. Recent progress has also
made ordered mesoporous carbon available through an organic-organic self
assembly mechanism. There are several phases with different structures (Ia3d, p6mm, Im-3m, Fd-3m and Fm-3m) with pore diameters ranging from 2 to
90 nm [48–51]. The use of triblock copolymers has made it possible to enlarge pore sizes but the crystallinity is low for material containing the largest
pores [52]. The use of anionic surfactants gave rise to larger structural diversity with structures containing chiral features [53]. Mesoporous materials, like
the zeolites, are named after the affiliation of first reported synthesizes and is
thus not related to the structure. Due to the large surface areas of mesoporous
materials they can have a high interaction with gases/liquids and thus have
many applications within filtration, separation, heterogenous catalysis and gas
storage [54].
Gas adsorption is commonly used to characterize porous materials. By examining the shape of the hysteresis between the adsorption and desorption
branches the type of interaction between gas molecule and solid surface can
be deduced [55]. The sorption-desorption isotherm is analyzed to determine
both surface area and pore size.
What we ideally want to know is the pore size and shape and the connectivity between pores. Disorder is commonly occurring and makes the characterization more complicated. In paper V the studies of FDU-16 and FDU-18
by TEM, with electron crystallography and tomography, as well as SEM combined with cross-section polishing and gas adsorption is summarized.
42
3.2.2
Experimental
Synthesis
FDU-14
The number 14 denotes the bicontinuous cubic mesostructure with the Ia-3d
symmetry. It was synthesized by using triblock copolymer P123 as a template
and phenol/formaldehyde as a carbon precursor. First, 2.0 g of phenol and
7.0 mL of formaldehyde solution (37 wt %) were dissolved in 50 mL of 0.1
M NaOH solution. Then the mixture was stirred at 70◦ C for 30 min. For the
synthesis of mesoporous carbon FDU-14, 4.8 g of P123 was dissolved in 50
mL of water. Then 60 mL of the precursor solution added to the above mixture
with stirring. Yellow precipitation was observed after about 24 h. The final
product was collected by sedimentation separation and filtration, washed with
water, and dried in air. The obtained sample was calcined at 800◦ C for 3 h in
a nitrogen flow. [48]
FDU-15
The number 15 denotes the 2D hexagonal mesostructure with the 2d p6mm
symmetry. It was synthesized by the EISA method using triblock copolymer
F127 as a template and phenol/formaldehyde as a carbon precursor. For producing the carbon precursor 8.0 g of phenol was melted at 40-42◦ C in a flask
and mixed with 1.7 g of 20 wt % NaOH aqueous solution under stirring and
14.16 g of formalin (37 wt % formaldehyde) was added. The pH was adjusted
to 7.0 with 2.0 M HCl solution. Water was removed and it was dissolved in
ethanol (20wt %). For the synthesis 1.0 g of F127 was dissolved in 20.0 g
of ethanol and 5.0 g of carbon precursor was added. This was stirred until
a homogeneous solution was obtained. The ethanol was evaporated at room
temperature followed by heating at 100◦ C for 24 h. The as-made products,
transparent films, were scraped from the dishes and crushed into powders.
The sample was calcined at 900◦ C for 4 h in a nitrogen flow. [48]
FDU-16
The mesoporous carbon FDU-16 sample with body-centered cubic symmetry
(Im-3m) was synthesized by a solvent EISA method with triblock copolymer
Pluronic F127 as a template in an ethanol solution. In a typical preparation,
1.0 g of F127 was dissolved in 20.0 g of ethanol. Then 10.0 g of phenolic
resol precursors in ethanol solution containing 1.22 g of phenol and 0.78 g of
formaldehyde was added. After stirring for 10 min, a homogeneous solution
was obtained. The solution was poured into a dish to evaporate ethanol at
room temperature for 5-8 h, followed by heating in an oven at 100◦ C for 24
h. The as-made products, transparent films, were scraped from the dishes and
crushed into powders. The obtained sample was calcined at 800◦ C for 3 h in
a nitrogen flow to obtain mesoporous carbon FDU-16. [48]
43
FDU-17
Propyleneoxide53 Polyethyleneoxide136 Propyleneoxide53
(0.30g)
was
dissolved in ethanol (5.0 g). Then 5.0 g resol precursor (containing phenol
(0.37 g, 3.8 mmol) and formaldehyde (0.23 g, 7.6 mmol)) was added with
stirring during 10 min. A transparent film was obtained by pouring the
solution into a dish and allowing the ethanol to evaporate at room temperature
for 5-8 h, then heating in an oven at 100-160◦ C for 24 h. The as-synthesized
product was collected and calcined at 350 or 450◦ C for 4 h at a heating rate
of 1 K/min under N2 to remove the templates. Mesoporous carbon was
obtained by direct carbonization of the corresponding mesoporous polymer.
The process was carried out in a tube oven at 600-1000◦ C for 4 h at a heating
rate of 1 K/min under N2 . [56]
FDU-18
Mesoporous carbon FDU-18 with face-centered cubic symmetry (Fm-3m) was
synthesized by using the labmade diblock copolymer polyethyleneoxide1 25b-polystyrene2 30 (molecular weight, 29 700 g/mol) as a template. The amphiphilic diblock copolymer was prepared via a simple method of atom transfer radical poly- merization (ATRP). Typically, 2.0 g of the resol precursor
in THF solution (containing 0.25 g of phenol and 0.15 g of form- aldehyde)
was added to 5.0 g of THF solution of PEO125-b-PS230 (containing 0.1 g
of copolymer) with stirring to form a homogeneous solution. The following
procedure was similar to that for the synthesis of FDU-16. [49]
Characterization
For the TEM observations, a small amount of the powder samples was crushed
in an agate mortar for up to an hour. Fine crystals were dispersed in n-butanol
by ultrasound and a droplet was put on a copper grid with a holey carbon film.
TEM images were taken at a low defocus value, around 3000 nm, which is a
beneficial defocus range for the mesoscale observations due to the need for information from the small scattering angles. Images were recorded with JEOL
2100 and 3010 LaB6 microscopes operated at 200 and 300 kV, respectively.
A JEOL JSM-7401F was used for the HRSEM imaging. For imaging of
the free powder, the samples were mounted on a brass stub with carbon paste.
