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Synthesis of gold nanorods and nanowires by a microwave–polyol method
Materials Letters 58 (2004) 2326 – 2330 www.elsevier.com/locate/matlet
Synthesis of gold nanorods and nanowires by a microwave–polyol method Masaharu Tsuji a,b,c,*, Masayuki Hashimoto b, Yuki Nishizawa b, Takeshi Tsuji a,b,c a
b
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan c CREST, Japanese Science and Technology, Kasuga, Fukuoka 816-8580, Japan Received 11 December 2003; accepted 20 February 2004 Available online 17 April 2004
Abstract HAuCl4 was reduced by ethylene glycol, in the presence of polyvinylpyrrolidone (PVP) under microwave (MW) heating in a continuous wave (CW) mode for 2 min. Dominant products were polygonal nanoplates and close-to-spherical nanoparticles of gold. In addition, small amounts of single crystalline gold nanorods and nanowires (0.5 – 3% of total number of products) with diameters of 20 – 100 nm and lengths of 0.6 – 5 Am were produced. The diameter and length of gold nanorods and nanowires could be controlled by changing the HAuCl44H2O/ PVP ratio. The formation mechanism of anisotropic gold nanostructures was discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Gold nanowire; Polyol method; Microwave heating; TEM
1. Introduction A variety of metallic nanostructures, including particles, prisms, plates, rods, and wires, have generated significant scientific and technological interest because of their unique optical properties as well as their novel chemical and catalytic properties. Among them, gold nanostructures with well-defined dimensions represent a particular class of interesting nanomaterials to synthesize and study because of their wide practical application. Compared with extensive experimental studies on the synthesis of gold nanoparticles and nanorods with well-defined dimensions [1 –6], little work has been made for the synthesis of gold nanowires. Here, we define nanorods and nanowires as materials with aspect ratios of 220 and >20, respectively. The most widely used method for generating metallic nanowires is template-directed synthesis that involves either chemical or electrochemical depositions [7 –9]. For the formation of needle and rod/wire shapes of gold, polycarbonate track-etched membranes [7], pores of alu* Corresponding author. Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan. Fax: +8192-583-7815. E-mail address: [email protected] (M. Tsuji). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.020
mina membranes [8], and HTAC (25 –30 wt.%) [9] were used as templates. The disadvantage of the templatedirected synthesis is that the templates must be removed in order to recover the individual nanowires. To avoid the step of template removal, a new convenient process must be developed. Microwave (MW) dielectric heating is a new promising technique for preparation of size-controlled metallic nanostructures due to its rapid heating and penetration. Gold nanoparticles have recently been synthesized under MW heating (480 –1100 W) by Tu and Liu [10], Jiang et al. [11], Pastoriza-Santos and Liz-Marza´n [12], and Liu et al. [13]. When HAuCl43H2O or HAuCl4 was reduced in methanol [10], ethanol [11], N,N-dimethylformamide [12], or water and sodium citrate [13] for 0.515 min, spherical nanoparticles with diameters below 85 nm were synthesized. Malikova et al. [14] synthesized triangular and hexagonal nanoplates of gold by reduction of HAuCl43H2O in N,Ndimethylformamide with salicylic acid, while Gao et al. [15] prepared them in aqueous solutions with the surfactant cetyltrimethylammonium bromide under conventional oilbath heating. These triangular and hexagonal nanoplates have also been synthesized more selectively by the photoreduction of NaAuCl4 and HAuCl4 in the presence of PVP or polyvinylalcohol [16,17].
