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Formation of PVP-Protected Metal Nanoparticles in DMF Isabel Pastoriza-Santos and Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, Universidade de Vigo, 36200 Vigo, Spain Received September 17, 2001. In Final Form: January 11, 2002 Poly(vinylpyrrolidone) (PVP) is a polymer capable of complexing and stabilizing Ag and Au nanoparticles formed through the reduction of Ag+ or AuCl4- ions with N,N-dimethylformamide. The reduction can be efficiently performed both at reflux and under microwave irradiation, but each of these methods leads to different nanoparticle morphology and colloid stability. The use of microwave irradiation provides an extra degree of control of the reduction process. The use of PVP with different polymer chain lengths leads to particles with similar sizes though with a different degree of stability. The colloids are also stable in ethanol for months, but only marginally stable in water.
Introduction The most widely used substances for the stabilization of metal nanoparticles are ligands and polymers, specially natural or synthetic polymers with a certain affinity toward metals, which are soluble in suitable solvents.1-4 Such substances can also control the reduction rate of the metal ions and the aggregation process of zerovalent metal atoms. It has been said5 that the preparation of polymerstabilized nanoparticles (through chemical methods) basically involves two processes: reduction of metal ions into neutral atoms and coordination of the polymer to the metal nanoparticles. The polymers also control the aggregation of the metal atoms in solution. In practice, the reduction can take place either after or before the interaction between the metallic moieties and the polymers. In the former case (after), a complex between metal ions and polymer is formed, followed by the reduction of the metal ions on the polymer, so that the obtained metal atoms retain their interaction with the polymer. However, if the reduction precedes the interaction, no complex formation takes place between metal ions and polymer, so that the growth of the metal particles cannot be properly controlled by the polymers and the protection effect is only obtained after the particle has been formed. Since the time when Faraday presented the first preparation method of metal nanoparticles in an aqueous medium,6 a large number of methods have been developed for the synthesis of metal nanoparticles, involving the use of different protecting agents, such as gelatin,6 synthetic polymers,7,8 citrate ions,9,10 or organometallic ligands.11,12 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Hirai, H. J. Macromol. Sci., Chem. 1979, A13, 633. (2) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1985, 3, 127. (3) Zhao, B.; Toshima, N. Kobunshi Ronbunshu 1989, 46, 551. (4) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht, 1986; p 87. (5) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (6) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145. (7) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 3268. (8) Dunworth, W. P.; Nord, F. F. J. Am. Chem. Soc. 1950, 72, 4197. (9) Turkevich, J.; Kim, G. Science 1970, 169, 873. (10) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (11) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (12) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655.
During the past decade, Toshima’s group developed and optimized an effective technique for the preparation of colloidal dispersions through reduction with an alcohol at reflux, in the presence of a protecting agent of polymeric nature.13-18 Using poly(vinylpyrrolidone) (PVP) as a stabilizing agent, they managed to obtain stable dispersions of nanoparticles of gold, platinum, and other metals in ethanol. However, in ref 18, these authors explicitly mention that stable silver dispersions cannot be prepared using the same procedure. Microwave irradiation19-21 is one of the novel techniques developed during the last years for the synthesis of solid materials. Since its first application in synthetic chemistry,22 it has been rapidly extended toward the areas of analytical chemistry23 and organic reactions.24,25 Additionally, it has been successfully used for the synthesis of semiconductor quantum dots26 and Pt colloids.27 The main advantage of microwave irradiation is that it produces a uniform heating of the solution, so that a more homogeneous nucleation is obtained as well as a shorter crystallization time, as compared to conventional heating, and it is therefore very useful for the formation of monodisperse metal colloids. Further advantages are short thermal induction period, absence of convection processes, easy control, and low cost. (13) Yonezawa, T.; Sutoh, M.; Kunitake, T. Chem. Lett. 1997, 619. (14) Hirai, H.; Nakao, Y.; Toshima, N.; Adachi, K. Chem. Lett. 1976, 905. (15) Hirai, H.; Nakao, Y.; Toshima, N. Chem. Lett. 1978, 545. (16) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1978, A12, 1117. (17) Lu, P.; Teranishi, T.; Asakura, K.; Miyake, M.; Toshima, N. J. Phys. Chem. B 1999, 103, 9673. (18) Hirai, H.; Yukimichi, N.; Toshima, N. J. Macromol. Sci., Chem. 1979, A13, 727. (19) Sheppard, L. M. Ceram. Bull. 1988, 67, 1656. (20) (a) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. Rev. 1991, 20, 1. (b) Mingos, D. M. P.; Baghurst, D. R. Br. Ceram. Trans. J. 1992, 91, 124. (c) Mingos, D. M. P. Chem. Ind. 1994, 596. (21) Rao, K. J.; Ramesh, P. D. Bull. Mater. Sci. 1995, 18, 447. (22) Liu, S. W.; Wightman, J. P. J. Appl. Chem. Biotechnol. 