Block copolymer as surface modifier to monodisperse patchy silica

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Block copolymer as surface modifier to monodisperse patchy silica nanoparticles for superhydrophobic surfaces Shuo Lou, Junzheng Wang, Xiaohong Yin, Wenxiu Qu, Yuexiao Song, and Feng Xin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00166 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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Block

copolymer

monodisperse

patchy

as

surface silica

modifier

to

nanoparticles

for

superhydrophobic surfaces Shuo Lou †, Junzheng Wang*, †, Xiaohong Yin ‡, Wenxiu Qu†, Yuexiao Song †, Feng Xin*, †, and Fahim Abdo Ali Qaraah † †

‡

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of

Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China

ACS Paragon Plus Environment

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ABSTRACT: Monodisperse patchy silica nanoparticles (PSNPs) less than 100 nm are prepared based on the seed-regrowth method using a PEO-PPO-PEO-type block copolymer as surface modifier. Well-defined patches are controllably synthesized through area-selective deposition of silica onto the surface of seeds. After colloidal PSNPs are further modified with trimethylchlorosilane, the advancing and receding contact angles of water for PSNPs are 168± 2° and 167±2°, respectively. The superhydrophobic and transparent coatings on the various types of substrates are obtained by a simple drop-casting procedure. Additionally, almost the same superhydrophobicity can be achieved by using colloidal PSNPs via redispersing the powder of superhydrophobic PSNPs in ethanol.

KEYWORDS:

block

copolymer,

patchy,

silica

nanoparticles,

trimethylchlorosilane,

superhydrophobic.


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INTRODUCTION Superhydrophobic surfaces, defined as surfaces with a water contact angle larger than 150° and a water roll-off (tilt) angle less than 10°, have been extensively investigated over the last decade. Water droplets roll off (with some slip) easily from superhydrophobic surfaces and take dust particles along with them, providing outstanding self-cleaning, low adhesion, and drag reduction properties.1-4 Thus, they are of interest in a wide range of applications, such as antifouling,5 anti-corrosion,6,7 anti-icing,8,9 oil-water separation,10,11 energy conversions,12,13 and protections of electronic devices,14,15 etc. Generally,

the

wettability

of

a

solid

surface

is

determined

by

the

intrinsic

hydrophobicity/hydrophilicity of the material and surface roughness. Two theoretical models illustrate surface wetting properties: (i) Wenzel’s model describes the relationship between the contact angle of a liquid droplet and the rough surface;16 (ii) Cassie-Baxter’s model demonstrates that a gas phase may be trapped in the grooves of a rough surface, resulting in a solid-liquid-air interface.17 There are two main approaches to prepare superhydrophobic surfaces. The one is to construct surface roughness on initially hydrophobic materials by various methods including wax solidification,18 vapor deposition,19 plasma etching,20 and photolithography,21 etc. The other one is to modify a rough hydrophilic surface with low-surface-energy chemicals or deposit superhydrophobic colloids onto many types of surfaces of substrates such as paper, glass, wood, fiber, and metals, etc.22-26 Inspired by rose petals, lotus leaves, and other organisms in nature, raspberry-like patchy nanoparticles (NPs) are promising candidates for the fabrication of superhydrophobic surfaces due to their hierarchical structure.27-31 Small silica NPs (70 nm) were covalently grafted onto the big ones (700 nm) to form raspberry-like silica NPs, which were


