Dipping versus Spraying: Exploring the Deposition Conditions for

The fact that the film obtained with only a few (10%) of the rinsing steps has a larger thickness increment per layer than the film obtained with rins...
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Dipping versus Spraying: Exploring the Deposition Conditions for Speeding Up Layer-by-Layer Assembly A. Izquierdo,† S. S. Ono,†,‡ J.-C. Voegel,§ P. Schaaf,† and G. Decher*,†,| Institut Charles Sadron, CNRS, UPR 22, 6 rue Boussingault, F-67083 Strasbourg Cedex, France, R&D Center, Mitsui Chemicals, Incorporated, 580-32 Nagaura Sodegaura Chiba, 299-0265 Japan, INSERM Unite´ 595, Fe´ de´ ration de Recherches, Odontologie, Universite´ Louis Pasteur, 11 rue Humann, F-67085 Strasbourg Cedex, France, and Faculte´ de Chimie, Universite´ Louis Pasteur, 1 rue Blaise Pascal, F-67008 Strasbourg Cedex, France Received October 20, 2004. In Final Form: March 21, 2005 Polyelectrolyte film fabrication by successive spraying of polycation and polyanion solutions is described and compared to classic dipping. The poly(styrenesulfonate)/poly(allylamine) system is examined in detail. The influence of various parameters such as spraying time, polyelectrolyte concentration, and effect of film drying during multilayer construction is investigated. It is found that film deposition by spraying is easily controlled and very reliable. The thickness of the multilayers grows linearly with the number of deposition cycles similarly to what is observed when dipping substrates or when polyelectrolyte solutions flow over a surface. The assembly of films is very fast and leads to films with small surface roughness as estimated by atomic force microscopy and X-ray reflectometry. Spray deposition allows achieving regular multilayer growth even under conditions for which dipping fails to produce homogeneous films (e.g., extremely short contact times). Moreover, because drainage constantly removes a certain quantity of the excess material arriving at the surface, one can even skip the rinsing step and, thus, speed up even further the whole buildup process.

1. Introduction The sequential deposition of polycations and polyanions on solid surfaces leads to the buildup of polyelectrolyte multilayer films. The method was described in 1992 by Decher and co-workers1-3 and has received considerable attention since.2,4-19 Usually the substrate is brought * To whom correspondence should be addressed. † Institut Charles Sadron. ‡ Mitsui Chemicals, Incorporated. § Fe ´ de´ration de Recherches, Odontologie, Universite´ Louis Pasteur. | Faculte ´ de Chimie, Universite´ Louis Pasteur. (1) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003. (2) Decher, G. Science 1997, 277, 1232-1237. (3) Decher, G.; Schmitt, J. Thin Solid Films 1992, 210/211, 831835. (4) Decher, G. Templating, Self-Assembly and Self-Organization. In Comprehensive Supramolecular Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (5) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Sashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. (6) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32-39. (7) Donath, E.; Sukhorukov, G.; Caruso, F.; Davis, D. P.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201-2205. (8) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (9) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430442. (10) Ho, P. K. H.; Kim, J. S.; Burroughes, J. H.; Becker, H.; Li, S. F. Y.; Brown, T. M.; Cacialli, F.; Friend, R. H. Nature 2000, 404, 481-484. (11) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C. Langmuir 2001, 17, 7414-7424. (12) Struth, B.; Eckle, M.; Decher, G.; Oeser, R.; Simon, P.; Schubert, D. W.; Schmitt, J. Eur. Phys. J. E 2001, 6, 351-358. (13) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (14) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guildi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190-194. (15) Freemantle, M. Chem. Eng. News 2002, 80, 44-48. (16) Jessel, N.; Atalar, F.; Lavalle, P.; Mutterer, J.; Decher, G.; Schaaf, P.; Voegel, J. C.; Ogier, J. Adv. Mater. 2003, 15, 692-695. (17) Scho¨nhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 86-95. (18) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413-418.

alternatively in contact with a polyanion and a polycation solution either by dipping or by flowing the solutions over the surface to be coated. In 2000 Schlenoff et al.20 reported the construction of a poly(diallyldimethylammonium)/ poly(styrenesulfonate) (PDAD/PSS) multilayer film by spraying the respective solutions on the substrate. They found that such a film had a composition and thickness similar to those of a multilayer film constructed by the conventional deposition method in solution. Unfortunately, this early work has not been pursued much, although spraying is convenient, fast, and more generally applicable for the coating of large surfaces. Here we present a more detailed study on the fabrication of multilayer films by dipping versus spraying, including the incorporation of strong polyelectrolytes. It will become clear that spraying is a method as general as solution dipping, that it has many advantages, and that it can easily be exploited. Intentionally we do not employ sophisticated spraying equipment but rather simple household devices, thus, demonstrating the reliability of spray deposition. We present a general picture of spray deposition focusing on the influence of the major parameters that are, at first sight, expected to play a role in this new multilayer buildup process. What are the fundamental differences between solution dipping and spraying? The layer-by-layer (LbL) deposition process in solution consists of bringing a given substrate alternatively in contact with a polyanion and a polycation solution. Each deposition step is followed by a rinsing step to remove the excess polyelectrolytes in contact with the surface. This way one avoids the formation of polyelectrolyte complexes in the solution, especially in the vicinity of or loosely bound to the substrate. When the rinsing solution is replaced by the one containing the polyelectrolyte of opposite charge using mild agitation only, one should expect to form a zone with very low (19) Hu¨bsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985. (20) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Langmuir 2000, 16 (26), 9968-9969.

