Motional Resistance Evaluation of the Quartz Crystal Microbalance to

Article pubs.acs.org/Langmuir

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 Alejandro Cuenca, Jerónimo Agrisuelas,* Raquel Catalán, José J. García-Jareño, and Francisco Vicente Department of Physical Chemistry, University of Valencia, C/Dr. Moliner, 50, 46100 Burjassot, Spain 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, SO42− 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.

1. INTRODUCTION Hyphenated techniques based on spectroscopic and electrochemical methods lead to new perspectives, strengthening the knowledge and facilitating the characterization of interesting materials and, in particular, the interfacial region of electrode| solution systems. The quartz crystal microbalance (QCM) has been employed for chemical, physical, and biological sensing applications taking advantage of the resonant wave generated across the crystal thickness at the excitation frequency of piezoelectric quartz.1−3 Usefully, if a quartz crystal resonator is the working electrode, instantaneous values of the resonance frequency (f r) and motional resistance (Rm) can be obtained using a quartz crystal microbalance with motional resistance monitoring (QCM-R). From f r increments (Δf r), mass variations (Δm) associated with the electrochemical process can be obtained using the Sauerbrey equation4 Δfr = −

2fq 2 A ρq μq

of the coating provides information about their viscoelastic nature in a similar way that seismic waves are used to understand the interior structure of the earth. Mechanical/ structural and viscoelastic transformations of adsorbed coatings can be extracted when the quartz crystal resonator is modeled as an electrical equivalent circuit, known as a Butterworth−Van Dyke (BVD) equivalent circuit, and the lumped elements are fitted to experimental admittance−frequency (Y−f r) curves.5,6 A BVD equivalent circuit consists of two branches that represent a static capacitance (Cs) in parallel with a motional (or resonant) branch with the motional inductance, Lm, the motional capacitance, Cm, and the motional resistance, Rm, in series. The complete electrical admittance of the loaded quartz resonator is Y (ω) = jωCp +

Δm

1 R m + jωLm +

1 jω C m

= jωCp +

(2)

(1)

where ω = 2πf r is the angular frequency, j = (−1) , and Zm is the motional impedance. 1/2

where fq is the fundamental resonance frequency of quartz in air and A is the piezoelectrically active area. ρq and μq are the density and the shear modulus of quartz, respectively. On the other hand, the acoustic energy of the wave inserted in adsorbed coatings on resonant electrodes involves mechanical deformations. The resonant or acoustic response © 2015 American Chemical Society

1 Zm(ω)

Received: June 18, 2015 Revised: August 17, 2015 Published: August 19, 2015 9655

DOI: 10.1021/acs.langmuir.5b02233 Langmuir 2015, 31, 9655−9664

Article

Langmuir

The electrochemistry of copper is used as a model with broad technological applications.29−31 In particular, the dissolution of copper and their alloys have generated much scientific interest because of corrosion and important practical applications such as electropolishing.32−38 Taking advantage of the fact that motional resistance provides information about the electrode| solution interface,39 we expect the evolution of passive layers to be reflected in changes in motional resistance during copper dissolution. In this work, we discuss the evolution of motional resistance during copper dissolution in diluted sulfuric aqueous solution. For that, a copper layer was electrodeposited on a gold|quartz resonator electrode of QCM-R. The first aim of this work was to understand motional resistance changes during forced experimental conditions. Copper dissolution was induced by a potentiodynamic technique (linear voltammetry) hyphenated with QCM-R and vis−NIR spectroscopy. In this manner, we have simultaneous information on a number of time-dependent magnitudes as transferred charge (Q), mass from f r (m), molecular absorbance between 350 and 1100 nm (Aλ), and motional resistance (Rm). The second aim of this work was to investigate copper corrosion in a more natural acidic medium by QCM-R with the support of electrochemical impedance spectra (EIS) at open circuit potential. In both situations, the analysis of Rm evolution evidenced novel and interesting aspects of the dissolution process of copper in dilute sulfuric solutions.

Each element of the electroacoustic model has a relationship among the mechanical properties of the three-layer (quartz| coating film|solution) resonator.7,8 However, Zm(ω) is not simply the sum of those for the individual layers for such a composite system.9 According to the viscoelastic model of Granstaff and Martin,10 Zm(ω) can be modeled by considering the characteristics of the quartz and of the solution constants such as Zm(ω) =

hq 2 2

4eq A

Zf +

Zs + Zf tanh(θf hf ) Zf + Zs tanh(θf hf )

