Gold Nanoparticle Internal Structure and Symmetry Probed by Unified

Letter pubs.acs.org/NanoLett

Gold Nanoparticle Internal Structure and Symmetry Probed by Unified Small-Angle X‑ray Scattering and X‑ray Diffraction Coupled with Molecular Dynamics Analysis Blaise Fleury,*,† Robinson Cortes-Huerto,† Olivier Taché,† Fabienne Testard,† Nicolas Menguy,‡ and Olivier Spalla† †

CEA Saclay, DSM/IRAMIS/NIMBE/LIONS, UMR CEA/CNRS 3685, 91191 Gif-sur-Yvette CEDEX, France Université Pierre et Marie Curie, IMPMC, UMR 7590 CNRS, Campus Jussieu, 75252 Paris CEDEX 05, France

‡

S Supporting Information *

ABSTRACT: Shape and size are known to determine a nanoparticle’s properties. Hardly ever studied in synthesis, the internal crystal structure (i.e., particle defects, crystallinity, and symmetry) is just as critical as shape and size since it directly impacts catalytic efficiency, plasmon resonance, and orients anisotropic growth of metallic nanoparticles. Hence, its control cannot be ignored any longer in today’s research and applications in nanotechnology. This study implemented an unprecedented reliable measurement combining these three structural aspects. The unified small-angle X-ray scattering and diffraction measurement (SAXS/XRD) was coupled with molecular dynamics to allow simultaneous determination of nanoparticles’ shape, size, and crystallinity at the atomic scale. Symmetry distribution (icosahedraIh, decahedraDh, and truncated octahedraTOh) of 2−6 nm colloidal gold nanoparticles synthesized in organic solvents was quantified. Nanoparticle number density showed the predominance of Ih, followed by Dh, and little, if any, TOh. This result contradicts some theoretical predictions and highlights the strong effect of the synthesis environment on structure stability. We foresee that this unified SAXS/XRD analysis, yielding both statistical and quantitative counts of nanoparticles’ symmetry distribution, will provide new insights into nanoparticle formation, growth, and assembly. KEYWORDS: Gold nanoparticle, nanoparticle defects, nanoparticle morphology, crystal structure, symmetry, SAXS, XRD, PDF, molecular dynamics

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However, when investigating a nanocrystal’s symmetry and crystallinity, high-resolution TEM (HR-TEM) or scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) is more efficient than scattering techniques.17,18 Moreover, aberration-corrected TEM or STEM offers new capabilities with unprecedented spatial resolution.19−21 Still, the need of sample tilting to identify unambiguously each particle requires a huge amount of time for a statistically representative analysis (at least several hundreds of particles). It thus prevents the use of microscopy to analyze the particles’ crystallinity and symmetry routinely.21 On the contrary, scattering is a technique of choice as it analyzes on average billions of particles at the same time. However, this improvement in statistics is at the expense of a more elaborate analysis. Usually, a Rietveld approach is applied to nanomaterials for data treatment in reciprocal space.22,23 Recently, analysis in direct space was reconsidered due to high synchrotron

he late twentieth century was a propitious era for the quest of chemical reactions yielding new nanoparticles (e.g., oxides, semiconductors, metals).1 Focus then was set on mastering the particles’ shapes and sizes. Decades of intense research later, this is almost achieved for most any kind of materials, but the control over the internal crystal structure of nanoparticles remains a difficult task.2−4 The issue lies in the fact that structural defects in single nanoparticles or particle symmetry strongly affect their properties4,5 (e.g., for catalysis and plasmonics)6−9 and their ability to self-assemble into supracrystals.10,11 By taking advantage of this structure− property relation in self-assembly, several nanoparticle-based electronic devices, such as transistors, inverters, light-emitting diodes, and solar cells, were developed.12−15 Nevertheless, the control over the nanoparticles’ internal structure during their preparation remains a major challenge and requires, as a scientific keystone, a statistically representative characterization of their crystallinity.16 The two actual standards for nanoparticle analysis, transmission electronic microscopy (TEM) and X-ray scattering, are both sensitive to electronic density and particle shape. © XXXX American Chemical Society

