Materials Science and Engineering C 38 (2014) 192–196
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Reduction and aggregation of silver ions in aqueous citrate solutions Ridhima Chadha, Nandita Maiti, Sudhir Kapoor ⁎ Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 13 March 2013 Received in revised form 23 December 2013 Accepted 22 January 2014 Available online 31 January 2014 Keywords: Metal nanoparticles TEM UV–visible spectroscopy Radiolysis
a b s t r a c t Radiolytic reduction of Ag+ ions and the subsequent formation of Ag clusters were studied in aqueous citrate solutions. Pulse-radiolysis studies show that the presence of citrate in the solution affects the early processes, via complexation of Ag+ ions with the carboxyl moieties of the citrate. The ratio of citrate to Ag+ determines the kinetic consequences of the reduction and agglomeration processes. The complexation reduces somewhat the rate of reduction by hydrated electrons. However, when all the ions are complexed to the citrate, the surface plasmon absorption band becomes broader, albeit small, but nevertheless it provides extreme stability to the formed nanoparticles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently an extensive research has been focused on the physical and chemical properties and potential applications of metal nanoparticles. In this respect, coinage metals such as silver and gold have been explored extensively. The main reason for this is due to the fact that they exhibit strong surface plasmon resonance absorption located in a convenient part of the spectrum ∼400 nm for Ag and ~520 nm for Au [1–5]. The other significant reason is that they are relatively unreactive in nature, and the ease with which their nanoparticles can be synthesized. These properties have encouraged the researchers to explore the applications of silver and gold nanoparticles in various areas such as solar cell enhancement [6], biosensing [7], surface-enhanced Raman spectroscopy (SERS) [8,9], etc. To stabilize the nanoparticles various surfactants, polymers, ligands, etc. have been used to prevent the aggregation [1–5]. Similarly, various methods for reducing metal ions have been reported in literature [1–6,10–13]. In this context, the citrate reduction of HAuCl4 in hot water, initially developed by Turkevich et al. [11] and later refined by Frens [12] is the most commonly used method to synthesize quasispherical Au nanoparticles with controlled sizes in the range of 10 to 40 nm, depending on the ratio of citrate to HAuCl4. Similarly, method developed by Lee and Meisel [13] is commonly used for citrate stabilized Ag nanoparticles. In these methods, citrate serves the dual purpose, as a reducing agent and a stabilizer. Very recently, the role of citrate ions in controlling the morphology of the particles has been discussed [14–19]. As mentioned above, many experiments have shown the growth of silver nanocrystals in solution using either citric acid or sodium citrate in which citric acid, (or citrate) is used as a reductant and as a stabilizer. ⁎ Corresponding author at: Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Tel.: +91 22 25590298; fax: +91 22 25505151. E-mail address: [email protected] (S. Kapoor). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.041
However, the exact role of citrate ions still remains debatable. For instance, in photo illumination by visible light it is demonstrated that the citrate ions act as stabilizers [20,21] for generating monodispersed silver nanoprisms. Also, using γ-irradiation of AgClO4 solutions it was shown that citrate ions do not act as reducing agents but solely as stabilizers of the colloidal particles [22]. It is also demonstrated that citrate ions influence the particle growth by complexing with positively charged Ag+ 2 dimers [23] and nanoparticles [14]. Very recently, the role of citrate in the formation of anisotropic particles is investigated [24]. It has been suggested that citrate is not the crucial component in the evolution of Ag nanoplates. Inspired by the work we have made an attempt to study the effect of rate of reduction on the formation and stability of the particles in the presence of citrate. Radiolysis is a clean method where, reduction can be carried out without adding any reductant from outside. In this study, we systematically investigate the role of citrate ions in the complexation as well as in the stabilization of silver nanoparticles. Functions of citrate ions in the formation and growth of silver clusters and eventually to the nanoparticles were probed using time resolved kinetic spectrophotometry, UV–vis spectrometry and by transmission electron microscopy (TEM) techniques. The possible mechanisms in the formation of silver nanoparticles are finally discussed. The novelty of this study is that using faster rate of reduction it is shown that the small and nearly monodisperse Ag particles can be prepared using citrate.
