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Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface
Accepted Manuscript Title: Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface Authors: Bing Wang, Yinqun Hua, Yunxia Ye, Ruifang Chen, Zhibao Li PII: DOI: Reference:
S0169-4332(17)32170-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.169 APSUSC 36703
To appear in:
APSUSC
Received date: Revised date: Accepted date:
8-4-2017 14-7-2017 19-7-2017
Please cite this article as: Bing Wang, Yinqun Hua, Yunxia Ye, Ruifang Chen, Zhibao Li, Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.169 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface
Bing Wanga, Yinqun Huaa,b,*, Yunxia Yeb, Ruifang Chenb, Zhibao Lia
a
School of Material and Science Engineering, Jiangsu University, Zhenjiang 212013, China
b
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China
Correspondence Email: [email protected]
Graphical abstract
Highlights
Superhydrophobic solar glass was efficiently fabricated using a picosecond laser.
Transformation from superhydrophilicity to superhydrophobicity.
With the increase of laser line interval, the contact angles of sample surfaces ranged from 172° to 156°.
The average transmittance of the superhydrophobic glass is up to 87.28% in the visible spectrum.
Abstract
The contact angle for chloroform on the microstructured glass is 163° underwater.
In this paper, we fabricate superhydrophobic solar glass by microstructuring of the surface
using picosecond laser pulses and then coating the surface with a layer of fluoroalkylsilane molecules. The glass surface shows the hierarchical structures containing both micro- and nanoscale roughness after laser irradiation. The increase in glass surface roughness makes the surface exhibit superhydrophilicity with a water contact angle of 0°. After surface modification with 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane, the glass surface exhibits stable superhydrophobicity. By varying the laser line intervals, we can regulate the surface morphology and the wetting properties. With the increase of laser line interval, the contact angles of the glass surfaces range from 172° ± 1° to 156° ± 2° and the sliding angles can be ignored. Before chemical surface modification, we also find that the glass surface of hierarchical structures is superoleophobic underwater. The contact angle for chloroform is 163° ± 2° and the sliding angle is 1.5° ± 0.4°. In addition, the as-prepared glass surface shows excellent transparency and its average transmittance is up to 87.28% in the visible spectrum. Furthermore, we find that the water droplet on the modified original glass surface looks like a lens which has zoom function. Keywords: picosecond laser; superhydrophobic solar glass; groove-shaped arrays; superoleophobic; contact angle; wettability transformation 1
Introduction Wettability of material surface is an important property governed by both its surface chemistry and
micro/nano-structures [1-6]. Research shows that the contact angle higher than 120° has never been achieved by lowering the surface energy of a smooth and flat substrate [7]. Wettability is distinguished through contact angle value, and it is generally considered that a surface is called hydrophilic when the contact angle on the solid surface is lower than 90°. If the contact angle is larger than 90° but smaller than 150°, the surface is called hydrophobic. When the contact angle on the solid surface is higher than 150° and the sliding angle is lower than 5°, the surface is called superhydrophobic [8,9], whereas the surface is called superhydrophilic when the contact angle on the surface is lower than 10°. Superhydrophobicity is an important property for water-repellent [10], self-cleaning [1,11,12], drug release [13,14], anti-icing [15] and anti-corrosion [16,17], etc. Generally speaking, there are two ways for the preparation of superhydrophobic material. Considering the surface chemistry, it is well known that wetting behavior of solid surface is determined by its outermost molecular-level structure [18,19], which is one way to control the wettability of material surface. The other way to change the wettability is to fabricate microstructure on material surface [2].
The surface with micro/nano-structures can be fabricated by many techniques, such as hydrothermal method [20], anodization method [21], sol-gel [22,23], laser etching [24-26], laser interference texturing [27-29], chemical bath deposition [30]. The traditional methods of fabricating micro/nano-structures on the glass surface include mechanical etching, lithography [31], coating technology [32], plasma etching [33]. However, these methods are still limited by complicated preparation process, expensive instrument, and vacuum environment. In comparison, laser fabrication technique has many advantages such as noncontact, short action time and ultrahigh peak power. The mechanism of ultrashort pulse ablating transparent materials has been studied by many researchers.
Picosecond laser ablates transparent
material, which is a complex process. In general, it is difficult for transparent material to produce linear absorption of the picosecond laser pulse energy. The nonlinear ionization mechanisms of transparent material mainly include avalanche ionization [34,35] and multiphoton ionization [36,37]. When a single pulse irradiates on the glass surface, the avalanche ionization occurs due to the nonlinear absorption of material surface. The formation of high density plasma is the main cause of material ablation. Ablated material is removed away, which leads to the permanent damage of material surface [38]. In this paper, we reported a novel, simple and easily-controlled method to fabricate a superhydrophobic glass surface by microstructuring of the surface using a picosecond laser and further chemical modification to lower the surface energy. The influence of different laser line intervals on the surface morphology and wettability of the glass was studied. The mechanism of superhydrophobicity was also discussed in this work. 2
Experimental
2.1 Materials Solar glass (20 mm × 20 mm × 2 mm) with refractive index of 1.51 and transmittance of 91.03% at 1064 nm was purchased from Wuhan Chaofeng Glass Co., Ltd. 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane (FAS-17), toluene, acetone, ethanol, chloroform and dichloromethane were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Nitrogen (N2) gas was purchased from Suzhou Jinhong Gas Co., Ltd. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane was used as low surface energy material and solar glass was used as substrate. The glass substrate was cleaned with toluene, acetone, ethanol sequentially for 30 min in an ultrasonic cleaner, and then dried in the stream of N2.
