Electrochemical-STM Analysis of Platinum-Complex Adstructures and Platinum Nanoparticles on Au(hkl) Electrode Surfaces for Oxygen Reduction Reaction

Abstract

One of the major goals of surface science researches is to clarify atomic scale characteristics of the structures of electrode surfaces during reaction processes, since many electrochemical reactions on electrode surfaces are structure-sensitive [1]. Correlational studies between surface structure and catalytic activity typically employ two approaches, the researches based on (1) single-crystal surfaces with steps and (2) nanoparticles supported on electrodes. The electrocatalytic properties of nanoparticles, especially Pt, have been studied to gain fundamental knowledge on the behavior of electrocatalysts. The development of Pt nanoparticles with appropriate activities, which is tailored as a function of parameters such as the structure of catalyst surfaces, the particle size and shape of the catalyst, and the nature of the substrate, is important for technological application of the electrocatalysts. We previously reported the electrodeposition of Pt from spontaneously adsorbed Pt complexes with various halide ligands on Au(111) [2]. However, the effect of substrate structures on adlayer formation and deposition of spontaneously adsorbed Pt complexes is still obscure, and a systematic investigation of the interplay of deposition conditions, Pt adlayer morphology, and electrocatalytic behavior of electro-reactions such as oxygen reduction reaction (ORR) is required.In this study, we observed the structures and electrochemical deposition of Pt complex adlayers, PtCl4 2- and PtBr4 2- on Au(100), (111), and (110) surfaces, by using electrochemical scanning tunneling microscopy (EC-STM) in 0.1 M HClO4 solution, and evaluated the catalytic activities for ORR on the Pt nanoparticles-modified Au electrodes [3].Single-crystal Au electrodes were prepared by the method of Clavilier et al. as substrates. Adlayers of K2PtCl4 were prepared by immersing the Au electrodes for 1 minute into a 100 μM K2PtCl4 + 0.1 M HClO4 solution. Subsequently, the sample was washed with water, and then immersed in 0.1 M HClO4, following the previously report [2]. Electrochemical measurements were carried out using an HAB-151A potentiostat (Hokuto Denko) and EC-STM images were obtained with a Nanoscope E (Digital Instruments [currently Bruker Co.]). The STM tip was made from a Pt/Ir wire, and the tips were then coated with a transparent nail polish to minimize the faradic current.Figure 1 shows the cyclic voltammograms for the deposition of the K2PtCl4 adlayer in 0.1 M HClO4 on Au(100). The potential was first swept negatively from 0.95 V, and a large reduction peak due to the reduction of adsorbed PtCl4 2- adlayer and deposition of Pt nanoparticles was observed at 0.72 V only in the first scan. The result reveals the formation of a stable PtCl4 2- monolayer on Au(100), deposition of Pt(0) from the Pt complexes, and no remaining Pt complexes in the electrolyte solution. The estimated coverage of PtCl4 2- on Au(100) calculated from the charge density of a reduction peak, ca. 46 μC cm-2, is 0.12. After Pt deposition, a broad redox peak corresponding to hydrogen adsorption and desorption on Pt appeared around 0.10 ~ 0.30 V, which was not observed on bare Au electrodes.The high-resolution STM image in Fig. 2(a) shows the adlayer structure of PtCl4 2- as bright pinwheel-like spots on the Au(100) electrodes observed by EC-STM at 0.95 V. The structure of the ordered PtCl4 2- adlayer exhibited a square arrangement with a nearest-neighbor Pt complexes distance of 0.63 nm, which corresponds to √5 times the Au-Au distance. Our result suggested that the PtCl4 2- adlayer on Au(100) formed a (√5 × √5)R26.7° structure, whose model with a unit cell is depicted in Fig. 2(b). The coverage of PtCl4 2- calculated from the model is 0.20, and the coverage is higher than that calculated from CV measurements because of the co-adsorption of chloride anions originated from the coordinated chloride of the PtCl4 2- complexes.Then, we stepped the electrode potential to 0.40 V and observed the Pt nanoparticles deposited on Au(100) formed from PtCl4 2 adlayer. Figure 3 shows an STM image of the Pt deposited Au(100) surface, where island-like spots with 0.2 ~ 0.5 nm in height appeared and were distributed randomly on terraces and step edges. Pit-like defects were also observed on the terrace because of surface etching by chloride ion derived from the PtCl4 2- complex.In our talk, we will introduce the results of ORR on the Pt nanoparticles-deposited Au(100) surface and compare to that on Pt deposited Au(111) and Au(110) surfaces.