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Mechanically robust, humidity-resistant, thermally stable high performance antireflective thin films with reinforcing silicon phosphate centers
Solar Energy Materials and Solar Cells 170 (2017) 95–101
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Mechanically robust, humidity-resistant, thermally stable high performance antireflective thin films with reinforcing silicon phosphate centers Tong Lia,b, Junhui Hea,
MARK
⁎
a Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Antireflection Robustness Durability Hierarchical nanopores Silicon phosphate
High performance, function durability, thermal stability and mechanical robustness have long been pursued for advanced antireflective thin films. In the current work, we developed a novel and effective approach to fabricate mechanically robust, humidity-resistant, thermally stable high-performance antireflective thin films with reinforcing silicon phosphate centers from an acid-catalyzed hybrid silica sol. The thin film has a hierarchically nanoporous structure, resulting in favorable antireflective properties. After being coated with the antireflective thin film, the maximum and average transmittances of K9 glass increase from 92.1% and 92.0% (400–800 nm) to 99.7% and 98.8% (400–800 nm), respectively. In addition, the thin film shows favorable humidity-resistance and heat-resistance, and these extraordinary performances are attributed to its rich heat-resistant and hydrophobic groups. Moreover, mechanical strength measurements indicate the thin film has extraordinary 5H pencil hardness and 5A adhesion to substrate because of the formation of silicon phosphate centers in the thin film, which significantly reinforce the thin film. These high-performance antireflective thin films are promising in solar energy utilization, especially in solar cells.
1. Introduction Light reflections from optical materials surfaces are undesirable in consequence of a loss of transmitted light. Antireflective (AR) thin films can reduce light reflections and have significant application prospects in solar cells, solar collectors, smart windows, display panels and lenses [1–8]. The principle of antireflection is the light interference between air and substrate. Theoretically, a homogeneous single-layer AR thin film with zero-reflection need a reflective index of (nans)1/2 and a thin film thickness of a quarter wavelength of incident light, where na, nc and ns are the refractive indices of air, thin film and substrate, respectively [9]. As the refractive indices of air and commercial glass are 1 and 1.52, respectively, the optimal refractive index of AR thin film should be ca. 1.23 [9–11]. However, the refractive indices of available substances are generally higher than 1.35 [9,12,13]. Thus, nanoporous and nanoarray structures are mainly used to obtain low refractive index thin films. To date, various approaches have been developed to obtain nanoporous [14–17] and nanoarray [9,11,18] materials with low refractive index, such as reactive ion etching (RIE) [5,19,20], acid or base chemical etching [21,22], nanoparticles deposition [23–26] and template
⁎
Corresponding author. E-mail address: [email protected] (J. He).
http://dx.doi.org/10.1016/j.solmat.2017.05.068 Received 4 April 2017; Received in revised form 27 May 2017; Accepted 29 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
methods [27,28]. Although the RIE method generically provides a nanoarray structure surface, resulting in excellent broadband AR properties, it is restricted by its expensive and complex equipment as well as small-area productions and low outputs [5,19,29]. The acid or base chemical etching approaches represent top-down routes to preparation of AR thin films, but the thin film via the base etching is mechanically weak and the acid etching method is time-consuming [21,22]. The assembly of nanoparticles on substrates has been widely investigated because of its design flexibility of film structure, but the obtained films usually show weak robustness and need further enhancing their mechanical strength [30]. The template methods are one of the general approaches to nanoporous structures. Specially, the acid-catalyzed silica system with micelles as template is a potential facile and low cost route to practical AR thin films with good mechanical strength [27,31]. As a result, various robust silica-based AR thin films have been prepared by this method [10,27,28]. Nevertheless, adsorption of water from air could seriously reduce the transmittance of those mesoporous AR thin films [32]. Therefore, it is still a big issue to obtain AR thin films simultaneously with favorable durability and mechanical robustness. Recently, an antireflective film was prepared with outstanding 6 H
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Fig. 1. Top-view (a) and cross-section (b) SEM and TEM (c and d) images of thin film. Inset in (c) is a selected area diffraction pattern. Histograms of pore size distributions (e and f) obtained from (c and d), respectively.
pencil hardness via crosslinking silica nanoparticles in the film by Si-OP as linkage, though its long-term humidity-resistance needs to be further investigated [33]. Very recently, we introduced methyl into AR thin films, resulting in favorable humidity-resistance and mechanical robustness [10]. This system is a compromise between humidity-resistance and mechanical robustness, and the mechanical robustness still need improvement for practical applications. Thus, it is still a significant issue to fabricate both robust and durable AR thin films. In this paper, we demonstrate an approach to fabrication of highperformance humidity-resistant AR thin films with reinforcing silicon phosphate centers from an acid-catalyzed hybrid silica sol. The thin films possess hierarchical nanopores, which are remarkably different from conventional single nanoporous thin films via acid-catalyzed silica sol with CTAB as template [10]. The hierarchically porous structure of thin films result in excellent broadband AR properties. The hydrophobic methyl groups present in the thin films effectively prevent water from
entering their mesopores. Moreover, the silicon phosphate centers forming in the film structure endow the thin films with extraordinary mechanical robustness.