Received: June 12, 2015 Revised: August 7, 2015

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organic solutions of (i) chloro(triphenylphosphine)gold(I) and dodecanethiol (1:2 molar ratio) and (ii) borane tert-butylamine complex (10 equiv) were mixed at 100 °C and stirred for 1 h. Each solution was then washed by centrifugation and redispersed in toluene. Depending on the dielectric properties of the organic solvent used during the synthesis, different particle sizes could be obtained (see Methods). The four samples (Au22, Au33, Au54, and Au63) were imaged by TEM and characterized by X-ray scattering (Figure 1 and Supporting Information 1).

brilliance, and the use of pair distribution function (PDF) analysis for nanoparticles dramatically increased, including liquid samples. 24,25 Yet, care is necessary as limited experimental q-range and q-resolution may introduce biases toward either shape or internal structure. In addition, all these studies are qualitative, as the concentration of particles analyzed is not taken into account. The identification of crystallinity and symmetry distributions is of prime importance when dealing with metallic nanoparticles, as it could have a strong impact on the understanding of (i) some of their intrinsic or collective properties in areas such as catalysis, magnetism, or plasmonics;8,26−28 (ii) shape transition and symmetry breaking in seeded-growth synthesis (nanorods, bipyramids, etc.);26,29−31 and (iii) facet stabilization due to environmental effects in wet synthesis (solvent, ligand, etc.).32 Gold nanoparticles were chosen for the present studies since they constitute a corpus of extensively studied model nanoparticles. Gold nanoparticles have several stable internal structures. The three main ones are icosahedron (Ih), decahedron (Dh), and truncated octahedron (TOh). For the smallest particles ( 1 Å−1 region, whereas the particle size shows up at q ≈ 0.1 Å−1.

The comprehensive structural characterization of nanoparticles required three effective orders of magnitude in qrange to provide (i) the shape and size and (ii) the internal structure. This was usually done by performing two separate experiments disconnected in q-range with different normalization constants.16 The originality of the present work was the use of an additional setup to access the intermediate q-range to connect XRD to SAXS and normalize scattering intensities (see Methods and Supporting Information 2). Complete characterization of nanoparticles became possible with the unification of SAXS and XRD on a single diagram in absolute scattering units. Both TEM and scattering techniques revealed the close-tospherical shape and sharp size dispersion. Fitting only the SAXS part of the data using a Gaussian polydisperse sphere model (Figure 1c and Supporting Information 1) yielded the following sizes: ∼21.8 ± 6 Å, ∼33.0 ± 5.2 Å, 54.4 ± 5.6 Å, and ∼63.0 ± 6.4 Å in radius for the Au22, Au33, Au54, and Au63 samples, respectively (in good agreement with the TEM size B

