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

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In a crystal containing twin defects, the crystal lattices continue across the twin

boundaries without a break. Another similar defect, the antiphase defect, is formed

by a shift of the crystal by half a unit cell along the antiphase boundary. This defect

can also contribute to strong image contrast as shown in Figure 10.3 b.

10.6.3

Point Defects

Point defects occur where an atom is missing, or is replaced by an impurity

atom or is in an irregular place in the structural lattice. Point defects include self -

interstitial atoms and interstitial impurity atoms in a random arrangement.

Since the defects are very small and disordered, the corresponding diffraction

intensities in SAED and image contrast in HRTEM are normally too low to be

visible. On the other hand, some point defects can cause a local lattice distortion

and the real defect areas are much larger than a single atom. For example, a

vacancy may induce relocation of the atoms towards it or an interstitial atom may

push the surrounding atoms away from their ideal positions. Even a substitutional

atom at an ideal lattice position can cause a local distortion owing to its different

property (size, charge, etc) from those of the atoms in a bulk oxide. In these cases,

it is possible to see the image contrast created by these defects.

An example of such expanded point defects was demonstrated in the perovskite -

based La

n − 4

3 n +2

series [62, 63] . When the n value is small, the amount of

excess oxygen per perovskite unit x

in the compound is large. For example, x

0.5 in La

( n = 4), x

= 0.4 in La

SrTi

( n = 5), x

= 0.333 in La

and so on. To accommodate these excess oxygen anions, the perovskite structures

are broken into blocks so that excess oxygen can be accommodated in between the

blocks. When the n value increases, the dimensions of the blocks increase and the

overall structure approaches that of a perovskite phase. When n is 12, a cubic phase

10.6 Defects in Oxides 467

Figure 10.14 (a) TEM and (b) HRTEM images of YBa

on the [001] projection, showing twin defects. The inset of (a)

is a schematic model of twin defects. In (b), the a axis across

the twin planes is marked by the white lines.

468 10 Transmission Electron Microscopy

is synthesized. However, in this compound, the average value of excess oxygen is

still signiﬁ cant, x

= 0.1667 per perovskite unit. These oxygen atoms occupy inter-

stitial sites in the perovskite structure. Since oxygen anions are too large to stay

in any spare space in the perovskite matrix, local distortion must be induced. This

distortion gave us an opportunity to see directly the disordered nature of these

oxygen anions in the compound (Figure 10.15 ). The evidence is crucial in under-

standing the properties of materials developed for application in solid - state fuel

cells [63] .

10.7

Porous Materials

10.7.1

Microporous Zeolites

Microporous zeolites with pore diameters normally smaller than 1 nm (mainly

aluminum silicates) are important industrial materials for catalysis, gas absorption

and so on. Most known crystal structures of zeolites containing corner - sharing

SiO

and AlO

tetrahedra, were determined by XRD. The principal role of HRTEM

in studying these materials in the last 30 years was to directly image the open

framework structures and therefore to discover the local microstructures. However,

for some new zeolites with very complicated, low symmetry structures, the pore

arrangements revealed by HRTEM are indeed essential to the ﬁ nal solution of the

structures [64, 65] . One example is the determination of a TNU complex zeolite

structure by combining powder diffraction and electron microscopy [66] .

Figure 10.15 HRTEM image of n = 12 member in the

n − 4

3n +2

series. Randomly distributed local defects are

visible as dark spots. The inset is an enlarged image of a

single defect.

Zeolite - β , as an excellent catalyst, is one of the most useful high - silica zeolites.

The crystal structure of zeolite - β has been comprehensively determined. However,

the previously elucidated structures contain no open bonds and cannot explain

why a large number of silanol groups are present in zeolite - β .

A HRTEM investigation of zeolite - β gave a reasonable explanation [67] . Figure

10.16 a is a HRTEM image of zeolite - β . The basic pore structure contains large

pores, each surrounded by a group of small pores (Figure 10.16 b). There are two

polytypes, A and B, with different modes of stacking (Figure 10.16 d). These two

polytypes can intergrow together, forming many layered defects in a disordered

manner. It was found that the two stacking variants can grow in the same layer

from different sites and are unable to connect each other at the boundary, leading

to the formation of larger pore defects as shown in Figure 10.16 c and e. In the

defective large pores, open bonds are created, which serve as the sites for accom-

modating silanol groups [67] . The observation of these defects by HRTEM also

shed light on the crystallization mechanism of zeolite - β .

