Electrostatic Interactions between Polyelectrolytes and a Titania

Layer-by-layer film growth was monitored by UV−vis spectrophotometry, ellipsometry, and by a quartz crystal .... Chemistry of Materials 2002 14 (5),...
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Electrostatic Interactions between Polyelectrolytes and a Titania Precursor: Thin Film and Solution Studies Xiangyang Shi, Thierry Cassagneau, and Frank Caruso* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Received August 17, 2001. In Final Form: October 21, 2001 A facile approach, based on the polyelectrolyte-mediated electrostatic adsorption of a water-soluble titania precursor, titanium (IV) bis (ammonium lactato) dihydroxide (TALH), is presented for the formation of multilayered thin films of defined thickness and composition. The thin films were formed by the alternate deposition of TALH and positively charged polyelectrolyte [poly (diallyldimethylammonium chloride), PDADMAC, poly (allylamine hydrochloride), PAH, and chitosan, CH] utilizing electrostatic interactions for film formation. Layer-by-layer film growth was monitored by UV-vis spectrophotometry, ellipsometry, and by a quartz crystal microbalance (QCM). The UV-vis, ellipsometric, and QCM data all showed that a uniform amount of TALH was deposited with each alternate deposition cycle of polyelectrolyte and TALH, indicating that TALH binds to positively charged polyelectrolytes. Using PDADMAC/TALH as a model system, the influence of drying on multilayer film formation was examined by QCM. These experiments showed that water-equilibrated multilayers facilitated the binding of TALH to PDADMAC. The electrostatic nature of the binding between TALH and positively charged polyelectrolytes was confirmed by fluorescence studies in aqueous solution: Removal of the anionic probe, pyrenetetrasulfonic acid (4-PSA), precomplexed to PDADMAC in aqueous solution, was observed upon the addition of TALH. PDADMAC/TALH multilayers were also constructed on colloid particles in order to gain insight into the binding behavior between the deposited species. The results suggest that multilayer film formation is facilitated via the deposition of oligomeric species of TALH. The approach presented here may be exploited for the fabrication of novel advanced titania-based materials (e.g. thin films, porous structures, and composite colloids).

Introduction In the past decade, tremendous attention has been paid to the production of thin films and colloid particles comprising titanium dioxide. The impetus for such research has been the range of useful optical, electrical, and chemical properties afforded by titanium dioxide; for example, high refractive index, excellent transmittance of visible light, high relative dielectric constant, and desirable solar energy conversion and photocatalytic properties.1-3 Numerous methods exist for preparing thin titania films, including sol-gel chemistry,4 chemical vapor deposition,5 dip-coating processes,6 and spray pyrolysis.7 Although the above techniques allow the formation of titania films for use in various applications (photocatalysis, sensing, catalysis etc), the production of thin films with tailored thickness is often difficult to achieve. Recently, * To whom correspondence should be addressed. Tel.: +49 331567 9410. Fax: +49 331 567 9202. E-mail: frank.caruso@ mpikg-golm.mpg.de. (1) Hangefeldt, A.; Gra¨tzel, Chem. Rev. 1995, 95, 49. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Negishi, N.; Takeuchi, K.; Ibusuki, T. J. Sol.-Gel Sci. Technol. 1998, 13, 691. (4) (a) Watanabe, T.; Fukayama, S.; Miyauchi, M.; Fujishima, A.; Hashimoto, K. J. Sol.-Gel Sci. Technol. 2000, 19, 71. (b) Garzella, C.; Comini, E.; Tempesti, E.; Frigeri, C.; Sberveglieri, G. Sens. Actuators, B 2000, 68, 189. (c) Imai, H.; Hirashima, H. J. Am. Ceram. Soc. 1999, 82, 2301. (d) Moriguchi, I.; Maeda, H.; Teraoka, Y.; Kagawa, S. Chem. Mater. 1997, 9, 1050. (5) (a) Tuan, A.; Yoon, M.; Medvedev, V.; Ono, Y.; Ma, Y.; Rogers, J. W. Thin Solid Films 2000, 377, 766. (b) Byun, D.; Jin, Y.; Kim, B.; Lee, J. K.; Park, D. J. Hazard. Mater. 2000, 73, 199.(c) Han, Y. K.; Lee, T. G.; Yom, S. S.; Son, M. H.; Kim, E. K.; Min, S. K.; Lee, J. Y. J. Korean Phys. Soc. 1998, 32, S1697. (d) Lee, J. S.; Song, H. W.; Lee, W. J.; Yu, B. G.; No, K. Thin Solid Films 1996, 287, 120. (e) Nitta, T.; Nishitani, K.; Hanabusa, M. Jpn. J. Appl. Phys. 1995, 34, L1500. (6) Negishi, N.; Takeuchi, K.; Ibusuki, T. J. Sol.-Gel Sci. Technol. 1998, 13, 691. (7) Natarajan, C.; Fukunaga, N.; Nogami, G. Thin Solid Films 1998, 322, 6.