1971, 21, 168. (23) Pare´, J. R. J.; Be´langer, J. M. R.; Stafford, S. S. Trends Anal. Chem. 1994, 13, 176. (24) Abramovitch, R. A. Org. Prep. Proced. Int. 1991, 23, 683. (25) Mingos, D. M. P.; Whittaker, A. G. In Microwave Dielectric Heating Effects in Chemical Synthesis, Chemistry under Extreme or Non-Classical Conditions; Van Eldik, R., Hubbard, C. D., Eds.; Wiley: New York, 1996; p 11. (26) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (27) Yu, W.; Tu, W.; Liu, H. Langmuir 1999, 15, 6.
10.1021/la015578g CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
PVP-Protected Metal Nanoparticles
In this paper, we describe in detail the preparation of Ag nanoparticles by reduction of Ag+ ions in/by N,Ndimethylformamide (DMF), in the presence of PVP. A systematic study is presented on the influence of several parameters, such as the use of reflux conditions or microwave irradiation to accelerate the reaction, PVP concentration, or the molecular weight of the PVP used. Additionally, the formation of gold nanoparticles using the same procedure is also reported. Experimental Section Chemicals. AgClO4 (99.999%) and HAuCl4 (Aldrich) were used as precursors of Ag and Au particles, respectively. N,NDimethylformamide (g98%) (Fluka) was used as both solvent and reducing agent. Poly(vinylpyrrolidone) (PVPK15, MW 10 000, and PVPK30, MW 40 000) was purchased from Fluka and used as the stabilizer. Absolute ethanol, EtOH (99.8%) (Scharlau), and Milli-Q water were used as solvents. All of the chemicals were used as received from the suppliers. Particle Preparation and Sample Handling. The preparation of silver particles takes place by simple addition of the corresponding aqueous solution (10%) of silver perchlorate (always in microliter volumes) to a PVP solution in DMF ([PVP] ) 0.76-7.6 mM). The polymer concentration was varied so that the ratio between silver salt ([AgClO4] was either 0.76 or 0.2 mM) and PVP concentrations acquired one of the following values: 1.0, 0.45, or 0.1. The preparations at reflux (156 °C) were carried out in a two-neck round beaker placed on a heating mantle and equipped with a cooler to keep the reflux. In these experiments, the PVP solution in DMF was allowed to reach the reflux temperature and subsequently the silver salt solution was added. This addition was performed taking all necessary precautions to avoid degradation of the plastic tips of the micropipet by DMF vapor (highly corrosive and at a high temperature). For the same reason, the reaction vessel was closed with a glass stopper. In the case of the reactions induced by microwave irradiation, the reaction mixtures (in 15 mL volumes), with the appropriate concentrations of PVP and silver salt, were introduced at room temperature in a conventional microwave oven (Panasonic, 1100 W) and were heated in 10 s pulses (since a longer exposure time resulted in small explosions within the samples), cooling after each pulse in an ice bath for several minutes. Once the reaction was initiated, aliquots were taken for measurement of UV-visible spectra at selected times (1 mm optical path length quartz cuvettes were used due to the high extinction coefficient of colloidal silver). For the study of the stability in other solvents, Milli-Q water and absolute ethanol were used. Such experiments were performed by simple dilution (volume ratio 1:20) of an aliquot of the DMF sample into the corresponding solvent, and the time evolution was monitored through UV-visible spectroscopy using 10 mm optical path length quartz cuvettes. Characterization Techniques. Transmission Electron Microscopy (TEM). TEM was carried out with a Philips CM20 microscope operating at 200 kV, equipped with an elemental analysis system by X-ray energy dispersion (EDS) EDAX PV9900 with a detector Super Ultrathin Window. For the characterization of the silver particles by TEM, either carbon-coated or FORMVAR-carbon-coated copper grids were used. To deposit the particles, the grids were dipped directly into solution, which was previously diluted with ethanol to avoid the dissolution of the polymer layer on the grids by DMF and to decrease the concentration of PVP for a better contrast. UV-Visible Spectroscopy. UV-visible spectra were measured with a Hewlett-Packard HP8453 diode-array spectrophotometer equipped with a thermostated multiple sample holder.
