4 Characterisation of the Colloidal...

4 Characterisation of the ColloidalSuspension

This chapter will discuss in detail different aspects of the colloidal suspensions, forexample the formation of the colloids, their size and morphology and their chemical andstructural composition. This will shed light on the behaviour of different activatorsolutions and point out the important parameters of the activator.

The nature of the colloidal suspension has been studied with SAXS, which allowsobserving the ensemble of colloids in their natural environment and with CRYO-TEM,which allows obtaining a local view of the colloidal morphology. Structural andchemical analysis of individual colloids has been performed with HRTEM, EDX andEELS.

4.1 Sample Preparation for the TEM

The sample preparation for the TEM is delicate since only very thin samples can beobserved. TEM samples should not be thicker than about 50 nm. This requires specialpreparation procedures. The procedures applied in this work will be described brieflybelow. Most of the time, the samples have been supported by a Cu grid with a holeycarbon membrane.

4.1.1 Deposited Colloids

For the colloids in suspension, two different methods of sample preparation have beenapplied. Either, a microscopy grid with a holy carbon membrane has been put into thesuspension. Some colloids have been absorbed onto the carbon membrane. The grid hasthen been dried and was ready for observation. This technique has been used notably toobserve single colloids at the border of holes in the membrane. The second techniqueconsisted in spreading a sub-monolayer of colloids onto a membrane on a Cu grid. Aglass fibre has been used as a brush to deposit the colloids. This technique wasparticularly suited for the observation of the morphology and the crystallography of thecolloids on the membrane, for which a sub-monolayer coverage has been essential.

4.1.2 Quench freezing for the Cryo-TEM

A special technique is necessary in order to observe liquid samples in a TEM. It consistsin quench-freezing thin liquid films of the suspensions in liquefied ethane [5].. Theywere observed at -180°C in a Gatan cryoholder, using a Philips CM200 'Cryo'microscope operated at 80 kV, under low dose conditions. The Cryo-experiments have

74 4 Characterisation of the Colloidal Suspension

been carried out at CERMAV (Grenoble) by Jean-Luc Putaux. By quench-freezing thesuspension, a local view can be obtained by Cryo-Transmission Electron Microscopy.

The main steps of the sample preparation are pointed out in Figure 4-2. The importantstep is the rapid freezing of the thin liquid film in ethane, which guarantees that the icerests in an amorphous state and does not crystallise. Then the sample is permanentlykept at low temperatures in a special sample holder and introduced into the CRYO-TEM.

1. 2.

Figure 4-1: A glass fibre is used as a brush to deposit a sub-monolayer of colloids on a carbonmembrane on a microscopy grid:

77 K

1. 2. 3. 4.

Figure 4-2: Sample preparation for the Cryo-TEM: 1. A droplet of the liquid is deposited on the Cu grid.2. It is freezed in liquid ethane. 3. The grid is placed on a cryo sample holder and 4.transferred into the Cryo-TEM

4.2 Analyse of the Colloids in Suspension - Stability, Morphology 75

4.2 Analyse of the Colloids in Suspension - Stability, Morphology

4.2.1 CRYO-TEM

Figure 4-3 shows a TEM micrograph of a quench frozen solution of Futuronconcentrated and of Futuron 2x diluted. The thickness of the frozen liquid is between50-100 nm. Therefore, not all of the colloids are perfectly in the focal plane of themicroscope, which makes the size analysis difficult, since Fresnel effects can changeslightly the apparent size of the particles.

Colloids of about 2 nm with a narrow size distribution are visible in the concentratedactivator solution. The dilute solution, where the pH value increased considerably,consists of regions with particles of 1-8 nm which are well separated and seem to repeleach other, and "empty" regions. The particles seem to be embedded in a gel-likestructure. The repulsion between the particles suggests that they are charged.

Figure 4-3: Cryo-TEM micrograph of the concentrated colloidal suspension (left) and micrograph of asuspension diluted with H2O (Futuron 2x diluted)(right).

