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Synthesis of monodisperse Ag–Au alloy nanoparticles with large size by a facile fabrication process
Materials Chemistry and Physics 131 (2011) 136–141
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of monodisperse Ag–Au alloy nanoparticles with large size by a facile fabrication process Weiwei Zhang, Liqing Huang ∗ , Jian Zhu, You Liu, Jun Wang Non-equilibrium Condensed Matter and Quantum Engineering Laboratory, The Key Laboratory of Ministry of Education, School of Science, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 19 December 2010 Received in revised form 29 June 2011 Accepted 29 July 2011 Keywords: Alloys Chemical synthesis Energy dispersive analysis of X–rays Electron microscopy Optical properties
a b s t r a c t The fairish monodisperse Ag–Au alloy nanoparticles (NPs) with various composition and size were synthesized by a facile fabrication procedure. UV–visible (UV–vis) spectroscopy and transmission electron microscopy (TEM) confirmed the formation of homogeneous alloy NPs. The diameters of the resultant monodisperse spherical Ag–Au alloy NPs are 21–32 nm and the long-axis size of the resultant monodisperse nonspherical Ag–Au alloy NPs are about 28 nm. The possible formation mechanism is suggested by analyzing the dynamic absorption spectra, TEM and energy dispersive X-ray (EDX) spectroscopy of the alloy nanoparticles at the different time. Ag–Au alloy NPs synthesis method developed in this paper has several advantages: (1) the synthesis process is very simple. Only sodium citrate was used as both reducing agent and stabilizing agent and only one step heating process was needed. (2) The resultant Ag–Au colloidal dispersion is free from AgCl deposition and (3) the fairish monodisperse and large size (>20 nm) Ag–Au alloy NPs can be obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bimetallic silver–gold NPs have received enormous attention due to their unique electronic, optical [1–6] and catalytic [7,8] properties distinct from those of pure mono-metal NPs. For example, localized surface Plasmon absorption for Ag–Au alloy NPs can be tuned systematically over a broad range by simply varying the alloy composition [1–6] and the combination of desirable features of both Ag and Au is useful to a diverse range of applications, especially in sensing [9,10] and surface-enhanced Raman spectroscopy (SERS) [11–19]. In addition, Ag–Au alloy NPs are more catalytically active than monometallic Ag or Au NPs in the oxidation of CO at low temperatures [8]. A number of physical and chemical techniques have been developed to produce Ag–Au alloy NPs. The physical methods include evaporation–condensation [20], laser ablation [21], sputter deposition [22], etc. Among the chemical methods, the most common one is the thermal chemical methods [2,4,23–32], including co-reduction [2,4,23–26], galvanic replacement reaction [27–29], digestive ripening [30] and other complex methods [31,32]. Chen et al. produced Ag–Au alloy NPs with the mean diameter of 4–22 nm using hydrazine as reducing agent in water-in-oil microemulsions [24]. Mallin and coworkers yielded Ag–Au alloy NPs with average diameters of 5–7 nm using sodium borohydride
and sodium citrate as a reducing and stabilizing agent, respectively [2]. The Ag–Au alloy NPs of various compositions with the average diameter less than 10 nm were synthesized by the galvanic replacement reaction [27,29]. Lu and coworkers reported the replacement reaction preparation of hollow Au–Au alloy NPs with diameter of less than 20 nm [28]. And the synthesis of Ag–Au alloy particles with diameter of 15 nm from core/shell structure Ag/Au was reported by digestive ripening [30]. The Ag–Au alloy NPs of different shape and composition with diameter of less than 10 nm were produced by more complex methods [32]. Despite a myriad of methods of preparation, the synthesis of large size, monodisperse Ag–Au alloy NPs by the facile fabrication procedure remains a challenge. The diameters of monodisperse alloy particles prepared by current methods are less than 22 nm and the synthesis process is relative complicated. Also, some preparation process needs to adopt an additional measure in order to take out the AgCl by-production [29]. In this paper, the synthesis of fairish monodisperse Ag–Au alloy NPs with large size by a novel facile fabrication procedure was studied. The size, morphology and optical properties of the resultant Ag–Au alloy NPs were characterized by TEM and UV–vis spectroscopy, respectively. The formation mechanism of the alloy NPs was also discussed. 2. Experimental 2.1. Materials and instruments
∗ Corresponding author. E-mail address: [email protected] (L. Huang). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.079
Silver nitrate (AgNO3 ) and chlorauric acid (HAuCl4 ·4H2 O) from Chinese Shanghai Chemical Company and sodium Citrate from Chinese Tianjin BaiShi Chemical
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141
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Fig. 1. (a) UV–vis absorption spectra of the resultant Ag–Au alloy with varying initial Ag:Au molar ratios (1:0, 0.75:0.25, 0.67:0.33, 0.5:0.5, 0.33:0.67, 0.25:0.75, 0:1). (b) Plot of the peak wavelength against the initial gold mole fraction.
