Surface Extraction Process During Initial Oxidation of Pt(111): Effect of Hydrophilic/Hydrophobic Cations in Alkaline...

The surface oxidation states of the metal electrodes affect the activity, selectivity, and stability of the electrocatalysts. Oxide formation and reduction on such electrodes must be comprehensively understood to achieve next-generation electrocatalysts with outstanding performance and stability. Herein, the initial electrochemical oxidation of Pt(111) in alkaline media containing hydrophilic and hydrophobic cations is investigated by X-ray crystal truncation rod (CTR) scattering, infrared (IR) spectroscopy, and nanoparticle-based surface-enhanced Raman spectroscopy (SERS). Structural determination using X-ray CTR revealed surface buckling and Pt extraction at the initial stage of surface oxidation, depending on the cationic species. Vibrational spectroscopy is performed to identify the potential- and cation-dependent formation of three oxide species (IR-active OH_ad, Raman-active OH_ad/O_ad(H₂O), and Raman-active O_ad). Hydrophilic alkali metal cations (Li⁺) inhibit surface roughening via irreversible oxide formation. Hydrophilic Li⁺ can strongly stabilize IR-active OH_ad, hindering the extraction of Pt surface atoms. Interestingly, bulky hydrophobic cations such as tetramethylammonium (TMA⁺) cation also reduce the extent of irreversible oxidation despite the absence of IR-active OH_ad. Hydrophobic TMA⁺ inhibits the formation of Raman-active OH_ad/O_ad(H₂O) associated with Pt extraction. In contrast, the moderate hydrophilicity of K⁺ has no protective effect against irreversible oxidation. Moderate hydrophilicity enables the coadsorption of Raman-active OH_ad/O_ad(H₂O) and Raman-active O_ad. The electrostatic repulsion between Raman-active OH_ad/O_ad(H₂O) and neighboring Raman-active O_ad promotes Pt extraction. These results provide insights into controlling the surface structures of electrocatalysts using cationic species during the oxide formation and reduction processes.