Langmuir 1991, 7, 1546-1549
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Structural Study of Gold Single Crystals Produced by Colloidal Methods R. Hernrindez, G. D h ,A. VBzquez, J. Reyes-Gasga, and M. Jos&Yacam&n' Znstituto de Ftsica, Universidad Nacional Autbnoma de Mkxico, Apartado Postal 20-364, Delegacibn Alvaro Obregbn, 01000 Mkxico D. F., M6xico Received June 26,1990.I n Final Form: January 15,1991 We report the growth of monometallic crystals from reduction of metal sols. In particular the Au system is reported in this work. A number of techniques are applied to the characterization of the crystallites. It is shown that particles from a few nanometers to crystals of 100pm can be prepared with different shapes and morphologies. Particularly conspicuous shapes are thoses with a 5-fold symetry axis such as icosahedron or decahedron.
Introduction It is well-known that colloidal particles of noble metals can be produced by reduction of their metallic ions in solution. Some important works on this area are that of Turkevich et al.1who optimized the preparation conditions and that of Sermon et aL2who introduced the important idea of preparing catalysts by using monodispersed sols and succeeded in producing particles of Pt-Au alloys supported on graphite. Their work opened interesting possibilities of preparing a wide range of new catalysts under more controlled conditions with more defined shapes. On the other hand it has been known for some time that small particles prepared by colloidal methods tend to form anomalous structures such as decahedral, i~osahedral,~-~ or other types of structures with a 5-fold symmetry axis. As is well-known, a 5-fold axis is not consistent with the periodicity of crystalline materials. The recent discovery of quasicrystals in alloys which show a true 5-fold symmetry axis6 makes the study of alloys of noble metals even more attractive. Since these quasicrystalline materials have order but no translational symmetry and their properties are unlike those of the noble metals, they could provide a new type of alloy material with a large number of potential applications. The colloidal methods also produce crystals in the range of hundreds of micrometers, which might be useful in many experiments. We have undertaken in our laboratory a series of studies of the growth and characterization of colloidal monometallic and bimetallic particles. In the present paper we report only the growth of Au particles.
Experimental Methods Particles of Au were prepared by reduction of an aqueous solution of HAuC4 with a 40% trisodium citrate solution at different temperatures. I n order to produce larger particles, a slow reduction up to 20 h was carried out at 40 "C. TEM studies were performed by use of a JEOL 100-CX equipped with STEM attachment and a JEOL4000-EX ultra(1)Turkevich, J.; Sevenson, P. C.; Hillier, J. Discuss.Faraday SOC. 1955, 11, 55.
( 2 ) Sermon, P.; Keryou, K.; Thomas, J. M.; Millward, C. R. Mater. Res. SOC.Symp. h o c . 1988, 3, 13. (3) Kerlen, A.; Tholen, A. In Proceedings of 43rd Annual EMSA Meeting; Bailey, O., Ed.; S.Fco. Press: 1985; p 389. (4) Duff, D. G.; Curtis, A. C.; Edwards, P. P.; Jefferson, D. A.; Johnson, B. F. G.; Kirkland, A.; Logan, D. E. Angew. Chem. 1987,26,678. ( 5 ) Briue, M.; Gillet, M. Thin solid Films 1988, f f7,149. (6) D. Schecktman, I. Blech, D. Graties and J. W. Cahn. Phys. Reu. Lett. 1984, 53, 1951.
0743-7463/91/2407-1546$02.50/0
high-resolution microscope. Image processing of pictures was carried out, with an Innovion image display system attached to a Vax 780 computer. The chemical composition of the samples was obtained by X-ray microanalysis in the TEM using an EDX system. Some images were obtained with a high-resolution field emission JEOL-SEM microscope (JSM-840F).
Results A general aspect of the particles prepared by using the methods described above is shown in Figure 1A. Most of the particles shown in parts B and C of Figure 1 clearly have a 5-fold symmetry axis. The particles are twinned as suggested by the change in contrast in different parts of the particles. This twinned nature was also confirmed by dark field studies. It should be noted that the shape of twinned particles obtained by this method is substantially different from that obtained by standard evaporation methods, which tend to be more regular in shape. This is in agreement with the results of Kerlen and Tholen3 who reported the star-shaped structure which corresponds to an irregular decahedron. When growth takes place at a slower rate, larger particles can be obtained. In fact after a growth process up to 20 h and subsequent precipitation at 40 "C,crystals with sizes up to 100 pm can be produced. Several shapes of crystals are observed in this case. One of these shapes corresponds to a decahedron as shown in Figure 2A). The size of the crystallite is 30 pm. Parts B and C of Figure 2 show different orientations and views of this type of crystal. Decahedral particles have been long studied on the literature since the report by Ino.' The structure of these crystals is particularly interesting since the decahedral shape is inconsistent with the cubic symmetry of gold. A number of authors have discussed this problem in the case of small particles,"1° in the -100-500 A size range. However, very limited data have been produced for crystals of the -100 pm size. Large crystals of this type were reported before by Disgard et al." We have also found crystallites with icosahedral shape such as the ones shown in Figure 3 and hexagonal platelets as the ones shown in Figure 4. Finally we have also observed a crystallite with cubo-octahedral shape and
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(7) S. Ino, J . Phys. SOC. of Jpn. 1966,21, 346.
