Fabrication and characterization of transparent superhydrophilic/superhydrophobic
silica nanoparticulate thin
films
Yan-Hong Lin
a, Kai-Ling Su
a, Ping-Szu Tsai
b, Feng-Lin Chuang
a, Yu-Min Yang
a,⁎
aDepartment of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
bDepartment of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan
a b s t r a c t
a r t i c l e i n f o
Article history: Received 27 August 2010
Received in revised form 22 February 2011 Accepted 22 February 2011
Available online 2 March 2011 Keywords:
Transparent Superhydrophilic Superhydrophobic
SiO2nanoparticulate thinfilm
The realization of transparent and superhydrophilic/superhydrophobic surfaces by silica nanoparticulate thin films was exploited in this work. An aqueous electrostatic layer-by-layer assembly process was utilized to fabricate nanoparticulate thinfilms with adhesion/body/top layer structure on glass substrates by using SiO2
nanoparticles and polyelectrolytes. The effects of volume ratio of differently sized silica nanoparticle solutions for the body layer deposition on transmittance in visible light region and surface wettability of the nanoparticulate thinfilms were systematically studied. The experimental results revealed that both optical transparency and superhydrophobicity/superhydrophilicity can be achieved on the same SiO2
nanoparticu-late thinfilm by using appropriate volume ratios of differently sized silica nanoparticle solutions for body layer deposition, and with and without silane treatment in the fabrication process. The high contrast of wettability that can be achieved by this way suggests the possibility of the creation of superhydrophilic/ superhydrophobic patterning and superhydrophilic–superhydrophobic gradient on the same surfaces.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
While a surface is usually called“superhydrophobic” when the advancing contact angle of water on it is larger than 150° with a very low contact angle hysteresis (the difference between advancing and receding contact angles)[1,2], a surface is called“superhydrophilic” when the advancing contact angle of water on it is less than 5°[3–6]
or the time for complete wetting by small droplets of water has been observed to be less than 0.5 s [6,7]. It is well-known now the wettability of naturally occurring solid surfaces with liquids is governed by both the chemical properties and the microstructure of the surface. Plant leaf surfaces that exhibit unusual wetting char-acteristics of superhydrophobicity have been investigated and documented [8,9]. In these leaves (including the lotus leaf) even dew and fog, but especially rain, lead to the complete removal of particulate contamination. This self-cleaning or Lotus effect[8,10]is caused by both the hierarchical roughness of the leaf surface from micrometer-sized papillae having nanometer-sized branch like protrusions and the intrinsic material hydrophobicity of a surface layer of epicuticular wax covering these papillae[11,12]. The natural surfaces have inspired researchers to consider biomimetic approaches for generating superhydrophilic, superhydrophobic, and superhydro-philic/superhydrophobic artificial surfaces. A recent review of the
design and creation of surfaces with special wettability such as superhydrophilicity and superhydrophobicity is available[13]. More-over, the preparation of artificial superhydrophilic/superhydrophobic patterned surfaces can then be possibly realized based on these methods.
While surface roughness is necessary for surface wettability, surface roughness must be minimized to reduce the light scattering so that light transparency can be achieved. Establishment of the appropriate surface structure length scale, therefore, is crucial to the fabrication of thinfilms that exhibit both properties. Xiu et al.[14]
demonstrated that a sol–gel process using a eutectic liquid can be invoked to form superhydrophobic, optically transparentfilms on glass slides. Previous investigations of the optical transparency of superhydrophobicfilms have also been reviewed by Xiu et al.[14]. Among them, an electrostatic layer-by-layer (ELbL) processing scheme that can be utilized to create transparent superhydrophobic films from SiO2nanoparticles of various sizes has been demonstrated
by Bravo et al. [15]. Their films consisted of three main parts: adhesion, body, and top layers. Bilayers of cationic and anionic polyelectrolytes were deposited to create the adhesion-promoting multilayer. For polyelectrolyte–silica body layers, two different sized silica nanoparticles were used. Furthermore, top layers containing only one-sized silica nanoparticles were also added to enhance the two-scale roughness necessary for superhydrophobicfilms. The final assembly was rendered superhydrophobic with sintering and silane treatment. Optical transmission levels above 90% throughout most of the visible region of the spectrum were realized in optimized coatings. Advancing water droplet contact angles as high as 160° with low
Thin Solid Films 519 (2011) 5450–5455
⁎ Corresponding author. Tel.: +886 6 2757575x62633; fax: +886 6 2344496. E-mail address:[email protected](Y.-M. Yang).
