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Sunlight-driven synthesis of anisotropic silver nanoparticles
Chemical Engineering Journal 260 (2015) 99–106
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Sunlight-driven synthesis of anisotropic silver nanoparticles Bin Tang a,b,⇑, Lu Sun a,b, Jingliang Li b, Mingwen Zhang b, Xungai Wang a,b,⇑ a b
School of Textile Science and Engineering, Wuhan Textile University, Wuhan 430073, China Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Sunlight is used to prepare
Sunlight
anisotropic silver nanoparticles. High concentration of citrate
promotes the formation of pyramidal silver nanoparticles during photoinduced reaction. UV light from sunlight plays an important role in shape conversion.
Silver seed
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 11 August 2014 Accepted 16 August 2014 Available online 21 August 2014 Keywords: Solar radiation Silver nanoprism Silver nanodecahedron Shape conversion Photoinduction
Silver nanodecahedron
a b s t r a c t Photoinduced shape conversion of silver nanoparticles was realized using sunlight. The silver seeds were transformed to silver nanoprisms under sunlight when the concentration of citrate was low (65.0 10 4 M). Nevertheless, sunlight converted the obtained silver nanoprisms to silver nanodecahedrons when the concentration of citrate in reaction system was increased. It was found that the ultraviolet light from sunlight played a vital role in the shape conversion from nanoprism to nanodecahedron. Lighting power density did not influence the shape conversion except for reaction rate. Besides, the silver nanodecahedrons were synthesized in the mixed solution of AgNO3 and citrate in absence of silver seeds through irradiation by simulated sunlight. The mechanism on the sunlight induced synthesis of silver nanoparticles was discussed. Anisotropic silver nanoparticles including nanoprisms and nanodecahedrons were obtained through controlling the citrate concentration and irradiation time by sunlight from green light source. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles with different morphologies have attracted increasing attention in the past decades because of their great potential for applications in surface enhanced spectroscopy [1–3], biological and chemical sensing [4,5], and diagnosis and therapy of diseases [6]. These applications are derived from the unique
⇑ Corresponding authors at: Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia. E-mail addresses: [email protected] (B. Tang), [email protected]. au (X. Wang). http://dx.doi.org/10.1016/j.cej.2014.08.044 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
Silver nanoprism
optical properties of silver nanoparticles, which are known as localized surface plasmon resonance (LSPR) [7–10]. The LSPR of noble metal nanoparticles is sensitive to their size, shape, composition and surroundings [11–15]. The LSPR spectra can be effectively tuned by controlling the shape and size of silver nanoparticles. Many methods have been devised to prepare anisotropic silver nanoplates and tailor their morphologies, such as photoinduction [16,17], thermal treatment [18–20], and ion etching [21–23]. Among them, the photoinduction approach as an effective synthesis route for silver nanoprisms was first reported by Mirkin et al. [17]. Subsequently, many approaches based on photoinduction have been used to synthesize the anisotropic silver
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nanoparticles, such as hexagon [24], disk [25,26], tetrahedron [27,28], decahedron [29–31], and bipyramid [32,33]. Nevertheless, the light sources used to excite the reaction systems were generally expensive and delicate, including laser, ultraviolet (UV) light and light-emitting diode (LED). In the synthesis methods, sodium borohydride as reducing agent is generally used reduce silver ions to form silver nanoparticles [34–36]. In addition, trisodium citrate is also extensively employed as stabilizing and reducing agents in synthesis reactions of silver nanoparticles [16,17,34,37–39]. Sunlight is the largest clean renewable source of energy and solar radiation is non-toxic, non-polluting and traceless in chemical processes [40,41]. Gold nanoparticles were synthesized by sunlight in the previous reports [42–44]. There have been a few reports about the synthesis of silver nanoparticles using sunlight as the photoinducing source [45–48]. Yin et al. found that the dissolved organic matter (DOM) in natural water could reduce silver ions to nanoparticles under sunlight. They suggested that the reduction was mediated by superoxide from photoinduction of the phenol group in DOM and the dissolved O2 enhanced the formation of silver nanoparticles. Silver nanoparticles were synthesized using sunlight as inducing light source in the presence of an anionic surfactant (sodium dodecyl sulfate, SDS) [48] or an ethanol extract of Andrachnea chordifolia [49]. However, the silver nanoparticles prepared from induction of sunlight were generally spherical or amorphous. It was demonstrated that shape-controlled synthesis of anisotropic silver nanoparticles was significant in application related with LSPR property, such as surface enhanced Raman scattering (SERS) and biological sensing. Herein, a simple and green synthesis method was developed to achieve photoinduced synthesis of anisotropic silver nanoparticles using sunlight as an irradiation source. Evolution of LSPR of silver nanoparticles under sunlight irradiation was monitored through UV–vis extinction spectra. Moreover, the morphologies of silver nanoparticles were characterized at different periods during photoinduced reaction. The influence of citrate concentration and inducing light on the photoinduction process and shape of silver nanoparticles was also investigated. The mechanism of the sunlight induced synthesis of silver nanoparticles was discussed. 2. Experimental 2.1. Materials AgNO3 (>99%), trisodium citrate (P99.0%) and sodium borohydride (>98%) were purchased from Sigma–Aldrich. All the chemicals were of analytical grade and used as received. 2.2. Characterization Extinction spectra of silver nanoparticle solutions and transmission spectra of optical filters were recorded using a Varian Cary 3E UV/vis spectrophotometer. Emission spectra of light sources were obtained using Ocean Optics USB4000 spectrometer. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM2100 and a JEOL JEM-2100F with an acceleration voltage of 200 kV, respectively. Specimens for TEM analysis were prepared by dripping a drop of silver nanoparticle solution onto the carbon coated copper grids and drying them in air at room temperature. 2.3. Photoinduced synthesis of anisotropic silver nanoparticles First, silver seeds were prepared by dropwise addition of sodium borohydride solution (8.0 10 3 M, 1.0 mL) to an aqueous solution of AgNO3 (10 4 M, 100 mL) in the presence of trisodium citrate
