Rapid room-temperature synthesis of silver ...

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Rapid room-temperature synthesis of silver nanoplates with tunable in-plane surface plasmon resonance from visible to near-IR† Zongwei Cao,ab Hongbing Fu,*a Longtian Kang,a Liwei Huang,ab Tianyou Zhai,ab Ying Maa and Jiannian Yao*a Received 14th January 2008, Accepted 12th March 2008 First published as an Advance Article on the web 31st March 2008 DOI: 10.1039/b800691a We report a rapid (within 15 minutes), simple, green, inexpensive, versatile, and reproducible method for the synthesis of Ag triangular and hexagonal nanoplates in aqueous phase under ambient atmosphere. The method involves reducing silver oxide (Ag2O) with hydrazine (N2H4) in the presence of trisodium citrate (TSC) and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) in aqueous phase. In our system, TSC molecules serve as colloidal stabilizers to prevent as-prepared colloids from aggregating, while EDTA molecules serve as a ligand to monomers. The complexation of EDTA to Ag+ not only significantly slows the reduction kinetics of Ag+ by N2H4, but also kinetically controls the formation and growth of nanoplates. By varying the amount of EDTA, the shape (triangular and hexagonal) and edge length of nanoplates have been readily controlled, providing a surface plasmon resonance (SPR) response tunable from visible to near infrared. Most importantly, the SPR response is almost a linear function of the quantity of EDTA. Silver nanocrystals with a required SPR response can be provided, even without considering the actual nature of the Ag colloids. Recent results suggest that this chelation-mediated kinetic control over the sizes and morphologies of nanostructures can also be applied for other metal nanostructures.

1. Introduction Over the past decades, silver nanostructures have been the focus of intensive research as a result of their unique SPR properties. The coupling of light to the resonant motion of the free electron plasma in the metal allows silver nanostructures to enhance and confine electromagnetic fields. Such plasmonic materials are therefore promising not only as active substrates for surfaceenhanced Raman scattering, as near-field optical probes and contrast agents for biomedical imaging, but also for photonic applications in nonlinear optics.1 The precise wavelength of SPR of metal nanostructures depends on several parameters, among which the particle size and shape, and the dielectric environment are probably the most important.2 Therefore, much effort has been devoted to the shape-controlled synthesis of silver nanostructures, including zero-dimensional spherical3 or polyhedral4 nanodots, one-dimensional nanorods and wires,5 and twodimensional nanoplates.6 Most recently, particular emphasis has been placed on triangular nanoplates,6d,e as metal nanostructures with sharp corners and edges are capable of generating maximum electromagnetic-field enhancement.2,7 Although there have been no reports of generalized synthetic routes and mechanisms thus far, several research groups have

a Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing, 100080, P.R. China. E-mail: [email protected] b Gradate School of Chinese Academy of Science, Beijing, 100039, P.R. China † Electronic supplementary information (ESI) available: TEM images of nanoplates, UV-vis spectra of different concentrations of TSC. See DOI: 10.1039/b800691a

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developed diverse methods to generate triangular and circular nanoplates of silver in a number of different solvents, such as soft templates for plate growth,6a–c and photochemically6d,e or thermally6f induced transformation of small spherical nanoparticles into triangular nanoplates. In addition, polygonal silver nanoprisms were also synthesized by boiling AgNO3 dissolved in N,N-dimethyl formamide in the presence of poly(vinylpyrrolidone) (PVP).8 Very recently, Metraux and Mirkin reported a straightforward approach to Ag triangular nanoprisms with tailorable thickness by reducing AgNO3 with NaBH4 in the presence of TSC, H2O2 and PVP.9 Xia et al. demonstrated that the hydroxyl end group of PVP could also serve as a mild reductant for kinetically controlled synthesis of Ag triangular nanoplates.10 Kelly’s group showed a rapid method for controlling morphology of stable nanoparticles by only varying the concentration of TSC.11 However, it still remains a grand challenge to produce Ag nanoplates with controllable SPR according to specific requirements, via a facile and clean method. Herein, we report a rapid (within 15 minutes), simple, green, inexpensive, versatile, and very reproducible method for the synthesis of Ag triangular and hexagonal nanoplates in aqueous phase under ambient atmosphere. The method starts by reducing Ag2O with N2H4 in the presence of TSC and EDTA in the aqueous phase under ambient atmosphere. The shape (triangular or hexagonal) and edge length of the as-prepared nanoplates can be readily controlled only by adjusting the quantity of EDTA, providing a SPR response tunable from visible to near infrared. Most importantly, the SPR response is almost a linear function of the quantity of EDTA (Fig 1B). Namely, silver nanocrystals with a particular SPR response can be provided according to special requirements, even without considering the actual nature of the Ag colloids. J. Mater. Chem., 2008, 18, 2673–2678 | 2673

