Influence of the Surface Roughness of Platinum Electrodes on the Calibration of the Electrochemical Quartz-Crystal Nanobalance.

Abstract

The electrochemical quartz-crystal nanobalance (EQCN) is an in situ technique that measures mass changes (Δm) associated with interfacial phenomena. Analysis of Δm sheds light on the mass balance (in addition to the charge and energy balances) and provides new insight into the nature of electrochemical processes. 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). The value of Cf is determined by physical parameters of the crystal and refers to an atomically smooth surface. However, real resonators are not smooth and electrodes have their intrinsic roughness. Thus, the conversion of Δf to Δm should be done using an experimentally determined characteristic constant (Cf,exp) for a given value of the surface roughness factor (R). Here, we calibrate the system using Ag electrodeposition on Pt electrodes of gradually increasing R; the latter is adjusted through Pt electrodeposition. The surface morphology of the Pt substrates prior to and after Ag electrodeposition is examined using atomic force microscopy. The values of Cf,exp are determined by analyzing the slopes of charge density versus Δf plots for the Ag electrodeposition. They are different than Cf and increase logarithmically with R. The Cf and Cf,exp values are used in a comparative analysis of the mass changes (δΔm) for complete cyclic voltammetry profiles covering the 0.05-1.40 V range. This reveals that the employment of Cf instead of Cf,exp provides inaccurate values of δΔm, and the magnitude of the discrepancy increases with R.