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further covered by poly (dimethyl-siloxane) layer to obtain superhydrophobic surfaces.32 Similarly, raspberry-like polymer particles were produced via grafting small glycidyl-bearing particles (212 nm) onto the big ones (332 nm), leading to the formation of superamphiphobic coatings after further fluorination.33 Moreover, Carcouët et al. studied the influences of roughness on wetting properties of raspberry-like silica-silica particles, where the size of small silica particles (SS) ranged from 15 to 45 nm and the size of large silica particles (LS) ranged from 130 to 400 nm. It was demonstrated that contact angle hysteresis (CAH) on the surface was strongly connected to both the size of SS and the size ratio (r) of LS to SS. While superhydrophobic surfaces can be obtained only at the condition of the size of SS greater than 45 nm and r larger than 6.34 However, most raspberry-like patchy particles need a tedious fabrication process in which commonly expensive and toxic fluorine-containing reagents are used. It is also hard to generate uniform patchy NPs with dual-scale roughness in sub-100 nm scale range. Here we present a novel approach to preparation of patchy silica nanoparticles (PSNPs) via the regrowth of silica nanospheres (SNSs) by using a commercially available block copolymer Pluronic F127 (PEO100-PPO70-PEO100) as surface modifier in a liquid-liquid (tetraethoxysilane (TEOS)-water) biphasic system (Scheme 1). The uniform and size-tunable PSNPs with welldefined patches can be achieved in sub-100 nm scale. After modification of hydrophilic PSNPs with trimethylchlorosilane (TMCS), PSNPs exhibit outstanding superhydrophobic property with advancing and receding contact angles of 168±2° and 167±2°, respectively. This novel type of superhydrophobic PSNPs dispersed in ethanol can be coated on various flat and curved substrates, including but not limited to glass, silicon, and paper to obtain superhydrophobic surfaces. Our method can fabricate large-scale superhydrophobic surfaces by drop-casting


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without any special equipment. Meanwhile, it can also maintain superhydrophobicity when substrates are coated with colloidal PSNPs from redispersion of powder of superhydrophobic PSNPs in ethanol. Scheme 1. The formation process of superhydrophobic PSNPs and superhydrophobic surfaces.

EXPERIMENTAL SECTION Reagents and chemicals. Arginine (Arg, 98%) and Pluronic F127 were purchased from Sigma-Aldrich. Tetraethoxysilane (99%) and trimethylchlorosilane (99%) were purchased from Shanghai Aladdin Co., Ltd., China. Ethanol (99.5%), hexane (99%), chromic acid (98%), and ammonium hydroxide (NH4OH, 25~28%) were purchased from Tianjin Jiangtian Chemical Technology Co., Ltd., China. The glass, silicon wafer, and printer paper were purchased from the local market. Milli-Q water (18.2 MΩ·cm) was used for all experiments. All chemicals were used as received without further purification. Synthesis of SNSs and PSNPs. SNSs were prepared by the Yokoi method (100 nm) 38. In a typical Yokoi method, arginine (41.7 mg) was dissolved in deionized water (34.65 g), then TEOS (2.7 g) was added to the water-Arg solution. The biphasic


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reaction was carried out at 60 °C for 24 h under magnetic stirring (500 rpm) in a water bath. SNSs with a size of ca. 22 nm were obtained. Larger SNSs were obtained by regrowth of the SNS seeds in a dispersion containing TEOS, water, and arginine. In a typical Stöber method, ethanol (16.25 mL), deionized water (24.75 mL), and ammonium hydroxide (9 mL) were mixed in a beaker under magnetic stirring (1100 rpm). TEOS (4.5 mL) was added to the above mixture quickly. The reaction was performed under magnetic stirring (400 rpm) for 2 h. SNSs with a size of ca. 403 nm were obtained. The size of SNSs can be tuned by varying the addition amount of ethanol and ammonium hydroxide. PSNPs were prepared via the Pluronic F127-directed seed-regrowth method.36 Arginine (33.6 mg) was dissolved in deionized water (25 g), followed by the addition of Pluronic F127 (1.0 g) and the 80 nm SNSs seed suspension (5.8 wt % SiO2, 10 g). TEOS was added to the mixture step by step (≤1.0 g) over 24-48h. The biphasic reaction was carried out at 60 °C for 24 h under magnetic stirring (500 rpm) in a water bath. Preparation of hydrophobic and superhydrophobic films. The modification of a smooth surface with TMCS was achieved as follows. The glass substrate was soaked in chromic acid overnight to remove the surface contamination and activate hydroxyl groups. The substrate was ultrasonically cleaned with water, followed by rinsing with deionized water, and dried at 60 °C for 2 h in oven. Finally, the pre-treated substrate was soaked in a hexane solution containing sufficient TMCS. The reaction was carried out at 50 °C for 24 h. The hydrophobic treatments of PSNPs were achieved by modification of PSNPs with TMCS in a hexane solution. In a typical procedure, the PSNPs suspension (5.8 wt %, 95 nm, 16 g) was centrifugated, and then washed with ethanol (twice) and hexane (once). Subsequently, they were