a surface sol-gel technique that provides film thickness control at the nanometer level for the fabrication of titania films has been employed.8-10 The experimental process typically involves multiple adsorption of the metal alkoxide onto hydroxylated surfaces, with intermediate solvent rinsing, hydrolysis of the adsorbed alkoxide, and drying. An alternative and flexible strategy that affords nanometer control over film thickness is the layer-by-layer (LbL) self-assembly method.11 By utilizing the electrostatic attraction between alternately deposited oppositely charged species, the construction of tailor-designed, structurally complex, multicomposite multilayer assemblies is realizable.11 The LbL technique has been employed for the fabrication of titania thin films by using TiO212 or capped TiO2 nanoparticles,13 or titania nanosheet crystallites14 as layer components interspersed with polyelectrolyte. The LbL method has also recently been applied to construct multilayered thin shells on colloid particles. Nanocomposite core-shell colloids with tailored optical,15-17 mag(8) Kleinfeld, E. R.; Ferguson, G. S. Mater. Res. Soc. Symp. Proc. 1994, 351, 419. (9) (a) Yonzawa, T.; Matsune, H.; Kunitake, T. Chem. Mater. 1999, 11, 33. (b) Ichinose, I.; Kawakami, T.; Kunitake, T. Adv. Mater. 1998, 10, 535. (c) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1996, 9, 1296. (d) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831. (10) Fang, M.; Kim, C. H.; Martin, B. R.; Mallouk, T. E. J. Nanoparticle Res. 1999, 1, 43. (11) Decher, G. Science 1997, 227, 1232. (12) (a) Liu, Y.; Wang, A.; Claus, R. J. Phys. Chem. B 1997, 101, 1385. (b) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (13) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (14) Sasaki, T.; Ebina, Y.; Watanabe, M.; Decher, G. Chem. Commun. 2000, 2163. (15) Caruso, F.; Spasova, M.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M. Adv. Mater. 2001, 13, 190. (16) Yang, W.; Trau, D.; Renneberg, R.; Yu, N. T.; Caruso, F. J. Colloid Interface Sci. 2001, 234, 356. (17) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Mo¨hwald, H. Colloids Surf., A 1998, 137, 253.

10.1021/la011310d CCC: $22.00 © 2002 American Chemical Society Published on Web 01/11/2002

Polyelectrolytes and a Titania Precursor

netic,18 and biocatalytic19 properties have been prepared. Polymer-core titania-shell particles with tunable shell thickness and composition have also been constructed.20 Removal of the core from the coated particles by either chemical or thermal means affords hollow colloids of the shell materials (titania, silica, iron oxide, polymer, composites etc.) with technological promise.15,18,20-22 In the studies that utilize nanoparticles,15,18,20,21 the quality and final properties of the films and colloids depend on the properties of the preformed nanoparticles used. Titania precursors, particularly alkoxides, are widely used to attain titania coatings on a variety of surfaces and particles. The water sensitivity of titanium alkoxides, resulting in rapid hydrolysis and condensation, limits their range of application and can often lead to the formation of nonuniform coatings. In contrast, titanium (IV) bis (ammonium lactato) dihydroxide (TALH) is relatively stable at ambient temperature in water. Therefore, TALH represents an attractive precursor for the construction of titania-based thin films from aqueous solutions. It has already been reported that TALH can be used as a new base material in preparing catalysts,23 in making electrodes for medical treatment and diagnosis,24 and in generating UV-protective films on various surfaces by using a biomimetic approach for the preparation of titania films.25 Recently we employed the LbL assembly method to prepare monodisperse polymer-core titania-shell particles with defined diameters and shell thicknesses by the layered deposition of TALH and polyelectrolyte onto colloid particles.26 Subsequent calcination of the core-shell particles afforded hollow titania spheres of anatase or rutile, depending on the calcination temperature.26 The creation of tailored titania coatings on colloids and planar supports derived from TALH by the LbL strategy will be dependent on obtaining an understanding of the binding interactions between TALH and polyelectrolytes. Therefore, in the present study we examine the binding of TALH with different polyelectrolytes (molecular structures are shown in Figure 1) via the LbL formation of TALH/ polyelectrolyte films on planar substrates (quartz, silicon, and quartz crystal microbalance electrodes (QCMs)). In addition, the interaction of TALH and polyelectrolyte in solution is investigated by fluorescence spectroscopy by utilizing the anionic probe pyrenetetrasulfonic acid (4PSA) (Figure 1). Previous studies have focused on the binding of 4-PSA with polyelectrolytes in solution,27 with polyelectrolyte multilayer films assembled onto colloids,28 or with polyelectrolytes on planar substrates.29 These (18) (a) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.;Caruso, R. A. Chem. Mater. 2001, 13, 109. (b) Caruso, F.; Susha, A.; Giersig, M.; Mo¨hwald, H. Adv. Mater. 1999, 11, 950. (19) (a) Caruso, F.; Schuler, C. Langmuir 2001, 16, 9595. (b) Schu¨ler, C.; Caruso, F. Makromol. Chem., Rapid Commun. 2000, 21, 750. (20) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (21) (a) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Chem. Mater. 1999, 11, 3309. (b) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (22) (a) Pastoriza-Santos, I.; Scho¨ler, B.; Caruso, F. Adv. Functional Mater. 2001, 11, 122. (b) Caruso, F.; Schu¨ler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394. (c) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201. (23) Hanprasopwattana, A.; Rieker, T.; Sault, A. G.; Datye, A. K. Catal. Lett. 1997, 45, 165. (24) Lawrence, K. US Patent 4692273, 1998. (25) Baskaran, S.; Song, L.; Liu, J.; Chen, Y. I.; Graff, G. L. J. Am. Ceram. Soc. 1998, 81, 401. (26) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. Adv. Mater. 2001, 13, 740. (27) Caruso, F.; Donath, E.; Mo¨hwald, H.; Georgieva, R. Macromolecules 1998, 31, 7365. (28) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317.