2. Experimental Chemicals. Tetraethyl orthosilicate (TEOS, 98%) and methytrimethoxysilane (MTMS, 97%) were purchased from Alfa Aesar. Phosphoric acid (85%), Cetyltrimethyl ammonium bromide (CTAB), hydrochloric acid (38%), and absolute ethanol (99.5%) were obtained from Beihua Fine Chemicals. Ultrapure water with a resistivity higher than 18.2 MΩ•cm was used in all experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). Preparation of sol and thin film. Hybrid sols were synthesized as follows: MTMS/TEOS/HCl/H2O/EtOH with a molar ratio of 0.5:0.5:0.004-0.007:3-6:41 were mixed, and then 1.5–2.5 wt% CTAB 96
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FTIR) spectra were recorded on a Varian Excalibur 3100 spectrometer. X-ray photoelectron spectroscopy (XPS) analyses of samples were carried out on an ESCALab220i-XL. 3. Results and discussion 3.1. Morphology and structure of thin films The morphology and structure of thin films were characterized by SEM and TEM in detail. Top-view SEM image (Fig. 1a) shows that the overall thin film has a very dense and smooth surface. Cross-sectional SEM image (Fig. 1b) reveals that the thin film has a uniform thickness of 114 ± 3 nm. TEM images (Fig. 1c and d) further reveal the thin film has a hierarchically nanoporous structure. As shown in Fig. 1c and e, nanopores with diameter of ca. 13.4 nm are homogeneous dispersed in the thin film. A zoom-in image (Fig. 1d and f) of the thin film further reveals that countless nanopores with diameter of ca. 1.4 nm are closely packed in the framework of thin film. Moreover, the selected area diffraction pattern (Inset in Fig. 1c) indicates the thin film includes polycrystalline components, which may derive from the formation of polycrystalline silicon phosphate in the thin film [33–35]. The surface morphology of thin film was also investigated by AFM analyses. Fig. 2a and b show that various shallow pits with average depth of ca. 3.2 nm (a height profile is shown in Fig. 2c as example) are dispersed on the film surface. The Root-Mean-Square (RMS) roughness of the film surface was estimated to be ca. 0.7 nm from an area of 2×2 µm2. Apparently, the thin film has a super-smooth surface.
Fig. 2. (a) Two- and (b) three-dimensional AFM images of thin film (c) height profile along the orange line in (a).
and 1.5–4 wt% H3PO4 were added into the mixture followed by stirring at room temperature for 2–4 h. Then the sol was deposited on glass substrates at room temperature with a relative humidity of 15–40%. Finally, the as-prepared thin films were solidified at 60 °C, ultrasonically cleaned by water and calcined at 500 °C for 1 h. The control sample was prepared with a MTMS/TEOS molar ratio of 0.4/0.6, and is marked as M4 thin film. Characterization. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-4800 field emission scanning electron microscope operated at 5 kV. The thin films were coated with a layer of gold by ion sputtering before SEM observations. Transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Small pieces of thin film were scratched from substrate, dispersed in ethanol by sonication for 10 min, and added onto a carbon-coated copper grid. The TEM grid with thin film pieces was dried at 60 °C overnight before TEM observation. The morphology and roughness of thin films were characterized by atomic force microscopy (AFM) on an MM8-SYS scanning probe microscope (Bruker AXR). Transmission spectra were recorded using a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co.). Fourier transform infrared (ATR-
3.2. Chemical composition of hybrid thin film XPS was employed to analyze the chemical composition of thin film. As shown in Fig. 3a, O, C and Si are the main elements of the thin film, and possess a molar percentage of 45.8%, 28.0% and 25.3%, respectively. The P element has a molar percentage of 0.9% in the thin film. The inset curve in Fig. 3a shows the binding energy of P is 134.1 ± 0.1 eV, which corresponds to the binding energy of silicon phosphate [33,36]. Furthermore, FTIR was used to investigate a powder sample that had been prepared through the same process as the thin film. As shown in Fig. 3b, the peak appearing at 1250 cm−1 is ascribed to the symmetric deformation of Si–CH3, the peak at 1320 cm−1 arises from the stretching vibration of P˭O, the peak at 1113 cm−1 results from the combination of P–O stretching of P–O–P and P–O–Si bridging units, and the peaks at 1024 cm−1 and 802 cm−1 are attributed to the stretching vibrations of Si-O-Si. Hence, the XPS and FTIR analyses indicate the thin film consists of methyl-silica and silicon phosphate centers. As silica is amorphous, the polycrystalline composition (as deduced from the inset of Fig. 1c) should be the silicon
Fig. 3. (a) XPS spectra of thin film. (b) FTIR spectrum of powder sample prepared using the same process as the thin film.
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Fig. 4. (a) Transmission and reflection spectra of thin film on K9 glass in contrast to bare K9 glass. The insets are digital photos of K9 glass (left) and AR thin film on K9 glass (right) on a flower picture, respectively. (b) Transmission spectra of thin film on slide glass at different stages of preparation. Fig. 5. TEM images of thin film after calcination.
Fig. 6. (a) TG-DSC profiles of powder sample prepared in the same procedure as thin film. (b) Transmission spectra of thin film coated slide glass prepared from a MTMS/TEOS molar ratio of 0.5/0.5 before (spectrum 1) and after (spectrum 2) damp heat test in contrast to bare glass.
phosphate centers in the thin film.
transmittance of 92.3% (1099 nm) and an average transmittance of 92.0% (400–800 nm) and 92.1% (380–1100 nm). On the other hand, the thin film has low reflectance with a minimum reflectance of 0.8% (540 nm) and an average reflectance of 1.7% (400–800 nm) and 2.9% (380–1100 nm), in contrast to K9 glass with a minimum reflectance of 8.7% (750 nm) and an average reflectance of 9.1% (400–800 nm) and 9.0% (380–1100 nm). As the efficiency of solar cells depend on the incident light flux, the average transmittance enhancement of 6.8% (400–800 nm) and 5.5% (380–1100 nm) would surely improve the efficiency of solar cells with such thin films.
3.3. Optical properties of thin films The optical properties of thin film were investigated. As shown in Fig. 4a, the thin film exhibits excellent broadband AR properties in comparison with bare substrate. The thin film coated K9 glass has favorable transmittance with a maximum transmittance of 99.7% (537 nm) and an average transmittance of 98.8% (400–800 nm) and 97.6% (380–1100 nm), in contrast to K9 glass with a maximum 98
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transmittance of 90.4% (400–800 nm). After calcination, TEM images (Fig. 5) of the thin film reveals that worm-like mesopores are closely packed in the thin film, and the pore size was roughly estimated to be ca. 1.4 nm. According to the previous reports [10], CTAB micelles with size of ca. 2 nm formed in the thin films because of the evaporationinduced self-assembly when the sol was deposited on the substrate. The calcination process removed CTAB micelles from the thin film, resulting in the nanopores and thus antireflective effect. The washing treatment further led to an increase in transmittance. In contrast to the TEM images (Fig. 5) of thin film (with nanopores of ca. 1.4 nm) before washing, TEM images (Fig. 1c and d) show the film after washing has a hierarchical nanoporous structure. The nanopores with size of ca. 13.4 nm are dispersed in the thin film besides the smaller pores with size of 1.4 nm. Thus, the porosity of the thin film must have increased after removal by washing of components within the nanopores with diameter of ca. 13.4 nm. As a result, the transmittance of the thin film further increases to 97.3%.
