Formation Mechanism of Gold Nanoparticles Synthesized by

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Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymer Using In Situ Quick XAFS and SAXS Masafumi Harada, and Syoko Kizaki Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01168 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on January 28, 2016

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Crystal Growth & Design

Cover Page Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymer Using In Situ Quick XAFS and SAXS Masafumi Harada* and Syoko Kizaki Department of Health Science and Clothing Environment, Faculty of Human Life and Environment, Nara Women’s University, Nara 630-8506, Japan

ABSTRACT: Photoreduction process of [AuCl4]- precursors in poly(N-vinyl-2-pyrrolidone) solution was investigated by utilizing a combination of in-situ quick XAFS (QXAFS) and smallangle X-ray scattering (SAXS) measurements to elucidate the nucleation and subsequent aggregative particle growth of the colloidal gold nanoparticles (Au NPs). The sequence of stages for reduction-nucleation and association process (autocatalytic surface growth and aggregative particle growth) of Au atoms to generate Au NPs was observed during the photoreduction. QXAFS analysis revealed that the reduction of [AuCl2]- species to Au0 atoms is a slower step than that of [AuCl4]- to [AuCl2]-, and the reduction of [AuCl2]- to Au0 atoms and the association of Au0 atoms to produce Au nucleates concurrently proceeds during the short-duration photoirradiation. A sigmoidal curve expressed by the solid-state kinetic model, specifically the Avrami-Erofe’ev model, was applicable to demonstrate the aggregative particle growth. Complementary in-situ SAXS results showed that Ostwald ripening-based growth of Au NPs is not observed after the termination of aggregative growth stage in the long-duration photoirradiation.

AUTHOR INFORMATION: Corresponding Author Prof. Dr. Masafumi Harada Tel: +81-742-20-3466; Fax: +81-742-20-3466 E-mail address: [email protected] (M. Harada)

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Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymer Using In Situ Quick XAFS and SAXS Masafumi Harada* and Syoko Kizaki Department of Health Science and Clothing Environment, Faculty of Human Life and Environment, Nara Women’s University, Nara 630-8506, Japan CORRESPONDING AUTHOR FOOTNOTE: Prof. Dr. Masafumi Harada Tel: +81-742-20-3466; Fax: +81-742-20-3466 E-mail address: [email protected] (M. Harada) ABSTRACT: Photoreduction process of [AuCl4]- precursors in poly(N-vinyl-2-pyrrolidone) solution was investigated by utilizing a combination of in-situ quick XAFS (QXAFS) and smallangle X-ray scattering (SAXS) measurements to elucidate the nucleation and subsequent aggregative particle growth of the colloidal gold nanoparticles (Au NPs). The sequence of stages for reduction-nucleation and association process (autocatalytic surface growth and aggregative particle growth) of Au atoms to generate Au NPs was observed during the photoreduction. QXAFS analysis revealed that the reduction of [AuCl2]- species to Au0 atoms is a slower step than that of [AuCl4]- to [AuCl2]-, and the reduction of [AuCl2]- to Au0 atoms and the association of Au0 atoms to produce Au nucleates concurrently proceeds during the short-duration photoirradiation. A sigmoidal curve expressed by the solid-state kinetic model, specifically the Avrami-Erofe’ev model, was applicable to demonstrate the aggregative particle growth. Complementary in-situ SAXS results showed that


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Ostwald ripening-based growth of Au NPs is not observed after the termination of aggregative growth stage in the long-duration photoirradiation.

KEYWORDS:

colloidal

dispersions;

gold

nanoparticles;

polymer-stabilized

particles;

photoreduction; autocatalytic surface growth; in-situ quick XAFS; in-situ SAXS Introduction Gold nanoparticles (Au NPs) have attracted much attention due to their unique properties and numerous promising applications in biotechnology, catalysis, and optoelectronics.1,2 The electronic, optical, and chemical/biological properties are significantly dependent on the size, shape, and size monodispersity. Photochemical reduction under γ- or UV-irradiation has been used as a method for the preparation of Au NPs in recent publications.3,4 The advantages of photoreduction methods are that they have excellent spatial and temporal control, avoid the use of harmful strong reducing agents, and are often room temperature procedures. It has been recognized that the color of [AuCl4]disappears before Au NPs absorbance appears, indicating the presence of at least one intermediate state. Mills et al.3 showed that disproportionation of Au+ is responsible for formation of Au NPs in polymer solutions. Gachard et al.4 described the stability of Au+ as the only possible intermediate with the appropriate lifetime to demonstrate the observed absorption behavior. The LaMer mechanism5,6 (classical nucleation and growth) and Ostwald-ripening (OR) mechanism

7

are generally used to describe nanoparticle formation. These classical models of

nucleation burst have been challenged in the case of citrate reduction where the size and size distribution are influenced by the reaction conditions (pH, temperature, solvent, and concentration of the reducing agents).8,9 Using a slow reduction process to facilitate the time-resolved studies, Sardar and Shumaker-Parry10 reported that 1-octadecanethiol protected Au NPs prepared using 9borabicyclo[3.3.1]nonane (9-BBN) as a reducing agent are highly single crystalline through the synthesis and appear to be formed by a diffusion-controlled Ostwald-ripening growth mechanism. On the other hand, an alternative model called as the aggregative growth model has been used to describe Au NPs growth.11-14 In this model, the initial nucleation and growth is constituted of a number of critically sized aggregates of smaller nanocrystallites in an aggregative nucleation step. Nanoparticle growth is then achieved by the coalescence of these aggregates and is characterized by polycrystallinity of the growing particles,15 bimodal size distributions in the early stage of 3 ACS Paragon Plus Environment


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growth,13,16 and sigmoidal growth kinetics.13,17-19 The crystallinity of the nanoparticles implies that the growth of the particles follows a classical diffusion controlled Ostwald-ripening mechanism. However, in a recent report by Buhro and co-workers,13,14 two different growth mechanisms for formation of thiol-protected Au NPs were described: (i) aggregative and (ii) Ostwald ripening. An aggregative growth mechanism produces primarily polycrystalline Au NPs, while single crystalline Au NPs are expected if the growth process follows an Ostwald-ripening mechanism. A technique sensitive to the time evolution around an atomic species is essential to fill the gap of the knowledge about the initial stage of nanoparticle growth. Fortunately, due to the deep penetration of X-ray and atomic species identity, time-resolved in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful method to carry out a direct investigation for the local structural change during the reactive processes in a bulk or a solution system.20-22 A second powerful method, small-angle X-ray scattering (SAXS), is most suited to determine the in situ size and size distribution of nanocrystals during the growth process. This offers a direct probe, since the scattering cross section depends directly on the shape and size of nanoparticles.23-26 In this paper we used a method that analyzes in situ QXAFS and SAXS employing synchrotron radiation to monitor precursor reduction and nanoparticle formation during the photoreduction process of [AuCl4]-. This leads to gain information on the composition of elemental oxidation states and on particle shape and sizes during the photoreduction. The goal of this paper is to study the kinetics of nanoparticle nucleation and growth, in an effort to identify their time evolutions and better understanding the aggregative growth mechanism. Experimental Section Materials. Colloidal dispersions of Au NPs were produced by the photoreduction of mixed solutions containing Au3+ (or Au+) precursors and poly(N-vinyl-2-pyrrolidone) (PVP) as a protective polymer. Tetrachloroauric(III) acid (hydrogen tetrachloroaurate(III), HAuCl4 ･ 4H2O, Nacalai Tesque), gold(I) chloride (AuCl, Aldrich), ethanol (Nacalai Tesque), and distilled water were used as received. The polymers with different molecular weight, PVP (K-15, average M.W.= 10000), PVP (K-30, average M.W.= 40000), and PVP (K-90, average M.W.= 630000), were supplied from Tokyo Kasei Kogyo Co., Ltd. These polymers were denoted as PVP10000, PVP40000 and PVP630000, respectively, according to their number average molecular weight.


