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Analytical electron tomography mapping of the SiC pore oxidation at thenanoscale†

Ileana Florea,a Ovidiu Ersen,*a Charles Hirlimann,a Lucian Roiban,a Adrien Deneuve,b Matthieu Houll�e,b

Izabela Janowska,b Patrick Nguyen,c Charlotte Phamc and Cuong Pham-Huub

Received 28th June 2010, Accepted 21st July 2010

DOI: 10.1039/c0nr00449a

Silicon carbide is a ceramic material that has been widely studied because of its potential applications,

ranging from electronics to heterogeneous catalysis. Recently, a new type of SiC materials with

a medium specific surface area and thermal conductivity, called b-SiC, has attracted overgrowing

interest as a new class of catalyst support in several catalytic reactions. A primary electron tomography

study, performed in usual mode, has revealed a dual surface structure defined by two types of porosities

made of networks of connected channels with sizes larger than 50 nm and ink-bottled pores with sizes

spanning from 4 to 50 nm. Depending on the solvent nature, metal nanoparticles could be selectively

deposited inside one of the two porosities, a fact that illustrates a selective wetting titration of the two

types of surfaces by different liquids. The explaining hypothesis that has been put forward was that this

selectivity against solvents is related to the pore surface oxidation degree of the two types of pores. A

new technique of analytical electron tomography, where the series of projections used to reconstruct the

volume of an object is recorded in energy filtered mode (EFTEM), has been implemented to map the

pore oxidation state and to correlate it with the morphology and the accessibility of the porous

network. Applied, for the first time, at a nanoscale resolution, this technique allowed us to obtain 3D

elemental maps of different elements present in the analysed porous grains, in particular oxygen; we

found thus that the interconnected channel pores are more rapidly oxidized than the ink-bottled ones.

Alternatively, our study highlights the great interest of this method that opens the way for obtaining

precise information on the chemical composition of a 3D surface at a nanometer scale.

Introduction

Silicon carbide is a ceramic material that has been widely studied

because of its potential applications, ranging from electronics to

heterogeneous catalysis.1 Alpha silicon carbide (a-SiC) is the

most common crystalline form of the material exhibiting a

wurtzite like hexagonal crystalline structure. The beta poly-

morph of silicon carbide (b-SiC), that exhibits a zinc blend

crystal structure, has attracted a growing interest during the last

decades as a new class of catalyst support owing to its relatively

large specific surface area (>20 m2 g�1) as compared to a-SiC

synthesized via the Acheson process that leads to low specific

surface areas (0.1–1 m2 g�1).2 The SiC material is also thermally

conductive compared to traditional catalyst supports such as

alumina or silica and that could allow a rapid heat transfer

throughout the catalyst body and thus, prevent hot spots

formation in exothermic reactions which is detrimental for the

reaction selectivity.1 The medium specific surface area allows for

the deposition of metallic active phases with reasonably high

aInstitut de Physique et Chimie des Mat�eriaux de Strasbourg, CNRS -Universit�e de Strasbourg (UDS) UMR 7504, 23, rue du Loess, BP 43,67037 Strasbourg cedex 02, France. E-mail: ovidiu.ersen@ipcms.u-strasbg.fr; Fax: +33(0) 03 88 10 72 48; Tel: +33(0) 03 88 10 70 28bLaboratoire des Mat�eriaux, Surfaces et Proc�ed�es pour la Catalyse, CNRS -Universit�e de Strasbourg (UDS), UMR 7515, 25, rue Becquerel, 67087Strasbourg cedex 08, FrancecTechnical Center, Sicat, 1, rue du Broetch, 67700 Otterswiller, France

† Electronic supplementary information (ESI) available: SI-1–SI-4. SeeDOI: 10.1039/c0nr00449a

2668 | Nanoscale, 2010, 2, 2668–2678

dispersion while the good thermal conductivity of the substrate

strongly influences the selectivity of the catalyst for exothermic

reactions like the Fischer–Tropsch synthesis (FTS),3 n-butane

partial oxidation,4 H2S selective oxidation5 or methanol dehy-

dration to dimethylether.6 These properties are attributed to the

synthesis method based on a gas-solid reaction involving SiC

vapour and solid carbon at moderate temperature (about

1400 �C) that avoids the sintering of the material, as it is the case

of high-temperature synthesis.7 Previous analysis based on gas

adsorption and mercury intrusion methods have shown that this

specific preparation generates a dual porosity inside the material,

made of mesopores (4–50 nm) and macropores (>50 nm), giving

rise to two types of SiC surfaces. One should expect that the

reactivity of these spatially separated surfaces, especially towards

oxygen, depends strongly on the morphology, the accessibility

and the hydrophilic-hydrophobic character of the corresponding

pores. A recent electron tomography study has shown that beta

SiC exhibits a dual surface structure corresponding to a porosity

made of a network of connected channels (C-pore) with typical

sizes larger than 50 nm and ink-bottled pores (R-pore) with

sizes spanning from 4 to 50 nm.8 The channel pores have been

observed to be easily wetted by water, i.e. they have a hydrophilic

surface behaviour, whereas the ink-bottled pores are wetted by

alcoholic or organic solvents, i.e. their surface has a hydrophobic

behaviour. From this observation the hypothesis has been put

forward that this selective wetting against solvents could be

related to the oxidation degree of the two classes of pores:

hydrophilic surface containing a higher concentration of the

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oxygenated functional groups whereas hydrophobic surface