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and EDTA. The final products were re-dispersed in water for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization. Characterization The samples for SEM (Hitachi, S-4300) and TEM (JEOL JEM2010) were prepared by dropping one drop of the colloidal suspension on a silicon wafer and a carbon-coated copper grid, respectively, and dried in air at room temperature. The highresolution transmission electron microscopy (HRTEM, JEOL JEM-2010, equipped with a CCD camera) measurements were performed at an accelerated voltage of 200 kV. The electron diffraction (ED) experiment was carried out on a single nanoplate observed in the TEM micrograph; the crystal spacing (dhkl) values were calculated directly by measuring the distance between the opposite spots on the same circumference of the ED pattern. The UV-vis spectra of aqueous solutions of EDTA, TSC and Ag nanoplates (i.e. reaction mixture) were measured in a 1 cm quartz cell for the wavelength range 200–1100 nm by using a UV-vis spectrophotometer (Shimadzu UV-1601PC). Samples for the UV-vis spectra measurement were taken before the centrifugation cycles.

3. Results Optical properties of the silver nanoplates Fig. 1 (A) The UV-vis spectra of as-prepared silver colloids in the presence of different quantities of EDTA, from left to right, VEDTA ¼ 10, 30, 50, 70, 100 mL, respectively. The inset shows the real colour of colloid suspensions in 1 cm cuvette. (B) The working curve of in-plane SPR wavelength of nanoplates vs. VEDTA added to the sample.

2. Experimental Chemicals The A.R. grade silver oxide (Ag2O), hydrazine hydrate (N2H4$H2O), TSC and EDTA were purchased from Shanghai Reagent Co., China and used without further purification. Ultrapure water with a resistivity of 18.2 U m was produced using a Milli-Q apparatus (Millipore). Synthesis of silver nanoplates 1.0 g Ag2O was put into 500 mL water with vigorous stirring, then left in darkness for one day. An upper layer of transparent and saturated Ag2O solution was obtained (the solubility of Ag2O in water is 0.053 g L1 at 80 C).12 In our experiments, synthesis of silver colloids was carried out at room temperature in air under stirring. Typically, an aqueous saturated solution of silver oxide (0.2 mM, 25 mL), TSC (30 mM, 1.5 mL) and EDTA (0.1 M, 0 to 110 mL) was pre-combined. To this mixture, hydrazine hydrate (0.15 M, 100 mL) was rapidly injected. The reactions were finished completely within 15 min. Depending on the volume quantities of EDTA used, the final color of Ag colloids ranged from pink to turquoise. The colloidal suspension was then transferred into a tube and centrifuged at 12000 rpm for 10 min, and the obtained deposit was rinsed with water. The centrifugation/rinse cycle was conducted totally three times in order to remove the excess TSC 2674 | J. Mater. Chem., 2008, 18, 2673–2678

