pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis of Small Atomic Copper Clusters in Microemulsions Carlos Vazquez-Vazquez,*,† Manuel Ba~nobre-Lopez,† Atanu Mitra,†,§ M. Arturo Lopez-Quintela,*,† and Jose Rivas‡ †
Facultade de Quı´mica, Departamento de Quı´mica Fı´sica, Avenida das Ciencias, s/n, 15782 Santiago de Compostela, Spain, and ‡Facultade de Fı´sica, Departamento de Fı´sica Aplicada, Campus Sur, 15782 Santiago de Compostela, Spain. § Present address: Department of Chemistry, Sree Chaitanya College, Habra, West Bengal, India Received January 9, 2009. Revised Manuscript Received June 1, 2009 We report evidence of the formation of small atomic copper clusters, Cun, by the microemulsion technique, and how their size can be controlled by adjusting the percentage of the reducing agent used. Copper clusters were characterized by UV-visible spectrophotometry and atomic force microscopy. Photoluminescent copper clusters, Cun, with n j 13, can be obtained using very low percentages of the reducing agent (0.10. When increasing R, a red-shift in the two absorption bands is obtained. When enough reducing agent is added, that is, for R > 0.50, the second band broadens and shifts to ∼560 nm, which is the reported value for the copper plasmonic band.18 For R g 0.60, the colloidal solution is rather turbid and it is difficult to determine the position of the absorption bands. For R g 0.35, λ1max reaches a constant value of ∼390 nm; and for 0.15 e R e 0.30, λ2max also shows a constant value of ∼460 nm. However, for R > 0.30, λ2max shifts to larger wavelengths and finally reaches a limiting value of ∼560-570 nm, a wavelength that corresponds to the plasmonic band of copper. All these features are clearly shown in Figure 2a, where the absorption maxima are plotted as a function of R. The peak positions were determined by Lorentzian fitting. Figure 2b shows the absorbance ratios at selected wavelengths (295, 390, 460, and 560 nm) plotted as a function of R. In this graph, the evolution of the copper clusters can be clearly seen. In addition, smaller clusters act as seeds and become larger. For R e 0.25, the absorbance at longer wavelengths is much smaller compared to that at shorter wavelengths, indicating the presence of small clusters mainly. If we compare the absorbance ratios at consecutive wavelengths, it is evident that different species are present all along the synthesis: copper clusters responsible for the absorption band at 295 nm transform to larger clusters (with absorption band centered at 390 nm), these ones become larger and give an absorption band centered at 460 nm, and then these clusters finally turn into copper nanoparticles, showing a plas-
(37) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1994, 10, 4726. (38) Zhao, M.; Sun, L; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877.
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monic band at 560 nm. The clusters’ evolution is proved when we observe the steps in the curve Abs(560 nm)/Abs(295 nm). In order to study the UV-visible changes at low R values in a more detailed way, a new set of experiments were carried out for R e 0.20, using 0.5% (w/v) freshly prepared NaBH4 aqueous solution (Figure 1b). This detailed experiment shows a more complex behavior, with UV-visible absorption bands centered at approximately 295, 330, 360, 420, 480, and 580 nm. These absorption bands can be related to different copper clusters, as it will be discussed later. To get a more direct evidence of the existence of these small clusters, several aliquots of 5 μL of the microemulsion (in which R ≈ 0.05, 0.10, 0.20, 0.30, and 0.40) were deposited on mica surfaces for AFM study. Then the samples were washed thoroughly with Milli-Q water and dried under nitrogen atmosphere. Figure 3 shows the AFM analyses of the samples with low R values (R ≈ 0.05 (a), 0.10 (b), and 0.20 (c)), and Figure 4 shows the AFM analyses of the samples at intermediate R values (R ≈ 0.30 (a) and 0.40 (b)). In these figures, we can see the AFM images, the corresponding section analysis showing the height profiles of some clusters, and the size histograms obtained after counting a large number of particles (N). These histograms were fitted to log-normal distribution functions. In the case of R ≈ 0.05, the size histogram is completely different and only an average value has been calculated. For R ≈ 0.05 (Figure 3a), very small clusters are observed throughout the image and the section analysis of the AFM line profiles. The corresponding