In Situ Electrochemical Quartz Crystal Microbalance Study of Potential

Langmuir 1999, 15, 813-818

813

In Situ Electrochemical Quartz Crystal Microbalance Study of Potential Oscillations during the Electrodeposition of Cu/Cu2O Layered Nanostructures Eric W. Bohannan, Ling-Yuang Huang, F. Scott Miller, Mark G. Shumsky, and Jay A. Switzer* Department of Chemistry and Graduate Center for Materials Research, University of MissourisRolla, Rolla, Missouri 65409-1170 Received July 7, 1998. In Final Form: November 16, 1998

Application of a constant cathodic current to an electrode in an alkaline Cu(II) lactate solution results in oscillation of the electrode potential during the electrodeposition of copper/cuprous oxide layered nanostructures. The electrochemical quartz crystal microbalance (EQCM) is used for in situ phase analysis measurements of the nanoscale layers and the results are compared with bulk XRD measurements. The EQCM is also used to estimate the layer thicknesses and overall modulation wavelength of the nanostructures. We propose that Cu2O is deposited during the positive spikes in electrode potential, while a composite of Cu and Cu2O is deposited during the more negative plateau region of the oscillation. The modulation wavelength calculated from the EQCM is in good agreement with that observed with scanning electron microscopy. The EQCM is shown to be a useful tool for estimating layer thicknesses and phase compositions for layers that are too thin to be examined by other instrumental techniques.

Introduction Nanoscale materials are of interest in that the optical, electrical, chemical, and mechanical properties may be tuned by changing the size of the material. Our approach to the synthesis of nanoscale materials is electrodeposition. We have previously shown that compositional superlattices in the Pb-Tl-O system can be electrodeposited simply by pulsing the applied current or potential during deposition.1,2 We have also prepared defect-chemistry superlattices of thallium(III) oxide in which cation interstitials are favored during deposition at low overpotential and oxygen vacancies are favored at high overpotential.3 Our recent work has centered on the electrochemical self-assembly of copper/cuprous oxide layered nanostructures electrodeposited from alkaline copper(II) lactate solution. In this system, the electrode potential spontaneously oscillates during application of a constant cathodic current, resulting in nanoscale multilayers of Cu and Cu2O.4 The nanoscale Cu2O in the material exhibits the optical behavior of quantum dots, showing a blue-shift in the optical absorption edge.5 We have previously shown by Auger depth profiling and scanning tunneling microscopy that these materials are layered.6 The electrical properties of the material are highly anisotropic. Parallel

transport is rigorously ohmic and shows percolation behavior, while perpendicular transport exhibits negative differential resistance, due to resonant tunneling through the nanoscale Cu2O layers.6 The Cu2O layers in these films are estimated to be less than 3 nm thick, making them difficult to analyze by electron microscopic techniques, especially in terms of microanalysis of the phase composition. In the present work, we use the excellent sensitivity of the electrochemical quartz crystal microbalance (EQCM) as a mass detector to monitor in situ the electrodeposition of the Cu/Cu2O layered nanostructures. The EQCM is a versatile nanogravimetric device with detection limits in the sub-nanogram range.7 It has been used in a wide variety of applications such as studies of the underpotential deposition of metals,8 as a sensor for heavy metal ions,9 in studies of protein-DNA interaction,10 and recently in conjunction with the scanning electrochemical microscope.11 The piezoelectric device can be used in electrochemical cells as the working electrode, allowing the determination of minute mass changes by monitoring the frequency of the quartz oscillator. The Sauerbrey equation (eq 1), describes the

∆f )

-2fo2 ∆m (Fµ)1/2A

(1)

* To whom correspondence should be addressed. E-mail: [email protected].

relationship of mass and frequency at the EQCM surface, where ∆f is the measured frequency shift, fo is the resonant