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Preparation and Characterization of Colloidal Dispersions of Au NPs. Photoreduction of ionic precursor HAuCl4･4H2O (i.e., Au3+ ions) without photo-activator was carried out to prepare the colloidal Au NPs. In a typical procedure of the Au NPs, PVP40000 (2.23 g, 20.1 mmol per monomeric unit) was completely dissolved into 10 mL of ethanol solution. Aqueous solution (10 mL) dissolving 0.244 mmol or 0.488 mmol of HAuCl4･4H2O was then poured in it and stirred for 1 min. The final metal concentration in the mixture was [Au] = 12.2 or 24.4 mM. The photoirradiation of the mixed solution was subsequently performed using a 500 W superhighpressure mercury lamp under constant stirring. To examine the effect of polymer molecular weight on the morphology of Au NPs and their stabilization, PVP10000 or PVP630000 were used in the same solvent. The reduction of [AuCl4]- and the formation of Au NPs were monitored by observing changes in the absorption spectra centered at ~ 220 nm for the gold(III) chloride solutions and those centered at ~ 540 nm originating from the surface plasmon of the Au NPs using a UV-visible spectrometer (Hitachi, U-3010). To adjust the concentration of metal for the UV absorption measurements, 0.1 mL of the obtained samples were diluted in 3 mL of the 1:1 mixture solution of water and ethanol. TEM micrographs of these dispersions were obtained using a JEM-2000FX instrument operated at 200 kV of the acceleration voltage. A high-resolution carbon-supported copper mesh was used to support the samples of dispersions. The diameter of each particle was determined from enlarged photographs, and the average diameter was estimated by counting at least 200 particles from the photograph. In-situ QXAFS Measurement and Data Analysis.

In situ QXAFS experiments were

performed at the BL-9C and 12C in the Photon Factory, High Energy Accelerator Research Organization (PF, KEK). The storage ring was operated at 2.5 GeV. A Si(111) single crystal was used to obtain a monochromatic X-ray beam. In situ QXAFS data of the Au L3-edge (11921 eV) were collected in a transmission mode. The beam size was 1 × 2 mm2 at the sample position. The energy of X-ray was calibrated with Au foil as a reference. Ionization chamber for incident beam intensity (I0) and transmitted beam intensity (I) were filled with flows of Ar(15)N2(85) gas and Ar gas, respectively. Each spectrum (ca. 2000 points) was measured for 10 s by scanning the energy in the range of 11700 – 12920 eV. The dispersions were produced in quartz reactor (path length = 1 cm), sealed with Kapton films (thickness: 50 µm). The direction of the photoirradiation from the lamp was perpendicular to that of the direct X-ray beam. When the photoirradiation was started, 5 ACS Paragon Plus Environment


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QXAFS measurement of the sample solutions was launched with continuous stirring. The QXAFS spectrum was recorded every 20 s under the photoirradiation for up to 60 min. Data analysis of the XAFS spectra was performed using the REX2000 program. The k3weighted EXAFS oscillation in the range of 30-160 nm-1 was Fourier transformed, and curve fitting analyses were performed in the k range of 40-150 nm-1. In the curve fitting analyses, the empirical parameters (phase shift and amplitude functions) of Au-Cl and Au-Au bond were extracted from HAuCl4 ･4H2O and Au foil, respectively.27 On the other hand, for the XANES analysis, each spectrum was normalized by the edge height. To confirm the kinetics and dynamics of Au NPs formation, the intensity evolution of the white line (11917 eV assigned to Au3+-Cl bond) and the intrinsic peak (11943 eV assigned to Au+-Cl or Au-Au bond) in photoreduction time was checked on the basis of the dispersive kinetic equations, using the similar procedure.22 In Situ SAXS Measurements and Data Analysis.

In order to examine the kinetics of

association process (nucleation, growth, and aggregation) of Au atoms, in situ time-resolved SAXS experiments were performed at room temperature at the BL45XU beamline in SPring-8. A CCD camera with 6-in. image intensifier was set to a sample detector distance of 2.5 m, operating at a photon energy of 13.8 keV. The CCD camera was used to record 2D images that were processed to determine the scattering intensity, I(q), as a function of the modulus of the scattering vector q = (4π/λ) sin(θ/2), where θ is the scattering angle and λ is the wavelength of X-ray. The q range was set to 0.15 - 4.0 nm-1 in the present study. Silver behenate (CH3(CH2)20-COOAg), with a d-spacing of 58.38 Å, was used as a standard to calibrate the angular scale of the 2D image. The accumulation time for each exposure was 5 sec. The observed scattering intensities were background corrected by subtracting air scatterings. Then the 2D images were radially averaged and corrected for transmitted intensities. For in situ SAXS measurements, the colloidal dispersions were synthesized in a stainless steel reactor (inner volume of 13.5 mL) with four optical windows. Two windows were single crystal diamond (optical path length : 1 mm) for the incident X-ray beam and the other two were quartz for the photoirradiation. The direction of the photoirradiation was perpendicular to that of the incident X-ray beam. The sample solution was continuously stirred and photoirradiated during the SAXS experiments. The time interval for monitoring of a single SAXS spectrum was typically 60 sec during the photoirradiation for up to 120 min. Analysis of SAXS data was done according to the same procedure,26 as shown in the Supporting Information. The software IGOR Pro was used to perform the modeling and fitting of the SAXS profiles to evaluate the average radius of spherical 6 ACS Paragon Plus Environment

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Au NPs (R0), the total number of particles (np), the standard deviation (σR/R0) in radius, and the total volume of the spherical Au NPs (Vp) converted from Au3+ ions. The time dependence of R0, np, σR/R0, and Vp, is acquired by the model fitting using eqs. (S2) – (S7) in the Supporting Information. Results and Discussion Formation of the Colloidal Dispersions of Au NPs. Colloidal Au NPs were successfully produced by the photoreduction of HAuCl4･4H2O in the presence of PVP. The solution retained the red color originating from colloidal Au NPs for a couple of weeks. The reduction of [AuCl4]- and the formation of Au NPs were monitored by observing changes of UV-vis absorption. Figure 1 shows representative UV-vis absorption spectra of the colloidal Au NPs in the presence of PVP40000 before and after the photoirradiation. The absorption band centered at ~540 nm originating from the localized surface plasmon resonances (LSPR) was observed,28 while the peaks below 400 nm (216 and 320 nm) were assigned to ligand-to-metal charge transfer (LMCT) ( π → σ* ) Cl pπ → 5dx2-y2 bands of the various chlorohydroxoaurate species AuClxOH4-x- that may be present depending on the solution pH.29,30 Importantly, this indicates that Au NPs were formed following the reduction of [AuCl4]-. The LMCT band around 300-400 nm disappeared after photoirradiation for 30 min. The plasmon absorption peak at ~540 nm then appeared, and its intensity gradually became higher until the irradiation time of 60 min. However, over the irradiation time of 60 min, the increase of the baseline was observed due to the aggregate formation of Au NPs. The weak absorption around 650 nm was also observed, which could be attributed to the enhanced plasmon resonances of larger aggregates.31,32 Similar UV-vis absorption spectra were obtained for the colloidal Au NPs in the presence of PVP10000 and PVP630000, as shown in Figure S1 in the Supporting Information. Worth emphasizing here is that the Au NPs prepared in the presence of PVP10000 was more unstable than those prepared in the presence of PVP630000, resulting in the precipitation of the former, which is consistent with the disappearance of LSPR band over the photoirradiation time of 10 min.



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Figure 1. UV-vis absorption spectra from the colloidal Au NPs ([Au] = 24.4 mM) produced by the photoreduction in the presence of PVP40000. The time evolution of absorption spectra was indicated during the photoreduction for up to 120 min. The morphology of the resultant Au NPs was determined by TEM microscopy. Figure 2 shows TEM images of the colloidal Au NPs prepared in the presence of PVP40000 and PVP10000. In the case of PVP40000, at the photoirradiation time as short as 3-5 min, small and spherical Au NPs having an average diameter of 5.8 nm (3 min) and 9.6 nm (5 min) was observed. After the photoirradiation time of 15 min, a small portion of tremendously larger NPs (~ 30 nm) was dispersed among the small NPs. Some triangular and hexagonal shapes were also observed, but only a small percentage. Over the photoirradiation of 15 min, however, no more aggregates were observed and the average diameter slightly increased; 14.5 nm (15 min), 16.2 nm (30 min) and 16.0 nm (60 min). On the other hand, in the case of PVP10000, the average diameter of Au NPs rapidly increased to 9.2 nm during the first 3 min probably due to the higher rate of nanoparticle formation. At the photoirradiation time of 5 - 15 min, gradual increase in size of the Au NPs (average diameter of ~10.4 nm (5 min) and ~12.8 nm (15 min)) were observed, and the NPs were irregular in shape and had somewhat twinned structures. With further increase of photoirradiation time, the individual NPs aggregated more and more, and finally changed to larger aggregates (or precipitates with diameters of up to ca. 1 - 2 µm) as shown in the inset of Figures 2(i) and 2(j). Noteworthy is that the aggregation/precipitation of Au NPs was eventually observed when the sample solutions were stored under ambient conditions for a couple of days. Additional TEM images of other colloidal Au NPs are provided in Figure S2 in the Supporting Information. 8 ACS Paragon Plus Environment

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(a)

