Preparation of gold nanosheets using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers via photoreduction

Materials Science and Engineering B 140 (2007) 182–186

Preparation of gold nanosheets using poly(ethylene oxide)–poly (propylene oxide)–poly(ethylene oxide) block copolymers via photoreduction Sang-Ho Cha, Jong-Uk Kim, Ki-Hyun Kim, Jong-Chan Lee ∗ School of Chemical and Biological Engineering, Seoul National University, Shilim-9-Dong, Gwanak-Gu, Seoul 151-744, Republic of Korea

Abstract Gold nanosheets having single crystalline structure were successfully synthesized using the bulk phase mixture of HAuCl4 and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers through the irradiation of a glow lamp for 5 days. When the molar ratio of propylene oxide to ethylene oxide block units in the block copolymer is about 1.75, mostly gold nanosheets were obtained. Gold nanosheets with an average width of 8 and 5 ␮m were obtained from the when the molar ratio of gold salt to the ethylene oxide units in the block copolymer were 1/80 and 1/160, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Gold; Amphiphilic polymers; Photoreduction; Optical properties

1. Introduction The synthesis of anisotropic gold nanocrystals such as nanorods [1], nanowires [2] and nanosheets [3] has attracted much attention due to their shape and size dependent optical [4], magnetic [5] and mechanical properties [6]. Especially, gold nanosheets have been synthesized very recently using several synthetic methods for their possible applications as conductivity tips [7] and flat substrates [8] in scanning tunneling microscopy. For example, gold nanosheets with the lateral size ranging from 100 nm to 1.8 ␮m and various shapes such as triangle, truncated triangle and hexagon have been synthesized by heating the aqueous solution of HAuCl4 with linear polyethylenimine (LPEI) [9] or by microwave irradiation of an ethylene glycol solution containing gold precursors and polyvinylpyrrolidone (PVP) [10], where the polymeric surfactants, LPEI and PVP, were known to control the crystal growth rate of various faces by the adsorption and desorption processes on the metal surfaces. Although these gold nanosheets with different shapes and sizes have been successfully synthesized from a variety of methods, clear explanation for such synthesis has not been made yet. There are still a lot of challenges to develop new methods for the shape-controlled synthesis of gold nanosheets with a high

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yield, which might lead to mass production for the practical applications. We reported a novel synthetic method for gold nanowires and nanosheets using the bulk phases of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer [11]. The gold nanowires were prepared from the mixture of the block copolymer and HAuCl4 containing a small amount of water through UV irradiation at room temperature where the bulk phases of block copolymer has a lamellar structure. The gold nanosheets were prepared by heating the polymer mixture at 70 ◦ C where the mixture has an isotropic state. Therefore, the structures of the bulk phase and the reduction conditions affect the shape of the resulting nanocrystals. Accidentally, we found that nanosheets can also be prepared from the block copolymer mixture having a lamellar structure in very high yield by sunshine or the irradiation of a glow lamp. In this paper we report the details of such synthesis including the effect the composition of the block copolymers. 2. Experimental Gold salt, hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 ·3H2 O), was purchased from Aldrich. PEO–PPO–PEO block copolymers (Pluronic P85, P103, P105, L121 and P123) were purchased from BASF. Tetrahydrofuran was dried by refluxing it with sodium and benzophenone followed by distillation.