In our experiments, silver nanostructures were synthesized by reducing Ag2O with N2H4 in the presence of TSC and EDTA in aqueous phase. The Ag2O, N2H4, and TSC were kept constant with molar ratios of Ag2O to N2H4 ¼ 1 : 3 and Ag2O to TSC ¼ 1 : 9, respectively. The N2H4 is over-dosed, thus the total amounts of Ag0 generated in different samples are actually the same. Interestingly, we found that the color of the as-prepared silver colloids changes dramatically by varying the volume quantities of EDTA (VEDTA), for example, red, blue, and turquoise (see the inset in Fig. 1A), for VEDTA ¼ 10, 30, and 50 mL, respectively. The in-plane dipole SPR band has been shown to be a good indicator of general nanoplate architecture.2,6d Therefore, the UV-vis spectra provide a quick evaluation of nanoplates formed. As shown in Fig. 1A, each spectrum of as-prepared colloids exhibits three SPR bands, for which the three peaks in the spectrum are located at approximately 500–1100 nm, 403 nm and 330 nm (see Fig. 1A). These values are similar to those already reported for triangular nanoplates.6d,e,g The one at 403 nm is probably due to the spherical nanoparticles, while the shoulder around 330 nm and the band at the longer wavelength side in the range of 500–1100 nm are attributed to the out-of-plane quadrupole and in-plane dipole SPR modes of silver nanoplates, respectively. It can be seen from Fig. 1A that the in-plane dipole SPR bands of nanoplates present a notable red-shift from 530 to 1000 nm with increasing VEDTA from 10 to 100 mL. Fig. 1B shows that the wavelength of this SPR band varies almost linearly as a function of VEDTA added in the sample. The wide tuning range from visible to near infrared provides a working curve for the synthesis of silver colloids with a specific SPR response to a specific requirement, even without considering the actual nature of the Ag colloids. This journal is ª The Royal Society of Chemistry 2008


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Microscopy characterization and structural analysis of the silver nanoplates

Table 1 Average side lengths, thicknesses, and band maxima of Ag nanoplates obtained with different quantities of EDTA

TEM and SEM were used to analyze the morphology of the resulting silver colloids prepared at different values of VEDTA. Consistent with spectroscopic results, the colloids are a mixture of spherical particles and nanoplates. Fig. 2A depicts that triangular nanoplates of 60 8 nm in edge length and spherical nanoparticles of 30 nm in diameter were obtained at VEDTA ¼ 30 mL. The triangular nanoplates can selfassemble in a one-dimensional way somewhere on the TEM grid (ESI† Fig. S1A), yielding an analysis of their thickness around 11.2 1.5 nm. As VEDTA increases to 50 mL, triangular nanoplates become more truncated with a mean edge length of 85 8 nm (Fig. 2C), while the thickness decreases to 9.0 0.8 nm (ESI† Fig. S1B). Meanwhile, spherical nanoparticles grow to 40 nm in diameter. Fig. 2D shows the SEM image of as-prepared silver colloids at VEDTA ¼ 70 mL. Spherical nanoparticles are scarcely seen; hexagonal nanoplates are the dominant products with an edge length of 100 20 nm (see the TEM image of Fig. 2E) and a thickness of 8.5 0.7 nm (ESI† Fig. S1C). Upon increasing

