+
+
172
Langmuir 1997, 13, 172-175
Synthesis, Characterization, and Nonlinear Optical Properties of Copper Nanoparticles H. H. Huang,*,† F. Q. Yan,† Y. M. Kek,† C. H. Chew,† G. Q. Xu,*,† W. Ji,*,‡ P. S. Oh,‡ and S. H. Tang‡ Department of Chemistry, National University of Singapore, Singapore 119260, and Department of Physics, National University of Singapore, Singapore 119260 Received June 5, 1996. In Final Form: October 25, 1996X In this paper, copper nanoparticles were prepared by the reduction of copper(II) acetate in water and 2-ethoxyethanol using hydrazine under reflux. The synthesized nanoparticles exhibit a distinct absorption peak in the region 572-582 nm. The average size variation from 6.6 to 22.7 nm in ethoxyethanol and from 15.5 to 30.2 nm in water was achieved by the addition of various amounts of a protective polymer (poly(N-vinylpyrrolidone)). The nonlinear optical properties of the copper colloids were first measured using the Z-scan technique. The χ(3)/R0 values obtained were found to be of the magnitude of 10-11-10-12 esu cm, which are in good agreement with the reported values obtained for copper nanoparticles embedded in glass.
1. Introduction In recent years, much interest has been generated in the studies of nanoparticles. The novelty of such particles lies in their unique properties which, as a result of their small size, differ largely from bulk properties.1 For example, metal nanoparticles exhibit quantum size effects which arise because in such small particles, the electronic energy levels do not form a continuous set but are discrete in nature.2 In view of these special properties, many investigations into their potential applications have been carried out, such as heterogeneous catalysis3 as well as nonlinear optical devices.4 A number of different ways have been developed to prepare metal nanoparticles. Some of these methods include photoreduction5 and reduction using various reducing agents in association with protective polymers or surfactants.3 The nanoparticles of various single metals such as Ag,5,6 Au,7 Cu,3,8-11 Pt,12,13 Pd,12,13 and Ru12 as well as alloys like Pd-Cu,14,15 and Pd-Au,16 and Pd-Pt17-19 * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Physics. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Siegel, R. Nanostruct. Mater. 1993, 3, 1. (2) Chakravorty, D.; Giri, A. K. in Chemistry of Advanced Materials; Rao, C. N. R., Ed.; Blackwell Scientific Publications: Boca Raton, Fla., 1993. (3) Hirai, H.; Wakabayashi, H.; Komiyama, M. Bull. Chem. Soc. Jpn. 1986, 59, 367. (4) Puech, K.; Blau, W.; Grund, A.; Bubeck, C.; Cardenas, G. Opt. Lett. 1995, 20, 1613. (5) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909. (6) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (7) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301. (8) Savinova, E. R.; Chuvilin, A. L.; Parmon, V. N. J. Mol. Catal. 1988, 48, 217. (9) Ershov, B. G.; Janata, E.; Michaelis, M.; Henglein, A. J. Phys. Chem. 1991, 95, 8996. (10) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (11) Tanori, J.; Duxin, N.; Petit, C.; Lisiecki, I.; Veillet, P.; Pileni, M. P. Colloid Polym. Sci. 1995, 273, 886. (12) Duteil, A.; Queau, R.; Chaudret, B. Chem. Mater. 1993, 5, 341. (13) Toshima, N.; Takahashi, T. Bull. Chem. Soc. Jpn. 1992, 65, 400. (14) Esumi, K.; Tano, T.; Torigoe, K.; Meguro, K. Chem. Mater. 1990, 2, 564. (15) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5, 254. (16) Liu, H. F.; Mao, G. P.; Meng, S. J. J. Mol. Catal. 1992, 74, 275. (17) Toshima, N.; Kushihashi, K.; Yonezawa, T.; Hirai, H. Chem. Lett. 1989, 1769.