(5.0 10 4 M, 10 3 M, 2.5 10 3 M and 5.0 10 3 M) under vigorous stirring. Yellow silver seed solutions were obtained. After stirring for 3 min, the silver seed solutions in quartz beakers covered with quartz dishes were then placed under simulated sunlight for different periods inside a Suntest instrument with an air-cooled xenon lamp (SUNTEST XLS+ from Atlas Material Testing Technology LLC). The color of silver colloids changed from yellow to green, blue, purple, brownish red or brownish yellow during the irradiation process. The natural sunlight induction of silver seeds was performed in glass flask on clear days in August, which is winter in Geelong, Australia. 3. Results and discussion 3.1. Photoinduced synthesis under natural sunlight The color evolution of the silver solutions with different concentrations of citrate under irradiation of natural sunlight is shown in Fig. S1. After the yellow silver seed solutions were irradiated for 3 h, the solutions changed to blue or green, which was due to the transformation of silver seeds under sunlight. As can be found, the initial concentration of citrate in solution influenced the evolution of colors of silver nanoparticle solutions. Moreover, the final colors of silver nanoparticle solutions were obviously different when the citrate concentrations of solution varied (Fig. S1). The extinction spectra of silver nanoparticles were recorded at different time intervals in the process of sunlight irradiation (Fig. 1). New extinction bands of solutions with different concentrations of citrate appeared after the silver seed solutions were irradiated for 3 h, which indicates that the anisotropic silver nanoparticles were produced during irradiation by sunlight. It was noted that the extinction band around 400 nm attributed to silver seeds decreased in intensity dramatically in the photoinduced process. The change rate of this extinction band increased with an increase in citrate concentration of silver nanoparticle solutions, implying that citrate influenced visibly the synthesis of silver nanoparticles induced by sunlight. It is difficult to investigate exactly the evolution of silver nanoparticles under sunlight, as the condition of natural sunlight, especially lighting power, may be different each day. The spectrum of natural sunlight was measured to be in the range of 300–800 nm (Fig. 2A). The main emission band is located in 450–600 nm in the natural sunlight spectrum. The emission spectrum from the Suntest instrument is shown in Fig. 2B, very similar to the spectrum of natural sunlight. Therefore, the light from the Suntest instrument can be employed as simulated sunlight for the sunlight induced synthesis of silver nanoparticles to gain insight into the interaction of sunlight and silver nanoparticles. The lighting power of simulated sunlight can be controlled to eliminate the influences from instability of natural sunlight on photoinduced reaction. 3.2. Photoinduced synthesis under simulated sunlight Fig. S2 shows the photographs of silver nanoparticle solutions with different concentrations of citrate under irradiation of simulated sunlight with 500 W/m2 of lighting power density. The color evolution of silver nanoparticle solutions obtained by simulated sunlight was similar to that of silver nanoparticle solutions synthesized by natural sunlight. Fig. 3 depicts the extinction spectra of silver nanoparticle solutions from photoinduction of simulated sunlight. A new extinction band around 580 nm appeared in the extinction spectra of silver nanoparticles with 5.0 10 4 M of citrate after the solution was irradiated for 1 h (Fig. 3A). The intensity of this new band increased as irradiation time increased. Meanwhile, the extinction band around 400 nm attributed to silver
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Fig. 1. Evolution of extinction spectra of silver nanoparticle solutions during natural sunlight induced process corresponding to Fig. S1. The concentration of citrate in solution: (A) 5 10 4 M, (B) 10 3 M, (C) 2.5 10 3 M and (D) 5.0 10 3 M.
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Fig. 2. Spectra of natural sunlight (A) and simulated sunlight from the Suntest instrument (B).
seeds decreased in intensity significantly. The characterization of extinction spectra of silver nanoparticle solutions indicates that the silver seeds were transformed into anisotropic silver nanoparticles, which is similar to the case of natural sunlight (Fig. 1). When the silver solution was irradiated for 3 h, the extinction band of silver seeds disappeared and the extinction spectrum displayed the characteristic LSPR bands of silver nanoplates (nanoprism and nanodisk) (Fig. 3A) [17,26]. The LSPR band at long wavelength blue-shifted when light irradiation was prolonged. Moreover, a shoulder band arose in the LSPR band around 480 nm, which reveals that the silver nanoparticles were transformed to different shape. TEM was employed to observe the changes in morphologies of silver nanoparticles during photoinduced process. Fig. 4 shows the TEM images of silver nanoparticles with 5.0 10 4 M of cit-
rate during irradiation process. Some silver nanoparticles in Fig. 4A exhibited anisotropy when the silver nanoparticle solution was irradiated for 1 h. The anisotropy of partial silver nanoparticles resulted in the appearance of the new LSPR band at long wavelength. However, most of silver nanoparticles were still the silver seeds. Lots of silver nanoprisms were formed when the silver nanoparticle solution was irradiated for 2 h (Fig. 4B). There were still some small nanoparticles in the visual field. Silver nanoprisms dominated in TEM images, accompanying with some nanodisks, after 5 h of irradiation (Fig. 4C). When the silver nanoparticles were irradiated for 8 h, a few of pyramidal silver nanoparticles were seen in the TEM images (Fig. 4D), which suggests that the shape conversion from triangular to pyramid happened. Nevertheless, the proportion of pyramidal nanoparticles in the samples was small. It was inferred that the shoulder extinction
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Fig. 3. Evolution of extinction spectra of silver nanoparticle solutions under simulated sunlight with 500 W/m2 of lighting power density corresponding to different concentrations of citrate: (A) 5 10 4 M, (B) 10 3 M, (C) 2.5 10 3 M and (D) 5.0 10 3 M.
