Synthesis of Gold Nanoparticles in Liquid Polyethylene Glycol by

4 Feb 2011 - ... Takeshi Morita, Satoshi Takahashi, Kei Onishi, and Keiko Nishikawa*. Graduate School of Advanced Integration Science, Chiba Universit...
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Synthesis of Gold Nanoparticles in Liquid Polyethylene Glycol by Sputter Deposition and Temperature Effects on their Size and Shape Yoshikiyo Hatakeyama, Takeshi Morita, Satoshi Takahashi, Kei Onishi, and Keiko Nishikawa* Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan

bS Supporting Information ABSTRACT: Combining the use of liquid polyethylene glycol (PEG) as a capture medium with the sputter deposition technique, we developed the easy and simple preparation of gold nanoparticles (Au NPs) in liquid PEG with neither chemical reactions nor additional stabilizers. PEG was selected because of its ability to stabilize NPs and its environmental friendliness. We used PEG with an average molecular weight of 600 because it is in the liquid state at room temperature and has a vapor pressure low enough to endure the sputtering operation. Structural characterizations were performed using transmission electron microscope (TEM) observations, small-angle X-ray scattering (SAXS) measurements, and UV-vis absorption measurements. It is revealed that the particle size and shape are strongly dependent on the preparation temperature of PEG. Close investigation of the temperature-dependent properties of PEG suggests that the collision frequency of sputtered Au particles (atoms and small clusters) is one of the most important factors for the determination of particle size and shape. We discuss the stabilization effects of the capture media from the viewpoint of the structures of the NPs and their formation processes by comparing the present NPs with our recent results of Au NPs prepared in an ionic liquid, 1-butyl3-methylimidazolium tetrafluoroborate. We also investigated the heat-treatment effect on NPs that were previously generated in PEG at 20 °C. It is established that the size of the NPs can be controlled by postheating and that this effect on the previously generated NPs is quite different from the temperature effect during initial preparation.

1. INTRODUCTION The sputter deposition of metals onto the surface of particular types of liquids generates metal nanoparticles (NPs) in the liquids.1-4 In particular, if ionic liquids are used as the capture media, complicated devices are not necessary and metal/alloy NPs are obtained through a very simple operation.4,5 Moreover, this method is regarded as an excellent process for the generation of clean NPs because they can be synthesized in ionic liquids as the capture media with neither chemical reactions nor additional stabilizing agents. Ionic liquids are expected to have several fields of application because they have many unique physicochemical properties. Among these properties, their extremely low vapor pressures6-8 are useful, which enable them to be treated under high-vacuum conditions. As a result, ionic liquids are regarded as fruitful liquid media for vacuum sciences and technologies.9-15 This property brings out the best in the formation of metal NPs by the sputtering deposition technique. In addition to low vapor pressures, their ability to stabilize metal NPs makes ionic liquids useful media for generating these particles.16,17 Because of this stabilization by constituent ions, metal NPs can be dispersed in ionic liquids without requiring other stabilizers. Since the first report,4 developments in preparative techniques and studies on the formation process and factors determining the size and shape are still in progress.5,18-21 r 2011 American Chemical Society

The stabilization capability of ions is an important factor for the above-mentioned method. For example, the addition of other media into the ionic liquids dispersing metal NPs causes aggregation of the NPs. Although methods of supporting NPs on some substrates are discussed for certain applications,22-25 NPs must usually be treated in ionic liquids. Ordinarily, ionic liquids are expensive compared to other solvents such as liquid polymers or substances based on glycerol.7 Thus, if metal NPs can be prepared in cheaper liquids by sputter deposition, it would be very beneficial considering their large use. It is thought that metal/alloy NPs can be synthesized into a liquid whose vapor pressure is low enough to permit the sputtering operation. Polyethylene glycol (PEG), with a small molecular weight, is in the liquid state at room temperature and its vapor pressure is known to be low. In this study, we used liquid PEG having an average molecular weight of 600 and a vapor pressure of 0.2 Pa even at 90 °C.7 This is much lower than the Ar pressure in our previous experiments involving sputtering in the range 2060 °C. Consequently, we selected liquid PEG in place of ionic liquids, as the capture medium in the sputter deposition technique. This is because PEG is economical, environmentally Received: November 2, 2010 Revised: December 28, 2010 Published: February 04, 2011 3279

