Fusion Growth of Gold Nanoparticles Induced by the Conformational

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Fusion Growth of Gold Nanoparticles Induced by the Conformational Change of a Thermoresponsive Polymer Studied by Distance Distribution Functions Takeshi Morita,*,† Kenta Kurihara,† Osamu Yoshida,‡ Hiroshi Imamura,† Yoshikiyo Hatakeyama,†,§ Keiko Nishikawa,† and Nobuo Uehara‡ †

Division of Nanoscience, Graduate School of Advanced Integration Science, Chiba University, Yayoi, Inage-ku, Chiba 263-8522, Japan ‡ Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan ABSTRACT: The growth kinetics of gold nanoparticles induced by the conformational collapse resulting from the coil−globule transition of thermoresponsive polymer was studied using small-angle X-ray scattering (SAXS) method. The growth process of aggregates and nanoparticles formed during the reaction was determined on the basis of distance distribution functions calculated from the SAXS profiles without having to approximate or simplify the actual structures. The size of aggregates and nanoparticles reached a maximum not in the final stage of the reaction but in the middle stage, indicating that shrinkage of the particles occurred in the growth process. The surface roughness increased up to the middle stage of the reaction and then significantly decreased in the final stage. Furthermore, the dependence of polymer concentration was also investigated. It was found that small particles were generated prior to the formation of main products, and size of the small particles was estimated to be ca. 7 nm in diameter. The results reveal that the formation process progresses mainly based on fusion of the small particles induced by the structural collapse of polymer. Finally, the growth mechanism is discussed in detail.

1. INTRODUCTION Gold nanoparticles constitute an important class of nanomaterials.1−3 Chemical synthesis techniques such as a seed-mediated growth method4−7 are useful for synthesizing nanoparticles with a certain size and dispersion by adjusting synthesis parameters. The reduction step of gold ions is typically applied in the chemical synthesis methods in aqueous solutions. Uehara and coworkers discovered a novel chemical synthesis method for gold nanoparticles from gold nanoclusters without the need for the reduction step.8 In this method, nanoparticle formation, hereafter referred to as thermal-induced nanoprocessing, proceeds induced by structural collapse of a thermoresponsive polymer via coil−globule transition.9−11 Poly(N-isopropylacrylamide) (p-NIP)-conjugated gold nanoclusters were effectively used for this method. The nanoclusters were grown to be nanoparticles with diameters of several tens of nanometers by heat treatment up to 90 °C, followed by cooling below phase-transition temperature of the polymer. The method has a significant advantage of not requiring the reduction process of gold ions to grow nanoparticles. An understanding of the growth kinetics of nanoparticles is essential for the development of structural controls and synthesis methods. Investigation of the growth kinetics based on structural © XXXX American Chemical Society

evaluation of metal nanoparticles has been performed using timeresolved experimental methodologies such as UV−vis absorption, wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), X-ray absorption fine structure (XAFS), energy-dispersive XAFS (DXAFS), X-ray absorption near edge structure (XANES), and combinations of these and other techniques. For spherical gold nanoparticles, the time dependence of growth and the reaction model has been discussed in detail by Rao et al.12,13 The progress of particle generation with high rate constants has been reported using in situ observation of SAXS/ WAXS and UV−vis absorption spectroscopy.14 The mechanisms of nanoparticle formation by photoreduction of Rh(III) and Pd(II) in a polymer solution have been investigated using in situ SAXS and DXAFS measurements,15,16 and the mechanism for silver particles has been investigated using time-resolved SAXS measurement.17 These studies address the process of nucleation as well as of particle growth. Characteristics such as size, shape, Received: November 4, 2012 Revised: May 29, 2013