Company, were used as received. Doubly deionized water (>18 M) was produced by a LABCONCO water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned by AS10200BDT Ultrasonic cleaner from AUTO SCIENCE and then washed in aqua regia, followed by copious washing with deionized water before drying in an oven. An OCEAN’s HR2000+ high resolution fiber spectrometer and a DH-2000 deuterium tungsten halogen sources (200–1100 nm) were used to monitor and record dynamic absorption spectra. The constant temperature heater with magnetic force stirrer was from Hangzhou Instrument Company. 2.2. Preparation of silver and gold NPs The gold and silver NPs were prepared by the citrate-reduction procedure of Frens [33]. For Au NPs: 2.1 ml 2.4 mM HAuCl4 was mixed with 47.9 ml H2 O in a quartz beaker. The solution was stirred and heated to boiling, and then 1.0 ml 35 mM sodium citrate solution was added to the boiling solution. The solution was further boiled for 30 min and was then left to cool to room temperature. The vigorous stirring was maintained during the heating process. For Ag NPs: the initial reaction solution contained 2.5 ml 2.0 mM AgNO3 and 47.5 ml H2 O. A 1.0 ml 35 mM sodium–citrate was used as reducing agent and the heating duration was about 40 min. The other conditions were same as those of Au NPs preparation process. When exposed in air at room temperature, the gold colloidal dispersion could be stable for about 1 month, but the silver colloidal dispersion could only be stable for about 2 weeks. 2.3. Preparation of Ag–Au alloy NPs The aqueous solution of AgNO3 and sodium–citrate was heated to boiling for some time, and then the HAuCl4 aqueous solution was added to it just before the obvious absorption band of Ag NPs appearing. The dynamic UV–visible absorption spectra were detected by a fiber spectrometer. While the solution was further boiled for about 13 min and was then left to cool to room temperature. The vigorous stirring was maintained during the whole reaction process. The total molar amount of Ag and Au was fixed for all resultant Ag–Au NPs (5 × 10−6 mol L−1 ). The Ag–Au alloy NPs of various composition were synthesized by fixing the molar ratio of the total metal and reduction agent (0.125:0.875) and varying the initial Ag:Au molar ratios (0.75:0.25, 0.60:0.40, 0.67:0.33, 0.50:0.50, 0.33:0.67, 0.40:0.60, 0.25:0.75). While the Ag and Au molar ratio was fixed, by varying the molar ratio of the total metal and reducer, the Ag–Au NPs of various size were synthesized (Ag:Au = 0.75:0.25, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75, 0.4:0.6; Ag:Au = 0.50:0.50, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75; Ag:Au = 0.25:0.75, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75, 0.4:0.6).
NPs are shown in Fig. 1(a). Only one absorption band is observed and the maximum wavelength of absorption spectrum red-shifts from 420 nm (Ag NPs) to 520 nm (Au NPs) with the increase in the Au mole fractions. In Fig. 1(b) the change of maximum wavelength of the absorption band with the initial gold mole fractions is plotted and a linear relationship is found. These results indicate that the resultant NPs are alloy other than core–shell NPs [1,34], or a mixture of Ag and Au NPs [2]. Fig. 2 shows the digital photos of the resultant Ag–Au alloy colloidal dispersion with varying initial Ag:Au molar ratios after being deposited for different time. The color changes from yellow to orange and to red with the increasing in Au mole fraction and does not appear notable change with the deposited duration, indicating the formation of the stable Ag–Au alloy NPs with various compositions. The TEM micrographs of the resultant Ag–Au alloy NPs with varying initial Ag:Au molar ratio are displayed in Fig. 3(a)–(e) and the inset in each TEM is the corresponding size histogram. The monodispersity of the alloy NPs is good and the average diameters are 30.83 nm, 26.62 nm, 21.58 nm, 23.32 nm and 24.88 nm, respectively. The change of Ag–Au alloy NPs mean diameter with the Au mole fractions is illustrated in Fig. 3(f). The mean diameter decreases firstly and then increases slightly with the increasing in Au mole fraction. The least one was obtained at Au mole fraction 0.5. This size change character will be discussed in the later study on formation mechanism of the Ag–Au NPs.
2.4. Characterization The size, morphology and composition of the resultant Ag–Au alloy NPs were determined by a JEOL JEM-2100 TEM. The UV–vis spectra of the resultant Ag–Au alloy colloidal solution were measured with a Perking Elmer Lambda750 s UV–visible spectrophotometer at room temperature.
3. Results and discussion 3.1. The Ag–Au alloy NPs of various compositions The UV–vis absorption spectra of the resultant NPs with varying initial Ag:Au molar ratios together with those of Ag and Au
Fig. 2. Photos of the resultant Ag–Au alloy NPs with varying initial Au mole fractions (form left to right 0.25, 0.33, 0.50, 0.67, 0.75). The photo of (a) obtained after being deposited for several days, (b) obtained after being deposited for 2 months.
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Fig. 3. TEM images and size change plot of Ag–Au alloy NPs with varying initial Ag:Au molar ratios and fixing reducing mole fraction 0.825. (a) 0.75:0.25, (b) 0.67:0.33, (c) 0.50:0.50, (d) 0.33:0.67, (e) 0.25:0.75. The inset shows the size histogram of the NPs. (f) Plot of average diameters change with the initial gold mole fraction.
3.2. Ag–Au alloy NPs of various sizes The size of the Ag–Au alloy NPs can be controlled by tuning the molar ratio of the metal salt and reducing agent at a fixed Ag:Au molar ratio. At different fixed Ag:Au molar ratios (0.75:0.25, 0.50:0.50, 0.25:0.75), the UV–vis absorption spectra of the assynthesized NPs with various reducing agent mole fractions are shown in Fig. 4(a)–(c), respectively. The dependence of maximum wavelength of the absorption band on the reducing agent mole fractions at different fixed Ag:Au molar ratio is shown in Fig. 5. Only one absorption band was observed and the peak wavelength of absorption spectrum is red-shifted with the decrease in the reducing agent mole fractions, indicating the formation of the Ag–Au alloy NPs with various sizes. For the Au mole fractions 0.75, the amount of red-shifted is about 21 nm, while Au mole fractions equal or small
than 0.50, the red shift is about 12 nm. The TEM micrographs of the resultant Ag–Au alloy NPs with varying Au mole fractions (0.25, 0.50, 0.75) at the lower sodium citrate mole fractions (0.75, 0.6) are shown in Fig. 6. The insert in each TEM is the corresponding size histograms. For the Au mole fractions 0.25, 0.5 and reducing agent mole fractions 0.75, the resultant NPs are spherical and of fairish good monodispersity, the average diameters are 32.63 nm and 29.67 nm, respectively. The average diameter increases with the decrease in the reducing agent mole fractions. When the reducing agent mole fractions decrease form 0.875 to 0.75, the diameter is increase about 2 nm [the diameter of Fig. 6(a) – the diameter of Fig. 3(a)] and 8 nm [the diameter of Fig. 6(b) – the diameter of Fig. 3(c)], respectively. The amount of red-shifted and diameter change indicate that the tunable range of the resultant NPs size is broader for rich gold
Fig. 4. UV–vis absorption spectra of the resultant NPs with varying metal:reducer and initial Ag:Au molar ratios.