10.1021/la047407s CCC: $30.25 © 2005 American Chemical Society Published on Web 06/29/2005

Deposition Conditions for Speeding Up LbL Assembly

polyelectrolyte concentration (“depletion zone”) close to the surface of the substrate. We discuss this issue in the introduction in more or less speculative form to set up the scenario for our present manuscript. We do not attempt to verify the concept of a depletion layer in our specific case. This thin “depletion layer”, which is poorly controlled, is formed for two reasons. The first is hydrodynamic phenomena that prevent the whole rinsing solution to be replaced instantaneously up to the deposition surface, and the second is gradual depletion of polyelectrolyte by adsorption on the surface. Depending on the solution viscosity and on the diffusion constant of the polyelectrolyte, which are functions of parameters such as the chemical nature and the degree of polymerization of the polyelectrolyte, its concentration, the concentration of added salt, and the temperature, the depletion zone is expected to vary in thickness and to possess a gradient of polyelectrolyte concentration which would essentially be zero close to the surface and increasing toward the free solution. The chains have then to diffuse through this depletion zone before reaching the surface. The deposition process is, thus, usually diffusion-controlled when the solution in contact with the surfaces is at rest and controlled both by convection and by diffusion when the solution is stirred or constantly flowed over the substrate. It takes typically several tens of seconds or even minutes before the adsorption becomes homogeneous over the whole surface and the adsorption process becomes wellcontrolled. Although some reports on speedy multilayer deposition exist in the literature,21 one tends to extend the deposition time until reaching the plateau of adsorption and one tends to favor extensive rinsing to avoid the cross-contamination of the dipping solutions. Other fast methods for LbL deposition include spin coating, but this method is also restricted with respect to substrate size and planarity.22-24 Thus, it becomes very difficult to go below a few minutes between two consecutive polyelectrolyte deposition steps, and frequently deposition times are on the order of 15-20 min and rinsing times are on the order of 5-10 min, even for polyelectrolytes that deposit very well such as PSS and poly(allylamine) (PAH). Such deposition speed varies not only with physical parameters but also with the chemical nature of the components that have an influence on the sticking probability. For some cases, the adsorption time-dependent monolayer thickness has been studied.25,26 However, the equilibrium thickness of a single adsorbed layer is not necessarily related to the film quality of the final assembly. For spray deposition we have chosen a vertical ((15°) orientation of the receiving surface and a horizontal ((20°) direction for the orientation of the major axis of the spray cone (Figure 1). Thus, we have a perpendicular orientation between the spray axis and the receiving surface and all liquid arriving at the surface can drain with maximum speed. More precisely, all liquid being applied will not accumulate on the surface but will flow away quickly, driven by gravity, and only a thin film of liquid will initially remain on the surface and then typically dry within minutes. Thus, this setup provides for mild but permanent (21) Lvov, Y.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc 1995, 117, 6117-6123. (22) Chiarelli, P. A.; Johal, M. S.; Casson, J. L.; Roberts, J. B.; Robinson, J. M.; Wang, H.-L. Adv. Mater. 2001, 13 (15), 1167-1171. (23) Cho, J.; Char, K.; Hong, J.-D.; Lee, K.-B. Adv. Mater. 2001, 13 (14), 1076-1078. (24) Lee, S.-S.; Hong, J.-D.; Kim, C. H.; Kim, K.; Koo, J. P.; Lee, K.-B. Macromolecules 2001, 34 (16), 5358-5360. (25) McAloney, R. A.; Goh, M. C. J. Phys. Chem. B 1999, 103 (49), 10729-10732. (26) Mermut, O.; Barrett, C. J. J. Phys. Chem. B 2003, 107, 25252530.

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Figure 1. Experimental setup for multilayer film deposition by spraying. A: Perpendicular spray of polyelectrolytes on a vertically oriented substrate indicating the spray cone, the distance between the nozzle and the receiving surface, and the direction of the draining liquid. B: Spray on a sheet of paper that changes its color as a function of the quantity of water delivered. This allows distinguishing two regions: a central zone (zone 1) surrounded by a ring (zone 2). Zone 1, the central core, is perfectly wet while the surrounding ring (zone 2) is sparsely covered by fine droplets. C: Fluorescent images taken from representative areas of zone D and ring d by fluorescence microscopy of a multilayer film with the deposition sequence PEI(PSS/PAH)2PSS/PAH-FITC. This film was fabricated in two steps. PEI(PSS/PAH)2-PSS was assembled maintaining the substrate vertically and the spray horizontally, whereas the layer of PAH-FITC was deposited with the substrate oriented horizontally and the spray vertically to avoid drainage effects during the deposition of the fluorescence label.

agitation as driven by the draining solution. Because the thickness of the adhering solution is very thin, it is to be expected that any spray droplet arriving at the surface will immediately fuse with the liquid film and replace liquid draining off. It should, thus, be safe to assume that, in this case, the concentrations of the adsorbing species in the adhering liquid film and in the reservoir in the spray tanks are always identical, even in the close vicinity of the receiving surface. This means in consequence that, as a result of arriving spray droplets and drainage, the depletion zone that should form close to the deposition surface and the diffusion of the adsorbing species through this zone should only play a minor role, if any. This is in marked contrast to the dipping method. It also suggests that contact times of the liquid containing the adsorbing molecules and the surface could be very short so that the time interval between two consecutive deposition steps can be significantly reduced, when compared to the deposition by dipping. Moreover, because drainage constantly removes a certain quantity of the excess material arriving at the surface, one can even skip the rinsing step and, thus, speed up even further the whole buildup process. As we will see, a deposition cadence of a few seconds can be reached without affecting the quality of the films. A film composed of 20 layer pairs can, thus, be constructed in less than 4 min as compared to about 2.5 h or typically even much more for the conventional dipping method! To develop a general picture of the spraying procedure, we report in Table 1 the different parameters that enter into the process. We divide them into those which we can control and those which we cannot control with our spray devices. As will be demonstrated further down, even the limited control that can be achieved with household spraying equipment is sufficient to fabricate films of excellent homogeneity. In this paper we will focused on the influence of some of these parameters such as the spraying time and the polyelectrolyte concentration of the

