Synthesis of highly monodisperse Pd nanoparticles using a binary surfactant combination and sodium oleate as a reductant

This study presents the synthesis of monodisperse Pd nanoparticles (NPs) stabilized by sodium oleate (NaOL) and hexadecyltrimethylammonium chloride (CTAC). The synthesis was conducted without traditional reductants and Pd-precursors are reduced by NaOL. It was confirmed that the alkyl double bond in NaOL is not the only explanation for the reduction of Pd-precursors since Pd NPs could be synthesized with CTAC and the saturated fatty acid sodium stearate (NaST). A quantitative evaluation of the reduction kinetics using UV-Vis spectroscopy shows that Pd NPs synthesized with both stabilizer combinations follow pseudo first-order reaction kinetics, where NaOL provides a faster and more effective reduction of Pd-precursors. The colloidal stabilization of the NP surface by CTAC and NaOL is confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) analysis.


Experimental parameters
In Table S1, the experimental parameters that were used in the synthesis of the Pd NPs are listed.

High resolution scanning transmission electron microscopy (HRSTEM) imaging of Pd nanoparticles
Through the observation in the TEM, it is discovered that most of the Pd NPs are polycrystals and only several Pd NPs are single-crystals. A HRSTEM image of a single-crystal Pd NP is shown in Figure S1 (a). It can be seen that there are stacking faults and twins in the singlecrystal Pd NPs. The {111} and {200} lattice planes and their interplanar spacings are marked in the HRSTEM image ( Figure S1 (a)). Figure S1 (b) shows a HRTEM image of a single-crystal Pd NP and its corresponding FFT pattern is displayed in Figure S1 (c), indicating that the zone axis of this crystal unit is [-112]. The spots resulted from (220), (311) and (1-11) lattice planes are marked in Figure S1 (c). Corresponding FFT pattern of (b), indicating that the zone axis of this crystal unit is [-112]. The spots resulted from (220), (311) and (1-11) lattice planes are pointed by red arrows.

Synthesis optimization of Pd NPs
For the development of the Pd NP synthesis a thorough evaluation of reaction parameters and the influence on the particle size and morphology was performed.

Reaction time
Optimization of the reaction time was performed by synthesizing Pd NPs for different times.

Synthesis in oil bath
The Pd NPs were synthesized in an oil bath at 100 C. Pd NPs show reproducible shapes and morphologies compared to the optimal synthesis conditions when synthesized for 2h ( Figure   S4a) and 4h ( Figure S4b).

Pd NPs stabilized with NaOL and CTAB
The effect on the shapes of Pd NPs when replacing CTAC with CTAB in the stabilizer mixture is shown in Figure S5. The shapes of Pd NPs consist of a mixture of cubes, bars and "arrows". Figure S5. TEM image of Pd NPs prepared according to the optimal reaction conditions except using CTAB and NaOL as stabilizers.

Pd NPs stabilized with NaST and CTAC
Pd NP were synthesized with NaST and CTAC as stabilizers. The shapes are a mixture of triangles, twinned and polycrystalline Pd NPs ( Figure S6). Figure S6. a) TEM image of Pd NPs prepared with optimal reaction conditions except using NaST and CTAC as stabilizers. b) Histogram of size distribution of Pd NPs with average size of 13 nm  19 %.

Pd NPs stabilized with NaOL
Pd NPs stabilized with NaOL were synthesized in an oil bath ( Figure S7). The NPs were stabilized with different concentrations of NaOL and showed poor colloidal stability and visual precipitation ( Figure S7a). TEM image of the small Pd NPs synthesized with 39.8 mM NaOL ( Figure S7b).

Pd NPs stabilized with NaST
For evaluation of the reduction mechanisms in the formation of Pd NPs NaOL was replaced by NaST. The initial precursor suspension with 8 mM NaST is turbid ( Figure S8a image of Pd NPs after 4h reaction, sampled from the black precipitate at the surface of the solution shows Pd NPs and the presence of many particle seeds ( Figure S8c).

Qualitative characterization using FTIR and 1 H NMR
The Pd NPs and the stabilizers CTAC and NaOL were analyzed with FTIR ( Figure S10) and with 1 H NMR in D 2 O ( Figure S11)