Ultramicroscopy 12 (1983) 9-18 9 North-Holland Publishing Company

PHENOMENOLOGICAL IN-S ITU T E M GAS E X P O S U R E S T U D I E S O F P A L L A D I U M P A R T I C L E S ON M g O AT R O O M T E M P E R A T U R E

K. H E I N E M A N N , T. O S A K A * and H. POPPA

Stanford NASA Joint Institute for Surface and Microstructure Research, Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA

Received 27 May 1983

Palladium was deposited, inside a controlled-vacuum specimen chamber of a transmission electron microscope (TEM), onto clean (100) and (111) MgO surfaces at room temperature (RT) and subjected to low-pressure gas and RT-annealing treatments. Exposure to oxygen for 3 min at 2 × 10 -a mbar (1.5 x 10-a Torr) produces significant flattening of the particles (originally 1.5 nm mean size), much coalescence, and distinct crystallographic shapes of the coalesced particles. A similar yet somewhat less pronounced effect was found for exposure to nitrogen under otherwise identical conditions. Exposure to CO produced even less flattening but enhanced the particle mobility. In contrast to these findings, exposure to water vapor or hydrogen yielded no notable changes except for a slightly increased particle mobility and coalescence activity, when compared to control experiments under 1 x 10 s mbar background pressure conditions that left the deposit virtually unaffected. RT-annealing under "conventional" TEM vacuum conditions (high 10-7 mbar range) also produced much coalescence to three-dimensional particles. The results are tentatively explained in terms of the influence of the admitted gas on the palladium particle/MgO substrate interaction. However, although special care was taken to minimize the influence of the imaging electron beam in these experiments, we cannot entirely rule out that some sort of interaction between the electron beam and the chemisorbed species occurred and affected the particles.

1. Introduction

We have recently demonstra ted in high-resolu- t ion in-situ studies that very small vapor-deposited catalytically active metal particles in the 1-2 nm size range on metal oxide substrates such as MgO and a lumina can undergo significant changes when they are exposed to gases such as oxygen or air, or even when allowed to "annea l " at room tempera- ture (RT) under 10 -8 mbar (1 mbar = 0.75 Torr) vacuum condit ions [1]. These studies have under- l ined the unique advantage of well controlled in- situ TEM measurements when characterizing changes of as-deposited small particles of catalyti- cally active materials. Such informat ion should be complementary to results obta ined by other cata- lytic model studies where good spatial resolution is not possible, but where, for example, surface ele-

* On leave from Waseda University, Tokyo, Japan.

mental analysis or analysis of chemisorbed species can be obta ined [2-6].

This paper reports on cont inued in-situ gas exposures of as-deposited, 1 to 2 n m size pal- lad ium particles on MgO to air, oxygen, nitrogen, hydrogen, CO, and water vapor at RT.

2. Experimental

A detailed description of the experimental facil- ities for this research, a TEM equipped for in-situ

experimentat ion, was presented in an earlier paper [7]. The heart of the in-situ microscope' facility is a dynamical ly he l ium-cryo-pumped specimen cham- ber with a Pd-vapor source, a quartz crystal micro- balance for deposi t ion rate and deposit thickness monitor ing, and a gas jet for establishing local pressures up to 0.01 mbar at the site of the speci- men (chamber base pressure in the mid l0 -9 mbar range). Entrance and exit of the imaging electron

0304-3991 /83 /0000-0000 /$03 .00 © 1983 Nor th -Hol land

10 K. Heinemann et al. / Phenornenological in -situ T E M gas exposure of Pd on MgO

beam is through small circular apertures that also connect the specimen chamber with the rest of the microscope which operates in the high 10 .7 mbar range.

Clean MgO substrate areas were prepared im- mediately prior to the Pd deposit ion by electron-beam flash heating of chemically pre- thinned MgO disks [8,9]. Both the {111} and the {100} surfaces of MgO created in the flash-heating process were used as substrates. Palladium was deposited at RT and a rate of 3 × 10 ~3 a t o m s / c m - s to a total average thickness of 0.3 nm (if unity condensation probability is assumed). Previous re- sults [1] had indicated that particles in the desired 1-2 nm size range are obtained under these condi- tions, and that effects of mobility and coalescence as well as changes of the particle shapes are readily detected.