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Table 1. Parameters that Are Known and Controlled or Unknown and Uncontrolled with Our Spraying Device parameters that are known and controlled

parameters that are unknown or poorly controlled

spray distance

droplet size (function of nozzle, pressure, distance, humidity, angle of spray cone and polydispersity) spraying time and other droplet speed and characteristic times entering speed distribution in the deposition process concentration of spray volume polyelectrolyte in droplets (if droplet evaporates, spray rate particularly during flight) number density of droplets concentration of polyelectrolyte in the spray jet in the droplets (if droplet volume is constant variation of parameters at/during the flight) within spray cone

film-buildup solutions. We will also compare the buildup of films in which a rinsing step is performed after each polyelectrolyte deposition with those in which these steps are skipped. It is to be expected that this latter case will be of importance with respect to some technological perspectives such as ink-jet printing.27 2. Materials and Methods a. Polyelectrolyte Solutions. Poly(sodium 4-styrenesulfonate) (PSS, MW ) 70 000 g/mol), poly(allylamine hydrochloride) (PAH, MW ) 15 000 g/mol) were purchased from Aldrich (batch P106005MU, 24903CB, 111K5114, 9745904), and poly(ethylenimine) (PEI, Lupasol WF, MW ) 25 000 g/mol) was from BASF. All solutions were prepared using ultrapure water (Milli-Q plus system, Millipore) with a resistivity of 18.2 MΩ‚cm. Polyelectrolyte solutions were always freshly prepared by direct dissolution of the respective adequate amounts. Thus, PEI at 1 mg/mL, PSS at 0.6 mg/mL, and PAH at 0.27 mg/mL were prepared, respectively. Each polyelectrolyte, except PEI, was dissolved in Milli-Q water containing 0.5 M NaCl, whereas PEI was dissolved in pure Milli-Q water. The final polyelectrolyte concentrations were all around 3 × 10-3 monomol/L expressed in units of monomers per volume unit (monomol ) moles of the respective monomer repeat unit). b. Atomic Force Microscopy (AFM). Silicon wafers (Polylabo, Strasbourg, France) were used and cleaned with CH2Cl2, followed by 1:1 MeOH/HCl(aq) solution and then by H2SO4, and finally extensively rinsed with Milli-Q water and dried under nitrogen. AFM images were obtained in contact mode with a Nanoscope IIIa from Digital Instruments (Santa Barbara, CA). Cantilevers with a spring constant of 0.03 N/m and silicon nitride tips were used (model MLCT-AUHW, Park Scientific, Sunnyvale, CA). Several scans were performed over a given surface area. These scans were aimed at producing reproducible images to ascertain that there was no sample damage induced by the tip. Deflection and height mode images were scanned simultaneously at a fixed scan rate (between 2 and 4 Hz) with a resolution of 512 × 512 pixels. For the PSS/PAH system, rms (root-mean-square) values of the surface roughness were calculated from images corresponding to an area of 10 × 10 µm2. Then a 5 × 5 µm2 square was displaced on the image, and the rms values were calculated for each position. Finally, we obtained a minimal and a maximal rms value for each image. c. Ellipsometry. Measurement of the film thickness was carried out with a PLASMOS SD 2100 instrument operating at the single wavelength of 632.8 nm and a constant angle of 45°. The refractive index of all polyelectrolyte films was assumed to be constant at n ) 1.465. While this procedure will lead to slightly incorrect values with respect to the absolute film thicknesses, it allows for the quick and precise determination of the relative film thicknesses. Thickness values obtained with the assumption of a fixed refractive index for all films are of better precision than required for the comparison of film growth data as in this report. (27) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 9 (124), 2101.

