Synthesis by Laser Ablation as a Function of Permittivity - American

Jul 24, 2012 - Research Organization of Science & Engineering, Ritsumeikan University, 1-1-1, Noji-Higashi, Kusatsu, Shiga 525-8577, Japan. ABSTRACT: ...
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Fractal of Gold Nanoparticles Controlled by Ambient Dielectricity: Synthesis by Laser Ablation as a Function of Permittivity Ken-ichi Saitow,*,†,‡ Yoshinori Okamoto,‡ and Yohko F. Yano§,⊥ †

Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima 739-8526, Japan ‡ Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima 739-8526, Japan § Research Organization of Science & Engineering, Ritsumeikan University, 1-1-1, Noji-Higashi, Kusatsu, Shiga 525-8577, Japan ABSTRACT: The nanosecond pulsed laser ablation of a gold plate at the excitation wavelength of 532 nm was conducted in the supercritical fluid of dipolar trifluoromethane (CHF3). The generation of gold nanoparticles was investigated as a function of the fluid density that corresponded to changes in the permittivity and thermal properties over the wide ranges. The analysis of data from electron microscopy and small-angle X-ray scattering (SAXS) revealed that the principal product is a gold nanonetwork with length up to a few 10 μm and a branched structure. The gold nanonetwork consisted of gold nanospheres with an average diameter of 20 nm and a mass fractal structure. The mass fractal dimension of the nanonetwork changed by the fluid density, and its dimension was attributed to the number of the nanospheres. It was clarified that not the fluid thermal properties but the ambient dielectricity and polarization energy during the ablation are responsible for the morphology and number of gold nanoparticles. This is the first report of (i) fractal structure of gold nanoparticles created by the ablation, (ii) ablation mechanism in dipolar and nondipolar fluids, and (iii) a chemical-free nanoparticle synthesis in a dipolar fluid.

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INTRODUCTION Gold as a material has attracted considerable attention in natural and applied sciences, industrial applications, and human history. Furthermore, nanosized gold materials have properties that are not observed in bulk gold.1 Namely, the gold nanomaterials have been utilized as a substrate for singlemolecule detection by surface-enhanced Raman scattering (SERS)2 or metal-enhanced fluorescence (MEF),3 as optomaterial of ultrasensitive bio- and medical sensors,4,5 as catalysts for CO oxidation,6 and as photothermal material for cancer therapy.7 According to recent research on SERS,8 MEF,2 and photochemical reaction,9 the enhancement effect of the electric field due to plasmon is significantly promoted using a structured ensemble of nanoparticles. For instance, the SERS intensity is enhanced 107 times when an ensemble of nanospheres is used instead of a single nanosphere.10 Recently, gold nanoparticles with well-defined shape and size have been synthesized by reducing gold ions in HAuCl4 aqueous solution,11 and the products are dispersed as a single nanoparticle in the solution. The reduction reaction is the most popular synthesis method of single gold nanoparticle, but the product surface is capped and contaminated by various chemicals (i.e., reductants, surfactants, or ions). In particular, the chemicals cover the surface of the gold nanoparticle in which the enhancement effect occurs and disturb the assembly of nanoparticles in which the significant enhancement appears. © 2012 American Chemical Society

Accordingly, gold nanomaterials with no capped surface are needed to understand the enhancement effect as well as to generate the assembled structure that produces the giant enhancement. Pulsed laser ablation (PLA) is a physical synthesis method and does not require many chemicals.12 A laser pulse above the ablation threshold irradiates a metal target, and nanoparticles are ejected from the surface of the target via relaxation processes such as melting, evaporation, electron emission, plasma generation, and fragmentation. Recently, the PLA technique has become a popular tool for generating both nanoparticles and nanostructures.13−15 During the past decade, PLA of gold in solution using a nanosecond pulsed laser has been explored by many researchers.13−21 Koda et al. first conducted PLA of gold in solution and investigated the generation (fragmentation) processes of gold nanoparticles.16 Kondow and Mafune synthesized gold nanoparticles by PLA in sodium lauryl sulfate (SDS) solution.17 A network structure of gold nanoparticles was found to form in a less concentrated SDS solution, whereas a size-reduced nanosphere was formed above the critical micelle concentration. Compagnini and coworkers performed PLA of gold in various neat liquids.18,19 The Received: April 28, 2012 Revised: June 12, 2012 Published: July 24, 2012 17252