having less oxygenated functional groups. Such surface behav-

iour is very similar to the one occurring on carbon nanotubes

surfaces where the hydrophilic and hydrophobic character can be

controlled at will as a function of the chemical or thermal

treatment.9 In the catalysis field, the oxidative resistance of the

support is of tremendous importance, as at an extensive oxida-

tion degree the influence of pore migration and plugging and

active phase encapsulation become predominant contributing to

an activity loss. The oxygen concentration on the support surface

is also of great importance as it allows the anchorage and

dispersion of the metal or oxide active phase on the support

surface. However, the oxygen concentration, i.e. the thickness of

the oxide layer, on each type of pore is not easy to map using

traditional techniques at hand. Indeed, high oxygen concentra-

tion or thick layers of oxide could plug the pore or encapsulate

the active phases that are deposited on the surface deteriorating

the overall catalyst activity. Up to now, no technique was able to

give a direct access to such pore surface behavior. It is thus of

great interest to find a new and appropriate characterization

technique allowing to selectively map the pore oxidation degree

in the two classes of pores of beta SiC, as well as to correlate this

parameter to the morphology and to the accessibility of the

porous network at a nanoscale spatial resolution. Comparing the

various existing techniques able to provide specific information

at a nanometer scale, it appears that one of the best ways to

acquire 3D compositional mapping is the Energy Filtered TEM

(EFTEM) tomography. In that regard, the first goal of this paper

is to demonstrate the feasibility of the electron tomography in

the analytical mode with a resolution down to the nanometer.

Applied in a quantitative way, this technique was subsequently

used to determine the pore oxidation degree, or in other words,

the thickness of the oxide layer, at the surfaces defined by the two

types of pores present in b-SiC. The characterization of such pore

surfaces could be helpful for the understanding of the support

behavior towards oxidation reactivity which, in turn, would

represent a powerful tool for controlling the active phase local-

ization and its accessibility as a function of the pore encapsula-

tion by the oxide layer. The developed technique could also be

further applied to the study of other one-dimensional catalyst

supports, i.e. inner versus outer surface reactivity of carbon

nanotubes, as well as to other porous catalytic systems, either

pure or doped with foreign elements, in order to build-up the

relationship between the catalytic performance and surface

composition.

Emergence and state-of-the-art of the EFTEM tomography

Generally, to obtain chemically selective information at the

nanoscale, the electron energy loss spectroscopy (EELS) tech-

nique is commonly used and allows the mapping of the elemental

composition (in both image and spectroscopic mode), the

chemical state or the phase distribution.10–12 Related to the Si–C–

O-based materials, a typical example of EELS analysis concerns

the investigation of the chemical bonding in bulk ceramics

prepared by pyrolysis technique as a function of O content.13

However, by recording individual images or spectra, this tech-

nique remains a 2D analysis tool because the projection effect

gives only access to thickness-integrated information along the

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direction of observation. Interpretation needs the setting of

hypothesis about the studied nano-object nature and can be

sometimes difficult or ambiguous, especially for objects with

complex 3D morphologies or random spatial distribution of their

components. From the morphological point of view, the analysis

of objects at a nanometer scale implies the use of electron

microscopy in the tomographic mode, which produces 3D

reconstructions of the said objects and thus, eliminates the

hypothetical interpretations. Electron tomography in a bright-

field mode has been extensively employed in the field of biology

research and it has only recently been translated in the field of

materials at a nanoscale, since the pioneering work published by

de Jong and co-workers.14 More recently, a new experimental

mode has been implemented, using for the volume reconstruc-

tion, images recorded with a high angle annular dark-field

detector in the scanning mode (STEM-HAADF). This mode is

indeed extremely valuable to characterize crystalline systems,

because it avoids diffraction contrast and allows the recording of

images mainly sensitive to the atomic number. However, the

difference in atomic number, especially for light elements, of the

chemical elements present in the sample frequently leads to

insufficient contrast preventing the use of this density-sensitive

method. To overcome this drawback, a third chemically selective

mode was recently implemented based on energy-filtered TEM

(EFTEM) imaging to acquire the tilt projection series used

subsequently to compute the reconstruction. The individual

images are formed across a spectrometer that selects inelastic

scattered electrons with a given energy loss allowing the collec-

tion of signals emitted from a specific chemical element amongst

the others. The EFTEM tomography method has been first used

to study nanocrystals15 or composite materials.16 Recent results17

were obtained from tilt series of EFTEM images on the basis of

an appropriate choice of materials (constituting elements with

very distinct energy losses: Fe, Ni, O) and with a compromise

between data acquisition speed and resolution. In another work,

filtered images recorded in the plasmon region of an EELS

spectrum have been used to reconstruct a 3D signal sensitive to

local changes in the specimen density.18 Some specific applica-

tions have also recently been published in the field of biology,

either in zero-loss mode selecting only elastically scattered elec-

trons in order to enhance the contrast,19 or in a more quantitative

way to provide 3D maps of phosphorous in nucleic acid, by

recording L2,3 pre-edge and post-edge images.20 However,

exploiting a unique 3D chemical map is not accurate enough,

since the method is very sensitive to various parameters,

including extraction of the background (for a proper extraction,

it is certainly better to use two pre-edge images than only one),

change in specimen thickness when tilting, and data processing.

A more valuable analysis can be performed when all the chemical

elements present in the specimen are mapped altogether, yielding

relative 3D chemical maps with unknown parameters. Thomas

et al.21 have shown that it is possible to perform chemical tomo-

graphy using the spectroscopic mode instead of the image mode

of EELS spectroscopy. This technique, referred to as 4D

STEM-EELS tomography, requires a 360� rotation holder and

pillar-shaped samples to minimize artifacts, thus limiting its

applications in a routine way. However, in the more convenient

EFTEM mode the current spatial resolution reported up to now

is limited to only a few tens of nm. In this framework, reaching

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the nanometer range represents a great breakthrough for

extending this technique to full characterization of complex

nano-objects. In this way, correlations between the distributions

of the chemical elements can be obtained, as for example the

oxygen distribution in the vicinity of a surface having a complex

3D morphology.