(3)

where h is the layer thickness, ρ is the density of quartz (q), the coating film (f), or the solution (s), eq is the quartz piezoelectric constant, Zs is the acoustic impedance of the solution, and Zf = (ρfGf)1/2 is the acoustical impedance of the coating film. θf = (ρf/Gf)1/2 is the complex propagation constant of the coating film where Gf = G′f + jG″f is the complex shear modulus of the coating film, G′f is the storage modulus, and G″f is considered to be the loss modulus. Despite Rm being an electric magnitude, it can be interpreted by an electromechanical analogy.7 Rm is related to the energy loss of acoustic waves generated from piezoelectric electrode resonance. The energy loss depends on internal frictions and mechanical losses in the mounting system.11 Therefore, Rm also has a relationship among the mechanical properties of the three-layer resonator, and it can be expressed as7,8,12 2

Rm = +

π ηq hq 8K 2c66ξ22A hq 2 2

4eq A

+

2. EXPERIMENTAL SECTION

hq 2 ⎛ ω 2ρf 3/2 hf 2Gf′ ⎞ ⎟ ⎜ 4eq 2A ⎜⎝ (Gf″)2 − (Gf′)2 ⎟⎠

2.1. Materials. CuSO4 (99%) and H2SO4 (96%) were used. All chemicals were reagent grade and were used as received from Scharlau. High-purity water was obtained from a Millipore Milli-Q Simplicity water-purification system. 2.2. QCM-R Calibration. Before the experiments, QCM-R (RQCM, MAXTEK Inc.) was calibrated by means of galvanostatic copper electrodeposition by applying −2 mA for 150 s.36 The calibration solution was 0.5 M CuSO4 and 0.1 M H2SO4 (pH 1.92), which gave an experimental Sauerbrey constant equal to 8.29 × 108 Hz g−1 cm2. This value is close to 9.35 × 108 Hz g−1 cm2, the theoretical Sauerbrey constant extracted from eq 1. 2.3. Copper Deposition. Copper was deposited on 29.58 mm2 gold-plated quartz commercial electrodes (6 MHz AT cut quartz crystal, Matel-Fordahl, France) which were used as working electrodes. The geometric surface of working electrodes was measured by image analysis after digitalizing the electrode surface. A Pt filament was the counter electrode. Both electrodes were placed in a typical twoelectrode electrochemical cell and immersed in a 0.5 M CuSO4/0.1 M H2SO4 aqueous solution (pH 1.92). Then, copper was deposited galvanostatically on the working electrode by applying −2 mA for 150 or 1000 s by means of an AUTOLAB potentiostat-galvanostat (PGSTAT302). The current efficiency of copper deposition was calculated from the consumed charge during the experimental time (150 or 1000 s). Then, the expected mass from consumed charge and the real mass from QCM-R data were compared. The current efficiency of copper deposition was always higher than 97% at both experimental times. 2.4. Linear Vis−NIR Spectroelectrogravimetry. Copper dissolution was controlled by linear voltammetry by means of the AUTOLAB potentiostat−galvanostat. The three-electrode electrochemical cell was a 2 × 2 cm2 high-transmittance glass cell from Hellma OG quality equipped with a Pt wire and a Ag|AgCl|KCl(sat) (RE-1C, Bas Inc., Japan) reference electrode. All potentials given in this paper are referred to the Ag|AgCl|KCl(sat) reference electrode (+0.197 V vs SHE at 298 K). The modified working electrode with a copper layer deposited during 150 s was immersed in a 0.1 M H2SO4 aqueous (pH 0.94) solution. The potential ranged from −0.2 to 0.8 V at 10 mV s−1. Ar was bubbled into the cell for 5 min, and an inert atmosphere was maintained during the experiment. The cell

ωρη s s 2

(4)

where η is the viscosity, K is the electromechanical coupling factor for no-loss quartz, c66 is the piezoelectrically stiffened elastic constant for no-loss quartz, ξ22 is the quartz permittivity, and eq is the piezoelectric stress constant for quartz. QCM-R takes advantage that Rm is the real part of motional impedance extracted from the acoustic impedance of the BVD circuit. Thus, Rm can be analogically obtained without fitting the acoustic impedance spectra to the BVD circuit as in network analyzers. The main advantage of QCM-R is that Rm is acquired fast enough to be analyzed with other analogical signals such current, absorbance, and mass. Rm has been used to obtain viscoelastic information on materials deposited on the QCM resonator and the interfaces.12−19 Metal electrodissolution processes tend to accumulate some hydrated metal cations and electrolyte anions on the electrode surface.20,21 The anion- or cation-selective character will be a function of the electrolyte composition. Anions of large size and charge, such as SO42−, favors the development of cation-selective interfacial structure, whereas highly mobile and/or relative small charge, such as Cl− and NO3− anions, favors the development of anion-selective interfacial structures.20,22−28 In the first case, the transition from an active metal surface to a passive metal surface by means of the precipitation of more or less hydrated hydroxides is favored. In the second case, the salt layer precipitation is relatively favored. In all cases, the degree of passivation and composition of the interfacial region depends on the solubility of the salts and hydroxides. 9656