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strong polar molecule, and its dipole moment is as large as 1.65 D.32 The obtained results can be compared with those in a nondipolar fluid. (ii) The physicochemical properties vary as a function of the fluid pressure by a factor of 100. Note that the permittivity of supercritical CHF3 is significantly affected by pressure33 and becomes 5 times larger than that of supercritical CO2, as shown in Figure 1. (iii) Fluid CHF3 is typically used to achieve the supercritical state because of its low critical constant (Tc = 299.3 K, Pc = 4.8 MPa), which makes it easy to handle.34 Here, we present the synthesis of gold nanoparticle by PLA by changing the density, permittivity, and thermal properties of supercritical CHF3 at the isothermal condition. The size, morphology, and quantity of gold nanoparticles were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). The principal product was a long-range gold nanonetwork with a length up to a few 10 μm that consisted of numerous nanospheres with an average diameter of 20 nm. The gold nanonetwork has a branching structure and showed a mass fractal structure with dimensions that changed as a function of fluid density during PLA. The nanonetwork was generated by the fragmentation of large nanosphere with an average diameter of 400 nm. It was found that the ambient dielectricity and polarization energy is crucial to the morphology and quantity of PLA-generated gold nanoparticles.

morphology of the gold nanoparticle changed as a function of the alkane solvent, and the aspect ratio of the gold nanoparticle decreased as the chain length of the alkane increased.19 Meneghetti and Amendora conducted PLA in toluene, acetonitrile, tetrahydrofuran, and dimethyl sulfoxide.15,20,21 The generated gold nanoparticles were stable for hours to weeks and their size changed as a function of the solvent, whereas PLA in toluene gave a gold/graphite particle with a core/shell structure.21 Clearly, the gold nanoparticle generation is strongly affected by the environment of PLA, and thus studying the solvent dependence becomes crucial. Supercritical fluids have attracted much attention in studying solvent dependence because fluid pressure and/or density varies significantly physicochemical properties without the change of solvent, e.g., heat capacity, thermal conductivity, dielectric constant, viscosity, and so on, as shown in Figure 1.22,23 We have investigated supercritical fluids by Raman

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EXPERIMENTAL METHODS We have developed an instrument to fabricate nanoparticles, described elsewhere.25−29 Briefly, the instrument consists of a high-pressure cell, optics, and a Q-switched frequency-doubled Nd:YAG laser. The temperature of the fluid in the cell is controlled by a set of heaters, a proportional-integral-derivative (PID) controller, and a thermocouple. The pressure of the fluid is increased with an HPLC pump. The Nd:YAG laser serves as the PLA light source and is operated at an excitation wavelength of 532 nm, an energy of 19 mJ/pulse, a repetition rate of 20 Hz, a fluence of 0.8 J/cm2, and a pulse width of 8 ns. A gold plate immersed in supercritical CHF3 was irradiated with the laser for 10 min at T = 305.3 K, corresponding to a reduced temperature Tr = T/Tc = 1.02, where Tc is the critical temperature. The pressure for PLA ranged from 2.91 to 5.39 MPa. The fluid density was calculated from the empirical equation of state of supercritical CHF3,35 using the measured values of P and T. The density ranged from 0.105 to 0.421 g/ cm3, and it was expressed as 0.2 ≤ ρr = ρ/ρc ≤ 0.8, where ρr and ρc are the reduced and critical density, respectively. To prepare samples for electron microscopy, the gold nanoparticles generated at each fluid density were deposited on a substrate in supercritical CHF3. The substrates for field emission scanning electron microscopy (FE-SEM, Hitachi S5200) and transmission electron microscopy (TEM, JEOL JM1010) were a carbon disk and a grid covered by carbon film, respectively. After sedimentation of the gold nanoparticles in the fluid, the substrates were retrieved from the cell and used in the FE-SEM and TEM measurements. The sedimentation time of the gold nanoparticles was calculated using the pressuredependent viscosity of the supercritical CHF3. Briefly, the sedimentation time was commonly adjusted to the time required for a 100 nm diameter sphere to sink to a depth of 1 cm for all densities.29 The SAXS measurements were conducted using synchrotron X-ray radiation at the BL40B2 beamline of SPring-8 at the Japan Synchrotron Radiation Institute, Harima. The exper-

