Optical studies of polyvinylpyrrolidone reduction effect on free and complex metal ions

Optical studies of polyvinylpyrrolidone reduction effect on free and complex metal ions Caixia Kan, Weiping Cai, a) Cuncheng Li, and Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academia of Sciences, Hefei 230031, People s Republic of China (Received 11 May 2004; accepted 27 August 2004) Polyvinylpyrrolidone (PVP) reduction effect on free and complex Ag + and Au 3+ ions was studied from optical measurements by adding a metal precursor (K-30), commonly used as a stabilizer, to PVP. It was found that PVP has a strong reduction effect on free ionic metal, such as Ag + ion in AgNO 3, but much weaker on complex ionic metals, AuCl 4 in HAuCl 4 and Ag(NH 3 ) 2 + in Ag(NH 3 ) 2 OH. This is explained based on the coordinative field of polar group in PVP molecules. I. INTRODUCTION Since the time when Faraday presented the first preparation method for metal nanoparticles in an aqueous medium, 1 a large number of methods have been developed for the synthesis of metal nanoparticles, involving use of different protecting agents, such as polymers and alkanethiol with different alkyl chain lengths. 2 6 Among the polymers available, the synthetic polymer polyvinylpyrrolidone (PVP) is widely used because of its chemical stability, avirulence, and dissolubility in many solvents. Due to its affinity toward metals, PVP polymer was often used to obtain stable dispersions of metal (especially Au, Ag and platinum group) nanoparticles synthesized by many methods, such as irradiation, reflux, and addition of chemical reductants. 7 14 Generally, the preparation of PVP polymer-stabilized nanoparticles (through chemical methods) involves two processes: reduction of metal ions into neutral atoms, which form clusters, and coordination of the polymer to the metal clusters. In experiments, the focus is generally fastened on the protective effect of PVP by varying its amount (the molar ratios of metal to PVP usually varies from 0.1 to 4). Even if PVP had a reduction effect on metal ions, such an effect was usually ignored (or could not be detected) because in previous works PVP was added to solution as a stabilizer usually together with reducing agents. Recently, we studied the reduction effect of PVP on metal ions (such as Au 3+ and Ag + ) simply by mixing PVP with the metal ion solutions from optical measurements and found that PVP has a strong reduction effect on the free metal ions but weak on the complex metal ions. The formation amount of Ag or a) Address all correspondence to this author. e-mail: wpcai@issp.ac.cn DOI: 10.1557/JMR.2005.0039 Au in PVP assumes a roughly linear increase with time, and the formation rate of Ag is three orders of magnitude higher than that of Au in PVP. The details are reported in the following. II. EXPERIMENTAL PVP-K30 (molecular weight 40,000), AgNO 3 and HCl AuCl 3 H 2 O were purchased from the Chemical Reagent Company of Shanghai, China, and used as received. Distilled water was used as the solvent. Ag(NH 3 ) 2 OH was obtained at our lab through the following reactions 2AgNO 3 + 2NH 4 OH Ag 2 O + 2NH 4 NO 3 + H 2 O, Ag 2 O + 4NH 4 OH 2 Ag(NH 3 2 OH + 3H 2 O. AgNO 3 (0.2 ml) and Ag(NH 3 ) 2 (OH) aqueous solutions (both [Ag + ] and [Ag(NH 3 ) 2 ] + are 0.1 M/L) were mixed with 400 mg PVP (molar ratio of Ag/PVP is 2). The ph values were 7 and 8 9, respectively for AgNO 3 and Ag(NH 3 ) 2 (OH) solutions. Similarly, the mixtures of HAuCl 4 /PVP were obtained by dropping 0.2 ml HAuCl 4 aqueous solutions (with [HAuCl 4 ] of 0.08 M/L and 0.02 M/L, ph 1 3) to 400 mg PVP (Au/PVP molar ratios are 1.6 and 0.4, respectively). All these mixtures were prepared on well-cleaned glass slides (25 25 1.2 mm 2 ). The mixtures become transparent gelatins in a short time (<5 min) and lose fluidity. All samples have a similar size ( 0.2 mm in thickness and 20 25 mm 2 ). In addition, the solutions (0.2 ml) of AgNO 3, Ag(NH 3 ) 2 OH, and HAuCl 4 were dropped on separate glass substrates without PVP, followed by evaporation at room temperature (about 30 C) for a long time ( 20 h) or drying at 80 C for 5 h for reference. 320 J. Mater. Res., Vol. 20, No. 2, Feb 2005 2005 Materials Research Society

