Electrochemical Quartz Crystal Microbalance in a Channel Flow Cell

J. Phys. Chem. C 2007, 111, 3669-3674

3669

Electrochemical Quartz Crystal Microbalance in a Channel Flow Cell: A Study of Copper Dissolution Clare M. Galvani,† Andrew Graydon,‡ D. Jason Riley,*,† and David York‡ School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, UK and Procter and Gamble Technical Centres Ltd., Newcastle Upon Tyne, NE12 9TS, UK ReceiVed: September 20, 2006; In Final Form: January 8, 2007

We describe how a quartz crystal microbalance (QCM) may be incorporated in a channel flow system (CFC). It is demonstrated that although deposition is nonuniform the increase in mass loading of the electrode can be related to the change in the resonance frequency of the system. The QCM-CFC is used to investigate the corrosion of copper in acidic sulfate solutions. It is demonstrated that the rate of corrosion in the system is limited by mass transport of oxygen at low volumetric flow rates and by the oxidation of an adsorbed copper(I) intermediate at high volumetric flow rates. Concentration profiles of reactants and products in the QCMCFC have been calculated, using the backward implicit finite difference method to solve the convectivediffusion equation and rate constants for the oxidation of the Cu(I) species determined

Introduction In this paper we describe how a quartz crystal microbalance (QCM) may be mounted in a channel flow cell (CFC) to allow interrogation of heterogeneous reaction mechanisms under conditions of well-defined nonuniform flow. It is demonstrated, using the electrodeposition of copper that despite the nonuniform accessibility of the interface and resultant nonuniform massloading of the crystal there is a linear relationship between the mass of material deposited and the change in the resonance frequency of the crystal. We illustrate the advantages of a QCM-CFC in heterogeneous reaction studies with reference to the dissolution of copper in sulfuric acid solutions. The novel methodology yields new insight into the mechanism of copper dissolution and backward implicit finite difference modeling of the reaction kinetics and hydrodynamics permits determination of the rate of oxidation of the Cu(I) intermediate to Cu(II). Below, we review the advantages of CFCs in studying hetrogenous reactions and describe how QCMs may be employed to monitor the progress of reactions. We then discuss proposed mechanisms for the dissolution of copper in sulfate solutions. Construction and calibration, using the electrodeposition of Cu2+, of a QCM-CFC is then detailed. Finally the results of studies of the dissolution of copper in sulfuric acid solutions using the QCM-CFC are discussed. Heterogeneous reactions involve not only interfacial chemistry but also the transport of reactants, products and intermediates to and from the interface. Thus to interrogate the kinetics of such reactions it is necessary to perform experiments under conditions in which both the transport of species to the interface can be easily varied and the local concentration of species accurately determined via solution of the convective-diffusion equation. Rotating-disk and wall-jet configurations are examples of hydrodynamic systems1 that can be employed for the study of heterogeneous processes. Both these systems are uniformly * Corresponding author. New address: Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ., UK. E-mail: [email protected]. † University of Bristol. ‡ Procter and Gamble Technical Centres Ltd.

accessible (i.e., the chemical environment and diffusion layer thickness are constant across the substrate surface). In contrast, in a channel flow cell the reacting surface is nonuniformly accessible, the surface concentration and hence the mass transport-limited flux of species varies down the length of the substrate.2-4 The solution at the upstream edge of the substrate contains predominantly reactants, while at the downstream edge solution-phase reaction products are also present. This nonuniformity in surface concentration means that CFC systems offers excellent discrimination into the mechanisms of heterogeneous reactions.5,6 The QCM is commonly employed to monitor the progress of heterogeneous reactions, being used to determine the rate of change of mass, with ng s-1 sensitivity at a reacting interface.7-11 Reactions that have been monitored using QCMs in stationary solution include corrosion,12 self-assembly,13,14 polymer hydration,15 DNA duplexing,16 and antibody recognition.17-19 Greater insight into some heterogeneous reactions has been achieved using QCMs in rotating-disk20-23 and wall-jet configurations.24 While QCMs have been incorporated into simple flow systems to facilitate changes of solution,25-27 the employment of gravimetric devices in laminar flow systems is more challenging. Here we report the fabrication and commissioning of a QCM-CFC and illustrate how it may be employed to gain new insight in to the dissolution of copper in sulfuric acid solutions at open circuit potentials. Copper is a technologically important material and its dissolution has been studied for many decades. Replacement of aluminum with copper in microelectronic interconnects has led to renewed interest in the electrodeposition and corrosion of this divalent metal.28 Electrochemical studies indicate that the dissolution of copper occurs via a two step electron-transfer process, the Bockris-Mattson mechanism:29-31 fast

