Insight into the OrganicâInorganic Hybrid and Microstructure Tailor

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Insight into the Organic-Inorganic Hybrid and Microstructure Tailor Mechanism of Sol-Gel ORMOSIL Antireflective Coatings Xin-Xiang Zhang, Wensheng Lin, Jiaxian Zheng, Yingying Sun, Bibo Xia, Lianghong Yan, and Bo Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10294 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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The Journal of Physical Chemistry

Insight into the Organic-Inorganic Hybrid and Microstructure Tailor Mechanism of Sol-Gel ORMOSIL Antireflective Coatings Xinxiang Zhang†,±,*, Wensheng Lin†, ±, Jiaxian Zheng†, Yingying Sun†, Bibo Xia‡, Lianghong Yan§, Bo Jiang‡ † College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, China. ‡ Key Laboratory of Green Chemistry & Technology, College of Chemistry, Sichuan University, Chengdu, 610064, China. § Research Center of Laser Fusion, China Academy of Engineering Physical, Mianyang, 621900, China.

*

Corresponding

author:

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+86-591-83715175;

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+86-591-83715175;

E-mail:

[email protected] ±

These authors contribute equally to this work.

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Abstract: Having a deep insight into the organic-inorganic hybrid (OIH) and microstructure tailor mechanism of sol-gel ORMSOSIL antireflective (AR) coating is of great interest in the field of sol-gel science and technology. VTES/TEOS ORMOSILs sols were prepared via sol-gel process by using vinyltriethoxysilane (VTES) and tetraethoxysilane (TEOS) as co-precursors. 29

Si MAS NMR was applied to deduce the OIH mechanism between VTES and TEOS. The

results of N2 adsorption-desorption analysis indicated that OIH could tailor effectively the pore structure. The OIH of VTES and TEOS significantly increased the porosity. With this convenient sol-gel process, the refractive index of VTES/TEOS ORMOSIL AR coatings could be regulated gradually between 1.12 and 1.21 by varying the VTES/TEOS molar ratio. The OIH mechanism and the drying process of these VTES/TEOS ORMOSIL AR coatings were proposed after a detail discussion in the relationship between microstructure and optical properties.

1. Introduction In 1968, Stӧber developed a method for preparation of spherical silica particles with uniform size from alkyl silicates catalyzed by ammonia.1 The particle size of silica prepared by the Stӧber method could be controlled in the range of less than 0.05 µm to 2 µm in diameter. Since then, especially from 1980s, the preparation and application of Stӧber silica particles had gained great attention.2-16 One of the most important application of Stӧber silica particles is the sol-gel silica antireflective (AR) coating.6,17-25 According to the published papers, for the first time, Solaga deposited Stӧber silica particles onto the solar collector windows as AR coating.17-19 The sol-gel silica AR coating prepared by Stӧber method consists of silica particles of ~20 nm size that are randomly stacked on the substrate surface. This loose stack gives sol-gel silica AR coating with excellent transmittance. The inter-particle and particle interior porosity give the sol-gel silica AR coating with a refractive index of about 1.23.26 This index is perfect for use as the single-layer


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quarter-wavelength AR coating on glass substrates because it is close to square root of the indices of the glass substrates. In addition, there is no light scattering as the wavelength of incident light higher than ~200 nm (λ>>particle size). Therefore, the transmittance of glass substrate coated with sol-gel silica AR coating is excellent of higher than 99.8%. However, there is a serious drawback for sol-gel silica AR coating. Sol-gel silica AR coating is porous and polar and tends to absorb water and plasticizers of engineering materials from working environment. This will increase the refractive index and hence degrade the AR stability.27 Therefore, lots of researchers focus on improving the AR stability of sol-gel silica AR coating. A two-step postdeposition treatment of sol-gel silica AR coating was applied in NIF, which was proposed by Thomas.28 With Thomas method, both the internal and external surface of the silica particles are covered by nonpolar methyl groups, and the AR stability was significantly improved. Inspired by Thomas, surface modification

21, 29

and organic-inorganic hybrid (OIH) was applied to enhance the AR stability of sol-gel

silica AR coating and became one of the hotspots for sol-gel silica AR coating in the past few

decades. The OIH silica was named as ORMOSIL (organically modified silicate). ORMOSIL was generally realized by using Si(OR)4 and SiR'n(OR)4-n as co-precursors or from hybrid of silica30-31 and reactive polymers32-33. It had been reported that ORMOSIL AR coatings possessed the advantages of improved hydrophobicity and hence good AR stability, improved mechanical property and higher laserinduced damage threshold.34 Our research team have carried out in-depth study on the sol-gel silica AR coatings, especially ORMOSIL AR coating used in high-power laser fusion system.26,33,35-37 In addition to the improved AR stability, we have also pointed out that the OIH could realize AR coating with very low refractive index without supercritical drying and template-adding.37 We have deduced that this phenomenon is arose by two collapsing forces during the drying process. However, until now, this deduction has not been demonstrated by the experiment. It is urgently desirable to pay more attention to the OIH and microstructure tailor mechanism of ORMOSIL AR coating. This will not only contribute theoretically to