EDTA/mL SPR/nm Shape Edge length/nm Thickness/nm

30 596 Triangle 40 8 11.2 1.5

50 700 Triangle 85 8 9.0 0.8

70 851 Hexagon 100 20 8.5 0.7

90 852 Hexagon 145 20 13.6 1.2

VEDTA further, larger hexagonal nanoplates can be generated, for example, with an edge length of 145 20 nm at VEDTA ¼ 90 mL (Fig. 2G) and 170 20 nm at VEDTA ¼ 110 mL (Fig. 2H). These results demonstrated that both the shape and edge length of Ag nanoplate can be readily adjusted by simply varying the value of VEDTA, though the total amounts of Ag0 generated in different samples are actually the same. According to the discrete dipole approximation (DDA) model,13 the shape evolution of nanoplates from triangular to hexagonal, the increase in their edge length and the modest decrease in their thickness account for the wide spectral range exhibited by nanoplate colloids prepared at different values of VEDTA (Fig. 1A and Table 1).9 Fig. 2B and F display typical ED patterns for triangular and hexagonal nanoplates, respectively, recorded by directing the electron beam perpendicular to the triangular or hexagonal flat faces of an individual nanoplate. In both cases, the diffraction spots are arranged in a 6-fold rotational symmetry, implying that the triangular or hexagonal faces are actually presented by the {111} planes.14a This is consistent with the discussion by Wang in a review article.14b The diffraction spots marked with a triangle, a square and a circle in Fig. 2B and F are due to the {422}, {220}, and 1/3{422} Bragg reflections of face-centered cubic (fcc) Ag ˚ , respectively. Note that with the d-spacing of 0.8, 1.4, and 2.5 A for an fcc lattice, the former two reflections of {422} and {220} are allowed, however, the 1/3{422} reflection is generally forbidden. The forbidden 1/3{422} reflection has also been observed in Pd or Au nanostructures in the form of thin plates or films bounded by atomically flat surfaces.15 Effects of TSC and EDTA

Fig. 2 TEM (A, C and E) and SEM (D, G and H) images of silver colloids prepared at VEDTA ¼ (A) 30, (C) 50, (D) and (E) 70, (G) 90, (H) 110 mL. The scale bars represent 100 nm in (A), (C) and (E), and 200 nm in (D), (G), and (H), respectively. (B) and (F) are ED patterns taken from individual single triangular and hexagonal nanoplates by directing the electron beam perpendicular to the flat faces. The diffraction spots marked with a triangle, a square and a circle could be indexed to the allowed {422}, {220}, and the forbidden 1/3{422} reflections of face-centred cubic (fcc) Ag. See text for details.


A study focused on the effect of EDTA and TSC concentration on the synthesis of nanoplates gave insight into their roles in the process. When N2H4 was not injected into a mixture of an aqueous saturated solution of silver oxide (0.2 mM, 25 mL), TSC (30 mM, 1.5 mL) and EDTA (0.1 M, 0 to 110 mL), even after stirring for 3 h the mixed solution was still colorless. This result indicates that Ag+ can not be reduced by TSC or EDTA at room temperature. When we decreased the quantity of TSC, while keeping the other experimental conditions fixed, silver nanoplates were also observed. However, the as-prepared nanoplates tended to aggregate and deposit. Moreover, we found that the quantity of TSC used in the synthesis had little effect on the peak of SPR of as-prepared nanoplates (ESI† Fig. S2). Hence, it is rational to conclude that TSC molecules function as a stabilizer ligand on nanoplates to prevent them from aggregating. Without EDTA in the reaction mixture, silver nanoplates were not acquired and silver nanoparticles were the main product, even after prolonging the time of stirring. These experimental phenomena showed that EDTA played an important role in the growth of the silver nanoplates. J. Mater. Chem., 2008, 18, 2673–2678 | 2675

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The reaction of Ag2O and N2H4 is represented by eqn (1):


2Ag2O + N2H4 / 4Ag0(s) + 2H2O + N2(g)

(1)

While this equation highlights the simplicity of the reaction, it sheds little light on the mechanism by which the silver nanoplates are formed. The solubility of Ag2O in water (0.053 g L1 at 80 C) is described approximately by eqn (2) with a solubility constant Ksp ¼ 1.9 108:12 Ag2O + H2O / 2Ag+ + 2OH

(2)

The pH of a silver oxide saturated solution in water is measured to be 10. Using the above Ksp value, the major contributor of Ag2O solubility is the Ag+ ion, which has a concentration of roughly 2 104 M. It is known that one mole of EDTA can form complexes with 1–4 mole of Ag+, but the one-to-one form, i.e., (EDTA$Ag)3, prevails as shown by eqn (3), of which the equilibrium constant is logK ¼ 7.32 at pH > 10:16 (EDTA$Ag)3 4 Ag+ + EDTA4

(3)