Cu 98 Cu(I) + e slow

Cu(I) 98 Cu(II) + e

(1) (2)

For reaction in aqueous chloride solutions, reaction32 eq 1 is split into two steps: electron transfer to a copper atom to yield

10.1021/jp066173d CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007

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Galvani et al.

an adsorbed Cu(I) intermediate, followed by desorption to yield CuCl2-(aq). In noncomplexing sulfate solutions, two mechanisms have been advanced: (i) the oxidation of an adsorbed Cu(I) intermediate33,34 and (ii) the dissolution of the Cu(I) species and its subsequent oxidation.35 Rotating-ring-disk electrode measurements indicate that Cu(I) solution species may play an important role, while more recent electrogravimetric studies point to either parallel Cu(I) and Cu(II) dissolution or an adsorbed Cu(I) intermediate only. In an oxygenated sulfate solution, the cathodic reaction in copper corrosion is the reduction of oxygen. Both two33,34,36-38 and four35,39-42 electron reduction of oxygen at copper have been reported as

O2 + 2H+ + 2e f H2O2

(3)

O2 + 4H+ + 4e f 2H2O

(4)

For the open circuit dissolution of copper in oxygenated acidic solutions, Andersen proposed that in addition to the anodic and cathodic processes, oxygen may also react directly with the Cu(I) intermediate Figure 1. (a) Cross sectional and (b) top view of the QCM-CFC. +

2Cu(I) + 2H + O2 f 2Cu(II) + H2O2

(5)

4Cu(I) + 4H+ + O2 f 4Cu(II) + H2O

(6)

or

thus explaining why the dissolution of copper does not follow the mixed potential concept. Andersen33 and Zelinsky43,44 state that at an open circuit the dissolution of copper is limited by the mass transport of oxygen. Here we report studies of the corrosion of copper in sulfuric acid solution at open circuit potentials in the QCM-CFC. It is demonstrated that the rate of dissolution of copper is mass transport independent at high volumetric flow rates, indicating that the anodic process is rate limiting. Modeling of the dependence of the QCM signal on volumetric flow rate, using the backward implicit finite difference (BIFD) method,45,46 allows the rate constant for the electron-transfer step to be determined. Experimental Planar AT-cut, 2.5 cm diameter, blank 9 MHz crystals optimized for 25 °C were obtained from Testbourne Ltd. Square electrodes consisting of a Cr adhesion layer ca. 20 nm and an Au upper layer ca. 100 nm were vacuum-deposited on the front face (5 mm × 5 mm) and the rear face (7 mm × 7 mm) of the crystal. A channel of dimensions 16 mm × 8 mm × 0.5 mm, volume of 0.64 × 10-9 m3, was fabricated from Delrin. To ensure laminar flow across the surface of the microbalance, deeper inlet and outlet recesses were cut at each end of the flow channel. The flow cell was constructed by placing the quartz crystal above the channel and securing with a low melting-point wax. The completed cell was connected to a gravity fed flow system. The solution volumetric flow rate was controlled by placing capillaries of different diameters at the terminus of the flow system and altering the height of the reservoir relative to the exit point. The volumetric flow rates were determined by measuring the time required for a set volume of solution to flow through the system. The flow regime was such that the Reynolds numbers were low (in the range 0.5-50), and hence the flow was laminar at all volumetric flow rates. Frequency measurements were recorded using a Maxtek research quartz crystal