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the sol-gel science and technology but also promote practically the application of ORMOSIL AR coatings. In this work, VTES were utilized as organic precursors to co-condense with TEOS to

prepare ORMOSIL AR coatings. As expected, OIH of VTES and TEOS significantly decreased the refractive index of VTES/TEOS AR coating from 1.21 to 1.12. The microstructure and properties of ORMOSIL AR coatings were investigated systematically, and after a in-depth discussion about the relationship between microstructure and properties of ORMOSIL AR coatings, a deep insight into the OIH and microstructure tailor mechanism of ORMOSIL AR coating was provided.

2. Experimental section 2.1 Materials High pure TEOS was purchased from Kermel (Tianjin, China). Analytical grade ammonia water with NH3 content of 25~28% and hydrofluoric acid with HF content of 40% were purchased from Sinopharm Chemical Reagent Co., Ltd. Analytical grade anhydrous ethanol was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. VTES was purchased from Xiya Reagent Research Center (Shandong, China). 2.2 Preparation of sols 2.2.1 Preparation of VTES/TEOS ORMOSIL sols The sols were prepared by the Stӧber method. TEOS, VTES, ethanol and ammonia were added in a sealed glass container and then magnetically agitated for 2 h at 30°C. After that, the sols were aged at 25°C for 7 d. The molar ratio of (TEOS+VTES):H2O:NH3:EtOH is 1:2.0:0.7:37.7. The formulas for all sols was shown in Table 1. The sols were named as V0, V0.2, V0.4 and V0.8 as the molar ratio of VTES to TEOS was 0, 0.2, 0.4 and 0.8, respectively.


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Table 1. Formula for VTES/TEOS ORMOSIL sols Sample

TEOS /g

VTES /g

EtOH /g

NH3·H2O /g

V0

15.6

0

130

3.6

V0.2

13.9

2.5

130

3.6

V0.4

11.9

4.3

130

3.6

V0.8

9.3

6.8

130

3.6

2.2 Preparation of VTES/TEOS ORMOSIL AR coating Fused silica substrates with diameter of 30 mm and thickness of 3 mm were dipped into 0.5% HF solution for several seconds and then immediately washed with deionized water. Finally, the substrates were washed with anhydrous ethanol and wiped carefully with cleanroom wiper. Before deposition, the VTES/TEOS ORMOSIL sols were diluted with equal weight of anhydrous ethanol to lower the viscosity of ORMOSIL sols. The ORMOSIL AR coatings were obtained by dipping substrates in the diluted sols and then withdrawing out at a constant speed. The obtained ORMOSIL AR coatings were heated at 120°C for 1 h. 2.3 Characterization The AR coatings were coated onto the substrates using the SYDC-100N Dip Coater from SANYAN (Shanghai, China). The humidity was controlled to about 30% with nitrogen atmosphere. The ORMOSIL xerogels were obtained by rotary evaporation with a further heattreatment at 120 for 2 h. FTIR spectra of ORMOSIL xerogels were analyzed by Bruker Tensor 27 using KBr pellet method with transmission mode in the mid-IR range from 400 to 4000 cm-1. XPS spectra of ORMOSIL AR coatings were obtained by using a Thermo SCIENTIFIC ESCALAB 250Xi with Al X-radiation (K alph, hv=1486.8 eV). The power was 150 W (15 kV and 10 mA), and the vacuum of the ssampe chamber was 2*10-9 mbar.

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Si MAS NMR

experiments of ORMOSIL xerogels were performed on a Bruker Avance III 400 WB (11.75 T)


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spectrometer operating at a frequency of 400.58 MHz using a 4.0 mm probe. N2 adsorption/desorption experiments were carried out on Autosorb IQ gas adsorption analyzer from Quantachrome Instruments, and the specific surface area and pore size distribution was analyzed

using

the

Barrett-Joyner-Halenda

(BJH)

method.

