Controlled growth of silver nanoparticles in a hydrothermal process

China Particuology 5 (2007) 206–212

Controlled growth of silver nanoparticles in a hydrothermal process Juan Zou a,b , Yao Xu c,∗ , Bo Hou c , Dong Wu c , Yuhan Sun c a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China c CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Received 21 November 2006; accepted 17 March 2007

Abstract A two-step synthesis was used to control the shape of silver nanoparticles. First, a few spherical silver nanoparticles, ∼10 nm in size, were prepared via reduction of Ag+ ions in aqueous Ag(NH3 )2 NO3 by poly(N-vinyl-2-pyrrolidone) (PVP). Then, in a subsequent hydrothermal treatment, the remaining Ag+ ions were reduced by PVP into polyhedral nanoparticles, or larger spherical nanoparticles formed from the small spherical seed silver nanoparticles in the first step. The morphology and size of the resultant particles depend on the hydrothermal temperature, PVP/Ag molar ratio and concentration of Ag+ ions. By using UV–visible spectroscopy (UV–vis), transmission electron microscopy (TEM) and powder X-ray diffraction (XRD), the possible growth mechanism of the silver nanoparticles was discussed. © 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Hydrothermal process; Morphology-evolvement; Nanoplates

1. Introduction Metal nanoparticles have been the focus of intensive research in the past several decades due to their potential applications in fabricating electronic, optical, optoelectronic, and magnetic devices that may perform better than using their bulk counterparts (Favier, Walter, Zach, Benter, & Penner, 2001; Halperin, 1986; Hu, Odom, & Lieber, 1999; Kamat, 2002; Templeton, Wuelfing, & Murray, 2000). The properties of metal nanoparticles depend both on their preparation and on the dielectric properties of the surrounding medium (Haynes & Van Duyne, 2001), which affects the size (Kreibig & Genzel, 1985), shape (Yu, Chang, Lee, & Wang, 1997) and state of aggregation (Sun & Xia, 2003). Shape-control has been proved to be as effective as size-control in fine-tuning the functions of metal nanoparticles (Falicov & Somorjai, 1985). The surfuce plasmon resonance (SPR) and fluorescence features of gold or silver nanoparticles have been shown to be highly sensitive to their morphological aspect-ratios (Murphy & Jana, 2002). The number of SPR bands and effective spectral ranges for surface-enhanced Raman scattering (SERS) have also been demonstrated to be dependent on

∗

Corresponding author. Tel.: +86 351 404 9859; fax: +86 351 404 1153. E-mail address: [email protected] (Y. Xu).

the shape of silver nanoparticles (Dick, McFarland, Haynes, & Van Duyne, 2002). Thus, synthesis has been focused on wellcontrolled shapes of gold and silver nanoparticles (Sun & Xia, 2003; Dick et al., 2002). Several methods can be applied to synthesize silver nanoparticles with well-defined shapes. Among these methods, solution-phase methods have the potential to obtain nanostructured metals with well-defined morphologies and high yields, as compared to the gas-phase approach (Graoui, Giorgio, & Henry, 1998; Harris, 1986). For examples, highly anisotropic silver nanodisks were obtained via seed-mediated growth method in the presence of cetyltrimethylammonium bromide (CTAB) soft templates (Maillard, Giorgio, & Pileni, 2003), and polygonal (mainly triangular) silver nanoprisms were synthesized by boiling AgNO3 in N,N-dimethyl formamide, in the presence of poly-(vinylpyrrolidone) (Pastoriza-Santos & Liz-Marzan, 2002). Among most of the solution-phase methods to transform small silver nanospheres into nanoprisms, a heat-induced process was more or less needed (Metraux & Mirkin, 2005; Pastoriza-Santos & Liz-Marzan, 2002). Generally, boiling and refluxing were used as the heat-induced process in the shapeevolution of silver nanoparticles, but the hydrothermal method has seldom been applied as a heat-induced process, though this method can provide a facile and energy-efficient route to prepare well-crystallized metal-oxide nanoparticles (Gervais,

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doi:10.1016/j.cpart.2007.03.006

J. Zou et al. / China Particuology 5 (2007) 206–212

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Table 1 Synthesis components Sample

Ag+ (mmol/L)

Molar ratio of PVP monomer unit to Ag+

Hydrothermal temperature (◦ C)

Morphology of silver nanoparticles Truncated and nontruncated triangular plates, (∼40%), polyhedral Truncated and nontruncated triangular plates, (∼10%), polyhedral, circular disks and spherical Polyhedral (∼20%), circular disks and spherical Polyhedral (∼5%), circular disks and spherical Truncated and nontruncated triangular plates, (∼50%), polyhedral Circular disks and spherical Nanorods (∼5%), polyhedral, smaller spherical Truncated and nontruncated triangular plates, (∼5%), polyhedral Aggregative polyhedral

