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Photochemical Formation of Silver Nanoparticles in Poly(N-vinylpyrrolidone) H. H. Huang,*,† X. P. Ni,† G. L. Loy,‡ C. H. Chew,† K. L. Tan,§ F. C. Loh,§ J. F. Deng,| and G. Q. Xu*,† Department of Chemistry, National University of Singapore, Singapore 0511, Department of Zoology, National University of Singapore, Singapore 0511, Department of Physics, National University of Singapore, Singapore 0511, and Department of Chemistry, Fudan University, Shanghai, People’s Republic of China Received June 5, 1995. In Final Form: August 30, 1995X Silver nanoparticles have been prepared by photoreduction of silver nitrate with 254 nm UV light in the presence of poly(N-vinylpyrrolidone). The effects of PVP concentration on the particle size, the UV-vis absorption peak, and the rate of the photoreduction process were studied. The average particle size ranged from 15.2 to 22.4 nm, with the corresponding UV-vis absorption peak position at 404-418 nm in 1-0.25 wt % PVP. The rate of the photoreduction process was observed to increase with the PVP concentration. X-ray photoelectron spectroscopic studies further revealed that the polymer interacts with silver particles through the oxygen atom in the >CdO group. A negative shift of binding energy in the Ag 3d5/2 for silver nanoparticles was observed.
1. Introduction Synthesis of nanoparticles is an interesting field in solidstate chemistry.1 Because of their small size, these crystallites exhibit novel material properties which largely differ from the bulk properties.1c The chemical reactivity of small metallic particles is strongly dependent on particle size. For preparation of metal particles, metal ions have often been reduced in protective colloids. As protective media for colloids, water-soluble polymers such as poly(vinyl alcohol), poly(N-vinylpyrrolidone), and poly(methyl vinyl ether) have been used.2-6 Silver nanocrystallites are particularly interesting due to their roles as substrates in studies of surface-enhanced Raman scattering,7 nonlinear optical properties,8 and catalysis.9,10 Esumi et al.11 have prepared colloidal silver in the presence of copolymers of vinyl alcohol and N-vinylpyrrolidone using various reductants. It was found that fine particles cannot be easily obtained in homopolymers. Hirai et al.5 obtained colloidal silver by reduction of silver nitrate with methanol and sodium hydroxide in the presence of * To whom correspondence shoud be addressed. † Department of Chemistry, National University of Singapore, Singapore 0511. ‡ Department of Zoology, National University of Singapore, Singapore 0511. § Department of Physics, National University of Singapore, Singapore 0511. | Department of Chemistry, Fudan Universtry, Shanghai, People’s Republic of China. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) (a) Turkevich, J.; Stevenson, P.; Hillier, J. J. Phys. Chem. 1953, 57, 670. (b) Turkevich, J.; Hillier, J. Anal. Chem. 1949, 21, 475. (c) Ozin, G. A. Adv. Mater. 1992, 4, 612. (2) Dunworth, W. P.; Nord, F. F. Adv. Catal. 1954, 6, 125. (3) Kiwi, J., Gra¨tzel, M. J. Am. Chem. Soc. 1979, 101, 7214. (4) Brugger, P. A.; Guendet, P.; Gra¨tzel, M. J. Am. Chem. Soc. 1981, 103, 2923. (5) Hirai, H. J. Macromol. Sci., Chem. 1979, A13, 633. (6) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1979, A13, 727. (7) Pileth, W. J. J. Phys. Chem. 1982, 86, 3461. (8) Hache, F.; Richard, D.; Flytzanis, C. J. Opt. Soc. Am. 1986, B3, 1647. (9) Henglein, A.; Lillie, J. J. Phys. Chem. 1981, 85, 1246. (10) Henglein, A. J. Phys. Chem. 1980, 84, 3461. (11) Esumi, K.; Ishizuki, N.; Torigoe, K.; Nakamura, H.; Meguro, K. J. Appl. Poly. Sci. 1992, 44, 1003.