The

samples

for

N2

adsorption/desorption experiments is bulk xerogel, which are obtained by heat-treating the sols in 120 for 6 h. The surface morphology of the coating was analyzed by scanning electron microscope (SEM, ZEISS Z500). The dispersive curves of refractive index were determined using an ellipsometer from 206 nm to 879 nm (Horiba, UVISEL). Water contact angles were measured by a Krüss DSA100 (Germany). 3. Results and discussion 3.1 Chemical structure characterization of VTES/TEOS ORMOSIL FTIR spectra of ORMOSIL xerogels with increasing VTES/TEOS molar ratio were recorded and shown in Figure 1. For pure xerogel without VTES addition, there are absorption bands at 3445 cm-1, 2985 cm-1, 2926 cm-1, 2853 cm-1, 1080 cm-1, 966 cm-1, 781 cm-1 and 454 cm-1.38-39 The adsorption bands at 3445 cm-1 and 966 cm-1 are attributed to Si-OH bond. Absorption bands at 2985 cm-1, 2926 cm-1 and 2853 cm-1 are assigned to the vibration of un-hydrolyzed ethoxy groups. The absorption bands at 1080 cm-1, 781 cm-1 and 454 cm-1 are corresponding to the asymmetric stretch, symmetric stretch and bend vibration of -Si-O-Si- bond, respectively. For VTES/TEOS ORMOSIL xerogels, with the exception of these absorption bands, other absorption bands appear at 3076 cm-1, 2958 cm-1, 1606 cm-1, 1411 cm-1 and 1276 cm-1. Absorption bands at 3076 cm-1 and 2958 cm-1 correspond to asymmetrical and symmetrical stretching vibrations of =CH2. The absorption bands at 1606 is associated with C=C vibration of terminal olefin while that at 1411 cm-1 is assigned to vinyl CH2 in-plane deformation. The


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existence of vinyl groups indicates the formation of VTES/TEOS xerogels by covalent bond. In addition, the intensity for absorption bands of vinyl groups increases gradually with increasing VTES/TEOS molar ratio, indicating that more and more vinyl groups were incorporated into the ORMOSIL.

Figure 1. FTIR spectra of ORMOSIL xerogel with VTES/TEOS molar ratio of 0 (a), 0.2 (b), 0.4 (c) and 0.8 (d) For quantitative elemental analysis, XPS measurements were carried out and the survey spectra are presented in Figure 2. The extrapolated atom ratios are displayed in Table 2. From Figure 2 and Table 2, we can see that the surface of samples are mainly composed of silicon, oxygen and carbon. For pure silica xerogel, the carbon amount was found to be 13.69%, which was attributed to the unhydrolyzed -OCH2CH3 groups. This is also verified from the FTIR measurements, in which the absorption bands at 2854-2976 cm-1 corresponded to the -CH2- and CH3 of unhydrolyzed -OCH2CH3 groups. For ORMOSIL xerogel with molar ratio of VTES to TEOS of 0.2 and 0.4, carbon amount was found to be 38.7% and 40.18%. The increase in the carbon amount demonstrates that VTES was hydrolyzed and incorporated into the silica to realize ORMOSIL.


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Figure 2. XPS spectra of ORMOSIL xerogel with VTES/TEOS molar ratio of 0 (V0), 0.2 (V0.2), and 0.4 (V0.4) Table 2. Atom amount of the investigated pure silica xerogel and ORMOSIL (extrapolated from the XPS survey spectra) Atom amount (%) Sample Si

O

C

V0

26.51

59.79

13.69

V0.2

17.92

43.38

38.7

V0.4

18.6

41.22

40.18

3.2 Pore structure of VTES/TEOS ORMOSIL AR coatings The pore structure of ORMOSIL xerogel was investigated by nitrogen adsorption-desorption isotherms analysis to further verify the OIH mechanism of VTES/TEOS ORMOSIL. Nitrogen adsorption-desorption isotherms and corresponding BJH pore size distribution of ORMOSIL xerogel with different VTES/TEOS molar ratio are represented in Figure 3(a) and (b). All of the nitrogen adsorption-desorption isotherms are a type IV isotherm, typical of mesoporous solids. In addition, for all isotherms, the adsorption and desorption lines overlap completely in the low


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relative pressure range, while the hysteresis loop exists in the high relative pressure region (P/P0>0.5). The hysteresis loop of pure silica xerogel is type H2, while that of ORMOSIL xerogel shows an intermediate situation between H1 and H2. Hysteresis types H1 and H2 are characteristic of particulate materials formed by aggregation of spherical SiO2 particles. H1 presents a better connectivity between pores than the H2 isotherm. Thus, it means that the connectivity of pores is increasing as VTES/TEOS molar ratio is raised. The volume adsorbed of pure silica xereogel is 437 cm3/g, which increases significantly to 817, 918 and 728 cm3/g, respectively, when the VTES/TEOS molar ratio increases to 0.2, 0.4 and 0.8. This indicates a significant increasement in pore volume. As illustrated in Table 3, the specific surface areas of ORMOSIL xerogels prepared under the VTES/TEOS molar ratios of 0, 0.2, 0.4 and 0.8 are determined to be 695, 940, 965 and 958 m2/g, accompanied with pore volumes of 0.77, 1.39, 1.57 and 1.26 cm3/g, respectively. In particular, the porosity calculated from pore volume is 63.0% for V0 while it is 77.5% for V0.4. This is further demonstrated by the photograph of xerogels as shown in Figure 3(c). As the VTES/TEOS molar ratio increased to be higher than 0.4, the samples represent a characteristic soft blue for silica aerogel. It can be deduced that the refractive index of ORMOSIL thin films will be significantly decreased after incorporating VTES into the silica AR coating.


