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ARTICLES One-Pot Synthesis of Silver Nanoplates and Charge-Transfer Complex Nanofibers Jianhui Yang, Haishui Wang, and Hongjie Zhang* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: June 14, 2008
We describe a facile one-pot process to synthesize Ag nanoplates by reducing silver nitrate with 3,3′,5,5′tetramethylbenzidine (TMB) at room temperature. The silver nanoplates were highly oriented single crystals with (111) planes as the basal planes. TMB can be readily oxidized to charge-transfer (CT) complex between TMB, as a donor, and (TMB)2+, as an acceptor. The π-π interaction of the neutral amine (TMB) and diiminium structure (dication, TMB2+) result in the formation of one-dimensional CT complex nanofiber. The shape of silver nanostructures and the size of CT complex nanofibers could be controlled by tuning the initial molar ratio of TMB to Ag. This wet-chemical route provides a new concept for simultaneous preparing metal and CT complex nanostructures. Introduction In the past decades, metallic nanoparticles have attracted considerable attentions owing to their potential application in catalysis, electronics, optoelectronics, plasmonics, information storage, optical sensing, biological labeling, imaging, and surface-enhanced Raman scattering.1-5 The intrinsic properties of metal nanoparticles can be tuned by controlling their size and shape.6-9 Recently, the dramatic effect is placed on tuning the novel shape-dependent properties of these nanostructures in contrast to the size-dependent, because different crystal surfaces may possess different physical and chemical properties, such as different surface atom densities and electronic structures. This is particularly true for silver nanostructures, and therefore the synthesis of silver nanostructures with well-controlled morphology is important for uncovering their morphologydependent properties and for achieving their potential practical applications.10-14 Various approaches have been reported for synthesizing silver nanoparticles with various shapes, including nanocubes,15,16 nanodisks,17,18 nanoprisms,19,20 nanoplates,21-23 nanorods,24 nanowires,25 nanobelts,26 and branched nanocrystals.27,28 Although these methods are successful to produce well-defined silver nanostructures, there is still a challenge to develop facile routes for the controlled synthesis of silver nanostructures. Amines have been used in metal nanoparticles synthesis as both reducing agents and as stabilizers after metal nanoparticles formation.29 A variety of amines have been explored, including simple primary amines,30 amino acids,23 and multifunctional amines including polymers.31 Amines are a particularly attractive class of reducing agents because of their nearly universal presence in biological and environmental systems. 3,3′,5,5′Tetramethylbenzidine (TMB), in which the amino groups are protected methylation of the adjacent ring positions, is a new * To whom correspondence should be addressed. E-mail: hongjie@ ciac.jl.cn. Tel: +86-431-85262127. Fax: +86-431-85698041.
safe color reagent.32 In comparison with traditional color reagents such as benzidine and o-phenylenediamine (OPD), TMB has high detection sensitivity, good stability, and security. Animal experiment and Ames bacteria automatic mutation experiment show that TMB is noncarcinogenic and is not mutationale. Presently TMB has replaced strong carcinogen benzidine and other carcinogenic ramification of benzidine to be applied to fields such as clinical assay, test of legal medical expert, criminal detection, and environmental monitoring. However, to the best of our knowledge, the reducing ability of TMB has not been applied to preparing metal nanostructures. In the present work, we successfully synthesize Ag nanoplates by reducing silver nitrate with TMB at room temperature. TMB can be readily oxidized to charge-transfer (CT) complex between TMB, as a donor, and (TMB)2+, as an acceptor.33-35 The proposed structure of the CT complex is shown in Scheme 1. The π-π interaction of the neutral amine (TMB) and diiminium structure (dication, TMB2+) results in the formation of onedimensional CT complex nanofiber. We could control the shape of silver nanostructures from nanoplates to nanoflowers and the size of CT complex nanofibers by tuning the initial molar ratio of TMB to Ag. These CT complex nanofibers can be easily separated from Ag nanoplates by a simple centrifugal progress. According to previous reports, these CT complex nanofibers show remarkable paramagnetism, high anisotropic conductivity, and other interesting properties, which are expected to have wide applications in organic ferromagnets, organic superconductors, and sensors.35-38 Experimental Section Materials. 3,3′,5,5′-tetramethylbenzidine (TMB) was purchased from Sigma. Silver nitrate (AgNO3) and absolute ethanol were purchased from Beijing Chemical Reagents Industry. All chemicals were used as supplied without further purification. Deionized and doubly distilled water was used throughout the experiment.