2.2 Preparation of superhydrophobic solar glass surface An Edgewave laser with an amplified Nd: YVO4 laser system (λ = 1064 nm) delivering ν = 10 ps pulses at a repetition rate of 400 kHz was used. The output power used is 14 W. The average pulse energy is 35 μJ (the calculated laser fluence is 11.15 J/cm2) and the spot size is 20 μm. The laser beam was shone on each sample surface fifty times at a scanning speed of 300 mm/s. In order to fabricate periodic microgrooves, the laser beam was focused vertically on the top of the glass surface as depicted in Fig. 1(a). An x-y scanning galvanometer (Scanlab, hurry SCANІІ 14) was used to control the laser beam over the glass surface in a pattern of lines in atmospheric environment. In order to induce a gradual evolution of the surface topography and wettability, six samples were irradiated with different laser line intervals. The picosecond laser beam was shone on sample surfaces periodically with a scanning step, i.e., period of 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, respectively. The periodic microgrooves were fabricated throughout the sample surface of 10 mm × 10 mm area. Before emeasuring th contact angle on the glass surface, the samples were cleaned in ultrasonic bath inside deionized water for 1 h and then placed inside plasma cleaner (SD-PL-2000, China) to remove dust or other impurities. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane and ethanol with a mass ratio 1:50 were added in mixed solvent under stirring at ambient temperature for 1 h. The samples were immersed into fluoroalkylsilane solution at 60 °C for 2 h and dried in a drying chamber at 120 °C for 60 min. 2.3 Characterization The morphology of the glass surface after laser irradiation was characterized by using field emission scanning electron microscope (FE-SEM) (JEOL, JSM-7001F) and three dimensional (3D) laser scanning microscope (VHX-1000, Keyence, Japan). The chemical features were determined by energy dispersive spectrometer (EDS, X-MaxN20, Oxford), X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250, America) and fourier transform infrared spectrometer (Nicolet Corporation, Nicolet Is50, America), respectively. Contact angles and sliding angles were measured by using a dynamic contact angle measuring device (Dataphysics Corporation, DCAT11, Germany) under ambient temperature. Deionized water droplets of about 5 μL were carefully dropped onto the sample surface through a syringe. All the contact angles were measured at five different positions. The transmittance of the sample was measured by UV-Vis-NIT spectrophotometer (UV-3600Plus, Japan). 3
Results and discussion
The SEM images of sample surfaces irradiated by picosecond laser at different laser line intervals are illustrated in Fig. 1(b). The SEM images and 3D morphology of the microstructured glass surface (at laser line interval of 60 μm) are illustrated in Fig. 2. As seen in Fig. 2(a) and Fig. 2(d), the treated surface has multiple parallel microgrooves with a period of 60 μm that corresponds to the horizontal step between two vertical scanning lines. Magnified SEM images in Fig. 2(b) and (c) show that extensive nano- and fine micro-structures superimpose onto both the valleys and ridges of the microgrooves. Closer examination of the microgrooves (Fig. 2b) reveals them to be rough-sided, V-shaped features. Fig. 2(b) also shows that the profile of microgrooves has extensive nanostructures. The nanostructures formed are in the shape of nanoprotrusions and nanocavities. In addition, the height of the nanoprotrusions is about 300 nm. From Fig. 2(c), it can be seen that the slag on the glass surface is very loose and has many nano holes. This hierarchical surface structures containing both micro- and nanoscale roughness can store more air and form a layer of air cushion between the liquid and the glass substrate. Fig. 2(d) shows the 3D morphology corresponding to Fig. 2(a) and Fig. 2(e) shows the profiles along the solid line in Fig. 2(d). As seen in Fig. 2(e), the depth of the microgrooves is about 26 μm. The contact angles of the original glass surface and the microstructured glass surface were measured separately. Fig. 3(a) shows that the contact angle on the original glass surface is 57°, indicating the original glass to be a hydrophilic material. After modification with 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane, the water contact angle on the glass surface increases to 92° (Fig. 3b). While after laser irradiation, all the glass surfaces exhibit superhydrophilicity with the contact angle of 0° (Fig. 3c). Compared with original glass surface, increasing the glass surface roughness makes the surface hydrophilicity increment [20]. The most fundamental reason for superhydrophilicity is the capillary effect [39]. When a water droplet contacts the surface of the groove-shaped arrays, water penetrates into the gaps of two adjacent pillars. Consequently, the water droplet can spread rapidly. On the other hand, surface chemical composition is another important factor to regulate surface wettability. It is well known that a solid surface is readily wetted when the solid free energy value is close to the liquid free energy [40,41]. After modification with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane, the water contact angles of the sample surfaces change with the laser line interval due to their different surface morphologies. At laser line interval of 50 μm and 75 μm, the contact angles are 172° ± 1° and 156° ± 2° (Fig. 3d-e), respectively. The result is consistent with the previous findings that the micro/nanoarchitectures of glass with low surface energy prepared by hydrothermal method exhibited
