One-Step Synthesis of Gold and Silver Hydrosols Using Poly(N-vinyl

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One-Step Synthesis of Gold and Silver Hydrosols Using Poly(N-vinyl-2-pyrrolidone) as a Reducing Agent Cristina E. Hoppe,* Massimo Lazzari, Iva´n Pardin˜as-Blanco, and M. Arturo Lo´pez-Quintela Laboratory of Magnetism and Nanotechnology, Institute of Technological InVestigations, Dpt. Physical Chemistry, UniVersity of Santiago de Compostela, Santiago de Compostela, Spain ReceiVed April 3, 2006. In Final Form: May 30, 2006 Synthesis of gold and silver hydrosols was carried out in a one-step process by reduction of aqueous solutions of metal salts using poly(N-vinyl-2-pyrrolidone) (PVP). Both kinds of metal nanoparticles were obtained without the addition of any other reducing agent, at low temperatures and using water as the synthesis solvent. Shape, size, and optical properties of the particles could be tuned by changing the employed PVP/metal salt ratio. It is proposed that PVP acts as the reducing agent suffering a partial degradation during the nanoparticles synthesis. Two possible mechanisms are proposed to explain the reduction step: direct hydrogen abstraction induced by the metal ion and/or reducing action of macroradicals formed during degradation of the polymer. Initial formation of the macroradicals might be associated with the metal-accelerated decomposition of low amounts of peroxides present in the commercial polymer.

Introduction In the last years, synthesis and stabilization of colloidal nanocrystals have been the subject of active investigation, mainly due to the unique properties of these systems associated with the change from the macro- to the nanoscale.1-4 Gold and silver nanoparticles (NPs) have received special attention because of their catalytic,5 electronic,6 and optical properties7,8 making them very attractive in fields such as sensing, bioconjugation, and SERS enhancement.9,10 In particular, the development of stable water-dispersible metallic NPs has concentrated many efforts, by considering potential biological applications and environmental effects associated with the use of organic solvents.11,12 To avoid aggregation in the aqueous phase, a protective agent is normally used to coat and stabilize the particles. Many of these agents also play a crucial role in the control of the size and shape of the nanocrystals.13,14 One of the most frequently used protective agents in metal nanoparticles synthesis is poly(N-vinyl-2-pyrrolidone) (PVP). This water-soluble polymer has been extensively used as protecting agent against agglomeration of metal colloids in the well-known polyol process.15,16 In this synthetic procedure the alcohol (normally ethylene glycol) is considered to act both as solvent and as reducing agent of the metal ions. PVP has also been used in the reduction of silver and gold in water or ethylene * Corresponding author. Fax: +34 981 595012. Tel: +34 981 563100. E-mail: [email protected]. (1) Wang, X.; Zhuang, J.: Peng Q.; Li, Y. Nature 2005, 437, 121. (2) Yin, Y.; Alivisatos, P. Nature 2005, 437, 664. (3) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49. (4) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (5) Mallik, K.; Witcomb, M. J.; Scurrell, M. S. Appl. Phys. A 2005, 80, 797. (6) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (7) Liz-Marza´n, L. Langmuir 2006, 22, 32. (8) Mulvaney, P. Langmuir 1996, 12, 788. (9) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (10) Cao, Y. C.; Jin, R.; Nam, J.; Thaxton, C. S.; Mirkin C. A. J. Am. Chem. Soc. 2003, 125, 14676. (11) Wang, Y.; Wong, J. F.; Teng, X.; Lin, X. Z.; Yang, H. Nano Lett. 2003, 3, 1555. (12) Raveendran, P.; Fu, J.; Wallen S. L. J. Am. Chem. Soc. 2003, 125, 13940. (13) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11, 454. (14) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (15) Sun, Y.; Xia Y. Science 2002, 298, 2176. (16) Silvert, P. Y.; Herrera Urbina R.; Duvauchelle, N.; Vijayakrishnan, V.; Tekaia-Elhsissen, K. J. Mater. Chem. 1996, 6, 6573.