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More recently, we have prepared polygonal gold nanoplates by reduction of HAuCl44H2O in ethylene glycol in the presence of PVP under MW heating in a CW or pulse mode [18 –20]. A mixture of triangular, square, hexagonal, and close-to-spherical gold nanoplates was synthesized. One reason for the lack of formation of polygonal nanoplates in the previous MW – polyol methods [10 – 13] may be lower reaction temperatures in methanol (b.p. 65 jC), ethanol (78 jC), water (100 jC), and N,N-dimethylfolmamide (156 jC) than that in ethylene glycol (198 jC) used in our study. Sun et al. [21] have succeeded in the synthesis of uniform silver nanowires by reducing AgNO3 with ethylene glycol in the presence of seeds and PVP under oilbath heating. They found that both morphology and aspect ratios of these silver nanostructures could be varied from nanoparticles and nanorods to long nanowires by adjusting the reaction conditions, including the ratio of PVP to silver nitrates. By analogy with their study for silver, similar anisotropic nanostructures may be created in the case of gold. In the present study, we have attempted to synthesize gold nanorods and nanowires by a MW – polyol method in a one pot. In addition to isotropic polygonal nanoplates and close-to-spherical nanoparticles, small amounts of anisotropic gold nanorods and nanowires could be synthesized under CW MW heating for only 2 min. It was found that the diameter and length of gold nanorods and nanowires can be controlled by changing the HAuCl44H2O/PVP ratio. To the best of our knowledge, this is the first synthesis of gold nanorods and nanowires by a MW – polyol method within a few minutes. The great advantage of this technique is that templates are unnecessary as well as a short crystallization time.
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graphs, PVP was separated from gold nanostructures by centrifugation. In this case, the reaction mixtures was diluted with water and centrifuged at 13,000 rpm for 60 min. Obscure TEM photographs were obtained without this procedure due to a heavy coverage of PVP.
3. Results and discussion At first, gold nanostructures were synthesized without adding PVP in order to examine the effects of PVP. Thus, only large spherical gold particles with diameters of 100– 300 nm were produced in solution, as shown in TEM photograph (Fig. 1). These large spherical gold particles must be formed by aggregation of small particles due to the lack of surfactant PVP. Fig. 2(a) – (d) shows TEM photographs of products obtained at four different HAuCl44H2O/PVP molar ratios at a constant PVP concentration of 1.03 mmol dm 3 . At the lowest HAuCl44H2O/PVP ratio of 1.2103, a small triangular, square, rhombic, hexagonal, and close-to-spherical particles with diameters of about 20– 40 nm are produced [Fig. 2(a)]. It should be noted that a nanowire with a diameter of 10 nm and a length of 600 nm is produced. At an HAuCl44H2O/PVP ratio of 4.7103 [Fig. 2(b)], larger triangular, square, rhombic, hexagonal, and close-to-spherical particles with diameters of 20 –100 nm were produced. In addition, a few nanorods and nanowires with a diameter of f20 nm and lengths of 0.3 –1.4 Am were obtained. At an HAuCl44H2O/PVP ratio of 9.3103 [Fig. 2(c)], besides larger triangular and hexagonal nanoplates and spherical particles, larger nanorods and nanowires with diameters of 40 –80 nm and lengths of 0.4 –2.0 Am are observed. At the highest HAuCl44H2O/PVP ratio of 1.9102 [Fig. 2(d)], in addition to small and large nanoplates and spherical nanoparticles, a long nanowire with a
2. Experimental An MW oven was modified by installing a condenser and thermocouple through holes of the ceiling and a magnetic stirrer coated with Teflon at the bottom. A 100-ml glass flask was placed in an MW oven and connected to a condenser. A resolved solution of HAuCl44H2O (20 –160 mg: 0.048 – 0.384 mmol) in ethylene glycol (20 ml: 0.357 mol) containing PVP (average molecular weight: 40,000, 2.288 or 4.576 g: corresponding to 20.6 or 41.2 mmol in term of monomeric units, respectively) was irradiated by MW in a CW mode (Shikoku Keisoku: 400 W). PVP acts as a stabilizer of small gold nanostructures. For the solution irradiated by CW MW, the temperature increased linearly and abruptly with a fast heating rate and reached 196 jC after 1 min. The solution was kept at 196 jC for 1 min. Thus, the total irradiation time of MW was only 2 min. Product particles were characterized by using transmission electron microscopy (TEM: JEOL JEM-200CX) and UV-visible absorption spectroscopy (Shimadzu UV-2450). Before measurements of TEM photo-
Fig. 1. TEM photographs of gold nanoparticles obtained by pulse MW heating after 2 min without adding PVP.