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Langmuir temperature was stabilized at 298 K with Peltier modules controlled by a homemade thermostat circuit. A light beam from a balanced deuterium, halogen light source (Ocean Optics DH-2000-BAL) was reflected on the working electrode surface. During the linear voltammetry, 43 vis−NIR absorbance spectra per second were collected via a PC with a diode array Maya2000 portable fiber optic spectrophotometer (Ocean Optics) between 350 and 1100 nm. The color response reflected on the diode array from the QCM-R electrode was received as the number of counts at each wavelength, nλcounts, which is directly converted into light intensity at a given wavelength, Iλ. In this work, an apparent time derivative of absorbance at a specific wavelength (Aλ) is calculated for comparing with mass and electrical charge changes by means of Lambert−Beer’s law

⎛ Iλ ⎞ Aλ = − log⎜⎜ λ ⎟⎟ = −log(I λ) + log(I0λ) ⎝ I0 ⎠

(5)

d( −log(I λ) + log(I0λ)) d(ln(I λ)) dAλ 1 dI λ = =− =− dt dt ln(10)dt 2.303I λ dt (6) where Iλ0 is the light intensity of the light source. Since the intensity of reflected light, Iλ, is proportional to the number of counts, nλcounts, received by the diode at this wavelength, it can be written as λ dncounts dAλ 1 dI λ 1 =− = − λ dt dt 2.303I λ dt 2.303ncounts

(7)

2.5. Electrochemical Impedance Spectroscopy at Open Circuit Potential. The modified working electrode with a copper layer deposited during 1000 s was immersed in 0.1 M H2SO4 aqueous (pH 0.94) solution during 40 h. During the experimental time, the resonance frequency, potential, current, and motional resistance were monitored by means of the QCM-R. EIS measurements were carried out repeatedly with the AUTOLAB potentiostat−galvanostat at open circuit potential. The amplitude of the sinusoidal potential perturbation was 10 mV rms in a frequency range from 65 kHz to 10 mHz with five points per decade. A typical three-electrode electrochemical cell was used. The counter electrode was the Pt filament, and the reference electrode was Ag|AgCl|KCl(sat) (RE-1C, Bas Inc., Japan). To avoid the diffusion of chloride ions from the reference electrode to the electrolyte, the reference electrode separated from the solution by only a salt bridge was placed in the electrochemical cell when the impedance measurements were carried out. Experimental impedance data were fitted to the proposed equivalent circuit by means of a nonlinear least-squares procedure based on the quasi-Newton algorithm for function optimization. A stream of Ar was bubbled for 5 min when the electrochemical cell was assembled, and afterwards an Ar atmosphere was maintained during the whole time of the experiment (40 h) to minimize the impact of oxygen on copper dissolution. The temperature of the cell was stabilized at 298 K with Peltier modules controlled by a homemade thermostat circuit.

Figure 1. Current density and mass variation (A) and dm/dt and ΔRm (B) during the linear sweep voltammetry of copper in a 0.1 M H2SO4 aqueous solution (pH 0.94) at a scan rate of 10 mV s−1.

electrochemical dissolution of copper involves a decrease in deposited mass on the resonator electrode of 0.33 mg cm−2 to achieve the bare electrode. The thickness of the copper layer, about 390 nm, was estimated by taking into account the mass loss and the copper density (8.91 g cm−3).40 A bare resonator electrode could be sensitive to the surrounding medium at a distance of about 200 nm.8,17 Taking into account that the Au| Cu layers behave as a rigid layer coupling the acoustic wave of the resonator, Rm can give information to approximately 590 nm from the gold surface. This distance will be reduced as the copper layer diminishes. A bare gold resonator in 0.1 M H2SO4 aqueous solution has an Rm value of 290 Ω. This value was considered to be a reference to calculate ΔRm in all figures. Figure 1b shows the evolution of the time derivative of mass (dm/dt) and ΔRm during the copper electrodissolution. The copper layer on a resonator electrode involves an increase in ΔRm with respect to a bare electrode of about 65 Ω. Consequently, we can expect a reduction in ΔRm as the layer of copper diminishes. Before this electrochemical process, ΔRm shows the expected value for a copper layer on a resonator from −0.2 to 0 V. The onset potential where dm/dt decreases caused by the

3. RESULTS AND DISCUSSION 3.1. Potential-Induced Copper Dissolution. Linear voltammetry was employed to induce the electrodissolution of the copper layer on a resonator electrode in a 0.1 M H2SO4 aqueous solution. Under these controlled experimental conditions, we pretend to understand the meaning of Rm changes by taking advantage of simultaneous data provided by the vis−NIR spectroelectrogravimetry hyphenated technique. Figure 1a shows the voltammetric and electrogravimetric behavior during the electrochemical process. As expected, the loss of mass is associated with the evolution of current. The 9657