Therefore, the effective concentration of silver ion, [Ag+], decreases as VEDTA increases. As a result, the rate of reduction reaction (4): 2Ag+ + N2H4 + 4OH / 2Ag0 + N2 + 4H2O

(4)

can be substantially slowed.17 This also assures that the reaction rate remains constant when the concentration of EDTA is fixed. Once the nucleus is formed, the subsequent growth of the nanoplate is supplied by the continuous generation of Ag0 via the reaction chain of (3) and (4), in which (EDTA$Ag)3 serves as a reservoir for Ag+. Therefore, it is EDTA, as a ligand on monomers, that controls the effective concentration of Ag+ so as to slow the reduction reaction (4), providing control over the formation and growth of nanoplates. Growth kinetics of silver nanoplates To further investigate the formation process of Ag nanoplates, the temporal evolution of SPR of shape-specific nanoplates was probed by employing UV-vis spectrometry. Fig. 3A illustrates a typical sequence of spectra monitored by using a sample with 50 mL EDTA added. During the initial stage of approximately 4 minutes (Fig. 3B), only a small band around 425 nm associated with spherical nanoparticles appears. This is followed by a sudden development of a band around 520 nm associated with nanoplates. Then SPR bands of both spherical nanoparticles and nanoplates grow rapidly in a parallel way (Fig. 3B). The SPR appears first near to 425 nm, and then is blue-shifted to 403 nm. Simultaneously, the SPR at 520 nm is red-shifted to 750 nm. Therefore, it is reasonable to assume the formation of small particles in the very beginning of the reaction, some of which developed by isotropic growth into large spheres, and the others by anisotropic growth forming the nanoplates. Fig. 3C displays the maximum absorbance of SPR bands of both spherical nanoparticles (squares) and nanoplates (circles) as a function of time. Slow stage I followed by a relatively fast stage II growth can be clearly identified for both. This is essential to achieve the controlled synthesis of inorganic nanocrystals.18 As 2676 | J. Mater. Chem., 2008, 18, 2673–2678

Fig. 3 (A) The temporal evolution of UV-vis spectra during the formation of silver colloids in the presence of VEDTA ¼ 50 mL. (B) The spectra in the early stage. (C) The maximum absorbance of SPR bands of both nanoparticles (squares) and nanoplates (circles) as a function of time. (D) The UV-vis spectra of VEDTA ¼ 45 mL, (a) before and (b) after another addition of 25 mL Ag2O saturated solution. Spectrum (c) was obtained by multiplying curve (b) by a factor of 1.95.

one can see from Fig. 3C, the growth rate of nanoplates is faster than that of nanoparticles in stage II, and this may be the reason why nanoplates are the main products. Fig. 3C also indicates the reaction is complete in ten minutes. Fig. 3D shows a UV-vis spectrum monitored by using a sample with 45 mL EDTA added: SPR at 678 nm (red line) can be observed. It is consistent with the linear relation and confirms that the SPR is a function of the quantity of EDTA. When another 25 mL Ag2O saturation solution was added into the above colloidal solution, spectrum b was obtained. As the original colloidal solution was diluted, the peak intensities in curve b deceased as compared with those in curve a. If curve b was multiplied by a fact of 1.95 (the diluted multiple), curve c was This journal is ª The Royal Society of Chemistry 2008


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obtained. One can see from curves a and c that the peak at 403 nm is not shifted and its intensity has not changed, however, the peak at 678 nm shifted to 721 nm and its intensity increased. These results indicate that the newly added Ag+ mainly grew into silver nanoplates rather than spherical nanoparticles, suggesting that the growth rate of nanoplates is quicker than that of spherical nanoparticles in stage II. This is consist with the result obtained from Fig. 3C, though the extinction co-efficients of nanoplates and nanoparticles are not the same.