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contact angle hysteresis (b10°) were obtained for the optimized multilayer thinfilms.
As far as the microstructure of a surface is concerned, surface roughness is known to enhance the hydrophilic and hydrophobic properties [16–20]. Moreover, two-scale surface roughness is also known to be necessary for superhydrophobicity [21–25]. It is noteworthy that the volume ratio of the large sized nanoparticles to small sized nanoparticles in the mixture solution for the body layer deposition by Bravo et al.[15]wasfixed at 1:1. As the property of body layers is crucial to the surface roughness of the thinfilm fabricated, a question is therefore raised that what the composition effect of the two-sized nanoparticle solution for the body layer deposition is on the transmittance and surface wettability of the as-fabricated nanoparti-culate thinfilms.
In this work, the effect of the volume ratio of large to small sized silica nanoparticles in the mixture solution during the body layer deposition was systematically studied with the aim to elucidate the relationship between the volume ratio of differently sized nanopar-ticles and the transmittance and surface wettability of the nanoparti-culate thinfilms prepared with and without silane treatment. This problem is important in that the creation of superhydrophilic/ superhydrophobic patterning and superhydrophilic –superhydropho-bic gradient on the same transparent surface may then become possible with the ensuing transparent and superhydrophilic/super-hydrophobic properties.
2. Experimental section
The methodology used by Bravo et al.[15]was basically followed for preparing transparent and superhydrophobic silica nanoparticu-late thinfilms in this work. However, 8 compositions in the two-sized silica nanoparticle solution in addition to two one-sized silica nanoparticle solutions for the body layer deposition were examined. Meanwhile, the transparency and wettability of the as-fabricated nanoparticulate thin films before silane treatment were also investigated.
2.1. Materials
Ludox SM-30 colloidal silica 30 wt.% (7 nm diameter silica particles), Ludox TM-40 colloidal silica 40 wt.% (22 nm diameter silica particles), poly(allylamine hydrochloride) (PAH, Mw = 70,000 g/mol), and poly (acrylic acid) (PAA, Mw = 100,000 g/mol) were purchased from Sigma-Aldrich. Dodecyltrichlorosilane (C12H25Cl3Si, 99% pure) was purchased
from Fluka and used as the self-assembled monolayer component without further purification. Chloroform (99.9% pure), methanol (99.8% pure), and hydrochloric acid (30%) were supplied by Mallinckrodt. Sulfuric acid (95–97% pure) was obtained from Fluka. A standard buffer with pH 9.18, hydrogen peroxide (30%) and sodium chloride (99.8%) were purchased from Riedel-de Haën. All experiments were conducted with pure water that was passed through a Milli-Q plus purification system (Millipore, USA) with a resistivity of 18.2 MΩ cm.
2.2. Fabrication of multilayers
For thefilm deposition, a 24 mm×40 mm×0.1 mm glass micro-scope slide was thoroughly cleansed with rinsing liquids as indicated by a measured contact angle of less than 5° with water, The ELbL assembly process consisted of immersing the glass substrate repet-itively into different aqueous solutions by a computer-controlled dip coater.
As shown inFig. 1(a), the multilayers were composed of three main blocks including adhesion, body and top layers. The procedure and timing for the assembly of each bilayer remained constant during the entire process except for that of top layers. First, the glass substrate was dipped into the cationic solution for 15 min, followed
by one 2 min and 1 min rinsing steps using water. Then, the glass substrate was dipped into the anionic solution for another 15 min, followed by the same rinsing steps. For the assembly of top layers, however, only 5 min dipping time was used.