In fact, these particles are attributed to the formation of SnO2-complexes, which areembedded in a gel-like structure of SnO4. The formation of such complexes, as well asthe gel-like structure of SnO4, is predicted from the Pourbaix diagrams (Figure 4-4),when the pH value is increased to above ~2 [76]. Figure 4-5 gives a schematicrepresentation of what should occur in dilute suspensions.


Figure 4-4: Diagram of the equilibrium potential vs. pH of the tin-water system at 25°C. The hydroxidesSn(OH)2 and Sn(OH)4 have been considered. After [76]

Sn(OH)2

Sn(OH)2

Sn(OH)2

Sn(OH)4

Pd/Sn colloidsSn2+ Sn2+

Sn2+Sn4+

Sn4+

Sn4+

Sn4+

pH increases above ~2

Figure 4-5: Schematic representation of the possible process occurring during dilution with water: Byincreasing the pH value above 2, Sn(OH)2 particles with a large size distribution,embedded in a gel-like structure consisting of Sn(OH)4., have been formed


4.2.2 SAXS

4.2.2.1 Sample Preparation

X-rays get absorbed when passing by the sample. The transmitted intensity I should bein the rage of 90-10 % of the incident intensity I0. The absorption coefficients µ [cm-1]for the different elements present in Futuron at a given X-ray energy have been takenfrom [77]. It has been transformed to the mass absorption coefficient µ(m)= µ/ρ with ρthe density of the pure element. The average mass absorption coefficient has beencalculated as follows: µ(av)=µ(Pd)*n(Pd)+µ(Sn)*n(Sn)+…, with n(X) the weightpercentage of element X. Multiplying µ(av) with the density of the suspension leads tothe absorption coefficient of this suspension. The needed thickness of the sample canthen be deduced from the formula

I= I0. exp(-µx) ( 4.1 )

The concentrated suspensions (Futuron concentrated and Futuron plus concentrated)absorb heavily and have therefore to be not thicker than 1 mm. For the dilutesuspensions, cells of 2 mm could be taken. Small rings made of PVC with a thickness of1 or 2 mm have been prepared. Both sides have to be closed with a Kapton-film. Thesuspension has been injected with a shot by a small hole drilled in the PVC ring. Thishole has then been closed with glue.

4.2.2.2 Results

Small angle X-ray scattering provides information on the size of the colloids, their sizedistribution and in some cases also about the solid-liquid interface between the colloidsand the surrounding medium [4]

The average radius of the ensemble of the colloids has been found by fitting thetheoretical diffusion amplitude (equation 3.13) of a spherical particle to theexperimental data.

From the large q range where a linear behaviour in the Guinier plots is observed, it canbe seen that the colloids in the Futuron plus concentrated suspension (colloidconcentration twice as high as for the normal suspension) are highly monodispersed(Figure 4-6); the diameter calculated with equation 3.13 is 2.12 nm for this solution. Itis in perfect agreement with the CRYO-TEM observations reported in section 4.2.1,where a mean particle size of about 2 nm, with a narrow distribution, could be deducedfrom Figure 4-3.

For the other suspensions (i.e., Futuron concentrated, Futuron (nc) H2O and Futuron 2xor 3x diluted), the SAXS experiments reveal different modifications of the organisationof the colloids.

In the Futuron concentrated solution, the mean diameter of the colloids remains of thesame order (2.52 nm as deduced from Figure 4-7), still with a narrow distribution.However, a significant peak at q2 ≈ 4 nm-2 is observed in the spectrum, which can only


