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Langmuir 2006, 22, 2856-2862
Nanoporosity-Driven Superhydrophilicity: A Means to Create Multifunctional Antifogging Coatings Fevzi C¸ . Cebeci,§ Zhizhong Wu,‡ Lei Zhai,‡ Robert E. Cohen,*,∧ and Michael F. Rubner*,‡ Departments of Materials Science and Engineering and Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed NoVember 23, 2005. In Final Form: January 13, 2006 Multifunctional nanoporous thin films have been fabricated from layer-by-layer assembled silica nanoparticles and a polycation. The resultant multilayer films were found to exhibit both antifogging and antireflection properties. The antifogging properties are a direct result of the development of superhydrophilic wetting characteristics (water droplet contact angle 150° and low contact angle hysteresis) or that are completely and instantaneously wet by water (superhydrophilic state; water droplet contact angle 18 MΩ‚cm) were performed with a VCA-2000 contact angle system (AST Products, Inc., MA). A water drop of approximately 0.5 µL was placed on the multilayer coated surfaces using a syringe. Contact angle values were calculated from dynamic video files that captured at 60 frames/s using the software provided (VCA Optima XE Version 1.90) by the manufacturer. A Barnstead Thermolyne 47900 furnace was used to calcinate the films at 500° for 4 h. Thickness measurements for multilayers assembled on glass were done with a Tencor P10 surface profilometer using a 2 µm stylus tip and 6 mg stylus force. Profiling was achieved by scratching a portion of the film down to the substrate. More than four measurements were performed for each film, and the average value is reported.
Experimental Section Materials and Chemicals. Poly(allylamine hydrochloride) (PAH) (Mw ) 70 000), poly(sodium 4-styrene sulfonate) (PSS) (Mw ) 70 000), and the colloidal silica nanoparticles Ludox SM-30 (30 wt% SiO2 suspension in water, average particle size of 7 nm, and specific surface area of 345 m2 g-1), Ludox HS-40 (40 wt% SiO2 suspension in water, average particle size of 12 nm, and specific surface area of 220 m2 g-1), and Ludox TM-40 (40 wt% SiO2 suspension in water, average particle size of 22 nm, and specific surface area of 140 m2 g-1) were obtained from Sigma-Aldrich (St. Louis, MO). The natural solution pH of the nanoparticle solutions is 10, 9.8, and 9.0 for the 7, 12, and 22 nm diameter particles, respectively. Poly(acrylic acid) (PAA) (25% aqueous solution, Mw ) 90 000) was obtained from Polysciences (Warrington, PA). Deionized water (>18 MΩ‚cm, Millipore Milli-Q) was exclusively used in all aqueous solutions and rinsing procedures. Thin Film Assembly. Sequential adsorption of polyelectrolyte multilayers was performed on either glass slides or silicon wafers by using a nanoStrata dipping unit; the dipping time for polymers and nanoparticles was 15 min followed by three rinses in MQ water: one 2 min and two 1 min rinses. All the PAH, PAA, and PSS solutions (0.01 M based on repeat unit) were adjusted to dipping pH with HCl or NaOH. Nanoparticles were prepared with five different concentrations (0.005, 0.01, 0.03, 0.05, and 0.10 by wt%) and were adjusted to pH 3.0, 4.0, 6.0, 8.0, or 9.0. The assembly pH of a multilayer is indicated in the following example: PAH 4.0/SiO2 4.0, where the number following the material assembled represents the pH of the dipping solution. Characterization. Transmission and reflectance measurements were performed using a Varian Cary 5E spectrophotometer at a (28) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. (29) Rouse, J. H.; Ferguson, G. S. J. Am. Chem. Soc. 2003, 125, 15529. (30) Sennerfors, T.; Bogdanovic, G.; Tiberg, F. Langmuir 2002, 18, 6410. (31) Bogdanvic, G.; Sennerfors, T.; Zhmud, B.; Tiberg, F. J. Colloid Interface Sci. 2002, 255, 44. (32) (a) Marston, N.; Vincent, B. Langmuir 1997, 13, 14. (b) Kovtyukhova, N.; Ollivier, P. J.; Chizhik, S.; Dubravin, A.; Buzaneva, E.; Gorchinskiy, A.; Marchenko, A.; Smirnova, N. Thin Solid Films 1999, 337, 166. (c) Kovtyukhova, N.; Martin, B. R.; Mbindyo, J. K. N.; Smith, P. A.; Razavi, B.; Mayer, T. S.; Mallouk, T. E. J. Phys. Chem. B 2001, 105, 8762. (d) He, J.-A.; Mosurkai, R.; Samuelson, L. A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169. (33) Hattori, H. AdV. Mater. 2001, 13, 51. (34) Koo, H. Y.; Yi, D. K.; Yoo, S. J.; Kim, D.-Y. AdV. Mater. 2004, 16, 274. (35) Ahn, J. S.; Hammond, P. T.; Rubner, M. F.; Lee, I. Colloids Surf., A 2005, 259, 45. (36) Kommireddy, D. S.; Patel, A. A.; Shutava, T. G.; Mills, D. K.; Lvov, Y. M. J. Nanosci. Nanotechnol. 2005, 5, 1081.
