Quantifying the Nucleation and Growth Kinetics of Microwave

Dec 1, 2015 - The in situ X-ray diffraction technique reported in this work is promising ... Liheng Wu , Adriana Mendoza-Garcia , Qing Li , and Shouhe...
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Letter pubs.acs.org/NanoLett

Quantifying the Nucleation and Growth Kinetics of Microwave Nanochemistry Enabled by in Situ High-Energy X‑ray Scattering Qi Liu,† Min-Rui Gao,† Yuzi Liu,† John S. Okasinski,‡ Yang Ren,‡ and Yugang Sun*,† †

Center for Nanoscale Materials and ‡X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The fast reaction kinetics presented in the microwave synthesis of colloidal silver nanoparticles was quantitatively studied, for the first time, by integrating a microwave reactor with in situ X-ray diffraction at a highenergy synchrotron beamline. Comprehensive data analysis reveals two different types of reaction kinetics corresponding to the nucleation and growth of the Ag nanoparticles. The formation of seeds (nucleation) follows typical first-order reaction kinetics with activation energy of 20.34 kJ/mol, while the growth of seeds (growth) follows typical self-catalytic reaction kinetics. Varying the synthesis conditions indicates that the microwave colloidal chemistry is independent of concentration of surfactant. These discoveries reveal that the microwave synthesis of Ag nanoparticles proceeds with reaction kinetics significantly different from the synthesis present in conventional oil bath heating. The in situ X-ray diffraction technique reported in this work is promising to enable further understanding of crystalline nanomaterials formed through microwave synthesis. KEYWORDS: Microwave nanochemistry, in situ high-energy X-ray diffraction, silver nanoparticles, first-order reaction kinetics, self-catalytic reaction kinetics

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nanocrystal formation in large, statistically relevant solutions. In comparison with conventional X-rays, using high-energy X-rays to probe real-time nanophase evolution of colloidal nanoparticles in solutions is advantageous because of their deep penetration in liquid solutions and weak absorption by the reactants. This reduces the background interference to improve probing sensitivity and eliminates the possibility of X-rayinduced side reactions, respectively.14−20 Herein, we evaluate the in situ X-ray diffraction technique by focusing on the synthesis of colloidal Ag nanoparticles that has been extensively explored with the use of polyol reduction of AgNO3 at elevated temperatures in the presence of polyvinylpyrrolidone (PVP) that serves as capping ligand for stabilizing the resulting Ag nanoparticles.21−29 Quantitative analysis of the in situ X-ray diffraction data reveals that the typical microwave synthesis of colloidal Ag nanoparticles includes four well-separated steps, that is, incubation of the nucleation process, formation of seeds, growth of seeds, and completion of the reaction, which are dominated by different reaction kinetics. For example, the formation of seeds follows typical first-order reaction kinetics, while self-catalytic reaction kinetics dominates the growth of seeds. Synthesis of colloidal Ag nanoparticles has been carried out with microwave-assisted polyol reduction of AgNO3 at different elevated temperatures. The elevated temperature is critical to

icrowave chemistry, the science of activating chemical reactions through microwave irradiation, represents a greener way to synthesize materials because some attributes of microwave-heating including shorter reaction time, lower energy consumption, high-level thermal management (e.g., uniform temperature distribution), and high product yield.1−3 These advantages have enabled microwave heating to be widely used in organic synthesis for petrochemistry, drug discovery, etc., while applying microwave heating to the synthesis of nanomaterials (so-called “microwave nanochemistry”) only emerged in the past decade.4−7 In principle, microwave nanochemistry is suitable for synthesizing all of the colloidal nanoparticles available with conventional heating methods, although only a limited number of nanomaterials have been demonstrated. In addition, because of the unique characteristics of microwave heating, microwave nanochemistry is also capable of synthesizing nanoparticles that are difficult (or unrealized) with other methods.8−11 As a result, microwave nanochemistry represents a promising strategy for synthesizing nanomaterials with tailored properties. However, the fast reaction kinetics present in microwave heating, and the design of microwave reactors makes in situ studying the real-time evolution of colloidal nanoparticles very difficult. This challenge significantly hinders our understanding of the reaction kinetics as well as the precise control over properties of the synthesized nanoparticles.12,13 In this Letter, we report, for the first time, the integration of a microwave reactor with a high-energy X-ray synchrotron beamline that enables one to capture the rapid kinetics of © 2015 American Chemical Society

