Mechanistic Study of the Electrodeposition of Nanoporous Self

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Langmuir 2006, 22, 10545-10553

10545

Mechanistic Study of the Electrodeposition of Nanoporous Self-Assembled ZnO/Eosin Y Hybrid Thin Films: Effect of Eosin Concentration† Aure´lie Goux,‡ Thierry Pauporte´,*,‡ Tsukasa Yoshida,§ and Daniel Lincot*,‡ Ecole Nationale Supe´ rieure de Chimie de Paris - ENSCP, UniVersite´ Pierre et Marie Curie - Paris 6, Laboratoire d’Electrochimie et de Chimie Analytique - UMR 7575 CNRS-ENSCP-Paris 6, ENSCP-11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France, and Gifu UniVersity, Yanagido 1-1, Gifu 501-1193, Japan ReceiVed April 30, 2006. In Final Form: August 1, 2006 ZnO films prepared by one-step electrodeposition in the presence of dissolved eosin molecules present an internal nanoporous hybrid structure resulting from self-assembling processes occurring in solution between ZnO and eosin components. This study aims to better understand the underlying growth mechanism, which is still unexplained. The films were deposited by cathodic electrodeposition from an oxygen-saturated aqueous zinc chloride solution. The effects of the addition of 10 to 100 µmol‚L-1 eosin Y, as a sodium salt, on the growth rate and film properties, were systematically studied while all other parameters remained constant (concentrations of zinc salt and supporting electrolyte, applied potential of -1.4 V versus the mercurous sulfate electrode (MSE), temperature of 70 °C, rotating disk electrode at 300 rotations per min, and a glass-coated tin oxide electrode). It is shown that the addition of eosin provokes the formation of a nanoporous “cauliflower” structure whose nodule size and composition depend on the eosin concentration in the bath. The growth rate of the hybrid films increases markedly with the eosin concentration. The ZnO and eosin contents of the films are determined for each concentration by chemical analysis. Comparing with thickness determinations, it is shown that the total porosity increases up to 60-65% in volume fraction toward an eosin concentration of 100 µmol‚L-1. The empty pore volume fraction increases up to about 30% at an eosin concentration of about 20 µmol‚L-1 and then decreases. These correlations have been precisely established for the first time. It is shown that the global composition is fixed by the relative rate of deposition for zinc oxide, which is constant, and for the relative rate of eosin inclusion, which is proportional to the concentration in solution. This is explained on the basis of different steps in the growth mechanism, in particular, a diffusion effect limitation for both oxygen and eosin. This variation explains part of the increase in the growth rate. Another contribution is related by the structural effect on the nanoscale leading to the formation of the interpenetrated porous network. Competition between empty and eosin-filled parts of the pore network is evidenced. The formation of the porous network structure could be governed by a diffusion-limited aggregation mechanism. The system may represent a reference case of competing reactions in the electrochemical self-assembly of hybrid nanostructures.

Introduction Interest in the synthesis of metallic or semiconducting nanostructures has increased tremendously during the past decade, as reviewed in several books,1-3 because of the unique electrical, magnetic, optical, chemical, and energetic properties as compared to those of bulk materials. In this field, electrochemistry has emerged as a remarkable method for the creation of nanostructures, involving in particular recent advances in the electrodeposition of semiconductors.4 In the first class of electrochemical processes, nanostructures are created by the corrosion and reaction of a metallic or semiconducting electrode. This was the case in the formation of porous membranes of alumina in the first period,5 now extending rapidly toward other valve metals such as †

Part of the Electrochemistry special issue. * Corresponding author. E-mail: [email protected]; [email protected]. ‡ Ecole Nationale Supe ´ rieure de Chimie de Paris. § Gifu University. (1) Fendler, J. H., Ed. Nanoparticles and Nanostructured Films; Wiley-VCH: Weinheim, Germany, 1998. (2) Hodes, G. Electrochemistry of Nanomaterials; Hodes, G., Ed.; WileyVCH: Weinheim, Germany, 2001. (3) Licht, S. Semiconductor Electrodes and Photoelectrochemistry. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds; Wiley-VCH: Weinheim, Germany, 2002; Vol. 6. (4) Lincot, D. Thin Solid Films 2005, 487, 40 and references therein. (5) Hulteen, J. C.; Martin, C.R. Template Synthesis of Nanoparticles in Nanoporous Membranes. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, Germany, 1998; Chapter 10, p 236.

titanium6,7and tungsten.8,9 In the latter cases, the semiconducting properties of the oxidized phase can be used directly for photoelectrochemical processes.9 Porous semiconductors of Si, II-VI, and III-V compounds can also be formed by electrochemical oxidation.10,11 In the second class of electrochemical processes, nanostructures are formed by direct electrodeposition from dissolved precursors into templates, in particular that using porous membranes (alumina or track-etched polymer membranes) for the deposition of nanowire arrays of metals and various semiconductors,5,12,13 including ZnO.14 Deposition in selfassembled colloidal templates is also developing successfully.13 (6) Swilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1999, 45, 921. (7) Beranek, R.; Tsuchiya, H.; Sugishima, T.; Macak, J. M.; Taveira, L.; Sugimoto, S.; Kisch, H.; Schmuki, P. Appl. Phys. Lett. 2005, 87, 343114. (8) Mukherjee, N.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Mater. Res. 2003, 18. (9) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (10) Kelly, J. J.; Vanmaekelbergh, D. Porous Etched Semiconductors: Formation and Characterization. In Electrochemistry of Nanomaterials; Wiley-VCH: Weinheim, Germany, 2001; Chapter 4, p 103 and references therein. (11) Levy-Cle´ment, C. Macroporous Microstructures Including Silicon. In Nanoparticles and Nanostructured Films; Wiley-VCH: Weinheim, Germany, 1998; Chapter 3.2, p 185 and references therein. (12) Hodes, G. Preparation of Nanocrystalline Semiconductors. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds; Wiley-VCH: Weinheim, Germany, 2002; Chapter 3.1, p 173 and references therein. (13) Bartlett, P. N. In Electrochemical Society Interface; Winter 2004; p 28 and references therein. (14) Leprince-Wang, Y.; Wang, G. Y.; Zhang, X. Z.; Yu, D. P. J. Cryst. Growth 2006, 287, 89.