Figure 1. Physicochemical properties of supercritical CHF3 and CO2 at the same thermodynamic condition. Red solid circles and blue open triangles represent the data for supercritical CHF3 and CO2, respectively: (a) heat capacity (ref35), (b) thermal conductivity (ref 32), (c) viscosity (ref 35), and (d) relative permittivity (ref 33). The data are collected at the isotherm of reduced temperature Tr = T/Tc = 1.02, where Tc is the critical temperature, as a function of reduced density ρr = ρ/ρc, where ρc is the critical density.

spectroscopy,24a−h dynamic light scattering,24i−l terahertz absorption,24m,n and recently developed a new synthesis method of nanomaterials.25−29 Specifically, this was the first time that PLA in supercritical fluid is used to generate nanoparticles.25 In previous studies, we performed PLA of a silicon crystal25−28 and a gold plate in supercritical CO2. In the case of silicon, RGB-light-emitting26 or white-light-emitting silicon nanocrystals27 were fabricated. Luminescence colors and intensities were varied by manipulating the fluid pressure during PLA. In the case of gold, a gas-like structure of supercritical CO2 produced a gold nanonecklace, whereas a liquid-like structure produced large, 500 nm diameter gold nanospheres.28,29 Either the fragmentation or solidification of the gold liquid droplet was responsible for the quantity and morphology of nanoparticles. Recently, several research groups have adopted PLA in supercritical fluids and have reported the generation of nanostructures and nanomaterials.30,31 In the present study, we conducted PLA of gold in supercritical trifluoromethane (CHF3). The reasons for choosing supercritical CHF3 are: (i) The CHF3 molecule is a 17253

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imental configuration has been described elsewhere.36 Briefly, the X-rays scattered from the sample were collected on a RAXIS IV++ (Rigaku Co., Japan) as an imaging plate detector, and the scattering intensity was recorded as a function of the scattering angle. The magnitude of the scattering vector q was obtained by measuring the diffraction patterns of a collagen, and the camera length was set to 4.103 m. For the SAXS measurements, the gold nanoparticle generated by PLA was deposited on a Kapton film immersed in supercritical CHF3. After PLA, the Kapton film was taken out of the cell and attached to the sample holder normal to the incident X-ray beam. The X-rays scattered from the gold nanoparticle onto the Kapton film were recorded. In addition, the signal from a blank Kapton film was also recorded to compensate for the background signals. The purity of CHF3 (Taiyo Nissan Co.) and the gold plate (Tanaka Co.) was 99.995% and 99.95%, respectively. The critical constants of CHF3 are reported to be Tc = 299.3 K, Pc = 4.83 MPa, and ρc = 0.527 g/cm−3.34 We represent the density of CHF3 by the reduced density ρr = ρ/ρc, as shown in the top axis of Figure 1a.

images were measured as displayed in Figures 2c and 2d. The images were captured at ρr = 0.2 and ρr = 0.8, respectively, and the densities are the same with those measured by FE-SEM. Thus, it was revealed that the networks consist of nanospheres with a diameter of about 20 nm and have branched structures. We performed SAXS experiments to evaluate the size and morphology of the gold nanoparticles. The scattering intensity of the nanoparticles was formulated with the following equations37 I(q , R 0 , M ) = |F(q , R 0 , M )|2 M 1 ⎛ M ⎞ = ⎜ ⎟ |Δρ|2 Γ(M ) ⎝ 2R 0 ⎠

∫0

∞

e−MR / R 0(2R )−1 + M

|Ω(q , R )|2 dR

Ω(q , R ) =

4π [sin(qR ) − (qR ) cos(qR )] q3

(1)