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Figure 2. TEM images for the colloidal Au NPs ([Au] = 24.4 mM) produced by the photoreduction for (a) 3 min, (b) 5 min, (c) 15 min, (d) 30 min, and (e) 60 min in the presence of PVP40000, and for (f) 3 min, (g) 5 min, (h) 15 min, (i) 30 min, and (j) 60 min in the presence of PVP10000. Inset: Representative TEM image of the aggregates of the primary Au NPs. Further TEM images are available in Figure S2 in the Supporting Information. 9 ACS Paragon Plus Environment


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Photoreduction of [AuCl4]- Complexes to Au0 Atoms. Figure 3(a) shows a series of in situ Au L3-edge XANES spectra during the formation of Au NPs, together with aqueous solutions of HAuCl4 and AuCl as well as Au foil. The photochemical reactions induced by synchrotron X-ray irradiation hardly ever influenced the quality of XAFS spectra in relatively high concentration ([Au] = 24.4 mM). The XANES spectrum of HAuCl4 has a sharp and narrow absorption band (11917.5 eV) around the edge, which is called the white line; it corresponds to the electronic transition from the 2p3/2 core level state to the vacant 5d3/2 and 5d5/2 states. The XANES spectrum of the Au foil exhibits almost no white line because of the fully filled 5d state of Au0 atom. As the photoreduction progresses, we can observe that the XANES spectra exhibited a clear decrease in intensity for the white line peak and a slow increase for the intrinsic peak (11943.2 eV).33,34 In fact, between 11930 and 11947 eV the intensity increased in course of time, between 11947 and 11965 eV it decreased, and so forth, thus the Au3+ species was converted to Au0. It should be noted that 11930, 11947, and 11965 eV are clearly isosbestic points, which are usually obtained when high quality XAFS spectra are collected for a system subjected to an A → B transformation, indicating that there is a continuous change from Au3+ to Au0 species via Au+ species with negligible amount of any transient species. In the EXAFS Fourier transforms of the colloidal Au NPs as shown in Figure 3(b), the peak located at 0.2 nm before the photoirradiation is assigned to an Au-Cl bond (a bond length of 0.228 nm), as compared with an aqueous solution of HAuCl4･4H2O with high Au concentration (Figure 3(c)). With an increase of photoirradiation time, the peak intensity assigned to Au-Cl bond decreases. The other peak at 0.270 nm is observed after longer photoirradiation, which is assigned to a metallic Au-Au bond of Au foil (Figure 3(c)). This indicates the formation of Au NPs. It is noted that between the reduction time of 400 and 1800 sec the peak intensity at 0.2 nm remains almost constant, and obviously attributes to the Au+-Cl- bond compared with that of the AuCl powder. This fact confirms the change in the “3+” valence state of [AuCl4]-, which, during the reaction with ethanol, are probably transformed stepwise via “1+” valence state of [AuCl2]-, and finally to the “0” value, which is characteristic of metallic Au0 atoms. To obtain quantitative structural parameters around Au atoms in photoreduction, a least-squares curve fitting was performed using REX 2000 program. In the curve-fitting analysis, the peaks in the range of 1.2 – 3.4 Å were attributed to Au-Cl and Au-Au bonds without any contribution of the Au-


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(a)

(b)

(c)

(d)

Figure 3. Evolution as a function of the photoreduction time for (a) representative Au L3edge XANES spectra and (b) Fourier transformed spectra of k3-weighted Au L3-edge EXAFS for the colloidal Au NPs ([Au] = 24.4 mM) produced in the presence of PVP40000, together with the spectrum of reference compounds of Au foil ( ), aqueous HAuCl4･4H2O solution ( ), and AuCl powder ( ). (c) k3-weighted Fourier transforms of Au foil ( ), ), and AuCl powder ( ). (d) Coordination numbers aqueous HAuCl4･4H2O solution ( as a function of the reduction time for Au-Cl and Au-Au bond for the colloidal Au NPs. O bond, which coincides with the fitting procedures in the previous work.27 This indicates that the hydrolysis reaction of [AuCl4]- to [Au(OH)4]- complex does not occur in the case of relatively high Au concentration.33,34 The evolution of coordination number (CN) contributed to Au-Cl and Au-Au bonds is displayed in Figure 3(d). With an increase of photoreduction time, the CN of the Au-Cl bond decreased while the bond length of Au-Cl kept unchanged (2.28 ± 0.03 Å). Even after the photoreduction of 3600 s the CN (0.6) of Au-Cl bond was still observed, suggesting some Au+ species still remain in solution. On the other hand, the CN of the Au-Au bond appeared after the photoreduction of 600 s and continued to increase as 3.6 (1800 s), up to ca. 8.5 (3600 s). The time dependence was not found for the bond length (2.88 ± 0.03 Å) and Debye-Waller factor of Au-Au bond, which were accorded with those of bulk Au metal. 11 ACS Paragon Plus Environment


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The in-situ EXAFS results (Figures 3(b) and 3(d)) are consistent with the observation of the UV-vis spectra shown in Fig. 1. The decrease of the Au-Cl peak intensity in FT by a short-duration photoirradiation is due to the transformation of [AuCl4]- to [AuCl2]-, which corresponds to the decrease of the CN of the Au-Cl bond from 4.0 to 1.8. This is consistent with the complete disappearance of the LMCT band around 300-400 nm during the short-duration photoirradiation. Over the photoirradiation time of 600 sec, the increase for the LSPR band is noticeably observed around 500-600 nm and CN of the metallic Au-Au bond is obviously detectable. Association of Au0 Atoms to Generate Au NPs. Further inspection of the isosbestic points in Figure 3(a) suggests that the photoreduction proceeds through a stepwise mechanism, where Au3+ ion is first reduced to Au+ ion and then reduced further to Au0 atom. It is expected that the association of Au0 atoms with excess Au+ ions is followed by their coalescence into Au nanoclusters (Au nucleates). After the formation of Au nanoclusters, these nanoclusters will be further aggregated to yield larger Au NPs. Although important investigations3,4 using the photoirradiation method have focused on the reduction steps of the reaction Au3+ → (Au2+) → Au1+ → Au0, Au NPs formation by photoreduction is little understood. Hence, in order to investigate the association process of Au0 atoms in the Au NPs formation, we performed in situ time-resolved SAXS experiments of the colloidal Au NPs ([Au] = 24.4 mM). Time evolution of the SAXS profiles (I(q)) of the colloidal Au NPs are shown in Figures S3 in the Supporting Information. The observed intensity I(q) in q < 3.0 nm-1 for dispersions after longer photoirradiation is stronger than that after the shorter photoirradiation, revealing that the increase of scattering intensity should originate from the formation of Au NPs. Furthermore, the remarkable increase in the small q-region (q < 0.3 nm-1) is found, which is owing to the formation of assemblies of polymer-Au ion complexes and/or polymer-Au metal particle complexes. Specifically, Guinier region is not detected in this region so that the mass-fractal structure is anticipated to be constructed. Thus, to quantitatively evaluate the mass-fractal dimensions, the unified Guinier/power-law approach has been performed for the SAXS analysis, as described in the Supporting Information. Figures 4(a) and 4(b) show the excess scattering intensities of Iexc(q) against scattering vector q in the logarithmic scale in course of reduction times in case of PVP40000 and PVP10000, respectively. A monotonically decreasing broad curve at the q-region (0.5 < q < 3.0 nm-1) is found for the photoirradiation time longer than 5 min. This feature indicates a form factor of spherical particles (dashed lines), as shown in our earlier work.26 To the contrary, the deviation from the 12 ACS Paragon Plus Environment

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Iexc(q) / a.u.