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The Journal of Physical Chemistry C friendly, biocompatible, and commonly used in many areas.26 It is obvious that the most important factor for selecting PEG was that it is frequently used as a stabilizer for metal NPs. PEG, especially if the terminals of the chain are chemically modified by a specific ligand, is preferred as a stabilizer for metal NPs in aqueous media.27-31 In addition to the chemical synthesis of NPs using PEG, there are some previous studies on the combinational use of PEG with the deposition technique. One example is the generation of thin PEG films dispersed with Au NPs, where Au was vapor-deposited onto the thin melt of PEG with NH2terminal groups (molecular weight, ca. 2000).32,33 As far as we know, our study using a combination of the sputter deposition technique and liquid PEG is the first trial for the preparation of metal NPs. In the present trial, we succeeded in the easy and simple preparation of pure Au NPs in liquid PEG using the sputter deposition technique and also in the control of particle size by regulating the treatment temperature and by postheat treatment. In this study, we performed structural characterization using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and UV-vis absorption spectroscopy, and compared the results with our earlier findings of Au NPs prepared in an ionic liquid, 1-buthyl-3-methylimidazolium tetrafluoroborate (C4mimþ/BF4-).20 We discuss the factors, which determine the structure of the NPs and their formation processes by considering the differences in the size and shape of the NPs generated in PEG and in C4mimþ/BF4-.

2. EXPERIMENTAL SECTION 2.1. Preparation of Au NPs. In this trial, we selected liquid PEG as the capture medium for sputtered Au particles instead of the ionic liquids used in our previous work.19,20 PEG was purchased from Wako Pure Chemical Industries, Ltd. Its molecular weight and melting temperature were guaranteed to be 560-640 and 15-20 °C, respectively. Using field desorption mass spectroscopy (FD-MS), we ascertained that the degree of polymerization (n) ranged from 10 to 17 with the main species of n = 13 and 14. We also checked by an NMR measurement that the PEG had not functional groups such as carboxylic acid groups, which would affect the capability of stabilizing Au NPs. To remove contamination by volatile substances, especially water, the sample was dried for 24 h at 60 °C under a vacuum of 10-2 Pa with continuous stirring, and was then kept under an Ar atmosphere until the operation of the sputter deposition. After drying, on visual inspection, we found no decrease of PEG in the vessel. The water content, as determined by Karl Fischer titration (DL32, Mettler Toledo International Inc.), was less than 130 ppm. Au NPs in PEG were prepared using the same operation of sputter deposition as that used with ionic liquids.19,20 Our sputter coater (SC-704, SANYU Electron Co. Ltd.) was modified by attaching a device circulating temperature-regulated water into the base of the deposition for temperature control of the sample.20 Moreover, a device circulating temperature-regulated water into the sputtering target holder was recently attached to prevent increase in temperature of the target. The temperature was maintained at 20 °C during the sputter deposition. The deposition was performed at a voltage of 1 kV and a current of 20 mA under an Ar pressure of 16-19 Pa at constant temperatures of 20, 30, 40, 50, and 60 °C for PEG capture. These conditions were almost similar to those used in the earlier case with ionic liquids,20 with the exception of the cooling system of