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3.3. Effect of citrate on morphologies of silver nanoparticles The concentration of citrate in solution affected obviously the optical properties and morphologies of silver nanoparticles during simulated sunlight photoinduced reaction. The evolution of the
extinction spectra of silver nanoparticles was different when the concentration of citrate in solution was changed (Fig. 3). The intensity of the shoulder band in the extinction spectra after the same exposure period increased with an increase in citrate concentration. When the concentration of citrate was 10 3 M, the shoulder band around 480 nm increased in intensity, compared with that corresponding to 5 10 4 M of citrate (Fig. 3A and B). Meanwhile, the intensity of the extinction band around 570 nm decreased. It is
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noted that the extinction band at 480 nm became a main band in the final extinction spectra of silver nanoparticle solutions when the concentration of citrate increased to 2.5 10 3 M and 5.0 10 3 M (Fig. 3C and D). The changes of the extinction spectra arose from the shape conversion of silver nanoparticles. These results reveal that proportion of pyramidal nanoparticles in the samples increased as the citrate concentration was increased. In addition, extinction spectra of silver nanoparticle solution with lower concentration of citrate (2.5 10 4 M) were recorded during irradiation of simulated sunlight (Fig. S3). There is no new shoulder extinction band appearing in the extinction spectra even that the solution was irradiated for 8 h, implying that no pyramidal nanoparticles were formed at low concentration of citrate. TEM characterization was performed to further investigate the effect of citrate on morphologies of silver nanoparticles in the simulated sunlight induced reaction. The silver nanoparticles with a high concentration of citrate (2.5 10 3 M and 5.0 10 3 M) became platelike (Fig. S4B and C) after only 1 h irradiation, which is different from the case with 10 3 M of citrate (Fig. S4A). Comparing the case with 5 10 4 M of citrate, the proportion of pyramidal nanoparticles increased obviously after 8 h of irradiation when the concentration of citrate was increased to 10 3 M (Fig. 5A). Almost all silver nanoparticles, after 8 h of light irradiation, changed to pyramidal nanoparticles (including nanodecahedrons) when the concentration of citrate was 2.5 10 3 M (Fig. 5B), which suggests that high concentration of citrate can increase the yield of pyramidal nanoparticles in the photoinduced process of simulated sunlight. Fig. 6 displays the TEM images of silver nanoparticles with 5.0 10 3 M of citrate. A few silver pyramidal nanoparticles were observed in the image of silver nanoparticles after 2 h of irradiation (Fig. 6A). The decahedral nanoparticles were dominant products after 5 h of irradiation (Fig. 6B and Fig. S5). The decahedral silver nanoparticles did not change visibly in shape when the irradiation time was increased to 8 h (Fig. 6C), except for a small change in the spectra of silver nanoparticles when the irradiation time was prolonged (Fig. 3D). The results further proved that the high concentration of citrate promoted the formation of decahedral silver nanoparticles in short time under simulated sunlight. Besides, a typical fivefold twinned structure was observed in the TEM image of final silver nanodecahedron (insert in Fig. 6C), which is consistent with the previous reports [29,50]. Combining extinction spectra and TEM images of silver nanoparticles in the present study, it can inferred from previous studies that the extinction bands at 403 and 477 nm in the extinction spectrum of silver nanoparticles with 5.0 10 3 M of citrate after 8 h irradiation (Fig. 3D) should be assigned to the transverse and longitudinal plasmon modes of silver nanodecahedrons [31,51]. Additionally, the exposure time of silver nanoparticle solution with 5.0 10 4 M of citrate was increased to 28 h under simulated sunlight, which provided inspection whether all the silver nanoparticles could be converted to pyramidal nanoparticles under
Fig. 5. TEM images of silver nanoparticles with (A) 10
low concentration of citrate through prolonged light irradiation. The corresponding extinction spectrum of silver nanoparticles presents a main LSPR band at 455 nm and a shoulder band at 395 nm (Fig. S6A). A number of platelike silver nanoparticles exists in the TEM image of silver nanoparticles even irradiated for 28 h (Fig. S6B). The results indicate that citrate concentration in solution plays a significant role in conversion from nanoplates to pyramidal nanoparticles during sunlight induced synthesis of silver nanoparticles. 3.4. Effect of inducing light source The influences of lighting power density of light source were inspected. Fig. S7 displays the extinction spectra of silver nanoparticle solutions with different concentration of citrate under 250 W/m2 of simulated sunlight. The evolution of extinction spectra was similar to the case corresponding to 500 W/m2 of lighting power density. However, the change rate of extinction intensity of silver seed LSPR band decreased. These results reveal that lighting power density only influenced the reaction rate. To gain insights into the effect of simulated sunlight on the photoinduced reaction of silver nanoparticles, different optical filters (Filter I–IV) were used to cover the reaction vessels during irradiation process to investigate influences from the different parts of sunlight. The transmission spectra of the optical filters (Filter I–IV) are shown in Fig. 7A–D. The corresponding extinction spectra of silver nanoparticle solutions with 2.5 10 3 M of citrate covered with the optical filters after irradiation for 8 and 16 h are presented in Fig. 7E–H. Filter I totally blocks the light with wavelength less than 385 nm (Fig. 7A). The extinction spectrum of silver nanoparticle solution covered with Filter I displayed the characteristic LSPR bands of silver nanoplates after irradiation for 8 h (Fig. 7E). All the silver nanoparticles were platelike in the TEM image (Fig. S8A), which is different from the case without filter covering (Fig. 5B). A prolonged light exposure (16 h) only led to formation of very few decahedral silver nanoparticles (Fig. S8B) though some changes occurred in extinction spectra of silver nanoparticles (Fig. 7E). It can be inferred that the light with short wavelength is crucial for formation of pyramidal nanoparticles in the sunlight induced process. Filter II bocks all the light in the region of 340– 405 nm and the partial light less than 340 nm (Fig. 7B). As can be seen, the intensity of simulated sunlight in the range of 300– 340 nm is very low (Fig. 2B). The light transmitting from Filter II mainly belongs to visible light. The extinction spectra (Fig. 7F) and TEM images (Fig. S8C and D) demonstrate that the silver nanoparticles with 2.5 10 3 M of citrate obtained after irradiated for 8 and 16 h covered with the Filter II were nanoplates. Similarly, the extinction spectra of silver nanoparticles obtained from Filter III and IV only imply the conversion from silver seeds to silver nanoplates (Fig. 7C and D). And no pyramidal nanoparticles were observed in TEM characterization (Fig. S9). The results
