Silica Hybrid Antireflective Coatings

J. Phys. Chem. C 2010, 114, 19979–19983

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Sol-Gel Preparation of PDMS/Silica Hybrid Antireflective Coatings with Controlled Thickness and Durable Antireflective Performance Xinxiang Zhang,† Haiping Ye,† Bo Xiao,† Lianghong Yan,‡ Haibing Lv,‡ and Bo Jiang*,† Key Laboratory of Green Chemistry & Technology, College of Chemistry, Sichuan UniVersity, Chengdu 610064, China, and Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: October 14, 2010

Thick silica antireflective (AR) coatings with controlled thickness and durable AR performance were prepared by a base-catalyzed sol-gel process using tetraethyl orthosilicate (TEOS) as precursor and hydroxyl-terminated polydimethylsiloxane (PDMS) as a modifier. The addition of PDMS greatly increased the controllable viscosity range of the silica sol but did not obviously affect the particle size. This phenomenon is attributed to a “compulsive aggregation” process of the sol, which involves the formation of “PDMS bridges” between silica particles in the sol. The mechanism of “PDMS bridge” formation is proposed based on sol viscosity, sol particle size changes, and FTIR identification. The increased controllable viscosity range provided a convenient way to prepare AR coatings with controlled thickness and therefore with controlled wavelength of maximum transmittance. The introduction of PDMS into the silica sol also increased the hydrophobicity and hence the durability of the AR coatings in wet environments. Introduction Antireflective (AR) coatings have gained considerable attention for their applications in optical devices such as automotive windows, solar cells, and laser systems.1-3 In order to prepare a good AR coating, it is necessary to control coating thickness and its reflective index, which must satisfy the following principle: the optical thickness of the coating should be λ/4, where λ is the wavelength of the incident light, and nc ) (na × ns) 0.5, where nc, na, and ns are the refractive indices of the coating, air, and substrate, respectively.4,5 The refractive index of fused silica is 1.45, which implies that the refractive index of AR coating must be about 1.21 to obtain zero reflectance. However, the lowest refractive index among homogeneous onephase materials is above 1.35 for magnesium fluoride.6 The refractive index can be lowered by introducing nanopores (n ) 1) into AR coatings.7–9 So far, several methods have been applied to generate nanoporous AR coatings, including a polymer blend and sintering approach,10,11 phase separation and etching,12,13 reactive twin-magnetron sputtering,14 a layer-bylayer deposition method,15 and a sol-gel process.16 Among these methods, the sol-gel process becomes attractive because of its advantage in large area deposition and large-scale production with controllable microstructure for film preparation.7,17 For many years, quarter-wave porous silica AR coatings prepared by sol-gel processes have been used on the fused silica of laser systems. The sol-gel AR coatings consist of a layer of silica particles that are randomly stacked on the substrate surface. The interparticle and particle interior porosity (n ) 1) decrease the refractive index of the coating to the square root of the indices of the substrates. For AR coatings used in laser systems, the wavelengths of maximum transmittance should be set to 1064, 532, or 355 nm that can be realized with a film thickness of * Corresponding author. Tel.: +86 28 85418112; fax: +86 28 85412907; e-mail:[email protected]. † Sichuan University. ‡ China Academy of Engineering Physics.

about 220, 110, or 73 nm, respectively. For the dip-coating process, the film thickness was easily controlled by varying the dip-coating speed, sol concentration, and sol viscosity.18 However, it is difficult to prepare thick AR coatings from pure silica sols by varying dip-coating speed and sol concentration because of their low viscosity. In this work, PDMS was added as modifier and the effect of PDMS on sol particle size and viscosity was studied. The addition of PDMS greatly increased the viscosity and controllable viscosity range of the sol, which provided a convenient way to prepare thick AR coatings with controlled thickness. Antireflective durability is another important property of AR coatings. Owing to the high porosity (about 50-60%) and hydroxyl groups existing on the base-catalyzed silica particles,19 AR coatings are hydrophilic, possess high specific surfaces areas, and therefore easily adsorb water molecules from the environment. The adsorbed water displaces air in the pores of AR coatings, resulting in an increase of the refractive index and hence optical thickness of the coating, which causes a shift of the maximum transmittance wavelength. This maximum shift decreases the effective transmittance at the desired wavelength. Kim et al. suggested that the hydrophobic nature of silica thin films is closely linked to the durability of the AR coating.20 There have been many attempts to prepare silica AR coatings with improved hydrophobicity by the introduction of organic molecules or polymers bearing hydrophobic groups into the coatings.21-24 Mosquera et al.25 have prepared a PDMS/silica hybrid material by cocondensation of tetraethoxysilane (TEOS) and hydroxyl-terminated polydimethylsiloxane (PDMS) in the presence of a nonionic surfactant (n-octylamine). This crack-free and hydrophobic material has been applied to stone restoration. Many studies about the preparation and characterization of silica/ PDMS materials have been published.26–28 In our work, a convenient method was proposed to prepare AR coatings with controlled thickness and AR durability by utilizing PDMS as a