have been synthesized and characterized. However, the physical properties of metal nanoparticles are still not well understood. In this paper, copper nanoparticles were synthesized by reduction (by hydrazine) of copper(II) acetate in water or 2-ethoxyethanol in the presence of poly(N-vinylpyrrolidone) (PVP). By the addition of different amounts of PVP, copper nanoparticles of varying sizes were obtained. The characterization of the particles was accomplished by the use of transmission electron microscopy (TEM) as well as UV-vis spectroscopy. Nonlinear optical properties of the copper nanoparticles were studied using the Z-scan technique. 2. Experimental Section 2.1. Materials. Copper(II) acetate (analytical reagent) was obtained from Merck, and poly(N-vinylpyrrolidone) with a weightaverage number of 40 000 was from Fluka. The solvent of 2-ethoxyethanol was purchased from Tokyo Kasei Kogyo Co., Ltd. All reagents and solvents were used as received. Deionized water was prepared with a Milli-Q water purification system. 2.2. Synthesis of Copper Nanoparticles. The copper nanoparticles in 2-ethoxyethanol were synthesized by the following method. One milliliter of 0.01 M copper(II) acetate in ethanol was added to 5 mL of 2-ethoxyethanol containing various weight percentages of PVP (0.2, 0.5, and 1.0 wt %). The Cu2+ ions in the reaction mixture were then reduced to copper metal by the introduction of an excess of hydrazine monohydrate under refluxing conditions. The reaction was carried out in an inert atmosphere of N2 gas to prevent the reoxidation of the copper metal by atmospheric oxygen. The Cu nanoparticles in water were prepared in a similar way except that water was used as the solvent. This method of preparation employed in this series of experiments has enabled the successful synthesis of polymerprotected copper nanoparticles in a relatively short time interval of 30-40 min. 2.3. Characterization. The optical spectra of copper colloidal dispersions in 2-ethoxyethanol or water with various concentrations of PVP were recorded using an HP 8452A diode array spectrophotometer at a path length of 1 cm. The average sizes of the copper nanoparticles formed were determined by transmission electron microscopy. A drop of the sample was placed on a copper grid coated with a thin film of Formvar and dried. Electron mirographs were taken with a JEOL LEM-100CXII electron microscope at an accelerating voltage of 100 kV. The particle sizes were determined from the maximum length of the particles. (18) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.; Meguro, K. Langmuir 1991, 7, 457. (19) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448.
+
+
Copper Nanoparticles
Langmuir, Vol. 13, No. 2, 1997 173
2.4. Nonlinear Optical Properties Measurements. The Z-scan technique was developed by Stryland and his co-workers.20 It is a simple, highly sensitive, single beam experimental technique for determining the sign and magnitude of the nonlinear refraction and nonlinear absorption. For third-order nonlinear optical materials, it can provide us the information on the third-order nonlinear absorption coefficient (R2) and thirdorder nonlinear refractive index (n2), which are related to the imaginary and real part of the third-order susceptibility (χ(3)), respectively. In this experiment, the third-order nonlinear optical properties of colloidal copper in both solvents (2-ethoxyethanol and water) were determined using the Z-scan technique. Our results of Z-scan analysis with aperture showed that n2 did not contribute to the third-order nonlinear response. Thus, the χ(3) measured in this experiment is totally attributed to R2
χ(3)(esu) ) ((9 × 108)0n02c2R2)/(4πω) where 0 is the permittivity of vacuum, c is the speed of light, n0 is the refractive index of the medium, and ω ) 2πc/λ, in which λ is the wavelength of the laser pulse. The laser used was a frequency-doubled Q-switched Nd:YAG DCR-3 laser with a wavelength of 532 nm and a pulse duration of 7 ns. The energy detectors used were Laser Precision RJP735. For Z-scan measurements with aperture, an aperture of 1.0 mm diameter was employed. The samples were subjected to Z-scan analysis in quartz cells with a path length of 1 cm. All samples were freshly prepared just before analysis and kept under N2 until use.