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poly(N-vinylpyrrolidone). Esumi et al.12 have also reported the preparation of bimetallic Ag-Pd colloids by UV irradiation in the presence of poly(N-vinylpyrrolidone), but stable pure Ag colloids were not formed. In this work, stable colloidal silver has been prepared by reduction of silver nitrate with 254 nm UV light in the presence of poly(N-vinylpyrrolidone). The silver colloids have been characterized by transmission electron microscopy (TEM) and UV-vis spectra. The interaction between the polymer chain and silver nanoparticles was studied by X-ray photoelectron spectroscopy (XPS). 2. Experimental Section 2.1. Materials. Silver nitrate (AgNO3, analytical reagent) was obtained from Merck. Poly(N-vinylpyrrolidone) (PVP) with a weight-average number of 40 000 (Scientific Polymer Products) was used as received. Deionized water was prepared with a Milli-Q water purification system. 2.2. Preparation of Colloidal Silver. A 3 mL portion of aqueous AgNO3 (0.23-2.3 mM) containing 0.25-1 wt % PVP was placed in a rectangular quartz vessel (1 × 1 × 3.2 cm). The sample was placed in a photochemical reactor (Rayonet) and irradiated by 254 nm UV light with four 40 W low-pressure mercury lamps at room temperature. The incident intensity of 254 nm light was 1.28 × 1015cm-2 s-1, as measured by a Melles Griot energy meter. 2.3. Characterization of Colloidal Silver. Optical spectra were recorded using a HP 8452A diode array spectrophotometer at 1 cm of path length. The quartz cell was the same as that used for photolysis. To obtain the distribution of particle size, a drop of the sample was placed on a copper grid coated with a thin layer of Formvar. Electron micrographs were taken with a JEOL JEM-100CXII electron microscope operating at 100 kV. The particle sizes were determined from the maximum length of the particles. X-ray photoelectron spectra were recorded using a VG ESCA/SIMSLAB MKII spectrometer with a Mg KR radiation source. The spectra were fitted using Gaussian curves of constant full-width at half-maximum (fwhm). The amount of reduced silver at various irradiation times was determined as follows. After irradiation, the samples were dialyzed in a semipermeable tube (Spectra/Por dialysis tube) with a large excess of water for 24 h, and the colloid particles remaining were dissolved in concentrated HNO3 and then diluted to 25 mL. Quantitative analysis of silver in the solution was conducted with a Shimadzu AA-670 spectrophotometer. (12) Torigoe, K.; Esumi, K. Langmuir 1993, 9, 1664.
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Figure 1. UV-vis spectra of silver colloids prepared from UV reduction of 0.23 mM AgNO3 in various PVP concentrations: (a) 1 wt %, (b) 0.5 wt %, (c) 0.25 wt %.
3. Results and Discussion 3.1. Effects of PVP and AgNO3 Concentrations. UV-vis absorption spectra have been proved to be quite sensitive to the formation of silver colloids.13 An intense absorption peak around 400 nm was generally observed and attributed to the surface plasma excitation of silver particles. Figure 1 compares the absorption spectra obtained for three different PVP concentrations after complete photoreduction using 254 nm light. The peak positions for curves a, b, and c were determined to be at 404, 412, and 418 nm, corresponding to PVP concentrations of 1, 0.5, and 0.25 wt %, respectively. That the peak is shifted toward red on decreasing the PVP concentration is clearly demonstrated in the figure. Mie14 and more recently Wang et al.15 have carried out theoretical studies on the dependence of the UV absorption maximum on the size of metal spheres. The general trend they found is that the absorption peak shifts toward longer wavelengths as particles become bigger.16 Our spectra in Figure 1 are quite different from the absorption features attributed to multiple polarization17 or dipole-dipole interaction18,19 in large aggregates of small particles. The aggregation of colloidal silver causes a decrease in the intensity of the main peak around 400 nm and also results in a long tail at the long-wavelength side of the peak.16,26 Furthermore, (13) (a) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (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. (d) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014. (e) Kerker, M.; Siiman, O.; Wang, D. S. J. Phys. Chem. 1984, 88, 3186. (14) Mie, G. Ann. Phys. 1908, 25, 377. (15) Wang, D. S.; Kerker, M.; Chew, H. Appl. Opt. 1980, 19, 2135. (16) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (17) Kreibig, U.; Zacharias, P. Z. Phys. 1970, 231, 128. (18) Kreibig, U.; Quinten, M.; Schoenauer, D. Physica A 1989, 157, 244. (19) Quintin, M.; Kreibig, U. Surf. Sci. 1986, 172, 557. (20) Fornasiero, D.; Grieser, F. J. Colloid Interface Sci. 1991, 141, 168. (21) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014. (22) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K. J. Chem. Soc., Faraday Trans. 1 1991, 87 (12), 1905. (23) Chastain, J. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation Physical Electronics Division: Eden Prairie, MN, 1992. (24) Hammond, J. S.; Winograd, N. J. Eletronanal. Chem. 1977, 123, 80. (25) Hada, H.; Yonezawa, Y.; Yoshida, A.; Kurakake, A. J. Phys. Chem. 1976, 80, 2728. (26) Fornasiero, D.; Grieser, F. J. Colloid Interface Sci. 1991, 141, 168.