superhydrophobicity [20]. This hierarchical structure of material surface shows that superhydrophilic and superhydrophobic properties depend on the surface chemical composition [20,33,39]. Thus, a conclusion can be drawn that the superhydrophobicity of material surface is governed by both its chemical composition and geometrical structure. Fig. 3(f) shows an optical image of water droplets at different locations on the superhydrophobic glass surface, which indicates uniform surface superhydrophobicity. The volume of each water-drop is about 15 μL. According to Arkles theory [42], when fluoroalkylsilane contacts with glass, it first hydrolyzes to form siloxane molecules. Then dehydration reaction has occurred between siloxane and Si-OH groups of the glass substrate. The specific reaction process is shown in Fig. 4. Fig. 5(a) shows the FTIR spectrum of glass surface. The peaks were identified at 1251cm-1, 1096cm-1, 900cm-1 and 755cm-1 corresponding to C-F stretching mode, Si-O-Si antisymmetric stretching mode, Si-O bending vibration mode, Si-O symmetric stretching mode, respectively. EDS analysis was used to further investigate the composition of the surface. As shown in Fig. 5(b), the strong peaks corresponding to Si, O, Na, Mg, Ca and C could be clearly observed. Meanwhile, the presence of F in the modified glass surface indicates that the fluoroalkylsilane was deposited on the glass surface. The surface composition and chemical bonds were further investigated by X-ray photoelectron spectroscopy (XPS) and the results are shown in Fig. 5(c) and (d). Fig. 5(c) shows that the surface composition of the glass hasn’t changed after laser irradiation. The wide-scan spectrum of the modified glass surface is mainly composed of F 1s, O 1s, and C 1s, Si 2p signals (Fig. 5c). Compared with original and microstructured glass surface, an evident peak located at 688.5 eV is ascribed to the core level XPS spectrum of F 1s, which indicates the existence of fluorine species in the modified glass surface [43]. Fig. 5(d) shows that the C 1s core-level spectrum of original and microstructured glass surfaces can be curve-fitted into one peak component with binding energies (BE) at 284.8. However, the C 1s core-level spectrum of modified glass surface can be curve-fitted into three peak components with binding energies (BE) at 284.8, 291.5 and 293.8 eV, attributable to C-H, C-CF2 and C-CF3 species, respectively [44-46]. The characteristic peaks of C-CF2 and C-CF3 are consistent with the previous studies [39, 43-45]. Through the above analysis, the results indicate that 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane has been successfully deposited on the microstructured glass surface after chemical treatment. Fig. 3(c) shows that the glass surface exhibits superhydrophilicity with the contact angle of 0° after laser irradiation. After modification with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane, the glass surface
exhibits stable superhydrophobicity (Fig. 3d). However, the water contact angle on the modified original glass surface just increases to 92°. Thus, it can be concluded that the combination of low surface energy of fluoroalkyl silanes and micro/nano-structures leads to the superhydrophobicity. Transparency is another important property for the use of superhydrophobic glass surface. In order to detect the transparency of the as-prepared glass, we took the photograph of the sample above a picture to assess the transparency of the microstructured glass, which is illustrated in Fig. 6(a). Although the picture under the microstructured glass is a little fuzzy compared to original glass surface, still we can see the picture clearly. We have further measured the transmittance of the as-prepared glass surface using UV-Vis-NIT spectrophotometer, as illustrated in Fig. 6(b). The transmittance of the as-prepared glass surface in the visible spectrum was over 87% at the laser line interval of 75 μm, which decreased by 3% compared with the original glass. With the laser line interval decreased from 65 μm to 55 μm, the transmittance of the glass surface decreased accordingly, from 84.59% to 78.62%. We also found the microstructured glass is superoleophobic underwater (Fig. 7a). Blue boundary indicates a chloroform droplet on the microstructured glass surface in water. The contact angles for chloroform and dichloromethane (chloroform and dichloromethane droplets of about 8 μL) on the microstructured glass surface underwater were 163° ± 2° and 158° ± 2° (Fig. 7b), respectively. In addition, we found that water droplet on the modified original glass surface looks like a lens which has zoom function. Fig. 7(c) shows that water droplet on the original glass surface doesn’t has zoom function, while water droplet on the modified original glass surface can magnify the picture (Fig. 7d). As for the details of contact angle hysteresis, five states are possible for superhydrophobic surfaces: Wenzel’s state, Cassie’s state, the so-called “Lotus” state, the transitional state between Wenzel’s and Cassie’s states, and the “Gecko” state [47]. Based on the research results on the “lotus effect”, the wettability of solid surfaces mainly depends on two factors: surface chemical composition and structural roughness [48]. The superhydrophobic glass surface as we prepared shows the hierarchical structures containing both micro- and nanoscale roughness. The contact angle on the as-prepared glass surface is higher than 150° and the sliding angle can be ignored. Apparently, the as-prepared glass surface is in “Lotus” state (Fig. 8a) [47]. In composite wetting state, the area fraction of micro-pillar on the surface is defined as follows [26,38]:
Ss
f1
al
(a b ) l
Sp
a
(1)
ab
While for a nano-pillar on a micro-meter pillar in Fig. 8(b), the area fraction of nano-pillar on the surface can be written as [48]: f2
Ss '
Sp '
where
l
a1 l ( a 1 b1 ) l
a1 a 1 b1
,
a1
(2)
a 1 b1
is the length of the contact area.