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Figure 1. Molecular structures of the polyelectrolytes used, the assumed structure of the nonionized form of TALH (the arrows indicate donated electron pairs),42 and the molecular structure of the fluorescent probe employed.

studies have demonstrated that the high excimer-tomonomer ratio (IE/IM) of 4-PSA fluorescence is due to the electrostatic association of the probe molecules and polyelectrolytes.27-29 In the current work, variations in IE/IM for 4-PSA prebound to polyelectrolyte in solution upon the addition of TALH were used to identify the binding between TALH and polyelectrolyte. Electrophoresis measurements on PDADMAC/TALH-coated polystyrene (PS) particles were also undertaken and confirmed the electrostatic binding of TALH species to polyelectrolyte. Experimental Section Materials. Poly (diallyldimethylammonium chloride) (PDADMAC), Mw < 200 000, poly(allylamine hydrochloride) (PAH), Mw 8000-11,000, poly (sodium 4-styrenesulfonate) (PSS), Mw 70,000, and polyethyleneimine (PEI), Mw 55,000 were obtained from Aldrich. PSS was dialyzed against Milli-Q water (Mw cutoff 14,000) and lyophilized before use. Chitosan (CH) was purchased from Sigma. Chitosan sulfate (CHS) (Mw ) 200,000) was a gift from X. Qiu (Max Planck Institute of Colloids and Interfaces). All polyelectrolytes were used as aqueous solutions at a concentration of 1 mg mL-1 (containing 0.5 M NaCl). 1, 3, 6, 8-Pyrenetetrasulfonic acid, tetrasodium salt (4-PSA) was obtained from Molecular Probes, Eugene, Oregon, USA. The doubly negatively charged titania precursor, titanium (IV) bis (ammonium lactato) dihydroxide (TALH, Figure 1) (50-wt % in aqueous solution) was purchased from Aldrich and was diluted to 5-wt % with water shortly before use. 2-Mercaptoethylamine hydrochloride (MEA) (98%) was purchased from Aldrich. Sodium chloride was obtained from Merck. The water (resistivity higher than 18.2 MΩ cm) used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system. Negatively charged, sulfate stabilized polystyrene (PS) spheres of diameter 640 nm were prepared by H. Zastrow (MPI) as described previously.30 (29) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (30) Furusawa, K.; Norde, W.; Lyklema, J. Kolloid Z. Z. Polym. 1972, 250, 908.