S1–160

8.000

6

160

S2–160

8.000

20

160

S3–160 S4–160 S5–160

8.000 8.000 60.000

40 80 6

160 160 160

S6–160 S1–100 S1–180

6.000 8.000 8.000

6 6 6

160 100 180

S1–240

8.000

6

240

nanorods nanorods

nanorods

nanorods

Smith, Pottier, Jolivet, & Babonneau, 2001; Hu, Guan, & Yan, 2004; Wang & Li, 2002), metal-sulfide nanoparticles (Chen et al., 2004) and metal/carbon nanocables (Yu et al., 2004). So, in this work, we first used the hydrothermal method to fabricate silver nanoparticles with different morphologies. Furthermore, the effects of various parameters on the size and shape of silver nanoparticles were investigated, including the hydrothermal temperature, PVP/Ag ratio and the concentration of Ag+ ions, in order to determine the optimal condition for synthesizing silver nanoplates. Moreover, the possible morphology-evolving mechanism of silver nanoparticles was also discussed. 2. Experimental 2.1. Raw materials Silver nitrate (AgNO3 , AR), aqueous ammonia (28 wt.%, AR) and PVP (Acros, MW = 1,300,000) were used without further purification in this experiment. All glassware was treated with chromate lotion, followed by repeated washing with distilled water before drying in an oven. 2.2. Sample preparation In a typical procedure, measured amounts of AgNO3 and PVP were separately dissolved in 30 mL water to prepare two aqueous solutions, as shown in Table 1. Ammonia was added dropwise to the aqueous solution of AgNO3 under magnetic stirring until a clear colorless solution was obtained, and then the above solution was mixed with the aqueous solution of PVP. After stirring for 1 h, the mixture was transferred to a Teflonlined autoclave, and the hydrothermal reaction was carried out at different temperature for 36 h. The resultant samples were vacuum-dried at room temperature. 2.3. Characterization UV–visible spectra of the samples were recorded by a Shimadzu 3150 spectrophotometer. Morphology of particles was

Fig. 1. TEM image (a) and UV–vis spectrum (b) of silver nanoparticles before hydrothermal treatment.

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Fig. 2. TEM images of silver nanoparticles prepared at hydrothermal temperature of (a) 100 ◦ C, (b) 160 ◦ C, (c) 180 ◦ C and (d) 240 ◦ C.

observed on a H-600-2 (Hitachi) transmission electron microscope (TEM) using an accelerating voltage of 70 kV. And X-ray powder diffraction (XRD) patterns were derived from a Rigaku D/max-rA 2500 X-ray diffractometer with Cu K␣ radiation (40 kV, 30 mA). 3. Results and discussion 3.1. Formation of silver nanoparticles before hydrothermal treatment As soon as the fresh aqueous solution of Ag(NH3 )2 NO3 was mixed with the aqueous solution of PVP, the color of the mixture changed from colorless to black, then to wine-colored after stirring for half an hour. Fig. 1a shows the morphology of sample

Fig. 3. XRD pattern of silver nanoparticles S1–160.

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Fig. 4. TEM images of (a) S1–160, (b) S2–160, (c) S3–160 and (d) S4–160.

S1–160 before hydrothermal treatment. This sample is mostly composed of spherical nanoparticles with an average diameter of ∼10 nm together with some large aggregated silver particles. The corresponding UV–vis spectrum of S1-160 is shown in Fig. 1b. There is a single absorption peak at ca. ∼436 nm, which represents the characteristic surface plasmon resonance of spherical silver nanoparticles (He et al., 2001). This 436 nm peak is very broad, and significant absorption exists at >700 nm, all consistent with an aggregated silver sol. It can be concluded that the interaction between Ag(NH3 )2 + ions and PVP in the aqueous solution resulted in the reduction of Ag+ ions, demonstrating the function of PVP as a reducing and particle-stabilizing agent, just as reported previously (Jiang, Li, Xie, Gao, & Song, 2004; Jose et al., 2002). However, to realize complete reduction of the Ag+ ions, long-time UV light- or heat-induction is necessary.

So, in our first step, at room temperature and in short durations, reduction of Ag+ ions by PVP is rarely quantitative, and large quantities of unreacted Ag+ ions would remain. 3.2. Influence of hydrothermal temperature on the size and shape of silver nanoparticles After the second-step hydrothermal treatment, all residual Ag+ ions were reduced by PVP, and the small spherical silver nanoparticles formed in the first step were converted to larger silver particles of various shapes. The effect of hydrothermal temperature was investigated from 100 to 240 ◦ C for the samples S1–100, S1–160, S1–180 and S1–240. At 100 ◦ C (Fig. 2a), the silver nanoparticles are mainly irregular nanoplates with some nanorods, surrounded by dots of still smaller spherical particles.