Results and Discussion This section has been divided into four subsections. In the first one, we study the process of formation of silver particles at reflux and under microwave irradiation. The influence of the length of the polymer chain on the reaction rate and on the average size and stability of the particles is discussed in the second subsection. Then we study the
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stability of the dispersions in water and ethanol. The final subsection deals with the formation of Au nanoparticles in similar conditions to those used for the formation of Ag and describes studies on the influence of the polymer chain length and the stability of Au dispersions in water and ethanol. A. Formation of Silver Nanoparticles from AgClO4. In previous studies,28,29 we have shown that the rate of reduction of Ag+ ions by DMF is strongly temperature dependent. Although the reduction can take place at room temperature, the rate is much higher when the temperature is increased. In particular, we have observed in such studies that the optimal conditions for the synthesis of nanoparticles with a homogeneous size distribution are found at reflux. This is due to the high boiling point of DMF (156 °C) and the ease of keeping a constant temperature. As an alternative procedure, reduction experiments have also been carried out in which heating was induced through microwave irradiation. In such cases, the conditions are not so aggressive as at reflux, while heating is equally homogeneous within the whole volume of the solution. Presented here are results on the reduction of silver perchlorate in PVP solutions in DMF at reflux and by microwave irradiation, studying for each method the influence of PVP concentration. Reduction at Reflux. In the first set of experiments, a constant AgClO4 concentration of 0.76 mM was used. The effect of PVP K15 (MW 10 000) concentration was studied using three different Ag+/PVP molar ratios (1.0, 0.45, and 0.1). The effect on the reaction rate was studied by monitoring the process through UV-visible spectroscopy. In the case of a molar ratio of 1.0, we observe an important drop of the absorbance within 2 min, due to rapid aggregation. Plotted in Figure 1 are the spectra measured for the higher PVP concentrations, showing the characteristic plasmon band for silver.30-33 For a ratio of 0.45, there is a blue shift of the plasmon band during the initial stages of the reaction due to the adsorption of Ag+ ions (which are in excess in solution), but as the reduction proceeds, the concentration of these ions decreases and therefore increases the electron concentration on the surface.34 Once the reduction has been completed, the high temperature can easily promote particle aggregation or Ostwald ripening, which is reflected in a red shift and a new shoulder (which arises from the plasmon resonance of the aggregates). Conversely, for a molar ratio of 0.1, the band broadens and a shoulder shows up, but the maximum position only slightly blue-shifts. For a larger amount of PVP, the time necessary to finalize the reduction (understanding that this happens when the absorbance at the peak is around 1.25) increases, which means that the reduction rate decreases as the concentration of stabilizer increases. On the other hand, the formation of a shoulder takes place at shorter times as the amount of PVP increases. Thus, for a molar ratio of 0.45 the shoulder is formed after 2 min, while for a molar ratio of 0.1 it becomes evident after just 15 s. This (28) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 1999, 15, 948. (29) Pastoriza-Santos, I.; Serra-Rodrı´guez, C.; Liz-Marza´n, L. M. J. Colloid Interface Sci. 2000, 221, 236. (30) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. (31) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (32) (a) Kerker, M.; Siiman, O.; Wang, D. S. J. Phys. Chem. 1984, 88, 3186. (b) Kerker, M.; Wang, D. S.; Chew, H. Appl. Opt. 1980, 19, 4159. (c) Wang, D. S.; Kerker, M. Phys. Rev. B 1981, 24, 1777. (33) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (34) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. Chem. Soc. 1991, 92, 31.