For cross-sectioned samples, the powders were mounted on the bottom side
of a silicon wafer, followed by the cross-sectioning by an Ar ion using JEOL
SM-09010 operated at 4 kV for 10 h and an emission of 0.050 mA.
More details on the characterization of FDU-16 and FDU-18 are given in
paper V.
3.2.3
Results and discussion
For the SEM studies cross-sections of the particles were prepared by the CP
to be able to see the internal structure. The cross-sectioned surfaces showed
44
only a smooth surface from the start. By comparing with the cracked surfaces,
where pores could be seen it was concluded that the pores was a results of
damage in the CP. It became apparent that the sample was melting during the
observations. This problem was mitigated by reducing the accelerating voltage
of the Ar-beam from 6 kV to 4 kV. In the successful cross-sectioned particles,
seen in Figure 3.8, it was possible to see small pores in a regular manner,
confirmed by a Fourier diffractogram.
The TEM-images confirms the presence of the different phases, Figure 3.7.
For FDU-16 and FDU-18 the images were used for crystallographic reconstructions [57]. For this the structure factors from images along principal zone
axes are used after correction for the CTF. Mesoporous materials can give
only a small number of reflections because of the large size of the scattering
moieties (pores). Therefore the scattering amplitude decreases very quickly
with scattering angle. The materials inevitably contain local structural fluctuations and since the reconstruction gives the average structure of the periodic
features in the material, the quality will suffer from a high degree of disorder.
For the two cage-type structures, FDU-16 and FDU-18, a crystallographic reconstruction was made where the pores in both materials can be seen but the
connectivity was not visible. This could be an indication that the connectivity
was fluctuating highly in the materials and thus not present in the reconstruction since it is not ordered.
Due to the many structural fluctuations of the sample, the crystallographic
reconstruction is not satisfactory as a method to fully understand the structure.
Another way to use TEM-images is to make tomographic reconstruction, for
this however a complete tilt-series of images is required. With this technique
there is no demand on crystallinity and the lack of it will not affect the resolution in the reconstruction, which is one of the great benefits of tomography. The reconstructions provides the local structure with variations, in this
study the pore diameter and pore connectivity. Even though tomographic reconstruction some major drawbacks such as radiation damage and a quite poor
resolution of the pores of a few nanometeres. It provided information of the
shape of the pores and show how much these were fluctuating both in size and
shape. Furthermore some connectivities between the pores could be observed,
as indicated in Figure 3.9.
The comparison of the techniques displayed a big variation of the values
obtained by the different characterization methods for the pore size measurement. Each method provides different types of structural information and the
pore sizes are ranging from 3.1 to 8.4 nm for FDU-16 and 15.5 to 19.8 nm for
FDU-18.
45
Figure 3.7: TEM images of the series of FDU specimen with the FD inserted. a) FDU14 taken along [111]-direction b) The 2D FDU-15 structure taken along the pore direction. c) FDU-16 along [001]-direction. d) Images the FDU-17 along [001] direction
where the direction does not show any visible pores. Both e) and f) shows FDU-18
from [110]-direction with the difference that the latter contains stacking faults.
46
Figure 3.8: SEM studies of a cross-sectioned FDU-16 particle containing pores measured from the images to be 7.7 ±1.4 nm. The top image A) shows the cross-sectioned
particle, where the arrow is indicating the cross-sectioned surface and the square
where image B) is an taken. A) is taken at accelerating voltage 3.0 kV, work distance
3.0 mm and magnification 7.5 K. The cross-section in B) shows that the porosity is
visible at the high magnification. B) Taken with observation conditions: accelerating
voltage 3.0 kV, work distance 3.0 mm and magnification 200 K. FD of the image is
inserted in B) to verify the periodicity of the pores.
47
Figure 3.9: Tomographic reconstruction of cage-type FDU-16. A) with the full volume
shown in and enlargements of slice images in B and C. The white and black arrows
are indicating a connecting channel between mesopores in the structure and two pores
that have merged together, respectively.
48
3.3 Ordered
mesoporous
Pd-nanoparticles
silica-carbon
with
The results presented in this section has partially been published in paper I.
3.3.1
Introduction
In this section a carbon-silica nanocomposite mesoporous material loaded
with palladium metal particles is investigated. This material has been
proven to be a good candidate as a heterogeneous palladium catalyst in
water-mediated coupling reactions of aryl chlorides. Mesoporous crystals
are good candidates for incorporating metal nanoparticles to prevent the
metal particles from agglomerating. The position and size of the metal
nanoparticles are important to be investigated for ensuring that the particles
are formed inside of the mesoporous channels and that they are of a preferred
size which are both crucial for their catalytic activity. A nanocomposite
material can have the functions from two materials and hence gives unique
properties. The composite material also improved the catalytic properties.
It was shown that in the catalysis process the composite material with
consisting of a mixture of silica and carbon preforms more efficiently than
the corresponding carbon or silica materials [58].
In this part of the project conventional TEM-imaging and STEM-HAADF
was used to image the nanocomposite material. Since STEM-HAADF will
show z-contrast this is a good method to visualize both the particles that builds
up the mesoporous crystal and the palladium metal particles.
3.3.2
Experimental
The material is synthesized by an evaporation-induced triconstituent
co-assembly method where a soluble resol polymer is used for the organic
precursor, the inorganic precursor prehydrolysed TEOS and as template
triblock copolymer F127. Once a carbon-silica nanocomposite material
is synthesized, the silica material is obtained by combustion or a carbon
material by etching with HF. The synthesis procedure described more
thoroughly by Wan et al. [58]
JEOL JEM-2100FXII microscope, equipped with a field emission gun and
operated at 200 kV was used to obtain STEM-BF and STEM-HAADF images.
The probe size was 0.2 nm and the inner and outer collection semiangles for
the HAADF detector were 76 and 203 mrad, respectively.
3.3.3
Results and discussion
This material has columnar pores following 2D p6mm symmetry which was
confirmed by the stripe-like features shown in TEM images taken along [110]
49
(Figure 3.10 c) and hexagonal pattern along [001] (Figure 3.10 a). For the
conventional bright-field TEM images along [001] some particles in the tunnels can be spotted but the contrast is homogeneous. This is different from the
TEM-images in the [110]-direction where smaller particles of various contrasts is shown. That is however not very clear from the conventional TEMimages.
