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Role of the Critical Micelle Concentration in the Electrochemical Deposition of Nanostructured ZnO Films under Utilization of Amphiphilic Molecules Cathrin Boeckler, Torsten Oekermann,* Armin Feldhoff, and Michael Wark Leibniz UniVersita¨t HannoVer, Institute of Physical Chemistry and Electrochemistry, Callinstrasse 3-3A, 30167 HannoVer, Germany ReceiVed May 31, 2006. In Final Form: August 17, 2006 A detailed understanding of micelle formation that occurs above a critical micelle concentration (cmc) is a crucial point for the surfactant-assisted preparation of porous materials such as molecular sieves. However, the role of the cmc in the surfactant-assisted electrodeposition of porous oxides is widely unknown. In this study, we investigated the electrodeposition of ZnO films under utilization of alkyl sulfates and alkyl sulfonates with different chain lengths. Cmc values of the surfactants were measured directly in the electrodeposition bath by surface tension measurements. Subsequently, we performed electrodeposition with surfactant concentrations from above the cmc down to concentrations well below the cmc. Beside a lamellar ZnO phase already known from earlier studies, a second nanoparticular ZnO phase was found at concentrations below the cmc.
Introduction Porous crystalline oxide semiconductor films are of interest for numerous applications such as in membranes, sensors, and dye-sensitized solar cells.1,2 Electrodeposition of such films is a low-temperature method, which has several advantages, e.g., use of less energy in the production process and the possibility to deposit films on flexible plastic substrates. ZnO films can be electrodeposited cathodically from aqueous zinc salt solutions in the presence of oxidants such as NO3- or O2.3,4 The electrodeposition of nanoporous ZnO was first demonstrated by addition of water-soluble organic dye molecules with anionic groups such as -SO3- or -COO-, which adsorb on the growing ZnO surface, to the electrodeposition bath.5,6 Especially ZnO films co-deposited with the dye eosin Y from O2-saturated ZnCl2 solution were found to be highly porous and exhibit spongelike structures that can be described as porous single crystals of ZnO.7 Due to the reduced number of grain boundaries, these films exhibit much better electron transport properties than porous ZnO films prepared from nanoparticles. This can be beneficial for the use of the films in dye-sensitized solar cells.8 More recently, addition of amphiphilic molecules with anionic groups such as sodium dodecyl sulfate was also found to lead to the formation of porous ZnO.9-11 A lamellar structure with alternating layers of ZnO and amphiphilic molecules was reported for films electrodeposited in the presence of surfactants such as * Corresponding author. Email:
[email protected] (1) Gra¨tzel, M. Prog. PhotoVolt. Res. Appl. 2000, 8, 171. (2) Hagfeld, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (3) Izaki, M.; Omi, T. Appl. Phys Lett. 1996, 68, 2439. (4) Peulon, S.; Lincot, D. AdV. Mater 1996, 8, 166. (5) Yoshida, T.; Miyamoto, K.; Hibi, N.; Sugiura, T.; Minoura, H.; Schlettwein, D.; Oekermann, T.; Schneider, G.; Wo¨hrle D. Chem. Lett. 1998, 7, 599. (6) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214. (7) Yoshida, T.; Pauporte, T.; Lincot, D.; Oekermann, T.; Minoura, M. J. Electrochem. Soc. 2003, 150, C608. (8) Oekermann, T.; Yoshida, T.; Minoura, H.; Wijayantha, K. G. U.; Peter, L. M. J. Phys. Chem. B 2004, 108, 8364. (9) Choi, K. S.; Lichtenegger, H. C.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 12402. (10) Tan, Y.; Steinmiller, E. M. P.; Choi, K. S. Langmuir 2005, 21, 9618. (11) Michaelis, E.; Wo¨hrle, D.; Rathousky, J.; Wark, M. Thin Solid Films 2006, 497, 163.
