A New Mechanism of Stabilization of Large Decahedral Nanoparticles

Article pubs.acs.org/JPCC

A New Mechanism of Stabilization of Large Decahedral Nanoparticles Gilberto Casillas, J. Jesús Velázquez-Salazar, and Miguel Jose-Yacaman* Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States S Supporting Information *

ABSTRACT: The stability of decahedral shaped nanoparticles depends upon size. Ino and Marks introduced new mechanisms for the stabilization of decahedra nanoparticles that involves the faceting and formation of surfaces different from {111}. These mechanisms have relevance for small size nanoparticles; however, they do not thoroughly explain how decahedral particles can grow up to 300 nm or more. Here, we report new mechanisms that help stabilize very large decahedra. With the use of aberration-corrected scanning transmission electron microscopy, we observed the formation of high index facets, determined to be of the {511} family, on all five sides of the particles. Surface dislocations strings are also observed. In addition, surface reconstruction of the {001} surfaces can also be observed in two different orientations: with hexagonal strings along the [110] and [410] directions.

1. INTRODUCTION Nanoparticles of metals with icosahedral and decahedral shape have attracted the interest of scientists for almost a century.1−10 Particles such as the decahedron (Dh) or the icosahedron (Ic) show a 5-fold symmetry axis; however, this axis does not reflect a real 5-fold symmetry, but rather, the packing of 5 or 20 tetrahedra in a twin relationship. Therefore, unlike quasicrystals, no large single crystal formed by decahedra or icosahedra can be grown (showing real 5-fold symmetry).11,12 A decahedral particle will be formed by tetrahedra put together by five twins leaving a gap of 7.35 degrees (Figure 1). That gap will be filled by introducing stress in the nanoparticle. Quite

naturally, when the Dh increases its size, the internal stress will increase; the same situation will be valid for the icosahedral structure. However, although calculations of the total energy versus size13−17 indicate that decahedral and icosahedral nanoparticles become unstable as the sizes increase and should not be observed, the reality is very different. Decahedral (and icosahedral) particles can be observed at sizes that deliberately defy the predictions made by the theory. Figure 2 shows an example of gold decahedral particles of different sizes from a few nanometers to micrometers. Decahedral nanoparticles appear to be formed from the nano- to the mesoscopic range. In his pioneering work, Ino18 introduced (001) facets on the Dh structure (the so-called “Ino” decahedron) (Figure 3a), as a way to stabilize the total energy of the particle. A more advanced version was introduced by Marks, who proposed a Dh containing (111) wedges at the twin boundaries (the so-called “Marks” decahedron) (Figure 3b).19 The latter work clearly shows that faceting constitutes a very important aspect of the stability of particles shape during growth. The main argument presented by Marks was the introduction of a modified Wulff construction, which included the energy of the twin boundaries.19 In addition, it is well-known that the {001} and {111} surfaces of gold (mostly of the 5d metals) show varieties of reconstructions due to strain or energetic reasons.20,21 As previously stated, the “Marks” and “Ino” shapes allow the nanoparticles to lower their total energy. However, as the nanoparticles become bigger, other mechanisms become influential in the decrease of the total energy. Throughout Received: February 4, 2012 Revised: March 29, 2012 Published: March 31, 2012

Figure 1. (a) Geometrical angular deficiency of 7.35° of a 5-fold particle formed by perfect face-centered cubic (fcc) tetrahedra. © 2012 American Chemical Society

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Figure 2. (a−f) Gold decahedra of sizes ranging from a few nanometers up to a micrometer. Images a−e correspond to HAADF images, while f corresponds to an SEM image.

synthesis. Immediately afterward, the solution was withdrawn from the heat source and was cooled down very rapidly in ice. The products were collected by centrifugation at 4000 rpm for 5 min and washed with ethanol several times. The resulting particles were drop-casted onto 3 mm copper grids for their subsequent characterization. The particles were characterized by transmission electron microscopy (TEM), Cs-corrected scanning TEM (STEM) and scanning electron microscopy (SEM). TEM images were collected in a JEOL 2010F operated at 200 kV, equipped with a field emission gun (FEG) and with an AMT CCD camera that had a minimum exposure time of 0.067 s. High angle annular dark field (HAADF) STEM images were collected with a JEOL JEM-ARM200F operated at 200 kV equipped with a probe Cs probe corrector. Collection angles were set to 68 and 280 mrad for inner and outer semiangles. The convergence angle was set to 32 mrad resulting in a probe size of 0.095 nm. SEM images were collected in a Hitachi S5500 operated at 30 kV equipped with a cold-FEG with a resolution of 0.4 nm.