4. Discussion Growth of nanoplates with controlled shapes and sizes by the chelated-mediated method described above allows the preparation of silver nanoplates whose in-plane SPR can be tuned from visible to near IR only by simply varying the concentration of chelate (namely EDTA). It must be noted that all other parameters remain constant. As the concentration of EDTA is the only parameter varied in the synthesis, it is thought to play a key role in determining the sizes and morphologies of nanoplates. In general, the surface energy of nanoplates is much higher than the thermodynamically favored shapes so that their formation requires kinetic control.10 In our experiment, the concentration of EDTA not only determined the effective concentration of silver ions but also affected the reduction rate (i.e. growth kinetics). When the concentration of EDTA was varied, the reduction rate changed. As a result, the percentage of nanoplates of products and the sizes and morphologies of nanoplates also changed. Therefore, it is reasonable that the formation of nanoplates was controlled by the growth kinetics. Although a number of methods of synthesis of silver nanoplates6 have been demonstrated, photochemically or thermally induced transformation of small spherical nanoparticles into triangular nanoplates, most of these methods were slow enough to ensure kinetic control. The method presented here is new, rapid, versatile, and capable of producing silver nanoplates with controllable sizes and morphologies. The presence of a range of sizes and morphologies in the product suggests that the formation of small particles in stage I consists of a mixture of single crystals and twinned crystals.6c,11 Spherical particles of products are formed when single crystals grow isotropically. Twinned crystals are readily formed in silver and gold as well as in silver halides, where the stacking fault energy is lower than in most metals, decreasing the energy required to form a twin plane.19 Most recently, Lofton and Sigmund suggested that the mechanism controlling the crystal habits of fcc Ag follows that used for fcc silver halide crystals.20 The mechanism relies on the presence of twin planes creating favorable sites for the addition of adatoms, leading to accelerating anisotropic growth. This mechanism may apply in the presence synthesis. Indeed, as one can see from Fig. S1 (ESI†), grooves or ridges could be observed running parallel to the top face. when the edges of the as-prepared nanoplates were magnified. The exact nature of the features could not be clearly determined, but they appeared to be the reetrant grooves created by the emergence of twin planes on the fast growing surfaces.21 Furthermore, the fast growth of nanoplates was also confirmed (Fig. 3). Finally, the diffraction spots of 1/3{422}, which are forbidden in a single-crystal fcc metal, can be found for both This journal is ª The Royal Society of Chemistry 2008

Fig. 4 HRTEM images at the corners of (A) triangular and (C) hexagonal nanoplates recorded along the [111] zone axis. (B) and (D) are the high magnification images of (A) and (C), respectively.

triangular and hexagonal nanoplates. Fig. 4A and C show the HRTEM images at the corners of triangular and hexagonal nanoplates, respectively, recorded along the [111] zone axis. One ˚ in both cases, can see that the fringes are separated by 2.5 A corresponding to the 1/3{422} reflection. The above results indicate that twin planes on {111} type crystal faces are present and directing the growth of nanoplates.

5. Conclusions In conclusion, we developed a rapid room-temperature synthesis method of Ag triangular and hexagonal nanoplates by reducing Ag2O with N2H4 in the presence of TSC and EDTA in aqueous phase. In our system, TSC molecules serve as colloid stabilizers to prevent the as-prepared colloids from aggregating, while EDTA molecules serve as a ligand to monomers. The ratio of EDTA to Ag+ significantly changes the rate of reaction, tuning the product morphology and size. The shape (triangular or hexagonal) and edge length of nanoplates were readily controlled by adjusting the quantity of EDTA, providing a SPR response tunable from visible to near infrared. Most importantly, the SPR of nanoplates can be easily tuned from visible to near infrared as required, which could be expected to extend greatly the applications of silver. We also offer a facile and clean method to produce silver nanoplates with controllable size and shape. This method is very simple, inexpensive, green and straightforward. Our very recent results suggested that this chelation-mediated kinetic control over nanoparticle morphology can also be applied for other metal nanostructures.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 90301010, 20373077, 90606004), the J. Mater. Chem., 2008, 18, 2673–2678 | 2677

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Chinese Academy of Sciences (‘‘100 Talents’’ program), and the National Research Fund for Fundamental Key Project 973 (2006CB806200).

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