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141
Fig. 5. Plot of the absorption peak wavelength against the reducer mole fractions at varying initial Ag:Au molar ratios.
composition NPs. While for Au mole fractions greater than 0.50 and reducing agent mole fractions 0.6, the resultant NPs are nonspherical and also of fairish good monodispersity, the length of major axis is about 28 nm long, indicating that the size tuning range is limited by tuning the metal to reducing molar ratio. 3.3. Formation mechanism of Ag–Au alloy NPs To understand the dynamics of the formation of the resultant Ag–Au alloy NPs, UV–vis spectra were recorded at 5 s intervals after
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HAuCl4 aqueous solution were added into the boiling solution of the AgNO3 and sodium–citrate. Representative dynamic absorption spectra are shown in Fig. 7 for sodium citrate and Au mole fractions 0.875 and 0.25, respectively. At the early stage of the process, a very weak absorption band at about 418 nm was observed and disappeared at the final stage. This absorption band (as indicated with a red dashed arrow in Fig. 7) is ascribed to the surface plasmon resonance (SPR) of the Ag NPs and suggests that the small Ag NPs were formed. Within 25 s after adding the HAuCl4 aqueous solution to the boiling solution, a broad weak absorption band at about 537 nm appeared and then shifted to shorter wavelength accompanying with the increase in absorbance during the reaction process (as indicated with a red solid arrow in Fig. 7). This blueshifted absorption band is ascribed to the SPR of the Ag–Au alloy NPs with increasing Ag content and suggests that the Ag–Au alloy NPs with increasing gradually Ag composition were formed. Fig. 8 shows the changes of maximum wavelength of the absorption band with increasing in reaction time at different Au mole fractions and the changes during the first 60 s are showed in the insert. It shows that the blue-shifted was rapidly at the first and then slowly. And the greater the Au mole fractions were, the slower the blue shift was. As the Au mole fraction was less than 0.5, the maximum wavelength of the absorption band of the initial absorption band was all about 538 nm, but as the Au mole fractions greater than 0.5 (0.67, 0.75), it increased with the increase in Au mole fractions. According to the above spectra varying characteristics (the change of the absorption band number, maximum wavelength of the absorption band, absorbance and blue-shifted rate with
Fig. 6. TEM images of Ag–Au alloy NPs with varying initial Au and reducing mole fraction. (a) 0.25, 0.75. (b) 0.50, 0.75. (c) 0.75, 0.6. The inset shows the size histogram of the NPs.
Fig. 7. Time dependent absorption spectrum of alloy nanoparticles. (Normalized and moved along the Y axis, (a) the time interval of each line is 5 s, (b) the time interval of each line is 1 min, (c) the time interval of each line is 10 s.)
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Fig. 8. The changes of absorption peak wavelength with increasing in reaction time at different Ag:Au mol ratios.
reaction time and Au mole fractions), we put forward a possible formation mechanism of the Ag–Au NPs, schematically depicted in Fig. 9. Before adding HAuCl4 aqueous solution, a certain number of Ag atoms and very small Ag NPs had been formed by sodium citrate reducing Ag+ ions in the boiling solution. Once the HAuCl4 aqueous solution was added to the boiling solution (Fig. 9(a)), the AuCl− 4 ions were reduced rapidly by citrate, formed Ag atoms and
NPs synchronously. The reduction reaction was so rapidly that the AuCl− 4 ions were reduced completely and the reduced Au atoms aggregated rapidly to form the Au nuclei almost at the same time (Fig. 9(b)). After that, the Ag atoms both remaining and reduced subsequently by citrate deposited on the surface of the Au nuclei and by alloying process at high temperature (ca. 100 ◦ C) the Ag–Au NPs with increasing in Ag composition were formed (Fig. 9(c)). By properly controlling the metal to reducer molar ratios, the Ag+ ions were reduced completely and used to form the alloy NPs, no obvious AgCl deposition was produced in the resultant Ag–Au colloidal dispersion. The size of the Au nuclei is dependent on the molar ratios of the metal:reducer and Ag:Au. While the metal:reducer mole fraction is fixed (for example, 0.125:0.875), the size of the Au nuclei is varying with the Au mole fractions and there is a critical Au mole fractions (0.5). For the Au mole fractions is less than the critical mole fractions, the size do not change obviously with the Au mole fractions (the maximum wavelength of the initial absorption band is nearly same for various Au mole fractions), but for the Au mole fractions is greater than the critical value, the size of the Au nuclei increases with the increasing in Au mole fractions (the maximum wavelength of the initial absorption band red shifts with the increasing in Au mole fractions). Since the total mole amount of metal was fixed, the size of the resultant Ag–Au alloy NPs is dependent on the size of the Au nuclei and the Au mole fractions. For the Au mole fractions is smaller than the critical value, the number of the Au nuclei increase with the increasing in the Au mole fractions and the size of the resultant Ag–Au NPs decrease. And for the Au mole fractions greater than
Fig. 9. The possible mechanism for formation of alloy nanoparticles.
Fig. 10. The EDX analysis (above) and TEM images of the alloy nanoparticles at different time. (a) 30 s, (b) 60 s, (c) 120 s, (d) 300 s, (e) resultant alloy NPs at 15 min.