glycol/water mixtures.17,18 Although some authors have previously mentioned the potentiality of PVP to act as a reducing or nucleating agent,19-22 to our knowledge, this polymer has not been used in the development of a general strategy to obtain stable metal hydrosols in aqueous solutions. In most of the works related to the synthesis of nanoparticles in the presence of PVP, this polymer is only considered to play the role of stabilizer or protecting agent. Only in very recent papers dealing with the preparation of some special nanostructures, the reducing action of PVP has been reported. Umar and Oyama used PVP for growing gold nanoplates onto ITO substrates and propose that PVP might play a reducing function.23 Zhou et al. reported on the formation of icosahedral gold nanocrystals24 by a thermal strategy in which PVP solutions are heated with a gold precursor, whereas Deivaraj et al. attributed the formation of silver triangular nanoplates in pyridine solution to the reducing action of PVP.19 In any case, a possible explanation for the reducing action of this polymer still remains unclear. Some examples of the use of macromolecules in the reduction of metal salts to obtain NPs can be found in the literature. Hussain et al.25 reported the preparation of gold and silver nanoparticles by reducing aqueous solutions of the respective metal salts with poly(sodium acrylate), which also acts as capping agent. Li et al. obtained Ag, Au, and Pt nanoparticles using water-dispersible conducting polymer colloids composed of polyaniline and a conventional polyelectrolyte.26 Longenberger and Mills27 used poly(ethylene glycols) and poly(vinyl alcohols) to transform Au, Pd, and Ag complexes into metal NPs. In this last case, the reducing action is explained assuming that Au(III) complexes, initially bound to pseudocrown (17) Patel, K.; Kapoor, S.; Dave, D. P.; Mukherjee, T. J. Chem. Sci. 2005, 117, 53. (18) Eustis, S.; Hsu, H.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811 (19) Deivaraj, T. C.; Lala, N. L.; Lee, J. Y. J. Colloid. Interface Sci. 2005, 289, 402. (20) Silvert, P. Y.; Herrera Urbina R.; Tekaia-Elhsissen, K. J. Mater. Chem. 1997, 7, 293. (21) Carotenuto, G.; Nicolais, L. Polym. Int. 2004, 53, 2009. (22) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 2002, 18, 2888. (23) Umar, A. A.; Oyama, M. Cryst. Growth Des. 2006, 6, 818. (24) Zhou, M.; Chen, S.; Zhao, S. J. Phys. Chem. B 2006, 110, 4510. (25) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831. (26) Li, W.; Jia, Q. X.; Wang, H. Polymer 2006, 47, 23. (27) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475.

10.1021/la060885d CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006

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ether structures of the poly(ethylene glycols), are reduced by the polymer chains in a process in which oxidation products are formed. A similar mechanism is proposed by Sakai and Alexandridis28 for the formation of gold nanoparticles in airsaturated aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer. Mallick et al. synthesized silver nanoparticles by UV irradiation of aqueous solutions of methoxypoly(ethylene glycol) (MPEG).29 They proposed that ultraviolet radiation generates free radicals that act as the reducing agent toward the silver ion. It is clear from the last three examples that partial degradation of a polymer constitutes an alternative via for the synthesis of metal colloids. Some degradation studies carried out on PVP aqueous solutions have shown that this polymer can be oxidized at mild conditions and that this reaction is accelerated by metal salts which can be reduced during the process.30-31 These facts make oxidation of PVP a possible explanation for its reducing power in the synthesis of metal nanoparticles. In this work we describe the synthetic procedure to obtain gold and silver NPs using PVP acting as both reducing and capping agent (i.e. without the addition of any other reducing agent). The method involves a simple one-step process in which the polymer and the salt precursor are mixed in aqueous solution to give very stable gold and silver hydrosols. Furthermore, we show that simple changes in the reaction conditions allow the tuning of shape and size of NPs. We propose that PVP plays the role of reducing agent in the reaction, suffering a partial degradation during the process. Two possible mechanisms are put forward to explain the reduction step: direct hydrogen abstraction induced by the metal ion and/or reducing action of macroradicals formed during degradation of the polymer. Experimental Section Materials. Two different batches of poly(N-vinyl-2-pyrrolidone) with an average molecular weight (in g/mol) of 10 000 (PVP10 and PVP K15) were purchased from Aldrich and used as received. Both samples were used along the experiments. Two additional samples with Mw ) 40 000 (PVP40, Aldrich) and Mw ) 360 000 (PVP K90, Fluka) were used to analyze the effect of the molecular weight on the reducing power of the polymer. Aqueous solutions of the polymer were prepared by weighting the solid and dissolving it in water at room temperature. As metallic precursors of the NPs, silver nitrate, AgNO3, and hydrogen tetrachloroaurate(III) trihydrate, HCl4Au‚ 3H2O (Sigma-Aldrich, purity g 99.5 wt %), were used without further purification. Aqueous solutions of these salts were prepared with concentrations varying in the 0.01-0.05 M range. To check the possible interference of impurities in the reaction, a PVP sample (Mw ) 10 000) was purified by repeated dissolution in water and precipitation with large amounts of acetone, followed by drying of the solid at 70 °C for 1 day. NaOH and HCl diluted solutions, used for pH adjustments carried out for stability tests, were prepared from standard volumetric solutions (0.0974 N, Aldrich). Purification of the Products. Reaction products containing large amounts of PVP were purified as a previous step to their analysis by transmission electron microscopy (TEM) or X-ray diffraction (XRD). Excessive amounts of polymer interfere in the process of imaging in TEM measurements and degrade the quality of the XRD spectra (due to the broad peaks associated to the diffraction of noncrystalline PVP). Purification was carried out by using two different methods referred in the literature: (1) exchange of the (28) Sakai, T.; Alexandridis, P. J. Phys. Chem. B 2005, 109, 7766. (29) Mallick, K.; Witcomb, M. J.; Scurrel, M. S. J. Mater. Sci. 2004, 39, 4459. (30) Kaczmarek H.; Kaminska, A.; Swiatek, M.; Rabek, J. F. Angew. Makromol. Chem., Appl. Macromol. Chem. Phys. 1998, 261-262, 109. (31) Sionkowska A.; Wisniewski M.; Skopinska J.; Vicini S.; Marsano E. Polym. Degrad. Stabil. 2005, 88, 261.