10.1021/la061199h CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006

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One can also mention the use of lyotropic liquid crystal soft templates directly self-assembled at the substrate surface, allowing the formation of 3D nanoporous structures in a very elegant way.13 Besides this category of electrodeposition processes involving solid or soft preexisting templates, 2D or 3D nanostructures can be electrodeposited by playing with substrate modifications for semiconducting dots,15 electrochemical atomic layer epitaxy (ECALE),16 or periodic modification of the electrochemical interface conditions17 for the formation of superlattices. Another class of electrochemical processes to generate nanostructures is the direct addition of structure-directing agents, as complex molecules or ions, to the deposition bath. This approach, which was not present in recent reviews,1-3 has emerged recently in the field of oxide electrodeposition, with zinc oxide as a leading example.18-37 It can be considered to be the electrochemical analogue of crystallization processes from solutions in the presence of molecular or biological additives, which are well known in natural processes and are under intense investigation with respect to the growth of biomimetic nanostructures.27-29 This opens new perspectives for the electrodeposition of semiconductor (or metal) nanostructures, also in the direction of hybrid organic/inorganic materials. Hybrid materials deserve an impressive amount of attention in the solgel area.30 The aim of this article is to deal with the case of the electrodeposition of zinc oxide, which was discovered in 1996 for both films31,32 and microcolumn arrays.31 The electrodeposition of ZnO in the presence of additives was introduced in 199818 and has received increasing attention since this time.18-26 Despite the fact that spectacular results have been obtained in terms of nanostructures with different organizations (occlusions, platelets, nanopores, and onion rings), compositions, and properties, there is a need to improve the understanding of the underlying electrochemical growth mechanisms. (15) Rajshwar, K.; de Tacconi, N. R.; Chanthamarakshan, C. R. Curr. Opin. Solid State Mater. Sci. 2004, 8, 173. (16) Stickney, J. L. In Electroanalytical Chemistry: A Series of AdVances; Bard, A. J., Ed.; Dekker: New York, 1999; p 75. (17) Switzer, J. A. Electrodeposition of Superlattices and Multilayers. In Electrochemistry of Nanomaterials; Hodes, G., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Chaper 3, p 67. (18) Yoshida, T.; Miyamoto, K.; Hibi, N.; Sugiura, T.; Minoura, H.; Schlettwein, D.; Oekermann, T.; Schneider G.; Wohrle, D. Chem. Lett. 1998, 27, 599. (19) Yoshida, T.: Terada, K., Schlettwein, D.; Oekermann, T., Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214. (20) Choi, K. S.; Lichtenegger, H. C.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 12402. (21) Yoshida, T.; Terada, T.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. Electrochemistry 2002, 70, 470. (22) Yoshida, T.; Pauporte, T.; Lincot, D.; Oekermann, T.; Minoura, H. J. Electrochem. Soc. 2003, 150, C608. (23) Pauporte, T.; Yoshida, T., Froment, M.; Lincot, D. J. Phys. Chem. B 2003, 107, 10077. (24) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; Who¨rle, D.; Minoura, H. Chem. Commun. 2004, 400. (25) Oekermann, T.; Yoshida, T.; Boeckler, C.; Caro, T.; Minoura, H. J. Phys. Chem. B 2005, 109, 12560. (26) Pauporte´, T.; Bedioui, F.; Lincot, D. J. Mater. Chem. 2005, 15, 1552. (27) Meldrum, F. C. Oriented Growth of Nanoparticles at Organized Assemblies. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Chapter 2, p 24. (28) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; McDermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821 and references therein. (29) Sarikaya, M.; Tamerler, C.; Jen, A. K.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577 and references therein. (30) Sanchez, C.; Lebeau, B., Chaput, F.; Boilot, J. P. AdV. Mater. 2003, 15, 1969. (31) Peulon, S.; Lincot, D. AdV. Mater. 1996, 8, 166. (32) Izaki, M.; Omi, T. Appl. Phys. Lett. 1996, 68, 2439. (33) Peulon, S.; Lincot, D. J. Electrochem. Soc. 1998, 145, 864. (34) Pauporte´, T.; Lincot, D. Appl. Phys. Lett. 1999, 75, 3817. (35) Pauporte´, T.; Lincot, D. J. Electroanal. Chem. 2001, 517, 54. (36) Goux, A.; Pauporte´, T.; Chivot, J.; Lincot, D. Electrochim. Acta 2005, 50, 2239. (37) Goux, A.; Pauporte´, T.; Lincot, D. Electrochim. Acta 2006, 51, 3168.

Goux et al.

In this article, we will focus our attention on the study of the growth of ZnO in the presence of various concentrations of eosin Y molecules in solution as the key parameter specifically addressing the relations between the growth mechanism and the properties of the films (structure, porosity, and composition). We will show that the growth involves complex electrochemical self-assembling processes between eosin species and zinc oxide, leading to a highly interpenetrated structure and a porous network. Important experimental correlations will be evidenced for the first time and discussed in terms of the possible underlying growth mechanism and structural evolutions.