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demonstrate that short-wavelength light, especially UV light (<400 nm), is necessary to conversion from nanoplates to pyramidal nanoparticles in the sunlight induced synthesis of silver nanoparticles. Furthermore, UV lamp was employed to irradiate silver seed solution to investigate role of UV light in the shape conversion of silver nanoparticles. The emission spectrum of UV lamp shown in Fig. S10 presents a main emission band in the range of 280– 360 nm. The extinction band of the seed solution with 2.5 10 3 M of citrate blue-shifted to 405 nm from 393 nm and increased in intensity after irradiation for 6 h under UV lamp (Fig. 8A and Fig. S11). It was found from TEM images that decahedral silver nanoparticle were obtained after UV light irradiation (Fig. 8B), which reveals that UV light can induce the formation of
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It was possible that photothermal effect promoted the shape conversion of silver nanoparticles in the process of the photoinduced reaction. The temperature of reaction solution was measured to be around 40 °C when the irradiation was performed by simulated sunlight. In order to clarify the influence of heat effect from irradiation, the silver nanoparticle solution (initial solution) from 2 h irradiation of simulated sunlight was wrapped with aluminum foil to eliminate the light effect. The extinction spectrum
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of initial silver nanoparticle solution after 3 h irradiation changed slightly in wavelength (Fig. S12), which implies that the heat originated from light irradiation did not contribute to shape conversion of silver nanoparticles during simulated sunlight induced reaction. Moreover, the silver nanoparticles from 2 h irradiation by simulated sunlight was heated for 3 h at 45 °C. The extinction bands around 500 nm of the initial silver nanoparticle solution blueshifted after heat treatment (Fig. S12). However, the characteristic profile of the LSPR extinction spectrum did not changed obviously. These results further testify that heat treatment did not transform silver nanoplates into pyramidal nanoparticles. The synthesis of anisotropic silver nanoparticles under sunlight irradiation contained two stages: fabrication of the silver nanoplates and conversion from nanoplates to pyramidal nanoparticles. The morphology transformation of silver nanoparticles can be controlled by the wavelength of irradiating light [52,53]. The silver nanoplates were formed in the first stage of simulated sunlight induced process. It has been suggested in previous studies that silver seeds in the presence of citrate were transformed to silver nanoplates including nanoprisms and nanodisks through irradiation by light with long wavelength (>500 nm) [29,54,55]. It was thought that the silver seed solution contained platelike and multi-twinning seeds. In this study, it was proposed that the visible light (long-wavelength light) in the sunlight induced fast growth of nanoplates from platelike seeds in the first stage. Compared with multi-twinning seeds, the platelike silver seeds were easily excited by light irradiation [52,53]. The silver nanoplates were fabricated at first from transformation of silver seeds under long-wavelength light from sunlight. Subsequently, in the second stage, the shape conversion from nanoplates to decahedral nanoparticles from irradiation of UV light (short-wavelength light) in sunlight dominated the photoinduced process. It was suggested that the nanoplates were less stable relative to the decahedral nanoparticles [31]. The silver nanoplates dissolved under further irradiation and the released silver incorporated into the growing multi-twinning seeds, leading to the shape conversion from nanoplates to nanodecahedrons. Besides the effect of light, citrate is essential to the shape conversion of silver nanoparticles under sunlight irradiation. It was demonstrated that citrate as a capping agent stabilized silver nanoparticles and as a reducing agent reduced silver ions from oxidization of silver nanoparticles in solution [39,56]. It was suggested that LSPR excitation of silver nanoparticles from light irradiation induced the formation of so-called ‘‘hot holes’’ on nanoparticles that were then filled with electrons from the oxidization of adsorbed citrate [30,51,56]. The silver ions from oxidative etching of silver in the presence of O2 were reduced on specific facets of nanoparticles by the electrons from photo-oxidation of citrate, resulting in growth and shape conversion of silver nanoparticles [30].
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3.6. Seed-free sunlight induced synthesis of silver nanoparticles Seed-mediated photoinduced method is one of the most effective routs for synthesis of anisotropic silver nanoparticles. In the present study, in addition to seed-mediated strategy, the synthesis of silver nanoparticles in the absence of silver seeds was also realized under irradiation of simulated sunlight. The mixed solution of AgNO3 (10 4 M) and citrate (2.5 10 3 M) was irradiated for 12 h under simulated sunlight. The extinction spectrum of solution after light irradiation displays characteristic LSPR bands of silver nanodecahedrons (Fig. 9A), implying that the silver nanodecahedrons were prepared using AgNO3 as precursor in the presence of citrate through irradiation of simulated sunlight. Furthermore, TEM characterization demonstrated that most of the prepared silver nanoparticles were decahedral in shape (Fig. 9B). The results indicate that simulate sunlight irradiation could give rise to production of silver nanodecahedrons from mixture of AgNO3 and citrate in the absence of silver seeds, which is similar to work of Yang et al. [51]. Significantly, compared with monochromatic light from blue light-emitting diodes (LEDs), it is convenient, energy-saving and environmentally friendly to use sunlight to induce the seedfree synthesis of silver nanodecahedrons.
4. Conclusions Silver nanoprisms and nanodecahedrons were synthesized from silver seeds through shape conversion induced by sunlight. The concentration of citrate in solution influenced the shapes of silver nanoparticles in the sunlight induced synthesis process. Silver nanoprisms dominated in the products when reaction system contained citrate with a low concentration (5 10 4 M). Decahedral silver nanoparticles were obtained from silver nanoplates when concentration was increased to 5 10 3 M. UV light from sunlight was proved to be crucial in the synthesis of pyramidal silver nanoparticles. In addition, silver nanodecahedrons were prepared from the mixed solution of AgNO3 and citrate in the absence of silver seeds by irradiation of simulated sunlight. Sunlight-induced synthesis of silver nanoparticles is significant for application of green light source in preparation of nanoparticles.
