Fabrication of Composite Films Containing Zirconia and Cationic

Obtained results pave the way for the electrodeposition of other polymer-ceramic composites. The deposits were studied by thermogravimetric analysis, ...
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Langmuir 2004, 20, 2921-2927

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Fabrication of Composite Films Containing Zirconia and Cationic Polyelectrolytes Xin Pang and Igor Zhitomirsky* Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 Received November 23, 2003. In Final Form: January 12, 2004 Composite films were prepared by electrophoretic deposition of poly(ethylenimine) or poly(allylamine hydrochloride) combined with cathodic precipitation of zirconia. Films of up to several micrometers thick were obtained on Ni, Pt, stainless-steel, graphite, and carbon-felt substrates. When the concentration of polyelectrolytes in solutions and the deposition time were varied, the amount of the deposited material and its composition can be varied. The electrochemical intercalation of yttria-stabilized zirconia particles into the composite films has been demonstrated. Obtained results pave the way for the electrodeposition of other polymer-ceramic composites. The deposits were studied by thermogravimetric analysis, X-ray diffraction analysis, scanning electron microscopy, and atomic force microscopy. The mechanisms of deposition are discussed.

Introduction Organic-inorganic composites have attracted substantial attention because of the potential of combining advanced properties of organic and inorganic components.1 Formation of composite films based on polyelectrolytes and inorganic nanoparticles has become the subject of extensive experimental work. Many important studies focused on layer-by-layer assembly,2,3 which utilizes electrostatic interactions between oppositely charged species. This technique offers researchers a strategy for forming a wide variety of advanced organic-inorganic composites.4 Considerable attention has been given to the fabrication of novel composites based on cationic polyelectrolytes such as poly(diallyldimethylammonium chloride) (PDDA), poly(ethylenimine) (PEI), and poly(allylamine hydrochloride) (PAH). There is a growing interest in the application of composite materials in humidity sensors,5 batteries,6 microelectronic devices,7 magnetic memory,8 and quantum dot devices.9 Novel composite materials containing inorganic nanoparticles embedded * Author to whom correspondence should be addressed. Phone: (905) 525-9140 extension 23914. Fax: (905) 528-9295. E-mail: [email protected]. (1) (a) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (b) Collins, D. E.; Slamovich, E. B. Chem. Mater. 1999, 11, 2319. (c) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (2) (a) Decher, G.; Hong, J. D. Macromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232. (3) (a) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877. (b) Cassagneau, T.; Gue´rin, F.; Fendler, J. H. Langmuir 2000, 16, 7318. (c) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515. (d) Jiang, X.; Hammond, P. T. Langmuir 2000, 16, 8501. (e) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (4) (a) Tyan, H.-L.; Liu, Y.-C.; Wei, K.-H. Chem. Mater. 1999, 11, 1942. (b) Dante, S.; Hou, Z.; Risbud, S.; Stroeve, P. Langmuir 1999, 15, 2176. (c) Dutta, A. K.; Jarero, G.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 176. (d) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (e) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1996, 8, 1575. (5) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1995, 7, 2327. (6) (a) Fojas, A. M.; Murphy, E.; Stroeve, P. Ind. Eng. Chem. Res. 2002, 41, 2662. (b) Zhang, L.; Dutta, A. K.; Jarero, G.; Stroeve, P. Langmuir 2000, 16, 7095. (7) (a) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H.-N.; Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526. (b) Fang, M.; Kim, C. H.; Mallouk, T. E., Chem. Mater. 1999, 11, 1519. (c) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (8) Liu, Y.; Wang, A.; Claus, R. O. Appl. Phys. Lett. 1997, 71, 2265.