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Figure 1. Time evolution of UV-visible absorption spectra after addition of AgClO4 0.76 mM onto PVP solutions in DMF at reflux. The Ag+/PVP molar ratios used are (a) 0.45 and (b) 0.1. For each spectrum, time at reflux is indicated.
tendency seems to indicate that the reduction takes place in solution rather than on the surface of PVP (i.e., the reduction is so fast that the polymer chains cannot complex the silver ions and stabilize in an efficient manner the large density of particles formed, avoiding their precipitation). Another plausible explanation is that a lower PVP concentration may be insufficient to provide steric stabilization, thus resulting in nanoparticle aggregation. From the TEM images (see an example in Figure 2) of the particles present in solution during the initial and final stages of the reaction, it can be concluded that the red shift of the plasmon band coincides with a clear increase of particle size and in general of the polydispersity of the colloid. It was also found that the average particle size does not depend notably on the Ag+/PVP molar ratio (see Table 1). We studied as well the influence of the starting AgClO4 concentration by repeating the experiments with [AgClO4] ) 0.2 mM and varying PVP concentration to achieve the same molar ratios of 1.0, 0.45, and 0.1. In general, the plasmon bands are more stable with time (see an example in Figure 3 for the lower PVP concentration), which means that it takes longer for the band to drop and the red shift is less marked. TEM images show a similar trend to that observed for higher concentrations; that is, the average particle size and polydispersity vary during the reaction and the size difference between the initial and final stages decreases as the amount of PVP increases (Table 1). This is a further confirmation that the reduction is completed in just a few seconds, and by keeping the system at reflux, aggregation and particle growth are induced. In the experiments performed at reflux, as the reaction proceeds there is an increase in absorbance below 400
Pastoriza-Santos and Liz-Marza´ n
Figure 2. TEM micrographs for a 0.76 mM Ag colloid with Ag+/PVP molar ratio ) 0.1 and reaction times at reflux of 15 s (a) and 4 h (b). Table 1. Summary of Particle Sizes and Standard Deviations for Ag Nanoparticles Formed under Different Experimental Conditions in DMFa [Ag+] (mM)
a
method
[Ag+]/[PVP] polymer
0.76 0.76
reflux reflux
1.0 0.45
PVPK15 PVPK15
0.76
reflux
0.1
PVPK15
0.20
reflux
1.0
PVPK15
0.20
reflux
0.45
PVPK15
0.20
reflux
0.1
PVPK15
0.76
reflux
1.0
PVPK30
0.76
reflux
0.45
PVPK30
0.76
microwave
1.0
PVPK15
0.76
microwave
0.45
PVPK15
0.76
microwave
0.1
PVPK15
diameter (nm) 6.61 ( 2.05 7.11 ( 3.20 27.01 ( 11.05 5.99 ( 4.10 19.48 ( 6.98 9.49 ( 7.01 21.73 ( 12.19 14.02 ( 5.24 22.00 ( 10.76 7.41 ( 3.65 13.75 ( 5.36 5.65 ( 3.11 10.94 ( 6.43 5.06 ( 2.13 6.47 ( 4.31 2.92 ( 0.84 3.64 ( 2.38 3.38 ( 1.51 3.30 ( 2.51 3.42 ( 1.06 3.31 ( 1.73
Values are given for initial and final stages of the reaction.
nm, and this effect is more intense as the PVP concentration increases. Experiments were performed to check whether this absorbance increase is related to the presence of PVP, by refluxing PVP solutions in DMF with no added silver salt and water. The results show the formation of a band centered between 350 and 400 nm. This band is more intense as the amount of PVP is increased, and the same results are obtained for PVP solutions in DMSO. As will be shown below, when the reduction is induced with microwave irradiation this effect is not observed, so we can suggest as a reason PVP degradation under reflux conditions in DMF.
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Figure 3. Time evolution of UV-visible spectra after addition of AgClO4 0.20 mM onto PVP solutions in DMF at reflux. Ag+/ APS ) 1. Time at reflux is indicated.
