CRYSTAL GROWTH & DESIGN
Visual Detection of Crystallographic Orientations of Face-Centered Cubic Single Crystals
2002 VOL. 2, NO. 1 73-77
Maria Luisa Foresti,* Ferdinando Capolupo, Massimo Innocenti, and Francesca Loglio University of Florence, Department of Chemistry, Via G. Capponi 9, 50121 Florence, Italy Received July 4, 2001;
Revised Manuscript Received November 8, 2001
ABSTRACT: A simple method to single out, with the naked eye, the position of several crystallographic orientations of a face-centered cubic metal, such as silver or gold, is described. The method is based on the observation that the preferential etching corresponding to some crystallographic orientations causes the formation of well-defined geometrical figures, such as triangles, squares, and circles. A number of faces are easily identified, since on a globeshaped single crystal the different geometrical figures recur on its surface according to the rules of the face-centered cubic system. Introduction It is well-known that many heterogeneous reactions are affected by the structure of the solid surfaces.1 In electrochemistry, the importance of using single crystals has been strongly stressed in the literature.2-9 The most important interfacial parameters depend on the crystallographic orientation of the metal used as an electrode: the potential of zero charge, for example, is strictly connected to surface properties such as the electronic work function8,10-12 and the surface energy.8,13,14 The actual polycrystalline electrode surface consists of many facets and grain boundaries; hence, its surface properties depend on the distribution of the various crystallographic orientations, which in turn depends on how the metal surface was prepared.13,15-18 The methods available to grow single crystals are widely described in ref 8. The most frequently employed methods are the growth from metal vapor and the growth from metal melt. The first technique consists of evaporating or sputtering the metal on a suitable substrate obtaining deposits of a few hundred nanometers in thickness. By this procedure, mono-oriented crystalline electrodes with an electrochemical behavior close to that of massive conventional single crystals are generally formed. The second technique consists of melting the metal and forcing the solidification to start at some seed. This latter technique is generally carried out with the Bridgman method for metals such as gold, silver, and copper. Alternatively, a metal bead is formed by melting the extremity of a wire. This method was pointed out by Clavilier to obtain single-crystal platinum electrodes.7,19 Crystals grown from the melt have to be oriented by the X-ray back reflection Laue method8 or through the use of a laser beam.7,19 Then, the face of interest is somehow isolated by cutting the crystal either with a saw or by eliminating part of the crystal by abrasion. This step is followed by the mechanical polishing of the surface to a mirror finish with progressively finer papers * To whom correspondence should be addressed. University of Florence, Department of Chemistry, Via G. Capponi 9, 50121 Florence, Italy. Fax: +39 55 244102. Phone: +39 55 2757541. E-mail: foresti@ unifi.it.
and with alumina powder or diamond paste of different grades. The final step of the surface preparation for electrochemical measurements consists of a cleaning treatment performed by flame annealing or chemical polishing.8,19,20 This paper describes a simple method to single out, with the naked eye, the position of several crystallographic orientations of a face-centered cubic metal, such as silver or gold. Experimental Section Gold and silver single crystals were melt by the Bridgman technique. The graphite crucible represents an evolution of one previously described.21 It consists of two parts that, when held together, form an inner hole of 1 cm in diameter or more (Figure 1). The crucible is put in an outer graphite cylinder where it is exactly contained. This set is vertically placed in a quartz tube positioned in the helix of an induction oven (Figure 2). The quartz tube is held by means of an independent stand to avoid any possible contact with the helix. The helix was moved up or down at an adjustable rate by means of an oleopneumatic piston which ensured the complete absence of vibration. To induce the solidification, the helix is moved up, and if the cooling step is sufficiently slow, a single crystal is obtained. To this end, the helix must be moved up very slowly. However, we found that a rate of about 0.2 mm s-1 represents a good compromise between the necessity of a slow cooling process, which allows the solidification to start from a seed, and the necessity of a short period of time to reduce the probability of any undesirable external vibration. As a matter of fact, at this rate a single crystal is obtained in about 3-4 min. The optimum rate is easily reproduced by means of a pressure gauge connected to the oleopneumatic system. The calculated weight necessary to form a sphere of 1 cm in diameter is 5.7 g for silver and 9.9 g for gold. An amount of metal exceeding the calculated weight of 1-2 g is put at the top of the crucible. This way, an upper stem is formed that ensures the complete formation of the sphere. According to the theory of alloy solidification,22 on freezing, the solute atoms tend to remain in the liquid rather than solidify with the solvent atoms at the crystal-liquid interface. Thus, during the cooling, the impurities migrate to the top and are then eliminated by cutting the stem. Both the graphite crucible and the oleopneumatic system were designed and constructed in the workshop of our department. For silver single crystals, etchings of increasing strength were performed with mixtures of H2O2 and NH4OH composi-
10.1021/cg015537i CCC: $22.00 © 2002 American Chemical Society Published on Web 12/11/2001
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Foresti et al.
