Transformation from Silver Nanoprisms to Nanodecahedra in a

Article pubs.acs.org/JPCC

Transformation from Silver Nanoprisms to Nanodecahedra in a Temperature-Controlled Photomediated Synthesis Haitao Wang, Xianliang Zheng, Jianli Chen, Dechao Wang, Qiyu Wang, Tianyu Xue, Chang Liu, Zhao Jin, Xiaoqiang Cui,* and Weitao Zheng* Department of Materials Science, Key Laboratory of Automobile Materials of MOE, and State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The photomediated transformation of silver nanoparticles is both synthetically useful and mechanistically intriguing. Temperature effects on photochemical synthesis of silver nanoparticles are investigated. The morphology of final products is strongly dependent on the reaction temperature: nanodecahedra are formed at a low temperature of 20 °C; nanoprisms are formed at a higher temperature of 40 °C; and a mixture of shapes results at 30 °C. An interesting transformation process is observed at a lower temperature of 20 °C: silver nanoprisms are grown first and then transformed into nanodecahedra completely. We propose that silver seeds in a type of multitwinning are more stable than the platelike structure at lower temperature during the photochemical growth process. The transformed silver nanodecahedra exhibit greatly superior enhancement of Raman scattering compared to silver nanoprisms. These findings may provide a new insight on photomediated synthesis of silver nanostructures and suggest a new way of thinking about control over the morphology of nanoparticles.

1. INTRODUCTION Silver nanoparticles have been of great interest in the past decade due to their unique physical and chemical properties.1−3 Such particles have been used in the development of many important applications in the fields of optical,4 catalysis,5,6 surface-enhanced Raman scattering (SERS),7−10 and biological diagnostics.11,12 Controlling the shape and size of silver nanoparticles is considered essential in manipulating their localized surface plasmon resonance (LSPR) properties and applications.4,13−15 Therefore, it is not surprising that considerable interest has been shown in the development of synthetic methods for preparing these nanostructures and investigating their size- and shape-dependent properties. To date, the majority of the research has been focused on the preparation of isotropic silver particles with controlled size7,16−18 and shape such as prism,19−21 disk,22 rod,23 icosahedra,24 decahedra,25,26 octahedra,17 wire,27 cube,18 and bipyramid.28 These nanoparticles have been widely used in the field of biosensors and medical therapy.29,30 Photomediated31 and thermal32 synthesis are two main approaches that have been employed in the synthesis of silver nanoparticles. Thermal synthesis usually requires strong reducing agents and frequently high temperatures.13,33 It was a milestone when Mirkin and co-workers19 first proved that silver spherical nanoparticles can be transformed into triangular prisms by a photochemical method. Their series work has demonstrated that light can direct and/or drive the growth of silver nanoprisms by photoexciting silver nanoparticle colloids with wavelengths that overlap the dipole and quadrupole SPR modes of the final silver nanoprisms.19,20,31Several mechanisms © 2012 American Chemical Society

that involve crystal face-blocking and anisotropic surface energetics that create preferential growth on various crystal facets have been proposed.31 For instance, Mirkin and coworkers19 have identified three distinctive stages in nanoprism formation: induction, growth, and termination.19 Brus and coworkers34 propose that the inhomogeneous deposition of silver layer degenerates plasmon resonance splitting into transverse and longitudinal modes that often display different absorption coefficient in the photomediated synthesis, resulting in anisotropic growth. A series of efforts have been attempted to control the shape and size of silver nanoparticles by adjusting the wavelength,20 pH,35 and capping agents36 in photomediated synthesis. Temperature plays an important role in a thermal synthesis. Xia and co-workers37 reported the morphology of nanoparticles (and thus their optical properties) could be readily controlled by changing the reaction temperature. It has been demonstrated that an increase in reduction rate due to an increase of the reaction temperature can promote growth toward thermodynamically favorable structures.16 However, there are few reports on the temperature effect during a photomediated synthesis procedure. In this study, we investigate an interesting phenomenon of controlling the morphology of silver nanoparticles by simply tuning the reaction temperature. This report of temperature effects may be able to provide new insight on photomediated synthesis of silver nanostructures. The process was investigated Received: May 21, 2012 Revised: October 19, 2012 Published: November 1, 2012 24268