The STEM-HAADF images have a contrast difference between the phases
in the material, due to the fact that elements with higher atomic number
scatters more in wider angles. The intensity of palladium is higher than that
from carbon and silica which contains elements of lower atomic number.
Therefore the STEM-HAADF images provide a possibility to locate the
Pd-nanoparticles within the structure. In the images there are lighter spots
inside the tunnels and these are believed to consist of palladium. These
were well dispersed inside the pores with no obvious aggregation, which
is important to establish since the activity can be significantly reduced by
agglomeration. The mean particles size is of a few nanometers estimated
from the image and is smaller than the pore size. Therefore, the particles
should be easily accessible for chemical reactions within catalysis.
50
Figure 3.10: Mesoporous silica-carbon nanocomposite material incorporated with Pdnanoparticles imaged with conventional TEM (a and c) and STEM-HAADF (b and d)
of directions [001] (a and b) and [110] (c and d). Note that in b and d the Pd-particles
can be seen with brighter contrast.
51
3.4
Mesoporous LTA
The study presented in this section has partially been published in paper III.
3.4.1
Introduction
LTA is one of the most well known zeolites and its structure has been thoroughly studied [59]. The pores of conventional LTA are only 1.14 nm. Zeolites
belong to the family of crystalline aluminosilicate and was first synthesized
already in 1956. In an attempt to improve the diffusion and accessibility, the
properties of the very common zeolite LTA has been improved by incorporation of mesopores during the synthesis by using organo silane surfactants
(OSS) as structure directing agent (SDA). Introducing mesoporoes into the
structure improves the accessibility and the diffusion of gases and liquids is
improved. By this increase in the number of reaction centers between the catalyst and the reactants, the reaction rate can be increased by orders of magnitude [60].
In this project the aim was to investigate the effect of the OSS on the growth
and the nature of the mesopores generated inside the crystals. SEM in combination with cross-section polishing was used for the characterization. This is
since SEM can show the local internal structure in the crystals when combined
with cross-section polishing.
3.4.2
Experimental
The alumina source was sodium aluminate. 3-(trimethoxysilyl) propylhexadecyldimethyl ammonium chloride was used as OSS. These reactants were
mixed with sodium hydroxide and distilled water with the molar ratio of 100
SiO2 /333 Na2 O/ 67.0 Al2 O3 /20 000 H2 O/n OSS at room temperature. Hydrothermal synthesis was used with heating at 373 K for 4 h under vigorous
magnetic stirring. The product (solid precipitate during crystallization) was
collected by filtration. The synthesis procedure is described more in detail by
Cho et al [61].
The samples were observed in JEOL JSM-7401F (Stockholm university)
glued onto a brass stub with a carbon paste or directly on a carbon stub. SEM
studies were also made on a specimen prepared with JEOL SM-09010 CP
operated at 6.0 kV, 6 hrs with a current of 0.120 µ A mounted in the way
shown in Figure 2.3. The observations in SEM are made at low accelerating
voltages of a few kV.
3.4.3
Results and discussion
During the observation in SEM problems with charge-up of the material was
encountered. The material was also beam-sensitive and got easily damaged.
This is expected for zeolite materials, which are known to be hard to observe
52
Figure 3.11: LTA formation moieties for different times in the synthesis imaged with
SEM. A shows only smaller particles and no presence of the final cubic particles.
In B we see how the smaller moieties seen in A are arranged on the surface of the
LTA particles. After 4 hours (C and D) the crystals have been fully formed and the
smaller moieties shown in A and B cannot be seen anymore. C shows how the variation
in size of the LTA particles. D the corner of a particle in higher magnification with
some defects present. The images were taken at accelerating voltage of 0.5 kV, work
distance of 3.0 nm and magnifications of a) 50K, b) 25K, c) 5K and d) 50K.
in electron microscopes. To minimize the energy of electrons low accelerating
voltages of 0.5 to 3.0 kV was used for the observation. GB can be used while
imaging samples that suffers from charge build-up but it has the downside
of being more time consuming which is not compatible with beam-sensitive
materials. The charge build up on the material usually makes the detectors
unable to produce good quality images. Fast scaning was used when it was
impossible to take the images in a conventional way. This imaging method
however requires that there is no drift in the system.
The conventional LTA samples at different synthesis procedures were imaged to understand the formation steps, as shown in Figure 3.11. The specimen taken from the crystallization step at 1 h 50 min only consisted of small
moieties of 20-50 nm. 30 minutes later, at 2 h 20 min, larger particles were
formed, but smaller moieties were still present. Finally, after 4 h the crystallization was completed and only large crystals were detected (Figure 3.11).
The topography of the conventional LTA crystals show a clear difference to
those prepared in the presence of OSS, as seen in Figure 3.12. SEM images
show that the surface of the crystals consisted of small moieties when OSS was
used (Figure 3.13) which reminds of those present in the earlier crystallization
53
Figure 3.12: SEM image of A) LTA particle synthesized in a conventional way showing typical morphology and a smooth surface. With the use of OSS in the synthesis
(B) the particle keeps the same overall morphology but the surface is completely jaggered. Observation conditions: accelerating voltage of 0.5 kV, working distance of 3.0
nm and magnification 20K.
Figure 3.13: SEM images showing crystals synthesized in the presence of OSS with
B) as an enlargement of A) where the smaller moieties show a clear resemblance to
that of those in the early crystallization steps of standard LTA. Observation conditions:
accelerating voltage of 1.0 kV, A) working distance of 6.0 mm and magnification 13K
and B) working distance 1.5 mm and magnification 200K.
steps for standard LTA, as indicated in Figure 3.11 a and b. This means that
the OSS induced the mesopores by influence of the arrangement of the smaller
moieties.
In Figure 3.14 the SEM images of the cross-sectioned particles displays
very clearly mesopores incorporated into the structure. Furthermore, the crosssections of the crystals shows that there were two types of mesopores. In the
middle of the crystals the pores were larger than that of the pores on the sides.
Pores further out from the middle were tunnel shaped and directional towards
the middle. With a smaller amount of OSS in the synthesis, the crystals contained less pores which were difficult to observe. In Figure 3.14, BE images
of the cross-section of the crystals reveal the pores located on the surface or
underneath as dark contrasts.