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the target. Liquid PEG (2 cm3) was spread on a stainless steel plate (15.9 cm2) horizontally set in the sputter coater. The surface of PEG was located at a distance of 25 mm from the Au foil target (99.99% purity). The sputtering time was 50 min. The sample turned dark red in color and no precipitate was apparently observed. This indicates the success in forming Au NPs in PEG with sputter deposition. The density and viscosity of PEG were measured using a density meter (DMA4500, Anton Paar) and a viscosity meter (AMVn, Anton Paar) respectively in the range 20-90 °C. Using the results of density measurements of PEG samples containing Au NPs, the Au concentration was determined to be about 40 mmol/dm3 as Au atoms. 2.2. SAXS. Small-angle scattering intensities were measured with a SAXS apparatus (NANO-Viewer, RIGAKU Corporation). Using a multilayer mirror, X-rays emitted from a rotating Cu target of an X-ray generator were monochromatized to λ = 0.154 nm (λ, wavelength of the X-ray) and focused to a beam with a diameter of 0.4 mm at the sample position. The camera length was set to 417 mm. Overall X-ray paths, with the exception of the sample position, were maintained in a vacuum of about 10 Pa to avoid X-ray scattering from the air. The SAXS intensity was measured with a 2D detector (PILATUS, DECTRIS Ltd.); the positional resolution of the detector was 172 μm. The observable q-region in the measurements was 0.24-5.5 nm-1, where the scattering parameter q (absolute value of scattering vector q) is defined as q = (4π sin θ)/λ (2θ, scattering angle of X-rays). The intensity of the incident X-rays was monitored with a microionization chamber (REPIC Corporation) backed up by a picoammmeter (6485, Keithley Instruments Inc.). To experimentally determine the absorption factor of a sample,34 the intensities of the transmitted X-rays of the Au NPs dispersed in PEG and the pure PEG were measured every time before the SAXS intensity measurements using a photo diode (S3590-19, Hamamatsu Photonics K. K.) backed up by a picoammmeter (6485, Keithley Instruments Inc.), which was used instead of the PILATUS detector. For SAXS measurements, each sample was packed in a holder with polyetherimide thin film windows (SUPERIO UT F type, Mitsubishi Plastics Inc.) immediately after preparation by the sputter deposition. The sample holder had an inside diameter of 6 mm and thickness of 0.3 mm. The thickness was adjusted by a Teflon spacer. Because of the hygroscopic property of PEG, all sampling operations were performed under an Ar atmosphere. The SAXS measurement for each sample was performed at room temperature because the NPs generated at higher temperatures were ascertained to keep their original size and shape during the cooling operation. The accumulation time for the measurement of scattering intensity from the sample was 30 min. Details of the data corrections applied were reported in our previous publication.19 For further investigation on the effect of rising temperature on the size and shape of the NPs, we performed SAXS measurements with in situ heating for the Au NPs previously generated at the lowest temperature of 20 °C. The SAXS intensities were measured up to a temperature of 110 °C. 2.3. TEM and UV-vis Spectra. When preparing samples for TEM experiments, we confirmed by visual observation that the mixing of the PEG dispersing Au NPs with a large amount of methanol did not cause aggregation of the NPs. In other words, the Au NPs once stabilized by PEG, dispersed stably in methanol for one day. However, the color of the solution changed from 3280

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Figure 1. (A) Colors of Au NPs (PEG) prepared at 20 and 60 °C. Both solutions are diluted with PEG from the initial concentration of about 40 mmol/ dm3 to 1/100. TEM images of Au NPs prepared by sputter deposition into liquid PEG. Prepared at (B) 20 °C and (C) 60 °C.

reddish-purple to bluish-violet after several days, which indicated the occurrence of a slow aggregation of Au NPs. On the basis of this observation, TEM samples were prepared by dissolving the Au NPs dispersed in PEG into methanol at room temperature and immediately dropping onto collodion-coated copper grids. All grids were dried in a vacuum-drying apparatus for about 24 h. The size and shape of Au NPs were observed using a TEM (JEM-2100F, JEOL Ltd.) with an acceleration voltage of 120 kV. TEM observations were performed for the samples of Au NPs generated at 20 and 60 °C, and for the heat-treated sample at 110 °C. UV-vis absorption spectra were measured using a spectral photometer (U-3900H, HITACHI Co. Ltd.) with a quartz sample holder of 0.1 mm optical path, immediately after the sputter deposition.

3. RESULTS AND DISCUSSION 3.1. Structure Studies on Au NPs Generated in Liquid PEG. Figure 1 shows typical TEM images of the Au NPs

prepared in PEG at 20 and 60 °C. Apparent structural differences are observed by comparing the images. The sputter deposition at 20 °C resulted in the formation of nanosized spherical particles. On the other hand, the Au NPs prepared at 60 °C were larger, and anisotropic particles were observed. This difference indicates that the formation process of the particles is strongly affected by the temperature of the capture medium. To obtain more detailed information on the structure and the formation process of the Au NPs, the SAXS profiles were analyzed. Figure 2 displays the SAXS profiles of the Au NPs generated at different temperatures after corrections for the intensity fluctuation of the incident X-rays, background intensities, and

Figure 2. SAXS patterns of Au NPs generated in liquid PEG at 20, 30, 40, 50, and 60 °C. For clarity, the curves are displaced vertically with successive multiplication by 10.

absorption effects. Because we selected the scattering intensity of the pure PEG as the background, the scattering profiles shown in Figure 2 are the scattering intensities of the Au NPs themselves. The profiles change depending on the temperature, reflecting the size and shape of Au NPs. The black lines in Figure 2 are theoretical fitting curves. Curve fitting was performed assuming that the NPs were spherical and that the size distribution was expressed by Γ distribution, details of which are described in our previous article.19 As shown in Figure 2, the theoretical curves based on the above-mentioned assumptions 3281

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Figure 3. Particle-size distributions of Au NPs generated in PEG at different temperatures.