Acknowledgments This research was supported by the National Natural Science Foundation of China (NSFC 51273153), and the Central Research Grants Scheme and Alfred Deakin Postdoctoral Research Fellowship scheme at Deakin University.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.08.044.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Sunlight-driven synthesis of anisotropic silver nanoparticles Bin Tang a,b,⇑, Lu Sun a,b, Jingliang Li b, Mingwen Zhang b, Xungai Wang a,b,⇑ a b
School of Textile Science and Engineering, Wuhan Textile University, Wuhan 430073, China Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Sunlight is used to prepare
Sunlight
anisotropic silver nanoparticles. High concentration of citrate
promotes the formation of pyramidal silver nanoparticles during photoinduced reaction. UV light from sunlight plays an important role in shape conversion.
Silver seed
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 11 August 2014 Accepted 16 August 2014 Available online 21 August 2014 Keywords: Solar radiation Silver nanoprism Silver nanodecahedron Shape conversion Photoinduction
Silver nanodecahedron
a b s t r a c t Photoinduced shape conversion of silver nanoparticles was realized using sunlight. The silver seeds were transformed to silver nanoprisms under sunlight when the concentration of citrate was low (65.0 10 4 M). Nevertheless, sunlight converted the obtained silver nanoprisms to silver nanodecahedrons when the concentration of citrate in reaction system was increased. It was found that the ultraviolet light from sunlight played a vital role in the shape conversion from nanoprism to nanodecahedron. Lighting power density did not influence the shape conversion except for reaction rate. Besides, the silver nanodecahedrons were synthesized in the mixed solution of AgNO3 and citrate in absence of silver seeds through irradiation by simulated sunlight. The mechanism on the sunlight induced synthesis of silver nanoparticles was discussed. Anisotropic silver nanoparticles including nanoprisms and nanodecahedrons were obtained through controlling the citrate concentration and irradiation time by sunlight from green light source. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles with different morphologies have attracted increasing attention in the past decades because of their great potential for applications in surface enhanced spectroscopy [1–3], biological and chemical sensing [4,5], and diagnosis and therapy of diseases [6]. These applications are derived from the unique
⇑ Corresponding authors at: Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia. E-mail addresses: [email protected] (B. Tang), [email protected]. au (X. Wang). http://dx.doi.org/10.1016/j.cej.2014.08.044 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
Silver nanoprism
optical properties of silver nanoparticles, which are known as localized surface plasmon resonance (LSPR) [7–10]. The LSPR of noble metal nanoparticles is sensitive to their size, shape, composition and surroundings [11–15]. The LSPR spectra can be effectively tuned by controlling the shape and size of silver nanoparticles. Many methods have been devised to prepare anisotropic silver nanoplates and tailor their morphologies, such as photoinduction [16,17], thermal treatment [18–20], and ion etching [21–23]. Among them, the photoinduction approach as an effective synthesis route for silver nanoprisms was first reported by Mirkin et al. [17]. Subsequently, many approaches based on photoinduction have been used to synthesize the anisotropic silver
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nanoparticles, such as hexagon [24], disk [25,26], tetrahedron [27,28], decahedron [29–31], and bipyramid [32,33]. Nevertheless, the light sources used to excite the reaction systems were generally expensive and delicate, including laser, ultraviolet (UV) light and light-emitting diode (LED). In the synthesis methods, sodium borohydride as reducing agent is generally used reduce silver ions to form silver nanoparticles [34–36]. In addition, trisodium citrate is also extensively employed as stabilizing and reducing agents in synthesis reactions of silver nanoparticles [16,17,34,37–39]. Sunlight is the largest clean renewable source of energy and solar radiation is non-toxic, non-polluting and traceless in chemical processes [40,41]. Gold nanoparticles were synthesized by sunlight in the previous reports [42–44]. There have been a few reports about the synthesis of silver nanoparticles using sunlight as the photoinducing source [45–48]. Yin et al. found that the dissolved organic matter (DOM) in natural water could reduce silver ions to nanoparticles under sunlight. They suggested that the reduction was mediated by superoxide from photoinduction of the phenol group in DOM and the dissolved O2 enhanced the formation of silver nanoparticles. Silver nanoparticles were synthesized using sunlight as inducing light source in the presence of an anionic surfactant (sodium dodecyl sulfate, SDS) [48] or an ethanol extract of Andrachnea chordifolia [49]. However, the silver nanoparticles prepared from induction of sunlight were generally spherical or amorphous. It was demonstrated that shape-controlled synthesis of anisotropic silver nanoparticles was significant in application related with LSPR property, such as surface enhanced Raman scattering (SERS) and biological sensing. Herein, a simple and green synthesis method was developed to achieve photoinduced synthesis of anisotropic silver nanoparticles using sunlight as an irradiation source. Evolution of LSPR of silver nanoparticles under sunlight irradiation was monitored through UV–vis extinction spectra. Moreover, the morphologies of silver nanoparticles were characterized at different periods during photoinduced reaction. The influence of citrate concentration and inducing light on the photoinduction process and shape of silver nanoparticles was also investigated. The mechanism of the sunlight induced synthesis of silver nanoparticles was discussed. 2. Experimental 2.1. Materials AgNO3 (>99%), trisodium citrate (P99.0%) and sodium borohydride (>98%) were purchased from Sigma–Aldrich. All the chemicals were of analytical grade and used as received. 2.2. Characterization Extinction spectra of silver nanoparticle solutions and transmission spectra of optical filters were recorded using a Varian Cary 3E UV/vis spectrophotometer. Emission spectra of light sources were obtained using Ocean Optics USB4000 spectrometer. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM2100 and a JEOL JEM-2100F with an acceleration voltage of 200 kV, respectively. Specimens for TEM analysis were prepared by dripping a drop of silver nanoparticle solution onto the carbon coated copper grids and drying them in air at room temperature. 2.3. Photoinduced synthesis of anisotropic silver nanoparticles First, silver seeds were prepared by dropwise addition of sodium borohydride solution (8.0 10 3 M, 1.0 mL) to an aqueous solution of AgNO3 (10 4 M, 100 mL) in the presence of trisodium citrate