Figure 1. Crucible used to grow globe-shaped single crystals.
Figure 3. Photograph of a globe-shaped silver single crystal, 1 cm in diameter, corresponding to the (111) face.
Figure 4. Photograph of a globe-shaped silver single crystal, 1 cm in diameter, corresponding to the (100) face.
Figure 2. Oleopneumatic system for coil movement. tions ranging between 5:1 and 10:1. For gold, mild etching was obtained by concentrated aqua regia.
Results and Discussion The method is based on the observation that preferential etching corresponding to some crystallographic orientations causes the formation of well-defined geometrical figures, such as triangles, squares, and circles. Thus, when etching a silver or a gold globe-shaped single crystal, the different geometrical figures recur on its surface according to the rules of the face-centered cubic system. As it will be shown, the formation of triangles is observed corresponding to the (111) face, squares corresponding to the (100) face, and rectangles corresponding to the (110) face. This phenomenon is pointed out by the photographs of a silver single crystal 1 cm in diameter (Figures 3 and 4).
Figure 5. Photograph of a globe-shaped silver single crystal 1 cm in diameter where more than one geometrical figure is shown (a), and a scheme is drawn from the photograph (b).
It must be noted that, due to optical effects, the figures of the photos do not coincide exactly with those observed with the naked eye. In particular, the triangle observed with the naked eye is much smaller than that of the photo, whereas the square observed corresponding to the (100) face becomes a circle. Analogously, the rectangle which indicates the (110) face becomes an ellipse. Figure 5a shows the photograph where the single crystal is rotated to evidence more than one geometrical figure. For a better interpretation, the scheme in Figure 5b has been drawn from the photograph. Here, the small
Visual Detection of FCC Single Crystals
Crystal Growth & Design, Vol. 2, No. 1, 2002 75
Table 1. Angles between the Face of Reference and the Low-Index Facesa {111} {433} 8.07° {755} 9.45° {322} 11.42° {533} 13.65° {211} 19.47° {100} 54.73° a
{100} {331} 22.00° {551} 27.21° {110} 35.26°
{210} 26.56° {110} 45.00°
Ref 8.
Figure 6. Geometrical figures evidenced by a stronger etching on the same silver single crystal.
Figure 8. SEM image of an Ag(111) surface. The triangle are the fingerprints of this surface. Mark in the figure corresponds to 1 µm.
Figure 7. Planar representation of the scheme of Figure 5b, with the (111) face placed in the center.
triangle of the (111) face is well observed at the center of a larger triangular figure. From the scheme, the connection between the different figures is evident: the triangular figures are connected by the ellipse, whereas the circle lies at the center of four triangles. It is also easy to verify that the distances in millimeters measured on the surface of the sphere between the (100), (111), and (110) faces reflect the angles between them, that is, 45° between the (100) and (110) faces, 54.73° between the (100) and (111) faces, and 35.26° between the (111) and (110) faces (Table 1).8 With progressively stronger etchings, further faces are observed (Figure 6). Large circular areas are seen around the (100) face, and small circles are seen between the (111) and (110) faces. Moreover, a sort of triangle is formed as delimited by the (111) face and two of the large circles (gray areas in Figure 6). A still better representation is obtained by transferring the preceding scheme on the plane (Figure 7). In drawing this scheme, the figures were not distorted. As a result, only the central area, where the different faces are in contact, reflects the true structure, whereas the outer part is only indicative. However, it is evident that the central figure has a 3-fold symmetry, and, therefore, it can be identified with the (111) face. Moreover, the face identified by the ellipse lies just in the middle between two (111) faces and has a 2-fold symmetry. Thus, it can be identified with the (110) face. Incidentally, triangles are generally observed by the optical microscope on Ag(111) surfaces with a low degree
Figure 9. Planar representation of the scheme of Figure 5b, with the (100) face placed in the center.