Even being beneficial as a characterization technique in the

materials field, the EFTEM technique implementation and

application at the nanoscale remain slow to show up. Similarly as

in the case of classical EFTEM imaging, the first concern about

the technique is the high doses of electrons needed to obtain

series of chemical projections with a significant signal to noise

ratio (SNR). Furthermore, in the tomographic mode, in order to

obtain relevant 3D chemical maps, the number of projections (N)

must be higher than 50 and each projection is obtained from

three filtered images. Even if the sample under study is not

known to be sensitive to electron irradiations, long exposition

times can produce small damages accumulation leading to non-

reliable 3D reconstructions. The second obstacle in this tech-

nique is that the image intensities in the so-called ‘‘chemically

selective projections’’ must satisfy the projection requirement for

tomography that stipulates that the signal must vary mono-

tonically through the structure to be suitable for reconstruc-

tion.21 This consideration generally requires the acquisition of

three energy-filtered images for each chemical element at each tilt

angle, two before the chosen edge to estimate the background

and one on the edge from which the background will be extrac-

ted. Once the tilt series of chemical projections and the corre-

sponding elemental reconstructions are computed, the combined

analysis of the obtained 3D maps for the major elements present

in the specimen becomes possible. The main practical difficulties

and drawbacks of the technique are further discussed in the

Experimental Methods section.

In this context, the challenge was here to adapt the acquisition

and the data treatment to insure accurate 3D chemical maps

constructions with a nanometer resolution which was not achieved

so far. Indeed, as predicted by some authors,16,22 improving the

spatial resolution seems to be possible if the sampling of the

angular range is performed with an increased pixel and angular

resolution. Considerable work has therefore been devoted in this

study to carefully optimise the acquisition parameters, computing

and analysis processes in order to achieve chemical mapping with

a nanometer-scale resolution. In particular, for the system studied

in this work, the analytical electron tomography is the unique

method which allows performing a quantitative analysis of the

oxygen content on the two types of surfaces. More generally, it is

thought that such result could be extremely helpful for studying the

real chemical composition of the active phases in the heterogeneous

catalysis field, especially their composition with spatial resolution

on the topmost surface where the catalytic steps do occur, and to

determine the relationship between the chemical composition of

the active phases and the performance of the catalyst.

Results

Classical analysis by 3D-TEM and XPS: selective localization of

nanoparticles versus porous structure and surface chemistry

In this work the b-SiC material was synthesized by allowing solid

carbon and SiO to react in an argon flow at a moderate

2670 | Nanoscale, 2010, 2, 2668–2678

temperature (�1400 �C).23 The low synthesis temperature avoi-

ded specific surface area loss through surface pore diffusion. The

as-synthesized material was further calcined in air at 800 �C for 2

h in order to remove residual carbon inside the matrix. Using the

Brunauer, Emett, and Teller technique (BET), we measured

a specific surface area of 25 m2 g�1, a reasonably large value due

to the presence of both meso- and macropores.

A meticulous analysis using 3D-TEM imaging revealed that

the resulting ceramic material possesses two types of porosities

with different surface wetting behavior.8 The main results

obtained by analyzing the 3D reconstruction computed from

a tomographic record following the procedure described in the

Experimental Method section are summarized in Fig. 1A, B and

C, which highlight the presence of both channel pores with

typical size larger than 50 nm on average and ink-bottled pores

with sizes spanning from 4 to 50 nm. In particular, Fig. 1B shows

a typical slice through the reconstructed volume on which both

the interconnected channel pores (C-pores) and the ink-bottled

pores (R-pores) are observable. In Fig. 1C, a modelling of the 3D

reconstruction of the SiC grain is shown that gives a better

intuitive perception of the two different types of pores defining

the two spatially separated surfaces.

The averaged surface oxygen content on the two surfaces which

is the core of our questioning has been first characterized using

XPS analysis. The XPS Si2p spectrum recorded on the SiC sample

after synthesis and air calcination at 800 �C is presented in Fig. 1D.

The decomposition of the raw spectrum indicates that at least three

Si-based species are present on the SiC surface. The Si2p peak

located at low binding energy, i.e. 101 eV, could be attributed to the

Si atoms engaged in the SiC phase, whereas the two other Si2p

peaks located at higher binding energies were attributed to the Si

atoms engaged in the SiCxOy and SiO2 phases.24 According to the

relative intensity of the different Si2p peaks, one can conclude that

oxygen-containing Si compounds were present in a large amount

on the topmost surface of the SiC material after the calcination

step. Silicon carbide is well-known to be highly reactive with

oxygen and, thus, soon after its exposure to air the SiC surface was

steadily covered with a thin layer of oxygen-containing species,

i.e. SiCxOy and SiO2.25–27 Mozdzierz et al.28 have proposed the

following chemical formula for the silicon oxycarbide phase:

SiO1.52C0.6–1.05. In the present work, the concentration of these

various species on the SiC surface after air calcination at 800 �C,

determined from the XPS decomposition, is 34 at. % for SiC, 33 at.

% for SiCxOy and 33 at. % for SiO2. Such results are in good

agreement with the HRTEM analysis that reveals an amorphous

layer on the SiC surface (Fig. 1E), as well with the previously

reported results on the oxidation state of the SiC surface upon air

calcination.24 The next question addresses the localization of the

various species either on the whole surface of the material or onto

specific localizations depending on the nature of the pores.

