Comprehensive Study of the Influence of Surface Roughness of Platinum Electrodes on their Interfacial Mass Changes as Examined Using the Electrochemical Quartz-Crystal Nonobalance

Abstract

The electrochemical quartz-crystal nanobalance (EQCN) is an electroanalytical technique used to measure in-situ minute mass changes (Δm) associated with interfacial electrochemical phenomena. The results acquired by combining the EQCN and electrochemical techniques provide a new insight into the nature of electrochemical processes and contribute to the understanding of their mechanisms at the atomic/molecular level. The EQCN has been used for over three decades; however, its limits of detection (LOD) and quantification (LOQ) have never been studied. We propose a methodology for determining the values of LOD and LOQ for Pt electrodes in aqueous H2SO4 solutions. The values of LOD and LOQ decrease as the concentration of the electrolyte decreases. The EQCN measures changes in frequency (Δf) of a quartz-crystal resonator, which are converted into Δm using the Sauerbrey equation containing the characteristic constant (Cf); Cf is determined by physical parameters of the quartz and refers to an atomically smooth surface. However, real electrodes are not atomically flat and it is unknown how surface roughness factor (R) affects Δm. We examine Δm associated with H adsorption/desorption and surface oxide formation/reduction on Pt electrodes. The value of R is fine-tuned through Pt electrodeposition and the surface morphology is examined using atomic force microscopy. The results reveal linear relationships between Δm and R, and their extrapolation to R = 1.00 leads to the determination of Δm for atomically flat polycrystalline Pt electrodes. The values of Δm for R = 1.00 are analyzed in terms of the number of electrolyte components interacting with each Pt surface atom. A modified Sauerbrey equation, which takes into account the surface roughness, is proposed. Also, we calibrate the system using Ag electrodeposition, and the experimental characteristic constant (Cf,exp) values are determined by analyzing the slopes of charge density versus Δf plots for the Ag electrodeposition. They are different than the value of Cf and increase logarithmically with R. The Cf and Cf,exp values are used in a comparative analysis of Δm for complete cyclic-voltammetry profiles. It reveals that the employment of Cf instead of Cf,exp provides inaccurate values of Δm, and the magnitude of discrepancy increases with R.