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Characterization of Pt microcrystals using high resolution electron microscopy
Ultramicroscopy 20 (1986) 15-20 North-Holland, Amsterdam
15
CHARACTERIZATION OF Pt MICROCRYSTALS USING HIGH RESOLUTION ELECTRON MICROSCOPY N.J. LONG Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, USA
R.F. MARZKE Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
M. McKELVY Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, USA
and
W.S. GLAUNSINGER Department of Chemistry, Arizona State University, Tempe, Arizona 85287, USA Received 14 January 1986, in revised form 25 March 1986; received at editorial office 13 June 1986
Small colloidal particles of platinum have been produced with and without a protective agent (polyvinylpyrrofidone), and have been studied by high resolution electron microscopy. Initially, the protected particles have been found to be single crystals in the size range 5-30 A. After continued beam exposure, the protective coating's effect is lost and the particles coalesce, forming both larger single crystals and multiply-twinned crystals. In contrast, a sample of the unprotected particles shows extensive initial linking and twinning, but no further beam-induced agglomeration.
1. Introduction
Our study of small Pt particles by high resolution electron microscopy (HREM) is motivated by interest in both their chemical and magnetic properties. The former stem primarily from the chemically active surface of the platinum metal itself, which is capable of chemisorbing nearly all molecular species at moderate temperatures, and in so doing dissociating many into highly reactive surface intermediates. Fundamental reactions catalyzed by platinum in this way include oxidation, hydrogenation, dehydrogenation, isomerization and hydrogenolysis [1]. The magnetic properties of small Pt particles, on the other hand, may be related either to their surfaces [2,3] or to quan-
tum size effects in their electronic structure [4-6] which are affected by atoms within the particles as well as by those on their surfaces. To assist in furthering our understanding of these properties use was made of HREM, which should be capable of direct imaging of small regions of the surfaces of individual particles. The greatly increased resolution required for this is constantly being pursued at many laboratories, but despite recent advances, HREM cannot yet yield direct information about either the nature of the chemically active sites on the surfaces of small particles or about the electronic states which contribute to their magnetism. What can be learned about supported metallic particles using HREM is nonetheless considerable and important, and it includes their size distribu-
0304-3991/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
16
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
tions, as well as information about their growth and behavior under strong electron-beam irradiation, as we show in this article.
2. Pt microcrystal samples and their characterization by magnetic methods An extensive development program of methods for the preparation of colloidal platinum, using chemical reduction of chloroplatinic acid as the basic approach [7-9], has led to the production of new types of Pt microcrystal samples, two of which were used for this work. These preparations yield samples with a wide range of average particle diameters (22 to 196 ,A), and with very high metal loading (weight percentages of Pt in the final product 28 to 50%). Both of these features are important for the characterization of the particles by magnetic techniques, while primarily the first, particle size, affects catalytic activity in structuresensitive, or "demanding" reactions [10]. The second property, high metal loading, is frequently avoided in the conventional supported-metal catalysts used in industry because of the danger of sintering the catalytic particles at high reaction temperatures, with a consequent loss of active surface area of the expensive transition metal. Thus our samples do not resemble commercial catalysts in this respect, but they have the advantage of being much easier to characterize using electron microscopy. They are of two kinds: those with, and those lacking, an organic protective or supporting material for the Pt colloid. The function of this type of support, which is usually an organic material such as gelatin or PVP (polyvinylpyrrohdone), is primarily to keep the particles well separated from one another and secondly to help maintain uniformity in particle size. On an amorphous carbon grid the organic supports allow EM imaging of the metallic particles embedded in them with a resolution of better than 2 A, considerably better than is achievable with conventional silica or alumina supports. Accordingly, Pt particles were prepared from an aqueous solution of chloroplatinic acid and protective agent PVP, by the reduction of the former through the gradual addition of sodium