For the dual-scale roughness 2-D pillar texture structures, the solid-liquid contact area fraction (
fs
) of
the substrate can be generally expressed as [48,49]: f s f1 f 2
a a b
a1 a 1 b1
a
(3)
a b
According to Cassie-Baxter’s equation [50], the static apparent contact angle can be obtained as follows: cos
ca
a a b
(1 c o s s ) 1
(4)
Defining a surface characteristic value, periodic space
A ( A b / a)
, we can obtain the following
equations:
ca
a a rc c o s (1 c o s s ) 1 a rc c o s a b
1 (1 c o s s ) 1 1 A
(5)
Since we fabricated groove-shaped arrays on the glass surface using the same pulse energy in the experiment,
f2
can be treated as an invariant constant. In composite wetting state,
the increase of laser line interval, but
ca
decreases as the
fs
fs
increases with
increases. Therefore, the contact angle
decreases with increasing of laser line interval. In the experiment, the contact angles of the glass surfaces ranged from 172° ± 1° to 156° ± 2° with the increase of laser line interval. This variation tendency is consistent with the Eq. (5). It has been reported that a hierarchical surface structure containing both micro- and nanoscale roughness elements is the optimum surface morphology to achieve superhydrophobicity [51,52]. The micro-groove arrays are composed of microscale pillars and nanoscale roughness features on their surfaces, which is favorable to obtain a superhydrophobic surface [39]. 4
Conclusions In summary, we fabricated superhydrophobic solar glass surface by microstructuring using a
picosecond laser and coating the surface with a layer of fluoroalkylsilane molecules. After laser irradiation, the hierarchical structures containing both micro- and nanoscale roughness were formed on the glass surface. We also investigated the influence of laser line interval on the static and dynamic wetting properties of the glass surface. By varying the laser line intervals, we obtained the maximal water contact angle of 172° ± 1°. In addition, the as-prepared glass shows excellent transparency and the
average transmittance of the as-prepared glass surface is higher than 87% in the visible spectrum. We also measured the static and dynamic contact angles for chloroform and dichloromethane on the microstructured glass underwater. The laser-assisted technique that we proposed is a simple and easilycontrolled method for fabricating glass-based superhydrophobic surface.
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Appendices
Fig. 1. (a) Picosecond laser processing system. (b) SEM images of the periodic microgrooves fabricated on solar glass surface at different laser line intervals.
Fig. 2. SEM images and 3D morphology of the microstructured glass surface (at laser line interval of 60 μm). (a) Periodic microgrooves; (b) Micrograph of microgroove and profile of the microgroove; (c) Micrograph of slag on the glass surface; (d) 3D morphology corresponding to Fig. 2(a); (e) Profiles along the solid line in Fig. 2(d).
Fig. 3. Water contact angles of glass surfaces: (a) Original glass surface; (b) Modified original glass surface; (c) Microstructured glass surface; (d) Microstructured glass surface after silanization (at laser line interval of 50 μm); (e) Microstructured sample surface after silanization (at laser line interval of 75 μm); (f) Photograph of water droplets on the superhydrophobic glass surface; (g) Contact angles on the as-prepared glass surface as a function of corresponding laser line intervals.
Fig. 4. Reaction of the fluoroalkylsilane and the glass substrate.
Fig. 5. (a) FTIR spectrum and (b) EDS spectra of the glass surface. (c) Widen scan of the glass surface. (d) C 1s core-level spectra of the glass surface.
Fig. 6. Transparency of the microstructured sample. (a) Photograph of the microstructured sample (at laser line interval of 75 μm) above a printed picture (blue boundary indicates the microstructured sample area, whereas the remaining part is the unstructured sample area); (b) Transmittance of the samples.
Fig. 7. (a) Optical image of chloroform droplet on the microstructured glass surface in water; (b) The contact angles for chloroform (top) and dichloromethane (down) on the microstructured glass surface underwater; (c) Photograph of water droplet on the original glass surface; (d) Photograph of water droplet on the modified original glass surface.