3. Results and Discussion 3.1. UV-vis Optical Absorption and Size Distributions. Copper colloidal dispersions successfully prepared in liquid media (water and 2-ethoxyethanol) in the presence of a protective polymer (PVP) appeared as red solutions ranging from bright red to reddish brown, depending on the polymer concentration. These preparations were highly unstable in the presence of air and were rapidly oxidized to give a clear yellow solution, indicating copper being oxidized from zero to +2 oxidation state. If kept in air-tight containers and allowed to stand overnight, the color of the preparations deepened probably due to the coagulation of the nanoparticles and precipitation was observed, especially for those with lesser amounts of PVP. The copper colloidal dispersions prepared in water and 2-ethoxyethanol were similar in appearance. However, the dispersions in 2-ethoxyethanol seemed to be oxidized more readily when exposed to the atmosphere while those in water precipitated out much faster, leaving behind a colorless solution. To avoid the reoxidation and precipitation of colloidal copper, all the samples in this experiment were freshly prepared in the inert atmosphere of N2. Another noteworthy point was that higher PVP concentrations were required to obtain stable colloidal dispersions in water as compared to those in 2-ethoxyethanol. The lowest concentration of PVP used for preparing stable colloidal copper in 2-ethoxyethanol was 0.2 wt %, but a concentration of 0.5 wt % PVP was required for a similar preparation in water. PVP molecules were thought to interact with metal nanoparticles via their carbonyl oxygens found on the side chains of the polymer.5 This interaction could be weaker in water than in 2-ethoxyethanol, thus resulting in poorer protective properties of the polymer in water. The weakening of the above said interaction can be attributed to the stronger interactions between the polymer and water molecules than those between polymer and 2-ethoxyethanol molecules. However, the exact nature of this has yet to be elucidated. (20) Sheikbahae, M.; Said, A. A.; Vanstryland, E. W. Opt. Lett. 1989, 14, 955.
Figure 1. UV-vis absorption spectra of Cu nanoparticles in (a) water and (b) 2-ethoxyethanol as a function of PVP concentration. The copper nanoparticles in 2-ethoxyethanol were synthesized by refluxing a reaction mixture containing 1 mL of 0.01 M copper(II) acetate in ethanol, 5 mL of PVPcontaining 2-ethoxyethanol, and 0.2 mL of 1 M hydrazine monohydrate under the inert atmosphere of N2 gas. The Cu nanoparticles in water was prepared in a similar way except for the change of solvent to water in the reaction system.
Figure 1 shows the UV-vis absorption spectra of the copper colloidal dispersions prepared in water and 2ethoxyethanol with various concentrations of PVP. For the colloids prepared in 2-ethoxyethanol, the absorption maxima were at 574 nm (for 0.2 wt %) and 572 nm (for 0.5 and 1 wt % PVP). In water however, the absorption peak was shifted to the longer wavelength of 580 nm (0.5 wt % PVP) and 582 nm (1 and 1.5 wt % PVP). This range of absorption is in good agreement with the reported values with absorption maxima from 570 to 590 nm, attributable to the plasma excitation in copper colloids.8 Figures 2 and 3 show the size distributions of the copper colloids synthesized in 2-ethoxyethanol and water with different concentrations of PVP. It can be readily seen that the size distribution becomes narrower and also shifts toward the smaller diameter as the concentration of PVP is increased. This trend was observed in water as well as in 2-ethoxyethanol. In 2-ethoxyethanol, the mean particle
+
174
+
Langmuir, Vol. 13, No. 2, 1997
Huang et al.
Figure 4. A typcial Z-scan plot of the transmittance versus the sample position, Z for Cu nanoparticles synthesized with 0.2 wt % PVP in 2-ethoxyethanol (diluted by a factor of 2.5). The scan data (open circles) were taken with 532 nm laser pulses of 7 ns duration at the irradiance of 27 MW/cm2. The solid curve is a theoretical fit with R0 ) 141.9 m-1 and R2 ) -6.0 × 10-10 m W-1.
Figure 2. Particle size distributions of Cu nanoparticles in water at PVP concentrations of (a) 0.5 and (b) 1.5 wt %. The conditions for preparation are the same as those in Figure 1.