borohydride. Log normal size distributions [11] were observed for the particles thus formed, with small average diameters ( - 2 8 ~,) and standard deviation equivalents of approximately 11 ,~. Without the use of PVP, the same reaction yields particles which have an average apparent size of nearly 60 ,A, and are typically joined together in long chains. The form of their size distributions has not been determined, but their standard deviations appear to be quite large. Samples of platinum particles prepared by these methods have been characterized using magnetic techniques (see ref. [3]). The most prominent magnetic property of the samples observed to date is their large electronic paramagnetism, which typically appears at temperatures below 100 K and is seen in all colloidal platinum formed by chemical reduction in water. The resulting magnetic susceptibilities follow a Curie-Weiss law at low temperatures, with small, negative Curie temperatures, indicating the presence of weak antiferromagnetic couplings between individual magnetic moments assumed to be responsible for the paramagnetism. The nature of the moments is not known. Transition-metal impurity levels in both the starting reagents and the final products of the preparation reactions are nearly two orders of magnitude too low to account for them. If they arise from individual particles via quantum size effects, there must be many more particles of extremely small sizes (< 10 ,A) in our samples than we are presently able to detect by EM. On the other hand, the measured Curie constants for samples with widely differing dispersions, i.e. ratios of the number of Pt atoms on the surfaces of the particles to the total number of Pt atoms in the sample, show that an approximately linear relationship exists between the constants and the dispersion values, the latter being either measured from gas adsorption or estimated from EM. This may indicate that the magnetism has origins in the surface itself (see ref. [3]). Chemisorption of hydrogen and oxygen gases on the surfaces of unprotected samples, however, turns out to affect the magnetism only slightly, so that any surface moments in such samples cannot simply be due to free radicals chemisorbed on their outer surface layers. The many questions that have arisen dur-
17
N.J. Long et aL / Characterization of Pt microcrystals using H R E M
ing our inquiry into the origins of the moments thus provide strong incentives for characterizing our small-particle samples to the highest degree possible by methods additional to magnetic measurements, in particular by electron microscopy.
3. Characterization of Pt partide samples by
HREM In the present work both types of colloids were examined in a JEM 4000EX, using the instrument's high resolution performance to image the lattice structure of the small crystals and to observe dynamic changes using a TV imaging system combined with a video recorder. This gave information about the structure of the particles, and also, in the case of the protected colloids, about the processes by which they agglomerated as the protective coating was decomposed by the electron beam.
3.1. Unprotected Pt microcrystals Fig. 1 is a low magnification image of a typical chain-like arrangement, which is composed of small single and twinned crystals. Clearly the widespread linking of the Pt grains will reduce the total surface area of the platinum, which explains the lower dispersion values obtained by gas adsorption for these unprotected samples (-0.16), as compared to those expected ( - 0.3) for isolated grains with average size 35 ]k, discernible at higher magnifications (see for example fig. 2). Fig. 2a
,,,
i~ .
Fig. 2. HREM'image of unprotected Pt particles, showing (a) twin relationships and (b) incoherent boundary.
Fig. 1. Electron micrograph of unprotected Pt particles.
also shows the frequently encountered twin or near-twin relationship of microcrystals comprising the grains. For example, crystals 1 and 2 are clearly twin related, as are 3 and 4, but 2 and 3 do not have a common (111) plane and thus form a grain boundary. This boundary has some correspondence to the (111) planes, but the details of this will be left to a future study.
18
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
At closer inspection it is also possible to find in the unprotected platinum samples larger single crystal regions ( - 100 A), where imperfect matching occurs at grain boundaries, creating interface defects and thus a misfit strain energy. Also observed in fig. 2 are slightly bent lattice planes (bent through 2-3°), where again some accommodation of misfit has been made, but where there is insufficient structural information available from these micrographs alone to show in detail how the grains are actually related.
i:
3.2. Protected Pt microcrystals The spatial distribution of the protected Pt particles on holey carbon films can be one of two types. Fig. 3 illustrates both, showing individual particles well isolated from one another, and also large numbers of particles embedded in long filaments of organic matter. These filaments are not observed in all micrographs. They are assumed to result from the freeze-drying procedure used to deposit the colloids on the carbon films. Whereas the unprotected particles were relatively stable under the electron beam current densities used here ( - 20 A / c m 2), the same cannot be said of the protected particles. On exposure to the electron beam, following an initial period of several seconds the protected particles began to move rapidly and to coalesce. By video recording the TV image it was possible to observe the early stages of particle motion and rearrangement. The initial period of irradiation is apparently required for the beam to strip the protective agent from the par-
Fig. 4. Images from single frames of a video tape ~ u e n c e , illustrating coalescence of PVP-proteeted Pt particles.