10.1021/jp802604d CCC: $40.75 2008 American Chemical Society Published on Web 07/31/2008
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SCHEME 1
Synthesis of Silver Nanoplates and Charge-Transfer Complex Nanofibers. In a typical experiment, 1 mL of 10 mM AgNO3 aqueous solution was rapidly added into 2 mL of 10 mM TMB ethanol solution under vigorous stirring at room temperature. The color of the reaction mixture started to change from green to dark purple after the reaction had proceeded. A large amount of floccule was observed several hours later, which was collected by centrifugation. There two layers of precipitations were formed due to distinct density. It is suggested that the bottom gray precipitation is Ag nanoplates and the top purple precipitation is CT complex nanofibers by the following experiments. Characterization. X-ray diffraction (XRD) analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 200 mA) radiation. Scanning electron microscopy (SEM) was performed with a XL30 ESEM FEG scanning electron microscopy at an accelerating voltage of 20 kV. An energy-dispersive X-ray (EDX) spectroscopic detecting unit was used for elemental analysis. Transmission electron microscope (TEM) measurements were made on a JEOL JEM-2010F transmission electron microscope operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on an ESCLAB MKII X-ray photoelectron spectroscopy, using Mg as the exciting source. The UV-vis absorption spectra were collected on a TU-1901 spectrophotometer. Results and Discussion The crystal structure of the floccule was characterized using XRD analysis. Figure 1 presents the XRD pattern of the asprepared sample. According to previous reports, the peaks located at 10.5, 17.6, and the broad peak centered at ∼25° can be ascribed to the formation of charge-transfer (CT) complex between TMB and (TMB)2+.34 All other peaks located at 38.1, 44.3, 64.4, 77.4, and 81.6° are assigned to the diffractions of
Figure 1. XRD pattern of the as-prepared product.
(111), (200), (220), (311), and (222) of face-centered cubic (fcc) silver, respectively. The cell parameter calculated from this XRD pattern is 4.087 Å, very consistent with the value reported in the literature (JCPDF No. 04-0783 a ) 4.086 Å). It is obvious that there is a sharp peak indexed as (111) plane, which indicates that the top face plane of silver nanostructures was the (111) plane and the (111) planes of silver nanostructures are highly oriented parallel to the supporting substrate.23 Observation from the XRD pattern indicates that the as-prepared product consist of charge-transfer (CT) complex and metal Ag. It is well known that XPS is valuable for detecting the composition and structure of samples.39 The XPS surveys (Figure 2A) shows the occurrence of carbon, oxygen, nitrogen, and silver signals. Figure 2B displays the Ag 3d3/2 and Ag 3d5/2 peaks, which we identify at 374.5 and 368.5 eV, respectively. These values are comparable to the bonding energy of metal silver,40 which we also observed in our XRD spectrum (Figure 1). The N1s high-resolution XPS spectrum (Figure 2C) can be deconvoluted into three components. The origin of the line at binding energy of 406.5 is ascribed to nitrate. The result supports the idea that the counterion is NO3-. The two peaks at binding energy of 401.4, 399.2, and with a line width of 1.60 eV are assigned to the charged protonated imine and neutral amine structures, respectively.41,42 These observations suggest that the product contains a CT complex between neutral amine TMB, as a donor, and the charged protonated imine (TMB)2+, as an acceptor, and two NO3- ions.34 The diiminium cation (TMB)2+ has a quinone structure, so that it can act as an electron acceptor. Transfer of an electron from TMB to (TMB)2+ generates unpaired electrons in the complex. The result is consistent with those obtained by the XRD characterization. On the basis of the above analysis, it could be deduced that the as-prepared product consists of charge-transfer (CT) complex and metal Ag. Due to the density of silver is distinctly bigger than that of organic CT complex, we could separate them each other by centrifugation. It is suggests that the bottom gray precipitation is Ag nanoplates and the top purple precipitation is CT complex nanofibers by the following experiments due to the differences of density and color. Figure 3A,B illustrates the low- and high-magnification SEM images of the