Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

436 9 Thermal Analysis and Calorimetric Methods

9.5

Conclusion

We hope that the impression left by this chapter will be that of a ﬁ eld with exciting

prospects for the future, as the sophisticated use of powerful tools such as micro-

calorimetry linked to other techniques begins to make connections with compu-

tational approaches to the chemistry of solid surfaces.

Adsorption microcalorimetry is probably the most direct method for describing

in detail both the quantitative and the energetic features of surface sites. The ability

of the microcalorimetric technique to readily reveal subtle differences among

samples is worth stressing. Even though one has to be cautious about the nature

of the acidic and basic sites, calorimetric information on the concentration and

strength of the sites can be used with conﬁ dence for interpreting the catalytic

behavior.

Metal oxides are largely used as catalysts in the chemical industry, and many

organic reactions such as cracking, isomeration, alkylation and partial oxidation

are performed over such catalysts. Hence, a deep knowledge of the surface proper-

ties is of great importance for the development of catalysts with suitable and tai-

lored acid – base characteristics.

Because no single technique can provide an exhaustive comprehension of the

acid – base picture of oxide surfaces, the joint use of calorimetric and spectroscopic

techniques (FTIR, XPS) is necessary for full characterization.

XPS displays great analytical power in the detection of surface species in provid-

ing their oxidation state. Moreover, the N

1s

or S

2p

XP spectra of chemisorbed N -

probes (NH

3

, pyridine) or S - probes (SO

2

) on surface acid – base sites make it

possible to distinguish between Br ø nsted and Lewis sites and to evaluate their

relative concentrations. Vibrational spectroscopy (FTIR) is also a powerful tech-

nique for evaluating the qualitative nature of the surface sites. Nevertheless, we

have demonstrated in this chapter that calorimetry, although a time - consuming

technique, has great power in determining in a single experiment the number,

strength and strength distribution of the active sites of oxides. Moreover, ﬂ ow

calorimetry methods (DSC) have also demonstrated the possibility of characteriz-

ing the redox properties of solid surfaces from an energetic point of view, even if

they compete in this area with more classical systems such as TPR - TPO reactors

equipped with thermal conductivity detectors.

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Transmission Electron Microscopy

Wuzong Zhou

443

10

10.1

Introduction

The identiﬁ cation of the electron by J.J. Thomson in 1897 [1] was one of the most

important scientiﬁ c discoveries of all time, and changed human life completely.

In 1924, the theory of the wave motion of the electron was established by Louis

de Broglie [2] . From the wave motion of the electron comes one of the most fasci-

nating and powerful applications of the electron, namely the electron microscope.

The ﬁ rst electron diffraction experiments were performed by G.P. Thomson in

1927 [3] , proving Louis de Broglie ’ s theory. The ﬁ rst electron microscope was

constructed by Ruska and Knoll in 1931.

In the 1940s, it was realized that interaction between electrons and atoms might

change the phase of an electron wave and make possible the observation of single

atoms using phase contrast. This is the theoretical foundation of high - resolution

transmission electron microscopy ( HRTEM ). It was also established by O. Scher-

zer [4] that using under - focus conditions could overcome the effect of lens defects

and therefore increase resolution. This is the practical foundation of HRTEM. In

the early 1970s, the resolution of HRTEM was improved to 0.3 nm as demon-

strated by the observation of lattice fringes in the images of Ti

2

Nb

10

O

29

crystals [5] .

Thus transmission electron microscopy ( TEM ) has a history going back over 70

years and HRTEM one of about 40 years.

HRTEM investigation of solid oxides is one of the most active research ﬁ elds in

solid - state and materials science. The structures of metals are relatively simple;

for example, many metals have one of two common structures – cubic close packing

( ccp ) and hexagonal close packing ( hcp ). When a metal is oxidized forming oxides,

the different oxidation states of metal cations and various types of metal – oxygen –

metal bonding states result in a large number of new structures and many possible

structural modiﬁ cations, including the formation of superstructures and defects.

HRTEM has proved to be essential in studying the structures of solid oxides, and

its importance in materials science and nanoscience today is obvious.

Although the theory of electron optics was established many years ago, technical

developments of TEM continue. It is well known that a crystal structure can be

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

444 10 Transmission Electron Microscopy

accurately determined by X - ray diffraction ( XRD ) or neutron diffraction methods.

However, these diffraction techniques collect structural information from a large

number of unit cells (typically 10

15

). In other words, the structure determined by

these techniques is an average structure. Diffraction methods can only give indi-

rect structural information. On the other hand, HRTEM is unique for directly

imaging three - dimensional microstructures of solids. Many local structures on the

nanometer scale can be observed. For nano - scale materials, such as nanoparticle

catalysts, nanowires and nanotubes of metal oxides, HRTEM is particularly useful,

since the dimensions of the nanomaterials are very small and information from

conventional diffraction methods is limited.