Received: November 6, 2015 Revised: November 24, 2015 Published: December 1, 2015 715

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Figure 1. Time-resolved HEXRD patterns recorded in the course of microwave synthesis of Ag nanoparticles at 140 °C. (A) Scheme illustration of the in situ HEXRD experiment setup. (B) Contour plot of the HEXRD patterns at different reaction times. The green arrows highlight the time when the temperature ramp ended and the temperature reached 140 °C. The standard XRD pattern of Ag (ICDD PDF 04−001−3180) was plotted as a reference (top sticks). (C) HEXRD patterns at a number of key times. The wavelength of the high-energy X-ray was 0.1771 Å.

(220) reflections of Ag nanoparticles, and no crystalline impurities are detected. The intensity of the diffraction peaks quickly increases with the reaction time, while the positions of diffraction peaks remain unchanged, indicating that the reduction of AgNO3 is the only reaction to continuously grow the Ag nanoparticles. Since the peak area in HEXRD pattern is proportional to the mass of the corresponding crystalline materials with a pure crystalline phase, the variation of peak area as a function of reaction time can be used to track the reaction kinetics of nucleation and growth involved in the microwave reaction. The quantitative analysis method is described with details in the Supporting Information (Figures S1−S3). Figure 2, panel A presents the peak area of the Ag(111) reflections at different reaction times, showing a time-dependent trajectory that can be well fitted with a sigmoid function, except at the early reaction stage of t ≤ 15 min. The sigmoidal profile indicates that the growth of Ag nanoparticles at t > 15 s follows a typical selfcatalytic reaction mechanism.18,37 The deviation, that is, the lower value of peak area than the fitted sigmoid function (y = 11.757 + (−11.757/(1 + e(x−22.2204)/2.84443))) at t < 16 s, indicates that the condensation of Ag atoms into Ag nanocrystallites experiences a different mechanism from the self-catalytic reaction process. Clearly, the synthesis involves four well-defined stages (i.e., induction period of nucleation, formation of seeds, growth of seeds, and completion of synthesis) with specific characteristics according to the timedependent trajectory shown in Figure 2, panel A. In region I of nucleation induction period (i.e., t = 0−10 min), no apparent diffraction peaks of Ag are observed even after the solution

enhance the reducing ability of ethylene glycol and thus the kinetics of forming Ag nanoparticles. Compared with conventional heating (e.g., oil bath or heating mantle), microwave heating not only remarkably improves the energy delivery efficiency to reduce reaction time, but also significantly changes reaction kinetics and selectivity, often in favorable ways. The accelerated reaction kinetics in combination with the enclosed geometry of microwave cavity (so-called “black box”) leads to the extreme difficulty to in situ monitor the complex evolution process of colloidal nanoparticles. Consequently, though microwave synthesis of Ag nanoparticles or other nanoparticles has already been reported,11,30−36 the reaction kinetics remained poorly understood due to the lack of real-time experimental data and a related theory. Therefore, we developed in situ high-energy X-ray diffraction (HEXRD) to probe the reaction kinetics involved in the synthesis of Ag nanocrystals through microwave chemistry. Figure 1, panel A illustrates the in situ synchrotron HEXRD experiment setup, and Figure 1, panel B presents the 2D contour plot of the time-resolved HEXRD patterns recorded from the microwave synthesis of Ag nanoparticles at 140 °C. No diffraction signals from crystalline Ag are observed even after the solution temperature reaches 140 °C at 6 min. A clear signal starts to appear at t = 11 min, that is, 5 min after the temperature is stable at 140 °C, indicating the nucleation process for the formation of stable seeds is relatively slow and takes time even at high temperature. This change is clearly illustrated with the HEXRD pattern shown in Figure 1, panel C, which highlights HEXRD patterns at a number of key reaction times. The observed peaks are ascribed to the (111), (200), and 716

