Study of the Nucleation and Growth Mechanisms in the

ARTICLE pubs.acs.org/JPCC

Study of the Nucleation and Growth Mechanisms in the Electrodeposition of Micro- and Nanostructured Cu2O Thin Films S. Bijani,*,† R. Schrebler,‡ E. A. Dalchiele,§ M. Gab
as,† L. Martínez,† and J. R. Ramos-Barrado† †

Departamento de Física Aplicada I, Laboratorio de Materiales y Superficie (Unit associated to C.S.I.C.), Facultad de Ciencias, Campus de Teatinos, s/n, Universidad de M
alaga, 29071 M
alaga, Spain ‡ Instituto de Química, Facultad de Ciencias, Universidad Cat
olica de Valparaíso, Av. Brasil, 2950, Valparaíso, Chile § Instituto de Física, Facultad de Ingeniería, Herrera y Reissig 565, C.C. 30, 1100 Montevideo, Uruguay ABSTRACT: In previous works, the electrochemical deposition method has been used to prepare pure Cu2O films onto titanium substrates from an aqueous cupric lactate solution. Recently, they have been shown to react reversibly with Li. The phase composition, the microstructure, and especially the surface morphology, crystal or grain size, and thickness of these films can be varied by changing the electrodeposition parameters. Because the characteristics of these films determine the electrochemical response toward Li+, a study of their kinetics and the mechanisms of the nucleation and growth will help us to understand fully their different reversibility behavior. Using three different applied potential values (
150,
400, and
575 mV), pure Cu2O thin films with varying surface morphologies and grain or crystal sizes were electrodeposited. Two- and three-dimensional nucleation models (instantaneous or progressive) under chargetransfer or diffusional growth control were used to describe the experimental potentiostatic current density
time transients to study the nucleation and growth mechanisms of as-prepared Cu2O films as a function of the surface morphology. The results obtained suggest a 2D layer-by-layer growth in parallel to a dependence-time 3D progressive nucleation process under charge-transfer control for the Cu2O thin films synthesized at Ed =
150 and
400 mV versus SCE. For the case of the films deposited at Ed =
575 mV versus SCE, the main contribution corresponds to a 3D progressive nucleation with diffusional control.

1. INTRODUCTION Electrochemical deposition is one of the most attractive methods for the synthesis of thin films. It provides advantages such as a low synthesis temperature, low manufacturing costs, and a high level of purity in the products. Also, electrodeposition allows the stoichiometry, thickness, and microstructure of the films to be controlled by adjusting the deposition parameters. A variety of materials can be prepared in this way.1
3 Among these materials, Cu2O is an attractive material for its preparation through this technique because of its multiple applications.4
7 Electrodeposition of Cu2O thin films is very well-documented by many authors.8,9 However, an in-depth study of these films deposited at low temperature (30 °C) and bath pH 9 had not yet been reported. In some of our works on this subject,10,11 we describe the preparation of highly uniform electrodeposited Cu2O thin films, with variable particle size, morphology, and thickness, by changing some of the electrodeposition parameters, such as, applied potential (Ed), bath temperature, and bath pH. From those studies, we concluded that the applied potential has a very significant influence on the surface morphology and grain size of the deposited Cu2O films. Depending on the applied potential, three different surface morphologies with different preferred crystallographic orientations were found at a low bath temperature (30 °C) and bath pH 9: a highly crystalline micrometer granular morphology (obtained at Ed =
150 mV r 2011 American Chemical Society

vs a saturated calomel electrode (SCE)), a submicrometer and compact granular morphology (obtained at Ed =
400 mV vs SCE), and a porous granular morphology with a cauliflower-like appearance (obtained at Ed =
575 mV vs SCE). The usefulness of the electrodeposition method for designing electrodes for lithium batteries is well-documented, particularly for tin alloy-based materials12,13 and more recently for 3d transition-metal oxides (Fe, Co, Ni, and Cu).14
19 Cu2O thin films with varying surface morphologies were synthesized in our laboratory with different thicknesses and tested as electrodes for Li-ion batteries.20,21 Their electrochemical response, tested with different charge and discharge step experiments, revealed a relationship between the Cu2O reactivity toward Li and the morphological and textural characteristics of the electrodes. The results obtained indicated that with a certain number of cycles the reversibility of the reduction process improved when the film was thinner. For a given thickness, identical behavior was observed as the porosity and particle size increased and decreased, respectively. Subsequent bulk analysis on the composition of these electrodes corroborated the above relationship establishing a deep insight into the reversible reaction mechanism of these films toward lithium.22 Received: September 5, 2011 Published: September 14, 2011 21373