Fig. 7. Transmission spectra of the M4 thin film on slide glass prepared from a MTMS/ TEOS molar ratio of 0.4/0.6 before and after damp heat test in contrast to bare glass.
3.4. Durability of thin films
Furthermore, the mechanism of high-performance antireflection was investigated in respect of the sequential stages of preparation. Transmission spectra of slide glass were recorded sequentially after: (1) depositing the sol on slide glass, (2) then calcining the thin film at 500 °C for 1 h and (3) finally washing the thin film by the mixture of water and ethanol (10:1, v/v). In other words, the transmission spectra correspond to the as-deposited thin film, the thin film after calcination and the final AR thin film, respectively. As shown in Fig. 4b, the transmittance of the thin film on substrate increases along with the preparation procedure. In detail, the average transmittance of coated slide glass increases from 91.7% to 94.6–97.3% in the range of 400–800 nm, in contrast to glass substrate with an average
Thermal stability is a significant aspect for AR thin films, which would influence their durability for practical applications, especially in solar cells. As the –CH3 group could generally be oxidized at high temperature in the environment, the thermal stability of ≡Si–CH3 in the thin film was investigated. Fig. 6a shows TG-DSC curves of the powder sample prepared in the same process as the thin film. The weight of sample underwent a 19% loss in the range of 200–343 °C, and exothermic peaks appeared in the range of 235–345 °C, which are believed to originate from decomposition of CTAB and dehydration between ≡Si–OH. The 2.1% weight loss in the range of 542–615 °C in the TG curve and the exothermic peak at 556–589 °C in the DSC curve are mainly ascribed to oxidation of methyl groups to CO2 and water,
Fig. 8. (a, b) SEM images of thin film after scratching with a 5H pencil. (c) SEM image of thin film after adhesion test. (d) Schematics of structure and chemical composition of thin film, the black arrows point to both large and small pores, which correspond to the TEM images of Fig. 1c and d.
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silica sol. The thin film has a hierarchically nanoporous structure with reinforcing silicon phosphate centers homogeneously dispersed in the film structure. The thin film shows favorable antireflective properties. When coated with the antireflective thin film, the maximum and average transmittances of K9 glass increase from 92.1% and 92.0% (400–800 nm) to 99.7% and 98.8% (400–800 nm), respectively. In addition, the thin film has extraordinary humidity-resistance and thermal stability, which are attributed to the good thermal stability of rich hydrophobic ≡Si-CH3 groups. Moreover, the thin film has an extraordinary 5H pencil hardness and 5A adhesion to substrate, because the silicon phosphate centers well cross-link the entire thin film. Hence, the current work provides a promising approach to the fabrication of AR thin films simultaneously with high broadband transmittance, function durability, thermal stability and high mechanical strength. These high-performance antireflective thin films are promising in solar energy utilization, especially in solar cells.
indicating that the ≡Si–CH3 groups are stable up to 556 °C. These results demonstrate that the ≡Si–CH3 groups have good thermal stability in the process of calcination at 500 °C for 1 h, and the thin film would be thermally stable for general applications. Humidity-resistance is also a crucial aspect of AR thin films for durable applications. In ambient environment, mesoporous AR thin films would generally adsorb moisture, leading to lowering of their porosity and transmittance. The adsorbed water molecules would also corrode the films at elevated temperatures, and thus influence their transmittance. Hence, it is necessary that the thin films have good humidity-resistance, which could be assessed by damp heat test (100 °C, 100%RH). Fig. 6b shows the transmission spectra of the thin film on slide glass before (spectrum 1) and after (spectrum 2) damp heat test in contrast to bare slide glass substrate. After damp heat test, the maximum transmittance (554 nm) of the thin film on slide glass decreases from 99.3% to 98.5% and the average transmittance (400–800 nm) decreases from 97.7% to 97.0%. The remaining excellent transmittance after the test and only slight decrease in transmittance demonstrate the excellent humidity-resistance of the thin film, suggesting durable application prospects. Moreover, the reason of this excellent humidity-resistance was discussed. As demonstrated previously [10], if the molar ratio of -CH3 in the film is less than the critical value, the AR thin film would not be humidity-resistant. In fact, when the molar ratio of -CH3/Si in the precursor decreases from 0.5 to 0.4, the obtained M4 thin film do not show humidity-resistance, whose average transmittance decreases from 97.3% to 90.3% after damp heat test (Fig. 7). The failure of M4 thin film in damp heat test demonstrates the thin film would not be humidity-resistant unless it has sufficient –CH3 groups.