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Substrate Preparation. Quartz substrates (Hellma Optik GmbH, Germany) and silicon wafers (Silchem Handelsgesellschaft mbH, Germany) were cleaned by using the RCA protocol (i.e., by immersing them in a 5:1:1 (vol %) H2O/H2O2/NH3 mixture at 70 °C for ca. 10 min),31 followed by extensive rinsing with Milli-Q water. A prelayer of PEI was adsorbed onto the quartz substrates by immersing the slides for 15 min into a 1 mg mL-1 aqueous solution of PEI containing 0.5 M NaCl, rinsing the substrates by dipping them three times in water for 1 min, and then drying them with N2. The silicon substrates were primed with a PDADMAC layer by exposing them to PDADMAC solution (1 mg mL-1, containing 0.5 M NaCl) for 15 min, rinsing the surface with water (three times for 1 min), and then drying them with N2. 9-MHz QCM electrodes (Kyushu Dentsu, Japan) were prepared as outlined in previous publications.32 To remove influences of the QCM (gold) surface on film formation, the QCM electrodes were modified with mercaptoethylamine, MEA.33 They were then coated with a PDADMAC layer as outlined above for the silicon substrates. The QCM experiments were performed by exposing only one side of the electrode surface to solution. Preparation of Multilayer Films. Thin, alternating TALH/ polyelectrolyte multilayer films were assembled onto the substrates by the sequential deposition of TALH (5-wt % in water) and polyelectrolyte (15 min each). To investigate the effect of drying on the formation of the multilayer films, two different drying conditions were employed: N2 drying after deposition of each layer, and only drying after total film growth. Intermittent water washing steps (3 × 1 min) were used in all cases. Colloid Particles. PS particles coated with PDADMAC/PSS/ PDADMAC (PE3) primer multilayers were further coated by the alternate deposition of TALH and PDADMAC as outlined previously.26 The TALH/PDADMAC-coated colloid particles were refluxed at 100 °C for 24 h. Refluxing converts the TALH precursor to titania nanoparticles (anatase form).26 UV-Visible Spectrophotometry. Multilayer film growth on quartz substrates was followed by using a Hewlett-Packard, Agilent 8453 spectrophotometer. Ellipsometry. Film growth on silicon substrates was monitored by null ellipsometry (Multiskop instrument, Optrel, Berlin, Germany; 2-mM HeNe Laser; λ ) 632.8 nm; angle of incidence ) 70°). A film refractive index of 1.68 was experimentally determined for films greater than 20 nm in thickness. Therefore, thicknesses for all other films (comprising at least 2 layers) were calculated assuming a refractive index of 1.68. Quartz Crystal Microgravimetry. A detailed description of the QCM system can be found elsewhere.32 According to the Sauerbrey equation32,34,35 for a 9-MHz QCM (fundamental frequency of 9 × 10 6 Hz) with an electrode area of 0.16 cm2, the following relationship is obtained between the adsorbed mass, ∆m, and the change in resonant frequency, ∆F:

∆m(ng) ) - 0.9 × ∆F(Hz)

(1)

Equation 1 indicates that a decrease in frequency of 1 Hz corresponds to a mass increase of 0.9 ng on the electrode surface. Microelectrophoresis. Electrophoretic mobilities of the coated PS particles were measured with a Malvern Zetasizer 4. All measurements were performed on coated PS particles redispersed in air-equilibrated pure water. Steady-State Fluorescence. Fluorescence measurements were undertaken using an LS-50B fluorescence spectrophotometer with excitation and emission bandwidths set at 5 nm. All measurements were performed on air-equilibrated solutions at room temperature. The 4-PSA probe was dissolved in water at a concentration of 5 × 10-3 M. An aliquot of the 4-PSA aqueous solution (5 × 10-3 M) was added to freshly prepared polyelec(31) Kern, W. Semicond. Int. 1984, 94. (32) (a) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (b) Caruso, F.; Rinia, H. A.; Furlong, D. N. Langmuir 1996, 12, 2145. (33) Cassagneau, T.; Mallouk, T. E.; Fendler J. H. J. Am. Chem. Soc. 1998, 120, 7848. (34) (a) Guilbault, G. G. In Applications of Piezoelectric Quartz Crystal Microbalances; Lu, C., Czanderna, A. W., Eds.; Elsevier: New York, 1984; p 251. (b) Lucklum, R.; Henning, B.; Hauptmann, P.; Schierbaum, K. D.; Vaihinger, S.; Gopel, W. Sens. Actuators 1991, A25, 705. (35) Sauerbrey, G. Z. Phys. 1959, 155, 206.

Figure 2. UV-vis absorption spectra of TALH/PDADMAC multilayer films grown on quartz substrates: solid and dashed lines correspond to spectra after the deposition of TALH and PDADMAC, respectively. The bold-lined spectrum (top) represents a LbL assembled (TALH/PDADMAC)5 multilayer film that was dried only after the last layer was deposited (to exclude the effect of drying between each deposition step). The absorbance data shown take into account the presence of the film on both sides of the substrate. A PEI/PSS/PDADMAC multilayer film on a quartz substrate was used as the reference (primer film). The inset displays the absorbance of the film at 250 nm as a function of the number of deposited layers. The odd layer numbers correspond to TALH adsorption and the even layer numbers to PDADMAC deposition. trolyte solutions and after 2 min stirring the fluorescence spectrum was recorded. Aliquots of a TALH solution were then added to the 4-PSA-polyelectrolyte solutions to cover a range of TALH concentrations. The solutions were stirred prior to the fluorescence spectra being recorded. Equilibrium fluorescence intensities were observed over the time frame of the experiments (30 min).