The total amount of the SiCxOy and SiO2 species present on

the topmost surface of SiC is determined by submitting the

sample to a soda (20 wt. %) treatment at 80 �C that removes the

oxygen-containing species. According to the results the oxygen

concentration on the SiC material was found to be 3.5 wt. %. The

complete removal of the oxygen-containing species on the SiC

after the soda treatment was already shown in a previous

reported study.29 Similar results have also been observed in our

previous work using a Zeta potential technique.8

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 (A) Example of a 2D TEM image from the tilt series recorded in Bright Field mode on a typical grain of SiC. The scale bar is 100 nm. (B) Typical

slice through the volume reconstruction of the sample showing the presence of the two types of pore: interconnected channel pores and ink-bottled pores.

(C) Modeling of the reconstruction showing the global morphology of the pores and their spatial distribution. Inset: Closer view of the porous structure.

(D) XPS Si2p spectrum of the SiC sample after synthesis and after air calcination at 800 �C to remove the unreacted carbon. (E) High resolution TEM

image showing the presence of an oxide layer on the C-pore surface.

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Another accurate determination of the oxygenated groups

present on a SiC surface and, in turn, of the hydrophilic/hydro-

phobic character of the material surface, has recently been

reported by Keller et al.30,31 using H/D exchange between gaseous

D2O and a SiC material. H/D titration to selectively titrate the

oxygen sites present on the fresh SiC surface is given to be 4.0 �0.3 mmol/m2 (Table 1). On SiC treated with soda solution at

80 �C, no H/D exchange was observed confirming the oxygen-

free nature of the material. The same SiC sample after oxidizing

in air at higher temperature, i.e. 1000 �C has its whole surface

covered with a layer of oxygen-containing species similar to that

of pure silica, i.e. 11.7� 1.0 mmol/m2 for SiC-oxidized and 11.6�0.7 mmol/m2 for silica. This is confirmed by XPS results reported

in the literature indicating a homogeneous surface coverage of

SiC with a SiO2 layer.32,33 Oxidizing SiC material in air at 1000 �C

leads to complete surface oxidation and in that case, only

hydrophilic surfaces are detected. However, depending to the

hydrophilic or hydrophobic nature of the original surface, the

thickness of the oxygen-containing layer can be different.

Characterizing the nature of the surface is the objective of the

present EFTEM tomographic analysis.

Taking into account the fact that on the oxidized SiC sample

the totality of the surface is covered with a homogeneous thin

layer of oxygen, one can estimate the oxygen surface (hydrophilic

surface) coverage on the fresh SiC sample to be 34%. The

oxygen-free pure SiC surface (hydrophobic surface) represents

then 66% of the total surface of the SiC material. According to

This journal is ª The Royal Society of Chemistry 2010

these results one can state that SiC material, after synthesis and

air calcinations at 800 �C, exhibits a hybrid surface containing

both oxygen and oxygen-free fractions; contrary to bulk SiC

covered with a SiO2 shell that only exhibits hydrophilic surface

character as does bulk SiO2. These results are somewhat different

from those obtained using tomography analysis, i.e. 48 and 52%,

respectively. In this particular case, they were deduced by

assigning, in a first approximation, the hydrophilic and hydro-

phobic surfaces to the R-pore and C-pore surfaces.8

Another typical finding which has been inferred from our

previous observations is that, when this system was used as

a catalytic support and Pd nanoparticles were deposited inside

the porous network by means of an incipient wetness

impregnation method, the localization of Pd seems to be

closely depended on the nature of the solvent, ethanol or

water, which selectively wets one or the other porous system.8

To confirm this observation, additional 3D analysis were

performed on a similar system composed by Fe3O4 nano-

particles inserted in the SiC matrix using ethanol as solvent

(see ESI, SI-1).† Once again, the particles were mostly local-

ized in the R-porosity when ethanol was used as a solvent. As

a consequence, we concluded that the selective localization of

the active phase deposited by impregnation is only due to

a selective wetting titration of the two surfaces by different

liquids, the channel C pores being wetted by water whereas the

ink-bottled R pores were preferentially wetted by alcoholic or

organic solvents.

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Table 1 Number of –OH surface groups in mmol/g and mmol/m2 present on the different SiC samples determined from the H/D exchange experimentsand fraction of the SiC surface, i.e. C-pore and R-pore, containing oxygenated species. The hydrophilic versus hydrophobic surface exposure is derivedfrom the surface oxygen coverage

Fresh SiC Soda treated SiC Oxidized SiC (1000 �C - 2h) SiO2

Number of –OH surface groups(mmol/g)

115 � 10 0 282 � 25 1390 � 85

Number of –OH surface group(mmol/m2)

4.0 � 0.3 0 11.7 � 1.0 11.6 � 0.7

Surface coverage by oxygenatedspecies (%)

34 � 4 0 100 � 8 100

Hydrophilic-hydrophobic surfaceestimation from H/D titration(%)

34–66 0–100 100–0 100–0

Hydrophilic-hydrophobic surfaceestimation from tomography (%)

48–52

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In order to explain such a difference, we propose the following

hypothesis about the pore surface reactivity: the channelled pores

are mostly decorated with oxygenate functional groups exhibit-

ing a hydrophilic behavior, while the ink-bottled pores that seem

to have less affinity with oxygen exhibit a hydrophobic character.