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141 Table 1 Composition of Ag–Au alloy NPs formed at different time of the growing process. Time
30 s
60 s
120 s
300 s
15 min
%Ag %Au
29.14 70.86
33.75 66.25
39.27 60.73
49.21 50.79
52.87 47.13
the critical value, the number of the Au nuclei decrease with the increasing in Au mole fractions, the size of the resultant Ag–Au alloy NPs increase with the increasing in Au mole fractions. This effect of Au mole fraction on the alloy NPs size is in agreement with the experimental results of the resultant Ag–Au alloy NPs, which is shown in Fig. 3(f). The above possible formation mechanism can be shown with the schematic diagram in Fig. 9. Fig. 9(a) shows the formation of the Ag atoms and NPs by citrate reducing Ag+ ions. Fig. 9(b) shows the formation of the Au nuclei rapidly by Ag atoms, Ag NPs and citrate reducing AuCl− 4 synchronously, which assures the monodispersity of the resultant Ag–Au NPs. Fig. 9(c) shows the formation of the Ag–Au alloy NPs by depositing the Ag atoms reduced by citrate on the surface of Au nuclei and alloying at high temperature. The sodium citrate was only used to be both reducing agent and stabilizing agent in the whole synthesizing process. The TEM and EDX analysis were performed for the alloy nanoparticles (composition molar ratio Ag:Au = 0.5:0.5, reducing agent mole fractions 0.75) at different time of the growing process. Fig. 10(a)–(e) are the TEM images and the EDX spectra of alloy NPs formed at 30 s, 60 s, 120 s, 300 s and 15 min, respectively, after adding HAuCl4 solution. The TEM images show relative uniform contrast for all formed NPS and do not show any banding. The composition analyzing by the EDX spectra are shown in Table 1, which indicates that the amount of silver increase from 29.14% to 52.87% with the time increased from 30 s to 15 min. These facts indicate that resultant NPs are Ag–Au alloy NPs other than core–shell NPs [35], or a mixture of Ag and Au NPs [2]. 4. Conclusion The fairish monodisperse spherical Ag–Au alloy NPs with various compositions and diameters were synthesized by a facile synthesis process. Analyzing based on dynamic absorption spectra and TEM images shows that both the Ag:Au molar ratio and reducing agent mole fraction can affect the formation of alloying nanoparticles by controlling the number of the Au nuclei and alloying process. There are several advantages of the Ag–Au alloy NPs
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synthesis method developed in this paper. Firstly only sodium citrate was used and only one step heating process was needed. Then the resultant Ag–Au colloid solution is free from AgCl deposition. And the last one is that the monodisperse and large size (>20 nm) Ag–Au alloy NPs can be obtained. Acknowledgment This work was supported by the Industry Key Technologies R&D project in Shaanxi Province of China (Program No. 2009K06-23). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
P. Mulvaney, Langmuir 12 (1996) 788. M.P. Mallin, C.J. Murphy, Nano Lett. 2 (2002) 1235. J. Zhu, Low-dimens. Syst. Nanostruct. 27 (2005) 296. A. Pal, S. Shah, S. Devi, Colloids Surf. A-Physicochem. Eng. A 302 (2007) 51. J. Zhu, J. Phys. Chem. C 113 (2009) 3164. J. Zhu, Nanoscale Res. Lett. 4 (2009) 977. J.P. Wilcoxon, B.L. Abrams, Chem. Soc. Rev. 35 (2006) 1162. L.H. Liu, A.Q. Wang, Y.S. Chi, H.P. Lin, C.Y. Mou, J. Phys. Chem. B 109 (2005) 40. R. Jha, A.K. Sharma, J. Opt. A-Pure Appl. Opt. 11 (2009) 045502. A.K. Sharma, B.D. Gupta, Nanotechnology 17 (2006) 124. R.A. Alvarez-Puebla, J.P. Bravo-Vasquez, J. Colloid Interface Sci. 333 (2009) 237. Y. Yang, J. Shi, G. Kawamura, M. Nogami, Scr. Mater. 58 (2008) 862. X.Q. Zou, E.B. Ying, S.J. Dong, J. Colloid Interface Sci. 306 (2007) 307. S.E. Hunyadi, C.J. Murphy, J. Mater. Chem. 16 (2006) 3929. Y.L. Wang, H.J. Chen, S.J. Dong, E.K. Wang, J. Chem. Phys. 125 (2006) 044710. R.A. Alvarez-Puebla, G.A. Nazri, R.F. Aroca, J. Mater. Chem. 16 (2006) 2921. Y. Cui, B. Ren, J.L. Yao, R.A. Gu, Z.Q. Tian, J. Phys. Chem. B 110 (2006) 4002. R.A. Alvarez-Puebla, D.J. Ross, G.A. Nazri, R.F. Aroca, Langmuir 21 (2005) 10504. E. Hao, S.Y. Li, R.C. Bailey, S.L. Zou, G.C. Schatz, J.T. Hupp, J. Phys. Chem. B 108 (2004) 1224. G.C. Papavassiliou, J. Phys. F-Met. Phys. 6 (1976) L103. F. Hajiesmaeilbaigi, A. Motamedi, Laser Phys. Lett. 4 (2007) 133. K.I. Okazaki, T. Kiyama, K. Hiralara, N. Tanaka, S. Kuwabata, T. Torimoto, Chem. Commun. (2008) 691. S. Link, Z.L. Wang, M.A. EI-Sayed, J. Phys. Chem. B 103 (1999) 3529. D.H. Chen, C.J. Chen, J. Mater. Chem. 12 (2002) 1557. O.M. Wilson, R.W.J. Scott, J.C. Garcia-Matinez, R.M. Crooks, J. Am. Chem. Soc. 127 (2005) 1015. Z.Y. Li, J.P. Wilcoxon, F. Yin, Y. Chen, R.E. Palmer, R.L. Johnston, Faraday Discuss. 138 (2008) 363. Y.S. Shon, G.B. Dawson, M. Porter, R.W. Murray, Langmuir 18 (2002) 3880. X.M. Lu, H.Y. Tuan, J.Y. Chen, Z.Y. Li, B.A. Korgel, Y.N. Xia, J. Am. Chem. Soc. 129 (2007) 1733. Q.B. Zhang, J.Y. Lee, J. Yang, C. Boothroyd, J.X. Zhang, Nanotechnology 18 (2007) 245605. A.B. Smetana, K.J. Klabunde, C.M. Sorensen, A.A. Ponce, B. Mwale, J. Phys. Chem. B 110 (2006) 2155. C. Wang, S. Peng, R. Chan, S.H. Sun, Small 5 (2009) 567. Q.B. Zhang, J.P. Xie, J. Liang, J.Y. Lee, Adv. Funct. Mater. 19 (2009) 1387. G. Frens, Nat. (Lond.) Phys. Sci. 241 (1973) 20. K. Mallik, M. Mandal, N. Pradhan, Nano Lett. 1 (2001) 319. S. Sloufova, F. Lednicky, A. Gemperle, Langmuir 16 (2000) 9928.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of monodisperse Ag–Au alloy nanoparticles with large size by a facile fabrication process Weiwei Zhang, Liqing Huang ∗ , Jian Zhu, You Liu, Jun Wang Non-equilibrium Condensed Matter and Quantum Engineering Laboratory, The Key Laboratory of Ministry of Education, School of Science, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 19 December 2010 Received in revised form 29 June 2011 Accepted 29 July 2011 Keywords: Alloys Chemical synthesis Energy dispersive analysis of X–rays Electron microscopy Optical properties
