Effects of External Electric Field on Film Growth, Morphology, and

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Effects of External Electric Field on Film Growth, Morphology, and Nanostructure of Polyelectrolyte and Nanohybrid Multilayers onto Insulating Substrates Guojun Zhang,* Limin Dai, Lei Zhang, and Shulan Ji Center for Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China

bS Supporting Information ABSTRACT: Electric-field-enhanced layer-by-layer (LbL) assembly of polyelectrolyte and nanohybrid multilayers onto insulating rigid substrates was successfully accomplished using two independent capacitor cells. The influence of external electric field on the multilayer formation was intensively investigated by UV-vis adsorption spectrometry, profilometry, atomic force microscopy, and small-angle X-ray diffraction. This approach was also attempted as a means of fabricating selective layer on flat sheet polymeric porous substrates to obtain the dense composite membranes with high pervaporation performance.

’ INTRODUCTION During the past decades, the versatile layer-by-layer (LbL) assembly method has been widely used for creating advanced organic or organic/inorganic hybrid thin films. Such films are typically prepared by the consecutive deposition of oppositely charged polyelectrolytes (PEs) from bulk solutions onto solid supports.1,2 LbL assembly can be used to combine a wide variety of species including nanoparticles (NPs),3 carbon nanotubes,4 and nanowires5 with polymers, thus merging new properties of each type of material. A variety of functional materials have been utilized in the LbL construction to make these structures suitable for prospective applications, such as microcapsules,6 macroscopic sacs,7 sensitive solar cell films,4,8 biosensors,9 superhydrophobic surfaces,10 fuel cells,11 and separation membranes.12-22 In recent years, there has been great interest in modifying the film features, such as the thickness, nanostructure, and morphology, by changing the charge density of the building polymers, the ionic strength of the adsorption solutions, and the quality of the solvent.23 It is well recognized that it is very important to control these properties of PE and nanohybrid films for their potential applications. In spite of precise control over thickness and functionality, it is well-known that the LbL process involves numerous deposition and rinsing steps, and this complexity greatly limits the potential applications of multilayer films.24 To overcome this challenge, spray LbL technique has recently received considerable interest because it could dramatically decrease the process time to a few seconds per layer instead of 20-30 min.25-28 Meanwhile, a few attempts have also been r 2011 American Chemical Society

made to simplify the LbL process by using external forceenhanced LbL technique in recent years, which mainly depends on the decrease in the process cycle rather than the deposition time per layer. For example, we have developed a dynamic pressure-driven LbL assembly to construct various PE multilayers onto polymeric porous substrates. It has been demonstrated that the external pressure can accelerate the LbL deposition and in turn allow the reduction of the deposition cycles for the preparation of defect-free dense membranes.19-22 Gao and co-workers have developed an electric-field-directed layer-by-layer assembly (EFDLA) method for fabricating patterns of LbL self-assembled films on conducting substrates.29,30 The basic idea is to use an electric field to direct the spatially selective deposition of LbL self-assembled films. Wu et al. have successfully fabricated horseradish peroxidase nanotubes based on EFDLA and template synthesis.31 With regard to the EFDLA technique, the PEs were usually directly deposited onto a conducting working indium-tin-oxide electrode.29-31 This meant that the working electrode was used as the film substrate as well. However, most of the substrates are insulating or semiinsulating materials. The typical substrates such as quartz slides, silicon wafers, and polymeric porous membrane cannot directly be used as electrode for EFDLA. Therefore, it is necessary to seek a new approach to implement the electric-field-enhanced assembly onto these substrates. In addition, it has been demonstrated Received: October 13, 2010 Revised: January 18, 2011 Published: January 31, 2011 2093