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Fig. 2. TEM photographs of gold nanostructures obtained by MW heating after 2 min. HAuCl44H2O/PVP molar ratios were (a) 1.2103, (b) 4.7103, (c) 9.3103, and (d) 1.9102 at a PVP concentration of 1.03 mmol dm3, and (e) 9.3103 at a PVP concentration of 2.06 mmol dm3. (f) Electron diffraction pattern of the longest nanowire shown in panel (e).
diameter of 70 nm and a length of 3 Am is observed. These results indicated that the size of particles and plates and the diameter and length of nanorods and nanowires generally increased with increasing the HAuCl44H2O/PVP ratio. The relative number ratio of nanorods and nanowires to the total number of products increased from about 0.5% to 2% with increasing the HAuCl44H2O/PVP molar ratio from 1.2103 to 9.3103, then decreased to 1% at the highest HAuCl44H2O/PVP ratio of 1.9102. As shown later, the reduction of HAuCl4 was not completed and a small amount of reagent remained in the solution at the highest HAuCl44H2O/PVP ratio of 1.9102. When more PVP was added, the reduction was completed. Fig. 2(e) shows the TEM photograph obtained at an HAuCl44H2O/PVP ratio of 9.3103 under a higher PVP concentration of 2.06 mmol dm3. Besides large triangular and hexagonal nanoplates and spherical particles, a few long nanowires with diameters of 60 –100 nm and lengths of 2– 5 Am are produced. The relative number ratio of nanorods and nanowires to the total number of products was about 3%, which was larger than 1% under the condition of Fig. 2(d). The formation of anisotropic gold nanostructures probably starts by the following reactions according to reduction
mechanism of metallic ions in ethylene glycol proposed by Fievet et al. [22]: CH2 OH CH2 OH ! CH3 CHO þ H2 O;
ð1aÞ
6CH3 CHO þ 2AuðIIIÞ ! 2Au þ 6Hþ þ 3CH3 COCOCH3 : ð1bÞ As a result of the above reduction process, nucleation and growth processes of gold yield a mixture of polygonal plates and close-to-spherical nanoparticles. These nanoplates and nanoparticles were well dispersed because of the presence of a polymeric surfactant PVP that could chemically adsorb onto the surfaces of gold solid probably through an interaction between an N – C=O group and Au. The surface energies of large particles are lower than those of smaller ones. It is therefore expected that some small nanoparticles are grown to larger ones via an Ostwaldripening process [23]. With the assistance of PVP, some of the large nanoparticles were grown into anisotropic rodshaped and wire structures. PVP acts as a kinetically controller of the growth rates of different crystalline faces through adsorption and desorption. The PVP adsorbed on specific crystalline surfaces could significantly decrease
M. Tsuji et al. / Materials Letters 58 (2004) 2326–2330
their growth rates and lead to a highly anisotropic growth [24]. Small gold nanoplates and nanoparticles were consumed with increasing the reaction time, and then nanorods, nanowires, and more stable larger nanoparticles were produced. In the oil-bath heating, besides small amounts of short nanorods with aspect ratios below 5 and nanoplates, large spherical particles (>100 nm) were dominantly produced [18 – 20]. No evidence for the formation of nanowires was observed in the oil-bath heating. Particles and plates in the oil-bath heating had less sharp edges than those formed under CW MW heating due to lower crystallization. Size and morphology of gold nanostructures obtained under MW heating were different from those obtain by the oil-bath heating. It was therefore concluded that irradiation effects of MW such as MW dielectric heating of Au3+ and Au, superheating, and/or nonthermal acceleration of chemical reaction, contribute to the nucleation and growth of Au nanostructures. Higher crystallization leading to single crystalline Au nanorods and nanowires under MW heating must be induced by bursting nucleation due to rapid and homogeneous dielectric heating, which cannot be achieved by oilbath heating. We found here that the HAuCl44H2O/PVP ratio plays an important