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Reduction and morphology of silver nanoparticles via liquid–liquid method
Applied Surface Science 226 (2004) 422–426
Reduction and morphology of silver nanoparticles via liquid–liquid method Minmin Caia,*, Juilin Chenb, Ji Zhoua a
State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Received 14 May 2003; received in revised form 14 May 2003; accepted 31 October 2003
Abstract 1-Hexanethiol-capped silver nanoparticles of about 10–20 nm in diameter were prepared using a liquid–liquid two-phase method. With the UV-Vis absorbance spectra, it was found that large particles were formed at the beginning of reaction and then disintegrated into small ones. In late reaction stage, the small particles became a bit larger with time and kept its size after 5 h, which was stable in 3 weeks. The reaction rate was strongly influenced by initial silver salt concentration in a first-order fashion and by reaction temperature. The transmission electron microscopy (TEM) images showed that size distribution and the morphology of fabricated alkylthiol-capped silver particles were different when they were prepared under different solvent, indicating that the reduction and capping process may depend on various solvents. # 2003 Elsevier B.V. All rights reserved. PACS: 61.46.þw Keywords: Silver nanoparticles; Alkylthiol capping; Formation kinetics; Morphology
1. Introduction Metal nanoparticles have gained much attention in recent years due to their significant properties which are quite different from those of bulk substance and, consequently, their further potential applications in optical and electronic devices. Many groups are now actively involved in the synthesis of these metal nanoparticles and a variety of preparation methods
*
Corresponding author. Tel.: þ86-10-62784674; fax: þ86-10-62771160. E-mail address: [email protected] (M. Cai).
can be found in the literature, such as radiation chemical reduction, chemical reduction in an aqueous medium with or without stabilizing polymers, chemical or photo reduction in reverse micelles, etc. [1–3]. For most of these present works, many investigations are focused on the control of particles’ size and their self-assembly into two-dimensional or three-dimensional superlattice structure. Kinetics of evolution and control of morphology, which play a very important role in fabrication and self-assembly, on the other hand, acquire relatively less considerations. Recently, He et al. and Chen et al. investigated the evolution and superlattice of the organic thiol-capped gold or silver clusters [4,5]. Liz-Marzan and Phillise reported the
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.046
M. Cai et al. / Applied Surface Science 226 (2004) 422–426
stability and reduction kinetics of silver ions in ethanol when certain surfactants were also present [6]. In this study, we demonstrate the fabrication of silver nanoparticles which are capped with alkylthiol to prevent it from aggregation. We will present here the evolution kinetics and solvent influence to nanoparticles’ morphology based on the maximum absorbance wavelength of the 1-hexanethiol-capped silver nanoparticles and the time evolution in toluene or chloroform with UV-Vis absorbance spectroscopy. The features of nanocrystals are characterized with transmission electron microscopy (TEM).
2. Experiment 2.1. Fabrication of Ag nanoparticles Thirty milliliters of aqueous silver ions solution (0.01 M AgNO3 ) was mixed with 50 ml of toluene or chloroformic solution of phase transfer catalyst (0.02 M (CH3 CH2 )4 NBr) and stirred violently for 1 h until the organic phase color became gray white. Subsequently, add 10–15 drops (about 1 ml) of 1hexanethiol to mixture solution. After the solution was stirred for another 30–60 min, 30 ml fresh aqueous sodium borohydride used as reductant was slowly dropped into the solution and the reaction continued under stirring for 5 h before the organic phase (containing nanoparticles colloid) was collected into a beaker. Wash the colloid suspension with ethanol for three or four times to eliminate the excess reagent and catalyst. Finally, size-selective centrifugation was employed to separate various sizes of Ag nanocrystals and remove the residual byproduct.