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Figure 3. Nitrogen adsorption-desorption isotherms (a), pore size distributions from the adsorption branches by the BJH method (b) and photograph of xerogels (c) Table 3 Structure parameters of ORMOSIL xerogel with different VTES/TEOS molar ratio Porosity' b

(nm)

Porosity a (%)

0.77

4.44

63.0

59.0

940

1.39

5.92

75.4

69.5

P0.4

965

1.57

6.49

77.5

71.5

P0.8

958

1.26

5.25

73.5

77.5

Surface area

Pore volume

Pore size

(m2/g)

(cm3/g)

P0

695

P0.2

Sample

a b

(%)

porosity calculated from pore volume

porosity calculated from refractive index

3.3 OIH mechanism of VTES/TEOS ORMOSIL Figure 3 and Table 3 reveal that the porosity of VTES/TEOS can be adjusted by varying the VTES/TEOS molar ratio. To explain this phenomenon, the OIH mechanism of VTES and TEOS should be investigated. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) is a powerful tool for investigating structure in silica-based materials because it is sensitive to the bonding environment of the Si nucleus. In this work, Qn and Tn notation were used in a similar way to previous authors.40 Figure 4 shows the

29

Si MAS NMR spectra of the pure silica, V0.2

and V0.4 xerogels. As can be seen from Figure 4, two Qn (n=1-4 in theory) peaks were observed at -111.55 ppm (Q4, (SiO)4Si), -104.64 ppm (Q3, (SiO)3SiOR, R=H or OCH2CH3). For VTES/TEOS ORMOSIL xerogel, an additional Tn (n=1-3 in theory) peak at -83.04 ppm (T3, (SiO)3SiVi) was found.


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Figure 4. 29Si MAS NMR spectra of V0, V0.2 and V0.4 Results from

29

Si MAS NMR spectra not only confirmed that the OIH reaction was realized

successfully but also was helpful to deduce the OIH mechanism between VTES and TEOS. SN2Si mechanism of hydrolysis of TEOS was proposed by Iler41 and Keefer42 that, under basic conditions, OH- displaced OR- with inversion of the silicon tetrahedron. The steric hindrance of ethoxyl is not convince to the hydrolysis of TEOS. However, as one of the ethoxyl groups was replaced by the hydroxyl group, the further hydrolysis of ethoxyl groups is very easy. So, it is generally agreed that 3~4 ethoxyl groups of TEOS hydrolyzed which react into silica particles by condensation. This is demonstrated by 29Si MAS NMR spectrum in which there are only Q4 and Q3 peaks for pure silica xerogel. For VTES/TEOS ORMOSIL, because there is only an additional T3 peak, it is convincing that most of VTES is hydrolyzed into ViSi(OH)3 and cocondenses with Si(OH)4 and EtOSi(OH)3 from TEOS. If VTES is hydrolyzed into ViSi(OH)2(OEt) or ViSi(OH)(OEt)2, there will be another T2 or T1 peak in

29

Si MAS NMR

spectra for VTES/TEOS OMROSIL. And that is, most of VTES was incorporated into the silica network by co-condensation of VTES and TEOS. Therefore, there are more and more vinyl groups incorporated into the ORMOSIL. The schematic representation of preparation


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VTES/TEOS ORMOSIL sols as well as the drying process of resultant thin films with different VTES/TEOS molar ratio were shown in Figure 5 (a). 3.4 Drying process and microstructure tailor mechanism of VTES/TEOS ORMOSIL In order to have a deep insight into the relationship between the pore structure and properties and hence the microstructure tailor mechanism of ORMOSIL AR coatings, it is inevitable to discuss the drying process of them. The drying process of sol-gel silica AR coatings is very complicated. In 1980's, Scherer43-44 had given an assessment of the driving forces causing shrinkage during drying of organometallic gels, which were capillary pressure, condensation reaction of hydroxyl and alkoxy groups on surface of silica particles, and osmotic pressure. In addition, Scherer43 ascribed the capillary pressure to the contraction of the dry region to reduce the solid-vapor interfacial area, contraction of the wet region to reduce the solid-liquid interfacial area and contraction of the wet region driven by the redistribution pressure. Back to this work, the drying process of sol-gel silica AR coatings can be divided into two stages: sol-gel transition and drying of gel to the solid thin film.37 Apparently, the drying process of VTES/TEOS ORMOSIL AR coatings in this work can also refer to that of organometallic gels proposed by Scherer.43-44 In the first step, the solvent evaporates and the distance between silica particles decreases. As the particle distance decreases gradually, the repulsive forces corresponding to the surface charge turns into the attractive forces, which results in the sol-gel transition. In silica gel, ethanol was encapsulated into the 3D network of silica particles. As shown in Figure 5 (b), after sol-gel transition, the porosity of is very high. As shown in the Process I in Figure 5 (b), at the early stage of drying, the gel encounters the contraction derived from condensation of -OH and -OEt groups on the surface of adjacent silica particles. FTIR spectra have demonstrated the existence -OH and -OEt groups in ORMOSIL, and