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R0 4.59 4.60 4.63 4.67 4.70 4.72 4.76

Df 2.81 2.91 2.90 3.08 2.98 2.94 2.49

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σR/R0 0.40 0.41 0.47 0.47 0.47 0.47

Df 2.79 2.51 2.21 2.46 2.40 2.38

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5

Diameter / nm

10

15

20

25

30

Diameter / nm

Figure 4. Evolution of the excess scattering intensity (Iexc(q)) from the Au NPs ([Au] = 24.4 mM) and their aggregates prepared in the presence of (a) PVP40000 and (b) PVP10000 in a time interval of several min. The solid lines show the predicted curve best-fitted with the experimental curves calculated by the unified Guinier/power-law approach, and the dashed lines indicate the predicted curve best-fitted with the experimental curves by the spherical model fitting. The fractal dimension Df is indicated. Particle size distribution (P(R)) estimated from the model fitting for the colloidal Au NPs prepared in the presence of (c) PVP40000 and (d) PVP10000 is also shown. scattering is observed at the smaller q-region (0.1 < q < 0.3 nm-1), implying that this excess scattering should originate from the power-law scattering for the aggregates of the primary Au NPs. The best fitted curves (solid lines) are shown in the whole q range (from 0.1 to 3.0 nm-1), which are obtained by the unified Guinier/power-law approach. This indicates the data analysis based on the unified Guinier/power-law approach is required at the smaller q-region, and the theoretical curves (dashed lines) obtained from the spherical model are well fit to the experimental data (0.5 < q < 3.0 nm-1), as is similar to the unified Guinier/power-law approach. The average particle radius R0, the standard deviation (σR/R0) in radius, and the fractal dimension Df obtained from the model fits are also shown in Figures 4(a) and 4(b) (See also Figures 6(a) and 6(d)). The values of R0 drastically 13 ACS Paragon Plus Environment


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increase up to ~4.60 nm (in case of PVP40000) and ~4.62 nm (in case of PVP10000) within the photoreduction of 10 min, and those of standard deviation σR/R0 are close to 0.40. With increase of photoirradiation time up to 60 min, the values of R0 increase to ~4.76 nm (for PVP40000) and ~5.44 nm (for PVP10000), indicating the association process to form larger Au NPs. The σR/R0 values approach to 0.47, and the values of Df slightly decrease as the photoirradiation time increases for both PVP40000 and PVP10000. No remarkable change of scattering profiles is observed after 60 min as shown in Figures S3 in the Supporting Information. In addition, the time evolutions in particle size distribution (P(R)) are shown in Figure 4(c) and 4(d), which are derived by the spherical model fitting. Remarkable change in particle size distribution is not observed after the photoirradiation time of 30 min. Kinetic Modeling of the Au NPs Formation during Photoirradiation. Photoirradiation for the LMCT bands of the [AuCl4]- complex in alcohol-water solution resulted in the reduction of Au3+ to metallic Au NPs with the simultaneous oxidation of the alcohol. For example, an earlier report by Gachard et al.4 presented the synthesis of Au NPs from 2-propanol under γ-irradiation. 2-Hydroxy2-propyl radicals (ketyl) were formed upon intermolecular H-abstraction by OH radicals in solution, and the ketyl radicals initiated a one-electron reduction of the [AuCl4]- complex following a disproportionation of [AuCl3]- species to produce [AuCl2]- species. Further irradiation of [AuCl2]species initiated the formation of metallic Au NPs. This disproportionation reaction mechanism in the reduction process of the [AuCl4]- complex to Au0 atom is experimentally confirmed by some previous reports.3,4,28,35 The disproportionation reaction is a rate-determining process in the formation of Au NPs, and thus the rate of formation of the Au NPs is determined by the fast disproportionation reaction, that is, 2[AuCl3]- → [AuCl2]- + [AuCl4]-. In our previous work,27 it has been found that, the electronic structure of the photoirradiated samples is composed of three stable states, [AuCl4]-, [AuCl2]-, and Au0 atoms. The reduction of [AuCl2]- species to Au0 atoms is a slower process than that of [AuCl4]- to [AuCl2]-, and the reduction of [AuCl2]- species to Au0 atoms and the association of Au0 atoms to form seed Au particles proceeds concurrently. On the underlying basis of these above hypotheses, we believe that the observed Au NPs are formed from the reduction due to the fast disproportionation of [AuCl3]- to [AuCl2]- and [AuCl4]-, and then the subsequent photoreduction of [AuCl2]-, with a mechanism similar to that proposed by Belloni,4,35 El-Sayed,28 Scaiano36 and their co-workers to interpret generation of Au NPs by the photochemical reduction of HAuCl4. Consistent with this observation, Buntine et al.37 demonstrated that the relative yields of Au3+, Au+, and Au0 species vary in a manner explained by interconversion 14 ACS Paragon Plus Environment

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steps where the photoreduction by soft X-ray exposure proceeds via the Au+ intermediate. Therefore, under moderate UV-irradiation conditions, some kind of Au+ intermediate could be generated in the initial nucleation stage prior to the complete reduction of Au3+ ions to Au0 atoms. It is well known that the kinetics of reduction-nucleation and aggregative particle growth process of nanocrystal can be quantitatively evaluated by the dispersive kinetic equations13,14,19 specifically for Avrami-Erofe’ev (AE) model.38-40 We can use an “Avrami equation” to fit the sigmoidal profile for the mechanistic studies of nanocrystals (eq 1),

α = 1 − exp[−(kt ) n ]

(1)

where α is the product crystal fraction in the system at a given conversion time t. k is the rate constant for this system, and n is the system dimensionality. To simplify the data analysis, as reported in the previous paper,22 normalized X-ray absorbance µ(E) of HAuCl4 precursor during the photoreduction is shown in Figure 5(a), which is derived from the time evolution for intensity of the white line peak (11917.5 eV) contributed to Au3+-Cl bond and the intrinsic peak (11943.2 eV) contributed to Au+-Cl and Au-Au bond. At the photoreduction time of 0 s (that is, before photoreduction), we define that the amount of unreduced Au3+ species is abbreviated as [Au3+]0, assuming that Au3+ species are nearly reduced at the photoreduction time of 600 s and the unreduced Au3+ amount would be proportional to the difference (R0 – R600). Similarly, the amount of unreduced Au3+ at the photoreduction time of t sec, which is defined as [Au3+]t, would be proportional to the difference (Rt – R600). Here, Rt and R0 are regarded as white line intensity µ(E = 11917 eV) value at irradiation time of t and 0, respectively. Hence the value of [Au3+]t/[Au3+]0 is considered as that of [(Rt – R600)/(R0 – R600)], as seen in the left axis of Figure 5(b). The similar definition can be done to evaluate the kinetics of Au+ ions (the right axis of Fig. 5(b), abbreviated as [Au+]t) and Au0 atoms (the left axis of Fig. 5(c), abbreviated as [(Au0)m]t). During the first 200-300 s, kinetic data of [Au3+]t/[Au3+]0 (i.e., the consumption of Au3+ species) directly mirrors kinetic data of [Au+]t/[Au+]600 (i.e., the generation of Au+ species) in t < 600 s. Due to the lack of induction time, it is suggested that the predominant transformation from Au3+ to Au+ species is not autocatalytic and it is a zero order reaction



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Table 1. Summary of Fitting Results Derived from the AE Equation and the Two-Step FW Equation Sample

k (s-1)

AE model n

k1 (s-1)

FW model k2 (s-1M-1)

Au(12.2mM) / PVP10000

1.6×10-3

2.00

3.41×10-4

3.99×10-1

Au(24.4mM) / PVP10000

2.0×10-3

1.70

6.87×10-4

1.86×10-1

Au(12.2mM) / PVP40000

5.0×10-4

1.71

1.74×10-4

9.46×10-2

Au(24.4mM) / PVP40000

5.8×10-4

1.80

1.75×10-4

5.96×10-2

Au(12.2mM) / PVP630000

6.7×10-4

1.50

3.09×10-4

9.49×10-2

Au(24.4mM) / PVP630000

7.5×10-4

1.44

3.87×10-4

4.72×10-2

(a)

(b)

(c)

Figure 5. (a) Evolution of the normalized absorbance µ(E) at the intrinsic peak (located at 11917 and 11943 eV) for the colloidal Au NPs ([Au] = 24.4 mM). (b) Plots of [Au3+]t/[Au3+]0 and [Au+]t/[Au+]600 versus photoreduction time (t) based on the temporal change in µ(E = 11917 eV) and µ(E = 11943 eV). (c) Plots of [(Au0)m]t/[(Au0)m]3600 versus photoreduction time. Curve fitting to the experimental data was performed using the AE model (green curve) expressed by the solid-state kinetics and FW model (red curve) by the solution-phase reaction kinetics. 16 ACS Paragon Plus Environment

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AuCl4-

+

CH3CH2 OH

AuCl4-

+

CH3CHOH

hν

AuCl42- + CH3CHOH + H+

AuCl42- + CH 3 C H

+

(2)

H+

(3)

O AuCl 2- + CH3CHOH + H+

AuCl 42- + CH3CH2 OH AuCl4-

AuCl2-

+

+

hν

2 CH3CH2 OH

CH3CHOH

(Au0)n+ AuCl2-

+

k0

0

2H+ + 2Cl- (5)

H+

+

2Cl-

+

(6)

O

CH3CHOH

0

(4)

+

(7)

k2

(Au0)n+1 + CH 3 C H 0

2Cl-

AuCl2- + 2 CH3CHOH

Au0 + CH 3 C H

k1

+

+

H+

+

2Cl-

O

(Au )n + (Au )m → (Au )n+m

(8)