microbalance that applied an ac voltage of 125 mV to the piezoelectric crystal.47 A schematic of the QCM-CFC is shown in Figure 1. Electrochemical measurements were performed under potentiostatic control. During all electrochemical experiments, a three electrode system was used, the working electrode was the front electrode of the crystal (i.e., the electrode in contact with the electrolyte solution), the pseudoreference electrode employed was a gold wire at the inlet of the flow cell, and the counter electrode employed was a platinum foil at the outlet of the flow cell. Copper deposition was used to calibrate the system (see below). In the electrochemical QCM-CFC setup, copper was electrodeposited and the Faradaic current and change in resonance frequency monitored. The experiments were performed under hydrodynamic conditions using volumetric flow rates in the range 5 × 10-3 to 0.5 cm3 s-1. Solutions of 0.1 mol dm-3 H2SO4(aq) containing 2.5, 5, 7.5, or 10 mmol dm-3 CuSO4 were employed as the electrolyte. Electrodeposition was performed at constant potential, and the potential versus the pseudoreference was chosen such that the current flowing was limited by mass transport of Cu2+ ions to the electrode. The solutions were degassed in argon prior to deposition and the QCM apparatus held in an argon atmosphere. The dissolution of freshly deposited copper in oxygenated 0.1 mol dm-3 H2SO4(aq) solutions containing 2.5, 5, 7.5, or 10 mmol dm-3 CuSO4‚5H2O was studied using the QCM-CFC. The change in mass loading of the QCM was monitored as the solution flowed over the interface. Volumetric flow rates in the range 5 × 10-3 to 0.5 cm3 s-1 were employed. Results and Discussion In a QCM, the sensitivity to mass loading varies across an electrode surface.48,49 The local mass sensitivity for a square electrode on a rectangular AT cut quartz crystal (note the assupplied circular crystal is sealed to a rectangular duct) is discussed in ref 50. The sensitivity is greatest at the center but nonzero at the electrode edge. In a conventional QCM experiment, mass loading is assumed always to be uniform, and in a gravimetric analysis the mass sensitivity is averaged over the electrode area. In a CFC, deposition on the electrode is

Electrochemical QCM-CFC Study of CU Dissolution

Figure 2. The change in the resonance frequency of the QCM-CFC with time in an argon environment. The electrolyte employed was deaerated 0.1 mol dm-3 H2SO4(aq) containing 10 mmol dm-3 CuSO4. Each plot corresponds to a different electrolyte flow rate. In zone 1 the working electrode was at open circuit, in zone 2 a potential was applied to drive mass transport-limited electrodeposition of copper, and in zone 3 the electrode was again at open circuit.

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3671 dissolution of copper at the open circuit potential. In zone 2, for any given volumetric flow rate after a brief induction period when the potential is first applied the rate of change of ∆f is constant. The gradient, -d(∆f)/dt, increases with solution volumetric flow rate. In electrodeposition experiments, changes in resonance frequency arise due to increased mass loading of the electrode and changes in the viscosity of the solution near the interface.51 In a channel flow cell at steady state, the local current is constant, hence once the steady-state concentration profile is achieved the viscosity of the solution at the interface will be invariant. Therefore, the constant rate of change of ∆f during electrodepsoition arises from the nonuniform increase in mass loading of the crystal. In the laminar channel flow cell used in this work, the transport-limited current, iL, and the transport-limited rate of change of mass for copper electrodeposition based on Faraday’s law may be calculated assuming the Le´veˆque approximation2,6 (linearization of the parabolic flow profile near the channel walls) is valid.

iL ) -0.925nF[Cu2+]D2/3 Vf1/3 (h2d)-1/3 wxe2/3

(7)

and

(

)

d(∆m) dt

Figure 3. The flow-rate dependence of the limiting current and the rate of change of resonance frequency for the electrodeposition of copper from a deaerated 0.1 mol dm-3 H2SO4(aq) solution containing 10 mmol dm-3 CuSO4 in an argon environment.

nonuniform. Thus, prior to using the system to study dissolution it is necessary to establish how the resonance frequency of the system changes with mass loading. To achieve this, mass transport-limited electrodeposition of copper from an acidified copper sulfate solution under flow was considered. Typical plots of ∆f versus time, t, during copper electrodeposition for various volumetric flow rates are shown in Figure 2. The plot is split in to three time zones; in zone 1 (t < 0.5 min) the gold electrode is at open circuit, zone 2 (0.5 min < t < 1.5 min) corresponds to the period in which a constant potential at which the current is mass transport limited was applied to the working electrode, and zone 3 (t > 1.5 min) the copper-coated gold electrode is again at open circuit. The constant resonance frequency in zone 1 indicates that the QCM system is stable under the flow regimes employed. The slow increase in resonance frequency in zone 3 suggests that in the absence of oxygen there is a very slow

