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Formation of ordered arrays of gold nanoparticles from CTAB reverse micelles
July 2001
Materials Letters 49 Ž2001. 282–286 www.elsevier.comrlocatermatlet
Formation of ordered arrays of gold nanoparticles from CTAB reverse micelles Jun Lin a,) , Weilie Zhou b, Charles J. O’Connor b a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renmin Street, Changchun 130022, People’s Republic of China b AdÕanced Materials Research Institute, College of Science, UniÕersity of New Orleans, New Orleans, LA 70148, USA Received 26 September 2000; accepted 20 October 2000
Abstract In this presentation, a reverse micelle technique was described to create colloid gold nanoparticles and their self-organization into superlattices. Gold nanoparticles were prepared by the reduction of HAuCL 4 in CTABroctaneq 1-butanolrH 2 O reverse micelle system using NaBH 4 as reducing agent. Dodecanethiol ŽC 12 H 25 SH. was used to passivate the gold nanoparticles immediately after formation of the gold colloid. After re-dispersing in toluene under ultrasonication, a supernatant containing nearly monodispersed dodecanethiol-capped gold nanoparticles was obtained. Self-organization of the gold nanoparticles into 1D, 2D and 3D superlattices was observed on the carbon-coated copper grid by TEM. UV–vis absorption spectra were also used to characterize the gold colloids with and without dodecanethiol capping. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Gold nanoparticles; From reduction of HAuCl 4 in reverse micelle; Self-organization; TEM
1. Introduction Due to their specially small size Ž1–100 nm., metal and semiconductor nanoparticles possess properties Žoptical, catalytic, electrical, magnetic, etc.. that are greatly different from either the corresponding bulk materials or the individual atoms of which they are composed w1–3x. In recent years, more and more attention has been paid to the organization of these metal and semiconductor nanoparticles into two- and three-dimensional superlattices w4–10x. Novel collective properties will be produced due to ) Corresponding author. Tel.: q86-431-5682-828 ext. 5353; fax: q86-431-5698-041. E-mail address: [email protected] ŽJ. Lin..
the interactions of the individual nanoparticles in ordered arrays w4,11x. The basic requirement to initiate ordered self-assembly is to develop stable colloids containing monodispersed nanoparticles with a controlled size distribution and morphologies, inherent van der Waals attraction between the particles, and dispersion forces w12x. It is now established that size monodisperse lyophobic nanocrystals readily self-assemble into ordered hexagonal close-packed arrays upon solvent evaporation w13–15x. Size-selective precipitation is one of the most commonly used methods for narrowing the size distribution, and passivation of the nanoparticle surfaces with organic ligands such as alkylthiol and trioctylphosphine can prevent their aggregation. Following these strategies, many 2D and 3D superstructures of semiconductor
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 8 5 - 2
J. Lin et al.r Materials Letters 49 (2001) 282–286
nanoparticles w11,13x and metal nanoparticles w10,14,16–18x have been produced by self-assembly. Gold colloids have been studied for many years due to their unique physical and chemical properties, and their wide potential applications w4x. Self-organization of gold nanoparticles into 2D and 3D superlattices has also been realized in several systems, such as the gold–dithiol system w19x, two-phase system w20x, gas-phase system followed by solution-encapsulation w21x, A–B diblock polymer system w22x. In most of the above cases, gold nanoparticles were capped with alkanethiols and had a narrow size distribution. Interestingly enough, self-organization of two distinct-sized gold nanoparticles Žcapped by alkanethiols. into ordered raft has been reported by Kiely et al. w23x recently. Although the gold nanoparticles were prepared by Barnickel and Wokaun w24x in CTAB Žcetyltrimethylammonium bromide.rnhexanolrH 2 O system, neither morphology nor selforganization for the gold nanoparticles were given therein. As a compensation of previous work w24x, we describe a simple and practical method for selforganization of gold nanoparticles into ordered arrays directly from CTABroctaneq 1-butanolrH 2 O reverse micelle system without any selective precipitation and washing procedures in this paper.
