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J. Phys. Chem. B 2006, 110, 7208-7212
Tuning of Copper Nanocrystals Optical Properties with Their Shapes C. Salzemann, A. Brioude, and M-P. Pileni* Laboratoire L.M.2.N., U.M.R. C.N.R.S. 7070 UniVersite´ Pierre et Marie Curie (Paris VI), B.P.52, 4 place Jussieu, F-752 31 Paris Cedex 05, France ReceiVed: January 9, 2006; In Final Form: February 17, 2006
Copper nanocrystals are obtained by chemical reduction of copper ions in mixed reverse micelles. A large excess of reducing agent favors producing a new generation of shaped copper nanocrystals as nanodisks, elongated nanocrystals, and cubes. By using UV-Visible spectroscopy and numerical optical simulations we demonstrate that the optical properties are tuned by the relative proportions of spheres and nanodisks.
I. Introduction During these last two decades careful studies of structural properties of free nanoclusters were carried out. From these studies, it is concluded that, for materials characterized by a fcc structure in bulk phase as gold, silver, and copper small icosahedra, intermediate size decahedra and large truncated octahedral clusters are produced.1 The growth mechanism of colloidal nanocrystals is not well established.2 This is mainly due to the fact that, to prevent against coalescence, the nanocrystals have to be coated during and after the formation process,3-5 and it is difficult to determine to what extent the action of the passivating agents can modify the structure.6 It is well established that the size and shape of the nanocrystals of noble metals (e.g., silver, gold) tune their optical properties driven by the dipolar and multipolar plasmon resonances.7 For nanocrystals with a size of few nanometers, the multipolar resonances can be neglected. In the case of particles having various shapes, several plasmon resonance modes appear associated with the various orientations of the particle axes. These different oscillation modes can be observed when the particles are randomly dispersed. Hence, the optical properties of particles depend on the particle shape, the polarization of light, and the distributions of the particle orientation.8,9 In 1988, we used reverse micelles as nanoreactors to produce nanocrystals differing by their sizes.10 This technique permits the development of various types of materials into semiconductors and metallic nanocrystals.2,3 Because the crystallinity of these nanoparticles is very high, even at very low size, their edges are well defined and only the polycrystals are spherical. This technique allows the copper nanocrystals remain stable for some time without any coating. However, similar data were obtained with coated silver nanocrystals.11 A careful study of copper nanoparticles by high-resolution electron microscopy12 demontrates the formation of nanocrystals having an average size less than 10 nm and characterized by various shapes as cuboctahedron and decahedron. Other shapes, such as nanoprisms and elongated nanocrystals, are produced. The optical spectra of an assembly of these nanocrystals are characterized by the resonance centered at 560 nm and attributed to the well-known Mie resonance for spherical particles. However, a shoulder around 650 nm is observed. From a kinetic study, the absorption centered at 650 nm appears first and then * Corresponding author.