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dx.doi.org/10.1021/jp310906b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION 2.1. Preparation of Gold Nanoclusters. A solution of gold nanoclusters and gold nanoparticles containing 0.043 g-Au/L was passed through a column of silica gel, and a solution containing only nanoclusters with diameters 2 to 3 nm was prepared. We improved the procedure of preparation of nanoclusters for removing byproducts such as nanoparticles with extinction at around 520 nm using the gel chromatography compared with that of our previous study.8 Figure 1 shows

and their dispersion of nanoparticles generated by sputtering method have been examined by SAXS measurements.18,19 For rod-shaped nanoparticles, the growth along the length of ZnO nanorods has been investigated on the basis of crystallite size calculated from peak width of WAXS.20 The growth kinetics of ZnO nanorods, investigated using in situ SAXS and transmission electron microscope (TEM) observations, has been reported by Biswas et al.21,22 In a seed-mediated growth method, the growth kinetics of Au and AuCu nanorods have been investigated using in situ SAXS and UV−vis absorption measurements.23 Furthermore, the growth rate of gold nanorods and their aspect ratio dependence have been evaluated using time-resolved distance distribution functions calculated from SAXS profiles without having to approximate or simplify the actual shape of nanorods.24 The formation process of gold nanorods, including reduction step of Au ions, has been investigated using a new method combining time-resolved SAXS and XANES.25 The change and transition of rod-like shape in the growth process of nanorods have been clarified by combined cryo-TEM method.26 For the growth aspects of nanoprocessing, the dependences of macroscopic parameters on the process have been reported by Uehara et al.8 However, the growth mechanism in nanoprocessing has not yet been clarified from the viewpoint of structural investigation of aggregates and nanoparticles formed during the process. As outlined above, SAXS techniques have been widely applied for investigation of the growth kinetics of nanoparticles as well as their basic characteristics. However, SAXS structural investigations, including time-resolved measurements, generally assume that the focused nanoparticles have a rigid structure such as a spherical or rod-like shape. However, TEM observation showed that aggregates formed during nanoprocessing had extremely varied and complex structure, similar to those occasionally observed in nanoparticle syntheses. We assert that structural analysis considering theoretical functions based on assumption with respect to certain shape is not adequate for investigation on growth kinetics of this type of reaction. In the present study, the formation process was investigated using distance distribution functions P(r) obtained from SAXS profiles. The P(r) value gives the most reliable information about the structure of a scattering center (scatterer) in real space, because neither approximation nor assumption is used in the function. In addition, the value of P(r)/4πr2 is proportional to the probability that a vector of length r can be positioned in the space with the same electron density. From this meaning of the P(r) function, r at which P(r) = 0 is exactly equal to the maximum length of scatterer such as an aggregate or a nanoparticle in the present case. The maximum length evaluated by the function reliably corresponds to particle size of the aggregates and the nanoparticles, even though various differently shaped particles exist in the system. Here the growth process of aggregates and nanoparticles formed during the reaction was studied on the basis of the P(r) functions calculated from SAXS profiles without neither approximation nor simplification of the actual structures. Through this investigation combined with UV−vis absorption spectroscopy and TEM observation, the growth mechanism of nanoprocessing is discussed from the structural viewpoints in mesoscale size.

Figure 1. Change of UV−vis absorption spectra of nanocluster solutions before and after passed through the silica gel column. Inset shows the enlarged figure of the spectrum after passed through the column. The extinction around at 520 nm was not observed in the spectrum after passing.

comparison of UV−vis absorption spectra between before and after passing through the column. The improvement was confirmed by observations of both UV−vis absorption and TEM. 2.2. Synthesis of Gold Nanoparticles. Screw tubes were used as reaction vessels. In each screw tube, 1.5 mL of the nanocluster solution and 2.5 mL of an aqueous solution containing 2.0 wt % p-NIP were simultaneously mixed and heated at 90 °C using a hot water bath. Every 10 min, the containers were removed individually and immediately cooled for 1 h in an ice bath to stop the reaction. In this manner, sample solutions at reaction times of 10, 20, 30, 40, and 50 min were prepared. Aqueous solutions containing 0.5 wt % p-NIP were also heated at 90 °C for 60, 120, 180, 240, 300, 360, 420, and 480 min. For evaluating concentration dependence of the polymer, sample solutions containing 0.25, 0.50, 0.75, 1.0, and 2.0 wt % pNIP were also prepared and heated at 90 °C for 50 min, followed by cooling for 1 h. Other procedures of the preparation and the synthesis are the same as those reported in our previous paper.8 2.3. SAXS, UV−vis Absorption, and TEM. SAXS experiments were performed using apparatus at the BL-15A and 6A stations,27,28 Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK), Tsukuba. An X-ray beam was monochromatized to the wavelength λ = 1.50 Å using a bent monochromator and a bent mirror. An imaging plate (IP; FUJIFILM BAS-MS2025) with a positional resolution of 100 μm was used as a detector. X-ray signals recorded on the IP were read out by an IP reader (FUJIFILM BAS-2500), under the same condition of fading characteristic of the IP. The camera length was set at 2.4 m, and the observable s-region was 0.0090 to 0.30 B