54
Figure 3.14: SEM images of the cross-sections of two mesopore incorporated LTA
crystals synthesized with different ratios of OSS. A) shows darker contrast that arises
from the pores beneath the surface and B) has a large amount of pores on the surface.
Observation conditions: A) accelerating voltage 2.0 kV, working distance 2 mm and
magnification of 20K, B) accelerating voltage 3.0 kV, working distance 2 mm and
magnification of 25K.
55
3.5
Hollow spheres containing Au-nanoparticles
The results presented in this section has partially been published in paper II
and IV.
3.5.1
Introduction
The hollow sphere material, also referred to as yolk-shell catalysts, is constructed by a porous shell of TiO2 , ZrO2 or amorphous carbon that encapsulates a Au nanoparticle as shown in Figure 3.15. Gold of nanoscale size is
important as a catalyst in oxidation processes [62–64]. The hollow spheres act
as a carrier that prevents the Au nanoparticle from agglomerating. [65–69]
The aim here is to determine the size of the hollow spheres, thickness of the
porous walls, size of the particles building up the walls and sizes of the Aunanoparticles. It is also important to get a clear image on whether or not the
Au-nanoparticles are separated by the hollow spheres, since this is linked to
the catalytic activity. For these reasons it is important to visualize the insides
of the hollow spheres. Cross-section polishing can, in a more systematic way,
provide insight to the see inside the hollow spheres. Due to the sizes of the
hollow spheres and the other moieties, ranging from a few to several hundreds
of nm, SEM is a good technique. The challenge is to optimize the experimental
conditions for visualizing both the hollow spheres and the Au-nanoparticles.
3.5.2
Experimental
Synthesis
Au@SiO2 intermediate material
The Au@SiO2 intermediate material was prepared according to a previous
method reported by Arnal et al [66]. Millipore water was refluxed under stirring and 25 mL of a HAuCl4 (2.54×10−1 M) solution was added, followed
Figure 3.15: Schematic model of the hollow sphere material. A porous outer shell
of nanoparticles of TiO2 , ZrO2 or amorphous carbon is encapsulating a single Au
nanoparticle.
56
by addition of 12.5 mL of a sodium citrate solution (10 mg/mL). The resulting solution was refluxed for 30 min and finally cooled to room temperature.
Subsequently, 0.325 mL of an aqueous polyvinylpyrrolidone solution (12.8
mg/mL) was added and the resulting mixture was stirred overnight to allow
complete adsorption of the polymer on the gold surface. The solution was
then centrifuged (10000 rpm, 50 min) to remove the supernatant. The volume of the concentrated colloid was adjusted to 6 mL by dilution with water
and vigorously stirred for 5 min. In the next step, an ammonia solution (18.9
mL ethanol premixed with 0.84 mL of aqueous ammonia solution) was added
immediately, followed by addition of a tetraethyl orthosilicate (TEOS) solution (1.19 mL TEOS in 12.8 mL ethanol). The reaction mixture was stirred
overnight at room temperature and then centrifuged (10000 rpm, 30 min) and
washed (2 × water, 2 × ethanol).
Au@ZrO2 and Au@TiO2 material
The covering of the intermediate material with a zirconia shell and the removal of the SiO2 spacing layer was achieved as reported earlier [66]. The
intermediate material (Au@SiO2 ) was dispersed in 25 g ethanol in a 100 mL
flask sealed with a septum and heated under vigorous stirring to 30◦ C. Then
0.125 mL of an aqueous lutensol solution (430 mg lutensol in 11 g water) was
added. After 1 h stirring 0.45 mL zirconium butoxide or 6 mL of titanium
butoxide was added and the reaction was allowed to proceed overnight. Following, the material was washed four times with water and aged for 3 days
for ZrO2 and 6 days for TiO2 before calcination. The material was calcined in
air by heating from room temperature to 900◦ C with a heating rate of 2 K/min
followed by natural cooling [67, 68].
Au@C material
The exotemplates (Au@SiO2 @ZrO2 ) were evacuated under vacuum at 250◦ C
overnight in order to remove adsorbates from the porous material and subsequently kept under argon for 30 minutes. The pores were filled with a mixture (furfuryl alcohol/catalyst (oxalic acid): 100/1) via the incipient wetness
method, added dropwise in three steps under vigorous shaking by hand. The
solid was forcefully crushed with a spatula against the internal wall of the
glass flask for about 10 minutes. The monomers inside the pore system were
left to diffuse at 50◦ C for 24 h. Afterwards, the system was heated to 90◦ C
for 24 h under air in order to allow the polymerization of the monomers. The
polymer was thermally carbonized under argon by heating the sample with a
heating rate of 5 K/min to 850◦ C and kept at the final temperature for 3 h.
Removal of spacing layers
Finally, the silica spacing layer is removed by treatment with 1 M NaOH solution at 50◦ C. The resulting core-shell particles were washed four times with
water, once by ethanol and once by methyl tert-butyl ether before drying at
57
50◦ C. The zirconia was removed by the treatment with stoichiometric amounts
of HF for 6 hours at room temperature.
Characterization
SEM studies of the hollow sphere materials Au@TiO2 , Au@ZrO2 and Au@C
were done with a JEOL JSM-7401F on samples combined with the crosssection polisher JEOL SM-09010 operated at 4 kV, 10 hrs with a current of
0.50 µ A. For SEM observations the powders were mounted on brass stubs.
For the cross-section polishing, the powders of the materials were mounted
in the way described in Figure 2.3. Accelerating voltage of the electron beam
used during the imaging was between 0.5 and 15.0 kV and EDS analyses were
made at 10.0 kV.
3.5.3
Results and discussion
Overall openings in the hollow spheres were observed, which means that the
pores are connected to each other. This is good for the overall diffusion but
can also be a disadvantage since the Au-nanoparticles might have a possibility
to migrate and agglomerate. For imaging at low magnification at about 40-50
kX there were no problems encountered during the observation of the three
types of specimen. However at higher magnifications problems with charging
up occurred.