(2)

where I is the scattering intensity, q is the scattering vector, R0 is the average radius of the spherical particles, M is the shape parameter, F is the static structure factor, Γ is the gamma function of the size distribution, Δρ is the difference between the electron densities of the nanoparticle and the fluid, R is the radius of a spherical particle, and Ω(q,R) is the form factor of the spherical particle with radius R at q. The scattering vector is expressed by q = 4π sin θ/λ, where λ is the wavelength of the Xrays (λ = 1.50 Å). We used eq 1 to analyze the scattering profile of the gold nanoparticles. Figure 3a shows a typical SAXS data of the gold nanoparticles generated by the density of ρr = 0.4. The red open circles and black solid line represent the experimental data points and the fitting using eq 1, respectively. The red solid, green dot, and blue dashed lines are fitting curves of nanospheres with average diameters of 17, 45, and 410 nm, respectively. Figure 3b shows the size distribution functions of the number of nanospheres, which are obtained from the red solid, green dots, and blue dashed lines of Figure 3a. Thus, the three different distributions are observed. Figure 3c displays the averaged diameters of the three distribution functions as a function of fluid density during PLA. Figure 3d shows the relative abundance of the nanospheres as a function of fluid density during PLA. The relative abundances are estimated from the ratio of integrated intensities of distribution functions, as shown in Figure 3b. According to these results, the scattering X-rays well characterize the gold nanospheres having three different sizes. Obviously, the size of the nanoparticles has three components with R0 ≅ 20, 40, and 400 nm with respective percentages of 99%, 1%, and 10−3%. Accordingly, it was found that the principal component in the products is the nanospheres with a diameter of 20 nm within the current experimental range. This conclusion was consistent with the electron microscopy observations; that is, the principal component of the gold nanonetwork is the nanospheres with a diameter of 20 nm. To characterize the morphology of the gold nanonetworks, we examined the mass fractal structure by analyzing the SAXS data. According to the Porod theory, the mass fractal structure can be characterized with eq 338

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RESULTS AND DISCUSSION Figure 2 shows typical electron microscope images of gold nanoparticles generated by PLA in supercritical CHF3. Figures

Figure 2. Electron microscope images of gold nanoparticles generated by pulsed laser ablation in supercritical CHF3. FE-SEM images of gold nanoparticles generated by pulsed laser ablation in supercritical CHF3 at (a) ρr = 0.2 and (b) ρr = 0.8, where ρr = ρ/ρc and ρc are the reduced and critical densities, respectively. TEM images of gold nanoparticles generated by pulsed laser ablation in supercritical CHF3 at (c) ρr = 0.2 and (d) ρr = 0.8.

2a and 2b show FE-SEM images of gold nanoparticles generated at low (ρr = 0.2) and high (ρr = 0.8) densities, respectively. The data at ρr = 0.2 show a nanonetwork with length of up to a few 10 μm, whereas the quantity of the network significantly decreases at ρr = 0.8. To verify the morphologies of the products at low and high densities, TEM

log I(q) = log I(q) − D log q

(3)

where I(q) is the scattering intensity and D the mass fractal dimension. Figure 4a shows the SAXS data measured at the 17254

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Figure 3. Data obtained from small-angle X-ray scattering of gold nanoparticles generated by pulsed laser ablation in supercritical CHF3. (a) Scattering profile of gold nanoparticles generated at ρr = 0.4 as a function of the scattering vector q, where ρr = ρ/ρc and ρc are the reduced and critical densities, respectively. Red open circles and black solid line represent the data points and fitting curve, respectively. Red solid, green dot, and blue dashed lines are each fitting curve of the nanospheres with an average diameter of 17, 45, and 410 nm, respectively. (b) Size distribution functions of the number of nanospheres of the three different sizes. The distribution functions of red, green, and blue symbols are obtained from the fitting curves of red solid, green dot, and blue dashed lines shown in (a). (c) Average sizes of the gold nanospheres as a function of the reduced density. Red, green, and blue symbols are given by each average size calculated from the distribution functions, as shown in (b). (d) Relative abundances of the three nanospheres with the different sizes. Red, green, and blue symbols are the abundances calculated from the ratio of integrated intensities of size distribution functions, as shown in (b).