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Å−1, with the scattering parameter s = 4π sin θ/λ (2θ: scattering angle). The accumulation time for each exposure was 300 s. An in situ beam monitoring apparatus29,30 was constructed for the SAXS beamlines, and the absorption factor of the sample, which exponentially affects the precision of absorption correction, was measured using the apparatus simultaneously during the SAXS measurements. SAXS profile from a sample holder filled only with water was determined and used for subtraction of background intensity. UV−vis absorption spectra were measured using a UV−vis spectrometer (Hitachi U-3900H). TEM observation was performed under an acceleration voltage of 120 kV using an electron microscope of field-emission type (JEOL JEM-2100F). The measurements of SAXS and UV−vis absorption were performed within 2 h of completion of the synthesis. TEM grids were prepared by placing drops of the solutions on carboncoated grids within 2 h of completion of the synthesis and drying them under room temperature for 1 day.

3. RESULTS AND DISCUSSION 3.1. Formation Process Based on Distance Distribution Functions. 3.1.1. Calculation of Distance Distribution Function. Figure 2 shows typical TEM images for aggregates

Figure 3. SAXS intensities during nanoprocessing as a function of reaction time. The SAXS profiles are shifted for clarity. The solutions containing 2.0 wt % p-NIP were heated to 90 °C.

where A is the normalization coefficient to reduce the maximum value of P(r) to 1. In the present calculation, the exponential term exp(−Bs2) (B: damping factor) was applied to remove the termination effect of the direct transform, paying careful attention to prevent excessive smoothing of the functions. The other procedures for the calculation are the same as those reported in our previous papers.24,33,34 3.1.2. Polymer Concentration Dependence. Figure 4 shows the dependence of polymer concentration on P(r) calculated from SAXS profiles. As previously mentioned, the value of r at which P(r) = 0 equals the maximum length of scatters. In the present investigation, the maximum length reliably corresponds to the size of aggregates and nanoparticles formed during the reaction. Their size increased with an increase in polymer concentration up to 1.0 wt % and then decreased at 2.0 wt %. The concentration dependence on the particle size is analogous to that obtained from UV−vis absorption spectra in our previous study.8 In the case of low polymer concentration, the polymer plays a role of progress of the reaction, resulting in the formation of the larger nanoparticles. The polymer serves as a dispersant of nanoparticles for stabilization in high polymer concentration, resulting in a decrease in the size. Shoulder peaks at ∼3 nm were meaningfully observed in P(r) at polymer concentrations of 0.25 and 0.50 wt %. These peaks attribute to the composition of P(r) for particles with diameters 6.5 and 7.3 nm, respectively, because the P(r) profiles calculated from an empirical equation35 with these maximum lengths correspond to the peaks, as shown in Figure 4. The results indicate that small particles with diameter ca. 7 nm were generated prior to the main steps of formation of aggregates and final products. The behavior has been pointed out in our previous investigation on the basis of hydrodynamic radius estimated from dynamic light scattering (DLS).8 The estimated size of small particles using DLS is ca. 8 nm, and the value is close to that evaluated by P(r) function using SAXS data. The generation of the small particles was certainly revealed directly based on scattering of X-rays.

Figure 2. Typical TEM images of particles generated by nanoprocessing: (a) 10, (b) 30, (c) 30, and (d) 50 min. The solutions containing 2.0 wt % p-NIP were heated to 90 °C.

and nanoparticles generated by the nanoprocessing as a function of reaction time. The reaction proceeded under 2.0 wt % in pNIP concentration and 90 °C in heating temperature. Figure 3 shows reaction-time dependence of SAXS intensities during the nanoprocessing under the same condition. An increase in the intensity in the small s region suggests that the formation and growth of particles from nanoclusters occurred in the solution. Distance distribution function P(r) is calculated from the direct Fourier transform of the SAXS profile I(s) as follows:31−33 P(r ) = A

∫0

∞

I(s)sr sin(sr ) exp( − Bs 2) ds

(1) C



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Figure 5. Distance distribution functions, P(r), obtained from the measured SAXS profiles for various reaction times. Arrows indicate the maximum length of the particles generated by nanoprocessing. The solutions containing 2.0 wt % p-NIP were heated to 90 °C.

