Electrochemical Scanning Tunneling Microscopic Observations of Pt Nanostructures Prepared by Ultra-High Vacuum Deposition Methods
Carbon-supported Pt (Pt/C) is widely used as electrocatalysts for polymer electrolyte fuel cells (PEFC). However, in highly acidic environments under applying electro-potentials, e.g., PEFC operating conditions , catalytic activity of Pt/C is gradually degraded due to aggregation and dissolution of Pt nanoparticles as well as corrosion of carbon support. Therefore, electrochemical (EC) structural stability of Pt nanoparticles on carbon support is an important subject for practical application of the Pt/C catalyst. However, in-situ, real-time observation of degradation process of the practical Pt/C catalyst is generally difficult. In this study, we prepared two types of model nanostructures of Pt to investigate dynamic behavior in EC environments. One is Pt nanoparticles on highly-oriented pyrolytic graphite (HOPG) (Pt/HOPG) prepared by arc-plasma deposition (APD) method. Another is molecular beam epitaxially (MBE) grown Pt monolayer on Au(111) (Pt/Au(111)). In-situ observations for both Pt nanostructures by using electrochemical scanning tunnelling microscopy (EC-STM) are conducted under applying electro-potentials.
Sample preparations: All samples were fabricated in ultra-high vacuum (UHV). Mass of Pt was estimated by a quartz microbalance installed in deposition chambers.
(1)APD-Pt/HOPG：Pt nanoparticles were deposited by an arc-plasma gun on 773K-HOPG substrate at arc-voltage of 100V.
(2)MBE-Pt1ML/Au(111)：1ML thick Pt epitaxial layer was formed on Au(111) at room temperature by MBE .
EC-STM observations: EC-STM observations were conducted in 0.1M HClO4. A tip (Pt-Ir) was approached to the sample surfaces at applying potential of Ework=+0.4V (vs. RHE). Then, the potential was increased up to +1.0V and then decreased back to +0.4V. EC-STM images were collected at fixed several voltages during the potential sweep. EC-STM observations were also conducted at +0.4V after applying potential cycles between +0.6V and + 1.2V. Applied potentials are presented with respected to RHE.
Results and discussion
Fig.1 shows EC-STM images for Pt nanoparticles (0.404μg/cm2) on HOPG recorded at specific voltages. At +0.4V (a), The Pt nanoparticles were highly oriented on HOPG. The Pt nanoparticles diffused on the substrate with increasing applied voltages, and at +1.0V (b), roughness in height increased relative to the image recorded at +0.4V, revealing that EC-oxidation of the Pt nanoparticles by the positive potential sweep. After negative potential swept from +1.0V to +0.4V (c), a mean particle size was slightly increased. The result suggests that reduction of the oxidized Pt nanoparticles formed by the +1.0V potential sweep correlates with aggregation of the Pt nanoparticles.
This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
 Y.Shao-Horn, et. al., Top Catal., 46, (2007) 285.
 Y. Iijima et al., J. Electroanal. Chem., 685, (2012) 79.