Mechanically Stable Antireflection and Antifogging Coatings

Sep 4, 2008 - Yan Wang , Tianqi Li , Siheng Li , and Junqi Sun ... Yan Li , Xiaoyan Chen , Qianqian Li , Kai Song , Shihui Wang , Xiaoyan Chen , Kai Z...
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Langmuir 2008, 24, 10851-10857

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Mechanically Stable Antireflection and Antifogging Coatings Fabricated by the Layer-by-Layer Deposition Process and Postcalcination Lianbin Zhang, Yang Li, Junqi Sun,* and Jiacong Shen State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed June 10, 2008. ReVised Manuscript ReceiVed July 22, 2008 Complexes of poly(diallyldimethylammonium chloride) (PDDA) and sodium silicate (PDDA-silicate) are alternately deposited with poly(acrylic acid) (PAA) to fabricate PAA/PDDA-silicate multilayer films. The removal of the organic components in the PAA/PDDA-silicate mulilayer films through calcination produces highly porous silica coatings with excellent mechanical stability and good adhesion to substrates. Quartz substrates covered with such porous silica coatings exhibit both antireflection and antifogging properties because of the reduced refractive index and superhydrophilicity of the resultant films. A maximum transmittance of 99.86% in the visible spectral range is achieved for the calcinated PAA/PDDA-silicate films deposited on quartz substrates. The wavelengths of maximum transmittance could be well tailored by simply changing the deposition cycles of multilayer films. The usage of PDDA-silicate complexes allows for the introduction of high porosity to the resultant silica coatings, which favors the fabrication of antireflection and antifogging coatings with enhanced performance. Meanwhile, PDDA-silicate complexes enable rapid fabrication of thick porous silica coatings after calcination because of the large dimensions of the complexes in solution. The easy availability of the materials and simplicity of this method for film fabrication might make the mechanically stable multifunctional antireflection and antifogging coatings potentially useful in a variety of applications.

Introduction Antireflection (AR) coatings can effectively enhance the transmittance of light and remove ghost images of optical surfaces and therefore are indispensable for applications in photovoltaic and displaying devices and all kinds of optical lenses.1 The principle of antireflection, which was presented by Fresnel,2 is the destructive interference between light reflected from the coating-substrate and the air-coating interfaces. One of the simple film structures for antireflection is a single-layer coating with a low refractive index. Generally, when the refractive index (nc) for an ideal homogeneous AR coating meets the condition of nc ) (nans)1/2, reflection will be suppressed at the wavelength near the quarter-wavelength optical thickness, where na and ns are the refractive indices of the air and the substrate, respectively. For a glass substrate (ns ) 1.5), the refractive index of AR material should be 1.22. However, natural materials with such a low refractive index are either rare or expensive to obtain in thin film form. As a substitute, nanoporous materials are usually chosen as AR coatings, since the introduction of the nanopores can reduce the refractive index of the coatings and satisfy the AR requirement. In recent years, many methods have been developed to produce nanoporous materials for use as AR coatings, including plasma-enhanced chemical vapor deposition,3 oblique-angle * To whom correspondence should be addressed. Fax: 0086-431-85193421. E-mail: [email protected]. (1) (a) Ho, P. K. H.; Thomas, D. S.; Friend, R. H.; Tessler, N. Science 1999, 285, 233. (b) Ishikawa, H.; Honjo, Y.; Watanabe, K. Thin Solid Films 1999, 351, 212. (c) Ohsaki, H.; Kokubu, Y. Thin Solid Films 1999, 351, 1. (2) Macleod, H. A. Thin Film Optical Filters, 2nd ed.; Adam Hilger Ltd.: Bristol, U.K., 1986. (3) Martinu, L.; Poitras, D. J. Vac. Sci. Technol., A 2000, 18, 2619. (4) Xi, J. Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M. F.; Lin, S. Y.; Liu, W.; Smart, J. A. Nat. Photonics 2007, 1, 176. (5) (a) Walheim, S.; Scha¨ffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (b) Ibn-Elhaj, M.; Schadt, M. Nature 2001, 410, 796. (6) (a) Cao, M.; Song, X.; Zhai, J.; Wang, J.; Wang, Y. J. Phys. Chem. B 2006, 110, 13072. (b) Lee, C.; Bae, S. Y.; Mobasser, S.; Manohara, H. Nano Lett. 2005, 5, 2438.