L

) 0.925[Cu2+]D2/3 Vf1/3 (h2d)-1/3 wxe2/3 M (8)

where n is the number of electrons transferred, F is Faraday’s constant, D is the diffusion coefficient of Cu2+, Vf is the volumetric flow rate of the solution, h is cell half-height, d is the width of the cell, w is the electrode width, xe is the electrode length, and M is the molar mass of copper. For the plating solution containing 10 mmol dm-3 CuSO4, the simultaneously measured mass transport-limited current and rate of change of ∆f with time for electrodeposition of copper are plotted against Vf1/3 in Figure 3. The linearity of the plot of the iL versus the cube root of the volumetric flow rate and the agreement between the theoretical (solid line) and experimental limiting currents (symbols) suggests that the Le´veˆque approximation is valid, confirms the cell dimensions stated above, and shows that the shear oscillation of the working electrode during deposition does not alter significantly the mass transport. The plot of (d∆f/dt) against Vf1/3 is also linear indicating that under the conditions employed the change in resonance frequency is directly proportional to the change in mass loading. Comparison of the gradients of the lines displayed in Figure 3 yields a sensitivity factor, constant of proportionality between ∆f and ∆m, for the QCM-CFC of -0.157 Hz ng-1 cm-2. All the studies of copper dissolution at the open circuit potential use the experimentally determined proportionality factor. Typical plots of ∆f versus t during copper dissolution are shown in Figure 4. At each solution volumetric flow rate, the rate of copper corrosion reaches a steady-state value. The rate of corrosion increases with solution volumetric flow rate, Figure 5. For all CuSO4 concentrations studied, the rate of change of mass versus the cube root of volumetric flow rate shows two distinct regions: at low volumetric flow rates the rate of copper dissolution is proportional to Vf1/3, while at high volumetric flow rates the rate of dissolution is independent of Vf (i.e., the rate of copper dissolution is mass transport controlled at low volumetric flow rates and under kinetic control at high volu-

3672 J. Phys. Chem. C, Vol. 111, No. 9, 2007

Galvani et al. the transport of oxygen to the interface and that two copper atoms react per oxygen molecule. Given that the final product of dissolution is Cu2+(aq), this stoichiometry indicates that the corrosion reaction is

2Cu + O2 + 4H+ f 2Cu2+ + 2H2O

Figure 4. The variation in the rate of copper dissolution with flow rate for a 0.1 mol dm-3 H2SO4(aq) 10 mmol dm-3 CuSO4 solution in a naturally aerated solution.

(10)

The observation (see Figure 5) that at high volumetric flow rates the rate of dissolution is totally independent of Vf points to the rate determining step of the chemical reaction being zerothorder with respect to the transport of any solution based species, suggesting the anodic reaction is rate determining. The BockrisMattson mechanism29 for copper dissolution, eqs 1 and 2, states that oxidation of the Cu(I) intermediate is the rate-determining step. The volumetric flow rate independence of the anodic reaction suggests the Cu(I) intermediate is adsorbed on the electrode surface in agreement with recent results reported by Gabrielli32 and co-workers. Thus the results indicate that the reaction mechanism for the corrosion of copper in oxygenated acidified sulfate solutions, containing a background of Cu2+ ions, is

Cu f Cu(I)ads + e

(11)

k0

Cu(I)ads 98 Cu2+(aq) +e

(12)

diffusion

O2 + 4H+ + 4e 98 H2O

(13)

where k0 is the zeroth-order rate constant for the oxidation of the adsorbed Cu(I) intermediate. To further verify this mechanism and obtain rate constants for the rate determining step of the reaction, the concentration profile of species and the rate of change of mass in the QCMCFC were determined using the BIFD method to solve the convective-diffusion equation. Detailed discussions of the application of the BIFD method to the CFC configuration can be found in refs 39-40. In the QCM-CFC, the rate of change of oxygen concentration is given by the expression

[

]

