Durable Superhydrophobic Cotton Textiles with Ultraviolet-blocking

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Materials and Interfaces

Durable Superhydrophobic Cotton Textiles with Ultraviolet-blocking Property and Photocatalysis Based on Flower-like Copper Sulfide Lihui Xu, Xuanyu Zhang, Yong Shen, Ying Ding, Li Ming Wang, and Yu Sheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00254 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Industrial & Engineering Chemistry Research

Durable Superhydrophobic Cotton Textiles with Ultraviolet-blocking Property and Photocatalysis Based on Flower-like Copper Sulfide

Lihui Xu*, Xuanyu Zhang, Yong Shen, Ying Ding, Liming Wang and Yu Sheng

College of Fashion and Textiles, Shanghai University of Engineering Science, Shanghai 201620, People’s Republic of China *

Corresponding Author. E-mail: [email protected].

ABSTRACT: Durable superhydrophobic cotton textiles with ultraviolet-blocking property and photocatalytic activity were prepared by coating polydimethylsiloxane (PDMS)/copper sulfide (CuS) based on the copper sulfide with hierarchical mesoporous microstructure like flower, which was composed of self-assembled nanosheets. The treated textile showed the exceptional superhydrophobic surface whose water contact angle and shedding angle was 157.7 ± 0.9° and 7.2 ± 0.2º, respectively, due to the simultaneous introduction of hierarchical porous surface topography constructed by flower-like CuS and low surface tension PDMS layer on cotton fabric. The obtained superhydrophobic cotton fabric was also proved to possess high ultraviolet-blocking performance with a large ultraviolet protection factor (UPF) of 511.09 and good photocatalytic activity. The prepared superhydrophobic fabrics showed excellent

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ultraviolet-durability and laundering durability attributed to stable and firm PDMS adhesive layer. This simple approach can show great potential for wide application and prospect.

Keywords: superhydrophobic; ultraviolet-blocking; photocatalysis; flower-like CuS; durability


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1. INTRODUCTION In nature, many plants leaves like lotus leaves show superhydrophobicity which have a water contact angle up to 150°.1,2 Water droplets deposited on lotus leave surface with a small tilt angle lower than 10° easily roll away. The phenomenon is known as “lotus effect”,3,4 owing to the low surface tension waxy layer together with uneven microstructures of protrusions reported by Barthlott and Neinhuis in 1997.5 Learning from nature, research interests on fabrication of superhydrophobic surfaces have grown tremendously during the last decade mainly as they can be widely used for selfcleaning,6 anticorrosion,7 antiicing,8 dragreduction,9 non-wetting10,11 and oil-water separation.12 Lately, flexible superhydrophobic fabrics have become the focus study because of breathability, light weight, easy source of inexpensive raw materials, and potential applications.13,14

With

the

rapid

development

of

outdoor

textiles,

ultraviolet(UV)-blocking fabrics have received increasing attention because of harmful ultraviolet radiation. With people's increasing demand for products that ensure a comfortable and healthy life, multifunctional textiles have got more great interests continuously,15,16 in which superhydrophobic and ultraviolet-blocking fabrics are favored because they can be extensively used in industry, health-care, military and daily life.9,17,18 Generally speaking, superhydrophobicity can be obtained by the cooperation of topographical rough structures and low surface tension coatings. So far, a variety of techniques like sol-gel technique,19 chemical vapor deposition,20,21 plasma treatment22,23 and electrospinning24 have been employed to construct surfaces with superhydrophobicity.



However, the majority of techniques have some restrictions, like severe conditions (e.g. rigorous chemical processing),25 high cost of materials (e.g., fluorochemicals, carbon nanotubes, graphene),26 poor durability and tedious fabrication.27 To prepare superhydrophobic surfaces, topographical rough microstructures are usually obtained by introducing inorganic nanophase materials, like silica particles,28 carbon nanotubes,15 ZnO nanorods,29 and TiO2 particles,30 combined with a hydrophobic treatment. TiO2 nanoparticles and ZnO nanorods are good candidate materials and they also have excellent UV-shielding property. Therefore, some researchers prepared bifunctional superhydrophobic and UV-blocking fabrics with TiO2 or ZnO as well as low surface energy materials like fluoroalkylsilane, fluorocarbons or silicones.17,29 However, the powerful photocatalytic activity of TiO2 or ZnO as semiconductor characteristic material must be seriously considered.31,32 And various organic supports or coverings contacted with TiO2 or ZnO surface can be degraded in the presence of UV radiation.33,34 For the preparation of superhydrophobic and UV-blocking textiles with TiO2 or ZnO, TiO2 or ZnO showed photodegradation under UV light for fibers and low surface energy materials like hexadecyltrimethoxysilane which were contacted with TiO2 or ZnO surfaces, which would cause the treated textiles to achieve hydrophilicity during UV irradiation.35 Therefore, with regard to superhydrophobicity, UV-blocking durability is a key technique. In order to obtain superhydrophobic textile with UV-blocking durability, there had been several reports in suppression of photoactivity by depositing SiO2 on ZnO or TiO2 surface.36 Xue et al.37,38 reported the fabrication of fabrics with


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superhydrophobicity and UV-blocking performance using ZnO/SiO2 core/shell nanomaterial followed by hexadecyltrimethoxysilane hydrophobic modification. Wang et al.39,40 obtained superhydrophobic and ultraviolet-shieding cotton textile by using ZnO@SiO2 core/shell nanomaterial and hydrophobization with octadecyltrimethoxysilane. Nevertheless, the ZnO/SiO2 core/shell nanomaterial involved tedious fabrication. Otherwise, Zhao et al.41 prepared superhydrophobic cotton textiles with UV-shieding property with organic ultraviolet absorber. But some organic UV absorbers may be potentially hazardous to environment and humans. Furthermore, some studies have reported the preparation of superhydrophobic and UV-shielding cotton textiles with CeO2 sol or nano-Al sol followed by hydrophobic modification.42,18 But CeO2 and nano-Al showed relatively poor UV shielding and only had UV shielding for a certain band. Furthermore,

some

researchers

reported

the

preparation

of

simultaneously

superhydrophobic and photocatalytic material using TiO2 because of its excellent photocatalysis

and

chemical

stability.

