J. Phys. Chem. C 2008, 112, 11661–11666
11661
Hydrothermal Synthesis of Free-Floating Au2S Nanoparticle Superstructures Chi-Liang Kuo and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: May 6, 2008; ReVised Manuscript ReceiVed: June 3, 2008
We describe the formation of gold sulfide (Au2S) nanoparticle superstructures via hydrothermal synthesis approach at 175 °C using HAuCl4 and Na2S reagents and cetyltrimethylammonium bromide (CTAB) surfactant. Uniform 2-4 nm Au2S nanoparticles were found to assemble into a densely packed lamellar phase structure, and the resulting material displays a cross-sheet-like morphology. The sheets are 125-250 nm in length and can be suspended in solution. XRD, TEM, and EDS characterization of the nanoparticle samples confirmed the composition of the nanoparticles as Au2S. Some isolated faceted gold nanoparticles were also observed. XPS data also support the formation of Au2S nanoparticles. UV-vis absorption spectra showed only absorption features from the Au2S nanoparticles, and an indirect band gap value of 1.77 eV was obtained. Sufficiently high concentrations of Na2S and CTAB in the reaction mixture were concluded to be necessary to promote the growth of Au2S nanoparticles, and reduce the production of gold nanocrystals. Maintenance of the superstructures after boiling in water suggests the absence of CTAB molecules on the particle surfaces. FTIR, XPS, and NMR results indicate thermolysis of CTAB, and long alkyl chains interact with the Au2S nanoparticles and direct their assembly into a lamellar phase structure. Introduction Many studies have focused on the synthesis of metal chalcogenide nanoparticles and their assembled structures in the past few years. Hence, methods for the preparation of monodisperse Ag2S and Cu2S nanoparticles with some degree of shape control are available.1,2 In contrast to the more abundant reports on the formation of Ag2S and Cu2S nanocrystals, very few studies have addressed the synthesis of Au2S nanoparticles and verified their existence. Pure and dispersed Au2S nanoparticles have exclusively been produced by using Au(I) complexes such as Na3Au(SO3)2 and KAu(CN)2.3–5 Au2S has also been shown to exhibit p-type semiconductivity.6 Formation of goldcoated Au2S nanoparticles via the reaction of HAuCl4 and Na2S at room temperature has been proposed to explain the observation of evolution of an near-infrared absorption band during their synthesis.7–9 Zhang and co-workers conducted extensive examinations of the products formed in this reaction, and disputed against the formation of Au2S/Au core/shell structures, concluding that only Au nanoparticles were generated.10,11 The above studies suggest that it remains a significant challenge to prepare Au2S nanoparticles by using the typical Au(III) complex. Thus, a major purpose of this work is to find reaction conditions which favor the formation of gold sulfide nanostructures with use of HAuCl4 as the gold source. Another interesting aspect of the metal sulfide nanoparticle research is the construction of three-dimensional (3D) superstructures or superlattices via the assembly of these sulfide nanocrystal building blocks. Spherical Ag2S nanoparticles have been demonstrated to self-assemble into colloidal spheres,12 tetrahedral colloidal crystals,13 and other organized morphologies through surface-capping molecules.14 Pea-shaped Ag2S nanocrystals can also densely assemble into wire-like superstructures.15 Similarly, Cu2S nanoparticles and nanodisks can be directly assembled or stacked to form chain-like and other linear * To whom correspondence should be addressed. E-mail: hyhuang@ mx.nthu.edu.tw.