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Langmuir electrodissolution of copper is about 0.05 V. However, the electrochemical process causes an increase in ΔRm. Later, the peak potential of copper electrodissolution takes place at 0.385 V (Figure 1a), whereas the peak potential of dm/dt is at 0.44 V. Despite this, ΔRm reaches the highest value at 0.46 V. Finally, ΔRm has values close to the bare resonator electrode (0 Ω) when the copper electrodissolution is finished. To elucidate the meaning of ΔRm variations during the copper electrodissolution, current density, mass, and vis−NIR spectroscopic signals are simultaneously analyzed. Figure 2

Figure 2. Three-dimensional surface of the dAλ/dt−mass−current density−potential−time−wavelength for the linear sweep voltammetry of a coated copper electrode in 0.1 M H2SO4 aqueous solution (pH 0.94) at a scan rate of 10 mV s−1.

Figure 3. Mass vs charge plot (A) and F(dm/dQ) profile (B) obtained during the linear sweep voltammetry of copper in a 0.1 M H2SO4 aqueous solution (pH 0.94) at a scan rate of 10 mV s−1.

shows the spectroscopic signal from 350 to 1100 nm during the induced electrochemical reactions. Between 600 and 1100 nm, dA/dt increases from 0.2 to 0.4 V. On the contrary, dA/dt decreases between 350 and 600 nm in the same potential range. The spectroscopic signal changes when the amount of Cu(II) by the electrodissolution of copper is enough to allow the formation of blue hydrated species close to the surface electrode.41 This fact is in accordance with the differences between mass and ΔRm. The onset potentials where mass decreases and current density increases are similar (Figure 1a). However, ΔRm involves significant changes around 0.2 V, like the spectroscopic signal in Figure 2. From 0.4 V to more anodic potentials, the changes in dA/dt over the whole spectrum are due to changes in light reflected on the copper surface to gold surface and the disappearance of previously formed species on the electrode surface. The effect of this change in the electrode surface on ΔRm will be thoroughly explained below. Figure 3a shows the charge dependence of the dissolved mass during copper electrodissolution. We can observe two linear tendencies. Considering Faraday’s law, the first slope corresponds to −26 g mol−1. This value is close to the expected values for copper electrodissolution involving two-electron transfer (−32 g mol−1). The second slope corresponds to −54

g mol−1. Therefore, entities heavier than Cu2+ are unstacked from the surface electrode. A better understanding of the nature of the electrochemical processes may be achieved by the instantaneous mass/charge ratio (F(dm/dQ)) evolution on the potential scale. F(dm/dQ) gives instantaneous information about the mechanism of electrochemical reactions involving changes in mass on the electrode surface considering Faraday’s law42 ⎛ dm /dt ⎞ ⎛ dm ⎞ F⎜ ⎟= ⎟ = F⎜ ⎝ dQ ⎠ ⎝ j ⎠

∑ νi

δiMi ni

(8)

where Mi is the molar mass of species i, which are involved in the electrochemical process to balance ni electrons, and νi represents a percentage (per unit) of the charge balanced by the participation of these species. δi is −1 when the species are removed from the surface during oxidation involving a mass decrease. On the contrary, δi is +1 when the species are deposited on the surface during oxidation involving a mass increase. In the simplest reactive system involving one electron and one monovalent species, the absolute value of F(dm/dQ) coincides with Mi throughout the potential scan. However, F(dm/dQ) can change throughout the potential scan depend9658


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Langmuir ing on νi for each participating species in complex electrochemical systems.43 In Figure 3b, F(dm/dQ) shows slightly lower values than those expected for the dissolution of copper to Cu2+ at 0.33 V (dashed line in the figure marks −32 g mol−1) as in Figure 3a. This fact points to a simultaneous adsorption of anions from solution. Two anions could be adsorbed, SO42− and OH−. Considering eq 8 −23 g mol−1 = νCu2+

−64 g mol−1 +Manion + νanion 2 2

(9)

If SO42− is the anion involved, then νSO42− would be 0.11 whereas νOH− would be 0.22 with OH− participation. From these results, we cannot discard any anion during this process, as will be discussed later. However, it is possible that OH− adsorption is preferred to form the passive layer. Cu(II) and anion accumulation is supported by changes in dA/dt around 0.3 V in Figure 4a, where dA400 nm/dt, dA500 nm/ dt, and dA600 nm/dt were selected as significant visible wavelengths for studying the color changes on the electrode surface. At 400 nm, the decreases in dA/dt is due to the vanishing of the copper color on the surface and the emerging color due to the species attached to the electrode surface as confirmed by the increasing dA600 nm/dt. At 500 nm, dA/dt increases due to the formation of an intermediate situation on the copper surface before 0.3 V.41 After this potential, this situation disappears because dA500 nm/dt becomes negative. At this point, it is interesting to introduce the concept of electrochromic efficiency at any wavelength per unit area (F(dA/dq))44,45 ⎛ dA ⎞ ⎛ dA /dt ⎞ ε F⎜ ⎟ = F⎜ ⎟= ⎝ j ⎠ ni ⎝ dq ⎠