For

example,

Lee

et

al.31

prepared

superhydrophobic surfaces with photocatalytic activity using mixtures of hydrophobic polydimethylsiloxane (PDMS)-coated nano-SiO2 particles and N-doping of TiO2. Crick et al.43 reported the formation of a material that was simultaneously both superhydrophobic and photocatalytic via aerosol-assisted cold-walled chemical vapor deposition (AACVD) using PDMS polymer and functionalized nano-TiO2 particles. However, as far as we know, little research reported the fabrication of superhydrophobic surfaces with UV-blocking and photocatalytic properties.



The semiconducting copper sulfide (CuS) has attracted considerable attention because of its excellent physical and chemical characteristics.44,45 In our previous report,46 the hierarchical flower-like CuS was prepared via solvothermal reaction. The obtained flower-like CuS was confirmed to have excellent photocatalytic activity. And the textile treated with the prepared CuS was also preliminarily proved to show UV-blocking property. However, little research had been conducted in applying hierarchical flower-like CuS to construct topographical rough structure for the preparation of superhydrophobic surfaces. The flower-like CuS with rough hierarchical structures consisted of many self-assembled nanosheets and there were lots of mesopores between nanosheets. The specific rough hierarchical surface topography and large fraction of air trapped between nanosheets of flower-like CuS which could effectively repel the water penetration may be more favorable to superhydrophobic surfaces. Due to the UV-shielding property and photocatalytic performance of flower-like CuS, the prepared superhydrophobic surfaces based on flower-like CuS also had UV-blocking property and photocatalytic performance. In this research, a simple approach was reported to prepare superhydrophobic fabrics with ultraviolet-blocking property and photocatalytic activity using hierarchical flower-like CuS and polydimethylsiloxane (PDMS) adhesive layer. PDMS with its essential low surface energy is a good candidate for fabrics treatments, as it is nontoxic, cheap and durable and it is worth mentioning that PDMS is also highly stable under UV light and could suppress photodegradation.9,31 The hierarchical flower-like copper sulfide


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was obtained via facile solvothermal reaction. The PDMS/CuS dip coating solution was prepared and used to coat cotton textiles via dip-cure approach. The prepared fabric showed outstanding UV-blocking property, good photocatalytic activity and durable superhydrophobicity which could withstand UV light and laundering with shearing forces. Cotton fabrics were highly covered with flower-like CuS and PDMS adhesive layer via the simple process in favor of fabricating stable textiles with superhydrophobicity, UV-shielding and photocatalytic property. This process is facile and may be potentially applied in various fields. The superhydrophobic textiles with UV-shielding property and photocatalytic activity could show widespread application in protective clothing, industrial fabrics (automotive, etc.), medical fabrics, military fabrics and everyday life uses, like umbrellas, covering fabrics, tent, camp textiles, household fabrics and advertising fabrics.

2. EXPERIMENTAL 2.1. Materials. The substrates were white woven pure cotton textiles (504 ends/236 picks), which have warp yarns of 29 tex, weft yarns of 36 tex and weight per unit area of 172.2 g/m2. Methylene blue, copper chloride, thiourea, 1,2-propanediol(PG), as well as isopropyl alcohol were analytical and provided by Sinopharm Chemical Reagent Limited Company (China). Polydimethylsiloxane (PDMS, Sylgard-184) and the corresponding curing agent were provided by Dow Corning Corporation of the United States.



2.2. Synthesis of flower-like copper sulfide. The copper sulfide was synthesized via simple solvothermal method according to our previous report.46 Briefly, a solution of copper chloride was first prepared by mixing copper chloride (0.02 mol) and PG (120 mL). The copper chloride solution was stirred vigorously at 120 °C for 20 min. Then, the thiourea solution (0.08 mol thiourea in 100 mL of PG ) was slowly dropped into the above copper chloride solution with constant agitation. After further stirring for 30 min, the reaction system was kept at 170°C for 5 h in Teflonlined stainless steel autoclave. After cooling, washing and drying, the flower-like copper sulfide was synthesized.

2.3. Treatment of cotton textiles. The dip-coating solution was prepared by adding flower-like CuS (4%), PDMS (4%) and curing agent (the weight ratio of PDMS/curing agent was 10: 1) into isopropyl alcohol and dispersed ultrasonically. Then the PDMS/CuS dip-coating solution was obtained and used to treat cotton textiles via dip-cure process. Pristine cotton textiles were dipped in PDMS/CuS dip-coating solution and ultrasonicated for 30 min with ultrasonic frequency of 50 KHz and ultrasonic power of 130W. After drying at 80 °C for 5 min and curing at 160 °C for 60 min, the treated textiles were obtained.

