CRYSTAL GROWTH & DESIGN
Selective Synthesis of Gold Cuboid and Decahedral Nanoparticles Regulated and Controlled by Cu2+ Ions
2008 VOL. 8, NO. 3 906–910
Jianhua Sun,†,‡ Mingyun Guan,† Tongming Shang,‡ Cuiling Gao,† Zheng Xu,*,† and Jianmin Zhu§ State Key Laboratory of Coordination Chemistry, School of Chemistry and Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, Jiangsu Key Laboratory of Precious Metals Chemistry, Jiangsu Teachers UniVersity of Technology, Changzhou 213001, P. R. China, and Laboratory of Solid State Microstructures, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed July 10, 2007; ReVised Manuscript ReceiVed October 31, 2007
ABSTRACT: It is found that Cu2+ is an effective agent for the controlled preparation of shaped gold nanoparticles. As the concentration of Cu2+ increases from 0 mM to 0.2 mM to 1.6 mM, the shape of the gold nanoparticles changes from rod to cuboid to decahedron. A possible mechanism based on selectively retarding the growth rate of the {111} plane is proposed. Introduction In the past decades, the synthesis of metal nanoparticles has been an intense research subject owing to their potential application in catalysis, photography, electronics, optoelectronics, plasmonics, information storage, optical sensing, biological labeling, imaging, and surface-enhenced Raman scattering.1 The intrinsic properties of metal nanoparticles can be tuned by controlling their size, shape, crystallinity, and structure. Recently, shape control has received considerable attention, because different crystal surfaces may possess different physical and chemical properties, such as different surface atom densities and electronic structures; therefore, they can exhibit quite different chemical reactivities.2,3 So, establishing a reliable method for producing nanoparticles with the desired shape and facet has been an important task. To date, shape-controlled syntheses have been reported for a number of metals and alloys, including Co, Ag, Au, Pt, Pd, Rh, Ir, and FePt, most of them involving the reduction of a salt or the thermal decomposition of organometallic4 precursors assisted with surfactant, polymers, biomolecules, and coordination ligands. The seed-mediated growth method is one of those successful synthesis methods. Gold nanorods, nanocubes, and nanostars have been prepared with this method.5–7 However, there is little work in the literature on the synthesis of shaped gold nanoparticles with sizes less than 50 nm. Herein, we report a shape-controlled synthesis of gold nanoparticles with sizes of 20–40 nm. By adjusting the amount of Cu2+, cuboid and penta-twinned decahedron structures were selectively fabricated with a high yield and a good size uniformity. On the basis of the structure analysis results of the gold nanoparticles and the evaluation of the vis-NIR spectrum evolution with the reaction time, the role played by Cu2+ was proposed.
appeared as a brownish-yellow color. The solution was stirred for another 2 min and kept at 25 °C for 4 h before being used. This was called the seed solution. Cuboid Nanoparticles. A CTAB solution (20.00 mL, 0.020 M) was mixed with 5.00 mL of 2.00 × 10-3 M HAuCl4 at 25 °C, into which 50.0 µL of 0.010 M CuSO4 was added. Then, 3.00 mL of a 0.10 M freshly prepared ascorbic acid (AA) solution was added. The solution turned from orange to colorless. This was called the growth solution. Then, 5.0 µL of the seed solution was added into the growth solution at 25 °C. The color of the solution gradually changed to violet-red within 10–30 min. The solutions were centrifuged at 12 000 rpm for 10 min and washed with deionized water twice. The precipitates were dispersed in deionized water for further measurements. Decahedral Nanoparticles. A procedure similar to the one for the synthesis of the cuboid nanoparticles was used, except that 400 µL of 0.010 M CuSO4 was used instead of 50.0 µL. Characterization. All transmission electron microscopy (TEM) images were obtained by using a JEOL model JEM-200CX microscope at an acceleration voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) measurements were conducted with an EDS system attached to the same microscope. The samples for the TEM and EDS studies were prepared by drying a drop of the aqueous suspension of particles on carbon-coated copper grids or nickel grids under ambient conditions. The field-emission scanning electron microscopy (FESEM) measurements were made on a JEOL model JESM-7001F field-emission microscope. High-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were taken on a JEM-4000EX high-resolution transmission electron microscope using an accelerating voltage of 400 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD-3A X-ray diffractometer equipped with a Cu KR radiation source (0.15417 nm). The vis-NIR absorption spectra of the prepared solutions were collected on a Shimadzu UV-3100 spectrophotometer.
