Two-Dimensional Effects on the Oxygen Reduction Reaction and Irreversible Surface Oxidation of Metallic Ru Nanosheets and Nanoparticles

Abstract

Nanosheets have attracted increasing attention owing to their electrochemical properties. While the relationship between the activity and stability of metal nanoparticles has been widely reported, the activity−stability relationship of metallic nanosheets has not been characterized. Herein, we report on this relationship for Ru nanosheets and discuss its difference with the relationship for Ru nanoparticles. The oxygen reduction reaction activity of the Ru nanosheets was 130% higher than that of the nanoparticles, which was attributed to the larger electrochemically active surface area of the nanosheets. In addition, the activity of the nanosheets after potential cycling was ca. 40 times higher than that of the nanoparticles. Based on in situ X-ray absorption spectroscopic (XAS) measurements and X-ray absorption near-edge structure (XANES) spectra simulations by first-principles theoretical calculations, the average coordination number of the Ru atoms on the nanosheet surface was larger than that on the nanoparticle surface; thus, we concluded that this property contributed to the higher stability of the nanosheets. This finding provides supporting evidence for the cause of the high intrinsic activity and stability of metallic nanosheets.

Platinum nanosheets synthesized via topotactic reduction of single-layer platinum oxide nanosheets for electrocatalysis

“…In addition, improved stability of Pt catalysts has been reported for morphology-controlled catalysts 39 , 40 . Although most studies did not discuss the mechanism of the improved stability, we have recently reported the mechanism of the high oxidation tolerance of Ru NSs 29 . Based on the combination of in situ XAS measurements and XANES spectral simulations, the CN ave. of the Ru atoms on the surface of the nanosheet was found to be larger than that on the surface of the nanoparticle; thus, the Ru NSs showed higher oxidation tolerance.…”

“…The sample was immersed in a methanol/water solution to improve the wettability of the hydrophobic catalyst layer. A polychlorotrifluoroethylene-based in situ electrochemical cell was used for the in situ XAS analysis, as described in previous studies 29 , 45 . Pt wire and a RHE were used as the counter and reference electrodes, respectively.…”

“…We recently reported that metal nanosheets are more electrocatalytically active and stable than their nanoparticle equivalents 29 . Consequently, 2D Pt can be a potential candidate for a fuel cell electrocatalyst.…”

Synthesis and electrocatalytic performance of ultrathin noble metal nanosheets

“…Both Takimoto and Fukuda obtained Ru nanosheets by reducing RuO 2 , but their reduction methods slightly differed. Takimoto et al 73 prepared ultrathin nanosheets containing only two atomic layers by electrochemical reduction, obtaining nanosheets with a thickness of 0.6 nm (Fig. 10g and h).…”

“…Fig. 10 (a) TEM, (b) HRTEM, and (c) AFM images of Ru nanosheets; (d-f) TEM images of the intermediate products obtained at 1 h, 3 h, and 10 h, respectively; 72 (g) TEM and (h) AFM images of Ru nanosheets;73 (i) AFM image of Ru nanosheets 74. …”