Specific Context of ZnO Electrodeposition Studies Zinc oxide films are deposited from aqueous solution in the presence of zinc(II) ions by the cathodic reduction of oxygen precursors such as molecular oxygen,31,33,36,37 nitrate ions,18,32 and hydrogen peroxide14,35 at an electrode surface. In the case of dissolved oxygen, for instance, the deposition takes place via the following overall reaction:31,33

Zn2+ + 0.5O2 + 2e- f ZnO + H2O

(1)

This reaction involves two consecutive steps with an intermediate generation of hydroxide ions,33,37 which increases the pH at the interface and then provokes a precipitation reaction with zinc ions:33

0.5O2 + H2O + 2e- f 2OH-

(2)

2OH- + Zn2+ f ZnO + H2O

(3)

Because of the confinement of the supersaturation near the electrode surface, the precipitation takes place by a heterogeneous mechanism at the surface. This is the reason why the electrodeposition reaction is sometimes called an electroprecipitation reaction. A detailed mechanistic study has been carried out in the absence of additives.33 In the absence of additives, wellcrystallized films or nanocolumns arrays can be formed, with possible epitaxial growth.34,38 The addition of organic elements to zinc oxide deposition in solution (either chemical or electrochemical) has been carried out for polymers,40-45 proteins,46 complexing agents,47,48 surfactants,20,49-54 and organic dyes.18-25,55-64 (38) Liu, R.; Vertegek, A. A.; Bohannan, E. W.; Sorenson, T. A.; Switzer, J. A. Chem. Mater. 2001, 13, 508. (39) Wegner, G.; Baum, P.; Mu¨ller, M.; Norwig, J.; Landfester, K. Macromol. Symp. 2000, 175, 349. (40) Kovtyukhova, N. I.; Gorchinskiy, A. D.; Waraksa, C. Mater. Sci. Eng. B 2000, 69-70, 424. (41) Neves, M. C.; Trinidade, T.; Timmons, A. M. B.; Pedrosa de Jesus, J. D. Mater. Res. Bull. 2001, 36, 1099. (42) Taubert, A.; Ku¨bel, C.; Martin, D. C. J. Phys. Chem. B 2003, 107, 2660. (43) Li, Y.; Yang, M. J.; She, Y. Talanta 2004, 62, 707. (44) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26. (45) Yeo, K. H.; Teh, L. K.; Wong, C. C. J. Cryst. Growth 2006, 287, 180. (46) Topoglidis, E.; Cass, A. E. G.; O’Regan, B.; Durrant, J. R. J. Electroanal. Chem. 2001, 517, 20. (47) Vayssie`res, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (48) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (49) Tan, Y.; Steinmiller, E. M. P.; Choi, K. S. Langmuir 2005, 21, 9618. (50) Jing, H. J.; Li, X. L.; Lu, Y.; Mai, Z. H.; 4.J., Li, M. J. Phys. Chem. B 2005, 109, 2881. (51) Du, J.; Liu, Z.; Huang, Y.; Gao, Y; Han, B.; Li, W.; Yang, G. J. Cryst. Growth 2005, 280, 126. (52) Wang, C.; Shen, E.; Wang, E.; Gao, L., Kang, Z.; Tian, C.; Lang, Y.; Zhang, C. Mater. Lett. 2005, 59, 2867. (53) Gomes, A.; da Silva Pereira, M. I. Electrochim. Acta 2006, 51, 1342.

Electrodeposition of ZnO/Eosin Y Hybrid Thin Films

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Figure 1. Formula of eosin Y as the sodium salt molecule.

The electrodeposition in the presence of dyes has been studied in depth for applications in the field of dye-sensitized solar cells, as an alternative to the classical process using sintered colloidal titanium oxide films introduced by Gra¨tzel et al.65,66 The codeposition of ZnO/dye was obtained with different types of dyes: metallic complexes such as metallophthalocyanines18,26,55,57 or organic compounds such as tetrabromophenol blue56 and eosin Y.19,21,25,61,62 The dye molecules not only fill created interstices of the oxide but also are strongly anchored to the inorganic matrix by means of sulfonate or carboxylate groups. Highly porous structures are obtained, especially with eosin, which can be used directly in photoelectrochemical cells.19,21,25,61,62 The best results are nevertheless obtained by removing the molecules absorbed in the pores and reintroducing them in a more controlled manner.62 A maximum incident photon-to-current conversion efficiency (IPCE) of 90% at 520 nm is reached after dye readsorption.62 Experimental Section The deposition solutions contained 5 mM ZnCl2 (Merck, reagent grade) and 0.1 M KCl (Merck, reagent grade). Eosin Y, at concentrations between 0 and 0.1 mM, was introduced as the soluble sodium salt, Na2EY (Kanto, 85%). The corresponding molecule is given in Figure 1. The solutions were prepared with Milli-Q-quality water. Then, they were saturated with molecular oxygen, and slight O2 bubbling in the bath was maintained during the deposition process. The substrates were F-doped SnO2-conducting glass, which were cleaned in an ultrasonic bath for 5 min in acetone and 5 min in ethanol and then treated for 3 min in 45% nitric acid. The substrates were fixed to a rotating electrode support, and the depositions were performed at a constant rotation speed of 300 rpm. The potentiostat was an Autolab PGSTAT 12. The electrochemical cell was a classical three-electrode cell placed in a thermoregulated bath. The deposition temperature was 70 °C. The counter electrode was a platinum wire, and the reference electrode was a saturated mercurous sulfate electrode (MSE) (with a potential of +0.65 V vs NHE) that was placed in a separate compartment maintained at room temperature. The applied potential was -1.4 V versus MSE. The working electrode surface area in contact with the solution was 0.79 cm2. (54) Michaelis, E.; Wo¨hrle, D.; Rathousky, J.; Wark, M. Thin Solid Films 2006, 497, 163. (55) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Wo¨hrle, D.; Sugiura T.; Minoura H. Chem. Mater. 1999, 11, 2657. (56) Yoshida, T.; Yoshimura, J.; Matsui, M.; Sugiura, T.; Minoura, H. Trans. Mater. Res. Soc. Jpn. 1999, 24, 497. (57) Schlettwein,; Oekermann, D. T.; Yoshida, T.; Tochimoto, M.; Minoura, H. J. Electroanal. Chem. 2000, 481, 42. (58) Yoshida, T.; Minoura, H. AdV. Mater. 2000, 12, 1219. (59) Karuppuchamy, S.; Yoshida, T., Sugiura, T.; Minoura, H. Thin Solid Films 2001, 397, 63. (60) Okabe, K.; Yoshida, T.; Sugiura, T.; Minoura, H. Trans. Mater. Res. Soc. Jpn. 2001, 26, 523. (61) Pauporte´, T.; Yoshida, T.; Goux, A.; Lincot, D. J. Electroanal. Chem. 2002, 534, 55. (62) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; Wo¨hrle, D.; Minoura, H. Chem. Commun. 2004, 4, 400. (63) Oekermann, T.; Karuppuchamy, S.; Yoshida, T.; Schlettwein, D.; Wo¨hrle, D.; Minoura, H. J. Electrochem. Soc. 2004, 151, C62. (64) Oekermann, T.; Yoshida, T.; Boeckler, C.; Caro, J.; Minoura, H. J. Phys. Chem. B 2005, 109, 12560. (65) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (66) Nazeeruddin, N. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382.