size histogram shows the predominance of copper clusters of ca. 0.2-0.4 nm in size, with an average value of 0.29 ( 0.08 nm. This value is very close to the atomic size of copper atoms (see the Discussion section). For R ≈ 0.10 (Figure 3b), the clusters have grown in size and most of them are in the size region between 0.4 and 0.8 nm. The size histogram was fitted to a log-normal distribution to obtain the center (xc = 0.75 ( 0.05 nm) and the width (w=0.39 ( 0.07 nm) of the distribution. For R ≈ 0.20 (Figure 3c), the main percentage of them has shifted to the size region between 0.6 and 0.8 nm. The size histogram was fitted to a log-normal distribution, and the center of the distribution was close to the previous one: xc=0.77 ( 0.05 nm. The width of the distribution was w=0.34 ( 0.06 nm. For R ≈ 0.30 (Figure 4a), the main percentage of them has shifted to the size region between 1.0 and 1.2 nm. The size histogram was fitted to a log-normal distribution, and the center and the width of the distribution were xc=1.17 ( 0.03 nm and w = 0.33 ( 0.03 nm, respectively. For R ≈ 0.40 (Figure 4b), a broad range of sizes was observed with almost the same relative frequency, between 1.0 and 3.2 nm. The size histogram was fitted to a log-normal distribution, and the center and the width of the distribution were xc =2.14 ( 0.16 nm and w=0.40 ( 0.11 nm, respectively. If we check carefully the size histograms, it is observed that the percentage of larger clusters is larger as the R value increases. For example, the percentage of clusters that show height above 1.2 nm is 8.8% in the case of R ≈ 0.10, 17.1% for R ≈ 0.20, 48.4% for R ≈ 0.30, and 89.1% for R ≈ 0.40. This result indicates that we have a mixture of copper clusters and that the larger clusters are formed at the expense of the growth of smaller clusters. Figure 5 shows a TEM micrograph corresponding to the colloidal copper solution collected at the end of the reduction, where a large excess of reducing agent was added: R=5.65. The particle shape is almost spherical, and the TEM size distribution histogram was fitted to a Gaussian profile. The average size of the copper nanoparticles is xc=2.89 ( 0.02 nm, and the width of the distribution is w=1.06 ( 0.04 nm. Langmuir 2009, 25(14), 8208–8216
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Figure 1. (a) UV-visible absorption spectra at different R values. Vertical lines show the evolution of the bands with the addition of reducing agent. (b) UV-visible absorption spectra at small R values, with R e 0.20.
Figure 2. (a) Maxima of absorption bands as a function of the relative NaBH4 added, R. The vertical dashed line shows the appearance of the second absorption band. (b) UV-visible absorbance ratios at different wavelengths as a function of the R values.
In addition, we have also studied the fluorescence of the copper clusters at different stages of the reduction (Figure 6). We have observed that only some kind of clusters show fluorescent properties, in particular, only the smaller ones. If we excite the sample at λexc=290 nm, fluorescence emission centered at λemi=333 nm is observed. In addition, several shoulders are present in the excitation and emission spectra, indicating that not only one species is the responsible of the fluorescence. When more reducing agent is added, the fluorescence decreases and finally disappears at R>0.25. This fact indicates that only small clusters could be responsible for the fluorescence. When the samples were excited at longer wavelengths, no fluorescence was observed, indicating that larger copper clusters are not fluorescent. Fluorescence was measured after several months in order to check the stability of the clusters. In the case of the sample with R=0.061, the photoluminescence intensity was kept almost invariant after 6 months, and only a slight change in the relative intensities of the emission peaks of Figure 6 (attributed to small transformations among the fluorescent clusters) is observed.
Discussion Geometrical and electronic closed-shell structures have been used to explain the origin of especially stable metal clusters (magic numbers).30 Although electronic effects seem to predominate in gas metal clusters,39,40 the situation is not clear for metal clusters (39) Haberland, H. Clusters of Atoms and Molecules Springer: Berlin, 1994. (40) Ellis, D. E. In Physics and Chemistry of Metal Cluster Compounds: Model Systems for Small Metal Particles; Jongh, L. J., Ed.; Kluwer Academic: Dordrecht, The Netherlands, 1994; Chapter 4.