Figure 4. UV-vis absorption spectra of Au NPs in PEG normalized at the height of the plasmon band. The insert shows the temperature dependence of the wavelength of plasmon maximum.

reproduced well the experimental scattering profiles of Au NPs generated at 20-50 °C. The experimental curve at 60 °C with small swellings shown by arrows could be reproduced only by assuming an interference interaction of the particles. This indicates that the NPs start to gather without cohering at this temperature; the same trend is also confirmed by the TEM image. The derived particle-size distributions are shown in Figure 3. The curves indicate the distribution of the number of particles versus the diameter and are normalized by the area to highlight the widths of dispersion. The value at the peak-top, dpeak, corresponds to the diameter of the most abundant NPs. Comparing the particle sizes determined by TEM with those by SAXS, it is noted that the particle sizes determined by the former are larger than those determined by the latter, although the quantitative trends, such as the existence of NPs with various sizes and the situation of gathering of NPs, are the same.

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Figure 5. Temperature dependences of the diameter determined from the peak-top (dpeak) and full-width at half-maximum (Wfwhm) of the distribution curves for Au NPs (PEG) (red circles) and that for Au NPs (C4mimþ/BF4-) (blue squares). Closed marks and open cones correspond to dpeak and Wfwhm, respectively.

Referring to almost the same particle sizes determined by TEM and SAXS observations for the sample, which makes neither time-dependent change nor some changes caused by operations for measurements,35,36 it is concluded some cohesion of the Au NPs occurs by the operation and/or during the preparation of the samples for the TEM observation. Figure 4 shows UV-vis absorption spectra, which are normalized at the peak-top of the surface plasmon resonance absorption to nullify the differences in the sample thickness and the concentration of Au. As the preparation temperature increased, absorbance decreased in the shorter wavelength region and increased in the longer wavelength region. It is known that the absorption in the shorter wavelength region corresponds to the existence of very small clusters.37,38 This decrease suggests that small clusters are not stable at higher temperatures. In fact, clusters smaller than 2 nm in diameter are sparsely observed in the TEM image of the sample prepared at 20 °C; most NPs grow larger at 60 °C. It is reported that, at longer wavelengths, the absorbance is enhanced by the generation of larger and anisotropic particles.39-41 The temperature dependence of these UV-vis spectra corresponds to the results by TEM and SAXS analyses. Moreover, the plasmon maxima shift to longer wavelengths with increasing preparation temperature (insert in Figure 4). The generation of larger NPs at the higher temperature is also indicated by this band shift. 3.2. Comparison of Au NPs generated in PEG and C4mimþ/BF4-. We now compare the Au NPs generated in PEG with those in ionic liquid C4mimþ/BF4-. Hereafter, we describe the NPs generated in PEG and C4mimþ/BF4- as NPs (PEG) and NPs (C4mimþ/BF4-), respectively. Two values are used to represent the distribution curve obtained by SAXS experiments, namely dpeak and the full width at half-maximum of the curve (Wfwhm). Figure 5 shows temperature dependences of dpeak and Wfwhm for NPs (PEG) and for NPs (C4mimþ/ BF4-).20 In our previous study on the temperature dependence of the size of Au NPs (C4mimþ/BF4-),20 we concluded that one of the most effective factors to determine particle size is the diffusion velocity of sputtered particles in the capture liquid. 3282

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Figure 6. T/η dependences of dpeak (closed marks) and Wfwhm (open marks) for Au NPs (PEG) (red circles) and for Au NPs (C4mimþ/ BF4-) (blue squares).