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account with a Rietveld-like approach or a model of a fcc arrangement including microstrains and stacking faults.44 Still, these approaches could not discriminate between a population of large particles with small crystal domains and a larger population of smaller single crystalline particles (See Figures S6 and S7). In the case reported here, SAXS, being sensitive to the shape and size, was sufficient to eliminate the wrong solutions. Performing the analysis of both the outer dimension and the internal structure, on the same sample, implied that scattering by polycrystalline particles could not be misinterpreted and fitted by smaller but crystalline particles. We propose a new method based on the analysis of scaled scattered intensity in the whole q-range to proceed quantitatively (i.e., getting an absolute number of particles in solution). The principle is to fit the experimental data with the scattering intensity obtained by a combination of stable particles obtained from molecular dynamics. The strength of the method comes from the use of a set of nanoparticles going beyond simple fcc nanoparticles and matching HRTEM particle identification (Figure 2 and Supporting Information 4). This unified analysis requires the scaling of the measured intensities16 (see Methods, Supporting Information 2 and 3), which could be obtained through the link between the two qrange domains. From a computational perspective, gold nanoparticles were obtained by using the motif optimization approach (see Supporting Information 5).42 Then, the positions of atoms were employed to calculate the unified X-ray scattering diagrams using the Debye equation (i.e., the SAXS and the XRD parts simultaneously). Calculated X-ray scattering diagrams from ∼6500-atom stable structures (i.e., TOh, Dh, and Ih) are compared in Figure 2, panel c. After normalization at q = 0 Å−1, the diagrams only differ in the XRD region (q > 1 Å−1), while no clear differences are seen in the small-angle region, only sensitive to the particle overall shapes. Going from TOh to Dh and finally Ih, the coherence length decreases (i.e., yielding wider and less intense Bragg peaks), while again, average external sizes of particles are similar. The calculated unified scattering diagram being different for these three nanoparticles, making a comparison with the experimental diagram was thus a powerful method to extract their contributions. As a matter of fact, none of the experimental diagrams could be reproduced by a unique size and structure, and in each of the cases, a linear combination of at least two of the structures was mandatory to obtain a satisfactory fit (Figure 3). This very successful fit linking SAXS and XRD obtained for the first time was already an achievement. The only part that slightly deviated from the experiment, concerning the amplitude of the formfactor oscillations (q ≈ 0.15−0.8 Å−1), could be attributed to facets and size dispersion. After fitting, it appeared that Au54 sample was made of ∼26% Dh, 74% Ih, and 0% TOh. In addition, through scaling in absolute scattering units, the SAXS/XRD global fitting had also the strong advantage to produce the concentration for each type of particles as reported in Table 1. This quantitative analysis has been successfully applied to the three other samples (Table 1 and Figures S11− S14). Similar results were obtained with a mixture of icosahedral and decahedral structures, Ih particles being obtained in majority. An extended HRTEM analysis on Au63 sample validated these results, as 96.5% of particles were found to be flawed (Supporting Information 4). Few monocrystalline nanoparticles

distributions, Supporting Information 1). Focusing on the Au54 sample, the X-ray scattering diagram presented a crossover regime around q = 6.0 × 10−2 Å−1 followed by four form-factor oscillations, which emphasized a narrow size dispersion (Figure 1c). Moreover, the presence of dots on the 2D SAXS image (light blue dots in Figure 1b), leading to a peak at q = 8.4 × 10−2 Å−1 on the corresponding diagram (Figure 1c), highlights these particles’ trend to self-assemble into supracrystals in solution. However, this contribution was minor for these nanoparticles and could be discarded in this model.37 Beyond 1 Å−1, Bragg peaks of the gold structure resolved up to the twelfth reflection (531) (q = 9.1 Å−1) are the signature of the internal structure of the nanocrystals. For Au54 (Figure 1c), the crystal coherence length obtained from the first Bragg peak was no longer than 2π/Δq = 30.0 Å (with Δq the fwhm of the (111) reflection), while the typical size obtained from SAXS was larger (∼54.4 Å). Similar results were obtained for the other samples (Au22, Au33, and Au63) for which the coherence length, obtained from XRD, was always smaller than the SAXS size. However, as the nanoparticle size reduced, crystal coherence length became almost equal to the external size (19 Å for Au22). This comparison between sizes obtained by fitting SAXS data and XRD crystal coherence length highlighted that nanoparticles could not be single crystals. Bragg peaks’ width could not be directly related to the nanoparticle size.38 To strengthen the argument, we have shown in Supporting Information 3 the calculated scattering diagram of monocrystalline nanoparticles whose external sizes were a match for SAXS size. As expected, it clearly fit the SAXS (external size) but did not fit the XRD part (internal structure) as the Bragg peaks were both too sharp and too intense. It led us to this qualitative conclusion: the majority of the particles are polycrystalline. HRTEM images supported this hypothesis, but they showed a mixture of monocrystalline, Ih, Dh, and undetermined particles (Figure 2 and Supporting Information 4).39 Thus, the option of fitting by a particle made of a unique crystallite could not be valid.38,40 Earlier studies40−42 based on powder XRD concluded that a single structure was not enough to reproduce the experimental patterns of nanoparticles.43 For gold, a combination of three geometrical structures (TOh, Ih, and Dh) had to be taken into

Figure 2. Left panel: HRTEM of gold nanoparticles found in sample Au54 after its deposition on a carbon grid, (a) Monocrystalline nanoparticle, (b) icosahedral nanoparticle. (c) SAXS/XRD diagrams calculated for TOh (black line), Dh (red line), and Ih (blue line) relaxed particles (MD). Inset: zoom of the XRD part and schematic structure of TOh, Dh, and Ih from JMol software (Jmol: an opensource Java viewer for chemical structures in 3D. http://www.jmol. org/). C

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Figure 4. (a) Experimental atomic PDF for Au54 sample (black crosses) and best fit (red line) obtained from the combination of (b) small and (c) large spheres, respectively. For short distances, crystal coherence is kept for both particles (short green arrows), whereas it is only kept for larger ones at higher r values (long green arrows). The dashed vertical line on the graph marks out roughly the crossover regime.