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prepared by wet-chemical techniques. At this moment, it seems that there is an agreement for the predominance of electronic effects in thiol-protected Au clusters. For example, H€akkinen has recently shown that most of the thiol-protected Au clusters obtained so far can be described by the formula n*=NνA-M - z, where n* is the shell-closing electron count of the metallic core, N is the number of the core metal atoms A, νA is the atomic valence, M is the number of electron-localizing (or electronwithdrawing) ligands (assuming here a withdrawal of one electron per each ligand), and z is the overall charge on the complex (LsANXM)z (with Ls being the weak ligands that may be required for completion of the steric protection of the core surface).41 This formula gives rise to the corresponding electronic close-shell structures of clusters having 2, 8, 18, 34, 58, 92, 138, ... electrons. However, for Au clusters protected with other “weaker” capping molecules, the predominance of the electronic effects is not clear. For example, geometrical effects are dominant in phosphineprotected Au clusters, with the case of Au55 being the most studied one.42-45 It seems then that the strong S-Au bond can “favor” the electronic effects. This could be a possible explanation of the totally empirical so-called thiol “etching” of clusters: a kind of reorganization and splitting of clusters when they are aged in the presence of thiols, which was very nicely shown by Tsukuda and (41) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gr€onbeck, H.; H€akkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157. (42) Schmid, G. Chem. Rev. 1992, 92, 1709. (43) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, 3081. (44) Sawitowski, T.; Franzka, S.; Beyer, N.; Levering, M.; Schmid, G. Adv. Funct. Mater. 2001, 11, 169. (45) Torma, V.; Reuter, T.; Vidoni, O.; Schumann, M.; Radehaus, C.; Schmid, G. Chem. Phys. Chem. 2001, 8/9, 546.
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Figure 3. Continued. 8212 DOI: 10.1021/la900100w
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Figure 3. AFM images of copper clusters at small R values: (a) 0.05, (b) 0.10, and (c) 0.20. The section analysis of solid lines (red and green) in the image is given on the right of the image. The cluster size distribution is given below the AFM image (N is the number of specimens used for the histograms), and the results were fitted to log-normal distribution functions (except for R ≈ 0.05).
co-workers.30,46-49 The case of Ag and Cu clusters is however totally unknown. In these cases, even the influence of electronic effects for thiol-protected clusters has not been reported. In our case, the sizes obtained by AFM and TEM can be correlated to different copper clusters if the closed-shell cluster model is considered. The interatomic distance in copper nanocrystals (Fm3m, cubic lattice parameter a = 3.6149 A˚) at room temperature is 2.556 A˚.50 Therefore, taking into account the closed-shell cluster model, the different cluster sizes can be estimated. The number of copper atoms forming the first six closed shells is 13, 55, 147, 309, 561, and 923 for the cuboctahedral and icosahedral models.51 Table 1 summarizes the diameters for the different closed-shell clusters. Theoretical calculations support the fact that, in the size range 10 e n e 55, most of the clusters adopt the icosahedral structure which can be derived from the 13-atom icosahedron and the 55-atom icosahedron by adding or removing atoms.52 In addition, comparing the total energies of optimized structures of (46) Tsunoyama, H.; Nickut, P.; Negishi, Y.; Al-Shamery, K.; Matsumoto, Y.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4153. (47) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (48) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464. (49) Shichibu, Y.; Negishi, Y.; Tsunoyama, H.; Kanehara, M.; Teranishi, T.; Tsukuda, T. Small 2007, 3, 835. (50) JCPDS-IUCr, PDF file no. 04-0836. (51) Koga, K.; Tadeo, H; Ikeda, T.; Ohshima, K. Phys. Rev. B 1998, 57, 4053. (52) Kabir, M.; Mookerjee, A.; Bhattacharya, A. K. Phys. Rev. A 2004, 69, 043203.