Also drawing from our previous experience [1], we selected the following standard experimental procedure for gas exposures: The palladium de- position was started within 5 min after preparation by electron-beam "cleavage" of a suitable speci- men (support) area. The total electron dose neces- sary for verification of the suitability of this area for a deposi t ion/exposure/anneal ing experiment was intentionally kept very low (less than 0.2 A s/cm2; 1 A s / c m 2= 6242 electrons/nm2). The imaging electron beam was off during the deposi- tion. Two minutes after the end of the deposition, the imaging beam was turned on (current density about 0.02 A/cm2), and a high-resolution micro- graph defocus series of typically 4 exposures was completed within 90 s. The electron beam was then turned off, and gas was introduced into the chamber through the jet. A pressure of 10 -4 mbar, as calculated from the jet and pumping geometries and the chamber and manifold pressures [7], was maintained for 3 min at the site of the specimen. After closing the gas inlet valve, the chamber pressure recovered within typically 5 min to the low 10 ~ mbar range. After a waiting period of about 20 h (a time previously established to practi- cally complete any changes in the particle mor- phology upon gas exposure [1]), the electron beam was again turned on, and a second micrograph defocus series was taken within 2 min. All experi- ments were performed at room temperature.

For the air exposure experiments, the controlled gas inlet was replaced by admission of (unfiltered) laboratory air into the chamber to atmospheric pressure, which was maintained for 3 min. After 20 h pumpdown, when the next defocus series was taken, the chamber pressure had reached about 3 × 10 -s mbar. This experimental sequence obvi- ously prohibited chamber bakeout.

One series of experiments was performed under simulated conventional TEM vacuum conditions (CTEM series). In this case, the valve between the He-cryopump and the specimen chamber was closed after the first micrograph series had been taken, causing the residual chamber pressure to rise (close) to equilibrium with the pressure in the TEM column. No gas was admitted through the jet in these experiments. After 20 h, the ionization gauge in the chamber registered 4 × 10 7 mbar total pressure.

3. Results

Control experiments with background-pressure annealing only, without admitting a gas, but under conditions otherwise identical to the gas exposure experiments, reproduced the modest degree of par- ticle mobility and coalescence that was reported in our earlier study [1]. A typical example is shown in fig. 1. Here, as well as in figs. 2-6, the upper and lower sections show MgO {111) and MgO{100) substrate areas, respectively. We observed no flat- tening or spreading of the particles, but there was a distinct increase of the number of particles that exhibited discrete shapes rather than essentially round profiles. The squares in fig. 1 indicate exam- ples of such shape changes, the circles denote mobility events.

In contrast to the above results for background- pressure annealing, annealing under less stringent vacuum conditions (fig. 2), such as typically pre- vail in the column of standard high-resolution microscopes (CTEM experiments), yielded quite different results. The particle mobility and coales- cence activity was extremely high, and especially in the (100) substrate areas coalescence resulted in particles with no distinct shapes. The area cover- age of the substrate with particles decreased, indi-

K. Heinemann et al. / Phenomenological in -situ TEM gas exposure of Pd on MgO 11

o * * * t l **

~0 , ~ t o l l l l l

{tool

Fig. 1. P d / M g O background-annealing control experiment. Left: immediately after deposition; right: 20 h later. Room temperature. For more details and exact exposure and e-beam conditions, see text; conditions for figs. 2 -6 are the same, except when specified otherwise, namely: top, (111); bottom (100) substrate areas. Squares indicate formation of crystallographic shapes, circles show mobili ty/coalescence events.

Fig. 2. Simulation of influence of regular TEM vacuum conditions (mid 10 7 mbar range, CTEM experiments): much mobility and coalescence (circles).

] 2 K. t l e i n e m a n n el al. / P h e n o m e n o l o g l c a l m -s~tu T E M gas e.~poxure o/ P d rm M g O

caring an increased height of the coalesced par- ticles, probably approaching hemispherical shapes.

Still very much, yet distinctly less coalescence was noted in air annealing experiments (fig. 3), when compared to the C T E M case. A moderate number of mobility events (circles in fig. 3) results from air annealing, some particles develop crystal- lographic shapes (squares in fig. 3), and substrate surface steps become slightly more decorated, as is shown by the arrows in fig. 3b. With fig. 3c we show an example of the effect of additional an- nealing at modestly increased temperature. The specimen temperature had been increased to 120°C for chamber bakeout for 4 h, while maintaining

the pressure in the high 10 ~ mbar range. It is apparent that a moderate additional amount of coalescence has taken place, leading to particles with increased height, and that some of the step decorat ion of fig. 3b has been lost.