bottom precipitation deposited onto silicon wafers taken from different region, respectively. The low-magnification SEM image (Figure 3A) reveals that the bottom precipitation mainly consist of nanoplates with hexagonal and truncated triangular in geometry (∼70-80%). Small quantities of spherical and unregulated aggregates are also observed as byproduct. The edge length and thickness of these nanoplates are measured at about 500 and 100 nm, respectively. The thickness of the nanoplates was measured standing against the substrate (indicated by arrows in Figure 3A) and the distance between two planes of the plate standing against the silican substrate (Figure 3B). The chemical composition of the nanostructures was further determined by
One-Pot Synthesis of Ag Nanoplates
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EDX spectroscopy. The EDX spectrum (Supporting Information, Figure S1) obtained from the particles only shows the peak corresponding to Ag. The low-intensity peaks around the main silver peak originated from Ag, which were caused by different energy-level differences of the silver atom. In combination with the results of XRD and XPS, we can conclude that these nanoplates were pure metallic Ag. Because of the bigger size of the plate and the presence of particles with unregulated shape, the UV-vis absorption spectrum shows a complex pattern in the region between 350-600 nm (Supporting Information, Figure S2A). Figure 4A and B present a typical TEM image of a single Ag nanoplate and high-resolution TEM image obtained from an area close to the edge of the nanoplate, respectively. The bandlike patterns shown on the surface of the nanoplate in Figure 4A are due to deformation or bending of the nanoplate during the irradiation of the electron beam.43 The high-resolution TEM image (Figure 4B) of these nanoplates suggests that they were single crystalline. The inset in Figure 4B shows the selected area electron diffraction (SAED) pattern taken by aligning the electron beam perpendicular on the nanoplate, indicating that
Figure 3. (A) Low- and (B) high-magnification SEM images of Ag nanoplates.
Figure 2. XPS spectra of (A) surveys, (B) Ag3d, and (C) N1s.
each nanoplate was a single crystal. The diffraction spots could be indexed according to the face-center-cubic (fcc) structure of silver. The spot array, diagnostic of a hexagonal structure, is from the [111] orientation of an individual Ag nanoplate lying flat on the substrate with its top perpendicular to the electron beam and the top and bottom faces of the silver nanoplate were bounded by the (111) planes.19,23 The morphology and chemical composition of the top precipitation were confirmed with SEM and EDX methods. The top image in Figure 5 displays the SEM image of the top precipitation from which it can be seen that the sample exclusively consists of a large quantity of nanofibers with a diameter of several hundreds nanometers and length of several micrometers. The EDX spectrum of these nanofibers (the bottom image in Figure 5) shows the presence of C, N, and O. Quantitative analysis reveals that the atomic ratio of C/N/O is very close to the stoichiometry of charge-transfer complex shown in Scheme 1. We can conclude that the top precipitation obtained in our research consist of charge-transfer complex nanofibers. The investigation of UV-visible absorption spectra further affirm our predication. The UV-vis spectrum (Supporting Information, Figure S2B) of the top precipitation dispersed in water displays two bands near 370 and 650 nm, which were ascribed to a CT complex consisting of the neutral diamine (TMB) as a donor and the dication (TMB2+) as an acceptor. The proposed structure of the CT-complex is shown in Scheme 1 in which the acceptor assumes a diiminium or quinoid structure and acts as a strong electron acceptor. AgNO3 is a powerful oxidant with a highly electropositive character of Ag (+0.799 V) and can be easily reduced with various reducing agents. In this synthesis, TMB is performing multiple tasks. TMB has strong electron donating properties and
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Figure 4. (A) TEM image of a single Ag nanoplate and (B) high-resolution TEM image taken from the edge of the nanoplate. The inset in (B) shows the corresponding SAED patterns. In the [111] zone axis, the diffracted spots are indexed to (200) and 1/3(422) (inner spot) Bragg reflections, respectively.