The material with TiO2 was imaged without any problems and also the
shape of the NPs in the wall was clearly viewed in magnifications of 400 k
times (as shown in Figure 3.16 b). For Au@ZrO2 there were problems with
charge-up and the high-resolution images were more blurry which makes the
wall material harder to resolve. The charge-up of the material also causes
the detectors to not function properly. This is why it is not possible to work
at higher magnifications for the Au@ZrO2 material. For Au@C the problem
was that the material was deforming due to the beam during observation. This
didn’t occur momentarily but reduced the observation time to less than 15
minutes. Thus fewer adjustments were made in order to reduce the beam damage. Also for this material, similar to Au@ZrO2 , it was not possible to get the
particles in the walls clearly resolved.
The SEM images were used to measure the size of the different components
of the materials gathered in Table 3.2. For the diameter measurements of the
hollow spheres about 100 hollow spheres were measured. For the measurements of the wall thickness, diameters of the nanoparticles in the wall and the
Au nanoparticles, the sampling volume was in the order of about 10 measuring
points.
The cross-sections from the CP were visualized with both SE- and BEimaging, see Figure 3.17. For TiO2 material this was highly successful as
preparation technique. In the case of ZrO2 and amorphous carbon the walls of
the hollow spheres were deformed in the process. Since the surfaces of the hol58
Table 3.2: Average size of components of the hollow sphere materials calculated from
HRSEM images for Au@TiO2 , Au@ZrO2 and Au@C material.
Au@TiO2
Au@ZrO2
Au@C
Diameter of hollow sphere (nm)
190 ± 12.0
115.2 ± 9.4
108 ± 10.0
Wall thickness of hollow sphere (nm)
19.0 ± 4.5
11.3 ± 2.6
9.8 ± 2.2
Diameter of wall material particle (nm)
19.5 ± 4.6
-
Diameter of Au nanoparticle (nm)
18.2 ± 1.5
14.2 ± 2.1
13.6 ± 1.6
low spheres were smoother than those of the unprepared specimen. They were
also slightly elongated. The observation conditions for the samples mounted
directly on the stubs were compared to those mounted for the CP. The way
the specimen was mounted had an effect on the conditions for imaging with
high resolution. This is not surprising since the mounting of the specimen
influences the magnetic fields in the microscope and the conductivity of the
sample. Charging on sample surface seldom occurs when the sample is polished in the CP. It was found that the best imaging conditions, when mounted
on a carbon stub, was an accelerating voltage of 0.5 kV. For the powders prepared by CP a higher accelerating voltage could be used, ranging from 3.0 kV
and up to 15.0 kV.
The contrast in the BSE images relates to the atomic number and hence
gives us element information. These images can locate the Au-nanoparticles,
however the interaction volume from where the BSE are generated is larger
than that of SE, which means that the spatial resolution is reduced in the BSE
image. This is why it is useful to collect both BSE and SE electrons in order
to observe both surface topology and chemical information.
To further investigate the presence of the Au-nanoparticles EDS was used,
see Figure 3.18. In EDS a high beam current must be used, in order to have
good signal-to-noise ratio in the EDS spectra. With the higher beam current,
the size of the Au-particles is smaller than the probe size and thus the actual
position of the nanoparticles can not be obtained accurately. The EDS-spectra
taken from the Au-nanoparticle and the TiO2 wall show an increase of Au Mα
peak, at 2.1213 keV in the spectrum of the Au-nanoparticle. By doing EDS
mapping, using an energy window at the Au Mα peak, the positions of several
Au-nanoparticles were determined. The particles the in closed hollow spheres
were not detected in the EDS measurements, although they were observed in
the BE-images. It is because the X-rays emitted did not reach the surface from
underlying hollow spheres.
59
Figure 3.16: SE-images of the hollow sphere materials: A) and B) Au@TiO2 , C) and
D)Au@ZrO2 and E) and F) Au@C. Images to the right are taken at a higher magnification. The images shows the porous hollow spheres forming a shell for the Aunanoparticles. In A and B arrows are indicating where the sphere has an opening to
the next sphere. In E and F it is possible to observe Au-particles inside some of the
spheres. Observation conditions: a) magnification 120kX, accelerating voltage of 0.5
kV (gun voltage 2.0 kV and specimen bias 1.5 kV) and working distance 2.0 nm, b)
magnification 400kX, accelerating voltage of 0.5 kV (gun voltage 2.0 kV and specimen bias 1.5 kV) and working distance 2.0 nm, c) magnification 100kX, accelerating
voltage of 0.5 kV and working distance 1.5 nm, d) magnification 220kX, accelerating
voltage of 0.5 kV and working distance 1.5 nm, e) mag. 200kX, accelerating voltage
of 1.0 kV and working distance 1.9 nm, f) magnification 400kX, accelerating voltage
of 0.5 kV (gun voltage 2.0 kV and specimen bias 1.5 kV) and working distance 1.9
nm.
60
Figure 3.17: SE- and BSE-images of the same area taken in the SEM of cross-sections
produced by the CP by the Au@TiO2 , Au@ZrO2 and Au@C materials. The spheres
are systematically opened by the sample preparation technique. From these images
we can see the Au-nanoparticles inside the hollow spheres more clearly than from
the unpolished samples in Figure 3.16. The images clearly displays the difference
between an SE- and BSE-image where the BSE-image visualizes the Au-nanoparticles
with a brighter contrast since they scatter more electrons. Observation conditions:
a and b) magnification 80 K, accelerating voltage 3.0 kV and working distance 3.0
nm c) magnification 150 K, accelerating voltage 3.0 kV and working distance 1.5
nm d) magnification 120 K, 3.0 kV and working distance 1.5 nm e) magnification
100 K, accelerating voltage 5.0 kV and working distance 2.0 nm f) magnification 75
K, accelerating voltage 5.0 kV and working distance 2.0 nm. Asahina Shunsuke is
acknowledged for image C and D.
61
Figure 3.18: Compositional information using BSE-imaging in A) and in B) the EDS
signal from Au Mα peak is used for an EDS-image of the same area. The identical
positions of the Au nanoparticles are marked with blue rings. Au Mα peak, at 2.1213
keV, used for the elemental mapping in B.
62
3.6
MOF-5 with Ni-metal particles
The results described in this section have at the time of writing not yet been
published.
3.6.1
Introduction
Metal organic frameworks (MOFs), are also referred to as coordination polymers, have a metal-containing part of formally positive charge (referred to
as connectors) and an organic part with formally negative charge (linkers).
The first publications with MOF type structure are made already in 1959
by Kinoshita et al. [70–72] with an increased interest for the field due to
the possible applications for gas separation and storage and catalysis [73].