be attributed to agglomerates of particles in a well-ordered state. According to theBragg law, the peak position corresponds to an inter-particle distance of 2.7 nm, whichindicates that the particles may not be directly in contact but protected by a shell ofcounter-ions. Using the Debye-Scherrer formula, the FWHM of this peak leads to adiameter of these agglomerates of colloids of about 13 nm (for details, see AppendixA). This assumes, however, a perfect crystallographic structure and uniform sizedistribution of the 'colloid crystals'. It was not possible to reproduce the peak with thisFWHM by applying models of biphasic systems with a gas-like and a liquid-likecolloidal phase, which justifies treating the agglomerates as colloidal crystals. Thediameter of these agglomerates has nevertheless to be treated with caution. Theexplanation for the appearance of these aggregates can be found in [25]. and will bedetailed in the general discussion (i.e. section 6.1). Such agglomerates are not easilyseen in the CRYO-TEM image of Figure 4-3 (left image), owing to the projectionproblem in the thickness of the TEM foil (estimated to more than 50 nm, which is largerthan the agglomerate size, i.e. 13 nm). We cannot however exclude that a partialevaporation of the liquid occurs during the preparation procedure of the CRYO-TEMsamples, which would then lead to a solution more concentrated than expected. Such aneffect would, according to SAXS results obtained on the 'Futuron plus concentrated'sample, give rise to a more homogeneous distribution of the particle (e.g. see Figure4-6).

The Shipley 9F suspension showed a similar behaviour to the Futuron plusconcentrated suspension. The colloidal diameter was slightly smaller (1.82 nm). Thepeak indicating the formation of clusters could not be observed (see Appendix A).

Figure 4-8 shows that changing the pH value drastically changes the suspension. Byadding H2O, the pH has been increased from 0 to above ~2; under these conditions, theSAXS Guinier-plots show a large distribution of the particles size in all cases, since theregion between q2=0 nm-2 and q2=2 nm-2 can not be approximated by a straight line anymore. The spread in size distribution was already and consistently seen in the Cryo-TEM experiments (Figure 4-3), which further confirms that the CRYO-TEMobservations were well representative of the dilute suspensions.

4.2.2.3 In Situ Experiments

The initial acid solutions of PdCl2 and SnCl2 have been mixed and heated to 90°C in theX-ray beam. SAXS spectra have been taken permanently during this heating process.Time resolved in situ experiments are represented in Figure 4-9, Figure 4-10 and Figure4-11: the time interval between each spectrum is about 1 min. The spectrum at 11 min34 s is equal to that at the start (0 min 0 s).


0 2 4 6 8 10-1

0

1

Concentrated colloidal suspension

Linear fit

ln(I

) (a

rb.

un

its)

q² (nm-2)

Figure 4-6: The Guinier plot of the concentrated colloidal suspension shows a uniform size distributionof the colloids.

0 2 4 6 8 10

-1

0 "Normal" Colloidal suspension Linear fit

ln(I

) (a

rb.

un

its)

q² (nm-2

)

Figure 4-7: The Guinier plot of the colloidal suspension shows a uniform size distribution of the colloids.An agglomeration of several colloids is responsible for the peak at q²=4 nm-2.


0 2 4 6 8 10-4

-3

-2

-1

0 First dilution Second dilution Third dilution

ln(I

) (a

rb. u

nits

)

q² (nm-2)

Figure 4-8: Guiner-plot of the colloidal suspension diluted with water. The lack of any linear regionindicates a large size distribution of the particles.

The maximum spectrum is that at 14 min 59. At the beginning, only scattering at high qvalues has been detected which has been attributed to complexes in the solution. About12 minutes after the heating has switched on, the spectrum evolves and shows thetypical scattering of colloids. Again, the small peak of agglomerated colloids can beobserved as in the case of Futuron concentrated. This is therefore a reproduciblephenomenon for Futuron concentrated. After 16 minutes, the intensity decreasedslightly which has been attributed to a loss of the Futuron suspension in the observationcell.

4.2.2.4 Large peak at large q

The spectra of the Futuron concentrated exhibit a large peak with a maximum atq=5.7 nm-1. The original mixture of PdCl and SnCl has a similar peak which can beattributed to the formation of complexes (Figure 4-12), but with a maximum at aslightly lower q-value of q=4.8 nm-1. By heating the solution, the maximum of thispeak evolves towards the higher value before the particles are formed. Also for theFuturon concentrated this broad peak at q=5.7 nm-1 is attributed to complexes. Apossible influence of the counter-ions around the particles can not be detected due tothat peak (see Figure 3.13 about the diffusion intensity of Core-Shell colloids).