Scheme 1. Formation mechanism of the Au NPs prepared from aqueous ethanol solution of HAuCl4 in the absence of photo-activator. (i.e., -d[AuCl4-]/dt = k0) because it is continuous and rapid process under UV-irradiation, which has been observed during the transformation from Pt4+ to Pt2+ in the Pt nanoparticle formation.22 It should be noted that, at the end of this period (400-600 s), the Au3+ ions have just completely disappeared and Au+ is therefore the only stable product. Its formation rate would be thus equal to the Au3+ disappearance rate. To the contrary, over the irradiation time of 600 s, kinetic curve of [(Au0)m]t/[(Au0)m]3600 (i.e., the formation of Au-Au bond) exhibits a pseudosigmoidal profile for the nucleation and subsequent autocatalytic surface growth, as shown in Figure 5(c). Here we apply the classical solid-state kinetics (AE model) for the description of the aggregative growth of Au nuclei to analyze the pseudosigmoidal profile. The sigmoidal growth profile of Au NPs in the time regime (~ 3600 s) is well fit by the AE model (the solid green curve) with k = 5.8×10-4 s-1 and n = 1.80 (Table 1). 17 ACS Paragon Plus Environment


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On the other hand, an alternative analytical FW model,41,42 on the basis of the chemically, mechanistically based kinetics, can be applied to differentiate nucleation and subsequent autocatalytic surface growth processes. Herein the overall reaction scheme of [AuCl4]- precursor in aqueous ethanol solution under the photoirradiation is proposed. We consider the two-step consecutive reduction process of Au ions (i.e., Au3+ → Au+, and subsequently, Au+ → Au0) as well as the association process (i.e., nucleation and subsequent autocatalytic surface growth) of Au0 atoms in Scheme 1. The initial photochemical step by excitation of [AuCl4]- species involves electron transfer from Cl- to the metal center due to the LMCT of the absorption bands, generating Cl· radicals and [AuCl3]- intermediate species.27,43 Then, Cl· radicals interact with solvent ethanol by hydrogen abstraction to form hydroxymethyl radicals (CH3ĊHOH) (reaction 2). These radicals virtually exhibit high reducing ability and take part in the reduction of [AuCl4]- species to yield acetaldehyde (CH3CHO) (reaction 3). A subsequent propagation reaction generates CH3ĊHOH radicals through the reduction of [AuCl4]2- species by the solvent ethanol (reaction 4). Consequently, from the sum of reactions 2 to 4, the transformation of [AuCl4]- to [AuCl2]- under the photoirradiation yields CH3ĊHOH radical, H+ and Cl- (reduction process of Au3+ to Au+ ions, rate constant k0, reaction 5).43,44 Continuous photoirradiation of [AuCl2]- makes Au+ ions reduce to Au0 atoms under the existence of CH3ĊHOH radicals (rate constant k1, reaction 6). Afterwards, autocatalytic surface growth (rate constant k2, reaction 7) followed by the particle growth (reaction 8) produces larger Au NPs. Based on the kinetically important reactions 5 to 8, if we assumed to have a distinct rate of production in each reaction, two-step FW model41,42 has been applied to fit the kinetic data (the solid red curve in Figure 5(c)). As a result, the rate constants are obtained as k1 = 1.75×10-4 s-1 (rate constant of reaction 6) and k2 = 5.96×10-2 s-1M-1 (rate constant of reaction 7), respectively, both of which are also shown in Table 1. This insight also ruled out the operation of Ostwald ripening in the later stages of Au NPs growth, so that the autocatalytic surface reaction is more adequately involved in nucleation and aggregative particle growth process. It is most likely that the sigmoidal growth kinetics is not a nucleation-driven process and is not consistent with the LSW model for Ostwald ripening. On the basis of these considerations, it is thought that the reduction of [AuCl4]- proceeds via the oxidation of ethanol solvent through polymer-[AuCl4]- complex where the reduction of [AuCl4]follows the sequence [AuCl4]- → [AuCl2]- → Au0. The rate determining process is the reduction of 18 ACS Paragon Plus Environment

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[AuCl2]- which can be produced from the fast reduction of [AuCl4]- by hydroxymethyl radicals or the rapid disproportionation ( 2[AuCl3]- → [AuCl2]- + [AuCl4]- ), followed by the Au0 atom coalescence forming Au nanoclusters (Au nucleates). After the nucleation of Au0 atoms had launched, further [AuCl2]- reduction was enhanced via an autocatalytic process.41,42 Hence there appear to be a two-step mechanism involved in the Au nanocluster formation: the first step is a reduction of [AuCl4]- to isolated Au0 atoms and the second step is an autocatalytic surface growth of Au0 atoms to form Au nanoclusters (or small Au NPs). The aggregative growth of Au NPs in solution is a combination of the nucleation-dominated autocatalytic process (i.e., reactions 6 and 7) and subsequent aggregative particle growth via dynamic coalescence45 of the small Au NPs (i.e., reaction 8). Dependence on the Molecular Weight of PVP for the Formation Kinetics. In order to investigate the dependence of the kinetics and the characteristics of the generated Au NPs upon the molecular weight of PVP, we have performed further experiments (See Figures S4, S5 and S6). The AE parameters (k and n) and the average FW rate constants (k1 and k2) are evaluated by the fitting to XANES data (Figures S7(b), S7(d), S8(b), S8(d), and S8(f)). These parameters are listed in Table 1. The rate constant k (= 2.0 × 10-3 s-1) estimated from the fitting of the AE model in Au(24.4mM)/PVP10000 is much larger than those both in Au(24.4mM)/PVP40000 (= 5.8×10-4 s-1) and in Au(24.4mM)/PVP630000 (= 7.5×10-4 s-1). When decreasing the [Au] concentration from 24.4 to 12.2 mM, the value of k tends to decrease in three kinds of PVP samples. For the dimensionality n, there are subtle differences in n (ranging from 1.44 to 2.00) obtained from the AE model. However, these results are indicative of the aggregative particle growth with the sigmoidal kinetics. Furthermore, the average rate constants (k1 for nucleation and k2 for autocatalytic growth) in

Au(24.4mM)/PVP10000

is

larger

than

those

in

Au(24.4mM)/PVP40000

or

Au(24.4mM)/PVP630000. It is apparent that the value of k1 and k2 increases with decrease in molecular weight of PVP, which is closely related to the behavior of AE parameter k. In other word, the FW rate constant k1 and k2 tends to become larger while the AE rate constant k becomes larger. This indicates that the particle growth kinetics is strongly dependent on the molecular weight of PVP. The characteristic structural parameters (such as R0, np, σR/R0, and Vp) obtained from the SAXS analysis are shown in Figure 6 as a function of reduction time t. In the case of PVP10000 sample, within the first 2 min, the rapid formation of Au NPs with an average radius R0 of 4.58 nm is 19 ACS Paragon Plus Environment


6.0

10

(a)

10

3 2

PVP(Mw=10000) PVP(Mw=40000)

1

R0 / nm

1

2

4

6 8

10

2

2

4

6 8

3

2

4

6 8

10

time / sec

5.0

4.5

4.0

PVP(Mw=10000) PVP(Mw=40000)

4

10

number of particles / a.u.

R0 / nm

4

10

1000

2000

3000

8

4000

5000

(b)

4

2

10

-8 8 6 4

PVP(Mw=10000) PVP(Mw=40000)

2

10

0

-7 6

7 6 5

5.5

-9

6000

0

1000

2000

3000

4000

5000

6000

time / sec

time / sec 0.6

100

(c)

(d)

80 0.5

σR / R0

volume fraction / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40

0.4

0

PVP(Mw=10000) PVP(Mw=40000)

PVP(Mw=10000) PVP(Mw=40000)

20

0.3

0

1000

2000

3000

4000

5000

6000

0

1000

2000

3000

4000

5000

6000

time / sec

time / sec

Figure 6. Time evolution versus photoreduction time t for (a) average radius (R0), (b) number of particles (np), (c) volume fraction (Vp), and (d) standard deviation (σR/R0) in radius of the metallic Au NPs ([Au] = 24.4 mM) produced in the presence of PVP40000 or PVP10000. observed. Afterward, the R0 gradually increases from 4.60 nm (t = 5 min), through 4.62 nm (t = 10 min), to 4.80 nm (t = 20 min) and it continues up to 5.44 nm (t = 60 min). The number of particles (np) continuously increases during the first 40 min and then begins to drop after 40 min. Similarly, in PVP40000 sample, R0 increases to 4.58 nm within 2 min, through 4.59 nm (t = 5 min), 4.60 nm (t = 10 min) to 4.67 nm (t = 30 min) and its value constantly increases to 4.86 nm during the further irradiation up to 90 min, but the np increases during the first 48 min and subsequently begins to decrease. The slope of log R0(t) vs log t, as displayed in the inset of Figure 6(a), is much less than 1/3, suggesting that the relation of R0(t) ~ t1/3 is not satisfied in the present study.26 This result ensures that the diffusion-limited Ostwald ripening-based growth does not occur in the formation process. Interestingly, a decrease in number of particles after the irradiation time of 40 min takes place, accompanied by an increase in particle size during the initial 60 min. The volume fraction (Vp) value has maximum (approximately 100%) at the irradiation time of 40 – 60 min, 20 ACS Paragon Plus Environment