Figure 2. Transmission electron micrographs and corresponding size histograms of silver particles obtained by UV reduction of 0.23 mM AgNO3 in various PVP concentrations: (a) 1 wt %, (b) 0.5 wt %, (c) 0.25 wt %. Table 1. Geometrical and Optical Parameters of Silver Colloids Prepared in a 0.23 mM AgNO3 Solution with Different PVP Concentrations PVP wt %
λmax
D (nm)
σ
1 0.5 0.25
404 412 418
15.2 18.9 22.4
3.2 3.8 6.3
theoretical calculations18 have shown that the dipolar adsorption is dominant for particles smaller than 20 nm. Above 30 nm, dipolar scattering and quadrupolar adsorption play important roles, accompanied by a significant broadening in the absorption peak. However, in our experiments only one symmetric absorption peak around 400 nm was observed, which is mainly attributable to primary dipolar excitation. This is further confirmed by our TEM results on the average particle sizes (Table 1). To confirm this correlation between absorption maximum and particle size, TEM micrographs were taken on those three different colloidal samples (Figure 2). To obtain size distributions of silver colloids, approximately 200300 particles were counted and then combined into histograms (Figure 2). The standard deviation, σ, was calculated based on the experimentally determined distributions and is listed in Table 1. The average sizes of silver colloids are 15.2, 18.4, and 22.4 nm, corresponding to the UV-vis absorption peaks at 404, 412, and 418 nm, respectively. This experimental result is in good agreement with the theoretical calculations.16 Furthermore, our work is also consistent with other experimental studies. Grieser et al.20 obtained 8 nm silver colloids with an absorption peak at 397 nm. Siiman21 prepared 13 nm colloidal silver with an absorption peak centered at 400 nm. Our results also revealed that the particle size and distribution are strongly dependent on PVP concentrations, which is clearly seen in Table 1 and Figure 2. On
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Figure 4. Amount of reduced silver as a function of irradiation time in different concentrations of PVP.
Figure 3. Size histograms of silver particles obtained by UV reduction of various concentrations of AgNO3 in 1 wt % PVP: (a) 0.38 mM, (b) 1.15 mM, (c) 2.30 mM. Table 2. Dependence of Amount of Silver Reduced and Particle Size on Irradiation Time in a 0.23 mM AgNO3 Solution Containing 0.5 wt % PVP irradiation time (min)
amount of reduced silver (µmol)
D (nm)
σ
30 60 120 200
0.09 0.24 0.43 0.61
10.4 15.2 18.9
4.2 4.1 3.8
decreasing PVP concentration from 1 to 0.25 wt %, the average particle size changes from 15.2 to 22.4 nm. In addition, the width of the distribution, σ, increases from 3.2 to 6.3 nm. PVP is used as a protective medium for the colloids in this experiment. It affects the molecular motion of reduced silver and subsequently limits the aggregation of colloids. Therefore, it is expected that smaller colloids are formed in a more concentrated PVP solution. The dependence of silver particles on AgNO3 concentration was also studied using TEM. The size distributions for different AgNO3 concentrations are shown in Figure 3. The experimental result indicates that the concentration of AgNO3 does not affect the particle distribution significantly. However, the diameter of particles increases from 15.6 to 18.3 nm as the AgNO3 concentration changes from 0.38 to 2.3 mM, which is not remarkable compared with the effect of polymer concentration seen in Figure 2. 3.2. Time-Dependence Studies. The photochemical formation of silver colloids was monitored by taking UVvis absorption spectra as a function of irradiation time. After a brief exposure of 254 nm (10 min), an absorption peak at about 400 nm can be clearly observed. On increasing the irradiation time, the absorption peak gradually shifts toward longer wavelengths and its intensity is steeply enhanced. This is characteristic of an increase in the concentration as well as the size of silver colloids. To further verify this point, the average particle
size and the total amount of reduced silver were measured at different photoreduction times, shown in Table 2. Two trends can be noticed. The amount of silver reduced and the extent of silver aggregation increase with the irradiation time of 254 nm light. The average particle size changes from 10.4 to 18.9 nm after 60 and 200 min exposures, respectively. The distribution of particles seems to be narrower with longer irradiation, as evidenced by the changes in σ values. After a certain irradiation time, 200 min in this particular case, the UV absorption spectrum reaches a state of stagnation. No further changes either in intensity or in maximum absorption wavelength can be observed. This is probably due to the completion of the photoreduction process. The amount of silver reduced after 200 min exposure was measured to be 0.61 µmol, which is close to the initial value of 0.69 µmol of Ag+ contained in the original solution. The rate of the photoreduction process was also studied as a function of PVP concentration. The amount of reduced silver versus time is plotted in Figure 4. It is readily noticed