Figure 3. (a) Ino decahedron model. (b) Marks decahedron model.

the paper, the mechanisms that very large nanoparticles use to stabilize the total energy will be reported. We found that high index facets are stable at high temperatures, and surface reconstruction is an important contributing mechanism to stabilize the particles at larger sizes.

2. EXPERIMENTAL METHODS Gold nanoparticles were synthesized by the polyol method. Five milliliters of ethylene glycol (EG) were stirred heated at 280 °C to reflux for 10 min. Solutions of HAuCl4.3H2O (0.08 g) in EG (5 mL), and polyvinylpyrrolidone (PVP; 0.208 g) in EG (5 mL) were prepared. At the same time, both the PVP and the HAuCl4.3H2O solutions were added to boiling EG drop by drop. Magnetic stirring was applied throughout the entire

3. RESULTS The morphologies of the synthesis are polydisperse, with 50% of the decahedra nanoparticles ranging from 300 to 800 nm. Figure 4a shows a low-magnification SEM image of the resulting decahedral nanoparticles where it is possible to

Figure 4. (a) SEM image showing several decahedra with high index facets. (b) SEM image of a faceted decahedron close to its 5-fold axis. (c) HAADF image of a decahedra oriented in its 5-fold axis exhibiting high index facets on all five sides. 8845

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Figure 5. (a) HAADF image of the “zipper” on the decahedron. (b,c) Magnified area of the marked squares in panel a. (d) Model of a (511) facet.

Figure 6. (a) Resulting decahedron after washing the solution. Area enclosed by the white circle was exposed for several seconds to a strong parallel electron beam resulting in the reconstruction shown in panels b and c. (b) Types I and II strings along the [110] axis. (c) Hexagonal reconstruction with strings rotated 30° away from the [110] orientation.

Figure 7. Series of HRTEM images of a reconstruction process along the [110] orientation on a tetrahedra subunit. Total time passed during the sequence is 66 s. In each frame, the time is specified on the left.

formerly was at high temperature. This indicates that the most stable shape is different at high temperatures and that high index facets become stable. It is worth mentioning that this shape is also possible due to the stabilizing effect of the PVP; however, if removed, a different morphology is obtained. Washing the solution several times with acetone removes the excess PVP, resulting in decahedra with smother features. Figure 6a shows the resulting decahedra with no high index facets and very round contours at the twin boundaries. Hexagonal strings along the [110] orientation on the (001) terraces near the steps (Figure 6b) were observed, as well as strings along the [410] direction (Figure 6c). It has been proven that this reconstruction is more stable than the unreconstructed {001} surfaces,23 so it is expected to see a reconstructed surface. There were two distinguished dislocations: dislocations formed by five and three columns of atoms.

observe several facets on the edges as well as a defect in the center of every decahedron. These features are more easily seen in Figure 4b. By using Cs-corrected STEM, it is possible to get atomic resolution imaging and determine the planes corresponding to these facets. Figure 5 shows high-resolution HAADF images of the facets. The angle between the facets and the (001) planes is 14.8° on average. While in a perfect crystal a (511) facet makes an angle of 15.79° (Figure 5e), with respect to a (001) plane, this is the best match. The slight difference can be attributed to the rounded edges that connect the facets due to surface tension,22 as well as the internal strain existent in the lattice of the decahedron. The fact that these high index facets are stable at room temperature is quite surprising. The reason for this stability is that, due to the fast cooling from 280 °C to room temperature, the atoms of the nanoparticles do not have time to rearrange, thus, leaving the morphology as it 8846

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Figure 8. (a) Reconstruction model of hexagonal strings of type I along the [110] direction (left). (b) Reconstruction model with strings along the [410] direction (right).

These types of dislocations, as described by Pao et al.,24 refer to the “arc” formed by five columns of atoms as a dislocation of type I and the one formed by three columns of atoms as a dislocation of type II. Both dislocations occur due to the insertion of an extra row of atoms to the (001) surface; however, they have different consequences in the stability of the surface. While type I dislocations increase the tensile strain, type II decreases it. On the other hand, they have the opposite effect on the surface energy: while type I decreases the surface energy, type II increases it.24