superstructures.2,16 In addition, the preparation of crystals and free-floating sheets with respectively bimetallic (Au and Ag)19 and CdTe18 nanocrystal building blocks, and the investigation of the interparticle forces driving their assembly have received considerable attention. Large-scale organized structures of Au2S nanoparticles, however, have not been described before, presumably because of the challenge to synthesizing uniform and densely packed gold sulfide particles. Here we report the formation of monodisperse Au2S nanoparticles by a hydrothermal synthesis approach using HAuCl4 as the gold source. Interestingly, these nanoparticles spontaneously assemble into cross-sheet-like or plate-like superstructures, which can be suspended in solution. Each sheet or plate contains multiple layers of Au2S particles arranged in a lamellar phase structure. The identity of the nanoparticles and the composition of the materials holding these particles together to form the superstructures were carefully studied by various spectroscopic and electron microscopic techniques. Reaction conditions favorable for the preparation of Au2S nanoparticles and their selfassembled structures were determined. Optical characterization of the Au2S particles was also performed. Experimental Section For the typical synthesis of the Au2S nanoparticle superstructures, a volume of 30 mL of aqueous solution containing 1.0 × 10-4 M tetrachloroauric acid (HAuCl4, Aldrich, g99.9%) and 0.10 M cetyltrimethylammonium bromide surfactant (CTAB, Aldrich, >99.0%) was prepared. To this solution was added 400 µL of 0.15 M sodium sulfide (Na2S · 9H2O, Aldrich, g98.0%) solution and the mixture was stirred. Then the solution was transferred to a Teflon-lined stainless autoclave with a capacity of 43 mL for the hydrothermal synthesis of Au2S nanoparticles at 175 °C in an oven. After 8 h, the autoclave was removed and cooled to room temperature. The resulting mixture was centrifuged at 8500 rpm for 60 min to collect the brownish black precipitate. Next, the precipitate was washed with deionized water and centrifuged three times at 6000 rpm
10.1021/jp8039669 CCC: $40.75 2008 American Chemical Society Published on Web 07/16/2008
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Figure 1. (a) FE-SEM image of the cross-sheet-like or plate-like Au2S nanoparticle superstructures. A few gold nanoparticles can also be observed. (b) A high-magnification FE-SEM image of individual cross-sheet-like Au2S nanoparticle superstructures. (c) FE-SEM image of an icosadecahedral gold nanoparticle found in the sample.
3000F electron microscopes equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. X-ray photoelectron spectroscopy (XPS) analysis of the Au2S nanoparticles was conducted on a ULVAC-PHI Quantera SXM photoelectron spectrometer. UV-vis absorption spectra were taken with a JASCO V-570 spectrophotometer. A Perkin-Elmer Spectrum RX I spectrometer was used to obtain the Fourier transform infrared (FT-IR) spectra. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Mercury spectrometer (400 MHz, 1H, CDCl solvent). 3 Results and Discussion
Figure 2. XRD pattern of the Au2S nanoparticle superstructures supported on a clean Si (100) substrate. Because of the small size of the substrate used, reflection peaks from the Al sample holder were also recorded. The inset shows the small-angle XRD pattern of the sample. The peak at 1.0° 2θ is from the X-ray source.
for 10 min to remove excess CTAB and unreacted reagents. The products were stored in deionized water for subsequent characterization. The identity of the Au2S nanoparticle products was verified by using a Shimadzu XRD-6000 X-ray diffractometer (XRD) with Cu KR radiation. Field emission scanning electron microscopes (FE-SEM, Hitachi S4700 and JEOL JSM-7000F operated at 15 kV) and transmission electron microscopes (TEM, JEOL JEM-2000 FXII operated at 160 kV and JEOL JEM3000F operated at 300 kV) were used to make morphological and detailed structural observation of the samples. Elemental analysis of the samples was performed on JSM-7000F and JEM-
The products formed after the hydrothermal synthesis were first examined by scanning electron microscopy. Figure 1 shows the FE-SEM images of the cross-sheet-like or cross-plate-like Au2S nanoparticle superstructures. The fact that these superstructures are composed of aggregated Au2S nanoparticles will become clear after the TEM characterization. Interestingly, the products exhibit an unusual morphology with two or more sheets arranged roughly perpendicular to each other. The sheets are about 125-250 nm in length for the reaction conditions used, and are around 20-35 nm in thickness. These cross-sheet-like structures are free-floating or suspended in solution, and need to be centrifuged at high rpm for 1 h to collect the precipitate. The products formed also include a few highly faceted particles, which were confirmed to be gold nanocrystals. Figure 1c displays the SEM image of an icosahedral gold nanocrystal found in this sample. Over 95% of the nanostructures observed are the Au2S nanoparticle superstructures. It is possible that gold nanoparticles were produced via a redox reaction between HAuCl4 and hydroxyl ions formed through the reaction of Na2S and water, as the resulting solution is highly basic. AuCl2- may also be present from the reduction of AuCl4-, and account for the colorless solution observed after mixing the reagents. Under the present hydrothermal synthesis conditions, the equilibrium may shift to produce S2– from HS-, which reacts with AuCl2-
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Figure 3. (a) TEM image of the cross-sheet-like Au2S nanoparticle superstructures. A faceted gold nanoparticle is also present here. (b) TEM image of individual Au2S nanoparticle superstructures revealing a lamellar phase arrangement of the Au2S nanoparticles. (c) SAED pattern of the area in panel a and the surrounding region.
Figure 4. (a) EDS spectrum of the Au2S nanoparticle superstructures in the square region of the inset FE-SEM image. No gold nanoparticles can be found in this region. (b) EDS spectrum of a single Au2S nanoparticle superstructure taken from the square region of the inset high-magnification TEM image.
to form Au2S nanoparticles. The final solution also becomes neutral or slightly basic (pH ∼7-8 as tested with a pH paper).