Zeolites are crystalline oxides. However, HRTEM imaging of these materials

usually suffers from some difﬁ culties. Although unit cells of zeolites are large, the

10.7 Porous Materials 469

Figure 10.16 (a) HRTEM of an edge of a

zeolite β crystallite, annotated to show the

stacking directions in different parts.

Nanodomains related to polytypes A and B

are indicated. Defects are visible in the center

of the image, where domains with different

stacking directions meet; (b) Fourier - averaged

image of a domain of type B, with structural

units outlined; (c) Image of two double pore

defects, showing different stacking directions

and outlining secondary structural details on

either side of the defects; (d) Models of the

polytypes A (tetragonal) and B (monoclinic);

(e) Model of the observed defects, obtained

by stacking in the two ways onto a single layer

and continuing to the third similar layer.

470 10 Transmission Electron Microscopy

separations of atomic planes in zeolites are very small (see Figure 10.16 d). It is

impossible to observe the atomic fringes that we often see from normal non -

porous metal oxide crystals. The image contrast patterns of zeolites only show the

pore networks. The second difﬁ culty is that many zeolites, especially those contain-

ing Al, are beam sensitive. The specimen should be dried before being inserted

into the microscope column. The application of charge - coupled device s ( CCD )

allows signiﬁ cant reduction in the exposure time to the electron beam, thus reduc-

ing specimen damage, since CCDs are much more sensitive to electrons than

photographic detectors [68] . The third difﬁ culty is low image contrast because the

elements in zeolite frameworks are normally Al, Si and O, without any heavy

metals. Using a small objective aperture may increase the contrast. When a zeolite

contains heavy metal cations, image contrast can be signiﬁ cantly improved [69] .

10.7.2

Mesoporous Silica

Since Mobil scientists reported their synthesis of the M41S series materials in

1992 [70] , tremendous progress has been made in the synthesis of new mesopo-

rous phases and in potential applications of these materials. Unlike crystalline

zeolites, the frameworks of these materials are amorphous. The so - called ordered

mesoporous silicas normally refer only the ordering of the mesopores. Strictly

speaking, they are not “ crystals, ” which must have short - range ordering, although

the term “ crystal ” is still used for these materials in many relevant papers because

no better term is available. The unit cell dimensions of the mesoporous silicas are

much larger than those of the microporous zeolites, and the X - ray powder diffrac-

tion method often gives insufﬁ cient reﬂ ection peaks. TEM therefore becomes the

most powerful technique for structural characterization of these materials [65,

71 – 74] , although a complete solution of the structures relies on a combination

of various techniques such as XRD, TEM, NMR, gas adsorption – desorption and

so on.

TEM images of mesoporous silica can only show the pore arrangement on a

nanometer scale. Therefore, a low magniﬁ cation is sufﬁ cient and the multiple

scattering problem in HRTEM at high magniﬁ cations becomes insigniﬁ cant.

When the pores are large enough, the mass – thickness contrast dominates in the

formation of TEM images and the explanation of the images is relatively easy,

especially when the shapes of pores are simple, for example cylindrical pores in

MCM - 41 [75] and SBA - 15 [76, 77] and spherical pores in SBA - 2 [60, 78] , SBA - 12

[76] , SBA - 16 [77] , FDU - 12 [79, 80] and so on. Figure 10.17 shows TEM images of

FDU - 12 with a pore diameter of about 20 nm. The face centered cubic structure

of the pores can be determined. The pore size can be directly estimated from the

image of an edge of a particle (Figure 10.17 c).

From other techniques (e.g. gas adsorption/desorption), it is known that the

spherical pores in these mesoporous silicas are connected by some small channels.

A similar property has been found in SBA - 15 in which the straight cylindrical

pores are connected by randomly located small channels. TEM cannot show these

channels directly owing to their extremely low contrast. One possible way to reveal

these channels is to ﬁ ll the pores with a heavy metal. The morphology of the as -

produced metal nanoparticles is the negative replica of the pores in mesoporous

silica [79] . TEM images of the metal nanoparticles show that some of these

nanoparticles are connected by small bridges, which reﬂ ect the small channels in

the original mesoporous silica.