Results and Discussion Multilayered Thin Films. The LbL assembly of PDADMAC and TALH was undertaken on quartz substrates, and multilayer film growth was followed by UVvis spectrophotometry. A pronounced, broad peak at 250 nm, which is characteristic of TALH absorption is seen in the spectra (Figure 2). (PDADMAC does not show any absorption.) No significant change in the UV-vis spectra occurs with subsequent PDADMAC absorption, indicating that TALH remains adsorbed in the film. The UV-vis absorption spectra show a regular increase in TALH absorbance as a function of bilayer number (inset), reflecting the uniform growth of the PDADMAC/TALH multilayer films. These data confirm that PDADMAC can serve as an ‘electrostatic glue’ for the formation of TALHbased multilayer films, highlighting the binding between TALH and PDADMAC. The influence of drying on film formation was examined by preparing multilayer films and drying them only after the last layer was deposited. The UV-vis data show that approximately 50% more TALH can be deposited in the multilayer films when drying is avoided between deposition of each layer. A range of other polyelectrolytes (PEI, PAH, PSS, CH, and CHS) was also employed as electrostatic self-assembling layer components for the sequential deposition with TALH onto quartz substrates. Only the positively charged polyelectrolytes (PAH, PEI, and CH) mediated the growth of TALH multilayer films (Figure 3). The selection of CH and CHS was based on their similar molecular structures. CHS, negatively charged, does not induce TALH multilayer formation, whereas its positively

Polyelectrolytes and a Titania Precursor

Figure 3. UV-vis absorbance as a function of layer number for the LbL assembly of TALH and PEI, CH, PAH, CHS, or PSS. A PEI/PSS bilayer deposited onto quartz substrates was used as the reference (primer film) for all multilayers, except for the PSS/TALH system where a single PEI layer was used. For the PEI/TALH, CH/TALH, and PAH/TALH systems, the large increments in absorbance correspond to the TALH adsorption steps.

Figure 4. Ellipsometric thickness of thin films prepared by the sequential deposition of PDADMAC and TALH. The odd layer numbers correspond to PDADMAC deposition and the even layer numbers to TALH.

charged counterpart, CH, does. The differences in the amount of TALH deposited depended on the intermediate polyelectrolyte layer employed: The TALH binding amount on planar surfaces for the different polyelectrolytes examined, as assessed from the UV-vis data, is PEI > PDADMAC > PAH ≈ CH. Such a trend most probably reflects differences in the charge density, structure, and conformation of the adsorbed polyelectrolyte, resulting in variations in their binding capacity for TALH. CHS and PSS showed no significant binding of TALH; this can be explained by the similar charge of TALH and these polyelectrolytes. The growth of PDADMAC/TALH multilayer thin films on silicon wafers was also monitored by ellipsometry. (PDADMAC was chosen as the model polyelectrolyte.) A regular increase in film thickness was observed with the deposition of each bilayer (Figure 4). Stepwise PDADMAC and TALH deposition on a single substrate showed an average thickness increase of 0.4 ( 0.1 nm for PDADMAC and 4.2 ( 0.3 nm for TALH. The average refractive index

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Figure 5. QCM frequency shifts for the LbL assembly of TALH and PDADMAC on MEA-modified gold QCM electrodes to form multilayer films. The even layer numbers correspond to TALH deposition and the odd layer numbers to PDADMAC deposition. Only one side of the QCM was used for measurement.

determined ellipsometrically for films thicker than 20 nm (10 layers) was 1.68. This value is higher than that of polyelectrolyte layers (∼1.47)36 and indicates the loading of TALH into the films. (TALH is a titanium containing species and hence it is expected to have a refractive index higher than polymers.) The preparation of PDADMAC/ TALH multilayer films on a number of silicon wafers yielded identical results (within experimental error), confirming the regularity of film growth and the reproducibility of the deposition process from sample to sample. The TALH thickness measured implies that titanium oligomers were deposited from solution, a finding that is in agreement with our earlier study using colloid particles as substrates for TALH/PDADMAC multilayer growth.26 TALH monomeric species may also have been deposited along with the oligomers. The LbL assembly of TALH/PDADMAC multilayer films was also undertaken on QCM electrodes. Figure 5 shows the frequency change (-∆F) of the QCM electrodes due to the LbL assembly of TALH and PDADMAC. The QCM frequency regularly decreased with an increasing number of adsorption cycles, demonstrating an increase of mass on the electrode surface with each deposited layer. The average -∆F for each TALH and PDADMAC layer is 115 ( 4 Hz and 6 ( 1 Hz, respectively. These data correspond to a mass increase of 103 ng and 5 ng for each TALH and PDADMAC layer, respectively. Assuming a density of about 1.6 × 106 g m-3 for TALH37 and using the -∆F values, a thickness of approximately 4 nm per TALH layer is calculated. This thickness is in excellent agreement with that determined ellipsometrically, lending further evidence for the deposition of titanium oligomers. The PDADMAC layer thickness calculated from the QCM data, 0.3 nm, assuming a film density of 1.2 × 106 g m-3, also agrees well with that measured ellipsometrically. To obtain additional information on the binding of TALH to polyelectrolytes, preformed multilayers on QCM electrodes were used as matrixes for TALH adsorption. TALH was adsorbed onto preassembled (PDADMAC/PSS)5/ PDADMAC and (PDADMAC/PSS)5 multilayer films (positive (PDADMAC) and negative (PSS) outermost layers, (36) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (37) The density of TALH was calculated from the known density of a 50-wt % TALH aqueous solution (1.222 g cm-3).