Fig. 8. (a) Noncomposite and composite (the “Lotus” state) wetting states; (b) geometrical parameters.
S0169-4332(17)32170-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.169 APSUSC 36703
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Please cite this article as: Bing Wang, Yinqun Hua, Yunxia Ye, Ruifang Chen, Zhibao Li, Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.169 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Transparent superhydrophobic solar glass prepared by fabricating groove-shaped arrays on the surface
Bing Wanga, Yinqun Huaa,b,*, Yunxia Yeb, Ruifang Chenb, Zhibao Lia
a
School of Material and Science Engineering, Jiangsu University, Zhenjiang 212013, China
b
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China
Correspondence Email: [email protected]
Graphical abstract
Highlights
Superhydrophobic solar glass was efficiently fabricated using a picosecond laser.
Transformation from superhydrophilicity to superhydrophobicity.
With the increase of laser line interval, the contact angles of sample surfaces ranged from 172° to 156°.
The average transmittance of the superhydrophobic glass is up to 87.28% in the visible spectrum.
Abstract
The contact angle for chloroform on the microstructured glass is 163° underwater.
In this paper, we fabricate superhydrophobic solar glass by microstructuring of the surface
using picosecond laser pulses and then coating the surface with a layer of fluoroalkylsilane molecules. The glass surface shows the hierarchical structures containing both micro- and nanoscale roughness after laser irradiation. The increase in glass surface roughness makes the surface exhibit superhydrophilicity with a water contact angle of 0°. After surface modification with 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane, the glass surface exhibits stable superhydrophobicity. By varying the laser line intervals, we can regulate the surface morphology and the wetting properties. With the increase of laser line interval, the contact angles of the glass surfaces range from 172° ± 1° to 156° ± 2° and the sliding angles can be ignored. Before chemical surface modification, we also find that the glass surface of hierarchical structures is superoleophobic underwater. The contact angle for chloroform is 163° ± 2° and the sliding angle is 1.5° ± 0.4°. In addition, the as-prepared glass surface shows excellent transparency and its average transmittance is up to 87.28% in the visible spectrum. Furthermore, we find that the water droplet on the modified original glass surface looks like a lens which has zoom function. Keywords: picosecond laser; superhydrophobic solar glass; groove-shaped arrays; superoleophobic; contact angle; wettability transformation 1
Introduction Wettability of material surface is an important property governed by both its surface chemistry and
micro/nano-structures [1-6]. Research shows that the contact angle higher than 120° has never been achieved by lowering the surface energy of a smooth and flat substrate [7]. Wettability is distinguished through contact angle value, and it is generally considered that a surface is called hydrophilic when the contact angle on the solid surface is lower than 90°. If the contact angle is larger than 90° but smaller than 150°, the surface is called hydrophobic. When the contact angle on the solid surface is higher than 150° and the sliding angle is lower than 5°, the surface is called superhydrophobic [8,9], whereas the surface is called superhydrophilic when the contact angle on the surface is lower than 10°. Superhydrophobicity is an important property for water-repellent [10], self-cleaning [1,11,12], drug release [13,14], anti-icing [15] and anti-corrosion [16,17], etc. Generally speaking, there are two ways for the preparation of superhydrophobic material. Considering the surface chemistry, it is well known that wetting behavior of solid surface is determined by its outermost molecular-level structure [18,19], which is one way to control the wettability of material surface. The other way to change the wettability is to fabricate microstructure on material surface [2].
The surface with micro/nano-structures can be fabricated by many techniques, such as hydrothermal method [20], anodization method [21], sol-gel [22,23], laser etching [24-26], laser interference texturing [27-29], chemical bath deposition [30]. The traditional methods of fabricating micro/nano-structures on the glass surface include mechanical etching, lithography [31], coating technology [32], plasma etching [33]. However, these methods are still limited by complicated preparation process, expensive instrument, and vacuum environment. In comparison, laser fabrication technique has many advantages such as noncontact, short action time and ultrahigh peak power. The mechanism of ultrashort pulse ablating transparent materials has been studied by many researchers.
Picosecond laser ablates transparent
material, which is a complex process. In general, it is difficult for transparent material to produce linear absorption of the picosecond laser pulse energy. The nonlinear ionization mechanisms of transparent material mainly include avalanche ionization [34,35] and multiphoton ionization [36,37]. When a single pulse irradiates on the glass surface, the avalanche ionization occurs due to the nonlinear absorption of material surface. The formation of high density plasma is the main cause of material ablation. Ablated material is removed away, which leads to the permanent damage of material surface [38]. In this paper, we reported a novel, simple and easily-controlled method to fabricate a superhydrophobic glass surface by microstructuring of the surface using a picosecond laser and further chemical modification to lower the surface energy. The influence of different laser line intervals on the surface morphology and wettability of the glass was studied. The mechanism of superhydrophobicity was also discussed in this work. 2
Experimental
2.1 Materials Solar glass (20 mm × 20 mm × 2 mm) with refractive index of 1.51 and transmittance of 91.03% at 1064 nm was purchased from Wuhan Chaofeng Glass Co., Ltd. 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane (FAS-17), toluene, acetone, ethanol, chloroform and dichloromethane were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Nitrogen (N2) gas was purchased from Suzhou Jinhong Gas Co., Ltd. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane was used as low surface energy material and solar glass was used as substrate. The glass substrate was cleaned with toluene, acetone, ethanol sequentially for 30 min in an ultrasonic cleaner, and then dried in the stream of N2.