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icosahedral and cuboctahedral Cun (n=55, 147, and 309), it was reported that icosahedral copper clusters are more stable.53 When comparing these cluster sizes with the AFM size histograms, it can be seen that at R ≈ 0.05 (Figure 3a) the main clusters present have only one copper atom height, which points to the presence of small planar clusters Cun (n < 13). Further additions of reducing agent favor the formation of closed-shell structures rather than planar ones. For R ≈ 0.10 (Figure 3b), the main clusters present are formed by 1 and 2 shells: Cu13 and Cu55, with Cu13 being the most abundant one. For R ≈ 0.20 (Figure 3c), it is observed in addition the presence of cluster formed by 3 shells (Cu147). For R ≈ 0.30 (Figure 4a), it is observed that the center of the distribution is shifted to almost 1.2 nm, indicating that the most abundant cluster is Cu55; in addition, the presence of clusters formed by 4 shells (Cu309) is pointed out by the increase of the relative frequencies between 2.0 and 2.6 nm. When R is increased from 0.10 to 0.30, the relative abundance among different clusters changes and the relative frequency of Cu13 decreases with regard to the larger ones, Cu55 and Cu147. In the case of R ≈ 0.40 (Figure 4b), the relative frequencies are more important in the range 1.6-2.6 nm, indicating that the most abundant clusters are the formed by 3 and 4 shells (Cu147 and Cu309, respectively). On the other hand, the presence of a small amount of copper clusters at large sizes in the AFM pictures can be explained by the aggregation of smaller clusters when drying the drop deposited on (53) Taneda, A.; Kawazoe, Y. Mater. Trans., JIM 1999, 40, 1255.
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Figure 4. AFM images of copper clusters at intermediate R values: (a) 0.30 and (b) 0.40. The section analysis of solid lines (red and green) in the image is given on the right of the image. The cluster size distribution is given below the AFM image (N is the number of specimens used for the histograms), and the results were fitted to log-normal distribution functions.
the mica surface (e.g., sizes above 2.2 nm for R ≈ 0.20 in Figure 3c). This assumption is supported by the absence of a plasmonic band in the UV-visible spectra at such low R values (Figure 1a). 8214 DOI: 10.1021/la900100w
Comparing further the theoretical cluster sizes with the TEM sizes (Figure 5), it can be determined that at the end of the reduction the clusters have 5 closed shells (Cu561), Langmuir 2009, 25(14), 8208–8216
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Figure 5. TEM micrograph and size histogram of Cu nanoparticles at the end of the reduction. Sample size: 200 nanoparticles.
Figure 6. Fluorescence of the copper clusters at different stages of reduction. Left curves are the excitation spectra that show emission at 333 nm, while the right curves are the emission spectra when an excitation wavelength of 290 nm is used. Inset: Picture taken when the sample with R = 0.061 (stored in the refrigerator during 6 months) was excited with light of wavelength λ=310 nm. Table 1. Copper Cluster Diameters for Different Closed-Shell Cuboctahedra or Icosahedra closed shells
no. of atoms
diameter (nm)
1 2 3 4 5 6
13 55 147 309 561 923
0.7668 1.2780 1.7892 2.3004 2.8116 3.3228
although a small amount of Cu309 and Cu923 clusters are also present. Taking into account that the AFM results show the presence at low R values (0.10 e R e 0.20) mainly of Cu13, Cu55, and Cu147 clusters (i.e., 1, 2, and 3 shells, respectively), the fact that no plasmonic band is observed at R ≈ 0.30-0.40, and that the TEM results (measured at the end of the reaction) match mainly with Cu561 (clusters of 5 closed shells), we can conclude then the plasmonic band appears for the copper clusters of 309 atoms (Cu309), that is, those corresponding to 4 closed shells. As a consequence, the UV-visible spectra evolution (Figure 2) can be explained if we consider that λ3¥ corresponds to copper clusters of 309 atoms (Cu309, which can be considered as “conventional” copper nanoparticles); λ2¥ corresponds to Cu147; and λ1¥ to Cu55. This allows us to explain the fact that the second absorption band is not observed if the first one is not present; that Langmuir 2009, 25(14), 8208–8216
is, it means that the presence of smaller clusters is a need for the existence of larger ones, as it can be observed in Scheme 1. Following this cluster size assignation, the presence of Cu13 could be related to the absorption band that appears at 360 nm, while the absorption bands at shorter wavelengths (330, 295 nm,...) could be assigned to smaller clusters, Cun (n