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The Journal of Physical Chemistry C Following on from the above, it is of interest to know what the nucleation and growth mechanisms (NGMs) are involved in the electrodeposition of Cu2O thin films to explain the relationship between the different surface morphologies exhibited by these films and the electrodeposition parameters (mainly the applied potential). The growth control of the electrodeposition of a thin film may be carried out by different methods.23 In this Article, we study the nucleation and growth process using a classical electrochemical technique, such as the potentiostatic current density
time transient measurements, which allows us to define and characterize the mechanisms and the kinetics of the deposition process in a quantitative way. This technique has been widely used for this purpose by many authors,24
31 offering the great advantage of being easy and straightforward to interpret when applying the many different and useful theoretical methods developed for this purpose. The aim of the present study is to investigate the mechanisms of nucleation and growth of pure Cu2O thin films with different surface morphologies electrodeposited onto titanium substrates through experimental potentiostatic current density
time transients. The structural and phase composition, as well as the surface morphology of the films, were determined from X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, respectively.

2. EXPERIMENTAL DETAILS Cu2O electrodeposition was accomplished by using a conventional three-electrode single-compartment electrochemical cell. Cu2O films were electrodeposited in the potentiostatic mode on titanium substrates (plates 1 cm2 in surface area). Titanium substrates were previously polished with emery paper, rinsed with deionized water, immersed in 24% HF for 20 s, and finally, rinsed with water. A platinum sheet was used as the counterelectrode and an SCE was used as the reference electrode. The electrolytic bath contained 0.4 M copper(II) sulfate pentahydrate and 3 M lactic acid as the chelating agent; its pH was adjusted to 9 with sodium hydroxide. The bath temperature was kept constant at 30 °C. In the electrodeposition experiments, the working electrode was polarized from the open circuit potential to different constant potentials, Ed. During Cu2O film electrodeposition, the current density was registered by means of an Autolab Potentiostat-Galvanostat (model PG STAT30). The cathodic current densities registered are expressed as positive quantities in this work. Several applied potentials, Ed, were tested, but three different applied potential values (
150,
400, and
575 mV vs SCE) were selected to obtain Cu2O films with different microstructure and morphology. A more detailed description of the method is found in a previous paper.11 The structure and phase composition of the electrochemically deposited films was identified by XRD on a Siemens D5000 diffractometer using Cu Kα radiation. The surface morphology of the films was examined in a JEOL JSM-5410 SEM with an accelerating voltage of 35 kV. 3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Characterization. 3.1.1. Structural Determination by X-ray Diffraction. In the potentio-

static mode, the electrodeposition potential required for the synthesis of a determined compound is usually unknown. In this

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Figure 1. X-ray diffraction profiles of electrodeposited Cu2O films at Ed =
150,
400, and
575 mV versus SCE. Cu2O JCPDS pattern (no. card 70-2076) is included. Ti peaks are marked with an asterisk.

situation, linear sweep voltammetric measurements help us to identify any possible oxidation
reduction processes experienced by the system of interest and to choose the appropriate potential. From the results shown in previous works, the potential for the formation of stable Cu2O ranges from
150 to
600 mV.10,11 Once the deposition conditions were established, pure Cu2O films of variable morphology were obtained by changing the electrodeposition potential. As reported in our previous paper,11 three different surface morphology types were observed at each different applied potential value, that is, at Ed =
150,
400, and
575 mV versus SCE. In that study, we concluded that the applied potential has a very significant influence on the surface morphology and grain size of pure Cu2O films deposited under specified conditions. Figure 1 illustrates the typical XRD profiles of Cu2O films deposited at the three mentioned applied potential values. The JCPDS pattern (no. card 78-2076) corresponding to a polycrystalline pure Cu2O powder is also shown in this Figure.32 Apart from the reflections corresponding to the titanium substrate, the diffraction peaks corresponding to these samples are ascribed to pure Cu2O, whereas the formation of no other crystalline phase is observed, confirming the purity of the samples. To determine the preferred crystallographic orientation of these films, we estimated the intensity ratio I(111)/I(200) values for the two most intense peaks corresponding to the three XRD profiles as previously reported.11 In that study, we also concluded that the films deposited at Ed =
150 and
400 mV versus SCE present a preferred crystallographic orientation in the [100] direction whereas the samples synthesized at Ed =
575 mV versus SCE are slightly oriented along the [111] direction. 3.1.2. Morphological Study by Scanning Electron Microscope. Figure 2 shows the SEM top images of the aforementioned surface morphologies. The films deposited at Ed =
150 mV versus SCE show a micrometer granular morphology (highly crystalline) formed by regular, well-faceted, polyhedral crystallites whose largest polyhedral edge is ∼800 nm in length (Figure 2a). The films deposited at Ed =
400 mV vs SCE show a submicrometer and compact granular morphology similar to that observed at
150 mV (Figure 2b) but with a smaller grain size (800


