Tuning Precursor Reactivity toward Nanometer-Size Control in Palladium Nanoparticles Studied by in Situ Small Angle X-ray Scattering

Abstract

Synthesis of monodisperse nanoparticles (NPs) with precisely controlled size is critical for understanding their size-dependent properties. Although significant synthetic developments have been achieved, it is still challenging to synthesize well-defined NPs in a predictive way due to a lack of in-depth mechanistic understanding of reaction kinetics. Here we use synchrotron-based small-angle X-ray scattering (SAXS) to monitor in situ the formation of palladium (Pd) NPs through thermal decomposition of Pd−TOP (TOP: trioctylphosphine) complex via the "heat-up" method. We systematically study the effects of different ligands, including oleylamine, TOP, and oleic acid, on the formation kinetics of Pd NPs. Through quantitative analysis of the real-time SAXS data, we are able to obtain a detailed picture of the size, size distribution, and concentration of Pd NPs during the syntheses, and these results show that different ligands strongly affect the precursor reactivity. We find that oleylamine does not change the reactivity of the Pd−TOP complex but promote the formation of nuclei due to strong ligand−NP binding. On the other hand, TOP and oleic acid substantially change the precursor reactivity over more than an order of magnitude, which controls the nucleation kinetics and determines the final particle size. A theoretical model is used to demonstrate that the nucleation and growth kinetics are dependent on both precursor reactivity and ligand−NP binding affinity, thus providing a framework to explain the synthesis process and the effect of the reaction conditions. Quantitative understanding of the impacts of different ligands enables the successful synthesis of a series of monodisperse Pd NPs in the broad size range from 3 to 11 nm with nanometer-size control, which serve as a model system to study their size-dependent catalytic properties. The in situ SAXS probing can be readily extended to other functional NPs to greatly advance their synthetic design.

The role of nanoparticle size and ligand coverage in size focusing of colloidal metal nanoparticles

“…Despite observing continuous nucleation in many colloidal synthesis studies, the nal polydispersity can be as low as 10-20%. 7,22,44,59,67 As shown in Fig. 2, there is a continuous formation of Pd nanoparticles (increase in the concentration of nanoparticles) that overlaps with the nanoparticles growth (increase in the average diameter) from t ¼ 0 to 0.2 h, which is consistent with our previous work.…”

“…S4 †). The overlap of nucleation and growth while reaching a narrow nal size distribution is in fact very common and has been reported for Au, 39,71 Pd, 7,44 Ir, 48 and CdSe. 23 Fig.…”

“…7,[39][40][41] Additionally, the direct observation during synthesis can provide evidence for, or against the central hypotheses in the different proposed nucleation and growth models. For example, in contrast to the LaMer burst nucleation, temporal overlap of nucleation and growth has been observed in the synthesis of several colloidal nanoparticles including Ir, 42,43 Pd, 7,40,44,45 Rh, 46 and CdSe. 47 Moreover, growth was shown to be reaction-instead of diffusion-limited while the nal nanoparticles had a narrow size distribution.…”

Catalytic Oxidation of Methane: Pd and Beyond

“…Further advances in solvothermal synthesis are required to allow fast and reliable synthesis of the desired NPs (e.g. new in‐situ diagnostic methods, flow micro‐reactors) . Another critical step in the preparation of the final supported catalysts is the removal of the organic ligands employed in the solvothermal synthesis, typically carried out by UV/ozone treatment and flash calcination, which should be controlled to avoid the disruption of the pre‐formed nanostructures …”

In Situ Synchrotron X‐ray Characterization Shining Light on the Nucleation and Growth Kinetics of Colloidal Nanoparticles

“…These parameters can also be obtained from the Monte‐Carlo fitting of the SAXS patterns using Equation (8), where all the parameters are randomly chosen with little or no empirical knowledge . For example, Wu et al used in situ SAXS to study the synthesis of monodisperse Pd CNPs in octadecene containing Pd(acac) 2 (acac=acetylacetonate) as precursor reactant and various amounts of trioctylphosphine(TOP), oleic acid, and oleylamine as surfactant ligand . The time‐dependent SAXS patterns of a typical synthesis reaction containing TOP surfactant only (Pd:TOP molar ratio=1:5) are presented in Figure a. Analyzing the SAXS patterns produced the concentration (number of NPs in 1 mL solution) (Figure b), size, and size distribution (Figure c) of the NPs.…”