283
of the gold colloid was taken out and transferred to a 20-ml vial. One-half milliliter of dodecanethiol C 12 H 25 SH was added into this vial, which immediately resulted in the formation of precipitates. In this way, most of the gold nanoparticles were extracted from the reverse micelle system by dodecanethiol due to its high affinity with gold. The addition of 13.5 ml toluene into the precipitates and ultrasonication for 40 min leaded to partly re-dissolving of the precipitate, i.e. the formation of a suspension. This suspension was placed on table for several days, producing a clear and homogenous red-purple supernatant and a black precipitate. The supernatant mainly contains the smaller C 12 H 25 SH-capped gold nanoparticles ŽC 12 H 25 S–Au. dissolved in toluene, as well as a small amount of other organic precursors. The precipitate mainly consists of larger C 12 H 25 SH-capped gold particles Žblack. and CTAB. The supernatant was used to form superlattices of gold nanoparticles and was characterized by absorption spectra. A supernatant without C 12 H 25 SH addition was prepared as well for the purpose of comparison. Transmission electronic microscopies ŽTEMs. were obtained on a JEOL ŽModel 2100. transmission electron microscope. UV–vis absorption spectra were recorded on a CARY 500 Scan UV–vis–NIR spectrophotometer.
2. Experimental Gold nanoparticles were prepared by the reduction of HAuCl 4 in CTABroctaneq 1-butanolrH 2 O reverse micelle system using NaBH 4 as the reducing agent. Typically, 0.3 g of CTAB, 0.148 ml of 0.056 M HAuCl 4 aqueous solution, 1.0 g octane Žsurfactant. and 0.25 g 1-butanol Žco-surfactant. were mixed together and stirred vigorously for 10 min until a homogenous phase was obtained wcalled reverse micelle Ža.x. Reverse micelle Žb. was prepared in the same way except that the same volume of 0.32 M NaBH 4 aqueous solution replaced the HAuCl 4 solution. Reverse micelle Žb. was slowly added to reverse micelle Ža. under ultrasonic agitation. A darkred color developed with release of gas during this process, indicating the oxidation–reduction reaction occurred with the formation of gold colloid. Due to the high stability of the gold colloid, its color remains unchanged for several months. A 1 ml aliquot
3. Results and discussion Self-organization of gold nanoparticles into superstructures was obtained by evaporation of the above-prepared C 12 H 25 SH-capped gold supernatant on carbon-coated copper grids. Three kind of superstructures have been observed by transmission electron microscopy ŽTEM., as shown in Fig. 1 Ža. one-dimensional nanowire Ž1D wire., Žb. two-dimensional nanoarray Ž2D monolayer. and Žc. three-dimensional superlattices Ž3D multilayers., respectively. It is interesting to find the formation of 1D nanowire of gold clusters in the current system ŽFig. 1a.. The chain has a length larger than 1 mm. The average size Ždiameter. of gold clusters in the wire is about 5.3 nm, and the nearest center-to-center distance between the clusters is 7.5 nm. This leads the
284
J. Lin et al.r Materials Letters 49 (2001) 282–286
Fig. 1. TEM micrographs of the C 12 H 25 SH-capped gold nanoparticles self-organized into 1D Ža., 2D Žb. and 3D Žc. superstructures.