Izquierdo et al. For each substrate studied, several points were measured to obtain the average value for the film thickness and to determine the film homogeneity. d. Fluorescently Labeled Polyelectrolytes. Fluorescein isothiocyanate (FITC)-labeled PAH (MW ) 65 000 g/mol; λabs/em ) 495/525 nm, from Sigma, batch 013K2528) was prepared according Hermanson.28 The excess of FITC not bound to PAH was removed by dialysis (Spectra Por, molecular weight cutoff 25 000 g/mol) against a 0.5 M NaCl solution. The degree of substitution of the PAH-FITC was calculated to be 0.013 fluorophores per PAH monomer, corresponding to about 9 per polymer chain. e. Fluorescence Microscopy. Fluorescence images were taken with a Zeiss Epiplan microscope using a conventional filter set for fluorescein and a Nikon COOLPIX 4500 camera with a MDC2 relay lens. Onto a glass plate (2 cm × 10 cm) horizontally placed, a PAH-FITC solution is sprayed vertically at a distance of 10 cm after the deposition of a PEI(PSS/PAH)2 film. The plate is then photographed over the whole surface, and one can discriminate two distinct zones (see Figure 1). f. Small-Angle X-ray Reflectivity (SAXR). SAXR was measured with a Philips X-PERT MPD (θ-θ). The X-ray source was a Philips high-intensity ceramic sealed tube (3 kW), and the wavelength was Cu KR (1.5418 Å). Reflectivity scans were taken in θ/θ geometry after aligning the height of the sample and its orientation for maximum reflectivity. All measurements were performed on multilayer films prepared on slides cut from silicon wafers. Data were quantitatively evaluated using the Parratt algorithm29 as implemented by Russel30 using numerical leastsquares fitting procedures (ProFit, Quansoft, Zu¨rich, Switzerland). g. Film Buildup by Spraying. All (PSS/PAH)n multilayer films were deposited on a PEI precursor layer. The PEI solution was sprayed for 3 s onto the surface, followed by 27 s of further contact time during drainage. Then the PEI monolayer was sprayrinsed for 20 s with pure water with an additional contact/drying time of 10 s. Once this PEI layer was deposited, the (PSS/PAH)n multilayer was constructed. The spray deposition was carried out by using air pump spray cans made of polypropylene and polyethylene purchased from Roth (ref, 0110.1; size, height 217 × diameter 55 mm; nozzle, 0.40 mm). Four different spray cans were used for each experiment, containing the PEI, PAH, PSS, and rinsing solutions, respectively. Each can was filled with liquid and pressurized by pumping cycles in such a way that the spray rate remained approximately constant over the entire film deposition process. The spray rate turned out to be around 0.6 mL/s with pump flasks as they were received from the manufacturer. Spraying was carried out perpendicularly to the receiving surface which was fixed in a vertical orientation. This allows drainage of the fluid along the surface as outlined above (Figure 1A). The only exception from this geometry was employed for applying PAHFITC because the fluorescent images in Figure 1C required a horizontal setup of the substrate to avoid draining of liquid which would have caused smearing of the fluorescence. To characterize the aerosol, we analyzed the homogeneity of the surface covered by the droplets originating from the spray. This was realized by spraying pure water on a paper sheet that darkens when turning wet. The paper was selected such as not to take up liquid actively to avoid broadening of the wetted area. In this case the spray (pure water) was applied for about 1 s. This corresponds to the shortest reproducible spraying time that is experimentally accessible with a manually operating spraying device. Figure 1B shows that the wetted area is more or less circular and is characterized by two zones: the central region (zone 1) of the paper is entirely wet whereas the outer region (ring 2) is sparsely covered by fine droplets. One can define a minimal diameter of each zone as shown in Figure 1B. The sizes of both zones are a function of the angle of the spray cone (which cannot be varied with our equipment) and the distance between the nozzle and the receiving surface; these data are shown in (28) Hermanson, G. T. Bioconjugate Techniques; Academic Press: 1996. (29) Parratt, L. G. Phys. Rev. 1954, 95, 359. (30) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171.

Deposition Conditions for Speeding Up LbL Assembly

Figure 2. Effect of the spray distance on the deposition of water on the receiving surface. The central zone d (O) indicates the formation of a homogeneous liquid film whereas the outer ring D (0) corresponds to incomplete wetting of the surface. The lines connecting the data points only serve as a guide to the eye. Figure 2. In combination with the spray rate, these parameters determine the quantity of liquid delivered per unit of surface area and time. Furthermore, images such as those presented in Figure 1B allow estimating the variations of the spray rate within the spray cone from the central axis to its outer boundary. As mentioned above, our devices yield a homogeneous central region d and a partially covered ring D. Figure 2 shows clearly that, for our spraying devices, the spraying distance must be smaller than 25 cm to achieve homogeneous wetting of the surface without moving the nozzle. In consequence, we always worked at relatively short spraying distances between 5 and 10 cm. These conditions can be adapted easily to other spraying devices within a short time.

3. Results and Discussion PSS/PAH constitutes a reference system for polyelectrolyte multilayer deposition for which a wealth of data is already available. It has the characteristic that films grow linearly with the number of deposition steps by the conventional dipping method. We took it also as a model system to investigate the influence of various parameters that control the deposition of films by the sequential spraying procedure. A typical multilayer buildup process by spraying is schematically represented in Scheme 1. We describe the general procedure for depositing PSS/ PAH films starting after the adsorption of a first layer of PEI. As graphically represented in Scheme 1, t0 is the time at which the deposition of each layer starts. t1 is the

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period of time in seconds during which a polyelectrolyte (PSS or PAH) is applied by spraying. t2 is the period of time in seconds during which spraying is suspended but drainage and evaporation of water continue. t3 is the time in seconds during which the film is spray-rinsed with water (or buffer). t4 is the period of time in seconds during which spray rinsing is suspended but drainage and evaporation of water continue. The total deposition time per layer (tlayer) is, thus, tlayer ) t1 + t2 + t3 + t4. Film deposition is realized by repeating the sequence as defined above for each polyanion and each polycation layer. Figure 3 represents a typical evolution of the film thickness obtained in such a way with t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s). The concentrations of the polyanion and the polycation solutions were both equal to 3 × 10-3 monomol/L (monomol ) moles of the respective monomer repeat unit). The film thicknesses as a function of the layer number were measured by ellipsometry after the final rinse (t4) of the respective layer and drying of the film in a stream of nitrogen. The continuous increase of the film thickness clearly demonstrates that the films grow regularly (O). Moreover, as for the “classical” dipping method (4) the thickness increases linearly with the number of deposition cycles. This result is confirmed by UV/vis spectrophotometry experiments which show that the PSS content of the multilayers also increases linearly with the number of deposition steps (Figure 3b). These observations are in full agreement with Schlenoff’s findings.20 As already discussed, the spray deposition method allows deposition to speed up further by omitting the rinsing step (t3) and the final waiting step (t4). Figure 3a (0) shows that the deposition is regular even in the absence of rinsing steps for the deposition of both polyanion and polycation layers. It should be noted, however, that the thickness measurements themselves required rinsing and drying the film, so that effectively rinsing (t3) and waiting (t4) were applied four times during the deposition of the total number of 20 pairs of layers. The fact that the film obtained with only a few (10%) of the rinsing steps has a larger thickness increment per layer than the film obtained with rinsing after each deposition step can be easily understood and indicates that during the rinsing step some weakly bound polyelectrolyte chains or even some free chains close to the surface which become part of the multilayer film are certainly removed from the film/ liquid interface. The thickness of films prepared by spraying is also comparable to that of a multilayer constructed by the conventional dipping method using the same polyelectrolyte solutions. It is observed that the films constructed by conventional dipping (4) have a larger thickness increment per layer than those built by the

Scheme 1. Standard Procedure Used for the Fabrication of Polyelectrolyte Multilayer Films by Sprayinga

a t0, t1, t2, t3, and t4 are process times given in seconds. Grey zones correspond to periods in which the spray is suspended; however, drainage and evaporation of water continue.