According to this assumption one should expect the oxygen

content on the surface of the C pores to be larger than the oxygen

content in the R channel pores after a thermal treatment in air,

which typically leads to an enhanced layer of oxygenated

compounds on the surface. That unveils a difference in oxygen

affinity between the two pores surfaces. However, no study

dealing with the 3D surface mapping of the pore oxidation

degree at the nanoscale has been reported up to now and it is the

purpose of this work to bridge this gap.

The existence of a difference in chemical composition between

the R-pore and C-pore surfaces is primarily suggested by accu-

rately analyzing the SiC volume reconstruction obtained by

classical tomography in the vicinity of the surface (for more

details, see ESI, SI-2).† Indeed, a relatively thick fringe-like

contrast can be observed at the SiC surface and especially at that

defined by the C-pores. The difference in contrast with bulk SiC

could be related to a slightly different chemical composition, and

could be associated to a relatively thick oxide layer present on the

SiC surface. The presence of such fringes, especially at the C-pore

surface, suggests a difference in oxidation rate between the two

pore surfaces, in agreement with the previous hypothesis.

Fig. 2 EELS analysis on a typical SiC aggregate with an R-pore roughly pe

mainly on the two surfaces. (A) General STEM image recorded in the annu

magnification showing the scan direction of the electron beam (in green), for t

(in blue) and oxygen relative concentration deduced from the EELS spectra (in

the R-pore.

2672 | Nanoscale, 2010, 2, 2668–2678

Analysis by STEM-EELS: 2D chemical composition

In order to obtain more chemical selective information on the

oxidation state at the two surfaces, we first performed 2D

analytical TEM measurements using the STEM-EELS mode. In

that regard, several EELS spectra were recorded for various

positions of the electron beam focused probe (0.25 nm in diam-

eter). Fig. 2A shows an image recorded in the dark-field mode of

the analyzed grain. The electron beam of the microscope was

then scanned along a line (green line on Fig. 2B) crossing a SiC

aggregate presenting a roughly cylindrical R-pore having its

surface perpendicular to the beam. The scan direction was

chosen in order to chemically investigate small areas going from

the outside of the SiC aggregate to the inside of the considered

R-pore, allowing the simultaneous probing of the two types of

surfaces. A typical EELS spectrum recorded with a 0.3 eV energy

dispersion showing the K-edge of oxygen is shown in the ESI (SI-

3).† The as-deduced relative concentration of oxygen for the

probed areas is drawn on Fig. 2C and clearly shows that it is

present on the two surfaces (R-pore and C-pore). However, this

information remains insufficient to precisely compare the

concentration of oxygen present on the two surfaces, due partly

to the poor quality of the signal to noise ratio (SNR) which does

not allow quantitative analysis. But the main reason is that

chemical fluctuations can occur through the direction of the

electron beam, because the analyzed surfaces are certainly

rpendicular to the electron beam, suggesting the presence of the oxygen

lar dark-field (ADF) mode. (B) STEM-ADF image taken with a higher

he recording of the successive EELS spectra. (C) Mass-sensitive intensity

black), along this direction going from the outside of SiC to the inside of

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not perfectly perpendicular to this direction and have also a 3D

morphology.

3D oxygen mapping in SiC through EFTEM tomography

Fig. 3A and B present a typical 2D projection of the analyzed SiC

fragment recorded in the Zero Loss mode and the corresponding

EELS spectrum recorded with a 0.1 eV energy dispersion on the

whole object, showing the K-edges of carbon and oxygen. The

positions and widths of the three energy windows used to record

the energy filtered images are also represented for the oxygen, as

well as the corresponding chemical signal (see inset), after the

background extraction. Fig. 3C and D show three energy filtered

images recorded at the O K-edge (left) and the resulting 2D

oxygen elemental map. In the same manner a 2D mapping of

carbon is obtained for each tilt angle (the corresponding images

are given in the ESI, SI3).† At the end of a recording series and

elemental mapping calculation, three correlated tilt series are

obtained: one with the mean density and the two others giving

the elemental projections of the oxygen and carbon. From these

tilt series two 3D reconstruction volumes are calculated that

contain two mappings of the elements of interest. Alternatively,

a third reconstruction corresponding to the mean density volume

Fig. 3 (A) Image of a b-SiC nanoparticle recorded in the Zero Loss mode on

spectrum recorded on the whole object, illustrating the C K-edge (at 284 eV

windows used to acquire the tilt series of filtered images for the oxygen. The ins

Corresponding filtered images recorded at the K-edge of the oxygen at a given

filtered images, after the background extraction using the three-window meth

This journal is ª The Royal Society of Chemistry 2010

is obtained from the Zero Loss tilt series, from which a typical

image is presented in Fig. 4A.

A representative slice extracted from the mean density recon-

struction is shown in Fig. 4B. In such image the thickness of the

slice, given by the pixel size in the initial projection images (1.1

nm), is thin enough to insure that one can assume the absence of

overlapping effects along the sample thickness.

Analyzing the reconstruction corresponding to the mean

density we can easily identify the two types of surfaces proposed

by the analyzed SiC fragment. We observe thus a R-pore inside

the grain that contributes to the R-porosity, whereas the external

surface of the grain defines its C-porosity. Comparing the

amount of oxygen present on the two surfaces by analyzing only

the slices from the oxygen volume is not quite convincing, as the

signal to noise ratio (SNR) remains relatively low in a unique

chemical reconstruction and thus, leads to a relatively large error

on the oxygen localization. However, since the three volumes are

spatially correlated, they can be analyzed simultaneously, by

superposing the 3D maps of carbon and oxygen in order to

obtain a 3D relative map of SiOx compared to SiC. A cross-

section extracted from the obtained 3D relative map taken

through the R-pore is presented in Fig. 4C, with SiC colored in

red pixels and SiOx colored in green pixels. As can be observed,

the SiC grain selected for the analytical tomographic analysis. (B) EELS

) and O K-edge (at 532 eV), as well as the characteristics of the energy

ert shows the resulting oxygen signal after the background extraction. (C)

tilt angle. (D) Elemental projection of the oxygen obtained from the three

od. The scale bar is 100 nm for all images.