Acknowledgements Financial supports are greatly appreciated from the National Natural Science Foundation of China (Grant Nos. 21571182, 21271177) and a Chinese Academy of Sciences Grant (CXJJ-14-M38). Thanks to Ms. Kaikai Wang for helping collect the digital photos. References [1] M.K. Hedayati, M. Elbahri, Antireflective coatings: conventional stacking layers and ultrathin plasmonic metasurfaces, Mini-Rev., Mater. 9 (2016) 1–22. [2] S. Kim, U.T. Jung, S.K. Kim, J.H. Lee, H.S. Choi, C.S. Kim, M.Y. Jeong, Nanostructured multifunctional surface with antireflective and antimicrobial characteristics, ACS Appl. Mater. Interfaces 7 (2015) 326–331. [3] X. Zhang, J. He, Antifogging antireflective thin films: does the antifogging layer have to be the outmost layer? Chem. Commun. 51 (2015) 12661–12664. [4] T. Li, J. He, L. Yao, Z. Geng, Robust antifogging antireflective coatings on polymer substrates by hydrochloric acid vapor treatment, J. Colloid Interface Sci. 444 (2015) 67–73. [5] E.E. Perl, W.E. McMahon, R.M. Farrell, S.P. DenBaars, J.S. Speck, J.E. Bowers, Surface structured optical coatings with near-perfect broadband and wide-angle antireflective properties, Nano Lett. 14 (2014) 5960–5964. [6] H.J. Gwon, Y. Park, C.W. Moon, S. Nahm, S.J. Yoon, S.Y. Kim, H.W. Jang, Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells, Nano Res. 7 (2014) 670–678. [7] K. Manabe, S. Nishizawa, K.H. Kyung, S. Shiratori, Optical phenomena and antifrosting property on biomimetics slippery fluid-infused antireflective films via layer-by-layer comparison with superhydrophobic and antireflective films, ACS Appl. Mater. Interfaces 6 (2014) 13985–13993. [8] J.D. Chen, L. Zhou, Q.D. Ou, Y.Q. Li, S. Shen, S.T. Lee, J.X. Tang, Enhanced light harvesting in organic solar cells featuring a biomimetic active layer and a selfcleaning antireflective coating, Adv. Energy Mater. 4 (2014) 1301777. [9] L. Yao, J. He, Recent progress in antireflection and self-cleaning technology – From surface engineering to functional surfaces, Progress. Mater. Sci. 61 (2014) 94–143. [10] T. Li, J. He, A facile hybrid approach to high-performance broadband antireflective thin films with humidity resistance as well as mechanical robustness, J. Mater. Chem. C 4 (2016) 5342–5348. [11] J.G. Cai, L.M. Qi, Recent advances in antireflective surfaces based on nanostructure arrays, Mater. Horiz. 2 (2015) 37–53. [12] S. Cai, Y. Zhang, H. Zhang, H. Yan, H. Lv, B. Jiang, Sol-gel preparation of hydrophobic silica antireflective coatings with low refractive index by base/acid two-step catalysis, ACS Appl. Mater. Interfaces 6 (2014) 11470–11475. [13] L.H. Yan, N. Liu, S.N. Zhao, H.W. Yan, H.B. Lu, X.D. Yuan, Effect of hydrophobic modification on the durability and environmental properties of porous MgF2 antirefiective films, Acta Metall. Sin.-Engl. Lett. 27 (2014) 649–655. [14] G. Zhan, H.C. Zeng, Integrated nanocatalysts with mesoporous silica/silicate and microporous MOF materials, Coord. Chem. Rev. 320–321 (2016) 181–192. [15] V. Malgras, Q. Ji, Y. Kamachi, T. Mori, F.-K. Shieh, K.C.W. Wu, K. Ariga, Y. Yamauchi, Templated synthesis for nanoarchitectured porous materials, Bull. Chem. Soc. Jpn. 88 (2015) 1171–1200. [16] W.R. Erwin, H.F. Zarick, E.M. Talbert, R. Bardhan, Light trapping in mesoporous solar cells with plasmonic nanostructures, Energ. Environ. Sci. 9 (2016) 1577–1601. [17] E. Yamamoto, K. Kuroda, Colloidal mesoporous silica nanoparticles, Bull. Chem. Soc. Jpn. 89 (2016) 501–539. [18] T. Aytug, J.T. Simpson, A.R. Lupini, R.M. Trejo, G.E. Jellison, I.N. Ivanov, S.J. Pennycook, D.A. Hillesheim, K.O. Winter, D.K. Christen, S.R. Hunter, J.A. Haynes, Optically transparent, mechanically durable, nanostructured superhydrophobic surfaces enabled by spinodally phase-separated glass thin films, Nanotechnology 24 (2013) 315602. [19] M. Toma, G. Loget, R.M. Corn, Flexible Teflon nanocone array surfaces with tunable superhydrophobicity for self-cleaning and aqueous droplet patterning, ACS Appl.
3.5. Mechanical properties of thin films As mechanical properties of thin films are a key factor for practical applications, pencil hardness test and adhesion test were used to access the mechanical properties of the thin film. The pencil hardness test was carried out according to the ASTM D3363-05 standard. Fig. 8a shows that the thin film has few scratches and the majority of the thin film nearly remained intact after 5H hardness test. The high magnification SEM image (Fig. 8b) of scratched areas revealed that only a minor part of the scratched area was deformed on the surface of the thin film. Thus, the thin film could withstand scratches by a 5H pencil, and has the 5H pencil hardness. The adhesion of thin film to substrate was assessed by the tape test (ASTM D3359 standard). According to the ASTM D3359 standard, an X-cut was made through the thin film; a 3M No. 710 pressure-sensitive tape was applied over the cut and then removed. Adhesion is usually assessed qualitatively on the 0 (worst) to 5A (best) scale. If no peeling or removal is observed, the thin film has the highest 5A grade adhesion. As shown in Fig. 8c, the edges of cuts were smooth and no pieces of the thin film were detached from the substrate after the tape test, indicating that the thin film, with 5A grade adhesion, has excellent adhesion to the substrate. The reason for the mechanical strength of the thin film must be discussed. In contrast to the methyl-silica AR thin film with 3H pencil hardness [10], the phosphoric methyl-silica thin film in this work shows better mechanical strength of 5H pencil hardness. The chemical composition analysis revealed that the silicon phosphate had formed in the thin film, while the methyl-silica AR thin film only contains the Si-O-Si compound. Hence, the polycrystalline silicon phosphate strengthened the thin film [34], (Fig. 8d) which is more favorable than films requiring post-strengthening or post-crosslinking [37–39]. 4. Conclusion In summary, we developed a novel and effective approach to fabricate mechanically robust, humidity-resistant, thermally stable highperformance antireflective thin film from an acid-catalyzed hybrid 100
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Mechanically robust, humidity-resistant, thermally stable high performance antireflective thin films with reinforcing silicon phosphate centers Tong Lia,b, Junhui Hea,
MARK
⁎
a Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Antireflection Robustness Durability Hierarchical nanopores Silicon phosphate
High performance, function durability, thermal stability and mechanical robustness have long been pursued for advanced antireflective thin films. In the current work, we developed a novel and effective approach to fabricate mechanically robust, humidity-resistant, thermally stable high-performance antireflective thin films with reinforcing silicon phosphate centers from an acid-catalyzed hybrid silica sol. The thin film has a hierarchically nanoporous structure, resulting in favorable antireflective properties. After being coated with the antireflective thin film, the maximum and average transmittances of K9 glass increase from 92.1% and 92.0% (400–800 nm) to 99.7% and 98.8% (400–800 nm), respectively. In addition, the thin film shows favorable humidity-resistance and heat-resistance, and these extraordinary performances are attributed to its rich heat-resistant and hydrophobic groups. Moreover, mechanical strength measurements indicate the thin film has extraordinary 5H pencil hardness and 5A adhesion to substrate because of the formation of silicon phosphate centers in the thin film, which significantly reinforce the thin film. These high-performance antireflective thin films are promising in solar energy utilization, especially in solar cells.