Figure 2. (A) Effect of eosin Y concentration on current density transient curves: (a) 0, (b) 20, (c) 40, (d) 50, and (e) 100 µM. (B) Variation of the current density after 900 s with eosin Y concentration. Other parameters are ZnCl2, 5 mM; KCl, 0.1 M; applied potential, -0.75 V/NHE; rotating electrode at 300 rpm; temperature, 70 °C. The film morphology was observed by field emission scanning electron microscopy (FESEM) (LEO 1530 GEMINI) after metallization of the samples with 2 to 3 nm of platinum. To determine the eosin Y content in the films, the UV-visible absorption spectra were recorded in the visible near-infrared (NIR) wavelength region with a two beams from a Cary 100 Varian spectrophotometer. A part of the films of known thickness and area was dissolved in 5 mL of 10% ammonia, allowing the dissolution of eosin as monomers. The solutions were introduced into a polymer cell with 1-cm-long optical path. The reference was a cell filled with a 10% ammonia solution. The zinc content in the films was also measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) after dissolution of the layer in 0.7 mL of a 10% HCl solution.

Results Electrochemical Study. The current density transient curves after the application of a constant potential of -0.75 V versus NHE are presented in Figure 2A for different concentrations of the dye. In the absence of dye, the cathodic current density increases until reaching a maximum as the potential is applied and then decreases to reach a plateau. This phenomenon has already been described in the literature.67 It is typical of an instantaneous nucleation process followed by the tridimensional growth of the grains. The maximum in the current density and the following decrease are due to the appearance of the crystallites (67) Pauporte´, T.; Lincot, D. Electrochim. Acta 2000, 45, 3345.

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Goux et al.

Figure 4. Variation of the grain diameter with eosin concentration in a deposition bath. Conditions are the same as for Figure 2. Table 1. Film Thicknesses after 900 s of Deposition as a Function of Eosin Y Concentration in the Deposition Bath

Figure 3. (a) SEM top view (15 kV, with a conventional detector of secondary electrons) of a pure ZnO film. (b-h) FESEM crosssectional and top views (3 kV for the top sight and 5 kV for the cross section, with an in-lens detector of secondary electrons) of films deposited during 900 s from solutions containing different eosin Y concentrations: (b, c) 10, (d, e) 30, and (f-h) 100 µM.

and to their coalescence. In the presence of eosin, the nucleation peak disappears completely, indicating a change in the nucleation step corresponding to a faster coalescence process on the surface. The current density increases gradually toward a plateau, which is higher than without eosin. This can be related to the catalytic effect of eosin for oxygen reduction studied in detail in a previous paper.22 The current densities attained after 900 s in the presence of eosin are weakly dependent on its concentration in the bath with a value of around 1.5 mA‚cm-2

Structural Properties of the Films Selected FESEM views corresponding to some of the previous experiments are shown in Figure 3. They illustrate the effect of the eosin concentration on the morphology of the films. In the absence of eosin (Figure 3a), zinc oxide films are made of individual hexagonal crystallites with flat surfaces, as observed classically.22,31 With the introduction of eosin into the solution, the shape of the grain changes dramatically. They lose their hexagonal shape and present rounded shapes as already reported in our previous work.22,23 What is new in the present case is the presentation of the evolution of the morphology specifically related to the eosin concentration in the bath: 10 µM (Figure 3b and c), 30 µM (Figure 3d and e), and 100 µM (Figure 3f-h). Considering first the cross sections (Figure 3b, d, and f), it appears that layers are composed of individual elongated grains with a fibrous inner structure. Previous work has shown that these grains are individual single crystals23,24 and they can even grow epitaxially on single-crystalline substrates.23 From 30 µmol‚L-1

[o-EY2-] (µmol‚L-1)

film thickness (µm)

10 20 30 40 50 100

1.2 1.5 1.7 1.8 2.3 3-3.5

of eosin, a few cracks due to stress in the films appear, and above 50 µmol‚L-1, the layers are less adherent to the substrate. On the top views (Figure 3c, e, g, and h), it can be observed that the grains present an “orange peel” aspect that reflects the outer surface of the fibrous structure, with extremities of pores (black dots) and zinc oxide (white dots). The observation of pores is already present at 10 µM (Figure 3c), which is less evident on the corresponding cross section. From a qualitative observation, it appears that the surface density of pore/fiber extremities does not change markedly with eosin concentration, in particular between 20 and 100 µmol‚L-1. The most striking changes concern the shape and the size of the grains: the hemispherical shape of the top of the grains becomes more and more perfect at higher concentrations (Figure 3e and g). For the highest concentration of eosin, the grains are cylindrical with an almost perfect hemispherical outer surface (Figure 3f) whose diameter is much larger than at the lowest concentrations. The diameter of the grains increases markedly with concentration as shown in Figure 4. It seems that the variation is almost linear with the eosin concentration in the bath. However, the key influence of the eosin concentration is an increase in the thickness of the layer as a function of eosin concentration as shown in Table 1. Figure 5 presents the corresponding variation of the growth rate with additional determinations, especially for concentrations of eosin lower than 10 µmol‚L-1. The growth rate variation is higher for concentrations lower than about 10 µmol‚L-1 with a slope of 4 µm‚h-1‚µM-1 and tends to decrease for higher concentrations with a slope that is about 3 times lower. Note that the growth rate at 100 µmol‚L-1 is about 6 times higher than for pure ZnO films.