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Langmuir that LbL-assembled PE multilayer membranes are particularly suitable for the pervaporation separation of solvent-water systems in past studies.14-22 However, high bilayers are usually required to obtain defect-free selective layers for high pervaporation performance. This presents an unacceptable constraint for its industrial applications. Considering the charge properties of PEs, we extended the electric-field-enhanced assembly to fabricate the PE composite membranes for the pervaporation separation of an ethanol-water mixture.32 This was the first example to construct the PE onto polymeric porous substrate using electricfield-enhanced assembly. In this case, the assembly was accomplished in a specially designed capacitor setup. The polymeric substrate was separated from both electrodes. After that, Qian’s group further investigated electric-field-enhanced LbL self-assembled multilayer membranes for separating isopropanolwater mixtures.33,34 In addition to these studies, electric-fieldenhanced LbL assembly onto insulating rigid substrates such as quartz slides and silicon wafers remains largely unexplored. Meanwhile, the changes of PE and nanohybrid multilayer growth speed, nanostructure, and morphologies during the electric-fieldenhanced LbL assembly are still unknown. Herein, we report on the development of electric-field-enhanced LbL assembly of PE and nanohybrid multilayer films onto insulating rigid substrates. Furthermore, we extend this technique to rapidly fabricate the dense composite membrane with high pervaporation performance. To accomplish the electric-field-enhanced LbL assembly, two independent capacitor cells were specially designed and used for the assembly of polycation and polyanion, respectively (Figure S1 in the Supporting Information). In each cell, two stainless steel electrodes are positioned on both sides, parallel to the quartz substrate. The gap between substrate and electrode slides was filled with PEs or PE-coated NP solutions. Then a voltage was applied to the electrodes to generate an electric field. The influence of external electric field on the growth speed, nanostructure, and morphologies of multilayer films was systematically investigated and discussed. Additionally, the electric-field-enhanced assembly of PE and nanohybrid multilayers on polymeric substrate was also attempted. The pervaporation performance of the resulting multilayer membrane was evaluated by using ethanol-water mixtures.

’ EXPERIMENTAL SECTION Materials. Poly(sodium styrene sulfonate) (PSSNa; Mw 70 000) and poly(diallyldimethyl ammonium chloride) (PDDA; Mw 100 000200 000) were purchased from Aldrich. ZrO2 NPs (particle size < 100 nm) were also supplied by Aldrich in the form of a 5 wt % aqueous dispersion. Sodium hydroxide, ethanol, hydrogen peroxide, and sulfuric acid were provided by Beijing Chemical Factory. Quartz substrates for UV-vis measurements were purchased from Beijing Kinglass Quartz Co. Ltd. The flat sheet polyacrylonitrile (PAN) ultrafiltration (UF) membranes with a nominal molecular weight cutoff of 30 000-40 000 were supplied by Sepro Membranes. Preparation of PSS-Coated ZrO2 NPs. To obtain PSS-coated ZrO2 NPs for subsequent assembly, 8.0 mL of ZrO2 colloid dispersion (5 wt %) was added into 1.2 wt % PSS solution. The pH value was maintained at 6.0. The mixed solution was first sonicated for 30 min. The dispersion was then centrifuged at 10 000 rpm for 10 min. The supernatant was replaced with ultrapure water so as to remove the excess free PSS chains from the surface of NPs, and this step was repeated three times. The preliminary PSS-coated ZrO2 colloid was redispersed by

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sonication for 30 min and followed by the centrifugation and supernatant exchange steps same as above. Finally, the stable PSScoated ZrO2 NPs suspension was obtained after the third sonication dispersion.

