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The preparation of hydrophobic silver nanoparticles via solvent exchange method
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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 158–161
The preparation of hydrophobic silver nanoparticles via solvent exchange method Doowon Seo a , Wonjung Yoon a , Sangjoon Park a , Jihyeon Kim b , Jongsung Kim a,∗ a
Department of Chemical and Bio Engineering, Kyungwon University, Gyeonggi-do 461-701, Republic of Korea Department of Chemical & Biochemical Engineering, Dongguk University, Seoul 100-715, Republic of Korea
b
Received 31 October 2006; accepted 30 April 2007 Available online 2 June 2007
Abstract We have prepared hydrophobic silver nanoparticles capped by oleic acid via solvent exchange method. AgNO3 was reduced by NaBH4 to produce Ag nanoparticles in aqueous solution with various pH conditions and oleic acid as stabilizer. By the addition of H3 PO4 , the carboxylate group was converted to carboxylic acid, and this induced the reorientation of oleic acid, and provided hydrophobic silver nanoparticles. The oleic acid-capped silver nanoparticles were characterized by UV–vis spectrophotometer and Fourier transfer infrared spectrometer (FTIR). Spherical silver particles with uniform size of 8 nm were obtained from silver nitrate under basic aqueous solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Phase transfer; Silver hydrosol; Silver organosol
1. Introduction In recent years, the interest in the synthesis and preparations of nanometer-sized metal nanoparticles in organic solvent has increased because of their potential application in the field of electronics and photonics due to their peculiar size-dependent optical and electronic properties [1–5]. A variety of methods have been used for the preparation of metal nanoparticles. Silver nanoparticles can be doped in sol–gel matrix by precipitation method with HCl as catalyst [6]. Metallic silver nanoparticles having diameters from 5 to 10 nm in supercritical CO2 were prepared using an optically transparent, water-inCO2 microemulsion [7]. Gold, nickel, and silver nanowires were prepared within a polycarbonate membrane template by a combination of electroplating and photolytic method [8]. Gold nanorods were prepared by organic solvent reduction [9]. The preparation of nanoparticles in organic solvent is generally based on the extraction of metal ions from the aqueous phase with phase transfer agent, and subsequent reduction in the organic phase in the presence of capping agents [10,11]. This method yields good control over the particle size and dispersity but very
∗
Corresponding author. Tel.: +82 31 750 5361; fax: +82 31 750 5363. E-mail address: [email protected] (J. Kim).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.086
little effect on controlling the shape of the particles [12]. Solvent exchange method has become one of the prominent synthetic roots of nanoparticles, which can provide an effective way to prepare uniform-sized nanoparticles in organic solvents [13,14]. In this study, we have prepared colloidal silver nanoparticles in cyclohexane by using solvent exchange method, and their conformation and size dependence on pH of the aqueous solution were investigated. The silver nanoparticles stabilized by the oleic acid were produced in aqueous phase by the reduction of silver nitrate, and transferred into organic solvent by the reorientation of oleic acid. The silver colloids were characterized by UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and Turbiscan. 2. Experiment 2.1. Materials Silver nitrate (AgNO3 ), sodium borohydride (NaBH4 ), oleic acid (9-octadecenoic acid), phosphoric acid (H3 PO4 ), sulfuric acid (H2 SO4 ), ammonium hydroxide (NH4 OH), and cyclohexane were purchased from Aldrich and used without further purification. Double-distilled water was used when necessary.