deposition,4 nanophase separation,5 chemical etching,6 UV or thermal decomposition of porogen,7 sol-gel processes,8 the adsorption of colloidal layers,9 and so forth. The layer-by-layer (LbL) assembly technique, which was developed by Decher and co-workers in the early 1990s,10 has been proven to be a simple and inexpensive way to fabricate various kinds of coatings with a tailored chemical composition and architecture on the micro- and nanoscales.11,12 Compared with the above-mentioned methods, the LbL assembly technique holds great promise in the fabrication of AR coatings. First, a wide range of materials can be used to fabricate AR coatings. Second, the LbL assembly technique enables the deposition of AR coatings with large areas on nonflat surfaces. Third, the LbL assembly technique allows for easy control of the film thickness, and therefore, wavelengths for maximum transmittance could be tailored by simply changing the number of film deposition cycles. Up to now, a variety of AR coatings have been successfully fabricated on the basis of the LbL assembly technique by using materials including linear polyelectrolytes,13 nanoparticles or (7) (a) Fu, G.-D.; Yuan, Z.; Kang, E.-T.; Neoh, K.-G.; Lai, D. M.; Huan, A. C. H. AdV. Funct. Mater. 2005, 15, 315. (b) Joo, W.; Park, M. S.; Kim, J. K. Langmuir 2006, 22, 7960. (c) Kim, H. C.; Wilds, J. B.; Kreller, C. R.; Volksen, W.; Brock, P. J.; Lee, V. Y.; Magbitang, T.; Hedrick, J. L.; Hawker, C. J.; Miller, R. D. AdV. Mater. 2002, 14, 1637. (8) (a) Uhlmann, D. R.; Suratwala, T.; Davidson, K.; Boulton, J. M.; Teowee, G. J. Non-Cryst. Solids 1997, 218, 113. (b) Penard, A. L.; Gacoin, T.; Boilot, J. P. Acc. Chem. Res. 2007, 40, 895. (c) Chen, D. Sol. Energy Mater. Sol. Cells 2001, 68, 313. (9) Hattori, H. AdV. Mater. 2001, 13, 51. (10) Decher, G. Science 1997, 277, 1232. (11) (a) Hammond, P. T. AdV. Mater. 2004, 16, 1271. (b) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (c) Caruso, F. Chem.sEur. J. 2000, 6, 413. (12) (a) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848. (b) Lutkenhaus, J. L.; Hammond, P. T. Soft Matter 2007, 3, 804. (c) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (d) Zhang, X.; Chen, H.; Zhang, H. Chem. Commun. 2007, 1395. (e) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319. (13) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59.

10.1021/la801806r CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

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nanowires with a low refractive index,14,15 copolymer micelles,16 and so forth. Recently, the fabrication of multifunctional AR coatings has become an attractive issue because of the additive functions besides the increase of light transmittance. For instance, we successfully fabricated broad-band superhydrophobic antireflection coatings in the near-infrared region by integrating AR and superhydrophobic properties into one coating.17 The superhydrophobic property endows the AR coatings with a waterrepellent ability and enables the application of the AR coatings under humid environments. However, a troublesome problem encountered by an eyeglass-wearer is that eyeglasses often fog up after coming indoors on a cold day. A superhydrophilic coating, on which the water contact angle is less than 5° within 0.5 s as soon as the water droplet contacts such surfaces, can solve this problem. Superhydrophilic coatings can significantly suppress the fogging behavior by the rapid spread and flow of water droplets on their surface and therefore eliminate the light scattering caused by water droplets. Titania-based materials have been widely used as antifogging coatings because the surface of titania becomes highly hydrophilic under UV illumination.18 However, there are still some problems concerning the indoor application of such titania coatings because the realization of superhydrophilicity requires UV illumination as intense as natural sunlight. Recent efforts showed that the dependence of the UV illumination intensity can be reduced, but UV illumination is still a prerequisite for superhydrophilic titania coatings.19 Actually, there exists a textured-surface-induced superhydrophilicity. According to Que´re´ and Wenzel’s studies,20 surface roughness will make the surface of a hydrophilic material more wettable. For highly porous coatings the surface roughness factor becomes infinite. It is possible to achieve superhydrophilicity by introducing nanopores into hydrophilic coatings.21 Therefore, it is expected that highly porous AR coatings constructed from hydrophilic materials could become superhydrophilic and possess the antifogging property. In fact, AR coatings with an antifogging capability are highly desired in daily used eyeglasses, swimming goggles, periscopes, lenses in laparoscopic and gastroscopic surgery,22 and so forth. Recently, Cohen, Rubner, and co-workers reported the fabrication of multifunctional AR and antifogging coatings by LbL deposition of silica nanoparticles with polycations or TiO2 nanoparticles.23 Porous coatings with superhydrophilic properties were successfully fabricated, which endowed the resultant coatings with AR and antifogging properties. The AR properties can be easily tuned throughout the visible range by simply varying the number of deposited layers. (14) Rouse, J. H.; Ferguson, G. S. J. Am. Chem. Soc. 2003, 125, 15529. (15) Podsiadlo, P.; Sui, L.; Elkasabi, Y.; Burgardt, P.; Lee, J.; Miryala, A.; Kusumaatmaja, W.; Carman, M. R.; Shtein, M.; Kieffer, J.; Lahann, J.; Kotov, N. A. Langmuir 2007, 23, 7901. (16) Cho, J.; Hong, J.; Char, K.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9935. (17) Zhang, L. B.; Li, Y.; Sun, J. Q.; Shen, J. C. J. Colloid Interface Sci. 2008, 319, 302. (18) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (b) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. AdV. Mater. 1998, 10, 135. (19) (a) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 2000, 12, 1923. (b) Tokudome, H.; Miyauchi, M. Chem. Lett. 2004, 33, 1108. (c) Hosono, E.; Matsuda, H.; Honma, I.; Ichihara, M.; Zhou, H. Langmuir 2007, 23, 7447. (20) (a) Bico, J.; Thiele, U.; Que´re´, D. Colloids Surf., A 2002, 206, 41. (b) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (21) (a) Liu, X.; He, J. J. Colloid Interface Sci. 2007, 314, 341. (b) Ogawa, T.; Murata, N.; Yamazaki, S. J. Sol-Gel Sci. Technol. 2003, 27, 237. (c) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (22) Ohdaira, T.; Nagai, H.; Kayano, S.; Kazuhito, H. Surg. Endosc. 2007, 21, 333. (23) (a) Cebeci, F. C.; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856. (b) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 2305.