After the mixtures on the substrates became gelatin, optical absorption spectra were immediately measured as a function of time on a Cary 5E UV-Vis-NIR, Tokyo, Japan, spectrophotometer over the wavelength range from 200 to 1000 nm at the room temperature. Microstructure of the samples was examined by transmission electron microscope (TEM; JEOL-2010, Tokyo, Japan). III. RESULTS After the mixtures loaded on the substrates became gelatin, the color evolves from colorless to yellow and violet for both AgNO 3 PVP and Ag(NH 3 ) 2 (OH) PVP, but the former changes much more quickly. Figure 1 shows optical absorption evolution with time. For the AgNO 3 PVP sample [Fig. 1(a)], an absorption peak around 425 nm corresponding to the surface plasmon resonance (SPR) of Ag nanoparticles 15,16 appears and rapidly increases with time, accompanied by a small blue shift, indicating the reduction of Ag + ions and formation of Ag particles in the system. However, in the case of Ag(NH 3 ) 2 (OH) PVP system, the SPR of Ag nanoparticles is detectable after 90 min, as shown in Fig. 1(b). The inset of Fig. 1(b) demonstrates the optical absorption spectrum of this composite after 20 h, which is comparable to that of the former sample after about 1 h. The corresponding reference samples show no SPR of Ag nanoparticles after holding at room temperature for a long time (>20 h) or even after drying at 80 C for about 5 h, as illustrated in Fig. 1(c). We can thus deduce that PVP polymer has a reduction effect on the metal ions and that the reduction effect is strong on free Ag + ions but much weaker on its complex ions. For the HAuCl 4 PVP system, the color of the sample changes gradually from yellow to colorless, and the colorlessness turns very slowly to wine red and blue. Figure 2(a) shows time evolution of the optical absorption spectra for the 0.02 M HAuCl 4 PVP composite. In the initial stage, there is an absorption peak around 320 nm. This peak directly originates from the d-d transition of the AuCl 4 ions, which is a unique fingerprint for the AuCl 4 ions and has been reported. 17 This peak decreases with time and disappears after 24 h, indicating reduction of AuCl 4 ions. About two weeks later, another peak around 550 nm, which originates from SPR of Au nanoparticles, appears and enhances. 15 As for the HAuCl 4 PVP composite with higher Au + ion concentration (0.08 M), the spectrum evolves similarly but more slowly. The SPR of Au particles is located around 580 nm, as illustrated in Fig. 2(b). Correspondingly, the reference sample without PVP shows no SPR of Au nanoparticles but the almost unchanged absorption peak from AuCl 4 ions, as shown in Fig. 2(c), which also provides indirect proof of the reduction effect of PVP on Au 3+ ions. FIG. 1. Time evolution of optical absorption spectra for the mixture (a) AgNO 3 -PVP and (b) Ag(NH 3 ) 2 OH PVP with [Ag + ] of 0.1 M. (c) Optical spectra of AgNO 3 and Ag(NH 3 ) 2 OH without PVP, sprayed on glass slides after evaporation at room temperature for >20 h (solid) and 80 C (dash) for 5 h. TEM examination was performed for the samples with Ag and Au after optical measurement. TEM observation indicates that the formed Ag nanoparticles are nearly spherical with size distribution of 10 20 nm for both samples of AgNO 3 PVP and Ag(NH 3 ) 2 (OH) PVP, while the formed Au particles have different shapes with larger size, up to hundreds of nanometers, as shown in Fig. 3. Further analysis indicates that the peak height for Ag and Au SPR assumes roughly linear evolution with J. Mater. Res., Vol. 20, No. 2, Feb 2005 321

FIG. 3. TEM images of (a) Ag particles in the AgNO 3 PVP sample and (b) Au particles in HAuCl 4 PVP with [HAuCl 4 ] of 0.08 M after final optical measurement. FIG. 2. Time evolution of optical absorption spectra for HAuCl 4 PVP mixture with [HAuCl 4 ] of (a) 0.02 M and (b) 0.08 M. (c) Optical spectra of HAuCl 4 solution (0.02 M) sprayed on glass slide after holding at room temperature for 7 days (the curve is almost unchanged after a longer time). time during the period studied, as shown in Fig. 4. Since the peak height of SPR is direct proportional to amount of metal nanoparticles, 15 Fig. 4 also demonstrates the formation kinetics of Ag or Au metal. Obviously, from the slope values of plots in Fig. 