- Email: [email protected]
Formation and characterization of silver nanoparticles in aqueous solution via ultrasonic irradiation
Ultrasonics Sonochemistry 21 (2014) 542–548
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Formation and characterization of silver nanoparticles in aqueous solution via ultrasonic irradiation Chaodong He, Lanlan Liu, Zeguo Fang, Jia Li, Jinbao Guo, Jie Wei ⇑ College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China Beijing Engineering Research Center of Syntheses and Applications of Waterborne Polymers, Beijing 100029, P.R. China
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 3 September 2013 Accepted 4 September 2013 Available online 13 September 2013 Keywords: Silver nanoparticles Sonochemical synthesis Ultrasonic irradiation Aqueous solution
a b s t r a c t In this study, a simple and green method to synthesize silver nanoparticles (Ag NPs) in aqueous solution via ultrasonic irradiation has been developed. Ultrafine Ag NPs with average diameter of 8 nm were obtained through sonicating aqueous solution of sodium hydroxide (NaOH, 0.1 mM) with adding silver nitrate solution (AgNO3, 5.88 mM) drop by drop. In pure aqueous solution, the reactive route related to hydroxyl radicals ( OH) is presented. Furthermore, in alkaline aqueous solution, the effects of hydroxyl ions (OH) on formation of Ag NPs are discussed detailedly. The formation of Ag NPs was tracked by surface plasmon resonance (SPR) band of ultraviolet–visible (UV–Vis) spectrum; the morphology of the obtained Ag NPs was characterized through transmission electron microscopy (TEM); energy dispersive X-ray spectroscopy (EDX) and X-ray powder diffraction (XRD) confirmed the formation of metallic Ag NPs. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles (Ag NPs) have been paid enormous attention in various areas, such as catalysts [1], antibacterial agents [2], surface enhanced Raman spectroscopy (SERS) [3] and biochemical sensing [4], because of their novel physicochemical properties [5] differing significantly from macroscopic metal phases. Nowadays, Ag NPs can be prepared through various ways, such as chemical reduction [6], photochemical method [7], laser irradiation [8], microwave irradiation [9], ultrasonic irradiation [10], and electrochemical method [11]. However, in most of them, either organic solvents, toxic reducing agents and stabilizers, which had potential environmental and biological risks, or more than one reactive step, were needed. Sonochemical method, since discovered, has been studied for yielding kinds of nanomaterials, especially noble metal nanoparticles, such as silver [12,13], gold [14,15], platinum [16,17], and lead [17]. Among these researches, polyol method under ultrasonic condition has been applied widely due to the relatively high reductive ability of polyol. In our previous research [18], ethylene glycol (EG) was utilized to fabricate Ag NPs/carbon nanotubes (CNTs) composite with sonochemical method, in which it served as a dual role:
reducing to silver ions (Ag+) to Ag NPs and dispersing CNTs. However, EG has several drawbacks of high boiling point and high viscosity to limit its application in fabricating nanocomposite composed of Ag NPs and 2-D nanomaterials, such as graphene oxidation (GO). Compared with EG, water is less viscous to be the solvent of GO. Moreover, water also has similar hydroxyl group as EG does to reduce Ag+ [19]. Therefore, it’s worthy of study on the reductive ability of water under ultrasonic condition. In this paper, water was used to reduce Ag+ to form Ag NPs via ultrasonic irradiation without adding any reductant and surfactant. During this process, high temperature and pressure resulting from the broken of cavitation bubbles caused by ultrasonic irradiation play extensive roles. After analyzing the route of the reaction, further study was conducted and revealed that hydroxyl ions (OH) could accelerate the formation of Ag NPs in sonochemical synthesis for helping to yield nucleation sites and donating electrons to Ag+. Additionally, OH served as capping group can keep obtained Ag NPs stable for a long time.
2. Materials and methods 2.1. Reagents
⇑ Corresponding author. Postal address: 6 Mailbox, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, PR China. Tel.: +86 10 64454598; fax: +86 10 64427628. E-mail address: [email protected] (J. Wei). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.003
All reagents including silver nitrate (AgNO3), sodium hydroxide (NaOH), nitric acid (HNO3) and ammonia (NH3H2O) with analytical grade were purchased from Beijing chemical reagent, China,
543
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
and used without further purification. Deionized (DI) water was used during the preparation of samples. 2.2. Synthesis of Ag NPs 2.2.1. Synthesis of Ag NPs in pure aqueous solution 10 mL silver nitrate aqueous solution (58.8 mM) was put into a penicillin bottle sealed with plastic wrap. Then the solution was sonicated for 2 h with an ultrasonic reactor (Cleaning bath Auto Science As510B, 150 W, 50 kHz). The power transferred to the solution was 1.61 W measured by means of the calorimetric method [20]. During the ultrasonic reaction, the solution temperature was maintained constant for 25 ± 1 °C with the help of cooling water surrounding the reactor cell. 2.2.2. Synthesis of Ag NPs in alkaline aqueous solution 100 mL NaOH aqueous solution (0.1 mM) was put into a 250 mL glass beaker. The beaker was sonicated by the ultrasonic reactor for 12 h with adding 0.1 mL AgNO3 aqueous solution (5.88 mM) into the solution every 10 min. The sample was tested by UV–Vis spectrophotometer and pH Meter every 1 h during the whole process (0–14 h) to track the formation of Ag NPs. Another 20 mg NaOH was added into the solution after 12 h ultrasonic reaction. Then extra 2 h sonication was performed to the solution without adding other AgNO3 aqueous solution. After then 5 mL diluted ammonia (3%, wt%) was added into the solution to react with remaining AgO2 resulting from the decomposition of AgOH. At last, the product was collected with 0.45 lm microporous membrane. The fresh made sample was washed with DI water three times and then dried in vacuum oven for 12 h. The obtained Ag NPs on the membrane weighted 4.07 mg. The control experiments in acidic (pH < 7) and neutral (pH = 7) aqueous solution were conducted as described above except that initial pH values for the solutions were adjusted by adding nitric acid or nothing and no additional step was performed on the certain solutions after 12 h reaction.
Fig. 1. UV–Vis absorption spectra for reaction solution before sonication and after sonication with 2 h.
