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Study of the Effects of Annealing on the Morphology of Platinum Cluster size on Highly Oriented Pyrolytic Graphite by Scanning Tunneling Microscopy
G u n i , L ei al. (Editors), New Frontiers in Colalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights resewed
STUDY OF THE EFFECTS OF ANNEALING ON THE MORPHOLOGY OF PLATINUM CLUSTER SIZE ON HIGHLY ORIENTED PYROLYTIC GRAPHITE BY SCANNING TUNNELING MICROSCOPY S. Lee, H. Permana and K
Y. S. Ng
Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202, USA
Abstract The effects of annealing on the morphological transformation of platinum clusters supported on graphite was investigated by scanning tunneling microscopy. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealed for 4 hours. Further annealing shows a transformation from spherical to elongated shape. Introduction The morphology of metal clusters plays an important role in determining the catalytic activity and selectivity of many structure sensitive reactions. Thus, a fundamental understanding of the particle size and the shape of supported metal clusters is essential and has been the focus of a number of studies, mainly by TEM and SEM [I-41. Recently, with the development of non-destructive scanning tunneling microscopy (STM) [ 5 ] , small clusters and surface structures can be observed in real 3-dimension space with atomic resolution. Ganz et al. [6] were among the pioneers in demonstrating STM as a very promising tool for the study of metal clusters supported on highly oriented pyrolytic graphite (HOPG). Their work was followed by a number of STM investigations on Pt cluster size, statistical analysis of bond length and bond angle of Pt clusters, effects of adsorbed Pt clusters on HOPG surface structure, and the effect of substrate functionalization on crystal size, distribution, morphology, and surface structure [7-1I]. However, the correlations among the degree of aggregation of Pt on the substrate, pretreatment conditions, and the transformation of shapes of the clusters have not been fully elucidated by STM. In this study, STM is applied to investigate the effects of annealing on the morphological transformation of vacuum vapor deposited Pt clusters on HOPG. Experimental Samples of platinum clusters supported on HOPG were prepared by a vacuum vapor deposition technique. Platinum was deposited onto newly cleaved HOPG by heating a platinum wire (Johnson Matthey, 99.99 % pure) in a tungsten boat at about 1000 OC in a tom The HOPG substrate was rotated at different angles to vacuum of 7 X control the amount of Pt deposited. The Pt-deposited samples were then annealed in a quartz tube furnace under flowing Ar at 600 OC. Four different samples were prepared and
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each sample was subjected to different annealing time of 0,4,12, or 24 hours. The samples were then imaged with a STM (Omicron) under ambient conditions without further treatment. All of the S T M images were obtained in a constant current mode, with a current of 1.5 nA and a sample bias of -15 mV. Typical scanning time was about 140 seconds for each image.
Results and Discussion Figure 1.a shows a three-dimensionalimage of an as-deposited Pt cluster adsorbed on HOPG without annealing pretreatment. The cluster appears as a bright spot in the upper left of the picture, and has a diameter of about 160 A. The cross section profile of this cluster in Figure 1.b shows that it is formed from aggregations of smaller clusters. The sizes and heights of the clusters ranges from 20 A to 50 A and 10 A to 40 A, respectively.
Figure 1.a. 3-dimensional STM image cluster of an vacuum vapor deposited Pt formed by on H O E without annealing.
Figure 1.b Cross section profile of Pt clusters shows that the large Pt cluster is aggregation of smaller clusters.
When the sample was annealed with Ar at 600 OC for 4 hours, clusters of about 150 A are observed [Figure 2.a]. The heights of the clusters are about 12 A. Interestingly, the Pt clusters are no longer an aggregation of small Pt clusters, the shape of Pt clusters has become more uniform and spherical, as shown in cross section profile [Figure 2.b] . This observation is consistent with those of Wang et al. [12] and Chojnacki and Schmidt [13] using TEM. They suggested that the catalyst morphology is influenced by the presence of gaseous species, annealing time and temperature. In their study, the particles were found to transform from cubes to spheres, and in the case of heating in H2, the spherical particles can transform back to cubes because of a significant anisotropy in surface energy produced [12].