Composition of the Films The quantities of zinc and eosin Y in the deposits are determined by ICP/AES and UV-visible spectrophotometry, respectively. The results for zinc as a function of eosin concentration are

Electrodeposition of ZnO/Eosin Y Hybrid Thin Films

Langmuir, Vol. 22, No. 25, 2006 10549

Figure 6. UV-visible spectra of a 10% ammonia solution of eosin Y obtained from the electrodeposited films from electrolysis bath containing (a) 10, (b) 30, (d) 40, and (e) 50 µmol‚L-1. Conditions are the same as for Figure 2. Figure 5. Variation of the growth rate of the films with the concentration of eosin Y in a deposition bath. Conditions are the same as for Figure 2. Table 2. Variation of the ZnO Occupied Volume with the Eosin Y Concentration in the Deposition Bath [o-EY2-] CZn2+ Sdeposit cZn film a nZn b ZnO occupied (µmol‚L-1) (µg‚cm-3) (cm2) (mol‚L-1) (mol‚cm-2) volume (%)c 10 20 30 40 50 100

107.2 106.8 95.0 87.8 117.6 261.4

0.196 0.196 0.196 0.196 0.196 0.393

48.78 38.88 30.52 26.64 27.92 23.73

5.85 × 10-6 5.83 × 10-6 5.19 × 10-6 4.8 × 10-6 6.42 × 10-6 7.12 × 10-6

72 56.5 45 39 40.5 35

Concentration of zinc in the film: cZn film ) (CZn2+Vsolution/MZnAdeNumber of moles of zinc by surface unity in the film: nZn ) cZn filmthdeposit. c ZnO occupied volume: %ZnO ) (cZn filmMZnO/dZnO) × 100, where CZn2+ is the concentration of Zn2+ ions determined by ICP/ AES (g‚cm-3), Vsolution is the volume of the titrated solution (cm3), dZnO is the density of zinc oxide (dZnO ) 5.606 g‚cm-3), Adeposit is the area of the deposit (cm2), thdeposit is the thickness of the deposit (cm), MZnO is the molar mass of ZnO (MZnO ) 81.4 g‚mol-1), and MZn the molar mass of zinc (MZn ) 65.4 g‚mol-1). a

positthdeposit). b

presented in Table 2. The methodology used is described in the caption. From the total thicknesses of the films, the molar concentrations of zinc in the films are calculated and reported in column 4. The concentration decreases with increasing eosin concentration in the solution. Assuming that zinc is present as zinc oxide, the volume fractions of ZnO in the films can be evaluated and are reported in column 6. From values approaching 100% for films grown without eosin (not shown), the fraction decreases markedly with increasing eosin concentration in the bath, with a value of only 72% for a 10 µM concentration of eosin. In addition to concentrations and volume fractions, the equivalent surface concentration of zinc is an important parameter. The values are reported in column 5. One can observe that contrary to previous parameters it does not vary markedly with eosin concentration. Concerning the determination of the dye concentrations, the UV-visible spectra obtained with the same set of samples are plotted in Figure 6. The shapes are typical of oxidized monomeric eosin. The intensity of the peak at 518 nm increases with the dye concentration in the deposition bath. The titrated eosin concentration is determined by the Beer-Lambert law with a molar extinction coefficient of 91 600 L‚mol-1‚cm-1 for EY.61 Table 3 presents the results of the calculations and the methodology used. The concentration of eosin in the film, reported in column

4, increases almost linearly with the concentration in solution up to values of about 0.8 M. In parallel, the volume fraction occupied by eosin increases up to about 55%, meaning that the majority of the film volume is now occupied by eosin molecules. Note that in order to evaluate the volume fraction a value of 2 (hypothesis) for the density of eosin has been considered. The fact that the sum of the eosin volume fraction (Table 3, column 6) and that of zinc oxide (Table 2, column 6) is lower than 100% indicates the presence of empty zones in the film. The corresponding volume fraction, obtained as the difference between 1 and the sum of the two numbers, is indicated in column 7. The addition of the eosin and empty space volume fraction represents the total volume fraction not occupied by ZnO, with values reported in column 8. Neglecting the intergrain empty volumes, this corresponds to the total porosity of the films. An impressive value of 65% is thus obtained with the higher concentrations of eosin in the bath. It is similar to values reported in the literature for other nanostructured films. The sum of the eosin and empty space volume fractions thus corresponds to the “recoverable” full porosity because eosin Y could be almost totally desorbed after deposition by a treatment in a dilute solution of NaOH.62 This indicates that empty and eosin-occupied parts probably belong to a unique porous network. To complete the results, as for zinc, the equivalent surface concentrations of eosin are calculated and reported in column 5.