gap between two particles to be about 2.2 nm. So far, only a few papers w7,10x have been published on the formation of nanowires of metal colloids, most of which were formed by template assistance. Generally speaking, formation of nanowires without template is not energetically favorable. One possible factor in the current case is that the carbon film used might have wrinkles which might act as template to assemble nanocrystals into wire structure. Additionally, the organic components Žsurfactant, co-surfactant and excess alkanethiol. left in the supernatant and the concentration of gold colloid might play some roles in the formation of nanowires, based on the fact that no such superstructure was formed at lower gold colloid concentration or without the organic components. Further studies are under way. The most frequently observed superlattices are ordered 2D arrays of gold nanoparticles. A representative TEM micrograph of such a 2D array is shown in Fig. 1b. Nearly monodispersed gold nanoparticles formed a closely packed hexagonal array with one
particle surrounded symmetrically by other six particles. The formation of the hexagonal 2D gold nanoarray can be attributed to equilibrium built between the van der Waals attractive forces and the steric repulsions from the thiol carbon chains Žof the gold nanoparticles.. Because these forces are isotropic, this equilibrium yields a compact organization in hexagonal 2D network. Similar to the 1D nanowire, the average size of the gold nanoparticles in the 2D array is about 5.8 nm, and the average center-to-center distance between adjacent clusters is 8.1 nm. So, the average gap between two clusters is 2.3 nm. The thickness of a self-assembled monolayer of dodecanethiol C 12 H 25 SH on Au Ž111. was determined to be 1.2 nm w25x. So, it is reasonable to assume that the gold clusters in our system are covered basically by a monolayer of dodecanethiol Žthe organic ligand layers cannot be observed by TEM, 1.2 nm = 2 s 2.4 nm, i.e. the gap between two particles is about twice the thickness of monolayer of C 12 H 25 SH on gold clusters.. From the
J. Lin et al.r Materials Letters 49 (2001) 282–286
viewpoints of ordering, domain area and defects, the 2D array of gold clusters in our system is comparable with that reported in Ref. w21x, and our reverse micelle method of preparing the gold particles is much simpler than that Žgas phase method. in Ref. w21x. Apart from the ID and 2D superstructures, 3D multilayers were also observed on the substrate by TEM, as illustrated by Fig. 1c. The formation of 3D superlattices can be resolved by careful examination of the TEM micrograph. At the edge of the TEM micrograph, it is easy to see the hexagonal-closepacked monolayer Ž2D. of gold nanoparticles. Great difference in electronic contract can be found between the central part and edge of the micrograph. This suggests the formation of multilayer of particles in the central part. In the second layer, chain arrangements of nanoparticles which appear as straight lines or ring structures can be seen clearly. The same structural pattern has been observed by Fink et al. w26x, which is attributed to the preference of occupying a twofold saddle site between two particles in the first layer rather than threefold hollow sites created where three basal plane particles meet. Finally, we mention that the dodecanethiol played a key role in the formation of the above superlattices. Fig. 2 shows the TEM micrograph of the gold supernatant without C 12 H 25 SH addition. It is seen from
285
Fig. 3. Absorption spectra of gold supernatant with Ža. and without Žb. the addition of C 12 H 25 SH.
Fig. 2 that the bare gold particles grow into single crystals with many different shapes Žcylinder, triangle, spherical, hexagon, trapezoid, pentagon. and sizes Ž7–40 nm.. These morphologies of gold particles are very similar to those of metallic Cu clusters prepared in another kind of microemulsion ŽAOT. system without C 12 H 25 SH capping w12x. In this case, the Au colloid cannot form ordered superstructures because of the wide size distribution and various shapes. However, with dodecanethiol capping, the gold clusters, which are stable with spherical shape, can form superlattices as observed above. The difference between the gold supernatant with and without dodecanethiol addition can also be reflected in the absorption spectra, as shown in Fig. 3. Without C 12 H 25 SH capping, the supernatant shows a broad surface plasmon absorption band with a maximum at 577 nm ŽFig. 3b.; while with C 12 H 25 SH capping, the supernatant exhibits a narrow absorption band with a maximum at 523 nm ŽFig. 3a.. This indicates that size distribution of the gold nanoparticles is indeed narrowed by C 12 H 25 SH capping in the supernatant. The absorption of the gold colloid without dodecanethiol capping occurred at longer wavelength can be attributed to the aggregation of large particles in the gold colloid w27x. All these agree with the TEM data.