Fig. 3. Low dose image of PVP-protected particles. Note that they are small and separate (cf. fig. 1).
ticles, leaving highly active, bare platinum surfaces. The latter, however, apparently do not form Pt-C bonds (with the carbon from the film) which are stable in the beam, because the particles remain active and able to move considerable distances over the substrate.
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
Fig. 5. Images recorded from single frames of a video recording of PVP-protected Pt particles, showing alignment of a small crystal with respect to a larger one, even though coalescence does not occur.
Fig. 4 is a s~uence taken from a video recording, of a 28 A particle moving up to a group of particles which have already coalesced into a string. The arriving crystal matches onto the others
19
in a twin relationship. The important observation is that it is already correctlyaligned in fig. 4a, at the beginning of the sequence; the crystal in some manner pre-aligns itself with respect to the crystal orientations of the group it approaches, before making close contact. This apparent alignment interaction at a distance was not an isolated observation, but was seen in many instances, including those in which several crystals merged to form one large single crystal domain. Fig. 5 is another example of this process, but with a different outcome: in fig. 5a a small ( - 1 5 A) crystal has moved close to a much larger particle and has become aligned with it. A short while later, instead of merging with the large particle, the crystal has moved off in the direction of a third particle. Whilst there are many possible explanations for the observed non-coalescence in this case, the main point remains the pre-alignment of one crystal with respect to another, which is visible well before any attempts at coalescence. The protected Pt particles differ from the unprotected ones in that they are probably all single crystals ranging in size from 5 to 30 A, and as such will have a high surface area and a low number of twin interfaces. (Because of the protective agent's effectiveness in coating the surfaces of the particles, however, measurements of the exposed surface area of particles in the protected samples by gas adsorption techniques yield very low values.) Another difference lies in the observation that the unprotected particles are nearly all joined together, frequently forming imperfect twins or misfit grain boundaries. The resulting strains at these interfaces might be thought likely io affect the chemical properties of the small crystals, since the strains are probably associated with catalytically active sites in much the same way as kinks or steps in single crystal surfaces are believed to be -[12]. Studies of the hydrogenation reaction of cyclohexene over other Pt catalysts show, however, that turnover frequencies are comparable for both supported and unsupported platinum [13]. Thus for this reaction, at least, there is no evidence that sites associated with particle interfaces behave differently from those associated with isolated particle surfaces. Crystals situated on the edge of the carbon film
20
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
were sometimes observed to grow by the incorporation of very small clusters of atoms which could be "seen" moving along the edge of the carbon. This cannot be readily illustrated in a figure but can be deduced from the original video recording.
4. Summary From this study we have obtained images with considerably better resolution than previously available of platinum particles prepared according to chemical reduction methods. These confirm and extend our original findings that the unprotected Pt coloids form long, chain-like strings of polycrystalline particles with an average size of about 35 A, and that the use of a protective agent keeps the particles in their colloidal state with sizes ranging between 5 and 30 ~,. New information was obtained as follows: (1) The grains of the unprotected sample are often in a twin or near-twin orientation relationship. Many of the twins are imperfect, and there is thus expected to be a significant amount of strain energy associated with these linked particle arrangements. (2) All evidence indicates that the grains of the protected sample are single crystals, free of twin interfaces and grain boundary misfit strains. (3) U p o n irradiation by the 400 kV electron beam, rapid motion and coalescence of particles occurs after two initial steps: (a) the apparent removal of the protective coating by the action of the beam and (b) prealignment of the crystalline axes of the particles, either in a direct or twin relationship.
Acknowledgements This work was performed at the ASU National Facility for High Resolution Electron Microscopy supported by N S F Grant number DMRo83 06501.