2.2 Preparation of superhydrophobic solar glass surface An Edgewave laser with an amplified Nd: YVO4 laser system (λ = 1064 nm) delivering ν = 10 ps pulses at a repetition rate of 400 kHz was used. The output power used is 14 W. The average pulse energy is 35 μJ (the calculated laser fluence is 11.15 J/cm2) and the spot size is 20 μm. The laser beam was shone on each sample surface fifty times at a scanning speed of 300 mm/s. In order to fabricate periodic microgrooves, the laser beam was focused vertically on the top of the glass surface as depicted in Fig. 1(a). An x-y scanning galvanometer (Scanlab, hurry SCANІІ 14) was used to control the laser beam over the glass surface in a pattern of lines in atmospheric environment. In order to induce a gradual evolution of the surface topography and wettability, six samples were irradiated with different laser line intervals. The picosecond laser beam was shone on sample surfaces periodically with a scanning step, i.e., period of 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, respectively. The periodic microgrooves were fabricated throughout the sample surface of 10 mm × 10 mm area. Before emeasuring th contact angle on the glass surface, the samples were cleaned in ultrasonic bath inside deionized water for 1 h and then placed inside plasma cleaner (SD-PL-2000, China) to remove dust or other impurities. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane and ethanol with a mass ratio 1:50 were added in mixed solvent under stirring at ambient temperature for 1 h. The samples were immersed into fluoroalkylsilane solution at 60 °C for 2 h and dried in a drying chamber at 120 °C for 60 min. 2.3 Characterization The morphology of the glass surface after laser irradiation was characterized by using field emission scanning electron microscope (FE-SEM) (JEOL, JSM-7001F) and three dimensional (3D) laser scanning microscope (VHX-1000, Keyence, Japan). The chemical features were determined by energy dispersive spectrometer (EDS, X-MaxN20, Oxford), X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250, America) and fourier transform infrared spectrometer (Nicolet Corporation, Nicolet Is50, America), respectively. Contact angles and sliding angles were measured by using a dynamic contact angle measuring device (Dataphysics Corporation, DCAT11, Germany) under ambient temperature. Deionized water droplets of about 5 μL were carefully dropped onto the sample surface through a syringe. All the contact angles were measured at five different positions. The transmittance of the sample was measured by UV-Vis-NIT spectrophotometer (UV-3600Plus, Japan). 3
Results and discussion
The SEM images of sample surfaces irradiated by picosecond laser at different laser line intervals are illustrated in Fig. 1(b). The SEM images and 3D morphology of the microstructured glass surface (at laser line interval of 60 μm) are illustrated in Fig. 2. As seen in Fig. 2(a) and Fig. 2(d), the treated surface has multiple parallel microgrooves with a period of 60 μm that corresponds to the horizontal step between two vertical scanning lines. Magnified SEM images in Fig. 2(b) and (c) show that extensive nano- and fine micro-structures superimpose onto both the valleys and ridges of the microgrooves. Closer examination of the microgrooves (Fig. 2b) reveals them to be rough-sided, V-shaped features. Fig. 2(b) also shows that the profile of microgrooves has extensive nanostructures. The nanostructures formed are in the shape of nanoprotrusions and nanocavities. In addition, the height of the nanoprotrusions is about 300 nm. From Fig. 2(c), it can be seen that the slag on the glass surface is very loose and has many nano holes. This hierarchical surface structures containing both micro- and nanoscale roughness can store more air and form a layer of air cushion between the liquid and the glass substrate. Fig. 2(d) shows the 3D morphology corresponding to Fig. 2(a) and Fig. 2(e) shows the profiles along the solid line in Fig. 2(d). As seen in Fig. 2(e), the depth of the microgrooves is about 26 μm. The contact angles of the original glass surface and the microstructured glass surface were measured separately. Fig. 3(a) shows that the contact angle on the original glass surface is 57°, indicating the original glass to be a hydrophilic material. After modification with 1H, 1H, 2H, 2Hperfluorodecyltriethoxysilane, the water contact angle on the glass surface increases to 92° (Fig. 3b). While after laser irradiation, all the glass surfaces exhibit superhydrophilicity with the contact angle of 0° (Fig. 3c). Compared with original glass surface, increasing the glass surface roughness makes the surface hydrophilicity increment [20]. The most fundamental reason for superhydrophilicity is the capillary effect [39]. When a water droplet contacts the surface of the groove-shaped arrays, water penetrates into the gaps of two adjacent pillars. Consequently, the water droplet can spread rapidly. On the other hand, surface chemical composition is another important factor to regulate surface wettability. It is well known that a solid surface is readily wetted when the solid free energy value is close to the liquid free energy [40,41]. After modification with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane, the water contact angles of the sample surfaces change with the laser line interval due to their different surface morphologies. At laser line interval of 50 μm and 75 μm, the contact angles are 172° ± 1° and 156° ± 2° (Fig. 3d-e), respectively. The result is consistent with the previous findings that the micro/nanoarchitectures of glass with low surface energy prepared by hydrothermal method exhibited