423
3. Results and discussion 3.1. Formation of alkylthiol-capped silver nanoparticles Fig. 1 shows a typical UV-Vis spectrum of 1hexanethiol-capped silver colloid suspension in toluene, in which the extinction band for silver nanoparticles appears at 476 nm with full width at half-maximum (fwhm) of 160 nm. This is the characteristic of 1-hexanethiol-capped silver colloid [7], while the extinction band of uncapped silver colloid has a maximum at 380 nm. One could see that the shift of maximum absorbance wavelength between capped and uncapped Ag nanoparticles is almost 100 nm. The red shift of the maximum absorbance wavelength could be explained as formation of the chemical band between sulfur ions and silver atoms of the nanocrystals, which provided the best evidence for 1-hexanethiol covering on Ag nanoparticles [8]. The formation process of 1-hexanethiol-capped silver nanocrystals was monitored by the UV-Vis absorbance spectrum. In this instance, we used the toluene as the solvent and 0.01 M silver nitrate as substrate. Fig. 2 shows the peak shift and the shape of absorbance spectra during the whole reaction. We set the time when all the reduction agent was added as
2.2. Experimental techniques UV-Vis absorbance spectra were determined in 10 mm optical path length quartz cuvettes with ThermoSpectronic UNICAM UV-500 PC double beam spectrophotometer. The sample prepared for measurement was pippetted out directly from the reaction solution during the particles formation or from ethanol solution of fabricated silver nanoparticles. Transmission electron microscopy was performed with JEOL JEM-200CX under 200 kV.
Fig. 1. Typical UV-Vis spectra of Ag nanoparticles capped with or without 1-hexanethiol.
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M. Cai et al. / Applied Surface Science 226 (2004) 422–426
Fig. 2. UV-Vis spectra of the Ag particles at different reaction time during the reaction (note: the vertical order did not show its exact absorbance value, left number was reaction time).
the starting of reaction. It can be seen that there was almost no peak at this time and the shape of spectrum was very similar with that of toluene, which indicated that the particles were not formed at the beginning of the reaction. With the passage of time, a clear peak appeared and its position changed from 489 nm at 75 min, 473 nm at 105 min, 465 nm at 135 min, to 418 nm at 165 min, 423 nm at 210 min. From this series of peak position changes, we may know some facts about the formation process of silver particles: large particles were formed at the first stage of reaction and then gradually disintegrated into small ones. In the late stage, however, the shift of peak position turned to the red shift, from 418 nm at 165 min to 423 nm at 210 min, rather than keeping the blue shift. This result gave the evidence that the particles became a bit larger again in the late stage and stopped its growth in size after 5 h with the peak of absorbance keeping at 476 nm (also see Fig. 1). We measured the absorbance spectrum of the fabricated silver colloid system at different shelf time so as to investigate its stability. From Fig. 3, we found that the absorbance spectra of silver colloid had no obvious difference in shape, position and symmetry in the first 2 weeks. After the third week, the fwhm of the spectrum started to become wider than before, and the peak position of absorbance spectrum had a slight red shift, which implied the onset of particles’ aggrega-
Fig. 3. UV-Vis spectra of the Ag nanoparticles stored in 3 weeks.
tion. So the silver nanoparticles could remain stable at least 2 weeks. 3.2. Kinetics of formation The process of silver nanoparticles formation was traced by UV-Vis spectra under the different substrate concentrations or reaction temperatures. Fig. 4 showed the time evolution of the absorbance at 400 nm. In Fig. 4 (left) the dependency on initial Ag concentration (at constant temperature 25 C) can be observed, while the right figure indicated the influence of the reaction temperature. One can observe that the formation rate was relatively larger under high temperature and decreased with time. From Fig. 4, it came out that the initial concentration of Ag salt determined the final maximum absorbance value, i.e. the final product volume of prepared silver nanocrystals, while the temperature settled the reaction rate. That the two plots started from almost the same point revealed that the reduction was greatly slow at the very beginning of the reaction, which agreed with that of Fig. 2. The absorbance time dependence in Fig. 4 could be fit by first-order rate equation with high reliability: At ¼ A1 ð1 ekt Þ
(1)
where At is the absorbance at time t, A1 the absorbance at a very long time, t the time difference, and k the first-order rate constant. The fit result was in Table 1.