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added to a hexane (30 g) solution containing TMCS (7 g). After ultrasonic treatment for 10 min, the reaction was carried out at 50 °C for 24 h under magnetic stirring (500 rpm) in a water bath. After washing with ethanol twice, part of the superhydrophobic PSNPs was ultrasonically dispersed in ethanol. The remained part of the superhydrophobic PSNPs was dried at 60 °C and ground to a fine powder using a mortar. The suspension was drop-cast onto a silicon substrate, and then the coated substrate was dried at room temperature overnight to form superhydrophobic film. In a typical procedure for water repellency test of superhydrophobic paper, superhydrophobic PSNPs powder (0.1 g) was added to ethanol (5 mL) to form PSNPs suspension through ultrasonic treatment for 5 min. The printer paper was soaked in the above PSNPs suspension for 5 s. The printer paper was taken out from the suspension and the excess liquid on the surface was removed by shaking. Then the paper sample was dried at room temperature overnight. Characterizations. Scanning electron microscopy (SEM) images were obtained using VE8800 (Keyence) and S-4700 (Hitachi) instruments. Transmission electron microscopy (TEM) images were obtained using JEM-2100F system. The root-mean-square roughness of the film was obtained using Atomic Force Microscope (AFM) instrument (NTEGRA Spectra, Russia). The optical transmittance of the film was measured in the visible range (400~800 nm) using UV2550 (Shimadzu, Japan). The FTIR spectra were obtained with Nicolet Nexus spectrometer. The static contact angles were measured using 3 µL water droplet on an optical contact angle meter (SL200KS, KINO, USA), which were obtained from the average values of measurements performed at five different positions on each surface. The dynamic contact angles were determined by calculating the difference between advancing CA and receding CA of water. This was achieved by the addition and withdrawal of water to a droplet using a microsyringe. In a


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typical procedure, a 3 µL water droplet was slowly increased to 6 µL, followed by sucking the water back from the droplet to 3 µL to determine the advancing CA and receding CA, respectively. Long-term and mechanical stability of the as-prepared superhydrophobic surfaces were evaluated by water-immersion experiment and sandpaper abrasion test, respectively. To evaluate the long-term stability of the coatings, the sample was immersed in deionized water for 264 h, and the wettability was measured after every 24 h. For the sandpaper abrasion test, the sample with a load pressure of 3 kPa was dragged on the sandpaper (1000 meshes) at a speed of 10 mm/s for a distance of 10 cm, and the wettability was measured after every 10 cycles.

RESULTS AND DISCUSSION Characterization of SNSs and PSNPs. The morphology and structure of SNSs and PSNPs as well as their films have been investigated by SEM and TEM. Uniform SNSs with sizes ranging from 50 nm to 403 nm are obtained (Figure 1a-e). The superhydrophobic PSNPs films are basically homogeneous in a large scale with a few small uncovered areas (Figure S1, See Supporting Information). Well-defined PSNPs with average size of 95 nm are found (Figure 1f), which are prepared by regrowth of 80 nm silica seeds with the aid of F127. The PEO segments of Pluronic F127 have the highest surface affinity for a hydrophilic surface, thus the micelles of triblock copolymer can adsorb onto the surface of SNSs (Scheme 1a).36, 37


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Figure 1. SEM and TEM images of silica NPs: (a) 50 nm SNSs, (b) 80 nm SNSs, (c) 95 nm SNSs, (d) 143 nm SNSs, (e) 403 nm SNSs, (f) PSNPs prepared from 80 nm SNSs, the insets show the corresponding size histogram obtained by statistical analysis of 90 PSNPs, (g) and (h) corresponding TEM images of PSNPs, the inset in (g) shows the corresponding size histogram of protrusions obtained by statistical analysis of 100 protrusions.