0 10 20 30 40 50 60

-2.8

-2.4

-2.0

00 min 00 s 03 min 32 s 04 min 40 s 05 min 48 s 06 min 59 s 08 min 10 s 09 min 18 s 10 min 18 s

Ln(I

)

q² (nm-2)

Figure 4-9: Futuron heated to 90°C, first part of the series of spectra, before the Pd/Sn colloids appear

0 5 10 15 20

-4

-3

-2

11 min 34 s 12 min 41 s 13 min 49 s 14 min 59 s

Ln(I

)

q² (nm-2)

Figure 4-10: Heated at 90°C, second part of the series of spectra. The formation of colloids can beobserved: the scattering intensity increases with time.

0 5 10 15

-6

-4

16 min 07 s 17 min 19 s 18 min 27 s 19 min 35 s 20 min 43 s 21 min 53 s

Ln(I

)

q² (nm-2)

Figure 4-11: Heated at 90°C, third part. The intensity decreases due to mass loss in the sample


4 6 8

0.4

0.6

0.8

1.0

Futuron + conc PdCl/SnCl mixture not heated

Inte

nsity

(a.

u.)

q (nm-1)

Figure 4-12: Comparison between the original solution (Futuron not heated) and Futuron plusconcentrated

4.3 Chemistry and Structure of the Colloids

4.3.1 Comparison of frozen and dried suspensions

A major disadvantage of the hydrochloric acid solution is that the colloids suspensioncould not be directly observed in HRTEM. Thus, dried samples were prepared bydepositing a small droplet of the suspension onto a membrane of holey carbonsupported by a Cu-grid. In order to verify that the structure of the colloids was notsignificantly modified during this preparation process, quench frozen solutions havebeen observed with a dedicated Cryo-TEM (see Figure 4-13). The comparison betweenthe frozen preparations and the deposited dried colloids showed no difference in the sizeand shape of the colloids [78]. It is therefore assumed that the colloids are not altered bythe drying procedure.

Figure 4-13: Comparison between dried colloids deposited on a carbon membrane (left) and a quenchfrozen preparation of the colloidal suspension (right)

4.3 Chemistry and Structure of the Colloids 83

4.3.2 Composition of agglomerates of colloids

If the microscopy grid has been immerged into the colloidal suspension, colloids areadsorbed in some regions as a sub-monolayer, in others as larger agglomerates as can beseen in Figure 4-14. The colloids adsorbed at the border of a hole are well suited for thechemical analysis with EELS, since no carbon signal from the substrate disturbs the Pdand Sn edges. The EDX spectrum of the cluster shows an atomic ratio Sn/Pd=0.65,indicating that the core of the particles contains significantly more Pd than Sn. Not allof the tin measured with EDX is part of the colloids, some of it is adsorbed as ionic tinfrom the solution. It has to be mentioned that the atomic ratio on this type of samples isrelatively inhomogeneous. Other regions, with more ionic tin adsorbed, exhibit a Sn/Pdratio much larger than 1.

Figure 4-14: Cluster of colloids adsorbed on a holy carbon film (left) and the EDX spectrum of thisregion (right). The atomic ratio Sn/Pd=0.65 from the EDX spectrum

4.3.3 Core and Surface of the colloids

Palladium-tin nano-colloids have been analysed with HRTEM and EELS.

HRTEM observations were done on the FEG-TEM instrument presented inchapter 3.1.3. Probes in the range 0.4 to 1 nm were used to acquire series of EELSspectra across individual particles (line-scan or line-spectrum technique) ; the probe wasmonitored either in a manual way (calibrated beam shifts with the microscope controls)or with the OXFORD semi-STEM (scanning TEM) device.

EELS spectra were obtained on the FEG-TEM microscope in the nano-probe mode.Usual corrections were performed on these experimental spectra before analysis (see[66], and below for the background subtraction), except the thickness deconvolution,since a low loss spectrum is not available at each point of the scan. However, mean lowloss spectra recorded on many colloids show that the effect of the thickness is negligibleas intuitively expected, since only regions with a thickness of some nanometers areinvestigated. For the same reason, the beam spreading within the thin colloid wasneglected.