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Figure 2. Analysis of peak area and peak width of the HEXRD patterns shown in Figure 1, panel B. (A, C) Variation of peak area of (A) Ag(111) and (C) Ag(200) reflections in the course of microwave reaction. The red curves represent the best fitting with sigmoidal functions. Inserts represent the linear fitting of ln([Ag+]/[Ag+]0) as a function of reaction time at early stage when crystalline Ag started to form. [Ag+]0 and [Ag+] represent the concentration of Ag+ in the reaction solution before and after reaction started, respectively. (B) Dependence of the lateral crystalline dimension of Ag nanocrystals along the crystalline directions on the reaction time. (D) The ratio of Ag(111) peak area to Ag(200) peak area as a function of time. The dotted horizontal line highlights the ratio of crystalline Ag with thermodynamically stable morphologies. The dotted vertical lines in panels A−C highlight the typical reaction stages (I, II, III, and IV) involved in the microwave nanochemistry.

temperature reaches 140 °C, indicating that forming Ag nuclei from homogeneous reaction solution takes time until the supersaturated concentration of Ag atoms in the reaction solution is high enough to drive spontaneous condensation of Ag atoms. If the Ag nuclei are too small in size, their crystalline structures fluctuate at elevated temperatures,38 leading to a difficulty to show well-defined HEXRD patterns. Continuous reaction can enlarge the Ag nuclei to reach the critical size, with which the crystalline structures are fixed to show clear signals in HEXRD patterns. For instance, diffraction signals start to be observed at 11 min, indicating that some Ag nuclei become large enough to prevent structural fluctuation. Such structurally stable nuclei are also called seeds that are formed in region II (i.e., t = 10−15 min). The concentration of free Ag+ ([Ag+]t, ionized from Ag NO3) in the reaction solution at different times can be calculated by subtracting the reduced Ag+ from the initial concentration (i.e., [Ag+]0 = 0.1 M) of AgNO3. In the period of 10−15 min, ln([Ag+]t/[Ag+]0) is linearly dependent on the reaction time (inset, Figure 2A), indicating that the formation of stable Ag seeds from reduction of Ag+ ions follows a typical first-order reaction with a rate constant of k = 1.33 × 10−4 s−1 (see Supporting Information for details). The linear relationship no longer exists at t > 16 min, indicating the occurrence of overgrowing the Ag seeds into larger nanoparticles with accelerated rate in region III (i.e., t = 16−36 min) as highlighted by the sigmoidal fitting (Supporting Information, Table S1). The acceleration in growth rate continues until t = 21 min, and then the growth rate monotonically decreases to zero at t = 36 min due to the consumption of precursor AgNO3 (Figure S4). In addition to peak area, the full-width-at-half-maximum (fwhm) of the XRD peaks can also be extracted. By factoring in

the contribution of the instrumental broadening, the corrected fwhm can be used to calculate the lateral dimension of nanocrystals according to Sherrer equation.16,18 Figure 2, panel B shows the average crystalline domain size in the as-grown Ag nanoparticles along the (111) direction as a function of reaction time. The stable Ag seeds observed at the earliest time (i.e., t = 11 min) exhibit an average crystalline domain size of 9.9 nm. In region II, the average crystalline size of the Ag seeds slightly increases to 15.4 nm, which represents approximate 2.8-times of volumetric increase of an individual nanoparticle in comparison with those formed at 11 min by assuming the nanoparticles are spherical. If the number of seeds in the reaction solution is constant, the overall mass of all Ag nanocrystals (proportional to peak area) should also increase by 2.8-times from 11 to 15 min. In fact, the measured peak area increases by 5.8-times in this period (Figure 2A), indicating that more stable seeds are continuously formed along with a slight increase in size. Once the size of the Ag seeds reaches a critical value (i.e., ∼15 nm), they can self-catalyze their growth with an acceleration of reduction of AgNO3. In region III (i.e., 15 min < t < 35 min) of growing Ag seeds, the average crystalline size in Ag nanoparticles quickly increases from 15.4 to 103.6 nm, which corresponds to 303-times of volumetric change of individual particles. According to the measured peak area presented in Figure 2, panel A, the overall volume of all Ag particles formed in the reaction solution increases by only 5.8times. Such significant difference indicates that the Ag nanoparticles coalescence via possible oriented attachment and fusion to form larger nanoparticles during the fast growth process. Finally, in region IV (t > 35 min), both the peak area and crystalline size exhibit plateaus, indicating the completion of chemical reaction and nanoparticle growth. 717