Electric-Field-Enhanced Assembly of PE Multilayers onto Substrates. Quartz slides were boiled in piranha solution (30:70 v/v H2O2: H2SO4) for 5 h, followed by rinsing with large amount of ultrapure water. The pretreated quartz slide was placed in between a specially designed capacitor setup (Figure S1 in the Supporting Information). As shown in Figure S1, the PE solution was added into a groove. The distance between cathode and anode is 15 mm. The two electrodes in each cell are connected to a DC power supply. Electricfield-enhanced assemblies of polycation and polyanion were performed using two independent cells. In each case, the direction of the DC electric field was the same as the migration direction of PE molecules from bulk solution to solid substrates. Electric field adjustment is to ensure the enhancement of electric field on the assembly of the respective polycation and polyanion. The action time of each electricfield-enhanced assembly step was maintained at 30 min. Prior to the next cycle, the films were extensively rinsed with ultrapure water and dried with a nitrogen flow. Alternating PDDA/PSS multilayer films could be obtained by repeating these steps in a cyclic fashion. In addition, PDDA/ PSS-ZrO2 nanohybrid multilayers were also conducted by replacing PSS with the corresponding PSS-coated ZrO2 aqueous dispersions during the electric-field-enhanced LbL assembly. Control experiments without electric field applied were also performed by sequentially exposing the substrates to PDDA and PSS solutions (or PSS-coated ZrO2 aqueous dispersions) for 30 min, respectively. Furthermore, Electric-field-enhanced assemblies of PE and nanohybrid multilayers onto the negatively charged hydrolyzed PAN flat sheet porous substrates were attempted. Prior to the assembly, the PAN UF membrane was hydrolyzed for at 65 °C for 30 min by immersing it into a 2 N NaOH aqueous solution.19-21 The hydrolyzed membranes were rinsed with ultrapure water and then loaded in a self-made capacitor setup.32 The PDDA and PSS layers were compulsorily constructed under the applied voltage of 10 V. The dynamic pressure was controlled at 0.1 MPa and the assembly time was maintained at 30 min. After each assembly step, the membrane was taken out, rinsed with adequate ultrapure water, and dried in an oven at 45 °C for about 2 h. Nanohybrid multilayer membranes were also prepared by replacing PSS aqueous solution with the PSS-coated ZrO2 aqueous solution during the electric-field-enhanced LbL assembly process. Pervaporation Experiments. The pervaporation performance of PE and nanohybrid multilayer membranes were evaluated using 95 wt % ethanol-water mixture. The measurement systems have been described in our previous studies.14,15 The temperature of the feed solution was 50 °C. Three parallel samples obtained from the same assembly conditions were examined under each pervaporation condition. The permeate vapor was trapped with liquid nitrogen. The downstream pressure was about 100 Pa. Fluxes were determined by measuring the weight of liquid collected in the cold traps during a certain time under steady-state conditions. The compositions of feed solutions and permeates were determined with gas chromatography (GC-14C, SHIMADZU). The separation factor R was calculated from the quotient of the weight ratio of component i and component j in the permeate, Yi/ Yj, and in the feed, Xi/Xj. R¼

Y i Xj Y j Xi

Characterization. The growth of the multilayers on quartz slides was monitored using a spectrophotometer (UV-2550, SHIMADZU). Film thickness was measured using a XP-1 profilometer (Ambios). A groove in the film was made using a razor blade. The depth of the groove, 2094

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Figure 1. (a) UV-vis spectra of electric-field-enhanced (PDDA/PSS)n films with n = 1-10 on quartz substrates (from bottom to top, applied voltage, 30 V). The inset shows the plots of the absorbance values at 226 nm versus the number of bilayers. (b) UV-vis absorption spectra of (PDDA/PSS)10, (PDDA/PSS-ZrO2)10, electric-field-enhanced (PDDA/PSS)10 (applied voltage, 15 V), (PDDA/PSS)10 (applied voltage, 30 V), and (PDDA/PSSZrO2)10 (applied voltage, 30 V) multilayer films assembled on quartz slides from bottom to top. which was measured by the profilometer stylus, was used to indicate the film thickness. Three thicknesses were obtained from three different places on the film, and the average was used to yield a data point. Atomic force microscopy (AFM) images were taken in tapping mode by using an atomic force microscoope (Pico ScanTM 2500). Small-angle X-ray diffraction experiments were conducted on a X-ray diffractometer (D8 ADVANCE, BRUKER/AXS, Germany).