(60.16 s) was fully formed and stuck together with the nearest dislocation (marked by open arrows). This shows that the formation of dislocations can be rather fast (less than 0.06 s) or rather slow (0.87 s). At the end of the sequence at 66 s we observe eight dislocations together separated from each other by a distance of 0.14 nm, which corresponds to the period of equilibrium of the (5 × 1) surface reconstruction, meaning that the surface of the decahedron reconstructed partially into a (5 × 1) reconstruction. This process lasted more than a minute, and only a distance of 1.12 nm of the (001) surface underwent reconstruction (see Movie 1 in the Supporting Information). However, by spreading the beam onto one full (001) surface (less intensity), the particle reconstructed in a different manner. Figure 6c shows an experimental image of the reconstruction where we can see an increased periodicity of the atoms in the outermost layer. This can also be explained as a hexagonal reconstruction in which the hexagonal strings are rotated 30° from the [110] orientation (which corresponds to the [410] direction). As mentioned before, this reconstruction has two possible orientations on a (001) surface. What is observed here is that, for wide (001) surfaces, the hexagonal strings align in the [410] direction (or the equivalent [140]). This shows that in decahedra nanoparticles, this orientation is more stable than the singular hexagonal strings along the [110] orientation due to the fact that the reconstructed surface is tens of nanometers large. Here it is quite difficult to determine the periodicity of the reconstruction, but a (5 × 1) reconstruction rotated along the [410] is a good approximation. Figure 8 shows schematics for both hexagonal orientations on the (001) surface of an Ino decahedron composed of type I dislocations. Figure 8a show the reconstruction with the hexagonal strings along the [110] orientation, while Figure 8b shows the same reconstruction but rotated 30° away from the [110] direction ([410] direction). In this orientation, it can be seen how the frequency of the atomic positions increases as shown in Figure 7c. Interestingly, {511} facets are also known to reconstruct in hexagonal layers,26 it was not, however, observed in this case, probably due to the small size of the facets. It is important to notice that reconstruction in the (001) surface is favored only in 5d metals since the energy gained by the surface density increase outweighs the mismatch energy loss. This is due to the fact that the surfaces of end-of-series (Ir, Pt, and Au) 5d metals are subject to twice as large tensile stress as that of isovalent 4d metals (Rh, Pd, and Ag).20 Decahedra of Pd or Ag should not present this reconstruction; therefore, they should present a different stabilization mechanism than Au decahedra.

4. DISCUSSION Since decahedra structures have intrinsic elastic tensile strain, in principle, type II dislocations should be more likely to be observed since it would lower the stress at the surface. However, the increase in surface energy will reduce the stability of the dislocation. Experimentally we observed both: while type I was more common near the edges of the {001} terraces, type II was more commonly seen in the center of wide {001} surfaces. This is consistent with the previous observations of Zandbergen et al.;25 they saw the same behavior on gold thin films. Their theoretical calculations showed that the lowest energy configuration for types I and II are on the edges and away from the edges, respectively; moreover, type I dislocations are more stable than type II. Figure 6b shows both types at the edges of (001) surfaces. Interestingly, most of the small terraces present these types of dislocations. Irradiation of the (001) surface of a decahedron by the electron beam caused the formation of several type I dislocations. Figure 7 shows a sequence of HRTEM images of an evolving (001) surface. At first (0 s) we only observe one (001) step, but as time passes, this outer (001) layer disappears, leaving behind a set of surface dislocations. Atoms of the outer layer seem to get injected into the next layer, while the rest of the atoms should go to the (111) surfaces of the decahedron. From 5.87 to 23.34 s we observe that the outer (001) layer retracts while leaving behind two type I dislocations. Interestingly, they never get together. After that, a set of type I dislocations will appear. At 33.02 s we observed a third dislocation, and 0.06 s later, a fourth dislocation appeared (marked by closed arrows). The atoms’ diffusion is faster than the 0.06 s interval between frames, which is the smallest exposure time available in our instrument. The formation of this dislocation was really fast; however, at 50.29 s we did observe the formation of a new dislocation a few interatomic distances away from the nearest dislocation, and 0.87 s later 8847