Hoppe et al. polymer by dodecanethiol (C12H25SH, DDT) in ethanolic solutions;21 (2) washing with water/acetone solutions.32 In the first technique, PVP coated nanoparticles were dissolved into a dilute ethanol solution of DDT (Aldrich, 98%) and left to stand for 2 h. Hydrophobic thiolcoated nanoparticles were separated from the PVP/ethanol solution by centrifugation and redispersed in heptane. In the second procedure, the particles were washed with an acetone/water solution (3 + 1) and then separated by centrifugation at 6000 rpm. This was repeated at least for three times to eliminate as much of PVP as possible. The first technique was used for XRD measurements, whereas the second one was applied for samples analyzed by TEM. Techniques. UV-visible spectra were measured with a diodearray Hewlett-Packard HP8452 spectrophotometer. The samples were placed in a 1 cm × 1 cm × 3 cm quartz cuvette and spectra recorded at room temperature. TEM was performed with a Philips CM-12 microscope operated at an accelerating voltage of 100 kV. Samples were prepared by dropping a dispersion of the particles on Formvar coated copper grids. Scanning electron microscopy (SEM) images were obtained using a Leica 440 scanning electron microscope. Analysis was performed on samples deposited onto glass supports. Films were prepared by immersion of glass slides (previously cleaned in aqua regia (HCl/ HNO3, 3:1) and rinsed with distilled water) in the hydrosols for 24 h and subsequent slow evaporation of the solvent at room temperature. XRD analysis was carried out on a Philips 1710 X-ray diffractometer using Cu KR radiation. 1H NMR spectra were recorded in CDCl at 50 °C on a Bruker 3 AMX spectrometer operating at 300 MHz.

Results and Discussion Synthesis of the NPs. Synthesis of nanoparticles was carried out by mixing selected volumes of aqueous solutions of AuCl4or Ag+ and PVP at room temperature. The final volume was adjusted by adding water. After the solution was shaken for homogenization, samples were heated to the selected reaction temperature (between 25 and 70 °C) and left standing for the reaction to proceed. Different molar ratios of repeating units to metal ions, R, were analyzed. These values were obtained by using the molecular weight of the polymer reported by the supplier, taking into account the mass of the repeat unit (i.e. 111 g/mol). Final concentrations of metal and PVP in each sample (expressed as the millimolar concentration of metal ions and the molar concentration of polymer chains, respectively), as well as reactions conditions and R values, are summarized in Table 1. Changes in the UV-Visible Spectra with Time. Roomtemperature synthesis of metal nanoparticles was followed by recording the changes in the UV-visible spectra with time, for different concentrations of gold and PVP. Figure 1 shows the evolution of the spectra for a solution containing [AuCl4-] ) 0.29 mM and [PVP] ) 0.02 M (sample A in Table 1). During an induction time of approximately 15 min, the absorbance of the sample remained almost invariable in all the wavelength range. This behavior was followed by the sudden development of a plasmon band located at 523 nm associated to the presence of spherical gold NPs.7 This band evolved with time, finally reaching a constant absorbance value of 1.1. Only minor increases in absorption occurred thereafter, and both the final absorbance value and the peak position remained constant even after storage for several weeks. In Figure 2, the temporal evolution of the plasmon band peak for different concentrations of gold in a 0.02 M PVP solution ([AuCl4-] ) 0.14, 0.29, and 0.57 mM, samples B, A, and C) can be observed. It is worth noting that changes in absorbance with time cannot be directly related to the kinetic change in (32) Chou, K.; Lai, Y. Mater. Chem. Phys. 2004, 83, 82.