various densities analyzed with the Porod theory. Each data show two lines and an inflection point. Specifically, the left and right linear features at the inflection point are characterized by mass and surface fractal regions, respectively. According to the linear feature at the left side of the inflection point, it was seen that the gold nanonetwork has the mass fractal structure. Thus, it was revealed that the mass fractal structure of gold nanoparticle is synthesized by pulsed laser ablation. Based on eq 3, the mass fractal dimension is obtained from the slope of the line. Figure 4b shows the mass fractal dimensions as a function of the CHF3 density. As the fluid density increases during PLA, the fractal dimension decreases. That is, the gold nanonetwork changes to a low-dimensional nanomaterial with the decrease of fluid density. Here, let us mention the growing process of mass fractal aggregation, briefly. According to in situ experiment of time-resolved dynamic light scattering, we tracked the gold nanoparticle size in supercritical CHF3 after PLA and investigated the time evolution of aggregation process. As a result, it was revealed that the gold nanoparticles dispersing in CHF3 form the aggregations in the CHF3 fluid and the nanonetworks with the size of micrometers start to grow from 15 min after PLA.39 From the material point of view, note that the current mass fractal structure is most probably useful to the SERS substrate that results in a giant enhancement factor because the nanonetwork is an ensemble particle with many nanogaps and a chemical-free surface. We did not evaluate here the surface fractal dimension because nearly no data points in the right-hand of the inflection point, as shown in Figure 4a, exist within the lower limit of the S/N ratio. Next, we discuss the density dependence of the mass fractal dimension. Here, the quantity of the generated gold nanoparticles was examined because the quantity depends on the

Figure 4. Fractal dimension analysis by small-angle X-ray scattering of the gold nanonetworks generated by pulsed laser ablation in supercritical CHF3. (a) Porod analysis of the scattering X-rays. (b) Fractal dimension of the gold nanonetwork as a function of the reduced density ρr = ρ/ρc, where ρc is the critical density.

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fluid density during PLA, as shown in Figures 2a and 2b. We estimated the quantity of the gold nanonetworks from the white areas in the FE-SEM images utilizing a software package, and this is described elsewhere.29 The results are shown in Figure 5. The red solid circles and blue open triangles represent

Figure 6. FE-SEM image of large gold nanoparticles (D ≅ 500 nm) generated by pulsed laser ablation in supercritical CHF3 at the reduced temperature Tr = T/Tc = 1.02, where Tc is the critical temperature, and at ρr = 1.7, where ρr = ρ/ρc and ρc are the reduced and critical densities, respectively. Figure 5. Density dependence of the quantity of the gold nanonetworks generated by pulsed laser ablation in supercritical CHF3 and CO2. Red solid circles and blue open triangles represent the quantities in supercritical CHF3 and CO2, respectively. The data of CO2 are obtained from our previous study in ref 29. All data are collected using the gold nanoparticles generated at the same thermodynamic states, that is, the isotherm of reduced temperature Tr = T/Tc = 1.02, where Tc is the critical temperature, as a function of reduced density ρr = ρ/ρc, where ρc is the critical density.

The precursor and fragmentation mechanism in supercritical CO2 has been ensured by the product quantity analysis as a function of the number of laser shots.29 Thus, it was concluded that the generation processes of nanonetwork occur similarly in supercritical CHF3 and CO2. However, the results of PLAs in CHF3 and CO2 show a different feature, as shown in Figure 5. That is, the quantity of network gradually changes as a function of the CO2 density, whereas it suddenly changes with increasing CHF3 density. Here, we discuss the physicochemical properties of supercritical CHF3 and CO2. First, we discuss the density dependence of permittivity ε because the ε of CHF3 is significantly different from that of CO2, and its value is 5 times larger than that of CO2, as shown in Figure 1d. On the basis of the Born theory, the large ε produces a large polarization energy as follows40