M. Cai et al. / Applied Surface Science 226 (2004) 422–426
425
Fig. 4. First-order fit of time evolution of Ag nanoparticles’ absorbance at 400 nm, left: T ¼ 25 C, right: [AgNO3 ] 0.01 M.
Table 1 First-order constant results from fits by Eq. (1) to the time evolution of the absorbance at 400 nm AgNO3 (mol l1 )
T ( C)
k (min1 )
A1
0.01 0.005 0.01
25 25 50
0.0102 0.0078 0.02184
3.93 3.04 3.92
3.3. Influence of solvent The silver ions were reduced in organic solution. So the polarity or other atmosphere of solvent may affect the formation of nanoparticles. For this consideration, we used chloroform instead of toluene as solvent to investigate this kind of dependence. From Fig. 5, we found out that although the peak of absorbance
Fig. 5. The difference of typical UV-Vis spectrum of silver nanoparticles prepared in chloroform or toluene.
Fig. 6. TEM image of 1-hexanethiol-capper silver nanoparticles (inset: electron diffraction analysis), left: prepared in toluene, right: prepared in chloroform.
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M. Cai et al. / Applied Surface Science 226 (2004) 422–426
spectrum of silver nanoparticles slightly shifted from 476 to 502 nm in chloroform, the fwhm in chloroform was relatively smaller, which indicated that the silver nanoparticles prepared in chloroform solvent were much more uniform in size. In the chloroform, the capping performance of alkylthiol was better than that in the toluene, which was consistent with the later TEM analysis. Fig. 6 was a typical TEM image showing the size distribution and morphology of the prepared silver nanoparticles. From the left figure where toluene as solvent, one could see that all the nanoparticles were smaller than 20 nm, but the size distribution was not so good and the shape was irregular. While in right one where chloroform as solvent, the morphology of nanoparticles was almost spherical and there were mainly about three different sizes of particles: 18, 12 and 9 nm, that is to say, the size distribution was relatively narrow. Consequently, the fwhm of absorbance spectrum was smaller (see Fig. 5). We may conclude accordingly that the 1-hexanethiol showed better capping performance in chloroform than in toluene.
4. Conclusion 1-Hexanethiol-capped silver nanocrystals were prepared by the two-phase liquid–liquid reduction system method. With the UV-Vis absorbance spectra, we proposed that the large particles were formed at the beginning stage of reaction and then disintegrated to small ones. While, in late reaction stage, the small particles became a bit larger again with time and kept
its size after 5 h, which was stable in 2–3 weeks. The reaction rate was strongly influenced by initial silver salt concentration (in a first-order fashion) and by temperature. It was also found that the solvent affected the reduction capping process and that the size distribution and morphology of 1-hexanethiol-capped silver nanoparticles were better in chloroform than that in toluene. Better size selection process, however, should be investigated and more work on the experiment and theory was needed to further understand the formation and capping mechanism.
Acknowledgements This work was supported by National Science Foundation of China under Grant Nos. 50172025 and 50272032, and the Ministry of Science and Technology of China through 973-Project under Grant No. 2001CB6104.
References [1] [2] [3] [4] [5] [6] [7] [8]
D.G. Duff, A. Balker, Langmuir 9 (1993) 2301. L.M. Liz-Marzan, I. Lado-Tourino, Langmuir 12 (1996) 3585. N. Alihara, K. Torigoe, K. Esumi, Langmuir 14 (1998) 4945. S. He, J. Yao, P. Jiang, D. Shi, Ha. Zhang, S. Xie, S. Pang, H. Gao, Langmuir 17 (2001) 1571. S. Chen, A.C. Templeton, R.W. Murray, Langmuir 16 (2002) 3543. L.M. Liz-Marzan, A.P. Phillise, J. Phys. Chem. 99 (1995) 15120. J.A. Creighton, D.G. Eaton, J. Chem. Soc. Faraday Trans. 87 (1991) 3881. M.P. Andrews, G.A. Ozin, J. Phys. Chem. 90 (1986) 2929.