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Nano Letters This four-stage process (i.e., induction period, formation of seeds, growth of seeds, completion of synthesis) is also consistent with the analysis of the (200) reflection peaks (Figure 2C and Figure S5). For instance, the time-dependent evolution of (200) peak area can also be well fitted with a sigmoid function except the data points at t ≤ 15 min (Figure 2C). With the concentration of Ag+ calculated from the (200) peak area, the reaction kinetics at t < 15 min also follows a firstorder reaction with a rate constant of 1.12 × 10−4 s−1, which is different from that determined from the (111) peak area. The difference in growth kinetics along different crystalline orientation implies the occurrence of kinetic anisotropic growth involved in the microwave synthesis of Ag nanoparticles. As shown in Figure 2, panel D, the ratio of peak area between the Ag(111) and the Ag(200) peaks varies as a function of time. The ratio first decreases from 2.63 to 1.8, and the change turns over at 18 min with a steady increase to reach the equilibrium value of 2.2 at 27 min. During the early stage of forming seeds (i.e., t ≤ 15 min), the (111)/(200) ratios are larger than the equilibrium value, indicating the surfaces of the Ag seeds are preferentially terminated with {111} facets (Figure S6). On the other hand, the continuous decrease of the (111)/(200) ratio suggests that the growth of the Ag seeds increases the percentage of the {100} facets in the nanoparticle surfaces by preferred anisotropic growth of the Ag nanoparticles along the directions. This trend stops at t = 18 min, and the anisotropic growth is then dominated by faster growing the Ag nanoparticles along the directions to enlarge the percentage of the {111} facets in the nanoparticle surfaces. Such anisotropic growth diminishes at ∼27 min, followed by an isotropic growth to further enlarge the Ag nanoparticles with both {100} and {111} surfaces (Figure S7). The Ag nanoparticles synthesized at 140 °C exhibit rough surfaces and high-density intraparticle grain boundaries (Figure S7), which are consistent with the possible orientated attachmentbased coalescence responsible for the nanoparticle growth in region III. Increasing temperature can enhance the reducing ability of ethylene glycol and thus promote the reduction of Ag+ ions as well as the growth of Ag nanoparticles. The influence of reaction temperature on the microwave-assisted polyol synthesis of Ag nanoparticles has been evaluated by probing the reactions at different temperatures (e.g., 130 °C, 140 °C, 160 °C, and 180 °C) with the in situ HEXRD technique. Figure 3, panel A compares peak area evolution of the Ag(111) peak as a function of reaction time recorded for different reaction temperatures (while the peak area evolution of Ag(200) shows the same trend, Figure S8). The peak area always reaches to a plateau with the same value regardless of the temperatures (e.g., 140 °C, 160 °C, or 180 °C), indicating that the reduction of AgNO3 can be completed at high enough temperatures, although the reaction time increases at lower temperatures. If the temperature is too low, such as 130 °C, 40 min is not long enough to complete the reduction of AgNO3 (blue triangles, Figure 3A). Similar to the synthesis at 140 °C shown in Figure 2, panel A, the trajectory of peak area recorded at higher temperatures can also be well fitted with sigmoidal functions corresponding to self-catalyzed reactions (Table S1). The apparent deviation from the sigmoidal curves at the early reaction stage implies different kinetic mechanism. Quantitative analysis of the early stage reaction rate reveals that the reduction of Ag+ ions always follows the typical first-order reactions with rate constants of 3.21 × 10−5 s−1 at 130 °C, 1.33