Figure 3. Experimental and calculated X-ray scattering diagrams for Au54. Best fit is obtained with a linear combination (cf Table 1) of calculated scattered intensities from atomic position of 74% Ih, 26% Dh, and 0% TOh structures, respectively, resulting from quenched molecular dynamics. Inset: zoom of the XRD part and schematic structure of Ih, Dh, and TOh drawn with Jmol.

slow or too fast. This fact is generally a signature of boundaries in the material as the crystal coherence is kept inside the boundaries and quickly lost beyond.47 To mimic the coherence length of the different crystals in the particles, the idea was to fit a population of spheres of fcc crystals to the data. For the three samples Au33, Au54, and Au63, we were able to fit with populations of small and large spheres as pictured in Figure 4, panels b and c. Without any assumption on the defects inside the nanoparticles, a main conclusion emerges: the samples contain different kind of particles. We concluded that twothirds of nanoparticles were made of small crystals (∼15 Å in diameter for all three samples), and one-third contained larger crystals (33, 52, and 63 Å, respectively) (Figures S15−S18). The PDF conclusion on the existence of these two populations could be compared to the unified method results based on predetermined structures. Although nothing suggests whether the small crystals obtained by PDF were grouped in polycrystalline nanoparticle, they could be linked to Dh, while the larger crystallites could be assigned to the Ih contribution. The obtained number ratio small/large was 2:1 in the PDF method, while the Dh/Ih in the unified method was 1:3, which may appear different at first, but since there are five small crystals in one Dh, the ratio becomes 5:3, which is closer to 2:1. The obtained results were in agreement for the three larger nanoparticles, whereas no mixture of small and large crystals could fit to the smaller particles’ PDF data (Au22). A reasonable fit has been done using one kind of particle with an increased atomic mobility (see Figure S15) showing that when particle size decreases, real crystallinity may become irrelevant. To compare the two approaches, the first strategy resulted in a more accurate quantitative analysis taking benefit of the possibility of well-defined structure calculation and the large

are visible (3.5%) in coexistence with 3.5% Dh, 7% Ih, and 86% unidentified particles exhibiting multiple planar defects. The unambiguous structure determination of a nanoparticle requires an image on which the particle is oriented along a specific zone axis. Although possible, this time-consuming task is not straightforward for few nanometer-sized particles and turns out to be unworkable for nanoparticles’ assemblies containing multiple structure types. Consequently, the TEM techniques are useful to identify different structures but may not be suitable for an analysis requiring a statistical significance. For example, the statistical error induced by underrepresentation of the sample (only 84 particles) leads to σ = 11%. Regarding the analysis of scattering data, a second strategy exists. It considers the fitting of the atomic PDF calculated from the experimental scattering data. Recently, it has been shown that PDF is a powerful technique to determine the 3D structure of nanocrystalline materials.24 This method was applied on the present data with a different model, which was not based on the knowledge of the particles’ structure. It consisted of calculating the PDF data by sine Fourier transforming the experimental XRD diagrams (q > 1 Å−1) with PDFGetX245 (see Figure 4a and Supporting Information 7). On the experimental PDF curves, two domains were visible. The first one, corresponding to short-range order, with strong marked peaks followed by the long-range order region with smoother peaks indicated that the crystal coherence quickly decreased with r but still persisted at higher r values (beyond vertical dashed line on Figure 4a). The data could be fitted using PDFGui46 assuming a spherical shape for the nanoparticles with a distorted fcc structure. Again, as in the preceding method, the fit could not be achieved by using only one kind of crystal size. The attenuation of peak heights on the PDF data with distance was either too