sodium dodecyl sulfate, alkyl phosphates, and alkyl sulfonates.9,10 This structure is quite different from that found in the sol-gelbased synthesis of mesoporous molecular sieves, where micelles of surfactants or block copolymers serve as templates for cubic or hexagonal pore arrangements.12 A detailed understanding of the micelle formation that occurs above a critical micelle concentration (cmc) is a crucial point for the tailoring of the porosity in such molecular sieves. Typically cmc values reported in the literature were obtained from concentration-dependent conductivity measurements at room temperature in pure solvent.13 Taking into account a cmc of 8.1 mM for dodecyl sulfate (DDS) in pure water, Tan et al.10 varied the concentration of DDS as an additive for the electrodeposition of ZnO in a broad range between 0.02 and 12.5 wt % ()0.7 mM-0.43 M). Assuming the use of surfactant in concentrations both below and above the cmc, no change of the film structure depending on the cmc was reported.10 However, elevated temperatures and high concentrations of foreign ions, as used in the electrodeposition of ZnO, are expected to change the cmc. Cmc values of most surfactants are known to pass through a minimum at about room temperature and increase toward higher temperatures,14 while an increase of the ionic strength of a solution decreases the cmc.15 A number of cmc values for many surfactants measured by different methods, at different temperatures and in different solutions, has been summarized by Mukerjee and Mysels.16 On the other hand, data for cmc values that were measured exactly under ZnO electrodeposition conditions are not available in the literature. An estimation of cmc values in the electrodeposition bath would be possible using empirical formulas that have been suggested to describe the dependence of the cmc on the temperature14 and the ionic strength17 of the solution. However, such investigations (12) Ying, J.Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem. 1999, 38, 58. (13) Do¨rfler, H. D. Grenzfla¨chen und Kolloidchemie: VCH: Weinheim, 1994; p 225. (14) Kim, H. U.; Lim, K. H. Bull. Korean Chem. Soc. 2003, 24, 1449. (15) Kadish, K. M.; Maiya, G. B.; Araullo, C.; Guilard, R. Inorg. Chem. 1989, 28, 2725. (16) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971. (17) Huang, Y. X.; Tan, R. C.; Li, Y. L.; Yang, Y. Q. Yu, L.; He, Q. C. J. Colloid Interface Sci. 2001, 236, 28.
10.1021/la0615544 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2006
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Table 1. Critical Micelle Concentrations of Different Additives Measured in Pure Water at 20 °C and in the Electrodeposition Bath at 70 °C cmc, mmol L-1
additive
formula
decyl sulfate (DS) dodecyl sulfate (DDS) dodecyl sulfonate (DDSO) hexadecyl sulfonate (HDSO)
CH3(CH2)9SO4CH3(CH2)11SO4CH3(CH2)11OSO3CH3(CH2)15OSO3-
in in the water, deposition 20 °C bath, 70 °C 18 8 2 0.4
8 0.6 0.6 0.2
have only been carried out within rather limited temperature and concentration ranges so far. In this study, we therefore determined the cmc values for several alkyl sulfates and alkyl sulfonates with different chain lengths experimentally directly in the electrodeposition bath. Subsequently, we performed electrodeposition of ZnO films in the presence of surfactant concentrations both above and below the cmc in order to elucidate the influence of the micelle formation on the structure and porosity of the deposited films. Experimental Section Measurement of the Critical Micelle Concentrations (cmc). Surface tension was measured with a PHYWE ring tensiometer. Film Preparation. Electrochemical deposition was carried out at -1.0 V vs SCE for 30 min in a stirred (300 rpm) O2-saturated aqueous solution of ZnCl2 (5 mM) and KCl (0.1 M) maintained at 70 °C. Glass coated with fluorine-doped tin oxide (FTO-glass) was used as substrate. Prior to the deposition it was ultrasonically cleaned in acetone and ethanol for 30 min each, etched in 45% nitric acid for 2 min, and finally rinsed with water. The cleaned FTO-glass sheets (size 2 cm × 4.5 cm) were connected as working electrodes in a three-electrode setup with a pure Zn wire as counter electrode and an Ag/AgCl reference electrode. Different sulfates and sulfonates (Table 1) were added to the bath as sodium salts in different concentrations between 50 µM and 10 mM (depending on the obtained cmc values). The deposited films were rinsed with water and dried at room temperature in air. Film Characterization. The surface morphology of the films was studied by scanning electron microscopy (SEM) in secondary electron contrast using a JEOL JSM-6700F field-emission microscope equipped with an Oxford Instruments INCA 300 energy-dispersive X-ray spectrometer (EDXS) with an ultrathin window that allows local elemental analysis. X-ray diffraction (XRD) was measured with a Philips X’pert diffractometer, using Cu KR radiation. For (scanning) transmission electron microscopy ((S)TEM) in brightfield modes, a JEOL JEM-2100F UHR field-emission instrument was used at 200 kV. Specimens for TEM investigations were obtained by tweezer-scratching ZnO films from the substrate onto 300 mesh copper-supported carbon films (Quantifoil). Kr adsorption for measurement of the porosity was carried out at 77 K with a Micrometics ASAP 2010 after desorption of the surfactant with ethanol in 20 h.
