Synthesis of Palladium Nanodendrites Using a Mixture of Cationic and Anionic Surfactants.

Xin Wen,Sarah Lerch,Eva Olsson,Ali Reza Tehrani-Bagha,Zhihang Wang,Bassem Aboudiab,Kasper Moth-Poulsen

doi:10.1021/acs.langmuir.9b03804

Abstract

Surfactants are used widely to control the synthesis of shaped noble-metal nanoparticles. In this work, a mixture of hexadecyltrimethylammonium bromide (CTAB), a cationic surfactant; sodium oleate (NaOL), an anionic surfactant; palladium chloride; and a reducing agent were used in the seed-mediated synthesis of palladium nanoparticles. By controlling the surfactant mixture ratio, we initially discovered that palladium nanodendrites with narrow size distribution were formed instead of the traditional nanocubes, synthesized with only CTAB. In order to investigate the optimal ratio to produce Pd nanodendrites with a high yield and narrow size distribution, samples synthesized with multiple molar ratios of the two surfactants were prepared and studied by transmission electron microscopy, dynamic light scattering, conductance, and ultraviolet–visible spectroscopy. We propose that the addition of NaOL alters the arrangement of surfactants on the Pd seed surface, leading to a new pattern of growth and aggregation. By studying the nanodendrite growth over time, we identified the reduction period of Pd2+ ions and the formation period of the nanodendrites. Our further experiments, including the replacement of CTAB with hexadecyltrimethylammonium chloride (CTAC) and the replacement of NaOL with sodium stearate, showed that CTA+ ions in CTAB and OL– ions in NaOL play the main roles in the formation of nanodendrites. The formation of palladium nanodendrites was robust and achieved with a range of temperatures, pH and mixing speeds.

Highlights

Palladium (Pd) nanoparticles have attracted considerable interest due to their hydrogen uptake ability and effective catalytic activity leading to a variety of applications, including catalysis, gas sensing, and hydrogen storage.1−4 In most hydrogen-based applications, the hydrogen atoms initially dissociate on the surface of Pd
We identified the optimal molar ratio for the surfactants that promotes the formation of Pd nanodendrites with high yield and narrow size distribution
In order to study the effect of the binary surfactant mixture on the shape and yield of Pd nanoparticles, a series of seedmediated growth synthesis experiments were performed with different molar ratios of CTAB and NaOL but maintaining the same total volume and initial molar concentrations of the surfactants (50 mM)

Summary

■ INTRODUCTION

Palladium (Pd) nanoparticles have attracted considerable interest due to their hydrogen uptake ability and effective catalytic activity leading to a variety of applications, including catalysis, gas sensing, and hydrogen storage.− In most hydrogen-based applications, the hydrogen atoms initially dissociate on the surface of Pd. For the first time, synthesis of Pd nanodendrites was achieved by controlling the molar ratio of CTAB and NaOL in surfactant mixtures. These Pd nanodendrites feature large surface areas, which have the potential to improve the material efficiency of Pd nanoparticles in sensing and catalytic applications. There is only CTAB (αCTAB = 1), Pd cubic seeds grow into larger Pd nanocubes; when 0.5 ≤ αCTAB ≤ 0.9, Pd nanodendrites and a few Pd crystals are formed; when 0 ≤ αCTAB ≤ 0.4, small Pd crystals are obtained and Pd seeds are not able to grow further This model identifies the micellar formation of the surfactants in solution as the reason for this dramatic shape change. A Jenway 570 pH meter was used to measure the pH value of the solutions

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES

Seed-Mediated Growth of Nearly Monodisperse Palladium