(1) Switzer, J. A.; Shane, M. J.; Phillips, R. J. Science 1990, 247, 444-446. (2) Switzer, J. A.; Raffaelle, R. P.; Phillips, R. J.; Hung, C.-J.; Golden, T. D. Science 1992, 258, 1918-1921. (3) Switzer, J. A.; Hung, C.-J.; Breyfogle, B. E.; Shumsky, M. G.; Van Leeuwen, R.; Golden, T. D. Science 1994, 264, 1573-1576. (4) Switzer, J. A.; Hung, C.-J.; Huang, L.-Y.; Miller, F. S.; Zhou, Y.; Raub, E. R.; Shumsky, M. G.; Bohannan, E. W. J. Mater. Res. 1998, 13, 909-916. (5) Switzer, J. A.; Hung, C.-J.; Bohannan, E. W.; Shumsky, M. G.; Golden, T. D.; Van Aken, D. C. Adv. Mater. 1997, 9, 334-338.

(6) Switzer, J. A.; Hung, C.-J.; Huang, L.-Y.; Switzer, E. R.; Kammler, D. R.; Golden, T. D.; Bohannan, E. W. J. Am. Chem. Soc. 1998, 120, 3530-3531. (7) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355-1379. (8) Niece, B. K.; Gewirth, A. A. Langmuir 1997, 13, 6302-6309. (9) Ng, S. C.; Zhou, X. C.; Chen, Z. K.; Miao, P.; Chan, H. S. O.; Li, S. F. Y.; Fu, P. Langmuir 1998, 14, 1748-1752. (10) Okahata, Y.; Niikura, K.; Sugiura, Y.; Sawada, M.; Morii, T. Biochemistry 1998, 37, 5666-5672. (11) Cliffel, D. E.; Bard, A. J. Anal. Chem. 1998, 70, 1993-1998.

10.1021/la980825a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998

814 Langmuir, Vol. 15, No. 3, 1999

Bohannan et al.

frequency of the quartz crystal, ∆m is the mass change, F is the density of quartz (2.648 g/cm3), µ is the shear modulus of quartz (2.947 × 1011 dyn/cm2), and A is the piezoelectrically active area. In the present work we use the EQCM for in situ phase analysis of Cu/Cu2O layered nanostructures. The EQCM is also used to estimate the layer thickness of the multilayers as well as the overall modulation wavelength. Experimental Section Electrochemical Quartz Crystal Microbalance. Commercially available 9 MHz AT-cut quartz crystals were used as the working electrodes for all experiments (Seiko EG&G model QA-AM9-PT). The quartz crystals have a coating on both sides, consisting of a 500 Å Ti layer beneath a 3000 Å layer of Pt. The EQCM electrodes were used as received from the manufacturer. The quartz crystals were installed in a Teflon holder so that only one electrode face with an area of 0.20 cm2 was exposed to the deposition solution. A quartz crystal analyzer (Seiko EG&G model QCA917) was used in conjunction with a Nicolet Pro10 oscilloscope to monitor changes in frequency during experiments. A gate time of 0.1 s and output range of either (2 kHz/10 V or (20 kHz/10 V was used with the analyzer. Electrodeposition. Solutions were prepared with reagentgrade Cu(II) sulfate pentahydrate, 85+% lactic acid, and sodium hydroxide in HPLC-grade water purchased from Aldrich. All electrochemical experiments were performed with an EG&G model 273A potentiostat/galvanostat with output recorded on a Nicolet Pro10 oscilloscope or on a computer using EG&G model 270 electrochemistry software. Either Pt EQCM electrodes or large area (1.6 cm2) Pt electrodes served as the working electrodes in conjunction with a copper wire counter electrode. Potentials are reported versus the saturated calomel electrode (SCE). The solution was stirred with a magnetic stir plate, and a constant temperature of 30 °C was maintained with a Fisher model 9100 circulator. Instrumental Methods. X-ray diffraction (XRD) experiments were performed with a Scintag 2000 diffractometer using Cu KR radiation. Phase analysis of the randomly oriented films was obtained by Rietveld analysis with RIQAS software. Instrumental broadening was determined with a LaB6 powder standard (SRM 660) obtained from NIST. Scanning electron microscopy experiments were performed with a Hitachi S-4700 scanning electron microscope (SEM). The cleaved EQCM electrode was imaged at 15 kV in the backscattered electron mode.