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Table 1. Reaction Experimental Conditions of Samples Synthesized by Reduction of Aqueous Solutions of Metal Ions with PVP sample

metal

[Mn+] (mM)a

[PVP] (M)b

Rc

polym batch

temp (°C)

A B C D E F G H I J K L M N O P Q

Au Au Au Au Au Ag Ag Ag Ag Au Au Au Au Au Ag Ag Au

0.29 0.14 0.57 0.29 0.29 0.07 0.58 1.75 2.90 2.00 2.00 2.00 2.00 2.00 26.0 26.0 6.0

0.020 0.020 0.020 0.009 0.013 0.020 0.020 0.020 0.020 0.052 0.039 0.025 0.020 0.014 0.020 0.0003 0.0014

6213 12870 3161 2796 4038 25740 3106 1030 621 2342 1757 1126 901 631 69 1 21

PVP K15 PVP K15 PVP K15 PVP K15 PVP K15 PVP K15 PVP K15 PVP K15 PVP K15 PVP 10 PVP 10 PVP 10 PVP 10 PVP 10 PVP K15 PVP K15 PVP K15

25 25 25 25 25 70 70 70 70 70 70 70 70 70 70 25 70

a

Millimolar concentration of metal ions in the reaction medium. Molar concentration of polymer chains in the reaction medium. c Molar ratio of repeating units to metal ions.

b

Figure 1. Evolution of the UV-visible spectra during reaction at room temperature of a sample prepared with [AuCl4-] ) 0.29 mM and [PVP] ) 0.02 M (sample A).

concentration of the formed NPs as its size changes during the reaction and produces a variation in the extinction coefficient.33 Owing to this effect, the real value of the NPs concentration at each time cannot be directly calculated from the absorbance value. Nevertheless, such changes allow comparing the global kinetic behavior of reactions in which not important differences in final shape and size are observed and have been frequently used to evaluate NPs formation.18,20 The curves show that the reaction follows an autocatalytic course, possibly due to the slow formation of reducing active species. The plasmon band appeared approximately at the same time for all the concentrations, indicating that, under these conditions, the induction time hardly depends on the gold concentration. After this time, a sudden increase of the plasmon band occurred indicating the formation of metal NPs. The maximum absorbance was attained approximately after 150 min from the beginning of the reaction. No significant changes in this final value could be observed even after storage for several weeks. (33) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410.

Figure 2. Evolution of the plasmon band intensity as a function of time for [PVP] ) 0.02 M and [AuCl4-] ) 0.14 (circles), 0.29 (squares), and 0.57 mM (triangles) (samples B, A, and C).

Figure 3. Evolution of the plasmon band intensity as a function of time for [AuCl4-] ) 0.29 mM and [PVP] ) 0.009 (circles), 0.013 (squares), and 0.02 M (triangles) (samples D, E, and A).

The effect of the polymer concentration on the evolution of the intensity of the plasmon band was also analyzed for a constant value of [AuCl4-] ) 0.29 mM and variable molar concentrations of PVP (0.009, 0.013, and 0.02 M, samples D, E, and A) (Figure 3). In this case, the decrease in PVP concentration has a clear retardation effect in the reaction kinetics, particularly by increasing the observed induction time. At the end of the reaction, the spectra of the samples obtained with gold molar concentrations of 0.14, 0.29, and 0.57 mM and a PVP molar concentration of 0.02 M (samples B, A, and C, respectively), showed one single peak centered at 523 nm typical of spherical gold colloids7 (Figure S1, Supporting Information). From the final absorbance values, and considering a small variation in the final size of the colloids under these conditions, an extinction coefficient  ) 3710 dm3 mol-1 cm-1 was estimated, close to the coefficient reported for Au nanoparticles/Au atom of 3200-4700 dm3 mol-1 cm-1.34 For decreasing amounts of PVP keeping constant the concentration of gold, the analysis of (34) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Vello´n, J. New J. Chem. 1998, 22, 1257.

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Figure 4. UV-visible spectra of samples containing 0.29 mM of AuCl4- and molar concentrations of PVP equal to 0.009 (solid line), 0.013 (dashed line), and 0.02 M (dotted line), obtained after complete reaction at room temperature (samples D, E, and A). The inset shows a TEM micrograph corresponding to the sample with the lowest PVP concentration.

the plasmon band position at the end of the reaction shows a slight red shift of the peak (Figure 4, samples A, D, and E) that could be associated with an increase in the mean particle size of the colloid7 due to the less effective protection of the more diluted PVP solutions. The analysis of the evolution of the plasmon band with time was also carried out for the reaction of PVP with silver nitrate solutions. At the same range of concentrations, silver reduction occurred at a much lower rate compared to gold, with several days needed for the development of the plasmon band. The lower reduction potential of this ion and the high negative electrochemical potential of Ag0 could be responsible for the slow formation of silver compared to gold.35 However, an increase in the reaction temperature or in the concentration of silver led to the rapid formation of NPs. The spectra obtained for a constant concentration of PVP ) 0.02 M and silver concentrations ranging from 0.07 to 2.9 mM are plotted in Figure 5 (samples F-I in Table 1). The curves were obtained at an intermediate reaction time of 7 h. The higher intensity in the plasmon band obtained for higher concentrations of the ion indicates that the reaction took place faster in more concentrated solutions in agreement with that observed for gold. Characterization of the Colloids. The noteworthy influence that the polymer/metal ratio has on the size and shape of metal nanoparticles obtained in the presence of PVP has been previously reported by several authors.13-15,20-21 Those results showed that, as a general trend, the formation of spherical nanoparticles with a narrow size distribution is favored at high PVP/metal ratios. Carotenuto and Nicolais24 reported an increase in the size of gold nanoparticles for lower PVP/AuCl4- ratios, whereas Silvert et al.23 found a decrease in size and polydispersity of silver colloids by increasing the PVP/metal ratio. This effect has been associated with a nearly isotropic addition of atoms on the metal seeds produced by the nonpreferential adsorption of the polymer onto the nanoparticles (at these high polymer/metal ratios) and to the efficiency of PVP to avoid aggregation.13,20 In the present case, PVP plays a double function as reducing agent and stabilizer, so it has to be taken into account that by increasing the PVP/ metal ratio, both the protecting action of the polymer solution (35) Liz-Marza´n, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120.