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Distance
Figure 2.a. 3-dimensional STM image of Pt clusters (circled) on H O E annealed in Ar at 600 OC for 4 hours.
(A)
Figure 2.b. Cross section profile of the Pt cluster (left cluster on Figure 2.a).
The effects of annealing time on transformation of shapes of the Pt clusters were further illustrated when the sample was annealed for 12 and 24 hours at 600 O C . Figure 3 is a scan of a 2000 A X 1500 A area of the surface of a sample after annealing in Ar at 600 O C for 12 hours. The diameters and the heights of Pt clusters ranges from 100 A to 150 A and 10 A to 15 A, respectively. The Pt clusters are still quite spherical. However, some clusters (bottom right of the picture) now appear to have a rounding of their comers. This transformation of shape is more pronounced when the sample is annealed for a longer time (24 hours) as shown in Figure 4. In this sample the spherical shape of the Pt clusters no longer exists, and the Pt clusters have become elongated. These elongated clusters are roughly 200 A long, 75 A wide and 15 A high. The average volume of these elongated clusters is within 15%compared with spherical clusters indicates that the elongated clusters are indeed the result of transformation of spherical clusters. After 24 hours of annealing, it is reasonable to assume that the clusters have attained their equilibrium shape at 600 OC. The equilibrium shape of small clusters is not completely resolved. Based on experimental and theoretical considerations, Drechsler [ 151 concludes that the equilibrium shape of small particles should be nearly spherical, but with the formation of carbon and oxygen species on the surface, polyhedra are observed. On the other hand, Wang et al. [12] suggest that for clean surface the equilibrium shape is polyhedral; while for adsorbate-covered surface, the equilibrium shape is spherical. In this study, we observed the transformation of irregular clusters (as-deposited) to uniform sphere (4 hours), to polyhedra with rounding corners (12 hours), and to elongated polyhedra (24 hours). Though high purity argon (99.9995%) was used during the annealing, it is possible that with a long annealing time, a small amount of hydrocarbon impurities were decomposed and adsorbed on the surface. This may explain the transformation of spherical shape (relatively clean) to polyhedral shape (with adsorbates) according to Drechsler [ 151. However, it can also be argued that the purging argon gas cleans the platinum surface. If this
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Figure 3. 3-dimensional STM image of Pt clusters on HOPG annealed in Ar at 600 O C for 12 hours.
Figure 4. 2-dimensional STM image of Pt clusters on H O E annealed in Ar at 600 OC for 24 hours.
is the case, then the transformation from spherical to polyhedral shape is the result of a cleaner surface. We are i n the process of using H2 and other gases under an in-situ environment to discern these possible explanations.
References 1. 2. 3 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14.
T.P. Chojnacki, K. Krause, and L.D. Schmidt, J. Catal., 128 (1991) 1419. M. Komiyama, S . Morita, and N. Mikoshiba, J. Micros., 152 (1988) 197. H.W. Salemink, H. Meir, R. Ellialtioglu, J.W. Gemtsen, and P. Muralt, Appl. Phys. Lett., 54 (1989) 1112. S. Gao and L.D. Schmidt, J. Catal., 115 (1989) 356. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett., 49 (1982) 57. E. Ganz, K. Sattler, and J. Clarke, J. Vac. Sci. Technol., A6 (1988) 419. M. Komiyama, J. Kobayashi, and S . Morita, J. Vac. Sci. Technol., B9 (1991) 829. U. Muller, K. Sattler, J. Xhie, N. Venkaswaran, and G . Raina, J. Vac. Sci. Technol., B9 (1991) 829. J. Xhie, K. Sattler, U. Muller, N. Venkaswaran, and G. Raina, J. Vac. Sci. Technol., B9 (1991) 833. X.C. Zhou and E. Gulari, Acta. Cryst., A47 (1991) 17. K.L. Yeung and E.E. Wolf, J. Vac. Sci. Technol., B9 (1991) 798. T. Wang, C. Lee, and L.D. Schmidt, Surf. Sci., 163 (1985) 181. T.P. Chojnacki and L.D. Schmidt, J. Catal., 115 (1989) 473. M. Drechsler, Surface Mobilities on Solid Materials, Vu Thien Binh (ed.), Plenum, New York, 1983.