400

25

125

280

>100

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The Journal of Physical Chemistry C

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Table 4. Kinetic Parameters Obtained by the Nonlinear Fitting of Equation 21 to the Experimental Potentiostatic Current Density
Time Transient Shown in Figure 8a (in the first 40 s), Ed =
575 mV versus SCE JB

Figure 8. (a) Comparison between the first part of the experimental potentiostatic current density
time transient (oooo) obtained at Ed =
575 mV versus SCE, with the corresponding theoretical curve (red dashed line) obtained using eq 23. The inset shows the(PN-3D)dif contribution extended along the whole time interval studied. (b) Numerical fit (----) of the combined nucleation model given by eq 29 along the whole time studied, together with the experimental transient (3333) obtained at Ed =
575 mV versus SCE. The individual contributions of the different types of nucleation that form eq 29 are also shown: (1) (PN3D)dif; (2) (PN-3D)ct; and (3) (PN-3D)dif and the background current density component, JB.

the two potential steps, Ed =
150 and
400 mV versus SCE, depicted in Figures 6b and 7b, respectively. It can be seen that when a more negative electric pulse is applied, the current density of the individual contributions is higher, and the times corresponding to the current density maximum are considerably shorter, as shown Table 3. Moreover, the corresponding kinetic parameters exhibit higher values, as it can be seen in Tables 1 and 2, which suggest that the nucleation and growth rate increase as the applied potential becomes more negative. Therefore, the Cu2O thin films electrodeposited at Ed =
400 mV versus SCE show the same 2D layerby-layer NGM as the films synthesized at Ed =
150 mV versus SCE but a more rapid (PN-3D)ct process than those observed at less cathodic overpotential. This could explain the fact that for both applied potential values the Cu2O crystals of the monolayers formed are oriented, according to XRD data, in the [100] direction, parallel to the deposition surface. These results are also corroborated by the SEM photographs corresponding to the

(PN-3D)dif

P5 μA 3 cm
2 3 s
1/2

P6 μA 3 cm
2

P7 μA 3 cm
2 3 s
1/2

P70 s
2

3.55  102

1.40  102

1.61  106

3.15  10
7

films synthesized at Ed =
400 mV versus SCE. (See Figure 2b.) In this image, we can observe a greater density of smaller and more homogeneously distributed nuclei than observed when the applied potential is less cathodic. The decrease in the grain size of the films deposited at this potential may be due to the increasing cathodic current density values recorded in the individual contributions (Figure 7) and, more specifically, to the high nucleation and growth rates (P1, P10 , Pi, and Pi0 values) obtained after fitting the experimental potentiostatic current density
time transient data using eq 17. (See Table 2.) In fact, it is well known that the use of high current density values, generally recorded by applying high applied potential values, leads to fine-grained polycrystalline growth.43 The preferential growth direction would indicate that at pH 9 there is a kinetically preferred growth direction for initially formed Cu2O crystallites; that is, the nucleus growth takes place at different rates on different crystal faces.44 In this way, a growth mechanism controlled by lattice incorporation enhances texture development. The layer-by-layer growth mechanism is reflected by the columnar morphology observed in SEM micrographs. In addition, columnar growth would progress with the coalescence of grains as inferred from AFM studies of the films deposited at Ed =
400 mV.10,21 3.2.4. NGM Study in Samples Synthesized at
575 mV versus SCE. At Ed =
575 mV versus SCE (Figure 8a), the experimental potentiostatic current density
time transient shows a different behavior from that obtained at more positive potential values. The transient shape points to two successive and well-differentiated processes. A high current density peak appears 40 s after electrodeposition begins. A hypothesis to explain this behavior may be based on the fact that at this potential value the formation of Cu2O (eq 2) competes with the formation of metallic Cu in eq 3.45 This reaction is dependent on the potential and is fast enough to take place on the electrode surface in the early stages of electrodeposition (short times). However, no presence of metallic Cu was detected by XRD and XAS in the films synthesized at this potential.22 Therefore, this effect could then be attributed to metallic Cu crystallites formed in the very initial stage of the process. After a few seconds, they could react chemically with the Cu(II)Lac2 complex to produce Cu2O according to the following Reaction 18. Cu þ CuðIIÞLac2 þ 2OH
T Cu2 O þ 2Lac
þ H2 O ð18Þ To corroborate this hypothesis, equilibrium constant data corresponding to reaction 18 have been calculated with the aim of determining if the above reaction is thermodynamically possible. In this case, the equilibrium constant is K 0f , Cu2 O ¼ Kf, Cuþ 3 21380

