Synthesis of Atomically Thin Pt Monolayer on HOPG

Abstract

High specific surface area is essential for Pt electrocatalyst. However, nanoparticles become less stable as the particle size decreases. Two-dimensional nanosheets are expected to have comparable or ever larger specific surface area than nanoparticles. Furthermore, nanosheets should be more stable than nanoparticles due to the two-dimensional bonding.  Synthesis of Pt nanosheets with 1 nm thickness has previously been reported1). However, non-reactive Pt atoms still exist in the core of nanosheets, thus the utilization of Pt may be low. The purpose of this research is the synthesis of Pt nanosheets with atomic-scale thickness (nanosheets with 1 or 2 Pt monolayers).  Highly oriented pyrolytic graphite (HOPG) was freshly cleaved using adhesive tape and then immersed into a diluted RuO2 nanosheet colloid for 2 minutes to coat RuO2 nanosheets on HOPG (RuO2 nanosheet/HOPG). Atomically thin Pt monolayer was deposited on the surface of RuO2 nanosheet/HOPG by vacuum evaporation2). Cyclic voltammetry was conducted to dissolve the RuO2nanosheets. The morphology and thickness were characterized by atomic force microscopy (AFM), and the chemical state of the nanosheets on HOPG was determined by X-ray photoelectron spectroscopy (XPS). Redox peaks at ~0.1, 0.65, 0.8 vs. RHE, which are characteristic of RuO2 nanosheets, disappeared after potential cycling between 0.05 and 1.55 V vs. RHE for 50 cycles, suggesting the dissolution of RuO2 nanosheets. AFM observation showed that the thickness of RuO2 nanosheets were 1.11±0.18 nm and the thickness of the as-deposited Pt on RuO2 nanosheets were 0.34±0.25 nm. Taking into account of the diameter of Pt atom as 0.28 nm, 1-2 monolayer of Pt was deposited on RuO2 nanosheets. The thickness of the nanosheet after cycling decreased to 0.40±0.08 nm, agreeing with the dissolution of the RuO2 support. The chemical state of Pt and Ru before and after electrochemical dissolution was studied by XPS. Pt 4f5/2 and Pt 4f7/2 peaks were observed before and after cycling, while Ru 3p1/2, Ru 3p3/2 peaks disappeared after cycling. Based on CV, AFM and XPS analysis, it is concluded that a Pt monolayer with atomic thickness was successfully prepared on HOPG by using RuO2 nanosheets as a structure directing scaffold and subsequent dissolution of RuO2nanosheets. 1) A. Funatsu, H. Tateishi, K. Hatakeyama, Y. Fukunaga, T. Taniguchi, M. Koinuma, H. Matsuura, and Y. Matsumoto, Chem. Commun., 50, 8503 (2014). 2) Q. Liu, C. Chauvin, and W. Sugimoto, J. Electrochem. Soc., 161, F360 (2014). Figure 1