2.4. Characterization.


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Scanning electron microscopy (SEM, S-4800, Hitachi, Japan) was used to characterize the morphology. The Brunauer-Emmett-Teller (BET) characterization was conducted by Micromeritics ASAP 2020. Surface chemical compositions of fabrics were tested with X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI, USA). Wettability of the cotton textile was analyzed with the contact angle of 5μL deionized water drop using DSA30 contact angle system (Kruss, Germany). Ten different points for each sample were measured and averaged. The water shedding angle (WSA) was also tested to analyze hydrophobicity of the textile according to the literature47 with 15μL water drop. Five different points for each sample were measured and averaged. The ultraviolet-blocking

property

of

the

cotton

was

evaluated

with

Textile

Ultraviolet-blocking Analyser (UV-2000F, LABSPHERE, USA). The photocatalytic performance was tested towards photodegradation of methylene blue solution under ultraviolet light. The cotton fabric treated with PDMS/CuS (2.5cm × 2.5cm) and the pristine cotton fabric (2.5cm × 2.5cm) was placed in methylene blue aqueous solution (10 mL, 15mg/L), respectively. They were called PDMS/CuS-Cot-MB, and Cot-MB, respectively. 10 mL of methylene blue aqueous solution was called MB. The three samples PDMS/CuS-Cot-MB, Cot-MB, and MB were placed under UV irradiation for 8h. For methylene blue solutions of these three samples, ultraviolet-vis spectrums were tested by Shimadzu UV-2600 UV-vis Spectrophotometer every certain time after UV irradiation. The photodegradation efficiency was evaluated by degradation rate according to our previous report.46 The laundering durability of the coated textile was measured by



AATCC Test Method 61-2006. The treated textile sample was put into detergent solution with 50 balls and was laundered at 40 °C for 45 min. One laundering was equivalent to five ordinary washing cycles. Afterwards hydrophobicity test was performed. The tensile and tearing strength of cotton fabric were tested based on Chinese Standard GB/T3923-1997 and GB/T39172-1997, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphology of Flower-like CuS. Flower-like CuS was prepared through simple solvothermal method at 170°C for 5 h using copper chloride, thiourea and 1, 2-propanediol (PG) as the solvent. SEM was applied to characterize CuS morphology. SEM images of obtained CuS are presented in Figure 1. The obtained CuS had well defined micro-sphere structure with the uniform diameter of 2-5 μm. In SEM image of the as-obtained CuS with high magnification (see Figure 1b), the flower-like CuS hierarchical structures composed of self-assembled nanosheets could be clearly observed. In addition, it was obvious to see that there were open-pore structures for the crossed nanosheets (Figure 1b), which can highly enhance the specific surface area of CuS microspheres. It was speculated that when the mixture of copper chloride and thiourea was under hydrothermal treatment, the ligand complex could be reduced to release free Cu2+ and S2− and a great deal of CuS nuclei were formed and further grew into nanosheets. As nanosheets with high surface energy were not terrifically stable, they could interconnect. They would finally transform into more stable


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flower-like microstructure by self-assembly based on the oriented attachment mechanism.46,48 Therefore, CuS with flower-like hierarchical mircostructures was obtained.

Figure 1. Scanning electron microscopy photos of flower-like CuS at diverse magnifications: (a) ×1500, (b) ×22000.

3.2. BET Characterization. The rough porous property of flower-like CuS used for preparation of superhydrophobic surfaces has great important impact. The Brunauer-Emmett-Teller (BET) characterization of prepared flower-like CuS was carried out. Figure 2a showed the nitrogen adsorption-desorption isotherm of the as-obtained flower-like CuS. It was clear that flower-like CuS had specific surface area of 15.79 m2/g. In addition, Figure 2a presented type IV isotherm curves with the typical hysteresis loop mainly caused by weak absorption desorption interactions and porous structures, indicating that the flower-like CuS particles possessed mesoporous structures. The pore size distribution of the as-obtained flower-like CuS was also estimated using the Barrett-Joyner-Halenda (BJH)



process. As shown in Figure 2b, the flower-like CuS showed a narrow pore size distribution ranging from 5 to 15 nm. Furthermore, the results showed that the crystal nuclei of copper sulfide grew uniformly and the obtained flower-like CuS had mesoporous structures between the compact self-assembly nanosheets. Because of rough porous structure of flower-like CuS, superhydrophobic surfaces may be prepared by the cooperation of specific surface porous topography constructed by flower-like CuS and low surface energy layer.

Figure 2. BET analysis of the as-obtained flower-like CuS: (a) nitrogen adsorption-desorption isotherm and (b) pore size distribution.

3.3. Effects of Dip-cure Parameters on Superhydrophobicity. In order to prepare superhydrophobic cotton textile, cotton textile was treated by coating the PDMS/CuS composite material via dip-cure process. The dip-cure parameters (e.g. CuS concentration, PDMS concentration, curing temperature or curing time)


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significantly affected the hydrophobicity of treated cotton textiles. The water contact angle (WCA) and water shedding angle (WSA) were used to investigate the superhydrophobicity. Figure 3 showed dip-cure parameters influences on the superhydrophobicity of treated textiles.

Figure 3. Dip-cure parameters influences on the superhydrophobicity of treated cotton fabrics: (a) CuS concentration, (b) PDMS concentration, (c) curing temperature and (d) curing time.