An example of the results for oxygen exposure is shown in fig. 4. Both in {111} and {100} substrate areas, strong changes have occurred. The number of mobility and coalescence events is extremely high, but the dominant new feature is a substan- tially increased substrate coverage by the coalesced particles. In some cases, the newly coalesced par- ticles exhibit discrete crystallographic shapes, such as shown in the outlined square.

{ l l l i

{ ool

a b c

Fig. 3. Air exposure. Left and center: much coalescence, some mobility toward steps, some discrete crystallographic shapes form (squares). Right: after chamber bakeout at 120°(. ".

K. Heinemann et al. / Phenomenological in - situ T E M gas exposure o f P d on M g O 13

e

S

0 #a ~ 0 m ~ t

I l l l l

{ ool

Fig. 4. Oxygen exposure: very much mobility and coalescence; strong flattening; some discrete crystallographic shapes form (square).

Table 1 The effects on Pd particles of exposure to various gases

Condition ~bA/~b a N A / N B Formation of Flattening 3D Coalescence Mobility Major (coverage) (density) crystallographic coalescence toward step observation

( l l l ) (100) ( l l l ) (100) shapes decoration

Fig. 1, U H V - T E M 1.0 1.0 1.0 0.9

Fig. 2, 1.05 1.0 0.8 0.7 CTEM

Fig. 3, air 0.9 1.0 0.85 0.8

Fig. 4, 1.9 2.2 0.7 0.6

02

Fig. 5, N 2 1.1 1.4 0.8 0.8

Fig. 6, 1.0 1.3 0.9 0.8 CO

H 2 ° 1.0 0.95 0.95 0.9

H 20 1.05 1.0 0.9 0.8

Some No Little

Some, mainly No Yes Much in (100) areas

Some No Much

Some Very Very much much

Some Much Moderate

Some Few Moderate particles

No No Moderate

No No Moderate

Y e s

Large distance

Large distance

Large distance

Coalescence

Coalescence

Flattening; coalescence

Flattening

Some flattening

Some mobility

Some mobility

14 K. t t e inemann et al. / Phenomenolo,a;ical m-vztu T E M gas expo~'ure ~1 t 'd on M ~ O

Significant particle flattening was also observed for nitrogen exposures (fig. 5). However, in this case the particle mobility and coalescence activity was lower. Flattening was also observed in CO-ex- posure experiments (fig. 6), but this case is dis- tinctly different from the oxygen and nitrogen exposure experiments in that only a minority of particles flatten. The development of some espe- cially well shaped particles was also noted with CO exposure (see the squares). In addition, we observed here many cases of particle mobility over distances of some 5 nm, such as shown in the hexagonally outlined area. Inside the circle in fig. 6, one can observe several coalescence events.

Other than a higher degree of coalescence and

particle mobility over large distances ( > 5 nm), water vapor exposure and hydrogen exposure yielded results that were essentially the same as those for the control (vacuum-annealing) experi- ments (fig. 1). There was no flattening, and the particle shapes remained rounded.

The most significant results are summarized in table 1. The area coverages before and after an- nealing, q'l~ and ~A, were measured with a Zeiss Videomat image analyzer. The respective particle number densities, N~ and Na, were obtained by manual counting, because contras t /shading diffi- culties rendered Videomat measurements too inac- curate.

{till

{tool

Fig. 5. Nitrogen exposure: some mobility and coalescence: much flattening: some discrete crystallographic shapes form (square),

K. Heinemann et al. / Phenomenological in-situ TEM gas exposure of Pd on MgO 15

Fig. 6. CO exposure: much coalescence (circles), mobility over 5 nm distance (hexagon), some discrete crystallographic shapes form (squares), some particles flatten.