A disadvantage of MOF is the instability of the material; they are stable in
most common organic solvents, but unstable in moisture and water. MOF5 shares this common property but is one of the most stable MOFs and the
structure (Figure 3.19) consists of ZnO4 -tetrahedral clusters as connectors and
1,4-benzenedicarboxylate linkers. These building blocks form a cubic Fm-3m
structure with pore sizes in the microporous region, of 11 and 15 Å [74]. A series of MOF-5 samples with nickel loading for improving the hydrogen sorption properties, are investigated. SEM combined with cross-section polishing
was used to characterize the material.
The materials are studied by SEM with the aim to investigate the location
and sizes of the incorporated nickel. Since the interest lies in the internal distribution of Ni, cross-section polishing is used. However, electron microscopy
study of MOF is always a challenge because of the instability nature of MOFs
as well as their poor conductivity that causes charges to build up during the
observation.
3.6.2
Experimental
Zn(NO3 )2 ·6H2 O (0.67 g, 2.25 mmol) and H2 BDC (1,4benzenedicarboxylate) (0.13 g, 0.75 mmol) were dissolved in
N-methylpyrrolidone (NMP) (30 mL). The reaction mixture was sealed in a
vial and heated at 95◦ C for 2 days to produce transparent pale yellow cubic
crystals. The prepared sample was washed with N,N-dimethylformamide
(DMF) (3 × 10 mL), and stored in DMF in a capped vial. Methylene
chloride was used for washing and storing (2 days) the MOF-5 crystals to
fully exchange the occluded DMF or NMP. After this the MOF-5 crystals
were evacuated under reduced pressure (<103 Torr) at 100◦ C for 12 hours.
The evacuated MOF-5 sample (0.20 g) was put in a vial (8 mL) under Ar
atmosphere, which in turn was put in a 250 mL round-bottomed flask. Into
the flask, nickelocene (Nc) was placed outside the vial containing the MOF-5
sample. The pressure inside the flask was reduced to 100 mTorr using a
vacuum line to make Nc sublime into the pores of MOF-5. The flask was
63
Figure 3.19: The structure of the cubic Fm-3m MOF-5 is constructed from a Zncontaining part, shown in a) with ZnO4 -tetrahedra drawn in blue with oxygen drawn in
red in the corners. The connector and the organic part, b) a 1,4-benzenedicarboxylate
linker which builds the structure shown in c).
heated at 95◦ C for 5 hours in an oven. When the flask was cooled to ambient
temperature the color of MOF-5 crystals turned black. Nc@MOF-5 (2.00 g)
was then placed into a 3-neck round-bottomed flask into which hydrogen
gas was supplied for 6 hours while heating the flask at 80◦ C in an oil bath.
After the hydrogenation process, the produced cyclopentane was removed
by further heating the flask at 80◦ C for 12 hours under vacuum. At this
stage, Ni@MOF-5a was produced. Further loading of Ni was accomplished
by repeating the same procedure as that for Ni@MOF-5a except for using
Ni@MOF-5a instead of MOF-5. That is, Nc was sublimed and transferred
into the remaining pores of Ni@MOF-5a, and the included Nc was reduced
by hydrogen to produce Ni-nanoparticles in Ni@MOF-5a. A similar
procedure was applied to Ni@MOF-5b to produce Ni@MOF-5c, where the
amount of nickel is increased with a containing the least amount and c the
highest.
Ni-doped MOF-5 samples were studied with both JEOL JSM-7600F
(KAIST) and JEOL JSM-7401F (Stockholm University) microscopes. In
this case a xylene-based carbon paste was found to induce less damage to
the sample so that was used when mounting the crystals. SEM studies were
furthermore made on a specimen prepared with CP operated at 4 kV, 10 hrs
with a current of 0.70 µ A. The material is known to be sensitive to air and
water so the transfer of the samples between the CP and the SEM has to be
made quickly. The EDS studies were made at an accelerating voltage of 3 kV
and therefore lower energy range, less than 1.5 kV, can be obtained. This
makes it possible to acquire the signals from C Kα at 0.2776 keV, O Kα at
0.5249 keV, Ni Lα at 0.8515 keV and Zn Lα at 1.0118 keV. The nickel and
64
Figure 3.20: The crystal habit of the MOF-5 sample is shown in A taken at a low
magnification, accelerating voltage of 2.0 kV and working distance of 8.1 mm. This
particle resembles a typical MOF-5 sample incorporated with nickel where the surface
contains cracks but follows a cubic crystal habit. B shows a cross-section of MOF-5
incorporated with nickel polished in the CP taken with at low magnification mode,
with an accelerating voltage of 3.0 kV and working distance of 7.7 mm. Here cracks
can be seen in the polished surface inside the crystal and also in areas of brighter
contrast.
carbon peaks yielded the best signals and thus these were mainly used for the
EDS-analyses.
3.6.3
Results and discussion
The size of the crystals is about 0.1 mm in diameter. Overall, the crystal habit
of the powders follow the original MOF-5 crystal shape as shown in Figure 3.20. All particles in the MOF-5 series contain cracks in various amounts.
Due to the non-conductive properties of the material the problem of charging
occurred in the microscope and the unmodified particles can only be imaged
when in low magnification mode.
In the cross sections study of the MOF-5 incorporated with nickel series,
Ni@MOF-5a to Ni@MOF-5c, the SE images show that the internal structure
of the crystals consists of areas with different contrast, see Figure 3.20. The
edge of the brighter areas are following the external borders and some of the
borders of the cracks. This leaves a darker band towards the edges. By EDSanalyses further information regarding the brighter areas could be obtained.
The signals used for the analyses are for carbon, oxygen, zinc and nickel.
For better understanding of the locations of nickel, point scans, line scans
(Figure 3.21) and mapping (Figure 3.22) using the signals from EDS were
performed. The analyses made with the different EDS techniques all showed
that brighter areas from the cross-sections contained significantly more nickel,
leading to the conclusion that Ni only mostly exists in the bright-contrast area
and that the MOF-5 is successfully doped with Ni.