(10)

where Δε is the increment of molar absorptivity between the reduced species and the oxidized species; consequently, Δε can be positive or negative. Constant values of F(dA/dq) throughout a potential range mean that only one species on an electrode surface is formed during the electrochemical process. This fact indicates that all oxidation charge involves the change in absorbance at the corresponding wavelength. Figure 4b shows the evolution of F(dA/dq) at 400, 500, and 600 nm. In general, F(dA/dq) points to a mixture of species formed on the copper electrode. However, F(dA500 nm/dq) is constant between 0.23 and 0.28 V, and F(dA400 nm/dq) is between 0.28 and 0.41 V. Therefore, two different species are formed on a copper surface during the electrodissolution of copper before complete dissolution around 0.46. At 0.46 V, dA600 nm/dt shows a negative peak, which exactly matches the potential peak of ΔRm. The increase in the density and viscosity of the interfacial region could be the cause of the highest value of ΔRm. A gelatinous layer of Cu(H2O)4(OH)2 should be the more stable compound formed because of the relative low solubility of this compound (Ksp = 2.2 × 10−20).40 The accumulation of Cu2+ on the copper surface attracts an increasing amount of OH−. Consequently, this fact involves local changes in pH on the copper surface.22,27,46 Under these circumstances, copper islands on a gold surface can be formed, causing both the tridimensional dissolution of copper and the enrichment of Cu(H2O)4(OH)2 in solution trapped between islands.14,47,48 Accordingly, the absorbance at 600 nm allows us to study the evolution of Cu(H2O)4(OH)2 in a passive layer.

Figure 4. dAλ/dt at 400, 500, and 600 nm (A) and F(dAλ/dq) at the same wavelengths (B), with ΔRm obtained during the linear sweep voltammetry of copper in a 0.1 M H2SO4 aqueous solution (pH 0.94) at a scan rate of 10 mV s−1.

Later, F(dm/dQ) reaches values close to −100 g mol−1 at 0.48 V. Around this potential, the electrodissolution of the last layer of copper takes place as corroborated by the changes in dA/dt over the whole spectrum due to the change in light reflected on the copper surface and on the gold surface. Consequently, −100 g mol −1 could be due to the detachment of Cu(H2O)4(OH)2 entities from the gold surface.41 At 0.484 V in Figure 4a, dA500 nm/dt shows a negative peak which could be due to the disappearance of color provided by hexaaquacopper(II) ions ([Cu(H2O)6]2+) close to the bare copper surface. [Cu(H2O)6]2+ formation is favored by the presence of SO42− in the acidic solution because CuSO4 has a higher solubility than Cu(H2O)4(OH)2.40 This hypothesis is supported by the evolution of dA500 nm/dt at the beginning of copper electrodissolution. Before the formation of the Cu(H2O)4(OH)2 layer, it is expected that [Cu(H2O)6]2+ is formed close to the bare copper surface because of the absence of sufficient OH−. This could explain the increase in dA500 nm/dt around 0.25 V. Consequently, the negative values of dA500 nm/ dt around 0.35 should be due to the formation of the Cu(H2O)4(OH)2 layer on the copper surface which involves a color change from the previous color provided by blue 9659


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Langmuir

Figure 5. Model of resonator electrode during the electrodissolution of copper (a) at the moment when there is not any electrochemical process, (b) at the moment of formation of the Cu(II) complex on the surface, (c) at the moment of copper island formation, and (d) at the moment of complete electrodissolution of copper.

[Cu(H2O)6]2+ on the bare copper surface. Both processes take place simultaneously because F(dm/dQ) shows values of around −23 g mol−1. Finally, Rm decreases as the sites occupied by copper and the passive layer are dissolved, reaching values of Rm corresponding to the bare electrode at more anodic potentials. The existence of traces of copper is supported by the values of F(dm/dQ) obtained after the dissolution of the last layer of copper, again close to −32 g mol−1. The different values of ΔRm before and after the dissolution of copper are due to the mass variation of copper deposited on the resonator attending the second term of eq 4. On the other hand, the third term of this equation explains the evolution of ΔRm depending on the interfacial region of the electrode. ΔRm provides information about the energy loss of acoustic waves in the normal direction from the piezoelectric quartz of the resonance electrode. A part of this energy loss is due to the copper layer deposited on the gold surface. The other one depends on the viscoelastic properties of the interfacial region, or in other words, it depends on the structure and composition of the interface. Figure 5 depicts the evolution of the copper|solution interface during the potential-induced electrodissolution where the following mechanism is proposed. In the first step (Figure 5a), Cu(s) − 2e− → Cu(II)(s)