4. Conclusion Fig. 2. TEM micrograph of the bare gold nanoparticles without C 12 H 25 SH capping.
Formation of ordered arrays of gold nanoparticles into 1D, 2D, and 3D superstructures can be realized
J. Lin et al.r Materials Letters 49 (2001) 282–286
286
directly and simply from CTABroctane q 1butanolrH 2 O reverse micelle system. Dodecanethiol modification of gold nanoparticle surfaces plays a key role in the formation of superlattices.
Acknowledgements We gratefully acknowledge the support of this work by AMRI through DARPA Grant No. MDA972-97-1-0003 and ABairen JihuaB of Chinese Academy of Sciences.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
A.P. Alivisatos, Science 271 Ž1996. 933. J.R. Heath, Science 270 Ž1995. 1315. Y. Wang, N. Herron, J. Phys. Chem. 95 Ž1991. 525. J.J. Storhoff, R.C. Mucic, C.A. Mirkin, J. Cluster Sci. 8 Ž1997. 179. H. Weller, Angew. Chem., Int. Ed. Engl. 35 Ž1996. 1079. J.H. Fendler, Chem. Mater. 8 Ž1996. 1616. G. Schmid, G.L. Hornyak, Curr. Opin. Solid State Mater. Sci. 2 Ž1997. 204. D. Bethell, D.J. Schiffrin, Nature 382 Ž1996. 581. G. Schmid, L.F. Chi, Adv. Mater. 10 Ž1996. 515. B.A. Korgel, D. Fitzmaurice, Adv. Mater. 10 Ž1998. 661.
w11x L. Motte, F. Billoudet, E. Lacaze, J. Douin, M.P. Pileni, J. Phys. Chem. B 101 Ž1997. 138. w12x M.P. Pileni, in: J.H. Fendler ŽEd.., Nanoparticles and Nanostructured Films, Wiley-VCH, Weinheim, 1998, 71. w13x C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 Ž1995. 1335. w14x S.A. Harfenist, Z.L. Wang, M.A. Alvarez, I. Vezmar, R.P. Whetten, J. Phys. Chem. 100 Ž1996. 13904. w15x M.D. Bentzon, J. Van Wonterghem, S. Morup, A. Tholen, Philos. Mag. B 60 Ž1989. 169. w16x A. Taleb, C. Petit, M.P. Pileni, Chem. Mater. 9 Ž1997. 950. w17x C. Petit, A. Taleb, M.P. Pileni, J. Phys. Chem. B 103 Ž1999. 1805. w18x S. Sun, C.B. Murray, J. Appl. Phys. 85 Ž1999. 4325. w19x M. Brust, D. Bethell, D.J. Schiffrin, J.K. Christopher, Adv. Mater. 7 Ž1995. 795. w20x K.V. Sarathy, G.U. Kulkarni, C.N.R. Rao, Chem. Commun. Ž1997. 537. w21x R.P. Andres, J.D. Bielfeld, J.I. Henderson, D.B. Janes, V.R. Kolagunta, C.P. Kubiak, W.J. Mahoney, R.G. Osifchin, Science 273 Ž1995. 1690. w22x J.P. Spatz, A. Roescher, M. Moeller, Adv. Mater. 8 Ž1996. 337. w23x C.J. Kiely, J. Fink, M. Brust, D. Bethell, D.J. Schiffrin, Nature 396 Ž1998. 444. w24x P. Barnickel, A. Wokaun, Mol. Phys. 69 Ž1990. 1. w25x P.E. Laininis, R.G. Nuzzo, G.M. Whitesides, J. Phys. Chem. 96 Ž1992. 5097. w26x J. Fink, C.J. Kiely, D. Bethell, D.J. Schiffrin, Chem. Mater. 10 Ž1998. 922. w27x U. Kreibig, L. Genzel, Surf. Sci. 156 Ž1985. 678.