Reduction and morphology of silver nanoparticles via liquid–liquid method Minmin Caia,*, Juilin Chenb, Ji Zhoua a
State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Received 14 May 2003; received in revised form 14 May 2003; accepted 31 October 2003
Abstract 1-Hexanethiol-capped silver nanoparticles of about 10–20 nm in diameter were prepared using a liquid–liquid two-phase method. With the UV-Vis absorbance spectra, it was found that large particles were formed at the beginning of reaction and then disintegrated into small ones. In late reaction stage, the small particles became a bit larger with time and kept its size after 5 h, which was stable in 3 weeks. The reaction rate was strongly influenced by initial silver salt concentration in a first-order fashion and by reaction temperature. The transmission electron microscopy (TEM) images showed that size distribution and the morphology of fabricated alkylthiol-capped silver particles were different when they were prepared under different solvent, indicating that the reduction and capping process may depend on various solvents. # 2003 Elsevier B.V. All rights reserved. PACS: 61.46.þw Keywords: Silver nanoparticles; Alkylthiol capping; Formation kinetics; Morphology
1. Introduction Metal nanoparticles have gained much attention in recent years due to their significant properties which are quite different from those of bulk substance and, consequently, their further potential applications in optical and electronic devices. Many groups are now actively involved in the synthesis of these metal nanoparticles and a variety of preparation methods
*
Corresponding author. Tel.: þ86-10-62784674; fax: þ86-10-62771160. E-mail address: [email protected] (M. Cai).
can be found in the literature, such as radiation chemical reduction, chemical reduction in an aqueous medium with or without stabilizing polymers, chemical or photo reduction in reverse micelles, etc. [1–3]. For most of these present works, many investigations are focused on the control of particles’ size and their self-assembly into two-dimensional or three-dimensional superlattice structure. Kinetics of evolution and control of morphology, which play a very important role in fabrication and self-assembly, on the other hand, acquire relatively less considerations. Recently, He et al. and Chen et al. investigated the evolution and superlattice of the organic thiol-capped gold or silver clusters [4,5]. Liz-Marzan and Phillise reported the
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.046
M. Cai et al. / Applied Surface Science 226 (2004) 422–426
stability and reduction kinetics of silver ions in ethanol when certain surfactants were also present [6]. In this study, we demonstrate the fabrication of silver nanoparticles which are capped with alkylthiol to prevent it from aggregation. We will present here the evolution kinetics and solvent influence to nanoparticles’ morphology based on the maximum absorbance wavelength of the 1-hexanethiol-capped silver nanoparticles and the time evolution in toluene or chloroform with UV-Vis absorbance spectroscopy. The features of nanocrystals are characterized with transmission electron microscopy (TEM).
2. Experiment 2.1. Fabrication of Ag nanoparticles Thirty milliliters of aqueous silver ions solution (0.01 M AgNO3 ) was mixed with 50 ml of toluene or chloroformic solution of phase transfer catalyst (0.02 M (CH3 CH2 )4 NBr) and stirred violently for 1 h until the organic phase color became gray white. Subsequently, add 10–15 drops (about 1 ml) of 1hexanethiol to mixture solution. After the solution was stirred for another 30–60 min, 30 ml fresh aqueous sodium borohydride used as reductant was slowly dropped into the solution and the reaction continued under stirring for 5 h before the organic phase (containing nanoparticles colloid) was collected into a beaker. Wash the colloid suspension with ethanol for three or four times to eliminate the excess reagent and catalyst. Finally, size-selective centrifugation was employed to separate various sizes of Ag nanocrystals and remove the residual byproduct.