STUDY OF THE EFFECTS OF ANNEALING ON THE MORPHOLOGY OF PLATINUM CLUSTER SIZE ON HIGHLY ORIENTED PYROLYTIC GRAPHITE BY SCANNING TUNNELING MICROSCOPY S. Lee, H. Permana and K
Y. S. Ng
Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202, USA
Abstract The effects of annealing on the morphological transformation of platinum clusters supported on graphite was investigated by scanning tunneling microscopy. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealed for 4 hours. Further annealing shows a transformation from spherical to elongated shape. Introduction The morphology of metal clusters plays an important role in determining the catalytic activity and selectivity of many structure sensitive reactions. Thus, a fundamental understanding of the particle size and the shape of supported metal clusters is essential and has been the focus of a number of studies, mainly by TEM and SEM [I-41. Recently, with the development of non-destructive scanning tunneling microscopy (STM) [ 5 ] , small clusters and surface structures can be observed in real 3-dimension space with atomic resolution. Ganz et al. [6] were among the pioneers in demonstrating STM as a very promising tool for the study of metal clusters supported on highly oriented pyrolytic graphite (HOPG). Their work was followed by a number of STM investigations on Pt cluster size, statistical analysis of bond length and bond angle of Pt clusters, effects of adsorbed Pt clusters on HOPG surface structure, and the effect of substrate functionalization on crystal size, distribution, morphology, and surface structure [7-1I]. However, the correlations among the degree of aggregation of Pt on the substrate, pretreatment conditions, and the transformation of shapes of the clusters have not been fully elucidated by STM. In this study, STM is applied to investigate the effects of annealing on the morphological transformation of vacuum vapor deposited Pt clusters on HOPG. Experimental Samples of platinum clusters supported on HOPG were prepared by a vacuum vapor deposition technique. Platinum was deposited onto newly cleaved HOPG by heating a platinum wire (Johnson Matthey, 99.99 % pure) in a tungsten boat at about 1000 OC in a tom The HOPG substrate was rotated at different angles to vacuum of 7 X control the amount of Pt deposited. The Pt-deposited samples were then annealed in a quartz tube furnace under flowing Ar at 600 OC. Four different samples were prepared and
1864
each sample was subjected to different annealing time of 0,4,12, or 24 hours. The samples were then imaged with a STM (Omicron) under ambient conditions without further treatment. All of the S T M images were obtained in a constant current mode, with a current of 1.5 nA and a sample bias of -15 mV. Typical scanning time was about 140 seconds for each image.
Results and Discussion Figure 1.a shows a three-dimensionalimage of an as-deposited Pt cluster adsorbed on HOPG without annealing pretreatment. The cluster appears as a bright spot in the upper left of the picture, and has a diameter of about 160 A. The cross section profile of this cluster in Figure 1.b shows that it is formed from aggregations of smaller clusters. The sizes and heights of the clusters ranges from 20 A to 50 A and 10 A to 40 A, respectively.
Figure 1.a. 3-dimensional STM image cluster of an vacuum vapor deposited Pt formed by on H O E without annealing.
Figure 1.b Cross section profile of Pt clusters shows that the large Pt cluster is aggregation of smaller clusters.
When the sample was annealed with Ar at 600 OC for 4 hours, clusters of about 150 A are observed [Figure 2.a]. The heights of the clusters are about 12 A. Interestingly, the Pt clusters are no longer an aggregation of small Pt clusters, the shape of Pt clusters has become more uniform and spherical, as shown in cross section profile [Figure 2.b] . This observation is consistent with those of Wang et al. [12] and Chojnacki and Schmidt [13] using TEM. They suggested that the catalyst morphology is influenced by the presence of gaseous species, annealing time and temperature. In their study, the particles were found to transform from cubes to spheres, and in the case of heating in H2, the spherical particles can transform back to cubes because of a significant anisotropy in surface energy produced [12].
1865
Distance
Figure 2.a. 3-dimensional STM image of Pt clusters (circled) on H O E annealed in Ar at 600 OC for 4 hours.
