Pt-based bimetallic nanoparticles (NPs) have been considered to be promising materials as electrocatalysts for improving the performance of fuel cells such as a direct methanol fuel cell because of decreasing CO poisoning on Pt surface and enhancing reactivity of Pt via a ligand effect. The compositional control of electrocatalyst surface is of significant importance to effectively utilize noble metals and to improve the electrocatalytic activities, because electrochemical reactions only occurs on the surface of particle catalysts. In our previous study, we have reported a facile method to synthesize noble metal NPs by metal sputter deposition on room temperature ionic liquids (RTILs) under vacuum condition.⁽¹⁾ This RTIL/metal sputtering technique was also useful to prepare core-shell-structured particles of Au@In₂O₃ and Pt@In₂O₃, which were prepared by the successive sputter deposition of noble metals and Indium into RTILs. Recently we have reported that a monolayer of Au nanoparticles was formed on the RTIL surface with single step deposition process when Au sputter deposition was carried out onto the surface of RTILs containing hydroxyl-functionalized cations.⁽²⁾In this study, we report the synthesis of Au core-AuPt alloy shell NPs monolayer by the sequential sputter deposition of Au and Pt onto hydroxyl-functionalized RTILs. The surface coverage of Pt atoms on individual Au NPs could be controlled with variation of Pt sputtering time.

 Original Au NPs monolayer was prepared by the sputter deposition of Au on the surface of 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate (HyEMI-BF4) used as a RTIL with a discharge current of 10 mA under argon pressure of 20 Pa for 150 s. The subsequent sputter deposition of Pt was carried out on thus-obtained Au NPs monolayer. The sputtering condition used for Pt deposition was the same as that used for Au deposition except for sputtering time.

The sputter deposition of Au changed the color of RTIL surface from colorless to reddish blue due to the Au film formation. By subsequent sputter deposition of Pt, the film on the solution surface tuned brownish. Extinction spectra of films showed an LSPR peak at 540 nm, the intensity of which was lessened with an increase in the Pt sputtering time and finally vanished after Pt sputter deposition for 80 min or more. This result suggested that the sputter deposition of Pt onto Au film caused surface modification of individual Au NPs. From TEM measurement, it was found that the films prepared by sputter deposition of Au and Pt were monolayers composed of spherical particles, the size of which increased with elapse of Pt sputtering time from 4.2 to 4.8 nm. Since pure Pt particles were not observed in the film, it was suggested that sputtered Pt species were simply deposited on individual Au particles floated on the RTIL surface without additional nucleation. The XPS measurement revealed that deposited Pt formed the surface layer composed of Au-Pt alloy on Au cores.

The cyclic voltammograms of Au-Pt bimetallic NPs monolayers transferred on HOPG electrodes were measured in an N₂-saturated 0.5 mol dm^-3 H₂SO₄aqueous solution, in which a cathodic peak assigned to the reduction of Au oxide monolayer appeared at 1.2 V vs. RHE along with a pair of redox peaks attributable to the adsorption/desorption of hydrogen on Pt surface at ca. 0.02-0.04 V vs. RHE. The electrochemical surface areas (ECSA) of Au and Pt were obtained by measuring charges flowed in reduction of Au oxide monolayer and in hydrogen desorption on Pt surface, respectively. The fraction of Pt ECSA was higher than the Pt content in particles, regardless of Pt sputtering time, indicating that Pt atoms were predominantly deposited on the surface of Au particles.

The methanol oxidation activity of the Au-Pt NPs monolayers was evaluated by a cyclic voltammetry in a 0.5 mol dm^-3 KOH aqueous solution containing 0.5 mol dm^-3methanol. The obtained voltammograms exhibited an anode peak at ca. 0.9 V vs. RHE, assignable to methanol oxidation (Fig. 1). The peak current intensity of methanol oxidation for Au-Pt bimetallic films was dependent on the Pt sputtering time but was larger than those of monometallic counterparts, indicating that the electrocatalytic activity for methanol oxidation was enhanced by the modification of Au NPs monolayer with AuPt alloy shell layer. The maximum intensity was obtained for the Au-Pt NPs film prepared with Pt sputtering time for 80 min.

References

1. T. Torimoto, et al., Appl. Phys. Lett., 89, 243117 (2006).
2. T. Torimoto, et al., Phys. Chem. Chem. Phys., 17, 13150 (2015).