Figure 1. X-ray diffraction pattern of a film grown to 10 C/cm2 at an applied current density of 1.0 mA/cm2. The working electrode was platinum (1.6 cm2) and the deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0. An asterisk indicates Pt substrate peaks.

Results and Discussion Phase Analysis Using the EQCM. Potential and current oscillations are observed quite frequently in electrochemical systems.12-15 The oscillations are often attributed to the adsorption and dissolution of a passivating film. Of particular interest in comparison to our system is the electrodissolution of copper in acetate buffer under potentiostatic conditions.16 In this system, Dewald et al. attribute the oscillations to the formation and dissolution of a basic copper acetate film followed by oxide passivation. Oscillations may also be observed during the electrodeposition of Zn and have been shown to result in a nanolaminated structure.17 In this case the oscillations (12) Hudson, J. L.; Tsotsis, T. T. Chem. Eng. Sci. 1994, 49, 14931572. (13) Basset, J. L.; Hudson, J. L. J. Phys. Chem. 1988, 92, 69636966. (14) Argoul, F.; Kuhn, A. J. Electroanal. Chem. 1993, 359, 81-96. (15) Matsuda, T.; Hommura, H., Mukouyama, Y.; Yae, S.; Nakato, Y. J. Electrochem. Soc. 1997, 144, 1988-1994. (16) Dewald, H. D.; Parmananda, P.; Rollins, R. W. J. Electrochem. Soc. 1993, 140, 1969-1973. (17) Yan, H.; Downes, J.; Boden, P. J.; Harris, S. J. J. Electrochem. Soc. 1996, 143, 1577-1583.

Figure 2. Potential oscillations observed at a series of current densities on 0.20 cm2 Pt EQCM electrodes. The deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0.

are attributed to the formation and subsequent reduction of a metal hydroxide layer at the electrode surface. We have previously shown by XRD that the phase composition of electrodeposited Cu/Cu2O films is strongly dependent on the applied current density.4,5 An XRD pattern for a multilayer film grown at 1.0 mA/cm2 is shown in Figure 1. Significant line broadening is observed for the copper and cuprous oxide peaks due to both particle size effects and residual strain. The phase composition of the films as measured by XRD can vary from pure Cu2O at an applied current density of 0.05 mA/cm2 to 89 mol % Cu at an applied current density of 1.5 mA/cm2. Here we compare bulk XRD measurements with in situ phase analysis measurements made with the EQCM. The initial potential oscillations observed at several current densities are shown in Figure 2. The oscillation period is relatively independent of the applied current

Electrodeposition of Cu/Cu2O Layered Nanostructures

Langmuir, Vol. 15, No. 3, 1999 815

Figure 3. Change in the frequency of the EQCM as a function of deposition time and applied current density. The working electrode was a 0.20 cm2 Pt electrode and the deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0.

density. The oscillations can persist for many hours in a stirred solution although there is a slow decrease in the amplitude of the oscillations as the deposition proceeds. An increase in the oscillation period is also observed for long deposition times, with the period increasing to a value approximately twice that of the initial oscillation period. The oscillation period is a strong function of pH, Cu(II) concentration, and temperature. Oscillations with periods of less than 1 s and greater than 300 s have been observed by varying these parameters. The EQCM was used to monitor the deposition in conjunction with the potential oscillations, and the output is shown in Figure 3. A decrease in the frequency of the quartz oscillator corresponds to an increase in mass added to the crystal. Assuming a uniformly distributed, rigidly elastic thin film,18 a decrease in the frequency of our quartz crystal of 1000 Hz corresponds to a mass increase of 1.1 µg. As the current density is increased, the mass increase occurs at a faster rate. The microbalance information can be used in conjunction with the applied current density to determine the phase composition of the film. Neglecting the complexation of copper by lactate ion, a simplified representation of the reactions occurring at the working electrode is shown below.