Results and Discussion Multilayer Assembly. Stable multifunctional superhydrophilic coatings can be easily created from layer-by-layer assembled films of negatively charged colloidal SiO2 nanoparticles and a suitable polycation. The realization of stable superhydrophilic coatings that exhibit both antifogging and antireflection properties requires optimization of a number of key parameters including the materials utilized (size of nanoparticles and polymer type), the solution processing conditions (nanoparticle and polymer concentration and assembly pH), and the multilayer architecture (number and sequence of assembled bilayers). Prior to the assembly of the nanoparticle-containing multilayers, bilayers comprised of alternating layers of PAH and sulfonated polystyrene (SPS) or PAA were assembled onto the surface to promote better adhesion to the glass substrate. Control experiments reveal that these “surface preparation layers” do not influence significantly the superhydrophilic behavior of the final multilayer assembly. The colloidal silica nanoparticles were then alternately assembled with PAH to complete the thin film coating. The layer-by-layer assembly process is driven by electrostatic interactions between the positively charged polycation (PAH) and the negatively charged SiO2 nanoparticles. Both PAH and the SiO2 nanoparticles exhibit pH-dependent charge densities. In the case of the nanoparticles, the surface charge density increases with increasing pH (due to ionization of surface silanol groups),37 whereas for PAH, the chain charge density decreases with increasing solution pH (due to deprotonation of ionized amine groups).38 Although not described in this paper, the addition of salt to the dipping solutions can also be used to mediate the effective charge of these materials. Three different sizes of commercial colloidal silica nanoparticles with particle diameters of 7 (Ludox SM-30), 12 (Ludox HS-40), and 22 nm (Ludox TM-40) were examined for their ability to promote stable superhydrophilic behavior. The pH of the PAH and SiO2 nanoparticle solutions was varied from 3.0 to about 10 to determine the specific processing conditions needed to create transparent, stable superhydrophilic films of the highest (37) Bergna, H. E.; Roberts, W. O. Silica. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: New York, 2002. (38) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116.
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quality. In the case of the larger-size SiO2 nanoparticles (12 and 22 nm), when the solution pH dropped below 9.0, multilayer films with more than eight bilayers of PAH/SiO2 became progressively more cloudy due to aggregation of the nanoparticles within the film. Since superhydrophilic behavior could not be obtained with eight bilayers or less, it was not possible to create high-quality multifunctional coatings from films assembled with these larger-diameter nanoparticles. As will become apparent shortly, a critical number of bilayers must be deposited to establish a nanocapillary effect and achieve stable superhydrophilic behavior. Multilayer films assembled with the smaller nanoparticles (7 nm), on the other hand, remained highly transparent even after the deposition of 16 PAH/SiO2 bilayers. Only when 24 or more bilayers had been deposited did these films begin to appear somewhat cloudy. At the lowest pH examined (pH 3.0), the low negative surface charge of the nanoparticles results in very limited nanoparticle deposition, and the superhydrophilic properties of such films are not stable with time. On the basis of these screening results, all multilayer films were assembled using 7 nm diameter nanoparticles with the pH of the nanoparticle and PAH solutions in the range of 4-9. The optimum solution pH for the assembly of the 7 nm diameter SiO2 nanoparticles was determined to be in the range of 7.5-9, although it is also possible to get high-quality multifunctional films at pH 4.0 (at a solution pH >4.0 and 144 h) may reflect the formation of siloxane bridges created by the condensation of surface silanol groups by thermal dehydroxylation.41 As a result, the SiO2 nanoparticles would be rendered more hydrophobic. The superhydrophilic behavior of films treated for long times at elevated temperatures could be completely recovered by treatment for 30 s in an oxygen plasma. This observation is consistent with the notion that the surface chemistry of the particles is being altered
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Figure 7. Thermal stability test of some superhydrophilic samples. Sample 1: 12 bilayers of PAH 7.5/SiO2 8.0 with 10 bilayers of PAH 7.5/PAA 3.5 as adhesion layers (0.03% by wt silica solution). Sample 2: 14 bilayers of PAH7.5/SiO29.0 and 4 bilayers of PAH 4.0/PAA 4.0 as adhesion layers (0.03% by wt silica). Sample 3: 16 bilayers of PAH 7.5/SiO2 9.0 and 4 bilayers of PAH 4.0/PAA 4.0 as adhesion layers (0.01% by wt silica solution). All films were fabricated from 7 nm nanoparticles.
Figure 8. (a) Comparison of the fogging behavior of a bare glass slide (right-hand slide) with a slide partially coated (left-hand side) with a superhydrophilic polyelectrolyte multilayer film (16 bilayers of PAH 7.5/SiO2 9.0 with 4 bilayers of PAH 4.0/SPS 4.0 as adhesion layers (0.03% by wt silica solution). Note the fogged top region of the left-hand slidesthis region was not coated with the superhydrophilic film. (b) Glass slide half coated (left side) with a superhydrophilic multilayer illustrating the nonuniform water dewetting behavior of normal glass compared to the uniform wetting behavior of the coated surface.