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For practical application of AR and antifogging coatings, several concerns should be taken into consideration. Among them, mechanical durability and adhesion are critically important. The superhydrophilic antifogging coatings, generally having a high surface energy, are easily fouled by contaminations and difficult to clean because the foulants are strongly adhered to the surface.24 For long-term applications, the AR and antifogging coatings need to have excellent adhesion to the underlying substrates and good mechanical durability for daily maintenance, such as cleaning. Meanwhile, the materials used to fabricate the AR and antifogging coatings should be easily available with a low cost. Sodium silicate is a common, inexpensive, and negatively charged chemical consisting of linear, branched, and ring-shaped oligomers which has been widely used as an industrial material.25 The usage of sodium silicate, accompanied by the cost-effective LbL assembly technique, will significantly depress the production cost of the AR and antifogging coatings. In this study, sodium silicate was first complexed with the polycation poly(diallyldimethylammonium chloride) (PDDA) to form positively charged complexes (noted as PDDA-silicate). Next, PDDA-silicate complexes were alternately deposited with poly(acrylic acid) (PAA) to obtain multilayer films of (PAA/PDDA-silicate)n on quartz substrates. Finally, the removal of the organic components in the multilayer films through calcination produced highly porous silica coatings with excellent mechanical stability and good adhesion to substrates. Quartz substrates with such porous silica coatings exhibit both AR and superhydrophilic antifogging properties. The unique advantages of using PDDA-silicate complexes to fabricate AR and antifogging coatings are as follows: First, PDDA-silicate complexes allow for the introduction of a high ratio of organic components into the as-fabricated multilayer films. Therefore, highly porous silica coatings with a lower refractive index and a higher transmittance can be easily fabricated after the removal of the organic components. Second, the large dimensions of PDDA-silicate complexes in solution enable the rapid fabrication of thick uniform films with a smooth surface, which is cost-effective for large-scale film production.

Experimental Section Materials. PDDA (20 wt %, Mw ) ca. 100000-200000), PAA (Mw ) ca. 2000), and sodium silicate solution were all purchased from Sigma-Aldrich and used as received. Water purified in a Milli-Q (Millipore) system was used during all the experiments. Quartz slides with a thickness of 1.0 mm were used for film deposition. Preparation of Complexes of PDDA and Sodium Silicate. To a stirring aqueous solution of PDDA (1.0 mg/mL) was added aqueous sodium silicate (34.4 mg/mL) dropwise. The ultimate volume ratio of PDDA and sodium silicate solutions was 60:5. The pH value of the aqueous PDDA-silicate complex solution was adjusted to 4.0 by using 1 M HCl. Fabrication of Antireflection and Antifogging Coatings. Slides of quartz were immersed in a slightly boiled piranha solution (3:1 mixture of 98% H2SO4 and 30% H2O2) for 20 min, rinsed with a copious amount of water, and dried with a N2 flow. Caution: Piranha solution reacts Violently with organic materials and should be handled carefully! The freshly cleaned quartz slides were first immersed in PDDA solution (1.0 mg/mL) for 20 min to render the substrate positively charged, followed by rinsing with water and drying with N2 flow. Then multilayer films of PAA/PDDA-silicate were deposited on the substrates according to the following general (24) Howarter, J. A.; Youngblood, J. P. Macromol. Rapid Commun. 2008, 29, 455. (25) (a) Trotman-Dickenson, A. F., Ed. ComprehensiVe Inorganic Chemistry, 1st ed.; Compendium Publishers: Elmsford, NY, 1973; Vol. 1, p 1413. (b) Zhang, L. B.; Chen, H.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2007, 19, 948.