3. Results and discussion
confinement. Therefore, no appearance of specific absorption band suggests that resulting Ag NPs is exceedingly small (average particle size less than 4 nm), as SPR peak is known to be broadened and depressed with decreasing particle size [23]. TEM was carried out further to observe whether Ag NPs formed or not during the process. Fig. 2A–C are TEM images and corresponding size distribution histogram of the sample sonicated for 2 h. Obviously, some nanoparticles with average size about 3.36 nm appeared after 2 h ultrasonic reaction. This result is consistent with the diameter analysis of UV–Vis spectrum in Fig. 1. Importantly, for in the whole experimental process no additional agent except AgNO3 was added, the nanoparticles should be Ag NPs. The strong signals at 3 and 3.16 keV in EDX spectrum (Fig. 2D) confirm the presence of element Ag. In addition, the atom ratio of Ag, N, and O of the sample is 21.6:2.4:3.4 (the inset of Fig. 2D), which is much higher than that of AgNO3. This demonstrates the formation of metallic Ag NPs in pure aqueous solution via ultrasonic irradiation. The signal of C is very likely to result from the carbon film on copper grid. During ultrasonic process, water plays the role of solvent as well as reductant. Although the reductive rate of Ag+ in pure water via ultrasonic irradiation is very low, this method can be utilized wildly for the use of simplest, non-toxic and environment friendly inorganic solvent. Therefore, the reactive route needs to be elucidated to find ways to enhance the reactive rate. As shown above, under ultrasonic condition Ag+ gets electron to be reduced into silver seed (Ag0). Based on early research [24–26], water molecules decompose to hydrogen radicals (H ) and hydroxyl radicals ( OH) under ultrasonic condition, and then H supply electron for the reduction of Ag+:
3.1. Synthesis of Ag NPs in pure aqueous solution
H2 O ! H þ OH; H þ Agþ ! Hþ þ Ag0
2.3. Characterization All of the samples were measured on U-3010 ultraviolet–visible (UV–Vis) spectrophotometer without dilution. Transmission electron microscopy (TEM) and were performed on Hong Kong FEI TEC nai Model G220 transmission electron microscope with an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDX) analysis was performed on EDAX AMETEK energy dispersive spectroscopy. The pH values for the solutions were tested by HANNA pH213 microprocessor pH meter. Ag NPs filtered on microporous membrane were characterized by X-ray powder diffraction (XRD) analysis performed on German AXS D8 ADVANCE diffractometer with CuKa radiation. All photographs of the samples were taken by Sony DSC-W570 camera.
Sonicate
To study the reductive effect of water under ultrasonic condition, UV–Vis spectrum of the reaction solution was carried out. Fig. 1 shows the UV–Vis spectrum of sample before and after sonication for 2 h. After sonicated for 2 h, there is no appearance of the specific surface plasmon resonance (SPR) absorption band of Ag NPs around 400 nm [21]. But the absorbance around 400 nm increased after the ultrasonic reaction, tiny while obvious, which means that some change occurred in the solution. Mulvaney [22] has reported that SPR peak of Ag NPs below 4 nm disappeared for the change in electronic structure that occurs with quantum
ð1Þ
Actually, only the valence of silver changed in the reductive reaction. In other words, other corresponding oxidative reaction should exist to keep the valence balance. During reactive process, a little bump appeared on the top of the penicillin bottle formed by the plastic wrap, which suggested that gas was produced in the bottle. For the presence of only water and AgNO3 aqueous solution in the reactor, the gas is probably oxygen. Therefore, the oxidative reaction is most likely to react in the following way as Eq. (2) shows:
HO þ OH ! H2 O2 ;
H2 O2 ! H2 O þ 1=2O2
ð2Þ
544
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
Fig. 2. TEM images of Ag NPs synthesized in pure water via sonication for 2 h (A and B); the corresponding size distribution histogram (C) and EDX spectra of the Ag NPs (D).
So the overall reaction reacts like Eq. (3):
3.2. Synthesis of Ag NPs in alkaline aqueous solution and the effect of OH in the reaction
Sonicate
2Agþ þ H2 O ! 2Ag0 þ 1=2O2 þ 2Hþ
ð3Þ
Design the overall reaction (Eq. (3)) as the following galvanic cell: 1
ðÞ Pt k H2 O;Hþ jO2 j Agþ ð5:88 103 mol kg Þj AgðþÞ
ð4Þ
The individual electrode reactions can be expressed as [27]:
H2 O ! 2Hþ þ 1=2O2 þ 2e
ðÞ Anodic reaction :
¼ þ1:2290 V
ðþÞ Cathodic reaction :
EhL ð5Þ
2Agþ þ 2e ! 2Ag EhR
¼ þ0:7994 V
ð6Þ
At standard condition (room temperature 25 °C with the pressure of 100 kPa), the electromotive force (EMF, E) of galvanic cell (Eq. (4)) is below zero, Eh ¼ EhR EhL ¼ 0:4296V < 0, which means this cell reaction cannot occur spontaneously. Fortunately, the extremely high temperature and pressure resulting from the broken of cavitation bubbles caused by the ultrasonic irradiation [28] make the galvanic cell reaction accessible. Calculated under ultrasonic condition (ESI 1.1), the EMF was 1.40 V, which indicates the successful overall cell reaction (Eq. (3)). This result is in agreement with the formation of Ag NPs demonstrated in above analysis of UV–Vis spectrum and TEM. In fact, OH plays extensively important role in this cell reaction. During the reaction, it offers electron to Ag+ accompanying with the exhaust of oxygen like anodic reaction shown in Eq. (3). But the production of oxygen is too slight to be detected during ultrasonic process. However, a similar reactive route involving Ag NPs, H2O2, OH and OH has been reported in He’s work [29]. To testify the effect of OH in the reaction, following experiments were designed.