Materials Letters 49 Ž2001. 282–286 www.elsevier.comrlocatermatlet
Formation of ordered arrays of gold nanoparticles from CTAB reverse micelles Jun Lin a,) , Weilie Zhou b, Charles J. O’Connor b a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renmin Street, Changchun 130022, People’s Republic of China b AdÕanced Materials Research Institute, College of Science, UniÕersity of New Orleans, New Orleans, LA 70148, USA Received 26 September 2000; accepted 20 October 2000
Abstract In this presentation, a reverse micelle technique was described to create colloid gold nanoparticles and their self-organization into superlattices. Gold nanoparticles were prepared by the reduction of HAuCL 4 in CTABroctaneq 1-butanolrH 2 O reverse micelle system using NaBH 4 as reducing agent. Dodecanethiol ŽC 12 H 25 SH. was used to passivate the gold nanoparticles immediately after formation of the gold colloid. After re-dispersing in toluene under ultrasonication, a supernatant containing nearly monodispersed dodecanethiol-capped gold nanoparticles was obtained. Self-organization of the gold nanoparticles into 1D, 2D and 3D superlattices was observed on the carbon-coated copper grid by TEM. UV–vis absorption spectra were also used to characterize the gold colloids with and without dodecanethiol capping. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Gold nanoparticles; From reduction of HAuCl 4 in reverse micelle; Self-organization; TEM
1. Introduction Due to their specially small size Ž1–100 nm., metal and semiconductor nanoparticles possess properties Žoptical, catalytic, electrical, magnetic, etc.. that are greatly different from either the corresponding bulk materials or the individual atoms of which they are composed w1–3x. In recent years, more and more attention has been paid to the organization of these metal and semiconductor nanoparticles into two- and three-dimensional superlattices w4–10x. Novel collective properties will be produced due to ) Corresponding author. Tel.: q86-431-5682-828 ext. 5353; fax: q86-431-5698-041. E-mail address: [email protected] ŽJ. Lin..
the interactions of the individual nanoparticles in ordered arrays w4,11x. The basic requirement to initiate ordered self-assembly is to develop stable colloids containing monodispersed nanoparticles with a controlled size distribution and morphologies, inherent van der Waals attraction between the particles, and dispersion forces w12x. It is now established that size monodisperse lyophobic nanocrystals readily self-assemble into ordered hexagonal close-packed arrays upon solvent evaporation w13–15x. Size-selective precipitation is one of the most commonly used methods for narrowing the size distribution, and passivation of the nanoparticle surfaces with organic ligands such as alkylthiol and trioctylphosphine can prevent their aggregation. Following these strategies, many 2D and 3D superstructures of semiconductor
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 8 5 - 2
J. Lin et al.r Materials Letters 49 (2001) 282–286
nanoparticles w11,13x and metal nanoparticles w10,14,16–18x have been produced by self-assembly. Gold colloids have been studied for many years due to their unique physical and chemical properties, and their wide potential applications w4x. Self-organization of gold nanoparticles into 2D and 3D superlattices has also been realized in several systems, such as the gold–dithiol system w19x, two-phase system w20x, gas-phase system followed by solution-encapsulation w21x, A–B diblock polymer system w22x. In most of the above cases, gold nanoparticles were capped with alkanethiols and had a narrow size distribution. Interestingly enough, self-organization of two distinct-sized gold nanoparticles Žcapped by alkanethiols. into ordered raft has been reported by Kiely et al. w23x recently. Although the gold nanoparticles were prepared by Barnickel and Wokaun w24x in CTAB Žcetyltrimethylammonium bromide.rnhexanolrH 2 O system, neither morphology nor selforganization for the gold nanoparticles were given therein. As a compensation of previous work w24x, we describe a simple and practical method for selforganization of gold nanoparticles into ordered arrays directly from CTABroctaneq 1-butanolrH 2 O reverse micelle system without any selective precipitation and washing procedures in this paper.