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Silica tends to deposit on the interspace among F127 micelles, leading to the formation of the nanosized protrusions with a narrow size distribution on the surfaces of silica seeds (Scheme 1b). TEM images (Figure 1g, h) show clearly that patchy structure is formed on the SNSs surface. With careful observation of PSNPs, one can find the nanosized protrusions with the average size of 11 nm on their surface (inset in Figure 1g). These results are consistent with those of SEM observations described above. No gap between the seed particles and the patchy part is observed. This suggests that the patchy structures are covalently attached to the silica surface by Si-O-Si bonds. The FT-IR spectra of the hydrophilic and superhydrophobic PSNPs are shown in Figure 2A. The peak at 2980 cm-1 can be ascribed to the C–H asymmetric stretching vibration. The peaks at 1460 cm-1 and 1380 cm-1 are ascribed to the bending of C–H bonds,39 suggesting that the –Si– (CH3)3 groups have been modified onto the surfaces of silica successfully. The wettability of the film is revealed by the water contact angle (WCA) on the surface. The variations of WCA and CAH on the films composed of different sizes of hydrophobic SNSs are displayed in Figure S2 and Figure S3, respectively. Uniform hydrophobic SNSs in the range of 80-143 nm show a relatively high WCA of ~150°, whereas superhydrophobic surfaces prepared from superhydrophobic PSNPs with 95 nm show an increase of the advancing WCA (168±2°) and the receding WCA (167±2°) as shown in Figure 2B. In contrast, the oil contact angle of superhydrophobic surfaces is close to 0° (Figure S4, See Supporting Information). The CAH is less than 1°, suggesting that using PSNPs with the dual-scale roughness is effective to improve superhydrophobic performance, similar to the superhydrophobic surface in nature.


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Figure 2. (A) FT-IR spectra of hydrophilic PSNPs (a) and superhydrophobic PSNPs (b). (B) Advancing water contact angle (a) and receding water contact angle (b) of the superhydrophobic film.

Two well-known models are used to describe the superhydrophobicity of the films. In Wenzel’s model16, the water droplets penetrate into the asperities of the surface and the WCA of a rough surface is connected with the surface roughness factor. The advancing WCA increases with increasing surface roughness factor, whereas the receding WCA remains almost unchanged, resulting in a high CAH. When the surface roughness factor increases to a critical value, air is trapped in the interstices of the rough surface, and water droplets contact with the composite surface that consists of solid and air pockets. Cassie and Baxter’s model17 is more suitable to


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describe the contact angle on the above composite surface. The intrinsic contact angle of smooth surface modified with TMCS is 109±1° (Figure S5, See Supporting Information). The films composed of smooth SNSs (50 nm and 403 nm) are in the Wenzel regime, while the films are prepared with smooth SNSs (80 nm, 95 nm, and 143 nm), Cassie-Baxter’s model works better to explain surface wettability. A special case of Cassie-Baxter’s superhydrophobic state- “lotus” state40 is suitable to describe PSNPs, since the dual-scale structure can capture more air between the solid-liquid interfaces. The surface roughness of the PSNPs coating was measured by AFM, the root-mean-square roughness (Rq) is about 38 nm (Figure S6, See Supporting Information). A more detailed study of the effects of the surface roughness (by adjusting the morphology of PSNPs with Pluronic F127) on the surface wettability is under progress. The stability of PSNPs powder and their application. Since the as-prepared film shows a high static contact angle and a low contact angle hysteresis, PSNPs are drop-cast to printer paper. Figure 3a clearly shows the nearly sphere-like water droplets that can stay on the superhydrophobic printer paper until they are disappeared completely due to evaporation. A water droplet (4 µL) on the surface of superhydrophobic PSNPs coated printer paper can maintain its sphere-like shape at least for 60 min, while the contact angle remains unchanged (Figure S7, See Supporting Information). In addition, the optical transmittance of the untreated glass substrate is about 90%, whereas the optical transmittance of the superhydrophobic glass substrate coated with PSNPs only decreases to 78% (Figure S8, See Supporting Information). The water repellency test of the superhydrophobic flat paper presents that the superhydrophobic paper can resist running water continuously (shown in the video S1 in Supporting Information). The coating process can be applied to various types of curved substrates. Water droplets become unstable on a superhydrophobic curved substrate and roll off the surfaces immediately once


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falling under gravitation (shown in the video S2 in Supporting Information). After redispersion of the dried PSNPs powder in ethanol (Figure 3b), stable colloidal PSNPs are obtained, which exhibit a pale blue color due to the Tyndall effect (Figure 3c). The superhydrophobicity of paper is maintained after drop-casting the above PSNPs dispersion onto printer paper (Figure 3d, e).