The Ni-domains follow the cracked borders, showing the highest contrast
as a band following the edge, 5-20 µ m. The inside of this border usually contained nickel in a fairly high amount, whereas the outside usually was quite
65
Figure 3.21: Elemental analysis of MOF-5 doped with Ni was done in order to find
out more about the composition of the brighter areas. EDS-line scan is here presented
where A) shows the position of it and the position of point scans that were used to
confirm the ratios of the elements. For the line scan four lines are plotted in B from
the signals: red) C Kα at 0.2776 keV, green) O Kα at 0.5249 keV, lilac) Ni Lα at
0.8515 keV and turquoise) Zn Lα at 1.0118 keV. By comparing the brightness in the
SEM-image along the line of the line scan it can be seen that the intensity is following
the amount of nickel.
Figure 3.22: EDS-mapping from SEM studies made at 3.0 kV: A) Shows a particle
cross-sectioned by CP which in B) has the overlaid EDS-signal from carbon, C Kα
at 0.2776 keV, in green and nickel, Lα at 0.8515 keV, in red. The elemental mapping
shows that there is nickel inside the particles but not in the outermost layer where
instead there is a strong signal from carbon .
low on nickel, suggesting that this nickel might have been washed away from
the outermost edge in the handling of the material. Since they follow the edges
of borders it means that some of the cracks were introduced in the material
during the synthesis step when Nc was introduced. It should be noted that
there are cracks present where the band doesn’t follow the border. These either wasn’t accessible to the solution when it was introduced or simply the
cracks were created later on.
From the images it was observed that regions of brighter contrast occasionally contained porosity, as shown in Figure 3.23 B. The pores are not arranged
periodically and size wise they are in the order of 10-40 nm, thus larger than
the pores of MOF-5. The formation of the pores could have been an artifact
produced either during the synthesis or cross-polishing. This porosity leads to
a brighter contrast due to an edge-effect. This means that it is not possible to
66
judge if the brighter contrast comes from high nickel content or porosity at
low magnification. Here the EDS-mapping is an important tool to distinguish
the areas from each other.
When looking at higher magnification, there are brighter areas that consist
of stains with sizes between 20 - 200 nm, which is much larger than the pore
sizes of the MOF-5 material. Nickel is expected to be located in the pores of
MOF-5 crystals and hence the size of the domains is surprising. The domains
could however consist of smaller agglomerated particles incorporated into the
MOF-5 structure. Unfortunately, the resolution limit for this material in the
SEM didn’t allow to resolve whether or not the particles are as small as the
pore size of the MOF-5.
The cross-section of a crystal contained quadratic domains, Figure 3.24, of
nickel which were differing from what had been observed earlier for the rest
of the crystals. One of these domains showed periodic porosity with a size of
100 nm, thus not corresponding to the MOF-5 type. This suggests that there
is another phase which is formed at synthesis conditions close to the MOF-5
phase.
In the study a series of MOF-5 crystals doped with different amounts of
nickel were investigated. It could be seen that with higher amount of nickel
doped in the MOF-5 also the level of nickel inside the MOF-5 crystals increased. For observation it was very clear that the nickel inside the material
made the imaging much easier which can be explained by the better conductivity of nickel. The undoped MOF-5, used as reference material in the study
could almost not be imaged due to charging, whereas areas of high nickel content could be viewed at magnifications as high as 100 kX without problems of
charging when imaging.
SEM studies of the polished cross-section surfaces were also much easier
than that for the unmodified crystals, where only low-magnification images
could be taken.
67
Figure 3.23: SE-images from the SEM of cross-section polished particles where A)
shows a surface with stains of bright areas containing nickel, taken at a magnification
of 50 kX, accelerating voltage of 3.0 kV and working distance of 7.9 mm. B) porosity
taken at magnification 100 kX, accelerating voltage of 1.0 kV and working distance of
3.0 mm. At a lower magnification both these areas can appear bright in comparison to
the rest of the particle, A) due to the nickel content and B) since the edge of the pores
glows due to an edge effect.
68
Figure 3.24: SE-images from the SEM of particles of MOF-5 incorporated with nickel
cross-section polished by CP. The inserted image displays the polished surface of a
whole crystal showing the rectangular type of domains, which is not typically seen in
MOF-5. The arrow points at the area which is enlarged. The surface of this enlargement is covered with pores (black) of 100 nm size ordered in a periodic way, which
can not be explained by the MOF-5 structure. The white stains in the image are nickel.
There is also an artifact from the polishing present across the particle leaving a "curtain effect". Images taken at accelerating voltage 1.0 kV, working distance 7.8 mm and
magnification 250 X for the inserted image and accelerating voltage 3.0 kV, working
distance 7.7 mm and magnification 25 kX.
69
3.7
Mesoporous MOF-1 with Ni-metal particles
The results described in this section have at the time of writing not yet been
published.
3.7.1
Introduction
This is a SEM study of the internal structure and external surfaces of mesoporous MOF-1, referred to as MesMOF-1, doped with nickel. This specimen
belongs to the MOF-family described more in detail in the introduction of the
previous section.
The MesMOF-1, is one of the few MOFs containing mesopores as large as
3.9 and 4.7 nm with structure refined in cubic F-43m and Fd-3m symmetries
with a huge unit cell of a =12.39 nm. The connectors consists of supertetrahodrons of Tb3+ ions linked together by trazine-1,3,5-tribenzoic acid and due to
the structural complexity further structural description and images are referred
to Park et al [75].
3.7.2
Experimental
Ni nanoparticles were prepared by gas-phase loading and subsequent reduction. After immobilizing nickelocene into the evacuated MesMOF-1 [75]. by
sublimation at 85◦ C for 1 to 3 h, treatment of the nickelocene-containing
MesMOF-1 with H2 at 95◦ C for 5 h yielded cyclopentane molecules and Ni
nanoparticles The remaining cyclopentane was mostly removed by washing
with methanol, leaving the Ni particles inside the pores of MesMOF-1. Three
different Ni@MesMOF-1 samples are termed 1a, 1b, and 1c, respectively,
corresponding to the precursor loading time of 1, 2, and 3 h, respectively.
The MesMOF-1 samples were glued onto a brass stub with a xylene-based
carbon paste observed in JEOL JSM-7401F (Stockholm University). SEM
studies were also made on a specimen prepared with CP operated at 4 kV,
10 hrs with a current of 0.50 µ A. EDS analyses were made in with accelerating voltage of 2.0 kV and the signals C Kα at 0.2776 KeV, O Kα at 0.5249
KeV, Tb Mα at 1.19 KeV and Ni Lα at 0.8515 KeV.