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demonstrating that the nucleation, autocatalytic surface growth and subsequent aggregative particle growth successively occurs until all Au3+ ions are consumed. Actually, after the photoirradiation time of 60 min, Au3+ ions are completely reduced to Au0 atom and the kinetic data for the Au-Au bond formation indicates a termination of the aggregative particle growth. In the same period of time (40 – 60 min), polydispersity (directly related to the standard deviation (σR/R0)) remains almost constant (0.47). The high polydispersity might exhibit the existence of a large amount of Au NPs with a radius around 4.8~5.4 nm, corresponding to LSPR centered at ~540 nm. According to the experimental results from XANES and SAXS analysis, the particle growth mechanism can be divided into three representative stages during the particle formation. The initial stage is the reduction of Au3+ to Au+ (reaction 5 in Scheme 1) completed within the first 10 min, clearly evidenced by XANES analysis. SAXS results also show that the reduction product of Au+ ions, i.e., very small amount of Au0 atoms and Au nanoparticles, is observed in this stage. In the second stage (between 10 and 40 min of the photoirradiation), nucleation-dominated autocatalytic surface growth (reactions 6 and 7 in Scheme 1) takes place. As indicated by the particle volume fraction Vp, about 90-100% of the Au3+ precursor is transformed into Au nuclei or Au nanoparticles within this period. This stage shows outstanding increase in the number of particles as well as an increase in the particle size for the small growing nanoparticles. In the subsequent third stage (at the latest after 40 min of the photoirradiation), aggregative particle growth (reaction 8 in Scheme 1) dominantly occurs. Specifically, an additional weak absorption around 650 nm in UV region (see Figure 1) appears although the spectrum shows the surface plasmon absorption band having a maximum around 560 nm. This distinct change certainly reveals the aggregation/precipitation of Au NPs in the latest period of the third stage. Noteworthy here is that such results of UV-vis measurements agree well with the results of TEM and SAXS. Actually, TEM images (Figures 2(c) to 2(e)) show the aggregation of Au NPs in which larger particles are clearly present along with smaller particles, and the multiple crystal facets and boundaries is observed. The polycrystalline nature for the as-grown Au NPs indicates the occurrence of the interparticle coalescence, this important evidence in support of the aggregative growth mechanism.11-13,19,22 Furthermore, the SAXS results also lead to an interpretation of the formation of aggregates/precipitates of Au NPs. As shown in Figures 6(a) and 6(b), even though in the photoirradiation time longer than 40 min, the number of particles np does significantly decrease with time. This behavior suggests the primary small Au NPs are collapsed into larger aggregates with a diameter of around 15 ~ 30 nm (Figures 2(d) and 2(i)), especially in the case of PVP10000. Finally, the formed large aggregates might be 21 ACS Paragon Plus Environment


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transformed into precipitates of Au NPs (noticeably observed in the case of PVP10000 as shown in the inset of Figures 2(i) and 2(j)), which is caused by fusion process of large aggregates and primary Au NPs.25 Use of Au(I)-Complexes to Generate Au NPs. There is plenty of literatures on the synthesis of Au NPs employing Au(III), in which HAuCl4･4H2O is the most common starting materials. However, formation of Au NPs starting from Au(I)-complex has rarely been reported.46,47 Due to the significant photochemical reactivity of Au(I)-complexes, the syntheses of Au NPs from aqueous AuCl solution were carried out under similar conditions in order to facilitate comparison of the Au(I) and Au(III) results. The evolution of Au L3-edge XANES spectra and the kitetic data of [(Au0)m]t/[(Au0)m]800 during the course of photoreduction is shown in Figures S9(a) and S9(b) in the Supporting Information. Apparently, the white line intensity of Au+ ion is relatively smaller than that of Au3+ ion (as shown in Figure 3(a)).21 While the Au+ ion is stable, it is rapidly reduced to Au0 atom within the first 60 sec. After the reduction of Au+ ion, a pseudosigmoidal profile related to the formation of Au-Au bond is seen, implying that the photoirradiation of the sample generates Au NPs with virtually induction period. This observation clearly reveals that the photoreduction of AuCl solution involves the nucleation and subsequent autocatalytic surface growth. By fitting the sigmoidal profile to the AE model, the parameters k = 2.1×10-3 s-1 and n = 2.30 (green curve) were obtained. When we fit the data by the FW model, average rate constants were obtained as k1 = 3.59×10-4 s-1 and k2 = 3.04×10-1 s-1M-1 (red curve). The AE and the FW model show essentially equal fitting results within experimental error. The rate constant k (= 2.1×10-3 s-1) obtained from the AE model in the case of AuCl/PVP40000 is much larger than that (= 5.8×10-4 s-1) in the case of HAuCl4/PVP40000. The average rate constants (k1 for nucleation and k2 for autocatalytic growth) of the former is larger than those of the latter. This demonstrates that the photoreduction of Au(I) is more accelerated in the absence of Au(III) species than that in the presence of Au(III) species which remains as unreduced species during the reduction of Au3+ to Au+. Value of n for the former (2.30) is larger than that for the latter (1.80) in terms of the dimensionality. In order to compare the growth of Au NPs synthesized from AuCl with those from HAuCl4, the UV-vis absorption spectra were measured at a designated irradiation time for the Au(I) reduction, as shown in Figure S9(c). The UV spectra with using AuCl reveals that Au(I) is the species in solution, since the characteristic band corresponding to the absorption of Au(III) at ~320 nm was very 22 ACS Paragon Plus Environment

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small.47 The slower Au NPs formation in the case of HAuCl4/PVP40000 is most likely attributed to the delay which is due to the reduction process starting from Au(III) to Au(I) and eventually to Au atoms. Over the irradiation time of 15 min, the weak absorption around 650 nm was observed due to the enhanced plasmon resonances of larger aggregates.31,32 From the TEM images of the Au NPs synthesized from AuCl, as shown in Figures S2(e) to S2(h), small and spherical Au NPs with an average diameter of 7.6 nm was obtained at the irradiation time of 5 min. The average diameter increased in course of time from ~9.3 nm (15 min), ~17.5 nm (30 min) up to ~21.6 nm (60 min), which are larger than those of the Au NPs synthesized from HAuCl4 (See Figures 2(d) and 2(e)). These Au NPs had twinned structures in the longer photoirradiation time. Therefore, regardless of the different starting compounds (AuCl or HAuCl4), the sigmoidal growth kinetics can be applied to interpret the Au NPs formation involving the nucleation, autocatalytic surface growth

and

aggregative particle growth as well as the aggregation/precipitation process. Aggregative Growth of the Au NPs in Polymer Solutions. Several studies have concluded that many solution-based syntheses of monodispersed nanoparticles do not conform to the LaMer mechanism. The deficiency is that the LaMer mechanism does not take into account for the aggregation of small nanocrystallite, which is shown to participate in the aggregative growth processes in many cases.9,11-14,22,24 In the latest work by Njoki and co-workers,11 it is demonstrated that the interparticle aggregative coalescence was an important factor that was operative in the size growth of Au NPs under thermally activated growth process in organic solvents. Along similar lines, Sabir et al.12 have indicated that the sigmoidal growth kinetics of Au NPs in triblock copolymer solution can be described by an aggregative nucleation and growth model, and the Au NPs morphology and size can be changed by adjusting the triblock copolymer/Au(III) ratio. On the other hand, another important mechanism that should be noted is that Ostwald ripening may occur at the completion of active growth regime under appropriate conditions for several nanoparticle systems.48,49 It is well known that Ostwald ripening is not a nucleation-driven process, and cannot account for the nucleation and aggregative growth behavior. In the present study, as already mentioned, from SAXS analysis of Au NPs we did not observe Ostwald ripening during the longduration photoirradiation. Instead, some structural parameters (R0, np, σR/R0, and Vp) obtained reflect some tendencies attributed to the autocatalytic surface growth41,42 and the aggregative particle growth mechanism.11-14,22