in a polyelectrolyte matrix10 are currently under investigation for catalytic, optical, and other applications. Composite films based on polyelectrolytes are also of great importance for nanoengineering of particle surfaces,11 surface modification of fibers, and fabrication of advanced fiber-reinforced composites.12 Polyelectrolyte multilayers were found to be very effective nucleating agents for hydroxyapatite crystals13 and magnetic nanoparticles.14 Electrophoretic deposition (EPD) of composite materials is an area of intense interest.15 Many important advantages of EPD can be cited,15 which make this technique important for various applications. EPD can be applied to polyelectrolytes and inorganic nanoparticles for the fabrication of advanced composite materials. Moreover, EPD could be combined with other electrochemical strategies.16 Previous research, motivated by the importance of polyelectrolytes in materials science and surface engineering, addressed the possibility of deposition of the composite films based on PDDA.17 Cathodic EPD of PDDA, (9) Dutta, A. K.; Ho, T.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 1042. (10) (a) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M.; Decher, G. Chem. Mater. 2001, 13, 4661. (b) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 2513. (c) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (d) Lvov, Y.; Munge, B.; Giraldo, O., Ichinose, I.; Suib, S. L.; Rusling, J. F. Langmuir 2000, 16, 8850. (e) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (f) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (11) (a) Caruso, F. Adv. Mater. 2001, 13, 11. (b) Shi, X.; Caruso, F. Langmuir 2001, 17, 2036. (12) Cinibulk, M. K. J. Am. Ceram. Soc. 1997, 80, 453. (13) Ngankam, P. A.; Lavalle, Ph.; Voegel, J. C.; Szyk, L.; Decher, G.; Schaaf, P.; Cuisinier, F. J. G. J. Am. Chem. Soc. 2000, 122, 8998. (14) (a) Shchukin, D. G.; Radtchenko, I. L.; Sukhorukov, G. B. J. Phys. Chem. B 2003, 107, 86. (b) Shchukin, D. G.; Radtchenko, I. L.; Sukhorukov, G. B. Mater. Lett. 2003, 57, 1743. (15) (a) Boccaccini, A. R.; Trusty, P. A. J. Mater. Sci. 1998, 33, 933. (b) Moreno, R.; Ferrari, B. Am. Ceram. Soc. Bull. 2000, 79, 44. (c) Van der Biest, O.; Vandeperre, L. J. Annu. Rev. Mater. Sci. 1999, 29, 327. (d) Kaya, C.; Boccaccini, A. R.; Trusty, P. A. J. Eur. Ceram. Soc. 1999, 19, 2859. (e) Zhitomirsky, I. Adv. Colloid Interface Sci. 2002, 97, 279. (f) Limmer, S. J.; Seraji, S.; Wu, Y.; Chou, T. P.; Nguyen, C.; Cao, G. Adv. Funct. Mater. 2002, 12, 59. (16) Russ, B. E.; Talbot, J. B. J. Electrochem. Soc. 1998, 145, 1253. (17) (a) Zhitomirsky, I.; Petric, A. Mater. Sci. Eng., B 2000, 78, 125. (b) Zhitomirsky, I.; Petric, A. Mater. Lett. 2000, 42, 273. (c) Zhitomirsky, I.; Petric, A. J. Mater. Chem. 2000, 10, 1215.