4. Discussion

The micrographs taken immediately following the completion of the depositions (left sides of figs. 1-6) show decoration of surface steps and random particle distribution in the step-free surface areas. Furthermore, a high degree of epitaxy was found [1]. It is generally assumed that this is indicative of nucleation on clean, homogeneous substrate surfaces [9,10]. The observation that UHV-annealing control experiments (fig. 1) yield only a minor influence on the as-deposited par-

ticles may lead to the conclusion that the partial pressure background levels prevailing during these 20 h control experiments do not markedly affect P d / M g O deposits in the nanometer size range. In our experiments, the background partial pressures of water, nitrogen (plus CO), and hydrogen were p ( H 2 0 ) < 1 X 10 -8, p(N2) < 4 X l 0 -9 , and p(H2) < 8 x 10 ~°mbar, respectively. All other con- stituents, including oxygen and CO, were < 5 x 10-~0 mbar each (fig. 7). Assuming unity sticking probability of residual gas molecules on a clean metal surface, the corresponding adsorption times

16 K. Heinemann et al. / Phenomenological tn- situ T E M gas exposure o f P d on M g O

t . - -

Z

:7 w

> B

18 MASS

o

Pt=2xlO -SmB

N2+CO

~ 2 CO2

_1__ I

28 44 NUMBER

Fig. 7. Typical residual gas spectrum shortly after Pd deposi- tion.

for monolayer coverage of water, nitrogen, and hydrogen are 100 s, 4 min, and 20 min, respec- tively. For the case of water vapor, this period (100 s) is of the same order of the time elapsed between the deposition and taking the first micro- graph series. It cannot, therefore, be excluded that the major effect that water vapor may have on small Pd particles had already taken place at the time of completing the first micrograph series and that further annealing with increased partial pres- sure of water has then only little additional effect on the particles. It should be noted, however, that we have also conducted experiments wherein the time elapsed between the end of the deposition and taking the first micrograph was varied be- tween less than 1 min and some 10 min. In these experiments, we never observed any consistent changes that might point toward an active role of water vapor within a time frame commensurate with its monolayer coverage period. Considering, furthermore, the fact that during the 1 rain Pd- evaporation period the water vapor pressure was increased, leading to about 1 langmuir (L) total exposure by the end of the deposition, we interpret (with due caution) our results as evidence that low-pressure water vapor exposure does not sub-

stantially affect P d / M g O deposits at room tem- perature (only a relatively minor coalescence and mobility activity is induced which is only slightly increased over background-pressure control ex- periments). Further work under improved vacuum conditions (lower water partial pressure, in partic- ular also during the depositions) is clearly needed to substantiate this conclusion. The arrival rates of the other background gas constituents, particularly oxygen and CO, are low enough that a measurable effect on the particles during a prolonged RT residual gas annealing experiment should not exist. The predominant effects noted in our study are strong particle flattening and adoption of discrete crystallographic shapes of the flattened particles during oxygen or CO exposure. This observation might be explained as a relative increase of the Pd/subs t ra te interaction which is already strong in the as-deposited condition (evidenced by the high degree of pseudomorphic epitaxy obtained for RT-deposits of P d / M g O [1], leading to ap- parently substantially increased Pd lattice parame- ters for small particles). One can argue that the interracial energy O'Pd/Mg O and the free-surface energy OMg o are much less likely to be affected by oxygen or CO adsorption than the free-surface energy of Pd (Opa). The observed flattening of the palladium particles would then be due mainly to a decrease of opd by adsorption during 02 and CO exposure.

We observed severe particle flattening and the formation of crystallographicaily well shaped par- ticles during coalescence already in previous work. For example, very pronounced spreading over the substrate surface was observed upon thermal an- nealing of Fe203 particles (10 nm size range) on sapphire [11]. In that case, the flattening actually proceeded to the degree that the originally poorly oriented island deposit formed flat islands of several 100 nm diameter and perfect epitaxial alignment. The significance of a relatively high overgrowth/substrate interfacial energy was un- derlined in these experiments by the fact that the annealed deposit had adopted the c~-phase of Fe20 > which was structurally more similar to the sapphire substrate than the original Fe203 y-phase. The formation of well shaped particles was also observed earlier in other epitaxial systems, such as

K. Heinemann et al. / Phenomenological in-situ TEM gas exposure of Pd on MgO 17

Fe/sapphire during growth coalescence [12] and for Au/sapphire during thermally induced epi- taxial realignment and coalescence of originally random island deposits [13].

It is tempting to rationalize the CTEM anneal- ing results (fig. 2) on the basis of the increased arrival rate of hydrocarbon molecules. It is con- ceivable that the contaminants having incessantly arrived at the surface (at a coverage rage of almost 1 monolayer per second) increased the ratio of opJOMg o and induced higher P d / M g O surface mobility, leading to the observed increased coales- cence activity and the formation of pronounced three-dimensional particles. Careful examination of fig. 2 actually reveals contrast variations within the coalesced particles. Such variations can be explained as being due to diffraction contrast, characteristic of multiply structured rather than single crystalline particles [ 14,15]. This would again be indicative of coalescence due to high surface mobility while the substrate/deposit interaction is low [16]. It should be mentioned, however, that there is some controversy about the validity of such conclusions [17].

