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small angle X-ray scattering

Chemistry of Materials

Chemistry of Materials Article Table 2. Average Pd NP Sizes Measured from TEM and SAXS reaction A B C D E F G SAXS (nm) 4.0 ± 0.4 3.8 ± 0.4 3.4 ± 0.4 5.5 ± 0.6 7.5 ± 0.5 9.2 ± 0.6 11.0 ± 0.9 TEM (nm) 3.7 ± 0.3 3.5 ± 0.3 3.3 ± 0.3 5.2 ± 0.7 7.3 ± 0.3 8.7 ± 0.6 10.5 ± 0.9 Figure 3. Representative SAXS patterns (colored plots) and the corresponding fits (black plots) of Pd NPs at different reaction times in the presence of (a) TOP 1.25 mmol + oleylamine 0.65 mmol (reaction B); (b) TOP 1.25 mmol + oleylamine 2.50 mmol (reaction C). The scattering intensities are offset for clarity. Time t = 0 s is set when nucleation starts and the nucleation temperature is at ∼230 °C for both reactions. Although oleylamine changes the nucleation rate, the change is small which leads to limited tunability of final particle size. We also tested the effect of final reaction temperature on the formation of Pd NPs and found that it has minimal or no effect on particle size and size distribution, as long as it is above the critical nucleation temperature that enables burst nucleation. For example, there is no obvious difference in the final size of Pd NPs ex situ synthesized using the same ligand mixture from reaction C but at different final reaction temperatures of 240 and 290 °C (Figure S4). In contrast to oleylamine, TOP substantially changes the reactivity of Pd precursor. It has been demonstrated that Pd(II) forms a stable Pd II −(TOP) 4 complex and a large excess of TOP retards the thermal decomposition of the Pd−TOP complex. 53 Taking advantage of the in situ SAXS, we quantitatively studied the effect of TOP by doubling its amount (molar ratio of Pd:TOP increases from 1:5 to 1:10, reaction D). We observed the formation of Pd NPs at 255 °C rather than 230 °C due to the shift in the reaction equilibrium of the decomposition of Pd−TOP complex into Pd NPs, which requires higher energy to activate it. Quantitative analysis of the SAXS data (Figure S5) shows that the formation rate of Pd NPs is reduced by a factor of 5 (Figure 4c). Due to a depressed precursor reactivity in the presence of large excess of TOP which cannot support a ∼230 °C, indicating that oleylamine does not affect the formation of monomer species or the chemistry of the Pd− TOP complex. The Pd NP size and concentration are derived from the fits of the SAXS results, and their quantitative comparisons are shown in Figure 2a−c. Similar to the case without oleylamine, a rapid nucleation occurred in the first ∼50 s, followed by a slow growth of NP from ∼2.8 nm in size. The similar yield of Pd NPs over reaction times suggests that oleylamine does not affect precursor reactivity (Figure 2c). However, slightly higher nucleation rate (1.37 × 10 14 mL −1 s −1 ) was observed when 0.63 mmol of oleylamine was used (reaction B), generating more nuclei. Since the precursor amount is constant, the more nuclei formed, the smaller the final Pd NPs, as confirmed by the smaller size measured from SAXS (3.8 ± 0.4 nm) and TEM (3.5 ± 0.3 nm) (Figure 2d−f and Table 2). The difference in the nucleation rate in the presence of oleylamine is likely due to stronger binding between oleylamine and the Pd 0 nuclei, which facilitates the formation of nuclei by stabilizing them with reduced nuclei− solvent interfacial tension. This observation is also supported by the fact that the TOP ligand on the Pd surface can be easily exchanged with oleylamine (Figure S3) and further corroborated by previous studies involving NMR. 53 Further increasing the amount of oleylamine from 0.63 to 2.50 mmol correspondingly increases the nucleation rate to 1.86 × 10 14 mL −1 s −1 (Figure 2), leading to even smaller Pd NPs with an average diameter of 3.4 ± 0.4 nm from SAXS (Figure 2d−f). Figure 4. Quantitative analysis of (a) size, (b) concentration, and (c) yield of Pd NPs as measured via in situ SAXS during the syntheses with different amounts of TOP. (d) Representative TEM image of the final Pd NPs. (e) SAXS pattern (colored plot) and the corresponding fit (black plot) of the as-synthesized Pd NPs. (f) Size and size distribution measured from TEM (histogram) and SAXS (dotted plots, normalized to the maximum of the histogram). 1130 DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

Chemistry of Materials Article Figure 5. (a, b) Quantitative analysis of (a) size and (b) yield of Pd NPs measured via in situ SAXS at varying molar ratios of oleic acid:TOP (reactions E−G). (c) Represenative TEM images of the dispersed Pd NPs synthesized from reaction E (upper), reaction F (middle), and reaction G (lower). Scale bars are 20 nm. (d) SAXS patterns (colored plots) and corresponding fits (black line) of the as-synthesized Pd NPs. (e) Size and size distribution measured from TEM (histogram) and SAXS (dotted plots, normalized to the maximum of the histogram). Results from reactions E, F, and G are shown in red, blue, and purple, respectively. Figure 6. Classical modeling of the formation kinetics of Pd NPs using different ligands. (a) Only TOP (reaction A). (b) A mixture of TOP and oleylamine (reaction C). (c) A large excess of TOP (reaction D). (d) A mixture of oleic acid and TOP (reaction F). The red lines are from the classical model, and the size and size distribution are experimental results from SAXS. large amount of monomer, the nucleation rate (1.01 × 10 13 mL −1 s −1 ) is more than an order of magnitude smaller compared to that for reaction C (1.86 × 10 14 mL −1 s −1 ), and the nucleation process lasts much longer (∼290 s) compared to the rapid nucleation (∼50 s) for reaction C (Figure 4a,b). As a result, fewer nuclei are formed (1.9 × 10 15 mL −1 vs 8.5 × 10 15 mL −1 ) and the final Pd NPs are larger in diameter (5.5 ± 0.6 nm for reaction D vs 3.4 ± 0.4 nm for reaction C) from SAXS, Figure 4e,f), which are also confirmed by ex situ TEM (Figure 4d). It is worth mentioning that, due to much longer nucleation process in reaction D, the particle size distribution is wider (Figure 4d), thus highlighting the importance of controlling precursor reactivity for burst nucleation in order to obtain narrow size distribution. Recently we reported that, in stark contrast to conventional colloidal synthesis, the Pd NPs rapidly crystallize into threedimensional superlattices in the presence of oleic acid rather than dispersed NPs, and weak binding between oleic acid and Pd NPs plays a critical role in the crystallization. 41 We found that, in the presence of oleic acid (1:1 molar ratio of oleic 1131 DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

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