After the alkaline aqueous solution was sonicated for 2 h, an absorption peak around 410 nm corresponding to SPR peak of Ag NPs appeared in Fig. 3A. This phenomenon indicates that Ag NPs formed easily under alkaline condition compared with under acidic and neutral condition (ESI Figs. S1 and S2). Meanwhile, the color of the sample also transferred from colorless to pale yellow clearly (inset of Fig. 3A), which resulted from the formation of Ag NPs [13]. After the solution was sonicated for 12 h, its color turned into dark brown further for much more formation of Ag NPs. Since the peak height of SPR is direct proportional to amount of metal nanoparticles [30], the increment of absorbance for the solution at 410 nm (Fig. 3A) suggests that Ag NPs formed enormously after 12 h as well. Fig. 3B shows that pH values for the solution decreases evidently as the ultrasonic reaction progresses. This can be attributed to two possibilities: (1) the reaction of OH and (2) the hydrolysis of Ag+. However, when the solution was sonicated for 9 h, the SPR peak height for Ag NPs does not increase any more (Fig. 4B) due to the limited number of OH. Meanwhile, the varying rate of pH values for the solution after 9 h (Fig. 3B, slope of green solid curve) decreases distinctly compared with that of before 9 h (Fig. 3B, slope of red solid curve). This suggests that the reaction of OH is mainly responsible for the decrement of pH values for the solution other than the hydrolysis of Ag+. In addition, other proof, that the varying trend of SPR peak height with time for Ag NPs (Fig. 4B the red solid and green solid curve) is the same as the varying trend of pH values with time for the solution (Fig. 3B the red solid and green solid curve), also demonstrates this point. To verify this point further, another 20 mg NaOH was added into the solution after reacted for 12 h, which leads the sudden increase of pH values for the solution shown in Fig. 3B. And then extra 2 h sonication was performed to the solution without adding any more AgNO3 aqueous solution. Clearly, the absorbance for the solution at 410 nm begins to increase again as ultrasonic reaction progresses (Fig. 4A). This suggests that Ag NPs begin to form
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
545
Fig. 3. Time evolution (0–12 h) of the UV–Vis absorption spectra and the color (the inset, left to right) for the reaction solution (A, pH > 7); Time evolution (0–14 h) of the pH values for the reaction solution (B, pH > 7). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Time evolution (12–14 h) of the UV–Vis absorption spectra and the color (the inset, left to right) for the reaction solution after adding 20 mg NaOH into it (A); Time evolution (0–14 h) of SPR peak height with time for Ag NPs (B) [data from Figs. 3A and 4A]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
again. More importantly, the variety rate of pH values for the solution after 12 h (Fig. 3B, slope of blue solid curve) is almost the same as that of before 9 h (Fig. 3B, slope of red solid curve). This indicates that OH is very crucial to the synthesis of Ag NPs in aqueous solution via ultrasonic irradiation. Therefore, it is reasonable to believe that OH really reduces Ag+ during ultrasonic process. Under acidic and neutral condition, no clear movement is occurred in the UV–Vis spectrum and the pH values for both the solutions are almost stable (ESI Figs. S1 and S2). These also prove that alkaline condition is much preferred in the synthesis of Ag NPs in aqueous solution via ultrasonic irradiation. TEM images of obtained Ag NPs (Fig. 5A and B) show that much more Ag NPs are obtained in alkaline aqueous solution compared with in pure aqueous solution (Fig. 1A and B). The photograph of Ag NPs filtered on microporous membrane is delivered in Fig. 6A. Fig. 5D shows typical XRD pattern of the as-prepared sample in alkaline aqueous solution. The five diffraction peaks, 38.07°, 44.29°, 64.50°, 77.48°, 81.55°, can be readily indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of the face-centered-cubic (fcc) structured metallic silver (JCPDS No. 42-0783). The diameter of Ag NPs (8.9 nm) calculated from the XRD peak of the (1 1 1) reflection using well-known Scherrer formula is well consistent with the mean sizes (8 nm) measured from TEM images [31]. As described above, OH plays significant role in forming Ag NPs. Scheme 1 responds to the reactive route of Ag NPs in alkaline aqueous solution via ultrasonic irradiation. As is known, AgNO3 is unstable under alkaline condition to produce AgOH, and then AgOH decomposes to Ag2O. Ag2O can also hydrolyze into complex
of Ag(OH)x [32], such as Ag(OH) and AgðOHÞ 2 [6]. The formation of Ag NPs can be separated into three steps: reduction of Ag+, nucleation of Ag0, and growth of the crystal [33,34]. Neglecting the growth of crystal, the formation of Ag NPs can be expressed as follows [35]: K1
Agþ ! Ag0 K2
nAg0 ! Agn
ð7Þ ð8Þ
where K1 and K2 represent overall reduction and nucleation rates, respectively. Here we analyze that there might be three reasons why the formation rate of Ag NPs in alkaline aqueous solution is higher than in acidic and neutral aqueous solution: (1) In same condition, the intermediate complex Ag(OH)x possess bigger reduction rates (K1) than AgNO3 [6,35]. After Ag2O hydrolyzed into Ag(OH)x, it accepts electron releasing from OH or OH to form Ag0 during ultrasonic process. (2) As shown in Scheme 1, Ag2O particle, solid insoluble impurity, serves as nucleation sites for forming Ag NPs at the initial reaction stage [35]. Furthermore, the appearance of these nucleation sites decreases the required supersaturated concentration of Ag0 to yield the first nucleation site for changing the nucleation process from homogenous to heterogeneous [36]. This leads the faster nucleation process which is equivalent to increasing nucleation rate (K2).
546
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
Fig. 5. TEM images of Ag NPs synthesized in alkaline aqueous solution via sonication 14 h (A and B); the corresponding size distribution histogram (C) and XRD pattern (D) of the Ag NPs filtered on microporous membrane.
Fig. 6. Photograph of filtered Ag NPs on microporous membrane (A) and the obtained nano-silver colloid solution after 7 h ultrasonic reaction kept for 2 months (B) and 12 h ultrasonic reaction kept for 24 h (C) with same initial concentration of NaOH in both solutions.
Scheme 1. Reactive scheme for the formation of Ag NPs in alkaline aqueous solution via ultrasonic irradiation.
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
(3) OH can consume H+ formed during the reduction of Ag+ to increase the reduction rate (K1) [35]. In fact, under alkaline condition, the former galvanic cell reaction (Eq. (3)) can be redesigned as follows: 1
ðÞ PtjH2 O;OH jO2 j Agþ ð5:88 107 mol kg Þj AgðþÞ
ð9Þ
The individual electrode reactions can be expressed as [27]:
2OH ! H2 O þ 1=2O2 þ 2e
ðÞ Anodic reaction :
¼ þ0:4010 V ðþÞ Cathodic reaction :
ð10Þ
2Agþ þ 2e ! 2Ag EhR
¼ þ0:7994 V Overall reaction :
EhL
2Agþ þ 2OH ! H2 O þ 1=2O2 þ Ag
ð11Þ ð12Þ
At standard condition, the EMF of galvanic cell shown in Eq. (9) is above zero, Eh ¼ EhR EhL ¼ 0:3984V > 0, which indicates the cell reaction can occur spontaneously thermodynamically. The high temperature and pressure caused by the sonication will dramatically accelerate the reactive speed dynamically. Meanwhile, the yielded O2 during the ultrasonic process promoted the production of H2O2 as reported before [37]. These might be the main reasons for faster formation of Ag NPs in alkaline aqueous solution via ultrasonic irradiation. In addition, OH has the same effect as the traditional surfactant does. As shown in Fig. 6B and C, the sample containing more OH for reacting only 7 h stayed stable more than 2 months; while the other sample with less OH for reacting 12 h precipitated only 24 h later. It’s because enough OH surrounded around Ag NPs provides an energy barrier to counteract the van der Waals’ force between nanoparticles to prevent the aggregation [36].
4. Conclusion In summary, we observed the formation of Ag NPs in pure aqueous solution without adding any reductant and surfactant under ultrasonic condition. During this process, water serves as solvent as well as reductant. In alkaline aqueous solution, ultrafine Ag NPs with diameter of 8 nm were achieved. Under this condition, OH plays role of electron donors as OH does in pure aqueous solution. It also has the effects of helping to yield the nucleation sites and acting as surfactant. This method to synthesize Ag NPs may have broad biomedical and antibacterial application for no addition of environmentally hazardous reagents and the relatively small diameter of the obtained Ag NPs. Moreover, it can be applied in fabricating composite due to the simple process and condition. A further study on Ag NPs/GO composite fabricated by this method is now in preparation to be published elsewhere. Acknowledgment The authors are grateful for the financial support by National Natural Science Foundation of China (Grant No. 51173013) and Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110010110007). 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.ultsonch.2013. 09.003.
