Motional Resistance Evaluation of the Quartz Crystal Microbalance to Study the Formation of a Passive Layer in the Interfacial Region of a Copper|Diluted Sulfuric Solution

Abstract

A hyphenated technique based on vis–NIR spectroscopy and electrochemical quartz crystal microbalance with motional resistance monitoring was employed to investigate the dissolution of copper in acid media. Changes in motional resistance, current, mass, and absorbance during copper dissolution allow the evolution of the interfacial region of copper|diluted sulfuric solution to be understood. In particular, motional resistance is presented in this work as a useful tool to observe the evolution of the passive layer at the interface. During the forced copper electrodissolution in sulfuric solution, SO4(2–) favors the formation of soluble [Cu(H2O)6]2+. On the contrary, OH– involves the formation of Cu(H2O)4(OH)2, which precipitates on the electrode surface. The high viscosity and density of Cu(H2O)4(OH)2 formed on surface causes an increase in motional resistance independently of resonance frequency changes. During the copper corrosion in a more natural acidic environment, the results of electrochemical impedance spectra at open circuit potential indicate that corrosion is controlled by the diffusion of copper to the solution at short experimental times. However, copper diffusion is hindered by the formation of a passive layer on the electrode surface at long experimental times. During the copper corrosion, motional resistance shows an oscillatory response because of an oscillatory formation/dissolution of the passive later. Vis–NIR spectroscopy and electrochemical quartz crystal microbalance with motional resistance monitoring give new perspectives for reaching a deep understanding of metal corrosion processes and, in a future, other interfacial processes such as the catalysis or adsorption of (bio)molecules.

Oscillatory Changes of the Heterogeneous Reactive Layer Detected with the Motional Resistance during the Galvanostatic Deposition of Copper in Sulfuric Solution

“…The high viscosity and density of Cu(H 2 O) 4 (OH) 2 formed in the heterogeneous reactive layer causes an increase in ΔR m (Figure 7b). 25 Simultaneously, the presence of protons close to the copper surface produces the chemical dissolution of Cu(H 2 O) 4 (OH) 2 ,…”

“…R m has provided valuable information about the evolution of the hydrated passive layer at the interfacial region of copper|diluted sulfuric solutions during copper dissolution when it was analyzed with other analogical signals such as current, absorbance, or mass. 25 In general, EQCM is employed as a gravimetric sensor to evaluate changes in the mass caused by electrochemical reactions. The resonance frequency increments (Δf r ) of the quartz|gold electrode are associated with the increments of mass per unit area (Δm) through the Sauerbrey equation: 26 ρ μ…”

“…Perusing the bibliography, we find some calibration methods involving silver deposition, 27,28 copper deposition, 29 or iodine monolayer deposition. 30 One calibration method with excellent results is the galvanostatic deposition of metallic copper from a 0.5 M CuSO 4 /0.1 M H 2 SO 4 aqueous solution at a constant current density of −8 mA cm −2 during 150 s. 25 In this work, we focus our attention on the evolution of the electrode|solution interface throughout R m changes during the galvanostatic copper deposition in the above-mentioned calibration solution. On one hand, we aim to add valuable information about the compositional changes of the liquid loading on the electrode surface.…”

“…If vis-NIR spectroscopy is assembled together with the EQCM-R, the evolution of electromagnetic responses informs about the accumulation of colored species on electrode surface. R m has provided valuable information about the evolution of the hydrated passive layer at the interfacial region of copper|diluted sulfuric solutions during copper dissolution when it was analyzed with other analogical signals such as current, absorbance, or mass …”

“…Perusing the bibliography, we find some calibration methods involving silver deposition, , copper deposition, or iodine monolayer deposition . One calibration method with excellent results is the galvanostatic deposition of metallic copper from a 0.5 M CuSO 4 /0.1 M H 2 SO 4 aqueous solution at a constant current density of −8 mA cm –2 during 150 s …”