Figure 6. (a) Still images from video contact angle measurements for a first and second drop (0.5 µL) of water, (b) time-dependent change in contact angle for a first water drop as a function of the number of deposited bilayers, and (c) time-dependent change in contact angle for a second water drop as a function of the number of deposited bilayers. Multilayer film: PAH 7.5/SiO2 8.0 (7 nm nanoparticles in a 0.03 wt% solution) with PAH 4.0/SPS 4.0 adhesion layers.
at elevated temperature. From these tests, however, we cannot rule out the possibility that the coatings become contaminated in the oven. It should be noted that the antireflection properties of these films did not change as a result of these various heat treatments, indicating that the nanoporous nature of the film remains unaffected. In addition, no significant differences in the stability of the wetting behavior were observed between asprepared and calcinated samples (calcinated samples exhibit the same wetting behavior as noncalcinated materials).
As expected, a surface with a water droplet contact angle of essentially zero exhibits antifogging characteristics due to the fact that the nearly instantaneous, sheetlike wetting by water prevents light scattering water droplets from forming on the surface. Figure 7 indicates that antifogging behavior persists as long as the water droplet contact angle is below about 7°. The images in Figure 8a illustrate the antifogging behavior. This figure shows two glass sides, one with a superhydrophilic coating and the other without such a coating. Both slides were cooled in a refrigerator at about -18 °C and then moved into humid laboratory air. The uncoated slide fogged immediately, whereas the portion of the slide coated with a superhydrophilic multilayer film remained clear. Another interesting characteristic of a superhydrophilic coating is its ability to prevent dewetting by water. As soon as a typical glass slide is withdrawn from water, the well-known dewetting phenomenon takes place. In sharp contrast, a glass slide coated with a superhydrophilic multilayer remains fully wet after removal from water and stays in this state until the water evaporates (see Figure 8b).
Discussion and Conclusions Wetting Behavior. The wetting behavior of a surface is determined by both its chemical composition and micro/ nanotexture. It is well established, for example, that for a given
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chemical composition, increasing the roughness of a surface can render it more hydrophobic or more hydrophilic depending on the wettability of the material in the smooth state (Wenzel-type roughness effect).24,26 With suitable engineering of surface roughness/texture, it is also possible to obtain materials that exhibit extreme wetting characteristics including both superhydrophobic and superhydrophilic behavior.25,26 Superhydrophobic behavior (Cassie-Baxter type; water droplet contact angle >150° and low contact angle hysteresis)1-6 occurs when air becomes entrapped in the microstructure: in this state, water droplets simply roll on the surface. Near complete wetting on the other hand (superhydrophilic behavior; water droplet contact angle
1 r
(2)
To test these interesting predictions, it will be necessary to identify liquids with advancing contact angles on flat glass substrates ranging from 20° to about 90° (water has a contact angle of 10-20° on flat glass). Since most liquids, including many hydrocarbon oils, readily wet glass, this is not a simple proposition. We are currently exploring ways to conduct these or related experiments. Multifunctional Properties. The creation of a nanoporous thin-film coating with a low refractive index also gives rise to antireflection behavior. The optical thickness of the coating (product of refractive index and thickness) can be tuned by simply varying the assembly conditions and number of deposited bilayers. To obtain multifunctional coatings that exhibit both antifogging and antireflection capability, it is necessary to achieve the critical thickness needed for superwetting behavior and the optical thickness needed for the maximum suppression of reflections over a desired wavelength range. The layer-by-layer processing approach is ideally suited for this task as it can be used to create a family of thin-film coatings that provides antireflection capability that spans the visible to near-infrared regions of the spectrum. In principle, this range could be further extended by using nanoparticles that do not absorb light in the spectral region of interest. As has been demonstrated with a different type of layer-by-layer assembled nanoporous thin film,39 it is also possible to realize antireflection properties over an even wider range of wavelengths by simply assembling a multilayer heterostructure that mimics a more ideal gradient refractive index profile. An interesting question to consider is what effect humidity has on the antireflection properties of the coating. If we assume that the nanopores become completely filled with water with a refractive index of 1.33, using an estimated pore fraction of 65%, model simulations show that the maximum transmission would decrease from 99.8% to 97.0%. Thus, even in this extreme situation, the coating still functions as a reasonable antireflection coating. In conclusion, it has been demonstrated that, through the judicious choice of materials, assembly conditions, and number of deposited layers, it is possible to create layer-by-layer assembled multilayer thin films of silica nanoparticles with controllable levels of nanoporosity. The low refractive index and nanoporosity-driven superwetting behavior of these coatings result in stable antifogging and antireflection properties. Acknowledgment. This work was supported in part by the DARPA BOSS Program and the MRSEC Program of the National Science Foundation under Award No. DMR 02-13282. This work also made use of the Shared CMSE Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under Award No. DMR 02-13282. F.C¸ .C. acknowledges the Scientific and Technical Research Council of Turkey (TUBITAK) for a NATO-A2 Science Fellowship. Supporting Information Available: Plot of RMS values as a function of the number of deposited bilayers. This material is available free of charge via the Internet at http://pubs.acs.org. LA053182P
