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small angle X-ray scattering

Chemistry of Materials

Chemistry of Materials Article Table 3. Fitting Parameters Used for Modelling the Nucleation and Growth Kinetics reaction A B C D E F G γ (J m −2 ) 1.155 1.150 1.142 1.245 1.160 1.175 1.192 ξ 100000 115000 135000 80000 56700 50000 40000 k r /T (nm s −1 K −1 ) 0.407 0.355 0.305 0.566 0.714 0.799 0.985 acid:TOP), Pd NPs start to form at 200 °C, which is ∼30 °C lower than the standard reaction A with only TOP. Oleic acid acts like a “catalyst” for the thermal decomposition of the Pd− TOP complex, which is probably due to either the active involvement of oleic acid in the thermal decomposition of Pd− TOP complex or the formation of a Pd(oleate) x (TOP) y complex that decomposes at lower temperatures compared to pure Pd−TOP complex. 58−60 The reduced reaction temperature resulted in a smaller nucleation rate, and consequently, larger particles of 6.8 ± 0.4 nm, which is ∼2.8 nm larger than the Pd NPs synthesized without oleic acid (reaction A). 41 By increasing the amount of oleic acid, we observed similar in situ assembly of Pd NPs into face-centered cubic superlattices, followed by postgrowth of Pd NPs inside the superlattices as described before 41 (Figure S6−S9). The mean diameter and the yield of Pd NPs as a function of reaction time derived from SAXS are shown in Figure 5a,b. Interestingly, increasing the molar ratio of oleic acid:TOP results in even larger Pd NPs with narrow size distributions. Pd NPs of 7.5 ± 0.5 nm, 9.2 ± 0.6 nm, and 11.0 ± 0.9 nm average diameters from SAXS were synthesized using 3:1, 7.5:1, and 12.5:1 molar ratios of oleic acid:TOP, respectively (Figure 5c−e). We ascribe the large tunability of size to different nucleation rates caused by different binding affinity of oleic acid/TOP on Pd NPs. To gain more insights on the formation of Pd NPs with different ligands, we modeled the nucleation and growth kinetics using classical models 61 in which three steps are involved: (i) decomposition of Pd precursor into monomers; (ii) reaction of monomers to form nuclei; and (iii) growth of nuclei with addition of monomers (see details in SI). Fitting the experimental results to these models provides useful insights on the nucleation kinetics and the growth-limiting mechanism of Pd NPs in the presence of different ligands. The nucleation rate is dependent on the degree of supersaturation, reaction temperature, and interfacial tension between NP and the solvent. The growth rate can be expressed by the equation ⎡ 2γV ⎤ DV C ⎣S − exp r ( m d m 0 rRT ) ⎦ = , where D is the diffusion coefficient of dt r+ D/ kr Pd monomers, V m is the molar volume of Pd NPs, C 0 is monomer solubility, S is the dimensionless parameter describing the supersaturation of Pd monomer in the solution, r is the radius of Pd NPs, γ is interfacial tension between Pd NP and the solvent, k r is the surface reaction rate of the incorporation of monomers into the NPs, R is gas constant (8.314 J K −1 mol −1 ), and T is the reaction temperature. 62−64 This equation is converted into dimensionless form * * dr S− exp(1 / r ) = dτ * r + ξ with three dimensionless parameters: the * RT reduced radius r = γ r, the reduced kinetic length 2 V m D RT RT ( ) ξ = , and the reduced time τ = ( kr 2γVm γ VC ) Dt 2 V m 0 .By m adjusting the parameters, we obtained a good fit to experimental results in both nucleation and growth regimes of reaction A with γ = 1.155 J m −2 , k r /T = 4.07 × 10 −1 nm s −1 K −1 , and ξ = 1.0 × 10 5 (Figure 6a). The large ξ ≫ 1 indicates 2 that k r·r ≪ D, suggesting that the growth of Pd NPs is limited by surface reaction between monomers and Pd NPs rather than by diffusion of monomers to the particle surface. 62 As discussed before, oleylamine does not change the Pd−TOP precursor reactivity, and thus good fits of the growth kinetics were obtained showing similar nucleation and growth mode (Figures 6b and S10 and Table 3). The slight decrease of the γ value in the presence of oleylamine is in line with our hypothesis that stronger binding ligand covers NPs more densely reducing the NP−solvent interfacial tension. However, in the presence of large excess of TOP, a classical growth kinetic model does not provide a good fit to the experimental results due to substantial decrease of precursor reactivity (Figure 6c), suggesting that the particle growth is limited neither by the monomer diffusion nor by surface reaction but instead by the thermal activation of Pd precursor, which is confirmed by our experimental observations. In the case with oleic acid, the classical model fits well with the experimental results (Figures 6d and S10), suggesting the same surface-reaction limited growth mechanism. The larger γ value in the case of oleic acid suggests less dense ligand coverage of Pd NPs, which was confirmed by weaker binding between oleic acid and Pd NPs compared to oleylamine or TOP. 41 The same trend of larger γ with oleic acid compared to oleylamine was also observed in the Au NPs. 16,64 We should emphasize that increased value of γ with oleic acid affects the nucleation and growth kinetics much more considerably, thus offering wider tunability of the particle size. These results together with the quantitative results from in situ SAXS highlight the importance of tailoring the precursor reactivity and ligand−NP binding affinity to rationally tune the final NP size. 65,66 4. CONCLUSIONS Using in situ synchrotron-based SAXS, we have systematically studied the effect of different ligands (i.e., oleylamine, TOP, and oleic acid) on the synthesis of Pd NPs. Through quantitative analysis we have shown that nucleation kinetics is strongly dependent on the Pd−TOP precursor reactivity and ligand−NP binding affinity, which determines the final particle size and quality. Due to the formation of thermally stable Pd− TOP complex, an excess amount of TOP significantly retards the precursor decomposition and slows down the nucleation rate by more than an order of magnitude, and larger and more polydisperse NPs are synthesized. Oleylamine does not affect the reactivity of Pd−TOP precursor but slightly facilitates the formation of nuclei due to stronger binding between oleylamine and Pd NPs, leading to smaller NPs in the presence of oleylamine. In contrast, oleic acid strongly influences the reactivity of the Pd−TOP complex and the nucleation kinetics, and larger Pd NPs are synthesized in the presence of more oleic acid. The quantitative understanding of the nucleation kinetics with different ligands studied by in situ SAXS enables the synthesis of a library of monodisperse Pd NPs (polydispersity < 10%) with a wide size range from 3 to 11 nm. These welldefined Pd NPs serve as a model system for studying their sizedependent catalysis for methane combustion reaction which is 1132 DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