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With increase of hydrothermal temperature, the small particles have grown into larger nanoplates (Figs. 2b and c). However, when the temperature reaches 240 ◦ C the larger nanoplates form aggregates (Fig. 2d). In the range of the hydrothermal temperature investigated, the optimal is 160 ◦ C for maximal crop of nanoplates. All samples prepared at different temperatures exhibit similar XRD patterns, shown typically in Fig. 3 for silver nanoparticles formed at 160 ◦ C with peaks at 37.8◦ , 43.8◦ , 64.2◦ , 77.2◦ , and 81.8◦ , corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) diffractions for face centered cubic (fcc) silver phase (JCPDS File 04-0783). 3.3. Influence of PVP/silver molar ratio Inasmuch as PVP acts as a reducing as well as particlestabilizing agent in metal nanoparticles synthesis, the PVP/metal molar ratio is expected to affect the morphology of the products (Sun, Yin, Mayers, Herricks, & Xia, 2002), as shown in their TEM images (Fig. 4) for shape development from polyhedron to sphere, accompanied by size decrease. At low PVP/silver molar ratios, Ag particles consist mostly of triangular or polyhedral nanoplates (Fig. 4a). As PVP/silver molar ratio increases from 6 to 40, the quantity of polyhedral nanoplates decreases (Fig. 4b), with a gradual shift of particle shape to spherical (Fig. 4c and d). The corresponding UV–vis spectra of samples prepared with different PVP/silver molar ratios are shown in Fig. 5. While the color of the metal particles is determined by the sum of the effects of absorption and scattering of visible light (Kapoor, 1998), according to Mie’s theory (Jin et al., 2001), small spherical nanoparticles exhibit single surface plasmon resonance band, whereas anisotropic particles exhibit more bands in the UV–vis range due to the excitation of plasmon. For silver nanoplates, the 351 nm peak represents the out-of-plane quadrupole resonance; the 470 nm peak, the in-plane quadrupole resonance; the 710 nm peak, the in-plane dipole plasmon resonance; and the 414 nm peak, the out-of-plane dipole plasmon resonance (Jin et al., 2001). It has been demonstrated both theoretically and experimentally that the long wavelength in-plane dipole plas-

Fig. 5. UV–vis spectra of (a) S1–160, (b) S2–160, (c) S3–160 and (d) S4–160.

mon resonance is very sensitive to the aspect ratio (length to diameter) of particles (Jin et al., 2001; Link, Mohamed, & EISayed, 1999; Murphy & Jana, 2002). So the UV–vis spectra can serve to identify the shape evolvement. It can be seen that only sample S1–160 consisting mostly of nanoplates displays the 710 nm peak, corresponding to the in-plane dipole plasmon resonance. With increasing PVP/silver molar ratio, the peaks related to quadrupole resonances (351 and 470 nm), gradually weaken, while the peak (445 nm) corresponding to surface plasmon resonance of spherical silver nanoparticles becomes higher and more symmetric. The changing trend of UV–vis spectra indicates that the particle shape varies from plate to sphere with increasing PVP/silver molar ratio. The result of UV–vis is consistent with that observed by TEM. The role of PVP in shape evolvement is still not clear. One possible function of PVP is to kinetically control the growth rates of different faces by interacting with these faces through adsorption and desorption. Higher PVP/metal molar ratio would enhance isotropic growth of the metal nanoparticles (Bonet, Tekaia-Elhsissen, & Sarathy, 2000). For examples PastorizaSantoz and his co-workers used only low PVP concentration in their synthesis to obtain silver nanoprisms (Pastoriza-Santos & Liz-Marzan, 2002). The result obtained in this paper is consistent with what was mentioned above, and especially proves the important influence of PVP content on morphology under hydrothermal treatment. 3.4. Influence of the concentration of Ag+ ions To investigate the influence of the concentration of Ag+ ions on the morphology of silver nanoparticles, the hydrothermal temperature was fixed at 160 ◦ C and the PVP/silver molar ratio at 6:1, while different concentrations of Ag+ ions, i.e. 6, 8 and 60 mM, were studied (see the samples S6–160, S1–160 and S5–160 in Table 1). The TEM images of the three samples are shown in Fig. 6. The particle shapes of S5–160 and S1–160 are almost the same, both consisting mainly of nanoplates, although the concentration of Ag+ ions is drastically reduced from 60 to 8 mM. However, when the concentration of Ag+ ions goes down further to 6 mM (S6–160), the resulting nanoparticles consist mostly of spheres without any trace of nanoplates, indicating the existence of a critical concentration between 8 and 6 mM to effect such shape transition. The corresponding UV–vis absorption spectra of the three samples (Fig. 7) may also indicates the existence of such morphology transition. The UV–vis spectra of S5–160 are similar to that of S1–160, showing distinct quadrupole plasmon resonances. As mentioned above, the shape of the silver nanoparticles in S6–160 is greatly different from those of the other two, so the UV–vis spectra of S6–160 are distinctly different from those of S5–160 and S1–160, too. The serious decrease in aspect ratio of silver particles results in complete disappearance of the 710 nm peak for S6–160. A very strong peak appears at 438 nm for S6–160, which should be assigned to the presence of quasi-spherical particles.