2. Materials and methods 2.1. Materials Silver perchlorate (Aldrich), tert-butanol (Sisco, India) and sodium citrate (S.D. Fine Chemicals, India) were used as received. IOLAR grade N2 gas (purity ≥ 99.99%) used for purging solutions was
R. Chadha et al. / Materials Science and Engineering C 38 (2014) 192–196
2.2. Methods In a typical synthesis, 1 mL of AgClO4 solution (0.01 M) and 50 mL of sodium citrate solution (0.1 M) were added to the volumetric flask. The mixture turned to a light white suspension immediately, indicating the formation of a poorly soluble Ag–citrate complex (solubility 0.029 g/L at room temperature). Then, the solution was diluted to 100 mL using Millipore purified water. The complex dissolved completely and all the ratios of [citrate]/[Ag+] were prepared accordingly. Electron pulse radiolysis experiments were performed at the Bhabha Atomic Research Centre Laboratory with the 7 MeV electron linear accelerator [25]. Briefly, samples were irradiated in a 1 cm × 1 cm suprasil quartz cuvette kept at a distance of approximately 12 cm from the electron beam window, where, the beam diameter was approximately 1 cm. Upon generation of transient species, their absorption spectra and/or kinetics were measured using a pulsed Xe lamp as the light source. The absorption changes were measured using a monochromatorphotomultiplier tube arrangement while the photomultiplier output was collected with a digital oscilloscope and then stored on a computer for further analysis. An aerated 10−2 M KSCN solution was used for dosimetry, and the (SCN)•− 2 radical was monitored at 475 nm. The absorbed −4 dose per pulse was calculated [26] assuming Gε [(SCN)•− 2 ] = 2.6 × 10 m2 J−1 at 475 nm. Absorbed doses per pulse were of the order of 6 Gy (1 Gy = 1 J kg−1, for aqueous solution, 1 Gy corresponds to 1 J L−1). For practical purposes, the G-unit rather than the SI-unit for radiation chemical yields is used. The G-unit denotes the number of species formed or converted per 100 eV of absorbed energy in aqueous solution; G = 1 corresponds to 0.1036 μM per 1 J of absorbed energy in aqueous solution [27]. The radiolysis of water produces reactive free radicals, hydrated electrons, OH radicals and H atoms, and molecular products H2O2 and H2, according to the stoichiometry [28,29] as shown in reaction (1).
ð1Þ where, the numbers in parentheses represent the radiolytic yields, G-values, the quantity of species formed per Joule of energy deposited at pH 7 in μmol J− 1. Depending on the experimental conditions, one can get exclusively either reducing or oxidizing conditions by purging the solution either with N2 or N2O gas, respectively. In general, alcohols are added to the N2-bubbled media to scavenge •OH radicals and •H atoms, to get the reducing conditions. Hence, to isolate hydrated electron reactions, solutions were bubbled with N2 in the presence of tert-butanol as the tert-butanol radical produced (reaction 2) is inert and reduction proceeds via e− aq reaction only. The dose absorbed by the aqueous solution was kept low (dose = 6 Gy, total radical concentration = 1.6 μM per pulse) to minimize radical–radical reactions. •
•
•
OH= H þ ðCH3 Þ3 COH→H2 O=H2 þ CH2 ðCH3 Þ2 COH
ð2Þ
3. Results and discussion 3.1. Effect of citrate ions Citrate generally acts as a reducing agent for metal ions (Ag+ and Au3+) only at high temperature [11–13]. In radiolytic method, it is possible to initiate reduction of metal ions by primary species; hence, the role of other additives in the reaction mixture can be partitioned (or alienated) based on kinetic parameters. This helps in understanding the actual reaction mechanism and the role played by additives in the formation of nanoparticles. In order to investigate the role of citrate ions as a stabilizer and reductant, if any at room temperature, in the synthesis of silver nanoparticles, the molar ratio of [citrate]/[Ag+] was varied from 5 to 15, 50 and 500 and other parameters were kept constant. Fig. 1 shows the effect of citrate on the decay of e−aq for the reaction with Ag+ at constant concentration. It can be seen that the decay of e− aq decreases as the concentration of citrate increases due to the complexation of Ag+ with the citrate. To get the molar ratio of citrate/Ag+ at which nearly complete complexation of Ag+ occurs the variation in + the bimolecular rate constant of e− aq with Ag at different citrate concentration was plotted. It can be seen from Fig. 2 that nearly complete complexation occurs at molar ratio ≥500. The equilibrium constant for Ag+ with citrate can be determined as follows. The observed rate of formation of Ag0 can be expressed as Eq. (1) h i h i h i þ þ þ ¼ k f Ag þ kb Ag kobs Ag 0
f
ð1Þ
b
where, [Ag+]0 is the initial Ag+ concentration, kobs is the observed rate constant, and subscripts f and b denote free and citrate-bound species, respectively. From Fig. 2, we obtain kf = 2.2 × 1010 and kb = 1.1 × 1010 M−1 s−1. The fraction of bound Ag+ is given by h i h i þ þ = Ag f ¼ Ag b
ð2Þ
0
which can be rearranged to give f ¼ ðkobs −k f Þ=ðkb −k f Þ:
ð3Þ
Assuming a simple equilibrium for binding Ag+ to citrate, the equilibrium constant can be estimated from the results in Fig. 2 and using Eq. (4). 1=f ¼ 1 þ 1=fK½citrateg
ð4Þ
These results shown in Fig. 2 agree well with Eq. (4). The slope of the line yields K = 50 M−1. The binding of Ag+ with citrate was further 0.06 Citrate = 5.0 x 10 M Citrate = 0 M Citrate = 5.0 x 10 M
Δ O.D. (a.u.)
obtained from Indian Oxygen Limited. All solutions were prepared just before the experiments, wrapped with aluminum foil and kept in the dark to avoid any photochemical reactions. Water purified through a Millipore system was used.
193
0.04
Citrate = 5.0 x 10 M
0.02
2.3. Characterization of Ag nanoparticles UV–vis extinction spectra were collected on Jasco-650 spectrophotometer. Particle sizes were determined by TEM using a Zeiss-Carl, Libra-120 instrument. Specimen for TEM analysis was prepared on Lacey Formvar/carbon-coated 200 mesh copper grid from Ted-Pella. The 20 μL droplet of the sol was put on the grid and put on filter paper to remove excessive solution. The TEM image was taken by air-drying of the nanoparticles.