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Figure 4. Various films obtained by spraying under different experimental conditions. The symbols represent the following: (O) (PSS/PAH)n sprayed film whose thickness was measured after every layer pair deposition step; rinsing performed with Milli-Q water containing 0.5 M of NaCl. Only for each ellipsometry measurement was the film rinsed for an additional 10 s with pure Milli-Q water, to avoid crystallization of salt. (0) Same as the film represented by O but the rinsing solution was pure Milli-Q water. ()) Same as the film represented by 0, but after the deposition of 10 pairs of layers, the film is stored overnight before continuing the deposition process until the end. (4) Sprayed film built without any intermediate thickness measurement. The final film thickness was addressed only after deposition of 20 pairs of layers. For each experiment t1, t2, t3, and t4 were maintained to the values of t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s).

Figure 3. (a) Evolution of the thickness of a (PSS/PAH)n film with the number n of layer pairs constituting the film (n ) 20). The symbols represent the following: (O) film built by the spraying process characterized by t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s); (4) film built by the standard dipping method (the substrate is dipped for 20 min in the polyion solution followed by three rinsing steps of 100 s, 100 s, and 120 s; tlayer ) 1520 s); (0) film built by the spraying process without a rinsing step (t1 ) 3 s, t2 ) 27 s, t3 ) 0 s, and t4 ) 0 s; tlayer ) 30 s). Part b represents the dependence of the optical absorbance at 225 nm (O) on the number of deposited pairs of layers for a (PSS/PAH)n multilayer (n ) 20). The film was constructed on a fused quartz by using the same spraying conditions as in part a represented by the symbol O characterized by t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s) and concentration of polyions of 3 × 10-3 monomol/L.

alternating spraying procedure with rinsing (O) but a smaller one than the film grown without a rinsing step (0). The fact that the film thickness grows faster for multilayers formed by conventional dipping and grows slower by spraying can be attributed to the fact that the newly absorbing layers do not necessarily reach their equilibrium thickness at a short contact time. We will show later that, despite the fact that the adsorption occurs apparently at nonequilibrium, the film quality is excellent. While the ionic strength of the solution containing the polyelectrolytes is an important parameter for controlling the thickness of the adsorbed layer, we find that the composition of the rinsing solution seems to have no measurable influence on the film thickness. At least no difference in film thickness was noted for rinsing solutions

composed of pure water and of 0.5 mol/L NaCl. Among other variables, this is further shown in Figure 4. Different (PSS/PAH)n multilayers (n ) 20) were deposited keeping the characteristic times t1, t2, t3, and t4 and polyelectrolyte solutions constant while varying the composition of the rinsing solutions. In one case (O) rinsing was performed with a solution containing 0.5 M NaCl whereas in another case (0) pure water was used. In a third case, ()) the film was prepared like that represented by “0” until 10 layer pairs, dried and stored overnight. After 12 h of storage in an ambient atmosphere, the construction was continued up to the deposition of 20 layer pairs. Finally, the film (×) was constructed like that represented by “0” but without intermediate drying. The thickness of the film was only measured once at the end of the multilayer deposition. None of these substantial differences in the buildup conditions induced significant change film thicknesses of 1-20 layer pairs. All thickness values reported in Figures 3 and 4 correspond to an average value of five measurements performed at five different spots on a solid substrate of size 4 × 1 cm2 placed within zone 1 (Figure 1). The small error bars in Figures 3 and 4 indicate that all films were very homogeneous at the macroscopic level. The microscopic homogeneity of the films has also been verified by observing fluorescently labeled films (containing one layer of PAH-FITC) by fluorescence microscopy (Figure 1C). A film with the architecture glass/PEI(PSS/PAH)2PSS/ PAH-FITC appears very homogeneous in zone 1 whereas zone 2 is clearly not uniformly covered. These findings are further corroborated by AFM imaging and by X-ray reflectivity data of a PEI(PSS/PAH)20 sample to extend the morphological characterization to the nanoscale (Figure 5).

Deposition Conditions for Speeding Up LbL Assembly

Figure 5. SAXR, contact mode AFM, and cross-sectional profile trace of a PEI(PSS/PAH)20 film built by spraying with t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s). A: Data points correspond to experimental values, and the solid line corresponds to a numerical fit obtained for a single box model. B: AFM image dimensions are 10 × 10 µm2 with a maximum Z range of 10 nm. The horizontal black line indicates where the height profile was taken. C: Surface profile trace. The film is 42.53-nm thick as measured by SAXR reflectometry with a roughness of 0.99 nm.

Figure 5B represents a typical contact mode AFM image obtained for a film constructed by spraying with t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4 ) 10 s (tlayer ) 60 s). The profilometric height analysis (1.19 nm; Figure 5C) is in good agreement with the roughness obtained (0.99 nm) from SAXR (Figure 5A). All experimental data obtained for (PSS/PAH)n films underline that films obtained by fast spray deposition including a rinsing step (tlayer ) 60

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Figure 6. SAXR, contact mode AFM, and cross-sectional profile trace of a PEI(PSS/PAH)20 film made by dipping. The substrate is dipped alternately in a PSS (3 × 10-3 monomol/L) and a PAH (3 × 10-3 monomol/L) solution for 20 min each. Each polyelectrolyte deposition step is followed by a sequence of three rinsing steps in pure water (100 s, 100 s, 120 s; tlayer ) 1520 s). A: Data points correspond to experimental values, and the solid line corresponds to a numerical fit obtained for a single box model. B: Image dimensions are 10 × 10 µm2, and the maximum Z range is 90 nm. The horizontal black line indicates where the height profile was taken. C: Surface profile trace. The film is 55.22-nm thick with a roughness of 1.00 nm as measured by SAXR reflectometry.