Composition-Structure Relations Figure 7 presents the evolution of the eosin, the empty space, and the total porosity (eosin + empty space) volume fractions in the film as a function of the concentration of eosin Y in solution, as taken from Tables 2 and 3. The volume fraction of eosin appears to increase continuously, with a more or less linear variation, over the whole range of concentration. Very different behavior is observed for the volume fraction of empty space. It increases first up to 30 µmol‚L-1 and then decreases. The maximum value is about 30%. Above this value, the eosin and empty part volume fractions have opposite sign variations. At higher eosin concentrations, the empty part tends to go to zero. This is in good agreement with the observations made in previous reports that optimal performances of as-grown films in dye cells were obtained for dye concentrations near 20 µmol‚L-1, indicating the presence of open porosity, accessible to the electrolyte, and the fact that eosin molecules were already located within the pores and anchored on the ZnO surface.62 For higher concentrations, the performance is degraded because of the reduced

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Table 3. Variation of the Eosin Y and Eosin Y + Empty Space Occupied Volumes with the Eosin Y Concentration in the Deposition Bath [o-EY2-] (µmol‚L-1)

cEY (µmol‚L-1)

Sdeposit (cm2)

cEY film a (mol‚L-1)

nEY b (mol‚cm-2)

eosin Y occupied volume (%)c

empty space occupied volume (%)d

empty space and eosin Y occupied volume (%)

10 20 30 40 50 100e

0.49 2.21 2.14 3.60 5.37 18.56

0.196 0.196 0.196 0.196 0.196 0.393

0.10 0.26 0.32 0.51 0.60 0.79

1.2 × 10-8 3.9 × 10-8 5.44 × 10-8 9.18 × 10-8 1.38 × 10-7 2.37 × 10-7

7.2 17.8 22.2 35.3 41.2 54.5

20.8 25.2 32.8 25.7 18.3 10.5

28 43.5 55 61 59.5 65

a Concentration of eosin Y in the film: cEYfilm ) (cEYVsolution/Adepositthdeposit). b Number of moles of eosin Y by surface unity in the film: nEY ) cEY filmthdeposit. c EY occupied volume: %EY ) (cEY filmMEY/dEY) × 100. d Empty space occupied volume: %empty space ) 100 - %ZnO - %EY, where cEY is the concentration of eosin Y determined by UV-vis spectroscopy (mol‚L-1), Vsolution is the volume of the titrated solution (cm3), dEY is the density of eosin Y, which is supposed to be 2 g‚cm-3 but the exact value is unknown, Adeposit is the area of the deposit (cm2), thdeposit is the thickness of the deposit (cm), and MEY is the molar mass of eosin Y (MEY ) 692 g‚mol-1). e Thickness used for the calculations: 3 µm.

growth. The overall deposition mechanism can be associated with the following sequence of simplified steps: Diffusion Reactions

(a) O2(b) f O2(s) (b) EY2-(b) f EY2-(s) (c) Zn2+(b) f Zn2+(s), where the simplest 2+ form is taken for the sake of simplicity Electrochemical Reactions

(d) O2(s) + H2O + 2e f 2OH-(s) (e) EY2- + ne- f EY-2-n(s) with n ) 1 or 2 Hybrid Material Growth Reactions Figure 7. Volume fraction occupied in the film by eosin (0), empty space (4), and the sum of the two (O) as a function of eosin Y concentration in micromoles per L. Conditions are the same as for Figure 2.

accessibility of trapped eosin molecules, in agreement with the decrease of the empty volume fraction. After the desorption of eosin molecules, the total open porosity is restored,62 indicating that all of the eosin molecules were accessible to the selective dissolution treatment. For concentrations of eosin in solution lower than 10 µmol‚L-1 under the present conditions, we have observed that it was not possible to remove the eosin molecules by specific dissolution. This means that contrary to the structure obtained with higher concentrations the eosin molecules are occluded in the ZnO matrix and do not belong to an accessible pore network. Another important observation is that in the absence of Zn2+ in solution the electrodeposition of a pure eosin Y film is not possible. Therefore, the formation of the ZnO matrix is required for the stability of the structure. This hypothesis is supported by the fact that the deposits are less adherent, even powderlike, at high eosin concentration in the electrolysis bath (especially above 60 µM).

Discussion From the experimental results, it is possible to address some aspects of the underlying deposition mechanism. The overall deposition mechanism is made of a succession of elementary steps, involving diffusion from solution to the surface of reacting species (zinc ions, oxygen, and eosin) and electrochemical or chemical reactions at the interface leading to film

(f) Zn2+(s) + 2OH-(s) f ZnO + H2O (g) EY-2-n(s) f EY where bold letters correspond to solid species integrated into the growing film. Note that reaction g is probably not complete because the charge neutrality condition in the solid phase is not respected. This means that cationic species must be involved in the deposition process of reduced eosin (and also of oxidized eosin). Because Zn2+ has been already demonstrated to form complex ions with reduced eosin,61 we can expect that the charge neutrality involves Zn2+ cations. Reaction g can thus be rewritten similarly to reaction f, as

(

(h) EY-2-n(s) + 1 +

n 2+ Zn f EYZn1+(n/2) 2

)

The resulting film will have an effective composition of ZnxOy(EY)z. Note that this reasoning does not consider the internal structure of the film or the presence of empty space as pores. From the composition measurements of zinc and eosin given in Tables 2 and 3, it is convenient to calculate the corresponding fluxes. They are given in Table 4 and directly reflect the rates of reactions f and g. We can also calculate the corresponding cathodic current densities by applying Faraday’s law. For eosin, the two possibilities for one- and two-electron processes are given respectively in columns 3 and 4. For zinc, only the reaction with two electrons, corresponding to ZnO formation, is considered, giving the values in column 6. The equivalent current densities for eosin versus the eosin concentration in the deposition bath are plotted in Figure 8A. Clear linear variations are observed, demonstrating that the flux of eosin insertion into the film is proportional to its

Electrodeposition of ZnO/Eosin Y Hybrid Thin Films

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Table 4. Flux of Eosin Y and Zinc and Equivalent Current Densities Calculated from the Total Number of Mole of Eosin Y or Zinc in the Film [o-EY2-] (µmol‚L-1)

fluxEY a (mol‚cm-2‚s-1)

jEY 1e b (n ) 1 e-) (A‚cm-2)

jEY 2e b (n ) 2 e-) (A‚cm-2)

fluxZn a (mol‚cm-2‚s-1)

jZn 2e b (n ) 2 e-) (A‚cm-2)