0.00 0
2
4
6
8
Fig. 1. Decay traces of e−aq in N2-bubbled aqueous solution containing 1 × 10−4 M AgClO4, 0.1 M tert-butanol and various concentrations of citrate.
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0.75
Absorbance
Just after preparation Next day After 6 days
Fig. 2. Dependence of the rate constant for the reaction of e−aq with Ag+ on citrate concentration. Inset: Reciprocal fraction of complexed Ag+ vs. reciprocal citrate concentration.
confirmed by recording the kinetic traces of the growth process of silver atoms and its clusters. It is known in literature that in aqueous solution Ag0 atom shows absorption at 360 nm (reaction 3) [3,4], Ag+ 2 at 310 nm (reaction 4) [3,4], and Ag2+ at 270 nm (reaction 5) [5]. 3 þ
−
0
Ag þ eaq →Ag ð360 nmÞ
þ
Ag þ Ag →Ag2 ð310 nmÞ þ
þ
Ag2 þ Ag →Ag3
2þ
0.25
0.00 300
450
600
750
900
Wavelength, nm Fig. 4. Absorption spectra of Ag nanoparticles prepared by continuous electron pulse irradiation of N2-bubbled aqueous solution containing 1 × 10−4 M AgClO4, 5 × 10−2 M citrate and 0.1 M tert-butanol. Total dose = 4 kGy.
ð3Þ
þ
0
0.50
ð4Þ
ð270 nmÞ
ð5Þ
Fig. 3 shows the kinetic traces taken at wavelengths 360, 310 and 270 nm. It can be seen that significant change in the kinetic traces becomes visible as the [citrate]/[Ag+] molar ratio increased. This is probably caused by the formation of Ag+–citrate and its clusters–citrate
a
complexes, if, more citrate ions exist in the reaction solution which can decrease the reducing rate. As no transient absorption yield has increased in the presence of citrate it can be concluded that citrate ions are not reducing silver clusters within the time window of our experiment. It is known that citrate strongly coordinates with Ag+ to form Ag+– citrate complex (pK1 = 7.1) [30]. Hence, the pH of the solution was measured at all the studied [citrate]/[Ag+] ratios. It was observed that the pH of the solution varied from 7.2 to 8.1 at the studied extreme ratios of [citrate]/[Ag+]. Thus, it was presumed that in all the studied ratios citrate was present in deprotonated forms (pKa1 = 3.2, pKa2 = 4.8, pKa3 = 6.4 at room temperature), and small variation in different
b 0.06
360 nm
310 nm
0.02
0.00 0
5
10
Time, s
O.D. (a.u.)
O.D. (a.u.)
Citrate = 0 M Citrate = 1.5 x 10 -3 M Citrate = 5.0 x 10 -2 M
0.04
0.06
Citrate = 0 M
0.03
0.00
Citrate = 1.5 x 10-3 M Citrate = 5.0 x 10-2 M
0
5
10
Time, s
c 270 nm
O.D. (a.u.)
0.04
0.02 Citrate = 0 M Citrate = 1.5 x 10-3 M Citrate = 5.0 x 10-2 M
0.00
0
5
10
Time, s 2+ Fig. 3. Transient decay profiles recorded at (a) 360 nm for Ag0 (b) 310 nm for Ag+ following the pulse irradiation of N2-bubbled aqueous solution con2 and (c) 270 nm for Ag3 taining 1 × 10−4 M AgClO4 at different concentrations of citrate.
R. Chadha et al. / Materials Science and Engineering C 38 (2014) 192–196
195
Fig. 5. TEM images of silver nanoparticles in presence of (a) 5 × 10−3 M citrate and (b) 5 × 10−2 M citrate. Other conditions are same as in Fig. 4.