In Figure 3a, when the cotton textile was only modified by PDMS without CuS, the treated cotton textile had a WCA of 133.4 ± 1.3º and a WSA of 22.1 ± 0.2°,



demonstrating that the low surface energy alone was almost impossible to realize superhydrophobicity. From Figure 3a, with the increase of CuS concentration, WCAs of the treated cotton textiles were enhanced obviously, and WSAs of the treated cotton textiles were reduced gradually. When the CuS concentration was more than 4%, the WCA and WSA of the treated fabric reached 157.7± 0.9° and 7.2 ± 0.2º, respectively and changed little in Figure 3a. It was shown that the incorporation of flower-like CuS particles onto the fabric resulted in introduction of topographical rough structures on coated textile and improvement of superhydrophobicity. Because flower-like CuS with rough porous structure had a large amount of nanosheets and a lot of pores ranging from 5 to 15 nm between nanosheets, a lot of air trapped between nanosheets could preferably repel water wetting. When CuS concentration was up to 4%, the surface topographical rough structure of coated textile tended to perfection and the CuS load on cotton textile surface reached the maximum. Therefore, superhydrophobicity of the coated fabric remained unchanged almost. From Figure 3b, with increasing PDMS concentration from 1% to 4%, the WCAs of the coated cotton textile were enhanced steadily and WSAs gradually decreased. The higher PDMS concentration resulted in the lower surface tension of coated cotton textile. Thereby, hydrophobicity of cotton textile was improved. The surface tension of coated cotton textile with PDMS concentration of 4% had almost reached minimum and the WCA and WSA of treated cotton textile were 157.7± 0.9° and 7.2 ± 0.2º, respectively. Surface tension of treated cotton textile was almost constant with PDMS concentration up to 4%. And hydrophobicity of coated cotton textile was barely


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changed. From Figure 3c and Figure 3d, curing in the dip-pad-cure process was certainly worth for the enhancement of superhydrophobicity. Perhaps curing promotes formation of PDMS/CuS composite film and binding of PDMS/CuS onto the fibers of the textile. When the curing temperature was up to 160 °C and curing time was up to 60 min, the treated cotton fabric had excellent superhydrophobicity.

3.4. Superhydrophobicity Analysis of Coated Cotton Textile. The hydrophobicity of the untreated cotton textile, cotton textile treated with PDMS alone and cotton textile treated with the PDMS/CuS were evaluated as shown in Figure 4, respectively. As we all know, water droplets can spread rapidly on pure cotton textiles due to plentiful hydrophilic groups of cotton fiber. The water droplets vanished quickly on untreated cotton textile surface from Figure 4a and Figure 4d. The cotton textile coated with PDMS alone showed a WCA of 133.4° (Figure 4b, Figure 3a). For cotton textile treated by PDMS, water drop can not freely roll and show adhesion. Therefore, it failed to realize superhydrophobicity merely by lowering surface energy without specific surface topography. By contrast, the cotton textile treated with PDMS/CuS displayed outstanding superhydrophobicity. Its WCA was 157.7± 0.9° in Figure 4c, and its WSA was 7.2 ± 0.2º (Figure 3b). In Figure 5, water drop (15 μL) instantaneously rolled off cotton textile coated with PDMS/CuS (Video S1). And the coated cotton textile had an inclination angle of 7.2°. It was indicated that cotton textile coated by PDMS/CuS had outstanding dynamic hydrophobicity. As a result, superhydrophobic cotton textiles can be



prepared with PDMS/CuS via combination of rough porous microstructure constructed by flower-like CuS and PDMS film. The introduction of flower-like CuS could obviously improve the superhydrophobicity of coated cotton textile. Some reasons may be as follows. On the one hand, the flower-like CuS with rough hierarchical structures consisted of many self-assembled nanosheets. The nanosheets had typical quantum size effects, resulting in obvious enhancement of surface unevenness. On the other hand, for the flower-like CuS with big specific surface area, there were lots of mesopores between nanosheets. The large fraction of air trapped between nanosheets of flower-like CuS which could effectively repel the water penetration was more favorable to superhydrophobicity.


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Figure 4. Images of water droplets on cotton textiles: (a) WCA of untreated textile, (b) WCA of textile treated with PDMS alone, (c) WCA of textile treated by PDMS/CuS, (d) digital images of coloured water droplets on untreated textile surface (left), textile treated with PDMS alone (middle) and textile treated with PDMS/CuS (right).

Figure 5. Images of water drop which rolled off cotton textile coated with PDMS/CuS.



Figure 6 showed schematic diagram of preparation of superhydrophobic cotton fabrics. The CuS particles were obtained via solvothermal method using copper chloride and thiourea. Polydimethylsiloxane (PDMS) with identical terminating methyl could lower surface tension in an efficient manner. Moreover, PDMS is a good candidate for fabrics treatments, as it is less reactive, nontoxic, relatively cheap and more durable than most polymers. In this study, the PDMS/CuS dip coating solution was obtained by mixing flower-like CuS with PDMS. Then cotton textile was immersed in PDMS/CuS ultrasonically, dried and cured. Ultrasonication for coating of cotton textile was essential to increase superhydrophobicity. Probably ultrasonication promotes binding of PDMS and flower-like CuS onto the fibers of the textile. The prepared PDMS/CuS dip coating solution was also coated on the clean surface of aluminium foil in order to investigate the PDMS/CuS film. After drying and curing at 160 °C for 60 min, PDMS/CuS film was characterized by SEM. From the SEM image of the PDMS/CuS film in Figure 6, flower-like CuS particles with memberane-like substance were observed. After PDMS/CuS dip coating solution was dried and then cured, PDMS/CuS approached to each other, resulting in coalescing of PDMS/CuS and interdiffusion and winding of PDMS. Curing melted the PDMS polymer. But the CuS particles kept flower-like micro-spheres. Therefore, the dense PDMS/CuS film with low surface energy layer of PDMS onto the porous hierarchical flower-like CuS particles was formed. Similarly, when the cotton fabrics treated by coating PDMS/CuS, the dense PDMS adhesive film


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could be formed on surface of flower-like CuS and onto the fibers of the textile and PDMS/CuS composite film can be firmly combined with cotton textile (Figure 7).