Nanoscale, 2010, 2, 2668–2678 | 2673

Fig. 4 (A) 2D-TEM image at 0� tilt from the tilt series used to reconstruct the mean density volume. (B) Section through the reconstructed volumes

corresponding to the mean density. (C) Cross-section, taken at the same depth and angle with respect to the initial orientation as that of Fig. 4A, through

the 3D relative map of SiC (red) with respect to SiOx (green), in white the guide for the external surface and the pore. (D) Threshold sorting modeling of

the 3D SiC/SiOx map showing the spatial spread of the SiOx on the two surfaces; inset a: zoom in the vicinity of the R-pore showing the presence of less

oxygen on the R-pore surface; inset b: zoom near the external C-pore surface illustrating its high oxidation degree. (E) Global modeling of the analyzed

SiC grain.

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the determination of the relative spatial spread of the two

different chemical phases is much more accurate compared to the

results obtained by analyzing the individual elemental recon-

structions. In order to get a more intuitive view of the oxygen

distribution and to perform a quantitative analysis, a modeling

of the 3D relative map was realized using a criterion based on the

comparison of the intensities of individual elemental maps, pixel

by pixel. A typical view of the modeling is showed in Fig. 4D,

giving a clearer view on the difference in terms of oxidative

resistance between the R-pore and C pore surfaces (see insets

a and b). Note that the spatial resolution in this analytical

tomographic mode estimated by taking into consideration the

experimental conditions and specimen characteristics, is of the

order of 7 nm. In addition, the quantitative analysis of the 3D

relative map allows us to determine the mean concentration in

oxygen in the analyzed grain to be of the order of 10 wt. %, in

agreement with the global value obtained by the thermo-gravi-

metric analysis.

Discussion

High-temperature oxidative treatment slowly leads to the

complete oxidation of the SiC surface leading to a hydrophilic

behavior. However, the thickness of the oxygen-containing layer

largely varies in a great manner depending on the hydrophilic or

2674 | Nanoscale, 2010, 2, 2668–2678

hydrophobic nature of the original SiC surface. According to the

presented slice and to the 3D modeling, one can conclude that the

oxidation mostly starts on the surface of the C-pores whereas it is

less important on the surface exposed in the R-pores. That

clearly evidences, in the limit of our spatial resolution (7 nm),

a different oxidative behavior induced by the pore morphology.

Increasing the oxidation treatment duration leads to a more

complete transformation of SiC into SiO2 especially on the

C-pore surface. Classical TEM micrograph evidences the

progressive channel pore (C-pore) plugging by the extensive

growth of SiO2 away from the SiC surface which bridges the pore

gap (see ESI, SI-3).† Such pore plugging is also at the origin of

the overall specific surface area decrease after oxidation. It is

worth noting that during the oxidation process, the R-pore

surface also undergoes oxidizing as well but probably with

a lower rate due to the hydrophobic character of the original

surface.

The results obtained in this work clearly evidence the fact that

b-SiC having a dual porosity exhibits an atypical behavior

against oxidation. The interconnected channel pores (C-pore) are

more affected by the calcinations step due to their high oxygen

affinity provided by the presence of a higher defect density. The

C-pores result from the aggregation of several SiC grains

stacking along a channel where grain boundaries do generate

topological defects, which make the pores more reactive against

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oxygen. On the other hand, the ink-bottled pores are much less

affected by the oxidative treatment due to the following points:

(i) an ink-bottled aperture is less prone to oxygen diffusion and is

therefore less oxidized than the more accessible channel pores,

(ii) the less defective surface characteristics of the ink-bottled

pores prevents further oxygen adsorption and consecutive

oxidation. It appears that an active catalytic phase specifically

localized inside an ink-bottled pore is less affected during

oxidative treatments or reaction and this improves the catalyst

life-time.

The low oxygen content on the R-pore surface also signifi-

cantly modifies their wetting behavior against liquid media. It is

expected that organic liquid media will have a strong wetting

behavior with such a surface and thus, will significantly influence

the catalytic activity as well. Previous results have shown that

despite the larger size of palladium particles localized within the

R-pore, they display a higher hydrogenation activity for cinna-

maldehyde than the smaller ones localized on the C-pore surface.

Conclusion

In conclusion, it can be stated that the analytical electron

tomography technique is a powerful characterization tool for

imaging pores oxidation states in SiC, as the thickness of oxygen-

containing layers, and the connectivity of a porous material. In

this work it has been applied to porous b-SiC that was submitted

to a thermal treatment. It showed that interconnected channel

pores are more affected by an oxidative treatment due to the

presence of high defects density with a high affinity for oxygen

adsorption and to their open access, than ink-bottled pores that

exhibit a higher oxidative resistance owing to the presence of

a lower defects density and a constrained access. Such results are

of great interest for the subsequent use of the material in catal-

ysis. According to our results, the most accessible pore allowing

the reactants access to the active phase is the first to be plugged

after prolonged oxidative treatment due to the extensive devel-

opment of a thick layer of SiO2 over the pore channel, which

blocks the access to the reactants.