547
References [1] T. Maiyalagan, Synthesis, characterization and electrocatalytic activity of silver nanorods towards the reduction of benzyl chloride, Appl. Catal. A 340 (2008) 191–195. [2] C. Marambio-Jones, E.M.V. Hoek, A review of the antibacterial effects of silver nanomaterials and potential implications for human health and the environment, J. Nanopart. Res. 12 (2010) 1531–1551. [3] E. Solano-Ruiz, R. Sato Berrú, J. Ocotlán-Flores, J.M. Saniger, Synthesis of silver nanoparticles by sonochemical induced reduction application in SERS, J. Nano. Res-sw. 9 (2010) 77–81. [4] S.T. Dubas, V. Pimpan, Humic acid assisted synthesis of silver nanoparticles and its application to herbicide detection, Mater. Lett. 62 (2008) 2661–2663. [5] D. Kim, S. Jeong, J. Moon, Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection, Nanotechnology 17 (2006) 4019–4024. [6] D.D. Evanoff, G. Chumanov, Size-controlled synthesis of nanoparticles. 1. ‘‘Silver-only’’ aqueous suspensions via hydrogen reduction, J. Phys. Chem. B 108 (2004) 13948–13956. [7] H. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953. [8] J.P. Abid, A.W. Wark, P.F. Brevet, H.H. Girault, Preparation of silver nanoparticles in solution from a silver salt by laser irradiation, Chem. Commun. (2002) 792–793. [9] S. Kundu, K. Wang, H. Liang, Size-controlled synthesis and self-assembly of silver nanoparticles within a minute using microwave irradiation, J. Phys. Chem. C 113 (2009) 134–141. [10] A.R. Abbasi, A. Morsali, Synthesis and properties of silk yarn containing Ag nanoparticles under ultrasound irradiation, Ultrason. Sonochem. 18 (2011) 282–287. [11] H. Ma, B. Yin, S. Wang, Y. Jiao, W. Pan, S. Huang, S. Chen, F. Meng, Synthesis of silver and gold nanoparticles by a novel electrochemical method, Chem. Phys. Chem. 5 (2004) 68–75. [12] J. Zhu, S. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Shape-controlled synthesis of silver nanoparticles by pulse sonoelectrochemical methods, Langmuir 16 (2000) 6396–6399. [13] J.H. Byeon, Y.-W. Kim, A novel polyol method to synthesize colloidal silver nanoparticles by ultrasonic irradiation, Ultrason. Sonochem. 19 (2012) 209– 215. [14] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Formation of colloidal gold nanoparticles in an ultrasonic field: control of rate of gold(III) reduction and size of formed gold particles, Langmuir 17 (2001) 7717–7720. [15] G.H. Zhang, W.L. Guo, X.K. Wang, Sonochemical formation of nanocrystalline gold in aqueous solution, Mater. Res. Innov. 11 (2007) 201–203. [16] Y. Mizukoshi, E. Takagi, H. Okuno, R. Oshima, Y. Maeda, Y. Nagata, Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions: role of surfactants, Ultrason. Sonochem. 8 (2001) 1–6. [17] T. Fujimoto, S.-Y. Terauchi, H. Umehara, I. Kojima, W. Henderson, Sonochemical preparation of single-dispersion metal nanoparticles from metal salts, Chem. Mater. 13 (2001) 1057–1060. [18] H. Wang, J. Guo, J. Li, J. Wei, Fabrication of bimetallic nanoparticles/multiwalled carbon nanotubes composites for microelectronic circuits, Carbon 49 (2011) 779–786. [19] M. Tsuji, Y. Nishizawa, K. Matsumoto, M. Kubokawa, N. Miyamae, T. Tsuji, Effects of chain length of polyvinylpyrrolidone for the synthesis of silver nanostructures by a microwave-polyol method, Mater. Lett. 60 (2006) 834– 838. [20] T. Kimura, T. Sakamoto, J.-M. Leveque, H. Sohmiya, M. Fujita, S. Ikeda, T. Ando, Standardization of ultrasonic power for sonochemical reaction, Ultrason. Sonochem. 3 (1996) S157–S161. [21] H. Abe, K.P. Charlé, B. Tesche, W. Schulze, Surface plasmon absorption of various colloidal metal particles, Chem. Phys. 68 (1982) 137–141. [22] P.C. Mulvaney, Surface plasmon spectroscopy of nanosized metal particles, Langmuir 12 (1996) 788–800. [23] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley & Sons, New York, 2008. [24] Y. Nagata, Y. Watananabe, S.-I. Fujita, T. Dohmaru, S. Taniguchi, Formation of colloidal silver in water by ultrasonic irradiation, J. Chem. Soc. Chem. Commun. (1992) 1620. [25] N.A. Dhas, A. Gedanken, Sonochemical synthesis of molybdenum oxide and molybdenum carbidesilica nanocomposites, Chem. Mater. 9 (1997) 3144– 3154. [26] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [27] W.M. Haynes, D.R. Lide, T.J. Bruno, CRC Handbook of Chemistry and Physics 2012–2013, CRC Press, 2012. [28] A. Gedanken, Using sonochemistry for the fabrication of nanomaterials, Ultrason. Sonochem. 11 (2004) 47–55. [29] D. He, A.M. Jones, S. Garg, A.N. Pham, T.D. Waite, Silver nanoparticlereactive oxygen species interactions: application of a chargingdischarging model, J. Phys. Chem. C 115 (2011) 5461–5468. [30] U. Krebig, M. Vollmer, Theoretical considerations-single clusters: intrinsic size effects of the optical properties, in: U. Gonser, R.M. Osgood, M.B. Panish, H. Sakaki (Eds.), Optical Properties of Metal Clusters, Springer-Verlag, Berlin and Heidelberg, 1995, p. 83.
548
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
[31] V. Radmilovic, H.A. Gasteiger, P.N. Ross, Structure and chemical composition of a supported Pt–Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98–106. [32] X. Wang, H.-F. Wu, Q. Kuang, R.-B. Huang, Z.-X. Xie, L.-S. Zheng, Shapedependent antibacterial activities of Ag2O polyhedral particles, Langmuir 26 (2010) 2774–2778. [33] B.R. Pamplin, Crystal Growth, Pergamon Press, New York, 1975. [34] Y. Jiang, Forced hydrolysis and chemical co-precipitation, in: Z.L. Wang, Y. Liu, Z. Zhang (Eds.), Nanophase and Nanostructured Materials, Kluwer Academic, New York, 2003, p. 59.