10.1021/jp106192z  2010 American Chemical Society Published on Web 11/09/2010

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TABLE 1: Change in Film Thicknesses as a Function of PDMS Concentration and Aging Timea PDMS concentration (%)

aging time (day)

thickness (nm)

5 10 15 5 5 5

3 3 3 4 5 6

205.4 238.2 289.8 214.1 235.1 262.4

a

The speed of dip-coating is 150 mm/min.

modifier, and the mechanism for the formation of the PDMScontaining silica sol was investigated. Experimental Section Preparation of Silica Sols and AR Coatings. PDMS/silica sols were prepared by the Sto¨ber method.29 A solution of EtOH, PDMS (the molecular weight and hydroxyl group content are about 950 and 3.6%, respectively, Chenguang Research Institute of Chemical Industry, Chengdu, China), TEOS (Kermel, high pure, Tianjin Kermel Chemical Reagent Co., Ltd. Tianjin, China), H2O, and NH3 · H2O (13.4 mol/L) was first prepared and then immediately stirred for 2 h at 30 °C. This is the typical procedure for this simple one-pot reaction. The final concentration of SiO2 was 3% by weight, and the final molar ratio of TEOS:H2O:EtOH:NH3 was 1:3.25:37.6:0.17. The weight ratio of PDMS to SiO2 was varied from 0% to 15%. The resultant sols were aged in sealed glass containers at room temperature before deposition. All the sols were carefully filtered through 0.22 µm PVDF filters before the coating application. The silica sols were deposited on well-cleaned fused silica substrates by dip coating. The fused silica substrates were cleaned by ultrasonication in acetone for 10 min and wiping carefully before dip coating. The silica coatings were heat treated at 160 °C for 2 h under ambient atmosphere. Characterization. Infrared absorption spectra of the pure and 5%, 10%, and 15% PDMS-containing silicas were analyzed by FTIR (Bruker Tensor 27) using the attenuated total reflectance (ATR) method. All of PDMS-containing silicas were extracted exhaustively with ethanol for 4 days using Soxhlet apparatus, respectively. PDMS is a liquid, so the PDMS FTIR spectrum was characterized from a KBr pellet with a droplet of PDMS on it. We found that the particle size characterized from TEM was very close to the peak diameter (counted by number) performed on a Malvern Zetasizer nano-ZS. Consequently, the particle sizes of silica sols in this study were measured on a Malvern Zetasizer nano-ZS at 25 °C using ethanol as the dispersant. Viscosity measurements were carried out using a programmable rheometer (Brookfield, DV III Ultra) at 25 °C and at a shear rate of 132.0 s-1. The transmission spectra were measured with an UV-vis spectrophotometer (Unico, UV2800A). The thicknesses of AR coatings were determined by ellipsometry from 300 to 800 nm (Sentech SE850 UV). Contact angles were measured on a Kru¨ss DSA100 (Germany). Results and Discussion Film thickness is very important for AR coatings because it determines the wavelength of maximum transmittance. The change in film thickness of the AR coating as a function of PDMS concentration and aging time is shown in Table 1. The film thickness of PDMS-containing sols increases with aging time and PDMS concentration. Because the thickness of the

Figure 1. Change in viscosity of the silica sols with aging time at different PDMS concentrations.