Figure 3. Particle size distributions of Cu nanoparticles in 2-ethoxyethanol at PVP concentrations of (a) 0.5 and (b) 1.0 wt %. The conditions for preparation are the same as those in Figure 1.
sizes were found to be 22.7, 8.0, and 6.6 nm at the PVP concentrations of 0.2, 0.5, and 1 wt %, respectively. In contrast, the average particle size in water was measured to be 30.5, 22.7, and 14.5 nm for PVP concentrations of 0.5, 1, and 1.5 wt %, respectively. The particles synthesized in water are about 2.5 times bigger than those made in 2-ethoxyethanol at the same concentration of PVP, clearly indicating that the solvent has a significant effect on the average size of the particles. This result of
increasing in particle size in water is consistent with our observation that a higher minimum PVP concentration was required for forming stable Cu colloids in water. The possible reason might be that the interaction between the colloids and polymer could be weaker in water compared in 2-ethoxyethanol, resulting in poorer protective properties of PVP. By comparison of Figures 1, 2, and 3, it is interesting to note that the optical absorption peak of the Cu colloids does not shift significantly as the average particle size increasing from 6.6 to 22.7 nm in 2-ethoxyethanol or from 14.5 to 30.5 nm in water. In the case of Ag colloids,5 prepared by photochemical reduction, a red-shift of 14 nm was observed as the particle size changing from 15.2 to 22.4, which is consistent with the general trend that the peak shifts toward longer wavelength as particles become bigger.21 This difference in the extent of peak shift may be attributable to the different electronic nature of the Cu and Ag colloidal particles. The theoretical studies have further shown that the dipolar absorption is dominant for particles smaller that 20 nm.22 Above 30 nm, dipolar scattering and quadrupolar absorption play important roles, accompanied by a significant broadening in the absorption peak. Since there is no substantial peak broadening occurring in our spectra, it can be concluded that the observed absorption peak in our experiments is mainly contributed from the dipolar excitation. 3.2. Nonlinear Optical Properties of Copper Colloids. A typical Z-scan plot of transmittance as a function of sample position Z is shown in Figure 4. The experimental data were fitted with a theoretical curve with an appropriate value of R0 (linear absorption coefficient) and R2 (third-order nonlinear absorption coefficient). On the basis of the R2 obtained for each sample, the value of χ(3) can be readily calculated. The experimental results are shown in Tables 1 and 2. A linear relationship between the χ(3) value and the number density of the colloidal particles was observed (Table 1). The effect of PVP concentration on the χ(3) value is presented in Table 2. For the same solvent system, either water or 2-ethoxyethanol, the χ(3) increases with the PVP concentration. As shown in Figures 2 and 3, the colloidal particles become smaller (21) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (22) Kreibig, U.; Quinten, M.; Schoenauer, D. Physica A 1989, 157, 244.
+
+
Copper Nanoparticles
Langmuir, Vol. 13, No. 2, 1997 175
Table 1. Values of r0, r2, χ(3), and χ(3)/r0 for Cu Nanoparticles in 2-Ethoxyethanol with 1 wt % PVP as a Function of Dilution Factor dilution factors
R0 (1/m)
R2 (m/W) × 10-10
χ(3) (esu)
1.67 2 2.5 3 5
324.3 219.1 144.2 126.1 87.3
15 ( 2 12 ( 2 9.5 ( 2 8(0 6(0
4.27 × 10-11 3.42 × 10-11 2.71 × 10-11 2.28 × 10-11 1.71 × 10-11
χ(3)/R0 (esu cm) (average)
1.71 × 10-11
Table 2. Values of r0, r2, χ(3), and χ(3)/r0 for Cu Nanoparticles in 2-Ethoxyethanol and Water, Prepared as in Figure 1 and Diluted by a Factor of 2, versus PVP Concentrations PVP concentrations R0 R2 (m/W) (wt %) (1/m) × 10-10 0.2a 0.5a 1.0a 0.5b 1.0b 1.5b
192.1 202.2 219.1 228.3 240.7 266.7