forms of citrate has been neglected in calculating the binding affinities to the Ag+ ions and its subsequent clusters. Stable silver nanoparticles are formed when a deaerated aqueous solution of AgClO4 (1 × 10−4 M) containing different concentrations of citrate was subjected to continuous electron pulse irradiation. A representative absorption spectrum obtained in the presence of 5 × 10−2 M sodium citrate is shown in Fig. 4. A small amount of tert-butanol was added to scavenge hydroxyl radicals formed during the radiolysis. The aqueous electrons formed during radiolysis initiate the reduction of Ag+ ions (reaction 3) while the citrate ions present in the solution stabilize the colloidal suspension [22]. From the spectra presented in Fig. 4, we find the absorbance maximum shifts towards blue after 6 days of post irradiation. The observed blue shift with time in the absorption spectrum could be due to the precipitation of some aggregated particles in the solution though there was no clear visible precipitate. During the bombardment of aqueous solution by electron pulses, for preparing the silver particles, the possibility of citrate participation as reductant is possible; however, this can be neglected as the absorbance yield remains almost similar. Thus, the probability of reaction (6) at room temperature is quite low. 3−
þ
2−
þ
0
2−
C6 H5 O7 þ 2Ag →C5 H4 O5 þ H þ CO2 þ 2Ag ðC5 H4 O5 ¼
−
ð6Þ
−
OOCCH2 COCH2 COO Þ
Thus, our kinetic results of silver clusters and the absorption spectrum of silver nanoparticles clearly show that citrate ions do not participate in the reduction process in the absence of any reductant. It is noteworthy to mention here that e− aq does not exist in the solution after 5 μs at the maximum concentration of citrate (Fig. 1). TEM images of Ag nanoparticles obtained in the presence of 5 × 10−3 and 5 × 10−2 M sodium citrate are shown in Fig. 5. It can be seen that the formed particles are spherical in nature having the average diameter 5.0 ± 0.2 nm with a narrow size distribution, ranging from 4 to 6 nm. It can be noticed that there was no significant change in the size with increase in the concentration of sodium citrate. It is noteworthy to mention here that the formation of such small particles has not been reported earlier in literature using citrate as a stabilizer. It is known that citrate is a weak reductant and since the e− aq is a strong reductant, the Ag+ ions are reduced quite effectively and quickly during radiolysis. As the silver seeds are continuously produced, the citrate stabilizer present in the solution arrests the colloid growth. However, the role of citrate in capping the certain facets of growing silver clusters is diminished due to the fact that reduction of Ag+ ions and subsequent aggregation of its clusters is comparatively faster in pulse irradiation than that when citrate-induced reduction is being carried out at high temperature. This could be probably the reason for not observing different shape of the silver particles in the presence of citrate. The surface plasmon absorption band in the presence of citrate appears to be broader than its absence (Fig. S1). This could be due to the high ionic strength of the solution in the presence of high concentration
of citrate. The surfaces of the nanoparticles are generally surrounded by capping agents (e.g., citrate ions) producing a charged layer that serves as an electrostatic barrier to aggregation. Increasing the ionic strength of the solution by adding ionic species decreases the effective electric field between the charged nanoparticles and consequently, lowers this barrier. Our results corroborate the findings reported by Henglein and Giersig [22] who have shown that colloidal silver sols of long-time stability are formed in the γ-irradiation of AgClO4 solutions. It is also suggested that citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. Nevertheless, our findings clearly show that the formed particles by electron pulse irradiation are nearly spherical and show good monodispersity. As electron pulse irradiation generates nuclei at higher rate than γ-irradiation this lead to the formation of seeds at faster rate and hence, the formation of smaller particles. The above results show that in stabilizing the metal nanoparticles, the role of charged stabilizers may be different than neutral stabilizers. Working in this direction may be able to answer questions regarding the stability of the particles under different conditions. We have initiated the work in this direction. 4. Conclusion We have demonstrated the interaction of Ag+ ions and its clusters with citrate ions that eventually lead to the formation of small silver nanoparticles. The novelty of the work is that the weak capping ability of citrate can be overcome by generating seeds at faster rate that helps in stabilizing the smaller silver nanoparticles. Time-resolved kinetic studies show that binding of Ag+ ions and its clusters with the citrate moiety. The observed variation in the kinetic traces is due to the binding of Ag+ ions and its clusters with the citrate moiety. It was observed that the molar ratio of sodium citrate to Ag+ ions can greatly influence the reaction rate and, hence, the particle growth of silver nanoparticles. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2014.01.041. Acknowledgment The authors are grateful to Dr. S.K. Sarkar, Director, Chemistry Group and Dr. D.K. Palit, Head, Radiation & Photochemistry Division for their encouragement during the course of this study. The authors are also thankful to the entire LINAC team for providing the facility. References [1] [2] [3] [4] [5]
M.-C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293–346. M.A. El-Sayed, Acc. Chem. Res. 34 (2001) 257–264. A. Henglein, Chem. Rev. 89 (1989) 1861–1873. J. Belloni, Radiat. Res. 150 (1998) S9–S20. H. Remita, S. Remita, Metal clusters and nanomaterials: Contribution of radiation chemistry, in: J.F. Wishart, B.S.M. Rao (Eds.), Recent Trends in Radiation Chemistry, Word Scientific, Singapore, 2010, pp. 347–383.
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[6] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, J. Appl. Phys. 101 (2007) 093105-1–093105-8. [7] T. Huang, P.D. Nallathamby, D. Gillet, X.-H.N. Xu, Anal. Chem. 79 (2007) 7708–7718. [8] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957–2976. [9] R.F. Aroca, R.A. Alvarez-Puebla, N. Pieczonka, S. Sanchez-Cortez, J.V. Garcia-Ramos, Adv. Colloid Interface Sci. 116 (2005) 45–61. [10] B. Wiley, Y. Sun, Y. Xia, Langmuir 21 (2005) 8077–8080. [11] J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55–75. [12] G. Frens, Nat. Phys. Sci. 241 (1973) 20–22. [13] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391–3395. [14] X.C. Jiang, C.Y. Chen, W.M. Chen, A.B. Yu, Langmuir 26 (2010) 4400–4408. [15] X. Liu, H. Xu, H. Xia, D. Wang, Langmuir 28 (2012) 13720–13726. [16] N. Li, Q. Zhang, S. Quinlivan, J. Goebl, Y. Gan, Y. Yin, Chem. Phys. Chem. 13 (2012) 2526–2530. [17] X. Dong, X. Ji, H. Wu, L. Zhao, J. Li, W. Yang, J. Phys. Chem. 133 (2009) 6573–6576. [18] Z. Yang, K.T. Nguyen, H. Chen, H. Qian, L.P. Fernando, K.A. Christensen, J.N. Anker, J. Phys. Chem. Lett. 2 (2011) 1742–1746.