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Scheme 1. (a) Formation of PDDA and Sodium Silicate Complexes and (b) Schematic Illustration of the Fabrication of AR and antifogging coatings

protocol: (i) The substrates were immersed in a solution of PAA (1.0 mg/mL, pH 5.0) for 20 min, followed by rinsing with water for 1 min and drying with a N2 flow. (ii) The substrates were immersed in a solution of PDDA-silicate complexes for 20 min, followed by rinsing with water for 1 min and drying with a N2 flow. Steps i and ii were repeated until the desired number of deposition cycles was reached. Multilayer films of PAA/PDDA-silicate with n-cycle deposition is noted as (PAA/PDDA-silicate)n. Multilayer films of PDDA/silicate were fabricated by alternately immersing the quartz slides into aqueous solutions of PDDA and then sodium silicate for 20 min for each layer, with intermediate water washing and N2 drying. Calcination of the multilayer film covered quartz substrates was carried out in an oven at 600 °C for 4 h. Characterization. Transmittance measurements at normal incidence were performed using a Shimadzu UV-3600 spectrophotometer. QCM measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). Scanning electron microscopy (SEM) images were obtained on an XL30 ESEM FEG scanning electron microscope. The measurements of film thickness were obtained from their crosssectional SEM images. The mean value of the thickness was gathered by averaging at least five individual positions of a sample. The energy-dispersive X-ray (EDX) spectroscopy measurement was conducted on an EDAX Genesis 2000 X-ray microanalysis system attached to an XL30 ESEM FEG scanning electron microscope. Transmission electron microscopy (TEM) observations were carried out on a Hitachi H8100 instrument operated at 200.0 kV. The calcination was carried out using a Barnstead/Thermolyne 47900 furnace (Barnstead/Thermolyne Corp., Iowa) at 600 °C for a time period of 4 h. Water contact angle measurements were performed with a drop shape analysis system, DSA10-MK2 (Kruess, Germany), at ambient temperature. A water droplet of 1 µL was used as the indicator to characterize the wetting property of the coatings. The spreading process of the water droplet was recorded by a CCD camera with a frame rate of 25 frames/s. The electrophoresis studies of the PDDA-silicate complexes were carried out on a Malvern Nano-ZS zetasizer at room temperature. Atomic force microscopy (AFM) images were taken with a Nanoscope IIIa AFM Multimode system (Digital Instruments, Santa Barbara, CA) under ambient conditions. AFM was operated in the tapping mode by using silicon cantilevers with a force constant of 40 N/m.

Results and Discussion The typical procedures for the fabrication of PDDA-silicate complexes and AR and antifogging coatings are shown in Scheme

Figure 1. (a) QCM frequency decrease (-∆F) of alternative deposition of PAA (0) and PDDA-silicate complexes (9). (b) UV-vis transmission spectra of PAA/PDDA-silicate multilayer films with different deposition cycles on a quartz substrate.

1. PDDA-silicate complexes were obtained by dropwise addition of aqueous sodium silicate solution to aqueous PDDA solution under stirring. The ultimate monomer molar ratio of sodium silicate to PDDA in the complex solution was about 2:1. The resultant PDDA-silicate complexes have an average size of about 13.2 nm, as revealed by TEM measurements. Electrophoresis studies revealed that the ζ potential of the PDDA-silicate complexes in aqueous solution with a pH of 4.0 was +40 mV. The highly positive ζ potential indicated that sodium silicate was wrapped and stabilized by PDDA chains in the solution of PDDA-silicate complexes. The resultant PDDA-silicate complex aqueous solution is homogeneous, transparent, and stable for at least two months because of its high surface charge density. The positively charged PDDA-silicate complexes were then alternately deposited with PAA (1.0 mg/mL, pH 5.0) to fabricate PAA/PDDA-silicate multilayer films. Quartz crystal microbalance (QCM) measurements were first employed to monitor the deposition process of PAA/PDDA-silicate multilayer films on Ag-coated QCM resonators precoated with a PDDA layer. As shown in Figure 1a, the decreases in QCM frequency are plotted as a function of the film deposition cycles. In the primal several deposition cycles, the frequency decreased after the deposition of each PAA and PDDA-silicate complex layer, indicating the successful deposition of PAA/PDDA-silicate multilayer films. With more cycles of multilayer films deposited, an increased frequency decrease was obtained for the deposition of PAA layers. Meanwhile, an adsorption-desorption process was observed, in which PAA desorbed partially during the deposition of the PDDA-silicate complex layer. Despite the desorption process, one deposition cycle of the PAA/PDDA-silicate multilayer film leads to an average net frequency decrease of 2337.9 ( 301.6

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Figure 2. Dependence of the thickness of PAA/PDDA-silicate multilayer films on the number of deposition cycles before (9) and after (O) removal of organic components through calcination. Thickness data were collected from the cross-sectional SEM images of the multilayer coatings fabricated on quartz substrates.