References [1] See, for example, G.A. Somorjai, Chemistry in Two Dimensions: Surfaces(Cornel University Press, Ithaca, NY, 1981) chs. 8 and 9. [2] R.F. Marzke, W.S. Glaunsinger, K.B. Rawlings, P. Van Rheenen, M. McKelvy,J.H. Brewer, D.H. Harshman and R.F. Kiefl, in: Electronic Structure and Properties of Hydrogen in Metals, Eds. P. Jena and C.D. Satterthwaite (Plenum, New York, 1983) p, 647. [3] R.F. Marzke and W.S. Glaunsinger, in: Proc. Conf. on Characterization and Behavior of Materials with Submicron Dimensions, Ed. J.T. Waber (World Scientific, Singapore, 1985). [4] R.F. Marzke, W.S. Glaunsinger and M. Bayard, Solid State Commun. 18 (1976) 1025. [5] See review articles, for example: R. Kubo, A. Kawabata and S. Kobayashi, Ann. Rev. Mater. SOl. 14 (1984) 49; R, Marzke, Catalysis Rev. - Sci. Eng. 19 (1979) 43. [6] A more extensive and recent reviewis W.P. Halperin, Rev. Mod. Phys., in press. [7] P, Van Rheenen, PhD Thesis, Arizona State University (1981). [8] P. Van Khecnen, M. McKetvy, R. Maxzke and W.S. Glaunsinger, Inorganic Synthesis 24 (1986) 238. [9] P. Van Rhcenen, M. McKelvy and W.S: Glannsinger, J. Solid State Chem. Commun., in press; P. Van Rheenen, M. McKelvy and W.S. Glaunsinger, Sold State Commun. 57 (1986) 865. [10] See, for example, M. Boudart, J. Vacuum Sci. Technol. 12 (1975) 329, and references contained therein. [11] C.G. Granqvist and R.A. Buhrman, Solid State Commun. 18 (1976) 123; C.G. Crranqvist and R.A. Buhrman, J. Appl. Phys. 47 (1976) 2200. [12] See ref. [1], pp. 483-499. [13] DJ. O,Rear, Phl) Thesis, Stanford University (1980); see also E. Segal, R.J. Marion and M. Boudart, J. Catalysis 52 (1978) 45.
15
CHARACTERIZATION OF Pt MICROCRYSTALS USING HIGH RESOLUTION ELECTRON MICROSCOPY N.J. LONG Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, USA
R.F. MARZKE Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
M. McKELVY Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, USA
and
W.S. GLAUNSINGER Department of Chemistry, Arizona State University, Tempe, Arizona 85287, USA Received 14 January 1986, in revised form 25 March 1986; received at editorial office 13 June 1986
Small colloidal particles of platinum have been produced with and without a protective agent (polyvinylpyrrofidone), and have been studied by high resolution electron microscopy. Initially, the protected particles have been found to be single crystals in the size range 5-30 A. After continued beam exposure, the protective coating's effect is lost and the particles coalesce, forming both larger single crystals and multiply-twinned crystals. In contrast, a sample of the unprotected particles shows extensive initial linking and twinning, but no further beam-induced agglomeration.
1. Introduction
Our study of small Pt particles by high resolution electron microscopy (HREM) is motivated by interest in both their chemical and magnetic properties. The former stem primarily from the chemically active surface of the platinum metal itself, which is capable of chemisorbing nearly all molecular species at moderate temperatures, and in so doing dissociating many into highly reactive surface intermediates. Fundamental reactions catalyzed by platinum in this way include oxidation, hydrogenation, dehydrogenation, isomerization and hydrogenolysis [1]. The magnetic properties of small Pt particles, on the other hand, may be related either to their surfaces [2,3] or to quan-
tum size effects in their electronic structure [4-6] which are affected by atoms within the particles as well as by those on their surfaces. To assist in furthering our understanding of these properties use was made of HREM, which should be capable of direct imaging of small regions of the surfaces of individual particles. The greatly increased resolution required for this is constantly being pursued at many laboratories, but despite recent advances, HREM cannot yet yield direct information about either the nature of the chemically active sites on the surfaces of small particles or about the electronic states which contribute to their magnetism. What can be learned about supported metallic particles using HREM is nonetheless considerable and important, and it includes their size distribu-
0304-3991/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
16
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
tions, as well as information about their growth and behavior under strong electron-beam irradiation, as we show in this article.