a b s t r a c t The fairish monodisperse Ag–Au alloy nanoparticles (NPs) with various composition and size were synthesized by a facile fabrication procedure. UV–visible (UV–vis) spectroscopy and transmission electron microscopy (TEM) confirmed the formation of homogeneous alloy NPs. The diameters of the resultant monodisperse spherical Ag–Au alloy NPs are 21–32 nm and the long-axis size of the resultant monodisperse nonspherical Ag–Au alloy NPs are about 28 nm. The possible formation mechanism is suggested by analyzing the dynamic absorption spectra, TEM and energy dispersive X-ray (EDX) spectroscopy of the alloy nanoparticles at the different time. Ag–Au alloy NPs synthesis method developed in this paper has several advantages: (1) the synthesis process is very simple. Only sodium citrate was used as both reducing agent and stabilizing agent and only one step heating process was needed. (2) The resultant Ag–Au colloidal dispersion is free from AgCl deposition and (3) the fairish monodisperse and large size (>20 nm) Ag–Au alloy NPs can be obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bimetallic silver–gold NPs have received enormous attention due to their unique electronic, optical [1–6] and catalytic [7,8] properties distinct from those of pure mono-metal NPs. For example, localized surface Plasmon absorption for Ag–Au alloy NPs can be tuned systematically over a broad range by simply varying the alloy composition [1–6] and the combination of desirable features of both Ag and Au is useful to a diverse range of applications, especially in sensing [9,10] and surface-enhanced Raman spectroscopy (SERS) [11–19]. In addition, Ag–Au alloy NPs are more catalytically active than monometallic Ag or Au NPs in the oxidation of CO at low temperatures [8]. A number of physical and chemical techniques have been developed to produce Ag–Au alloy NPs. The physical methods include evaporation–condensation [20], laser ablation [21], sputter deposition [22], etc. Among the chemical methods, the most common one is the thermal chemical methods [2,4,23–32], including co-reduction [2,4,23–26], galvanic replacement reaction [27–29], digestive ripening [30] and other complex methods [31,32]. Chen et al. produced Ag–Au alloy NPs with the mean diameter of 4–22 nm using hydrazine as reducing agent in water-in-oil microemulsions [24]. Mallin and coworkers yielded Ag–Au alloy NPs with average diameters of 5–7 nm using sodium borohydride
and sodium citrate as a reducing and stabilizing agent, respectively [2]. The Ag–Au alloy NPs of various compositions with the average diameter less than 10 nm were synthesized by the galvanic replacement reaction [27,29]. Lu and coworkers reported the replacement reaction preparation of hollow Au–Au alloy NPs with diameter of less than 20 nm [28]. And the synthesis of Ag–Au alloy particles with diameter of 15 nm from core/shell structure Ag/Au was reported by digestive ripening [30]. The Ag–Au alloy NPs of different shape and composition with diameter of less than 10 nm were produced by more complex methods [32]. Despite a myriad of methods of preparation, the synthesis of large size, monodisperse Ag–Au alloy NPs by the facile fabrication procedure remains a challenge. The diameters of monodisperse alloy particles prepared by current methods are less than 22 nm and the synthesis process is relative complicated. Also, some preparation process needs to adopt an additional measure in order to take out the AgCl by-production [29]. In this paper, the synthesis of fairish monodisperse Ag–Au alloy NPs with large size by a novel facile fabrication procedure was studied. The size, morphology and optical properties of the resultant Ag–Au alloy NPs were characterized by TEM and UV–vis spectroscopy, respectively. The formation mechanism of the alloy NPs was also discussed. 2. Experimental 2.1. Materials and instruments
∗ Corresponding author. E-mail address: [email protected] (L. Huang). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.079
Silver nitrate (AgNO3 ) and chlorauric acid (HAuCl4 ·4H2 O) from Chinese Shanghai Chemical Company and sodium Citrate from Chinese Tianjin BaiShi Chemical
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141
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Fig. 1. (a) UV–vis absorption spectra of the resultant Ag–Au alloy with varying initial Ag:Au molar ratios (1:0, 0.75:0.25, 0.67:0.33, 0.5:0.5, 0.33:0.67, 0.25:0.75, 0:1). (b) Plot of the peak wavelength against the initial gold mole fraction.
Company, were used as received. Doubly deionized water (>18 M) was produced by a LABCONCO water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned by AS10200BDT Ultrasonic cleaner from AUTO SCIENCE and then washed in aqua regia, followed by copious washing with deionized water before drying in an oven. An OCEAN’s HR2000+ high resolution fiber spectrometer and a DH-2000 deuterium tungsten halogen sources (200–1100 nm) were used to monitor and record dynamic absorption spectra. The constant temperature heater with magnetic force stirrer was from Hangzhou Instrument Company. 2.2. Preparation of silver and gold NPs The gold and silver NPs were prepared by the citrate-reduction procedure of Frens [33]. For Au NPs: 2.1 ml 2.4 mM HAuCl4 was mixed with 47.9 ml H2 O in a quartz beaker. The solution was stirred and heated to boiling, and then 1.0 ml 35 mM sodium citrate solution was added to the boiling solution. The solution was further boiled for 30 min and was then left to cool to room temperature. The vigorous stirring was maintained during the heating process. For Ag NPs: the initial reaction solution contained 2.5 ml 2.0 mM AgNO3 and 47.5 ml H2 O. A 1.0 ml 35 mM sodium–citrate was used as reducing agent and the heating duration was about 40 min. The other conditions were same as those of Au NPs preparation process. When exposed in air at room temperature, the gold colloidal dispersion could be stable for about 1 month, but the silver colloidal dispersion could only be stable for about 2 weeks. 2.3. Preparation of Ag–Au alloy NPs The aqueous solution of AgNO3 and sodium–citrate was heated to boiling for some time, and then the HAuCl4 aqueous solution was added to it just before the obvious absorption band of Ag NPs appearing. The dynamic UV–visible absorption spectra were detected by a fiber spectrometer. While the solution was further boiled for about 13 min and was then left to cool to room temperature. The vigorous stirring was maintained during the whole reaction process. The total molar amount of Ag and Au was fixed for all resultant Ag–Au NPs (5 × 10−6 mol L−1 ). The Ag–Au alloy NPs of various composition were synthesized by fixing the molar ratio of the total metal and reduction agent (0.125:0.875) and varying the initial Ag:Au molar ratios (0.75:0.25, 0.60:0.40, 0.67:0.33, 0.50:0.50, 0.33:0.67, 0.40:0.60, 0.25:0.75). While the Ag and Au molar ratio was fixed, by varying the molar ratio of the total metal and reducer, the Ag–Au NPs of various size were synthesized (Ag:Au = 0.75:0.25, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75, 0.4:0.6; Ag:Au = 0.50:0.50, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75; Ag:Au = 0.25:0.75, metal:reducer = 0.125:0.875, 0.167:0.833, 0.25:0.75, 0.4:0.6).