Another group of mesoporous silicas, such as MCM - 48 and KIT - 6, have much

more complicated pore structures. The elucidation of these pore structures is not

straightforward. MCM - 48 is one of the ﬁ rst reported mesoporous phases [70] . Its

structure can be described using a gyroid minimal surface model, ﬁ rst suggested

in 1993 [81] , and conﬁ rmed a few years later using HRTEM [82, 83] and by newly

developed electron crystallography [71] . This minimal surface divides space into

two identical but separate compartments, forming a cubic unit cell containing

bicontinous pores with space group Ia - 3d. This is probably the best example to

demonstrate the successful application of HRTEM in the ﬁ eld of structural chem-

istry of mesoporous silicas.

10.7 Porous Materials 471

Figure 10.17 TEM images of mesoporous silica FDU - 12

viewed down the (a) [110] and (b) [233] directions.

image of FDU - 12 viewed down the [112] direction.

472 10 Transmission Electron Microscopy

Unlike zeolites, pure silica materials have no acidic sites and are of limited use

for catalysis and other applications. In order to produce acidic mesostructured

materials, aluminum has been successfully introduced into the framework of the

mesoporous MCM - 41 and can be detected by EDX. However, TEM indicated that

the degree of order of the mesopores in MCM - 41 was signiﬁ cantly reduced by

incorporation of even a small percentage of Al into the wall [84] .

Incorporation of many other cations into MCM - 41 to form porous mixed oxides

has been investigated extensively. The guest elements include Ti [85] , V [86] , Ga

[87] , Mn [88] , Fe [89] and Sn [90] . The doped materials often have a lower degree

of ordering compared with the undoped parent MCM - 41 silica.

Another method for creating active sites in MCM - 41 is introducing catalytically

active metal clusters on the inner surface of the pores. If the metal particles are

very small, for example in the form of individual atoms, and are evenly distributed

in the pores, HRTEM may not give enough image contrast. But the presence of

the metal can be conﬁ rmed by EDX elemental mapping. When larger clusters

( > 1 nm) form, they can be detected by HRTEM. It has been shown that direct

images can easily differentiate between clusters inside the pores and on the outer

surface of the mesoporous MCM - 41 [91] . In the case of Ag

anchored on the

inner surface of the pores, the clusters can be imaged as some dark dots even

though they are not ordered [92] . Ordered or partially ordered metal clusters inside

the mesopores result in a formation of the so - called rosary pattern of image con-

trast [93, 94] . In this case, an average distance of the cluster separation can be

estimated. The incorporation of metals inside the MCM - 41 pores usually has no

signiﬁ cant effect on the original silica framework. Recently, the clusters inside the

mesoporous materials have been better elucidated by electron tomography, a very

powerful 3D technique developed by Midgley and coworkers [95, 96] .

10.7.3

Porous Oxides

Mesoporous silicas are synthesized using some surfactants as soft templates. If

mesoporous silicas are used as hard templates and crystal growth of metal oxides

is allowed inside the pores, the pore system can be regarded as a nanoreactor. By

removal of the silica template, many porous single crystals of metal oxides have

been fabricated. The ﬁ rst such single crystal oxide, Cr

templated by SBA - 15,

was reported in 2003 [97] . Figure 10.18 a is a low - magniﬁ cation TEM image from

this specimen. The morphology and size of the particle seems to be the same as

those of the SBA - 15 template. Two sets of SAED patterns can be obtained simul-

taneously from the same particle, showing the single crystal nature of rhombohe-

dral Cr

(left inset of Figure 10.18 a) as well as the ordering of the mesostructure

(right inset, Figure 10.18 a). With higher magniﬁ cation, TEM images reveal that

the particles consist of an array of hexagonally arranged nanorods (Figure 10.18 b).

Further increasing magniﬁ cation allows us to view some small bridges connecting

the nanorods (Figure 10.18 c). The diameter of the nanorods is ca 8 nm and the

hollow space between the nanorods is visible as white fringes. HRTEM images

further corroborate the single crystal property of the specimen as shown in Figure

10.18 d. Any nanorod in a particle is part of the same crystal even when the nano-

rods are separated by a nanoscale space. It is assumed that the whole original

mesopore structure in SBA - 15 has been replicated by Cr

crystals.