2. Pt microcrystal samples and their characterization by magnetic methods An extensive development program of methods for the preparation of colloidal platinum, using chemical reduction of chloroplatinic acid as the basic approach [7-9], has led to the production of new types of Pt microcrystal samples, two of which were used for this work. These preparations yield samples with a wide range of average particle diameters (22 to 196 ,A), and with very high metal loading (weight percentages of Pt in the final product 28 to 50%). Both of these features are important for the characterization of the particles by magnetic techniques, while primarily the first, particle size, affects catalytic activity in structuresensitive, or "demanding" reactions [10]. The second property, high metal loading, is frequently avoided in the conventional supported-metal catalysts used in industry because of the danger of sintering the catalytic particles at high reaction temperatures, with a consequent loss of active surface area of the expensive transition metal. Thus our samples do not resemble commercial catalysts in this respect, but they have the advantage of being much easier to characterize using electron microscopy. They are of two kinds: those with, and those lacking, an organic protective or supporting material for the Pt colloid. The function of this type of support, which is usually an organic material such as gelatin or PVP (polyvinylpyrrohdone), is primarily to keep the particles well separated from one another and secondly to help maintain uniformity in particle size. On an amorphous carbon grid the organic supports allow EM imaging of the metallic particles embedded in them with a resolution of better than 2 A, considerably better than is achievable with conventional silica or alumina supports. Accordingly, Pt particles were prepared from an aqueous solution of chloroplatinic acid and protective agent PVP, by the reduction of the former through the gradual addition of sodium
borohydride. Log normal size distributions [11] were observed for the particles thus formed, with small average diameters ( - 2 8 ~,) and standard deviation equivalents of approximately 11 ,~. Without the use of PVP, the same reaction yields particles which have an average apparent size of nearly 60 ,A, and are typically joined together in long chains. The form of their size distributions has not been determined, but their standard deviations appear to be quite large. Samples of platinum particles prepared by these methods have been characterized using magnetic techniques (see ref. [3]). The most prominent magnetic property of the samples observed to date is their large electronic paramagnetism, which typically appears at temperatures below 100 K and is seen in all colloidal platinum formed by chemical reduction in water. The resulting magnetic susceptibilities follow a Curie-Weiss law at low temperatures, with small, negative Curie temperatures, indicating the presence of weak antiferromagnetic couplings between individual magnetic moments assumed to be responsible for the paramagnetism. The nature of the moments is not known. Transition-metal impurity levels in both the starting reagents and the final products of the preparation reactions are nearly two orders of magnitude too low to account for them. If they arise from individual particles via quantum size effects, there must be many more particles of extremely small sizes (< 10 ,A) in our samples than we are presently able to detect by EM. On the other hand, the measured Curie constants for samples with widely differing dispersions, i.e. ratios of the number of Pt atoms on the surfaces of the particles to the total number of Pt atoms in the sample, show that an approximately linear relationship exists between the constants and the dispersion values, the latter being either measured from gas adsorption or estimated from EM. This may indicate that the magnetism has origins in the surface itself (see ref. [3]). Chemisorption of hydrogen and oxygen gases on the surfaces of unprotected samples, however, turns out to affect the magnetism only slightly, so that any surface moments in such samples cannot simply be due to free radicals chemisorbed on their outer surface layers. The many questions that have arisen dur-
17
N.J. Long et aL / Characterization of Pt microcrystals using H R E M
ing our inquiry into the origins of the moments thus provide strong incentives for characterizing our small-particle samples to the highest degree possible by methods additional to magnetic measurements, in particular by electron microscopy.
3. Characterization of Pt partide samples by
HREM In the present work both types of colloids were examined in a JEM 4000EX, using the instrument's high resolution performance to image the lattice structure of the small crystals and to observe dynamic changes using a TV imaging system combined with a video recorder. This gave information about the structure of the particles, and also, in the case of the protected colloids, about the processes by which they agglomerated as the protective coating was decomposed by the electron beam.