NPs are shown in Fig. 1(a). Only one absorption band is observed and the maximum wavelength of absorption spectrum red-shifts from 420 nm (Ag NPs) to 520 nm (Au NPs) with the increase in the Au mole fractions. In Fig. 1(b) the change of maximum wavelength of the absorption band with the initial gold mole fractions is plotted and a linear relationship is found. These results indicate that the resultant NPs are alloy other than core–shell NPs [1,34], or a mixture of Ag and Au NPs [2]. Fig. 2 shows the digital photos of the resultant Ag–Au alloy colloidal dispersion with varying initial Ag:Au molar ratios after being deposited for different time. The color changes from yellow to orange and to red with the increasing in Au mole fraction and does not appear notable change with the deposited duration, indicating the formation of the stable Ag–Au alloy NPs with various compositions. The TEM micrographs of the resultant Ag–Au alloy NPs with varying initial Ag:Au molar ratio are displayed in Fig. 3(a)–(e) and the inset in each TEM is the corresponding size histogram. The monodispersity of the alloy NPs is good and the average diameters are 30.83 nm, 26.62 nm, 21.58 nm, 23.32 nm and 24.88 nm, respectively. The change of Ag–Au alloy NPs mean diameter with the Au mole fractions is illustrated in Fig. 3(f). The mean diameter decreases firstly and then increases slightly with the increasing in Au mole fraction. The least one was obtained at Au mole fraction 0.5. This size change character will be discussed in the later study on formation mechanism of the Ag–Au NPs.
2.4. Characterization The size, morphology and composition of the resultant Ag–Au alloy NPs were determined by a JEOL JEM-2100 TEM. The UV–vis spectra of the resultant Ag–Au alloy colloidal solution were measured with a Perking Elmer Lambda750 s UV–visible spectrophotometer at room temperature.
3. Results and discussion 3.1. The Ag–Au alloy NPs of various compositions The UV–vis absorption spectra of the resultant NPs with varying initial Ag:Au molar ratios together with those of Ag and Au
Fig. 2. Photos of the resultant Ag–Au alloy NPs with varying initial Au mole fractions (form left to right 0.25, 0.33, 0.50, 0.67, 0.75). The photo of (a) obtained after being deposited for several days, (b) obtained after being deposited for 2 months.
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Fig. 3. TEM images and size change plot of Ag–Au alloy NPs with varying initial Ag:Au molar ratios and fixing reducing mole fraction 0.825. (a) 0.75:0.25, (b) 0.67:0.33, (c) 0.50:0.50, (d) 0.33:0.67, (e) 0.25:0.75. The inset shows the size histogram of the NPs. (f) Plot of average diameters change with the initial gold mole fraction.
3.2. Ag–Au alloy NPs of various sizes The size of the Ag–Au alloy NPs can be controlled by tuning the molar ratio of the metal salt and reducing agent at a fixed Ag:Au molar ratio. At different fixed Ag:Au molar ratios (0.75:0.25, 0.50:0.50, 0.25:0.75), the UV–vis absorption spectra of the assynthesized NPs with various reducing agent mole fractions are shown in Fig. 4(a)–(c), respectively. The dependence of maximum wavelength of the absorption band on the reducing agent mole fractions at different fixed Ag:Au molar ratio is shown in Fig. 5. Only one absorption band was observed and the peak wavelength of absorption spectrum is red-shifted with the decrease in the reducing agent mole fractions, indicating the formation of the Ag–Au alloy NPs with various sizes. For the Au mole fractions 0.75, the amount of red-shifted is about 21 nm, while Au mole fractions equal or small
than 0.50, the red shift is about 12 nm. The TEM micrographs of the resultant Ag–Au alloy NPs with varying Au mole fractions (0.25, 0.50, 0.75) at the lower sodium citrate mole fractions (0.75, 0.6) are shown in Fig. 6. The insert in each TEM is the corresponding size histograms. For the Au mole fractions 0.25, 0.5 and reducing agent mole fractions 0.75, the resultant NPs are spherical and of fairish good monodispersity, the average diameters are 32.63 nm and 29.67 nm, respectively. The average diameter increases with the decrease in the reducing agent mole fractions. When the reducing agent mole fractions decrease form 0.875 to 0.75, the diameter is increase about 2 nm [the diameter of Fig. 6(a) – the diameter of Fig. 3(a)] and 8 nm [the diameter of Fig. 6(b) – the diameter of Fig. 3(c)], respectively. The amount of red-shifted and diameter change indicate that the tunable range of the resultant NPs size is broader for rich gold
Fig. 4. UV–vis absorption spectra of the resultant NPs with varying metal:reducer and initial Ag:Au molar ratios.