superhydrophobicity [20]. This hierarchical structure of material surface shows that superhydrophilic and superhydrophobic properties depend on the surface chemical composition [20,33,39]. Thus, a conclusion can be drawn that the superhydrophobicity of material surface is governed by both its chemical composition and geometrical structure. Fig. 3(f) shows an optical image of water droplets at different locations on the superhydrophobic glass surface, which indicates uniform surface superhydrophobicity. The volume of each water-drop is about 15 μL. According to Arkles theory [42], when fluoroalkylsilane contacts with glass, it first hydrolyzes to form siloxane molecules. Then dehydration reaction has occurred between siloxane and Si-OH groups of the glass substrate. The specific reaction process is shown in Fig. 4. Fig. 5(a) shows the FTIR spectrum of glass surface. The peaks were identified at 1251cm-1, 1096cm-1, 900cm-1 and 755cm-1 corresponding to C-F stretching mode, Si-O-Si antisymmetric stretching mode, Si-O bending vibration mode, Si-O symmetric stretching mode, respectively. EDS analysis was used to further investigate the composition of the surface. As shown in Fig. 5(b), the strong peaks corresponding to Si, O, Na, Mg, Ca and C could be clearly observed. Meanwhile, the presence of F in the modified glass surface indicates that the fluoroalkylsilane was deposited on the glass surface. The surface composition and chemical bonds were further investigated by X-ray photoelectron spectroscopy (XPS) and the results are shown in Fig. 5(c) and (d). Fig. 5(c) shows that the surface composition of the glass hasn’t changed after laser irradiation. The wide-scan spectrum of the modified glass surface is mainly composed of F 1s, O 1s, and C 1s, Si 2p signals (Fig. 5c). Compared with original and microstructured glass surface, an evident peak located at 688.5 eV is ascribed to the core level XPS spectrum of F 1s, which indicates the existence of fluorine species in the modified glass surface [43]. Fig. 5(d) shows that the C 1s core-level spectrum of original and microstructured glass surfaces can be curve-fitted into one peak component with binding energies (BE) at 284.8. However, the C 1s core-level spectrum of modified glass surface can be curve-fitted into three peak components with binding energies (BE) at 284.8, 291.5 and 293.8 eV, attributable to C-H, C-CF2 and C-CF3 species, respectively [44-46]. The characteristic peaks of C-CF2 and C-CF3 are consistent with the previous studies [39, 43-45]. Through the above analysis, the results indicate that 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane has been successfully deposited on the microstructured glass surface after chemical treatment. Fig. 3(c) shows that the glass surface exhibits superhydrophilicity with the contact angle of 0° after laser irradiation. After modification with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane, the glass surface
exhibits stable superhydrophobicity (Fig. 3d). However, the water contact angle on the modified original glass surface just increases to 92°. Thus, it can be concluded that the combination of low surface energy of fluoroalkyl silanes and micro/nano-structures leads to the superhydrophobicity. Transparency is another important property for the use of superhydrophobic glass surface. In order to detect the transparency of the as-prepared glass, we took the photograph of the sample above a picture to assess the transparency of the microstructured glass, which is illustrated in Fig. 6(a). Although the picture under the microstructured glass is a little fuzzy compared to original glass surface, still we can see the picture clearly. We have further measured the transmittance of the as-prepared glass surface using UV-Vis-NIT spectrophotometer, as illustrated in Fig. 6(b). The transmittance of the as-prepared glass surface in the visible spectrum was over 87% at the laser line interval of 75 μm, which decreased by 3% compared with the original glass. With the laser line interval decreased from 65 μm to 55 μm, the transmittance of the glass surface decreased accordingly, from 84.59% to 78.62%. We also found the microstructured glass is superoleophobic underwater (Fig. 7a). Blue boundary indicates a chloroform droplet on the microstructured glass surface in water. The contact angles for chloroform and dichloromethane (chloroform and dichloromethane droplets of about 8 μL) on the microstructured glass surface underwater were 163° ± 2° and 158° ± 2° (Fig. 7b), respectively. In addition, we found that water droplet on the modified original glass surface looks like a lens which has zoom function. Fig. 7(c) shows that water droplet on the original glass surface doesn’t has zoom function, while water droplet on the modified original glass surface can magnify the picture (Fig. 7d). As for the details of contact angle hysteresis, five states are possible for superhydrophobic surfaces: Wenzel’s state, Cassie’s state, the so-called “Lotus” state, the transitional state between Wenzel’s and Cassie’s states, and the “Gecko” state [47]. Based on the research results on the “lotus effect”, the wettability of solid surfaces mainly depends on two factors: surface chemical composition and structural roughness [48]. The superhydrophobic glass surface as we prepared shows the hierarchical structures containing both micro- and nanoscale roughness. The contact angle on the as-prepared glass surface is higher than 150° and the sliding angle can be ignored. Apparently, the as-prepared glass surface is in “Lotus” state (Fig. 8a) [47]. In composite wetting state, the area fraction of micro-pillar on the surface is defined as follows [26,38]:
Ss
f1
al
(a b ) l
Sp
a
(1)
ab
While for a nano-pillar on a micro-meter pillar in Fig. 8(b), the area fraction of nano-pillar on the surface can be written as [48]: f2
Ss '
Sp '
where
l
a1 l ( a 1 b1 ) l
a1 a 1 b1
,
a1
(2)
a 1 b1
is the length of the contact area.
