Aprotic ionic liquids have been recognized as the potential electrolytes for electrodeposition of various metals due to wide electrochemical potential window and intrinsic ionic conductivity. In addition, the ability of the ionic liquids to disperse variety of nanomaterials without any stabilizing reagents makes it possible to prepare metal nanoparticles simply by electrochemical reduction of metal species dissolved in the ionic liquids. We have already reported the electrochemical preparation of the metal nanoparticles of such transition metals as Fe, Co, Ni, Pd, Ag, Cd, Pt, and Au in some ionic liquids composed of bis(trifluoromethylsulfonyl)amide (TFSA^–) and dialkylpyrrolidinium cations.^1-10 The average size of the nanoparticles prepared in the ionic liquids was found to be a few nanometers. However, the factors determining the average size of the nanoparticles have not been clarified yet. In the present study, the effects of the cations and anions on the size distribution of Pd nanoparticles has been investigated in the TFSA^–- and FSA^– (bis(fluorosulfonyl)amide)-based ionic liquids.

The TFSA^–-based ionic liquids composed of 1-butyl-, 1-hexyl-, and 1-decylpyrrolidinium (BMP⁺, HMP⁺, and DMP⁺) with bromides of the corresponding organic cations were used as the electrolytes. BMPFSA was prepared by the metathesis reaction of BMPBr and LiFSA. PdBr₂ was dissolved in the electrolyte as a Pd(II) source. Glassy carbon was used as a working electrode. A rotating glassy carbon electrode was also used as a working electrode. A platinum wire was used as a counter electrode. A silver wire immersed in 0.1 M AgCF₃SO₃/BMPTFSA was used as a reference electrode. Pd nanoparticles were prepared by potentiostatic cathodic reduction using a double-compartment cell. The nanoparticles dispersed in the ionic liquids were characterized by transmission electron microscopy.

Potentiostatic cathodic reduction on a GC electrode in BMPTFSA containing [PdCl₄]^2– resulted in deposition of Pd metal on the electrode and formation of Pd nanoparticles dispersed in the ionic liquid. The amount of Pd deposited on the GC electrode in BMPTFSA decreased with lowering the electrode potential, indicating the adsorption of Pd atom formed at the electrode-electrolyte interface onto the electrode surface is considered to be inhibited by accumulation of BMP⁺ at the interface at negative potentials.¹ Pd nanoparticles were prepared using a rotating GC electrode with different rotating rates in order to examine the effects of the current density on the average size of Pd nanoparticles. The average size of Pd nanoparticles prepared at –2.5 V in 10 mM [PdBr₄]^2–/BMPTFSA was independent of the rotating rate, indicating the supply rate of Pd atoms at the electrode surface has no influence on the average size of Pd nanoparticles.¹⁰ The average size of Pd nanoparticles prepared in BMPTFSA, HMPTFSA, and DMPTFSA increased with an increase in the alkyl chain length of the cation.⁷ In addition, the average size of Pd nanoparticles prepared in BMPTFSA with excess Br^–or in BMPFSA was smaller than that in BMPTFSA. These results suggested that the average size of Pd nanoparticles is mainly affected not by the current density and the electrode potential but by the size of the cations and anions composing the ionic liquids.

References

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