dx.doi.org/10.1021/jp304941b | J. Phys. Chem. C 2012, 116, 24268−24273

The Journal of Physical Chemistry C

Article

by UV−vis spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM), and an evolution mechanism is proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. AgNO3 (99.8%) was obtained from Shanghai Reagent No. 1 Plant. NaBH4 (96.0%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Trisodium citrate (99.0%) was purchased from Beijing Chemical Plant. Rhodamine 6G (R6G) was obtained from Waldeck GmbH. All chemicals were used without further purification. 2.2. Instruments. A sodium lamp was purchased from Shanghai Yaming Lighting Co., Ltd. (the spectrum is shown in Figure S1 in Supporting Information). A temperaturecontrolled water-filled tank is designed and implemented to control the reaction temperature from 5 to 50 °C, in which a constant-temperature cycle device was purchased from Nanjing Xian’ou Instrument Manufacturing Co., Ltd. The UV−vis spectrometer (200−800 nm, CHEMUSB4000-UV/vis, Ocean Optics Inc.) was used to detect and monitor the evolution of the photochemical growth process. Transmission electron microscopic (TEM) images were taken with a Tecnai F20 instrument (FEI Co., Japan). Scanning electron microscopy (SEM) was performed on a JEOL JSM-6700 F instrument. Xray diffraction (XRD) patterns were obtained on a Bragg− Brentano diffractometer (D8_tools) in θ−2θ configuration with a Cu Kα line at 0.154 18 nm as a source. The Raman spectrum was detected with a Renishaw 1000 microspectrometer connected to a Leica microscope with an objective lens of 50× (NA = 0.5). The spectra were obtained under a laser power of 2 mW, an accumulation time of 10 s, and an excitation wavelength of 514.5 nm. 2.3. Sample Preparation. For preparation of silver seeds, ultrapure water (88 mL), AgNO3 (1 mL, 10 mM), and sodium citrate (10 mL, 10 mM) were combined in a 250 mL flask. The flask was immersed in an ice bath, and the solution was stirred for 30 min. Aqueous NaBH4 (0.8 mL, 10 mM, freshly prepared with ice-cold ultrapure water prior to injection) was added dropwise into the solution over the next 4 min. The resulting silver colloid was gently stirred for 2 min in ice bath. The reaction solution turned from colorless to bright yellow. This silver colloid exhibits an absorbance band with λmax at 400 nm. The bright yellow solution was then immediately exposed to the sodium lamp (220 W). The reaction temperature was controlled at 20, 30, 40, and 50 °C by the cycling device for a constant temperature during the photomediated synthesis. R6G with a concentration of 8 × 10−5 M was used as the probing molecule to estimate the abilities of silver colloid for Raman enhancement. Before Raman detection, R6G was mixed with silver colloid and incubated for about 30 min. Then the colloidal solution was drop-cast on the surface of a clean glass substrate and dried in the air under ambient temperature. The Raman spectrum of the obtained film was collected by the Raman spectrometer with an excitation line of 514.5 nm.