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In addition to the association processes (autocatalytic surface growth and aggregative particle growth) described above, there is an important process leading to the formation of assembly of polymer-Au ion complexes and/or polymer-Au metal particle complexes. Before the photoreduction, Au ions have weak interactions with polymer chains, resulting in the formation of aggregates of polymer-Au ion complexes. On the stage of photoreduction of polymer-Au ion complexes to form polymer-Au metal particle complexes, an association of primary small Au NPs into larger aggregates will concurrently occur in the polymer field. This polymer field may control the particle growth kinetics and growth mechanisms, which is dependent on the molecular weight of polymers. This work demonstrates that the reduction-nucleation, autocatalytic surface growth on nucleates, aggregative growth, and subsequent aggregation/precipitation (i.e., dynamic coalescence) are involved in the consecutive Au NPs growth process under the photoreduction. The reduction of Au3+ to Au+ ions occurs in the initial regime, and Au nucleates are concurrently produced. The reduction process is probably caused by the formation of reductive hydroxymethyl radicals. The subsequent reduction of Au+ to Au0 atoms takes place in the second regime where Au NPs growth (diameter is ~ 30 nm) is predominantly subject to autocatalytic surface growth processes of formed Au nucleates. Ostwald ripening is not predicted to occur after the termination of the autocatalytic surface growth period in our experimental time scale. In the final regime, aggregation/precipitation occurs, which is due to the aggregative growth and dynamic coalescence, resulting in the formation of larger aggregates (ca. 1 - 2 µm). Our recent study of the photochemical synthesis of Pd nanoparticles26 and Pt nanoparticles22 demonstrated that three processes – reduction-nucleation, autocatalytic surface growth (and aggregative growth) of nucleates, and Ostwald ripening – can participate in, despite consecutive time domains with little overlap. These earlier studies can be viewed as examples that illustrate the generality of particle formation mechanism outlined in Scheme 1. Conclusions Colloidal Au NPs protected by poly(N-vinyl-2-pyrrolidone) were prepared by photoreduction of the ionic solutions of HAuCl4･4H2O (Au3+ precursor) and AuCl (Au+ precursor). The combination of in-situ QXAFS and SAXS experiments can provide the acquisition and evaluation of kinetic data, i.e. the sigmoidal profile for the solid-state kinetic model (Avrami-Erofe’ev model) that allows a quantitative description of the reduction-nucleation and aggregative particle growth of the Au NPs. 24 ACS Paragon Plus Environment

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The sequence of stages for the reduction-nucleation, autocatalytic surface growth, and the association process (i.e., aggregative particle growth and dynamic coalescence) of Au atoms to produce Au NPs and their large aggregates were observed in the increase of photoreduction time. An important finding of these experiments is that the mechanism underlying the formation of Au NPs is constituted of three sequential stages. In the case of Au3+ reduction, the initial stage is the rapid reduction of Au3+ to Au+ essentially achieved by alcohol radicals R·. In the second stage after the complete reduction of Au3+ to Au+, nucleation, autocatalytic growth and aggregative growth process concurrently proceeds, leading to the generation of the primary Au NPs with a diameter of ~ 30 nm. In the third stage, the formation of aggregates/precipitates of primary Au NPs occurs, due to the aggregative growth and/or interparticle coalescence during the long-duration photoirradiation, which is elucidated by the appearance of weak absorption around 650 nm. Acknowledgments. In situ QXAFS measurements were performed at High Energy Accelerator Research Organization (KEK) with the approval (Proposal No. 2007G577, 2009G053, and 2011G005) of the Photon Factory Advisory Committee (PAC). We appreciate the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2010B1189 and 2011B1338) for the in situ time-resolved SAXS experiments performed in the Spring-8. TEM observation was supported by Kyoto University Nano Technology Hub in “Nanotechnology Platform Project” (No. A-12-KT0009). This work was also financially supported by Nara Women’s University Intramural Grant for Project Research. Supporting Information Available: UV-vis absorption spectra of the colloidal Au NPs prepared in the presence of PVP10000 and PVP630000; TEM images of the Au NPs obtained during the photoirradiation of the solution containing Au3+ or Au+ ions as well as PVP; Time evolution of the SAXS profile for the Au NPs; Evolution of the Au L3-edge XANES spectra, the Fourier transformed spectra of k3-weighted Au L3edge EXAFS, and the CNs of Au-Cl and Au-Au bond for the colloidal Au NPs prepared in the presence of PVP with different molecular weight, and their corresponding pseudosigmoidal kinetic data for the colloidal Au NPs. This material is available free of charge via the Internet at http://pubs.acs.org. References 25 ACS Paragon Plus Environment


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(1) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, QuantumSize-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293-346. (2) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Gold Nanoparticles: Past, Present, and Future. Langmuir 2009, 25, 13840-13851. (3) Quinn, M.; Mills, G. Surface-Mediated Formation of Gold Particles in Basic Methanol. J. Phys. Chem. 1994, 98, 9840-9844. (4) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. Radiation-induced and chemical formation of gold clusters. New J. Chem. 1998, 22, 1257-1265. (5) LaMer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847-4854. (6) LaMer, V. K. Nucleation in Phase Transitions. Ind. Eng. Chem. 1952, 44, 1270-1277. (7) Voorhees, P. W. The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 38, 231-252. (8) Bastús, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27, 11098-11105. (9) Polte, J.; Ahner, T. T.; Delissen, F.; Sokolov, S.; Emmerling, F.; Thünemann, A. F.; Kraehnert, R. Mechanism of Gold Nanoparticle Formation in the Classical Citrate Synthesis Method Derived from Coupled In Situ XANES and SAXS Evaluation. J. Am. Chem. Soc. 2010, 132, 1296-1301. (10) Sardar, R.; Shumaker-Parry, J. S. Spectroscopic and Microscopic Investigation of Gold Nanoparticle Formation: Ligand and Temperature Effects on Rate and Particle Size. J. Am. Chem. Soc. 2011, 133, 8179-8190. (11) Njoki, P. N.; Luo, J.; Kamundi, M. M.; Lim, S.; Zhong, C.-J. Aggregative Growth in the SizeControlled Growth of Monodispersed Gold Nanoparticles. Langmuir 2010, 26, 13622-13629. (12) Sabir, T. S.; Yan, D.; Milligan, J. R.; Aruni, A. W.; Nick, K. E.; Ramon, R. H.; Hughes, J. A.; Chen, Q.; Kurti, R. S.; Perry, C. C. Kinetics of Gold Nanoparticle Formation Facilitated by Triblock Copolymers. J. Phys. Chem. C 2012, 116, 4431-4441. (13) Shields, S. P.; Richards, V. N.; Buhro, W. E. Nucleation Control of Size and Dispersity in Aggregative Nanoparticle Growth. A Study of the Coarsening Kinetics of Thiolate-Capped Gold Nanocrystals. Chem. Mater. 2010, 22, 3212-3225. (14) Wang, F.; Richards, V. N.; Shields, S. P.; Buhro, W. E. Kinetics and Mechanisms of Aggregative Nanocrystal Growth. Chem. Mater. 2014, 26, 5-21. 26 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Bisson, L.; Boissiere, C.; Nicole, L.; Grosso, D.; Jolivet, J. P.; Thomazeau, C. C.; Uzio, D.; Berhault, G.; Sanchez, C. M. Formation of PalladiumNanostructures in a Seed-Mediated Synthesis through an Oriented-Attachment-Directed Aggregation. Chem. Mater. 2009, 21, 2668-2678. (16) Drews, T. O.; Katsoulakis, M. A.; Tsapatsis, M. A Mathematical Model for Crystal Growth by Aggregation of Precursor Metastable Nanoparticles. J. Phys. Chem. B 2005, 109, 23879-23887. (17) Finney, E. E.; Finke, R. G. Fitting and Interpreting Transition-Metal Nanocluster Formation and Other Sigmoidal-Appearing Kinetic Data: A More Thorough Testing of Dispersive Kinetic vs Chemical-Mechanism-Based Equations and Treatments for 4-Step Type Kinetic Data. Chem. Mater. 2009, 21, 4468-4479. (18) Finney, E. E.; Finke, R. G. Is There a Minimal Chemical Mechanism Underlying Classical Avrami-Erofe’ev Treatments of Phase-Transformation Kinetic Data? Chem. Mater. 2009, 21, 46924705. (19) Skrdla, P. J. Roles of Nucleation, Denucleation, Coarsening, and Aggregation Kinetics in Nanoparticle Preparations and Neurological Disease. Langmuir 2012, 28, 4842-4857. (20) Yao, T.; Sun, Z.; Li, Y.; Pan, Z.; Wei, H.; Xie, Y.; Nomura, M.; Niwa, Y.; Yan, W.; Wu, Z.; Jiang, Y.; Liu, Q.; Wei, S. Insights into Initial Kinetic Nucleation of Gold Nanocrystals. J. Am. Chem. Soc. 2010, 132, 7696-7701. (21) Ohyama, J.; Teramura, K.; Higuchi, Y.; Shishido, T.; Hitomi, Y.; Kato, K.; Tanida, H.; Uruga, T.; Tanaka, T. In Situ Observation of Nucleation and Growth Process of Gold Nanoparticles by Quick XAFS Spectroscopy. ChemPhysChem 2011, 12, 127-131. (22) Harada, M.; Kamigaito, Y. Nucleation and Aggregative Growth Process of Platinum Nanoparticles Studied by in Situ Quick XAFS Spectroscopy. Langmuir 2012, 28, 2415-2428. (23) Abécassis, B.; Testard, F.; Kong, Q.; Francois, B.; Spalla, O. Influence of Monomer Feeding on a Fast Gold Nanoparticles Synthesis: Time-Resolved XANES and SAXS Experiments. Langmuir 2010, 26, 13847-13854. (24) Polte, J.; Erler, R.; Thünemann, A. F.; Sokolov, S.; Ahner, T. T.; Rademann, K.; Emmerling, F.; Kraehnert, R. Nucleation and Growth of Gold Nanoparticles Studied via in situ Small Angle Xray Scattering at Millisecond Time Resolution. ACS Nano 2010, 4, 1076-1082. (25) Morita, T.; Kurihara, K.; Yoshida, O.; Imamura, H.; Hatakeyama, Y.; Nishikawa, K.; Uehara, N. Fusion Growth of Gold Nanoparticles Induced by the Conformational Change of a Thermoresponsive Polymer Studied by Distance Distribution Functions. J. Phys. Chem. C 2013, 117, 13602-13608. 27 ACS Paragon Plus Environment