EELS spectra were acquired with counting times of the order of 2 seconds, in anenergy-loss range of 250-750 eV including the Pd-M4,5 (starting at 330 eV) and the Sn-M4,5 edges (starting at 485 eV). These edges present the advantage of lying in arelatively low energy-loss range (compared to the Pd-L and Sn-L, respectively at about3.3 keV and 4.0 keV, and too far for a nanoprobe analysis), and on the same side withrespect to the carbon-K edge near 284 eV. One problem is however that these M edgesare not far above the C-K edge: they are therefore strongly influenced in the case ofparticles supported on a carbon film. The study was then restricted to colloids strictlylocated at the border of the carbon film in order to minimise the influence of the C-Kedge. For the spectra were the carbon-K edge was detected, a spectrum recorded on thecarbon film was used as a background model, and subtracted from the original spectra(see [79] Since (Sn,Cl) complexes are expected to stabilise the colloids (see above), itwould have been interesting to quantify a Cl edge, i.e. the L edge starting near 200 eV.The undesirable influence of the C-K edge has unfortunately led us to disregard the Cl-L edge, because this peak is lying at the opposite side of the C-K edge, which makes thebackground subtraction very difficult from about 200 to 600 eV in the presence ofcarbon.

An other problem is that the maximum of the oxygen K edge at 536 eV nearly coincideswith the maximum of the Sn-M edge, which may affect the analysis if the colloids arepartly oxidised. However, the Sn-M edge maximum is very broad while the O-K edgehas a significant fine structure, which makes it easy to detect oxygen (see thecomparison between the pure Sn spectrum and the SnO2 spectrum in Figure 4-15); agreat care has been paid regarding this point, and it was found that most colloids wereoxygen-free.

As a complement to the above discussion, it appeared interesting to compare the finestructures of the edges of interest, i.e. Pd-M and Sn-M, in compounds where thoseelements have different bonds with their neighbours. Hence, Appendix C shows variousreference spectra acquired on Pd-based (pure Pd, PdO, PdCl2) and Sn-based (Sn, SnO,SnO2, SnCl2) materials. From these results, it can be seen that the Pd and Sn edgesobserved in the colloids mostly resemble that obtained on the pure Palladium and Tinreferences, which further supports the conclusion that no significant oxidation occurred.

The linescans have been evaluated by comparing the experimental spectra toreconstructed ones obtained from a computer experiment on a model colloid; the fullprocedure has been detailed in Appendix B and can be summarised as follows. A core-shell colloid is geometrically modelled and filled randomly with atoms (e.g., Pd and Snatoms) according to the density of pure elements (respectively, 68 and 29.1atoms/nm3), and to the Sn/Pd atomic ratio deduced from the experiment (see below). Inthe computer experiment, a virtual scan is performed on a model colloid. At eachposition of the computed line-scan, the number of atoms in the electron beam arecounted; then, EELS spectra can be reconstructed by weighting normalised referencespectra of Pd and Sn (see Figure 4-15) with the number of atoms. The crucial


parameters of the calculation are (i): the scan parameters, i.e. the electron beam probesize expressed as the Full Width at Half Maximum (FWHM), the stepwidth betweenindividual spectra (these parameters being known from previous calibrations) and thestart position of the probe; (ii) the colloid geometry and chemistry, i.e. the core- andshell-radius, the respective concentrations of Sn and Pd species in the core and shellregions. Some of these parameters were deduced from the experiment: the total size ofthe particle could be estimated from HRTEM micrographs, and the mean composition(fixing the core concentrations) of the particle was measured from EDX analysis. Theshell was in fact considered as a pure Sn layer, which yields to describe any Sn surfaceenrichment.

300 400 500 6000

1

2

3

4

SnO2

Sn-M4,5

O-KPd-M

4,5

Inte

nsity

(a.

u.)