10.2

HRTEM and Related Techniques

10.2.1

Ray Diagram of TEM

The principles of TEM and HRTEM have been discussed in several textbooks [6,

7] . A TEM column can be described using a ray diagram as shown in Figure 10.1 ,

which is very similar to that for an optical microscope. The most important com-

ponents in an electron microscope are the electron source (normally called the

electron gun) and a group of electromagnetic lenses.

Electrons are emitted from the electron gun and accelerated by a potential dif-

ference (a typical value for HRTEM is 200 to 300 kV). These electrons are focused

by the condenser lenses to form a parallel beam. The electron beam strikes a

Figure 10.1 Ray diagram of TEM set - up.

specimen, creating different forms of energy. The transmitted and scattered elec-

trons are then recombined and focused by the objective lens to form a diffraction

pattern and an intermediate image at different stages. The beams then diverge

and are focused by the projective lenses to form the ﬁ nal image. Magniﬁ cation of

an image in a modern HRTEM can be ca 800 000 × to 1 500 000 × at maximum.

10.2.2

Electron Wavelength

In comparison with X - rays, an electron beam can have a very short wavelength,

which can be calculated according to the equation below when the relativistic cor-

rection is considered:

λ=

+

⎛

⎝

⎜

⎞

⎠

⎟

h

meV

eV

mC

21

2

OO

O

where h is Planck ’ s constant, m

o

the electron rest mass, e the charge on the

electron, V

o

the accelerating voltage and C the speed of light. Thus, an electron

beam accelerated by a voltage of 200 kV has a wavelength λ = 0.002 51 nm, which

is much shorter than the wavelength of X - rays used in conventional diffraction

experiments (e.g. Cu K

α

is 0.154 nm). This results in electron beams being very

powerful in detecting microstructures of solids.

10.2.3

Interaction between Electrons and Solids

Another characteristic of the high - energy electron beam is its very strong interac-

tion with specimens. At least ten different forms of energy can be expected from

such interaction as show in Figure 10.2 . The most useful energy forms for scan-

ning electron microscopy ( SEM ) are backscattered electrons and secondary elec-

trons. X - rays are used in energy dispersive X - ray spectroscopy ( EDX or EDS ).

Electron diffraction patterns and TEM images are formed by transmitted electrons

and scattered electrons.

It is also important to know that the interaction volume of electrons with a solid

specimen is much larger than the beam size. According to a Monte Carlo simula-

tion, in which the detailed history of an electron trajectory is calculated in a step-

wise manner, the interaction volume is a function of the accelerating voltage and

properties of the target specimen.

10.2.4

Formation of Image Contrast

There are three main formation mechanisms of TEM image contrast: mass –

thickness contrast, diffraction contrast and phase contrast. When electrons pass

10.2 HRTEM and Related Techniques 445

446 10 Transmission Electron Microscopy

through a particle, some are reﬂ ected and some are absorbed by the specimen.

Only some of the electrons can pass through the objective aperture, making a

contribution to the image contrast. If the electrons are scattered with a large angle,

they may not pass through the objective aperture and therefore will not contribute

to the image formation as shown in the inset of Figure 10.3 a. Thus, the region

with higher scattering power (heavier constituent elements or greater specimen

thickness) appears darker in contrast. For example, the edge of the particle in

Figure 10.3 a shows a lighter contrast in comparison with the central area. The

mass – thickness contrast can be enhanced by using a smaller objective aperture or

a lower accelerating voltage. TEM image contrast at low magniﬁ cations is normally

dominated by the mass – thickness contrast.

Crystals with different structures, the same crystals with different crystal orien-

tations with respect to the incident electron beam, and crystals with dislocations

or various grain boundaries will create different patterns of Bragg reﬂ ections. This

difference causes image contrast. Diffraction contrast is widely used to study dis-

locations and defects in crystals, because the regions around the defects have

Bragg angles differing from the perfect crystalline matrices and thus produce

contrast that can be observed. Figure 10.3 b is a HRTEM image obtained from a

high Tc superconducting RuSr

2

GdCu

2

O

8

[8] . The dark lines do not mean a mass –

thickness difference, but indicate antiphase grain boundaries.

Electron radiation is a wave, which can be characterized by two physical terms:

amplitude ( A ) and phase ( θ ): A e

i θ

. Since electron radiation is coherent, when two

waves meet, the resulting wave is a sum of the two waves, A e

i θ

= A

1

e

i θ

+ A

2

e

i θ

, and

the resultant intensity is not necessarily bigger than any individual wave intensity

but depends on the phase difference of the two waves. A phase - object refers to a

specimen so thin that when a coherent plane wave passes through it, only the

phase of the wave changes but the amplitude keeps its original value. Thus, there

Figure 10.2 Schematic drawing showing different forms of

energy created from an interaction between electron beam

and solid.

Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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