(8)Jos6-Yacamln, M.; Keineman, K.; Yang, C. Y.; Poppa, H.J. Cryst. Growth 1979,47, 187. (9) Renou, A.; Penisson, J. M. J . Cryst. Growth 1986, 78,357. (10) Marks, L. D.; Smith, D. J . Cryst. Growth 1981,54,425. (11) Disgard, C.; Mcurin, M.; Roberta, J. Met. Corresp. Ind. 1976,51, 255.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 7, 1991 1547
Structural Study of Gold Single Crystals
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Figure 2. SEM images of large crystallites with decahedral
Figure 1. Images of gold particle growth by colloidal methods: (A) general view of the particles; (B) a penta-twinned particle; (C) a star-shaped particle.
hexagonal facets, as shown in Figure 5. It should be noted in all the high-resolution images that the surfaces of the particles present some "bumps", as clearly distinguished on the top portion of the Figure 5. When these "bumps" are imaged in a lateral face, they appear as "black dots", which should not be confusedwith contamination. In fact in all the pictures taken with the SEM attachment in a JEOL100-CX microscope at GOKeV, the bumps are not visible. At this voltage the images are less sensitive to the surface topography, which could explain the abscense of
shape: (A) 5-fold orientation; (B) sideways view; (C) higher magnification and higher resolution image, showing the crystallite is flat.
bumps (as for instance Figure 2A). The fact that the samples are metallic gold is further shown by the EDX spectrum of Figure 6, which was taken with a raster over an area of 100 pm and shows only gold peaks. In order to clarify the nature of the interfaces of these crystals, we obtained higher magnification pictures of a 5-fold axis of an icosahedral particle as shown in parts A and B of Figure 7. Several features of the structure can be seen. There is a very extensive surface roughness in all facets, and steps appear in addition in all their facets. Figure 7C shows the region of one of the corners (indicated with an arrow) which shows an extended surface roughness.
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Discussion The stability of decahedral and icosahedral particles has been discussed from the thermodynamic point of view by Howie and Marks.12J3 According to those authors the competition between different shapes of particles does not imply that the icosahedral and decahedral can only occur below some size. Once these types of crystals are formed, its rearrangement to a single crystal will require extensive atomic movement. This appears very unlike for the growth conditions of the colloidal precipitation. Experimental observations have indicated that very small particles can change their shape from icosahedral to single crystal and get back to icosahedral14and it happens only when the electron beam of the TEM provides the proper energy. This does not seem to be likely in our case in which a temperature of 40 “C is used during the growth, and we did not observe any kind of changes during TEM observation. Therefore, a decahedral or icosahedral particle can grow indefinitely until reaching a size of 10100 pm. Our results tend to support the idea of Howie
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(12) Howie, A.; Marks, L. D. Philos. Mag. A 1984,49,95. (13) Marks, L. D. Philos. Mag. A. 1984, 49, 81. (14) Saito, Y.; Yatsuga, S.;Mihama, K.; Uyeda, R. Jpn. J . Appl. Phys. 1978,17, 1149.
Figure 4. Images of hexagonal platelets: (A) regularly shaped; (B) irregularly shaped.
and Marks12 that suggest that extensive facceting will promote the formation of &fold multiple twinned structures with either icosahedral or decahedral shape. Two new characteristics of the twin boundaries appear in our work: the surface of the particles are very rough showing “bumps” and the twin boundaries show large height steps. This is in opposition with the notches observed by Saito et al.14 and Marks et al.15 for twin boundaries in annealed small metal particles. A fact not observed previously is that in the case of decahedral particles, the lateral edges of the particles are not sharp but faceted, as shown in Figure 2C. It appears then that these particles are kinetically stabilized. In summaryour results show that it is possible to produce single crystals of gold in the hundreds of micrometers size range. These crystals can be prepared with a number of shapes, such as decahedral, icosahedral, and cubooctahedral. The surfaces of these shapes are very rough (15) Marks, L. D.; Smith, D. J.; Howie, H. Electron Microscopy and Analysis 1979; Mulvey, T., Ed.; Institute of Physics Conference Series No. 52; Institute of Physics: London, 1980.
Langmuir, Vol. 7,No. 7,1991 1549
Structural Study of Gold Single Crystals ----c-’rlr
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Figure 5. Two views of a cubo-octahedralcrystallite: (A) lateral view showing the hexagonal facets; (B) lateral view. Au
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Figure 7. Images of an icosahedral crystal: (A) general view; (B) region around the 5-fold axis.
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EDX spectra of a sample containing several crystal-
and appear as potential systems for uses in catalysis and several other areas.
Acknowledgment. We are indebted to Mr. Samuel Tehuacanero,Mr. Luis Rendbn, Mrs. Laura Cabrera, and Mr. A. SBnchez for technical help. M.J.Y. was a Guggenheim FoundationFellow during this work. This work was supported by CONACYT Grant 140-106 202-193 and P228CCOX891594. Registry No. Au, 7440-57-5.