Figure 6. Schematic diagram of preparation of superhydrophobic cotton fabrics and PDMS/CuS based on flower-like CuS.

When cotton textile was treated with PDMS/CuS, nanosheets of flower-like CuS particles on microscale cotton fiber surface could engender dual-size rough hierarchical microstructure like “lotus”. These hierarchical rough structures based on flower-like CuS particles were covered with hydrophobic PDMS polymer adhesive layer. The compact self-assembly nanosheets stacking and lots of mesopores between nanosheets generated a necessary unevenness for air pockets.3 If water droplets were placed on cotton fabric coated by PDMS/CuS, a lot of gas was embedded caused by a large number of mesopores between nanosheets of flower-like CuS. The water-air-solid interface was produced and the water drop wetting was efficiently repelled. Therefore, water droplets were only in



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contact with tops of hierarchical rough structures because of air pockets.19 In other words, water droplet could sit on surface and roll-off easily, achieving superhydrophobicity. The Cassie-Baxter equation could be also used to analyze hydrophobicity of cotton textile coated with PDMS/CuS.49 This equation is given below. (1) Where θCB represents the evident water contact angle of uneven solid surface, fls and flv represents the liquid/solid and liquid/air contact area divided by the projected area, respectively.

flv represents air fraction of the rough surface with micropores. θs and θv

(θv =180°) represent water contact angle of smooth solid surface and air surface, separately. With θv =180°, Equation (1) can be modified as follows. (2) As a large fraction of air was embedded caused by lots of mesopores between nanosheets of flower-like CuS, the air fraction

f lv was dramatically increased.

According to Equation (2), the apparent WCA θCB on the rough cotton surface was increased obviously. That is to say, if water droplets are placed on cotton textile surface coated with PDMS/CuS, a lot of air is embedded under water droplets, actively preventing water wetting. Thereby, simultaneous introduction of specific porous topography constructed by flower-like CuS and low surface energy PDMS adhesive layer due to PDMS/CuS coating for cotton textile led to successful preparation of superhydrophobic surfaces.


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3.5. SEM Analysis of Cotton Fabrics. The cotton textiles surface morphology was characterized with SEM. SEM photographs of the untreated cotton fabric and cotton fabrics treated by PDMS and PDMS/CuS were presented in Figure 7, respectively. From Figure 7a, some fibrils and grooves on the untreated cotton ﬁber can be clearly observed. As shown in Figure 7b, cotton fibers treated by PDMS had relatively smooth surface due to PDMS film. However, cotton fibers coated with PDMS/CuS was densely covered with well defined flower-like CuS micro-spheres and it was clearly observed that the CuS porous hierarchical structures were composed of self-assembled nanosheets (Figure 7c). In addition, the flower-like CuS particles covered with memberane-like substance can be seen. The reason for this could be probably as following. When cotton fabric was treated by the PDMS/CuS, curing melted the PDMS polymer. But the CuS particles kept flower-like micro-spheres. As a result, the dense PDMS film was formed on surface of flower-like CuS, implying that cotton fabric was successfully coated by PDMS/CuS with low surface tension PDMS film onto porous hierarchical flower-like CuS particles.



Figure 7. SEM images of (a) untreated cotton textile, (b) cotton textile coated by PDMS and (c) cotton textile coated by PDMS/CuS.


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3.6. XPS Characterization. The cotton textile was treated with PDMS/CuS to prepare superhydrophopbic surfaces. The prepared superhydrophobic textile was investigated by XPS. The corresponding XPS spectrums were presented in Figure 8. As for untreated textile, only C and O peaks at 288 eV and 534 eV were observed in Figure 8a. By comparison, from Figure 8a, for cotton textile treated with PDMS/CuS, two sharp peaks attributed to C1s and O1s could be detected, respectively. Two more characteristic Si2s and Si2p peaks successively appeared at 155 eV and 102 eV. Additional characteristic peaks of Cu2p and S2p emerged at 932 eV, 953eV and 163 eV. The Cu2p, S2p and Si2p peaks were also shown in Figure 8b, Figure 8c and Figure 8d. Furthermore, the C1s and O1s showed large increase. The reason for this is probably as following. Polydimethylsiloxane (PDMS) with a large number of methyl groups and silicon-oxygen-silicon bonds was bonded with flower-like CuS with Cu and S elements on cotton textile surface, bringing about the increase of carbon and oxygen. The XPS characterization also demonstrated successful coating of PDMS/CuS composite material onto the cotton fiber.



Figure 8. XPS spectrums of (a) untreated cotton textile and cotton textile treated with PDMS/CuS, (b) Cu2p spectra, (c) S2p spectra and (d) Si2p spectra of the cotton fabric treated by PDMS/CuS.

3.7. UV-blocking Property of Treated Cotton Textile. To evaluate UV-blocking ability of superhydrophobic cotton fabric, the effect of different CuS concentrations on the UV-blocking property of cotton textile treated by PDMS/CuS was investigated when the curing temperature was 160 °C, curing time was 60 min, and the PDMS concentration was kept as 4%. The UV-shielding performance of treated cotton fabric was evaluated by UVA transmittance and ultraviolet protection factor


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(UPF). From Figure 9, when CuS concentration was increased, UVA transmittances of treated cotton textiles were reduced gradually, and UPFs of the treated cotton textiles were enhanced obviously. When the CuS concentration was more than 3%, the UVA transmittance and UPF of the treated fabric changed little. However, in Figure 3a, with the increase of CuS concentration, superhydrophobicity of the treated cotton textile was enhanced. When the CuS concentration was more than 4%, superhydrophobicity of the treated fabric changed little. Taking into account combination of superhydrophobicity and ultraviolet resistance of the treated fabric, the CuS concentration was choosed to be 4% in our study.