Results and Discussion The reactions are as follows: NaBH4
Au3+ - CTAB 98 Auseed (seed solution)
(1)
Experimental Section Seed Solution (Synthesis of Gold Nanoparticles 90%, Figure 2A) and decahedral nanoparticles (∼70%, Figure 3A) were produced, respectively. In addition to controlling the shape of the gold nanoparticles, Cu2+ ions also slow down the rate of the reduction reaction. It takes 20 min for the growth solution to turn from colorless to violet-red at a Cu2+ concentration of 0.20 mM and ∼40 min at 1.60 mM, whereas the reaction takes less than 10 min in a Cu2+free control experiment. Obviously, Cu2+ ions play a critical role in controlling the reduction kinetics and, therefore, the morphology of the resulting gold nanoparticles. High-resolution TEM images of those gold seeds are shown in Figure 1. The diameters of the seeds are smaller than 5 nm. The clear lattice fringes indicate the single-crystalline nature of the gold seeds without any twin structure. The measured interplanar space for all the lattice fringes is 2.35 Å, which agrees with the {111} lattice plane of face-centered-cubic (fcc) gold. Figure 2A shows a typical TEM image of the sample prepared by adding 50 µL of 0.01 M CuSO4 solution. From this image, it is clearly seen that the as-prepared sample mainly consists of cuboid nanoparticles with a 20–40 nm edge-length. Their surfaces are smooth, but the corners and edges of these cuboids are slightly truncated. Furthermore, they self-assembled into ordered 2D arrays on a copper grid when the TEM specimen was prepared, because of the molecule interaction of the surfactants absorbed on the surface of the cuboids combined
Crystal Growth & Design, Vol. 8, No. 3, 2008 907
with their homogeneous size and shape. The stereo image was further determined by using FESEM (see Figure S2). The XRD spectrum in Figure 2E exhibits five diffraction peaks which can be indexed to the {111}, {200}, {220}, {311}, and {222} of fcc gold and indicate that the sample is composed of pure crystalline gold. It is worth noting that the intensity ratio of the {200} and {111} diffraction peaks in the XRD pattern is 0.72, which is 1.4-fold bigger than the value of a conventional powder sample (Joint Committee on Powder Diffraction Standards file no. 04-0784). The result is reasonable because the cuboids are bounded by enlarged {100} facets, which preferentially orient parallel to the substrate. This is confirmed by the TEM images. The microstructure of the gold cuboids was further characterized by HRTEM and SAED. The fringes in Figure 2D are separated by 2.00 Å, which can be attributed to the {200} lattice plane (2.04 Å) of fcc gold. The lattice planes continuously extend to the whole particles without stacking faults or twins, indicating their single-crystal nature. Figure 2C shows the corresponding SAED pattern obtained by directing the electron beam perpendicularly to one of the square faces of a cuboid (Figure 2B). The square symmetry of this pattern indicates that each gold cuboid is a single crystal bounded by {100} facets. Figure 3A shows a typical TEM image of the sample prepared by adding 400 µL of 0.01 M CuSO4 solution, in which 70% of the nanoparticles are decahedrons with a 20–30 nm edge-length. The decahedron is one of the basic multiply twinned structures, which consists of five tetrahedrons with {111} surfaces. The XRD pattern in Figure 3E exhibits five diffraction peaks corresponding to the {111}, {200}, {220}, {311}, and {222} surfaces of fcc gold, which indicate that the products are composed of pure crystalline gold. Figure 3D shows the HRTEM image of an individual decahedral nanoparticle. Five triangular facets of the gold decahedron can be clearly observed, wheras the other five are hidden in the back. This can be seen from the dark-field TEM image with the super contrast shown in Figure S3. The measured interplanar space for all the lattice fringes is 2.35 Å, which agrees with the {111} lattice plane of fcc gold. Figure 3C shows the corresponding SAED pattern with the electron beam directed along the 5-fold axis of the decahedron. Three sets of spots can be identified on the basis of the d-spacing: the outer set, with a lattice spacing of 1.44 Å, can be indexed to the {022} Bragg reflection; the middle set, with a spacing of 2.35 Å, is believed to originate from the {111} reflection; and the inner set, with a spacing of 2.00 Å, results from the {200} reflection of fcc gold. The complex diffraction pattern can be well interpreted by five twinned subunits with their common {110} axis along a 5-fold axis. Any of the adjacent subunits with an angle of 36° (180°/5) implies that the five triangular facets of the gold nanoparticles are all {111} facets. On the basis of these results, it is obvious that the gold decahedron is bounded by 10 {111} facets. In order to gain insight into the role of Cu2+ in the formation of the shaped gold nanoparticles, we carried out a series of experiments to study the influence of Cu2+ on the gold nanoparticles growth, in which the surface plasmon resonance (SPR) bands in the vis-NIR region were used to monitor the seed-mediated growth process, because the SPR of gold nanoparticles is sensitive to their size and shape. Vis-NIR spectra of gold nanoparticles with different shapes are shown in Figure 4A, which displays distinguishable SPR peaks: 544 and 836 nm for the transversal and longitudinal resonance modes of nanorods, 628 nm for the nanocuboid, and 558 nm for the decahedron. The spectral features of those three
908 Crystal Growth & Design, Vol. 8, No. 3, 2008
Sun et al.