’ RESULTS AND DISCUSSION It is well-known that electric field can accelerate and decelerate the deposition of charged species on an electrode. Previous studies have demonstrated that electric-field-directed LbL assembly could be achieved by using a conducting working electrode as substrate.29-31 However, for an insulating rigid substrate, because the substrate itself cannot be used as an electrode to generate the electric field, how to implement the electric-field-enhanced LbL assembly is relatively difficult. Therefore, so for, none of the work has dealt with the electric-fielddirected LbL assembly onto insulating rigid substrate. Differing from the above studies, the quartz substrate was first fixed into a specially designed groove and then placed in between the capacitor setup for assembly in the present work. Only a single side was allowed for contacting with the PE or PE-coated NP solution. Electric field was generated using two independent electrodes to fulfill the assembly. In order to verify the influence of the applied electric field on the LbL assembly of multilayers, the multilayer formation onto the quartz slides was monitored using ultraviolet-visible light (UV-vis) spectroscopy. As shown in Figure 1a, the absorption band at 226 nm was attributed to the aromatic group of PSS molecules. The linear increase in absorbance at 226 nm with the number of layer pairs indicates a progressive deposition of PDDA/PSS during the electric-fieldenhanced LbL assembly. However, it was noted from Figure 1b that the presence of electric field force accelerated the film growth in comparison with that using traditional LbL assembly without electric filed. In addition, the result also showed that PE multilayer grows much more rapidly under the higher applied voltage. The thickness of (PSS/PDDA)n multilayers deposited on a quartz slide as a function of bilayer number was also investigated (Figure S2 in the Supporting Information). By

comparing the three regression line slopes, it was further demonstrated that the growth rates obtained from 15 and 30 V electric-field-enhanced assemblies were much higher than those without electric field. It was also noted that the electric-fieldenhanced assembly resulted in significantly thicker films. For example, after enhanced assembly under 15 and 30 V, the total thickness of film with 10 bilayers increased from 30.0 ( 2.0 nm to 43 ( 2.0 nm and 56 ( 2.6 nm, respectively. This result signified that the average bilayer thickness could increase from 3.0 ( 0.2 nm to 4.3 ( 0.2 nm and 5.6 ( 0.3 nm. This trend is also observed in the growth of nanohybrid multilayers. An average thickness per bilayer of 6.0 ( 0.2 nm was calculated for the 30 V electric-field-enhanced assembly of a PDDA/PSS-ZrO2 nanohybrid multilayer. As a reference, the corresponding value without electric field being applied was only 3.8 ( 0.2 nm. Of note, the PDDA/PSS-ZrO2 film assembled with added nano-ZrO2 NPs is much thicker than a PDDA/PSS film. This is because the addition of ZrO2 NPs would inherently result in a thicker film due to the NPs’ own additional size to the layer. The topography and the surface roughness of the multilayer films were evaluated with AFM. Significant changes in the surface morphology were observed for the electric-field LbL assembled multilayers (Figure S3 in the Supporting Information). The values of mean roughness (Ra) were obtained based on a 20.0  20.0 μm2 scan area. The introduction of electric field resulted in much rougher surfaces. For example, the Ra obtained from a traditional LbL assembled (PSS/PDDA)10 multilayer film was only 7.7 nm. In contrast, under the applied voltages of 15 and 30 V, the corresponding Ra values could reach up to approximately 14.3 and 17.1 nm, respectively. This change is shown more obviously for the electric-field-enhanced nanohybrid multilayer. The Ra value varied from 24.4 to 84.7 nm after 30 V electric field action was introduced. It is well recognized that PE adsorption is a two step process: polymer chains are anchored to the substrate surface during a fast initial step and then relax to dense packing during a slower second step.35 Usually, the second step may take more time. The surface rearrangement of the adsorbed chains was completed in the second step. Particularly, in case that the adsorption time is long enough, the PE chain will be sufficiently stretched, which might hold a large space on the 2095