[19] H. Xia, S. Bai, J. Hartmann, D. Wang, Langmuir 26 (2010) 3585–3589. [20] R. Jin, Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901–1903. [21] R. Jin, Y. Cao, G.S. Metraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487–490. [22] A. Henglein, M. Giersig, J. Phys. Chem. B 103 (1999) 9533–9539. [23] Z.S. Pillai, P.V. Kamat, J. Phys. Chem. B 108 (2004) 945–951. [24] Q. Zhang, N. Li, J. Goebl, Z. Lu, Y. Yin, J. Am. Chem. Soc. 133 (2011) 18931–18939. [25] S. Kapoor, L. Varshney, J. Phys. Chem. A 101 (1997) 7778–7782. [26] G.V. Buxton, C.R. Stuart, J. Chem. Soc. Faraday Trans. 91 (1995) 279–281. [27] C. Von Sonntag, The Chemical Basis of Radiation Biology, Taylor and Francis, New York, 1987. [28] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, J. Phys. Chem. Ref. Data 17 (1988) 513–886. [29] J.W.T. Spinks, R.J. Woods, An Introduction to Radiation Chemistry, J. Wiley and Sons Inc., New York, Toronto, 1990. [30] K.K. Caswell, C.M. Bender, C.J. Murphy, Nano Lett. 3 (2003) 667–669.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Reduction and aggregation of silver ions in aqueous citrate solutions Ridhima Chadha, Nandita Maiti, Sudhir Kapoor ⁎ Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 13 March 2013 Received in revised form 23 December 2013 Accepted 22 January 2014 Available online 31 January 2014 Keywords: Metal nanoparticles TEM UV–visible spectroscopy Radiolysis
a b s t r a c t Radiolytic reduction of Ag+ ions and the subsequent formation of Ag clusters were studied in aqueous citrate solutions. Pulse-radiolysis studies show that the presence of citrate in the solution affects the early processes, via complexation of Ag+ ions with the carboxyl moieties of the citrate. The ratio of citrate to Ag+ determines the kinetic consequences of the reduction and agglomeration processes. The complexation reduces somewhat the rate of reduction by hydrated electrons. However, when all the ions are complexed to the citrate, the surface plasmon absorption band becomes broader, albeit small, but nevertheless it provides extreme stability to the formed nanoparticles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently an extensive research has been focused on the physical and chemical properties and potential applications of metal nanoparticles. In this respect, coinage metals such as silver and gold have been explored extensively. The main reason for this is due to the fact that they exhibit strong surface plasmon resonance absorption located in a convenient part of the spectrum ∼400 nm for Ag and ~520 nm for Au [1–5]. The other significant reason is that they are relatively unreactive in nature, and the ease with which their nanoparticles can be synthesized. These properties have encouraged the researchers to explore the applications of silver and gold nanoparticles in various areas such as solar cell enhancement [6], biosensing [7], surface-enhanced Raman spectroscopy (SERS) [8,9], etc. To stabilize the nanoparticles various surfactants, polymers, ligands, etc. have been used to prevent the aggregation [1–5]. Similarly, various methods for reducing metal ions have been reported in literature [1–6,10–13]. In this context, the citrate reduction of HAuCl4 in hot water, initially developed by Turkevich et al. [11] and later refined by Frens [12] is the most commonly used method to synthesize quasispherical Au nanoparticles with controlled sizes in the range of 10 to 40 nm, depending on the ratio of citrate to HAuCl4. Similarly, method developed by Lee and Meisel [13] is commonly used for citrate stabilized Ag nanoparticles. In these methods, citrate serves the dual purpose, as a reducing agent and a stabilizer. Very recently, the role of citrate ions in controlling the morphology of the particles has been discussed [14–19]. As mentioned above, many experiments have shown the growth of silver nanocrystals in solution using either citric acid or sodium citrate in which citric acid, (or citrate) is used as a reductant and as a stabilizer. ⁎ Corresponding author at: Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Tel.: +91 22 25590298; fax: +91 22 25505151. E-mail address: [email protected] (S. Kapoor). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.041
However, the exact role of citrate ions still remains debatable. For instance, in photo illumination by visible light it is demonstrated that the citrate ions act as stabilizers [20,21] for generating monodispersed silver nanoprisms. Also, using γ-irradiation of AgClO4 solutions it was shown that citrate ions do not act as reducing agents but solely as stabilizers of the colloidal particles [22]. It is also demonstrated that citrate ions influence the particle growth by complexing with positively charged Ag+ 2 dimers [23] and nanoparticles [14]. Very recently, the role of citrate in the formation of anisotropic particles is investigated [24]. It has been suggested that citrate is not the crucial component in the evolution of Ag nanoplates. Inspired by the work we have made an attempt to study the effect of rate of reduction on the formation and stability of the particles in the presence of citrate. Radiolysis is a clean method where, reduction can be carried out without adding any reductant from outside. In this study, we systematically investigate the role of citrate ions in the complexation as well as in the stabilization of silver nanoparticles. Functions of citrate ions in the formation and growth of silver clusters and eventually to the nanoparticles were probed using time resolved kinetic spectrophotometry, UV–vis spectrometry and by transmission electron microscopy (TEM) techniques. The possible mechanisms in the formation of silver nanoparticles are finally discussed. The novelty of this study is that using faster rate of reduction it is shown that the small and nearly monodisperse Ag particles can be prepared using citrate.