Energy Loss (eV)

Figure 4-15: Normalised reference EELS spectra for the Pd-M4,5 and Sn-M4,5 edges in pure Pd, pure Snand SnO2 (dotted line) particles

The unfixed parameters of the calculation were allowed to vary systematically withinrealistic ranges. For each set of geometrical and chemical configurations of the model,the atomic arrangement of the colloid was generated several times and averaged in orderto get a better statistical description of the colloids (see Appendix B).

Finally, the computer analysis leads to several hundreds of models, the reconstructedspectra of which are then compared to the experimental spectra, and sorted by order ofmerit with a least square fit procedure: the best matching model could then be found atthe minimum of the quality factor.

Figure 4-16 shows a typical HRTEM micrograph of Pd-Sn nano-colloids located at theborder of a hole on the carbon film; by chance, one of those particles exhibits a sharplattice contrast, which enables its crystallographic indexing by its diffractogram (FourierTransform of the image). The Pd3Sn2 hexagonal phase (S.G.: P63/mmc; a = 0.4390 nm,


c = 0.5655 nm) is unambiguously identified. An EELS line-scan, with a stepwidthbetween each spectrum of 0.35 nm and a probe size (FWHM) of 0.7 nm has beenperformed on this colloid (Figure 4-17 a).

The result of the evaluation of the line-scan with the method described above isillustrated by Figure 4-17 b) and Figure 4-18. In Figure 4-18 a), the quality factor isplotted versus the shell thickness, all other parameters of the model being optimised ateach point. This diagram clearly shows that the core-shell model is better than thehomogeneous model, i.e. a colloid with no shell (shell thickness = 0). In Figure 4-18 b),a similar plot is shown as a function of the Sn core concentration; the optimum is foundat 39 %, which agrees very well with the composition deduced from thecrystallographic analysis (i.e. Pd3Sn2, corresponding to [Sn] = 40 at. %).

Figure 4-16: Pd/Sn particle situated at the border of a hole on the carbon filmdiffractogram shows the [241] azimuth of Pd3Sn2

Three other colloids nicely located at the border of the carbon by the same procedure. Those particles were generally imageset of lattice fringes, but however not conveniently oriented withbeam; thus, the crystallographic indexing, such as presented been performed (it should be mentioned here that most of theclose-packed distances very close to that of the fcc Pd phcrystallographic indexing almost impossible under these condsummarised in Table 4.1. A slight Sn enrichment, correspondinhas been observed for all experiments. This corresponds roughatoms by Sn on low co-ordination sites at the surface, which arpoint of view, the first atoms that have to be replaced (see e.gof mixing of Pd and Sn is relatively large [53], it is not reasonpronounced surface segregation than the one observed.

2 nm

2-10

1-12

. The associated

film have been analysedd by a mono-dimensional respect to the electron

in Figure 4-16, could not (Pd,Sn) structures havease, which makes theiritions). The results are

g to a Sn sub-monolayer,ly to a replacement of Pd

e, from a thermodynamic[46][47]). Since the heatable to expect a more


300 350 400 450 500 550 600

300 350 400 450 500 550 600

Energy Loss (eV)

Figure 4-17: a): experimental linescan across the colloid shown in Figure 4-16, with a stepwidth of 0.35nm between each spectrum; b): reconstructed linescan from the best matching model (seeparticle 1 in Table 4.1). In both montages, the top spectrum has been acquired on thevacuum side, the bottom spectrum on the carbon film side

a)

Pd M4,5 Sn M4,5

b)


Table 4.1 : evaluation of linescans performed on different colloids

particlenumber average compositiona

Sn core concentration(at. %)

equivalent 100% Snshell thickness (nm)