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The gold salt (39.4 mg, 0.10 mmol) was dissolved in THF solution (5 mL) of Pluronic P85 (PEO26 PPO40 PEO26 , 0.70 g, 0.15 mmol), P103 (PEO17 PPO60 PEO17 , 1.18 g, 0.24 mmol), P105 (PEO37 PPO56 PEO37 , 0.70 g, 0.11 mmol), L121 (PEO5 PPO70 PEO5 , 3.59 g, 0.80 mmol) and P123 (PEO20 PPO70 PEO20 , 1.18 g, 0.20 mmol), respectively. The amounts of the block copolymers were decided to have the same mole ratio of Au3+ to ethylene oxide unit of 1/80. Then THF was evaporated using a rotary evaporator and vacuum drying at 30 ◦ C for 1 day. After all the THF was removed by vacuum distillation, the remaining mixture was exposed to the glow lamp (tungsten-filament incandescent lamp, 200 W, 220 V) to reduce the gold salts at room temperature for 5 days. The vial containing the mixture was rotated using a motor to irradiate the mixture uniformly. The mixture was subsequently purified through centrifugation using THF to remove the block copolymers and any remaining salts, and then the gold nanocrystals were obtained. The sizes and shapes of the gold nanosheets were studied by JEM-200CX transmission electron microscope (TEM) operating at 200 keV and JSM 6330-F scanning electron microscope (SEM). UV–vis spectra were obtained from the Agilent 8453E spectrometer. 3. Results and discussion Previously we reported that gold nanosheets could be prepared by heating a mixture composed of hydrogen tetrachloroaurate(III) trihydrate and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO20 PPO70 PEO20 , Pluronic P123) block copolymer at 70 ◦ C for 24 h [11]. While trying to find the optimum condition to prepare gram scale of gold nanosheets in high yield by changing reaction temperature and other reaction parameters, we accidentally found gold nanosheets were obtained in very large quantity from a vial containing the same block copolymer mixture placed next to the window in the sunshine for several days. However, the mixture with the same composition placed inside of the laboratory without the direct sunshine did not produce any gold nanocrysytals

Fig. 1. UV–vis absorption spectra of gold nanocrystals after glow lamp irradiation for: (a) 0 day, (b) 1 day, (c) 2 days, (d) 3 days, (e) 4 days and (f) 5 days on the gold complex prepared with Pluronic P123. The inset shows clearly that intensities of peaks due to the longitudinal plasmon resonance of gold nanosheets increased in the range of 500–900 nm.

Fig. 2. Planar (a) and side (b) SEM images of typical gold nanosheets prepared from 5-day glow lamp irradiation on the gold complex composed with Pluronic P123 and HAuCl4 .

Fig. 3. Typical TEM image of a single hexagonal gold nanosheet on TEM copper grid and corresponding selected area electron diffraction (SAED) pattern (inset).

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Fig. 4. SEM images of gold nanocrystal prepared by 5-day glow lamp irradiation on the gold complex composed with HAuCl4 and (a) Pluronic L121, (b) P85, (c) P105, (d) P103 and (e) P123 having the ratio of gold salt to ethylene oxide unit is 1/160, respectively.

indicating that reduction did not occur. In the previous work we also found that the same gold salt mixture irradiated with UV light produces gold nanocrystals composed of different shapes including nanoparticles and nanowires. Therefore, we thought that the reduction from the gold salt in the block copolymer to gold nanosheet is caused by the irradiation of the visible light

and/or infra red in the sunshine. As a handy way to produce the light in the laboratory we used a glow lamp to reduce the gold salt in the block copolymer mixture to gold nanocrystals. We monitored UV–vis absorption of the mixture with the irradiation of the glow lamp (Fig. 1). Before the irradiation, only one strong absorption band centered at ∼330 nm corresponding to

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Table 1 Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers used in this study Pluronic

Molecular weight

Number of propylene oxide (PO) unit

Number of ethylene oxide (EO) unit

PO/EO

Composition

P85a P105a P103b P123b L121a

4600 6500 4950 5800 4500

40 56 60 70 70

52 74 34 40 10

0.77 0.76 1.76 1.75 7

EO26 PO40 EO26 EO37 PO56 EO37 EO17 PO60 EO17 EO20 PO70 EO20 EO5 PO70 EO5

a b

Mixtures of gold nanocrystals are formed. Mostly gold nanosheets are formed.