The passive layer is placed between [Cu(H2O)6]2+ close to the copper surface and the copper surface (Figure 5c). This fact occurs between 0.28 and 0.41 V, where F(dA400 nm/dq) is constant because the passive layer completely hides the copper color. Equations 12 and 13 should be a competitive mechanism in the first stage. Later, the accumulation of Cu(H2O)4(OH)2 precipitate hinders the progress of eq 12 (Figure 5d). In Figure 5e, the maximum accumulation of Cu(H2O)4(OH)2 takes place when the copper surface is irregular enough to trap a passive layer between the islands of copper at 0.46 V, where Rm is higher. Trapping of the passive layer on the irregular copper surface affects Rm.48 However, this phenomenon could affect the apparent mass changes registered by QCM-R during copper electrodissolution. To evaluate this effect, |df r/dRm| can be calculated. Approximately 10 Hz Ω−1 is the characteristic |df r/ dRm| ratio of a net density/viscosity effect.8 The larger the absolute value of |df r/dRm|, the weaker the density/viscosity effect. Therefore, we expect a |df r/dRm| ratio close to 10 Hz Ω−1 if a viscous and dense passive layer affects the gravimetric measurements. Under these experimental conditions, we obtain values of between 50 and 430 Hz Ω−1 during the transition between the copper-coated electrode and the bare gold electrode (from 0 to 0.5 V). Thus, the resonance frequency changes recorded by QCM-R during the electrodissolution of copper are caused only by a real mass change.49 Moreover, these results point to a smooth copper surface because the values of |df r/dRm| are really far from 10 Hz Ω−1 but irregular enough to be detected by changes in Rm. Finally (Figure 5f), the passive layer is dissolved by the presence of protons from the acidic solution

(11)

where Cu(II) is accumulated on the copper surface around 0.05 V. Cu(II) accumulation does not affect Rm. Taking into account the characteristics of experimental conditions, the charge balance is achieved by the approximation of SO42− from solution (Figure 5b). This allows the formation of only [Cu(H2O)6]2+ close to the copper surface between 0.23 and 0.28 V, in accordance with a constant range of F(dA500 nm/ dq) in Figure 4b.

Cu(H 2O)4 (OH)2 ( ↓ ) + 2H+ + 2H 2O → [Cu(H 2O)6 ]2 + (aq) + 2H 2O

(14)

and the detachment of Cu(H2O)4(OH)2 entities from the gold surface between 0.55 and 0.8 V. 3.2. Copper Corrosion. The analysis of vis−NIR spectroelectrogravimetry data in the previous section has allowed us to relate ΔRm variations to the formation of Cu(H2O)4(OH)2 at the copper|solution interface. This information will be extrapolated to understand the passivation process of a copper layer immersed in 0.1 M H2SO4 aqueous solution. For that, 2.3 mg cm−2 of copper was deposited on a resonator electrode corresponding to a thickness of 2.6 μm. As expected, the deposited mass on the resonator electrode

Cu(II)(s) + SO4 2 −(aq) → CuSO4 CuSO4 + 6H 2O → [Cu(H 2O)6 ]2 + (aq) + SO4 2 −(aq) (12)

Rm increases moderately in this potential range. At the same time, Cu(II) is balanced by OH− to form the passive layer: Cu(II)(s) + 2OH−(aq) + 4H 2O → Cu(H 2O)4 (OH)2 ( ↓ ) (13) 9660


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Langmuir decreases owing to the dissolution of copper (Figure 6). After 40 h, the resonator recorded about 10% mass loss (0.25 mg

Figure 6. Mass variation during the copper corrosion in a 0.1 M H2SO4 aqueous solution (pH 0.94). The circles shown in this figure correspond to the measurements of the EIS.