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Fig. 6. TEM images of (a) S6–160, (b) S1–160 and (c) S5–160 with [Ag+ ] of 6 mM, 8 mM and 60 mM respectively.

These experimental results indicate that the concentration of Ag+ ions, in addition to the PVP/silver molar ratio, affect jointly the shape of silver nanoparticles derived from hydrothermal synthesis. 4. Conclusions

Fig. 7. UV–vis spectra of (a) S6–160, (b) S1–160 and (c) S5–160 with [Ag+ ] of 6, 8 and 60 mM, respectively.

A novel hydrothermal process to prepare polyhedral silver nanoparticles was developed using PVP as a reducing and particle-stabilizing agent. This preparative method is very simple. The shape evolvement of the resultant nanoparticles consists of two steps: spherical silver nanoparticles of ∼10 nm in diameter are firstly obtained from the reduction of part of the Ag+ ions by PVP, and then, the residual Ag+ ions were further reduced hydrothermally in a second step. Large polyhedral particles were

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formed from the spherical silver nanoparticles formed in the first step as seeds. It is found that 160 ◦ C is an optimal temperature to prepare silver nanoplates. And the morphology of silver nanoparticles can well be controlled by varying the reactant composition. The shape of nanoparticles changes from nanoplates to nanospheres with increasing PVP/silver ratio or with decreasing Ag+ ions concentration. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 200573128), the Natural Science Foundation of Shanxi Province, China (Grant No. 20051025), and the Natural Science Foundation for Young Scientists of Shanxi Province, China (Grant No. 2006021031). References Bonet, F., Tekaia-Elhsissen, K., & Sarathy, K. V. (2000). Study of interaction of ethylene glycol/PVP phase on noble metal powders prepared by polyol process. Bull. Mater. Sci., 23, 165–168. Chen, X. Y., Wang, Z. H., Wang, X., Zhang, R., Liu, X. Y., Lin, W. J., & Qian, Y. T. (2004). Synthesis of novel copper sulfide hollow spheres generated from copper (II)–thiourea complex. J. Cryst. Growth, 263, 570–574. Dick, L. A., McFarland, A. D., Haynes, C. L., & Van Duyne, R. P. (2002). Metal film over nanosphere (MFON) electrodes for surface-enhanced Raman spectroscopy (SERS): Improvements in surface nanostructure stability and suppression of irreversible loss. J. Phys. Chem. B, 106, 853–860. Falicov, L. M., & Somorjai, G. A. (1985). Correlation between catalytic activity and bonding and coordination number of atoms and molecules on transition metal surfaces: Theory and experimental evidence. Proc. Natl. Acad. Sci. U.S.A., 82, 2207–2211. Favier, F., Walter, E. C., Zach, M. P., Benter, T., & Penner, R. M. (2001). Hydrogen sensors and switches from electrodeposited palladium mesowire arrays. Science, 293, 2227–2231. Gervais, C., Smith, M. E., Pottier, A., Jolivet, J.-P., & Babonneau, F. (2001). Solid-state 47 49 Ti NMR determination of the phase distribution of titania nanoparticles. Chem. Mater., 13, 462–467. Graoui, H., Giorgio, S., & Henry, C. R. (1998). Shape variations of Pd particles under oxygen adsorption. Surf. Sci., 417, 350–360. Halperin, W. P. (1986). Quantum size effects in metal particles. Rev. Mod. Phys., 58, 533–606. Harris, P. J. F. (1986). Sulphur-induced faceting of platinum catalyst particles. Nature, 323, 792–793. Haynes, C. L., & Van Duyne, R. P. (2001). Nanosphere lithography: A versatile nanofabrication tool for studies of size-dependent nanoparticle optics. J. Phys. Chem. B, 105, 5599–5611.

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