Hz, confirming the successful deposition of PAA/PDDAsilicate multilayer films. Having established the successful alternative deposition of PDDA-silicate complexes with PAA on Ag-coated QCM resonators, multilayer films of PAA/PDDA-silicate were then deposited on quartz substrates. UV-vis transmission spectra were used to characterize the film deposition process. Figure 1b depicts the transmission spectra of PAA/PDDA-silicate multilayer films with different deposition cycles on a quartz substrate in the spectral range between 200 and 800 nm. In the shorter wavelength region, the transmittance of the quartz substrate covered with PAA/PDDA-silicate multilayer films decreased gradually with increasing number of film deposition cycles, while, in the longer wavelength region, the transmittance decreased only slightly. Starting from the ninth deposition cycle, obvious oscillations were observed in the whole spectral range. These oscillations are called Fabry-Pe´rot fringes, which arise from interferences between light beams transmitted and partially reflected at the film-air and film-quartz interfaces.26 The thicker the films are, the more oscillations that will appear in the spectrum. The number of oscillations increased with increasing number of film deposition cycles in the corresponding spectra, indicating that the film thickness increases with more cycles deposited. The as-prepared PAA/PDDA-silicate films are homogeneous and transparent, but have no AR function because of their dense film structures. The thicknesses of the PAA/PDDA-silicate multilayer films with different deposition cycles were determined from their corresponding cross-sectional SEM images. As shown in Figure 2, line a, the thickness of the PAA/PDDA-silicate films increases linearly along with an increase of the number of film deposition cycles except for the primal several cycles, with an average thickness increment of ca. 21.4 nm per deposition cycle. The linear thickness growth of the PAA/PDDA-silicate multilayer films provides a convenient way to tailor the desired film thickness just by simply varying the number of film deposition cycles. The film thickness increment of 21.4 nm per deposition cycle for PAA/PDDA-silicate multilayer films is much larger than that of multilayer films fabricated from alternative deposition of oppositely charged simplex polyelectrolytes. This means that PAA/PDDA-silicate multilayer films can be fabricated in a (26) (a) Lin, V. S. Y.; Motesharei, K.; Dancil, K. P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840. (b) Guan, Y.; Yang, S.; Zhang, Y.; Xu, J.; Han, C. C.; Kotov, N. A. J. Phys. Chem. B 2006, 110, 13484.

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relatively rapid way. The nonlinear film deposition in the beginning in Figures 1a and 2a originates from the influence of Ag and quartz substrate surfaces, which are slightly different from each other. PAA/PDDA-silicate multilayer films deposited on quartz substrates were calcinated in an oven at 600 °C for 4 h to remove the organic components. Energy-dispersive X-ray spectroscopy measurements were carried out to characterize film composition changes before and after the calcination. The complete disappearance of the nitrogen signal originating from PDDA in the calcinated films indicates that high-temperature calcination burned out completely the organic components. Pure silica coatings were finally obtained after calcination of PAA/ PDDA-silicate multilayer films27 (see the Supporting Information, Figure S2) As shown in line b of Figure 2, the thicknesses of the calcinated PAA/PDDA-silicate multilayer films with different deposition cycles exhibit a linear relationship with the number of film deposition cycles of their original films. An average increment of ca. 10.1 nm per deposition cycle was obtained for the calcinated PAA/PDDA-silicate films. The thickness of the calcinated PAA/PDDA-silicate films decreased by ca. 50% compared with that of the as-prepared PAA/PDDA-silicate films when the numbers of deposition cycles were equal, indicating the partial collapse of the films upon the removal of the organic components. The AR properties of the calcinated PAA/PDDA-silicate films were investigated by using the measurement of transmission spectra. Figure 3a shows the transmission spectra of quartz substrates covered with PAA/PDDA-silicate films of different numbers of deposition cycles (n ) 6, 9, 10, 11, 12, 15) after calcination. The transmission spectrum of bare quartz was also provided for comparison. The bare quartz substrate has a transmittance below ∼93% in the spectral range between 300 and 800 nm. The transmittance of quartz substrates coated with PAA/PDDA-silicate films was dramatically enhanced after the postcalcination treatment, indicating the AR property of the resultant silica coatings. For a calcinated (PAA/PDDA-silicate)12 film, a maximum transmittance of 99.86% was achieved at a wavelength of 570 nm. The wavelength of maximum transmission of calcinated PAA/PDDA-silicate films can be well tuned by changing the coating thickness, which in turn can be achieved by simply varying the number of film deposition cycles. For calcinated PAA/PDDA-silicate films with 9, 10, 11, and 12 deposition cycles, the maximum transmittances all exceed 99.5%, with the wavelengths of maximum transmission being 358, 405, 495, and 570 nm, respectively. The calcinated (PAA/ PDDA-silicate)6 film has a thickness of 63.2 nm, which is too thin to achieve a satisfactory AR property in the present spectral range. AR coatings in the NIR region have potential applications in areas such as night vision technology, NIR analysis, NIR sensors, and so forth. By further increasing the number of film deposition cycles, the thickness of calcinated PAA/PDDA-silicate films increased too, which satisfied the application of these films as AR coatings in the NIR region. In Figure 3b, the transmission spectra of (PAA/PDDA-silicate)n (n ) 15, 20, and 25) films before and after calcination are presented. These calcinated PAA/ PDDA-silicate coatings showed AR properties in the NIR region, confirming that the highly porous film structures remain unchanged for thick films. A maximum transmittance in excess of 99.5% was obtained for a calcinated (PAA/PDDA-silicate)20 film in the NIR region. It should be mentioned that the transmittance peaking at a wavelength of about 1400 nm resulted (27) He, J. H.; Fujikawa, S.; Kunitake, T.; Nakao, A. Chem. Mater. 2003, 15, 3308.