Figure 6. SEM images of silver nanoparticles (A, B) and chargetransfer complex nanofibers (C) obtained with an initial TMB to silver of 1:1, respectively. The inset in panel A shows the corresponding SAED patterns.
Figure 5. SEM image (top) and EDX spectrum (bottom) of chargetransfer complex nanofibers. Silicon signal is due to the silican wafer substrate.
acts as a reducing agent here. Either TMB or the CT complex (TMB and TMB2+) are stabilizers, and the nitrogen in these molecules could selectively absorb on the different crystal facets and affect their growth rate, thus resulting in the formation of a special Ag nanostructure. When TMB/Ag molar ratio was changed from 2:1 to 1:1 under otherwise identical conditions, a large amount of flowerlike Ag nanostructures are obtained (Supporting Information, Figure S3). It is clear that a single flower consists of a double-fold snowflake back-to-back (Figure 6A and B). Each fold has one bud on the center circled by six ordered petals. The angle between two adjacent petals is
measured to be about 60°. The SAED patterns, inset in Figure 6A, reveal that only hexagonal diffraction spots patterns are observed. These high qualities of diffraction spots confirm the single crystal feature of double-fold snowflakelike silver nanostructures oriented along the six equivalent [110] directions on a (111) basal plane.44,45 The results indicate that the concentration of TMB plays an important role in the formation of anisotropic silver nanostructures. At the same time, the diameter of charge-transfer complex nanofibers (Figure 6C) distinctly is reduced as the initial concentration of TMB decreases. It suggests that the molar ratio of TMB to Ag would be high enough to exercise shape control of silver nanostructures and size-control of CT complex nanofibers. The reduction of AgNO3 by TMB leads to the formation of Ag nanostructures with the occurrence of charge-transfer complex. According to previous reports, the compounds of aniline species preferentially forms as nanofibers during chemical oxidative polymerization through intermolecular π-π interaction.46,47 In the present case, it is expected that these intermolecular π-π interactions of the
One-Pot Synthesis of Ag Nanoplates neutral amine (TMB) and diiminium structure (dication, TMB2+) result in the formation of one-dimensional charge-transfer complex nanostructure. Conclusion In summary, analytical reagent TMB has been used for the first time in the synthesis of silver nanoplates at room temperature. The results show that the as-prepared silver nanoplates were (111)-oriented single crystals. The oxidation product charge-transfer complex (TMB and TMB2+) nanofibers are also obtained along with silver nanostructures. In our research, TMB molecules provide both reduction and recognition to form silver nanostructures. The shape of silver nanostructures and the size of CT complex nanofibers could be controlled by tuning the initial molar ratio of TMB to Ag. This wet-chemical route provides a new concept for simultaneous preparing metal and charge-transfer complex nanostructures.48 Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 20372060, 20340420326, 20490210, and 90306001) and the MOST of China (“973” Program, Grant 2006CB601103). Supporting Information Available: EDX, UV-vis absorption spectra, and SEM images. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (3) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. Mater. Res. Soc. Bull. 2001, 26, 985. (4) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (5) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (6) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (7) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (8) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209. (9) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (10) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755. (11) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. ReV. B 2002, 64, 235402. (12) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544.
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