s) are as homogeneous and smooth as those obtained by conventional dipping (Figure 6). However, spray deposition allows building films even much faster than the conventional dipping method. We have, for example, built a PEI(PSS/PAH)20 film within 4 min (t1 ) 3 s, t2 ) 0 s, t3 ) 3 s, t4 )0 s; tlayer ) 6 s; Figure 7). Such a film has a final thickness of 38.68 nm with a roughness of 1.10 nm as determined by SAXR. While this film is still homogeneous,

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Figure 7. SAXR, contact mode AFM, and cross-sectional profile trace of the PEI(PSS/PAH)20 film made by the spraying method in 4 min with t1 ) 3 s, t2 ) 0 s, t3 ) 3 s, and t4 ) 0 s (tlayer ) 6 s). A: Data points correspond to experimental values, and the solid line corresponds to a numerical fit obtained for a single box model. B: Image dimensions are 10 × 10 µm2, and the maximum Z range is 20 nm. The horizontal black line indicates where the height profile was taken. C: Surface profile trace. Film thickness and roughness, respectively, equal 38.68 and 1.10 nm.

it shows a slightly increased roughness in comparison with films made with tlayer ) 60 s. Obviously, for the polyelectrolytes discussed here and for the “primitive” spraying equipment employed, deposition times on the order of 6 s/layer tend to the lower time where high-quality films can be obtained. Table 2 summarizes the results obtained for the multilayer films prepared using different conditions. While ellipsometry indicates that all five fabrication conditions lead to the deposition of polyelectrolytes, not all of them lead to the formation of homo-

Izquierdo et al.

Figure 8. A: X-ray reflectometry of two PEI(PSS/PAH)20 films obtained respectively in 4 (blue curve) and 40 min (orange curve) time by dipping. The film made in 4 min was built by dipping the substrate for 3 s alternately in a PSS (3 × 10-3 monomol/L) and a PAH (3 × 10-3 monomol/L) solution. Each polyelectrolyte deposition step is followed by a sequence of 3 s of rinsing steps in pure water. The film made in 40 min was built by dipping the substrate 30 s alternately in a PSS (3 × 10-3 monomol/L) and a PAH (3 × 10-3 monomol/L) solution. Each polyelectrolyte deposition step is followed by a sequence of 5 × 3 s of rinsing steps in pure water. B: AFM image corresponds to the film built in 40 min. The image dimensions are 10 × 10 µm2, the maximum Z range is 200 nm, and the roughness is 31.31 nm. C: Cross-sectional profile trace. X-ray reflectometry curves do not show the characteristic Kiessig fringes.

geneous and smooth films. While spraying is efficient even at short (40 min) and ultrashort (4 min) deposition times for the buildup of the entire 20 pairs of layers, dipping does not lead to regular film growth at short (40 min) deposition times. Ellipsometry and AFM (Figure 8) show large fluctuations in the film thickness on different spots

Deposition Conditions for Speeding Up LbL Assembly

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Table 2. Film Thickness Data of Two Films Obtained by Spraying in 40 and 4 min Time and Three Films Obtained by Dipping in 1013, 40, and 4 min Timea film prepared by dipping in ∼17 h

film prepared by spraying in 40 min

film prepared by dipping in 40 min

dipping (robot) 1200 0 320 (2 × 100 + 1 × 120) 0 1520 20 1013

Film Deposition Conditions spraying (manual) dipping (robot) 3 30 27 0 20 15 (3 × 5) 10 15 60 60 20 20 40 40

ellipsometric film thickness (nm)

65.20 ( 0.80

Ellipsometry Data 45.55 ( 0.74 51.46 ( 3.49

film thickness (nm) surface roughness (nm) electron density (Å-3)

55.22 1.00 0.39

42.5 0.99 0.39

preparation t1 (s) t2 (s) t3 (s) t4 (s) tlayer (s) number of layer pairs total film preparation time (min)

a

SAXR Data no Kiessig fringes no Kiessig fringes no Kiessig fringes

film prepared by dipping in 4 min

film prepared by spraying in 4 min

dipping (robot) 3 0 3

spraying (manual) 3 0 3

0 6 20 4

0 6 20 4

90.34 ( 23.36

44.12 ( 0.27

no Kiessig fringes no Kiessig fringes no Kiessig fringes

38.68 1.10 0.41

All films were prepared by carrying out 20 deposition cycles corresponding to 40 layers.