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Figure 4. Top-view SEM images of a (PAA/PDDA-silicate)12 film deposited on a quartz substrate before (a) and after (b) calcination. Crosssectional SEM images of a (PAA/PDDA-silicate)12 film deposited on a quartz substrate before (c) and after (d) calcination.

Figure 3. (a) UV-vis transmission spectra of calcinated (PAA/ PDDA-silicate)n (n ) 0, 6, 9, 10, 11, 12, and 15) films deposited on quartz substrates after calcination. (b) Near-infrared (NIR) transmission spectra of (PAA/PDDA-silicate)n (n ) 15, 20, and 25) films deposited on quartz substrates before (na, dashed line) and after (nb, solid line) calcination. Note that both sides of the quartz substrates are coated with the multilayer films. (c) Photograph of quartz substrates exposed to sunlight. Bottom: a quartz substrate with both sides covered with a calcinated (PAA/PDDA-silicate)12 film. Top: a control quartz substrate without any film deposition.

from the quartz itself. The transmission spectra in Figure 3a,b demonstrated that a maximum transmission at the demanded wavelengths can be conveniently tailored by simply varying the number of film deposition cycles. The AR property of the calcinated (PAA/PDDA-silicate)12 film was also exhibited straightforwardly in Figure 3c. The control quartz slide without any film deposition and the one with both sides coated with the calcinated (PAA/PDDA-silicate)12 films were exposed to sunlight. The clear words below the quartz with the AR coatings demonstrate the increased transparency and reduced reflection of the coated quartz slide, whereas the control substrate reflects the sunlight, and the words below are not as clear as those under the quartz coated with AR coatings. The structures of PAA/PDDA-silicate films before and after calcination were investigated by SEM measurements. The asprepared (PAA/PDDA-silicate)12 film has a compact film

structure, although a few disconnected pores with sizes between 15 and 50 nm can be observed in its top-view SEM image (Figure 4a). The calcinated (PAA/PDDA-silicate)12 film is highly porous (Figure 4b). Dense pores with sizes between 30 and 50 nm are clearly observed for the calcinated film. Organic components of PDDA and PAA acted as porogen for the formation of these nanopores. The cross-sectional SEM images of the (PAA/ PDDA-silicate)12 film before and after calcination are presented in parts c and d, respectively, of Figure 4. The cross-sectional SEM image indicates clearly that the film before calcination is compact and without any observable pores within the resolution achieved here. The as-prepared (PAA/PDDA-silicate)12 film has a constant thickness of 285.7 nm. The thickness of the calcinated film was reduced by ca. 55%, with a value of 121.8 nm. The reduced thickness resulted from the collapse of the film upon the removal of the organic templates, as mentioned above. Moreover, the cross-sectional SEM image reveals that the calcinated films are composed of loosely stacked silica nanoparticles. Actually, silica coatings with a three-dimensional porous structure were finally obtained after calcination of PAA/ PDDA-silicate films. The homogeneous nanopores introduced into the calcinated PAA/PDDA-silicate films reduced the effective refractive index of the coatings. Therefore, the highly porous silica films can be used as AR coatings. AFM images show that the root-mean-square (rms) roughness of the (PAA/ PDDA-silicate)12 film before and after calcination is 8.9 and 3.3 nm, respectively (see the Supporting Information, Figure S3). The calcinated film is smooth enough for use as AR coatings without any scattering of visible light. The hydrophilic silica material accompanied by the threedimensional porous structure further increased the hydrophilicity of the calcinated PAA/PDDA-silicate films. The water wetting behavior of the calcinated PAA/PDDA-silicate films was characterized by the water contact angle measurement by using a water droplet of 1 µL as the indicator. As shown in Figure 5a,b, as soon as the water droplet contacted the surface covered with the calcinated (PAA/PDDA-silicate)12 coating, it completely spread over the surface within 0.24 s, suggesting that a superhydrophilic surface was achieved. The wettability of the calcinated PAA/PDDA-silicate is thickness-dependent. It takes 0.5 s for a water droplet to completely spread over the surface of a calcinated (PAA/PDDA-silicate)6 film. The superhydrophilic property of the calcinated PAA/PDDA-silicate films originates from their highly porous film structure. Wenzel’s equation can well explain the superhydrophilic property of the calcinated PAA/

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Figure 5. (a, b) Still images from video contact angle measurements for a water droplet (1 µL) spreading on a quartz substrate covered with a calcinated (PAA/PDDA-silicate)12 multilayer film. (c) Photograph of a quartz substrate deposited on both sides with calcinated (PAA/ PDDA-silicate)12 films (upper) and a control quartz substrate without any film deposition (bottom) taken from a refrigerator (-18 °C) to the humid laboratory air (ca. 50% RH).