AR coating increases with sol viscosity,18 we focused on the effect of aging time and PDMS concentration on sol viscosity. As shown in Figure 1, as the aging time increases from 4 days to 17 days, the viscosity of the pure sol changes only slightly while that of 5%, 10% and 15% PDMS-containing sol increases from 1.95 mPa · s to 2.79 mPa · s, 2.79 mPa · s to 6.78 mPa · s, and 3.96 mPa · s to 12.90 mPa · s, respectively. Apparently, the PDMS concentration and aging time can be used to adjust the sol viscosity. Note that a very high PDMS content in the sol will make the sol viscosity grow too quickly, which will greatly limit the utility of the sol. Consequently, the PDMS concentration should be kept lower than 15%. Table 2 shows the change in peak diameter as a function of aging time of pure, 5%, 10%, and 15% PDMS-containing sols. There is little change in peak diameter as a function of PDMS concentration. Hench et al.30 suggested that the viscosity of a sol undergoing hydrolysis and polycondesation is time dependent and is related to the particle size. The larger the particle size, the higher the viscosity. In this work, the viscosity of PDMS-containing silica sol, especially high PDMS-containing silica sol, increased very fast while the silica particle size change is slight with aging time. To explain the apparent contradiction between sol viscosity and sol particle size, a “compulsive aggregation” process for PDMS-containing silica sol was proposed on the basis of Zukoski’s work.31,32 Zukonki proposed a controlled aggregation mechanism for sol nucleation and particle growth by aggregation of subparticles (a few nanometers in size). Once the aggregates or “particles” have reached a certain size, the sol attains a certain colloidal stability due to the surface charges, and the growth continues only by aggregation with the small subparticles and not by collisions with the larger particles. Figure 2 illustrates the growth mechanism of pure and PDMS-containing sols. The precursor, TEOS, is first hydrolyzed by mixing with water and ammonia and reacts to form Si-O-Si bonds. Dehydration of additional Si-OH tetrahedral centers occurs and eventually results in primary silica particles (subparticles). Thereafter, for pure sol, the particles grow by the process of subparticle aggregation. Once the particles reach a certain size, the pure sol tends to be stable due to surface charge repulsion. Consequently, the particle size and viscosity of pure silica sol increased very slightly with aging time. Zukoski’s theory explained very well the slow change in particle size and viscosity as a function of aging time. In our study, the rate of increase in viscosity of the PDMScontaining sols does not slow. When PDMS is mixed with TEOS, three different condensation processes may occur:25 (1)

PDMS/Silica Hybrid Antireflective Coatings

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TABLE 2: Change in Peak Diameter of Silica Sols as a Function of Aging Time and PDMS Concentration peak diameter at different aging times (nm) PDMS concentration, %

5 days

7 days

10 days

12 days

14 days

17 days

0 5 10 15

4.24 4.10 4.12 4.19

4.20 4.13 4.20 4.16

4.16 4.12 4.13 4.12

4.29 4.23 4.25 4.51

4.27 4.29 4.41 5.11

4.48 4.38 4.60 5.19

cocondensation between two components, (2) self-condensation of PDMS, or (3) hydrolysis of TEOS and the subsequent condensation of silanols. In the case of the base-catalyzed sol system, due to the fast hydrolysis and condensation of TEOS, the formation of silica particles is very fast. Subsequently, the self-condensation of PDMS and cocondensation take place simultaneously with catalysis by NH3 · H2O. To demonstrate the catalytic effect of ammonia, pure silica sol was refluxed for 24 h to remove the ammonia, and then PDMS was added to this ammonia-free pure sol. The viscosity of this PDMS-containing sol did not change with aging time and PDMS concentration, which indicates that ammonia has a catalytic effect on the dehydration between silica particles and PDMS. PDMS possesses a chain length of about 4 nm and selfcondenses to form PDMS with longer chain lengths. Regarding the molecule structure, PDMS contains two hydroxyl groups with a chain length longer than equilibrium spacing between silica particles. The hydroxyl groups of PDMS can react with hydroxyl groups on the surface of silica particles. As shown in process II of Figure 2, besides aggregating in “Zukoski’s way”, there is still a “compulsive aggregation” process. When the particles reach a “certain size”, these bigger particles cannot aggregate in “Zukoski’s way” because of their surface charges. However, these particles can be compulsively linked by PDMS despite the charge repulsion. In this “compulsive aggregation” process, PDMS plays a role as a “bridge” between silica particles. In the process of sol aging, the formation of “PDMS bridges” greatly increases the sol viscosity but does not cause obvious silica particle size change. In addition, the slight increase in particle size of 15% PDMS-containing sol (as shown in Table 2) is attributed to the formation of “PDMS coatings” on the particle surface. However, these “PDMS coatings” will not significantly increase the sol viscosity because they will create “hydrophobic coatings” which will restrict the further aggrega-