10.1021/la0361971 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/26/2004

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which is a strong cationic polyelectrolyte, has been combined with the electrochemical precipitation of inorganic nanoparticles at the cathode surface17 for the fabrication of the composite films. A deposition model has been developed based on the electrostatic attraction of PDDA and the oppositely charged colloidal particles formed at the electrode surface.17 Continuing work on the process resulted in the fabrication of superparamagnetic films based on Fe3O4 and PDDA.18 The composition, microstructure, and morphology of the films could be tailored by varying the bath composition and masstransport conditions for the organic and inorganic components. Right now, an important task is to further extend the use of new electrochemical methods to the electrodeposition of novel composite materials. The goal of the present paper is to achieve the electrodeposition of the composite films based on zirconia and weak cationic polyelectrolytes such as PEI and PAH. Experimental Section Commercial purity ZrOCl2‚8H2O (Fluka, Messerschmittstrasse, Germany), yttria-stabilized zirconia (YSZ, TZ-8Y, Tosoh, Sinnanyo-shi, Japan, crystallite size ∼22 nm), polyethylenimine (MW 70 000, Aldrich, Milwaukee, WI), and poly(allylamine hydrochloride) (MW 70 000, Aldrich, Milwaukee, WI) were used as starting materials. Transparent and stable solutions of 5 mM ZrOCl2 containing 0-1 g/L PEI in a methanol-water (5 vol % water) solvent were prepared for electrodeposition. Composite films were also obtained from the solutions containing particles of YSZ. The particle concentration in solutions was in the range of 0-4 g/L. Electrodeposition of PAH-zirconia composites was performed from aqueous 5 mM ZrOCl2 solutions containing 0-1.5 g/L PAH (pH 2.2-2.4). Cathodic deposits were obtained by the galvanostatic method on Pt, Ni, stainless-steel foils, graphite plates, and carbon fibers and felts (Lydall) at current densities ranging from 1 to 5 mA/cm2. The electrochemical cell for deposition included a cathodic substrate (15-30 cm2) centered between two parallel platinum counter electrodes. The volume of the EPD bath was 300 mL. Deposit weights were obtained by weighing the Pt substrates before and after the deposition experiments, followed by drying at room temperature for 24 h. After drying at room temperature, the deposits were scraped from the Pt electrodes for thermogravimetric (TG) and X-ray diffraction (XRD) analysis. The thermoanalyzer (Netzsch STH-409) was operated in air between room temperature and 1200 °C at a heating rate of 5 °C/min. The phase content was determined by XRD analysis with a diffractometer (Nicolet I2) using monochromatized Cu KR radiation at a scanning speed of 0.5°/min. The microstructures of the deposited films were studied using a Philips 515 scanning electron microscope (SEM). The surface topography of the prepared films was imaged using atomic force microscopy (AFM). An atomic force microscope (NanoScopeIIIa, Digital Instruments) was used in tapping mode.

Figure 1. Deposit weight versus deposition time for the deposits prepared from the 5 mM ZrOCl2 + 0.4 g/L PEI solutions on a Pt substrate at a current density of 5 mA/cm2.

Figure 2. TG data for the deposits obtained from the 5 mM ZrOCl2 solutions at a current density of 5 mA/cm2 (heating rate ) 5 °C/min).

Experimental Results Electrodeposition experiments performed from the 5 mM ZrOCl2 solutions containing 0-1 g/L PEI or 0-1.5 g/L PAH resulted in the formation of cathodic deposits. Figure 1 shows the deposit weight as a function of the deposition time for the 5 mM ZrOCl2 solutions containing 0.4 g/L PEI. Nearly linear dependence was observed. Similar dependencies were obtained for the solutions containing PAH. These data indicate that the amount of the deposited material can be controlled by a variation of deposition time at a constant current density. TG data for the deposits prepared from the 5 mM ZrOCl2 solutions in a methanol-water (5 vol % water) solvent exhibited weight loss, which can be attributed to the (18) Zhitomirsky, I.; Niewczas, M.; Petric, A. Mater. Lett. 2003, 57, 1045.

Figure 3. TG data for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.4 g/L PEI (a) and 1 g/L PEI (b) at a current density of 5 mA/cm2 (heating rate ) 5 °C/min).

dehydration of the deposits (Figure 2). Two distinct steps in the TG curve are distinguished. A sharp reduction of the sample weight was observed in the range of 20-150 °C. An additional step in the weight loss was recorded in the range of 310-350 °C. At higher temperatures, the weight changed gradually. The total weight loss at 1200 °C was found to be 18.4% of the initial sample weight. Figure 3 compares the results of the TG analysis for deposits obtained from the 5 mM ZrOCl2 solutions in a methanol-water (5 vol % water) solvent containing

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Figure 5. XRD pattern of a composite film prepared from the 5 mM ZrOCl2 solutions containing 1 g/L PEI and 4 g/L YSZ (b ) YSZ, 2 ) Ni substrate).