the Mie resonance peak. The comparison between the optical studies obtained experimentally and by DDA simulation shows that this 650 nm absorption resonance is the signature of decahedral nuclei present at the first step of the chemical reduction and decahedron, and nanoprisms and elongated nanocrystals at the end of the reaction.13 This study also demonstrated that cuboctahedron nanocrystals are characterized by the same absorption as Mie resonance for spheres and is centered at 560 nm. Hence, we show, for the first time, that the shape of the nucleus can be retained by a homogeneous growth in reverse micelles.13,14 Conversely to what is observed with other synthesis methods,15-20 the role of the surfactant molecules is minor when they are self-assembled in oil-rich colloidal solutions. This is probably due to the fact that, in such waterin-oil solutions, the surfactant molecules stay at the oil-water interface and are not free in the bulk phase. This markedly differs from the water-rich colloids where some of the surfactant molecules are self-assembled, whereas others remain free in the water phase.15-20 Very recently, we demonstrated12,21 that the influence of the supersaturation regime plays a drastic role in the control of the nanocrystals’ shapes. One of the major effects is the formation of more than 50% of copper nanodisks at very high supersaturation, whereas, at lower value, the major nanocrystals look like spheres with a formation of decahedron, cuboctahedron, and very few amounts of cubes and elongated particles. In this paper, because the syntheses are made in reverse micelles, the optical studies of assemblies of copper nanocrystals produced at various supersaturation regime conditions can be followed. Upon increasing the amount of reducing agent, in the saturation regime, the optical properties are tuned from the Mie resonance for spherical nanoparticles (560 nm) to that of nanodisks (650 nm). These data give a direct proof of the change in the optical properties of copper nanocrystals with their shape, and it reinforces the accuracy of the DDA simulation. II. Experimental Section II.1. Chemicals. AOT and hydrazine were from Sigma and isooctane from Fluka. The materials were used without further purification. Copper bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2, was prepared as described previously.22 II.2. Apparatus. Absorption spectra were recorded on a conventional Varian Cary 1 spectrophotometer. Transmission electron micrographs (TEM) were obtained using a Philips CM200 FEG microscope operating at 200 kV,
10.1021/jp0601567 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/17/2006
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Figure 1. A: Top, conventional electron microscopy study with R ) 15. Center: High-resolution transmission electron microscopy for triangular prisms, elongated particles, cylinders, and spheres. B. Corresponding absorption spectra.
Cs ) 1.35 mm with an information limit better than 0.18 nm. Together with overview images, high-resolution images and electron diffraction patterns were also obtained. II.3. Synthesis. The synthesis of copper nanocrystals in reverse micelles is described in detail in ref 23. A mixture of surfactants, 10-2 M Cu(AOT)2 and 8.10-2 M NaAOT, where AOT is bis(2-ethylhexyl)sulfosuccinate, is solubilized in isooctane and forms spherical reverse micelles.24 To produce copper nanocrystals, Cu(AOT)2 is reduced by various amounts of hydrazine (N2H4). The ratio R ) [N2H4]/[Cu(AOT)2] varies from 3 to 15, corresponding to changes in the hydrazine concentration from 3 10-2 M to 0.15 M. To obtain as consistent as possible data, we assumed that a molecule of water has the same volume as that of hydrazine, and both contribute similarly to the polar volume fraction. The overall polar volume fraction is kept constant at 2.6%. In absence of hydrazine, this corresponds to a water content, w, defined as [H2O]/[AOT] ) 10. After hydrazine addition, the solution immediately turns dark, indicating the reduction of Cu2+ to Cu0. Three hours later, the absorption spectrum of the solution is recorded. For the TEM study, a few drops of the colloidal solution are deposited on a carbon film supported by a copper grid and evaporated. To prevent oxidation, the reaction is carried out in an inert atmosphere. II.4. DDA Simulation. The discrete dipole approximation, DDA, is a computational procedure suitable for studying scattering and absorption of electromagnetic radiation by particles with sizes on the order of, or less than, the incident light wavelength, and especially for nonspherical shapes. The DDA has been widely developed in the past few years for both the specific study of applied nonspherical metallic nanoparticles and their aggregates.25 Recently, a number of groups compared the optical properties obtained from experiments and DDA simulations for silver nanoprisms,26silver nanodisks,27 gold nanorods,28 and copper nanocrystals of arbitrary shapes.13 Details of this computational method, first introduced by Purcell and Pennypacker29 and improved, in particular, by Draine et al.,30 are given in several recent papers. III. Results and Discussion The syntheses are made at fixed water content (w ) [H2O]/ [AOT] ) 10) and differ by the amount of reducing agent characterized by R, defined above.12 Because the solution