1. Introduction Light reflections from optical materials surfaces are undesirable in consequence of a loss of transmitted light. Antireflective (AR) thin films can reduce light reflections and have significant application prospects in solar cells, solar collectors, smart windows, display panels and lenses [1–8]. The principle of antireflection is the light interference between air and substrate. Theoretically, a homogeneous single-layer AR thin film with zero-reflection need a reflective index of (nans)1/2 and a thin film thickness of a quarter wavelength of incident light, where na, nc and ns are the refractive indices of air, thin film and substrate, respectively [9]. As the refractive indices of air and commercial glass are 1 and 1.52, respectively, the optimal refractive index of AR thin film should be ca. 1.23 [9–11]. However, the refractive indices of available substances are generally higher than 1.35 [9,12,13]. Thus, nanoporous and nanoarray structures are mainly used to obtain low refractive index thin films. To date, various approaches have been developed to obtain nanoporous [14–17] and nanoarray [9,11,18] materials with low refractive index, such as reactive ion etching (RIE) [5,19,20], acid or base chemical etching [21,22], nanoparticles deposition [23–26] and template
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Corresponding author. E-mail address: [email protected] (J. He).
http://dx.doi.org/10.1016/j.solmat.2017.05.068 Received 4 April 2017; Received in revised form 27 May 2017; Accepted 29 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
methods [27,28]. Although the RIE method generically provides a nanoarray structure surface, resulting in excellent broadband AR properties, it is restricted by its expensive and complex equipment as well as small-area productions and low outputs [5,19,29]. The acid or base chemical etching approaches represent top-down routes to preparation of AR thin films, but the thin film via the base etching is mechanically weak and the acid etching method is time-consuming [21,22]. The assembly of nanoparticles on substrates has been widely investigated because of its design flexibility of film structure, but the obtained films usually show weak robustness and need further enhancing their mechanical strength [30]. The template methods are one of the general approaches to nanoporous structures. Specially, the acid-catalyzed silica system with micelles as template is a potential facile and low cost route to practical AR thin films with good mechanical strength [27,31]. As a result, various robust silica-based AR thin films have been prepared by this method [10,27,28]. Nevertheless, adsorption of water from air could seriously reduce the transmittance of those mesoporous AR thin films [32]. Therefore, it is still a big issue to obtain AR thin films simultaneously with favorable durability and mechanical robustness. Recently, an antireflective film was prepared with outstanding 6 H
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Fig. 1. Top-view (a) and cross-section (b) SEM and TEM (c and d) images of thin film. Inset in (c) is a selected area diffraction pattern. Histograms of pore size distributions (e and f) obtained from (c and d), respectively.
pencil hardness via crosslinking silica nanoparticles in the film by Si-OP as linkage, though its long-term humidity-resistance needs to be further investigated [33]. Very recently, we introduced methyl into AR thin films, resulting in favorable humidity-resistance and mechanical robustness [10]. This system is a compromise between humidity-resistance and mechanical robustness, and the mechanical robustness still need improvement for practical applications. Thus, it is still a significant issue to fabricate both robust and durable AR thin films. In this paper, we demonstrate an approach to fabrication of highperformance humidity-resistant AR thin films with reinforcing silicon phosphate centers from an acid-catalyzed hybrid silica sol. The thin films possess hierarchical nanopores, which are remarkably different from conventional single nanoporous thin films via acid-catalyzed silica sol with CTAB as template [10]. The hierarchically porous structure of thin films result in excellent broadband AR properties. The hydrophobic methyl groups present in the thin films effectively prevent water from
entering their mesopores. Moreover, the silicon phosphate centers forming in the film structure endow the thin films with extraordinary mechanical robustness.