3.7.3
Results and discussion
Crystals of MesMOF-1 follow octahedral cubic morphology, as shown in Figure 3.25 A, which is in agreement to the previously reported cubic symmetry [75]. Some of the crystals were observed to be twin octahedras with truncated sides as shown in Figure 3.25 B. The particle size is in the range 0.3 0.5 mm and thus SEM is not the optimal imaging technique to visualize the
particles.
The surfaces and cross-sections of the undoped MesMOF-1 crystals were
present in Figure 3.26. There is lines on the surface of the crystal in Fig70
Figure 3.25: SEM images taken in low magnification mode at accelerating voltage of
1.0 kV and working distance of 8.0 mm of a particles of undoped MesMOF-1 showing
A) cubic morphology and B) a twin octahedral with truncated sides.
Figure 3.26: SEM images of A) the surface of a particle and B) the internal structure
of a particle polished with the CP. Both show lines: A) on the surface and B) internally.
The most common directions of the lines are marked with the triangle and the square.
The images are taken with A) accelerating voltage of 0.5 kV, working distance 7.9
mm and magnification of 25 kX, B) accelerating voltage of 3.0 kV, working distance
3.0mm and magnification 25 kX.
ure 3.26 A. For Figure 3.26 B the cross-sectioned surface shows lines. After
analysis of the lines it can be seen that they show tendencies of following
crystallographic planes. The lines of highest frequency are 60◦ apart, indicating that this contains a 3-fold symmetry and thus the direction is along [111].
The lines in the polished surface, image Figure 3.26 B, has a difference in
contrast from density fluctuations. For the case of the cross-section the lines
are instead 90 and 45 degrees apart, which indicates that it is along the [001]direction.
The surface of the CP processed Ni-doped sample showed regions of lighter
and darker contrast, where the lighter regions usually were formed as bands
following the edge of the particle, as shown in Figure 3.27. The sizes of the
bands were measured from the SEM images to be around 3 µ m. These lighter
bands were characterized by EDS-measurements (Figure 3.28) and it could be
concluded that the lighter contrast in the SE-images orginates from higher Ni
content. The inside of the band consists of Ni rich domains with an average
size of 80 nm.
71
Figure 3.27: SE-images obtained with SEM, obtained with accelerating voltage of 1.0
kV, working distance of 8.3 mm and magnifications A) 500 X B) and C) 10 kX, are
showing A) a cross-section of a MesoMOF-1c particle, with an arrow marking the
band of a higher Ni-density. B) shows an enlargement of the brighter band and C) the
area inside the band.
Figure 3.28: A) shows a typical EDS spectrum from an area with low concentration of
the brigher domains and B) is from an area of high concentration. The arrows marks
the Ni-peaks.
As in the previous section, where MOF-5 was doped with Ni, the results
here indicates that the material is indeed doped with Ni and the distribution is
concentrated on a band that follows the edge of the particle. Furthermore it is
noteworthy that the domains containing nickel are larger than the pore size of
the material.
72
4. Conclusions
It is a fact within material science that proper characterization is essential in
the development of new materials. This thesis shows that electron microscopy
provides invaluable structural information for a large variety of cavity containing materials. Papers V and VI clearly states that when combining multiple characterization techniques, they complement and reinforce each other
making the result more reliable. Which also means that relying on a single
technique can be hazardous for the characterization. Furthermore, it is shown
that novel materials require new developments in both observation and sample
preparation techniques.
For the Metal oxides in the Sb-W-Mo-O system the study presented in paper
VI reveals new structural information by combining several techniques. The
average crystal structure of a new antimony tungsten bronze, Sbx WO3+y , by
single crystal X-ray diffraction and total energy calculation. Furthermore local
fluctuations, both occupancy and positions, of Sb in the hexagonal pores were
obtained quantitatively by exit wave reconstruction combined with statistical
parameter estimation.
In the part concerning the ordered mesoporous cabon the study of FDU-16
and FDU-18 clearly shows the drawback of relying on a single characterization technique by combining and comparing the results of several. A combination of TEM (electron tomography and electron crystallography) and SEM
(with CP sample preparation) gives three dimensional average pore structures
as well as local variation of the pores and their arrangements.
The part about ordered mesoporous slilca-carbon with Pd-nanoparticles
shows that STEM-HAADF is very useful for localizing nanoparticles in mesoporous materials either within the walls or inside cavities. This structural information has been useful for understanding the catalytical properties and further development of the material.
The last sections concerning (i) zeolite LTA incorporated with mesopores,
(ii) metal organic frameworks doped with nickel and (iii) hollow spheres containing Au-nanoparticles, emphasizes the benefits of low voltage high resolution SEM combined with the sample preparation technique CP. During
the study, techniques and appropriate conditions for observations and sample
preparations have been developed. This work has made it possible to visualize
the internal fine structures of these materials and obtain chemical information.
73
5. Acknowledgements
First I want to express my gratitude to Osamu for being my main supervisor.
I have always felt that you believed and supported me. You also made it
possible for me to work on interesting projects and people all over the world.
I consider you to be my friend and enjoyed our philosophical discussions
about life while staying with you and Sachiko in Korea.
I wish to thank my co-supervisors: Prof. Margareta Sundberg, Dr.
Cheuk-Wai Tai and earlier Dr. Yasuhiro Sakamoto for your support and efforts
helping me to succeed with my PhD-degree. Especially I want to express my
gratitude to Cheuk-Wai for all the helpful comments on the thesis. For all the
technical support I would like to in particular thank Dr. Kjell Jansson and Jaja
Östlund. I have got the opportunity to work with beautiful novel materials and
wish to thank the following persons and their groups for providing these: Prof.
Dongyuan Zhao, Prof. Ferdi Schüth, Prof. Ryong Ryoo and Prof. Jaheon Kim.
Past and present group members, colleagues and collaborators throughout
the years have made my efforts both easier and more pleasant where
Changhong Xiao, Keiichi Miyasaka, Kristina Lund, Mirva Eriksson,
Mikaela Gustavsson, Sarah Haigh and Shunsuke Asahina are in particular
remembered.
A huge thank you to Anders, my family and my friends for the support.
75
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