423
3. Results and discussion 3.1. Formation of alkylthiol-capped silver nanoparticles Fig. 1 shows a typical UV-Vis spectrum of 1hexanethiol-capped silver colloid suspension in toluene, in which the extinction band for silver nanoparticles appears at 476 nm with full width at half-maximum (fwhm) of 160 nm. This is the characteristic of 1-hexanethiol-capped silver colloid [7], while the extinction band of uncapped silver colloid has a maximum at 380 nm. One could see that the shift of maximum absorbance wavelength between capped and uncapped Ag nanoparticles is almost 100 nm. The red shift of the maximum absorbance wavelength could be explained as formation of the chemical band between sulfur ions and silver atoms of the nanocrystals, which provided the best evidence for 1-hexanethiol covering on Ag nanoparticles [8]. The formation process of 1-hexanethiol-capped silver nanocrystals was monitored by the UV-Vis absorbance spectrum. In this instance, we used the toluene as the solvent and 0.01 M silver nitrate as substrate. Fig. 2 shows the peak shift and the shape of absorbance spectra during the whole reaction. We set the time when all the reduction agent was added as
2.2. Experimental techniques UV-Vis absorbance spectra were determined in 10 mm optical path length quartz cuvettes with ThermoSpectronic UNICAM UV-500 PC double beam spectrophotometer. The sample prepared for measurement was pippetted out directly from the reaction solution during the particles formation or from ethanol solution of fabricated silver nanoparticles. Transmission electron microscopy was performed with JEOL JEM-200CX under 200 kV.
Fig. 1. Typical UV-Vis spectra of Ag nanoparticles capped with or without 1-hexanethiol.
424
M. Cai et al. / Applied Surface Science 226 (2004) 422–426
Fig. 2. UV-Vis spectra of the Ag particles at different reaction time during the reaction (note: the vertical order did not show its exact absorbance value, left number was reaction time).
the starting of reaction. It can be seen that there was almost no peak at this time and the shape of spectrum was very similar with that of toluene, which indicated that the particles were not formed at the beginning of the reaction. With the passage of time, a clear peak appeared and its position changed from 489 nm at 75 min, 473 nm at 105 min, 465 nm at 135 min, to 418 nm at 165 min, 423 nm at 210 min. From this series of peak position changes, we may know some facts about the formation process of silver particles: large particles were formed at the first stage of reaction and then gradually disintegrated into small ones. In the late stage, however, the shift of peak position turned to the red shift, from 418 nm at 165 min to 423 nm at 210 min, rather than keeping the blue shift. This result gave the evidence that the particles became a bit larger again in the late stage and stopped its growth in size after 5 h with the peak of absorbance keeping at 476 nm (also see Fig. 1). We measured the absorbance spectrum of the fabricated silver colloid system at different shelf time so as to investigate its stability. From Fig. 3, we found that the absorbance spectra of silver colloid had no obvious difference in shape, position and symmetry in the first 2 weeks. After the third week, the fwhm of the spectrum started to become wider than before, and the peak position of absorbance spectrum had a slight red shift, which implied the onset of particles’ aggrega-
Fig. 3. UV-Vis spectra of the Ag nanoparticles stored in 3 weeks.
tion. So the silver nanoparticles could remain stable at least 2 weeks. 3.2. Kinetics of formation The process of silver nanoparticles formation was traced by UV-Vis spectra under the different substrate concentrations or reaction temperatures. Fig. 4 showed the time evolution of the absorbance at 400 nm. In Fig. 4 (left) the dependency on initial Ag concentration (at constant temperature 25 C) can be observed, while the right figure indicated the influence of the reaction temperature. One can observe that the formation rate was relatively larger under high temperature and decreased with time. From Fig. 4, it came out that the initial concentration of Ag salt determined the final maximum absorbance value, i.e. the final product volume of prepared silver nanocrystals, while the temperature settled the reaction rate. That the two plots started from almost the same point revealed that the reduction was greatly slow at the very beginning of the reaction, which agreed with that of Fig. 2. The absorbance time dependence in Fig. 4 could be fit by first-order rate equation with high reliability: At ¼ A1 ð1 ekt Þ
(1)
where At is the absorbance at time t, A1 the absorbance at a very long time, t the time difference, and k the first-order rate constant. The fit result was in Table 1.