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(5) Johnson, C. L.; Snoeck, E.; Ezcurdia, M.; Rodriguez-Gonzalez, B.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Hytch, M. J. Nat. Mater. 2008, 7, 120−124. (6) Jose-Yacaman, M.; Asencio, J.; Liu, H. B.; Gardea, J. J. Vac. Sci. Technol. B 2001, 19, 1091−1103. (7) Jose-Yacaman, M. A., M. Catal. Rev. 1992, 34, 55−127. (8) Marks, L. D. J. Cryst. Growth. 1983, 61, 556−566. (9) Marks, L. D.; Howie, A. Nature 1979, 282, 196−198. (10) de Wit, R. J. Phys. C: Solid State Phys. 1972, 5, 529. (11) Baletto, F.; Ferrando, R.; Fortunelli, A.; Montalenti, F.; Mottet, C. J. Chem. Phys. 2002, 116, 3856−3863. (12) García González, L.; Montejano-Carrizales, J. M. Phys. Status Solidi B 2000, 220, 357−362. (13) Cleveland, C. L.; Landman, U.; Schaaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L. Phys. Rev. Lett. 1997, 79, 1873− 1876. (14) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603. (15) Barnard, A. S. J. Phys. Chem. B 2006, 110, 24498−24504. (16) Wang, B.; Liu, M.; Wang, Y.; Chen, X. J. Phys. Chem. C 2011, 115, 11374−11381. (17) Barnard, A. S.; Young, N. P.; Kirkland, A. I.; van Huis, M. A.; Xu, H. ACS Nano 2009, 3, 1431−1436. (18) Ino, S. J. Phys. Soc. Jpn. 1969, 27, 941. (19) L.D, M. J. Cryst. Growth. 1983, 61, 556−566. (20) Fiorentini, V.; Methfessel, M.; Scheffler, M. Phys. Rev. Lett. 1993, 71, 1051−1054. (21) Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Bibérian, J. P.; Kesmodel, L. L.; BartoŠ, I.; Somorjai, G. A. Surf. Sci. 1981, 103, 189− 217. (22) Golovin, A. A.; Davis, S. H.; Nepomnyashchy, A. A. Phys. D (Amsterdam, Neth.) 1998, 122, 202−230. (23) Shi, H.; Stampfl, C. Phys. Rev. B 2008, 77, 094127. (24) Pao, C.-W.; Srolovitz, D. J.; Zandbergen, H. W. Phys. Rev. B 2007, 75, 195405. (25) Zandbergen, H. W.; Pao, C.-W.; Srolovitz, D. J. Phys. Rev. Lett. 2007, 98, 036103. (26) Kara, A.; Jayanthi, C. S.; Wu, S. Y.; Ercolessi, F. Phys. Rev. B 1995, 51, 17046−17062. (27) Egerton, R. F.; McLeod, R.; Wang, F.; Malac, M. Ultramicroscopy 2010, 110, 991−997. (28) Bauer, W.; Sosin, A. Phys. Rev. 1964, 135, A521−A526.

These observations are not expected to be altered by changing the electron beam energy. An 80 kV electron beam, for example, may transfer up to 1 eV of energy to a gold atom, whereas a 200 kV electron beam may transfer less than 3 eV to a gold atom.27 The maximum amount of energy transfer at 200 kV is less than 10% of the threshold energy for gold (36 eV28); therefore, we do not believe that the energy of the electron beam would change the results observed in this work.

5. CONCLUSIONS The fast cooling of the synthesis solution allows the structure at high temperatures to be frozen, which is also stable at room temperature. The high index facets were determined to be of the {511} family, which may present good catalytic activity. Furthermore, we presented experimental evidence that the {001} surfaces of the decahedra can undergo surface reconstruction of different types. Type I dislocations along the [110] direction were found to be more stable in the decahedra when compared to type II dislocations along the [110] direction. However, when more than one dislocation was present, that is the (5 × 1) surface reconstruction, the hexagonal strings along the [410] direction was more favorable when compared to the [110] direction. This work hints at the mechanisms of how large decahedra grow and stabilize in the mesoscopic range; however, further work needs to be done in order to fully understand 5-fold particles in this regime.

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ASSOCIATED CONTENT

S Supporting Information *

In situ TEM observation of a (5 × 1) reconstruction of a (001) surface of a decahedron with hexagonal strings along the [110] direction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the following agencies: The Welch Foundation Agency, project AX-1615: Controlling the Shape and Particles Using Wet Chemistry Methods and Its Application to Synthesis of Hollow Bimetallic Nanostructures; and the National Science Foundation (NSF) PREM Grant Number DMR-0934218: Oxide and Metal Nanoparticles: The Interface between Life Sciences and Physical Sciences. The authors would also like to thank the International Center for Nanotechnology and Advanced Materials (ICNAM) at UTSA, the RCMI Center for Interdisciplinary Health Research (CIHR), and the National Center for Research Resources for Project Award Number 2G12RR013646-1. The content of this Article is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources of the National Institutes of Health.

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REFERENCES

(1) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, M. J. J. Mater. Chem. 2006, 16, 3906−3919. (2) Hofmeister, H. Cryst. Res. Technol. 1998, 33, 3−25. (3) Howie, A.; Marks, L. D. Philos. Mag. A. 1984, 49, 95−109. (4) Ino, S. J. Phys. Soc. Jpn. 1969, 27, 941−953. 8848

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