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2.2. Preparation of silver colloids Silver nanoparticles were prepared using oleic acid as a stabilizer and keeping the molar ratios [AgNO3 ]:[NaBH4 ]:[oleic acid] = 4:16:1. In typical synthesis, the pH of silver nitrate aqueous solution was varied as 2, 7, and 11 using diluted sulfuric acid and ammonium hydroxide. Twenty millilitre of 1 × 10−3 M AgNO3 was added dropwise to equal volume of 4 × 10−3 M NaBH4 containing 2.5 × 10−4 M oleic acid, and mixed by vigorous stirring for 2 h. A brown-yellowish colloidal of silver hydrosol stabilized by oleic acid was obtained. Twenty millilitre of cyclohexane was added to equal volume of silver hydrosol to make biphasic mixture with distinct two layers. 0.16 ml of 0.1 M H3 PO4 was added to the biphasic mixture under vigorous stirring, and phase transfer was immediately induced. Hydrophobic silver colloids were obtained by phase transfer induced by H3 PO4 . As the silver particles became hydrophobic, the color of the cyclohexane layer became yellow while the aqueous layer became clear by the transfer of silver particles into the cyclohexane layer. 2.3. Characterization UV–vis spectra for silver colloids in aqueous phase and organic phase were obtained by Varian Cary 100 Conc UV–vis spectrometer in the range 300–550 run with a resolution of 1 nm and 0.5 s of exposure time. The transfer of the oleic acid-capped nanoparticles from aqueous solution into organic solution was monitored by measuring the absorbance change in each phase. The image of the silver nanoparticles after phase transfer was obtained by transmission electron microscopy (JEOL JEM2000EXII instrument) under an acceleration voltage of either 200 kV. A single drop of silver colloids was deposited with a micropipette onto a carbon-coated copper grid and allowed to dry under atmosphere for 5 min after which the extra solution was removed using a blotting paper. Fourier transform infrared spectra were recorded with a 2 cm−1 resolution in transmission mode in the 3500–1000 cm−1 range using Bruker Vector 22 FTIR spectrometer at room temperature. The silver colloids were drop-coated onto the KBr pellet and dried before the spectra were obtained. The dispersion properties of the silver colloids in cyclohexane were measured using Turbiscan (Turbiscan Lab Expert, Formulation). This apparatus allows the investigation of the dispersion stability and homogeneity of the silver nanoparticles in cyclohexane via periodical turbidity scan over sample height. 3. Results and discussion The phase transfer of the silver nanoparticles dispersed in aqueous medium into the organic medium is illustrated in Fig. 1. The figure shows hydrophilic silver colloids in aqueous solution and hydrophobic colloids in cyclohexane. When H3 PO4 is added to silver hydrosol, the carboxylate group in oleic acid is converted to carboxylic acid which caps the silver surface and hydrophobic tail is directed toward solvent media. The phase transfer was observed by the yellow coloration of the organic
Fig. 1. (a) Immiscible layers of the silver hydrosol stabilized by oleic acid (bottom) and cyclohexane before shaking. (b) Silver organosol in cyclohexane and clean aqueous solution at the bottom after the phase transfer induced by the conversion of carboxylate to carboxylic acid in oleic acid.
phase and color loss of the aqueous phase after H3 PO4 addition. Fig. 2 shows the UV–vis spectra of silver colloids in the aqueous phase without stabilizer and those in cyclohexane phase after phase transfer which were produced with various pH values of silver nitrate aqueous solution. More symmetric as the silver hydrosol capped by oleic acid was produced, the absorption maximum was red shifted from 388 to 415 nm. The figure also shows that after the phase transfer, the absorption spectra of the silver colloids in cyclohexane do not change much, but the particles produced with pH 11 (c) show the most symmetric and constant spectra with absorption maximum at 415 nm and a full width at half-maximum (FWHM) of 86 nm. This indicates that the silver nanoparticles produced with pH 11 are more uniform in size and have better dispersion stability. The TEM images of oleic acid-capped hydrophobic silver nanoparticles are shown in Fig. 3. The figure shows that when silver nitrate is reduced under acidic condition, the nanoparticles produced were not quite spherical with non-uniform size distribution, but under basic condition, the particles were produced with spherical shape and uniform size distribution. The mean diameter of the particles was 9, 6, and 8 nm from silver nitrate solution at pH 2, pH 7, and pH 