(5.0 10 4 M, 10 3 M, 2.5 10 3 M and 5.0 10 3 M) under vigorous stirring. Yellow silver seed solutions were obtained. After stirring for 3 min, the silver seed solutions in quartz beakers covered with quartz dishes were then placed under simulated sunlight for different periods inside a Suntest instrument with an air-cooled xenon lamp (SUNTEST XLS+ from Atlas Material Testing Technology LLC). The color of silver colloids changed from yellow to green, blue, purple, brownish red or brownish yellow during the irradiation process. The natural sunlight induction of silver seeds was performed in glass flask on clear days in August, which is winter in Geelong, Australia. 3. Results and discussion 3.1. Photoinduced synthesis under natural sunlight The color evolution of the silver solutions with different concentrations of citrate under irradiation of natural sunlight is shown in Fig. S1. After the yellow silver seed solutions were irradiated for 3 h, the solutions changed to blue or green, which was due to the transformation of silver seeds under sunlight. As can be found, the initial concentration of citrate in solution influenced the evolution of colors of silver nanoparticle solutions. Moreover, the final colors of silver nanoparticle solutions were obviously different when the citrate concentrations of solution varied (Fig. S1). The extinction spectra of silver nanoparticles were recorded at different time intervals in the process of sunlight irradiation (Fig. 1). New extinction bands of solutions with different concentrations of citrate appeared after the silver seed solutions were irradiated for 3 h, which indicates that the anisotropic silver nanoparticles were produced during irradiation by sunlight. It was noted that the extinction band around 400 nm attributed to silver seeds decreased in intensity dramatically in the photoinduced process. The change rate of this extinction band increased with an increase in citrate concentration of silver nanoparticle solutions, implying that citrate influenced visibly the synthesis of silver nanoparticles induced by sunlight. It is difficult to investigate exactly the evolution of silver nanoparticles under sunlight, as the condition of natural sunlight, especially lighting power, may be different each day. The spectrum of natural sunlight was measured to be in the range of 300–800 nm (Fig. 2A). The main emission band is located in 450–600 nm in the natural sunlight spectrum. The emission spectrum from the Suntest instrument is shown in Fig. 2B, very similar to the spectrum of natural sunlight. Therefore, the light from the Suntest instrument can be employed as simulated sunlight for the sunlight induced synthesis of silver nanoparticles to gain insight into the interaction of sunlight and silver nanoparticles. The lighting power of simulated sunlight can be controlled to eliminate the influences from instability of natural sunlight on photoinduced reaction. 3.2. Photoinduced synthesis under simulated sunlight Fig. S2 shows the photographs of silver nanoparticle solutions with different concentrations of citrate under irradiation of simulated sunlight with 500 W/m2 of lighting power density. The color evolution of silver nanoparticle solutions obtained by simulated sunlight was similar to that of silver nanoparticle solutions synthesized by natural sunlight. Fig. 3 depicts the extinction spectra of silver nanoparticle solutions from photoinduction of simulated sunlight. A new extinction band around 580 nm appeared in the extinction spectra of silver nanoparticles with 5.0 10 4 M of citrate after the solution was irradiated for 1 h (Fig. 3A). The intensity of this new band increased as irradiation time increased. Meanwhile, the extinction band around 400 nm attributed to silver
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Fig. 1. Evolution of extinction spectra of silver nanoparticle solutions during natural sunlight induced process corresponding to Fig. S1. The concentration of citrate in solution: (A) 5 10 4 M, (B) 10 3 M, (C) 2.5 10 3 M and (D) 5.0 10 3 M.
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Fig. 2. Spectra of natural sunlight (A) and simulated sunlight from the Suntest instrument (B).
seeds decreased in intensity significantly. The characterization of extinction spectra of silver nanoparticle solutions indicates that the silver seeds were transformed into anisotropic silver nanoparticles, which is similar to the case of natural sunlight (Fig. 1). When the silver solution was irradiated for 3 h, the extinction band of silver seeds disappeared and the extinction spectrum displayed the characteristic LSPR bands of silver nanoplates (nanoprism and nanodisk) (Fig. 3A) [17,26]. The LSPR band at long wavelength blue-shifted when light irradiation was prolonged. Moreover, a shoulder band arose in the LSPR band around 480 nm, which reveals that the silver nanoparticles were transformed to different shape. TEM was employed to observe the changes in morphologies of silver nanoparticles during photoinduced process. Fig. 4 shows the TEM images of silver nanoparticles with 5.0 10 4 M of cit-
rate during irradiation process. Some silver nanoparticles in Fig. 4A exhibited anisotropy when the silver nanoparticle solution was irradiated for 1 h. The anisotropy of partial silver nanoparticles resulted in the appearance of the new LSPR band at long wavelength. However, most of silver nanoparticles were still the silver seeds. Lots of silver nanoprisms were formed when the silver nanoparticle solution was irradiated for 2 h (Fig. 4B). There were still some small nanoparticles in the visual field. Silver nanoprisms dominated in TEM images, accompanying with some nanodisks, after 5 h of irradiation (Fig. 4C). When the silver nanoparticles were irradiated for 8 h, a few of pyramidal silver nanoparticles were seen in the TEM images (Fig. 4D), which suggests that the shape conversion from triangular to pyramid happened. Nevertheless, the proportion of pyramidal nanoparticles in the samples was small. It was inferred that the shoulder extinction
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Fig. 3. Evolution of extinction spectra of silver nanoparticle solutions under simulated sunlight with 500 W/m2 of lighting power density corresponding to different concentrations of citrate: (A) 5 10 4 M, (B) 10 3 M, (C) 2.5 10 3 M and (D) 5.0 10 3 M.
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Fig. 4. TEM images of silver nanoparticles with 5.0 10
LSPR band around nanoparticles.