Several synthetic methods have been developed and various transition metal

oxides, such as Cr

[97, 98] , WO

[99] , Co

[100] , Fe

[101] , In

[102] and

others, have been produced as porous single crystals. Most of them are known to

be catalysts. The general route for producing porous crystals of oxides using meso-

porous silica as template is by introducing a metal - containing precursor into the

silica pores and allowing the precursor to decompose, followed by crystal growth

of metal oxides inside the pores during a thermal treatment. Therefore, a crucial

step in the above process is the impregnation of the precursor. Five methods of

10.7 Porous Materials 473

Figure 10.18 TEM and HRTEM images of

porous crystals of Cr

templated by SBA - 15.

(a) Low magniﬁ cation image showing particle

morphology. The insets are SAED patterns

corresponding to the crystal structure of Cr

(left) and the one - dimensional ordered porous

structure (right); (b) TEM image at a higher

magniﬁ cation showing a group of parallel

nanorods; (c) TEM image with further

increasing magniﬁ cation showing small

bridges between the nanorods; (d) HRTEM

image showing perfectly oriented crystal

structure in the nanorods. The inset is the

corresponding SAED pattern from

multi - nanorods.

474 10 Transmission Electron Microscopy

impregnation have been previously developed: the so - called surface functionaliza-

tion method [97, 99] , the dual solvent method [98, 103] , the evaporation method

[100, 104, 105] , the one - step nanocasting method [102] and the most recently

reported solid – liquid method [106 – 108] , respectively. The most popular templates

used are SBA - 15, KIT - 6 and FDU - 12.

When KIT - 6 is used as a template, the typical morphology of the porous crystals

is spherical as shown in Figure 10.19 a and c and the particle size ( ∼ 300 nm) is

much smaller than the size of KIT - 6 particles (from several microns to tens of

microns). The reason for this phenomenon is a relatively low loading level

of chromium nitrate and a large shrink effect of the phase transformation of the

Cr - containing compounds [98] .

The most unusual feature of Figure 10.19 is that the crystal orientation has a

very close relation with the mesopore structure. For example, when one looks

Figure 10.19 TEM examination of porous

single - crystal of KIT - 6 templated Cr

viewed

down two principal zone axes. (a) TEM image

showing mesopore structure along the [111]

direction of the KIT - 6 related cubic unit cell

and (b) the corresponding SAED pattern

indexed to the rhombohedral unit cell of

crystalline Cr

. (c) TEM image showing the

mesopore structure along the [100] zone axis

of the KIT - 6 related unit cell and (d) the

corresponding HRTEM image on the

221

[]

zone axis of Cr

unit cell.

down the [111] zone axis of the KIT - 6 templated mesopore structure (Figure

10.19 a), seeing a hexagonal pattern of the pores, a hexagonal pattern of SAED

corresponding to the [001] zone axis of the rhombohedral Cr

can be obtained

without specimen tilting (Figure 10.19 b). Consequently, the [111] axis of the KIT -

6 - like cubic mesoporous unit cell and the [001] axis of Cr

are perfectly parallel.

When a particle is imaged on the [100] projection of the cubic mesoporous system

(Figure 10.19 c), a perfect square pattern of the mesopores can be seen. Figure

10.19 d is the corresponding HRTEM image recorded from a corner of the particle

in Figure 10.19 c. The image viewed down the

221

[]

axis of Cr

shows a contrast

pattern which is the closest to a square pattern from the Cr

crystal structure.

From this sample, many particles have these peculiar orientations, which seem to

have a close relation with the structural symmetry of KIT - 6. This unique phenom-

enon has not been observed with other samples of porous oxides. At present, the

reason for the formation of such coherence between crystal structure and the

symmetry of the nanopore system is not understood.

If we compare the image contrast pattern in Figure 10.19 c with the correspond-

ing one from KIT - 6, we can conclude that Cr

only replicated one of two inter-

laced pores in KIT - 6. Figure 10.19 a is a rare example showing replication of both

pores in the central area. On the other hand, TEM images from KIT - 6 - templated

showed that two interlaced pores were replicated in all the particles [100] .

Pore size and pore volume measurements by N

adsorption/desorption of these

two specimens conﬁ rmed the above conclusion. This implies that there are some

small channels connecting the two interlaced pores in KIT - 6.