consists of reverse micelles for any R-value, the absorption spectrum of the solution is recorded at the end of the chemical reduction. At a very high hydrazine concentration (R ) 15), nanocrystals with well-defined shapes are obtained.21 The percentage and the average size of each shape, determined using around 600 particles, are estimated by TEM. They were found to be 30% triangles (23 nm), 9% squares (19 nm), and 30% elongated nanocrystals, characterized by a length (L) of 23 nm and a width (w) of 13 nm, , in addition to 31% spheres (20 nm) (Figure 1A). As the TEM observation represents just a projected image, we have to take into account the third dimension. In the following, the squares are considered to be cubes. From a previous study,21 we know that the triangles and most of the elongated particles are, in fact, copper nanodisks. Due to their morphology, copper nanodisks can appear either as triangular prisms (Figure 1C) or elongated nanocrystals (Figure 1D), depending on the orientation. Under the [111] direction, copper nanodisks are viewed from above, then, a triangular shape is observed, whereas from the [110] direction, the nanocrystals are observed from their side, and in this case, ellipsoidal shapes are detected. Nevertheless, it must be noted that all the ellipsoidal shapes cannot be assigned as copper nanodisks observed on a profile view. A few elongated nanocrystals characterized by a 5-fold symmetry are also observed (Figure 1E).31 Distinguishing between these two shapes can only be done by a careful structural characterization. From the HRTEM studies of the various samples, we conclude that almost 20% of the ellipsoidal nanoparticles are, in fact, nanodisks, whereas 10% are elongated particles. From this, it is deduced that 50% of the produced particles are having 23-nm diameter, 13-nm thick nanodisks. The corresponding absorption spectrum is characterized by a peak centered at 625 nm and a broad band in the range of 500-600 nm (Figure 1B). The 625-nm peak is not expected for copper nanocrystals. However, as just stated, almost 50% of the nanocrystals are disks. We know that the absorption spectrum of nanodisks markedly differs from that of spherical nanocrystals.32 The simulated absorption spectrum of nanodisks characterized by 23 nm, 0, and 1.8 as size (s), truncature (TR), and aspect ratio (r) is centered at 650 nm (Figure 2A). Because the edges of the cubes are not well defined (Figure 1A), the simulations have been done for 19-nm cubooctahedral particles. The simulated absorption spectrum is characterized by a peak centered at 560 nm (Figure 2A). This
7210 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Figure 2. A: Simulated absorption spectrum of copper particles with different shapes: nanodisks (s ) 23 nm, r ) 1.8, TR ) 0), solid line; elongated particles (L ) 22, w ) 13, r ) 1.8), dotted line; cubooctahedra (19 nm), dashed line. B: Simulated absorption spectrum of 20 nm spherical copper particles. C: Comparison of the experimental (dashed line) and simulated spectra (solid line)
Salzemann et al. agrees with data obtained previously.13 The simulated resonance spectra of elongated (L ) 23 nm and w ) 13 nm), and that 20-nm spherical nanocrystals show peaks centered at 575 nm (Figure 2A) and 560 nm (Figure 2B), respectively. To demonstrate the influence of the different shape percentages on the absorption spectrum, we consider all the contributions of these shapes in the simulations. Taking into account the various percentages of nanocrystals with different shapes (50% nanodisks, 10% elongated particles, 31% spheres, and 9% cubooctahedra), the simulated absorption spectrum is in rather good agreement with the experimental one (Figure 2C). The difference between the experimental and simulated absorption spectra is attributed to the size distribution of nanocrystals produced. In fact, in simulation for the absorption spectrum, each shape is calculated for the average size and its distribution is not taken into account. From this, it appears that the band at 650 nm is due to the high proportion of copper nanodisks, whereas the broad absorption at a lower wavelength is due to spherical, cubic and elongated nanocrystals. Conversely to R equal to 15, at low hydrazine concentrations (R ) 3), the nanocrystals are mostly spherical (Figure 3A) with an average diameter of 12.3 nm. Careful observation shows the presence of nanocrystals with a slight deformation like pentagons, hexagons, and more or less well-defined shapes (Figure 3A). As discussed previously, the percentage and the average size of each shape have been estimated by TEM. The proportion of 12.3 nm spheres (79%) is clearly dominant, whereas only 8% triangles (14.8 nm), 3% cubes (13.5 nm), and 10% elongated nanocrystals (L ) 17.3 nm, w ) 10.5 nm) are observed. For the last of these nanocrystals, the HRTEM images indicate that these are characterized by a 5-fold symmetry with no detection of nanodisks on their sides. The corresponding absorption spectrum has a broad surface plasmon resonance centered at 560 nm with a shoulder around 650 nm (Figure 3B). As discussed previously, DDA simulations have been made (Figure 4). As expected, the simulated absorption spectrum of 12.3-nm spherical nanocrystals has a peak centered at 560 nm (Figure 4A). However, it must be noted that the width of the experimental resonance band shown in Figure 3B is rather large compared to that of the simulated band (Figure 4A). This is due to the fact that the not well-defined shapes present in low proportions in the colloidal solution (Figure 3A) contribute to
Figure 3. A: Left, conventional electron microscopy study with R ) 3. Bottom, High-resolution transmission electron microscopy for spheres, triangular prisms, and elongated particles. B: The corresponding absorption spectrum of the solution containing the particles with different sizes.