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the results from XPS reveals a decrease in oxygen element and an increase in carbon element with increasing VTES/TEOS molar ratio, that is, the content of reactive -OH and -OEt groups decrease while that of un-reactive vinyl groups increase as the VTES/TEOS molar ratio increases. Therefore, as shown in the Process I in Figure 5 (b), the shrinkage derived from condensation is inversely proportional to the VTES/TEOS molar ratio. It should be pointed out that, during Process I, there is solid-liquid interfaces and hence the contraction resulted from them is arisen. However, it can be excluded because the solid-liquid interfacial energy (γSL) is quite small.


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Figure 5. Schematic representation of preparation of ORMOSIL sols (a), drying process of gels (b), tension in narrow opening (c) and schematic representation of ORMOSIL thin films with different VTES/TEOS molar ratio compared with the TEM images (d)


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As shown in the Porcess II in Figure 5(b), a further evaporation of ethanol leads to an exposure of parts of silica particles' surface. Then the solid-vapor interface, whose specific energy (γSV) is much greater than γSL, occurs. This is one of the contractions in Process II. There is another factor for the contraction. The nitrogen adsorption-desorption isotherms revealed that the pores type is similar to an ink-bottle with narrow openings. The narrow opening is schematically represented in Figure 5 (c). There is ethanol filled in the narrow opening, and hence the tension (P) in the ethanol at the drying surface should be taken into account.

P=

2γ LV cos θ R

where γLV is the liquid-vapor surface tension, θ is the contact angle; R is the radius of curvature. Here, γLV and R are approximated to be constant, and then the tension is proportional to cosθ. The contact angle on the surface of silica particles is difficult to tested. Here, the contact angle on the surface of ORMOSIL AR coatings were tested. Of course, the contact angle on surface of silica particles and AR coatings is different. However, the change tendency of contact angle on surface of silica particles and AR coatings would be the same. It can be revealed from Figure 6 that the tension of ORMOSIL gel with VTES hybrid is much smaller than the pure silica gel. After comprehensive consideration of Process I, Process II and Process III, during drying process, V0 gel suffers from severe shrinkage, and the shrinkage is increasingly suppressed after incorporating more and more VTES into the ORMOSIL gels. This phenomenon is observed directly in the SEM images of ORMOSIL AR coatings as shown in Figure 5 (d). As shown in SEM images, all of ORMOSIL AR coatings are porous. More importantly, SEM images revealed that the pore size increased significantly with increasing VTES/TEOS molar ratio. The pore size of pure silica AR coating is only several nanometers while that of some of


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pores for V0.8 AR coating are almost 100 nm. This demonstrates convincingly the mechanism of OIH on the microstructure control. SEM images and the nitrogen adsorption-desorption isotherms analysis reveal a similar change in microstructure as a function of VTES/TEOS molar ratio, in which they seem to be more porous. But, there is also an evident difference in pore size. The pore size analyzed from the nitrogen adsorption-desorption isotherms is much smaller than that observed from SEM images. This is because the samples for the nitrogen adsorption-desorption isotherms test is the bulk xerogels which are obtained by heat-treating the sols at 120°C for 6 h. The drying of bulk xerogels would suffer from much greater capillary pressure than the xerogel thin films with thickness of 100-200 nm, and hence the pore size is much smaller.

Figure 6. Value of contact angle and corresponding cosine value of ORMOSIL thin films with different VTES/TEOS molar ratio 3.5 Optical properties of VTES/TEOS ORMOSIL AR coatings Results from SEM images and nitrogen adsorption-desorption isotherms analysis have revealed that the porosity of AR coatings increased with increasing VTES/TEOS molar ratio. The relationship between porosity and refractive index and the relationship between refractive index and transmittance can refer to Equation (1) and Equation (2), respectively.


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Porosity=1-(nc2-1)/(nd2-1) R=(1-T)/2=(nc2-ns)2/(nc2+ns)2

(1) (2)

where nc is the refractive index of AR coating and nd=1.46 is the refractive index of the dense silica thin film without pores; R and T is the reflection and transmittance of AR coating, ns is the refractive index of substrate. Based on these, the relationship between microstructure and properties is clear, and it can be deduced that the refractive index and hence the transmittance of AR coatings would decrease with increasing VTES/TEOS molar ratio. To verify the inference, the dispersion curve of refractive index and transmittance spectra of ORMOSIL AR coatings with varying VTES/TEOS molar ratio were recorded and shown in Figure 7 (b) and (a). As shown in Figure 7 (b), the refractive index of ORMOSIL AR coating at 725 nm decreased gradually from 1.21 to 1.12 as the molar ratio of VTES/TEOS increased from 0 to 0.8. The corresponding transmittance at central wavelength decreases from 99.99% to 98.6%.