Figure 4. XRD patterns for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.4 g/L PEI as prepared (a) and after annealing for 1 h at 200 (b), 400 (c), 600 (d), and 800 °C (e) (2 ) tetragonal zirconia, b ) monoclinic zirconia).

different amounts of PEI. The total weight loss at 1200 °C for deposits prepared from the solutions containing 0.4 g/L PEI and 1 g/L PEI was found to be 41.8 and 63.7 wt %, respectively. No appreciable weight change was observed at temperatures exceeding 550 °C. These results, coupled with the TG data for deposits prepared from the pure ZrOCl2 solutions (Figure 2), indicate the possibility of electrochemical codeposition of the PEI and zirconium species. The deposits prepared from the solutions containing PEI show weight loss (Figure 3), attributed to the dehydration of an inorganic phase, and additional weight loss related to the burning out of an organic phase. It may be concluded that the deposition process resulted in the formation of the composite films containing different amounts of PEI. The increase in the PEI concentration in solutions resulted in a higher PEI content in the deposits. The TG data shown in Figures 2 and 3 were used to evaluate the polymer content in the composite deposits. The content of an organic phase was evaluated to be 28.7 and 55.5 wt % for the deposits prepared from solutions containing 0.4 and 1 g/L PEI, respectively. The deposits were analyzed by XRD both before and after annealing in air at different temperatures. The fresh deposits and those heated at 200 °C were amorphous (Figure 4). Further increase of the annealing temperature resulted in the crystallization of tetragonal zirconia, which is the main crystalline phase at 400 and 600 °C. However, it is difficult to distinguish between the cubic and tetragonal zirconia phases owing to peak broadening. Faint peaks of monoclinic zirconia could also be distinguished in the XRD pattern obtained at 600 °C. When the deposits were heated to 800 °C, the XRD pattern displayed peaks of tetragonal and monoclinic zirconia. In this paper, electrodeposition was also performed using YSZ suspensions. YSZ particles were dispersed in a solution containing 5 mM ZrOCl2 and 1 g/L PEI. The prepared suspensions were relatively stable and allowed the formation of the cathodic deposits. Obtained films were characterized by XRD analysis. The X-ray data show peaks of YSZ and indicate the intercalation of the YSZ particles into the composite films (Figure 5). Figure 6 shows the TG data for the deposits prepared from the 5 mM ZrOCl2 aqueous solutions containing different amounts of PAH. The total weight loss at 1200

Figure 6. TG data for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.75 g/L PAH (a) and 1.5 g/L PAH (b) at a current density of 5 mA/cm2 (heating rate ) 5 °C/min).

°C for the deposits prepared from the solutions containing 0.75 and 1.5 g/L PAH was found to be 59.4 and 70.2 wt %, respectively. No appreciable weight change was observed at temperatures exceeding 600 °C. At this point, it is important to note that a weight loss of 32.6% was reported for the deposits obtained from the aqueous ZrOCl2 solutions.19 Therefore, the obtained results, coupled with the TG data for the deposits prepared from the aqueous ZrOCl2 solutions,19 indicate the possibility of electrochemical codeposition of the PAH and zirconium species. The deposits prepared from the solutions containing PAH show weight loss (Figure 6), attributed to the dehydration of an inorganic phase, and additional weight loss related to the burning out of an organic phase. From the TG data shown in Figure 6, it may be concluded that the deposition process resulted in the formation of the composite films containing different amounts of PAH. The increase in the PAH concentration in solutions resulted in a higher PAH content in the deposits. Obtained TG data coupled with the TG data for the zirconium species prepared from the aqueous solutions19 were used to evaluate the polymer content in the composite deposits. The content of an organic phase was evaluated to be 39.8 and 55.8 wt % for the deposits prepared from the solutions containing 0.75 and 1.5 g/L PAH, respectively. X-ray diffractograms of the fresh deposits prepared from the ZrOCl2 solutions containing 0.75 g/L PAH exhibit their (19) Zhitomirsky, I.; Gal-Or, L. J. Mater. Sci. 1998, 33, 699.