role in determining the morphologies of final products. A heavy coverage of PVP on the surfaces of nanoplates and nanoparticles results in an isotropic growth for all different faces leading to small nanostructures. With decreasing the coverage of PVP on the surfaces, the chance for the formation of anisotropic rod and wire structures increases. A decrease in coverage not only for the fastgrowing end faces, but also for the side surfaces of each nanowire occurs at high HAuCl44H2O/PVP ratios. Thus, thicker and longer nanowires are generally grown at high HAuCl44H2O/PVP ratios. Selected-area electron diffraction (ED) patterns of nanorods and nanowires were measured. A typical result obtained for the longest wire in Fig. 2(e) is shown in Fig. 2(f). The ED spot array gave the hexagonal symmetry, which was very similar to that reported previously for gold nanoplates [14,16]. Similar ED patterns were obtained for the other nanorods and nanowires. Based on these findings, it was concluded that product nanorods and nanowires are single crystals, and the incident electron beams are perpendicular to (111) facets of the nanowires. Fig. 3 shows UV-visible absorption spectra of reagent (a) and products (b) –(f). The reactant spectrum is composed of an absorption peak of AuCl4 at 328 nm. This peak either becomes weak in spectrum (e) or disappears in spectra (b), (c), (d), and (f). In the product spectra (b) – (f), broad surface plasmon bands of gold nanostructures appear in the 500 – 800 nm region. The observation of a weak peak at 328 nm in spectrum (e) indicates that the reduction of HAuCl4 was not completed at a high HAuCl44H2O/PVP ratio. It is known that a surface plasmon band of spherical gold nanoparticles appears in the 500600 nm region with a sharp
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Fig. 3. Absorption spectra of the original solution (a) and product solutions (b) – (f) obtained by MW heating after 2 min. The HAuCl44H2O/PVP molar ratios were (b) 1.2103, (c) 4.7103, (d) 9.3103, and (e) 1.9102, respectively, at a PVP concentration of 1.03 mmol dm3 and (f) 9.3103 at a PVP concentration of 2.06 mmol dm3. Sample solutions are diluted by a factor of 50.
peak at about 520 nm [4,12,14], while plasmon bands of gold polygonal plates are observed in the 550– 800 nm region [18]. Thus, strong bands in the 500 – 600 nm region observed in spectra (b) – (f) are ascribed to a surface plasmon band of spherical gold nanoparticles, while the longerwavelength bands above 600 nm are attributed to plasmon bands of polygonal gold plates. The plasmon band of polygonal gold plates increases and shifts to red from 610 to 680 nm with increasing the HAuCl44H2O/PVP ratio from 1.2103 to 9.3103, as shown in spectra (b), (c), and (d). Spectrum (e) is composed of a single peak at f570 nm, while spectrum (f) is composed of a broad band with a peak at f620 nm. These observations are consistent with the facts that dominant products in the former case are large spherical particles [Fig. 2(d)], while those in the latter case are large polygonal plates. It is known that gold nanorods and nanowires give peaks above 700-nm region [5]. Because the relative number ratio of gold nanorods and nanowires to the total number of products was small (<3%), its contribution to the observed absorption spectrum will be small.
4. Conclusion In summary, small amounts of single crystalline gold nanorods and nanowires with lateral dimensions of 20 – 100 nm and 0.6 – 5 Am long were synthesized in a one pot under CW MW heating (400 W) for only 2 min. The diameter and length of nanorods and nanowires could be varied by changing the HAuCl44H2O/PVP ratio. A further study is in progress in order to enhance the yields of nanorods and nanowires in the products.
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Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research No. 15651046 from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. We thank the Research Laboratory for High Voltage Electron Microscopy, Kyushu University for the use of TEM.