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141
Fig. 5. Plot of the absorption peak wavelength against the reducer mole fractions at varying initial Ag:Au molar ratios.
composition NPs. While for Au mole fractions greater than 0.50 and reducing agent mole fractions 0.6, the resultant NPs are nonspherical and also of fairish good monodispersity, the length of major axis is about 28 nm long, indicating that the size tuning range is limited by tuning the metal to reducing molar ratio. 3.3. Formation mechanism of Ag–Au alloy NPs To understand the dynamics of the formation of the resultant Ag–Au alloy NPs, UV–vis spectra were recorded at 5 s intervals after
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HAuCl4 aqueous solution were added into the boiling solution of the AgNO3 and sodium–citrate. Representative dynamic absorption spectra are shown in Fig. 7 for sodium citrate and Au mole fractions 0.875 and 0.25, respectively. At the early stage of the process, a very weak absorption band at about 418 nm was observed and disappeared at the final stage. This absorption band (as indicated with a red dashed arrow in Fig. 7) is ascribed to the surface plasmon resonance (SPR) of the Ag NPs and suggests that the small Ag NPs were formed. Within 25 s after adding the HAuCl4 aqueous solution to the boiling solution, a broad weak absorption band at about 537 nm appeared and then shifted to shorter wavelength accompanying with the increase in absorbance during the reaction process (as indicated with a red solid arrow in Fig. 7). This blueshifted absorption band is ascribed to the SPR of the Ag–Au alloy NPs with increasing Ag content and suggests that the Ag–Au alloy NPs with increasing gradually Ag composition were formed. Fig. 8 shows the changes of maximum wavelength of the absorption band with increasing in reaction time at different Au mole fractions and the changes during the first 60 s are showed in the insert. It shows that the blue-shifted was rapidly at the first and then slowly. And the greater the Au mole fractions were, the slower the blue shift was. As the Au mole fraction was less than 0.5, the maximum wavelength of the absorption band of the initial absorption band was all about 538 nm, but as the Au mole fractions greater than 0.5 (0.67, 0.75), it increased with the increase in Au mole fractions. According to the above spectra varying characteristics (the change of the absorption band number, maximum wavelength of the absorption band, absorbance and blue-shifted rate with
Fig. 6. TEM images of Ag–Au alloy NPs with varying initial Au and reducing mole fraction. (a) 0.25, 0.75. (b) 0.50, 0.75. (c) 0.75, 0.6. The inset shows the size histogram of the NPs.
Fig. 7. Time dependent absorption spectrum of alloy nanoparticles. (Normalized and moved along the Y axis, (a) the time interval of each line is 5 s, (b) the time interval of each line is 1 min, (c) the time interval of each line is 10 s.)
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Fig. 8. The changes of absorption peak wavelength with increasing in reaction time at different Ag:Au mol ratios.
reaction time and Au mole fractions), we put forward a possible formation mechanism of the Ag–Au NPs, schematically depicted in Fig. 9. Before adding HAuCl4 aqueous solution, a certain number of Ag atoms and very small Ag NPs had been formed by sodium citrate reducing Ag+ ions in the boiling solution. Once the HAuCl4 aqueous solution was added to the boiling solution (Fig. 9(a)), the AuCl− 4 ions were reduced rapidly by citrate, formed Ag atoms and
NPs synchronously. The reduction reaction was so rapidly that the AuCl− 4 ions were reduced completely and the reduced Au atoms aggregated rapidly to form the Au nuclei almost at the same time (Fig. 9(b)). After that, the Ag atoms both remaining and reduced subsequently by citrate deposited on the surface of the Au nuclei and by alloying process at high temperature (ca. 100 ◦ C) the Ag–Au NPs with increasing in Ag composition were formed (Fig. 9(c)). By properly controlling the metal to reducer molar ratios, the Ag+ ions were reduced completely and used to form the alloy NPs, no obvious AgCl deposition was produced in the resultant Ag–Au colloidal dispersion. The size of the Au nuclei is dependent on the molar ratios of the metal:reducer and Ag:Au. While the metal:reducer mole fraction is fixed (for example, 0.125:0.875), the size of the Au nuclei is varying with the Au mole fractions and there is a critical Au mole fractions (0.5). For the Au mole fractions is less than the critical mole fractions, the size do not change obviously with the Au mole fractions (the maximum wavelength of the initial absorption band is nearly same for various Au mole fractions), but for the Au mole fractions is greater than the critical value, the size of the Au nuclei increases with the increasing in Au mole fractions (the maximum wavelength of the initial absorption band red shifts with the increasing in Au mole fractions). Since the total mole amount of metal was fixed, the size of the resultant Ag–Au alloy NPs is dependent on the size of the Au nuclei and the Au mole fractions. For the Au mole fractions is smaller than the critical value, the number of the Au nuclei increase with the increasing in the Au mole fractions and the size of the resultant Ag–Au NPs decrease. And for the Au mole fractions greater than
Fig. 9. The possible mechanism for formation of alloy nanoparticles.
Fig. 10. The EDX analysis (above) and TEM images of the alloy nanoparticles at different time. (a) 30 s, (b) 60 s, (c) 120 s, (d) 300 s, (e) resultant alloy NPs at 15 min.