Results and Discussion Determination of Critical Micelle Concentrations. Cmc values of the surfactants in water at room temperature as well as in the electrodeposition bath were determined by surface tension measurements. This method is suitable especially for measurements in the electrodeposition bath, since it is not affected by the high concentration of foreign ions, which would for example be the case in conductivity measurements, which are often used to determine cmc values. The results of the surface tension measurements conducted in the electrodeposition bath at 70 °C are shown in Figure 1. Below the cmc, the surface tension of an aqueous surfactant-containing solution decreases with increasing
Figure 1. Surface tension of the electrodeposition bath at 70 °C containing different concentrations of (a) OS (1) and DS (0) and (b) DDSO (4), DDS (b), and HDSO ([).
surfactant concentration due to the formation of an adsorption layer at the solution/air interface. When the adsorption layer is saturated, the surface tension remains constant and additional surfactant molecules form micelles in the solution. This is seen in Figure 1 for decyl sulfate (DS) (Figure 1a) as well as dodecyl sulfate (DDS), dodecyl sulfonate (DDSO), and hexadecyl sulfonate (HDSO) (Figure 1b), proving that these surfactants form micelles. The cmc values determined from Figure 1 together with the respective values determined in pure water at room temperature are summarized in Table 1. The cmc values measured in pure water are all in good agreement with literature data,16 e.g., with the already mentioned value of 8.1 mM for DDS.18 It can be seen that the cmc values in the electrodeposition bath are significantly lower than those obtained in water. The rather high concentration of foreign ions more than compensates for the effect of the increased temperature on the cmc, which can be expected on the basis of the results of other authors, since an increase in the temperature by 50 K was usually found to increase the cmc by a factor ,2,14,16 while high salt concentrations typically decrease the cmc by almost 1 order of magnitude.16 Also in accordance with findings of others,16 sulfates and sulfonates with carbon chains of less than 10 C atoms showed no change of surface tension with increasing concentration and, therefore, no micelle formation in pure water, and the same result was found in the electrodeposition bath as seen for octyl sulfate (OS) as an example in Figure 1a. (18) Shanks, P. C.; Franses, E. I. J. Phys. Chem. 1992, 96, 1794.
Deposition of Nanostructured ZnO Films
Co-Deposition of ZnO with Sulfonates and Sulfates. For ZnO films deposited in the presence of MS, BS, HSO, and OS, which do not form micelles in the electrodeposition bath, no significant influence on the film morphology was observed in SEM images. All these films consisted of hexagonal ZnO crystals grown with the c-axis perpendicular to the FTO substrate, which are also seen in ZnO films electrodeposited without any additives.4,7 Thus, it can already be concluded that formation of micelles is a prerequisite for structure directing by amphiphilic molecules in the electrodeposition of ZnO. On the other hand, the micelle-forming molecules DS, DDS, DDSO, and HDSO significantly alter the morphology of the ZnO. Two different ZnO phases were found in all four cases, depending on the surfactant concentration in the electrodeposition bath: at concentrations above the cmc, a single phase was observed; at concentrations below the cmc, however, a second phase appeared. In the following the formation of the two phases is exemplarily shown for the surfactant decyl sulfate (DS) added to the electrodeposition bath in different concentrations (Figure 2). For all other surfactants similar series of SEM images of ZnO films were found, only the concentrations leading to the formation of the different phases vary according to differences in the cmc. Concentrations above the cmc result in disklike particles with the disks partly agglomerated (Figure 2d). Investigation by TEM revealed the presence of a lamellar structure of the disks (Figure 3a). The formation of such lamellar phase was also reported by other authors.9,10 EDXS results reveal high sulfur content for the lamellarstructured phase, proving the incorporation of surfactant layers between the ZnO sheets, as shown in the model in Figure 3b. The anionic headgroups of the surfactant molecules interact with the ZnO surface, forming alternating layers of surfactant molecules and ZnO during the deposition process. Obviously this lamellar microstructure is the reason for the formation of the disklike particles on the macroscopic scale. For all the tested surfactants at concentrations above the true cmc the lamellar structure was also detected by small-angle XRD measurements. The obtained patterns for DS, DDS, and HDSO are shown in Figure 4. The presence of high-order 00l reflection peaks is indicative for well-defined nanostructures. The 001 peaks appear at 3.3, 3.1, and 3.0 nm for the films deposited with HDSO, DDS and DS, respectively. As