Figure 3. Comparison of microwave syntheses of Ag nanoparticles at different temperatures. (A) Peak area evolution of the Ag (111) peak as a function of the reaction time (symbols) and the corresponding sigmoidal fittings (curves). (B) Linear fittings of ln([Ag+]/[Ag+]0) as a function of reaction time at different temperatures, highlighting the first-order reaction kinetics at the early reaction stage. The reaction rate constant (k) increases with reaction temperature. Inset shows ln(k) as a function of 1/T (T: absolute temperature scale with unit of Kelvin), in which the slope of linear regression line can be used to calculate the corresponding reaction activation energy (Ea) using the Arrhenius equation. (C) Plots of the lateral crystalline dimensions of the Ag nanoparticles along the (111) directions as a function of reaction time.

× 10−4 s−1 at 140 °C, and 1.30 × 10−3 s−1 at 160 °C, respectively (Figure 3B). The reaction is too fast at 180 °C, which prevents us from recording the first-order reaction kinetics by using the current experimental setup with temporal resolution of one XRD pattern/min. The dependence of reaction rate constant, k, on the reaction temperature, T, can be described by Arrhenius equation:

k = Ae−Ea / RT

(1,)

where R, A, and Ea represent the universal gas constant (8.314 J mol−1 K−1), the frequency factor, and the activation energy, respectively.39 The inset of Figure 3, panel B shows the linear relationship between ln(k) and 1/T with a slope that is used to 718

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55 000, Sigma-Aldrich) was used as a capping reagent to stabilize the synthesized Ag nanoparticles. In a typical synthesis, 3 mL of ethylene glycol solution of 0.3 M PVP (calculated based on the weight of repeating unit) was mixed with 3 mL of ethylene glycol solution of 0.2 M AgNO3 in a microwave reaction vessel, which was then placed in the microwave reactor. The temperature of reaction solution was increased from room temperature to 140 °C within 6 min followed by maintaining the temperature at 140 °C for 34 min. Once the microwave heating was turned off, the reaction solution was cooled down to room temperature with flow of pressurized air on the reaction vessel. The color of the reaction solution changed from colorless to pale yellow after microwave reaction, indicating the formation of Ag nanoparticles. Reaction temperature set point, concentration of AgNO3, and concentration of PVP were varied to evaluate their influence on the reaction kinetics and the resulting Ag nanoparticles. The reaction conditions compared in this work are highlighted in Table S3. The synthesized nanoparticles were characterized with a transmission electron microscope (JEOL 2100F) operated at an accelerating voltage of 200 kV. To probe in situ the reaction kinetics, the microwave reactor was modified to be compatible with the high-energy 1 ID-E beamline at the Advanced Photon Source (APS), Argonne National Laboratory. The schematic illustration of the experiment setup is shown in Figure 1, panel A. The high-flux, highenergy, synchrotron X-ray beam had a photon energy of 70 keV (wavelength of 0.1771 Å), which exhibited strong penetration power through large-volume reaction solutions to ensure the sensitive detection of low-concentration Ag nanocrystals. In a typical in situ synthesis, once the microwave heating started (normalized as time of 0), the beam shutter was opened to allow the incident X-ray beam (with size of 0.3 mm × 0.3 mm) to probe the evolution of Ag nanocrystals formed in the reaction solution. The scattered signals were simultaneously recorded using a GE 41RT area detector at a frequency of one pattern/min. The obtained 2D diffraction patterns were calibrated with a standard LaB6 sample and reduced using Fit2D software. The background was determined by measuring the scattering signals from the precursor reaction solution (before microwave heating) in the reaction vessel and was subtracted from all the X-ray diffraction patterns obtained during the reaction. For the quantitative analysis, the two most intensive Ag (111) and Ag(200) peaks were fit with a Gaussian function, and the peak areas were used to track the nucleation and growth kinetics involved in the formation of Ag nanoparticles (Figure S1). Fwhm values of these peaks were analyzed, and the Scherrer equation was employed to calculate the lateral crystalline dimensions of the Ag nanoparticle. The detailed procedure can be found in the Supporting Information (Figures S2 and S3). Rietveld refinements were also performed for the recorded XRD patterns for comparison, and the results of peak areas and fwhm values were same as those retrieved from Gaussian fitting with the Origin software (Figure S1).