From a general point of view, the development of this new and

powerful technique with a nanometer resolution will have an

important impact in the field of nanomaterials, as it allows one to

get access to two most critical parameters: (i) the 3D structure of

the sample in the tomographic mode and (ii) the chemical

composition of the analyzed material through the electron-

energy-loss signature. The technique used in the present work

can be employed for analyzing other catalytic systems, where the

access to the topmost surface chemical composition represents an

important parameter for understanding their catalytic perfor-

mances, as for instance intimate Si-Al catalysts or zeolite mate-

rials, or the chemical composition of various surfaces in carbon

nanotubes, inner versus outer, as a function of either a chemical

or a thermal treatment.

Experimental methods

Set-up of EFTEM tomography at the nanoscale

EFTEM tomography uses a tilt series of elemental projections to

compute an elemental selective reconstruction volume. For our

studied system, i.e. b-SiC, the elements of interest are oxygen,

This journal is ª The Royal Society of Chemistry 2010

present especially on the surface as SiOx compounds, and

carbon, present in the bulk as SiC. As the global concentration of

oxygen is rather low in an individual SiC aggregate since it is only

present as a native oxide layer on the surface, we performed our

analysis on oxidized SiC samples, containing 10% of oxygen in

weight in order to reach a reasonable signal to noise ratio (SNR)

and therefore minimizing the recording duration.

For the electron tomography analysis, the b-SiC samples were

crushed in a mortar into a very fine powder and dispersed in

ethanol in an ultrasonic bath for a few minutes. The solution was

then deposited on a copper grid recovered by a holey carbon

membrane where finally a drop of a solution containing gold

calibrated nanoparticles of 5 nm was deposited. These nano-

particles will be used as fiducial markers in the data treatment

process, in particular for the fine alignment of the tilt series

before the reconstruction.

The energy filtered images of the tilt series were obtained using

a JEOL 2100F (FEG) TEM/STEM electron microscope oper-

ating at 200 kV, equipped with a TRIDIEM post-column

imaging filter of the Gatan Company. The images acquired on

a 2048*2048 pixel cooled CCD detector were hardware-binned to

512*512 pixels with a pixel size of about 1.1 nm. The EFTEM

tomography software, implemented as a plug-in in Digital

Micrograph, provides automated acquisition of data by

controlling the spectrum energy offset and the width of the

energy selecting slit. In addition, the software allows to auto-

matically change the specimen tilt step by step, to correct the

defocusing and the specimen drift. In order to limit the irradia-

tion damage, a total of 57 angle positions were selected in the

angular range between�70 and 70�, with projections taken every

2.5�.

In order to ensure a monotonic dependence of the chemical

signal as a function of thickness and concentration, we used the

three-window method for the two elements of interest C and

O.34,35 This allows a proper extraction of the background in the

considered energy range and furnishes elemental projections with

a quite good approximation. In that regard, eight images were

recorded at each tilt angle: a first Zero-Loss image collecting the

elastic scattered electrons, three filtered images at the K-edge of

carbon (theoretical position 284 eV) used as a signature of SiC,

three energy filtered images at the K-edge of oxygen (theoretical

position 532 eV) used as a signature of SiOx, and finally a second

Zero-Loss image, used to check the immobility of the sample

during the acquisition procedure of the eight filtered images. The

energy positions of the filtered images for the carbon atoms were

244, 270 and 300 eV for the two pre-edge windows and the post-

edge window respectively, with an energy slit of 24 eV and an

exposure time of 5 s for each image; for the oxygen, the energy

positions were 470, 513 and 558 eV, with an energy slit of 40 eV

and an exposure time of each image of 12 s. A 120 mm diameter

objective aperture was used to record all these images.

After the acquisition process of the tilt series is finished, the

pre-processing steps and the reconstruction procedure were

performed using the IMOD and TOMOJ/EFTETJ softwares.36,37

In a first step, two series of chemical 2D maps (one for the oxygen

and the other for the carbon, so-called ‘‘elemental projections’’),

were computed from the tilt series of filtered images, tilt angle by

tilt angle. For one element, starting from the three tilt series

corresponding to the three chosen energy windows (previously

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aligned to correct for the specimen drift between their successive

acquisitions), a series of chemical projections were computed by

extracting from the post-edge image the background estimated

from the two pre-edge images using an exponential law. The tilt

series are further aligned, in order to set all the projections,

elemental or mass-sensitive, in the same frame of reference. Since

the signal to noise ratio (SNR) in the chemical projections of C

and O is relatively poor, the usual alignment procedure based on

cross-correlation or fiducial markers gives poor results.

However, in the Zero-Loss tilt series the signal to noise ratio

(SNR) is high enough as to allow the implementation of cross-

correlation algorithms. In addition, the gold fiducial markers

deposited on the TEM grid membrane supporting the object are

clearly observable in that case and can be used as reference points

to make a more accurate alignment of the tilt series. After

checking the stability of the sample during the acquisition

process of the successive images recorded at the same tilt angle

(by comparing the two Zero-Loss images recorded at the

beginning and at the end of the energy record set), the spatial

parameters used to align the Zero-Loss tilt series can be applied

to align the two elemental projection series. We obtained thus

three projection series aligned in a unique frame of reference: the

first one corresponds to the Zero-Loss images, which are mass-

sensitive, and the two others to the elemental projection series for

O and C, which are chemically sensitive. These three projection

series are spatially correlated one relative to the others and, as

a consequence, the corresponding reconstruction volumes are

also spatially correlated, allowing performing a combined anal-

ysis. To reduce the typical artifacts appearing in the recon-

struction, due to the limited maximum tilt angle and number of

projections,15 the three reconstructed volumes were computed

using iterative algorithms.38 With respect to the simple back-

projection method, the use of these algorithms is also strongly

recommended for the tilt series with poor signal to noise ratio

(SNR), as it gives a refined solution.39 To extract from the whole

reconstruction useful information, a segmentation procedure is

needed consisting on a threshold sorting of the voxels of the

reconstructed volumes as a function of their grey-level intensity.