cm−2). This result indicates that the dissolution of copper is, in general, a slow process despite the fast dissolution observed during the early hours of the experiment. At different times (circles in Figure 6), we have acquired electrochemical impedance spectra at open circuit potential to obtain information about the electrochemical processes. Figure 7 shows the results in the form of experimental Nyquist plots. In the high-frequency range, the capacitive semicircles show a similar flattening independently of experimental time. On the contrary, EIS data prove clear differences among the measurements in the low-frequency range. The angle formed between the real and imaginary parts of the impedance plot decreases from 45° with time. This fact points to diffusive behavior in the low-frequency range in EIS spectra at short times and their dependence on the structural changes of the interfacial region. As expected in metal corrosion,46,50 the chemical dissolution of copper takes place together with the self-formation of a passive layer during the time of the experiments. A Randles modified equivalent circuit included in Figure 7a was used to evaluate the electrochemical impedance.51 In the equivalent circuit, Ru is the uncompensated resistance associated with the electrolyte and the electrode contacts, Rct is the charge-transfer resistance, ZTL is the impedance of the transmission line, and CPEdl is assigned to the double-layer capacitance (ZCPEdl = 1/ Ae(jω)αdl with exponent αdl and pre-exponential factor Ae). However, other models also can explain our results; this equivalent circuit was chosen because the diffusion process can be modeled as a transmission line element (TL). TL allows us to obtain some information about the influence of the interfacial changes on the kinetics. The impedance of TL (ZTL) owing to the transport through a layer of finite thickness can be expressed as27,28 Z TL = σ

tanh

Figure 7. Nyquist plots of a coated copper electrode in 0.1 M H2SO4 aqueous solution (pH 0.94) at 250 s and 2 and 3 h (A) and 20 and 40 h (B). The electrode was immersed in the solution for 40 h and in an inert atmosphere (Ar). The amplitude of the sinusoidal potential perturbation was 10 mV rms. The frequency range was from 65 kHz to 10 mHz with five points per decade. The temperature of the cell was 298 K. In (A), it is the equivalent electric circuit considered for modeling the passive layer formed on the copper surface at open circuit potential. Ru is the uncompensated resistance, Rct is associated with the charge-transfer resistance, CPEdl is assigned to the doublelayer capacitance, and TL is a transmission line.

lim Z TL =

ω→∞

(16)

where σ/τD1/2 is the Warburg coefficient. This equation is mathematically equivalent to a CPE with a 0.5 exponent. The exponent of CPEdl associated with the interface electrode|solution was fixed at 0.85 to obtain the best fittings (R2 = 0.99). Therefore, it confirms this element as the doublelayer capacitance. As can be seen in Table 1, values of the regression parameters of other circuit elements depend on the exposition time of the electrode with the surrounding environment. At early corrosion stages, the angle formed between the real and imaginary parts of the impedance plot is close to 45° which is characteristic of net diffusion behavior because the surface of copper is free of Cu(H2O)4(OH)2. In this case, σ/τD1/2 should be 28.8 Ω cm2 s−0.5, which matches the

jωτD jωτD

σ 1−j τD 2ω

(15)

At high frequencies, eq 15 is approximated as 9661


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Langmuir Table 1. Values of Regression Parameters Obtained from the Fit of the Equivalent Circuit in Figure 7a to Experimental EIS during Copper Corrosion, Where αdl Was Always Fixed at 0.85a t (h)

Ru (Ω cm2)

CPEdl (μF cm−2 s−0.85)

0.07 2 3 20 40

3.3 3.3 3.3 3.3 3.3

14.3 13.5 13.9 14.2 21.3

Rct (Ω cm2) 48 73 106 215 208

σ/τD1/2 (Ω cm2 s−0.5) 28 39 52 129 126

Ei (mV)

Ef (mV)

32.2 26.4 23.2 20.9 21.8

31.3 25.7 22.4 20.0 21.2

a

Ru is the uncompensated for resistance, CPEdl is assigned to the double-layer capacitance, Rct is associated with the charge-transfer resistance, and σ/τD1/2 is the Warburg coefficient. Ei and Ef are open circuit potentials at the beginning and at the end of the impedance spectrum, respectively.

experimental value in Table 1 at 250 s (28 Ω cm2 s−0.5). After some time, this transport is more affected by the formation of a thin passive layer. Charge-transfer resistance and σ/τD1/2 increases to about 200 Ω cm2 and about 130 Ω cm2 s−0.5, respectively. First, it is important to note that this spontaneous process for the formation of a passive layer is not caused by the modulated electrical perturbation of each impedance experience since the open circuit potential changes less than 1 mV during the impedance experiment (Ei and Ef in Table 1). However, the open circuit potential over time changes owing to the transformation of the like-first electrode to a like-second electrode type, reversible to the anion. At the pH of the experiences, it is expected that the ion sulfate facilitates the formation of [Cu(H2O)6]2+ in the interfacial region (eq 12). However, the relatively low solubility of Cu(H2O)4(OH)2 should be the more stable compound formed on the electrode surface despite its continuous dissolution in the sulfuric aciddiluted medium (eq 14). The impedance spectra obtained from 20 h (Figure 7b) reflect at low frequencies the slowing down of the process of dissolution of the metal by the formation of the passive layer. The evolution of Rct in Table 1 supports this assumption. EIS results show the simplistic behavior of copper corrosion such as copper dissolution and passive layer formation, which hinders copper dissolution. However, the evolution of the copper|solution interface during the experiment is more complicated as revealed by ΔRm from acoustic impedance in Figure 8. To evaluate the changes in the copper|solution interface, we chose a period where there was not any perturbation. This period is between the EIS measurements at 3 and 20 h. Figure 8 and the inset show a detailed evolution of ΔRm during this period. In general, the accumulation of Cu(II) owing to the dissolution of copper together with OH− to form Cu(H2O)4(OH)2 on the copper surface causes the growth of the passive layer. The increase in viscosity and density in the interface involves the increase in ΔRm because the acoustic wave finds more difficulties in crossing the passive layer (Figure 8). An electrode of the pseudofirst kind whose equilibrium potential is a function of the concentration of the cation of the electrode metal in the solution evolves to an electrode of the pseudosecond kind with the equilibrium potential being a function of the concentration of anion in the solution. We found more difficulties in explaining the oscillating values of ΔRm. During approximately 100−200 s, ΔRm increases, and

