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9 months ago

small angle X-ray scattering

Chemistry of Materials

Chemistry of Materials success, important questions still unanswered are whether the measured nucleation and growth kinetics is valid in the lab-scale batch synthesis and how the kinetics can be utilized for designing better synthetic strategies toward well-defined NPs for desired applications. Here we report the in situ SAXS probing of the synthesis of Pd NPs through thermal decomposition of Pd−TOP complex in 1-octadecene solution using a “heat-up” method. 38−40 We use a custom-made flask reactor to mimic typical laboratory reaction conditions (e.g., high temperature, inert atmosphere) for solution-phase colloidal synthesis. 41 We chose to study the synthesis of Pd NPs because of their widespread catalytic applications in many important chemical reactions, including methane combustion reaction, 42 methane steam reforming reaction, 43 and electrochemical oxidation of formic acid. 44,45 Understanding the size−activity relationship is critical for designing better catalysts for those reactions. Although different synthetic approaches have allowed for the preparation of different-sized Pd NPs with narrow size distributions, 38,46−52 currently, there is still no general guideline for precisely controlling their size due to the lack of insights into the formation kinetics. By systematically studying the roles of different ligands (i.e., trioctylphosphine (TOP), oleylamine, and oleic acid) via in situ SAXS, we are able to quantitatively explore the reaction kinetics. We find that the Pd−TOP precursor reactivity is strongly affected by the type and amount of ligands used, which controls the nucleation kinetics and allows for the fine control of the final particle size. Theoretical models suggest that the growth of Pd NPs is limited by surface reaction between monomers and NPs or by thermal activation of Pd-precursor depending on the precursor reactivity. The mechanistic understanding of the effects of different ligands enables the synthesis of Pd NPs in the size range of 3 to 11 nm with nanometer-size control. We believe that the in situ SAXS characterization coupled with the versatile reactor geometry described here can be extended to accelerate the synthetic developments of various functional NPs. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Pd(acac) 2 (acac = acetylacetonate, 35% Pd) was purchased from Acros Organics. Oleic acid (90%), oleylamine (70%), and 1-octadecene (90%) were purchased from Sigma-Aldrich and degassed under vacuum at 100 °C for 1 h before use. TOP (97%) was purchased from Sigma-Aldrich and used without further purification. Hexanes and isopropanol were purchased from Fisher Scientific. 2.2. Syntheses of Pd NPs. The syntheses were performed in a custom-made flask reactor under inert Ar gas. Pd NPs were synthesized via thermal decomposition of Pd−TOP complex modified from previously reported procedures. 38,39 In a typical procedure, 1- octadecene (10 mL) and Pd(acac) 2 (0.25 mmol) were mixed via magnetic stirring under a gentle flow of Ar for 15 min. Then TOP was injected, immediately forming a light yellow Pd−TOP complex, 53 followed by the addition of oleylamine or oleic acid depending on experimental choices (Table 1). The mixture was kept at 60 °C for 30 min under the Ar flow to remove air and moisture and then heated to 280 °C using a heating rate of ∼15 °C min −1 (the heating rates for all the reactions are very similar) and kept at the final temperature for up to 30 min. After the reaction, the flask was cooled down to room temperature by removing the heating tape. The obtained Pd NPs were isolated by precipitation with 20 mL of 2-propanol followed by centrifugation at 8000 rpm for 3 min. The precipitated Pd NPs were further purified by another two cycles of precipitation (5 mL of hexanes as the solvent and 20 mL of 2-propanol as the antisolvent). After purification, the Pd NPs were redispersed in 5 mL of hexanes for 1128 Article Table 1. Different Combinations of Ligands for the Synthesis of Pd NPs in This Work a reaction TOP oleylamine oleic acid A 1.25 mmol - - B 1.25 mmol 0.63 mmol - C 1.25 mmol 2.50 mmol - D 2.50 mmol 2.50 mmol - E 1.25 mmol - 3.75 mmol F 1.25 mmol - 9.40 mmol G 1.25 mmol - 15.60 mmol a The amount of Pd(acac) 2 used in each synthesis is 0.25 mmol. further characterizations. Different combinations of ligands were studied to systematically evaluate their effect on reaction kinetics and are summarized in Table 1. For reactions E−G, due to large volume of oleic acid used, the amount of 1-octadecene used was tuned such that the total volume of 1-octadecene and oleic acid was 10 mL to maintain similar precursor concentration for these reactions. 2.3. In Situ SAXS Characterization. In Situ SAXS measurements were performed at Beamline 1-5 of Stanford Synchrotron Radiation Lightsource (SSRL) using our custom-made setup (Figure S1). 41 The X-ray path length within the reactor was 5 mm. The X-ray energy was 15.5 keV, and the beam spot size was 500 μm × 500 μm. The sampleto-detector distance was calibrated to be 741.6 mm using a silver behenate standard. For each data acquisition, an exposure time of 5 s was applied. A Rayonix SX165 CCD area detector was used to collect the two-dimensional (2D) scattering patterns. The 2D SAXS patterns were reduced to 1D data, calibrated to absolute scale using a glassy carbon standard (Figure S2a), 54 and were further analyzed using the Irena package (available at usaxs.xray.aps.anl.gov/staff/ilavsky/irena. html from the APS). 12 The size, size distribution, concentration, and volume fraction of the Pd NPs were modeled in Irena package using a spherical form factor (see details in the Supporting Information). The yield (Y) of Pd NPs was obtained based on the equation Y = ϕ/ϕ max , where ϕ is the volume fraction of total Pd NPs at a specific reaction time and ϕ max is the maximum volume fraction of Pd NPs in the solution at 100% conversion of Pd-precursor to Pd nanoparticles. 2.4. Ex Situ Characterization. The purified Pd NPs were characterized by TEM. TEM samples were prepared by drop-casting a dilute NP dispersion in hexane onto carbon-coated 300 mesh Cu grids. TEM images were collected on a FEI Tecnai transmission electron microscope operated with an accelerating voltage of 200 kV. The obtained superlattices of Pd NPs with the oleic acid ligand were imaged using a FEI Magellan 400 XHR scanning electron microscopy operating at 5 kV. Fourier transform infrared spectra of the NPs were recorded on a Nicolet iS50 spectrometer. 3. RESULTS AND DISCUSSION Pd NPs were synthesized through thermal decomposition of Pd−TOP complex in 1-octadecene (see Experimental Section for details). The synthesis procedure using only TOP as surfactant (reaction A in Table 1) is taken as standard. The Pd−TOP complex solution was heated up from 60 to 280 °C and kept at 280 °C for up to 30 min. During the reaction, SAXS patterns were acquired at an exposure time of 5 s for each pattern. The obtained 2D SAXS data were integrated into 1D data, and the background signal from the reactor and solvent were subtracted. As reaction temperature increases up to 230 °C, the scattering from nuclei with diameter larger than 1 nm appears. This temperature, defined as the nucleation temperature, is taken as t = 0 s. Representative background-subtracted SAXS patterns are shown in Figures 1a and S2b. As the reaction proceeds, the scattering intensity at low scattering vector q increases quickly, indicating increased particle size. Obvious oscillation peaks appear as well, suggesting the narrowing of the DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