Figure 7. Transmittance spectra (a) and dispersion curve (b) of ORMOSIL AR coatings with different VTES/TEOS molar ratio


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4. Conclusions An ORMOSIL sols and the resultant AR coatings were prepared by a simple sol-gel process by using TEOS and VTES as co-precursors. The OIH mechanism of VTES/TEOS ORMOSIL was deduced after a detail discussion of results of 29Si NMR. The pore structure of ORMOSIL can be tailored by varying the molar ratio of VTES to TEOS, and hence the refractive index of ORMOSIL AR coatings can be controlled gradually between 1.12 and 1.21. Finally, the microstructure tailor mechanism of VTES/TEOS ORMOSIL were proposed after a detail discussion in the relationship between microstructure and optical properties. 5. Acknowledgements The authors gratefully acknowledge the support from National Natural Science Foundation of China (61505029), Natural Science Foundation of Fujian Province of China (2015J05092) and Outstanding Youth Fund of Fujian Agriculture and Forestry University (XJQ201602). 6. References 1. Stӧber, W.; Fink, A.; Bohn E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62-69. 2. Comolli, R. Cytotoxicity of Silica and Liberation of Lysosomal Enzymes. J. Path. Bact. 1967, 93, 241-253. 3. Bridger, K.; Fairhurst, D.; Vincent, B. Nonaqueous Silica Dispersions Stabilized by Terminally-Grafted Polystyrene Chains. J. Colloid Interface Sci. 1979, 68, 190-195.


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4. Bogush, G. H.; Tracy, M. A.; Zukoski IV, C. F. Preparation of Monodisperse Silica Particles: cControl of Size and Mass Fraction. J. Non-Cryst. Solids 1988, 104, 95-106. 5. Coenen, S.; de Kruif, C. G. Synthesis and Growth of Colloidal Silica Particles. J. Colloid Interface Sci. 1988, 124, 104-110. 6. Thomas I. M. High Laser Damage Threshold Porous Silica Antireflective Coating, Appl. Opt. 1986, 25, 1481-1483. 7. Fox, J. R.; Kokoropoulos, P. C.; Wiseman, G. H.; Bowen, H. K. Steric Stabilization of Stӧber Silica Dispersions Using Organosilanes. J. Mater. Sci. 1987, 22, 4528-4531. 8. Wells, J. D.; Koopal, L. K.; de Keizer, A. Monodisperse, Nonporous, Spherical Silica Particles. Colloids Surfaces A 2000, 166, 171-176. 9. Giesche, H. Synthesis of Monodispersed Silica Powders II. Controlled Growth Reaction and Continuous Production Process. J. Eur. Ceram. Soc. 1994, 14, 205-214. 10. Markowitz, M. A.; Kust, P. R.; Deng, G.; Schoen, P. E.; Dordick, J. S.; Clark, D. S.; Gaber, B. P. Catalytic Silica Particles via Template-Directed Molecular Imprinting. Langmuir 2000, 16, 1759-1765. 11. Nozawa, K.; Gailhanou, H.; Raison, L.; Panizza, P.; Ushiki, H.; Sellier, E.; Delville, J. P.; Delville, M. H. Smart Control of Monodisperse Stӧber Silica Particles: Effect of Reactant Addition Rate on Growth Process. Langmuir 2005, 21, 1516-1523. 12. Yang, H.; Pi, P.; Cai, Z.; Wen, X.; Wang, X.; Cheng, J.; Yang, Z. Facile Preparation of Super-Hydrophobic and Super-Oleophilic Silica Film on Stainless Steel Mesh via Sol-Gel Process. Appl. Surface Sci. 2010, 256, 4095-4102.