Cu2+ + 2e- ) Cu

(2)

2Cu2+ + 2e- + 2OH- ) Cu2O + H2O

(3)

In both eqs 2 and 3, two equivalents of charge must be passed in order to produce 1 mol of either Cu or Cu2O. The ratio of mass increase to equivalents of charge passed can be used for the in situ determination of the instantaneous phase composition of the material. Assuming 100% current efficiency, the ratio of mass to equivalents of charge passed for cuprous oxide is 71.6 g/equivalent. For copper the ratio is 31.8 g/equivalent. Our measured equivalent weight obtained from the EQCM is converted to a phase composition by finding the appropriate ratio of the calculated equivalent weight values of Cu and Cu2O. Figure 4 compares the bulk phase composition measurements made with XRD to those obtained with the EQCM. In both cases a decrease in the percentage of Cu2O is observed as the current density is increased. At a given current density, however, the EQCM consistently gives a higher percentage of Cu2O than that measured by XRD. There are several (18) Hillier, A. C.; Ward, M. J. Anal. Chem. 1992, 64, 2539-2554.

Figure 4. Phase composition as a function of current density measured by EQCM and XRD. The electrodes for the XRD measurements were 1.6 cm2 Pt electrodes and those for the EQCM measurements were 0.20 cm2 Pt electrodes. The deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0.

possible explanations for this discrepancy. The XRD patterns were obtained on thick films grown to a charge density of 10 C/cm2 whereas the films examined with the EQCM were much thinner. This is necessary as the EQCM becomes less accurate as a mass detector as the mass added increases, whereas the XRD measurements require a larger amount of material for accurate results. Therefore, any change that occurs during bulk deposition, such as the period increase or amplitude decrease, will be reflected in the XRD measurements and not the EQCM measurements. Second, we are using the EQCM as a mass monitor, although the frequency response can be affected by other factors, such as strain, surface roughness, or viscosity variations. Any decrease in the frequency of the quartz oscillator due to compressive strain, for example, will cause our calculated phase composition of cuprous oxide to be high. Strain in Cu2O is observed by X-ray line broadening and will be discussed in the next section. Finally, we are proposing that there are extremely thin layers (1-3 nm) of Cu2O sandwiched between thicker layers of a composite of both Cu and Cu2O. The thin Cu2O layers may be completely obscured in the XRD pattern due to line broadening. This will result in a phase composition calculated from the XRD pattern that underestimates the amount of Cu2O present in the sample. Layer Thickness Calculation with the EQCM. We have shown previously by SEM, scanning tunneling microscopy (STM), and Auger depth profiling that the potential oscillations observed during the growth of our films result in a layered structure. Here, we propose that the positive spike in the potential oscillations corresponds to the deposition of an extremely thin Cu2O layer. The more negative plateau region forms a thicker composite layer of Cu and Cu2O. The EQCM provides a powerful tool to estimate the layer thickness of these films without an ex situ measurement. Figure 5A shows the first oscillation observed in a pH 9.0 solution with an applied current density of 1.5 mA/cm2. The EQCM output for the same time period is shown in Figure 5B. An increase in slope is observed in conjunction with the positive spike in potential, which corresponds to deposition of a larger mass in this region. This feature is illustrated more clearly when the derivative of the frequency data is shown, as in Figure 5C, with the change in frequency response converted to an instantaneous equivalent weight of the material being deposited. Examination of Figure 5C shows two distinct

816 Langmuir, Vol. 15, No. 3, 1999

Bohannan et al.

Figure 7. Williamson-Hall plot for Cu2O obtained from a film grown at 0.63 mA/cm2. The deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0. Instrumental broadening was accounted for using a strain-free LaB6 powder standard. The measured particle size for Cu2O was 33 nm while the strain was found to be 0.35%. The (110), (111), (200), and (220) reflections were used in the analysis and are labeled on the plot. Linear regression gave a correlation coefficient of 0.97.

Figure 5. (A) Initial potential oscillation observed at an applied current density of 1.5 mA/cm2. (B) Frequency shift of the EQCM over the same time period. (C) Derivative of the frequency response of the EQCM. The dotted lines show the region of the thin Cu2O layer. The working electrode was a 0.20 cm2 Pt electrode and the deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0.