8(1 10 ( 1 12 ( 2 5(0 2(0 6(0
χ(3) (esu)
χ(3)/R0 (esu cm)
2.28 × 10-11 2.85 × 10-11 3.42 × 10-11 1.42 × 10-11 1.56 × 10-11 1.71 × 10-11
1.19 × 10-11 1.41 × 10-11 1.56 × 10-11 6.22 × 10-12 6.48 × 10-12 6.41 × 10-12
in a wavelength range from 562 to 606 nm.4 In addition, gold metal particles (5-30 nm) embedded in glass had also been shown to exhibit third optical nonlinearity with a magnitude of 10-11 esu.23 However, for gold nanoparticles (with a diameter of 2.9 nm after heat treatment) in silica glass introduced via Au+ ion implantation at an acceleration energy of 1.5 MeV and a fluency level of 1017 ions /cm2, a large χ(3) value of 1.2 × 10-7 esu was observed using DFWM at 532 nm.24 Furthermore, copper nanoparticles of 5-25 nm diameters embedded in glass using Cu+ ion implantation had also been reported to have a comparatively large value of 10-8 esu.25 Compared with these results, our χ(3) values for copper colloids in solution are about 3-4 orders of magnitude lower. This difference in the absolute value of χ(3) is mainly attributable to the much higher number density of nanoparticles in their samples. In fact, the χ(3)/R0 values obtained in this experiment, 10-11 esu cm in 2-ethoxyethanol and 10-12 in water, are in good agreement with the reported values (10-12 to 10-11 esu cm) for copper nanoparticles embedded in glass26 and can be attributed to the surface plasma absorption of the copper colloids.
a PVP concentrations in 2-ethoxyethanol. b PVP concentrations in water.
as increasing the PVP concentration. In other words, the number density of the nanoparticles in the solution is expected to increase with the PVP concentration since the initial amount of Cu(II) is kept the same in our experiments. Therefore, the observed changes in χ(3) versus PVP concentration might be attributed to the effects of size as well as the number density of the Cu nanoparticles. To further eliminate the effect of the particle concentration, the value of χ(3)/R0 was obtained (Table 2). For one particular solvent, either water or 2-ethoxyethanol, the similar values were obtained at all different concentrations of PVP used in our experiments. This result indicates that the effect of particle size on the nonlinear optical response may not be significant in the size range of 10-30 nm. However, the value of χ(3)/R0 for the copper colloids prepared in 2-ethoxyethanol (10-11 esu cm) is noticeably higher than that (∼10-12 esu cm) in water. The physical origin for this observed difference is yet to be understood. The χ(3) values for Cu nanoparticles in this experiment were found to be of the same magnitude (10-12 to 10-11 esu) as those measured for 5 nm gold particles dispersed in acetone using degenerate four-wave mixing (DFWM)
4. Conclusion Copper colloidal dispersions have been successfully prepared by the use of a reducing agent under refluxing conditions. The UV-vis spectra of copper colloids in 2-ethoxyethanol and water showed distinct absorption peaks at 572 and 582 nm, respectively. The size variation of the particles from 6.6 (with 1 wt % PVP in 2-ethoxyethanol) to 30.5 nm (with 0.5 wt % PVP in water) was easily achieved by varying the amount of the protective polymer PVP in the solutions. The χ(3)/R0 values for copper colloids were first obtained to be of the magnitude of 10-12 to 10-11 esu cm. Acknowledgment. This work was supported by the National University of Singapore under Grant No. RP910681. LA9605495 (23) Ricard, D.; Roussignol, P.; Flytzanis, C. Opt. Lett. 1985, 10, 511. (24) Fukumi, K.; Chayahara, A.; Kadono, K.; Sakaguchi, T.; Horino, Y.; Miya, M.; Hayakawa, J.; Satou, M. Jpn. J. Appl. Phys. 1991, 30, L742. (25) Haglund, R. F.; Yang, L.; Magruder, R. H.; Becker, K.; Wittig, J. E.; Zuhr, R. A. Opt. Lett. 1993, 18, 373. (26) Ikushima, A. J.; Tokizaki, T.; Nakamura, A. J. Opt. Soc. Am. B 1994, 11, 1236.