11, respectively. Fig. 4 shows the FTIR spectra of the oleic acid-capped silver nanoparticles. In general, the asymmetric and symmetric stretching bands of CH2 and CH3 are appeared in the region 2920–2850 cm−1 , or region 2953–2872 cm−1 , respectively [15]. The frequency of the CH and COO− stretching mode was observed at around 3018 and 1560 cm−1 , respectively. CH2 and CH3 stretching mode was observed in the region 2969–2954 cm−1 . The figure confirms that oleic acid is attached to the surface of silver nanoparticle to provide hydrophobic property. Fig. 5 shows the backscattering flux profile of silver nanoparticles in hexane which were produced with various pH conditions measured by Turbiscan. The glass cell containing silver nanoparticles dispersed in hexane was placed in the apparatus and its
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Fig. 2. UV–vis spectra of the silver sol before and after phase transfer: curve 1, silver nanoparticles prepared in aqueous phase; curve 2, oleic acid-capped silver nanoparticles in cyclohexane immediately after phase transfer; curve 3, silver nanoparticles in cyclohexane after 24 h of phase transfer; curve 4, silver nanoparticles in cyclohexane after 72 h of phase transfer. Ag nanoparticles were prepared from silver nitrate solution solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
flocculation behavior was monitored every 30 min for 24 h. The backscattering intensity versus cell height was collected. Fig. 5(a) shows that more stable dispersion property can be observed from silver nanoparticles produced with pH 11, which
Fig. 3. TEM images of the hydrophobic Ag nanoparticles prepared from silver nitrate solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
coincides with the result of UV–vis observation. The figure also shows the decrease of backscattering flux at the top, which indicates the sedimentation of silver nanoparticles. Fig. 5(b) shows the time variation of mean value of backscattering flux. The fig-
D. Seo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 158–161
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ure shows that flocculation (decrease of flux with time) occurs in silver particles produced with pH 2 and 7, and better dispersion stability can be observed with pH 11. It is postulated that the particle size was minimum at pH7 because the zeta potential of the particle becomes higher as the solution becomes neutral [16]. The size of the particle depends on the absolute value of the zeta potential. 4. Conclusion
Fig. 4. FTIR spectra of the hydrophobic Ag nanoparticles prepared from silver nitrate solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
Oleic acid-capped silver nanoparticles have been prepared from reduction reaction of silver nitrate in aqueous phase at various pH values. The phase transfer from hydrophilic to hydrophobic state was induced by the local reorientation of oleic acid due to the conversion of carboxylate group to carboxylic acid group in oleic acid by the addition of H3 PO4 . When silver nitrate is reduced by NaBH4 in aqueous phase under basic condition (pH 11), nanoparticles were produced with spherical shape, uniform size distribution, and good dispersion stability. Acknowledgement This research was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (2006-05382). References
Fig. 5. (a) Turbidity scan over sample cell height containing Ag nanoparticles in cyclohexane and (b) time-dependent turbidity.
[1] A.J. Hoffman, G. Mills, H. Yee, M.R. Hoffman, J. Phys. Chem. 96 (1992) 5546. [2] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [3] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [4] N. Toshima, T. Yonezawa, New J. Chem. (1998) 1179. [5] H.S. Kim, J.H. Ryu, B. Jose, B.G. Lee, B.S. Ahn, Y.S. Kang, Langmuir 17 (2001) 5817. [6] P.W. Wu, B. Dunn, V. Doan, B.J. Schwartz, E. Yablonovitch, M.J. Yamane, Sol-Gel Sci. Technol. 19 (2000) 249. [7] M. Ji, X. Chen, C.M. Wai, J.L. Fulton, J. Am. Chem. Soc. 121 (1999) 2631. [8] J.J. Mock, S.J. Oldenburg, D.R. Smith, D.A. Schultz, S. Schultz, Nano Lett. 2 (2002) 465. [9] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065. [10] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [11] H. Nagasawa, M. Maruyama, T. Komatsu, S. Isoda, T. Kobayashi, Phys. Status Solidi A 191 (2002) 67. [12] W. Wang, S. Efrima, O. Regev, Langmuir 14 (1998) 602. [13] W. Wang, X. Chen, S. Efrima, J. Phys. Chem. B 103 (1999) 7238. [14] A. Kumar, H. Joshi, R. Pasricha, A.B. Mandale, M. Sastry, J. Colloid Sci. 264 (2003) 396. [15] L.H. Daly, N.B. Colthup, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975. [16] K.D. Kim, D.N. Han, H.T. Kim, Chem. Eng. J. 104 (2004) 55.
Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 158–161
The preparation of hydrophobic silver nanoparticles via solvent exchange method Doowon Seo a , Wonjung Yoon a , Sangjoon Park a , Jihyeon Kim b , Jongsung Kim a,∗ a
Department of Chemical and Bio Engineering, Kyungwon University, Gyeonggi-do 461-701, Republic of Korea Department of Chemical & Biochemical Engineering, Dongguk University, Seoul 100-715, Republic of Korea
b
Received 31 October 2006; accepted 30 April 2007 Available online 2 June 2007
Abstract We have prepared hydrophobic silver nanoparticles capped by oleic acid via solvent exchange method. AgNO3 was reduced by NaBH4 to produce Ag nanoparticles in aqueous solution with various pH conditions and oleic acid as stabilizer. By the addition of H3 PO4 , the carboxylate group was converted to carboxylic acid, and this induced the reorientation of oleic acid, and provided hydrophobic silver nanoparticles. The oleic acid-capped silver nanoparticles were characterized by UV–vis spectrophotometer and Fourier transfer infrared spectrometer (FTIR). Spherical silver particles with uniform size of 8 nm were obtained from silver nitrate under basic aqueous solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Phase transfer; Silver hydrosol; Silver organosol
1. Introduction In recent years, the interest in the synthesis and preparations of nanometer-sized metal nanoparticles in organic solvent has increased because of their potential application in the field of electronics and photonics due to their peculiar size-dependent optical and electronic properties [1–5]. A variety of methods have been used for the preparation of metal nanoparticles. Silver nanoparticles can be doped in sol–gel matrix by precipitation method with HCl as catalyst [6]. Metallic silver nanoparticles having diameters from 5 to 10 nm in supercritical CO2 were prepared using an optically transparent, water-inCO2 microemulsion [7]. Gold, nickel, and silver nanowires were prepared within a polycarbonate membrane template by a combination of electroplating and photolytic method [8]. Gold nanorods were prepared by organic solvent reduction [9]. The preparation of nanoparticles in organic solvent is generally based on the extraction of metal ions from the aqueous phase with phase transfer agent, and subsequent reduction in the organic phase in the presence of capping agents [10,11]. This method yields good control over the particle size and dispersity but very
∗
Corresponding author. Tel.: +82 31 750 5361; fax: +82 31 750 5363. E-mail address: [email protected] (J. Kim).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.086
little effect on controlling the shape of the particles [12]. Solvent exchange method has become one of the prominent synthetic roots of nanoparticles, which can provide an effective way to prepare uniform-sized nanoparticles in organic solvents [13,14]. In this study, we have prepared colloidal silver nanoparticles in cyclohexane by using solvent exchange method, and their conformation and size dependence on pH of the aqueous solution were investigated. The silver nanoparticles stabilized by the oleic acid were produced in aqueous phase by the reduction of silver nitrate, and transferred into organic solvent by the reorientation of oleic acid. The silver colloids were characterized by UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and Turbiscan. 2. Experiment 2.1. Materials Silver nitrate (AgNO3 ), sodium borohydride (NaBH4 ), oleic acid (9-octadecenoic acid), phosphoric acid (H3 PO4 ), sulfuric acid (H2 SO4 ), ammonium hydroxide (NH4 OH), and cyclohexane were purchased from Aldrich and used without further purification. Double-distilled water was used when necessary.