Hoppe et al.

Figure 5. UV-visible spectra obtained for a 0.02 M concentration of PVP and variable concentrations of silver after 7 h of reaction at 70 °C. Ag+ concentration from bottom to top: 0.07, 0.58, 1.75, and 2.9 mM (samples F-I).

and the reduction rate increase. As mentioned above, high PVP/ metal mass ratios give rise to an increase in the reduction rate that might favor the formation of lower sizes by increasing the rate of nucleation. It is then expected that changes in the PVP/ metal ratio cause a stronger effect on the shape and size of nanoparticles in comparison with that produced in synthesis in which an external reducing agent is used. It is important to note that the reducing action of PVP may also affect the kinetics of NPs formation when weak reducing agents are used and, in this way, it should be considered as a main variable in the control of the size and shape of NPs. With the aim to better analyze the effect of the R value on the shape and size of gold NPs, new synthetic conditions were selected with a wider variation of the R value (from 2342 to 631, samples from J-N in Table 1) and a higher temperature (70 °C) and concentration of gold to increase the reaction rate at the lower R values. Figure 6 shows TEM (samples J-M) and SEM (sample N) micrographs of the reaction products corresponding to R values of 2342, 1757, 1126, 901, and 631. In the representative powder XRD diffraction pattern (Figure 6f, sample L) the expected diffraction peaks for the face-centered cubic structure of gold are present. As can be observed, the PVP/metal molar ratio not only has an important effect on size but also affects the shape of the obtained particles. The gradual decrease of R led to a sharp increase in size and polydispersity of the structures, as well as to the formation of increasing amounts of polyhedral and platelike structures. At R ) 2342, a narrow distribution of spherical nanoparticles with an average size of 6.0 nm and a standard deviation of 1.5 nm could be observed. For lower R values, an increase in the amount of polyhedral nanoparticles was observed, as well as an increase in the average size of the structures (R values 1757 and 1126). Decreasing amounts of PVP, corresponding to R values of 901 and 631, increase the number of hexagonal and triangular platelike structures and promote a dramatic change in the size of the particles, reaching even micrometric dimensions (sample N). A comparable dependence of the shape with the R value has been recently reported by Zhou et al. by using a similar synthetic procedure for the synthesis of icosahedral and platelike structures.24 It has also been observed in the synthesis of gold plates carried out by the polyol process36 and by direct reduction of gold precursors with sodium citrate.37

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Figure 6. (a-d) TEM micrographs of samples obtained with [AuCl4-] ) 2 mM and decreasing R values of 2342, 1757, 1126, and 901 (samples J-M). The inset in (a) gives the size distribution of the obtained nanoparticles. (e) SEM micrograph of sample N (R ) 631). (f) Representative XRD pattern obtained for sample L (R ) 1126).

The mechanism of the formation of these metal platelike structures still remains a subject of controversy. Some authors attributed its formation to the preferential adsorption of certain surfactants or polymers onto preferred crystalline planes. This selective interaction might reduce the growth rate along these preferred directions, favoring the formation of {111} bounded structures as thin plates and icosahedral nanoparticles.13,24,37,38 On the other hand, Shankar et al. have recently proposed a different mechanism for the formation of triangular nanoplates synthesized by using lemongrass extract as a reducing agent. They propose that formation of platelike structures involves rapid reduction, assembly, and sintering of “liquidlike” spherical gold nanoparticles formed at the first stages of the reaction.39 Figure 7 shows TEM micrographs of the synthesis products obtained with silver at different R values (samples I, O, and P in Table 1). The representative powder XRD diffraction pattern (Figure 7d, sample I) shows the expected diffraction peaks for (36) Kan, C.; Zhu, X.; Wang, G. J. Phys. Chem. B 2006, 110, 4651. (37) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W.; Ha, D. H.; Yun W. S. Chem. Mater. 2005, 17, 5558. (38) Chu, H.; Kuo, C.; Huang, M. Inorg. Chem. 2006, 45, 808. (39) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482.