of crystallinity. The triangles tend to disappear with alternate mechanical and mild chemical polishings of the surface. Therefore, they probably correspond to defects that are progressively eliminated. On the other hand, they always appear after strong polishings of the surface and, hence, constitute a sort of fingerprint of the (111) plane. As a matter of fact, as the presence of defects is often unavoidable even on surfaces of high quality, some triangles are still observed at the highresolution of the scanning electron microscopy. As an example, Figure 8 shows a triangle with a side equal to 1 µm on the surface of a disk cut from a silver sphere corresponding to the (111) plane and accurately polished both mechanically and chemically. Of course, these triangles are of a different nature with respect to those evidenced by the etching on the sphere. However, their appearance underlies, once more, the 3-fold symmetry of this plane. The formation of triangularly shaped holes has also been observed during the dissolution of Ag adlayer grown on Au(111) substrates.23 Finally, let us consider the scheme where the circle is put in the center (Figure 9). Now, it is evident the 4-fold symmetry of this face that, therefore, can be identified by the (100) face. Large circular areas lie between the (100) and (110) faces: the distance in millimeters between the centers of these circular areas
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and the (100) and (110) faces, respectively, indicates that these figures correspond to the (210) faces. Analogously, the gray areas in Figure 9 lie between the (100) and (111) faces: the distance in millimeters between the centers of these areas and the (100) and (111) faces, respectively, indicates that these figures correspond to the (211) faces. Of course, the measurement in millimeters is only approximate, but all identifications were confirmed by X-ray back reflection Laue method, even if this technique is not a surface method. To this end, the sphere was placed in a suitable holder and manually oriented to position the selected face perpendicularly to the X-ray beam. As a matter of fact, the Laue pattern is able to indicate that the crystal is a single crystal. Therefore, all planes perpendicular to the X-ray beam have the same crystallographic orientation. The further figures, that is the small circle between the (111) and (110) faces, and the large triangular area surrounding the (111) face, are not more clearly distinguished with the naked eye, since they indicate more than one face. In particular, the measurement in millimeters places the small circle 24.5° from the (111) face. However, the observation by the optical microscope shows the presence of two adjacent faces, which we can tentatively identify by the (331) and (551) faces lying at 22.00° and 27.21°, respectively, from the (111) face. Analogously, when observed by the optical microscope, the large triangular area around the (111) face turns out to be formed by four concentric triangles forming a pyramid. More precisely, three adjacent steps parallel to the external edges were observed. The measurement in millimeters only indicates that the external edges lie at about 14° from the (111) face and can, therefore, be associated with the (533) faces. However, the X-ray analysis confirms the presence of four faces in the range between 13.65° and 8° from the (111) face: more precisely, the (533), (322), (755), and (433) faces (see Table 1). Therefore, we tentatively attributed formation of steps to a preferential etching corresponding to the above cited planes. These faces, according to the Lang notation, are formed by (111) terraces of 4 to 7 atoms and (100) steps. Now, the steps in Lang notation are monatomic and cannot be confused with the macroscopic steps that were observed by the optical microscope and that can reasonably be attributed to a preferential etching along the (100) direction. In conclusion, by reporting all figures represented in the scheme of Figure 9 on the Wulff net, where the twodimensional space is divided as to correspond to angles in the three-dimensional space, the standard (001) stereographic projection of a face-centered cubic crystal is obtained, even though it is limited to the identified faces (Figure 10).8,24 Conclusions The formation of well-distinguished geometrical figures on the surface of a globe-shaped fcc single crystal, as connected to different crystallographic orientations, confirms the observation already reported in the literature.2,25-27 However, up to now the imaging of the surfaces was performed with SEM or even with higher resolution methods, such as REM (reflectance electron microscopy) and TEM (transmission electron microscopy), and mainly refer to electrochemically grown
Foresti et al.
Figure 10. Stereographic projection of a face-centered cubic crystal, limited to the visually identified faces.