K Kps

ð19Þ

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Table 5. Kinetic Parameters Obtained by the Nonlinear Fitting of Equation 27 to the Experimental Potentiostatic Current Density
Time Transient Shown in Figure 8b, Ed =
575 mV versus SCE CURVE 1(PN-3D)dif P1* μA 3 cm


2


1/2

3s

2.02  104

CURVE 2(PN-3D)ct 0
2

P1* s

1.10  10
4

P2* μA 3 cm


2


1/2

3s

550.98

ð20Þ

ð21Þ

where Kps is the solubility product of Cu2O whose value is Kps = 1.99  10
15.47 Then, in eq 19, K0 f,Cu2O = 4.17  103, indicating that the reaction 18 is thermodynamically possible. In fact, the process associated with the current density in the first stage of the process (ca. 40 s) is itself a nucleation and growth process of a metal (inset in Figure 8a), in this case, metallic Cu, and it can be expressed as follows jTOTAL ¼ jB þ jðPN
3DÞdif

ð22Þ

P5 P7 jTOTAL ¼ pffiffi þ P6 þ pffiffið1
expð
P07 t 2 ÞÞ t t

ð23Þ

in which P5 and P6 are constant values and P7 and P0 7 involve kinetic and diffusive parameters π 3 n 3 F 3 D1=2 3 Cα F1=2

ð24Þ

and P07 ¼ A0 3 π 3 k 3 D

ð25Þ

where n 3 F and F have already been described above and D and Cα are the diffusion coefficient and the concentration in dissolution of the cupric lactate complex, respectively. Finally, in eq 25, A0 and k are described by the following mathematical expressions A0 ¼ A 3 Ndif

ð26Þ

and

  4 8 3 π 3 Cα 3 M 1=2 k¼ 3 F

ð27Þ

A is the rate constant of the nucleus formation and Ndif is the number of nuclei formed at t = 0 under diffusion control. Therefore, in eq 23, the first two terms correspond to the base current density attributed to the increase in the electrochemical reaction rate 1 because of the higher overpotential applied in the system. The latter term is due to a 3D progressive nucleation mechanism controlled by diffusion (PN-3D)dif. Figure 8a shows the simulation of the first portion of the experimental data obtained using eq 23. The values obtained of the kinetic parameters P5, P6, P7, and P0 7 are shown in Table 4. The inset in Figure 8a corresponds to the contribution of diffusion-controlled PN-3D but extended to the whole time interval studied. It shows that if only metallic Cu were deposited, then the observed current density should be much larger than the experimental

0
3

P3* μA 3 cm

6.10  10
8

K ¼ K 1 3 K 2 ¼ 1:5  10
2  9:5  10
4 ¼ 1:43  10
5

P7 ¼


2

P2* s

where K0 f,Cu+ = 5.8  10
7 (ref 46) and K is the dissociation constant of the complex CuðIIÞLac2 T Cu2þ þ 2Lac


JB

CURVE 3(PN-3D)dif 0
2

P5 μA 3 cm

P3* s

2.45  10
7

2.20  104


2


1/2

3s

P6 μA 3 cm
2

192

280

transient. Therefore, after a time of 40 s, only the Cu2O phase is formed, which has a lower conductivity than metallic Cu. The new process could be related to the pH increase in the electrode surface by the high rate of reaction 3.45 In fact, a high amount of lactate ion is liberated when the reduction takes place. The consequent restoration of the protonation equilibrium would cause a local increase in pH that enhances reaction 2 in detriment to reaction 3. For deposition times over 40 s (Figure 8b), it is possible to simulate the experimental potentiostatic current density
time transient using the following equation jTOTAL ¼