the quantities of the gold nanonetworks generated by PLA in supercritical CHF3 and CO2, respectively. The data for CO2 were taken from the previous our study.29 Here, the quantity of the nanonetworks was represented by the area they occupied in each FE-SEM image. As shown in Figure 5, the quantity of the gold nanonetwork generated in CHF3 significantly decreases as the density increases. The mass fractal dimension also decreases as the density increases, as shown in Figure 4. Consequently, the profile of the density dependence of the quantity is in good agreement with that of that of the fractal dimension. These results reveal that small quantities of gold nanoparticles result in a low fractal dimensions, and large quantities of gold nanoparticles generate high fractal dimensions. To investigate the density dependence of the quantity of gold nanonetwork, we discuss the generation mechanism of gold nanoparticle. As shown in Figures 2a and 2b, many gold nanonetworks were generated in supercritical CHF3 at ρr = 0.2, whereas they disappeared at ρr = 0.8, where a large 400 nm nanosphere appeared. Similar results were observed with PLA of gold in supercritical CO2, where a large nanosphere and nanonetwork are the principal products at the high and low density, respectively.29 It is reasonable to consider that a similar mechanism occurs in PLAs in supercritical CHF3 and CO2. That is, the small nanosphere and nanonetwork are generated by the fragmentation of large gold nanosphere that is a precursor. As shown in Figure 6, it was observed that many large gold nanospheres (D ≅ 500 nm) are generated by the ablation at the high fluid density of ρr = 1.7 in supercritical CHF3. According to SEM images analysis, the number of large nanosphere increased at the high CHF3 density (ρr > 0.7). Alternatively, the quantity of nanonetwork decreases with the increase of fluid density, as shown in Figure 5. This precursor and fragmentation model is the same as that in the previous study on gold nanoparticle generation in supercritical CO2.29

P=−

e2 ⎛ 1⎞ ⎜1 − ⎟ 4πε0d ⎝ εr ⎠

(4)

where P is the polarization energy, e the elementary electric charge, ε0 the vacuum permittivity, d particle size, and εr the relative permittivity. The larger polarization energy can stabilize the charged matter. Here, we calculated the values of P of a charged gold nanoparticle and/or nanodroplet with a diameter of 400 nm in supercritical CHF3 and CO2 as functions of fluid densities. Figure 7a shows that the magnitude of |P| in supercritical CHF3 is about 5 times larger than that in supercritical CO2, where |P| is the absolute value of P. The large |P| established at high density of CHF3 stabilizes the charged gold nanosphere (gold liquid droplet). The liquid droplet of large nanosphere can be cooled rapidly by faster cooling at the high density. Thus, many large nanospheres of solid are generated in high fluid density having large |P|, as shown in Figure 6. However, small |P| at low fluid density cannot stabilize the charged gold liquid droplet. Slow cooling at the low density cannot cool rapidly the liquid droplet. Thus, the large liquid droplet is fragmented in the fluid of small |P| at low density, and many small gold nanospheres and networks result in the generation, as shown in Figure 5. According to a previous study, it was observed that gold nanoparticles by PLA in supercritical CO2 are generated via multiphoton processes with energies that are greater than the ionization potential of bulk gold.29 Similar mechanisms are reported for gold and silver nanoparticles generated by PLA in liquids from fragmentation 17256

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nanoparticles by PLA in supercritical fluid. This conclusion was reached based on the experiments performed by using PLA in a dipolar fluid at the supercritical state as a function of permittivity.