Figure 1. UV−vis spectra and corresponding final Ag nanoparticle solutions obtained at different reaction temperatures.

characteristic peaks located at 340 and 650 nm (I, IV), which can be attributed to the out-of plane quadrupole resonance and in-plane dipole plasmon resonance of nanoprisms, respectively.19 A wider in-plane dipole absorbance band can be seen in the solution obtained at 50 °C, which should be due to the wide size distribution of nanoprisms.20 Surprisingly, the spectrum shows an absorption at 530 nm (III) due to the inplane dipole and several weak bands around 410 nm (II) due to in-plane quadrupole plasmon modes when the reaction temperature was controlled at 20 °C. This spectrum at 20 °C matches that of decahedra reported in a previous study.38 The product exhibited two LSPR peaks at 520 and 620 nm at a reaction temperature of 30 °C, which suggested that a mixture was obtained. These results indicate that the morphologies of final products changed when the reaction temperature was altered. The final solution color obtained depends upon the reaction temperature. The solution obtained at 20 °C displayed noticeable scattering, which indicated the presence of threedimensional nanoparticles.26 TEM images of four final samples obtained at various temperatures confirm the UV−vis spectra described above (as shown in Figure 2). The silver nanoparticle seeds synthesized from ice bath and without photomediated treatment is spherical with the average size of 5 nm (as shown in the inset of Figure 2A). Figure 2A shows the TEM image of the final colloids obtained at 20 °C, in which only decahedra can be observed. The mean edge length of nanodecahedra was about 51 nm, with the standard deviation calculated to be 5.03 nm (about 10%) (as shown in Figure S2 in Supporting Information). At a temperature of 40 °C, the main products were nanoprisms with truncated corners, and some small spherical nanoparticles can also be found (as shown in Figure 2C).39 Figure 2D shows that many more truncated nanoprisms and irregular particles were obtained at a temperature of 50 °C. We found that some of the nanoprisms were truncated. Some small particles were observed, which might be attributed to the etching process from acid, heat, and photoirradiation. Both prisms and decahedra were obtained at a temperature of 30 °C (as shown in Figure 2B). 3.2. Transformation from Nanoprisms to Nanodecahedra at Low Temperature. To further understand the temperature-dependent growth behavior during the photo-

3. RESULTS AND DISCUSSION 3.1. Different Morphology Growth Depends on Temperature. A UV−vis spectrum was used to investigate the whole photochemical reaction process. Figure 1 shows the UV−vis spectra and optical images of the silver nanoparticle solution obtained at different reaction temperatures. The spectra obtained at 40 °C exhibited typical nanoprisms 24269



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formed by the aggregation of small starting silver nanoparticle seeds.40 This phenomenon could be explained by the weak attractive forces arising after the excess borohydride ions constituting the particle repulsive layer were photooxidized in previous reports.40,41 Meanwhile, a new band appears and increases around 650 nm (peak A in Figure 3A), corresponding to the in-plane dipole bands of the nanoprisms. Another new band at 500 nm appeared and increased after 100 min (peak B). Finally, peak A disappeared and peak B red-shifted to around 530 nm (Figure 3B). Figure 3C shows the photoconversion kinetics of the absorbance at seeds and at peaks A and B. During the initial 220 min, the peak at 400 nm shows a quick decrease in intensity, concomitant with the increase of peaks A and B, which can be ascribed to the formation of nanoprisms and new-type nanostructure. After 4 h irradiation, peak A exhibits a gradual decrease until it completely disappears; meanwhile, peak B shows a gradual increase and consequently become the dominant band. The reaction is completed within 49 h. Figure 3 D−F shows the TEM images taken at different stages of the photomediated growth process. As shown in Figure 3D, the seeds were developed into prisms upon 1.5 h irradiation. However, with continuous irradiation of 12 h, the nanoprisms became truncated and transformed into nanodecahedra, as shown in Figure 3E. Finally, all silver nanoprisms were transformed into nanodecahedra under continued irradiation of 49 h at 20 °C (as shown in Figure 3F,G). 3.3. Morphology at Higher Temperature. We performed a series of experiments to gain insight into the role of reaction temperature in the photomediated growth process. The spectral evolution of the photomediated growth process was quite different from the case of 20 °C when temperature was adjusted to 40 °C, as shown in Figure 4. The reaction is very fast and only an increase of a new band around 650 nm was observed (peak A), which reaches a plateau after 2 h irradiation. Further irradiation caused a blue shift of the band position and narrowing in the bandwidth, concomitant with the appearance of a tailing peak at its long waveband. The results of