Figure 9. The effect of CuS concentrations on the UV-blocking property of cotton textile treated by PDMS/CuS.

The ultraviolet transmittances and UPF values of different cotton textiles were characterized, as presented in Figure 10 and Table 1. Figure 10 showed the ultraviolet transmittance curves of untreated cotton textile, cotton textile treated with PDMS, and



cotton textile treated by PDMS/CuS in wavelength ranging from 290 to 450 nm. From Figure 10, two almost overlapped transmittances curves of untreated cotton textile and cotton textile treated with PDMS were observed and their transmittances increased apparently when the wavelength was increased. From Table 1, the untreated cotton textile showed relatively high mean UVA and UVB transmittance of 9.60% and 3.95% and low UPF of 22.39, respectively. Both cotton textile treated with PDMS and untreated cotton textile had similar UV transmittances and UPF values. It was indicated that untreated cotton textile and cotton textile coated with PDMS had almost no UV-blocking ability. In contrast, it was clear that the UV transmittance of cotton textile coated by PDMS/CuS decreased dramatically, compared to untreated cotton textile and cotton textile coated by PDMS. In Figure 10, the UV transmittance of cotton textile coated by PDMS/CuS was almost constant when the wavelength was increased. From Table 1, cotton textile treated by PDMS/CuS showed very low mean UVA and UVB transmittances of 0.26% and 0.08% and the UPF value increased dramatically up to 511.09, obviously exceeding outstanding UPF rating (50+). Therefore, it was confirmed that cotton textile coated with PDMS/CuS had excellent UV-blocking ability due to the presence of flower-like CuS. There had been several reports on the preparation of superhydrophobic and UV-shielding coatings based on TiO2 or ZnO. Wang et al.36 fabricated superhydrophobic surfaces with UV-blocking property with TiO2 and SiO2 nanoparticles. For the prepared surfaces, the UVB as well as UVC blocking rate were 100% and UVA blocking rate was about 85%. Wang et al.39,40 obtained cotton fabrics with superhydrophobicity and


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ultraviolet-shielding property which had low ultraviolet transmittance (T < 5.8%) and high UPF of 101.51 by coating ZnO@SiO2 nanorods and hydrophobic modification with octadecyltrimethoxysilane. By comparison, it may be indicated that flower-like CuS particles coating in our study is superior to TiO2 and ZnO particles coating for UV shielding. The excellent ultraviolet-shielding performance can be attributed to good absorption and scattering of TiO2 or ZnO particles.37 Firstly, TiO2 or ZnO nanoparticles have outstanding UV absorption performance because they can efficiently separate electron and hole pairs. Additionally, they have typical quantum confinement effects. Secondly, the ultraviolet blocking performance was also due to the strong scattering of TiO2 or ZnO nanoparticles. For the present research, flower-like CuS particles were obtained by facile solvothermal method. In our previous report,46 the band gap of prepared CuS was only 1.45 eV by calculation according to its UV–vis absorption spectrum. This could be primarily caused by typical quantum confinement actions of low-dimensional nanosheets of flower-like CuS. Compared with TiO2 and ZnO particles, the as-obtained flower-like CuS was a material with more excellent ultraviolet shielding property mainly because of its stronger absorption and scattering for UV owing to its special flower-like hierarchical microstructures. The reasons may be as follows. First and foremost, it might be caused by its strong UV-absorption due to the flower-like CuS hierarchical microstructures composed of the open-pore crossed nanosheets which have big specific surface area. A large number of self-assembled nanosheets of flower-like CuS particles have typical quantum confinement effects, resulting in strong UV-absorption.



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Secondarily, the light scattering of flower-like CuS nanosheets and frequent light reflection in lots of mesopores between nanosheets contributed to the ultraviolet blocking performance of cotton fabric treated by PDMS/CuS.

Figure 10. UV transmittance curves of (a) untreated cotton textile, (b) cotton textile treated with PDMS, and (c) cotton textile treated with PDMS/CuS.

Table 1. UV Transmittance and Ultraviolet Protection Factor of Cotton textiles UV transmittance (%)

Ultraviolet protection

UVA

UVB

factor (UPF)

Untreated cotton textile

9.60

3.95

22.39

Cotton textile treated by PDMS

9.24

3.67

23.43

0.26

0.08

511.09

Cotton textile treated by PDMS/CuS


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3.8. Photocatalytic Activity. In our previous report,46 the as-prepared flower-like CuS was confirmed to have good photocatalytic property. In the present article, superhydrophobic cotton textile was fabricated with PDMS/CuS composite material via dip-cure process. The photocatalytic activity of the superhydrophobic cotton fabric treated by PDMS/CuS was evaluated towards photodegradation of methylene blue solution under UV light. The cotton textile coated by PDMS/CuS and the pristine cotton textile were added to MB aqueous solution which was called PDMS/CuS-Cot-MB, and Cot-MB, respectively. MB aqueous solution was called MB. The UV-vis absorption spectra and degradation rate of these three samples PDMS/CuS-Cot-MB, Cot-MB and MB under UV irradiation were shown in Figure 11 and Figure 12. It was clear that for the sample MB without cotton textile, absorption spectra had little change before and after ultraviolet irradiation for 8 h. The degradation rate of the sample MB was much lower, which was only 5.19% likely due to the slight self-decomposition of organic macromolecules in methylene blue. Therefore, it is worth mentioning that the self-decomposition of methylene blue can be negligible under UV irradiation. But as for the sample Cot-MB, the absorbance of methylene blue solution decreased a little and the degradation rate reached 13.09%. This may be likely due to the adsorption of cotton fibers for methylene blue macromolecular organics, leading to the slightly lower solution absorbance. In contrast, in Figure 11 and Figure 12, for the sample PDMS/CuS-Cot-MB, it was clear that the absorbance of methylene blue solution decreased dramatically as UV irradiation time prolonging, compared to the