Figure 8. Phase composition of films grown under potential control as measured by XRD and the EQCM. The electrodes for the XRD measurements were 1.6 cm2 Pt electrodes and those for the EQCM measurements were 0.20 cm2 Pt electrodes. Films grown for XRD were grown to a thickness of 10 C/cm2. The deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 9.0.

Figure 6. Block diagram of proposed Cu/Cu2O layered nanostructure.

regions in which materials with different equivalent weight are being deposited. The negative plateau region of potential corresponds to a composite of Cu and Cu2O being deposited with an equivalent weight of approximately 47 g/equivalent. This corresponds to a phase composition of approximately 39 mol % Cu2O in the composite layer. The region corresponding to the positive spike in potential and spike in equivalent weight is enclosed within the dotted lines. We attribute this region to the deposition of a thin Cu2O film. We show a representation of the proposed nanostructure in Figure 6. The spike in equivalent weight reaches a maximum at 115 g/equivalent. Pure Cu2O should give an equivalent weight of 71.6 g/equivalent. This discrepancy increases

as the deposition time increases. It is unlikely that this discrepancy is due to the incorporation of lactate in the film as samples examined by Auger depth profiling showed less than 0.2% carbon present on a weight basis. We attribute this discrepancy to compressive strain induced in the Cu2O due to the large lattice mismatch between Cu and Cu2O.4 Evidence for strain is also seen by examining line broadening observed in the XRD patterns of the films. With a technique developed by Williamson and Hall,19 X-ray line broadening can be used to determine strain and particle size in a given sample. The Williamson-Hall method considers the contributions of particle size and strain in X-ray line broadening to be additive.

βtotal ) βparticle size + βstrain

(4)

Broadening due to small particle size is given by the Scherrer equation while the contribution of broad(19) Williamson, G. K.; Hall, W. H. Acta Metall. 1953, 1, 22-31.

Electrodeposition of Cu/Cu2O Layered Nanostructures

Langmuir, Vol. 15, No. 3, 1999 817

Table 1. Layer Thickness and Phase Composition for Cu/Cu2O Layered Nanostructures As Estimated by the EQCM current density (mA/cm2)

Cu2O thickness (nm)

composite thickness (nm)

composite phase comp. (mol % Cu2O)

modulation wavelength (nm)

total phase comp (mol % Cu2O)

0.25 0.5 1.0 1.5 2.0

0.4 0.7 1.4 1.9 2.5

5 6 11 19 20

89 47 44 39 23

5.4 6.7 12.4 20.9 22.5

90 51 48 43 27

Figure 9. (A) Potential oscillations observed on a 0.20 cm2 Pt EQCM electrode at an applied current density of 0.4 mA/cm2. The deposition solution was 0.6 M CuSO4, 3 M lactate at a pH of 8.2. (B) Frequency shift of the EQCM over the same time period. (C) Backscattered SEM image of a cross section of the sample. There were 14 oscillations during sample growth. A modulation wavelength of 75 nm is calculated from the EQCM data in agreement with the value of 80 nm observed by SEM.

ening due to strain is obtained by differentiating the Bragg law.20

βtotal )

0.94λ + 4 tan θ (∆d/d) t cos θ

(5)

Multiplying both sides of eq 5 by cos θ gives the final form.

βtotal cos θ )

0.94λ + 4 sin θ(∆d/d) t

(6)

The diffraction angle is given by θ, βtotal is the measured fwhm (in radians) corrected for instrumental broadening, λ is the X-ray wavelength of the source (Cu KR ) 0.154 nm), t is the particle size, and the strain is represented by ∆d/d. Figure 7 shows a plot of β cos θ versus sin θ for Cu2O diffraction lines from an electrodeposited Cu/Cu2O film grown at 0.63 mA/cm2. The y-intercept is used to calculate a particle size of 33 nm, while a strain of 0.35% is obtained from the slope of the line. The large particle size indicates that the measured broadening is probably due mainly to Cu2O crystallites in the thicker composite layer. The strain measured by XRD may also be present in our EQCM measurements, which would lead to a high value for the Cu2O content in the composite layer, as measured by the EQCM. In future work we hope to examine this system using BT-cut quartz crystals, which (20) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978; Chapter 9.