on the substrate, and the accompanying SAXR measurements for dipped films did not show any Kiessig fringes31 neither for the deposition time of 40 min nor for that of 4 min (Figure 8A). Only deposition times of ∼17 h for 20 pairs of layers (standard classic dipping conditions) lead to high-quality films. Ellipsometry and SAXR agree that dipped films are thicker than sprayed ones, and there seems also to be little dependence of film thickness with the contact times if films are sprayed. As expected there is a systematic discrepancy for thickness measurements by ellipsometry and by SAXR because ellipsometry was carried out using an assumed refractive index of 1.465 for reasons of convenience. The choice of a fixed refractive index for different films is justified, for example, by the fact that different films possess very similar electron densities as obtained by SAXR. The excellent film quality obtained by spray deposition suggested studying the dependence of multilayer fabrication on the major deposition parameters such as the concentration of the polyelectrolyte solutions and the characteristic deposition times tn. For the variation of the polymer concentrations, the film architecture was always PEI(PSS/PAH)10 and the deposition times were chosen as for the previous experiments (t1 ) 3 s, t2 ) 27 s, t3 ) 20 s, and t4) 10 s; tlayer ) 60 s). Two sets of experiments were carried out, one in which the concentrations of both polyion solutions were varied simultaneously from 3 × 10-3 monomol/L to 0.015 × 10-3 monomol/L (dilution of C ) 3 × 10-3 monomol/L by factors of 1, 2, 10, 20, 50, 100, and 200) and one in which the solution of one polyelectrolyte was varied from 3 × 10-3 monomol/L to 0.015 × 10-3 monomol/L (dilution of C ) 3 × 10-3 monomol/L by factors of 1, 2, 10, 20, 50, 100, and 200) while the other one was kept constant to the value of 3 × 10-3 monomol/L (Figure 9). One observes that, in each case, the film thickness decreases with decreasing polyelectrolyte concentration. The dilution experiments were limited to polymer concentrations that would lead to a thickness increment per layer pair of 0.5 nm or more. At this increment one is already below monolayer coverage assuming the diameters of a polymer chain as minimum thickness of a monomolecular polymer layer.8 Even at this dilution, spray deposition is fairly efficient. Even if (31) Kiessig, H. Ann. Phys. 1931, 10, 769-788.

we would assume that all polymers in the volume of liquid arriving on the surface would adsorb and no chains would be lost with the draining liquid, the total quantity of polymer on the surface would be around 5 mg/m2 which is close to the value of 1 mg/m2 obtained under equilibrium conditions for the adsorption of a polyelectrolyte layers estimated for example by quartz crystal microbalance (QCM). The variation of the film thickness with the concentration can empirically be described by a logarithmic correlation. The experiment also shows in a first approximation that the thickness increment per layer depends essentially on the smaller of either the polyanion or the polycation concentration. While this is certainly an interesting observation we do not want to speculate on a mechanism responsible for this effect because several plausible explanations cannot easily be distinguished experimentally. We are planning to address this point in future work.

Figure 9. Thickness increase per layer pair for PEI(PSS/ PAH)10 films as a function of polyion concentration. Filled triangles (2) correspond to the case where the concentrations of both polymers PSS and PAH were varied but maintained equal, filled circles (b) correspond to the case in which the concentration of PSS was varied from 3 × 10-3 monomol/L (C/ 1) to 0.015 × 10-3 monomol/L (C/200) while the concentration of PAH was kept constant at 3 × 10-3 monomol/L; open squares (0) represent the case in which the concentration of PAH was varied from 3 × 10-3 monomol/L (C/1) to 0.015 × 10-3 monomol/L (C/200) while the concentration of PSS was kept constant at 3 × 10-3 monomol/L.

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Figure 10. Thickness increments per layer pair obtained from PEI(PSS/PAH)10 films plotted as a function of spraying time t1. The concentrations of PSS and PAH solution that are sprayed are equal to 3 × 10-3 monomol/L (O), 1.5 × 10-3 monomol/L (9), 3 × 10-4 monomol/L ()), and 10-4 monomol/L (1).

The second variable that was studied was the influence of the spraying time on the multilayer assembly. In this case the concentrations of both polyelectrolytes were kept the same. Figure 10 shows the evolution of the thickness increment per layer pairs for various polyelectrolyte concentrations. One observes that the film thickness increment increases with the spraying time t1 (t1 ) 1, 3, 6, and 9 s) while keeping the times t1 + t2 ) 30 s (contact time with polyion solution), t3 ) 20 s, and t4) 10 s constant (tlayer ) 60 s). At times of t1 below 3 s, the spraying time has a larger influence on the film thickness than for spraying times larger than 3 s. For the spraying times investigated here a leveling off of the thickness increment with increasing spraying time was not observed, indicating that, in all experiments described here, the newly adsorbing polyelectrolyte monolayer never reaches its equilibrium thickness. The shape of the curves shown in Figure 10 confirms earlier results on polyelectrolyte adsorption kinetics, namely, that the polyion adsorption is a twostep process in which a rapid initial adsorption is followed by a slower structural reorganization of the layer while reaching its final thickness. To elucidate the thickness per layer as a function of polyion availability (see also Figure 10) for the deposition method (dipping and spraying, see also Figure 3a), we conducted a series of additional experiments. Figure 11 shows how the thickness of an individual layer approaches a maximum value as a function of repeated spray cycles of the same polyion. In both cases (PAH and PSS) the final thickness is reached after about eight spray cycles, the initial spray cycle accounting for about 50% of the final thickness (case of PSS). Figure 11 also shows that the intermediate drying has no measurable effect on the layer thickness attained by repeated re-spraying of the PSS). For PAH the effect is much less pronounced; respraying PAH causes a very small thickness increase close to the error limit of our ellipsometer. At present we cannot explain these differences between PSS and PAH. Figure 12 describes the importance of rinsing during multilayer buildup. There are two sets of curves with individual thickness values for each polyion. It shows that polyions arriving on the surface after rinsing away the previous excess of the same polyion cause stronger adsorption than in the absence of rinsing. This points to the fact that rinsing

Izquierdo et al.

Figure 11. Influence of the number of spray cycles on monolayer thickness using t1 ) t2 ) t3 ) t4 ) 3 s. (O) Spraying and re-spraying of PSS on a precursor film of (PSS/PAH)6. After each cycle the sample is dried and the total film thickness is measured by ellipsometry. (0) Same as the film represented by O without intermediate drying and thickness determination. (4) Spraying and re-spraying of PAH on a precursor film of (PSS/PAH)6. After each cycle the sample is dried and the total film thickness is measured by ellipsometry. (b) Same as the film represented by 4 without intermediate drying and thickness determination.