Microwave-Induced Reduction. The process of particle formation was also monitored through UV-visible spectrometry for three different Ag+/PVP molar ratios (1.0, 0.45, and 0.1). Figure 4 shows the time evolution of the absorption spectra for a sample with [AgClO4] ) 0.76 mM. Comparison of these spectra with those in Figure 1 clearly shows that the microwave-induced process allows a better control on the reaction. With respect to the shift of the plasmon band during the reduction, we observe that for a molar ratio of 1.0, the band gradually blue-shifts (as was observed at reflux) basically until the maximum is achieved. However, for the lower ratios (higher PVP content) this shift is not observed; the maximum position remains basically constant along the whole reaction, probably because the plasmon band shifts before the first microwave pulse is finished (10 s). With respect to particle size, regardless of the amount of PVP the average size measured was basically identical. In the TEM images (see Figure 5 and Table 1), we observed mostly particles with sizes between 3 and 4.5 nm, though a small proportion of larger particles with diameters between 20 and 30 nm (ca. 3% of the total amount) was also found. In the experiments at reflux, during the initial stages the spectra showed a shoulder, due to particle aggregation, which is not observed using microwave irradiation, even at the final stages of the reaction. This makes us believe that under microwave irradiation the process of ion complexation by polymer chains is favored, so that the reduction takes place on the PVP chains rather than in solution as happened at reflux. Thus, the PVP chains complex Ag+ ions avoiding their adsorption on the metal particles formed and the corresponding red shift of the band. The formed particles stay on the polymer chains, and their ability to adsorb ions is also lowered. Furthermore, the fact that under microwave irradiation the colloids are more stable can be due to the milder and more uniform reaction conditions, which are less favorable for growth processes such as Ostwald ripening. The fact that under microwave irradiation silver salt and polymer are mixed before starting the irradiation can be responsible for the complexation of polymer and silver ions, while this does not happen at reflux since the salt is added when the PVP solution is already at reflux, so that the reduction can be very fast. We have therefore explored the reduction at reflux mixing silver salt and PVP prior to addition, but parallel reduction processes
Figure 4. Time evolution of UV-visible spectra after addition of AgClO4 0.76 mM onto PVP solutions in DMF under microwave irradiation. Ag+/APS molar ratios are (a) 1.0, (b) 0.45, and (c) 0.1. Irradiation time is indicated.
make the obtained results difficult to interpret. If AgClO4 is added to the PVP solution in DMF prior to heating, the reduction starts well before boiling temperature is reached,28 so that Ag nuclei form which are probably attached to PVP chains when boiling starts. If, alternatively, AgClO4 and PVP are mixed in water at high concentrations, so that a small volume is added to boiling DMF, PVP itself can act as a reductant, so that Ag nuclei are equally formed before addition to DMF. The corresponding spectra for both processes are accessible as Supporting Information (Figures S1 and S2). In these experiments, the stability of the colloids is higher than for the original experiments at reflux, suggesting that indeed the mixing order is of prime importance for the mechanism of particle formation, probably even more important than the heating method chosen. B. Influence of the Polymer Chain Length. A similar study was performed using a different stabilizer (PVPK30, MW 40 000) which only differs in the chain length. Bradley and co-workers35 studied the influence of (35) Bradley, J.-S.; Hill, E.-W.; Behal, S.; Klein, C. Chem. Mater. 1992, 4, 1234.
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Figure 5. TEM micrograph of Ag particles from 0.76 mM AgClO4 with Ag+/PVP ) 1 under microwave irradiation for 5 min.
Figure 7. Top: Extinction spectra calculated using Mie theory for 10 nm Ag particles dispersed in media with refractive indices for water, ethanol, and DMF. Bottom: Experimental spectra of silver particles in water, ethanol, and DMF.
Figure 6. Time evolution of UV-visible spectra after addition of AgClO4 0.76 mM onto PVPK30 solutions in DMF at reflux. Ag+/APS ) 0.45. Time at reflux is indicated.