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Scheme 1. Comparison of Two LbL Processes: (a) Traditional LbL Assembly; (b) Electric-Field-Enhanced LbL Assembly

underlying layer (Scheme 1a). Under the action of an electric field, more charged PE was compulsorily assembled onto the surface. Since there is not enough time for the adsorbed chain to fully stretch, the later adsorbed PE will occupy more adsorption points, which will squeeze the early adsorbed polymer (Scheme 1b). The amount of adsorbed polymer increases while the rearrangement of molecular fragments in the second step is strongly limited due to the change of conformation state. Therefore, external electric field not only can speed up the migration of charged PE from bulk solution to substrate but also accelerate the process of the polymer chain adsorption onto substrate. High electric field favors the rougher and thicker layers. Therefore, the electric-field-enhanced assembly might be recognized as an effective approach for the rapid fabrication of PE and nanohybrid multilayer films. Small-angle X-ray diffraction was conducted to further understand the influence of the electric field on the multilayer structure. It was noted that there were more diffraction peaks from traditional LbL assembled PDDA/PSS and PDDA/PSSZrO2 multilayers than those from electric-field-enhanced assembly (Figure 2). Particularly, no obvious diffraction peak was observed in the case of 30 V assembly. These results indicated that multilayers became more interpenetrated and much denser after electric-field-enhanced assembly. As shown in Scheme 1, more charged PE or PE-coated NPs were compulsorily assembled onto the substrate surface and squeezed into PE chains under the action of the electric field. Since the charged species continuously moved toward the oppositely charged electrode even after adsorbing on the sublayers, the PE or PE-coated NPs added would further compulsorily interpenetrate into underlying layers, which in turn cause much denser structure.

To examine the behaviors of the electric-field-enhanced assembly of PE and nanohybrid layers on polymeric substrates, further studies were conducted. Dense composite membranes were assembled by depositing multilayers onto hydrolyzed polyacrylonitrile (PAN) porous substrate. Since solvent dehydration is a difficult problem in many cases, such as fuel ethanol production and pharmaceutical manufacture, the resulting membranes were used for pervaporation separation of ethanolwater mixtures (Table 1). It was noted that the selectivity obtained from electric-field-enhanced PDDA/PAN membranes was significantly much higher than those without electric field applied. For example, in the case of separation of ethanol-water mixtures using electric-field-enhanced PDDA/PAN membranes, the water content could be enriched from 5.0% (in feed) up to 98.2% (in permeate), which meant that the separation factor (R) could reach at 1037 while the permeate flux was 224 g/(m2 3 h) (50 °C). As a reference, the assembled PDDA/PAN membrane without electric field had a separation factor of 444. The greater enhancement on the selectivity is generally attributable to much denser and thicker selective layers formed on the PAN substrates in the presence of electric field. More importantly, this approach greatly reduced the assembly cycles compared to the traditional static LbL assembly, which usually requires as many as 50-60 bilayers to obtain defect-free dense membranes.16,17 The membrane performance obtained from only one PDDA layer deposition is comparable to that from traditional LbL assembly of 60 bilayers.18 Therefore, the electric-field-enhanced assembly is a very effective alternative to simplify the LbL process and increase the convenience of assembly. It was also noted that the increase in the deposition cycle could further improve the selectivity of composite membrane using the electric-field-enhanced LbL assembly. However, the selectivity increases at the expense of 2096

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Figure 2. Small-angle X-ray diffraction pattern of (a) (PDDA/PSS)10 multilayer (applied voltage, 0 V), (b) PDDA/(PSS-ZrO2/PDDA)10 multilayer (applied voltage, 15 V), (c) (PDDA/PSS)10 multilayer (applied voltage, 30 V), (d) (PDDA/PSS-ZrO2)10 multilayer (applied voltage, 0 V), and (e) (PDDA/PSS-ZrO2)10 multilayer (applied voltage, 30 V) films on quartz substrates.