M. Cai et al. / Applied Surface Science 226 (2004) 422–426
425
Fig. 4. First-order fit of time evolution of Ag nanoparticles’ absorbance at 400 nm, left: T ¼ 25 C, right: [AgNO3 ] 0.01 M.
Table 1 First-order constant results from fits by Eq. (1) to the time evolution of the absorbance at 400 nm AgNO3 (mol l1 )
T ( C)
k (min1 )
A1
0.01 0.005 0.01
25 25 50
0.0102 0.0078 0.02184
3.93 3.04 3.92
3.3. Influence of solvent The silver ions were reduced in organic solution. So the polarity or other atmosphere of solvent may affect the formation of nanoparticles. For this consideration, we used chloroform instead of toluene as solvent to investigate this kind of dependence. From Fig. 5, we found out that although the peak of absorbance
Fig. 5. The difference of typical UV-Vis spectrum of silver nanoparticles prepared in chloroform or toluene.
Fig. 6. TEM image of 1-hexanethiol-capper silver nanoparticles (inset: electron diffraction analysis), left: prepared in toluene, right: prepared in chloroform.
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M. Cai et al. / Applied Surface Science 226 (2004) 422–426
spectrum of silver nanoparticles slightly shifted from 476 to 502 nm in chloroform, the fwhm in chloroform was relatively smaller, which indicated that the silver nanoparticles prepared in chloroform solvent were much more uniform in size. In the chloroform, the capping performance of alkylthiol was better than that in the toluene, which was consistent with the later TEM analysis. Fig. 6 was a typical TEM image showing the size distribution and morphology of the prepared silver nanoparticles. From the left figure where toluene as solvent, one could see that all the nanoparticles were smaller than 20 nm, but the size distribution was not so good and the shape was irregular. While in right one where chloroform as solvent, the morphology of nanoparticles was almost spherical and there were mainly about three different sizes of particles: 18, 12 and 9 nm, that is to say, the size distribution was relatively narrow. Consequently, the fwhm of absorbance spectrum was smaller (see Fig. 5). We may conclude accordingly that the 1-hexanethiol showed better capping performance in chloroform than in toluene.
4. Conclusion 1-Hexanethiol-capped silver nanocrystals were prepared by the two-phase liquid–liquid reduction system method. With the UV-Vis absorbance spectra, we proposed that the large particles were formed at the beginning stage of reaction and then disintegrated to small ones. While, in late reaction stage, the small particles became a bit larger again with time and kept
its size after 5 h, which was stable in 2–3 weeks. The reaction rate was strongly influenced by initial silver salt concentration (in a first-order fashion) and by temperature. It was also found that the solvent affected the reduction capping process and that the size distribution and morphology of 1-hexanethiol-capped silver nanoparticles were better in chloroform than that in toluene. Better size selection process, however, should be investigated and more work on the experiment and theory was needed to further understand the formation and capping mechanism.
Acknowledgements This work was supported by National Science Foundation of China under Grant Nos. 50172025 and 50272032, and the Ministry of Science and Technology of China through 973-Project under Grant No. 2001CB6104.
References [1] [2] [3] [4] [5] [6] [7] [8]
D.G. Duff, A. Balker, Langmuir 9 (1993) 2301. L.M. Liz-Marzan, I. Lado-Tourino, Langmuir 12 (1996) 3585. N. Alihara, K. Torigoe, K. Esumi, Langmuir 14 (1998) 4945. S. He, J. Yao, P. Jiang, D. Shi, Ha. Zhang, S. Xie, S. Pang, H. Gao, Langmuir 17 (2001) 1571. S. Chen, A.C. Templeton, R.W. Murray, Langmuir 16 (2002) 3543. L.M. Liz-Marzan, A.P. Phillise, J. Phys. Chem. 99 (1995) 15120. J.A. Creighton, D.G. Eaton, J. Chem. Soc. Faraday Trans. 87 (1991) 3881. M.P. Andrews, G.A. Ozin, J. Phys. Chem. 90 (1986) 2929.