PVP molecular weight on the size of Pd particles and found no significant effect. However, we have observed that the use of this polymer leads to results that are different in several aspects, which is discussed in what follows. Reflux. First, we compare the absorption spectra obtained with PVPK30 to those obtained using PVPK15, for a silver concentration of 0.76 mM. The more noticeable difference was found in the reaction rate, lower for K30 (compare Figure 6 with Figure 1a). Also, samples prepared with PVPK30 seem to be more stable, as indicated by the time at which the spectra start to show signs of particle aggregation and growth. The bands are sharper and more symmetrical, which reflects more uniform size distributions. In addition, with this stabilizer higher absorbance values are achieved and the plasmon bands are centered at lower wavelengths, which also points toward a smaller particle size. TEM measurements show that the average size for the particles at the initial stages is quite similar for both polymers, while in the final stages the particles obtained with K15 are much larger than those obtained with K30, where the size hardly varies during the process (see Table 1). The fact that the solutions are more stable and the particle size in the final stages is smaller for the larger polymer is directly related to the length of the polymer
chain, which avoids aggregation of the particles, keeping them away from each other. C. Stability in Water and Ethanol. Since for a number of applications it is necessary to use different solvents which are less aggressive than DMF, we tested the stability of silver nanoparticles in water and in ethanol. In general, the observed results were identical for all the experiments, and therefore we just show here the results for a representative sample. When the original DMF dispersions are diluted (20×) with either water or ethanol, a blue shift of the plasmon band is observed. The shift is stronger for the dilution with water than for that with ethanol. According to Mie theory for the optical properties of metal colloids,36,37 the maximum position of the plasmon band red-shifts when increasing the refraction index of the medium in which the particles are embedded. Since the refraction index for water is 1.333 while that for ethanol is 1.361 and that for DMF is 1.426, we expect exactly the trend found experimentally. To illustrate this effect, shown in Figure 7 are the extinction spectra calculated using Mie theory for 10 nm silver particles dispersed in media with the refraction indices for water, ethanol, and DMF. Shown in the same figure are experimental spectra, and although there are some quantitative differences, the tendency is exactly the same. With respect to colloid stability, we observed that the samples diluted with ethanol are in general more stable than those diluted with water. This means that for the aqueous dispersions, after a certain period of time (from a few hours to several days, depending on the amount of (36) Mulvaney, P. Langmuir 1996, 12, 788. (37) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.
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Figure 8. Time evolution of UV-visible spectra after addition of HAuCl4 0.76 mM onto PVP solutions in DMF at reflux. AuCl4-/ PVP ratios are (a) 4.0 and (b) 7.5. Time at reflux is indicated.
PVP, so that for larger amounts of stabilizer the stability is increased) the absorbance at the maximum starts to decrease, the position of the band red-shifts, and a shoulder at higher wavelengths shows up, which can be due to the larger polarity difference between DMF and water than between DMF and ethanol. Increasing polarity reduces the effectiveness of the polymer stabilizer. D. Formation of Au Nanoparticles. In a search for the generalization of this method of metal nanoparticle formation, a further study was performed on the formation of gold nanoparticles under similar conditions. Han et al.38 have recently reported on the preparation of Au particles in formamide, using PVP as the stabilizer. According to these authors, the reduction only takes place in the absence of oxygen and when induced by photochemical methods. In this section, we demonstrate that the reduction with DMF can take place in the presence of dissolved O2, and we discuss the influence of the molar ratio Au/PVP, the length of the polymer chain, and the stability of the obtained colloids in water and ethanol. Influence of PVP Concentration. In all these experiments, we used a constant HAuCl4 concentration of 0.76 mM and PVPK15 (MW 10 000). For this study, we chose three molar ratios Au/PVP (1.0, 4.0, and 7.5) and the reduction was performed at reflux. To study the effect on the reaction rate, the process was monitored through UVvisible spectroscopy. Shown in Figure 8 is the time evolution of the spectra for these three molar ratios. First, the gradual disappearance of the ligand-metal chargetransfer band (LMCT) characteristic of the gold salt, centered around 322 nm, can be observed. (This effect goes along with a change of the solution from pale yellow to colorless.) On the other hand, the most striking (38) Han, M. Y.; Quek, C. H.; Huang, W.; Chew, C. H.; Gan, L. M. Chem. Mater. 1999, 11, 1144.
Figure 9. Time evolution of UV-visible spectra of aliquots extracted at different reaction times during the formation at reflux of Au nanoparticles and subsequently diluted with water. Reflux times are 10 min (a), 2 h (b), and 4 h (c). For each figure, spectra were obtained over a total elapsed time of 5 h after dilution with water. Arrows in the figures indicate increasing time.
observation is the absence of a plasmon band, which can be attributed to the quantum effects that arise when Au particles become very small. Duff et al.39 showed that the plasmon band provides Au sols with diameters larger than ca. 5 nm with an intense red color, while sols with smaller particles do not show any peak or shoulder and therefore are orange-brown. In our case, the samples show during the first 2 h of the reaction an orange color which gets darker with time and the spectra show no plasmon band, so we could estimate a particle size below 5 nm, which was confirmed by TEM (see below). The plasmon band starts to become evident after a couple of hours at reflux in the sample with a lower amount of stabilizer, so we can state that, as was predicted by Buining et al.40 in their study on the preparation of Au colloids stabilized with mercaptopropyl-trimethoxy silane in ethanol, a larger concentration of stabilizer leads to a lower growth rate of (39) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301.