3.1. Unprotected Pt microcrystals Fig. 1 is a low magnification image of a typical chain-like arrangement, which is composed of small single and twinned crystals. Clearly the widespread linking of the Pt grains will reduce the total surface area of the platinum, which explains the lower dispersion values obtained by gas adsorption for these unprotected samples (-0.16), as compared to those expected ( - 0.3) for isolated grains with average size 35 ]k, discernible at higher magnifications (see for example fig. 2). Fig. 2a
,,,
i~ .
Fig. 2. HREM'image of unprotected Pt particles, showing (a) twin relationships and (b) incoherent boundary.
Fig. 1. Electron micrograph of unprotected Pt particles.
also shows the frequently encountered twin or near-twin relationship of microcrystals comprising the grains. For example, crystals 1 and 2 are clearly twin related, as are 3 and 4, but 2 and 3 do not have a common (111) plane and thus form a grain boundary. This boundary has some correspondence to the (111) planes, but the details of this will be left to a future study.
18
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
At closer inspection it is also possible to find in the unprotected platinum samples larger single crystal regions ( - 100 A), where imperfect matching occurs at grain boundaries, creating interface defects and thus a misfit strain energy. Also observed in fig. 2 are slightly bent lattice planes (bent through 2-3°), where again some accommodation of misfit has been made, but where there is insufficient structural information available from these micrographs alone to show in detail how the grains are actually related.
i:
3.2. Protected Pt microcrystals The spatial distribution of the protected Pt particles on holey carbon films can be one of two types. Fig. 3 illustrates both, showing individual particles well isolated from one another, and also large numbers of particles embedded in long filaments of organic matter. These filaments are not observed in all micrographs. They are assumed to result from the freeze-drying procedure used to deposit the colloids on the carbon films. Whereas the unprotected particles were relatively stable under the electron beam current densities used here ( - 20 A / c m 2), the same cannot be said of the protected particles. On exposure to the electron beam, following an initial period of several seconds the protected particles began to move rapidly and to coalesce. By video recording the TV image it was possible to observe the early stages of particle motion and rearrangement. The initial period of irradiation is apparently required for the beam to strip the protective agent from the par-
Fig. 4. Images from single frames of a video tape ~ u e n c e , illustrating coalescence of PVP-proteeted Pt particles.
Fig. 3. Low dose image of PVP-protected particles. Note that they are small and separate (cf. fig. 1).
ticles, leaving highly active, bare platinum surfaces. The latter, however, apparently do not form Pt-C bonds (with the carbon from the film) which are stable in the beam, because the particles remain active and able to move considerable distances over the substrate.
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
Fig. 5. Images recorded from single frames of a video recording of PVP-protected Pt particles, showing alignment of a small crystal with respect to a larger one, even though coalescence does not occur.
Fig. 4 is a s~uence taken from a video recording, of a 28 A particle moving up to a group of particles which have already coalesced into a string. The arriving crystal matches onto the others
19
in a twin relationship. The important observation is that it is already correctlyaligned in fig. 4a, at the beginning of the sequence; the crystal in some manner pre-aligns itself with respect to the crystal orientations of the group it approaches, before making close contact. This apparent alignment interaction at a distance was not an isolated observation, but was seen in many instances, including those in which several crystals merged to form one large single crystal domain. Fig. 5 is another example of this process, but with a different outcome: in fig. 5a a small ( - 1 5 A) crystal has moved close to a much larger particle and has become aligned with it. A short while later, instead of merging with the large particle, the crystal has moved off in the direction of a third particle. Whilst there are many possible explanations for the observed non-coalescence in this case, the main point remains the pre-alignment of one crystal with respect to another, which is visible well before any attempts at coalescence. The protected Pt particles differ from the unprotected ones in that they are probably all single crystals ranging in size from 5 to 30 A, and as such will have a high surface area and a low number of twin interfaces. (Because of the protective agent's effectiveness in coating the surfaces of the particles, however, measurements of the exposed surface area of particles in the protected samples by gas adsorption techniques yield very low values.) Another difference lies in the observation that the unprotected particles are nearly all joined together, frequently forming imperfect twins or misfit grain boundaries. The resulting strains at these interfaces might be thought likely io affect the chemical properties of the small crystals, since the strains are probably associated with catalytically active sites in much the same way as kinks or steps in single crystal surfaces are believed to be -[12]. Studies of the hydrogenation reaction of cyclohexene over other Pt catalysts show, however, that turnover frequencies are comparable for both supported and unsupported platinum [13]. Thus for this reaction, at least, there is no evidence that sites associated with particle interfaces behave differently from those associated with isolated particle surfaces. Crystals situated on the edge of the carbon film
20
N.J. Long et al. / Characterization of Pt microcrystals using H R E M
were sometimes observed to grow by the incorporation of very small clusters of atoms which could be "seen" moving along the edge of the carbon. This cannot be readily illustrated in a figure but can be deduced from the original video recording.