4, we can know that the formation rate of Ag particles in AgNO 3 PVP is three orders of magnitude higher than that of Au in PVP. The formation rate of Au particles in 0.02 M HAuCl 4 PVP sample is twice that of the 0.08 M HAuCl 4 PVP sample. In addition, the plots in Fig. 4 do not go through 0 on the time axis, which means that there is an incubation time before metal nanoparticles are formed during which reduction of metal ions occurs (see decrease of the peak at 320 nm in Fig. 2). Obviously, the incubation time is much longer for the sample HAuCl 4 PVP than it is for the samples AgNO 3 PVP and Ag(NH 3 ) 2 (OH) PVP. Additionally, we found that the reduction effect of PVP on metal ions, especially on the AuCl 4 ions, is temperature dependent. Higher temperature induces faster reduction. When the HAuCl 4 PVP system (0.02 M) was held at about 60 C for several minutes, the SPR of Au nanoparticles appeared in the optical spectrum. However, if the composite was held at a temperature lower than 15 C, the reduction phenomenon could not be detected from the optical spectrum, even after 60 days. IV. DISCUSSION From the experimental results above, we believe that PVP has obvious reduction effect on Ag + and Au + ions. 322 J. Mater. Res., Vol. 20, No. 2, Feb 2005

FIG. 5. Infrared transmission spectra for PVP and PVP-AgNO 3 after different time. FIG. 4. Evolution of the SPR peak height with time for (a) Ag PVP and (b) Au PVP [data from Figs. 1(a), 2(a), and 2(b), respectively]; (b) triangle: [HAuCl 4 ] is 0.08 M, circle: [HAuCl 4 ] is 0.02 M. It is known that PVP has an affinity toward many chemicals to form coordinative compounds because it has structure of a polyvinyl skeleton with strong polar group- (pyrrolidone ring). The reduction effect of PVP on the metal ions (M n+ ) can be explained in the following. Since the nitrogen and oxygen in the polar group of PVP have a strong coordinative field, when M n+ solution is mixed with PVP, the coordinative complex between metal ions and PVP will be formed. For Ag + ions, for instance, the reaction proceeds as 18,19 As we know from the organic chemistry, metal ions (such as Ag + ) can be reduced by carbonyl. For instance, R CHO+Ag + R HCO +Ag. Here correspondingly, PVP reacts with the metal ions, in which M n+ ions may receive electronic clouds from the ligand of N and C O in pyrrolidone ring to form atomic metal that grow into metal particles. During the reaction, PVP is modified, which has been confirmed by infrared spectra. Figure 5 shows the results of PVP and PVP-AgNO 3 film after different time. After the reaction, the peak of C N in PVP disappeared and the C O peak at 1663 was slightly widened and shifted to approximately 1635. For the Ag PVP system, the sp hybridized Ag + ions in the AgNO 3 /PVP coordinate complex get electrons from both oxygen and nitrogen atoms of PVP and form Ag atoms. After a short incubation time, the Ag atoms form crystal nuclei and grow further into Ag nanoparticles. However, in the case of the [Ag(NH 3 ) 2 ]OH PVP system, because most Ag + ions are embedded in the coordination units (NH 3 ), the [Ag(NH 3 ) 2 ] + ions are stable in basic solution. Nevertheless, when the [Ag(NH 3 ) 2 ]OH solution was added to the PVP polymer, free Ag + ions will be gradually produced due to the acidity of PVP solution [The ph value of PVP(k-30) solution (5%) ranges from 5 to 7, according to the product information given by supplier]. Therefore, the reduction process of the coordinate [Ag(NH 3 ) 2 ] + ions in the [Ag(NH 3 ) 2 ]OH can proceed, but it is slower than that of free Ag + ions in AgNO 3. As for the HAuCl 4 PVP system, because aqueous HAuCl 4 solution has a strong acidity, only very few Au 3+ ions were ionized from the stable AuCl 4 ions at room temperature (the probability of Au 3+ ionized from AuCl 4 ions increases with temperature). As a result, the complete reduction of Au 3+ in AuCl 4 needs a long time, which can be deduced from the slow decrease of the AuCl 4 ion absorption peak (Fig. 2). In addition, the higher HAuCl 4 concentration will induce stronger acidity in the aqueous solution, which will decrease the electronic transfer rate 20 and hence the reduction rate of Au 3+ ions [Fig. 2(b)]. The reduced Au 0 atoms will aggregate J. Mater. Res., Vol. 20, No. 2, Feb 2005 323