[35] S. Nishimura, D. Mott, A. Takagaki, S. Maenosono, K. Ebitani, Role of base in the formation of silver nanoparticles synthesized using sodium acrylate as a dual reducing and encapsulating agent, Phys. Chem. Chem. Phys. 13 (2011) 9335. [36] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [37] E.L. Mead, R.G. Sutherland, R.E. Verrall, The effect of ultrasound on water in the presence of dissolved gases, Can. J. Chem. 54 (1976) 1114–1120.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Formation and characterization of silver nanoparticles in aqueous solution via ultrasonic irradiation Chaodong He, Lanlan Liu, Zeguo Fang, Jia Li, Jinbao Guo, Jie Wei ⇑ College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China Beijing Engineering Research Center of Syntheses and Applications of Waterborne Polymers, Beijing 100029, P.R. China
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 3 September 2013 Accepted 4 September 2013 Available online 13 September 2013 Keywords: Silver nanoparticles Sonochemical synthesis Ultrasonic irradiation Aqueous solution
a b s t r a c t In this study, a simple and green method to synthesize silver nanoparticles (Ag NPs) in aqueous solution via ultrasonic irradiation has been developed. Ultrafine Ag NPs with average diameter of 8 nm were obtained through sonicating aqueous solution of sodium hydroxide (NaOH, 0.1 mM) with adding silver nitrate solution (AgNO3, 5.88 mM) drop by drop. In pure aqueous solution, the reactive route related to hydroxyl radicals ( OH) is presented. Furthermore, in alkaline aqueous solution, the effects of hydroxyl ions (OH) on formation of Ag NPs are discussed detailedly. The formation of Ag NPs was tracked by surface plasmon resonance (SPR) band of ultraviolet–visible (UV–Vis) spectrum; the morphology of the obtained Ag NPs was characterized through transmission electron microscopy (TEM); energy dispersive X-ray spectroscopy (EDX) and X-ray powder diffraction (XRD) confirmed the formation of metallic Ag NPs. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles (Ag NPs) have been paid enormous attention in various areas, such as catalysts [1], antibacterial agents [2], surface enhanced Raman spectroscopy (SERS) [3] and biochemical sensing [4], because of their novel physicochemical properties [5] differing significantly from macroscopic metal phases. Nowadays, Ag NPs can be prepared through various ways, such as chemical reduction [6], photochemical method [7], laser irradiation [8], microwave irradiation [9], ultrasonic irradiation [10], and electrochemical method [11]. However, in most of them, either organic solvents, toxic reducing agents and stabilizers, which had potential environmental and biological risks, or more than one reactive step, were needed. Sonochemical method, since discovered, has been studied for yielding kinds of nanomaterials, especially noble metal nanoparticles, such as silver [12,13], gold [14,15], platinum [16,17], and lead [17]. Among these researches, polyol method under ultrasonic condition has been applied widely due to the relatively high reductive ability of polyol. In our previous research [18], ethylene glycol (EG) was utilized to fabricate Ag NPs/carbon nanotubes (CNTs) composite with sonochemical method, in which it served as a dual role:
reducing to silver ions (Ag+) to Ag NPs and dispersing CNTs. However, EG has several drawbacks of high boiling point and high viscosity to limit its application in fabricating nanocomposite composed of Ag NPs and 2-D nanomaterials, such as graphene oxidation (GO). Compared with EG, water is less viscous to be the solvent of GO. Moreover, water also has similar hydroxyl group as EG does to reduce Ag+ [19]. Therefore, it’s worthy of study on the reductive ability of water under ultrasonic condition. In this paper, water was used to reduce Ag+ to form Ag NPs via ultrasonic irradiation without adding any reductant and surfactant. During this process, high temperature and pressure resulting from the broken of cavitation bubbles caused by ultrasonic irradiation play extensive roles. After analyzing the route of the reaction, further study was conducted and revealed that hydroxyl ions (OH) could accelerate the formation of Ag NPs in sonochemical synthesis for helping to yield nucleation sites and donating electrons to Ag+. Additionally, OH served as capping group can keep obtained Ag NPs stable for a long time.
2. Materials and methods 2.1. Reagents
⇑ Corresponding author. Postal address: 6 Mailbox, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, PR China. Tel.: +86 10 64454598; fax: +86 10 64427628. E-mail address: [email protected] (J. Wei). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.003
All reagents including silver nitrate (AgNO3), sodium hydroxide (NaOH), nitric acid (HNO3) and ammonia (NH3H2O) with analytical grade were purchased from Beijing chemical reagent, China,
543
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
and used without further purification. Deionized (DI) water was used during the preparation of samples. 2.2. Synthesis of Ag NPs 2.2.1. Synthesis of Ag NPs in pure aqueous solution 10 mL silver nitrate aqueous solution (58.8 mM) was put into a penicillin bottle sealed with plastic wrap. Then the solution was sonicated for 2 h with an ultrasonic reactor (Cleaning bath Auto Science As510B, 150 W, 50 kHz). The power transferred to the solution was 1.61 W measured by means of the calorimetric method [20]. During the ultrasonic reaction, the solution temperature was maintained constant for 25 ± 1 °C with the help of cooling water surrounding the reactor cell. 2.2.2. Synthesis of Ag NPs in alkaline aqueous solution 100 mL NaOH aqueous solution (0.1 mM) was put into a 250 mL glass beaker. The beaker was sonicated by the ultrasonic reactor for 12 h with adding 0.1 mL AgNO3 aqueous solution (5.88 mM) into the solution every 10 min. The sample was tested by UV–Vis spectrophotometer and pH Meter every 1 h during the whole process (0–14 h) to track the formation of Ag NPs. Another 20 mg NaOH was added into the solution after 12 h ultrasonic reaction. Then extra 2 h sonication was performed to the solution without adding other AgNO3 aqueous solution. After then 5 mL diluted ammonia (3%, wt%) was added into the solution to react with remaining AgO2 resulting from the decomposition of AgOH. At last, the product was collected with 0.45 lm microporous membrane. The fresh made sample was washed with DI water three times and then dried in vacuum oven for 12 h. The obtained Ag NPs on the membrane weighted 4.07 mg. The control experiments in acidic (pH < 7) and neutral (pH = 7) aqueous solution were conducted as described above except that initial pH values for the solutions were adjusted by adding nitric acid or nothing and no additional step was performed on the certain solutions after 12 h reaction.
Fig. 1. UV–Vis absorption spectra for reaction solution before sonication and after sonication with 2 h.