Table 1. Pervaporation Performance of the Composite Membranes Prepared by Two Methods water content

water content in

separation factor

total flux

in feed (wt %)

permeate (wt %)

(R)

(g/m2 3 h)

PDDA/h-PAN (without electric field) PDDA/h-PAN (applied voltage, 10 V)

5.0 5.0

95.90 98.20

444 1037

445 224

PSS/PDDA/h-PAN (applied voltage, 10 V)

5.0

98.43

1191

219

PDDA/PSS/PDDA/h-PAN (applied voltage, 10 V)

5.0

99.16

2243

197

PSS-ZrO2/PDDA/h-PAN (applied voltage, 10 V)

5.0

98.84

1619

213

PDDA/PSS-ZrO2/PDDA/h-PAN (applied voltage, 10 V)

5.0

99.77

8242

181

membrane

flux. Meanwhile, the experimental results showed a trend that the incorporation of ZrO2 NPs into multilayers appeared to possess much higher selectivity. For example, the PDDA/PSS-ZrO2/

PDDA/h-PAN nanohybrid membrane had a very high separation factor of 8242. When the inorganic NPs are embedded in organic matrices, these NPs modify transport properties 2097

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Langmuir without introducing gross defects into the membrane. The NPs act so as to create preferential permeation pathways for selective permeation while imposing a barrier for undesired permeation.

’ CONCLUSIONS In summary, we have successfully implemented the electricfield-enhanced LbL assembly onto insulating substrates using capacitor cells. It has been demonstrated that the growth speed, film thickness, morphology, and nanostructure of the multilayer film can be easily tuned by the applied voltage. The amount of adsorbed polymer increases due to the presence of electric field, which in turn strongly limits the rearrangement of molecular fragments due to the change of conformation state. Furthermore, it is also noted that electric-field-enhanced assembly may be used for fabrication of selective separation membranes. It is conceivable that this technique can be easily used to assemble different charge species onto other insulating substrates using capacitor cells, thus endowing the film with multifunctionality and extending their applications beyond separation membranes. ’ ASSOCIATED CONTENT

bS

Supporting Information. Schematic illustration of the cell used for electric-field-enhanced assembly; multilayer thickness as a function of bilayer number and AFM images. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: 86-10-67392393. E-mail: [email protected].

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’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 20806001), the National Basic Research Program of China (No. 2009CB623404), and the Scientific Research Common Program of Beijing Municipal Commission of Education (KM201010005016). ’ REFERENCES (1) Decher, G. Science 1997, 277, 1232. (2) Zhang, X.; Chen, H.; Zhang, H. Chem. Commun. 2007, 14, 1395. (3) Kong, B.; Geng, J.; Jung, H. Chem. Commun. 2009, 16, 2174. (4) Sgobba, V.; Troeger, A.; Cagnoli, R.; Mateo-Alonso, A.; Prato, M.; Parenti, F.; Mucci, A.; Schenetti, L.; Guldi, D. J. Mater. Chem. 2009, 19, 4319. (5) Podsiadlo, P.; Sui, L.; Elkasabi, Y.; Burgardt, P.; Lee, J.; Miryala, A.; Kusumaatmaja, W.; Carman, M.; Shtein, M.; Kieffer, J.; Lahann, J.; Kotov, N. Langmuir 2007, 23, 7901. (6) Feng, Z.; Wang, Z.; Gao, C.; Shen, J. Adv. Mater. 2007, 19, 3687. (7) Capito, R.; Azevedo, H.; Velichko, Y.; Mata, A.; Stupp, S. Science 2008, 319, 1812. (8) Leventis, H.; King, S.; Sudlow, A.; Hill, M.; Molloy, K.; Haque, S. Nano Lett. 2010, 10, 1253. (9) Hong, W.; Bai, H.; Xu, Y.; Yao, Z.; Gu, Z.; Shi, G. J. Phys. Chem. C 2010, 114, 1822. (10) Ji, J.; Fu, J.; Shen, J. Adv. Mater. 2006, 18, 1441. (11) Zhao, C.; Lin, H.; Cui, Z.; Li, X.; Na, H.; Xing, W. J. Power Sources 2009, 194, 168. (12) Li, X.; Feyter, S.; Chen, D.; Aldea, S.; Vandezande, P.; Prez, F.; Vankelecom, I. Chem. Mater. 2008, 20, 3876. 2098

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