the face-centered cubic structure of silver. Spherical NPs, with an average particle size of 18.8 nm and a standard deviation of 3 nm, were produced with [PVP] ) 0.02 M and [Ag+] ) 2.9 mM (R ) 621, sample I). By an increase of the silver concentration (lowering the R value), an increase in size, polydispersity, and amount of polyhedral NPs was found (samples O and P), in agreement with the behavior observed for gold. However, it should be noted that synthesis carried out with different metals at similar R values (e.g. samples I and N) did not conduct to the same kind of final morphology. Whereas small spherical NPs with a narrow size distribution were obtained for silver, mainly polyhedral NPs of big sizes and wide size distributions were obtained with gold. Although this behavior could be attributed to differences in the chemical nature of the metals and its relationship with the kinetic of the reaction, it should be also considered that synthesis of silver nanoparticles was carried out with a higher concentration of reactants, which could influence the relative rates of the steps involved in the formation of the NPs or even change completely the synthesis mechanism. Preliminary experimental results carried out with gold showed that even stronger differences in shape can be attained by changing the polymer and salt concentrations at a constant low R value

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Figure 7. (a) TEM micrograph of a sample obtained with [Ag+] ) 2.9 mM and [PVP] ) 0.02 M (sample I). The inset shows the size distribution of the obtained nanoparticles. (b, c) TEM images of samples obtained with [Ag+] ) 26 mM and R ) 69 (b) and 1 (c) (samples O and P). (d) Representative XRD pattern obtained for sample I (R ) 621).

(data not shown). In summary, although R seems to be an important value to optimize to tune shape and size of metal nanostructures, further research will have to be carried out to clarify the influence of all the variables on the obtained structures, as well as their relationship with the mechanism of the crystal growth. Stability of the Hydrosols. Stability of gold and silver hydrosols was analyzed by observation of the samples after storage in the reaction medium. Samples from A-J (Table 1) did not show any sign of flocculation after storage for several months. A partial stability was evidenced for the most concentrated gold dispersions at intermediate R values (samples K and L), showing a weak flocculation after few days. Samples M and N (with a much lower R value and bigger sizes) were not stable and precipitated after a short time following the synthesis. The lower stability of these hydrosols was attributed to the higher instability of the large nanocrystals and to the lower concentration of stabilizing agent. These results clearly evidenced the protecting role of PVP against aggregation. With the aim to analyze the effect of pH on the stability of the hydrosols, a diluted gold NPs sample (half dilution of sample A) was prepared and its pH adjusted at different values by using NaOH and HCl solutions. No instantaneous agglomeration was observed even in extreme acid (pH 1.5) or basic (pH ) 12.5) conditions. Basic dispersions remained stable for more than 2 weeks without any evidence of precipitate formation. Although acid samples did not show a comparable stability, beginning of aggregation was observed only after 5 days of storage at pH ) 1.5. Optical Properties as a Function of R. Figure 8 shows the UV-visible spectra of the final products corresponding to samples synthesized with [AuCl4-] ) 2 mM and decreasing R values of 2342, 1126, and 901 (samples J, L, and M). The gold nanospheres obtained for R ) 2342 presented spectra similar to that obtained for samples used for the kinetic analysis (see Figure 4). The

Figure 8. UV-visible spectra of samples synthesized with [AuCl4-] ) 2 mM and decreasing R values of 2342 (solid line), 1126 (dashed line), and 901 (dotted line), corresponding to samples J-M.

spectra show one peak centered at 523 nm typical of spherical gold nanoparticles with sizes lower than 20 nm.7 However, for samples obtained with R ) 1126, a band centered at 552 nm was observed. The origin of this change could be related to the bigger size of the particles and to the probable formation of aggregates because of the less effective protection of the obtained NPs in a more diluted PVP solution. On the other hand, the formation of anisotropic structures can also explain the high wavelengths features observed in the spectra. It is known that particles with an intrinsic anisotropy can exhibit multiple absorption bands associated with various resonance modes.7 Samples with even lower R values also showed a wide absorption at longer wavelengths, characteristic of the presence of big anisotropic