3. Results and discussion
confinement. Therefore, no appearance of specific absorption band suggests that resulting Ag NPs is exceedingly small (average particle size less than 4 nm), as SPR peak is known to be broadened and depressed with decreasing particle size [23]. TEM was carried out further to observe whether Ag NPs formed or not during the process. Fig. 2A–C are TEM images and corresponding size distribution histogram of the sample sonicated for 2 h. Obviously, some nanoparticles with average size about 3.36 nm appeared after 2 h ultrasonic reaction. This result is consistent with the diameter analysis of UV–Vis spectrum in Fig. 1. Importantly, for in the whole experimental process no additional agent except AgNO3 was added, the nanoparticles should be Ag NPs. The strong signals at 3 and 3.16 keV in EDX spectrum (Fig. 2D) confirm the presence of element Ag. In addition, the atom ratio of Ag, N, and O of the sample is 21.6:2.4:3.4 (the inset of Fig. 2D), which is much higher than that of AgNO3. This demonstrates the formation of metallic Ag NPs in pure aqueous solution via ultrasonic irradiation. The signal of C is very likely to result from the carbon film on copper grid. During ultrasonic process, water plays the role of solvent as well as reductant. Although the reductive rate of Ag+ in pure water via ultrasonic irradiation is very low, this method can be utilized wildly for the use of simplest, non-toxic and environment friendly inorganic solvent. Therefore, the reactive route needs to be elucidated to find ways to enhance the reactive rate. As shown above, under ultrasonic condition Ag+ gets electron to be reduced into silver seed (Ag0). Based on early research [24–26], water molecules decompose to hydrogen radicals (H ) and hydroxyl radicals ( OH) under ultrasonic condition, and then H supply electron for the reduction of Ag+:
3.1. Synthesis of Ag NPs in pure aqueous solution
H2 O ! H þ OH; H þ Agþ ! Hþ þ Ag0
2.3. Characterization All of the samples were measured on U-3010 ultraviolet–visible (UV–Vis) spectrophotometer without dilution. Transmission electron microscopy (TEM) and were performed on Hong Kong FEI TEC nai Model G220 transmission electron microscope with an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDX) analysis was performed on EDAX AMETEK energy dispersive spectroscopy. The pH values for the solutions were tested by HANNA pH213 microprocessor pH meter. Ag NPs filtered on microporous membrane were characterized by X-ray powder diffraction (XRD) analysis performed on German AXS D8 ADVANCE diffractometer with CuKa radiation. All photographs of the samples were taken by Sony DSC-W570 camera.
Sonicate
To study the reductive effect of water under ultrasonic condition, UV–Vis spectrum of the reaction solution was carried out. Fig. 1 shows the UV–Vis spectrum of sample before and after sonication for 2 h. After sonicated for 2 h, there is no appearance of the specific surface plasmon resonance (SPR) absorption band of Ag NPs around 400 nm [21]. But the absorbance around 400 nm increased after the ultrasonic reaction, tiny while obvious, which means that some change occurred in the solution. Mulvaney [22] has reported that SPR peak of Ag NPs below 4 nm disappeared for the change in electronic structure that occurs with quantum
ð1Þ
Actually, only the valence of silver changed in the reductive reaction. In other words, other corresponding oxidative reaction should exist to keep the valence balance. During reactive process, a little bump appeared on the top of the penicillin bottle formed by the plastic wrap, which suggested that gas was produced in the bottle. For the presence of only water and AgNO3 aqueous solution in the reactor, the gas is probably oxygen. Therefore, the oxidative reaction is most likely to react in the following way as Eq. (2) shows:
HO þ OH ! H2 O2 ;
H2 O2 ! H2 O þ 1=2O2
ð2Þ
544
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
Fig. 2. TEM images of Ag NPs synthesized in pure water via sonication for 2 h (A and B); the corresponding size distribution histogram (C) and EDX spectra of the Ag NPs (D).
So the overall reaction reacts like Eq. (3):
3.2. Synthesis of Ag NPs in alkaline aqueous solution and the effect of OH in the reaction
Sonicate
2Agþ þ H2 O ! 2Ag0 þ 1=2O2 þ 2Hþ
ð3Þ
Design the overall reaction (Eq. (3)) as the following galvanic cell: 1
ðÞ Pt k H2 O;Hþ jO2 j Agþ ð5:88 103 mol kg Þj AgðþÞ
ð4Þ
The individual electrode reactions can be expressed as [27]:
H2 O ! 2Hþ þ 1=2O2 þ 2e
ðÞ Anodic reaction :
¼ þ1:2290 V
ðþÞ Cathodic reaction :
EhL ð5Þ
2Agþ þ 2e ! 2Ag EhR
¼ þ0:7994 V
ð6Þ
At standard condition (room temperature 25 °C with the pressure of 100 kPa), the electromotive force (EMF, E) of galvanic cell (Eq. (4)) is below zero, Eh ¼ EhR EhL ¼ 0:4296V < 0, which means this cell reaction cannot occur spontaneously. Fortunately, the extremely high temperature and pressure resulting from the broken of cavitation bubbles caused by the ultrasonic irradiation [28] make the galvanic cell reaction accessible. Calculated under ultrasonic condition (ESI 1.1), the EMF was 1.40 V, which indicates the successful overall cell reaction (Eq. (3)). This result is in agreement with the formation of Ag NPs demonstrated in above analysis of UV–Vis spectrum and TEM. In fact, OH plays extensively important role in this cell reaction. During the reaction, it offers electron to Ag+ accompanying with the exhaust of oxygen like anodic reaction shown in Eq. (3). But the production of oxygen is too slight to be detected during ultrasonic process. However, a similar reactive route involving Ag NPs, H2O2, OH and OH has been reported in He’s work [29]. To testify the effect of OH in the reaction, following experiments were designed.