tion of silica particles. Thus, the fast increase in sol viscosity is mainly attributed to the formation of “PDMS bridges”. “PDMS bridges” play the key role in the “compulsive aggregation” of silica particles. These “PDMS bridges” were formed by the dehydration between hydroxyl groups of PDMS and silica particles. To confirm the dehydration between PDMS and silica particles, the FTIR spectra of PDMS, pure silica, and 5%, 10%, and 15% PDMS-containing silicas were measured. Before the FTIR spectral measurement, 5%, 10%, and 15% PDMS-containing silicas were extracted exhaustively with ethanol for 4 days using Soxhlet apparatus, respectively. PDMS that were not covalently bonded to the silica particles were extracted from the silica. As shown in Figure 3, all spectra show a very strong absorption band at 1062 cm-1 assigned to the stretching modes of the Si-O-Si bonds. The bands at 795 cm-1 are assigned to the symmetric Si-O stretching mode.33 In the spectra of PDMS-containing silica, further absorption peaks were revealed at 1268 cm-1 and 903 cm-1, attributed, respectively, to the Si-C stretching vibration and the Si-CH3 symmetric deformation of PDMS. These absorptions increase gradually as the PDMS concentration increases. This indicates that PDMS was covalently bonded to silica particles. The FTIR result convincingly shows that the PDMS is covalently linked to silica particles through Si-O-Si bonds by dehydration between PDMS and silica particles. Table 3 shows the effect of PDMS concentration and aging time on the maximum transmittance (Tmax) and the wavelength of the maximum transmittance (λmax). The maximum transmittance of AR coatings with increasing PDMS concentration shows no difference. Addition of PDMS affects the wavelength of the maximum transmittance. The wavelengths of the maximum transmittance of 5%, 10%, and 15% PDMS-containing AR coatings increase gradually from 875 to 1560 nm, which is

Figure 2. Schematic representation of the particle growth of pure (process I) and PDMS-containing (process II) sols.

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Figure 4. Water contact angles versus the concentration of PDMS: (a) 0%, (b) 5%, (c) 10%, and (d) 15%.

Figure 3. FTIR spectra of PDMS, pure, and 5%, 10%, and 15% PDMS-containing silicas.

TABLE 3: Effect of PDMS Concentration and Aging Time on Tmax and λmaxa PDMS concentration (%)

aging time (days)

Tmax (%)

λmax (nm)

5 5 5 5 10 15

3 4 5 6 4 4

99.8 99.9 99.9 100.0 100.0 99.9

840 875 915 975 1200 1560

a

The speed of dip-coating is 100 mm/min.

attributed to an increase in viscosity of the sols resulting in an increase in the coating thickness.34 For pure sol, owing to low viscosity and a small controllable viscosity range, the preparation of thick, single-layer AR coatings with controlled thickness is difficult. PDMS-containing sols prepared in this study have a wide controllable viscosity range and relatively high values of viscosity. This characteristic enables the preparation of thick, single-layer AR coatings with different thicknesses. The wavelength of maximum transmittance can reach 1560 nm even though the 15% PDMS-containing sol is only aged for 4 days, as shown in Table 3. Hydrophobicity can be represented by the water contact angle. Figure 4 shows the change in water contact angle of the AR coatings as a function of PDMS concentration. As the concentration of PDMS increases from 0% to 15%, the water contact angles of the AR coatings increase from 23.4° to 64.5°. This indicates an obvious increase in the hydrophobicity of the AR coatings after introducing PDMS. This increase in hydrophobicity is caused not only by the dehydration between PDMS and silica particles which decreases the number of hydroxyl groups but also by the introduction of hydrophobic methyl groups into the AR coatings. Antireflective durability is an important property for AR coatings. Figure 5 shows the maximum transmittance of pure and 15% PDMS-containing AR coatings as a function of the test time in a closed container with 95% relative humidity at room temperature. Clearly, the PDMS dramatically improves

Figure 5. Change in maximum transmittance as a function of test time.

the durability of AR coatings. The maximum transmittance of the 15% PDMS-containing AR coatings decreases from 99.9% to 99.6%, while that of the pure silica AR coating decreases from 99.9% to 98.7%. The incorporation of PDMS significantly improves the hydrophobicity of AR coatings, which prevents the adsorption of water in the pores of the coating, affording a much better antireflective durability in wet environments. Conclusion Silica AR coatings with controlled thickness and durable AR performance were prepared by a base-catalyzed sol-gel process using tetraethyl orthosilicate (TEOS) as a precursor and hydroxyl-terminated polydimethylsiloxane (PDMS) as a modifier. A “compulsive aggregation” mechanism for silica particle formation was proposed. This mechanism explains the behavior of silica sol in the presence of PDMS. The “compulsive aggregation” process involves the formation of “PDMS bridges” between silica particles by dehydration, which was confirmed by FTIR. The fast increase in viscosity of the silica sol enables the convenient preparation of AR coatings with controlled thickness. At the same time, introducing PDMS into the silica sol greatly increased the hydrophobicity of the AR coatings not only by the dehydration between PDMS and silica particles which decreases the number of hydroxyl groups but also by the introduction of hydrophobic methyl groups into the AR coatings. References and Notes (1) Muromachi, T.; Tsujino, T.; Kamitani, K.; Maeda, K. J. Sol-Gel Sci. Technol. 2006, 40, 267-–272.

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