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Figure 9. SEM picture of a composite deposit prepared from a 5 mM ZrOCl2 + 0.4 g/L PEI solution on a carbon felt (bar ) 10 µm). Figure 7. XRD patterns for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.75 g/L PAH as prepared (a) and after annealing for 1 h at 200 (b), 400 (c), 600 (d), and 800 °C (e) (2 ) tetragonal zirconia, b ) monoclinic zirconia).

Figure 8. SEM picture of a sectioned deposit obtained from a 5 mM ZrOCl2 + 0.4 g/L PEI solution on a graphite substrate (bar ) 10 µm).

amorphous nature (Figure 7). The deposits thermally treated at 400 °C exhibited a very broad peak near 2θ, ≈32°, but were essentially amorphous. In Figure 7 it can be seen that at 600 °C the deposits exhibit broad peaks, which could be attributed to tetragonal or cubic zirconia. At 800 °C, the transformation to the monoclinic phase was observed. Composite deposits of various thicknesses in the range of up to several micrometers were obtained on Pt, Ni, stainless-steel foils, graphite, and carbon-felt substrates. Figure 8 shows a SEM picture of a composite film containing PEI. The obtained films adhered strongly to the substrates. Moreover, SEM observations indicate that the method enables film formation on substrates of complex shape, such as carbon felt (Figure 9). It was observed that electrodeposition enabled uniform deposition. Parts a and b of Figure 10 show composite deposits on the individual carbon fibers prepared from the solutions containing different amounts of PEI. The deposit thickness was uniform along the fiber length. Deposits of different thicknesses were prepared by the variation of the deposition time (Figure 10b,c). SEM pictures indicate a dense and continuous morphology. The thickness of the deposits on the carbon fibers was in the range of up to 6 µm. Figure 11 compares the AFM images of the composite films on polished stainless-steel substrates obtained from the ZrOCl2 solutions containing PEI. The root-mean-

Figure 10. SEM picture of the composite deposits on individual carbon fibers, obtained from the 5 mM ZrOCl2 solutions containing 0.4 g/L PEI (a) and 1 g/L PEI (b and c) (bar ) 10 µm).

square (rms) surface roughness of the films prepared from the solutions containing 0.4 g/L PEI at a current density of 1.5 mA/cm2 was found to be 8.1 nm. High surface roughness of the films prepared by electrodeposition could be attributed to the gas evolution at the cathode surface during deposition. However, it was established that the surface roughness decreased with the increasing polymer

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motion of charged particles or macromolecules toward the electrode and their accumulation at the electrode surface. Deposit formation is achieved as a result of the coagulation of the colloidal particles and polymer macromolecules.15 EPD of the polyelectrolytes was combined with electrochemical precipitation of the colloidal particles, which were produced by the electrogenerated base method. Cathodic reactions that generate OH- are

2H2O + 2e- f H2 + 2OH-

(1)

O2 + 2H2O + 4e- f 4OH-

(2)

These reactions result in basic conditions at the electrode surface. The difference between the solution pH and that in the layer adjacent to the cathode increases with an increasing current density.20 It is important to note that the formation of the composite deposits is influenced by the behavior of the polyelectrolytes in basic conditions at the electrode surface. Cationic polyelectrolytes can be categorized into two groups: strong polyelectrolytes, for which the degree of ionization is independent of the solution pH, and weak polyelectrolytes, for which the degree of ionization is determined by the solution pH. Previous investigations were focused on the fabrication of the composites based on PDDA and metal oxides or hydroxides.17,18 PDDA is a strong cationic polyelectrolyte, which maintains a positive charge under basic conditions. As a result of the strong electrostatic repulsion of the PDDA macromolecules, no deposit formation was achieved from the aqueous PDDA solutions.17 However, codeposition of PDDA and the colloidal particles of metal oxides and hydroxides was observed.17,18 It was suggested that the deposit formation is driven by Coulombic attraction between two charged species: cationic PDDA and the negatively charged colloidal particles formed at the electrode surface. In this paper, composite materials were prepared using weak cationic polyelectrolytes, such as PEI and PAH. It is important to note that the PEI macromolecules achieve cationicity through protonation of the amine groups in acidic solutions21