Figure 8. Motional resistance variation during copper corrosion in a 0.1 M H2SO4 aqueous solution (pH 0.94).

then ΔRm takes a similar amount of time to recover the original value (inset of Figure 8). Moreover, the amplitude of changes increases as the passive layer grows (Figure 8). In chemical oscillating reactions, parallel reactions synthesize the same species. This fact could be the cause of oscillating values of ΔRm. Cu2+ in solution could be formed from the chemical oxidation of copper and the dissolution of Cu(H2O)4(OH)2 formed on the copper surface involving local changes in pH.22,27,46 Accordingly, the increases/decreases in ΔRm could be caused by the formation/dissolution of a passive layer on the electrode surface under this experimental condition (eqs 13 and 14, respectively). From these results, we can propose the following mechanism. The slow copper dissolution progressively accumulates Cu(H2O)4(OH)2 on the electrode surface. In this moment, the formation of the passive layer overwhelms the dissolution. At a certain point, the accumulation of enough Cu(H2O)4(OH)2 on the copper surface stops the copper corrosion. Consequently, acid solution favors passive layer dissolution. When Cu(H2O)4(OH)2 is low enough, copper corrosion starts up again and the cycle repeats. This mechanism could also take place during the potential-induced dissolution of copper in Figure 1b when ΔRm oscillates between 0.3 and 0.4 V. The slow voltammetric scan rate could favor this taking place.

4. CONCLUSIONS From the data analysis of vis−NIR spectroscopy and the electrochemical quartz crystal microbalance with motional resistance monitoring, we have proven that the measurement of motional resistance is useful in evaluating the formation of a passive layer during the dissolution of copper in an acidic medium. For that, a simultaneous analysis of current, mass, and absorbance was necessary as complementary measurements during the forced electrodissolution of copper. We can conclude that an increase in motional resistance means an increase in viscosity at the electrode|solution interface owing to the formation of a viscous passive layer. On the contrary, a motional resistance decrease is due to the dissolution of the passive layer carried out by the acidic nature of the solution. The results support that the accumulation of Cu(II) in the interfacial region enhances the formation of a passive layer on the electrode surface by the anion enrichment of the interfacial 9662


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Langmuir region. The implied anions in this process are SO42− and OH−. On the one hand, SO42− favors the conversion from Cu(II) on the electrode surface to solvated Cu2+ in solution ([Cu(H2O)6]2+). Soluble [Cu(H2O)6]2+ diffuses into solution when there is not a passive layer as corroborated by impedance data. On the other hand, Cu(H2O)4(OH)2 precipitates on the electrode surface when the proportion of Cu2+ in the interfacial region is great enough. The existence of a passive layer on the electrode surface hinders the transformation of Cu(II) formed on the electrode surface to [Cu(H2O)6]2+ in solution. Therefore, the passive layer reduces the dissolution rate on time as demonstrated by impedance analysis. In a corrosive environment, the slow dissolution of copper is hindered by the formation of a passive layer. However, Cu(H2O)4(OH)2 in the passive layer is not stable in acidic media, and the basic layer on the electrode reacts with protons in solution; consequently, this layer disappears. During copper corrosion in acidic media, an oscillatory reaction between the formation and dissolution of the passive layer takes place and is detected only by the oscillatory response of motional resistance. Although this work focuses on copper dissolution, vis−NIR spectroscopy hyphenated with electrochemical quartz crystal microbalance with motional resistance monitoring is a suitable technique for investigating the evolution of a passive layer in metal corrosion and other interfacial processes such as catalysis and the adsorption of (bio)molecules. Therefore, it suggests that there is a considerable volume of underexploited materials and experimental conditions further available.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was supported by CICyT project FEDERCTQ2011-28973/BQU. REFERENCES

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Motional Resistance Evaluation of the Quartz Crystal Microbalance to

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