samples Cot-MB and MB. After UV irradiation for 8 h, the methylene blue was almost completely photodegraded and the degradation rate of methylene blue came to more than 96.61%. It was revealed that superhydrophobic cotton textile coated by PDMS/CuS had good photocatalytic activity. It may be because that the good photocatalysis of flower-like CuS was due to its typical microstructures. Hierarchical flower-like CuS microstructures consisted of lots of nanosheets. These nanosheets with big specific surface area could provide many active spots for photodegration of methylene blue. Furthermore, they could substantially improve the charge transformation and efficiently separate electron and hole pairs of CuS.46


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Figure

11.

UV-vis

absorption

spectrums:

(a)

MB,

PDMS/CuS-Cot-MB under UV irradiation.


(b)

Cot-MB,

and

(C)


Figure 12. Degradation rate of three samples MB, Cot-MB, and PDMS/CuS-Cot-MB under UV irradiation.

In addition, the cotton fabric treated with PDMS/CuS displayed excellent superhydrophobicity based on the above analysis. The thorough photodegradation of superhydrophobic coatings under ultraviolet irradiation should be highlighted. In photocatalysis, flower-like CuS absorbed UV to produce electron–hole pairs. They could react with O2 and H2O reported to be adsorbed on the CuS nanosheets to form strong oxidizer such as O2•− along with •OH radicals. These radicals could take part in the degradation of organic contaminants. It seems hard that a surface has both photocatalysis and superhydrophobicity. The reasons are as follows. With regard to photocatalytic activity, water is necessary to be chemically interacted with photocatalyst surface. In contrast, for superhydrophobic property, water droplets are finitely interacted with superhydrophobic surface. Nevertheless, in view of photocatalytic activity, water is molecularly interacted with solid surface. However, for superhydrophobicity, water is


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macroscopicly interacted with solid surface.31 It was reported that water could be molecularly interacted with the superhydrophobic surfaces.31 In our study, the cotton fabric treated with PDMS/CuS was added to methylene blue aqueous solution to investigate photocatalytic activity. The cotton fabric treated with PDMS/CuS floated on the water without sinking due to its superhydrophobicity. Water chemisorption was initially prevented because of superhydrophobic coating, leading to the lower photodegradation of methylene blue. As ultraviolet illumination time prolonged, superhydrophobic surface had more molecular-level interaction with water and showed better photocatalysis. It is worth mentioning that low surface tension PDMS layer is highly stable under ultraviolet irradiation. But methylene blue are easily decomposed under UV light.31,50 Therefore, cotton fabric coated by PDMS/CuS exhibited superhydrophobic property with photocatalytic activity.

3.9. UV-durable Superhydrophobicity. The hydrophobic properties of the cotton fabric treated by PDMS/CuS under UV irradiation for different time were investigated to evaluate the UV-durability of superhydrophobic cotton fabric. As shown in Figure 13, cotton sample treated by PDMS/CuS maintained highly hydrophobicity with WCA almost above 150° and WSA below 10° after UV exposure for 80 h. It was indicated that under long-term UV irradiation, PDMS had hardly any photodegradation and the cotton fabric treated by PDMS/CuS exhibited excellent UV-durable superhydrophobicity. Similarly, it had been



reported that some low surface energy polymers with Si-O groups (chemical bonding energy 460kJ/mol) like polydimethylsiloxane (PDMS) were highly stable under UV light and could suppress the photocatalytic degradation of TiO2 nanoparticles (band gap 3.2eV, energy: 309 kJ/mol) and degradation of UV (energy: 314-419kJ/mol).31,50-52 Otherwise, it was also reported that the prepared flower-like CuS had low band gap (1.45 eV) and showed good photocatalysis under UV/visible irradiation in our group.46 It may be because that polydimethylsiloxane (PDMS) had relatively high chemical bonding energy and could suppress photodegradation by flower-like CuS under long-term UV illumination. Besides, as the dense PDMS film was covered on surface of flower-like CuS, when the cotton textile was treated with PDMS/CuS, flower-like CuS could not contact with the coated cotton fiber and showed almost no photodegradation for cotton fibers because of deposition of PDMS film on flower-like CuS surface. Therefore, under long-term UV irradiation, superhydrophobicity of the cotton fabric treated by PDMS/CuS was sustained and the treated cotton fabric exhibited excellent UV-durable superhydrophobicity. Thereby, it is significative to prepare superhydrophobic and ultraviolet-blocking fabrics. As noted, PDMS/CuS coating affected the colour of treated fabrics, mainly due to the black flower-like CuS. After PDMS/CuS coating, treated fabrics became blacken. Further research is needed for various applications of PDMS/CuS coating.


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Figure 13. Hydrophobic properties of the cotton fabric treated by PDMS/CuS under UV irradiation for different time.