Gold and SilVer Hydrosols

structures in agreement with that observed in TEM micrographs (see Figure 6). Proposed Mechanism of Metal Nanoparticles Formation. Different possibilities could be proposed to justify the reducing ability of PVP aqueous solutions toward metal ions. Taking into account that PVP used in this work is a commercial product and that was used as received, a first option to consider is the possible presence of a reducing impurity in the polymer. However, after purification of PVP as was described in the Experimental Section and subsequent reaction with gold under conditions similar to those used in sample A, NPs formation took place without any observable changes in the reaction rate or characteristics of the final product. Moreover, the reaction also occurred when a different batch of PVP was used (molecular weight 10 000, PVP10) and for PVP with higher molecular weights (PVP40, PVP K90). As the synthesis of these polymers is normally carried out under different conditions, depending on the molecular weight and the manufacturer, it is very improbable that the same reducing impurity can be present in all the analyzed PVP samples. It could also be argued that formation of NPs is attained by photoreduction of the ion-PVP complexes formed in aqueous solution. The ability of PVP to form metal complexes by the coordination of ions to the carbonyl group of the lactame ring has been previously reported.40 However, NPs were also obtained when experiments were carried out in darkness, showing that light was not necessary for the formation of the metal colloids. A third option to be analyzed is that the reaction could take place by the action of the terminal groups of the polymer chains. N-Vinylpyrrolidone is usually polymerized by radical polymerization. When this reaction is carried out in organic solvents, a low to medium molecular weight product is obtained, due to the fact that the solvent may act as a chain transfer agent. In this case, the nature of the end groups depends on the solvent used, although the analysis of common commercial grades of PVP has shown that hydroxyl-isopropyl, hydrogen, and hydroxyl are typical end groups found in these polymers.41 The highest molecular weight grades of PVP can be produced by solution polymerization in water using hydrogen peroxide as initiator. Under these conditions the polymer mainly contains hydroxyl end groups from the starting hydroxyl radicals. Considering that the principal end group found in PVP of different molecular weights is hydroxyl, its potential reducing action was analyzed. Although alcohols are often used as reducing agents in the synthesis of metal NPs,15-16,27 it has been observed that reduction of AuCl4- only occurs for certain alcohol concentrations higher than a threshold value. Sakai and Alexandridis tried to obtain nanoparticles using variable concentrations of different alcohols (methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, and glycerol) and 0.2 mM gold salt aqueous solutions. They found that formation of NPs did not take place at concentrations of the alcohol below 5 mM.28 However, our experiments showed that reactions carried out with a similar concentration of gold and concentrations of PVPK90 as low as 0.009 mM (a maximum concentration of end groups of 0.018 mM) also give rise to the formation of gold nanostructures. These findings are in contradiction with a reduction action of the hydroxyl terminal groups. A final alternative is to consider that reduction of the ions could be achieved by the PVP chain itself, with simultaneous oxidation of the polymer during the process. Several findings point to this possibility as the most probable. First, oxidation of PVP aqueous solutions is a process that easily occurs at mild (40) Jiang, P.; Li, S.; Xie, S.; Gao Y.; Song, L. Chem.sEur. J. 2004, 10, 4817. (41) Raith, K.; Ku¨hn, A. V.; Rosche, F.; Wolf, R.; Neubert, R. H. H. Pharm. Res. 2002, 19, 556.

Langmuir, Vol. 22, No. 16, 2006 7033

Figure 9. UV-visible spectra of a 0.02 M aqueous solution of PVP at 0 min (solid line), 7 h (dashed line), and 3 days (dotted line) from the beginning of heating at 70 °C.

conditions. In fact, yellowness of aqueous solutions of PVP after storage or heating has been reported in technical publications of the manufacturers and related to the partial oxidation of the polymer.42 Moreover, Kaczmarek et al. have reported that aqueous solutions of PVP can be photooxidized, at mild conditions, in reactions accelerated by metal salts which are reduced during the process. During this photooxidative degradation, formation of chromophore species takes place with characteristic absorption bands in the 200-400 nm range attributed to the presence of oxidation products.30 This degradation process showed to be accelerated by Fe3+, possibly by its participation in ion-radical reactions, for example, abstraction of hydrogen atoms from tertiary carbons or from allylic carbon atoms. We confirm this susceptibility to oxidation by measuring the UV-visible spectra of 0.02 M aqueous solutions of PVP (Mw ) 10 000) after heating at 70 °C for 3 days. The obtained curves can be seen in Figure 9. An increase of the absorbance in the 200-400 nm range and the evolution of a band centered at 444 nm could be observed after few hours. In agreement with results found in the literature,30 the evolution of these oxidation products showed to be much faster at higher temperatures or by the addition of small amounts of Cu2+ and Fe3+ salts (data not shown). A similar evolution of the UV-visible spectra was observed during the synthesis of metal nanoparticles. In the majority of the cases, the presence of the plasmon band overlapped the characteristic peak at 444 nm, but the presence of this band could be confirmed lowering the absorption of the particles by using low concentrations of metal ions (see Figure S2, Supporting Information). The origin of the high susceptibility of PVP to oxidation could be related to the presence of low amounts of peroxides that remained in the polymer after synthesis. The presence of these peroxides in all grades of PVP has been reported by manufacturers42 and was confirmed in our aqueous solutions by iodometric analysis. Thermal decomposition of these species could produce the formation of extremely reactive radicals that would react with the polymer to form new macroradicals. Following this initiation step, the most important processes that can take place during PVP degradation are reactions of main chain scission, hydrogen atom abstraction, or side ring abstraction.30 On the other hand, it is known that decomposition of peroxides to give (42) Buhler, V. In Kollidon: Poly(Vinylpyrrolidone) for the pharmaceutical industry; BASF Aktiengesellschaft: Ludwigshafen, Germany, 1998; pp 15-120.