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Page 20 of 32

13. Costa, C. A. R.; Leite, C. A. P.; Galembeck, F. Size Dependence of Stӧber Silica Nanoparticle Microchemistry. J. Phys. Chem. B 2003, 107, 4747-4755. 14. Tolbert, S. H.; McFadden, P. D.; Loy, D. A. New Hybrid Organic/Inorganic Polysilsesquioxane-Silica Particles as Sunscreens. ACS Appl. Mater. Interfaces 2016, 8, 31603174. 15. Wang, X.; Zhang, Y.; Luo, W.; Elzatahry, A. A.; Chen, X.; Alghamdi, A.; Abdullah, A. M.; Deng, Y.; Zhao, D. Synthesis of Ordered Mesoporous Silica with Tunable Morphologies and Pore Sizes via a Nonpolar Solvent-Assisted Stӧber Method. Chem. Mater. 2016, 28, 2356-2362. 16. Wang, H.; Lin, S.; Tang, A.; Singh, B. P.; Tong, H.; Chen, C.; Lee, Y.; Tsai, T.; Liu, R. Mesoporous Silica Particles Intergrated with All-Inorganic CsPbBr3 Perovskite Quantum-Dot Nanocomposites (MP-PQDs) with High Stability and Wide Color Gamut Sued for Backlight Display. Angew. Chem. Int. Edit. 2016, 55, 7924-7929. 17. Solaga, C. T. Optical Degradation of Antireflective Silica film on Solar Collector Windows, Appl. Opt. 1981, 20, 3464-3465 18. Cathro, K. J.; Constable, D. C.; Solaga, T. Durability of Porous Silica Antireflection Coatings for Solar Collector Cover Plates. Solar Energy 1981, 6, 491-496. 19. Cathro, K.; Constable, D.; Solaga, T. Silica Low-Reflection Coatings for Collector Covers, by a Dip-Coating Process. Solar Energy 1984, 32, 573-579. 20. Floch, H. G.; Belleville, P. F. Damage-Resistant Sol-Gel Optical Coatings for Advanced Lasers at CEL-V. J. Sol-Gel Sci. Technol. 1994, 2, 695-705.


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Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Manca, M.; Cannavale, A.; De Marco, L.; Aricὸ, A. S.; Cingolani, R.; Gigli, G. Durable Superhydrophobic and Antireflective Surface by Trimethylsilanized Silica Nanoparticles-Based Sol-Gel Processing. Langmuir 2009, 25, 6357-6362. 22. Hoshikawa, Y.; Yabe, H.; Nomura, A.; Yamaki, T.; Shimojima, A.; Okubo, T. Mesoporous Silica Nanoparticles with Remarkable Stability and Dispersibility for Antireflective Coatings. Chem. Mater. 2010, 22, 12-14. 23. Wang, J.; Zhang, C.; Yang, C.; Zhang, C.; Wang, M.; Zhang, J.; Xu, Y. Superhydrophilic Antireflective Periodic Mesoporous Organosilica Coating on Flexible Polyimide Substrate with Strong Abrasion-Resistance. ACS Appl. Mater. Interfaces 2017, 9, 5468-5476. 24. Wang, Y.; He, M. Y.; Chen, R. Y. Fabrication of Mechanically Robust Antireflective Films Using Silica Nanoparticles with Enhanced Surface Hydroxyl Groups. J. Mater. Chem. A 2014, 3, 1609-1618. 25. Zhang, X.; Lan, P.; Lu, Y.; Li, J.; Xu, H.; Zhang, J.; Lee, Y.; Rhee, J. Y.; Choy, K.; Song, W.

Multifunctional

Antireflection

Coatings

Based

on

Novel

Hollow

Silica-Silica

Nanocomposites. ACS Appl. Mater. Interaces 2014, 6, 1415-1423. 26. Cai, S.; Zhang, Y.; Zhang, H.; Yan, H.; Lv, H.; Jiang, B. Sol-Gel Preparation of Hydrophobic Silica Antireflective Coatings with Low Refractive Index by Base/Acid Two-Step Catalysis. ACS. Appl. Mater. Interfaces 2014, 6, 11470-11475. 27. Sun, J.; Zhang, Q.; Ding, R.; Lv, H.; Yan, H.; Yuan, X.; Xu, Y. Contamination-Resistant Silica Antireflective Coating with Closed Ordered Mesopores. Phys. Chem. Chem. Phys. 2014, 16, 16684-16693.