For the dual-scale roughness 2-D pillar texture structures, the solid-liquid contact area fraction (
fs
) of
the substrate can be generally expressed as [48,49]: f s f1 f 2
a a b
a1 a 1 b1
a
(3)
a b
According to Cassie-Baxter’s equation [50], the static apparent contact angle can be obtained as follows: cos
ca
a a b
(1 c o s s ) 1
(4)
Defining a surface characteristic value, periodic space
A ( A b / a)
, we can obtain the following
equations:
ca
a a rc c o s (1 c o s s ) 1 a rc c o s a b
1 (1 c o s s ) 1 1 A
(5)
Since we fabricated groove-shaped arrays on the glass surface using the same pulse energy in the experiment,
f2
can be treated as an invariant constant. In composite wetting state,
the increase of laser line interval, but
ca
decreases as the
fs
fs
increases with
increases. Therefore, the contact angle
decreases with increasing of laser line interval. In the experiment, the contact angles of the glass surfaces ranged from 172° ± 1° to 156° ± 2° with the increase of laser line interval. This variation tendency is consistent with the Eq. (5). It has been reported that a hierarchical surface structure containing both micro- and nanoscale roughness elements is the optimum surface morphology to achieve superhydrophobicity [51,52]. The micro-groove arrays are composed of microscale pillars and nanoscale roughness features on their surfaces, which is favorable to obtain a superhydrophobic surface [39]. 4
Conclusions In summary, we fabricated superhydrophobic solar glass surface by microstructuring using a
picosecond laser and coating the surface with a layer of fluoroalkylsilane molecules. After laser irradiation, the hierarchical structures containing both micro- and nanoscale roughness were formed on the glass surface. We also investigated the influence of laser line interval on the static and dynamic wetting properties of the glass surface. By varying the laser line intervals, we obtained the maximal water contact angle of 172° ± 1°. In addition, the as-prepared glass shows excellent transparency and the
average transmittance of the as-prepared glass surface is higher than 87% in the visible spectrum. We also measured the static and dynamic contact angles for chloroform and dichloromethane on the microstructured glass underwater. The laser-assisted technique that we proposed is a simple and easilycontrolled method for fabricating glass-based superhydrophobic surface.
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Appendices
Fig. 1. (a) Picosecond laser processing system. (b) SEM images of the periodic microgrooves fabricated on solar glass surface at different laser line intervals.
Fig. 2. SEM images and 3D morphology of the microstructured glass surface (at laser line interval of 60 μm). (a) Periodic microgrooves; (b) Micrograph of microgroove and profile of the microgroove; (c) Micrograph of slag on the glass surface; (d) 3D morphology corresponding to Fig. 2(a); (e) Profiles along the solid line in Fig. 2(d).
Fig. 3. Water contact angles of glass surfaces: (a) Original glass surface; (b) Modified original glass surface; (c) Microstructured glass surface; (d) Microstructured glass surface after silanization (at laser line interval of 50 μm); (e) Microstructured sample surface after silanization (at laser line interval of 75 μm); (f) Photograph of water droplets on the superhydrophobic glass surface; (g) Contact angles on the as-prepared glass surface as a function of corresponding laser line intervals.
Fig. 4. Reaction of the fluoroalkylsilane and the glass substrate.
Fig. 5. (a) FTIR spectrum and (b) EDS spectra of the glass surface. (c) Widen scan of the glass surface. (d) C 1s core-level spectra of the glass surface.
Fig. 6. Transparency of the microstructured sample. (a) Photograph of the microstructured sample (at laser line interval of 75 μm) above a printed picture (blue boundary indicates the microstructured sample area, whereas the remaining part is the unstructured sample area); (b) Transmittance of the samples.
Fig. 7. (a) Optical image of chloroform droplet on the microstructured glass surface in water; (b) The contact angles for chloroform (top) and dichloromethane (down) on the microstructured glass surface underwater; (c) Photograph of water droplet on the original glass surface; (d) Photograph of water droplet on the modified original glass surface.
Fig. 8. (a) Noncomposite and composite (the “Lotus” state) wetting states; (b) geometrical parameters.