For the deposition of adhesion layers, PAH and PAA were used to create the cationic solution (0.01 M based on the repeated unit, pH 4.0) and anionic solution (0.01 M based on the repeated unit, pH 4.0), respectively. Five bilayers of this polyelectrolyte pair were deposited and represented by the notation: [PAH/PAA]5. For the deposition of
body layers, PAH (0.01 M, pH 7.5) and two differently sized silica nanoparticles (22 and 7 nm) were used to create the cationic and anionic solutions, respectively. By preparing 0.03 wt.% solutions for each silica nanoparticle in a pH 9.18 buffer solution, anionic solutions of 22 nm, 7 nm, and eight mixtures of 22 nm and 7 nm silica nanoparticles were created by mixing different volume ratios (a : b = 4:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, and 1:6) of each 0.03 wt.% solution. The volume ratio of silica nanoparticles is always given as 22 nm:7 nm. Therefore, 10 solutions with 22 nm SiO2nanoparticle
mass fractions of 1, 0.8, 0.67, 0.5, 0.33, 0.25, 0.2, 0.17, 0.14, and 0 were correspondingly prepared for the deposition of body layers. Specif-ically, NaCl (0.1 M) was added to the solutions containing the mixtures of 22 nm and 7 nm silica nanoparticles. The following notation will be used to represent these multilayers:[PAH/(a:b)SiO2]x,
where x is the number of body bilayers assembled. Thinfilms with different values of x were fabricated and optimized. Only the results of x = 20, however, will be reported for the sake of simplicity. For the deposition of top layers, PAH (0.01 M, pH = 7.5) and 7 nm silica nanoparticles (0.03 wt.%) were used as the cationic and anionic solutions, respectively. Three additional bilayers containing 7 nm SiO2
nanoparticles were added on top of the [PAH/(a:b)SiO2]20multilayers
system. Eventually, nanoparticulate thinfilms [PAH/PAA]5+ [PAH/(a:
b)SiO2]20+ [PAH/7 nm SiO2]3with different volume ratios of 22 and
7 nm silica nanoparticles in the body layers were fabricated. 2.3. Sintering and silanization of nanoparticulate thinfilm
The as-assembled nanoparticulate thin films were heated in a furnace at 550 °C under air atmosphere for 4 h to obtain sintered, stable and organic-free silicafilms as shown inFig. 1(b). Moreover, the as-fabricated nanoparticulatefilms were post-treated by a self-assembled monolayer of dodecyltrichlorosilane. Typically, the nano-particle deposited glass substrates were dipped into a 0.25 wt.% solution of dodecyltrichlorosilane in chloroform at room temperature for 1 h. They were then removed from the silane solution, washed with chloroform twice, and dried with nitrogen gas.
2.4. Characterization of nanoparticulate thinfilm
UV–vis spectroscopy measurements were made by a spectropho-tometer (Cintra 10e, GBC, Australia). Surface morphologies and cross-sections of the nanoparticulate thin films were examined with a scanning electron microscope (SEM, Hitachi S4100, Japan). Surface morphologies of plain glass and the nanoparticulate thinfilms were also examined by using an atomic force microscopy (Digital Instru-ments Inc., NanoScope IIIa) via the tapping mode. A silicon tip on a cantilever with a length of 100μm (noncontact silicon cantilever, Silicon MDT Ltd.) was used. The root-mean-square (RMS) roughness of the surfaces can then be calculated.
The static and dynamic contact angles were measured with water by using a contact angle meter (GBX, PX610, France) and a dynamic contact angle analyser (Thermo Cahn, WinDCA 300, USA), respec-tively. The dynamic method, commonly referred to as the Wilhelmy technique, is the basis of the dynamic contact angle analysis of the fabricated nanoparticulate thinfilms. This is a dynamic approach in which the wetting force at the solid/liquid/gas interface is automat-ically recorded via an electrobalance as a function of immersion depth.
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The effects of volume ratio (a:b) of differently sized (22 nm and 7 nm) silica nanoparticle solutions for the body layer deposition on transmittance in visible light region and surface wettability of the nanoparticulate thinfilms were systematically studied. Actually, eight compositions of the two-sized silica nanoparticle solution in addition to 22 nm and 7 nm one-sized silica nanoparticle solutions for the body layer deposition were examined. After high temperature sintering at 550 °C for 4 h, average transmittance higher than that of plain glass (91%) and advancing contact angle lower than 5° were exhibited by the as-fabricated nanoparticulate thinfilms with a:b held at 1:4 and 1:5. On the other hand, transparent, advancing contact angle around 170°, and contact angle hysteresis less than 10° were exhibited by the as-fabricated nanoparticulate thinfilms with a:b held at 1:4, 1:5, and 1:6 after a silane post-treatment. Therefore, both optical transparency and superhydrophilicity/superhydrophobicity were shown to be realizable on the same SiO2 nanoparticulate thinfilm by using a:
b = 1:4 or 1:5, and with or without silane treatment in the fabrication process. The high contrast of wettability that can be achieved by this way suggests the possibility of the creation of superhydrophilic/ superhydrophobic patterning and superhydrophilic –superhydropho-bic gradient on the same surfaces.
Acknowledgments
This work was supported by the National Science Council of Taiwan through Grants NSC 97-2221-E-006-118 and NSC 98-2221-E-006-063-MY2.
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