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M of citrate irradiated for (A) 1 h, (B) 2 h, (C) 5 h and (D) 8 h under simulated sunlight (500 W/m2).
from
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3.3. Effect of citrate on morphologies of silver nanoparticles The concentration of citrate in solution affected obviously the optical properties and morphologies of silver nanoparticles during simulated sunlight photoinduced reaction. The evolution of the
extinction spectra of silver nanoparticles was different when the concentration of citrate in solution was changed (Fig. 3). The intensity of the shoulder band in the extinction spectra after the same exposure period increased with an increase in citrate concentration. When the concentration of citrate was 10 3 M, the shoulder band around 480 nm increased in intensity, compared with that corresponding to 5 10 4 M of citrate (Fig. 3A and B). Meanwhile, the intensity of the extinction band around 570 nm decreased. It is
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noted that the extinction band at 480 nm became a main band in the final extinction spectra of silver nanoparticle solutions when the concentration of citrate increased to 2.5 10 3 M and 5.0 10 3 M (Fig. 3C and D). The changes of the extinction spectra arose from the shape conversion of silver nanoparticles. These results reveal that proportion of pyramidal nanoparticles in the samples increased as the citrate concentration was increased. In addition, extinction spectra of silver nanoparticle solution with lower concentration of citrate (2.5 10 4 M) were recorded during irradiation of simulated sunlight (Fig. S3). There is no new shoulder extinction band appearing in the extinction spectra even that the solution was irradiated for 8 h, implying that no pyramidal nanoparticles were formed at low concentration of citrate. TEM characterization was performed to further investigate the effect of citrate on morphologies of silver nanoparticles in the simulated sunlight induced reaction. The silver nanoparticles with a high concentration of citrate (2.5 10 3 M and 5.0 10 3 M) became platelike (Fig. S4B and C) after only 1 h irradiation, which is different from the case with 10 3 M of citrate (Fig. S4A). Comparing the case with 5 10 4 M of citrate, the proportion of pyramidal nanoparticles increased obviously after 8 h of irradiation when the concentration of citrate was increased to 10 3 M (Fig. 5A). Almost all silver nanoparticles, after 8 h of light irradiation, changed to pyramidal nanoparticles (including nanodecahedrons) when the concentration of citrate was 2.5 10 3 M (Fig. 5B), which suggests that high concentration of citrate can increase the yield of pyramidal nanoparticles in the photoinduced process of simulated sunlight. Fig. 6 displays the TEM images of silver nanoparticles with 5.0 10 3 M of citrate. A few silver pyramidal nanoparticles were observed in the image of silver nanoparticles after 2 h of irradiation (Fig. 6A). The decahedral nanoparticles were dominant products after 5 h of irradiation (Fig. 6B and Fig. S5). The decahedral silver nanoparticles did not change visibly in shape when the irradiation time was increased to 8 h (Fig. 6C), except for a small change in the spectra of silver nanoparticles when the irradiation time was prolonged (Fig. 3D). The results further proved that the high concentration of citrate promoted the formation of decahedral silver nanoparticles in short time under simulated sunlight. Besides, a typical fivefold twinned structure was observed in the TEM image of final silver nanodecahedron (insert in Fig. 6C), which is consistent with the previous reports [29,50]. Combining extinction spectra and TEM images of silver nanoparticles in the present study, it can inferred from previous studies that the extinction bands at 403 and 477 nm in the extinction spectrum of silver nanoparticles with 5.0 10 3 M of citrate after 8 h irradiation (Fig. 3D) should be assigned to the transverse and longitudinal plasmon modes of silver nanodecahedrons [31,51]. Additionally, the exposure time of silver nanoparticle solution with 5.0 10 4 M of citrate was increased to 28 h under simulated sunlight, which provided inspection whether all the silver nanoparticles could be converted to pyramidal nanoparticles under
Fig. 5. TEM images of silver nanoparticles with (A) 10
low concentration of citrate through prolonged light irradiation. The corresponding extinction spectrum of silver nanoparticles presents a main LSPR band at 455 nm and a shoulder band at 395 nm (Fig. S6A). A number of platelike silver nanoparticles exists in the TEM image of silver nanoparticles even irradiated for 28 h (Fig. S6B). The results indicate that citrate concentration in solution plays a significant role in conversion from nanoplates to pyramidal nanoparticles during sunlight induced synthesis of silver nanoparticles. 3.4. Effect of inducing light source The influences of lighting power density of light source were inspected. Fig. S7 displays the extinction spectra of silver nanoparticle solutions with different concentration of citrate under 250 W/m2 of simulated sunlight. The evolution of extinction spectra was similar to the case corresponding to 500 W/m2 of lighting power density. However, the change rate of extinction intensity of silver seed LSPR band decreased. These results reveal that lighting power density only influenced the reaction rate. To gain insights into the effect of simulated sunlight on the photoinduced reaction of silver nanoparticles, different optical filters (Filter I–IV) were used to cover the reaction vessels during irradiation process to investigate influences from the different parts of sunlight. The transmission spectra of the optical filters (Filter I–IV) are shown in Fig. 7A–D. The corresponding extinction spectra of silver nanoparticle solutions with 2.5 10 3 M of citrate covered with the optical filters after irradiation for 8 and 16 h are presented in Fig. 7E–H. Filter I totally blocks the light with wavelength less than 385 nm (Fig. 7A). The extinction spectrum of silver nanoparticle solution covered with Filter I displayed the characteristic LSPR bands of silver nanoplates after irradiation for 8 h (Fig. 7E). All the silver nanoparticles were platelike in the TEM image (Fig. S8A), which is different from the case without filter covering (Fig. 5B). A prolonged light exposure (16 h) only led to formation of very few decahedral silver nanoparticles (Fig. S8B) though some changes occurred in extinction spectra of silver nanoparticles (Fig. 7E). It can be inferred that the light with short wavelength is crucial for formation of pyramidal nanoparticles in the sunlight induced process. Filter II bocks all the light in the region of 340– 405 nm and the partial light less than 340 nm (Fig. 7B). As can be seen, the intensity of simulated sunlight in the range of 300– 340 nm is very low (Fig. 2B). The light transmitting from Filter II mainly belongs to visible light. The extinction spectra (Fig. 7F) and TEM images (Fig. S8C and D) demonstrate that the silver nanoparticles with 2.5 10 3 M of citrate obtained after irradiated for 8 and 16 h covered with the Filter II were nanoplates. Similarly, the extinction spectra of silver nanoparticles obtained from Filter III and IV only imply the conversion from silver seeds to silver nanoplates (Fig. 7C and D). And no pyramidal nanoparticles were observed in TEM characterization (Fig. S9). The results