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28. Thomas, I. M.; Burnham, A. K.; Ertel. J. R.; Frieders, S. C. Method for Reducing the Effect of Environmental Contamination of Sol-Gel Optical Coatings. Proc SPIE 1999, 3492, 220-229. 29. Latthe, S. S.; Imai, H.; Ganesan, V.; Kappenstein, C.; Rao, A. V. Optically Transparent Superhydrophobic TEOS-Derived Silica Films by Surface Silylation Method. J. Sol-gel Sci. Technol. 2010, 53, 208-215. 30. Zhang, Y.; Wu, D.; Sun, Y.; Peng, S. Sol-Gel Synthesis of Methyl Modified Optical Silica Coatings and Gels from DDS and TEOS. J. Sol-Gel Sci. Technol. 2005, 33, 19-24. 31. Akamatsu, Y.; Makita, K.; Inaba, H.; Minami, T. Water-Repellent Coating Films on Glass Prepared from Hydrolysis and Polycondensation Reactions of Fluoroalkyltrialkoxylsilane. Thin Solid Films 2001, 389, 138-145. 32. Zhang, X.; Cao, C.; Xiao, B.; Yan, L.; Zhang, Q.; Jiang, B. Preparation and Characterization of Polyvinyl Butyral/Silica Hybrid Antireflective Coating: Effect of PVB on Moisture-Resistance and Hydrophobicity. J. Sol-Gel Sci. Technol. 2010, 53, 79-84. 33. Zhang, X.; Xia, B.; Ye, H.; Zhang, Y.; Xiao, B.; Yan, L.; Lv, H.; Jiang, B. One-Step SolGel Preparation of PDMS-Silica ORMOSILs as Environment-Resistant and Crack-Free Thick Antireflective Coatings. J. Mater. Chem. 2012, 22, 13132-13140. 34. Thomas, I. M. Effect of Binders on the Damage Threshold and Refractive Index of Coatings Prepared from Colloidal Suspensions. Proc. SPIE 1992, 1848, 281-289. 35. Zhang, X.; Ye, H.; Xiao, B.; Yan, L.; Lv, H.; Jiang, B. Sol-Gel Preparation of PMDS/Silica Hybrid Antireflective Coatings with Controlled Thickness and Durable Antireflective Performance. J. Phys. Chem. C 2010, 114, 19979-19983.


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36. Ye, L.; Zhang, Y.; Zhang, X.; Hu, T.; Ji, R.; Ding, B.; Jiang, B. Sol-Gel Preparation of SiO2/TiO2/SiO2-TiO2 Broadband Antireflective Coating for Solar Cell Cover Glass. Solar Energy Mater. Solar Cells 2013, 111, 160-164. 37. Zhang, X.; Cai, S.; You, D.; Yan, L.; Lv, H.; Yuan, X.; Jiang, B. Template-Free Sol-Gel Preparation of Superhydrophobic ORMOSIL Films for Double-Wavelength Broadband Antireflective Coatings. Adv. Funct. Mater. 2013, 23, 4361-4365. 38. Illescas, J. F.; Mosquera, M. J. Surfactant-Synthesized PMDS/Silica Nanomaterials Improve Robustness and Stain Resistance of Carbonate Stone. J. Phys. Chem. C 2011, 115, 14624-14634. 39. Mosquera, M. J.; Santos, D. M.; Rivas, T. Surfactant-Synthesized Ormosils with Application to Stone Restoration. Langmuir 2010, 26, 6737-6745. 40. Derouet, D.; Forgeard, S.; Brosse, J.; Emery, J.; Buzare, J. Application of Solid-State NMR (13C and

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Si CP/MAS NMR) Spectroscopy to the Characterization of Alkenyltrialkoxysilane

and Trialkoxysilyl-Terminated Polyisoprene Grafting onto Silica Microparticles. J. Polym. Sci. Part A: Polym. Chem. 1998, 36, 437-453. 41. Iler, R. K. The Chemistry of Silica; Wiley: New York, USA, 1979. 42. Better Ceramics through Chemistry; Ulrich, D. R., Brinker, C. J., Clark, D. E., Eds.; Elsvier-North-holland: Amsterdam, 1984. 43. Scherer, G. W. Drying Gels I. General Theory. J. Non-Cryst. Solids 1986, 87, 199-225. 44. Scherer, G. W. Drying Gels II. Film and Flat Plate. J. Non-Cryst. Solids 1986, 89, 217-238.


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TOC Graphic


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Figure 1. FTIR spectra of ORMOSIL xerogel with VTES/TEOS molar ratio of 0 (a), 0.2 (b), 0.4 (c) and 0.8 (d) 69x58mm (600 x 600 DPI)



Figure 2. XPS spectra of ORMOSIL xerogel with VTES/TEOS molar ratio of 0 (V0), 0.2 (V0.2), and 0.4 (V0.4) 67x55mm (600 x 600 DPI)


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Figure 3. Nitrogen adsorption-desorption isotherms (a), pore size distributions from the adsorption branches by the BJH method (b) and photograph of xerogels (c) 60x24mm (600 x 600 DPI)



Figure 4. 29Si MAS NMR spectra of V0, V0.2 and V0.4 70x59mm (600 x 600 DPI)


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Figure 5. Schematic representation of preparation of ORMOSIL sols (a), drying process of gels (b), tension in narrow opening (c) and schematic representation of ORMOSIL thin films with different VTES/TEOS molar ratio compared with the TEM images (d) 165x183mm (300 x 300 DPI)



Figure 6. Value of contact angle and corresponding cosine value of ORMOSIL thin films with different VTES/TEOS molar ratio 58x41mm (600 x 600 DPI)


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Figure 7. Transmittance spectra (a) and dispersion curve (b) of ORMOSIL AR coatings with different VTES/TEOS molar ratio 82x61mm (300 x 300 DPI)



TOC Graphic 44x24mm (600 x 600 DPI)


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