Crystal growth of metal oxides has also been performed in the pore systems of

some cage - containing mesoporous silicas, such as FDU - 12 [79, 80] and SBA - 16

[77] . SBA - 16 has a body centered cubic structure (space group

) and FDU - 12

is face centered cubic (space group

Fm m3 ). Accordingly, each spherical nanocage

is 8 coordinated by the neighboring nanocages in SBA - 16, but 12 coordinated in

FDU - 12. When crystals of oxides form in these two silica phases, it can be expected

that the structures of the products are 3D arrangements of solid nanospheres

connected to each other by some very short nanorods [107, 109] . The HRTEM

technique has been used to examine the yields and the single crystal properties of

these porous oxides. The small bridges are barely revealed in the TEM images.

However, it is possible to see a bridge at an edge of a particle where two adjacent

nanospheres can be identiﬁ ed. Another difﬁ culty in imaging the porous crystals

templated by these mesoporous silicas is that, since the crystal orientation has no

relation to the mesostructure, it is difﬁ cult to ﬁ nd a view direction parallel with

principal axes of both the crystal structure and the mesopore structure.

It seems to be possible that most transition metal oxides can be made in porous

crystals with different morphologies using various mesoporous silicas as tem-

plates. It is expected that these materials have potential in applications such as

catalysis, fuel cell, gas sensors and Li - batteries. Their physical properties would

fall in between nanoparticles and bulk specimens, although our knowledge about

these properties is still very limited.

10.7 Porous Materials 475

476 10 Transmission Electron Microscopy

10.8

Low - Dimensional Oxide Crystals

It has been well known that HRTEM is a powerful tool to investigate structures

of low - dimensional oxides, such as nanoparticles, nanowires, nanorods and nano-

tubes, while the information from powder diffraction of these low - dimensional

materials is normally very limited merely because their small crystallite sizes. For

the nanoscale oxides, HRTEM can give useful information on particle size, crystal

structure, particle morphology, structural defects and possible inter - particle

connections.

10.8.1

Nanoparticles

An important type of nanoscale oxide is the so - called quantum dot ( QD ) which

has size - dependent photoemission properties. Fu and coworkers have reported an

aqueous dispersion of ZnO QDs with strong blue emission (quantum yield of

76%). HRTEM images directly show the particle size of about 5 nm or below and

quite narrow size distributions [110] .

Nanoscale core – shell structures are also of great interest. When Zn/ZnO core –

shell nanoparticles are imaged by TEM at a low magniﬁ cation, the image contrasts

from the core and the shell may be signiﬁ cantly different due to a difference of

the mass – thickness (or density of Zn cations) between these two components

[111] . The structures of the core and shell are also different, leading to a large dif-

fraction contrast as well. Consequently, the thickness of the ZnO shells can be

measured directly from the images. The information helps in the reﬁ nement of

the synthetic conditions and allows the dimensions of nanoparticles to be control-

lable. HRTEM images can further reveal the crystal structures of ZnO in the shell.

As observed by Zeng and coworkers, the ZnO shells are polycrystalline [111] .

The overall properties of nanoparticle materials depend not only on their struc-

tures but also on their size. It is therefore crucial to have a narrow particle size

distribution for some applications. Since TEM often shows individual particles, it

is necessary to examine a large number of particles for a reasonably good estima-

tion of particle size distribution. A good example was shown by Park and cowork-

ers, who observed almost uniform sizes of spherical nanocrystals of MnO, CoO,

(magnetite) and Fe

(maghemite) from large areas of the specimens [112] .

Fe/Fe

core – shell nanotubes were imaged by HRTEM by Shavel and coworkers

[113] . The crystal structures of the iron core and the Fe

shell can be clearly

identiﬁ ed.

Magnetic nano - γ - Fe

is a useful material in nanodevices as it allows separation

of nanomaterials in a magnetic ﬁ eld. For example, Pang and coworkers simul-

taneously embedded ﬂ uorescent CdSe/ZnS QDs and magnetic nano - γ - Fe

into hydrazinized styrene/acrylamide (H

N - St - AAm) copolymer nanospheres to

produce cell - targeting multifunctional nanospheres [114] . Figure 10.20 shows a

typical spherical polymer particle of the specimen with QDs and Fe

nanoparti-