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respectively). For the (PDADMAC/PSS)5/PDADMAC multilayer, -∆F due to the outermost TALH layer is 953 Hz (or 858 ng) (data not shown), which is higher (∼10%) than the average -∆F obtained for each TALH layer when assembled in alternation with PDADMAC to form (PDADMAC/TALH)5 multilayer films (878 Hz or 788 ng; see earlier). This slight enhancement in mass uptake occurs even though a majority of the PDADMAC charged moieties are ionically associated with PSS,28 and can be explained by TALH binding to the outermost PDADMAC layer and, in addition, penetrating into the multilayer film and binding to charged groups of PDADMAC in layers underneath (i.e., within the film). Preliminary experiments have confirmed that TALH can penetrate into PDADMAC/PSS multilayers (with a PDADMAC outermost layer) and bind within the film: a linear relationship was observed for the amount of TALH deposited and the number of layers in preassembled PDADMAC/PSS multilayer films (a range of 2-12 layers was studied).38 However, exposure of (PDADMAC/PSS)5 multilayers to TALH solution (for times up to 24 h) did not yield a mass increase, indicating the absence of TALH binding when the outermost film layer is negatively charged. The absence of TALH binding to the multilayer films when PSS forms the outermost polyelectrolyte layer is consistent with earlier studies, which have shown that the anionic probe, 4-PSA, can bind to positively charged groups of PAH in preassembled PAH/PSS multilayer films only when PAH forms the outermost layer, with no binding detected when PSS is the topmost layer.28 The lack of TALH binding to films with a PSS outermost layer is likely to be due to the “charge neutralization” of the PDADMAC positive groups by the anionic groups of PSS in the multilayer film; when the outermost layer is negative the net fixed charge density of the system is negative, i.e., there exists an overall excess number of anionic charges.28 When PDADMAC is the outermost layer, the presence of PSS in the (PDADMAC/ PSS)5/PDADMAC film does not prevent binding (and infiltration) of TALH, probably resulting from the net positive charge density of the system (overall excess of cationic charges). It is also pointed out that topological constraints that exist in the multilayer film may also play a role in influencing the binding of TALH.28 On the whole, the above data show that exposing preformed polyelectrolyte multilayers (bearing an outermost positive polyelectrolyte) to TALH solution is a viable pathway to the preparation of thin films containing organometallic species. The influence of N2 drying on the multilayer film structure and TALH binding was also investigated by QCM measurements. (PDADMAC/TALH)5 and (PDADMAC/PSS)5/PDADMAC/TALH films prepared without N2 drying after deposition of each layer (i.e., the films were dried after deposition of the final layer) showed total additional frequency decreases of 20% and 35% compared with the corresponding films prepared with N2 drying after deposition of each layer. These data suggest that the waterequilibrated films (i.e., no drying between individual layer depositions) facilitate the deposition of TALH, in agreement with the UV-vis data (see earlier). As the deposition of additional polymer (in particular PDADMAC) would influence the quantity of TALH that could bind, experiments were conducted in order to assess any variations in the amount of polymer deposited with the different preparation procedures. Exposing an air-dried PDADMAC layer adsorbed on a substrate bound some 25% less TALH than an identically adsorbed PDADMAC layer without (38) Shi, X.; Caruso, F. Unpublished data.