283
of the gold colloid was taken out and transferred to a 20-ml vial. One-half milliliter of dodecanethiol C 12 H 25 SH was added into this vial, which immediately resulted in the formation of precipitates. In this way, most of the gold nanoparticles were extracted from the reverse micelle system by dodecanethiol due to its high affinity with gold. The addition of 13.5 ml toluene into the precipitates and ultrasonication for 40 min leaded to partly re-dissolving of the precipitate, i.e. the formation of a suspension. This suspension was placed on table for several days, producing a clear and homogenous red-purple supernatant and a black precipitate. The supernatant mainly contains the smaller C 12 H 25 SH-capped gold nanoparticles ŽC 12 H 25 S–Au. dissolved in toluene, as well as a small amount of other organic precursors. The precipitate mainly consists of larger C 12 H 25 SH-capped gold particles Žblack. and CTAB. The supernatant was used to form superlattices of gold nanoparticles and was characterized by absorption spectra. A supernatant without C 12 H 25 SH addition was prepared as well for the purpose of comparison. Transmission electronic microscopies ŽTEMs. were obtained on a JEOL ŽModel 2100. transmission electron microscope. UV–vis absorption spectra were recorded on a CARY 500 Scan UV–vis–NIR spectrophotometer.
2. Experimental Gold nanoparticles were prepared by the reduction of HAuCl 4 in CTABroctaneq 1-butanolrH 2 O reverse micelle system using NaBH 4 as the reducing agent. Typically, 0.3 g of CTAB, 0.148 ml of 0.056 M HAuCl 4 aqueous solution, 1.0 g octane Žsurfactant. and 0.25 g 1-butanol Žco-surfactant. were mixed together and stirred vigorously for 10 min until a homogenous phase was obtained wcalled reverse micelle Ža.x. Reverse micelle Žb. was prepared in the same way except that the same volume of 0.32 M NaBH 4 aqueous solution replaced the HAuCl 4 solution. Reverse micelle Žb. was slowly added to reverse micelle Ža. under ultrasonic agitation. A darkred color developed with release of gas during this process, indicating the oxidation–reduction reaction occurred with the formation of gold colloid. Due to the high stability of the gold colloid, its color remains unchanged for several months. A 1 ml aliquot
3. Results and discussion Self-organization of gold nanoparticles into superstructures was obtained by evaporation of the above-prepared C 12 H 25 SH-capped gold supernatant on carbon-coated copper grids. Three kind of superstructures have been observed by transmission electron microscopy ŽTEM., as shown in Fig. 1 Ža. one-dimensional nanowire Ž1D wire., Žb. two-dimensional nanoarray Ž2D monolayer. and Žc. three-dimensional superlattices Ž3D multilayers., respectively. It is interesting to find the formation of 1D nanowire of gold clusters in the current system ŽFig. 1a.. The chain has a length larger than 1 mm. The average size Ždiameter. of gold clusters in the wire is about 5.3 nm, and the nearest center-to-center distance between the clusters is 7.5 nm. This leads the
284
J. Lin et al.r Materials Letters 49 (2001) 282–286
Fig. 1. TEM micrographs of the C 12 H 25 SH-capped gold nanoparticles self-organized into 1D Ža., 2D Žb. and 3D Žc. superstructures.