2. Experimental Chemicals. Tetraethyl orthosilicate (TEOS, 98%) and methytrimethoxysilane (MTMS, 97%) were purchased from Alfa Aesar. Phosphoric acid (85%), Cetyltrimethyl ammonium bromide (CTAB), hydrochloric acid (38%), and absolute ethanol (99.5%) were obtained from Beihua Fine Chemicals. Ultrapure water with a resistivity higher than 18.2 MΩ•cm was used in all experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). Preparation of sol and thin film. Hybrid sols were synthesized as follows: MTMS/TEOS/HCl/H2O/EtOH with a molar ratio of 0.5:0.5:0.004-0.007:3-6:41 were mixed, and then 1.5–2.5 wt% CTAB 96
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FTIR) spectra were recorded on a Varian Excalibur 3100 spectrometer. X-ray photoelectron spectroscopy (XPS) analyses of samples were carried out on an ESCALab220i-XL. 3. Results and discussion 3.1. Morphology and structure of thin films The morphology and structure of thin films were characterized by SEM and TEM in detail. Top-view SEM image (Fig. 1a) shows that the overall thin film has a very dense and smooth surface. Cross-sectional SEM image (Fig. 1b) reveals that the thin film has a uniform thickness of 114 ± 3 nm. TEM images (Fig. 1c and d) further reveal the thin film has a hierarchically nanoporous structure. As shown in Fig. 1c and e, nanopores with diameter of ca. 13.4 nm are homogeneous dispersed in the thin film. A zoom-in image (Fig. 1d and f) of the thin film further reveals that countless nanopores with diameter of ca. 1.4 nm are closely packed in the framework of thin film. Moreover, the selected area diffraction pattern (Inset in Fig. 1c) indicates the thin film includes polycrystalline components, which may derive from the formation of polycrystalline silicon phosphate in the thin film [33–35]. The surface morphology of thin film was also investigated by AFM analyses. Fig. 2a and b show that various shallow pits with average depth of ca. 3.2 nm (a height profile is shown in Fig. 2c as example) are dispersed on the film surface. The Root-Mean-Square (RMS) roughness of the film surface was estimated to be ca. 0.7 nm from an area of 2×2 µm2. Apparently, the thin film has a super-smooth surface.
Fig. 2. (a) Two- and (b) three-dimensional AFM images of thin film (c) height profile along the orange line in (a).
and 1.5–4 wt% H3PO4 were added into the mixture followed by stirring at room temperature for 2–4 h. Then the sol was deposited on glass substrates at room temperature with a relative humidity of 15–40%. Finally, the as-prepared thin films were solidified at 60 °C, ultrasonically cleaned by water and calcined at 500 °C for 1 h. The control sample was prepared with a MTMS/TEOS molar ratio of 0.4/0.6, and is marked as M4 thin film. Characterization. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-4800 field emission scanning electron microscope operated at 5 kV. The thin films were coated with a layer of gold by ion sputtering before SEM observations. Transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Small pieces of thin film were scratched from substrate, dispersed in ethanol by sonication for 10 min, and added onto a carbon-coated copper grid. The TEM grid with thin film pieces was dried at 60 °C overnight before TEM observation. The morphology and roughness of thin films were characterized by atomic force microscopy (AFM) on an MM8-SYS scanning probe microscope (Bruker AXR). Transmission spectra were recorded using a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co.). Fourier transform infrared (ATR-
3.2. Chemical composition of hybrid thin film XPS was employed to analyze the chemical composition of thin film. As shown in Fig. 3a, O, C and Si are the main elements of the thin film, and possess a molar percentage of 45.8%, 28.0% and 25.3%, respectively. The P element has a molar percentage of 0.9% in the thin film. The inset curve in Fig. 3a shows the binding energy of P is 134.1 ± 0.1 eV, which corresponds to the binding energy of silicon phosphate [33,36]. Furthermore, FTIR was used to investigate a powder sample that had been prepared through the same process as the thin film. As shown in Fig. 3b, the peak appearing at 1250 cm−1 is ascribed to the symmetric deformation of Si–CH3, the peak at 1320 cm−1 arises from the stretching vibration of P˭O, the peak at 1113 cm−1 results from the combination of P–O stretching of P–O–P and P–O–Si bridging units, and the peaks at 1024 cm−1 and 802 cm−1 are attributed to the stretching vibrations of Si-O-Si. Hence, the XPS and FTIR analyses indicate the thin film consists of methyl-silica and silicon phosphate centers. As silica is amorphous, the polycrystalline composition (as deduced from the inset of Fig. 1c) should be the silicon
Fig. 3. (a) XPS spectra of thin film. (b) FTIR spectrum of powder sample prepared using the same process as the thin film.
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Fig. 4. (a) Transmission and reflection spectra of thin film on K9 glass in contrast to bare K9 glass. The insets are digital photos of K9 glass (left) and AR thin film on K9 glass (right) on a flower picture, respectively. (b) Transmission spectra of thin film on slide glass at different stages of preparation. Fig. 5. TEM images of thin film after calcination.
Fig. 6. (a) TG-DSC profiles of powder sample prepared in the same procedure as thin film. (b) Transmission spectra of thin film coated slide glass prepared from a MTMS/TEOS molar ratio of 0.5/0.5 before (spectrum 1) and after (spectrum 2) damp heat test in contrast to bare glass.
phosphate centers in the thin film.
transmittance of 92.3% (1099 nm) and an average transmittance of 92.0% (400–800 nm) and 92.1% (380–1100 nm). On the other hand, the thin film has low reflectance with a minimum reflectance of 0.8% (540 nm) and an average reflectance of 1.7% (400–800 nm) and 2.9% (380–1100 nm), in contrast to K9 glass with a minimum reflectance of 8.7% (750 nm) and an average reflectance of 9.1% (400–800 nm) and 9.0% (380–1100 nm). As the efficiency of solar cells depend on the incident light flux, the average transmittance enhancement of 6.8% (400–800 nm) and 5.5% (380–1100 nm) would surely improve the efficiency of solar cells with such thin films.
3.3. Optical properties of thin films The optical properties of thin film were investigated. As shown in Fig. 4a, the thin film exhibits excellent broadband AR properties in comparison with bare substrate. The thin film coated K9 glass has favorable transmittance with a maximum transmittance of 99.7% (537 nm) and an average transmittance of 98.8% (400–800 nm) and 97.6% (380–1100 nm), in contrast to K9 glass with a maximum 98
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transmittance of 90.4% (400–800 nm). After calcination, TEM images (Fig. 5) of the thin film reveals that worm-like mesopores are closely packed in the thin film, and the pore size was roughly estimated to be ca. 1.4 nm. According to the previous reports [10], CTAB micelles with size of ca. 2 nm formed in the thin films because of the evaporationinduced self-assembly when the sol was deposited on the substrate. The calcination process removed CTAB micelles from the thin film, resulting in the nanopores and thus antireflective effect. The washing treatment further led to an increase in transmittance. In contrast to the TEM images (Fig. 5) of thin film (with nanopores of ca. 1.4 nm) before washing, TEM images (Fig. 1c and d) show the film after washing has a hierarchical nanoporous structure. The nanopores with size of ca. 13.4 nm are dispersed in the thin film besides the smaller pores with size of 1.4 nm. Thus, the porosity of the thin film must have increased after removal by washing of components within the nanopores with diameter of ca. 13.4 nm. As a result, the transmittance of the thin film further increases to 97.3%.