After the alkaline aqueous solution was sonicated for 2 h, an absorption peak around 410 nm corresponding to SPR peak of Ag NPs appeared in Fig. 3A. This phenomenon indicates that Ag NPs formed easily under alkaline condition compared with under acidic and neutral condition (ESI Figs. S1 and S2). Meanwhile, the color of the sample also transferred from colorless to pale yellow clearly (inset of Fig. 3A), which resulted from the formation of Ag NPs [13]. After the solution was sonicated for 12 h, its color turned into dark brown further for much more formation of Ag NPs. Since the peak height of SPR is direct proportional to amount of metal nanoparticles [30], the increment of absorbance for the solution at 410 nm (Fig. 3A) suggests that Ag NPs formed enormously after 12 h as well. Fig. 3B shows that pH values for the solution decreases evidently as the ultrasonic reaction progresses. This can be attributed to two possibilities: (1) the reaction of OH and (2) the hydrolysis of Ag+. However, when the solution was sonicated for 9 h, the SPR peak height for Ag NPs does not increase any more (Fig. 4B) due to the limited number of OH. Meanwhile, the varying rate of pH values for the solution after 9 h (Fig. 3B, slope of green solid curve) decreases distinctly compared with that of before 9 h (Fig. 3B, slope of red solid curve). This suggests that the reaction of OH is mainly responsible for the decrement of pH values for the solution other than the hydrolysis of Ag+. In addition, other proof, that the varying trend of SPR peak height with time for Ag NPs (Fig. 4B the red solid and green solid curve) is the same as the varying trend of pH values with time for the solution (Fig. 3B the red solid and green solid curve), also demonstrates this point. To verify this point further, another 20 mg NaOH was added into the solution after reacted for 12 h, which leads the sudden increase of pH values for the solution shown in Fig. 3B. And then extra 2 h sonication was performed to the solution without adding any more AgNO3 aqueous solution. Clearly, the absorbance for the solution at 410 nm begins to increase again as ultrasonic reaction progresses (Fig. 4A). This suggests that Ag NPs begin to form
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
545
Fig. 3. Time evolution (0–12 h) of the UV–Vis absorption spectra and the color (the inset, left to right) for the reaction solution (A, pH > 7); Time evolution (0–14 h) of the pH values for the reaction solution (B, pH > 7). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Time evolution (12–14 h) of the UV–Vis absorption spectra and the color (the inset, left to right) for the reaction solution after adding 20 mg NaOH into it (A); Time evolution (0–14 h) of SPR peak height with time for Ag NPs (B) [data from Figs. 3A and 4A]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
again. More importantly, the variety rate of pH values for the solution after 12 h (Fig. 3B, slope of blue solid curve) is almost the same as that of before 9 h (Fig. 3B, slope of red solid curve). This indicates that OH is very crucial to the synthesis of Ag NPs in aqueous solution via ultrasonic irradiation. Therefore, it is reasonable to believe that OH really reduces Ag+ during ultrasonic process. Under acidic and neutral condition, no clear movement is occurred in the UV–Vis spectrum and the pH values for both the solutions are almost stable (ESI Figs. S1 and S2). These also prove that alkaline condition is much preferred in the synthesis of Ag NPs in aqueous solution via ultrasonic irradiation. TEM images of obtained Ag NPs (Fig. 5A and B) show that much more Ag NPs are obtained in alkaline aqueous solution compared with in pure aqueous solution (Fig. 1A and B). The photograph of Ag NPs filtered on microporous membrane is delivered in Fig. 6A. Fig. 5D shows typical XRD pattern of the as-prepared sample in alkaline aqueous solution. The five diffraction peaks, 38.07°, 44.29°, 64.50°, 77.48°, 81.55°, can be readily indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of the face-centered-cubic (fcc) structured metallic silver (JCPDS No. 42-0783). The diameter of Ag NPs (8.9 nm) calculated from the XRD peak of the (1 1 1) reflection using well-known Scherrer formula is well consistent with the mean sizes (8 nm) measured from TEM images [31]. As described above, OH plays significant role in forming Ag NPs. Scheme 1 responds to the reactive route of Ag NPs in alkaline aqueous solution via ultrasonic irradiation. As is known, AgNO3 is unstable under alkaline condition to produce AgOH, and then AgOH decomposes to Ag2O. Ag2O can also hydrolyze into complex
of Ag(OH)x [32], such as Ag(OH) and AgðOHÞ 2 [6]. The formation of Ag NPs can be separated into three steps: reduction of Ag+, nucleation of Ag0, and growth of the crystal [33,34]. Neglecting the growth of crystal, the formation of Ag NPs can be expressed as follows [35]: K1
Agþ ! Ag0 K2
nAg0 ! Agn
ð7Þ ð8Þ
where K1 and K2 represent overall reduction and nucleation rates, respectively. Here we analyze that there might be three reasons why the formation rate of Ag NPs in alkaline aqueous solution is higher than in acidic and neutral aqueous solution: (1) In same condition, the intermediate complex Ag(OH)x possess bigger reduction rates (K1) than AgNO3 [6,35]. After Ag2O hydrolyzed into Ag(OH)x, it accepts electron releasing from OH or OH to form Ag0 during ultrasonic process. (2) As shown in Scheme 1, Ag2O particle, solid insoluble impurity, serves as nucleation sites for forming Ag NPs at the initial reaction stage [35]. Furthermore, the appearance of these nucleation sites decreases the required supersaturated concentration of Ag0 to yield the first nucleation site for changing the nucleation process from homogenous to heterogeneous [36]. This leads the faster nucleation process which is equivalent to increasing nucleation rate (K2).
546
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
Fig. 5. TEM images of Ag NPs synthesized in alkaline aqueous solution via sonication 14 h (A and B); the corresponding size distribution histogram (C) and XRD pattern (D) of the Ag NPs filtered on microporous membrane.
Fig. 6. Photograph of filtered Ag NPs on microporous membrane (A) and the obtained nano-silver colloid solution after 7 h ultrasonic reaction kept for 2 months (B) and 12 h ultrasonic reaction kept for 24 h (C) with same initial concentration of NaOH in both solutions.
Scheme 1. Reactive scheme for the formation of Ag NPs in alkaline aqueous solution via ultrasonic irradiation.
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
(3) OH can consume H+ formed during the reduction of Ag+ to increase the reduction rate (K1) [35]. In fact, under alkaline condition, the former galvanic cell reaction (Eq. (3)) can be redesigned as follows: 1
ðÞ PtjH2 O;OH jO2 j Agþ ð5:88 107 mol kg Þj AgðþÞ
ð9Þ
The individual electrode reactions can be expressed as [27]:
2OH ! H2 O þ 1=2O2 þ 2e
ðÞ Anodic reaction :
¼ þ0:4010 V ðþÞ Cathodic reaction :
ð10Þ
2Agþ þ 2e ! 2Ag EhR
¼ þ0:7994 V Overall reaction :
EhL
2Agþ þ 2OH ! H2 O þ 1=2O2 þ Ag
ð11Þ ð12Þ
At standard condition, the EMF of galvanic cell shown in Eq. (9) is above zero, Eh ¼ EhR EhL ¼ 0:3984V > 0, which indicates the cell reaction can occur spontaneously thermodynamically. The high temperature and pressure caused by the sonication will dramatically accelerate the reactive speed dynamically. Meanwhile, the yielded O2 during the ultrasonic process promoted the production of H2O2 as reported before [37]. These might be the main reasons for faster formation of Ag NPs in alkaline aqueous solution via ultrasonic irradiation. In addition, OH has the same effect as the traditional surfactant does. As shown in Fig. 6B and C, the sample containing more OH for reacting only 7 h stayed stable more than 2 months; while the other sample with less OH for reacting 12 h precipitated only 24 h later. It’s because enough OH surrounded around Ag NPs provides an energy barrier to counteract the van der Waals’ force between nanoparticles to prevent the aggregation [36].
4. Conclusion In summary, we observed the formation of Ag NPs in pure aqueous solution without adding any reductant and surfactant under ultrasonic condition. During this process, water serves as solvent as well as reductant. In alkaline aqueous solution, ultrafine Ag NPs with diameter of 8 nm were achieved. Under this condition, OH plays role of electron donors as OH does in pure aqueous solution. It also has the effects of helping to yield the nucleation sites and acting as surfactant. This method to synthesize Ag NPs may have broad biomedical and antibacterial application for no addition of environmentally hazardous reagents and the relatively small diameter of the obtained Ag NPs. Moreover, it can be applied in fabricating composite due to the simple process and condition. A further study on Ag NPs/GO composite fabricated by this method is now in preparation to be published elsewhere. Acknowledgment The authors are grateful for the financial support by National Natural Science Foundation of China (Grant No. 51173013) and Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110010110007). 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.ultsonch.2013. 09.003.