According to the Stokes-Einstein relationship, the diffusion constant of a particle in a medium is proportional to T/η (η, viscosity of the medium). Similar to that for C4mimþ/BF4-, the values of η for PEG vary dramatically with temperature; changing from 167 to 13 cP as the temperature increases from 20 to 90 °C. As shown in the Supporting Information, the temperaturedependence curves of η for PEG and C4mimþ/BF4- are almost parallel, although the magnitude of the former is slightly larger than the latter. Figure 6 shows the plots of dpeak and Wfwhm for Au NPs (PEG) and Au NPs (C4mimþ/BF4-)20 as a function of T/η. It is noted from Figure 5 that the sizes (dpeak) and size deviations (Wfwhm) of both NPs (PEG) and NPs (C4mimþ/ BF4-) become larger as the temperature of the capture medium increases. This suggests that the collision frequency determined by the diffusion velocity is one of the most important factors in growing NPs (PEG) as well as NPs (C4mimþ/BF4-). However, the trends of Au NPs (PEG) and Au NPs (C4mimþ/BF4-) are different, as summarized below. First, the values of dpeak and Wfwhm for Au NPs (PEG) are considerably larger than those of Au NPs (C4mimþ/BF4-). Second, as shown in Figure 6, the dpeak and Wfwhm curves against T/η for Au NPs (PEG) change almost linearly in the temperature range 20-60 °C, whereas those for Au NPs (C4mimþ/BF4-) increase up to saturated values before the cohesion of smaller NPs that occurs at a higher temperature.20 If the mechanism of the generation of NPs could be limited only to the collision frequency of the sputtered Au particles, smaller NPs with less size distribution should be formed in PEG because of the larger value of η compared to that of C4mimþ/BF4-. However, the experimental results show the opposite effect. This implies that the stabilization mechanism by PEG or C4mimþ/BF4- is another important factor to determine the size and size distribution of NPs. In the case of ionic liquids, not limited to C4mimþ/BF4-, the stabilization mechanism is complicated. As candidate factors for determining the size and size distribution of NPs in addition to the collision mechanism, we can first consider the stabilization due to coordination by anions and/or cations around the NPs. Our preliminary experimental results imply that the anion effect is stronger than the cation effect, probably because the anions

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Figure 7. TEM image of Au NPs (PEG) after heat treatment up to 110 °C.

form the first coordination shell around the NPs.42 Dupont and co-workers also reported the possibility of strong interactions of NPs and anions, although the metal was not Au but Pt, and the generation method was different from the sputter deposition.43 The alkyl-chain length of the cations,19,21,44 the size of anions, the balance of anions and cations, and the direct interaction of the surface of NPs and imidazolium cations45-47 may be also closely related to the stabilization capability of the NPs. Occasionally, these stabilizers work as suppressors for the NPs to grow larger exceeding a characteristic size. As ascertained by FD-MS, the degree of polymerization of the PEG used in this study (average molecular weight, 600) is 13-14. It is quite possible that the oxygen atoms are coordinated to the surface of the Au NPs and that -CH2-CH2- chains work as stabilizers by surrounding the NPs. This stabilization mechanism seems to be the same as crown ethers for alkali metal ions. However, the stabilization capability is thought to be not sufficiently strong to suppress the growth of Au NPs. This is because the size and size distribution of NPs (PEG) are larger than NPs (C4mimþ/BF4-). In fact, the former is not so stable as the latter, as demonstrated by the fact that the color of the PEG solution dispersed with Au NPs generated at 20 °C changed from dark red to red-purple after a week, indicating the growth of the NPs. 3.3. Postheat-Treatment Effect on Structure of NPs Generated in PEG. Thus far, we have discussed the structure of Au NPs immediately after generation at several preparation temperatures. To investigate the heat-treatment effect on the Au NPs (PEG) generated once, we performed temperature-rising experiments. PEG dispersed with the Au NPs prepared at 20 °C was placed in a SAXS sample holder and the temperature was raised in increments of 10 °C up to 110 °C. The SAXS intensities were measured after the temperature had been held for about 10 min at each measuring point. After the SAXS measurements, the sample, heat-treated up to 110 °C, were used for the TEM observation. First, the results by TEM are presented. The TEM sample was prepared by the same procedure as described above. Figure 7 shows the TEM image of Au NPs (PEG) heat-treated up to 110 °C. After the heat treatment, spherical NPs were observed and their size increased compared to the initial size. It is also 3283