10 20 30 40 50 100

1.33 × 10-11 6.33 × 10-11 6.04 × 10-11 1.02 × 10-10 1.53 × 10-10 2.63 × 10-10

1.28 × 10-6 6.11 × 10-6 5.83 × 10-6 9.84 × 10-6 1.48 × 10-5 2.54 × 10-5

2.57 × 10-6 1.22 × 10-5 1.17 × 10-5 1.97 × 10-5 2.95 × 10-5 5.08 × 10-5

6.5 × 10-9 6.48 × 10-9 5.77 × 10-9 5.33 × 10-9 7.13 × 10-9 7.91 × 10-9

1.25 × 10-3 1.25 × 10-3 1.11 × 10-3 1.03 × 10-3 1.38 × 10-3 1.53 × 10-3

a Flux of eosin Y or zinc: fluxx ) (nx/t) where nx is the number of moles of eosin Y or zinc by surface unity in the film and t is the electrolysis time (t ) 900 s). b Equivalent current density of eosin Y or zinc: jx ) fluxxFn, where F is the Faraday constant and n is the number of electrons exchanged during the electrochemical reaction.

Figure 8. (A) Variation of the equivalent current density for eosin Y versus the dye concentration in the deposition bath, (O) calculated with n ) 1 electron and (0) n ) 2 electrons. (B) Variation of the equivalent current density, calculated with n ) 2 electrons, for zinc versus the dye concentration in the deposition bath. Conditions are the same as for Figure 2.

concentration in solution:

FE ) B[EY]b

(4)

Such a situation can be observed either from a limiting reaction related to pure diffusion control corresponding to reaction b), in which case the rate constant B is simply related to diffusion kinetics, or from a limiting reaction related to interfacial kinetics (electrochemical or adsorption/desorption processes). According to the voltamperograms registered on a rotating electrode of FTO (300 rpm) at 70 °C in a deaerated chloride solution of 100 µmol‚L-1 eosin Y, the current density is around 50 µA‚cm-2 with and without Zn2+ present in the bath.67 This value is close to the equivalent current density for the same eosin concentration

in solution if a two-electron mechanism is considered (Figure 8A). This would be more likely in agreement with a process limited by the eosin diffusion. However, the number of exchanged electrons as a function of the conditions is still not clear. In a previous article,61 it was assumed that the reduction of eosin Y involved one electron exchanged during the electrochemical reaction. This was because it was shown that after reduction in a one-electron charge-transfer process reduced eosin Y is complexed by zinc ions.61 Ongoing in situ ESR UV-visible spectroelectrochemistry studies suggest a reduction by a twoelectron process as the final reaction in the presence of zinc ions ([r-EY4-] product).68 Considering now the case of zinc, it appears immediatly from Table 2 that the number of moles deposited does not vary markedly with concentration of eosin. The corresponding flux is thus independent of the eosin concentration as shown in Table 4. Finally, the equivalent current densities of ZnO deposition are plotted in Figure 8B as a function of eosin concentration. It shows that it is almost constant but that the calculated current density values are in good quantitative agreement with the experimental ones recorded during the deposition (Figure 2A and B). One could note that in addition to ZnO deposition a small part of the current is associated with the reduction of eosin (reaction e). Considering the two-electron curve in Figure 8A allows us to evaluate this contribution to about 50 µA‚cm-2 for the highest concentration of eosin (100 µmol‚L-1). It is negligible as compared to the amplitude of the ZnO deposition current. The origin of the constant value of the ZnO deposition current can be attributed to either diffusion or kinetic limitations. However, owing to the concentration of 5 mmol‚L-1 of ZnCl2, which is much higher than that of dissolved oxygen (about 0.8 mM), the diffusion limitation of zinc ions through step c is not likely. As supported by previous studies and the fact that the absolute value is consistent with previous measurements, the flux of zinc oxide formation corresponds more likely from the diffusion limitations of oxygen through step a. Kinetic limitations are reduced due to the fact that oxygen reduction is also catalytically activated in the presence of eosin.22 However, one can note that the current densities in the present case are still lower than for oxygen reduction in pure KCl.37 As a consequence, the zinc oxide flux is given by

FZn ) A

(5)

with A being constant for a fixed hydrodynamic regime and bulk oxygen concentration. Equations 4 and 5 are key relations that allow us to simulate the global composition of the film, as expressed in the ZnO concentration ratio

R)

A A + B[EZ]b

(6)

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Figure 9. Schematic synthesis of the growth mechanism of ZnO/eosin hybrid thin films as a function of the eosin concentration under the conditions indicated in the caption of Figure 2. Oxygen reduction and eosin insertion are under diffusion control. r00 represent the equivalent growth rate of a bulk ZnO film without additives (line 1). Patterned zones in red and black together correspond to the contribution of the total porosity on the final growth rate of the film (distance between lines 3 and 1). The red patterned zone corresponds to the contribution of the porous network volume filled by eosin. The black patterned zone corresponds to the contribution of the empty part of the porous network. E1 represents the transition between occluded eosin and open porosity growth regimes, situated below a concentration of 10 µM in the present case. E2 represents the transition between adherent and nonadherent layer growth regimes, situated around 100 µM in the present case. (Top) Our speculative view of the evolution of the porous layer structure, associated with the progressive filling of the total porosity. Rectangular shapes of ZnO and pores are indicated for simplicity, but in reality complex and branched shapes are more likely.

with A being about 6.5 × 10-9 mol‚cm-2‚s-1 and B being about 0.3 cm‚s-1. The previous analysis corresponds to the classical approach to determine the global composition of a deposit, such as alloys involving composition variation. In some cases, surface sites can be introduced into these models. In the present case, it would be much more complicated. However, these treatments are limited to global composition modeling; they do not address the internal organization of the layer. They also do not consider the presence of empty space. In the absence of empty space, the volume of the deposit would be