In our case, for the chemical selective volumes, this procedure is

difficult to apply due to a poor resulting signal to noise ratio

(SNR). However, as all the volumes are spatially correlated, one

can use the segmentation obtained from the mean density volume

(with high signal to noise ratio (SNR)) and apply the mask of

the analyzed grain, which delimits the borders of the object, to

the elemental volumes. In that regard, one can consider for the

analysis of elemental maps only the voxels corresponding to the

full material. A relative 3D map can finally be deduced, by

superposing with different colors the intensities of the considered

elemental 3D maps.

To estimate the spatial resolution in the analytical tomo-

graphic mode, we have used a method simply based on the

superposition of the 2D analytical resolution and of that of

the tomographic approach. The first, which depends mainly on

the microscope aberrations and experimental conditions used

in the acquisition of the filtered images, was estimated by using

the relations given by Egerton,40 resulting in a resolution limit of

about 5 nm. Furthermore, as usual in the field of electron

tomography, the resolution of the tomographic approach was

determined by using the analytical formula incorporating the

2676 | Nanoscale, 2010, 2, 2668–2678

parameters of the acquisition process.15 Using this mathematical

approach, the value obtained for the tomographic-specific reso-

lution was about 5.5 nm. By combining this resolution with that

of the initial 2D elemental maps deduced previously, we found

a lower limit for the spatial resolution of the order of 7 nm, which

allows to comparatively analyze the presence of a silicon oxide

layer on the R-pore and C-pore surfaces.

Drawbacks and practical difficulties of the EFTEM tomography

technique

In addition to the usual artifacts of the electron tomography due

to an incomplete sampling of the angular range, another limi-

tation of the EFTEM tomography comes out from the poor

signal to noise ratio (SNR) in the chemical projections, which

degrades the accuracy of the reconstructions in terms of differ-

ence between the 3D solution and the chemical distribution in the

original object. To obtain significant results, it is recommended

to analyze simultaneously several complementary chemical

volumes of the same specimen, in a combined manner. Apart

from the problem related with the signal to noise ratio (SNR),

other main practical difficulty of this analytical mode is related to

the calculation of the elemental projections, in particular to the

extraction of the background from the pre-edge filtered images.

For instance, one must totally prevent the existence of a negative

signal in all the pixels of the 2D elemental maps corresponding to

a negative number of atoms. For this reason, with respect to the

other methods used to compute the 2D elemental maps as for

instance the jump-ratio method,34 the use of the three-windows

method is the most suitable. It requires recording three filtered

images for each chemical element, but allows a better estimation

of the background, pixel by pixel, and furnishes intensities in the

elemental projections that monotonically depend on the number

of atoms. The effect of the thickness variation when tilting the

sample must also be taken into account, as the plural inelastic

scattering affects the shape of the spectrum and consequently the

background calculation. Such an effect can be neglected in a first

approximation, provided that the three windows method is used

and especially if the thickness of the object does not change

drastically when tilting, as in the case of the grain analyzed in this

study. In addition, in order to fulfil the projection requirement

for the tomography technique, the effect of the plural scattering

on the 2D elemental maps must be neglected in a first approxi-

mation. This is generally the case when the sample thickness is

lower than the effective inelastic mean free path in any point of

the specimen and that for all angular positions of the tilt series.21

In that regard, one has to systematically compare the maximum

specimen thickness to the inelastic mean free path for the

analyzed specimen. However, if the maximum thickness is

reached only on a limited area of the specimen and it is not to

different from the estimated mean free path, the contribution of

the single scatterings to the total spectrum is still predominant40

and thus the projection requirement for the tomography is ful-

filled in a first approximation.

Concerning the pre-processing steps of the elemental tilt series

before the reconstruction, owing to their poor signal to noise

ratio (SNR), the alignment must be performed in an indirect

manner, using the parameters deduced from the alignment of the

Zero-Loss tilt series. These projections have high SNR and

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furthermore the presence of the fiducial markers allows to

precisely refining the rough cross-correlation solution. Finally,

a last experimental limitation is the presence of the irradiation

damages in the specimen, especially if long acquisition duration

is employed with the general aim to increase the SNR. The

optimization of all these parameters, from the acquisition

conditions to the parameters and methods for the data treatment

and analysis, is needed for obtaining realistic and significant 3D

chemical maps with a nanometer spatial resolution.

Acquisition parameters for the STEM-EELS mode

The STEM-EELS mode was locally employed to perform 2D

analytical measurement for accessing the pore oxidation degree.

The recording of the EELS spectra near the O-K edge was per-

formed using Gatan DigiScan in combination with Spectrum

Imaging (SI) plug-in in Digital Micrograph software. The

diameter of the probe, used to raster the sample in the chosen

direction was about 0.2 nm. The final spectrum was obtained

from 70 EELS SI spectra with 2 s exposure time per spectra using

a 2 mm spectrometer aperture and a dispersion of 0.3 eV/

channel.

Acknowledgements

This work was financially supported by SiCat Co. The SEM and

TEM experiments were carried out at the facilities of the IPCMS

(UMR 7504 CNRS). XPS experiments were carried out at the

facilities of the LMSPC (UMR 7515 CNRS) and P. Bernhardt is

gratefully acknowledged for performing the experiments.

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