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Figure 5. A: Conventional electron microscopy study at R ) 5 (A) and R ) 10 (B). C: Absorption spectra obtained with nanocrystals produced at various R values (R ) 3, 5, 10, 15).
Figure 4. A: Simulated absorption spectrum of 12.3-nm spherical copper particles. B: Simulated absorption spectrum of copper particles with different shapes: nanodisks (s ) 14.8 nm, r ) 1.7, TR ) 0), solid line; elongated particles (L ) 17.3, w ) 10.5, r ) 1.7), dotted line; cubooctahedra (13.5 nm), dashed line. C: Comparison of the experimental (dashed line) and simulated spectra (solid line).
the experimental absorption spectrum. By taking into account 79% spheres (12.3 nm), 10% elongated nanocrystals (L ) 17.3 nm, w ) 10.5 nm, r ) 1.7), 3% cubes (13.5 nm), and 8% of nanodisks (s ) 14.8 nm, TR ) 0, and r ) 1.7), we observe rather good agreement between the simulated and the experimental spectra (Figure 4C). This clearly demonstrates that the plasmon band at 560 nm is predominantly due to the high proportion of spheres compared to the proportion of nanodisks (very low), which leads to the absence of the peak at 650 nm. The same studies have been carried out for intermediate values of the hydrazine concentration (R ) 5, 10). At R ) 5, the nanocrystals are either aggregates or too dispersed, and it was not possible to evaluate the different percentages of the nanocrystal shapes (Figure 5A). The corresponding absorption spectrum shows the two bands at 560 and 650 nm (Figure 5C). At R ) 10, the TEM patterns show well-defined nanocrystals (Figure 5B). This makes it possible to determine the percentages and average sizes of the various shapes and the evaluation using more than 600 particles gives spheres (49%, 13.6 nm), triangles (20%, s ) 16 nm, r ) 1.6), cubes (10%, 13.6 nm), and elongated particles (20%, L ) 16.8 nm, w ) 10.5 nm, r ) 1.6). The corresponding absorption spectrum is similar to that of R ) 15
Figure 6. Simulated absorption spectra obtained by changing the ratio of spheres to nanodisks: 79% spheres, 8% nanodisks (solid line); 53% spheres, 31% nanodisks (dashed line); and 31% spheres, 50% nanodisks (dotted line).