the AuCl4 − solution is observed [12]. The intensity of this band decreased with increasing the glow lamp irradiation time indicating that the salt concentration decreases with time, and then this band disappears after 5-day irradiation. On the other hand, a new broad band from around 500 nm originated from the longitudinal plasmon resonance of gold nanocrystals developed with increasing the glow lamp irradiation time indicating the formation of gold anisotropic nanoscrystals [13]. These nanocrystals were found to be mostly nanosheets with the average width of about 8 ␮m and the thickness of about 100 nm as shown in the SEM image in Fig. 2a and b, respectively. These nanosheets are composed of different shapes such as triangle, pentagon and hexagon with small amount of spherical nanoparticles. However, we could not achieve the shape-controlled synthesis of gold nanosheets having only one shape as others [14–17]. A typical TEM image of a single gold nanosheet is presented in Fig. 3. Hexagonal symmetry diffraction spot in selected area electron diffraction (SAED) pattern (inset of Fig. 3) reveals that the gold nanosheet is a single crystal with a growth direction along with {1 1 1} plane. We studied the effect of the composition (propylene oxide/ethylene oxide ratio in the block copolymer) of the polymer on the formation of the gold nanocrystal using the mixtures prepared from other block copolymers such as Pluronic P85, P103, P105 and L121, where the ratio of HAuCl4 to ethylene oxide units in block copolymer was fixed to 1/80. The compositions and molecular weight of these polymers are shown in Table 1. When the block copolymer mixtures containing P85, P105 and L121 were irradiated with the glow lamp for 5 days, gold nanocrystals composed of mixtures of various shapes were obtained as shown in Fig. 4a–c, respectively. However when the mixture containing P103 (the PPO/PEO block ratio is 1.76, almost same with that of P123) was used, mostly gold nanosheets were obtained (see Fig. 4d). Therefore, the anisotropic growth of gold crystal for the formation of gold nanosheets can be affected by the hydrophobic/hydrophilic balance of block copolymer; the two-dimensional gold crystal growth is predominant when the PPO/PEO block ratio of block copolymer is around 1.75. We could control the size of gold nanosheets by changing the ratio of ethylene oxide units to gold salt. Fig. 4e shows the SEM image of gold nanocrystals obtained from the mixture of P123 and the gold salt having the ratio of HAuCl4 to ethylene oxide units of 1/160. Again mostly gold nanosheets were obtained while the average width (about 5 ␮m) of these nanosheets was found to be smaller than that obtained from the mixture with that

having the ratio of 1/80 shown in Fig. 2a. This suggests that the size of gold nanosheets can be further controlled by changing the ratio of ethylene oxide units to gold salt. The reason for the formation of nanosheets from the block copolymer mixture through the irradiation of glow lamp or sunshine is not clear. Irradiation by a glow lamp can increase the temperature of the mixture up to 40 ◦ C. When the mixture was heated to 40 ◦ C without the glow lamp irradiation, gold salt was not reduced, indicating that such heating does not affect the formation of gold nanosheets. It is already known that the ethylene oxide part in the block copolymer can work as a capping reagent to produce the nanocrystals with anisotropic shapes [18–21]. As mentioned above when the same mixture was irradiated with UV for 24 h, mixtures of nanowires and nanoparticles were obtained. For the direct comparison we irradiate this mixture with the glow lamp for 24 h. Then nanosheets were obtained again while their sizes were much smaller (the average width is less than 1 ␮m) and also the yield was very low (large amounts of gold salts were remained). Possibly the irradiation of the glow lamp giving the milder reduction condition compared with the UV light leads to the formation of larger size of gold nanocrystals such as the gold nanosheets. Other gold nanosheets by others also have been prepared under a mild reduction conditions using seaweed [22], sodium sulfide [23] and PVP [24], not using strong reducing agent such as sodium borohydride. 4. Conclusion Micrometer sized gold nanosheets having single crystalline structure were successfully synthesized using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO– PPO–PEO) block copolymers in the polymeric bulk phase through photoreduction using a glow lamp. When the molar ratio PPO/PEO block units is about 1.75, mostly gold nanosheets were obtained, while when the ratio is much larger or smaller (7.0 or 0.75) than this, a mixture of gold crystals with different shapes were obtained. The size of gold nanosheets was controlled by changing the ratio of the gold salt to block copolymer. Acknowledgements This work was supported by the Korea Science and Engineering Foundation through the HOMRC, the Research Institute of Engineering Science (RIES) at Seoul National University and MRSEC-NSF at University of Massachusetts. The authors

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