Probably the most unexpected and most dif- ficult to explain results were obtained for nitrogen exposure (fig. 5). Flattening and formation of crystallographic shapes were almost as pro- nounced in this case as for the oxygen exposure experiments. However, the heat of adsorption of nitrogen on Pd is very low, and more studies of this adsorption system are clearly needed to ex- plain this unexpected strong influence of nitrogen exposure. It is, for example, tempting to link this strong effect of nitrogen exposure, and possibly the minor influence of water vapor and hydrogen exposure, at least in part, to the electron beam, which, although kept at as low intensity as rea- sonably possible, reached several hundred elec- t rons /nm 2. s during the high-magnification TEM observations typically lasting for several tens of seconds. If adsorption of oxygen leads to a de- crease of opd and to particle spreading/flattening, it might, for example, be conceivable that the electron beam dissociates a n d / o r desorbs the (primary) adsorbate (N 2, H 2, H20 ) and creates sites for (secondary) adsorption of oxygen. This oxygen could be co-adsorbed with the N 2, H z, or

H 2 0 on the palladium from the O2-background pressure during the 20 h waiting period. For the nitrogen experiments, this would essentially result in a beam-induced nitrogen/oxygen place ex- change and nitrogen desorption. For hydrogen and water vapor, the adsorbed oxygen might react chemically with the beam-dissociated species; it would then not be available for adsorption on the Pd surfaces, and Pd would appear unaffected in the TEM image. More and better defined studies will have to follow to test these speculations. These will include systematic oxygen and nitrogen ex- posure experiments under further improved vacuum conditions and varying annealing periods.

One should note that the conclusion of "flatten- ing" of Pd particles is actually based on indirect TEM image interpretations, such as an observed increase of substrate surface coverage (table 1) and a rationalized concomitant decrease of the particle thickness, rather than on direct TEM contrast considerations. Recent studies of the morphology of small metal particles by high-resolution, uncon- ventional TEM methods have demonstrated that it would be an undue oversimplification to interpret images of small metal crystallites solely on the basis of their apparent image contrast [18-23]. Dynamic diffraction effects will affect the image contrast of particles as small as 2 nm to the extent that careful consideration of the orientation of the particles with respect to the electron beam is es- sential for conclusive contrast/thickness interpre- tations. These orientation relations are increas- ingly difficult to establish if the particle size de- creases. However, a general transition to the sim- plified contrast/thickness interpretation does exist for particles in the 1 nm thickness range, and we are reasonably confident that the decrease of par- ticle/substrate contrast observed concomitant with the increase of particle size upon oxygen, nitrogen, and CO exposure (figs. 4, 5, and 6) is indicative of a real decrease in particle thickness.

5. Conclusions

It has been shown that low-pressure exposure to various gases at room temperature can signifi-

18 K. Heinemann et al. / Phenomenological in -situ T E M gas exposure o f P d on M ~ O

cantly affect small palladium particles supported on MgO surfaces. Exposure to oxygen for 3 rain at 2 × 10 4 m bar produces a considerable amount of coalescence, flattening of the particles, and some distinct crystallographic particle shapes. A similar yet somewhat less pronounced effect is obtained upon nitrogen exposure, and CO exposure pro- duces only a few flattened particles and some coalescence and shape formation. Exposure to water vapor (in addition to the relatively high background water content) and hydrogen did not produce notable changes, except for some mobility and coalescence. Background-pressure annealing control experiments yield no significant changes, thus giving credence to the overall experimental conditions of the studies. An attempt was made to rationalize some of the results on the basis of gas exposure-induced changes of the particle surface free energy, and a correlation was established with results of earlier in-situ experiments on oxide sub- strates, where flattening and the formation of well shaped particles was also observed. We susPect that the imaging electron beam might have a role in our N 2, H2, and H 2 0 exposure results, and further studies will be needed to substantiate our speculations on this important aspect.

Acknowledgements

This work was performed at the N A S A Ames Research Center and was supported by a grant (NCC2-171) to Eloret Institute in Sunnyvale, California. One of us (T.O.) was supported by Waseda University, Tokyo, Japan, while on leave of absence during the course of this research. We also acknowledge financial support by Alcoa Foundation for this work.

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