Table 1. Number Fraction of Each Structure (Ih, Dh, and TOh), Number of Atoms in the Model Particle, and Absolute Number of Particles per mL Found with the SAXS/XRD Fitting Procedure sample Au22 Au33 Au54 Au63

Ih 49% 64% 74% 50%

(309 atoms) 7.6 × 1015 part/mL (1415 atoms) 9.1 × 1015 part/mL (5083 atoms) 4.2 × 1015 part/mL (8217 atoms) 3.0 × 1014 part/mL

Dh 43% 36% 26% 50%

(579 atoms) 6.7 × 1015 part/mL (1377atoms) 5.1 × 1015 part/mL (5933atoms) 1.5 × 1015 part/mL (8595 atoms) 3.0 × 1014 part/mL D

TOh 8% 0% 0% 0%

(1099 (1331 (5908 (8578

atoms) 1.2 × 1015 part/mL atoms) atoms) atoms) DOI: 10.1021/acs.nanolett.5b02924 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters accessible q-range for the fit. In the PDF analysis, polycrystallinity was not a direct result of the fit, and additional characterization was needed to provide a good estimation of the relative quantity of each kind of particles. However, since the experimental data are the same, a PDF analysis from the SAXS/ XRD unified diagram must give the same conclusions.48 In the literature,33,35,42 theory predicted icosahedra as the most stable structures in vacuum below 5 nm. Then, decahedron and fcc structures become more stable for larger particles. Hence, we can assume that the relative proportion of Dh and TOh compared to Ih will increase with particle size. Here, this trend is not that clear since the present particles were synthesized in organic solutions where surface energy or energy barrier could be affected by solvents or ligands.32 Still, the absence of TOh particles for those typical sizes agrees with theory. This could be extended with data from Goubet et al.37 where evidence was found of numerous monocrystalline particles of larger sizes using the same synthesis. Thus, the relative stability evolution with size from Ih to Dh and then TOh is verified for the larger nanoparticles. Concerning Au22 and Au33, their smaller size is due to higher reductant strength during synthesis (i.e., a faster reaction). Accordingly, kinetics played a major role compared to thermodynamics, and out-ofequilibrium particles were obtained. We infer that the impact of the environment on the particle mixture, especially the ratio between the different structures, was stronger for the smaller particles. Furthermore, such particles may keep their internal structure during growth because energy barriers between structures are too high. For example, in seeded-growth synthesis of anisotropic metallic nanoparticles, different seeds evolve in radically different objects (bipyramids, rods) after being injected into the same growth solution.49−51 Seeking reliable characterization tools for nanoparticle crystallinity, this study revealed how the use of a large qrange going from SAXS to XRD diagrams in absolute scattering units could be used to reveal simultaneously the shape, size, and internal structure of gold nanoparticles. To do so, the experimental data obtained on laboratory set-ups were unified, normalized, and then fitted with different stable structures identified with HRTEM and produced by molecular dynamics. External shape, size, and concentration being constrained by the SAXS patterns, the internal structures were described by quantitative numbers. The experimental technique presented here is most likely to be widely used and applied to other materials in the future, as the obtained unified diagrams are the signature of particles’ shape, size, and internal structure. Depending on the particles studied, different modeling will have to be implemented to allow characterizing and counting of nanoparticles in solution. This additional step in nanoparticle characterization can fill the gap between theoretical approaches based on specific structures and their applications. This will impact straight away nanoscience communities in different manners. First, structure− properties dependence of individual nanoparticles will be better determined as particles will be experimentally measured and fully characterized, not only their macroscopic properties. Second, since nanoparticles are now used in supracrystals and nanocrystal-based devices, it will be possible to precisely characterize the building blocks to prevent defective object fabrication and understand (un)expected properties. Third, results obtained on the present gold nanoparticles offer new possibilities concerning anisotropic nanoparticle formation and growth mechanism studies, which are both still debated in the