single crystals, which are generally considered quasi ideal single crystals. On the contrary, this note deals with crystals of 1 cm in diameter, and even more, grown by the classical Bridgman technique. The simple method described for the visual detection of the most important faces is approximate. However, the error is lower than 2%. Therefore, this procedure can be employed, at least for some of the identified faces, when a lower degree of accuracy is required or to speed up the exact orientation by X-rays. It is also interesting to note that SEM images of refs 26 and 27 point out the formation of concentric triangles forming a pyramid corresponding to the (111) face. These triangles seem to coincide with ours even for the detail that the vertexes lie on the direction of the (110) faces. Thus, they could reasonably coincide with the steps that we observed by the optical microscope. The method was pointed out and carefully checked for silver single crystals, for which, it is possible to somehow graduate and control the etching procedure. Preliminary tests suggest that it can easily be applied to copper single crystals. The method was also applied to gold single crystals. However, on gold only a mild etching was obtained, even by using concentrated aqua regia. Thus, only the geometrical figures corresponding to the low Miller indexes appeared. It must be noted that the etching process on silver could be continued to evidence more and more planes. However, a great number of figures on this relatively small surface makes impossible their identification. For the sake of completeness, we are just reporting that the progressive etching leads to a surface apparently disordered, until a new process of figure formation is started. We are also reporting that the figures formed on the surface are very stable, so that the crystal can be handled without particular care. Acknowledgment. The authors are deeply grateful to Dr. Antoinette Hamelin for having been introduced
Visual Detection of FCC Single Crystals
to the single crystal world and first trained in their preparation. They also thank Mr. Andrea Pozzi and Mr. Francesco Gualchieri of the Chemistry Department workshop for their technical assistance. The financial support of the Italian CNR and of the Murst is gratefully acknowledged. References (1) Somorjai, G. A. In Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (2) Piontelli, R. Electrochim. Met. 1966, 1, 1. (3) Damjanovic, A.; Setty, T. H.; Bockris J. O’M. J. Electrochem. Soc. 1966, 113, 429. (4) Bockris, J. O’M.; Razumney, G. A. In Fundamental Aspects of Electrocrystallization; Plenum Press: New York, 1967. (5) Schoeffel, J. A.; Hubbard, T. A. Anal. Chem. 1977, 49, 2330. (6) Budevski, E. B. In Comprehensive Treatise of Electrochemistry; Conway, B. E., Bockris, J. O’M., Yeager, E., Khan, S. U. M., White, R. E., Eds.; Plenum Press: New York, 1983; Vol. 7. (7) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (8) Hamelin, A. In Modern Aspect of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1985; Vol. 16, Ch. 1. (9) Bockris, J. O’M.; Khan, S. U. M. In Surface Electrochemistry; Plenum Press: New York, 1993; Ch. 2. (10) Frumkin, A. N. Sven. Kem Tiddskr. 1965, 77, 300. (11) Trasatti, S. In Trends in Interfacial Electrochemistry; Silva, A. F., Ed.; Reidel: Dordrecht, 1986; p 25. (12) Hamelin, A.; Vitanov, T.; Sevastianov, E.; Popov, A. J. Electroanal. Chem. 1983, 145, 225.
Crystal Growth & Design, Vol. 2, No. 1, 2002 77 (13) Hamelin, A.; Lecoeur, J. Surface Sci. 1976, 57, 771. (14) Bachetta, M.; Trasatti, S.; Doubova, L.; Hamelin, A. J. Electroanal. Chem. 1986, 200, 389. (15) Lecoeur, J.; Andro, J.; Parsons, R. Surface Sci. 1982, 114, 320. (16) Valette, G. J. Electroanal. Chem. 1982, 138, 37. (17) Lipkowski, J.; Nguyen van Huong, C.; Hinnen, C.; Parsons, R.; Chevalet, J. J. Electroanal. Chem. 1983, 143, 375. (18) Foresti, M. L.; Guidelli, R.; Hamelin, A. J. Electroanal. Chem. 1993, 346, 73. (19) Clavilier, J. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker Inc.: New York, 1999; p 231. (20) Hamelin, A.; Stoicoviciu, L.; Doubova, L.; Trasatti, S. J. Electroanal. Chem. 1988, 244, 133. (21) Hamelin, A.; Morin, S.; Richer, J.; Lipkowski, J. J. Electroanal. Chem. 1990, 285, 249. (22) Smallman, R. E. In Modern Physical Metallurgy; Butterworth Heinemann: Woburn, MA, 1985; pp 90-105. (23) Esplandiu, M. J.; Schneeweiss, M. A.; Kolb, D. M. Phys. Chem. Chem. Phys. 1999, 1, 4847. (24) Barret, C. S.; Massalski, T. B. In Structure of Metals: Crystallographic Methods, Principles and Data; MacGrawHill: London, 1966. (25) Kaischev, R.; Budevski, E.; Malinovski, J. Z. Phys. Chem. 1955, 204, 348. (26) Budevski, E.; Staikov, G.; Lorenz, W. J. In Electrochemical Phase Formation and Growth; VCH: New York, 1996; p 11. (27) Lehmpfuhl, G.; Uchida, Y.; Zei, M. S.; Kolb, D. M. In Imaging of Surfaces and Interfaces; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 1999; p 57.
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