The composition of the cross-sheet-like structures was verified by their XRD patterns (Figure 2). The (111), (200), (220), (311), and (222) reflection peaks of cubic Au2S can be clearly identified, confirming the formation of Au2S nanoparticles. Sharp XRD peaks from the faceted gold nanoparticles were also recorded, as expected. The peak widths for the Au2S nanoparticles, however, are relatively broad, suggesting their ultrasmall sizes. Their estimated particle size, calculated by using the Scherrer formula and the measured peak width at half-peak height of the (111) peak, is ∼5 nm.19 If the (200) peak were used, a particle size of ∼2 nm was obtained. Figure 3 gives the typical TEM images of the Au2S nanoparticle superstructures. Each cross-sheet-like structure is composed of a dense aggregation of tiny Au2S nanoparticles of largely 2-4 nm diameter (estimated from Figure 3b). Notably, dark striations can be seen over these superstructures with roughly perpendicular orientation to the plane of the underlying sheet, suggesting that the Au2S nanoparticles are arranged in a lamellar phase structure. Figure 3b also shows another sheet with layer-by-layer packing of nanoparticles oriented at some angle with respect to the underlying sheet of nanoparticle aggregates (the circled region). The observation made suggests that the entire superstructure should possess the same type of lamellar phase arrangement of Au2S particles. The interlayer spacing in the lamellar phase structure is 3-4 nm. The selectedarea electron diffraction (SAED) pattern of the area shown in Figure 3a and its surrounding region produces a ring pattern that can be indexed to those of Au2S and Au (Figure 3c). The diffraction rings from gold are attributed to the presence of some faceted gold nanocrystals. Consistent with the TEM observation, a peak appears at 2.4° 2θ (or d-spacing of ∼3.7 nm) in the small-angle XRD pattern of the superstructures (see the inset of Figure 2). This peak results from the lamellar packing alignment of the Au2S particles. Attempts to further analyze these Au2S nanoparticles under the high-energy electron beam of a 300 kV high-resolution TEM have led to continuous shape deformation and decomposition of the Au2S particles to Au nanoparticles, as evidenced by the SAED pattern and the measured lattice fringe distances of the resulting nanoparticles (see the Supporting Information). The spontaneous decomposition of Au2S to Au in the temperature range of 420-490 K has
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Figure 6. (a) UV-vis absorption spectrum of the Au2S nanoparticle superstructures. The inset illustrates a plot of (Rhν)2 vs hν for the determination of the direct band gap of the Au2S nanoparticles. (b) A plot of (Rhν)1/2 vs hν for the determination of the indirect band gap of the Au2S nanoparticles.
Figure 5. (a) Full XPS spectrum of the Au2S nanoparticle superstructures and the faceted gold nanocrystal byproduct. (b and c) Highresolution XPS spectra of the Au2S nanoparticle superstructures in the Au(4f) and S(2p) regions.
been reported.6 This thermal instability, coupled with the ultrasmall sizes of the Au2S nanoparticles synthesized here, makes them susceptible to a prolonged high-energy electron beam irradiation. To further verify the composition of the superstructures, their EDS spectra were taken. Figure 4a is the EDS spectrum of the superstructures obtained over the square region of the corresponding FE-SEM image shown. Faceted gold nanocrystals were avoided here. Signals for Au and S were recorded, and the atomic percentages of Au and S are 64.9%
and 35.1%, respectively. Similar EDS results were obtained by analyzing a portion of a superstructure under TEM (66.5% of Au and 33.5% of S, Figure 4b). The data also indicate that the cross-sheet-like structures are made up of Au2S nanoparticles. XPS measurements of the Au2S nanoparticle superstructures were also conducted. Figure 5 displays the full XPS spectrum and high-resolution spectra of the Au(4f) and S(2p) regions. The Au(4f7/2) peak appears at a binding energy of 84.0 eV. A value of 83.9 eV for the Au(4f7/2) peak has been found for Au2S nanoparticles of 4 nm diameter.3 Interestingly, the binding energy for the Au(4f7/2) peak of Au2S is the same as that of metallic gold (Au0).3,20 The binding energy of the S(2p3/2) peak is at 162.5 eV. The reported 4 nm Au2S nanoparticles have a value of 162.6 eV for this peak.3 The Au(4f7/2) and S(2p3/2) peak areas were determined for the quantitative elemental analysis of Au and S in the sample, and an atomic ratio of 7 to 3 was obtained. This ratio again confirms the presence of both Au2S and Au nanoparticles. We also characterize the optical properties of the Au2S nanoparticle superstructures. Figure 6 offers an UV-vis absorption spectrum of the sample dispersed in water. The solution color is brown (see the Supporting Information). The lack of a sharp absorption band is related to the believed indirect optical band gap of Au2S.3 The band gap of the Au2S nanoparticles, determined from a plot of (Rhν)2 vs hν, is 2.51 eV. If Au2S is considered an indirect semiconductor and a plot of (Rhν)1/2 vs
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Figure 7. (a and b) FE-SEM and TEM images of the cross-sheet-like structures after heating them in boiling water for several minutes. The insets show their respective high-magnification images.