Figure 2. (A) TEM image of gold cuboid nanopartilces. (B) and (C) TEM image and corresponding SAED pattern of an individual gold cuboid nanoparticle. (D) HRTEM image of the nanoparticle shown in (B). (E) XRD pattern (with Cu KR radiation) of the as-synthesized gold nanoparticles.
Figure 3. (A) TEM image of gold decahedral nanopartilces. (B) and (C) TEM image and corresponding SAED pattern of an individual gold decahedron. (D) HRTEM image of the nanoparticle shown in (B). (E) XRD pattern (with Cu KR radiation) of as-synthesized gold nanoparticles.
gold nanoparticles with different shapes are fairly consistent with previous experiments and theoretical simulations.2e,4d,8 It can be seen that, when the concentration of Cu2+ is increased from 0.0 mM to 0.2 mM, the longitudinal plasmon band disappears, indicating a transition trend from anisotropic to isotropic growth. Panels B and C of Figure 4 show the plots of the SPR absorbance of the nanoparticles and the maximum absorbance wavelength versus the reaction time at various Cu2+ ion concentrations, respectively (the absorption-spectra changes with
the reaction time are shown in Figure S4). On the basis of these two plots, we can conclude that the Cu2+ ions not only affect the reduction rate of Au+ to Au but also change the growth rate of the different facets. When Cu2+ ions are absent, the absorbance (Figure 4B, curve 1) increases quickly. At the same time, the SPR band red-shifts from 642 to 850 nm (Figure 4C, curve 1), indicating that the gold nanoparticles grow quickly, and the aspect ratio increases, indicating the 1D growth of the gold nanorods in this case.
Synthesis of Gold Cuboid and Decahedron
Crystal Growth & Design, Vol. 8, No. 3, 2008 909
Figure 4. (A) Vis-NIR spectra of the three different shapes of the gold nanoparticles prepared at various Cu2+ ion concentrations. (B) Plots of the SPR absorbance of nanoparticles versus reaction time at various Cu2+ ion concentrations (the monitored wavelengths are maximum absorbance wavelengths: 836, 628, and 558 nm for samples 1–3, respectively). (C) Plots of the maximum absorbance wavelength versus reaction time at various Cu2+ ion concentrations (0.0, 0.2, and 1.6 mM for samples 1–3, respectively).
However, when Cu2+ (0.2 mM) is added, the increase of absorbance distinctly slows down (Figure 4B, curve 2), and the SPR band red-shifts from 575 to 637 nm, without any change of the longitudinal plasmon band (Figure 4C, curve 2), which suggests that the reduction rate slows down and the 1D growth scarcely occurs. The gold cuboids with an aspect ratio less than 1.2 are formed in this case. When the concentration of Cu2+ increases, the growth rate of the gold nanoparticles is further slowed down, and the gold nanoparticles growth process takes more than 60 min at 1.6 mM Cu2+ (Figure 4B, curve 3). During the growth, the SPR wavelength is almost constant, indicating that the aspect ratio of the nanoparticles remains constant (Figure 4C, curve 3). It is commonly accepted that the shape of the fcc nanoparticles is mainly controlled by the ratio (R) of the growth rate along the {100} direction (G{100}) versus the growth rate along the {111} direction (G{111}) (R ) G{100}/G{111}).3,9 Tetrahedrons and decahedrons bound by the most stable {111} planes will be formed when R is about 1.73, and perfect cubes bound by the less stable {100} planes will be formed when R is about 0.58.4d,11 Therefore, the shape of the fcc nanoparticles can be controlled by adjusting the R value. Usually, the surfactant can play this role. Recently, the influence of silver ions on the growth of gold nanoparticles was found.7,10,11 Two growth mechanisms were proposed to explain the role played by Ag+ ions, 10,11 but they are not suitable for our case. We consider that the presence of Cu2+ ions along with CTAB in the solution might have significantly affected the surface energy of the gold various crystal facets because of the preferential adsorption of Cu2+ and CTAB on the different facets therefore affecting the growth
rate of the various facets during the crystal growth. The presence of copper on the surface of the gold nanoparticles is confirmed by the EDS analysis. Panels A and B of Figure S5 show the EDS spectra of the cuboid and the decahedron after extensive washing with deionized water, respectively. The spectra display strong peaks for gold and weak peaks for copper (carbon-coated nickel grids are used to support the samples), whereas no signal for bromine is detected. The molar ratio (Cu:Au) of the cuboid (1:19) is lower than the molar ratio of the decahedron (1:8). This result indicates that copper is preferentially binding to the surface of gold, and the more Cu2+ ions are added to the growth solution, the more copper particles bind to the surface of gold particles. As described above, the fine seeds produced in the presence of CTAB with a high concentration are faceted with the most stable {111} faces solvent accessible, because CTAB molecules appear to bind more strongly to the {100} than to the {111} faces.12 Thus, a higher CTAB concentration without Cu2+ ions favors the deposition of Au0 onto the {111} faces, leading to their appearance. On the other hand, multi-twinned particles including 5-fold twinned decahedral nanoparticles have been found in many fcc metals. It has been demonstrated that, in fcc metallic structures, the symmetry breaking by twinning introduces an anisotropy in the shape of the nanoparticles. This, coupled with the preferential binding of CTAB to {100} faces, leads to the formation of nanorods.13 When introduced into the growth solution, Cu2+ ions preferentially bind to the {111} faces because they are solvent-accessible and stop the growth in this direction in varying degrees depending on the concentrations of Cu2+ ions. Therefore, cuboids and decahedrons are obtained.