2. Materials and methods 2.1. Materials Silver perchlorate (Aldrich), tert-butanol (Sisco, India) and sodium citrate (S.D. Fine Chemicals, India) were used as received. IOLAR grade N2 gas (purity ≥ 99.99%) used for purging solutions was
R. Chadha et al. / Materials Science and Engineering C 38 (2014) 192–196
2.2. Methods In a typical synthesis, 1 mL of AgClO4 solution (0.01 M) and 50 mL of sodium citrate solution (0.1 M) were added to the volumetric flask. The mixture turned to a light white suspension immediately, indicating the formation of a poorly soluble Ag–citrate complex (solubility 0.029 g/L at room temperature). Then, the solution was diluted to 100 mL using Millipore purified water. The complex dissolved completely and all the ratios of [citrate]/[Ag+] were prepared accordingly. Electron pulse radiolysis experiments were performed at the Bhabha Atomic Research Centre Laboratory with the 7 MeV electron linear accelerator [25]. Briefly, samples were irradiated in a 1 cm × 1 cm suprasil quartz cuvette kept at a distance of approximately 12 cm from the electron beam window, where, the beam diameter was approximately 1 cm. Upon generation of transient species, their absorption spectra and/or kinetics were measured using a pulsed Xe lamp as the light source. The absorption changes were measured using a monochromatorphotomultiplier tube arrangement while the photomultiplier output was collected with a digital oscilloscope and then stored on a computer for further analysis. An aerated 10−2 M KSCN solution was used for dosimetry, and the (SCN)•− 2 radical was monitored at 475 nm. The absorbed −4 dose per pulse was calculated [26] assuming Gε [(SCN)•− 2 ] = 2.6 × 10 m2 J−1 at 475 nm. Absorbed doses per pulse were of the order of 6 Gy (1 Gy = 1 J kg−1, for aqueous solution, 1 Gy corresponds to 1 J L−1). For practical purposes, the G-unit rather than the SI-unit for radiation chemical yields is used. The G-unit denotes the number of species formed or converted per 100 eV of absorbed energy in aqueous solution; G = 1 corresponds to 0.1036 μM per 1 J of absorbed energy in aqueous solution [27]. The radiolysis of water produces reactive free radicals, hydrated electrons, OH radicals and H atoms, and molecular products H2O2 and H2, according to the stoichiometry [28,29] as shown in reaction (1).
ð1Þ where, the numbers in parentheses represent the radiolytic yields, G-values, the quantity of species formed per Joule of energy deposited at pH 7 in μmol J− 1. Depending on the experimental conditions, one can get exclusively either reducing or oxidizing conditions by purging the solution either with N2 or N2O gas, respectively. In general, alcohols are added to the N2-bubbled media to scavenge •OH radicals and •H atoms, to get the reducing conditions. Hence, to isolate hydrated electron reactions, solutions were bubbled with N2 in the presence of tert-butanol as the tert-butanol radical produced (reaction 2) is inert and reduction proceeds via e− aq reaction only. The dose absorbed by the aqueous solution was kept low (dose = 6 Gy, total radical concentration = 1.6 μM per pulse) to minimize radical–radical reactions. •
•
•
OH= H þ ðCH3 Þ3 COH→H2 O=H2 þ CH2 ðCH3 Þ2 COH
ð2Þ
3. Results and discussion 3.1. Effect of citrate ions Citrate generally acts as a reducing agent for metal ions (Ag+ and Au3+) only at high temperature [11–13]. In radiolytic method, it is possible to initiate reduction of metal ions by primary species; hence, the role of other additives in the reaction mixture can be partitioned (or alienated) based on kinetic parameters. This helps in understanding the actual reaction mechanism and the role played by additives in the formation of nanoparticles. In order to investigate the role of citrate ions as a stabilizer and reductant, if any at room temperature, in the synthesis of silver nanoparticles, the molar ratio of [citrate]/[Ag+] was varied from 5 to 15, 50 and 500 and other parameters were kept constant. Fig. 1 shows the effect of citrate on the decay of e−aq for the reaction with Ag+ at constant concentration. It can be seen that the decay of e− aq decreases as the concentration of citrate increases due to the complexation of Ag+ with the citrate. To get the molar ratio of citrate/Ag+ at which nearly complete complexation of Ag+ occurs the variation in + the bimolecular rate constant of e− aq with Ag at different citrate concentration was plotted. It can be seen from Fig. 2 that nearly complete complexation occurs at molar ratio ≥500. The equilibrium constant for Ag+ with citrate can be determined as follows. The observed rate of formation of Ag0 can be expressed as Eq. (1) h i h i h i þ þ þ ¼ k f Ag þ kb Ag kobs Ag 0
f
ð1Þ
b
where, [Ag+]0 is the initial Ag+ concentration, kobs is the observed rate constant, and subscripts f and b denote free and citrate-bound species, respectively. From Fig. 2, we obtain kf = 2.2 × 1010 and kb = 1.1 × 1010 M−1 s−1. The fraction of bound Ag+ is given by h i h i þ þ = Ag f ¼ Ag b
ð2Þ
0
which can be rearranged to give f ¼ ðkobs −k f Þ=ðkb −k f Þ:
ð3Þ
Assuming a simple equilibrium for binding Ag+ to citrate, the equilibrium constant can be estimated from the results in Fig. 2 and using Eq. (4). 1=f ¼ 1 þ 1=fK½citrateg
ð4Þ
These results shown in Fig. 2 agree well with Eq. (4). The slope of the line yields K = 50 M−1. The binding of Ag+ with citrate was further 0.06 Citrate = 5.0 x 10 M Citrate = 0 M Citrate = 5.0 x 10 M
Δ O.D. (a.u.)