3.10. Laundering-durable Superhydrophobicity. Washable ability of superhydrophobic textile coated by PDMS/CuS was investigated. Laundering process is complex including shearing forces between the coated textile and water.53 Figure 14 showed the WCA and WSA changes with different laundering cycles. As shown in Figure 14, with increase of laundering cycles the WCA decreased slightly and still maintained above 150° after 20 laundering cycles, and the WSA increased slightly and remained below 12° after 20 laundering cycles, demonstrating excellent laundering durability of the superhydrophobic coating. The uneven surface texture with low surface tension of the fabric coated by PDMS/CuS was retained, though a small amount of PDMS film started slightly to peel off from cotton textile surface, thus resulting in a slight decline of WCA. Polydimethylsiloxane (PDMS) adhesive layer can firmly adhere to the surface of the cotton fabric and flower-like CuS without using other



additives.9,17 Therefore, PDMS and flower-like CuS were interconnected after washing and still bonded together firmly on cotton fiber surface, contributing to the durability of treated cotton fabrics. The water droplets on washed textile could still freely roll and show no evident adhesion, which indicates the excellent laundering durability of the prepared superhydrophobic textile.

Figure 14. Hydrophobic properties of cotton fabric coated by PDMS/CuS with different laundering cycles.

3.11. Mechanical Strength Properties of Cotton Fabric. For the sake of investigating the effect of PDMS/CuS coating and UV irradiation on mechanical strength properties of the cotton fabrics, the mechanical strength properties of the pristine cotton textile, cotton textile coated with PDMS, cotton textile coated with PDMS/CuS before and after UV irradiation for 80 h were measured. From Table 2, compared with pristine cotton textile, tensile and tearing strength of cotton fabric treated by PDMS increased slightly. It may be because that PDMS dense protective film which


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was formed on fiber surface protected fiber structure and PDMS film also acted as adhesive layer, making cotton fibers more tightly bound to each other. The untreated cotton textile and cotton textile coated with PDMS/CuS had similar mechanical strength properties in Table 2. Thus, the PDMS/CuS coating treatment caused little loss in the fabric mechanical strength. By comparison, the mechanical strength of cotton fabric treated by PDMS/CuS after UV irradiation for 80 h showed a slight drop. But the adverse effects of the treatment with PDMS/CuS coating and UV irradiation on mechanical strength of the coated fabric were not significant. Some reasons may be as follows. On the one hand, PDMS with highly stable chemical structure could suppress photocatalytic degradation of flower-like CuS under long-term UV illumination. On the other hand, when the cotton textile was treated with PDMS/CuS, PDMS as intermediate adhesive layer could firmly adhere to the surface of the cotton fabric and flower-like CuS, avoiding direct contact between flower-like CuS and the coated cotton fiber. Therefore, flower-like CuS showed almost no photodegradation for cotton fibers. As a result, the treatment with PDMS/CuS coating and long-term UV irradiation of the cotton fabric treated by PDMS/CuS caused very limited or negligible reduction in mechanical strength properties.



Table 2. Tensile and Tearing Strength of Cotton Samples Tensile strength

(N)

Page 38 of 46

Tearing strength (N)

warp

weft

warp

weft

Pristine cotton fabric

478.5

429.6

38.2

31.3

Cotton fabric treated by PDMS

488.2

440.1

40.7

33.5

Cotton fabric treated by PDMS/CuS

477.8

427.3

36.9

30.1

Cotton fabric treated by PDMS/CuS after UV irradiation for 80h

451.3

410.7

30.4

22.6

4. CONCLUSIONS Durable superhydrophobic cotton textiles with ultraviolet-blocking property and photocatalytic activity were successfully prepared by simply coating PDMS/CuS via dip-cure process. The hierarchical flower-like CuS prepared via simple solvothermal reaction had compact self-assembly nanosheets and mesoporous structures as confirmed by SEM and BET. The coated cotton textiles under optimal conditions (e.g. CuS concentration 4%, PDMS concentration 4%, curing temperature 160 °C and curing time 60min) showed outstanding superhydrophobicity with water contact angle of 157.7± 0.9° and water shedding angle of 7.2 ± 0.2º, which could be ascribed to the cooperation of a specific surface porous topography by hierarchical flower-like CuS and low surface energy PDMS adhesive layer. SEM and XPS analysis confirmed the successful coating of PDMS/CuS onto the cotton fabric surface. The prepared fabric also showed excellent UV-blocking property with UPF of 511.09 and UVA transmittance of 0.26% and durable superhydrophobic properties under UV irradiation because flower-like CuS had high UV absorbance properties and PDMS with high chemical bonding energy covered on surface


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of flower-like CuS could suppress photodegradation by flower-like CuS. The PDMS/CuS treated fabric showed excellent photocatalysis towards methylene blue photodegradation under ultraviolet illumination. The obtained superhydrophobic fabrics also exhibited excellent laundering durability. The PDMS/CuS coating treatment and UV irradiation caused very negligible reduction in mechanical strength properties of the cotton fabric treated by PDMS/CuS. This facile approach reveals a promising potential to prepare durable superhydrophobic coatings with UV-blocking property and photocatalytic activity on many substrates which can be applied in kinds of fields.

ACKNOWLEDGMENTS The study was supported by National Natural Science Foundation of China (51703123) and Innovation Project of Shanghai Municipal Education Commission (12ZZ180).

Supporting Information Video of dynamic hydrophobicity of cotton textile coated with PDMS/CuS (Video S1).

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Graphical Abstract 36x26mm (600 x 600 DPI)


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Durable Superhydrophobic Cotton Textiles with Ultraviolet-blocking

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