V ) nZnOVm,ZnO + nEYVm,EY ) (AVm,ZnO + B[EZ]bVm,EY)t ) r0St (7) where r0 is the growth rate in the absence of empty space and Vm,X represents the volumetric masses of zinc oxide and eosin. It is a linear function of the concentration of eosin in solution as shown in the Figure. If we now consider the presence of empty space with volume Ve noted, then the total volume is

V t ) V + Ve

(8)

r ) r0 + re([EY])

(9)

The growth rate is

re([EY]) is a function of the spatial aspects of the growth (68) Goux, A.; Dunsch, L.; Pauporte´, T.; Lincot, D. To be submitted for publication.

mechanism, which cannot be addressed by the classical models directly. However, it is easily determined by experimental growth rate measurements as made in this work. In the present case, we have observed that this function presents a maximum for a concentration of eosin of about 30 µmol‚L-1. Figure 9 gives a generic overview of the results of this study in terms of the composition and structural evolution as a function of eosin concentration. It represents the variation of the growth rate and its relation to the internal components of the layer: ZnO, eosin, and empty space. Line 1 corresponds to the growth rate of the equivalent thickness of ZnO, which is zero, explaining horizontal line 1. The zone delimited by lines 1 and 2 corresponds to the growth rate of the equivalent thickness of eosin, which is proportional to eosin concentration. Thus, line 2 corresponds to the growth rate expressed in eq 7. The zone delimited by lines 2 and 3 corresponds to the growth rate of the empty space of the film, corresponding to re([EY]) introduced in eq 9. Line 3 corresponds to the overall growth rate r given in eq 9 and to the behavior observed experimentally (Figure 5). The growth rate of the equivalent thickness of the total porosity is given by the distance between lines 3 and 1. Unlike its empty part re([EY]), which presents a maximum, it is continuously increasing with the concentration of eosin. The internal structure of the films between ZnO, EY, and empty space components can be addressed further by considering the fact that ZnO is forming a continuous phase, which is the only way to explain the single-crystalline character of individual grains. This means that ZnO branches are all interconnected. We have also observed that no growth was observed without Zn in solution. As a consequence, the total thickness of the films must correspond to the height of the ZnO fibers or branches. As the

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Langmuir, Vol. 22, No. 25, 2006 10553

total volume of the film is increasing, the ZnO phase must become more and more porous as observed experimentally. From the SEM views, ZnO forms elongated fibers that can be approximated by columns as illustrated in Figure 9. At low eosin concentrations, the growth of columns is favored with mostly empty space. When the eosin concentration increases, eosin tends to fill the empty space created by the formation of the ZnO network as illustrated in Figure 9 in the case of high eosin concentration. The maximum in the empty space fraction is the result of competition between creating more empty space and an increased growth rate and the introduction of more and more eosin into the deposit. This explains very well the opposite variation of the eosin volume and the empty volume fractions observed experimentally in Figure 7. As a consequence, from the experimental results and without any detailed characterization of the internal structure of the films, it has been possible to propose interpretations involving the key characteristics of the nanostructure of the hybrid films and its evolution. The next step is now to address more precisely the processes on the molecular scale that are responsible for the nanostructure formation and self-assembly of the films. Eosin is very specific because it can be easily adsorbed onto the ZnO surface, via surface bonding, which should lead to growth frustration and poisoning, and on the other side, it catalyzes the reduction of oxygen and thus promotes the growth of ZnO in its immediate vicinity.22 It can act as an inhibitor and an accelerator of ZnO growth at the same time. Another aspect is the observation that the hybrid grains present rounded-top surfaces that look similar to the morphology of metallic nanostructures obtained by diffusion-limited aggregation by electrodeposition. This is shown in the case of copper.69 In the present case, the two components of the structure of the films are indeed deposited under a diffusion limitation that makes the DLA mechanism possible in the present case. Further studies are in progress.

from those obtained for pure ZnO films. Each grain is a single ZnO crystal with a fibrous nanoporous structure containing zinc oxide, eosin, and empty space. The variations of the characteristics of the films (morphology, composition, and growth rate) have been precisely determined as a function of the dye concentration in the solution. Strong experimental correlations have been established. It appears that the structure of the film is primarily dependent on the dye concentration. The flux of zinc oxide is independent of the dye concentration, which has been attributed to diffusion limitation effects of oxygen, corresponding more likely to a mixed kinetic regime. On the contrary, the flux of eosin into the film is shown to be proportional to the eosin concentration in the bath. It has been related to the diffusion limitation of eosin. The increased growth rate of the film is partially due to the increased eosin content and is predicted by a simple model. An additional component is related to the presence of empty space, which presents a maximum for an eosin concentration of about 30 µmol‚L-1. A very important outcome of this study is the introduction of the concept of total porosity, in which filling is the result of competition between eosin insertion in the pore and additional pore-formation mechanisms. The total porosity increases continuously to a level of about 65%. This total open porosity forms a unique porous network that can be filled either by the eosin or empty space. The electrodeposition of the ZnO/eosin system appears to be a very interesting case for the study of underlying mechanisms. Some key aspects have been evidenced in this work, but further studies are needed that extend the experimental window (oxygen concentration, hydrodynamic regime, and potential). Also, there is a need to address the characterization of the internal structure specifically, which has been shown in the present work to present well-defined behavior and may result from a digital limited aggregation mechanism. This hypothesis has been considered in a first attempt to modelize such a system by a statistical approach.70

Conclusions

Acknowledgment. We are very grateful to J.-L. Pastol for the FESEM measurements.

The ZnO/reduced eosin Y hybrid films have a “cauliflower” structure with round porous grains that are completely different (69) Schro¨ter, M. Die Fingermorphologie in der Elektrodeposition, ein komplexes Grenzfla¨chenpha¨nomen. Ph.D. Thesis, Uni-Magdeburg, 2003. See also http://chaos.utexas.edu/∼schroeter/.

LA061199H (70) Aarao Reis, F. D. A.; Badiali, J. P.; Pauporte´, Th.; Lincot, D. J. Electroanal. Chem., in press.