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2.2. Preparation of silver colloids Silver nanoparticles were prepared using oleic acid as a stabilizer and keeping the molar ratios [AgNO3 ]:[NaBH4 ]:[oleic acid] = 4:16:1. In typical synthesis, the pH of silver nitrate aqueous solution was varied as 2, 7, and 11 using diluted sulfuric acid and ammonium hydroxide. Twenty millilitre of 1 × 10−3 M AgNO3 was added dropwise to equal volume of 4 × 10−3 M NaBH4 containing 2.5 × 10−4 M oleic acid, and mixed by vigorous stirring for 2 h. A brown-yellowish colloidal of silver hydrosol stabilized by oleic acid was obtained. Twenty millilitre of cyclohexane was added to equal volume of silver hydrosol to make biphasic mixture with distinct two layers. 0.16 ml of 0.1 M H3 PO4 was added to the biphasic mixture under vigorous stirring, and phase transfer was immediately induced. Hydrophobic silver colloids were obtained by phase transfer induced by H3 PO4 . As the silver particles became hydrophobic, the color of the cyclohexane layer became yellow while the aqueous layer became clear by the transfer of silver particles into the cyclohexane layer. 2.3. Characterization UV–vis spectra for silver colloids in aqueous phase and organic phase were obtained by Varian Cary 100 Conc UV–vis spectrometer in the range 300–550 run with a resolution of 1 nm and 0.5 s of exposure time. The transfer of the oleic acid-capped nanoparticles from aqueous solution into organic solution was monitored by measuring the absorbance change in each phase. The image of the silver nanoparticles after phase transfer was obtained by transmission electron microscopy (JEOL JEM2000EXII instrument) under an acceleration voltage of either 200 kV. A single drop of silver colloids was deposited with a micropipette onto a carbon-coated copper grid and allowed to dry under atmosphere for 5 min after which the extra solution was removed using a blotting paper. Fourier transform infrared spectra were recorded with a 2 cm−1 resolution in transmission mode in the 3500–1000 cm−1 range using Bruker Vector 22 FTIR spectrometer at room temperature. The silver colloids were drop-coated onto the KBr pellet and dried before the spectra were obtained. The dispersion properties of the silver colloids in cyclohexane were measured using Turbiscan (Turbiscan Lab Expert, Formulation). This apparatus allows the investigation of the dispersion stability and homogeneity of the silver nanoparticles in cyclohexane via periodical turbidity scan over sample height. 3. Results and discussion The phase transfer of the silver nanoparticles dispersed in aqueous medium into the organic medium is illustrated in Fig. 1. The figure shows hydrophilic silver colloids in aqueous solution and hydrophobic colloids in cyclohexane. When H3 PO4 is added to silver hydrosol, the carboxylate group in oleic acid is converted to carboxylic acid which caps the silver surface and hydrophobic tail is directed toward solvent media. The phase transfer was observed by the yellow coloration of the organic
Fig. 1. (a) Immiscible layers of the silver hydrosol stabilized by oleic acid (bottom) and cyclohexane before shaking. (b) Silver organosol in cyclohexane and clean aqueous solution at the bottom after the phase transfer induced by the conversion of carboxylate to carboxylic acid in oleic acid.
phase and color loss of the aqueous phase after H3 PO4 addition. Fig. 2 shows the UV–vis spectra of silver colloids in the aqueous phase without stabilizer and those in cyclohexane phase after phase transfer which were produced with various pH values of silver nitrate aqueous solution. More symmetric as the silver hydrosol capped by oleic acid was produced, the absorption maximum was red shifted from 388 to 415 nm. The figure also shows that after the phase transfer, the absorption spectra of the silver colloids in cyclohexane do not change much, but the particles produced with pH 11 (c) show the most symmetric and constant spectra with absorption maximum at 415 nm and a full width at half-maximum (FWHM) of 86 nm. This indicates that the silver nanoparticles produced with pH 11 are more uniform in size and have better dispersion stability. The TEM images of oleic acid-capped hydrophobic silver nanoparticles are shown in Fig. 3. The figure shows that when silver nitrate is reduced under acidic condition, the nanoparticles produced were not quite spherical with non-uniform size distribution, but under basic condition, the particles were produced with spherical shape and uniform size distribution. The mean diameter of the particles was 9, 6, and 8 nm from silver nitrate solution at pH 2, pH 