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drying. Further, the alternate deposition of PDADMAC and PSS onto the QCM electrodes to construct (PDADMAC/PSS)5/PDADMAC multilayer films, followed by drying the film only after the final layer was deposited yielded -∆F ) 2302 Hz. This value is similar to that recorded for the same film prepared with intermediate drying steps between layer deposition (2353 Hz), indicating no significant variation in the amount of polyelectrolyte deposited by using either method. Therefore, the differences in the amount of TALH adsorbed likely reflect variations in the polyelectrolyte conformation and hence layer structure of the multilayer films with drying.39 These data are in accordance with our previous work where less dye was found to bind to polyelectrolyte multilayer films that were dried after deposition of each layer, as opposed to multilayer films that were dried after their formation.29 Whereas drying after each layer may compact the film structure, the water-equilibrated multilayer films can be in a more expanded state,40 thereby providing a larger internal volume and promoting TALH infiltration and binding. Hence, the amount of TALH deposited in polyelectrolyte multilayer thin films can be controlled by using different drying protocols. As already mentioned, the ellipsometric and QCM data suggest that titanium oligomeric (and possibly some monomeric) species were deposited from solution with each TALH adsorption step. Further experiments were conducted in order to confirm that small titanium species were actually deposited.41 Electrophoresis experiments were performed on (PDADMAC/PSS/PDADMAC/TALH)(abbreviated as PE3/TALH) and PE3/(TALH/PDADMAC)5coated polystyrene particles, and also on the same particles after refluxing for 24 h at 100 °C. The ζ-potential values for PE3/TALH-coated particles before and after refluxing are about -40 and -10 mV, respectively. This change in ζ-potential can be attributed to the conversion of TALH to small titania nanoparticles, as TEM analysis of the (nonrefluxed) polyelectrolyte/TALH-coated PS particles showed that the shell coating comprised of species no larger than a few nanometers in diameter, while refluxing causes transformation of TALH in the shell to crystalline (anatase) titania nanoparticles that remain adsorbed to the PS particles.26 Hence, the ζ-potential data indirectly suggest the deposition of titanium species. The ζ-potential value for PE3/(TALH/PDADMAC)5-coated particles with an outermost PDADMAC layer is, within experimental error, the same (+40 mV) regardless of thermohydrolysis, showing that the surface charge characteristics of the coated particles with an outermost PDADMAC layer are unaffected by elevating the temperature to 100 °C. TALH-Polyelectrolyte Interactions. TALH has octahedral coordination and its assumed molecular structure is depicted in Figure 1.42 Given the titanium valence state of +4 and the two ammonium groups, TALH has a double negative charge. The binding of TALH to the positively charged sites of polyelectrolytes may occur via the charged oxygen groups (associated with the ammonium ions) when TALH is in its ionized form. Binding through these sites would provide “bridging points” to positively charged polyelectrolytes and allow the LbL (39) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (40) Steiz, R.; Leiner, V.; Siebrecht, R.; v. Klitzing, R. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 163, 63. (41) Tapping mode atomic force microscopy (Nanoscope IIIa, Digital Instruments) measurements on air-dried samples confirmed that only small species were adsorbed from TALH solution onto a PEI-modified freshly cleaved mica substrate - no features greater than about 2 nm (in height) were observed. (42) Mo¨ckel, H.; Giersig, M.; Willig, F. J. Mater. Chem. 1999, 9, 3051.