[-CH2-CH2-NH-]n + H3O+ f [-CH2-CH2-NH2+-]n + H2O (3)

Figure 11. AFM images of the films prepared from the 5 mM ZrOCl2 solutions containing 0.4 (a) and 1 g/L PEI (b and c) at a current density of 1.5 (a and b) and 3 mA/cm2 (c), with a deposition time of 3 (a and c) and 5 min (b).

content in the deposits. For deposits prepared from solutions containing 1 g/L PEI, the rms surface roughness was found to be 0.91 and 0.88 nm for the current densities of 1.5 and 3 mA/cm2, respectively. Discussion In this paper, EPD of the cationic polyelectrolytes has been utilized for the fabrication of the composite films on cathodic substrates. The ability to deposit composite materials depends on a thorough knowledge of the factors that control the deposition mechanism and structural evolution of the films. Electrophoresis is related to the

The degree of ionization of PEI depends on the amount of acid added.21,22 It was pointed out22 that the PEI macromolecules had no charge before the titration with acid. The experimental data presented above indicate codeposition of the polyelectrolytes and zirconium species. In zirconyl chloride solutions, the formation of tetramers [Zr4(OH)8(H2O)16]8+ can be expected.23 It is important to note that the cationic species in solutions show a tendency to release protons.23 Zirconyl chloride solutions are highly acidic, and the following process was considered:23

[Zr4(OH)8(H2O)16]8+ f [Zr(OH)2+x‚(4 - x)H2O]4(8-4x)+ + 4xH+ (4) (20) Kuhn, A. T.; Chan, C. Y. J. Appl. Electrochem. 1983, 13, 189. (21) Baklouti, S.; Pagnoux, C.; Chartier, T.; Baumard, J. F. J. Eur. Ceram. Soc. 1997, 17, 1387. (22) Zhu, X.; Tang, F.; Suzuki, T. S.; Sakka, Y. J. Am. Ceram. Soc. 2003, 86, 189. (23) Clearfield, A. J. Mater. Res. 1990, 5, 161.

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It is suggested that PEI can be partially protonated in acidic zirconyl chloride solutions. As a result, the PEI macromolecules acquire a positive charge. It is known21,22 that protonated PEI has a positive charge over a wide pH range below pH 11. PAH is another weak cationic polyelectrolyte, which is positively charged in acidic solutions.4,7,10,14 It is expected that the electric field provides electrophoretic motion of the charged PEI and PAH macromolecules toward the cathode substrate. The decrease of the polyelectrolyte charge with an increasing pH at the electrode surface reduces the electrostatic repulsion of the polyelectrolyte macromolecules and promotes their deposition. It is important to note that the electrode reactions are not involved in EPD and relatively thick electrophoretic deposits can be obtained15 by this method. On the other hand, cathodic electrosynthesis of the oxide and hydroxide materials is based on the neutralization of the ionic species by the electrogenerated base (eqs 1 and 2) to form the colloidal particles at the electrode surface. The formation of an insulating layer prevents electrosynthesis of the zirconium species. As a result, the thickness of the composite monolayers was limited to several micrometers. Zirconium species can precipitate as zirconium hydroxide or hydrated zirconium oxide.24 The colloidal particles may become positively or negatively charged depending on the pH of the solution

Zr-OH + H+ f Zr-OH2+

(5)

Zr-OH + OH- f Zr-O- + H2O

(6)