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[9] K. Esumi, K. Matsuhisa, K. Torigoe, Langmuir 11 (1995) 3285. [10] W. Tu, H.J. Liu, J. Mater. Chem. 10 (2000) 2207. [11] Z.L. Jiang, Z.W. Feng, X.C. Shen, Chin. Chem. Lett. 12 (2001) 551. [12] I. Pastoriza-Santos, L.M. Liz-Marza´n, Langmuir 18 (2002) 2888. [13] F. Liu, C. Ker, Y. Chang, F. Ko, T. Chu, B. Dai, Jpn. J. Appl. Phys. 42 (2003) 4152. [14] N. Malikova, I. Pastoriza-Santos, M. Schierhorn, N.A. Kotov, L.M. Liz-Marza´n, Langmuir 18 (2002) 3694. [15] J. Gao, C.M. Bender, C.J. Murphy, Langmuir 19 (2003) 9065. [16] D. Ibano, Y. Yokota, T. Tominaga, Chem. Lett. 32 (2003) 574. [17] Y. Zhou, C.Y. Wang, Y.R. Zhu, Z.Y. Chen, Chem. Mater. 11 (1999) 2310. [18] M. Tsuji, M. Hashimoto, Y. Nishizawa, T. Tsuji, Chem. Lett. 32 (2003) 1114. [19] M. Tsuji, M. Hashimoto, Y. Nishizawa, T. Tsuji, Houshasen Kagaku 77 (2004) 8. [20] M. Tsuji, M. Hashimoto, Y. Nishizawa, T. Tsuji, Chemistry—A European Journal, to be published. [21] Y. Sun, Y. Yin, B. Mayers, T. Herricks, Y. Xia, Chem. Mater. 14 (2002) 4736. [22] F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32/33 (1989) 198. [23] A.R. Roosen, W.C. Carter, Physica, A 261 (1998) 232. [24] M. Almeida, L. Alcacer, J. Cryst. Growth 62 (1983) 183.
Synthesis of gold nanorods and nanowires by a microwave–polyol method Masaharu Tsuji a,b,c,*, Masayuki Hashimoto b, Yuki Nishizawa b, Takeshi Tsuji a,b,c a
b
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan c CREST, Japanese Science and Technology, Kasuga, Fukuoka 816-8580, Japan Received 11 December 2003; accepted 20 February 2004 Available online 17 April 2004
Abstract HAuCl4 was reduced by ethylene glycol, in the presence of polyvinylpyrrolidone (PVP) under microwave (MW) heating in a continuous wave (CW) mode for 2 min. Dominant products were polygonal nanoplates and close-to-spherical nanoparticles of gold. In addition, small amounts of single crystalline gold nanorods and nanowires (0.5 – 3% of total number of products) with diameters of 20 – 100 nm and lengths of 0.6 – 5 Am were produced. The diameter and length of gold nanorods and nanowires could be controlled by changing the HAuCl44H2O/ PVP ratio. The formation mechanism of anisotropic gold nanostructures was discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Gold nanowire; Polyol method; Microwave heating; TEM
1. Introduction A variety of metallic nanostructures, including particles, prisms, plates, rods, and wires, have generated significant scientific and technological interest because of their unique optical properties as well as their novel chemical and catalytic properties. Among them, gold nanostructures with well-defined dimensions represent a particular class of interesting nanomaterials to synthesize and study because of their wide practical application. Compared with extensive experimental studies on the synthesis of gold nanoparticles and nanorods with well-defined dimensions [1 –6], little work has been made for the synthesis of gold nanowires. Here, we define nanorods and nanowires as materials with aspect ratios of 220 and >20, respectively. The most widely used method for generating metallic nanowires is template-directed synthesis that involves either chemical or electrochemical depositions [7 –9]. For the formation of needle and rod/wire shapes of gold, polycarbonate track-etched membranes [7], pores of alu* Corresponding author. Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan. Fax: +8192-583-7815. E-mail address: [email protected] (M. Tsuji). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.020
mina membranes [8], and HTAC (25 –30 wt.%) [9] were used as templates. The disadvantage of the templatedirected synthesis is that the templates must be removed in order to recover the individual nanowires. To avoid the step of template removal, a new convenient process must be developed. Microwave (MW) dielectric heating is a new promising technique for preparation of size-controlled metallic nanostructures due to its rapid heating and penetration. Gold nanoparticles have recently been synthesized under MW heating (480 –1100 W) by Tu and Liu [10], Jiang et al. [11], Pastoriza-Santos and Liz-Marza´n [12], and Liu et al. [13]. When HAuCl43H2O or HAuCl4 was reduced in methanol [10], ethanol [11], N,N-dimethylformamide [12], or water and sodium citrate [13] for 0.515 min, spherical nanoparticles with diameters below 85 nm were synthesized. Malikova et al. [14] synthesized triangular and hexagonal nanoplates of gold by reduction of HAuCl43H2O in N,Ndimethylformamide with salicylic acid, while Gao et al. [15] prepared them in aqueous solutions with the surfactant cetyltrimethylammonium bromide under conventional oilbath heating. These triangular and hexagonal nanoplates have also been synthesized more selectively by the photoreduction of NaAuCl4 and HAuCl4 in the presence of PVP or polyvinylalcohol [16,17].