Polyelectrolytes and a Titania Precursor

formation of TALH/polyelectrolyte multilayers. However, given that divalent ions are typically poor candidates for alternate assembly with oppositely charged polyelectrolytes to form multilayer films, it is most probable that TALH association with oppositely charged polyelectrolytes is facilitated through the binding of titanium oligomers preformed in solution (as discussed earlier). Oligomers would have multiple contact points for binding - in this respect resembling a low molecular weight polyelectrolyte - and thereby assist subsequent polyelectrolyte deposition. This scenario is supported by the ellipsometric and QCM layer thicknesses measured for the TALH adsorption steps. Solution Fluorescence Studies. Electrostatic interactions are the main driving force for the interaction of a range of ionic probes with polyelectrolytes in solution.27,43 For example, the electrostatic binding of 4-PSA to oppositely charged polyelectrolytes in solution has recently been demonstrated.27 4-PSA binding was manifested by quenching of the monomer emission, and the appearance of excimer emission. Hence, the excimer to monomer fluorescence intensity ratio (IE/IM) of 4-PSA can be used to quantify the binding of 4-PSA with polyelectrolytes.27-29 In the current work, changes in the fluorescence behavior of 4-PSA, prebound to polyelectrolyte, upon the introduction of TALH were exploited to further study the binding behavior of TALH to positively charged polyelectrolytes. Prior to the binding of TALH with polyelectrolyte in solution, the concentrations of 4-PSA and polyelectrolyte were optimized so that the polyelectrolyte binding sites would be >75% saturated with 4-PSA. The exact concentration ratio was selected according to our previous work,27 corresponding to relative probe:polyelectrolyte concentrations that give IE/IM values below the maximum attainable value (data not shown). 4-PSA exhibits a high binding constant to polyelectrolytes such as PDADMAC and PAH; hence there is no detectable unbound (free) 4-PSA in solution under these experimental conditions.27 The reason for precomplexing 4-PSA and polyelectrolyte at concentrations that provide close to the maximum IE/ IM ratio was to provide maximum sensitivity in the fluorescence emission properties of 4-PSA with minor changes induced upon the addition of TALH. It was expected that if TALH would bind to the oppositely charged polyelectrolytes, then upon its introduction to aqueous solutions containing 4-PSA-polyelectrolyte complexes, removal of the 4-PSA from the bound state to bulk solution would occur, resulting in changes in its fluorescence emission properties. Such behavior was indeed observed: After the addition of TALH to a solution containing 4-PSA precomplexed with PAH, the 4-PSA monomer fluorescence increased while the excimer emission decreased (Figure 6a). This results in a net decrease in the IE/IM value (inset), reflecting the displacement of 4-PSA from the polymer chain into bulk solution by TALH. The displaced 4-PSA, which goes into solution gives an enhancement in the monomer emission because 4-PSA does not display excimer emission when uncomplexed in solution, not even at high concentrations.27 It should be noted that although the excimer emission steadily decreases with increasing TALH concentration (Figure 6a), some excimer emission is still observed over the concentration range studied. For the 4-PSA-CH combination (data not shown), the trend in the (43) (a) Hrdlovic, P.; Horinova, L.; Chmela, S. Can. J. Chem. 1995, 73, 1948. (b) Zimerman, O. E.; Cosa, J. J.; Previtali, C. M. J. Macromol. Sci., Pure Appl. Chem. 1994, A31, 859. (c) Becker, H. G. O.; Schu¨tz, R.; Kuzmin, M. G.; Sadovsky, N. A.; Soboleva, I. V. J. Prakt. Chem. 1987, 329, 87. (d) Becker, H. G. O.; Schu¨tz, R.; Kuzmin, M. G.; Soboleva, I. V. J. Prakt. Chem. 1987, 329, 95.

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Figure 6. (a) Fluorescence spectra of 4-PSA (10 µM) in a solution of 5 µg mL-1 PAH in the absence and presence of TALH. Excitation wavelength ) 350 nm. The arrow represents the direction of the spectra with increasing TALH concentration. The inset shows the IE (495 nm)/IM (383 nm) ratio for 4-PSA fluorescence as a function of TALH concentration. (b) Profile of the IE (500 nm)/IM (384 nm) ratio for 5 µM 4-PSA in 5 µg mL-1 PDADMAC solution as a function of TALH concentration. Excitation wavelength ) 350 nm.

data was similar to that obtained for the 4-PSA-PAH system. For the 4-PSA-PDADMAC system, the monomer emission increased upon the addition of TALH, whereas the excimer emission, although small, remained essentially constant. This results in an overall decrease in the IE/IM value (Figure 6b). (4-PSA-PDADMAC complexes display very weak excimer emission, hence accounting for the lower IE/IM values when compared to the 4-PSAPAH system.27) Control experiments showed that the fluorescence of 4-PSA is not affected by the addition of TALH (concentration range of 0 to 1000 µM). The above observations point toward a competitive binding effect between 4-PSA and TALH, suggesting that they can both bind ionically to the charged sites of the polyelectrolyte. This finding is in agreement with that previously reported for the competitive binding of ions (NaCl, phosphate ions) and 4-PSA for PAH and PDADMAC, where the addition of ions was found to displace 4-PSA prebound to the polyelectrolytes.27 In summary, the above data further provide evidence for the electrostatic interaction between TALH and positively charged polyelectrolytes.

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Conclusions The use of a water-soluble organometallic precursor (based on titanium) in the alternate assembly process with oppositely charged polyelectrolytes to form multilayered thin films has been investigated. The UV-vis, ellipsometry, and QCM results showed that regular deposition of TALH could be achieved, and coupled with the electrophoresis data, suggest that small titanium (oligomeric) species were deposited upon each TALH adsorption step. QCM experiments also revealed that water-equilibrated polyelectrolyte films enhanced the quantity of TALH bound. The electrostatic binding of TALH to positive polyelectrolytes in solution has also been confirmed by using fluorescence spectroscopy. Combination of the LbL

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technique with molecular precursors is an attractive strategy that can, in principle, be applied to other organometallic and inorganic species to prepare a variety of composite polymer/metal oxide thin films of well-defined thickness and composition. Acknowledgment. This work was supported by the BMBF and the Volkswagen Foundation. We thank C. Pilz (MPI) and Y. Zhang (MPI) for assistance with the ζ-potential and ellipsometry measurements, respectively. Valuable discussions with D. Wang (MPI) and H. Mo¨ckel (Hahn-Meitner-Institute, Berlin) are also acknowledged. LA011310D