Fig. 7. Transmission spectra of the M4 thin film on slide glass prepared from a MTMS/ TEOS molar ratio of 0.4/0.6 before and after damp heat test in contrast to bare glass.
3.4. Durability of thin films
Furthermore, the mechanism of high-performance antireflection was investigated in respect of the sequential stages of preparation. Transmission spectra of slide glass were recorded sequentially after: (1) depositing the sol on slide glass, (2) then calcining the thin film at 500 °C for 1 h and (3) finally washing the thin film by the mixture of water and ethanol (10:1, v/v). In other words, the transmission spectra correspond to the as-deposited thin film, the thin film after calcination and the final AR thin film, respectively. As shown in Fig. 4b, the transmittance of the thin film on substrate increases along with the preparation procedure. In detail, the average transmittance of coated slide glass increases from 91.7% to 94.6–97.3% in the range of 400–800 nm, in contrast to glass substrate with an average
Thermal stability is a significant aspect for AR thin films, which would influence their durability for practical applications, especially in solar cells. As the –CH3 group could generally be oxidized at high temperature in the environment, the thermal stability of ≡Si–CH3 in the thin film was investigated. Fig. 6a shows TG-DSC curves of the powder sample prepared in the same process as the thin film. The weight of sample underwent a 19% loss in the range of 200–343 °C, and exothermic peaks appeared in the range of 235–345 °C, which are believed to originate from decomposition of CTAB and dehydration between ≡Si–OH. The 2.1% weight loss in the range of 542–615 °C in the TG curve and the exothermic peak at 556–589 °C in the DSC curve are mainly ascribed to oxidation of methyl groups to CO2 and water,
Fig. 8. (a, b) SEM images of thin film after scratching with a 5H pencil. (c) SEM image of thin film after adhesion test. (d) Schematics of structure and chemical composition of thin film, the black arrows point to both large and small pores, which correspond to the TEM images of Fig. 1c and d.
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silica sol. The thin film has a hierarchically nanoporous structure with reinforcing silicon phosphate centers homogeneously dispersed in the film structure. The thin film shows favorable antireflective properties. When coated with the antireflective thin film, the maximum and average transmittances of K9 glass increase from 92.1% and 92.0% (400–800 nm) to 99.7% and 98.8% (400–800 nm), respectively. In addition, the thin film has extraordinary humidity-resistance and thermal stability, which are attributed to the good thermal stability of rich hydrophobic ≡Si-CH3 groups. Moreover, the thin film has an extraordinary 5H pencil hardness and 5A adhesion to substrate, because the silicon phosphate centers well cross-link the entire thin film. Hence, the current work provides a promising approach to the fabrication of AR thin films simultaneously with high broadband transmittance, function durability, thermal stability and high mechanical strength. These high-performance antireflective thin films are promising in solar energy utilization, especially in solar cells.
indicating that the ≡Si–CH3 groups are stable up to 556 °C. These results demonstrate that the ≡Si–CH3 groups have good thermal stability in the process of calcination at 500 °C for 1 h, and the thin film would be thermally stable for general applications. Humidity-resistance is also a crucial aspect of AR thin films for durable applications. In ambient environment, mesoporous AR thin films would generally adsorb moisture, leading to lowering of their porosity and transmittance. The adsorbed water molecules would also corrode the films at elevated temperatures, and thus influence their transmittance. Hence, it is necessary that the thin films have good humidity-resistance, which could be assessed by damp heat test (100 °C, 100%RH). Fig. 6b shows the transmission spectra of the thin film on slide glass before (spectrum 1) and after (spectrum 2) damp heat test in contrast to bare slide glass substrate. After damp heat test, the maximum transmittance (554 nm) of the thin film on slide glass decreases from 99.3% to 98.5% and the average transmittance (400–800 nm) decreases from 97.7% to 97.0%. The remaining excellent transmittance after the test and only slight decrease in transmittance demonstrate the excellent humidity-resistance of the thin film, suggesting durable application prospects. Moreover, the reason of this excellent humidity-resistance was discussed. As demonstrated previously [10], if the molar ratio of -CH3 in the film is less than the critical value, the AR thin film would not be humidity-resistant. In fact, when the molar ratio of -CH3/Si in the precursor decreases from 0.5 to 0.4, the obtained M4 thin film do not show humidity-resistance, whose average transmittance decreases from 97.3% to 90.3% after damp heat test (Fig. 7). The failure of M4 thin film in damp heat test demonstrates the thin film would not be humidity-resistant unless it has sufficient –CH3 groups.