∂2[O2] d[O2] (y - h)2 ∂[O2] ) DO 2 V 1 0 dt ∂x ∂y2 (h)2

Figure 5. Dependence of the rate of copper dissolution on the flow rate of the electrolyte, naturally aerated 0.1 mol dm-3 H2SO4(aq) containing 2.5, 5.0, 7.5, or 10 mmol dm-3 CuSO4. The fits to the data, solid lines, were calculated using the BIFD method to solve the convective-diffusion equation assuming that the cathodic reaction is mass transport limiting and oxidation of an adsorbed Cu(I) intermediate is the rate-determining step for the anodic reaction.

metric flow rates). Using the Le´veˆque approximation, the mass transport-limited rate of copper dissolution is given by the expression

(

)

d(∆m) dt

L

) 0.925[A]DA2/3 Vf1/3 (h2d)-1/3 wxe2/3 aM (9)

where [A] is the concentration of the rate-limiting species, DA is the diffusion coefficient of A, and a is the number of copper atoms oxidized per A that reaches the interface. Fitting eq 9 to the low volumetric flow rate experimental data displayed in Figure 5 shows that the rate of copper dissolution is limited by

(14)

where the Cartesian co-ordinates are as defined in Figure 1 and V0 is the solution velocity at y ) h. For the QCM-CFC employed in these experiments, (i) the concentration of [O2] upstream of the electrode is constant, 2.47 × 10-4 mol dm-3, and equal to that in the system reservoir, (ii) there is no reaction at the upper wall of the channel, (∂[O2]/∂y)2H ) 0, and (iii) the surface concentration of oxygen is zero if the reaction is transport limited, DO2 (∂[O2]/∂t)y)0 < k0/2, otherwise DO2 (∂[O2]/∂t)y)0 ) k0/2. Descretisation of eq 14 on an x-y grid of dimension 500 × 500 allows the concentration profile of oxygen in the QCM-CFC to be computed subject to the stated boundary conditions. The d(∆m)/dt is proportional to the flux of oxygen to the surface for that region of the electrode in which the reaction is mass transport controlled and proportional to k0 for the electrode area over which the reaction is kinetically controlled. To fit the experimental data, k0 is the only variable pararmeter as all other parameters were taken from literature or measured independently. The solid lines in Figure 5 represent the minimum mean square difference fits of the calculated rate of change of mass to the experimental. In contrast to Andersen33 and Zelinsky,43,44 who performed their investigations under poorly defined hydrodynamic condi-

Electrochemical QCM-CFC Study of CU Dissolution

Figure 6. Variation of the calculated rate constant for Cu(I) oxidation with the concentration of Cu2+ ions in the 0.1 mol dm-3 H2SO4(aq) solution.

tions, the rate of copper corrosion was found to depend on the concentration of Cu2+ ions in solution. The values of k0 determined for the 0.1 mol dm-3 sulfuric acid solutions containing 2.5, 5, 7.5, and 10 mmol dm-3 CuSO4 were 1.42 × 10-9, 1.65 × 10-9, 1.84 × 10-9, and 2.47 × 10-9 mol cm-2 s-1, respectively. The value of k0 increases linearly with the concentration, as shown in Figure 6. An increase in k0, the rate of oxidation of the adsorbed copper(I) species, is consistent with the positive shift in the open circuit potential with concentration. Conclusions It has been demonstrated that it is possible to mount a QCM in a channel flow cell to enable the study of heterogeneous processes under defined hydrodynamic flow. Investigation of copper corrosion in aerated sulfuric acid solutions indicate that the cathodic reaction involves the reduction of oxygen to water, and the anodic reaction is a two-step process with a surface adsorbed Cu(I) intermediate. In stationary solutions, the rate of corrosion is limited by the mass transport of oxygen but under forced convection the oxidation of Cu(I) may be rate limiting. Solution of the convective-diffusion equation using the backward implicit finite difference method has allowed the determination of the rate constant for Cu(I) oxidation. The value of k0 increases with the concentration of Cu2+ ions in the aqueous phase. It is anticipated that given the nonuniform accessibility of the reacting interface of a QCM-CFC and the resultant improved discrimination of reaction mechanism the technique will be used to study a range of heterogeneous reaction processes. Acknowledgment. The authors are grateful to the EPSRC and the Chemistry Innovation KTN for financial support. References and Notes (1) Unwin, (2) 155. (3) 1-20.

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