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More recently, we have prepared polygonal gold nanoplates by reduction of HAuCl44H2O in ethylene glycol in the presence of PVP under MW heating in a CW or pulse mode [18 –20]. A mixture of triangular, square, hexagonal, and close-to-spherical gold nanoplates was synthesized. One reason for the lack of formation of polygonal nanoplates in the previous MW – polyol methods [10 – 13] may be lower reaction temperatures in methanol (b.p. 65 jC), ethanol (78 jC), water (100 jC), and N,N-dimethylfolmamide (156 jC) than that in ethylene glycol (198 jC) used in our study. Sun et al. [21] have succeeded in the synthesis of uniform silver nanowires by reducing AgNO3 with ethylene glycol in the presence of seeds and PVP under oilbath heating. They found that both morphology and aspect ratios of these silver nanostructures could be varied from nanoparticles and nanorods to long nanowires by adjusting the reaction conditions, including the ratio of PVP to silver nitrates. By analogy with their study for silver, similar anisotropic nanostructures may be created in the case of gold. In the present study, we have attempted to synthesize gold nanorods and nanowires by a MW – polyol method in a one pot. In addition to isotropic polygonal nanoplates and close-to-spherical nanoparticles, small amounts of anisotropic gold nanorods and nanowires could be synthesized under CW MW heating for only 2 min. It was found that the diameter and length of gold nanorods and nanowires can be controlled by changing the HAuCl44H2O/PVP ratio. To the best of our knowledge, this is the first synthesis of gold nanorods and nanowires by a MW – polyol method within a few minutes. The great advantage of this technique is that templates are unnecessary as well as a short crystallization time.
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graphs, PVP was separated from gold nanostructures by centrifugation. In this case, the reaction mixtures was diluted with water and centrifuged at 13,000 rpm for 60 min. Obscure TEM photographs were obtained without this procedure due to a heavy coverage of PVP.
3. Results and discussion At first, gold nanostructures were synthesized without adding PVP in order to examine the effects of PVP. Thus, only large spherical gold particles with diameters of 100– 300 nm were produced in solution, as shown in TEM photograph (Fig. 1). These large spherical gold particles must be formed by aggregation of small particles due to the lack of surfactant PVP. Fig. 2(a) – (d) shows TEM photographs of products obtained at four different HAuCl44H2O/PVP molar ratios at a constant PVP concentration of 1.03 mmol dm 3 . At the lowest HAuCl44H2O/PVP ratio of 1.2103, a small triangular, square, rhombic, hexagonal, and close-to-spherical particles with diameters of about 20– 40 nm are produced [Fig. 2(a)]. It should be noted that a nanowire with a diameter of 10 nm and a length of 600 nm is produced. At an HAuCl44H2O/PVP ratio of 4.7103 [Fig. 2(b)], larger triangular, square, rhombic, hexagonal, and close-to-spherical particles with diameters of 20 –100 nm were produced. In addition, a few nanorods and nanowires with a diameter of f20 nm and lengths of 0.3 –1.4 Am were obtained. At an HAuCl44H2O/PVP ratio of 9.3103 [Fig. 2(c)], besides larger triangular and hexagonal nanoplates and spherical particles, larger nanorods and nanowires with diameters of 40 –80 nm and lengths of 0.4 –2.0 Am are observed. At the highest HAuCl44H2O/PVP ratio of 1.9102 [Fig. 2(d)], in addition to small and large nanoplates and spherical nanoparticles, a long nanowire with a
2. Experimental An MW oven was modified by installing a condenser and thermocouple through holes of the ceiling and a magnetic stirrer coated with Teflon at the bottom. A 100-ml glass flask was placed in an MW oven and connected to a condenser. A resolved solution of HAuCl44H2O (20 –160 mg: 0.048 – 0.384 mmol) in ethylene glycol (20 ml: 0.357 mol) containing PVP (average molecular weight: 40,000, 2.288 or 4.576 g: corresponding to 20.6 or 41.2 mmol in term of monomeric units, respectively) was irradiated by MW in a CW mode (Shikoku Keisoku: 400 W). PVP acts as a stabilizer of small gold nanostructures. For the solution irradiated by CW MW, the temperature increased linearly and abruptly with a fast heating rate and reached 196 jC after 1 min. The solution was kept at 196 jC for 1 min. Thus, the total irradiation time of MW was only 2 min. Product particles were characterized by using transmission electron microscopy (TEM: JEOL JEM-200CX) and UV-visible absorption spectroscopy (Shimadzu UV-2450). Before measurements of TEM photo-
Fig. 1. TEM photographs of gold nanoparticles obtained by pulse MW heating after 2 min without adding PVP.