Acknowledgements Financial supports are greatly appreciated from the National Natural Science Foundation of China (Grant Nos. 21571182, 21271177) and a Chinese Academy of Sciences Grant (CXJJ-14-M38). Thanks to Ms. Kaikai Wang for helping collect the digital photos. References [1] M.K. Hedayati, M. Elbahri, Antireflective coatings: conventional stacking layers and ultrathin plasmonic metasurfaces, Mini-Rev., Mater. 9 (2016) 1–22. [2] S. Kim, U.T. Jung, S.K. Kim, J.H. Lee, H.S. Choi, C.S. Kim, M.Y. Jeong, Nanostructured multifunctional surface with antireflective and antimicrobial characteristics, ACS Appl. Mater. Interfaces 7 (2015) 326–331. [3] X. Zhang, J. He, Antifogging antireflective thin films: does the antifogging layer have to be the outmost layer? Chem. Commun. 51 (2015) 12661–12664. [4] T. Li, J. He, L. Yao, Z. Geng, Robust antifogging antireflective coatings on polymer substrates by hydrochloric acid vapor treatment, J. Colloid Interface Sci. 444 (2015) 67–73. [5] E.E. Perl, W.E. McMahon, R.M. Farrell, S.P. DenBaars, J.S. Speck, J.E. Bowers, Surface structured optical coatings with near-perfect broadband and wide-angle antireflective properties, Nano Lett. 14 (2014) 5960–5964. [6] H.J. Gwon, Y. Park, C.W. Moon, S. Nahm, S.J. Yoon, S.Y. Kim, H.W. Jang, Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells, Nano Res. 7 (2014) 670–678. [7] K. Manabe, S. Nishizawa, K.H. Kyung, S. Shiratori, Optical phenomena and antifrosting property on biomimetics slippery fluid-infused antireflective films via layer-by-layer comparison with superhydrophobic and antireflective films, ACS Appl. Mater. Interfaces 6 (2014) 13985–13993. [8] J.D. Chen, L. Zhou, Q.D. Ou, Y.Q. Li, S. Shen, S.T. Lee, J.X. Tang, Enhanced light harvesting in organic solar cells featuring a biomimetic active layer and a selfcleaning antireflective coating, Adv. Energy Mater. 4 (2014) 1301777. [9] L. Yao, J. He, Recent progress in antireflection and self-cleaning technology – From surface engineering to functional surfaces, Progress. Mater. Sci. 61 (2014) 94–143. [10] T. Li, J. He, A facile hybrid approach to high-performance broadband antireflective thin films with humidity resistance as well as mechanical robustness, J. Mater. Chem. C 4 (2016) 5342–5348. [11] J.G. Cai, L.M. Qi, Recent advances in antireflective surfaces based on nanostructure arrays, Mater. Horiz. 2 (2015) 37–53. [12] S. Cai, Y. Zhang, H. Zhang, H. Yan, H. Lv, B. Jiang, Sol-gel preparation of hydrophobic silica antireflective coatings with low refractive index by base/acid two-step catalysis, ACS Appl. Mater. Interfaces 6 (2014) 11470–11475. [13] L.H. Yan, N. Liu, S.N. Zhao, H.W. Yan, H.B. Lu, X.D. Yuan, Effect of hydrophobic modification on the durability and environmental properties of porous MgF2 antirefiective films, Acta Metall. Sin.-Engl. Lett. 27 (2014) 649–655. [14] G. Zhan, H.C. Zeng, Integrated nanocatalysts with mesoporous silica/silicate and microporous MOF materials, Coord. Chem. Rev. 320–321 (2016) 181–192. [15] V. Malgras, Q. Ji, Y. Kamachi, T. Mori, F.-K. Shieh, K.C.W. Wu, K. Ariga, Y. Yamauchi, Templated synthesis for nanoarchitectured porous materials, Bull. Chem. Soc. Jpn. 88 (2015) 1171–1200. [16] W.R. Erwin, H.F. Zarick, E.M. Talbert, R. Bardhan, Light trapping in mesoporous solar cells with plasmonic nanostructures, Energ. Environ. Sci. 9 (2016) 1577–1601. [17] E. Yamamoto, K. Kuroda, Colloidal mesoporous silica nanoparticles, Bull. Chem. Soc. Jpn. 89 (2016) 501–539. [18] T. Aytug, J.T. Simpson, A.R. Lupini, R.M. Trejo, G.E. Jellison, I.N. Ivanov, S.J. Pennycook, D.A. Hillesheim, K.O. Winter, D.K. Christen, S.R. Hunter, J.A. Haynes, Optically transparent, mechanically durable, nanostructured superhydrophobic surfaces enabled by spinodally phase-separated glass thin films, Nanotechnology 24 (2013) 315602. [19] M. Toma, G. Loget, R.M. Corn, Flexible Teflon nanocone array surfaces with tunable superhydrophobicity for self-cleaning and aqueous droplet patterning, ACS Appl.
3.5. Mechanical properties of thin films As mechanical properties of thin films are a key factor for practical applications, pencil hardness test and adhesion test were used to access the mechanical properties of the thin film. The pencil hardness test was carried out according to the ASTM D3363-05 standard. Fig. 8a shows that the thin film has few scratches and the majority of the thin film nearly remained intact after 5H hardness test. The high magnification SEM image (Fig. 8b) of scratched areas revealed that only a minor part of the scratched area was deformed on the surface of the thin film. Thus, the thin film could withstand scratches by a 5H pencil, and has the 5H pencil hardness. The adhesion of thin film to substrate was assessed by the tape test (ASTM D3359 standard). According to the ASTM D3359 standard, an X-cut was made through the thin film; a 3M No. 710 pressure-sensitive tape was applied over the cut and then removed. Adhesion is usually assessed qualitatively on the 0 (worst) to 5A (best) scale. If no peeling or removal is observed, the thin film has the highest 5A grade adhesion. As shown in Fig. 8c, the edges of cuts were smooth and no pieces of the thin film were detached from the substrate after the tape test, indicating that the thin film, with 5A grade adhesion, has excellent adhesion to the substrate. The reason for the mechanical strength of the thin film must be discussed. In contrast to the methyl-silica AR thin film with 3H pencil hardness [10], the phosphoric methyl-silica thin film in this work shows better mechanical strength of 5H pencil hardness. The chemical composition analysis revealed that the silicon phosphate had formed in the thin film, while the methyl-silica AR thin film only contains the Si-O-Si compound. Hence, the polycrystalline silicon phosphate strengthened the thin film [34], (Fig. 8d) which is more favorable than films requiring post-strengthening or post-crosslinking [37–39]. 4. Conclusion In summary, we developed a novel and effective approach to fabricate mechanically robust, humidity-resistant, thermally stable highperformance antireflective thin film from an acid-catalyzed hybrid 100
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