literature. Last, in situ measurement of environmental stabilization of specific nanoparticle structures during their formation is now reachable. Methods. Gold Nanoparticle Syntheses. Gold nanoparticles were synthesized through an organic route adapted from Zheng et al.36 For each sample, two solutions were prepared using a specified solvent and then mixed. The first solution (A) was made of 25 mg of chloro(triphenylphospine)gold(I) and 25 μL of dodecanethiol in 5 mL of toluene (respectively, α,α,α-trifluorotoluene, respectively, chloroform) for Au54 and Au63 (respectively, Au33, respectively, Au22). The second solution (B) was made of 43.5 mg of t-butylamine borane complex dissolved in 3 mL of the same solvent. For the samples Au54 and Au33, A and B were mixed instantly and then heated at 100 °C under reflux for 1 h, whereas the mixture was kept at room temperature for the sample Au22. The last sample (Au63) was obtained by adding dropwise the solution B to the solution A, which was previously heated to 100 °C and kept at that temperature for 1 h. The clear solutions got tainted from brown to purple (Au22 to Au63) depending on the size of the nanoparticles. All samples were then washed twice by centrifugation at ∼0 °C. Ethanol was added to the sample solutions to allow a quicker precipitation of the gold particles. Particles were finally redispersed in toluene. SAXS/XRD. X-ray scattering diagrams were recorded in transmission on 2D detector in the laboratory using three different setups (SAXS, intermediate, and XRD). The first one gave access to a range of q = 9.10−3−5.10−1 Å−1 (q = 4π/λ sin θ, with 2θ the angle between the incoming and scattered beam) using a copper rotating anode (λ = 1.542 Å) and a sample− detector distance of ∼120 cm. The X-ray source was collimated via an Osmic mirror, and the scattered beam was collected on a Mar 300. The two other setups were built around a molybdenum rotating anode (λ = 0.709 Å) and offer a qrange of 3.3.10−2−3.0 Å−1 for the intermediate configuration and 1.1−9.8 Å−1 for the XRD configuration. The working distances were, respectively, ∼70 cm and ∼5 cm. The scattered beam was collected on a Mar 345 image plate for both configurations. The samples were prepared by filling 1.5 mm borosilicate capillaries for SAXS experiment, 3 mm borosilicate capillaries for intermediate experiment with solution. Concerning XRD samples, to increase the signal-to-noise ratio, drops of solutions were deposited on coverslips and dried. Destabilizing the colloidal solution using poor cosolvents such as ethanol was used to reduce coffee ring effect and allowed homogeneous deposition.52 Direct comparison between XRD measurement on gold nanoparticle solution in 3 mm capillary and dry sample on coverslip allowed us to conclude that there was no visible structural evolution for those particles through drying (see Figure S19). Scaling of the data was done using references such as tetradecanol and gold foil for q-scaling. Since the data obtained from the intermediate setup overlap the two other sets, the absolute intensity scaling using lupolen applies for the full diagram. Additional information is available in Supporting Information 2. The experimental data were not deconvoluted from the smearing with the resolution function. Indeed, this effect was very low, in particular in the XRD region, since the experiment configuration (monochromaticity, collimation, and thickness of the sample, in particular in the thin film configuration) was such that the Δq resolution was 0.01 Å−1, far below the width of the observed Bragg peaks. TEM. A single drop of diluted solution was deposited on Cucarbon grid for each sample and left to dry. HRTEM E

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observations were carried out at IMPMC using a JEOL JEM 2100F (JEOL, Japan) equipped with a Schottky emission gun, a UHR pole piece, and a Gatan S4000 CCD camera.

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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.5b02924. TEM and SAXS analyses; guide to X-ray scattering measurements; fitting with SAXS or XRD; HRTEM analysis; modeling and X-ray scattering calculations; Xray scattering measurements; PDF analysis; comparison of XRD data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: blaise.fl[email protected]. Present Address

R.C.-H., Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge the financial support from Agence Nationale de la Recherche (ANR) under Contract No. ANR11_BS10_006. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Luc Belloni for useful discussion regarding calculation techniques. We also thank Stéphan Rouzière from Laboratoire de Physique des Solides (LPS, Orsay, France) for the access to XRD measurement. Finally, we wish to thank Marion Renaud-Fabre for the idiomatic help.

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REFERENCES

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