Figure 3. (a) Photograph for droplets of colored water (dyed with methyl green and rhodamine 6G,) on the printer paper treated by hydrophobic PSNPs; (b) powder of hydrophobic PSNPs; (c) redispersion of powder of superhydrophobic PSNPs in ethanol; (d) photograph of water droplet on printer paper treated by superhydrophobic PSNPs from (c); (e) corresponding water contact angle of sample in (d).

In order to assess the reusability of superhydrophobic PSNPs, their powders collected from drying the as-synthesized PSNPs colloid were redispersed in ethanol and then drop-cast to printer paper. The WCA and CAH of the treated printer paper were measured during every redispersion process. Figure 4a shows the relationship between WCA and CAH changes and the redispersion cycles. The WCA remains almost unchanged, while CAH increases with the increasing cycle number. Interestingly, the CAH maintains a low value less than 4° for water even after cycled for 25 times.


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Figure 4. (a) WCA and CAH changes with the redispersion cycles; (b) evolution of WCA and CAH values for PSNPs coated glass substrate as a function of immersion time (in deionized water); and (c) WCA and CAH changes with the abrasion cycles.

The long-term stability of the PSNPs films is evaluated by immersing the samples in deionized water. As Kulinich et al.41 discussed in their research, heptadecafluorodecyl-trimethoxysilane layers lose their hydrophobic properties after contacting with water for ~200 h. The PSNPs coated glass substrate immersed in the deionized water for a period of time, and it is clearly observed that the sample can maintain superhydrophobic state even after immersion in water for 260 h (Figure 4b), exhibiting excellent long-term stability. Mechanical stability of the asprepared superhydrophobic surfaces is estimated by sandpaper abrasion treatment. The variation in the wettability of the PSNPs coated paper substrate along with the number of abrasion cycles is shown in Figure 4c. The WCA of the sample decreases to 150° after 20 cycles, and the CAH shows a slight increase. For further abrasion, the WCA values steadily decreases to 135° and CAH values gradually increased to 20° after 90 cycles.

CONCLUSION In summary, we have successfully prepared superhydrophobic surfaces based on uniform and size-tunable PSNPs in sub-100 nm scale with well-defined patches. The dual-scale structure can


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capture more air between the solid-liquid interfaces.27-34 In our process, using PSNPs is effective to improve the surface superhydrophobic performance, which can reach an advancing and receding WCAs of 168±2° and 167±2°, respectively. Additionally, PSNPs with a size less than 100 nm can reduce the scattering and reflection of light, which is conducive to the transmission of light and the construction of a transparent superhydrophobic surface. Our approach has the advantages of the use of economical TMCS instead of expensive fluorine-containing reagents and creation of superhydrophobic surface in large-scale on printer paper even using redispersion cycled PSNPs. The PSNPs shows great potential as new building blocks and nanocarriers for diagnosis and therapeutic applications.


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ASSOCIATED CONTENT Supporting Information. The supporting information includes the SEM images of the superhydrophobic surfaces composed of PSNPs, the WCA and CAH of the films made from SNSs, the evolution process of a n-hexane droplet on the surface of superhydrophobic PSNPs coated paper, the intrinsic contact angle on TMCS treated smooth surfaces, the AFM characterization of the film made from PSNP, the evolution process of a water droplet on the surface of hydrophobic PSNPs coated printer paper, the optical transmittance of coatings prepared by different nanoparticles, and the water repellency test video and the demonstration of superhydrophobic substrate with curved surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Tel: +86-22-2740-9533. Fax: +86-222740-9533. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC) (No. 21776206).


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Block copolymer as surface modifier to monodisperse patchy silica

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