(Figure 5C). Upon increasing R, the peak centered at 560 nm progressively decreases, whereas that centered at 650 nm simultaneously increases (Figure 5C). The peak centered at 560 nm is attributed to spherical and cubo-octahedral nanocrystals, whereas the peak centered at 650 nm is mainly due to nanodisks. This is rather consistent with the fact that the percentage of spheres progressively decreases from 79% to 31% and that of nanodisks and elongated particles increases from 18% to 60% upon increasing R from 3 to 15. Because it has not been possible to estimate the relative percentages of the various shapes of nanocrystals for R equal to 5, we cannot compare the simulated and experimental absorption spectra. However, to confirm the evolution of the optical response with the percentages of shapes, we consider in our simulations percentages between R ) 3 and R ) 15. We then consider 53% spheres (17 nm), 31% nanodisks (s ) 16 nm, TR ) 0, r ) 1.6), 10% elongated (L ) 17 nm, w ) 10.5 nm, r ) 1.6), and 6% cubic (13.6 nm) nanoparticles. Figure 6 clearly shows, by changing the relative ratio of spheres and nanodisks, that there is a change in the simulated spectra similar to that observed in Figure 5. IV. Conclusion From this study it is concluded that the experimental peak at 560 nm is due to the spheres and cubo-octahedra, whereas the
7212 J. Phys. Chem. B, Vol. 110, No. 14, 2006 peak at 650 nm is characteristic of copper nanodisks. By increasing the R value in the synthesis from 3 to 15, the percentage of nanodisks increases, whereas that of spheres decreases, leading to an inversion of the peak intensity of the two resonance bands centered at 560 and 650 nm. Hence, by controlling the relative proportions of nanodisks and spheres, it is possible to tune the optical properties. Nevertheless, the experimental resonance bands at 560 and 650 nm are quite broad due to the rather large size and shape distributions. From TEM observations and simulated and experimental spectra, it is clearly deduced that a slight change in shape introduces a marked change in the optical properties. This confirms previous data from which we claimed that the change in the optical properties during the nanocrystal growth is due to the particle shape.13 Acknowledgment. We thank Dr. I. Lisiecki for many fruitful discussions. References and Notes (1) Baletto, F.; Ferrando, R. ReV. Mod. Phys. 2005, 77, 371. (2) Pileni, M. P. Nat. Mater. 2003, 2, 145. (3) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (4) Pileni, M. P. Langmuir 1997, 13, 3266. (5) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murphy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedke, W. D.; Landman, U. AdV. Mater. 1996, 5, 428. (6) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266, 91. (7) Kreibig, U.; Vollmer, M. 1995 Optical Properties of Metal Clusters,Springer Series in Materials Science; Springer: Berlin, Germany, 1995; Vol 25, and references therein. (8) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677. (9) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (10) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (11) Courty, A.; Lisiecki, I.; Pileni, M. P. J. Chem. Phys. 2002, 116, 8074.
Salzemann et al. (12) Salzemann, C.; Lisiecki, I.; Urban, J.; Pileni, M. P. Langmuir 2004, 20, 11772 (13) Salzemann, C.; Lisiecki, I.; Brioude, A.; Urban, J.; Pileni, M.-P. J. Phys. Chem. B. 2004, 108, 13242 (14) Pileni M. P. J. Exp. Nanosciences 2006, in press. (15) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (16) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (17) Puntes, V. F.; Kristhnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (18) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Car, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Comm. 2001, 7, 617. (20) Jana, N. R.; Gearheart, L,; Murphy, C. J. J. Phys. Chem. B. 2001, 105, 4065. (21) Salzemann, C.; Urban, J.; Lisiecki, I.; Pileni, M. P. AdV. Funct. Mater. 2005, 15, 1277. (22) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (23) Lisiecki, I. et al. J. Am. Chem. Soc. 1993, 115, 3887; J. Phys. Chem. 1996, 100, 4160. (24) Copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2 is prepared by ion exchange with the sodium salt as described in Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (25) (a) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (b) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (26) Jin, R.; Cao, Y.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (27) Brioude, A.; Pileni, M. P. J. Phys. Chem. B. 2006, in press. (28) Brioude, A.; Jiang, X.; Pileni, M. P. J. Phys. Chem. B. 2006, in press. (29) Purcell, E. M.; Pennypacker, C. R.; Purcell, E. M.; Pennypacker, C. R. Astrophys. J. 1973, 186, 705. (30) (a) Draine, B. T.; Goodman, J. J. Astrophys. J. 1993, 405, 685. (b) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, 1491. (c) Goodman, J. J. Draine, B. T. Flatau, P. J. Opt Lett. B. 1991, 16, 1198. (31) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M. P.; J. Urban, Phys. ReV. B 2000, 67, 4968. (32) Germain, V.; Brioude, A.; Ingert, D.; Pileni, M. P. J. Chem. Phys. 2005, 122, 124707.