547
References [1] T. Maiyalagan, Synthesis, characterization and electrocatalytic activity of silver nanorods towards the reduction of benzyl chloride, Appl. Catal. A 340 (2008) 191–195. [2] C. Marambio-Jones, E.M.V. Hoek, A review of the antibacterial effects of silver nanomaterials and potential implications for human health and the environment, J. Nanopart. Res. 12 (2010) 1531–1551. [3] E. Solano-Ruiz, R. Sato Berrú, J. Ocotlán-Flores, J.M. Saniger, Synthesis of silver nanoparticles by sonochemical induced reduction application in SERS, J. Nano. Res-sw. 9 (2010) 77–81. [4] S.T. Dubas, V. Pimpan, Humic acid assisted synthesis of silver nanoparticles and its application to herbicide detection, Mater. Lett. 62 (2008) 2661–2663. [5] D. Kim, S. Jeong, J. Moon, Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection, Nanotechnology 17 (2006) 4019–4024. [6] D.D. Evanoff, G. Chumanov, Size-controlled synthesis of nanoparticles. 1. ‘‘Silver-only’’ aqueous suspensions via hydrogen reduction, J. Phys. Chem. B 108 (2004) 13948–13956. [7] H. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953. [8] J.P. Abid, A.W. Wark, P.F. Brevet, H.H. Girault, Preparation of silver nanoparticles in solution from a silver salt by laser irradiation, Chem. Commun. (2002) 792–793. [9] S. Kundu, K. Wang, H. Liang, Size-controlled synthesis and self-assembly of silver nanoparticles within a minute using microwave irradiation, J. Phys. Chem. C 113 (2009) 134–141. [10] A.R. Abbasi, A. Morsali, Synthesis and properties of silk yarn containing Ag nanoparticles under ultrasound irradiation, Ultrason. Sonochem. 18 (2011) 282–287. [11] H. Ma, B. Yin, S. Wang, Y. Jiao, W. Pan, S. Huang, S. Chen, F. Meng, Synthesis of silver and gold nanoparticles by a novel electrochemical method, Chem. Phys. Chem. 5 (2004) 68–75. [12] J. Zhu, S. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Shape-controlled synthesis of silver nanoparticles by pulse sonoelectrochemical methods, Langmuir 16 (2000) 6396–6399. [13] J.H. Byeon, Y.-W. Kim, A novel polyol method to synthesize colloidal silver nanoparticles by ultrasonic irradiation, Ultrason. Sonochem. 19 (2012) 209– 215. [14] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Formation of colloidal gold nanoparticles in an ultrasonic field: control of rate of gold(III) reduction and size of formed gold particles, Langmuir 17 (2001) 7717–7720. [15] G.H. Zhang, W.L. Guo, X.K. Wang, Sonochemical formation of nanocrystalline gold in aqueous solution, Mater. Res. Innov. 11 (2007) 201–203. [16] Y. Mizukoshi, E. Takagi, H. Okuno, R. Oshima, Y. Maeda, Y. Nagata, Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions: role of surfactants, Ultrason. Sonochem. 8 (2001) 1–6. [17] T. Fujimoto, S.-Y. Terauchi, H. Umehara, I. Kojima, W. Henderson, Sonochemical preparation of single-dispersion metal nanoparticles from metal salts, Chem. Mater. 13 (2001) 1057–1060. [18] H. Wang, J. Guo, J. Li, J. Wei, Fabrication of bimetallic nanoparticles/multiwalled carbon nanotubes composites for microelectronic circuits, Carbon 49 (2011) 779–786. [19] M. Tsuji, Y. Nishizawa, K. Matsumoto, M. Kubokawa, N. Miyamae, T. Tsuji, Effects of chain length of polyvinylpyrrolidone for the synthesis of silver nanostructures by a microwave-polyol method, Mater. Lett. 60 (2006) 834– 838. [20] T. Kimura, T. Sakamoto, J.-M. Leveque, H. Sohmiya, M. Fujita, S. Ikeda, T. Ando, Standardization of ultrasonic power for sonochemical reaction, Ultrason. Sonochem. 3 (1996) S157–S161. [21] H. Abe, K.P. Charlé, B. Tesche, W. Schulze, Surface plasmon absorption of various colloidal metal particles, Chem. Phys. 68 (1982) 137–141. [22] P.C. Mulvaney, Surface plasmon spectroscopy of nanosized metal particles, Langmuir 12 (1996) 788–800. [23] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley & Sons, New York, 2008. [24] Y. Nagata, Y. Watananabe, S.-I. Fujita, T. Dohmaru, S. Taniguchi, Formation of colloidal silver in water by ultrasonic irradiation, J. Chem. Soc. Chem. Commun. (1992) 1620. [25] N.A. Dhas, A. Gedanken, Sonochemical synthesis of molybdenum oxide and molybdenum carbidesilica nanocomposites, Chem. Mater. 9 (1997) 3144– 3154. [26] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [27] W.M. Haynes, D.R. Lide, T.J. Bruno, CRC Handbook of Chemistry and Physics 2012–2013, CRC Press, 2012. [28] A. Gedanken, Using sonochemistry for the fabrication of nanomaterials, Ultrason. Sonochem. 11 (2004) 47–55. [29] D. He, A.M. Jones, S. Garg, A.N. Pham, T.D. Waite, Silver nanoparticlereactive oxygen species interactions: application of a chargingdischarging model, J. Phys. Chem. C 115 (2011) 5461–5468. [30] U. Krebig, M. Vollmer, Theoretical considerations-single clusters: intrinsic size effects of the optical properties, in: U. Gonser, R.M. Osgood, M.B. Panish, H. Sakaki (Eds.), Optical Properties of Metal Clusters, Springer-Verlag, Berlin and Heidelberg, 1995, p. 83.
548
C. He et al. / Ultrasonics Sonochemistry 21 (2014) 542–548
[31] V. Radmilovic, H.A. Gasteiger, P.N. Ross, Structure and chemical composition of a supported Pt–Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98–106. [32] X. Wang, H.-F. Wu, Q. Kuang, R.-B. Huang, Z.-X. Xie, L.-S. Zheng, Shapedependent antibacterial activities of Ag2O polyhedral particles, Langmuir 26 (2010) 2774–2778. [33] B.R. Pamplin, Crystal Growth, Pergamon Press, New York, 1975. [34] Y. Jiang, Forced hydrolysis and chemical co-precipitation, in: Z.L. Wang, Y. Liu, Z. Zhang (Eds.), Nanophase and Nanostructured Materials, Kluwer Academic, New York, 2003, p. 59.
[35] S. Nishimura, D. Mott, A. Takagaki, S. Maenosono, K. Ebitani, Role of base in the formation of silver nanoparticles synthesized using sodium acrylate as a dual reducing and encapsulating agent, Phys. Chem. Chem. Phys. 13 (2011) 9335. [36] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [37] E.L. Mead, R.G. Sutherland, R.E. Verrall, The effect of ultrasound on water in the presence of dissolved gases, Can. J. Chem. 54 (1976) 1114–1120.