Chemistry of Materials Figure 1. (a) Representative SAXS data (colored plots) and the corresponding fits (black plots) of Pd NPs at different reaction times during the synthesis using only TOP as surfactant (reaction A, Pd:TOP molar ratio = 1:5). The scattering intensities are offset for clarity. Time t = 0 s is set when nucleation starts. (b) Quantitative analysis of the concentration of Pd NPs as a function of reaction time. (c) Size (dots) and size distribution (bars) of Pd NPs as measured by in situ SAXS. (d) Reaction temperature profile during the synthesis. Article NP size distribution. To obtain more information about size, size distribution, and concentration of the Pd NPs, the SAXS data were fitted using a spherical NP model in the Irena package (see SI for details). Figure 1b−d shows the quantitative analysis of the mean diameter, polydispersity, and concentration of Pd NPs as a function of reaction time. Based on the analysis of particle concentration, burst nucleation occurs in the first 50 s, during which the NP concentration increases with a constant nucleation rate of 1.22 × 10 14 mL −1 s −1 in the case of the standard reaction A (Table 1). Meanwhile, the polydispersity of the formed nuclei dropped significantly to ∼14%. The narrow size distribution of the nuclei is critical for their further simultaneous growth into uniform NPs. During this nucleation stage ∼27% of the Pd atoms in the precursor are incorporated into Pd NPs (Figure 2c). Between 50 to 115 s, the Pd NPs continue to slowly grow in size from 2.8 to 3.7 nm. Although a small concentration of new nuclei is formed during this period, the much slower nucleation rate (2.4 × 10 13 mL −1 s −1 ) guarantees that the fraction of newly formed nuclei is relatively small and the overall polydispersity was not increased but instead dropped to ∼10%. Between 115 and 160 s Pd NPs slowly grow to 3.9 nm at nearly constant particle concentration due to much decreased monomer concentration. After 160 s, there is a slight decrease in particle concentration (Figure 1b), suggesting the existence of NP−NP coalescence or Ostwald ripening, which is common in the colloidal synthesis of NPs. 18,24,55,56 After 5 min, Pd NPs with the final size of 4.0 ± 0.4 nm are obtained, and their size and concentration do not change during further aging process. The final Pd NPs were also characterized by TEM, as shown in Figure 2d. The size based on TEM characterization was measured to be 3.7 ± 0.3 nm, which is in good agreement with the SAXS measurement (Figure 2e,f and Table 2). To study the effect of different ligands on the synthesis of Pd NPs, we started by comparing the standard reaction condition (reaction A) with those where oleylamine, a widely used stabilizing ligand for metallic NPs, 57 is present. Real-time SAXS patterns using different amounts of oleylamine are shown in Figure 3. We observed a similar nucleation temperature at Figure 2. (a−c). Quantitative analysis of (a) size, (b) concentration, and (c) yield of Pd NPs with different amounts of oleylamine as measured via in situ SAXS. (d) TEM images of the final Pd NPs synthesized with different ligands. (e) SAXS patterns (colored plots) and the corresponding fits (black plots) 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). Results from reactions A, B, and C are shown in red, blue, and purple, respectively. 1129 DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

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