Figure 2. TEM images of final silver nanostructures at different temperatures: (A) 20 °C for 49 h; (B) 30 °C for 38 h; (C) 40 °C for 3 h; and (D) 50 °C for 1 h (scale bars = 100 nm). (Inset) TEM image of silver nanoparticles synthesized from ice bath without photomediated treatment (scale bar = 50 nm).

chemical reaction, the process was monitored and characterized by UV−vis and TEM. Figure 3 shows the UV−vis spectral evolution of the photoinduced growth of silver nanoparticles at 20 °C. The peak at 400 nm is a typical LSPR adsorption from silver nanoparticle seeds. The seeds prepared in this work consist of multitwinning and platelike nanocrystals.25 When the silver colloid was exposed to the sodium lamp, the 400 nm band showed a decrease in intensity and a small red shift, owing to the decline of the quantity of seeds and larger particles were

Figure 3. (A, B) Time-dependent UV−vis spectra of silver nanoparticles at 20 °C: (A) 0−220 min and (B) 4−49 h, (C) Corresponding absorbance at silver seeds and at peaks A and B as a function of illumination time. (D−F) TEM images after irradiation for (D) 1.5 h, (E) 12 h, and (F) 49 h. (G) SEM image from panel F. (Scale bars = 100 nm). 24270



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excite the plasmonic growth at 590 nm, which is far from the multitwinning seeds adsorption peak.43,44 This result showed that the temperature-controlled morphology transformation process is also dependent on the wavelength. Further detailed investigation on the effect of wavelength and temperature is ongoing in our group. 3.4. Mechanism Discussion. On the basis of the UV−vis and TEM results, we propose a mechanism of the temperature effects on the morphology control in a photomediated synthesis of the silver nanoparticles (as shown in Scheme 1). Two kind of Scheme 1. Proposed Photomediated Growth Pathways at Different Reaction Temperatures

Figure 4. (A) Time-dependent UV−vis spectra at 40 °C, showing the conversion of silver nanoparticle during 0−3 h. (B) Corresponding absorbance at silver seeds and at peak A as a function of illumination time.

silver seeds, platelike and multitwinning, exist in the seeds solution.25 It has been reported that the particle size and shape can be controlled by choosing the wavelength of light used to drive the photochemical growth.41 The platelike seeds are much more easily excited by photoirradiation than the multitwinning ones under irradiation by sodium lamp, which results in the fast growth of nanoprisms at all temperatures.41 A previous report has showed that multitwinning seeds are more stable at a lower temperature;26 therefore, we propose that multitwinning seeds can be continuously grown with photoirradiation at lower temperature in this work. Both the growing and etching rates of nanoprisms are proportional to temperature; therefore, the reaction at higher temperature will reach a plateau faster. The photoinduced fusion and fragmentation of noble metal nanoparticles occurs simultaneously under light irradiation.45 Hartland and co-workers46 demonstrated that silver nanoparticles fragment into smaller particles under light excitation and their aggregates fuse to form larger particles, owing to excited electron−hole pair effects. In this work, silver nanoprisms begin to dissolve upon further irradiation and the released silver atoms incorporate into the multitwinning seeds, resulting in a transformation from nanoprisms into decahedra at lower temperature.26,34 Therefore, lower temperature prescreens the multitwinning seeds, and then light drives the etching and redepositing of silver nanoparticles for the transformation of decahedra. 3.5. Surface-Enhanced Raman Spectroscopic Application. Diversely shaped silver nanoparticles have been applied as the substrates to enhance the normal Raman signals of probing molecules.22,26 Herein, as-prepared silver nanoprisms and nanodecahedra were used as the surface-enhanced Raman spectroscopy (SERS) substrates for the detection of rhodamine 6G (R6G). Figure 5 shows the SERS signals of R6G (8 × 10−5 mol/L) collected on substrates of glasses without silver, with silver nanoprisms, and with silver nanodecahedra colloid. Similar SERS signals of R6G were detected on both silver nanoprisms and nanodecahedra, but no obvious signals were