W. Zhang et al. / Materials Chemistry and Physics 131 (2011) 136–141 Table 1 Composition of Ag–Au alloy NPs formed at different time of the growing process. Time
30 s
60 s
120 s
300 s
15 min
%Ag %Au
29.14 70.86
33.75 66.25
39.27 60.73
49.21 50.79
52.87 47.13
the critical value, the number of the Au nuclei decrease with the increasing in Au mole fractions, the size of the resultant Ag–Au alloy NPs increase with the increasing in Au mole fractions. This effect of Au mole fraction on the alloy NPs size is in agreement with the experimental results of the resultant Ag–Au alloy NPs, which is shown in Fig. 3(f). The above possible formation mechanism can be shown with the schematic diagram in Fig. 9. Fig. 9(a) shows the formation of the Ag atoms and NPs by citrate reducing Ag+ ions. Fig. 9(b) shows the formation of the Au nuclei rapidly by Ag atoms, Ag NPs and citrate reducing AuCl− 4 synchronously, which assures the monodispersity of the resultant Ag–Au NPs. Fig. 9(c) shows the formation of the Ag–Au alloy NPs by depositing the Ag atoms reduced by citrate on the surface of Au nuclei and alloying at high temperature. The sodium citrate was only used to be both reducing agent and stabilizing agent in the whole synthesizing process. The TEM and EDX analysis were performed for the alloy nanoparticles (composition molar ratio Ag:Au = 0.5:0.5, reducing agent mole fractions 0.75) at different time of the growing process. Fig. 10(a)–(e) are the TEM images and the EDX spectra of alloy NPs formed at 30 s, 60 s, 120 s, 300 s and 15 min, respectively, after adding HAuCl4 solution. The TEM images show relative uniform contrast for all formed NPS and do not show any banding. The composition analyzing by the EDX spectra are shown in Table 1, which indicates that the amount of silver increase from 29.14% to 52.87% with the time increased from 30 s to 15 min. These facts indicate that resultant NPs are Ag–Au alloy NPs other than core–shell NPs [35], or a mixture of Ag and Au NPs [2]. 4. Conclusion The fairish monodisperse spherical Ag–Au alloy NPs with various compositions and diameters were synthesized by a facile synthesis process. Analyzing based on dynamic absorption spectra and TEM images shows that both the Ag:Au molar ratio and reducing agent mole fraction can affect the formation of alloying nanoparticles by controlling the number of the Au nuclei and alloying process. There are several advantages of the Ag–Au alloy NPs
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synthesis method developed in this paper. Firstly only sodium citrate was used and only one step heating process was needed. Then the resultant Ag–Au colloid solution is free from AgCl deposition. And the last one is that the monodisperse and large size (>20 nm) Ag–Au alloy NPs can be obtained. Acknowledgment This work was supported by the Industry Key Technologies R&D project in Shaanxi Province of China (Program No. 2009K06-23). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
P. Mulvaney, Langmuir 12 (1996) 788. M.P. Mallin, C.J. Murphy, Nano Lett. 2 (2002) 1235. J. Zhu, Low-dimens. Syst. Nanostruct. 27 (2005) 296. A. Pal, S. Shah, S. Devi, Colloids Surf. A-Physicochem. Eng. A 302 (2007) 51. J. Zhu, J. Phys. Chem. C 113 (2009) 3164. J. Zhu, Nanoscale Res. Lett. 4 (2009) 977. J.P. Wilcoxon, B.L. Abrams, Chem. Soc. Rev. 35 (2006) 1162. L.H. Liu, A.Q. Wang, Y.S. Chi, H.P. Lin, C.Y. Mou, J. Phys. Chem. B 109 (2005) 40. R. Jha, A.K. Sharma, J. Opt. A-Pure Appl. Opt. 11 (2009) 045502. A.K. Sharma, B.D. Gupta, Nanotechnology 17 (2006) 124. R.A. Alvarez-Puebla, J.P. Bravo-Vasquez, J. Colloid Interface Sci. 333 (2009) 237. Y. Yang, J. Shi, G. Kawamura, M. Nogami, Scr. Mater. 58 (2008) 862. X.Q. Zou, E.B. Ying, S.J. Dong, J. Colloid Interface Sci. 306 (2007) 307. S.E. Hunyadi, C.J. Murphy, J. Mater. Chem. 16 (2006) 3929. Y.L. Wang, H.J. Chen, S.J. Dong, E.K. Wang, J. Chem. Phys. 125 (2006) 044710. R.A. Alvarez-Puebla, G.A. Nazri, R.F. Aroca, J. Mater. Chem. 16 (2006) 2921. Y. Cui, B. Ren, J.L. Yao, R.A. Gu, Z.Q. Tian, J. Phys. Chem. B 110 (2006) 4002. R.A. Alvarez-Puebla, D.J. Ross, G.A. Nazri, R.F. Aroca, Langmuir 21 (2005) 10504. E. Hao, S.Y. Li, R.C. Bailey, S.L. Zou, G.C. Schatz, J.T. Hupp, J. Phys. Chem. B 108 (2004) 1224. G.C. Papavassiliou, J. Phys. F-Met. Phys. 6 (1976) L103. F. Hajiesmaeilbaigi, A. Motamedi, Laser Phys. Lett. 4 (2007) 133. K.I. Okazaki, T. Kiyama, K. Hiralara, N. Tanaka, S. Kuwabata, T. Torimoto, Chem. Commun. (2008) 691. S. Link, Z.L. Wang, M.A. EI-Sayed, J. Phys. Chem. B 103 (1999) 3529. D.H. Chen, C.J. Chen, J. Mater. Chem. 12 (2002) 1557. O.M. Wilson, R.W.J. Scott, J.C. Garcia-Matinez, R.M. Crooks, J. Am. Chem. Soc. 127 (2005) 1015. Z.Y. Li, J.P. Wilcoxon, F. Yin, Y. Chen, R.E. Palmer, R.L. Johnston, Faraday Discuss. 138 (2008) 363. Y.S. Shon, G.B. Dawson, M. Porter, R.W. Murray, Langmuir 18 (2002) 3880. X.M. Lu, H.Y. Tuan, J.Y. Chen, Z.Y. Li, B.A. Korgel, Y.N. Xia, J. Am. Chem. Soc. 129 (2007) 1733. Q.B. Zhang, J.Y. Lee, J. Yang, C. Boothroyd, J.X. Zhang, Nanotechnology 18 (2007) 245605. A.B. Smetana, K.J. Klabunde, C.M. Sorensen, A.A. Ponce, B. Mwale, J. Phys. Chem. B 110 (2006) 2155. C. Wang, S. Peng, R. Chan, S.H. Sun, Small 5 (2009) 567. Q.B. Zhang, J.P. Xie, J. Liang, J.Y. Lee, Adv. Funct. Mater. 19 (2009) 1387. G. Frens, Nat. (Lond.) Phys. Sci. 241 (1973) 20. K. Mallik, M. Mandal, N. Pradhan, Nano Lett. 1 (2001) 319. S. Sloufova, F. Lednicky, A. Gemperle, Langmuir 16 (2000) 9928.