hν is made, an indirect band gap value of 1.77 eV is obtained. Theoretical calculations have obtained a band gap range of 1.3-2.6 eV.8 Fluorescence from these Au2S nanoparticle samples was not observed, suggesting their indirect semiconductivity property. Note that practically no absorption from gold nanocrystals was observed, inferring that comparatively few gold particles were formed. Reaction conditions required for the formation of these crosssheet-like Au2S nanoparticle superstructures were examined. We found that such superstructures were produced at CTAB concentrations greater than 0.01 M (see the Supporting Information). At a lower CTAB concentration (e.g., 0.001 M), some micron-sized gold particles were observed along with the superstructures. Only micron-sized gold particles were generated at a CTAB concentration of 0.0001 M. The concentrations of Na2S in the reaction solution were also varied. We found that the cross-sheet-like structures can be synthesized at Na2S concentrations greater than 0.05 M (see the Supporting Information). At Na2S concentrations below 0.025 M, only quasispherical gold nanoparticles of about 20-70 nm were grown. The above results show that the addition of a certain amount of CTAB surfactant and Na2S in the reaction solution is necessary to promote the growth of the Au2S nanoparticle superstructures, and minimize the formation of gold nanocrystals. Finally, we consider the chemical species present around the Au2S nanoparticles allowing the formation of such lamellar
Figure 8. (a) FT-IR spectrum of the Au2S nanoparticle superstructures. (b and c) Selected regions of the FT-IR spectrum shown in panel a with bands arising from (b) C-H stretching vibrations and (c) C-H deformation vibrations. Higher percent transmittance in the 3200 to 3700 cm-1 region results from the subtraction of baseline spectrum of a KBr reference without predrying from a spectrum of the oven-dried Au2S nanoparticle superstructures.
packing order of particles. Since CTAB capping surfactant has been used here, a thermal treatment should disassemble the superstructures and release individual Au2S nanoparticles. Figure 7 provides the FE-SEM and TEM images of the products collected after heating the superstructures in boiling water for several minutes. The same cross-sheet-like Au2S nanoparticle superstructures with a lamellar phase arrangement of nanoparticles were obtained. This surprising result implies that CTAB molecules may have been decomposed under the present hydrothermal synthesis conditions, and other chemical species subsequently interact with the synthesized Au2S nanoparticles and direct their alignment. Although CTAB decomposition may
11666 J. Phys. Chem. C, Vol. 112, No. 31, 2008 seem unexpected, dodecanethiol molecules have been suggested to decompose under hydrothermal synthesis or normal heating conditions, and serve as the sulfur source for the formation of Cu2S and Ag2S nanocrystals.2a,13 To further identify the chemical species adsorbing on the Au2S nanoparticles, FT-IR spectra of the superstructures were taken and are given in Figure 8. The peaks at 2852 and 2923 cm-1 can be assigned respectively to the symmetric and asymmetric C-H stretching vibrations of the acyclic alkane -CH2- group.21 The peaks at 2868 and 2955 cm-1 are attributed to the symmetric and asymmetric C-H stretching vibrations of the aliphatic -CH3 group. These peak positions are similar to those reported for pure CTAB.22 However, the peaks at 1377 and 1461 cm-1 cannot be simply assigned to the symmetric and asymmetric C-H scissoring vibrations of the CH3-N+ moiety, which appear at 1430 and 1482 cm-1 for pure CTAB. Instead, these peaks should be more appropriately assigned to the C-H deformation vibrations of the aliphatic -CH3 group. Thus, CTAB molecules may be decomposed to form long aliphatic hydrocarbon chains and trimethylammonium species by C-N bond cleavage. Interestingly, a signal for nitrogen is not found in the XPS spectrum shown in Figure 5a. The C(1s) peak at 285.1 eV is characteristic of the alkyl group comprised largely of CH2 units.23 The 1H NMR spectrum of the superstructures also supports the above conclusion. Signals for the long alkyl chain were detected, but not for the methyl groups bonded to nitrogen (see the Supporting Information). Results of these spectroscopic