determine the activation energy as 20.34 kJ/mol. This represents the energy barrier to overcome to drive the reduction of Ag+ and condense the resulting Ag atoms into stable Ag seeds. The difference of reaction rate at different temperatures in region II leads to the formation of stable Ag seeds with different sizes. As shown in Figure 3, panel C, the maximum average crystalline size of the Ag seeds increases as reaction temperature increases (for example, 15.4 nm for 140 °C and 26.0 nm for 160 °C). Similarly the crystalline size of final product particles formed at higher temperatures is always larger than those formed at lower temperatures (e.g., 103, 116, and 142 nm for 140 °C, 160 °C, and 180 °C, respectively) (Figure S9A−C). Because of strong surface plasmon resonances in Ag nanoparticles, these as-synthesized Ag nanoparticles exhibit strong optical absorption in the UV− visible spectral region (Figure S9D). The absorption peak redshifts as particle size increases, which is consistent with previous observations. Overall, for the microwave-assisted synthesis of Ag nanoparticles, while increasing the synthesis temperature can significantly speed up the reaction, the formation of Ag nanoparticles still involves four welldistinguished steps, that is, induction of nucleation process, formation of seeds, growth of seeds, and the completion of synthesis. The reaction kinetics corresponding to the formation of Ag seeds and growth of seeds follows the typical first-order reaction mechanism and self-catalytic mechanism, respectively. Concentrations of the precursor AgNO3 and the surfactant PVP are also important parameters that can significantly influence reaction kinetics and the resulting Ag nanoparticles when the synthesis is driven with conventional oil bath heating.40−45 In contrast, increasing the concentration of AgNO3 in the microwave synthesis only slightly slows the reaction kinetics and increases the crystalline size of the Ag nanoparticles (Table S2 and Figure S10). The syntheses using different concentrations of PVP follow the same reaction kinetics and produce the same Ag nanoparticles when other reaction conditions are the same (Figure S11), indicating that the interactions between PVP and Ag nanoparticles may be very weak under microwave heating. The results clearly demonstrate the significant difference in the synthesis of colloidal nanoparticles using different heating methods. In summary, we have developed an in situ technique using high-energy, synchrotron X-rays as a probe, for the first time, to monitor reaction kinetics of microwave synthesis of colloidal Ag nanoparticles. The X-ray diffraction patterns provide quantitative information on nucleation and growth kinetics, revealing that seed formation (nucleation) and seed growth (growth) follow a typical first-order reaction and self-catalytic reaction, respectively. Different from the synthesis driven by conventional heating (e.g., oil bath), only temperature plays a profound influence on the microwave synthesis of Ag nanoparticles while the concentration of the precursor AgNO3 and the surfactant PVP exhibit minor effect. Such unprecedented in situ reaction kinetics of microwave nanochemistry promises to help us understand the complex nucleation and growth mechanisms involved in the formation of colloidal Ag nanoparticles and thus enabling better design and synthesis of nanoparticles with improved quality and properties. Experimental Section. Silver nanoparticles were synthesized through reduction of silver nitrate (AgNO3, SigmaAldrich) in ethylene glycol (Sigma-Aldrich) with the use of a Monowave 300 microwave reactor (Anton Paar). PVP (Mw ≈

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04541. Peak fitting of HEXRD patterns; correction of peak broadening; TEM and HRTEM images of Ag nano719

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particles formed at different times; peak area of Ag(200) reflections as a function of reaction time at different temperatures; TEM images of Ag nanoparticles formed at different temperatures; peak area of Ag(111) reflections as a function of reaction time at different precursor concentrations, reaction conditions, and sigmoidal fitting parameters (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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