4. Summary From this study we have obtained images with considerably better resolution than previously available of platinum particles prepared according to chemical reduction methods. These confirm and extend our original findings that the unprotected Pt coloids form long, chain-like strings of polycrystalline particles with an average size of about 35 A, and that the use of a protective agent keeps the particles in their colloidal state with sizes ranging between 5 and 30 ~,. New information was obtained as follows: (1) The grains of the unprotected sample are often in a twin or near-twin orientation relationship. Many of the twins are imperfect, and there is thus expected to be a significant amount of strain energy associated with these linked particle arrangements. (2) All evidence indicates that the grains of the protected sample are single crystals, free of twin interfaces and grain boundary misfit strains. (3) U p o n irradiation by the 400 kV electron beam, rapid motion and coalescence of particles occurs after two initial steps: (a) the apparent removal of the protective coating by the action of the beam and (b) prealignment of the crystalline axes of the particles, either in a direct or twin relationship.
Acknowledgements This work was performed at the ASU National Facility for High Resolution Electron Microscopy supported by N S F Grant number DMRo83 06501.
References [1] See, for example, G.A. Somorjai, Chemistry in Two Dimensions: Surfaces(Cornel University Press, Ithaca, NY, 1981) chs. 8 and 9. [2] R.F. Marzke, W.S. Glaunsinger, K.B. Rawlings, P. Van Rheenen, M. McKelvy,J.H. Brewer, D.H. Harshman and R.F. Kiefl, in: Electronic Structure and Properties of Hydrogen in Metals, Eds. P. Jena and C.D. Satterthwaite (Plenum, New York, 1983) p, 647. [3] R.F. Marzke and W.S. Glaunsinger, in: Proc. Conf. on Characterization and Behavior of Materials with Submicron Dimensions, Ed. J.T. Waber (World Scientific, Singapore, 1985). [4] R.F. Marzke, W.S. Glaunsinger and M. Bayard, Solid State Commun. 18 (1976) 1025. [5] See review articles, for example: R. Kubo, A. Kawabata and S. Kobayashi, Ann. Rev. Mater. SOl. 14 (1984) 49; R, Marzke, Catalysis Rev. - Sci. Eng. 19 (1979) 43. [6] A more extensive and recent reviewis W.P. Halperin, Rev. Mod. Phys., in press. [7] P, Van Rheenen, PhD Thesis, Arizona State University (1981). [8] P. Van Khecnen, M. McKetvy, R. Maxzke and W.S. Glaunsinger, Inorganic Synthesis 24 (1986) 238. [9] P. Van Rhcenen, M. McKelvy and W.S: Glannsinger, J. Solid State Chem. Commun., in press; P. Van Rheenen, M. McKelvy and W.S. Glaunsinger, Sold State Commun. 57 (1986) 865. [10] See, for example, M. Boudart, J. Vacuum Sci. Technol. 12 (1975) 329, and references contained therein. [11] C.G. Granqvist and R.A. Buhrman, Solid State Commun. 18 (1976) 123; C.G. Crranqvist and R.A. Buhrman, J. Appl. Phys. 47 (1976) 2200. [12] See ref. [1], pp. 483-499. [13] DJ. O,Rear, Phl) Thesis, Stanford University (1980); see also E. Segal, R.J. Marion and M. Boudart, J. Catalysis 52 (1978) 45.