investigations indicate that long alkyl chains (probably the C16 chain) are primarily responsible for the alignment of the Au2S nanoparticles. It is possible that their tight packing and association with the nanoparticles direct the assembly of the particles into a lamellar phase structure and the formation of the sheet-like morphology. Here strong van der Waals forces among the alkyl chains are expected to play a key role in driving the Au2S nanoparticle assembly. Conclusion Au2S nanoparticles with diameters of 2-4 nm have been successfully synthesized under hydrothermal synthesis conditions by using typical HAuCl4 and Na2S reagents and CTAB surfactant. These ultrasmall Au2S nanoparticles are arranged in a lamellar phase structure to form free-floating cross-sheet-like superstructures. Their identity has been confirmed with XRD, TEM, EDS, and XPS techniques. UV-vis absorption spectra of the samples showed only absorption from the Au2S nanoparticles, whereas strong absorption from gold nanoparticles has been observed when performing the reaction with the same reagents at room temperature. To enhance the growth of Au2S nanoparticles, and reduce the formation of gold nanocrystal byproduct, sufficient concentrations of Na2S and CTAB in the reaction mixture were found to be necessary. FT-IR, NMR, and XPS characterization of the Au2S nanoparticle superstructures revealed thermolysis of CTAB molecules under the hydrother-
Kuo and Huang mal synthesis conditions used, and long alkyl chains interact with the Au2S nanoparticles and organize their assembly into a lamellar phase structure. The synthetic approach adopted here represents a new and convenient method for the preparation of uniform Au2S nanoparticles and their densely assembled superstructures. Acknowledgment. We thank the National Science Council of Taiwan for financial support of this work (Grant NSC952113-M-007-031-MY3). Supporting Information Available: High-magnification TEM image of the superstructures, photographs of the nanoparticle solution, additional SEM images of the superstructures, and 1H NMR spectrum of the superstructures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lim, W. P.; Zhang, Z.; Low, H. Y.; Chin, W. S. Angew. Chem., Int. Ed. 2004, 43, 5685. (2) (a) Sigman, M. B.; Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050. (b) Du, X.-S.; Yu, Z.-Z.; Dasari, A.; Ma, J.; Meng, Y.-Z.; Mai, Y.-W. Chem. Mater. 2006, 18, 5156. (3) Morris, T.; Copeland, H.; Szulczewski, G. Langmuir 2002, 18, 535. (4) Majimel, J.; Bacinello, D.; Durand, E.; Valle´e, F.; Tre´guerDelapierre, M. Langmuir 2008, 24, 4289. (5) Yoshizawa, K.; Iwahori, K.; Sugimoto, K.; Yamashita, I. Chem. Lett. 2006, 35, 1192. (6) Ishikawa, K.; Isonaga, T.; Wakita, S.; Suzuki, Y. Solid State Ionics 1995, 79, 60. (7) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. ReV. B 1994, 50, 12052. (8) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (9) Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Kurzinger, K. Nano Lett 2004, 4, 1853. (10) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D.; Bridges, F.; van Buuren, A. J. Phys. Chem. B 2002, 106, 7005. (11) Schwartzberg, A. M.; Grant, C. D.; van Buuren, T.; Zhang, J. Z. J. Phys. Chem. C 2007, 111, 8892. (12) Wang, D.; Xie, T.; Peng, Q.; Li, Y. J. Am. Chem. Soc. 2008, 130, 4016. (13) Zhuang, Z.; Peng, Q.; Wang, X.; Li, Y. Angew. Chem., Int. Ed. 2007, 46, 8174. (14) Mott, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104. (15) Gao, F.; Lu, Q.; Zhao, D. Nano Lett. 2003, 3, 85. (16) Lu, Q.; Gao, F.; Zhao, D. Nano Lett. 2002, 2, 725. (17) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420. (18) Tang, Z.; Zhang, Z.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274. (19) West, A. R. Solid State Chemistry and its Applications; John Wiley & Sons: New York, 1992; pp 173-175. (20) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104. (21) Socrates, G. Infrared and Raman Characteristic Group Frequencies; John Wiley & Sons: Chichester, U.K., 2001. (22) (a) Sui, Z.; Chen, X.; Wang, L.; Chai, Y.; Yang, C.; Zhao, J. Chem. Lett. 2005, 34, 100. (b) Liu, X.-H.; Luo, X.-H.; Lu, S.-X.; Zhang, J.-C.; Cao, W.-L. J. Colloid Interface Sci. 2007, 307, 94. (23) Zhang, S.; Leem, G.; Srisombat, L.; Lee, T. R. J. Am. Chem. Soc. 2008, 130, 113.
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