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the particle size changed dramatically. For the sample exceeding 90 °C, the SAXS patterns could be fitted only by accounting for the effect of interparticle interference. That is, the NPs seem to gather around each other and start to aggregate. It is thought that the increase in the NP size with heat treatment up to 80 °C is due to the cohesion of very small NPs to form larger NPs because the former are more unstable against the heat treatment. After exceeding 90 °C, the agglutination of large and stable NPs seems to occur readily, and twin NPs are formed. This is consistent with the TEM observation as shown in Figure 7. These results create the possibility of controlling the structure of Au NPs by postheat treatment up to 80 °C after the synthesis, if larger and single spherical NPs are needed. Such growth is reported in the Au NPs generated in the ionic liquid C4mimþ/ PF6-.25 The mechanism of growth of the present NPs seems to be similar to that in C4mimþ/PF6-. Namely, the Au NPs, previously generated at 20 °C, easily cause coalescence between the NPs by heat treatment and result in the formation of larger single NPs. Figure 8. SAXS patterns of Au NPs (PEG) heat-treated in increments of 10 °C up to 110 °C. The initial sample was generated in PEG at 20 °C.

Figure 9. Particle-size distributions of Au NPs (PEG) heat-treated in increments of 10 °C up to 110 °C. Temperature dependences of dpeak (closed circles) and Wfwhm (open circles) are shown in the inset.

noted that there are a few large particles, which seem to be formed by agglutination between two particles. Most importantly, the size and shape of the heat-treated NPs are remarkably different from the Au NPs generated directly at 60 °C; the latter NPs had a complicated structure and were not spherical, as shown in Figure 1. It is indicated that there is much difference between the formation processes of the Au NPs generated at 60 °C and those generated by postheat treatment. For further details of the postheat-treatment effect, we analyzed the SAXS patterns shown in Figure 8. The SAXS pattern of the sample generated at 20 °C changed as the temperature was raised. The increase in the scattering intensity at the smaller q-range and its decrease at around q = 2 nm-1, strongly indicate the growth of the Au NPs. An analysis by curve fitting for the SAXS patterns was performed and the particle-size distributions obtained are shown in Figure 9. The particle size increased gradually with increasing heat-treatment temperature from 20 to 80 °C. Above 80 °C,

4. CONCLUSIONS We selected liquid PEG instead of an ionic liquid as the capture medium for the generation of Au NPs by the sputter deposition technique. The structure of Au NPs is dependent on the preparation temperature of the capture PEG. The size varies from about 2 to 8 nm depending on the temperature rise in the range of 20-60 °C. Whereas the shape of the NPs generated at 20 °C is spherical, the anisotropy increased with temperature increase. The size and size distribution of Au NPs (PEG) are larger and wider than those of Au NPs (C4mimþ/BF4-). In the syntheses in both PEG and C4mimþ/BF4-, the diffusion of the sputtered Au in the media and the stabilization capability of the formed NPs by the media are the two most important factors to determine the size and size distribution. The stabilization capability of PEG seems to be weaker than that of C4mimþ/BF4-. Heat treatment of the previously prepared NPs at 20 °C (PEG) in the range of 30 to about 80 °C causes the initial NPs to grow gradually while maintaining a spherical shape as single particles. This growth is probably caused by the cohesion of very small Au clusters to larger ones. Exceeding 90 °C, the agglutination of large and stable NPs occur and twin NPs are produced. In such a way, the preparation temperature and heat-treatment temperature affect the growing process of NPs (PEG) differently. In the generation of smaller Au NPs with uniform size, ionic liquids are preferable to PEG as the capture media for the sputter deposition technique. However, the use of PEG makes it possible to prepare Au NPs by a simple, easy, and inexpensive process. The prepared Au NPs (PEG) can be easily treated because they can be dissolved in water or alcohols without aggregation. Moreover, PEG is attractive as an environmentally friendly and biocompatible medium. Of course, these observations are not limited to liquid PEG or ionic liquids, as liquids with low vapor pressures are also candidates for the capture media of the sputter deposition technique to prepare NPs. Indeed, Dupont and his co-workers have recently reported the synthesis of Au NPs in castor oil by sputter deposition.48 ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures of densities of PEG600 and C4mimþ/BF4 -, and viscosities of PEG600 and C4mimþ/

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The Journal of Physical Chemistry C BF4-. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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