910 Crystal Growth & Design, Vol. 8, No. 3, 2008
Sun et al.
Acknowledgment. This work was supported by the National Nature Science Foundation of China (Grants. No. 90606005, No. 20490210, No. 20571040, and No. 20371026), Supporting Information Available: TEM, SAED, and HRTEM images of gold nanorods; FESEM image of gold cuboid nanopartilces; dark-field TEM image of the gold decahedral nanoparticles; vis-NIR absorption spectra of gold nanoparticles solution change with time; and EDS images taken form gold cuboids and decahedrons. This information is available free of charge via the Internet at http://pubs.acs.org.
References Figure 5. Schematic illustrating the shape control of gold by changing the concentration of Cu2+. R ) G{100}/G{111}.
For example, when the concentration of Cu2+ is 0.2 mM, Cu2+ ions are absorbed selectively on the {111} facets, and the growth rate of the {111} direction is decreased to some extent but is still faster than that of the {100} direction. In this case, R increases to ∼0.58, and the area of the {111} facet decreases, whereas that of the {100} facet increases and finally the gold cuboid is formed. When the concentration of Cu2+ increases to 1.6 mM, the growth rate of the {111} direction is further slowed down; therefore, R is further increased (∼1.73). As the growth rate in the {100} direction is larger than that in the {111} direction, the {111} plane enlarges; then, the tetrahedron bounded by the most stable {111} planes is formed. Multiple twinning in gold often occurs for particles above 8 nm in size by the coalescence of the primary particles with tetrahedral morphology, which leads to the formation of decahedrons.12,13 Conclusion In summary, we have demonstrated a new strategy to control the shape of gold nanoparticles by Cu2+ and CTAB preferential adsorption on the different facets. Gold cuboid and decahedral nanoparticles with a good uniformity (size of 20–40 nm) and a high yield have been synthesized at room temperature by introducing foreign Cu2+ ions into the growth solution. On the basis of selectively retarding the growth rate of the {111} direction, a preliminary mechanism has been proposed to account for the shape evolution of gold nanoparticles. The successful preparation of shaped gold nanoparticles examplifies the good shape control that can be achieved by carefully regulating the growth rate along the different crystallographic directions, and this method could be extended to the synthesis of shaped nanoparticles of other material systems.
(1) (a) Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; VCH: Weinheim, 1998. (b) Nanoscale Materials in Chemistry; Klabunde, K. J., Ed.; VCH: Weinheim, 2001. (c) Schmid, G. Chem. ReV. 1992, 92, 1709. (d) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (2) (a) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (b) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853. (c) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 5726. (d) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. ReV. B 2001, 64, 235402. (e) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (f) Muniz-Miranda, M. Chem. Phys. Lett. 2001, 340, 437. (g) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2154. (3) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (4) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (b) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (c) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (d) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673. (e) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (f) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (g) Chen, Y.; Gu, X.; Nie, C. G.; Jiang, Z. Y.; Xie, Z. X.; Lin, C. J. Chem. Commun. 2005, 4181. (5) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389–1393. (c) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (d) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (6) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (7) (a) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (b) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (8) (a) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755. (b) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (c) Yang, W. H.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1995, 103, 869. (9) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192. (10) Orendorff, C. J.; Murphy, C. J. J. Phys. Chem. B 2006, 110, 3990. (11) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (12) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (13) Yagi, K.; Takayanagi, K.; Kobayashi, K.; Honjo, G. J. Cryst. Growth 1975, 28, 117.
CG070635A