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CONCLUSIONS The nanosecond pulsed laser ablation of a gold plate at an excitation wavelength of 532 nm was conducted in dipolar supercritical CHF3. PLA in a dipolar fluid was conducted, and it was compared with PLA in nondipolar supercritical CO2. The generation of the gold nanoparticles was found to be a function of fluid density, and the particles were analyzed by FE-SEM, TEM, and SAXS. The following conclusions were obtained: (i) The principal morphology of the gold nanoparticles was a long-range gold nanonetwork with a length up to a few 10 μm. The nanonetwork consisted of gold nanospheres with an average diameter of 20 nm. According to the fluid density dependence of PLA, most of the nanonetwork was generated at low density. The nanonetwork and small nanospheres were generated by the fragmentation of the large nanosphere that was a precursor. At higher density, the large nanospheres with an average diameter 400 nm were generated alternatively. (ii) From the Porod analysis of the SAXS data, the gold nanonetwork showed a mass fractal structure. The mass fractal dimension decreased as the fluid density during PLA increased. The density dependence of the mass fractal dimension was attributed to the quantity of the gold nanospheres, and the density dependence of the quantity was attributed to the ambient dielectricity and polarization energy in PLA. (iii) The gold nanoparticles generated by PLA in dipolar supercritical CHF3 and nondipolar supercritical CO2 were compared. The generation processes of nanonetwork occurred similarly in supercritical CHF3 and CO2. However, not thermal properties but the permittivity of fluid was a key factor for characterizing the fragmentation process of gold nanoparticle. That is, the dielectricity and polarization energy of the fluid are responsible for the morphology and quantity of the nanoparticles. The current gold nanonetwork with fractal structure is most probably useful to the SERS, MEF, and biochip substrates that result in a giant enhancement because the nanonetwork is an ensemble particle with many nanogaps and a chemical-free surface.

Figure 7. Density dependences of (a) polarization energy and (b) cooling time of a gold nanoparticle with a diameter of 400 nm in supercritical CHF3 and CO2. Red solid circles and blue open triangles represent the data for supercritical CHF3 and CO2, respectively. All data are collected at the same thermodynamic states; that is, the isotherm of reduced temperature Tr = T/Tc = 1.02, where Tc is the critical temperature, as a function of reduced density ρr = ρ/ρc, where ρc is the critical density.

via Coulomb explosion of charged nanoparticles.41−45 Thus, the generation via a charged particle is consistent with the results of this study regarding the density and fluid dependence of PLAs in supercritical CO2 and CHF3. We examined the thermal properties of both fluids to discuss the thermal processes behind the generation of gold nanoparticles. Specifically, if a droplet was kept at a high temperature for a long time, a small nanoparticle would be generated by evaporation. We have calculated the cooling time as follows26,29,46,47 τ=

RAu 2ρAu 2 CAu 1 = k 9ρSCF CSCFλSCF

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(5)

where τ and k are the cooling time and cooling rate, RAu, ρAu, and CAu are the radius, density, and specific heat capacity of the gold,48 respectively, and ρSCF, CSCF, and ΛSCF are the density, specific heat capacity,35 and thermal conductivity33 of the supercritical fluid, respectively. Here, we calculated the τ of gold nanoparticle with a diameter of 400 nm. The density dependence of the calculated τ in supercritical CHF3 and CO2 is shown in Figure 7b. A negligible difference in τ is observed between supercritical CHF3 and CO2. Apparently, the cooling process is similar in both fluids. If the thermal process were responsible for the gold nanoparticles in supercritical fluid, the profile of the density dependence in CO2 should be in accord with that in CHF3. However, the experimental results indicate that the profiles of the density dependence on quantity of nanonetwork are not the same in supercritical CO2 and CHF3, as shown in Figure 5. Accordingly, it was concluded that it is not the thermal process but the dielectric property that is crucial for the morphology and quantity of the gold

AUTHOR INFORMATION

Corresponding Author

*Tel +81(Japan)-82-424-7487; fax +81(Japan)-82-424-7486; email [email protected]. Present Address ⊥

Department of Physics, Kinki University, 3-4-1 Kowakae, Higashiosaka City, Osaka 577-8502, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at the BL37XU in the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2008A1655). K.S. acknowledges the Funding Program for Next Generation World-Leading Researchers (GR073) of the Japan Society for the Promotion of Science (JSPS) and Professor Yoshio Okamoto of Nagoya University, the research 17257

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supervisor for “Structure Control and Function” of PRESTO of Japan Science and Technology Agency (JST). This study was supported by a Grant-in-Aid for Young Scientists (A) (16685001) and a Grant-in-Aid for Scientific Research (B) (21350015) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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