1 Pd3Sn2 39 0.10

2 ≈ Pd2Sn 31 0.15

3 ≈ Pd2Sn 30 0.15

4 Pd2Sn 33 0.10

aEDX measurement

Figure 4-19 shows the complete results, in terms of the Sn/Pd atomic ratio, for the line-scan analysed in Figure 4-17 and Figure 4-18. This diagram illustrates again the factthat the core-shell model is in better agreement with the experiment than thehomogeneous model. However, experimental data deviate from the deduced core-shellmodel for the first and six last probe positions: the first and last points have beenacquired on the edges of the particle, where the signal-to-noise ratio is very weak. In thesecond half of the line-scan, i.e. the six later points, the Sn enrichment seems to bestronger than that predicted by the best core-shell model; for those positions, originalspectra show an increasing C-K edge, which indicates that the probe touches the carbonsupporting film. The conclusion is that the carbon film itself may have absorbed Snions, or that the colloid is "glued" to the carbon support by Sn complexes. A similarresult has been obtained in the case of supported Pt/Sn-catalysts, the Pt particles beinganchored to their support by Sn [36] or Sn-O [35] bridges.

44

48

52

0.0 0.1 0.2 0.3

Shell thickness (nm)

34 36 38 40 42 4440

80

120

Core Sn concentration (at. %)

Qu

ality

Fa

cto

r (1

0-6)

Figure 4-18: Quality factors when changing: a): the shell thickness, b): the Sn core concentration


-2 -1 0 1 238394041424344454647

At.

% S

n

Probe position (nm)

Figure 4-19 : Experimental Pd/Sn ratio (full square dots) compared to that of a model colloid with (opentriangles) and without (open circles) Sn surface enrichment

There are only few cases where it is possible to determine the crystal structure fromHRTEM images. Figure 4-20 shows two colloids, where the Pd2Sn structure matchesbest, but it has to be mentioned that in this case some other Pd-Sn alloys have verysimilar crystal structures and it is difficult to identify the core compositionunambiguously. A small line-scan consisting of 4 spectra is presented in Figure 4-21. Itcorresponds to particle 2 in Table 4.1. The core Sn concentration is found to be 31%,close to the Pd2Sn composition.

The difficulty to find colloids situated at the border of the carbon film can be seen inFigure 4-22. Often, the colloids are oriented so that only one set of lattice planes isvisible. Here, the (111) planes of Pd can be seen. The EELS line-scan shows that thiscolloid consists of pure Pd. The small peak at about 530 eV is the Pd M3 edge and notthe O K edge. This has been verified by comparing the ratios of the M45 and the M3edge of the Pd reference with the experimental spectra.

The composition of individual Pd-Sn colloids with a diameter of 2-5 nm has beendeduced. The HRTEM and EELS analysis shows that their core consists in acrystallised PdxSn1-x alloy, x ranging from 0.6 to 1. Furthermore, the EELS line-spectrum technique in a nano-probe mode has evidenced a slight enrichment of Sn atthe external surface of the colloids, corresponding to less than a pure Sn monolayer.Although the experimental results are near the limits of performances of the TEMtechnique, this result has been obtained on several particles, which brings a first directevidence of the core-shell structure of such Pd-Sn colloids.


Figure 4-20: a) HRTEM image of a colloid, b) its FFT showing the diffraction pattern of the colloid, andc) a filtered image obtained by applying a mask to the FFT and performing an inverse FFT.The Pd2Sn crystal structure matches best the diffraction pattern.

300 400 500 600 700

Experiment

Cou

nts

(arb

. u

nits

)

Energy Loss (eV)

4

3

2

1

300 400 500 600 700

Cou

nts

(arb

. un

its)

Energy Loss (eV)

Reconstruction

4

3

2

1

Figure 4-21: Experimental and reconstructed EELS linescan of a particle. The reconstructed spectraevidenced a Sn surface enrichment corresponding to 0.15 nm and a core Sn concentrationof 0.31, thus close to Pd2Sn

300 400 500 600 700

d

c

b

a

Energy Loss (eV)

Figure 4-22: left: HRTEM image of a colloid showing Pd 111 lattice fringes; right: EELS line-scanperformed on this particle. It proves that no Sn is present in this particle


4.3.4 XPS

An activated surface without rinsing has been studied with XPS and EDX (see Table4.2). The sample is obtained from a drop of the Futuron suspension deposited and driedon an ABS substrate in vacuum (Futuron nc).