UV−vis spectrum and TEM (Figure 2C) indicate that the transformation will not happen at 40 °C, and only silver nanoprisms are obtained. The photoirradiation growth process at 30 °C is also investigated by UV−vis spectrum, as shown in Figure S3 (Supporting Information). The evolution of UV−vis is very similar to that processed at 20 °C at the first 4 h, which indicates a growth of nanoprisms. After 4 h, the intensity of peak A decreases with a blue shift, and peak B starts to increase. Peak A could not be completely transformed into peak B, although we prolonged the irradiation time to 38 h. Both peaks start to decrease due to aggregation after 23 h of irradiation. A mixture of decahedrons and nanoprisms was obtained at 30 °C. The photomediated reaction process at 50 °C is very similar to that at 40 °C but with a faster reaction rate, which reaches a plateau only after 1 h irradiation, as shown in Figure S4 (Supporting Information). Further irradiation after 1 h will result in the decrease and widening of peak A, which means the aggregation of the etched nanoprisms,42 as shown in the TEM image of Figure S5 (Supporting Information). It has been reported that the photomediated synthesis is sensitive to the irradiation wavelength.43 To investigate the effect of wavelength on the photomediated synthesis, we performed the experiment with light-emitting diodes (LEDs) at the wavelengths of 503 and 590 nm, which are close to the irradiation wavelength of sodium lamp (see Figure S1 in Supporting Information). The UV−vis spectra of photomediated synthesis at 503 and 590 nm are shown in Figures S6 and S7 (Supporting Information). The results showed that, at 20 °C, the transformation happens only under irradiation at 503 nm but not at 590 nm. This may be due to the lower irradiation energy of 590 nm than that of 503 nm.34,41 The other reason may be that irradiation is not able to 24271



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detected on the bare glass substrates, indicating the significant role of silver nanoparticles on the dramatic Raman enhancement. Characteristic bands of R6G can be observed in the spectrum: the bands at 1650, 1506, and 1360 cm−1 are assigned to aromatic C−C stretching mode; the bands at 1573 and 1310 cm−1 are assigned to N−H in-plane bend mode; and the 774 and 611 cm−1 bands are due to vibronic coupling.47 Interestingly, nanodecahedra present enhancement of 3 times higher than that of the nanoprisms. This can be attributed to the fact that SPR peaks of nanodecahedra are close to the laser wavelength to excite Raman hot spots.26,48 This makes silver nanodecahedra a highly promising SERS substrate for R6G detection.

4. CONCLUSION We have presented an interesting study on the transformation from silver nanoprisms to nanodecahedra in a temperaturecontrolled photomediated synthesis. The results show that the morphology of final products is strongly dependent on the reaction temperature: nanodecahedra are formed at a low temperature of 20 °C; nanoprisms are formed at a higher temperature of 40 °C; and a mixture of shapes results at 30 °C. A mechanism is proposed to interpret the temperature effects on morphology control in a photomediated synthesis. These findings suggest a new way of thinking about control over the morphology of nanoparticles in solution phase. ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

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Figure 5. Raman spectra of R6G adsorbed on the substrates of glasses without silver, with silver nanoprisms, and with silver nanodecahedra.

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S Supporting Information *

Seven figures, showing additional UV−vis and TEM results. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail [email protected] (X.C.) or [email protected] (W.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21075051, 21143008, and 50832001), Program for New Century Excellent Talents in University (NCET-10-0433), and the 211 and 985 projects of Jilin University, China. 24272



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Transformation from Silver Nanoprisms to Nanodecahedra in a

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