that a higher concentration of PVP results in a faster photochemical process, which implies the participation of PVP in silver reduction. PVP contains a functional group of >CdO which absorbs the 254 nm light. The excited species >CdO* can reduce Ag+ to Ag in the solution. An alternative reaction pathway could be the oxidation of ground state polymer carbonyl species by the excited Ag+, forming carboxylic acid. Previous studies have shown that the photoreduction of Ag+ in inorganic aqueous and alcoholic solutions was indeed initiated by the excited Ag+.22,25 However, our IR spectra obtained in both solution and solid forms after photoreduction did not show any vibrational features of carboxylic acid. Furthermore, the absorption intensity of the polymer at 254 nm was found to be 3 orders of magnitude higher than that of silver ions under our experimental conditions. These results suggest that the reaction between the photoexcited carbonyls and silver ions may play more significant roles in the photoreduction. This mechanism is consistent with previous work,22 in which Yonezawa and his co-workers demonstrated that the presence of acetone in the solution can speed up the photoreduction of silver ions. The main intermediate proposed by them was acetone ketyl radical formed by excitation of (CH3)2CO with 253.7 nm light. A similar reactive radical species could also be formed in our reaction system, but on polymer chains. 3.3. Interaction of Silver Colloids with PVP Chains. Studies on the nature of the interaction between silver particles and polymer chains are important in understanding the protected colloids. Polymers are expected to be adsorbed on the surface of silver particles
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Figure 5. X-ray photoelectron spectra of pure PVP.
to prevent further aggregation. In order to obtain some detailed knowledge about this chemisorption, X-ray photoelectron spectroscopic studies were carried out. The XPS spectra for pure PVP and silver colloids in PVP are shown in Figures 5 and 6, respectively. All the binding energies are referenced to C 1s (284.5 eV). For pure PVP, three peaks at 284.5, 285.5, and 287.1 eV can be fitted into the C 1s envelope. On the basis of the electronegativity of the substituents on the carbon atom, these peaks can be readily assigned. The more electronegative substituents would withdraw electron density from the carbon atom, which subsequently causes lowering in the initial state energy of C 1s and results in a higher binding energy of 1s electrons. The detailed assignment is listed in Table 3. The percentage area under each peak is a direct reflection of the relative abundance of the carbon species. Our experimental peak ratio is in good agreement with that expected from our assignment. The O 1s and N 1s spectra exhibit single peaks only at 530.7 and 399.3 eV, respectively, assigned to N and O atoms in N–C
O
In the system of PVP containing silver colloids, no changes in C 1s and N 1s photoemissions can be found, which implies that these atoms do not directly interact with silver particles. However, the O 1s photoemission spectrum can be resolved into two peaks. One is at 530.7 eV, which is similar to that of pure PVP; the other, at 531.6 eV, is attributed to the interaction between oxygen and silver particles. The peak of Ag 3d5/2 was found to be at 367.5 eV, which is significantly lower than 368.3 eV, the value for silver metal.23 The adsorption of the oxygen atom in +δ
C
–δ
O
on the silver particle will induce an image dipole on the particle surface, that is, partially positive charges present on the surface of the silver colloids. It is possible that the binding energies of O 1s and Ag 3d5/2 are predominantly governed by the final state effect. The electrostatic
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Figure 6. X-ray photoelectron spectra of silver colloids prepared from 0.23 mM AgNO3 in 1 wt % PVP. Table 3. Binding Energy Values of Pure PVP and Silver Colloids in PVP Prepared from 0.23 mM AgNO3 in 1 wt % PVP
interaction between the final ion with its environment will cause an upper shift in O 1s binding energy, but lower for Ag 3d5/2. This further confirms the previous observation of a negative shift of 0.65 eV in Ag 3d5/2 for very small Ag particles deposited onto Pt electrodes.24 Conclusion Silver colloids have been prepared by photoreduction of silver nitrate with 254 nm UV light in the presence of poly(N-vinylpyrrolidone). Both particle size and the UVvis absorption peak are strongly dependent on the PVP concentration. The average particle size ranged from 15.2 to 22.4 nm, with the corresponding UV-vis absorption peak position at 404-418 nm in 1-0.25 wt % PVP. In addition, PVP is seen to have a significant effect on the silver reduction and may actually participate in the process. The XPS results are consistent with the polymer interacting with silver particles through the oxygen atom in >CdO. A negative shift of binding energy in Ag 3d5/2 for silver nanoparticles was detected. Acknowledgment. This work was supported by the National University of Singapore under Grant No. RP910681. LA950435D