Figure 4. Distance distribution functions, P(r), obtained from the measured SAXS profiles as a function of polymer concentration. Green arrows indicate the maximum length of the particles generated by nanoprocessing. Dotted lines in the Figures for 0.25 and 0.50 wt % represent the theoretical values of P(r)35 calculated for particles with the maximum length of 6.5 and 7.3 nm, respectively. The solutions were heated to 90 °C for 50 min.

caused by structural fluctuation in the surface. The long tail of P(r) disappeared in the final stage. Figure 6 shows particle size, core size, and surface roughness of the particles formed during the reaction. As shown in Figure 6a, the core size was estimated from the diameter of a spherical portion combing P(r) function calculated from an empirical equation.35 The surface roughness is defined as the difference between particle size and that of core part, as illustrated in Figure 6. As shown in Figure 6b, the particle size decreased in the final stage, indicating that shrinkage of the aggregates occurred by fusion. As observed in Figure 6c, the fusion also caused a decrease in the core size in the early stage resulting from filling of voids and cavity between the small particles. The surface roughness increased up to the middle stage, and then sharply decreased in the final stage, as seen in Figure 6d. It was clearly evident that aggregates with surface roughness were generated up to the middle stage; then, the aggregates were fused and shrunken to be shaped single particles with smooth surface in the final stage. The presence of fusion growth in nanoprocessing was revealed from the viewpoint of structural investigation in detail. The process is based on relaxation of the surface energy of nanoclusters and aggregates composed of the small particles induced by the structural collapse of polymer. Figure 7 shows growth process in the lower-polymer concentration, 0.5 wt % of p-NIP. The aspect of the process is the same as that in the higher polymer concentration, 2.0 wt % pNIP, that is, aggregation of small particles, shrinkage by fusion, and then formation of shaped nanoparticles, as described in Figure 6. However, the shrinkage by fusion was observed in the middle, 180 min, and the final period, 420 min. The repeatability of the double shrinkage has been confirmed based on repeated

It is considered that the generation of small particles with diameter less than 10 nm is related to the size of collapsed polymer chain resulting from coil−globule transition. The structural change of p-NIP between random coil and globule state conformation has been reported in the literatures.11,36−38 The results indicate that the size of globular structure is extremely collapsed to be several tens of nanometers and is reduced by ∼1/10 compared with the size of random coil. The small particles observed in 0.25 and 0.50 wt % solutions were identified as aggregates generated by each collapsed polymer. In the lower concentration, the polymer chains were isolated in solution and the particles generated by the collapse were distinguished from each other. The polymer chains in the higher concentration solution were intertwined, and as a result, unified small particles were not observed in P(r) profiles owing to the structural complexity. 3.1.3. Reaction Time Dependence. Figure 5 shows the calculated P(r) as a function of reaction time. The aqueous solutions containing 2.0 wt % p-NIP were heated to 90 °C. As shown in Figure 5, the maximum length slightly increased with reaction time up to 30 min, the middle stage, and then decreased at 40 and 50 min, the final stage. The reaction time dependence indicates that the particle size reached a maximum not in the final stage but in the middle stage. Furthermore, the profiles of P(r) at reaction times of 20 and 30 min had longer tails than those at other reaction times. The long tail in P(r) is considered to be the presence of uneven surfaces in the aggregates because the probability that a vector of length r in P(r) function can be positioned gradually decreases below the maximum length D



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3.2. Formation Process Based on Enhanced Plasmon Resonance. Figure 8 shows UV−vis absorption spectra as a

Figure 8. UV−vis absorption spectra and their second derivative as a function of reaction time. The broad peaks centered at 700 nm were observed in UV−vis absorption spectra at 10, 20, and 30 min, the early and middle stages. The negative peaks at 580 nm were observed in second derivative at 40 and 50 min, the final stage, while the negative peak was not observed in the early and middle stages.