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Page 28 of 30

(26) Harada, M.; Tamura, N.; Takenaka, M. Nucleation and Growth of Metal Nanoparticles during Photoreduction Using In Situ Time-Resolved SAXS Analysis. J. Phys. Chem. C 2011, 115, 1408114092. (27) Harada, M.; Einaga, H. In Situ XAFS Studies of Au Particle Formation by Photoreduction in Polymer Solutions. Langmuir 2007, 23, 6536-6543. (28) Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. Gold Nanoparticle Formation from Photochemical Reduction of Au3+ by Continuous Excitation in Colloidal Solutions. A Proposed Molecular Machanism. J. Phys. Chem. B 2005, 109, 4811-4815. (29) Usher, A.; McPhail, D. C.; Brugger, J. A spectrophotometric study of aqueous Au(III) halidehydroxide complexes at 25-80 °C. Geochim. Cosmochim. Acta 2009, 73, 3359-3380. (30) Wang, S.; Qian, K.; Bi, X. Z.; Huang, W. Influence of Speciation of Aqueous HAuCl4 on the Synthesis, Structure, and Property of Au Colloids. J. Phys. Chem. C 2009, 113, 6505-6510. (31) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. What Controls the Optical Properties of DNA-Linked Gold Nanoparticle Assemblies? J. Am. Chem. Soc. 2000, 122, 4640-4650. (32) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. The Surface Chemistry of Au Colloids and Their Interactions with Functional Amino Acids. J. Phys. Chem. B 2004, 108, 4046-4052. (33) Pacławski, K.; Zajac, D. A.; Borowiec, M.; Kapusta, Cz.; Fitzner, K. EXAFS Studies on the Reaction of Gold(III) Chloride Complex Ions with Sodium Hydroxide and Glucose. J. Phys. Chem. A 2010, 114, 11943-11947. (34) Farges, F.; Sharps, J. A.; Brown, G. E., Jr. Local environment around gold(III) in aqueous chloride solutions: An EXAFS spectroscopy study. Geochim. Cosmochim. Acta 1993, 57, 12431252. (35) Dey, G. R.; El Omar, A. K.; Jacob, J. A.; Mostafavi, M.; Belloni, J. Mechanism of Trivalent Gold Reduction and Reactivity of Transient Divalent and Monovalent Gold Ions Studied by Gamma and Pulse Radiolysis. J. Phys. Chem. A 2011, 115, 383-391. (36) McGilvray, K. L.; Granger, J.; Correia, M.; Banks, J. T.; Scaiano, J. C. Opportunistic use of tetrachloroaurate photolysis in the generation of reductive species for the production of gold nanostructures. Phys. Chem. Chem. Phys. 2011, 13, 11914-11918.


Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Fong, Y.-Y.; Visser, B. R.; Gascooke, J. R.; Cowie, B. C. C.; Thomsen, L.; Metha, G. F.; Buntine, M. A.; Harris, H. H. Photoreduction Kinetics of Sodium Tetrachloroaurate under Synchrotron Soft X-ray Exposure. Langmuir 2011, 27, 8099-8104. (38) Avrami, M. Kinetics of Phase Change. I. General Theory. J. Chem. Phys. 1939, 7, 1103-1112. (39) Avrami, M. Kinetics of Phase Change. II. Transformation-Time Relations for Random Distribution of Nuclei. J. Chem. Phys. 1940, 8, 212-224. (40) Erofe’ev, B. V. A generalized chemical kinetics equation and its application to reactions involving the participation of solid substances. Dokl. Akad. Nauk SSSR 1946, 52, 511-514. (41) Watzky, M. A.; Finke, R. G. Transition Metal Nanocluster Formation Kinetic and Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow, Continuous Nucleation and Fast Autocatalytic Surface Growth. J. Am. Chem. Soc. 1997, 119, 10382-10400. (42) Watzky, M. A.; Finney, E. E.; Finke, R. G. Transition-Metal Nanocluster Size vs Formation Time and the Catalytically Effective Nucleus Number: A Mechanism-Based Treatment. J. Am. Chem. Soc. 2008, 130, 11959-11969. (43) Malone, K.; Weaver, S.; Taylor, D.; Cheng, H.; Sarathy, K. P.; Mills, G. Formation Kinetics of Small Gold Crystallites in Photoresponsive Polymer Gels. J. Phys. Chem. B 2002, 106, 7422-7431. (44) Kwolek, P.; Wojnicki, M. The kinetic study of photoreduction of tetrachloroaurate acid by methanol in acidic media. J. Photochem. Photobiol., A 2014, 286, 47-54. (45) Tiemann, M.; Marlow, F.; Hartikainen, J.; Weiss, Ö.; Lindén, M. Ripening Effects in ZnS Nanoparticle Growth. J. Phys. Chem. C 2008, 112, 1463-1467. (46) Elbjeirami, O.; Omary, M. A. Photochemistry of Neutral Isonitrile Gold(I) Complexes: Modulation of Photoreactivity by Aurophilicity and π-Acceptance Ability. J. Am. Chem. Soc. 2007, 129, 11384-11393. (47) Marin, M. L.; McGilvray, K. L.; Scaiano, J. C. Photochemical Strategies for the Synthesis of Gold Nanoparticles from Au(III) and Au(I) Using Photoinduced Free Radical Generation. J. Am. Chem. Soc. 2008, 130, 16572-16584. (48) Simonsen, S. B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Meinander, K.; Jensen, T. N.; Lauritsen, J. V.; Helveg, S. Effect of Particle Morphology on the Ripening of Supported Pt Nanoparticles. J. Phys. Chem. C 2012, 116, 5646-5653. (49) Simo, A.; Polte, J.; Pfänder, N.; Vainio, U.; Emmerling, F.; Rademann, K. Formation Mechanism of Silver Nanoparticles Stabilized in Glassy Matrices. J. Am. Chem. Soc. 2012, 134, 18824-18833. 29 ACS Paragon Plus Environment


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Table of Contents Graphics and Synopsis

Manuscript title: Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymer Using In Situ Quick XAFS and SAXS Author list: Masafumi Harada* and Syoko Kizaki

Reduction of Au(III) to Au(I)

Aggregative growth

Reduction-nucleation, and autocatalytic surface growth

Aggregative growth process

+ Formation mechanism of Au nanoparticles in an aqueous ethanol solution of polymer by photoreduction was investigated by means of in situ QXAFS and SAXS. The consecutive stages of reduction-nucleation, autocatalytic surface growth on nucleates, aggregative growth and aggregation/precipitation process were found to be involved in Au nanoparticle growth. Ostwald ripening-based growth was not observed after the termination of aggregative growth.


Formation Mechanism of Gold Nanoparticles Synthesized by

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