expected, d001 decreases with decreasing length of the hydrophobic chains. The lamellar structure and disklike particles were also formed at concentrations below but close to the cmc, as exemplified for DS in Figure 2c; however, a second ZnO phase appears in this concentration range. This second phase can be characterized as nanoparticulate ZnO with a particle size of 20-50 nm, as seen in the SEM and TEM images in Figure 5. EDXS measurements showed that these ZnO particles do not contain any sulfur, which means that no surfactant molecules are incorporated. Kr adsorption measurements at films templated with DS after desorption of the surfactant with ethanol showed a porosity of 76 cm2/cm2 for a film with lamellar structure, while no significant Kr uptake was detected for films consisting of the nanoparticels, indicating that the particles have no inner porosity. On the other hand, the almost spherical ZnO particles of the nanoparticulate phase are surprisingly small in comparison with ZnO deposited in the absence of surfactant molecules (Figure 2a), in which ZnO columns with lengths in the micrometer-regime and diameters of 200-300 nm are formed. To explain this remarkable result it can be assumed that adsorption of the surfactant molecules on the surface of the ZnO particles blocks the growth. As long as the molecules only reversibly adsorb to the surface, they cannot be detected in
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Figure 2. SEM images of ZnO deposited (a) without additive and with DS (b) at a concentration far below the cmc (2.5 mM), (c) at a concentration below but close to the cmc (6 mM), and (d) at a concentration above the cmc (10 mM).
the deposited films by EDXS measurements. It can also be concluded that surfactants with long carbon chains adsorb more strongly, since surfactants with short carbon chains did not lead to the formation of nanoparticles. A possible explanation for the stronger adsorption of surfactants with long chains is the higher gain of free energy from the hydrophobic interactions between the alkyl chains of surfactants adsorbed on the surface, since the compensation of the decrease in entropy by this gain in free energy is a prerequisite to enable strong adsorption of a surfactant on a surface.19 Other possible explanations are the higher electronpushing effect of a longer alkyl chain, which may lead to stronger interaction of the surfactant with Zn2+ ions at the ZnO surface and the lower solubility of surfactants with longer chains in the aqueous solution, which may lead to lower desorption rates. (19) Wa¨ngnerud, P.; Jo¨nsson, B. Langmuir 1994, 10, 3268.
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Figure 3. (a) TEM image of lamellar-structured ZnO electrodeposited with a DSS concentration of 6 mM. (b) Schematic model of the lamellar structure.
Figure 5. (a) SEM and (b) TEM micrographs of the nanoparticulate ZnO phase in a ZnO film deposited with a DDS concentration of 6 mM.
Figure 4. Small-angle XRD patterns of ZnO films deposited with HDSO, DDS, and DS at concentrations above the cmc.
It is further known that surface micelles can already form below the cmc because the surface forces increase the interfacial concentration of the amphiphiles. The concentration necessary for the formation of surface micelles was estimated to be about one-third to one-half of the cmc in solution.20 Since the lamellar structure starts to form in the same concentration range, the ability to form surface micelles seems to be a prerequisite for the formation of this phase. The formation of the surfactant layers can thus be regarded as a “flattening” of surface micelles or the combination of several surface micelles.
Conclusions Two ZnO phases were formed during the surfactant-assisted ZnO electrodeposition depending on the surfactant concentration. (20) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558.
The already known lamellar ZnO phase was found only for surfactant concentrations close to and above the cmc. The formation of micelles or surface micelles was therefore shown to be essential for the formation of the lamellar phase. A second, hitherto unknown nanoparticulate ZnO phase with nanocrystallites of about 20-50 nm size was found at surfactant concentrations below the cmc. At concentrations well below the cmc, where no surface micelles are seen, only the nanoparticulate phase is formed. The results confirm that the critical micelle concentration plays an important role for the texturing of electrodeposited ZnO films. We could show that the cmc is strongly influenced by the temperature and ionic strength of the electrodeposition bath. The cmc values were found to be significantly lower under electrodeposition conditions compared to the cmc in pure water at room temperature. Consequently, it is essential to measure the cmc directly in the electrodeposition bath under deposition conditions. Acknowledgment. The authors thank J. Caro for fruitful discussions and J. Rathousky, J. Heyrovsky Institute of Physical Chemistry, Prague, for the Kr adsorption measurements. LA0615544