obtained from Indian Oxygen Limited. All solutions were prepared just before the experiments, wrapped with aluminum foil and kept in the dark to avoid any photochemical reactions. Water purified through a Millipore system was used.
193
0.04
Citrate = 5.0 x 10 M
0.02
2.3. Characterization of Ag nanoparticles UV–vis extinction spectra were collected on Jasco-650 spectrophotometer. Particle sizes were determined by TEM using a Zeiss-Carl, Libra-120 instrument. Specimen for TEM analysis was prepared on Lacey Formvar/carbon-coated 200 mesh copper grid from Ted-Pella. The 20 μL droplet of the sol was put on the grid and put on filter paper to remove excessive solution. The TEM image was taken by air-drying of the nanoparticles.
0.00 0
2
4
6
8
Fig. 1. Decay traces of e−aq in N2-bubbled aqueous solution containing 1 × 10−4 M AgClO4, 0.1 M tert-butanol and various concentrations of citrate.
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0.75
Absorbance
Just after preparation Next day After 6 days
Fig. 2. Dependence of the rate constant for the reaction of e−aq with Ag+ on citrate concentration. Inset: Reciprocal fraction of complexed Ag+ vs. reciprocal citrate concentration.
confirmed by recording the kinetic traces of the growth process of silver atoms and its clusters. It is known in literature that in aqueous solution Ag0 atom shows absorption at 360 nm (reaction 3) [3,4], Ag+ 2 at 310 nm (reaction 4) [3,4], and Ag2+ at 270 nm (reaction 5) [5]. 3 þ
−
0
Ag þ eaq →Ag ð360 nmÞ
þ
Ag þ Ag →Ag2 ð310 nmÞ þ
þ
Ag2 þ Ag →Ag3
2þ
0.25
0.00 300
450
600
750
900
Wavelength, nm Fig. 4. Absorption spectra of Ag nanoparticles prepared by continuous electron pulse irradiation of N2-bubbled aqueous solution containing 1 × 10−4 M AgClO4, 5 × 10−2 M citrate and 0.1 M tert-butanol. Total dose = 4 kGy.
ð3Þ
þ
0
0.50
ð4Þ
ð270 nmÞ
ð5Þ
Fig. 3 shows the kinetic traces taken at wavelengths 360, 310 and 270 nm. It can be seen that significant change in the kinetic traces becomes visible as the [citrate]/[Ag+] molar ratio increased. This is probably caused by the formation of Ag+–citrate and its clusters–citrate
a
complexes, if, more citrate ions exist in the reaction solution which can decrease the reducing rate. As no transient absorption yield has increased in the presence of citrate it can be concluded that citrate ions are not reducing silver clusters within the time window of our experiment. It is known that citrate strongly coordinates with Ag+ to form Ag+– citrate complex (pK1 = 7.1) [30]. Hence, the pH of the solution was measured at all the studied [citrate]/[Ag+] ratios. It was observed that the pH of the solution varied from 7.2 to 8.1 at the studied extreme ratios of [citrate]/[Ag+]. Thus, it was presumed that in all the studied ratios citrate was present in deprotonated forms (pKa1 = 3.2, pKa2 = 4.8, pKa3 = 6.4 at room temperature), and small variation in different
b 0.06
360 nm
310 nm
0.02
0.00 0
5
10
Time, s
O.D. (a.u.)
O.D. (a.u.)
Citrate = 0 M Citrate = 1.5 x 10 -3 M Citrate = 5.0 x 10 -2 M
0.04
0.06
Citrate = 0 M
0.03
0.00
Citrate = 1.5 x 10-3 M Citrate = 5.0 x 10-2 M
0
5
10
Time, s
c 270 nm
O.D. (a.u.)
0.04
0.02 Citrate = 0 M Citrate = 1.5 x 10-3 M Citrate = 5.0 x 10-2 M
0.00
0
5
10
Time, s 2+ Fig. 3. Transient decay profiles recorded at (a) 360 nm for Ag0 (b) 310 nm for Ag+ following the pulse irradiation of N2-bubbled aqueous solution con2 and (c) 270 nm for Ag3 taining 1 × 10−4 M AgClO4 at different concentrations of citrate.