(A)
Figure 2.b. Cross section profile of the Pt cluster (left cluster on Figure 2.a).
The effects of annealing time on transformation of shapes of the Pt clusters were further illustrated when the sample was annealed for 12 and 24 hours at 600 O C . Figure 3 is a scan of a 2000 A X 1500 A area of the surface of a sample after annealing in Ar at 600 O C for 12 hours. The diameters and the heights of Pt clusters ranges from 100 A to 150 A and 10 A to 15 A, respectively. The Pt clusters are still quite spherical. However, some clusters (bottom right of the picture) now appear to have a rounding of their comers. This transformation of shape is more pronounced when the sample is annealed for a longer time (24 hours) as shown in Figure 4. In this sample the spherical shape of the Pt clusters no longer exists, and the Pt clusters have become elongated. These elongated clusters are roughly 200 A long, 75 A wide and 15 A high. The average volume of these elongated clusters is within 15%compared with spherical clusters indicates that the elongated clusters are indeed the result of transformation of spherical clusters. After 24 hours of annealing, it is reasonable to assume that the clusters have attained their equilibrium shape at 600 OC. The equilibrium shape of small clusters is not completely resolved. Based on experimental and theoretical considerations, Drechsler [ 151 concludes that the equilibrium shape of small particles should be nearly spherical, but with the formation of carbon and oxygen species on the surface, polyhedra are observed. On the other hand, Wang et al. [12] suggest that for clean surface the equilibrium shape is polyhedral; while for adsorbate-covered surface, the equilibrium shape is spherical. In this study, we observed the transformation of irregular clusters (as-deposited) to uniform sphere (4 hours), to polyhedra with rounding corners (12 hours), and to elongated polyhedra (24 hours). Though high purity argon (99.9995%) was used during the annealing, it is possible that with a long annealing time, a small amount of hydrocarbon impurities were decomposed and adsorbed on the surface. This may explain the transformation of spherical shape (relatively clean) to polyhedral shape (with adsorbates) according to Drechsler [ 151. However, it can also be argued that the purging argon gas cleans the platinum surface. If this
1866
Figure 3. 3-dimensional STM image of Pt clusters on HOPG annealed in Ar at 600 O C for 12 hours.
Figure 4. 2-dimensional STM image of Pt clusters on H O E annealed in Ar at 600 OC for 24 hours.
is the case, then the transformation from spherical to polyhedral shape is the result of a cleaner surface. We are i n the process of using H2 and other gases under an in-situ environment to discern these possible explanations.
References 1. 2. 3 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14.
T.P. Chojnacki, K. Krause, and L.D. Schmidt, J. Catal., 128 (1991) 1419. M. Komiyama, S . Morita, and N. Mikoshiba, J. Micros., 152 (1988) 197. H.W. Salemink, H. Meir, R. Ellialtioglu, J.W. Gemtsen, and P. Muralt, Appl. Phys. Lett., 54 (1989) 1112. S. Gao and L.D. Schmidt, J. Catal., 115 (1989) 356. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett., 49 (1982) 57. E. Ganz, K. Sattler, and J. Clarke, J. Vac. Sci. Technol., A6 (1988) 419. M. Komiyama, J. Kobayashi, and S . Morita, J. Vac. Sci. Technol., B9 (1991) 829. U. Muller, K. Sattler, J. Xhie, N. Venkaswaran, and G . Raina, J. Vac. Sci. Technol., B9 (1991) 829. J. Xhie, K. Sattler, U. Muller, N. Venkaswaran, and G. Raina, J. Vac. Sci. Technol., B9 (1991) 833. X.C. Zhou and E. Gulari, Acta. Cryst., A47 (1991) 17. K.L. Yeung and E.E. Wolf, J. Vac. Sci. Technol., B9 (1991) 798. T. Wang, C. Lee, and L.D. Schmidt, Surf. Sci., 163 (1985) 181. T.P. Chojnacki and L.D. Schmidt, J. Catal., 115 (1989) 473. M. Drechsler, Surface Mobilities on Solid Materials, Vu Thien Binh (ed.), Plenum, New York, 1983.