It is important to note that the isoelectric point of hydrous zirconia was reported25 to be 6.7. Therefore, it is suggested that the colloidal particles formed in basic conditions near the electrode surface are negatively charged. It is suggested that the deposit formation is achieved via heterocoagulation of the colloidal particles and polyelectrolyte macromolecules. The interaction of the polyelectrolytes and colloidal particles could be electrostatic, nonelectrostatic, or a combination of both.26 This interaction is a complicated phenomenon, influenced by the pH, ionic strength, and electric field. Recent studies indicate that the interaction of zirconia and PEI includes electrostatic attraction and hydrogen bonding.27 Adsorption of protonated PEI on the positively charged zirconia particles was observed at low pHs, below the isoelectric point of ZrO2. It was pointed out that this interaction is based on hydrogen bonding. The adsorption increases with an increasing pH. At high pHs, the interaction includes both mechanisms, electrostatic attraction and hydrogen bonding.27 It is known that polyamines form chelating complexes with various heavy-metal ions,28,29 and metal ions compete with protons to be bound to the polymers. Therefore, we cannot exclude the possibility that the PEI macromolecules may acquire positive charges by way of being bound to the charged zirconium species.29 In this case, the electric field (24) Huang, C.; Tang, Z.; Zhang, Z. J. Am. Ceram. Soc. 2001, 84, 1637. (25) Parks, G. A. Chem. Rev. 1965, 65, 177. (26) (a) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133. (b) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 146. (27) Wang, J.; Gao, L. Nanostruct. Mater. 1999, 11, 451. (28) (a) Kobayashi, S.; Tokunoh, M.; Saegusa, T.; Mashio, F. Macromolecules 1985, 18, 2357. (b) Kobayashi, S.; Hiroishi, K.; Tokunoh, M.; Saegusa, T. Macromolecules 1987, 20, 1496. (29) Geckeler, K. E.; Volchek, K. Environ. Sci. Technol. 1996, 30, 725.

provides electrophoretic motion of the PEI macromolecules and bonded cationic zirconium species toward the electrode surface. It is suggested that these species and the free zirconium species, which are not complexed by PEI, participate in cathodic reactions to form hydrated zirconium oxide or zirconium hydroxide. In this paper, electrodeposition of PEI was performed using a mixed methanol-water solvent. Note that the deposition process needs a certain amount of water for base generation in the cathodic reactions (eqs 1 and 2). The use of methanol as a solvent is important in order to reduce gas evolution and electrostatic repulsion15 of the polymer macromolecules. On the other hand, the polymer can be adsorbed on the surface of the colloidal particles when its solubility in the dispersion medium is low. It is in this regard that the thickness of the deposited polyelectrolyte multilayers30 increased with an increasing ethanol content in a mixed water-ethanol solvent. Therefore, another benefit of using a poor solvent is that it promotes the adsorption of the polyelectrolytes on the colloidal particles. Cathodic precipitation of the zirconium species is influenced by a solvent. Indeed, the weight loss for the deposits, prepared from a mixed methanol-water solvent, was found to be 18.4 wt %, compared to the weight loss of 32.6 wt % reported19 for deposits prepared from the aqueous solutions. It is in this regard that the precipitation of the zirconium species can result in the formation of hydrated zirconium oxide or zirconium hydroxide, which exhibited weight losses of 21.5 and 32.19 wt %, respectively.24 It is reasonable to expect that the electrosynthesis of the zirconium species is also influenced by the polyelectrolytes. It was observed that the phase content and crystallinity of some materials, prepared by chemical precipitation or electrosynthesis, are influenced by the polyelectrolytes.14,18 On the other hand, some composite materials cannot be considered as a simple mixture of organic and inorganic components. Therefore, a more detailed investigation, currently under way, is necessary to study the composition and microstructure of the composite films prepared in this paper. The method developed for the electrodeposition of the composite materials is a combination of two processes, EPD of polyelectrolytes and electrosynthesis of inorganic particles. The deposition rate W in the EPD can be described by the following equation:

W ) CµU/d

(7)

where C is the concentration of the colloidal particles or polymer macromolecules, U ) Uap - Udep, Uap is the applied voltage, Udep is voltage drop in the deposit, d is the distance between the electrodes, and µ is the electrophoretic mobility of the particles or polyelectrolytes.15,31 Constant-current EPD enables a constant electric field U/d in solutions and a constant deposition rate. On the other hand, Faraday’s law governs the electrosynthesis of inorganic materials. Therefore, at a constant-current mode, EPD and electrosynthesis enable a constant deposition rate. Indeed, the nearly linear deposit weight versus time dependencies were recorded for composite films, as shown in Figure 1. Equation 7 indicates that the increasing polymer concentration in solutions results in an increasing deposition yield. The increasing polymer concentration in solutions at a constant concentration of ZrOCl2 resulted (30) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (31) Ohshima, H. Colloids Surf., A 1995, 103, 249.

Fabrication of Composite Films

in an increasing polymer content in the composite deposits (Figures 3 and 6). Electrosynthesis enabled the formation of amorphous zirconium species. Crystallization of zirconia was observed after heat treatment at temperatures exceeding 400 °C, as shown in Figures 4 and 7. On the other hand, results from this paper indicate the possibility of electrophoretic codeposition of crystalline YSZ and PEI. It is suggested that the adsorption of PEI27 resulted in stable suspensions of positively charged YSZ particles. This strategy paves the way for the fabrication of other polymer-ceramic composites with important functional properties. Electrodeposition offers important advantages, such as the high deposition rate and process simplicity. Composite films can be formed on the substrates of a complex shape (Figures 9 and 10). The obtained results are important for the fabrication of thick ceramic films and composite polymer-ceramic films. The small amount of polyelectrolyte additives acting as a binder prevented deposit cracking and increased deposit adhesion. Sintering experiments resulted in the burning out of polymers and formation of zirconia films. X-ray data presented in Figures 4 and 7 indicate crystallization of the deposit at temperatures exceeding 400 °C. At higher concentrations of polyelectrolytes, composite polymer-ceramic materials can be prepared. It is expected that various composites with important functional properties will be prepared by the proposed method. Electrodeposition of the composite films is also important for the surface functionalization of the carbon fibers. It is expected that such coatings can be utilized for the design of the advanced ceramic matrix and polymer matrix composites. However, the use of electrodeposition for the fabrication of nanolaminates with precise control of layer thickness on the nanometric scale1-11 presents difficulties. AFM data from this paper indicate a relatively high surface roughness of the deposits prepared by electrodeposition. The uniformity of the

Langmuir, Vol. 20, No. 7, 2004 2927

deposits prepared by EPD is controlled by the electric field, and the results are controlled from the insulating properties of the deposited materials and electric field dependence of the deposition rate.15 However, the uniformity of the EPD deposits is limited by the size of the polymer particles or macromolecules used for deposition. It is suggested that the relatively high surface roughness of the deposits prepared by the combined method, based on EPD of the polyelectrolyte macromolecules and electrosynthesis of the colloidal particles, could also be attributed to hydrogen evolution at the cathode surface. Ongoing research is focused on the use of processing additives, which consume H+ in cathodic reactions and reduce gas evolution. Conclusions The electrochemical method has been developed for the fabrication of composite films containing weak cationic polyelectrolytes and zirconia. In this strategy, EPD of PEI or PAH has been combined with cathodic electroprecipitation of zirconia. Another approach has been developed based on EPD of the polyelectrolytes and YSZ particles. Films of up to several micrometers thick were obtained on various conducting substrates, including Pt, Ni, stainless steel, and graphite. The method enables uniform deposition on the substrates of complex shapes, such as carbon fibers and felts. Films of various thicknesses in the range of up to several micrometers were obtained by variation of the deposition time. It was demonstrated that the amount of an organic phase in the deposits can be changed by a variation of the PAH or PEI concentration in solutions. The obtained results pave the way for the fabrication of other composites based on weak cationic polyelectrolytes and metal oxides or hydroxides. LA0361971