7034 Langmuir, Vol. 22, No. 16, 2006

Hoppe et al.

showed a group of additional peaks located at values of chemical shifts between 4 and 6 ppm. According to the literature, these features could be associated to the presence of vinyl groups in the polymer chain.45 The presence of these groups is in agreement with the mechanism proposed by Kaczmarek et al. for the degradation of PVP in aqueous solutions.30

Figure 10. 1H NMR spectra in CDCl3 at 50 °C for PVP of Mw ) 10 000 (a) and the reaction product of AuCl4- and PVP with R ) 21 (b, sample Q). The inset shows a magnification of the product spectrum between 4.4 and 6.8 ppm.

In summary, our findings indicate that the reduction of metal ions to give metal nanoparticles is due to the reducing action of the polymer. The reduction process could be probably understood considering two main reactions: (1) direct abstraction of hydrogen atoms from the polymer by the metal ion; (2) reduction of the metal precursor by the organic radicals formed in (1) or during the metal-accelerated degradation of PVP.

Conclusions initiator radicals is catalyzed by the action of transition metal complexes.43 According to this, it seems probable that the addition of gold or silver salts could accelerate the decomposition of residual amounts of peroxides present in the polymer. This decomposition process would produce reactive radicals able to attack the PVP chain to give macroradicals. These species have proved to be the responsible for the reduction of metal ions in several synthetic methods used for the generation of metal nanoparticles.29,30 Consequently, a possible explanation for the formation of NPs is that reduction of metal occurs by action of organic radicals formed during the metal-accelerated degradation of PVP. The observed induction times in the kinetics curves and their dependence with the PVP concentration are also in agreement with a slow initial formation of organic radicals. It is also possible that some metal ions could directly participate in ion-radical reactions (as has been observed for Fe3+), for example, in the direct abstraction of hydrogen atoms from tertiary carbons or from allylic carbon atoms30 (see eqs 1 and 2). In any case, both types of reactions would involve the partial degradation of the polymer and the formation of new products. Additional evidence to the proposed mechanism was given by comparing the 1H NMR spectrum of PVP as received with that of a gold/PVP reaction product. In this case the reaction was carried out with a low PVP/metal ratio (R ) 21) to avoid the interference of nonreacted PVP in the analysis of the spectra. The spectra of both samples measured in CDCl3 at 50 °C are shown in Figure 10. Signals of PVP are in agreement with previous data reported for this polymer.44 However, the spectrum of the reaction product

Gold and silver stable hydrosols could be easily obtained using a one-step method based on the direct reduction, at low temperatures, of aqueous solutions of metal precursors with a nontoxic and physiologically compatible polymer. Simple variation of the PVP/metal ratio allowed the formation of structures with very different shapes and sizes which could find potential applications associated with their characteristic optical responses. The reduction of the metal salts to give nanoparticles was attributed to the presence of the polymer by considering two main possible reactions: (1) direct abstraction of hydrogen atoms from the polymer by the metal ion; (2) reduction of the metal precursor by organic macroradicals formed by degradation of PVP. The obtained results also show that great care has to be taken when PVP is used as capping agent in procedures involving other reducing agents, because this polymer can interfere itself in the reduction reaction. Acknowledgment. The authors thank the financial support of the “Ministerio de Ciencia y Tecnologı´a” (Spain) under Projects MAT2002-00824 and MAT2005-07554-C02-01. C.E.H. is thankful for postdoctoral grants from CONICET and Fundacio´n Antorchas (Argentina) and the 2006-IIF Marie Curie Grant (Proposal No. 021689, AnaPhaSeS). Supporting Information Available: Final spectra of samples obtained with [PVP] ) 0.02 M and variable concentrations of AuCl4and UV-visible spectra of gold diluted samples showing the 444 nm degradation peak of PVP. This material is available free of charge via the Internet at http://pubs.acs.org. LA060885D

(43) Ranby, B.; Rabek, J. F. In Photodegradation, photooxidation and photostabilization of polymers; Wiley: London, 1975. (44) Dutta, K.; Brar, A. S. J. Polym. Sci., Part A: Poly. Chem. 1999, 37, 3922.

(45) Pretsch, E.; Bu¨hlmann, P.; Affolter, C.; Herrera, A.; Martı´nez, R. In Structure Determination of Organic Compounds; Springer: New York, 2000.