Electrocatalytic Activity of Pt Nanoparticles Prepared in Biocompatible Ionic Liquids By Accelerated Electron Beam Irradiation

Abstract

A number of papers on the metal nanoparticle synthesis method have been reported to date because metal nanoparticles are expected to be key materials for further development of science and technology.1 Of these, from a practical application standpoint, the accelerated electron beam irradiation method,2 which can be conducted at existing industrial plants for sterilizing medical kits, has been attracting attention. In this decade, several approaches to synthesize metal nanoparticles in ionic liquid (IL) have been proposed by several research groups, too.3 Some scientists point out that IL behaves as an excellent medium for formation and stabilization of the metal nanoparticles. Recently we have succeeded in the metal nanoparticle preparation by the approach with combination of the accelerated electron beam irradiation method and the IL, which is called ionic liquid-accelerated electron beam irradiation (IL-AEBI) method.4However the past ILs used for the method are not green and expensive price. The aim of this study is to design the biocompatible and cost-effective ILs suitable for metal nanoparticle, especially Pt nanoparticle, preparation by the IL-AEBI method. After supporting the resulting Pt nanoparticles onto untreated multi-walled carbon nanotubes (Pt-MWCNTs), oxygen reduction reaction (ORR) electrocatalytic activity of the Pt-MWCNTs was also examined. Biocompatible and cost-effective ILs were prepared by a neutralization method. Carboxylic acids, e.g., propionic acid, hexanoic acid, levulinic acid, were slowly added into a choline bicarbonate aqueous solution to avoid unexpected thermal decomposition of the carboxylic acids by heat of neutralization. The crude IL solutions were purified prior to use. Solutions of 5.0 mM platinum acetylacetonate (Pt(acac)2) in biocompatible ILs were encapsulated in perfluoroalkoxyethylene container in a dry Ar gas-filled glove box with O2 and H2O <1 ppm. Accelerated electron beam irradiation experiments were carried out at an existing common industrial plant for sterilizing medical kits (accelerated energy: 4.8 MeV; beam current: 10 mA; absorbed dose: 6 kGy; irradiation time: ca. 2 sec.). Pt-MWCNTs were prepared by reference to our previous paper.5The resulting Pt nanoparticles supported on MWCNT was rinsed by acetonitrile. Characterization of all the samples prepared in this study was conducted by TEM, EDX, ICP, electrochemical measurements, etc. Solvated electrons and radicals are readily produced in various solvents by quantum beam irradiation including gamma-ray and electron beam. The generated reactive species are directly related to preparation of metal nanoparticles.2 However, as is often the case with ILs, it is hard to produce metal nanoparticles since usual ILs show a better radiation stability than conventional aqueous and organic solvents. Many years ago we revealed that only 1,3-dialkylimidazolium cation-based ILs yield a sufficient number of metal nanoparticles by the IL-AEBI because such ILs can release a limited amount of reactive species.4 It is important to know the radiation stability of biocompatible ILs in light of the application to the IL-AEBI method. NMR measurements for the neat ILs after the accelerated electron beam irradiation suggested the production of the reactive species during the irradiation. In fact, after the accelerated electron beam irradiation to the solutions of 5.0 mM Pt(acac)2 in the biocompatible ILs, existence of monodispersed Pt nanoparticles in the ILs was identified by TEM observation. Mean particle size of the Pt nanoparticle was ca. 2 ~ 3 nm. Possibility of the Pt nanoparticles as the electrocatalysts was investigated after preparing the Pt-MWCNTs.4 Surprisingly the mean particle size remained even after the preparation process at 473 K and the Pt nanoparticles were immobilized onto a basal plane of the MWCNTs. Electrocatalytic activity was examined by typical electrochemical analyses using the Pt-MWCNTs coated on a glassy carbon rotating disk electrode.5 As given in Table I, the characteristics are strongly dependent on the ionic liquid species. Given the variation in the mass activity, it is possible that the d-band center of Pt nanoparticles is influenced by the carboxylate anions. In conclusion, our designed biocompatible and cost-effective ILs are suitable reaction media for metal nanoparticle preparation by the IL-AEBI method. Acknowledgement Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology. References Nanoparticles: From Theory to Application, G. Schmid, ed., Wiley-VCH, Weinheim, 2004. J. Belloni, Cat. Today, 113, 141 (2006). J. Dupont and J. D. Scholten, Chem. Soc. Rev., 39, 1780 (2010). T. Tsuda, S. Seino, and S. Kuwabata, Chem. Commun.,6792 (2009); ibid, 48, 1925 (2012). K. Yoshii, T. Tsuda, T. Arimura, A. Imanishi, T. Torimoto, and S. Kuwabata, RSC Adv., 2, 8262 (2012). Figure 1