PDDA-silicate films: cos θa ) r cos θ, where θa is the apparent water contact angle on a rough surface, θ is the intrinsic contact angle as measured on a smooth surface, and r is the surface roughness, defined as the ratio of the actual surface area to the projected surface area.20b The intrinsic water contact angle on silica is reported to be about 20°.21a,23 The surface roughness r becomes infinite for the highly porous silica coatings derived from the calcinated PAA/PDDA-silicate films. Therefore, the highly porous silica coatings derived from calcinated PAA/ PDDA-silicate films exhibit superhydrophilic properties. Superhydrophilic coatings can significantly suppress the fogging behavior because condensed water droplets will spread flat almost instantaneously to form a thin sheetlike water membrane. In this way, light scattering by the condensed water droplets was eliminated. The thin water membrane will eventually disappear either by sliding down under the influence of gravity or by evaporation. The photograph in Figure 5c illustrates the antifogging behavior of the superhydrophilic silica coatings. A control bare quartz and a quartz with both sides coated with the porous AR and superhydrophilic silica coating fabricated from a calcinated (PAA/PDDA-silicate)12 film were cooled to about -18 °C in a refrigerator, and then both quartz slides were simultaneously exposed to the humid laboratory air (ca. 50% RH). As shown in the lower part of Figure 5c, the control quartz slide fogged immediately. The words below the control slide are blurred by the strong light scattering caused by the formed surface dewdrops. In sharp contrast, the quartz slide with the superhydrophilic silica coatings remains highly transparent and the words below are clearly seen (upper part of Figure 5c). Long-term stability is extremely important to practical application of the AR and antifogging coatings. The AR property did not show any decrease after storage of the porous silica coatings under ambient conditions for 2 months. It took a little longer time of ∼0.4 s for the water droplet to completely spread on the porous silica coating after storage under ambient conditions for 2 months. The resultant coating was still superhydrophilic, demonstrating the excellent stability of the antifogging property of the coatings after long-term storage. The mechanical stability and adhesion of the porous silica coatings to substrates were examined by uninterruptedly sonicating the quartz substrate covered with the calcinated (PAA/PDDA-silicate)12 films in water containing detergent. After 1 h of sonication in a sonicator with a power of 100 W, the AR and antifogging properties of the coatings remain unchanged. Furthermore, the adhesion of the coatings to the substrate was also examined via a standard Scotch tape peel test. The AR and antifogging coatings can resist the peeling of the Scotch tape. After the removal of the stained adhesives by actetone, the AR and antifogging properties show

Zhang et al.

Figure 6. UV-vis transmission spectra of a quartz substrate covered on both sides with calcinated (PAA/PDDA-silicate)12 (solid line) and (PDDA/silicate)30 films fabricated from aqueous PDDA solution with 1.0 M NaCl added (dashed line). The transmission spectrum of a bare quartz substrate is also provided for comparison.

no changes. The hardness of the AR and antifogging coatings was examined by the pencil hardness test. The pencil was pushed into the coating at a ∼45° angle under a sufficient force to crush the pencil tip. The pencil hardness of the calcinated multilayer films was found to be more than 5H. During the fabrication of the AR and antifogging coatings, calcination treatment not only removed the organic components and produced nanopores, but also cross-linked the silicates together via the formation of stable Si-O-Si bonds among them. The highly cross-linked film structure greatly improves the mechanical stability and adhesion of the porous silica coatings to substrates and guarantees the long-term application of the AR and antifogging coatings under conditions of daily maintenance. We found that PDDA-silicate complexes were indispensable for the fabrication of porous AR and antifogging coatings by the LbL assembly technique. The use of PDDA-silicate complexes instead of simplex sodium silicate can largely increase the ratio of the organic components in the LbL-deposited PAA/ PDDA-silicate films and therefore enhance the porosity of the calcinated films. To prove this, multilayer films of PDDA/silicate were fabricated from aqueous PDDA solution (1.0 mg/mL) and aqueous sodium silicate solution (2.7 mg/mL, the same concentration as that in PDDA-silicate complexes, pH 4) by the LbL assembly technique. The thickness of a (PDDA/silicate)40 film before and after calcination was about 57.4 and 45.2 nm, respectively. By adding NaCl to aqueous PDDA solution, thicker PDDA/silicate films with more PDDA deposited can be fabricated. In an optimized condition, a (PDDA/silicate)30 film fabricated from aqueous PDDA solution (1.0 mg/mL) with added 1.0 M NaCl shows a maximum transmittance of ca. 97.5% at a wavelength of 550 nm after removal of the PDDA components (Figure 6). The calcinated (PDDA/silicate)30 film has a thickness of 93.5 nm. These results demonstrate that the calcinated PDDA/ silicate films have a lower porosity than the calcinated PAA/ PDDA-silicate films. Meanwhile, the LbL assembly of PDDA-silicate complexes provides a relatively rapid way to construct thick multilayer films, as the fabrication of thick multilayer films by the LbL assembly technique is generally time-consuming. For instance, the (PAA/PDDA-silicate)12 film before and after calcination has a thickness of 285.7 and 121.8 nm, respectively, corresponding to a thickness of 23.8 and 10.2 nm for each deposition cycle. For the (PDDA/silicate)30 film fabricated from PDDA with added 1.0 M NaCl, the film thickness before and after calcination is 103.2 and 93.5 nm, respectively. This corresponds to a thickness of 3.4 and 3.1 nm for each