Figure 12. Evolution of the film thickness as a function of the rinsing conditions during re-spraying cycles of the same polyion. There are two sets of curves; the lower set corresponds to a single rinse after 10 re-spraying cycles (tlayer ) 66 s), and the upper set corresponds to rinsing after each re-spray cycle (tlayer ) 120 s). (O) (PSS/rinse)10 per layer with t1 ) t2 ) t3 ) t4 ) 3 s. (2) (PAH/rinse)10 per layer with t1 ) t2 ) t3 ) t4 ) 3 s. (b) ((PSS)10/rinse) per layer with t1 ) t2 ) t3 ) t4 ) 3 s. (4) ((PAH)10/rinse) per layer with t1 ) t2 ) t3 ) t4 ) 3 s. Films were dried and submitted to thickness measurements after completion of each re-spraying cycle. Data are given for each individual polyion.

causes not simply removal of weakly adsorbed material but also structural rearrangements leading to a better anchoring of the same polyion in the following re-spraying cycle. While repeated spraying without intermediate rinsing only causes minor increases in the layer thickness (∼15%) as compared to a single spray cycle, repeated spray/ rinse cycles of the same polyion lead to a significant increase of layer thickness (∼75%) exceeding even the layer thickness obtained by standard dipping. However,

Deposition Conditions for Speeding Up LbL Assembly Table 3. Summary Table of the Thickness Per Layer Pair Increment of Different Films Made by Dipping and Spraying for Different Deposition/Rinsing Conditions

method

layer pair composition (deposition/rinsing conditions)

tlayer (s)

thickness increment per layer pair (nm)

dipping

(PSS/rinse)/(PAH/rinse) (dipping ) 20 min, rinsing ) 320 s)

1520 3.26 ( 0.04

re-dipping

(PSS/rinse)10/(PAH/rinse)10 (dipping ) 20 min, rinsing ) 320 s)

15 200 5.88 ( 0.70

spraying

(PSS/rinse)/(PAH/rinse) (t1 ) 3 s, t2 )27 s, t3 )20 s, t4 ) 10 s) (PSS/rinse)/(PAH/rinse) (t1 ) 3 s, t2 )0 s, t3 )3 s, t4 ) 0 s)

re-spraying (PSS/rinse)10/(PAH/rinse)10 (t1 ) t2 ) t3 ) t4 ) 3 s) ((PSS)10/rinse)/((PAH)10/rinse) (t1 ) t2 ) t3 ) t4 ) 3 s)

60 2.28 ( 0.04 6 2.20 ( 0.02 120 3.87 ( 0.28 66 2.51 ( 0.14

performing additional dip/rinse cycles during deposition by dipping also causes a further increase in layer thickness. While after 10 spray/rinse cycles per polyion layer spraying yields a 3.87 ( 0.28 nm per layer pair, dipping with 10 dip/rinse cycles per polyion layer yields a still larger value of 5.88 ( 0.70 nm per layer pair. This shows that both deposition methods respond similarly to changing deposition parameters. However, films obtained by dipping are always thicker than films obtained by spraying even if sufficient material is provided during repeated re-spraying cycles or re-dipping cycles. Table 3 summarizes the result for the increment per layer pair for the different deposition/ rinsing conditions. 4. Summary and Conclusions Polyelectrolyte multilayer deposition can be simplified and sped up enormously by applying the polyion-containing liquid by spray rather than by bringing the receiving surface in contact with the solution by dipping. We fully confirm earlier work by Schlenoff with different polyelectrolytes. In addition, we outline the fundamental differences between spraying and dipping by discussing some hand-waving arguments. These speculative arguments provide a scenario to explain why dipping fails to produce homogeneous films at short and ultrashort deposition times while spraying leads to homogeneous and smooth films. The quality of films with a deposition time of 60 s per layer is equal or superior to films obtained by dipping with a deposition time of 1520 s per layer (classical dipping conditions). This corresponds to an

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acceleration of the LbL deposition by a factor of about 25. Deposition can be sped up even further by a factor of 10 (reaching a total acceleration of a factor of 250 with respect to dipping) while maintaining an acceptable quality of the film. Eventually, because drainage constantly removes a certain quantity of the excess material arriving at the surface, one can even skip the rinsing step and, thus, speed up even further the whole buildup process. There is a principal difference between spraying and dipping. From older work and QCM work, we know that PSS/PAH multilayers require about 15-20 min to approach the plateau of adsorption for each layer. In the present study we confirm this fact by showing that reducing the deposition time per layer to values of 4 min or less does not lead to homogeneous films. In contrast, films obtained by spraying require a deposition time of only 6 s per layer. We have provided some hand-waving arguments in the introduction where we outline the fundamental differences between spraying and dipping. These arguments are used as a base of explaining our experimental results. We have also shown that rinsing can play an important role during multilayer deposition. Repeated spray/rinsing or dip/ rinsing cycles during the adsorption of each individual layer increase the layer thickness in each case, presumably by structural rearrangements leading to the exposure of more anchoring sites on the surface. Interestingly, films prepared by dipping are always thicker than films prepared by spraying although in both cases it is safe to assume that enough material is provided to approach the plateau of adsorption. Both the effects of rinsing and the differences in film thickness between dipping and spraying are not understood at the present time. While we can speculate on their origin, for example, the formation of a depletion layer in the case of dipping, more work will be needed to elucidate the details. However, the huge acceleration of spraying versus dipping is also valid for the case of repeated re-spraying/re-dipping of the same polyion during the deposition of a single layer. Acknowledgment. We gratefully acknowledge the support of the CNRS through a postdoc program from the Ministe`re de l’Education et de la Recherche (Project Nο. NR204), BASF AG for donating the Lupasol, and Dr. Benoit Heinrich (IPCMS/GMO-Strasbourg) for his help with X-ray reflectivity experiments. This work was financially supported by the ACI “Nanoscience”. LA047407S