into clusters by diffusion in PVP and form Au nanoparticles after a longer incubation time, inducing the appearance and enhancement of the SPR. Obviously, the diffusion rate of Au atoms in PVP is much lower than that of Ag in PVP. V. CONCLUSIONS In summary, the reduction effect of PVP on free and complex metal ions has been reported from optical measurements. PVP has a strong reduction effect on free metal ions due to strong coordinative field of PVP, but a comparatively weak effect on the complex metal ions due to the metal ions embedded in coordination units in the complex. ACKNOWLEDGMENTS This work was co-supported by the National Natural Science Foundation of China (Grant No. 50271069), and national 973 Project of China (Grant No. G1999064501). REFERENCES 1. M. Faraday: Experimental relations of gold (and other metals) to light. Philos. Trans. R. Soc. London 147, 145 (1857). 2. M. Giersig and P. Mulvaney: Preparation of ordered colloid monolayers by electrophoretic deposition. Langmuir 9, 3408 (1993). 3. M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, and R. Whyman: Synthesis of thiol derivatised gold nanoparticles in a two phase liquid/liquid system. J. Chem. Soc. 801 (1994). 4. M.P. Pileni: Nanocrystal self-assemblies: fabrication and collective properties. J. Phys. Chem. B 105, 3358 (2001). 5. E. Hao and T. Lian: Buildup of polymer/au nanoparticles multilayer thin films based on hydrogen bonding. Chem. Mater. 12, 3392 (2000). 6. F.K. Liu, S.Y. Hsieh, T.C. Chu, and B.T. Dai: Synthesis of nanometer-sized poly (methyl methacrylate) polymer network by gold nanoparticle template. Jpn. J. Appl. Phys. 42, 4147 (2003). 7. N. Toshima: Metals reactions in homogenous solutions, in Fine Particles Synthesis, Characterization, and Mechanisms of Growth, edited by T. Sugimoto (Institute for Advanced Materials Proc., Marcel Dekker, New York, 2000), p. 439. 8. Y.G. Sun, B. Gates, B. Mayers, and Y.N. Xia: Crystalline Silver nanowires by soft solution processing. Nano Lett. 2, 165 (2002). 9. P.Y. Silvert, R.H. Urbina, and K.T. Elhsissen: Preparation of colloidal silver dispersions by the polyol process. Part 2: Mechanism of particles formation. J. Mater. Chem. 7, 293 (1997). 10. D.G. Duff, A. Baiker, and P. Edwards: New hydrosol of gold clusters. 1. Formation and particle size variation. Langmuir 9, 2301 (1993). 11. I.P. Santos and L.M. Marzán: Formation of PVP-protected metal nanoparticles in DMF. Langmuir 18, 2888 (2002). 12. Y.G. Sun, B. Mayers, T. Herricks, and Y.N. Xia: Polyol synthesis of uniform silver nanowires: A plausible growth mechanism and the supporting evidence. Nano Lett. 3, 955 (2002). 13. G. Carotenuto, S. DeNicola, and L. Nicolais: Spectroscopic study of the growth mechanism of silver microclusters. J. Nanopart. Res. 3, 469 (2001). 14. M.Y. Han, C.H. Quek, W. Huang, C.H. Chew, and L.M. Gan: A simple and effective chemical route for the preparation of uniform nonaqueous Au colloids. Chem. Mater. 11, 1144 (1999). 15. U. Krebig and M. Vollmer: Theoretical considerations single clusters: Intrinsic size effects of the optical properties, in Optical Properties of Metal Clusters, edited by U. Gonser, R.M. Osgood, Jr. M.B. Panish, and H. Sakaki (Springer-Verlag, Berlin and Heidelberg, Germany, 1995), p. 83. 16. C.F. Bohren and D.R. Huffman: Surface modes in small particles, in Absorption and Scattering of Light by Small Particles, edited by C.F. Bohren and D.R. Huffman (J. Wiley & Sons Inc., New York, 1983), p. 373. 17. K. Esumi, J. Hara, N. Aihara, K. Usui, and K. Torigoe: Preparation of aniosotropic gold particles using a Gemini surfactant. J. Colloid Interface Sci. 208, 578 (1998). 18. G.D. Zhou and L.Y. Duan: Structure and properties of coordination composites, in Structural Chemistry, edited by G.D. Zhou and L.Y. Duan (Beijing University Press, Beijing, China, 1995), p. 288. 19. Z.T. Zhang, B. Zhao, and L.M. Hu: PVP protective mechanism of ultrafine silver powder synthesized by chemical reduction processes. J. Solid State Chem. 121, 105 (1996). 20. M. Gratzel and A.J. Frank: Interfacial electron-transfer reactions in colloidal semiconductor dispersions. Kinetic analysis. J. Phys. Chem. 86, 2964 (1982). 324 J. Mater. Res., Vol. 20, No. 2, Feb 2005