Table 4.2: Futuron (nc): Peak positions and atomic ratios in the XPS spectra and atomic ratiosdetermined by EDX

XPS EDX

Peak Centre (eV) Pk Area [At %] Element [At %]

O 1s 532.2 5701.833 20.001 O K 10.83

Sn 3d5/2 487.4 30585.807 20.666 Sn L 29.68

Pd 3d5/2 337 5136.614 2.944 Pd L 1.7

C 1s 284.6 3743.159 33.502 C K 8.93

Cl 2p 199.2 6088.597 22.887 Cl K 48.77

The atomic ratios of this sample calculated from EDX and XPS spectra are:

Sn/Pd (XPS)= 7.02

Sn/Pd (EDX)= 17.5

Cl/Pd(XPS)=7.77

Cl/Pd(EDX)=28.7

The Sn/Pd ratio of the suspension is 62. Both, XPS and EDX results show therefore atoo low Sn content. In XPS, the Sn/Pd ratio is significantly lower than in EDX, whichshows that there are more colloids with the Pd-rich core at the surface than would beexpected from the average concentration of the suspension.

During the drying of the droplet of Futuron, some of the Sn ions may be evaporatedwhich leads then to the different atomic ratio. Notably SnCl4, which is present in theactivator solution, evaporates at air. If some of the tin chloride at the surface evaporatesin the vacuum chamber, the stable Pd rich colloids are more exposed to the surfacewhich explains the further difference between EDX and XPS.

These results show the difficulty of probing Cl in the electron microscope.

The evaluation of the oxidation state of the suspension from the XPS 3d peaks of Pdand Sn shows that Pd is metallic and about 60 % of the tin in Futuron (nc) is in anoxidised state.


This underlines the microscopic image of the suspension, with metallic Pd rich colloids,while some tin is in the metallic core, but most of it is in the suspension as Sn II or SnIV.

346 344 342 340 338 336 334 332

4500

5000

5500

6000

6500

7000

7500

Exp Pd 3d Peak Sum Background Peak 1 Peak 2

Co

unt

s

Binding Energy (eV)

500 498 496 494 492 490 488 486 484 482 480

2000

4000

6000

8000

10000

12000

14000

16000

Fut NC exp Sn Peak Sum Background Peak 1 Peak 2 Peak 3 Peak 4

Co

un

ts

Binding Energy (eV)

Figure 4-23: 3d 5/2 and 3/2 peaks of Pd (left) and Sn (right) of the Futuron (nc) sample

The parameters for the peaks used to fit the 3d spectra of Pd and Sn are listed in Table4.3:

Table 4.3 : Parameters used to fit the XPS-3d peaks of Pd and Sn

Peak Position(eV)

Area FWHM(eV)

%GL(%)

Pd:

1 336.921 6225.41 2.003 25

2 342.223 3630.286 1.935 12

Sn:

1 487.573 18947.1 2.074 0

2 496.073 13865.18 2.074 0

3 486.385 12719.36 2.454 54

4 494.885 5673.61 2.454 0

4.4 Summary Colloidal Suspension

The morphology and stability of the colloidal suspension has been studied with SAXSand Cryo-TEM. The suspension is very sensitive to the number of counter ions. TheFuturon concentrated shows agglomerates of about 100 colloids beneath the "free"colloids. Dilution with water creates new nanometric particles with a large size

4.4 Summary Colloidal Suspension 93

distribution, embedded in a gel-like structure. This has been attributed to Sn(OH)2

particles in a Sn(OH)4 gel.

The chemical composition and crystallographic structure of individual nanocolloids hasbeen analysed with HRTEM, EDX and EELS. The surface of the colloids have beenanalysed with a EELS line-scan technique and an evaluation procedure designed for thispurpose which compared the experimental line-scan with reconstructed ones on modelcolloids.

It could be shown that the core consists of a Pd-Sn crystalline phase with different Snconcentrations. At the surface, a slight Sn enrichment is observed. This correspondsroughly to a replacement of all Pd atoms at low co-ordination sites (corners, edges) bySn.

4 Characterisation of the Colloidal...

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