Stable Antireflection and Antifogging Coatings

deposition cycle before and after calcination, much thinner than those of PAA/PDDA-silicate films. The large porosity introduced and the rapid fabrication efficiency make the LbL deposition of PDDA-silicate complexes indispensable for the construction of AR and antifogging coatings in a convenient and economic way. Kunitake and co-workers investigated in detail the preparation of porous silica nanofilms by treating the PDDA/silicate films with O2 plasma and calcination.27 Their results showed that, instead of porous silica films, dense silica films were obtained through calcination at 450 °C. Therefore, calcination is unsuccessful for fabrication of porous silica films from PDDA/silicate multilayer films in their studies, demonstrating again the indispensable importance of using PDDA-silicate complexes for porous silica film fabrication conducted in this study. These authors did obtain highly porous silica films by O2 plasma treatment of PDDA/silicate multilayer films, but they also stated that their attempt to fabricate porous silica films with a thickness larger than 50 nm failed because O2 plasma flow could not penetrate such a thick film. O2 plasma penetration is thicknessdependent. Generally, the plasma flow could not penetrate a coating with a thickness larger than ca.100 nm, depending on the film composition and structure.28 Therefore, calcination was chosen in this study to fabricate porous silica AR and antifogging coatings.

Conclusions In the present study, we reported a facile and cost-effective method for the fabrication of mechanically stable antireflection and antifogging silica coatings by LbL deposition of PDDAsilicate complexes with PAA on quartz substrates followed by calcination. Calcination removed the organic components in PAA/ PDDA-silicate multilayer films and introduced three-dimensional nanopores in the resultant silica coatings. In this way, highly porous silica coatings with a reduced refractive index and superhydrophilic properties can be fabricated simultaneously, which can be used as multifunctional AR and antifogging coatings. Calcination cross-linked the resultant silica coatings via the formation of stable siloxane bridges, which endows the AR and antifogging coatings with high mechanical stability and excellent adhesion to the substrates. The unique advantages of using (28) (a) Huang, J.; Ichinose, I.; Kunitake, T.; Nakao, A Langmuir 2002, 18, 9048. (b) Kalachev, A. A.; Mathauer, K.; Ho¨hne, U.; Mo¨hwald, H.; Wegber, G. Thin Solid Films 1993, 228, 307. (c) Kalachev, A. A.; Wegber, G. Makromol. Chem., Macromol. Symp. 1991, 46, 229.

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PDDA-silicate complexes to fabricate AR and antifogging coatings are as follows: First, PDDA-silicate complexes allow for the introduction of a high ratio of organic components into PAA/PDDA-silicate multilayer films. Upon calcination, highly porous silica films with a lower refractive index and a high transmittance can be obtained. As a comparison, the AR coatings fabricated from calcination of LbL-assembled PDDA/silicate films have a lower transmittance. Second, PDDA-silicate complexes enable a relatively rapid fabrication of thick porous silica coatings after calcination because of the large dimensions of the complexes in solution. The rapid film fabrication, leading to lower production costs, and better control over the optical thickness is a contradictory pair that has to be compromised in the fabrication of AR coatings. Here the thickness increment of 10.1 nm per deposition cycle guarantees a relatively rapid film construction and fine control over the optical thickness. Additionally, the materials used in this study for the fabrication of AR and antifogging coatings are commercially available with low costs. The LbL assembly technique is particularly suitable for fabrication of AR and antifogging coatings on nonflat substrates with large areas without the requirement of elaborate instruments. Most importantly, the AR and antifogging coatings fabricated here are highly mechanically stable and have excellent adhesion to substrates, which guarantees the long-term application of the coatings. We believe that the AR and antifogging silica coatings with a high durability can be widely useful in the production of eyeglasses, swimming goggles, periscopes, lenses in laparoscopic and gastroscopic surgery, and so forth. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC Grant 20304004), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant No. 200323), National Basic Research Program (Grant 2007CB808000), the Program for New Century Excellent Talents in University (NCET) and the Jilin Provincial Science and Technology Bureau of Jilin Province (Grant 20070104). Supporting Information Available: Observation of the PDDA&silicate complexes by the TEM measurements, the composition determination of multilayer films of (PAA/PDDA&silicate)*12 before and after calcination via EDX spectroscopy measurements and AFM images of the multilayer films of (PAA/PDDA&silicate)*12 before and after calcinations. This material is available free of charge via the Internet at http://pubs.acs.org. LA801806R