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HAuCl4 solution was chosen (Au/PVP molar ratio ) 1.0) and aliquots were taken at different reaction times at reflux, which were then diluted with water and ethanol (in both cases we used a volume ratio sample/water or ethanol 1:3). Figure 9 shows the results of dilution with water. We can see in the spectra that when the dilution is performed at long reduction times, the samples are more stable. Thus, the aliquot taken after 10 min of reflux shows two bands, centered at 535 and 700 nm, which correspond to two particle populations of Au nanoparticles with different sizes, which grow with time. This effect goes along with a color change of the samples from orange/ yellow to purple. The aliquots taken after up to 2 h show a similar evolution, though the process of band formation gets slower. The samples transferred into water after 4 h of reduction stay completely invariable, even after several months, and no plasmon band is observed. Figure 10 shows TEM images corresponding to a sample taken after 15 min before (a) and after dilution with water (b) clearly showing that the Au particles grow when diluted with water. However, when the dilution was performed with ethanol no changes were observed in the absorption spectra, because of the higher stability (see subsection C above). These samples remained stable for several months.
Figure 10. TEM micrographs of Au nanoparticles from a sample extracted after 15 min at reflux (a) and a sample extracted after 10 min and diluted with water (b).
the particles and in turn a longer time before the appearance of the absorption plasmon band. Similar experiments have been carried out within a microwave oven, and the results are basically the same, with the only difference being that the spectral evolution is smoother and the process can be easily stopped at any intermediate stage. The spectra are provided as Supporting Information (Figure S3). If PVPK30 is used instead of PVPK15, similar results are obtained. In this case, the growth of the gold particles with a plasmon band takes place at comparatively larger PVP concentrations. Spectra for these experiments are also available in the Supporting Information (Figure S4). Stability in Water and Ethanol. Two independent motivations lead us to study the behavior of these systems when they are diluted with water or ethanol. On one hand, its stability can be important toward certain applications, and on the other hand, in case aggregation takes place, we can confirm the presence of gold nanoparticles in the starting colloid in DMF. For this experiment, a 0.76 mM (40) Buining, P. A.; Humbel, B. M.; Philipse, A. P.; Verkleij, A. J. Langmuir 1997, 13, 3921.
Conclusions The reduction of silver salts with DMF in the presence of PVP leads to the formation of Ag nanoparticles that are stable in solution. In general, reactions performed under microwave irradiation are much easier to control than reactions at reflux. Use of PVP with a larger chain length leads to sharper and more symmetrical UV-visible spectra as well as to more stable dispersions. The solutions can be diluted in ethanol without affecting their stability, but in water the solubility is only marginal, as reflected in a red shift and broadening of the spectra after a certain time. We have also prepared Au nanoparticles in DMF. The absence of a plasmon band suggests that the particles are very small. The stability of these colloids increases as the reaction proceeds, which was tested by transfer into water. At short reaction times, growth and aggregation are observed, while at longer times the particles are completely stable. Acknowledgment. This work has been supported by the Spanish Xunta de Galicia (Project No. PGIDT01PXI30106PR) and Ministerio de Educacio´n y Cultura (Project No. PB98-1088). I.P.S. acknowledges the Spanish Ministry of Education and Culture for a personal grant. The authors are indebted to J.B. Rodrı´guez from the CACTI of Vigo University for his assistance with TEM measurements. Special thanks go to Ana Sa´nchez for her assistance with last-minute test experiments. Supporting Information Available: Spectral evolution during Ag reduction at reflux with mixing of AgClO4 and PVP prior to heating and with simultaneous addition at reflux; spectral evolution during Au reduction under microwave irradiation with PVPK15 and at reflux with PVPK30. This material is available free of charge via the Internet at http://pubs.acs.org. LA015578G