Chemistry of Materials Article presented in a separate publication. 67 The in situ SAXS measurement coupled with the versatile flask reactor can be readily extended to a broad variety of functional NPs to accelerate their synthetic developments for both fundamental research ■ and technological applications. ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05186. Details on the SAXS data analysis, kinetic models on the formation of nanoparticles, and additional supplementary Figures S1−S10 (PDF) ■ AUTHOR INFORMATION Corresponding Authors *(C.J.T.) E-mail: tassone@slac.stanford.edu. *(M.C.) E-mail: mcargnello@stanford.edu. ORCID Jian Qin: 0000-0001-6271-068X Matteo Cargnello: 0000-0002-7344-9031 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Laboratory Directed Research and Development program, at SLAC National Accelerator Laboratory under Contract No. DE-AC02-76SF00515. In situ SAXS experiments were performed at the Beamline 1-5 at the Stanford Synchrotron Radiation Lightsource (SSRL) of SLAC, and use of the SSRL is supported by the DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02- 76SF00515. J.J.W. acknowledges support from the U.S. Department of Energy, Office of Sciences, Office of Basic Energy Sciences, to the SUNCAT Center for Interface Science and Catalysis. M.C. acknowledges support from the School of Engineering at Stanford University and from a Terman Faculty Fellowship. J.Q. acknowledges support from the 3M Non- Tenured Faculty Award and the Hellman Scholar Award. E.D.G. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant DGE-1656518. I.S.M. was supported by the Department of Defense through the National Defense Science & Engineering Graduate Fellowship Program and by the Fannie and John Hertz Foundation through a Hertz Foundation Fellowship. L.W. and C.J.T. thank T.J. Dunn from SSRL for his assistance during the experiments. The electron microscopy characterization was performed at the Stanford Nano Shared Facilities (SNSF) at Stanford University. ■ REFERENCES (1) Wu, L.; Mendoza-Garcia, A.; Li, Q.; Sun, S. Organic Phase Syntheses of Magnetic Nanoparticles and Their Applications. Chem. Rev. 2016, 116, 10473−10512. (2) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (3) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nat. 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(9) Liao, H. G.; Cui, L. K.; Whitelam, S.; Zheng, H. M. Real-Time Imaging of Pt 3 Fe Nanorod Growth in Solution. Science 2012, 336, 1011−1014. (10) Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells. Science 2012, 336, 61−64. (11) Pauw, B. R. Everything SAXS: Small-angle Scattering Pattern Collection and Correction. J. Phys.: Condens. Matter 2013, 25, 383201. (12) Ilavsky, J.; Jemian, P. R. Irena: Tool Suite for Modeling and Analysis of Small-angle Scattering. J. Appl. Crystallogr. 2009, 42, 347− 353. (13) Sun, Y.; Ren, Y. In Situ Synchrotron X-Ray Techniques for Real- Time Probing of Colloidal Nanoparticle Synthesis. Part. Part. Syst. Char. 2013, 30, 399−419. (14) Ingham, B. X-ray Scattering Characterisation of Nanoparticles. Crystallogr. Rev. 2015, 21, 229−303. (15) Li, T.; Senesi, A. J.; Lee, B. Small Angle X-ray Scattering for Nanoparticle Research. Chem. Rev. 2016, 116, 11128−11180. (16) Abecassis, B.; Testard, F.; Spalla, O.; Barboux, P. Probing in Situ the Nucleation and Growth of Gold Nanoparticles by Small-angle X- ray Scattering. Nano Lett. 2007, 7, 1723−1727. (17) Henkel, A.; Schubert, O.; Plech, A.; Sonnichsen, C. Growth Kinetic of a Rod-Shaped Metal Nanocrystal. J. Phys. Chem. C 2009, 113, 10390−10394. (18) Polte, J.; Ahner, T. T.; Delissen, F.; Sokolov, S.; Emmerling, F.; Thunemann, 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. (19) Polte, J.; Erler, R.; Thunemann, 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 X-ray Scattering at Millisecond Time Resolution. ACS Nano 2010, 4, 1076−1082. 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(24) Abecassis, B.; Bouet, C.; Garnero, C.; Constantin, D.; Lequeux, N.; Ithurria, S.; Dubertret, B.; Pauw, B. R.; Pontoni, D. Real-Time in DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

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