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demonstrate that short-wavelength light, especially UV light (<400 nm), is necessary to conversion from nanoplates to pyramidal nanoparticles in the sunlight induced synthesis of silver nanoparticles. Furthermore, UV lamp was employed to irradiate silver seed solution to investigate role of UV light in the shape conversion of silver nanoparticles. The emission spectrum of UV lamp shown in Fig. S10 presents a main emission band in the range of 280– 360 nm. The extinction band of the seed solution with 2.5 10 3 M of citrate blue-shifted to 405 nm from 393 nm and increased in intensity after irradiation for 6 h under UV lamp (Fig. 8A and Fig. S11). It was found from TEM images that decahedral silver nanoparticle were obtained after UV light irradiation (Fig. 8B), which reveals that UV light can induce the formation of
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of initial silver nanoparticle solution after 3 h irradiation changed slightly in wavelength (Fig. S12), which implies that the heat originated from light irradiation did not contribute to shape conversion of silver nanoparticles during simulated sunlight induced reaction. Moreover, the silver nanoparticles from 2 h irradiation by simulated sunlight was heated for 3 h at 45 °C. The extinction bands around 500 nm of the initial silver nanoparticle solution blueshifted after heat treatment (Fig. S12). However, the characteristic profile of the LSPR extinction spectrum did not changed obviously. These results further testify that heat treatment did not transform silver nanoplates into pyramidal nanoparticles. The synthesis of anisotropic silver nanoparticles under sunlight irradiation contained two stages: fabrication of the silver nanoplates and conversion from nanoplates to pyramidal nanoparticles. The morphology transformation of silver nanoparticles can be controlled by the wavelength of irradiating light [52,53]. The silver nanoplates were formed in the first stage of simulated sunlight induced process. It has been suggested in previous studies that silver seeds in the presence of citrate were transformed to silver nanoplates including nanoprisms and nanodisks through irradiation by light with long wavelength (>500 nm) [29,54,55]. It was thought that the silver seed solution contained platelike and multi-twinning seeds. In this study, it was proposed that the visible light (long-wavelength light) in the sunlight induced fast growth of nanoplates from platelike seeds in the first stage. Compared with multi-twinning seeds, the platelike silver seeds were easily excited by light irradiation [52,53]. The silver nanoplates were fabricated at first from transformation of silver seeds under long-wavelength light from sunlight. Subsequently, in the second stage, the shape conversion from nanoplates to decahedral nanoparticles from irradiation of UV light (short-wavelength light) in sunlight dominated the photoinduced process. It was suggested that the nanoplates were less stable relative to the decahedral nanoparticles [31]. The silver nanoplates dissolved under further irradiation and the released silver incorporated into the growing multi-twinning seeds, leading to the shape conversion from nanoplates to nanodecahedrons. Besides the effect of light, citrate is essential to the shape conversion of silver nanoparticles under sunlight irradiation. It was demonstrated that citrate as a capping agent stabilized silver nanoparticles and as a reducing agent reduced silver ions from oxidization of silver nanoparticles in solution [39,56]. It was suggested that LSPR excitation of silver nanoparticles from light irradiation induced the formation of so-called ‘‘hot holes’’ on nanoparticles that were then filled with electrons from the oxidization of adsorbed citrate [30,51,56]. The silver ions from oxidative etching of silver in the presence of O2 were reduced on specific facets of nanoparticles by the electrons from photo-oxidation of citrate, resulting in growth and shape conversion of silver nanoparticles [30].
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3.6. Seed-free sunlight induced synthesis of silver nanoparticles Seed-mediated photoinduced method is one of the most effective routs for synthesis of anisotropic silver nanoparticles. In the present study, in addition to seed-mediated strategy, the synthesis of silver nanoparticles in the absence of silver seeds was also realized under irradiation of simulated sunlight. The mixed solution of AgNO3 (10 4 M) and citrate (2.5 10 3 M) was irradiated for 12 h under simulated sunlight. The extinction spectrum of solution after light irradiation displays characteristic LSPR bands of silver nanodecahedrons (Fig. 9A), implying that the silver nanodecahedrons were prepared using AgNO3 as precursor in the presence of citrate through irradiation of simulated sunlight. Furthermore, TEM characterization demonstrated that most of the prepared silver nanoparticles were decahedral in shape (Fig. 9B). The results indicate that simulate sunlight irradiation could give rise to production of silver nanodecahedrons from mixture of AgNO3 and citrate in the absence of silver seeds, which is similar to work of Yang et al. [51]. Significantly, compared with monochromatic light from blue light-emitting diodes (LEDs), it is convenient, energy-saving and environmentally friendly to use sunlight to induce the seedfree synthesis of silver nanodecahedrons.
4. Conclusions Silver nanoprisms and nanodecahedrons were synthesized from silver seeds through shape conversion induced by sunlight. The concentration of citrate in solution influenced the shapes of silver nanoparticles in the sunlight induced synthesis process. Silver nanoprisms dominated in the products when reaction system contained citrate with a low concentration (5 10 4 M). Decahedral silver nanoparticles were obtained from silver nanoplates when concentration was increased to 5 10 3 M. UV light from sunlight was proved to be crucial in the synthesis of pyramidal silver nanoparticles. In addition, silver nanodecahedrons were prepared from the mixed solution of AgNO3 and citrate in the absence of silver seeds by irradiation of simulated sunlight. Sunlight-induced synthesis of silver nanoparticles is significant for application of green light source in preparation of nanoparticles.
Acknowledgments This research was supported by the National Natural Science Foundation of China (NSFC 51273153), and the Central Research Grants Scheme and Alfred Deakin Postdoctoral Research Fellowship scheme at Deakin University.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.08.044.
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