function of reaction time. The second derivative spectra were calculated using the Savitzky−Golay method39 and are shown for clarification of the spectral characteristics. The absorption at 530 nm was caused by the extinction of enhanced surface plasmon resonance of gold nanoparticles. For spherical nanoparticles, peak shift of the plasmon maximum and observed band in the spectrum depend on particle size, interparticle distance, and number of neighboring particles.40,41 Broad peaks in the absorbance spectra from 650 to 750 nm centered at 700 nm were observable at reaction times of 10, 20, and 30 min. The broad peaks were assigned to be the enhanced plasmon resonance of hot sites by neighboring particles. The peaks disappeared at the reaction time of 40 min, at which the shrinkage of particles occurred by fusion, as shown in Figure 6b. Therefore, the observed peak centered at 700 nm is evidence of the formation of aggregates composed of small particles. Valleys at 580 nm were observed in the second derivative spectra, as shown in Figure 8. The valley appeared at a reaction time of 40 min and became deeper at 50 min. If generation of the plasmon band at 580 nm is caused by generation of gold nanoparticles with spherical shape, then the particle size is calculated to be 116 nm in diameter.41 The size is not consistent with the present structural investigation based on P(r). As shown in Figure 3, the SAXS signal seems to satisfy small-angle resolution considering so-called Guinier region because log(s)− log(I(s)) plots of SAXS intensities show a flat profile in small s region. The peak at 580 nm was apparently assigned to be a sharpened component and a hot site caused by more enhanced plasmon resonance than that of single nanoparticle with smoothed spherical structure. Sharpened structure with spherical symmetry was observed via TEM observation, as shown in Figure 2d. On the contrary, the results of DLS in our previous study correspond to the size estimation from the viewpoint of generation of the larger particles distinguished from the particles with size of several tens of nanometers. However, hydrodynamic radius evaluated using DLS undesirably includes the size

Figure 6. Structure change of aggregates and nanoparticles formed during nanoprocessing. The solutions containing 2.0 wt % p-NIP were heated to 90 °C. (a) Evaluation of particle size, core size, and surface roughness from P(r) function, (b) particle size evaluated by the maximum length obtained from P(r), (c) core size, and (d) surface roughness estimated from difference between the particle size and that of core portion. The illustration shows the explanation of these parameters. Dotted line shown in panel a represents theoretical P(r) profile calculated from an empirical equation.35

Figure 7. Particle size evaluated from P(r) at the lower polymer concentration as a function of reaction time. The shrinkage by fusion was observed at 180 and 420 min. The solutions containing 0.5 wt % pNIP were heated to 90 °C.

SAXS measurements. The double shrinkage supports the fusion growth during the process. E



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information of polymer, and the tendency will become less ignorable with the particle size. We undertake clarification of the controversy in the future investigation on the basis of crystallite size evaluation using WAXS method. The present analysis indicates that the second derivative of UV−vis absorption spectra effectively decomposed the plasmon maximum band, which is useful to distinguish contributions of size and shape of the particles to red shift of the band in the absorption spectra.

4. CONCLUSIONS The formation process of gold nanoparticles induced by the conformational collapse of thermoresponsive polymer was clarified on the basis of distance distribution functions calculated from the direct Fourier transform. The growth mechanism of nanoprocessing is shown in Figure 9. The following tendencies

Figure 9. Schematic diagram for the growth mechanism of nanoprocessing.

are evident in the order of progress of the process: (1) generation of small particles with diameter ca. 7 nm, (2) formation of aggregates composed of the small particles with large surface roughness, (3) filling of voids between the small particles by fusion resulting in the shrinkage of particle, and (4) finally formation of shaped nanoparticles with smooth surface also by fusion.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81-43-290-3951. E-mail: [email protected]. Present Address §

Yoshikiyo Hatakeyama: Department of Physics, College of Humanities and Sciences, Nihon University, Sakurajosui, Setagaya-ku, Tokyo 156−8550, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to P.F. at KEK for providing the opportunity to perform the SAXS experiments. (no. 2011G540). This study was supported by Grants-in-Aid for Scientific Research Japan (C) (20550071) and (B) (20310040) and the Tokuyama Science Foundation. T.M. thanks the Global Center-of-Excellence Program ‘Advanced School for Organic Electronics’ for the financial support.

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Fusion Growth of Gold Nanoparticles Induced by the Conformational

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