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Fig. 2. TEM photographs of gold nanostructures obtained by MW heating after 2 min. HAuCl44H2O/PVP molar ratios were (a) 1.2103, (b) 4.7103, (c) 9.3103, and (d) 1.9102 at a PVP concentration of 1.03 mmol dm3, and (e) 9.3103 at a PVP concentration of 2.06 mmol dm3. (f) Electron diffraction pattern of the longest nanowire shown in panel (e).
diameter of 70 nm and a length of 3 Am is observed. These results indicated that the size of particles and plates and the diameter and length of nanorods and nanowires generally increased with increasing the HAuCl44H2O/PVP ratio. The relative number ratio of nanorods and nanowires to the total number of products increased from about 0.5% to 2% with increasing the HAuCl44H2O/PVP molar ratio from 1.2103 to 9.3103, then decreased to 1% at the highest HAuCl44H2O/PVP ratio of 1.9102. As shown later, the reduction of HAuCl4 was not completed and a small amount of reagent remained in the solution at the highest HAuCl44H2O/PVP ratio of 1.9102. When more PVP was added, the reduction was completed. Fig. 2(e) shows the TEM photograph obtained at an HAuCl44H2O/PVP ratio of 9.3103 under a higher PVP concentration of 2.06 mmol dm3. Besides large triangular and hexagonal nanoplates and spherical particles, a few long nanowires with diameters of 60 –100 nm and lengths of 2– 5 Am are produced. The relative number ratio of nanorods and nanowires to the total number of products was about 3%, which was larger than 1% under the condition of Fig. 2(d). The formation of anisotropic gold nanostructures probably starts by the following reactions according to reduction
mechanism of metallic ions in ethylene glycol proposed by Fievet et al. [22]: CH2 OH CH2 OH ! CH3 CHO þ H2 O;
ð1aÞ
6CH3 CHO þ 2AuðIIIÞ ! 2Au þ 6Hþ þ 3CH3 COCOCH3 : ð1bÞ As a result of the above reduction process, nucleation and growth processes of gold yield a mixture of polygonal plates and close-to-spherical nanoparticles. These nanoplates and nanoparticles were well dispersed because of the presence of a polymeric surfactant PVP that could chemically adsorb onto the surfaces of gold solid probably through an interaction between an N – C=O group and Au. The surface energies of large particles are lower than those of smaller ones. It is therefore expected that some small nanoparticles are grown to larger ones via an Ostwaldripening process [23]. With the assistance of PVP, some of the large nanoparticles were grown into anisotropic rodshaped and wire structures. PVP acts as a kinetically controller of the growth rates of different crystalline faces through adsorption and desorption. The PVP adsorbed on specific crystalline surfaces could significantly decrease
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their growth rates and lead to a highly anisotropic growth [24]. Small gold nanoplates and nanoparticles were consumed with increasing the reaction time, and then nanorods, nanowires, and more stable larger nanoparticles were produced. In the oil-bath heating, besides small amounts of short nanorods with aspect ratios below 5 and nanoplates, large spherical particles (>100 nm) were dominantly produced [18 – 20]. No evidence for the formation of nanowires was observed in the oil-bath heating. Particles and plates in the oil-bath heating had less sharp edges than those formed under CW MW heating due to lower crystallization. Size and morphology of gold nanostructures obtained under MW heating were different from those obtain by the oil-bath heating. It was therefore concluded that irradiation effects of MW such as MW dielectric heating of Au3+ and Au, superheating, and/or nonthermal acceleration of chemical reaction, contribute to the nucleation and growth of Au nanostructures. Higher crystallization leading to single crystalline Au nanorods and nanowires under MW heating must be induced by bursting nucleation due to rapid and homogeneous dielectric heating, which cannot be achieved by oilbath heating. We found here that the HAuCl44H2O/PVP ratio plays an important role in determining the