R. Chadha et al. / Materials Science and Engineering C 38 (2014) 192–196
195
Fig. 5. TEM images of silver nanoparticles in presence of (a) 5 × 10−3 M citrate and (b) 5 × 10−2 M citrate. Other conditions are same as in Fig. 4.
forms of citrate has been neglected in calculating the binding affinities to the Ag+ ions and its subsequent clusters. Stable silver nanoparticles are formed when a deaerated aqueous solution of AgClO4 (1 × 10−4 M) containing different concentrations of citrate was subjected to continuous electron pulse irradiation. A representative absorption spectrum obtained in the presence of 5 × 10−2 M sodium citrate is shown in Fig. 4. A small amount of tert-butanol was added to scavenge hydroxyl radicals formed during the radiolysis. The aqueous electrons formed during radiolysis initiate the reduction of Ag+ ions (reaction 3) while the citrate ions present in the solution stabilize the colloidal suspension [22]. From the spectra presented in Fig. 4, we find the absorbance maximum shifts towards blue after 6 days of post irradiation. The observed blue shift with time in the absorption spectrum could be due to the precipitation of some aggregated particles in the solution though there was no clear visible precipitate. During the bombardment of aqueous solution by electron pulses, for preparing the silver particles, the possibility of citrate participation as reductant is possible; however, this can be neglected as the absorbance yield remains almost similar. Thus, the probability of reaction (6) at room temperature is quite low. 3−
þ
2−
þ
0
2−
C6 H5 O7 þ 2Ag →C5 H4 O5 þ H þ CO2 þ 2Ag ðC5 H4 O5 ¼
−
ð6Þ
−
OOCCH2 COCH2 COO Þ
Thus, our kinetic results of silver clusters and the absorption spectrum of silver nanoparticles clearly show that citrate ions do not participate in the reduction process in the absence of any reductant. It is noteworthy to mention here that e− aq does not exist in the solution after 5 μs at the maximum concentration of citrate (Fig. 1). TEM images of Ag nanoparticles obtained in the presence of 5 × 10−3 and 5 × 10−2 M sodium citrate are shown in Fig. 5. It can be seen that the formed particles are spherical in nature having the average diameter 5.0 ± 0.2 nm with a narrow size distribution, ranging from 4 to 6 nm. It can be noticed that there was no significant change in the size with increase in the concentration of sodium citrate. It is noteworthy to mention here that the formation of such small particles has not been reported earlier in literature using citrate as a stabilizer. It is known that citrate is a weak reductant and since the e− aq is a strong reductant, the Ag+ ions are reduced quite effectively and quickly during radiolysis. As the silver seeds are continuously produced, the citrate stabilizer present in the solution arrests the colloid growth. However, the role of citrate in capping the certain facets of growing silver clusters is diminished due to the fact that reduction of Ag+ ions and subsequent aggregation of its clusters is comparatively faster in pulse irradiation than that when citrate-induced reduction is being carried out at high temperature. This could be probably the reason for not observing different shape of the silver particles in the presence of citrate. The surface plasmon absorption band in the presence of citrate appears to be broader than its absence (Fig. S1). This could be due to the high ionic strength of the solution in the presence of high concentration
of citrate. The surfaces of the nanoparticles are generally surrounded by capping agents (e.g., citrate ions) producing a charged layer that serves as an electrostatic barrier to aggregation. Increasing the ionic strength of the solution by adding ionic species decreases the effective electric field between the charged nanoparticles and consequently, lowers this barrier. Our results corroborate the findings reported by Henglein and Giersig [22] who have shown that colloidal silver sols of long-time stability are formed in the γ-irradiation of AgClO4 solutions. It is also suggested that citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. Nevertheless, our findings clearly show that the formed particles by electron pulse irradiation are nearly spherical and show good monodispersity. As electron pulse irradiation generates nuclei at higher rate than γ-irradiation this lead to the formation of seeds at faster rate and hence, the formation of smaller particles. The above results show that in stabilizing the metal nanoparticles, the role of charged stabilizers may be different than neutral stabilizers. Working in this direction may be able to answer questions regarding the stability of the particles under different conditions. We have initiated the work in this direction. 4. Conclusion We have demonstrated the interaction of Ag+ ions and its clusters with citrate ions that eventually lead to the formation of small silver nanoparticles. The novelty of the work is that the weak capping ability of citrate can be overcome by generating seeds at faster rate that helps in stabilizing the smaller silver nanoparticles. Time-resolved kinetic studies show that binding of Ag+ ions and its clusters with the citrate moiety. The observed variation in the kinetic traces is due to the binding of Ag+ ions and its clusters with the citrate moiety. It was observed that the molar ratio of sodium citrate to Ag+ ions can greatly influence the reaction rate and, hence, the particle growth of silver nanoparticles. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2014.01.041. Acknowledgment The authors are grateful to Dr. S.K. Sarkar, Director, Chemistry Group and Dr. D.K. Palit, Head, Radiation & Photochemistry Division for their encouragement during the course of this study. The authors are also thankful to the entire LINAC team for providing the facility. References [1] [2] [3] [4] [5]
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