gap between two particles to be about 2.2 nm. So far, only a few papers w7,10x have been published on the formation of nanowires of metal colloids, most of which were formed by template assistance. Generally speaking, formation of nanowires without template is not energetically favorable. One possible factor in the current case is that the carbon film used might have wrinkles which might act as template to assemble nanocrystals into wire structure. Additionally, the organic components Žsurfactant, co-surfactant and excess alkanethiol. left in the supernatant and the concentration of gold colloid might play some roles in the formation of nanowires, based on the fact that no such superstructure was formed at lower gold colloid concentration or without the organic components. Further studies are under way. The most frequently observed superlattices are ordered 2D arrays of gold nanoparticles. A representative TEM micrograph of such a 2D array is shown in Fig. 1b. Nearly monodispersed gold nanoparticles formed a closely packed hexagonal array with one
particle surrounded symmetrically by other six particles. The formation of the hexagonal 2D gold nanoarray can be attributed to equilibrium built between the van der Waals attractive forces and the steric repulsions from the thiol carbon chains Žof the gold nanoparticles.. Because these forces are isotropic, this equilibrium yields a compact organization in hexagonal 2D network. Similar to the 1D nanowire, the average size of the gold nanoparticles in the 2D array is about 5.8 nm, and the average center-to-center distance between adjacent clusters is 8.1 nm. So, the average gap between two clusters is 2.3 nm. The thickness of a self-assembled monolayer of dodecanethiol C 12 H 25 SH on Au Ž111. was determined to be 1.2 nm w25x. So, it is reasonable to assume that the gold clusters in our system are covered basically by a monolayer of dodecanethiol Žthe organic ligand layers cannot be observed by TEM, 1.2 nm = 2 s 2.4 nm, i.e. the gap between two particles is about twice the thickness of monolayer of C 12 H 25 SH on gold clusters.. From the
J. Lin et al.r Materials Letters 49 (2001) 282–286
viewpoints of ordering, domain area and defects, the 2D array of gold clusters in our system is comparable with that reported in Ref. w21x, and our reverse micelle method of preparing the gold particles is much simpler than that Žgas phase method. in Ref. w21x. Apart from the ID and 2D superstructures, 3D multilayers were also observed on the substrate by TEM, as illustrated by Fig. 1c. The formation of 3D superlattices can be resolved by careful examination of the TEM micrograph. At the edge of the TEM micrograph, it is easy to see the hexagonal-closepacked monolayer Ž2D. of gold nanoparticles. Great difference in electronic contract can be found between the central part and edge of the micrograph. This suggests the formation of multilayer of particles in the central part. In the second layer, chain arrangements of nanoparticles which appear as straight lines or ring structures can be seen clearly. The same structural pattern has been observed by Fink et al. w26x, which is attributed to the preference of occupying a twofold saddle site between two particles in the first layer rather than threefold hollow sites created where three basal plane particles meet. Finally, we mention that the dodecanethiol played a key role in the formation of the above superlattices. Fig. 2 shows the TEM micrograph of the gold supernatant without C 12 H 25 SH addition. It is seen from
285
Fig. 3. Absorption spectra of gold supernatant with Ža. and without Žb. the addition of C 12 H 25 SH.
Fig. 2 that the bare gold particles grow into single crystals with many different shapes Žcylinder, triangle, spherical, hexagon, trapezoid, pentagon. and sizes Ž7–40 nm.. These morphologies of gold particles are very similar to those of metallic Cu clusters prepared in another kind of microemulsion ŽAOT. system without C 12 H 25 SH capping w12x. In this case, the Au colloid cannot form ordered superstructures because of the wide size distribution and various shapes. However, with dodecanethiol capping, the gold clusters, which are stable with spherical shape, can form superlattices as observed above. The difference between the gold supernatant with and without dodecanethiol addition can also be reflected in the absorption spectra, as shown in Fig. 3. Without C 12 H 25 SH capping, the supernatant shows a broad surface plasmon absorption band with a maximum at 577 nm ŽFig. 3b.; while with C 12 H 25 SH capping, the supernatant exhibits a narrow absorption band with a maximum at 523 nm ŽFig. 3a.. This indicates that size distribution of the gold nanoparticles is indeed narrowed by C 12 H 25 SH capping in the supernatant. The absorption of the gold colloid without dodecanethiol capping occurred at longer wavelength can be attributed to the aggregation of large particles in the gold colloid w27x. All these agree with the TEM data.
4. Conclusion Fig. 2. TEM micrograph of the bare gold nanoparticles without C 12 H 25 SH capping.
Formation of ordered arrays of gold nanoparticles into 1D, 2D, and 3D superstructures can be realized
J. Lin et al.r Materials Letters 49 (2001) 282–286
286
directly and simply from CTABroctane q 1butanolrH 2 O reverse micelle system. Dodecanethiol modification of gold nanoparticle surfaces plays a key role in the formation of superlattices.
Acknowledgements We gratefully acknowledge the support of this work by AMRI through DARPA Grant No. MDA972-97-1-0003 and ABairen JihuaB of Chinese Academy of Sciences.
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