morphologies of final products. A heavy coverage of PVP on the surfaces of nanoplates and nanoparticles results in an isotropic growth for all different faces leading to small nanostructures. With decreasing the coverage of PVP on the surfaces, the chance for the formation of anisotropic rod and wire structures increases. A decrease in coverage not only for the fastgrowing end faces, but also for the side surfaces of each nanowire occurs at high HAuCl44H2O/PVP ratios. Thus, thicker and longer nanowires are generally grown at high HAuCl44H2O/PVP ratios. Selected-area electron diffraction (ED) patterns of nanorods and nanowires were measured. A typical result obtained for the longest wire in Fig. 2(e) is shown in Fig. 2(f). The ED spot array gave the hexagonal symmetry, which was very similar to that reported previously for gold nanoplates [14,16]. Similar ED patterns were obtained for the other nanorods and nanowires. Based on these findings, it was concluded that product nanorods and nanowires are single crystals, and the incident electron beams are perpendicular to (111) facets of the nanowires. Fig. 3 shows UV-visible absorption spectra of reagent (a) and products (b) –(f). The reactant spectrum is composed of an absorption peak of AuCl4 at 328 nm. This peak either becomes weak in spectrum (e) or disappears in spectra (b), (c), (d), and (f). In the product spectra (b) – (f), broad surface plasmon bands of gold nanostructures appear in the 500 – 800 nm region. The observation of a weak peak at 328 nm in spectrum (e) indicates that the reduction of HAuCl4 was not completed at a high HAuCl44H2O/PVP ratio. It is known that a surface plasmon band of spherical gold nanoparticles appears in the 500600 nm region with a sharp
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Fig. 3. Absorption spectra of the original solution (a) and product solutions (b) – (f) obtained by MW heating after 2 min. The HAuCl44H2O/PVP molar ratios were (b) 1.2103, (c) 4.7103, (d) 9.3103, and (e) 1.9102, respectively, at a PVP concentration of 1.03 mmol dm3 and (f) 9.3103 at a PVP concentration of 2.06 mmol dm3. Sample solutions are diluted by a factor of 50.
peak at about 520 nm [4,12,14], while plasmon bands of gold polygonal plates are observed in the 550– 800 nm region [18]. Thus, strong bands in the 500 – 600 nm region observed in spectra (b) – (f) are ascribed to a surface plasmon band of spherical gold nanoparticles, while the longerwavelength bands above 600 nm are attributed to plasmon bands of polygonal gold plates. The plasmon band of polygonal gold plates increases and shifts to red from 610 to 680 nm with increasing the HAuCl44H2O/PVP ratio from 1.2103 to 9.3103, as shown in spectra (b), (c), and (d). Spectrum (e) is composed of a single peak at f570 nm, while spectrum (f) is composed of a broad band with a peak at f620 nm. These observations are consistent with the facts that dominant products in the former case are large spherical particles [Fig. 2(d)], while those in the latter case are large polygonal plates. It is known that gold nanorods and nanowires give peaks above 700-nm region [5]. Because the relative number ratio of gold nanorods and nanowires to the total number of products was small (<3%), its contribution to the observed absorption spectrum will be small.
4. Conclusion In summary, small amounts of single crystalline gold nanorods and nanowires with lateral dimensions of 20 – 100 nm and 0.6 – 5 Am long were synthesized in a one pot under CW MW heating (400 W) for only 2 min. The diameter and length of nanorods and nanowires could be varied by changing the HAuCl44H2O/PVP ratio. A further study is in progress in order to enhance the yields of nanorods and nanowires in the products.
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Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research No. 15651046 from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. We thank the Research Laboratory for High Voltage Electron Microscopy, Kyushu University for the use of TEM.
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