7, and pH 11, respectively. Fig. 4 shows the FTIR spectra of the oleic acid-capped silver nanoparticles. In general, the asymmetric and symmetric stretching bands of CH2 and CH3 are appeared in the region 2920–2850 cm−1 , or region 2953–2872 cm−1 , respectively [15]. The frequency of the CH and COO− stretching mode was observed at around 3018 and 1560 cm−1 , respectively. CH2 and CH3 stretching mode was observed in the region 2969–2954 cm−1 . The figure confirms that oleic acid is attached to the surface of silver nanoparticle to provide hydrophobic property. Fig. 5 shows the backscattering flux profile of silver nanoparticles in hexane which were produced with various pH conditions measured by Turbiscan. The glass cell containing silver nanoparticles dispersed in hexane was placed in the apparatus and its
160
D. Seo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 158–161
Fig. 2. UV–vis spectra of the silver sol before and after phase transfer: curve 1, silver nanoparticles prepared in aqueous phase; curve 2, oleic acid-capped silver nanoparticles in cyclohexane immediately after phase transfer; curve 3, silver nanoparticles in cyclohexane after 24 h of phase transfer; curve 4, silver nanoparticles in cyclohexane after 72 h of phase transfer. Ag nanoparticles were prepared from silver nitrate solution solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
flocculation behavior was monitored every 30 min for 24 h. The backscattering intensity versus cell height was collected. Fig. 5(a) shows that more stable dispersion property can be observed from silver nanoparticles produced with pH 11, which
Fig. 3. TEM images of the hydrophobic Ag nanoparticles prepared from silver nitrate solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
coincides with the result of UV–vis observation. The figure also shows the decrease of backscattering flux at the top, which indicates the sedimentation of silver nanoparticles. Fig. 5(b) shows the time variation of mean value of backscattering flux. The fig-
D. Seo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 158–161
161
ure shows that flocculation (decrease of flux with time) occurs in silver particles produced with pH 2 and 7, and better dispersion stability can be observed with pH 11. It is postulated that the particle size was minimum at pH7 because the zeta potential of the particle becomes higher as the solution becomes neutral [16]. The size of the particle depends on the absolute value of the zeta potential. 4. Conclusion
Fig. 4. FTIR spectra of the hydrophobic Ag nanoparticles prepared from silver nitrate solution at: (a) pH 2, (b) pH 7, and (c) pH 11.
Oleic acid-capped silver nanoparticles have been prepared from reduction reaction of silver nitrate in aqueous phase at various pH values. The phase transfer from hydrophilic to hydrophobic state was induced by the local reorientation of oleic acid due to the conversion of carboxylate group to carboxylic acid group in oleic acid by the addition of H3 PO4 . When silver nitrate is reduced by NaBH4 in aqueous phase under basic condition (pH 11), nanoparticles were produced with spherical shape, uniform size distribution, and good dispersion stability. Acknowledgement This research was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (2006-05382). References
Fig. 5. (a) Turbidity scan over sample cell height containing Ag nanoparticles in cyclohexane and (b) time-dependent turbidity.
[1] A.J. Hoffman, G. Mills, H. Yee, M.R. Hoffman, J. Phys. Chem. 96 (1992) 5546. [2] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [3] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [4] N. Toshima, T. Yonezawa, New J. Chem. (1998) 1179. [5] H.S. Kim, J.H. Ryu, B. Jose, B.G. Lee, B.S. Ahn, Y.S. Kang, Langmuir 17 (2001) 5817. [6] P.W. Wu, B. Dunn, V. Doan, B.J. Schwartz, E. Yablonovitch, M.J. Yamane, Sol-Gel Sci. Technol. 19 (2000) 249. [7] M. Ji, X. Chen, C.M. Wai, J.L. Fulton, J. Am. Chem. Soc. 121 (1999) 2631. [8] J.J. Mock, S.J. Oldenburg, D.R. Smith, D.A. Schultz, S. Schultz, Nano Lett. 2 (2002) 465. [9] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065. [10] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [11] H. Nagasawa, M. Maruyama, T. Komatsu, S. Isoda, T. Kobayashi, Phys. Status Solidi A 191 (2002) 67. [12] W. Wang, S. Efrima, O. Regev, Langmuir 14 (1998) 602. [13] W. Wang, X. Chen, S. Efrima, J. Phys. Chem. B 103 (1999) 7238. [14] A. Kumar, H. Joshi, R. Pasricha, A.B. Mandale, M. Sastry, J. Colloid Sci. 264 (2003) 396. [15] L.H. Daly, N.B. Colthup, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975. [16] K.D. Kim, D.N. Han, H.T. Kim, Chem. Eng. J. 104 (2004) 55.