∑JðPN
3DÞ

jTOTAL ¼

∑ piffiffið1
expð
P0i t2ÞÞ i¼1 t

2

dif

þ jðPN
3DÞ ct þ jB

ð28Þ

P

P5 þ P3 ð1
expð
P03 t 3 ÞÞ þ pffiffi þ P6 t

ð29Þ

where Pi*, Pi*0 , P3*, and P3*0 are expressed by eqs 24, 25, 14, and 15, respectively, and P5 and P6 are constant values. In this part of the plot, it is observed that the main contribution corresponds to a 3D progressive nucleation under diffusional control, PN-3Ddif (curve 1 in Figure 8b). The other curves in Figure 8b correspond to a 3D progressive nucleation under charge-transfer control, PN-3Dct, (curve 2), another 3D progressive nucleation under diffusional control, PN-3Ddif, (curve 3), and a background current density, JB. The values obtained for the kinetic parameters P1* to P3*, P1*0 to P3*0 , and P5 and P6 are shown in Table 5. This change in the growth mechanism from 2D to 3D with respect to that observed at more positive potentials may explain the greater roughness of the electrodeposited film (SEM image in Figure 2c). The other two contributions could be present because of the surface heterogeneity of the film.

4. CONCLUSIONS Electrodeposition is an option for obtaining thin films for technological applications. In this work, potentiostatic current density-time transient measurements were recorded to study the NGMs of pure Cu2O thin films synthesized at different applied potentials as a function of the properties of the deposits, such as the surface morphology. The results obtained suggest a 2D layerby-layer growth with a dependence-time 3D progressive nucleation process under charge-transfer control for the Cu2O thin films synthesized at Ed =
150 and
400 mV versus SCE. This mechanism, proposed for both potentials, may explain the surface morphology observed in the SEM images. For the case of the films deposited at a more cathodic applied potential value, Ed =
575 mV versus SCE, the main contribution corresponds to a 3D progressive nucleation with diffusional control, as it can be deduced from the adjustment of the potentiostatic current 21381

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The Journal of Physical Chemistry C density
time transient. This fact may explain the greater roughness observed in the SEM images corresponding to these films.

’ AUTHOR INFORMATION Corresponding Author

*Tel: + 34-952137654. Fax: +34-952132382. E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge support from the European Social Fund and the Spanish Ministry of Innovation and Science (MICINN) through projects MAT2002-04477-C02-01, TEC2007-6099, and FUNCOAT-CSD2008-00023-CONSOLIDER INGENIO. R.S. thanks the Direcci
on de Investigaciones de la Pontificia Universidad Cat
olica de Valparaíso. E.A.D. also acknowledges the support received from PEDECIBA
Física and the CSIC (Comisi
on Sectorial de Investigaci
on Científica) of the Universidad de la Rep
ublica in Montevideo, Uruguay. ’ REFERENCES (1) Pilla, A. S.; Duarte, M. M. E.; Mayer, C. E. J. Electroanal. Chem. 2004, 569, 7–14. (2) Mahalingam, T.; John, V. S.; Sebastian, P. J. Mater. Res. Bull. 2003, 38, 269–277. (3) Bohannan, E. W.; Shumsky, M. G.; Switzer, J. A. Chem. Mater. 1999, 11, 2289–2291. (4) Mittiga, A.; Salza, E.; Sarto, F.; Tucci, M.; Vasanthi, R. Appl. Phys. Lett. 2006, 88, 163502. (5) Mahalingam, T.; Chitra, J-S.P.; Chu, J. P.; Moon, M.; Kwon, H. J.; Kim, Y. D. J. Mater. Sci.: Mater. Electron. 2006, 17, 519. (6) Hara, M.; Kondo, T.; Domen, K. Chem. Commun. 1998, 3, 357. (7) Fu, L. J.; Gao, J.; Zhang, T.; Cao, Q.; Yang, L. C.; Wu, Y. P.; Holze, R.; Wu, H. Q. J. Power Sources 2007, 174, 1197. (8) Rakshani, A. E.; Al-Jassar, A. A.; Varghese, J. Thin Solid Films 1987, 148, 191. (9) Wijesundera, R. P.; Hidaka, M.; Koga, K.; Sakai, M.; Siripala, W. Thin Solid Films 2006, 500, 241. (10) Bijani, S. Electrodeposition and Characterization of Cu2O Thin Films. Application as Electrodes in Lithium-Ion Batteries; Servicio de Publicaciones de la Universidad de M
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