should allow us to separate strain-induced frequency changes from those that are caused by changes in mass to the EQCM electrode.21,22 Films grown under potential control were examined in the potential region where the transition from Cu2O to composite occurs during galvanostatic deposition. Figure 8 shows the results of phase analysis of these films performed with the EQCM and XRD. XRD shows that films grown at potentials more positive than -0.47 V vs SCE are highly oriented (100) Cu2O films. At potentials more negative than -0.47 V vs SCE a composite of Cu and Cu2O is obtained. The phase composition during potentiostatic deposition was also determined with the EQCM using the first 200 s of the current transient in conjunction with the change in resonant frequency of the EQCM. The same transition is observed with the EQCM at -0.49 V vs SCE. On examination of Figure 5 once again it is seen that the same qualitative transition is observed during galvanostatic deposition. Deposition at potentials more positive than approximately -0.5 V vs SCE are ascribed to the deposition of Cu2O while a composite of Cu and Cu2O is obtained when the potential is more negative then -0.5 V. The Cu2O and composite layer thicknesses can be determined using the phase composition data obtained with the EQCM in conjunction with Faraday’s law: (21) EerNisse, E. P. J. Appl. Phys. 1972, 43, 1330-1337. (22) EerNisse, E. P. J. Appl. Phys. 1973, 44, 4482-4485.

818 Langmuir, Vol. 15, No. 3, 1999

t)

MQ FnFA

Bohannan et al.

(7)

where t is the layer thickness, M is the molecular weight (143.1 g/mol for Cu2O, 63.55 g/mol for Cu), Q is the charge passed, F is the density (6.0 g/cm3 for Cu2O, 8.92 g/cm3 for Cu), F is Faraday’s number (96485 C/equivalent), n is 2, and A is the electrode area (0.20 cm2). For the determination of the thickness of the composite layer, an average molecular weight and density are used based on the calculated phase composition. Table 1 shows the calculated layer thickness for the initial oscillations observed at a series of current densities. Also shown is the calculated phase composition of the composite layer and the overall phase composition of the material as calculated using the EQCM. The Cu2O layer thickness varies from 0.4 to 2.5 nm over this range of current densities whereas the composite layer thickness increases from 5 to 20 nm over the same range. The overall modulation wavelength can be varied 4-fold simply by changing the applied current density. Even larger modulation wavelengths can be obtained by lowering the pH of the solution. Figure 9A shows a portion of the potential transient obtained with an applied current density of 0.4 mA/cm2 at a solution pH of 8.2. Figure 9B shows the EQCM output over the same range. The calculated modulation wavelength for these oscillations is 75 nm. This is in good agreement with the modulation wavelength of 80 nm measured by SEM as shown in Figure 9C. We can only measure the modulation wavelength of the material, as the resolution of our SEM

is not sufficient to determine the thickness of only the Cu2O layer. Conclusions We have used the EQCM to monitor in situ the electrodeposition of Cu/Cu2O layered nanostructures. The phase composition of the films can be changed from pure Cu2O at low current densities to films that are mostly Cu at higher current densities as determined by both EQCM and XRD. Oscillations in the potential are observed during the deposition forming a layered structure in the growth direction of the film. The layer thicknesses and overall modulation wavelength of these films were determined using the derivative of the frequency response of the EQCM data. At a pH of 9.0, the Cu2O layer thickness varies from 0.4 to 2.5 nm as the current density is increased from 0.25 to 2.0 mA/cm2. These thicknesses are in the strong quantum confinement region and should lead to interesting optical and electrical properties of the films. The modulation wavelength calculated with the EQCM is in good agreement with what is observed with the SEM, confirming the usefulness of the EQCM as an in situ measurement tool. Acknowledgment. This work was supported by National Science Foundation Grant DMR-9704288 and by Office of Naval Research Grant N00014-96-0984. LA980825A